De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

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PAPER

De novo synthesis and properties of analogues of the self-assembling

chlorosomal bacteriochlorophyllsw

Olga Mass,^aDinesh R. Pandithavidana,^aMarcin Ptaszek,^aKoraliz Santiago,^a

Joseph W. Springer,^bJieying Jiao,^cQun Tang,^cChristine Kirmaier,^b

David F. Bocian,*^cDewey Holten*^band Jonathan S. Lindsey*^a

Received (in Gainesville, FL, USA) 12th July 2011, Accepted 9th August 2011

DOI: 10.1039/c1nj20611g

Natural photosynthetic pigments bacteriochlorophylls c, d and e in green bacteria undergo

self-assembly to create an organized antenna system known as the chlorosome, which collects

photons and funnels the resulting excitation energy toward the reaction centers. Mimicry of

chlorosome function is a central problem in supramolecular chemistry and artiﬁcial

photosynthesis, and may have relevance for the design of photosynthesis-inspired solar cells. The

main challenge in preparing artiﬁcial chlorosomes remains the synthesis of the appropriate

pigment (chlorin) equipped with a set of functional groups suitable to direct the assembly and

assure eﬃcient energy transfer. Prior approaches have entailed derivatization of porphyrins or

semisynthesis beginning with chlorophylls. This paper reports a third approach, the de novo

synthesis of macrocycles that contain the same hydrocarbon skeleton as chlorosomal

bacteriochlorophylls. The synthesis here of Zn(^II) 3-(1-hydroxyethyl)-10-aryl-13¹-oxophorbines

(the aryl group consists of phenyl, mesityl, or pentaﬂuorophenyl) entails selective bromination of

a 3,13-diacetyl-10-arylchlorin, palladium-catalyzed 13¹-oxophorbine formation, and selective

t

reduction of the 3-acetyl group using BH₃ꢀ BuNH₂. Each macrocycle contains a geminal dimethyl

group in the pyrroline ring to provide stability toward adventitious dehydrogenation. A Zn(^II)

7-(1-hydroxyethyl)-10-phenyl-17-oxochlorin also has been prepared. Altogether, 30 new

hydroporphyrins were synthesized. The UV-Vis absorption spectra of the new chlorosomal

bacteriochlorophyll mimics reveal a bathochromic shift of B1800 cm^ꢁ1of the

Q_yband in nonpolar solvent, indicating extensive assembly in solution. The Zn(II)

3-(1-hydroxyethyl)-10-aryl-13¹-oxophorbines diﬀer in the propensity to form assemblies

based on the 10-substituent in the following order: mesitylophenylopentaﬂuorophenyl.

Infrared spectra show that assemblies also can be formed in solid media and likely

involve hydrogen-bonding (or other) interactions of the ring E keto group.

protein scaﬀolds to deﬁne the proper relative distances and

Introduction

orientations for eﬃcient energy transfer. On the other hand, in

The light-harvesting antennas in photosynthetic organisms contain

green photosynthetic bacteria, the tetrapyrrole macrocycles

highly organized, three dimensional systems of pigments that

are organized in self-assembled structures (chlorosomes) with

little or no aid from a proteinaceous scaﬀolding.^2,3This highly

absorb light and funnel the resulting excitation energy to the

reaction centers.¹In plants and purple bacteria, photosynthetic

organized system encompassing as many as 250 000 pigment

pigments (chlorophylls and bacteriochlorophylls) are bound to

molecules collects light and supports rapid excitation energy

migration. Chlorosomes are perhaps Nature’s most spectacular

^aDepartment of Chemistry, North Carolina State University, Raleigh,

North Carolina 27695-8204, USA. E-mail: jlindsey@ncsu.edu

light-harvesting antennas and enable photosynthesis under

conditions of low ambient light intensity.

^bDepartment of Chemistry, Washington University, St. Louis,

It is important to note that the tetrapyrrole macrocycles in the

Missouri, 63130-4889, USA. E-mail: holten@wustl.edu

^cDepartment of Chemistry, University of California, Riverside,

California 92521-0403, USA. E-mail: david.bocian@ucr.edu

w Electronic supplementary information (ESI) available: Resonance

Raman data for ZnOP-He³M¹⁰and FbOP-He³M¹⁰in CH₂Cl₂

solutions and solid ﬁlms. ¹H NMR data for all new compounds. See

DOI: 10.1039/c1nj20611g

chlorosomes are termed bacteriochlorophylls but in fact are chlorins

(i.e., dihydroporphyrins) rather than true bacteriochlorins (i.e.,

tetrahydroporphyrins). The chlorosomal bacteriochlorophylls

(bacteriochlorophylls c, d, and e) are structurally heterogeneous

(i.e., not pure compounds) yet diﬀer only slightly from the plant

c

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resonance Raman spectroscopy,¹⁰and solid-state NMR

spectroscopy.^8,11,12The pattern of self-assembly of chlorosomal

bacteriochlorophyll molecules—which may diﬀer across diverse

organisms—remains a subject of intense debate. One of the

longstanding models proposed for the self-assembled architecture

is illustrated in Fig. 1.¹³In the proposed model, the hydroxy

oxygen of one macrocycle coordinates to the apical Mg(^II) site of

a second macrocycle, and the hydroxy proton forms a hydrogen

bond with the carbonyl oxygen from a third macrocycle. The

resulting two-dimensional lattice results in p–p interactions

between the chlorin macrocycles, which additionally stabilize

the self-assembled system. The organization shown in Fig. 1,

which implies long-range order of crystal-like periodicity, likely

represents an idealized limiting form given the heterogeneity of

the bacteriochlorophylls themselves. Several alternative models

have recently been proposed that retain key features of apical

coordination (and typically hydrogen bonding) yet pack the

macrocycles in other patterns.^12,14,15Structural issues that

remain poorly understood include the fundamental assembly

pattern of the tetrapyrrole macrocycles; the role and eﬀect of

length of diverse long-chain alcohols incorporated to form the

ester at the 17-propionate position;⁷the eﬀects of stereoisomeric

mixtures derived from the presence of R/S epimers of the

3-(1-hydroxyethyl substituent);¹⁶and the size, internal regularity,

and relative orientations of self-assembled domains of

bacteriochlorophylls within the chlorosome.⁷

The complexity and size of the chlorosomes has prompted a

large body of work on synthetic macrocycles to determine the

essential features for self-assembly.^13,17–19The groups of

Tamiaki^{13,18,20–25}and Wurthner^26–29extensively investigated

¨

semisynthetic chlorophyll and bacteriochlorophyll derivatives

to probe the inﬂuence of the nature and position of substituents

on the self-assembly pattern. Balaban^{13,14,16,17,19,30–34}explored

synthetic porphyrins bearing oxygenic (hydroxyalkyl and keto

groups, I, Chart 2) at the meso or b-positions, as well as cyano

substituents (II). Balaban also prepared chlorins by derivatization

of porphyrins (VIII).¹⁷Tamiaki also developed methodology for

converting octaethylporphyrin to porphyrin-based chlorosomal

bacteriochlorophyll mimics (III–VI).³⁵Almost all studies to date

of wholly synthetic macrocycles have employed zinc(^II) porphyrins,

though a recent study of magnesium(^II) porphyrins (analogues

of I) was reported.³⁶We synthesized porphyrin derivatives with

two oxygenic substituents at the meso or b-positions (VII).³⁷

However, X-ray structural studies of VII (5-hydroxymethyl

and 5,15-diphenyl; 3-hydroxymethyl and 5,15-di-p-tolyl)

revealed intermolecular interactions and packing patterns

quite diﬀerent from those suggested in Fig. 1. Balaban has

reported similar results.^14,16At a minimum, such results point

to the subtlety of structural features that engender assembly.

To build informative structural and functional mimics of the

chlorosomes requires access to model compounds that possess

essential molecular features of the natural bacteriochlorophylls

c, d and e. The minimum structural features appear to include

an a-hydroxyalkyl group and carbonyl group at the perimeter of

a metallochlorin. In the chlorosome neighboring tetrapyrrole

macrocycles are believed to be oriented such that their respective

Q_ytransition dipole moments are close to collinear, thereby

aﬀording strong exciton coupling. Because porphyrins are

planar oscillators whereas chlorins are linear oscillators,^38,39

Chart 1 Chief pigments of plants (top) and green bacteria (middle).

Nomenclature of 13¹-oxophorbine showing the reduced ring (D),

isocyclic ring (E), and spectroscopic axes (bottom).

photosynthetic pigments chlorophylls a and b (Chart 1).

Key structural diﬀerences include the presence of a mixture

of stereoisomeric 3-(1-hydroxyethyl) groups rather than the

3-vinyl unit; the absence of the 13²-methoxycarbonyl

substituent in the isocyclic ring; diverse substituents at the

R⁸, R¹², and propionate ester positions; and the presence of a

methyl group at position 20 (bacteriochlorophyll c and e).⁴

The structure of chlorosomes has been investigated by

diverse methods including cryoelectron microscopy,^5–7X-ray

scattering,^5,7,8absorption linear dichroism spectroscopy,⁹

c

2672 New J. Chem., 2011, 35, 2671–2690

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Fig. 1 One model for the assembly pattern of chlorosomal bacteriochlorophylls, showing coordination of the hydroxy group to the apical site on

magnesium, hydrogen bonding, and p–p stacking.

at the various b-pyrrolic and meso positions, and to install the

5-membered isocyclic ring (ring E, spanning positions 13 and

15).^40–45Chlorins that contain the isocyclic ring are termed

phorbines,⁴⁶which is the hydrocarbon skeleton for all

naturally occurring chlorophylls (including the chlorosomal

bacteriochlorophylls).⁴Herein we report extension of this

methodology to the synthesis of mimics of chlorosomal

bacteriochlorophylls. The resulting mimics are equipped with

the structural features that are believed to be essential for

self-assembly: (i) central metal with apical ligation site,

(ii) coplanar keto group, and (iii) a-hydroxyethyl group, with

the latter two substituents disposed on opposite sites of the

macrocycle. For appropriate electronic coupling to give rise to

eﬃcient energy transfer, as in the natural systems, the

a-hydroxyethyl and 13-keto groups are positioned along the

Q_yaxis. Three such 13¹-oxophorbines have been prepared,

which diﬀer only in the nature of a substituent (phenyl, mesityl,

or pentaﬂuorophenyl) at the 10-position. For comparison

purposes, a chlorin was prepared that contains a coplanar

keto group and an a-hydroxyethyl group located along the

Q_xaxis, which is perpendicular to the Q_yaxis (see lower right

structure in Chart 1). This de novo approach to the synthesis of

chlorin/13¹-oxophorbine macrocycles complements the

existing strategies for the semisynthesis of naturally derived

tetrapyrroles and the chemical synthesis of porphyrins.

Results and discussion

A. Synthesis of 13¹-Oxophorbines

(i) Retrosynthetic analysis. Our approach to chlorosomal

bacteriochlorophyll analogues relies on a de novo route to 3,13-

dibromochlorins.⁴⁰The route entails the directed formation of the

chlorin macrocycle by reaction of an Eastern half and a Western

half. Extension therefrom to the key precursor 3-acetyl-13¹-

oxophorbine FbOP-A³R¹⁰includes (a) palladium-mediated

Chart 2 Synthetic porphyrin and chlorin mimics of chlorosomal

bacteriochlorophylls.

acetylation of a 3,13-dibromochlorin (FbC-Br^3,13R¹⁰),⁴⁰(b) selec-

tive 15-bromination (under acidic conditions) of the resulting

it does not appear feasible to mimic the photophysical features of

the chlorosome with porphyrins alone, though valuable insights

concerning the structural requirements for assembly undoubtedly

can be gleaned by examination of diverse tetrapyrrole macrocycles.

Over the years we have been working to develop de novo

synthetic methods for preparing chlorins. A key feature of the

de novo methods is the ability to introduce substituents at will

3,13-diacetylchlorin,⁴⁴and (c) installation of the isocyclic ring by

palladium-mediated, intramolecular a-arylation.⁴¹The resulting

13¹-oxophorbine possesses two keto groups, one embedded in the

isocyclic ring and the second as the 3-acetyl substituent. Selective

reduction of the carbonyl group at the 3-position of the chlorin

will aﬀord the target 3-(1-hydroxyethyl)-13¹-oxophorbine FbOP-

He³R¹⁰(here we use ‘‘He’’ to denote the 1-hydroxyethyl unit).

c

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(ii) Chlorins FbC-Br^3,13R¹⁰. The requisite Western half

(8-bromo-2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin, 1) for the

syntheses of the target 13¹-oxophorbines has been prepared in

multigram quantities.⁴⁷The Eastern halves for the phenyl (2a)⁴⁸

and mesityl (2b)⁴⁰substituted target compounds also have been

reported. The synthesis of the pentaﬂuorophenyl substituted

Eastern half (2c) is shown in Scheme 1. Treatment of 1-formyl-

5-(pentaﬂuorophenyl)dipyrromethane⁴⁹with two equiv of

NBS at ꢁ78 1C aﬀorded the corresponding 8,9-dibromo

derivative 2c.

The dibromochlorin FbC-Br^3,13M¹⁰was recently prepared via a

streamlined procedure⁴⁵from 0.5 mmol quantities of 8-bromo-

2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin (1, Western half)⁴⁷and

8,9-dibromo-1-formyl-5-mesityldipyrromethane (2b, Eastern

half).⁴⁰Here, the same procedure at the 1.7 mmol scale aﬀorded

FbC-Br^3,13M¹⁰in 24% yield (versus 25% previously) (Scheme 2).

Dibromochlorin FbC-Br^3,13P¹⁰or FbC-Br^3,13Pf¹⁰bearing a

10-phenyl or 10-(pentaﬂuorophenyl) substituent was synthesized

in the same manner in 29% or 9% yield, respectively. It is

noteworthy that the three dibromochlorins were prepared in

0.13–0.53-g quantities, facilitating subsequent elaboration.

(iii) 3,13-Diacetylchlorins. The Pd-coupling of dibromo-

chlorin FbC-Br^3,13R¹⁰employed ﬁve mol equiv of tributyl-

(1-ethoxyvinyl)tin⁴¹in the presence of 20 mol% of (Ph₃P)₂PdCl₂

in CH₃CN/DMF (3 : 2) at 83 1C for 3.5–4 h. Hydrolysis with

10% aqueous HCl and puriﬁcation of the crude product gave

3,13-diacetylchlorin FbC-A^3,13R¹⁰(53–74% yield). Note that the

free base 3,13-diacetylchlorin FbC-A^3,13M¹⁰was previously

prepared from the zinc chelate ZnC-A^3,13M¹⁰via demetalation,⁴²

and the latter was obtained from the zinc chelate of the dibromo-

chlorin ZnC-Br^3,13M¹⁰via a multistep procedure that includes

demetalation, Pd-coupling, acidic work up, and remetalation.⁴⁰

(iv) Synthesis of 3-Acetyl-13¹-oxophorbines. Installation of

the isocyclic ring on the free base 3,13-diacetylchlorin

FbC-A^3,13R¹⁰was achieved by intramolecular ring closure of the

13-acetyl group and the 15-position of the chlorin macrocycle. The

15-bromo analogue of the 3,13-diacetylchlorin was synthesized

using a selective bromination strategy.⁴⁴The conditions of

selective bromination were studied with FbC-A^3,13M¹⁰. Thus,

treatment of FbC-A^3,13M¹⁰with 1 equiv of NBS in CH₂Cl₂/

TFA (10 : 1) resulted in incomplete reaction, aﬀording a B2 : 1

ratio of starting material/FbC-A^3,13M¹⁰Br¹⁵. The mixture was

not readily separable by column chromatography. Prolonged

reaction or higher temperature (50 1C, 1,2-dichloroethane/

TFA mixture) did not improve the outcome. The stepwise

treatment of FbC-A^3,13M¹⁰with 1.3 equiv of NBS in CH₂Cl₂/TFA

(10 : 1) at room temperature provided complete consumption of

Scheme 2 Synthesis of 3-acetyl-13¹-oxophorbine compounds.

the starting material and also yielded 30% of dibrominated

side product (according to the ¹H NMR spectrum of the crude

mixture). Analogous bromination of FbC-A^3,13P¹⁰and

FbC-A^3,13Pf¹⁰aﬀorded FbC-A^3,13P¹⁰Br¹⁵and FbC-A^3,13Pf¹⁰Br¹⁵,

Scheme 1 Preparation of the Eastern half for the pentaﬂuorophenyl-

substituted 13¹-oxophorbine.

c

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respectively (Scheme 2). With the 15-bromo-13-acetylchlorins

in hand, intramolecular ring closure⁴¹upon treatment with

(Ph₃P)₂PdCl₂(20 mol%) in the presence of 5–6 mol equiv of

Cs₂CO₃in THF at 85 1C in a sealed Schlenk ﬂask for 4 h

gave in 64–68% yield the desired 3-acetyl-13¹-oxophorbines

FbOP-A³R¹⁰.

number of such reductants under various conditions to achieve

selective reduction of the 3-acetyl moiety (Table 1).

Treatment of FbOP-A³M¹⁰with excess NaBH₄in THF/

MeOH (10 : 1) at room temperature resulted in fast reduction

of both keto groups to produce the corresponding diol

FbHP-He³M¹⁰(Table 1, entry 1). Reaction of FbOP-A³M¹⁰

with NaBH₄did not proceed at ꢁ78 1C (entry 2) whereas at

ꢁ20 1C to 0 1C (entry 3) a mixture of three products was

obtained: expected FbOP-He³M¹⁰(presumably as a racemic

mixture), 13¹-hydroxyphorbine FbHP-A³M¹⁰(presumably as

a racemic mixture) and 3,13¹-dihydroxyphorbine FbHP-He³M¹⁰

(presumably as a mixture of diastereomers). The expected

product could be separated by column chromatography;

however, it was diﬃcult to obtain fully reproducible results,

presumably due to the inaccuracy in temperature control and

the relative amount of sodium borohydride employed in these

vigorous, small-scale reactions. Stepwise treatment of

FbOP-A³M¹⁰with 2 mol equiv of a 1 M solution of BH₃ꢀTHF

in THF at 0 1C (entry 4) provided the expected FbOP-He³M¹⁰as

the major product in 42% isolated yield, together with small

amounts of FbHP-A³M¹⁰and FbHP-He³M¹⁰. The best

selectivity upon reduction was achieved when FbOP-A³M¹⁰

(v) Reduction of the 3-acetyl group. The selective reduction

of the 3-acetyl group of oxophorbine FbOP-A³M¹⁰to give

the macrocycle containing the 3-a-hydroxyethyl moiety

(FbOP-He³M¹⁰) is shown in Scheme 3. Several methods have

been described for the reduction (using NaBH₄,²⁴BH₃ꢀTHF,²⁰

t

BH₃ꢀMe₂S,²⁰BH₃ꢀ BuNH₂,²⁵and BH₃ꢀPhNEt₂)²⁰of the 3-acetyl

group in chlorophylls and their derivatives. We examined a

t

was treated with 6 mol equiv of BH₃ꢀ BuNH₂in anhydrous

CHCl₃at room temperature for 4.5 h (entry 5). Quenching the

reaction mixture with 5% aqueous HCl and stirring for 20 min

followed by organic extraction and column chromatography [silica,

CH₂Cl₂, then CH₂Cl₂/ethyl acetate (9 : 1)] gave FbOP-He³M¹⁰in

77% isolated yield; the side products 3-acetyl-13¹-hydroxyphorbine

FbHP-A³M¹⁰(10% yield) and diastereomers of 3,13¹-dihydroxy-

phorbine FbHP-He³M¹⁰(3% and 5% yield) also were isolated.

The workup procedure proved critical to obtain the desired

product: attempted quenching of the reaction with saturated

aqueous NH₄Cl followed by the same workup described above

resulted in full reduction of FbOP-A³M¹⁰to a diastereomeric

mixture of 3,13¹-dihydroxyphorbine FbHP-He³M¹⁰(presumably

t

because the excess BH₃ꢀ BuNH₂was not removed completely).

t

Application of the best conditions (6 mol equiv of BH₃ꢀ BuNH₂

in CHCl₃) to oxophorbines FbOP-A³P¹⁰and FbOP-A³Pf¹⁰

aﬀorded the corresponding free base analogues of the chloro-

somal bacteriochlorophyll mimics FbOP-He³P¹⁰and

FbOP-He³Pf¹⁰in 72% and 55% yield respectively (Chart 3).

(vi) Zinc insertion. Metalation of each of the three

hydroxyethyl–oxophorbines (FbOP-He³R¹⁰) with 15 mol equiv

of zinc acetate in CHCl₃/MeOH aﬀorded the corresponding

zinc(^II) chelate. Thus, ZnOP-He³P¹⁰, ZnOP-He³M¹⁰and

ZnOP-He³Pf¹⁰were obtained in good yield (Chart 4). The zinc

chelates (ZnOP-He³R¹⁰) are unstable in solution, especially in

chlorinated solvents (CH₂Cl₂, CHCl₃) resulting in demetalation

Scheme 3 Reduction to form a free base 3-(1-hydroxyethyl)-13¹-

oxophorbine.

Table 1 Reduction of FbOP-A³M¹⁰

Entry

Reductant

Solvent

Temp.

Product(s)

1

2

3

4

5

NaBH₄, excess

BH₃ꢀTHF

THF/MeOH

THF

RT

FbHP-He³M¹⁰

No reaction

ꢁ78 1C

ꢁ20 to 0 1C

0 1C

FbOP-He³M¹⁰+ FbHP-A³M¹⁰+ FbHP-He³M¹⁰(B1 : 1 : 1)

FbOP-He³M¹⁰(42%)^a

t

BH₃ꢀ BuNH₂

CHCl₃

RT

FbOP-He³M¹⁰(77%)^a

a

Isolated yield.

c

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Chart 3 Free base 3-(1-hydroxyethyl)-13¹-oxophorbines with various

R¹⁰substituents.

Chart 4 Target zinc(II) 3-(1-hydroxethyl)-13¹-oxophorbines.

Chart 5 Preferred sites of bromination of chlorins and oxochlorins.

and decomposition. Compound ZnOP-He³Pf¹⁰turned out to

be poorly soluble in organic solvents (CH₂Cl₂, THF, ethyl

acetate, MeOH, DMF), and hence the failure to obtain a

¹H NMR spectrum. For spectroscopic studies, the 3-acetyl-10-

phenyl-13¹-oxophorbine FbOP-A³P¹⁰also was converted to

the zinc chelate ZnOP-A³P¹⁰.

procedure⁵⁰[basic alumina (activity I) in toluene at 60 1C, DDQ

for 5 min at room temperature] (Scheme 4). Treatment of

OxoZnC-P¹⁰with one mol equiv of NBS in THF at room

temperature gave a mixture of several brominated products that

was not separable by column chromatography. Demetalation

of the mixture with TFA in CH₂Cl₂enabled separation of the

free base products. The major product was identiﬁed by

NOESY as OxoFbC-P¹⁰Br²⁰rather than the desired 7-bromo

derivative required for the target oxochlorin analogue of the

chlorosomal bacteriochlorophylls. On the other hand, demetalation

of OxoZnC-P¹⁰with TFA followed by treatment of the resulting

free base OxoFbC-P¹⁰with 1 mol equiv of NBS in THF at room

temperature aﬀorded the desired 7-bromo-17-oxochlorin

OxoFbC-Br⁷P¹⁰in 58% yield (Scheme 4). The strong

preference for 7- versus 8-substitution stems from shielding of

the latter site by the 10-phenyl substituent.⁴³The position of the

bromo group in OxoFbC-Br⁷P¹⁰was established by NOESY.

In summary, for the 10-phenyl-substituted macrocycles, the free

base oxochlorin exhibits a bromination pattern (7 4 15, 20)

that is distinct from that of the zinc oxochlorin (20 4 15, 7) and

the free base chlorin (15 4 7 4 20).

B. Synthesis of a 7-substituted oxochlorin

(i) Selective bromination of oxochlorins. Chlorins that

contain a keto group integral to the reduced ring (ring D) are

known as oxochlorins. Oxochlorins exhibit greater redox stability

than chlorins.⁵⁰Retrosynthetic analysis of a chlorosomal

bacteriochlorophyll analogue built around a 17-oxochlorin

macrocycle relies on: (1) regioselective bromination at the

7-position of the macrocycle; (2) selective reduction of the

7-acetyl group in the presence of the 17-keto group. Results

concerning the interplay of electronic and steric factors in

dictating the regioselectivity of bromination warrant mention

as a preface to the studies described below. First, bromination

of the chlorin FbC (which bears no substituents other than the

18,18-dimethyl group) with 1 mol equiv of NBS at room

temperature proceeds selectively at the 15-position given the

hindrance of the 20-position owing to the 18,18-dimethyl group,

while the second preferable sites of bromination are carbons 7 and

8 in equal extent (Chart 5).⁴⁴Second, the presence of aryl groups

at the 10- and 15-positions (e.g., FbC-M¹⁰P¹⁵) results in

preference for bromination at the 7-position.⁴³Third, the zinc

chelate of a 5,10-diaryloxochlorin (e.g., OxoFbC-Ar^5,10) under-

goes bromination selectively at the 20-position.⁵¹Fourth, the

20-position is 46-times more reactive than the 15-position in a

fully unsubstituted oxochlorin (OxoFbC) toward electrophilic

deuteration.⁵²Here, we investigated bromination of both the

zinc chelate and free base forms of a 10-phenyl-17-oxochlorin.

The 10-phenyl-17-oxochlorin (OxoZnC-P¹⁰) was prepared

in 52% yield from chlorin⁵³ZnC-P¹⁰via a two-step oxidation

(ii) Selective reduction. The 7-acetyl group was introduced

through palladium-coupling of OxoFbC-Br⁷P¹⁰with

tributyl(1-ethoxyvinyl)tin⁴¹in the presence of 14 mol% of

(Ph₃P)PdCl₂in CH₃CN/DMF for 3 h at 85 1C followed by

acidic hydrolysis to give 7-acetyl-oxochlorin OxoFbC-A⁷P¹⁰in

55% yield (Scheme 5). The reduction of the 7-acetyl group of

OxoFbC-A⁷P¹⁰was ﬁrst examined under conditions used

for selective reduction of the aforementioned 3-acetyl-13¹-

oxophorbines, namely treatment of OxoFbC-A⁷P¹⁰with 6 mol

t

equiv of BH₃ꢀ BuNH₂in CHCl₃at room temperature. The

conditions aﬀorded the 17-hydroxychlorin FbC-A⁷P¹⁰HO¹⁷in

70% yield. The mixture of diastereomers (FbC-He⁷P¹⁰HO¹⁷) was

isolated as a trace byproduct, but no presence of the desired

c

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Scheme 5 Reduction of a 7-acetyl-10-phenyl-17-oxochlorin.

upon treatment with excess NaBH₄in THF at room tempera-

ture aﬀorded OxoFbC⁰-He⁷P¹⁰in 71% yield. Deprotection

using reported conditions with some modiﬁcations⁵⁵(10%

aqueous HCl in THF at room temperature for 1.5 days) gave

the desired OxoFbC-He⁷P¹⁰in 50% yield. The oxochlorin

OxoFbC-He⁷P¹⁰was metalated with Zn(OAc)₂ꢀ2H₂O to

aﬀord OxoZnC-He⁷P¹⁰in 70% yield.

C. Chemical characterization

Altogether, 30 new macrocycles (chlorin, oxochlorin, oxophorbine

and derivatives thereof) were prepared and 4 macrocyclic

byproducts were isolated. Each macrocycle was characterized

by low-resolution (LD-MS) and high-resolution (ESI-MS)

mass spectrometry, and by UV-Vis absorption spectroscopy.

Each macrocycle also was characterized by ¹H NMR spectroscopy,

with the exception of ZnOP-He³Pf¹⁰, for which a solvent that

aﬀorded suﬃcient solubility was not identiﬁed. The hydroxyethyl

proton was not observed for any of the mesityl-substituted

chlorosomal mimics.¹³C NMR spectroscopy was attempted in

a number of cases; in many cases satisfactory spectra were

obtained whereas in other cases only a subset of the expected

set of signals was observed. Such cases are noted in the

Experimental section.

Scheme 4 Regioselective bromination of a 10-phenyl-17-oxochlorin.

7-(1-hydroxyethyl)-17-oxochlorin OxoFbC-He⁷P¹⁰was observed

by TLC analysis and ¹H NMR spectroscopy. The decreased

reactivity of the carbonyl group adjacent to the C₇QC₈double

bond was discussed previously by Tamiaki.²⁵The C₇QC₈double

bond is relatively isolated from the chlorin 18p-electron aromatic

system, which increases the conjugation of the carbonyl group.

The presence of diastereomers of FbC-He⁷P¹⁰HO¹⁷and the

absence of OxoFbC-He⁷P¹⁰together indicate that the 7-acetyl

group in OxoFbC-A⁷P¹⁰can be reduced with a stronger

reductant, albeit sacriﬁcing selectivity.

To overcome the problem of the reduction of the 7-acetyl

group of OxoFbC-A⁷P¹⁰, the 17-keto group was protected

(Scheme 6). Thus, following a procedure for ketalization,⁵⁴

OxoFbC-Br⁷P¹⁰was heated under reﬂux with ethylene glycol

and TsOHꢀH₂O in toluene for 20 h to aﬀord protected

oxochlorin OxoFbC⁰-Br⁷P¹⁰in 46% yield. Subsequent

Pd-mediated acetylation in the manner described for

OxoFbC-A⁷P¹⁰aﬀorded OxoFbC⁰-A⁷P¹⁰(84% yield), which

The synthetic oxophorbine bearing the 10-mesityl group

(ZnOP-He³M¹⁰) was characterized by the aforementioned set

of techniques as well as by ¹³C NMR and NOESY methods.

1

The H NMR spectrum of ZnOP-He³M¹⁰exhibits a doublet

at d = 2.04 ppm, attributed to the protons from the CH₃unit

of the 3-(1-hydroxyethyl) group, as well as a multiplet at

d = 6.15–6.70 ppm, attributed to the proton from the CH

unit of the same substituent. The NOESY spectrum exhibited

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1

satisfactory H and ¹³C NMR spectra, whereas the zinc chelate

was only characterized by ¹H NMR spectroscopy. (ii) The free base

analogue of the immediate precursor to the 10-(pentaﬂuorophenyl)

1

substituted oxophorbine (FbOP-A³Pf¹⁰) gave satisfactory H and

¹³C NMR spectra, whereas the zinc chelate target compound

(ZnOP-He³Pf¹⁰) was quite insoluble and was not characterized

by any form of NMR spectroscopy. (iii) The zinc chelate target

1

compound (OxoZnC-He⁷P¹⁰) was characterized by H NMR

spectroscopy. Regardless of the lack of NMR spectroscopic

characterization, the mass spectra were as expected throughout

the series of compounds that constitute each synthetic pathway.

D. Photophysical properties

The electronic absorption spectra of representative free base

oxophorbines are shown in Fig. 2. Reduction of the acetyl

group results in a signiﬁcant hypsochromic shift of the Q_yband

(603 cm^ꢁ1, l_max= 656 nm for FbOP-He³M¹⁰versus 683 nm for

FbOP-A³M¹⁰). The position of the wavelength maximum and

the relative intensity of the Q_yand B_xbands in FbOP-He³M¹⁰

are similar to those reported for 3-unsubstituted FbOP-M¹⁰.⁴⁴

The Q_yband maximum in isomeric FbHP-A³M¹⁰appears at

662 nm and has a lower relative intensity. The spectral band

positions and band intensity ratios are summarized in Table 2.

The ﬂuorescence yield and singlet excited-state lifetimes for

the target zinc oxophorbines (ZnOP-He³M¹⁰, ZnOP-He³P¹⁰

and ZnOP-He³Pf¹⁰), the corresponding free base oxophorbines

(FbOP-He³M¹⁰, FbOP-He³P¹⁰and FbOP-He³Pf¹⁰), and

several analogues are listed in Table 2. The various zinc

oxophorbines have similar ﬂuorescence yields (0.23–0.30)

and excited-state lifetimes (5.1–6.3 ns). The various free base

oxophorbines have comparable ﬂuorescence yields to each other

(0.26–0.32) and to the zinc chelates, but longer excited-state

lifetimes (9.9–13.0 ns). Note that the sets of zinc and free base

oxophorbines each include the parent compound for which the

only substituents are the geminal dimethyl groups (ZnOP and

FbOP);⁵⁶thus, the results indicate that the substituents at the

Scheme 6 Formation of the target 7-(1-hydroxyethyl)-17-oxochlorin.

the characteristic NOE between the CH in the 1-hydroxyethyl

substituent and aromatic proton at the 2-position of the

macrocycle.

Fig. 2 Absorption spectra (normalized at the B bands) in toluene at

room temperature of free base species including the 3-alkylphorbine

FbHP-He³M¹⁰(magenta), the 3-alkyl-13¹-oxophorbine FbOP-He³M¹⁰

(black), the 3-unsubstituted 13¹-oxophorbine FbOP-M¹⁰(blue), the

3-acetylphorbine FbHP-A³M¹⁰(green) and the 3-acetyl-13¹-oxophorbine

FbOP-A³M¹⁰(red).

In general, the three other target macrocycles with the requisite

structural features for self-assembly (zinc(^II) chelate and apposite

keto and 1-hydroxyethyl groups) proved diﬃcult to fully

characterize by NMR methods. (i) The free base analogue of

the 10-phenyl substituted oxophorbine (FbOP-He³P¹⁰) gave

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3- and 10-positions have little eﬀect on these photophysical

properties. Similarly, replacement of the ring keto

group with a hydroxy group gives compounds (FbHP-A³M¹⁰

and FbHP-He³M¹⁰) that have photophysical properties

comparable to the oxophorbines (FbOP-A³M¹⁰and FbOP-

He³M¹⁰).

of ZnOP-He³M¹⁰in THF or in n-hexane containing 0.3%

THF. The spectra have been normalized to have the same

integrated area (oscillator strength) across the spectral region

shown (to reﬂect the same total pigment content; see also

below).

E

In THF solution, ZnOP-He³M¹⁰exhibits a strong, sharp Q_y

band with maximum centered at 638 nm, characteristic for

monomeric species. In n-hexane containing 0.3% THF (v/v),

the Q_yband is intense and broad, and shifts to 720 nm

(a bathochromic shift of 82 nm, or 1785 cm^ꢁ1). The Soret (B)

band similarly shifts from 427 to 460 nm (33 nm; 1680 cm^ꢁ1). Such

spectral shifts, particularly that in the Q_yband, are characteristic

for self-assembled natural⁵⁸and semisynthetic^27,59analogues of

the chlorosomal bacteriochlorophylls in nonpolar solvents.

The monomeric form and formation of the assemblies of

ZnOP-He³M¹⁰were studied using a variety of conditions.

Fig. 4 shows spectra as a function of the concentration of this

compound in THF, in which solvent coordination to the central

zinc ion is expected to inhibit formation of the assembly. As

expected, the spectra can be ascribed to the monomeric species

throughout the concentration range studied; the absorbance at

637 nm depends linearly on ZnOP-He³M¹⁰concentration

(Fig. 4 inset). At the highest concentrations, which are far

above those normally used for photophysical characterization

studies, the sample is highly absorbant and virtually opaque,

yet remains a solution of the monomeric species.

Turning to the oxochlorins, the unsubstituted free base

compound OxoFbC studied previously⁵⁶and used here as a

benchmark has a ﬂuorescence yield (0.12) that is 2.4-fold

smaller than the average value for the free base oxophorbines

(0.29) and a singlet excited-state lifetime (8.4 ns) that is only

1.4-fold shorter than the oxophorbine average (11.6 ns). On

the other hand, zinc oxochlorin OxoZnC-He⁷P¹⁰, and the

parent OxoZnC reported previously,⁵⁶have an average

ﬂuorescence yield (0.031) and excited-state lifetime (0.84) that

are both about 7-fold smaller than those for the zinc oxophorbines

(Table 2). The latter values are comparable to those reported

previously for 5,10-substituted zinc oxochlorins.^50,57

E. Spectroscopic studies on self-assembly of ZnOP-He³M¹⁰

(i) Electronic absorption spectroscopy. The self-assembly of

ZnOP-He³M¹⁰(Chart 4) in solution was investigated using

UV-Vis absorption spectroscopy and a variety of solution

and preparative conditions, all at room temperature. The

parameters varied included concentration of the compound,

solvent (toluene, THF, n-hexane/THF mixture, Triton X-100

micellar solutions) and use of sonication. Somewhat limiting

cases are illustrated in Fig. 3, which shows absorption spectra

As illustrated in Fig. 3 (solid spectrum), the formation of

assemblies (aggregates) is apparent for ZnOP-He³M¹⁰in n-hexane

containing a small amount of THF to aid initial solubilization.

Fig. 5 shows absorption spectra for a 3.3 mg mL^ꢁ1(5.5 mM)

solution as a function of the percentage of THF in the medium.

With 7% THF the spectrum is that of the monomer and

essentially equivalent to that in 100% THF. With 3.7% THF,

the red-shifted absorption at 720 nm due to the assembly

begins to be seen (Fig. 5 inset). With 0.3% THF the monomer

feature at B640 nm is dramatically reduced as one would

expect due to greater formation of the assembly; however, a

proportionate rise in the intensity of the long-wavelength

feature at 720 nm is not observed, and in fact a diminution

is observed. (Similarly the absorption of the 1% THF solution

Table 2 Photophysical properties^a

B_maxQ_y(0,0) Q_y(0,0)

(nm) abs (nm) emis (nm)

b

c

I_B

I_Q_y

S_B

S_Q_y

Compound

F_f

t_f(ns)

Zinc chelates

ZnOP-He³M¹⁰426 642

ZnOP-He³P^{10 d}426 640

ZnOP-He³Pf¹⁰426 648

644

643

653

672

639

612

604

1.9 2.4 0.24 5.7

1.9 2.4 0.23 6.1

1.7 2.7 0.25 5.7

1.5 2.1 0.30 6.3

1.6 2.4 0.23 5.1

4.8 4.2 0.032 0.86

3.4 4.3 0.030 0.82

ZnOP-A³P¹⁰

449 665

419 637

e

ZnOP

OxoZnC-He⁷P¹⁰419 609

e

OxoZnC

411 602

Free base cmpds

FbOP-He³M¹⁰

FbOP-He³P¹⁰

FbOP-He³Pf¹⁰

FbOP-A³M¹⁰

FbOP-A³P¹⁰

FbOP-M^{10 g}

423 657

423 656

422 663

446^f683

446^f682

413 656

408 654

400 634

430 661

413 639

660

659

667

686

659

656

636

664

641

1.9 4.3 0.30 12.3

2.0 4.8 0.29 12.0

1.9 4.4 0.27 9.9

1.3 3.5 0.26 10.9

1.3 4.2 0.28 10.5

1.7 4.4 0.32 13.0

1.5 4.1 0.30 11.5

4.6 11.1 0.12 8.4

3.2 6.3 0.32 10.5

4.4 8.0 0.35 10.7

e

FbOP

OxoFbC

e

FbHP-A³M¹⁰

FbHP-He³M¹⁰

a

Data were acquired at room temperature for solutions in toluene,

except for ZnOP-He³P¹⁰, ZnOP-He³Pf¹⁰and ZnOP-A³P¹⁰, which

b

employed THF solutions. Ratio of the peak intensities of the Q_y(0,0)

band to the Soret (B) maximum, which could be either the B_y(0,0) or

c

B_x(0,0) band. Ratio of the integrated intensities of the Q_ymanifold

[Q_y(0,0), Q_y(1,0)] to the Soret manifold [B_y(0,0), B_y(1,0), B_x(0,0),

.

d

B_x(1,0)], for spectra plotted in cm^ꢁ1

in dilute solutions of the compound in THF and in toluene, with a

Comparable data are found

e

small amount of assembly present in the latter case. From ref. 56.

Fig. 3 Absorption spectra at room temperature of 3-(1-hydroxy-

ethyl)-13¹-oxophorbine ZnOP-He³M¹⁰in THF (dashed) and in n-hexane

containing 0.3% THF (solid).

f

Two peaks with comparable intensities are observed at 427 and

g

446 nm. From ref. 44.

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the assemblies (aggregates) fall out of solution and adhere to

the cuvette walls. The spectrum recorded depends on the

extent of mixing upon preparation and the subsequent time

to spectral acquisition. For example, the absorbance of the

assembly at 720 nm (Fig. 6A inset) for a 10 mg mL^ꢁ1solution

(red) is less than that for a 6.7 mg mL^ꢁ1solution even though a

larger amount of assemblies are produced in the former case,

but fall out of solution. Fig. 6B shows that assemblies (or some

fraction of them) can be returned to solution via sonication,

greatly enhancing the red-region feature. Due to such issues

concerning quantitation (and comparisons), the spectrum of

the monomeric species and that dominated by the assembly in

Fig. 3A are normalized to the same integrated intensity

over the near-UV and near-IR regions to correspond to

approximately the same total pigment content.

Several other points regarding the formation of assemblies

of ZnOP-He³M¹⁰can be noted. The assemblies formed in

n-hexane with a small amount of THF can be returned

signiﬁcantly (but typically not completely) to the monomeric

form by dilution in the same solvent. The assemblies can be

formed in aqueous Triton X-100 micellar solutions (the

compound is insoluble in water alone). The extent of formation

of the assemblies (as expected) depends on the concentration of

the oxophorbine and the detergent; starting near the critical micelle

concentration, appreciable assemblies are formed. Increasing the

pigment concentration shifts the system towards assembly,

while increasing the detergent concentration improves monomer

solubility and shifts the system towards the monomeric form.

Fig. 4 Absorption spectra as

a function of concentration of

ZnOP-He³M¹⁰in THF. The inset shows the absorbance at 637 nm

as a function of concentration along with a linear ﬁt. (Note that a

10 mg mL^ꢁ1solution of ZnOP-He³M¹⁰is 17 mM.)

is slightly smaller than with 2% THF.) These observations

reﬂect the inhomogeneous nature of the solutions with low

THF concentrations due to the large fraction of assemblies

that precipitate from solution and thus do not contribute to

the associated red-region absorption feature.

It was anticipated that the formation of the assemblies may

depend on interplay between the amount of THF in the

n-hexane solution and the concentration of the oxophorbine.

To complement the %THF dependence depicted in Fig. 5, a

study was performed in which the concentration of

ZnOP-He³M¹⁰was varied. The solvent consisted of n-hexane

containing 3% THF, which is the approximate proportion at

which the onset of formation of the assemblies was observed

for a 3.3 mg mL^ꢁ1solution of ZnOP-He³M¹⁰(Fig. 5). The

results are shown in Fig. 6A.

The 3.3 mg mL^ꢁ1solution in Fig. 6A is essentially the same

as that observed in pure THF (Fig. 3, dashed) and ascribed

to monomeric ZnOP-He³M¹⁰. Increasing concentrations

(6.7 mg mL^ꢁ1and above) show the presence of assemblies as

is evidenced by the absorption at 720 nm (Fig. 6A inset).

However, quantitation of assembly formation on the basis of

absorption in the red-region band is diﬃcult. As noted above,

Fig. 6 Absorption spectra of ZnOP-He³M¹⁰as

a function of

concentration in n-hexane containing 3% THF (A) and for two

diﬀerent preparations of a 3.3 mg mL^ꢁ1(5.5 mM) solution in n-hexane

containing 0.3% THF with and without sonication (B).

Fig. 5 Absorption spectra of a 3.3 mg mL^ꢁ1(5.5 mM) solutions of

ZnOP-He³M¹⁰as a function of %THF in n-hexane.

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Zinc oxophorbine ZnOP-He³M¹⁰is also soluble in toluene

to give solutions of the monomeric species. This point is

illustrated in Fig. 7 (dashed) for a 0.8 mg mL^ꢁ1(1.3 mM)

solution. Upon increasing the concentration about 5-fold

(Fig. 7 solid), the formation of the assembly is apparent by

the growth of the absorption at 720 nm relative to the 638-nm

Q_yband of the monomeric species (and by the sloping baseline

due to assembly particulates in the solution). Thus, for

ZnOP-He³M¹⁰the photophysical properties (ﬂuorescence

yields and excited-state lifetimes) of the monomeric species

could be investigated in dilute toluene solutions, as is our

standard protocol for synthetic chlorins and related macro-

cycles (Table 2).^42,56,57The replacement of the 10-mesityl

group with a phenyl group in ZnOP-He³P¹⁰aﬀords a complex

that is somewhat less soluble in toluene. This is evidenced for

very dilute (B1 mM) solutions by the presence of a red-shifted

feature similar to that found for ZnOP-He³M¹⁰in addition to

pronounced features due to the monomeric form (data not

shown). Incorporation of a 10-pentaﬂuorophenyl group in

ZnOP-He³Pf¹⁰aﬀords a compound that is virtually insoluble

in toluene and gives assemblies with little if any monomeric

species even for very dilute (o1 mM) solutions. On the other

hand, the analogous zinc oxochlorin OxoZnC-He⁷P¹⁰is highly

soluble in toluene. This compound remains in the monomeric

form without the appearance of spectral manifestations (red

shifted or broadened bands) or physical manifestations

(precipitates, cloudy solutions) of aggregation at far higher

concentrations than those at which ZnOP-He³M¹⁰or

ZnOP-He³P¹⁰readily form assemblies.

involving the aryl group. The greater extent of assembly for the

10-pentaﬂuorophenyl-substituted oxophorbine likely arises from

both decreased steric hindrance (versus 10-mesityl) and increased

intermolecular interactions (e.g., p-stacking) derived from the

electronic characteristics of the pentaﬂuorophenyl rings. On the

other hand, the free base analogues of all three zinc oxophorbines,

FbOP-He³M¹⁰, FbOP-He³P¹⁰and FbOP-He³Pf¹⁰(like oxochlorin

OxoFbC-He⁷P¹⁰) are soluble in toluene to give the monomeric

species. Collectively, the results demonstrate the ability to tune the

structural and electronic properties of the chlorosomal bacterio-

chlorophyll analogues toward the formation of assemblies for

further study of light-harvesting and energy-transport properties.

(ii) Vibrational spectroscopy. The assembly of ZnOP-He³M¹⁰

was further investigated using both resonance Raman (RR)

and IR spectroscopy. The focal point of these studies was the

13-keto substituent whose frequency is sensitive to hydrogen-

bonding interactions such as those that would occur in the

hypothetical assemblies shown in Fig. 1. The initial aim of the

vibrational spectroscopic studies was to probe the assemblies in

solution using RR spectroscopy that parallel the absorption

spectroscopic studies described above. However, examination

of the RR spectra of ZnOP-He³M¹⁰in solution (and in solid

ﬁlms) revealed that the band associated with the 13-keto

stretching vibration is too weak to identify with certainty

(Fig. S1, ESIw). Consequently, the focus turned to the IR spectra

of ZnOP-He³M¹⁰. The interpretation of the IR spectra of

ZnOP-He³M¹⁰in solution is severely compromised by inter-

ference from solvent bands. Thus, the IR spectra were acquired

of solids, both in KBr pellets and neat ﬁlms.

The pellet and ﬁlm IR spectra of ZnOP-He³M¹⁰are shown

in Fig. 8 (panel A). For comparison, the pellet and ﬁlm IR

spectra of the free base analogue, FbOP-He³M¹⁰, are also

shown in Fig. 8 (panel B). The key spectral region is

1650–1710 cm^ꢁ1, where the band associated with the 13-keto

stretching vibration is expected. [Note that the band in the

1720–1730 cm^ꢁ1region is not due to the 13-keto stretching

vibration as this band is observed for complexes that lack the

13-keto group (unpublished results).]

The above-noted results indicate that the propensity for

assembly formation increases in the following order

OxoZnC-He⁷P¹⁰

o

ZnOP-He³M¹⁰

o

ZnOP-He³P¹⁰

o

ZnOP-He³Pf¹⁰. The presence of the geminal dimethyl unit in

the same ring that bears the keto group in the zinc oxochlorin

must provide substantial steric constraints toward assembly

formation. This particular inhibition is removed in the zinc

oxophorbines, in which the geminal dimethyl group and keto

group are in separate rings. In turn, the greater assembly

formation for oxophorbines bearing a 10-phenyl versus 10-mesityl

complex can be understood by diminished steric hindrance

In the case of FbOP-He³M¹⁰, the pellet and ﬁlm samples

exhibit bands near 1685 cm^ꢁ1and 1705 cm^ꢁ1that are assignable

to the 13-keto stretching vibration. The relative intensity of these

bands depends on the concentration of the samples in the pellet

or in the solutions from which the ﬁlms were deposited. Previous

studies of chlorophylls have shown that 13-keto stretching

frequencies near 1705 cm^ꢁ1are associated with 13-keto groups

that are free from any type of interactions.⁶⁰On the other

hand, 13-keto stretching frequencies that are downshifted to

the 1685 cm^ꢁ1region are indicative of a subset of molecules in the

solid wherein the 13-keto group experiences hydrogen-bonding

interactions. The downshifted 13-keto stretching vibrations

observed for FbOP-He³M¹⁰could arise from hydrogen bonding

of the 13-keto group on one molecule with the a-hydroxyethyl

group of a neighboring molecule in the solid. This view is

supported by the observation that the relative amounts of free

versus interacting 13-keto groups in the solid samples are

dependent on concentration in the pellet or the solution from

which the ﬁlm was deposited.

Fig. 7 Absorption spectra of ZnOP-He³M¹⁰in toluene. The spectra

are normalized at the Soret (B) maximum to facilitate comparisons.

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interaction is stronger for the zinc chelate than for the free

base. Enhanced hydrogen bonding could arise if ZnOP-He³M¹⁰

forms an assembly with the types of interactions shown in Fig. 1.

In particular, ligation of the hydroxy oxygen of one molecule to

the zinc ion of a neighboring molecule would weaken the hydroxy

bond owing to transfer of electron density from the oxygen atom

to the metal ion. This in turn could result in a stronger hydrogen

bond between the hydroxy group and the 13-keto group of an

adjacent ZnOP-He³M¹⁰molecule.

Conclusions

The synthetic approaches developed previously to mimic

chlorosomal assembly and function rely on (i) modiﬁcation

of naturally occurring tetrapyrrole macrocycles (pioneered

chieﬂy by Tamiaki),¹⁸and (ii) synthesis and derivatization of

porphyrins (pioneered chieﬂy by Balaban).^17,19Here we have

introduced a third approach that relies on de novo synthesis of

chlorin macrocycles. While synthetically more intensive than

the aforementioned methods, greater control over structure

ultimately is available. This versatility is realized in the target

synthetic zinc oxophorbines that diﬀer in the 10-substituent

(mesityl, phenyl, pentaﬂuorophenyl); such compounds diﬀer in

the propensity to form assemblies, although all do so readily.

The ability to tune the steric and electronic characteristics of the

synthetic chlorosomal-bacteriochlorophyll mimics augurs well

for the use of these constructs for bioinspired light-harvesting

systems.

Experimental section

General

¹H NMR and ¹³C NMR (100 MHz) spectra were collected at

room temperature in CDCl₃unless noted otherwise. All

tetrapyrrole macrocycles were analyzed by laser desorption

mass spectrometry (LD-MS) in the absence of a matrix.

Electrospray ionization mass spectrometry (ESI-MS) data

are reported for the molecular ion or protonated molecular

ion. All commercially available materials were used as received.

All of the Pd-mediated coupling reactions were carried out

under argon using standard Schlenk-line procedures (e.g.,

three freeze-pump-thaw cycles were performed prior to and

after the addition of the palladium reagent, for a total of six

such cycles). Bromination reactions were performed using

freshly recrystallized NBS (from water). Anhydrous solvents

(CHCl₃, CH₂Cl₂, CH₃OH) were obtained from commercial

suppliers and used as received. Chromatography often was

performed with hexanes (a mixture of hexane isomers with bp

B68–70 1C). Sonication was performed in a benchtop bath.

Fig. 8 IR spectra at room temperature of (A) ZnOP-He³M¹⁰and (B)

FbOP-He³M¹⁰

.

The pellet and ﬁlm IR spectra of ZnOP-He³M¹⁰in the

13-keto region are quite diﬀerent from those of FbOP-He³M¹⁰.

In particular, the spectra of ZnOP-He³M¹⁰do not exhibit

bands attributable to the 13-keto stretching vibrations in the

1680-1705 cm^ꢁ1region. Instead, a band attributable to this

vibration is observed at near 1657 cm^ꢁ1. The frequency of this

band does not depend on the concentration in the pellet or

the solution from which the ﬁlm was deposited. The

much lower frequency for the 13-keto stretching vibration of

ZnOP-He³M¹⁰versus FbOP-He³M¹⁰indicates that the nature

of the interactions experienced by the 13-keto group are

diﬀerent for the two complexes in the solids. One plausible

explanation for the much lower 13-keto stretching frequency

observed for the zinc chelate is that the keto group of one

complex coordinates to the zinc ion of a neighboring complex.

Another plausible explanation is that the hydrogen-bonding

Noncommercial compounds

Compounds 8-bromo-2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin

(1),⁴⁷8,9-dibromo-1-formyl-5-phenyldipyrromethane (2a),⁴⁸

8,9-dibromo-1-formyl-5-mesityldipyrromethane (2b),⁴⁰1-formyl-

5-(pentaﬂuorophenyl)dipyrromethane⁴⁹and oxochlorin⁵¹

OxoZnC-P¹⁰were prepared following literature procedures.

c

2682 New J. Chem., 2011, 35, 2671–2690

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Mesityl-substituted compounds

was separated, dried (Na₂SO₄) and concentrated. Column

chromatography [silica, hexanes/CH₂Cl₂(1 : 2)] aﬀorded a

purple solid (96 mg, 60%): ¹H NMR (300 MHz) d ꢁ1.47

(br, 2H), 1.81 (s, 6H), 2.05 (s, 6H), 2.60 (s, 3H), 3.06 (s, 3H),

3.27 (s, 3H), 4.59 (s, 2H), 7.23 (s, 2H), 8.35 (d, J = 4.5 Hz,

1H), 8.50 (s, 1H), 8.83 (s, 1H), 8.86 (d, J = 4.5 Hz, 1H), 9.33

(s, 1H), 10.61 (s, 1H); LD-MS obsd 620.4; ESI-MS obsd

621.1854, calcd. 621.1860 [(M+H)⁺, M = C₃₅H₃₃BrN₄O₂];

l_abs(toluene) 425, 677 nm.

3,13-Dibromo-17,18-dihydro-10-mesityl-18,18-dimethylporphyrin

(FbC-Br^3,13M¹⁰). Following a streamlined procedure,⁴⁵

a

solution of 2b (762 mg, 1.70 mmol) and 1 (456 mg, 1.70 mmol)

in anhydrous CH₂Cl₂(45 mL) was treated with a solution of

TsOHꢀH₂O (1.62 g, 8.50 mmol) in anhydrous methanol

(11 mL) under argon. The red reaction mixture was stirred

at room temperature for 40 min. A sample of 2,2,6,6-tetra-

methylpiperidine (2.5 mL, 15 mmol) was added. The reaction

mixture was concentrated. The resulting solid was dissolved in

CH₃CN (170 mL) and subsequently treated with 2,2,6,6-

tetramethylpiperidine (5.5 mL, 33 mmol), Zn(OAc)₂(4.69 g,

25.5 mmol), and AgOTf (1.31 g, 5.10 mmol). The resulting

suspension was reﬂuxed for 20 h exposed to air. The crude

mixture was ﬁltered through a pad of silica (ethyl acetate), and

the ﬁltrate was concentrated. The resulting solid was dissolved

in CH₂Cl₂(70 mL), and the solution was treated dropwise with

TFA (1.90 mL, 24.7 mmol). After stirring for 2.5 h, saturated

aqueous NaHCO₃was slowly added to the reaction mixture.

The organic phase was washed (water and brine), dried

(Na₂SO₄) and concentrated. Column chromatography [silica,

hexanes/CH₂Cl₂(3 : 1)] aﬀorded a green solid (250 mg, 24%):

¹H NMR (300 MHz) d ꢁ1.80 (br, 2H), 1.84 (s, 6H), 2.04

(s, 6H), 2.60 (s, 3H), 4.62 (s, 2H), 7.24 (s, 2H), 8.40 (d, J =

4.4 Hz, 1H), 8.62 (s, 1H), 8.74 (s, 1H), 8.89 (d, J = 4.4 Hz,

1H), 8.94 (s, 1H), 9.07 (s, 1H), 9.82 (s, 1H); LD-MS obsd

613.3; ESI-MS obsd 615.0759, calcd. 615.0753 [(M+H)⁺,

M = C₃₁H₂₈Br₂N₄]; l_abs(toluene) 413, 651 nm. This

compound has been prepared previously in smaller quantity.⁴⁵

3-Acetyl-17,18-dihydro-10-mesityl-18,18-dimethyl-13¹-oxophor-

bine (FbOP-A³M¹⁰). Following a known procedure⁴¹with

modiﬁcation,

a

mixture of FbC-A^3,13M¹⁰Br¹⁵(94 mg,

0.15 mmol), Cs₂CO₃(248 mg, 0.759 mmol) and (Ph₃P)₂PdCl₂

(21 mg, 0.030 mmol) in freshly distilled THF (13 mL) under

argon was stirred for 4 h at 85 1C in a sealed Schlenk ﬂask. The

reaction mixture was concentrated, and the resulting crude

solid was dissolved in CH₂Cl₂. The organic phase was washed with

water, dried (Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated

to aﬀord a brown solid. Column chromatography [silica, hexanes/

CH₂Cl₂(1 : 2), then CH₂Cl₂] aﬀorded a purple solid (53 mg, 65%):

¹H NMR (400 MHz) d ꢁ1.64 (br, 1H), 0.72 (br, 1H), 1.87 (s, 6H),

2.03 (s, 6H), 2.58 (s, 3H), 3.23 (s, 3H), 4.30 (s, 2H), 5.15 (s, 2H),

7.23 (s, 2H), 8.43 (d, J = 4.4 Hz, 1H), 8.63 (s, 1H), 8.74 (s, 1H),

8.82 (d, J = 4.4 Hz, 1H), 9.31 (s, 1H), 10.56 (s, 1H); LD-MS obsd

539.8; ESI-MS obsd 541.2594, calcd. 541.2598 [(M+H)⁺, M =

C₃₅H₃₂N₄O₂]; l_abs(toluene) 427, 446, 683 nm.

3-(1-Hydroxyethyl)-10-mesityl-18,18-dimethyl-13¹-oxophorbine

(FbOP-He³M¹⁰). A sample of FbOP-A³M¹⁰(52 mg, 0.096 mmol)

t

in anhydrous CHCl₃(7 mL) was treated with BH₃ꢀ BuNH₂

3,13-Diacetyl-17,18-dihydro-10-mesityl-18,18-dimethylporphyrin

(FbC-A^3,13M¹⁰). Following a procedure for Stille coupling of

chlorins,⁴⁰a sample of FbC-Br^3,13M¹⁰(225 mg, 0.365 mmol),

tributyl(1-ethoxyvinyl)tin (620 mL, 1.84 mmol) and

(Ph₃P)₂PdCl₂(51 mg, 0.073 mmol) was stirred in CH₃CN/

DMF [18.5 mL (3 : 2)] under argon for 4 h at 83 1C in a

Schlenk line. The reaction mixture was treated with 10%

aqueous HCl (25 mL) at room temperature for 20 min.

CH₂Cl₂was added. The organic layer was separated, washed

(saturated aqueous NaHCO₃, water, and brine), dried

(Na₂SO₄), and concentrated. Column chromatography [silica,

hexanes/CH₂Cl₂(1 : 2)] aﬀorded a purple solid (147 mg, 74%):

¹H NMR (300 MHz) d –1.27 (br, 2H), 1.85 (s, 6H), 2.03

(s, 6H), 2.63 (s, 3H), 3.07 (s, 3H), 3.27 (s, 3H), 4.61 (s, 2H),

7.25 (s, 2H), 8.35 (d, J = 4.4 Hz, 1H), 8.82 (s, 1H), 8.87

(d, J = 4.4 Hz, 1H), 8.94 (s, 1H), 9.31 (s, 1H), 10.09 (s, 1H),

10.64 (s, 1H); LD-MS obsd 541.9; ESI-MS obsd 543.2751,

calcd. 543.2755 [(M+H)⁺, M = C₃₅H₃₄N₄O₂]; l_abs(toluene)

431, 687 nm. This compound has been prepared previously in

smaller quantity via a more elaborate route.⁴²

(49 mg, 0.56 mmol) at room temperature. After stirring for

4.5 h under argon, the reaction mixture was diluted with

CHCl₃and treated with 5% aqueous HCl (5 mL). The

reaction mixture was stirred vigorously for 20 min. The

organic phase was washed (5% aqueous HCl, water, saturated

aqueous NaHCO₃, and brine), dried (Na₂SO₄) and ﬁltered.

The ﬁltrate was concentrated to aﬀord a brown solid.

Column chromatography [silica, CH₂Cl₂, then CH₂Cl₂/ethyl

acetate (9 : 1)] aﬀorded a trace of unreacted starting material

(ﬁrst fraction), FbHP-A³M¹⁰(second fraction), title com-

pound FbOP-He³M¹⁰(third fraction), diastereomer 1 of

FbHP-He³M¹⁰(fourth fraction), and diastereomer 2 of

FbHP-He³M¹⁰(ﬁfth fraction).

Data for FbOP-He³M¹⁰(40 mg, 77%): ¹H NMR (300 MHz)

d ꢁ1.38 (br, 1H), 1.22 (br, 1H), 1.88 (s, 6H), 1.98 (s, 6H), 2.19

(d, J = 6.6 Hz, 3H), 2.58 (s, 3H), 4.21 (s, 2H), 5.08 (s, 2H),

6.38–6.45 (m, 1H), 7.22 (s, 2H), 8.39 (d, J = 4.4 Hz, 1H), 8.51

(s, 1H), 8.52 (s, 1H), 8.64 (d, J = 4.4 Hz, 1H), 8.76 (s, 1H),

9.44 (s, 1H), note that the hydroxy proton was not observed;

¹³C NMR d 21.4, 21.6, 26.1, 47.6, 48.5, 48.8, 65.4, 94.9, 101.4,

106.6, 115.9, 122.9, 126.4, 128.1, 132.73, 132.77, 133.1, 135.6,

135.8, 138.3, 138.9, 139.5, 142.2, 147.6, 150.0, 153.0, 154.6,

157.3, 177.8, 195.8; LD-MS obsd 542.2; ESI-MS obsd

543.2764, calcd 543.2755 [(M+H)⁺, M = C₃₅H₃₄N₄O₂]; l_abs

(toluene) 423, 657 nm.

3,13-Diacetyl-15-bromo-17,18-dihydro-10-mesityl-18,18-dimethyl-

porphyrin (FbC-A^3,13M¹⁰Br¹⁵). Following a known procedure⁴⁴

with modiﬁcation, a sample of FbC-A^3,13M¹⁰(140 mg,

0.258 mmol) in CH₂Cl₂/TFA [142 mL (10 : 1)] was treated

with NBS (36 mg, 0.26 mmol) at room temperature. After 1 h,

an additional amount of NBS (14 mg, 0.078 mmol) was added.

After 30 min, CH₂Cl₂was added. The mixture was washed

with saturated aqueous NaHCO₃and water. The organic layer

1

Data for FbHP-A³M¹⁰(5 mg, 10%): H NMR (300 MHz)

d ꢁ2.60 (br, 1H), ꢁ0.57 (br, 1H), 1.87 (s, 6H), 2.07 (s, 6H),

2.59 (s, 3H), 3.31 (s, 3H), 4.40 (s, 2H), 4.59 (d, J = 20 Hz, 1H),

c

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5.21 (dd, J = 20 Hz, J = 6.4 Hz, 1H), 6.44–6.46 (m, 1H),

7.24–7.26 (m, 2H), 8.46 (s, 1H), 8.46 (d, J = 3.8 Hz, 1H), 8.94

(s, 1H), 9.02 (d, J = 3.8 Hz, 1H), 9.41 (s, 1H), 10.9 (s, 1H),

note that the hydroxy proton was not observed; LD-MS obsd

542.6; ESI-MS obsd 543.2757, calcd 543.2755 [(M+H)⁺,

M = C₃₅H₃₄N₄O₂]; l_abs(toluene) 430, 661 nm.

stirred at room temperature for 40 min. A sample of

2,2,6,6-tetramethylpiperidine (4.5 mL, 27 mmol) was added.

The reaction mixture was concentrated. The resulting solid

was dissolved in CH₃CN (300 mL) and subsequently treated

with 2,2,6,6-tetramethylpiperidine (10.7 mL, 63.2 mmol),

Zn(OAc)₂(8.70 g, 47.4 mmol), and AgOTf (2.43 g, 9.46

mmol). The resulting suspension was reﬂuxed for 20 h exposed

to air. The crude mixture was ﬁltered through a pad of silica

(ethyl acetate), and the ﬁltrate was concentrated. The resulting

solid was dissolved in CH₂Cl₂(200 mL), and the solution was

treated dropwise with TFA (3.65 mL, 47.4 mmol). After

stirring for 2.5 h, saturated aqueous NaHCO₃was slowly

added to the reaction mixture. The organic phase was washed

(water and brine), dried (Na₂SO₄) and concentrated. Column

chromatography [silica, hexanes/CH₂Cl₂(3 : 1)] aﬀorded a

green solid (530 mg, 29%): ¹H NMR (300 MHz) d ꢁ1.80

(br, 2H), 2.04 (s, 6H), 4.64 (s, 2H), 7.72–7.75 (m, 3H),

8.08–8.11 (m, 2H), 8.57 (d, J = 4.4 Hz, 1H), 8.77 (s, 1H),

8.80 (s, 1H), 8.94 (d, J = 4.4 Hz, 1H), 9.96 (s, 1H), 9.12

(s, 1H), 9.87 (s, 1H); ¹³C NMR d 31.3, 52.1, 94.6, 95.5, 106.0,

124.6, 127.2, 128.1, 129.3, 132.7, 133.1, 134.2 (other signals

expected were not observed); LD-MS obsd 572.6; ESI-MS

Data for FbHP-He³M¹⁰(diastereomer 1) (2 mg, 3%): ¹H

NMR (400 MHz) d ꢁ2.98 (br, 1H), 1.25 (br, 1H), 1.87 (s, 3H),

1.88 (s, 3H), 2.07 (s, 6H), 2.27 (d, J = 5.6 Hz, 3H), 2.6 (s, 3H),

4.40 (m, 2H), 4.60 (d, J = 19.5 Hz, 1H), 5.23 (dd, J = 19.5 Hz,

J = 6.4 Hz, 1H), 6.46 (d, J = 6.4 Hz, 1H), 6.64–6.69 (m, 1H),

7.25 (s, 2H), 8.49 (s, 1H), 8.52 (d, J = 4.3 Hz, 1H), 8.88

(s, 1H), 8.94 (d, J = 4.3 Hz, 1H), 8.99 (s, 1H), 9.93 (s, 1H),

note that the two hydroxy protons were not observed; LD-MS

obsd 544.6; ESI-MS obsd 545.2910, calcd 545.2911 [(M+H)⁺,

M = C₃₅H₃₆N₄O₂]; l_abs(toluene) 413, 639 nm.

Data for FbHP-He³M¹⁰(diastereomer 2) (3 mg, 5%): ¹H

NMR (400 MHz) d ꢁ2.98 (br, 1H), 1.25 (br, 1H), 1.87 (s, 3H),

1.88 (s, 3H), 2.07 (s, 3H), 2.08 (s, 3H), 2.27 (d, J = 6.4 Hz,

3H), 2.60 (s, 3H), 4.40–4.41 (m, 2H), 4.60 (d, J = 19.0 Hz,

1H), 5.22 (dd, J = 19.0 Hz, J = 5.6 Hz 1H), 6.47 (d, J = 5.6

Hz, 1H), 6.64–6.69 (m, 1H), 7.25 (s, 2H), 8.49 (s, 1H), 8.52

(d, J = 4.3 Hz, 1H), 8.88 (s, 1H), 8.94 (d, J = 4.3 Hz, 1H),

8.99 (s, 1H), 9.94 (s, 1H), note that the two hydroxy protons

were not observed; LD-MS obsd 544.5; ESI-MS obsd

545.2917, calcd 545.2911 [(M+H)⁺, M=C₃₅H₃₆N₄O₂]; l_abs

(toluene) 414, 639 nm.

obsd 573.0281, calcd. 573.0284 [(M+H)⁺,

M

=

C₂₈H₂₂Br₂N₄]; l_abs(toluene) 400, 651 nm.

3,13-Diacetyl-17,18-dihydro-18,18-dimethyl-10-phenylporphyrin

(FbC-A^3,13P¹⁰). Following a procedure for Stille coupling of

chlorins,⁴⁰a sample of FbC-Br^3,13P¹⁰(150 mg, 0.260 mmol),

tributyl(1-ethoxyvinyl)tin (700 mL, 2.10 mmol) and

(Ph₃P)₂PdCl₂(36 mg, 0.052 mmol) was stirred in CH₃CN/

DMF [20 mL (3 : 2)] under argon for 4 h at 83 1C in a Schlenk

line. The reaction mixture was treated with 10% aqueous HCl

(20 mL) at room temperature for 20 min. CH₂Cl₂was added.

The organic layer was separated, washed (saturated aqueous

NaHCO₃, water, and brine), dried (Na₂SO₄), and concentrated.

Column chromatography (silica, CH₂Cl₂) aﬀorded a purple

Zn(^II)-3-(1-hydroxyethyl)-10-mesityl-18,18-dimethyl-13¹-

oxophorbine (ZnOP-He³M¹⁰). A sample of FbOP-He³M¹⁰

(6 mg, 0.01 mmol) in CHCl₃was treated with Zn(OAc)₂ꢀ2H₂O

(37 mg, 0.17 mmol) in methanol (1 mL). The reaction mixture

was stirred for 8 h at room temperature. The reaction mixture

was concentrated. The crude solid was dissolved in CH₂Cl₂,

and the organic phase was washed (saturated aqueous

NaHCO₃, water and brine), dried (Na₂SO₄) and ﬁltered. The

product, which adhered to the Na₂SO₄, was liberated by

washing with methanol. The CH₂Cl₂and methanol ﬁltrates

were combined and concentrated to aﬀord a bright green solid.

Column chromatography [silica, CH₂Cl₂/MeOH (25 : 1)]

aﬀorded a green solid (5 mg, 70%): ¹H NMR (400 MHz,

THF-d₈) d 1.87 (s, 3H), 1.88 (s, 3H), 2.01 (s, 6H), 2.04 (d, J =

6.6 Hz, 3H), 2.57 (s, 3H), 4.25 (s, 2H), 4.93 (s, 2H), 6.15–6.70

(m, 1H), 7.23 (s, 2H), 8.26 (d, J = 4.6 Hz, 1H), 8.30 (s, 1H),

8.39 (s, 1H), 8.60 (s, 1H), 8.61 (d, J = 4.6 Hz, 1H), 9.46

(s, 1H), note that the hydroxy proton was not observed;

¹³C NMR (THF-d₈) d 21.5, 21.7, 27.3, 30.8, 31.4, 47.3, 49.0,

65.5, 94.2, 104.1, 106.6, 120.5, 126.8, 128.2, 128.7, 129.4, 131.0,

135.6, 138.3, 139.0, 139.5, 142.2, 148.6, 149.2, 152.2, 154.2,

155.3, 174.3, 194.9 (other signals expected were not observed);

LD-MS obsd 604.5; ESI-MS obsd 605.1879, calcd. 605.1889

[(M+H)⁺, M = C₃₅H₃₂N₄O₂Zn]; l_abs(THF) 427, 641 nm.

1

solid (93 mg, 71%): H NMR (300 MHz) d ꢁ1.27 (br, 2H),

2.08 (s, 6H), 3.07 (s, 3H), 3.24 (s, 3H), 4.60 (s, 2H), 7.73–7.77

(m, 3H), 8.07–8.10 (m, 2H), 8.51 (d, J = 4.4 Hz, 1H), 8.82

(s, 1H), 8.91 (d, J = 4.4 Hz, 1H), 9.01 (s, 1H), 9.29 (s, 1H),

10.13 (s, 1H), 10.66 (s, 1H); ¹³C NMR d 29.9, 31.2, 46.5, 52.3,

96.4, 99.1, 109.2, 123.9, 127.2, 127.4, 128.3, 130.1, 131.9, 133.0,

133.7, 134.2, 134.5, 134.8, 135.4, 137.6, 138.1, 141.1, 154.4,

154.8, 165.8, 176.8, 196.9, 197.2; LD-MS obsd 500.3; ESI-MS

obsd 501.2281, calcd 501.2285 [(M+H)⁺, M = C₃₂H₂₈N₄O₂];

l_abs(toluene) 428, 686 nm.

3,13-Diacetyl-15-bromo-17,18-dihydro-18,18-dimethyl-10-phenyl-

porphyrin (FbC-A^3,13P¹⁰Br¹⁵). Following a known procedure⁴⁴

with modiﬁcation,

a

sample of FbC-A^3,13P¹⁰(100 mg,

0.200 mmol) in CH₂Cl₂/TFA [110 mL (10 : 1)] was treated

with NBS (46 mg, 0.26 mmol) at room temperature. After 1 h,

a further amount of NBS (35 mg, 0.20 mmol) was added. After

30 min, CH₂Cl₂was added. The mixture was washed with

saturated aqueous NaHCO₃and water. The organic layer was

separated, dried (Na₂SO₄) and concentrated. Column

chromatography (silica, CH₂Cl₂) aﬀorded a purple solid

Phenyl-substituted compounds

3,13-Dibromo-17,18-dihydro-18,18-dimethyl-10-phenylporphyrin

(FbC-Br^3,13P¹⁰). A solution of 2a (1.28 g, 3.16 mmol) and 1

(851 mg, 3.16 mmol) in anhydrous CH₂Cl₂(80 mL) under

argon was treated with TsOHꢀH₂O (3.00 g, 15.8 mmol) in

anhydrous methanol (20 mL). The red reaction mixture was

1

(75 mg, 65%): H NMR (300 MHz) d ꢁ1.42 (br, 2H), 2.04

(s, 6H), 3.05 (s, 3H), 3.25 (s, 3H), 4.59 (s, 2H), 7.71–7.74

c

2684 New J. Chem., 2011, 35, 2671–2690

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(m, 3H), 8.03–8.06 (m, 2H), 8.48 (d, J = 4.5 Hz, 1H), 8.61

(s, 1H), 8.82 (s, 1H), 8.88 (d, J = 4.5 Hz, 1H), 9.31 (s, 1H),

10.62 (s, 1H); ¹³C NMR d 29.9, 31.6, 34.7, 46.2, 55.6, 127.1,

127.6, 128.4, 128.6, 134.1, 134.4, 134.7, 141.2, 196.8, 202.1

(other signals expected were not observed); LD-MS obsd

579.6; ESI-MS obsd 579.1389, calcd. 579.1390 [(M+H)⁺,

M = C₃₂H₂₇BrN₄O₂]; l_abs(toluene) 425, 677 nm.

dried (Na₂SO₄), and ﬁltered. The title compound, which adhered

to the Na₂SO₄, was liberated by washing with methanol. The

CH₂Cl₂and methanol ﬁltrates were combined and concentrated

to aﬀord a bright green solid (6 mg, 80%): ¹H NMR (300 MHz,

THF-d₈) d 2.02 (s, 6H), 2.05 (d, J = 6.3 Hz, 3H), 4.27 (s, 2H),

4.85 (d, J = 4.4 Hz, 1H), 4.95 (s, 2H), 6.23–6.25 (m, 1H),

7.68–7.71 (m, 3H), 8.06–8.09 (m, 2H), 8.42 (s, 1H), 8.46

(d, J = 4.4 Hz, 1H), 8.51 (s, 1H), 8.62 (s, 1H), 8.67 (d, J =

4.4 Hz, 1H), 9.53 (s, 1H); LD-MS obsd 562.3; ESI-MS obsd

562.1343, calcd 562.1347 (C₃₂H₂₆N₄O₂Zn); l_abs(THF) 426,

640 nm.

3-Acetyl-17,18-dihydro-18,18-dimethyl-10-phenyl-13¹-oxo-

phorbine (FbOP-A³P¹⁰). Following a known procedure⁴¹with

modiﬁcation, a mixture of FbC-A^3,13P¹⁰Br¹⁵(60 mg, 0.10 mmol),

Cs₂CO₃(196 mg, 0.600 mmol), and (Ph₃P)₂PdCl₂(15 mg,

0.021 mmol) in freshly distilled THF (15 mL) under argon was

stirred for 4 h at 85 1C in a sealed Schlenk ﬂask. The reaction

mixture was concentrated, and the resulting crude solid was

dissolved in CH₂Cl₂. The organic phase was washed with water,

dried (Na₂SO₄) and ﬁltered. The ﬁltrate was concentrated to

aﬀord a brown solid. Column chromatography [silica, CH₂Cl₂/

ethyl acetate (20 : 1)] aﬀorded a dark purple solid (35 mg, 68%):

¹H NMR (400 MHz) d ꢁ1.62 (br, 1H), 0.74 (br, 1H), 2.02

(s, 6H), 3.24 (s, 3H), 4.28 (s, 2H), 5.10 (s, 2H), 7.73–7.76 (m, 3H),

8.09–8.13 (m, 2H), 8.63 (d, J = 4.4 Hz, 1H), 8.74 (s, 1H), 8.83

(s, 1H), 8.87 (d, J = 4.4 Hz, 1H), 9.32 (s, 1H), 10.57 (s, 1H);

¹³C NMR d 30.1, 31.2, 48.2, 48.3, 48.7, 96.7, 106.8, 107.6, 118.2,

126.8, 127.7, 128.2, 128.4, 129.8, 130.7, 133.9, 134.1, 134.4, 134.6,

134.9, 135.3, 139.0, 139.5, 141.2, 149.6, 153.9, 155.0, 158.5, 176.5,

195.6, 196.8; LD-MS obsd 498.5; ESI-MS obsd 499.2129, calcd.

499.2129 [(M+H)⁺, M = C₃₂H₂₆N₄O₂]; l_abs(toluene) 428, 446,

682 nm.

Zn(^II)-3-acetyl-17,18-dihydro-18,18-dimethyl-10-phenyl-13¹-

oxophorbine (ZnOP-A³P¹⁰). A sample of FbOP-A³P¹⁰(6 mg,

0.01 mmol) in CHCl₃(2 mL) was treated with Zn(OAc)₂

(37 mg, 0.20 mmol) in methanol (1 mL). The reaction mixture

was stirred for 8 h at room temperature. The reaction mixture

was concentrated. The crude solid was dissolved in CH₂Cl₂.

The solution was washed (saturated aqueous NaHCO₃, water

and brine), dried (Na₂SO₄), concentrated and chromato-

graphed [alumina, CH₂Cl₂/MeOH (25 : 1)] to aﬀord a green

solid (5 mg, 70%): ¹H NMR (300 MHz, THF-d₈) d 2.01

(s, 6H), 3.06 (s, 3H), 4.37 (s, 2H), 5.03 (s, 2H), 7.71–7.73 (m,

3H), 8.09–8.11 (m, 2H), 8.55 (d, J = 4.3 Hz, 1H), 8.63 (s, 1H),

8.68 (s, 1H), 8.85 (d, J = 4.3 Hz, 1H), 9.44 (s, 1H), 10.54

(s, 1H); LD-MS obsd 560.7; ESI-MS obsd 561.1257, calcd

561.1263 [(M+H)⁺, M = C₃₂H₂₄N₄O₂Zn]; l_abs(toluene) 449,

666 nm.

Pentaﬂuorophenyl-substituted compounds

3-(1-Hydroxyethyl)-18,18-dimethyl-10-phenyl-13¹-oxophorbine

(FbOP-He³P¹⁰). A sample of FbOP-A³P¹⁰(14 mg, 0.028 mmol)

8,9-Dibromo-1-formyl-5-(pentaﬂuorophenyl)dipyrromethane (2c).

Following a reported procedure,⁴⁸a solution of 1-formyl-5-

(pentaﬂuorophenyl)dipyrromethane⁴⁹(1.83 g, 5.39 mmol) in

anhydrous THF (54 mL) at ꢁ78 1C under argon was treated

with NBS (1.92 g, 10.8 mmol) in one batch. The reaction

mixture was stirred for 1 h at ꢁ78 1C, and then allowed to

warm up. When the temperature of the reaction mixture

reached ꢁ20 1C, hexanes and water were added. The contents

of the reaction ﬂask were transferred to a separatory funnel.

Ethyl acetate was added. The organic layer was separated,

dried (K₂CO₃) and concentrated without heating to aﬀord a

yellow solid. Column chromatography [silica, hexanes/

CH₂Cl₂/ethyl acetate (7 : 2 : 1)] aﬀorded a brown solid (1.44

g, 54%): mp 76–78 1C (dec.); ¹H NMR (THF-d₈) d 5.83

(s, 1H), 5.98 (d, J = 3.0 Hz, 1H), 6.10–6.12 (m, 1H),

6.85–6.87 (m, 1H), 9.42 (s, 1H), 11.12 (br, 1H), 11.43

(br, 1H); ESI-MS obsd 496.8920, calcd 496.8918 [(M+H)⁺,

M = C₁₆H₇Br₂F₅N₂O]. Note: Compound 2c is relatively

unstable but can be stored in the solid form at ꢁ20 1C under

argon for many weeks without decomposition.

t

in anhydrous CHCl₃(5 mL) was treated with BH₃ꢀ BuNH₂

(15 mg, 0.17 mmol) at room temperature. After stirring for

4.5 h under argon, the reaction mixture was diluted with

CHCl₃and treated with 5% aqueous HCl (5 mL). The

reaction mixture was stirred vigorously for 20 min. The

organic phase was washed (5% aqueous HCl, water, saturated

aqueous NaHCO₃, and brine), dried (Na₂SO₄) and ﬁltered.

The ﬁltrate was concentrated to aﬀord a brown solid. Column

chromatography [silica, CH₂Cl₂/ethyl acetate (20 : 1)] aﬀorded

a dark purple solid (10 mg, 72%): ¹H NMR (300 MHz)

d ꢁ1.38 (br, 1H), 1.16 (br, 1H), 1.95 (s, 6H), 2.20 (d, J = 6.9 Hz,

3H), 2.62 (br, 1H), 4.15 (s, 2H), 5.02 (s, 2H), 6.23–6.25 (m, 1H),

7.72–7.74 (m, 3H), 8.09–8.11 (m, 2H), 8.51 (s, 1H), 8.60 (d, J =

4.4 Hz, 1H), 8.70 (d, J = 4.4 Hz, 1H), 8.73 (s, 1H), 8.77 (s, 1H), 9.47

(s, 1H); ¹³C NMR d 25.9, 30.8, 30.9, 47.7, 48.5, 48.6, 65.3, 94.8,

101.6, 106.5, 117.3, 122.8, 127.5, 127.6, 128.4, 132.5, 132.8, 134.0,

134.3, 135.6, 139.6, 140.1, 142.2, 147.6, 149.7, 152.7, 154.3, 157.4,

177.8, 195.9; LD-MS obsd 500.9; ESI-MS obsd 501.2278, calcd

501.2285 [(M+H)⁺, M = C₃₂H₂₈N₄O₂]; l_abs(toluene) 423,

656 nm.

3,13-Dibromo-17,18-dihydro-18,18-dimethyl-10-(pentaﬂuoro-

phenyl)porphyrin (FbC-Br^3,13Pf¹⁰). Following a streamlined

procedure,⁴⁵a solution of 2c (1.06 g, 2.13 mmol) and 1

(570 mg, 2.13 mmol) in anhydrous CH₂Cl₂(56.8 mL) under argon

was treated with a solution of TsOHꢀH₂O (2.02 g, 10.6 mmol) in

anhydrous methanol (14.2 mL). The red reaction mixture

was stirred at room temperature for 40 min. A sample of

2,2,6,6-tetramethylpiperidine (2.70 mL, 16.0 mmol) was added.

Zn(^II)-3-(1-hydroxyethyl)-18,18-dimethyl-10-phenyl-13¹-oxo-

phorbine (ZnOP-He³P¹⁰). A sample of FbOP-He³P¹⁰(7 mg,

0.01 mmol) in CHCl₃(2 mL) was treated with Zn(OAc)₂

(44 mg, 0.24 mmol) in methanol (1 mL) for 8 h at room

temperature. The reaction mixture was concentrated. The

crude solid was dissolved in CH₂Cl₂, and the organic phase

was washed (saturated aqueous NaHCO₃, water and brine),

c

This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientiﬁque 2011

New J. Chem., 2011, 35, 2671–2690 2685

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The reaction mixture was concentrated. The resulting solid was

dissolved in CH₃CN (213 mL) and subsequently treated with

2,2,6,6-tetramethylpiperidine (7.2 mL, 43 mmol), Zn(OAc)₂

(5.88 g, 32.0 mmol), and AgOTf (1.64 g, 6.38 mmol). The resulting

suspension was reﬂuxed for 18 h exposed to air. The crude mixture

was ﬁltered through a pad of silica (ethyl acetate), and the ﬁltrate

was concentrated. The resulting solid was dissolved in CH₂Cl₂

(50 mL), and the solution was treated dropwise with TFA

(2.6 mL, 34 mmol). After stirring for 2 h, saturated aqueous

NaHCO₃was slowly added to the reaction mixture. The organic

phase was washed (water and brine), dried (Na₂SO₄) and

concentrated. Column chromatography [silica, hexanes/CH₂Cl₂

(3 : 2)] aﬀorded a green solid (132 mg, 9%): ¹H NMR (300 MHz)

d ꢁ2.32 (br, 1H), ꢁ2.00 (br, 1H), 2.05 (s, 6H), 4.68 (s, 2H), 8.51

(d, J = 4.3 Hz, 1H), 8.76 (s, 1H), 8.88 (s, 1H), 9.02 (s, 1H), 9.05

(d, J = 4.3 Hz, 1H), 9.23 (s, 1H), 9.94 (s, 1H); ¹³C NMR d 12.0,

14.7, 23.2, 28.1, 31.7, 32.1, 47.0, 52.7, 96.3, 96.9, 106.7, 115.6,

119.2, 125.3, 127.5, 131.0, 133.0, 135.0, 137.6, 140.2, 151.3 152.1,

165.0, 176.4; LD-MS obsd 662.4; ESI-MS obsd 662.9815, calcd

662.9813 [(M+H)⁺, M = C₂₈H₁₇Br₂F₅N₄]; l_abs(toluene) 400,

654 nm.

calcd 669.0919 [(M+H)⁺, M = C₃₂H₂₂BrF₅N₄O₂]; l_abs

(toluene) 432, 678 nm.

3-Acetyl-17,18-dihydro-18,18-dimethyl-10-(pentaﬂuorophenyl)-

13¹-oxophorbine (FbOP-A³Pf¹⁰). Following

a

known

procedure⁴¹with modiﬁcation, a mixture of FbC-A^3,13Pf¹⁰Br¹⁵

(45 mg, 0.067 mmol), Cs₂CO₃(110 mg, 0.336 mmol), and

(Ph₃P)₂PdCl₂(9 mg, 0.01 mmol) in freshly distilled THF

(5.6 mL) under argon was stirred for 4 h at 85 1C in a sealed

Schlenk ﬂask. The reaction mixture was concentrated, and the

resulting crude solid was dissolved in CH₂Cl₂. The organic

phase was washed with water, dried (Na₂SO₄) and ﬁltered. The

ﬁltrate was concentrated to aﬀord a brown solid. Column

chromatography (silica, CH₂Cl₂) aﬀorded a purple solid

1

(25 mg, 64%): H NMR (400 MHz) d ꢁ2.16 (br, 1H), 0.24

(br, 1H), 2.07 (s, 6H), 3.29 (s, 3H), 4.39 (s, 2H), 5.23 (s, 2H),

8.55 (d, J = 4.7 Hz, 1H), 8.78 (s, 1H), 8.94 (s, 1H), 9.00

(d, J = 4.3 Hz, 1H), 9.44 (d, J = 1.7 Hz 1H), 10.73 (s, 1H);

¹³C NMR d 14.4, 21.3, 26.1, 30.0, 31.0, 47.9, 48.4, 48.6, 60.6,

65.4, 96.4, 101.8, 107.6, 114.9, 123.4, 132.1, 133.6, 134.1, 136.1,

138.7, 141.7, 147.8, 149.3, 152.6, 155.0, 158.4, 177.9, 195.4;

LD-MS obsd 588.5; ESI-MS obsd 589.1661, calcd. 589.1657

[(M+H)⁺, M = C₃₂H₂₁F₅N₄O₂]; l_abs(toluene) 428, 445,

686 nm.

3,13-Diacetyl-17,18-dihydro-18,18-dimethyl-10-(pentaﬂuoro-

phenyl)porphyrin (FbC-A^3,13Pf¹⁰). Following a procedure for

Stille coupling of chlorins,⁴⁰a mixture of FbC-Br^3,13Pf¹⁰

(132 mg, 0.199 mmol), tributyl(1-ethoxyvinyl)tin (340 mL,

1.00 mmol) and (Ph₃P)₂PdCl₂(28 mg, 0.040 mmol) was stirred

in CH₃CN/DMF [10 mL (3 : 2)] under argon for 3.5 h at 83 1C

in a Schlenk line. The reaction mixture was treated with 10%

aqueous HCl (5 mL) at room temperature for 20 min. CH₂Cl₂

was added. The organic layer was separated, washed (saturated

aqueous NaHCO₃, water, and brine), dried (Na₂SO₄), and

concentrated. Column chromatography [silica, hexanes/

CH₂Cl₂(2 : 3)] aﬀorded a purple solid (62 mg, 53%; 95%

purity): ¹H NMR (400 MHz) d ꢁ1.73 (br, 1H), ꢁ1.62 (br, 1H),

2.04 (s, 6H), 2.18 (s, 3H), 3.28 (s, 3H), 4.66 (s, 2H), 8.47

(d, J = 4.4 Hz, 1H), 8.96 (s, 1H), 9.04 (d, J = 4.4 Hz, 1H),

9.06 (s, 1H), 9.38 (s, 1H), 10.25 (s, 1H), 10.77 (s, 1H);

¹³C NMR d 13.8, 17.7, 27.1, 28.1, 30.0, 30.2, 31.3, 46.6, 52.5,

97.8, 100.3, 128.2, 129.3, 131.2, 133.5, 134.4, 136.4, 137.4,

138.0, 153.5, 155.3, 166.5, 176.9 196.78, 196.86; LD-MS obsd

590.4; ESI-MS obsd 591.1816, calcd 591.1814 [(M+H)⁺,

M = C₃₂H₂₃F₅N₄O₂]; l_abs(toluene) 432, 688 nm.

3-(1-Hydroxyethyl)-18,18-dimethyl-10-(pentaﬂuorophenyl)-

13¹-oxophorbine (FbOP-He³Pf¹⁰). A sample of FbOP-A³Pf¹⁰

(25 mg, 0.043 mmol) in anhydrous CHCl₃(3 mL) was treated

t

with BH₃ꢀ Bu NH₂(22 mg, 0.25 mmol) at room temperature.

After stirring for 4 h under argon, the reaction mixture was

diluted with CHCl₃and treated with 5% aqueous HCl (5 mL).

The reaction mixture was stirred vigorously for 20 min. The

organic phase was washed (5% aqueous HCl, water, saturated

aqueous NaHCO₃, and brine), dried (Na₂SO₄) and ﬁltered.

The ﬁltrate was concentrated to aﬀord a brown solid. Column

chromatography [silica, CH₂Cl₂, then CH₂Cl₂/ethyl acetate

(9 : 1)] aﬀorded a brown solid (14 mg, 55%): ¹H NMR

(300 MHz) d ꢁ2.07 (br, 1H), 0.54 (br, 1H), 1.97 (s, 6H),

2.22 (d, J = 6.6 Hz, 3H), 2.64 (br, 1H), 4.23 (s, 2H), 5.10

(s, 2H), 6.43–6.50 (m, 1H), 8.52 (d, J = 4.5 Hz, 1H), 8.70

(s, 2H), 8.83 (d, J = 4.5 Hz, 1H), 8.88 (s, 1H), 9.63 (s, 1H);

LD-MS obsd 590.3; ESI-MS obsd 591.1814, calcd 591.1813

[(M+H)⁺, M = C₃₂H₂₃F₅N₄O₂]; l_abs(toluene) 422, 663 nm.

Zn(^II)-3-(1-hydroxyethyl)-10-(pentaﬂuorophenyl)-18,18-dimethyl-

13¹-oxophorbine (ZnOP-He³Pf¹⁰). A sample of FbOP-He³Pf¹⁰

(7 mg, 0.01 mmol) in CHCl₃(1.9 mL) was treated with

Zn(OAc)₂ꢀ2H₂O (40 mg, 0.18 mmol) in methanol (1.5 mL)

for 8 h at room temperature. The reaction mixture was diluted

with ethyl acetate/MeOH (10 : 1). The organic phase was

washed (saturated aqueous NaHCO₃, water and brine) and

concentrated to a minimum volume, which was freeze-dried

overnight. The resulting solid was treated with Et₂O and

sonicated. The resulting suspension was centrifuged, and the

supernatant (which contained the free base starting material)

was removed. The procedure (treatment with Et₂O, sonication,

centrifugation, and supernatant removal) was performed two

further times to aﬀord the title compound as a green solid

(6 mg, 80%). The title compound exhibited poor solubility in

organic solvents (e.g., CH₂Cl₂, ethyl acetate, THF, and DMF

3,13-Diacetyl-15-bromo-17,18-dihydro-18,18-dimethyl-10-

(pentaﬂuorophenyl)porphyrin (FbC-A^3,13Pf¹⁰Br¹⁵). Following a

known procedure⁴⁴with modiﬁcation, a sample of FbC-A^3,13Pf¹⁰

(60 mg, 0.10 mmol) in CH₂Cl₂/TFA [50.1 mL (10 : 1)] was treated

with NBS (18 mg, 0.10 mmol) at room temperature. After 1 h,

a further amount of NBS (5 mg, 0.03 mmol) was added. After

30 min, CH₂Cl₂was added. The mixture was washed with

saturated aqueous NaHCO₃and water. The organic layer

was separated, dried (Na₂SO₄) and concentrated. Column

chromatography [silica, hexanes/CH₂Cl₂(1 : 3)] aﬀorded a

brown solid (46 mg, 68%): ¹H NMR (400 MHz, 40 1C)

d ꢁ2.02 (br, 1H), ꢁ1.71 (br, 1H), 2.07 (s, 6H), 3.13 (s, 3H),

3.29 (s, 3H), 4.65 (s, 2H), 8.47 (d, J = 4.0 Hz, 1H), 8.61

(s, 1H), 8.99 (s, 1H), 9.02 (d, J = 4.7 Hz, 1H), 9.42 (s, 1H),

10.74 (s, 1H); LD-MS obsd 668.0; ESI-MS obsd 669.0912,

c

2686 New J. Chem., 2011, 35, 2671–2690

This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientiﬁque 2011

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and did not aﬀord a satisfactory ¹H NMR spectrum (DMF-d₇,

THF-d₈); LD-MS obsd 652.3; ESI-MS obsd 653.0923, calcd

653.0949 [(M+H)⁺, M = C₃₂H₂₁F₅N₄O₂Zn]; l_abs(THF)

427, 648 nm.

and brine), dried (Na₂SO₄) and concentrated. Column

chromatography [silica, hexanes/CH₂Cl₂(2 : 3)] aﬀorded a

purple solid (220 mg, 79%): ¹H NMR (400 MHz) d ꢁ2.76

(br, 1H), ꢁ2.28 (br, 1H), 2.10 (s, 6H), 7.76–7.79 (m, 3H),

8.16–8.18 (m, 2H), 8.78 (d, J = 4.4 Hz, 1H), 8.94 (dd, J = 4.6

Hz, J = 1.7 Hz, 1H), 9.06 (d, J = 4.4 Hz, 1H), 9.19–9.21

(m, 2H), 9.25 (s, 1H), 9.35 (dd, J = 4.6 Hz, J = 1.7 Hz, 1H),

9.92 (s, 1H), 9.98 (s, 1H); ¹³C NMR d 24.1, 50.2, 95.3, 96.8,

106.5, 126.3, 126.4, 127.2, 128.1, 128.4. 129.3, 133.9, 134.5,

134.6, 136.8, 137.7, 140.7, 146.4, 153.9, 168.6 (other signals

expected were not observed); LD-MS obsd 430.3; ESI-MS

obsd 431.1860, calcd. 431.1866 [(M+H)⁺, M = C₂₈H₂₂N₄O];

l_abs(toluene) 405, 637 nm.

Oxochlorins

Zn(^II)-17,18-dihydro-18,18-dimethyl-10-phenyl-17-oxoporphyrin

(OxoZnC-P¹⁰). Following a reported procedure for the

synthesis of oxochlorins,⁵¹a mixture of ZnC-P¹⁰(800 mg,

1.66 mmol) and basic alumina (activity I, 60 g) in 100 mL of

toluene was stirred overnight at 60 1C exposed to air. The

reaction mixture was ﬁltered and washed with CH₂Cl₂/CH₃OH

(19 : 1). The ﬁltrate was concentrated, and the resulting solid

was dissolved in toluene (600 mL). DDQ (940 mg, 4.15 mmol)

was added, and the mixture was stirred for 5 min at room

temperature. Triethylamine (20 mL) was added, and the

solvent was removed under reduced pressure. The residue

was immediately chromatographed (silica, CH₂Cl₂) to aﬀord

a purple solid (428 mg, 52%): ¹H NMR (400 MHz) d 2.08

(s, 6H), 7.74–7.76 (m, 3H), 8.11–8.14 (m, 2H), 8.75 (d, J = 4.0

Hz, 1H), 8.84 (d, J = 4.4 Hz, 1H), 9.01–9.03 (m, 3H), 9.05

(d, J = 4.4 Hz, 1H), 9.23 (d, J = 4.4 Hz, 1H), 9.65 (s, 1H),

9.80 (s, 1H); ¹³C NMR d 24.1, 50.6, 76.9, 95.2, 98.3, 108.5,

126.7, 128.1, 129.3, 129.8, 130.5, 132.1, 133.3, 133.9, 134.1,

142.2, 150.9, 151.6, 154.1, 165.2, 171.6 (other signals expected

were not observed); LD-MS obsd 491.9; ESI-MS obsd

492.0915, calcd. 492.0923 (C₂₈H₂₀N₄OZn); l_abs(toluene)

417, 606 nm.

7-Bromo-17,18-dihydro-18,18-dimethyl-10-phenyl-17-oxo-

porphyrin (OxoFbC-Br⁷P¹⁰).

A

solution of OxoFbC-P¹⁰

(100 mg, 0.232 mmol) in distilled THF (60 mL) under argon

was treated with NBS (41 mg, 0.23 mmol) for 45 min at room

temperature. CH₂Cl₂and saturated aqueous NaHCO₃were

added. The organic layer was washed (water, brine), dried

(Na₂SO₄) and concentrated. Column chromatography [silica,

hexanes/CH₂Cl₂(2 : 3)] aﬀorded a purple solid (68 mg, 58%):

¹H NMR (400 MHz, THF-d₈) d ꢁ2.69 (br, 1H), ꢁ2.27 (br,

1H), 2.06 (s, 6H), 7.78–7.81 (m, 3H), 8.15–8.18 (m, 2H), 8.74

(s, 1H), 8.92–8.94 (m, 1H), 9.33–9.35 (m, 2H), 9.51 (s, 1H),

9.57–9.59 (m, 1H), 9.89 (s, 1H), 10.22 (s, 1H); ¹³C NMR

d 24.1, 96.9, 127.3, 127.8, 128.1, 129.2, 129.5, 131.0, 134.8,

135.2, 137.3, 139.4, 141.9, 142.5, 148.3, 150.3, 170.6 (other

signals expected were not observed); LD-MS obsd 508.0;

ESI-MS obsd 509.0972, calcd 509.0970 [(M+H)⁺, M =

C₂₈H₂₁BrN₄O]; l_abs(toluene) 411, 637 nm.

20-Bromo-17,18-dihydro-18,18-dimethyl-10-phenyl-17-oxo-

porphyrin (OxoFbC-P¹⁰Br²⁰). A solution of OxoZnC-P¹⁰

(35 mg, 0.071 mmol) in distilled THF (44 mL) under argon

was treated with NBS (13 mg, 0.073 mmol) at room tempera-

ture. The reaction mixture was stirred for 30 min at room

temperature under argon. CH₂Cl₂and saturated aqueous

NaHCO₃were added. The organic layer was washed (water,

brine), dried (Na₂SO₄) and concentrated. Column chromato-

graphy did not aﬀord separation of the brominated products.

The mixture was dissolved in CH₂Cl₂(0.6 mL) and treated

with TFA (0.6 mL, 8 mmol) at room temperature for 2 h to

achieve demetalation. Saturated aqueous NaHCO₃was added.

The reaction mixture was extracted with CH₂Cl₂. The organic

phase was washed (water and brine), dried (Na₂SO₄) and

concentrated. Column chromatography [silica, hexanes/

CH₂Cl₂(3 : 2)] aﬀorded a purple solid (11 mg, 38%):

¹H NMR (400 MHz) d ꢁ2.66 (br, 1H), ꢁ1.85 (br, 1H), 2.20

(s, 6H), 7.78–7.80 (m, 3H), 8.16–8.18 (m, 2H), 8.71 (d, J = 4.3

Hz, 1H), 8.98 (d, J = 4.7 Hz, 1H), 9.09 (d, J = 4.3 Hz, 1H),

9.36–9.38 (m, 1H), 9.47–9.48 (m, 1H), 9.63 (d, J = 4.7 Hz, 1H),

9.94 (s, 1H), 10.13 (s, 1H); LD-MS obsd 508.2; ESI-MS obsd

509.0966, calcd 509.0972 [(M+H)⁺, M = C₂₈H₂₁BrN₄O]; l_abs

(toluene) 410, 645 nm.

7-Acetyl-17,18-dihydro-18,18-dimethyl-10-phenyl-17-oxo-

porphyrin (OxoFbC-A⁷P¹⁰). Following a procedure for Stille

coupling of chlorins,⁴⁰a mixture of OxoFbC-Br⁷P¹⁰(37 mg,

0.072 mmol), tributyl(1-ethoxyvinyl)tin (0.10 mL, 0.30 mmol)

and (Ph₃P)₂PdCl₂(8 mg, 0.01 mmol) in anhydrous CH₃CN/

DMF [3.6 mL (3 : 2)] under argon was stirred for 3 h at 85 1C

in a Schlenk ﬂask. The reaction mixture was treated with 10%

aqueous HCl at room temperature for 20 min. CH₂Cl₂was

added. The organic layer was washed (saturated aqueous

NaHCO₃, water, brine), dried (Na₂SO₄), and ﬁltered. The

ﬁltrate was concentrated. Column chromatography (silica,

CH₂Cl₂) aﬀorded a red solid (19 mg, 55%): ¹H NMR

(400 MHz) d ꢁ2.36 (br, 1H), ꢁ1.94 (br, 1H), 2.07 (s, 6H), 3.05

(s, 3H), 7.78–7.83 (m, 3H), 8.12–8.14 (m, 2H), 8.84 (d, J =

4.8 Hz, 1H), 9.08–9.12 (m, 3H), 9.16 (s, 1H), 9.39 (d, J =

4.4 Hz, 1H), 9.76 (s, 1H), 11.07 (s, 1H); LD-MS obsd 472.6;

ESI-MS obsd 473.1976, calcd 473.1976 [(M+H)⁺, M =

C₃₀H₂₄N₄O₂]; l_abs(toluene) 422, 639 nm.

7-Acetyl-17,18-dihydro-17-hydroxy-18,18-dimethyl-10-phenyl-

porphyrin (FbC-A⁷P¹⁰HO¹⁷). A sample of OxoFbC-A⁷P¹⁰

(7 mg, 0.02 mmol) in anhydrous CHCl₃(1.5 mL) under argon

t

17,18-Dihydro-18,18-dimethyl-10-phenyl-17-oxoporphyrin

(OxoFbC-P¹⁰).

was treated with BH₃ꢀ BuNH₂(7 mg, 0.08 mmol) at room

A

solution of OxoZnC-P¹⁰(320 mg,

temperature for 6 h. The reaction mixture was diluted with

CHCl₃and treated with 5% aqueous HCl (1 mL). The

reaction mixture was stirred vigorously for 20 min. The

organic phase was washed (5% aqueous HCl, water, saturated

aqueous NaHCO₃, and brine), dried (Na₂SO₄) and ﬁltered.

0.648 mmol) in CH₂Cl₂(20 mL) was treated with TFA (750 mL,

9.74 mmol) for 2 h at room temperature. Saturated aqueous

solution of NaHCO₃was added. The reaction mixture was

extracted with CH₂Cl₂. The organic phase was washed (water

c

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The ﬁltrate was concentrated to aﬀord a brown solid. Column

chromatography [silica, CH₂Cl₂, then CH₂Cl₂/ethyl acetate

(20 : 1)] aﬀorded three fractions, of which the ﬁrst was the title

compound and the latter two were trace amounts of the

doubly reduced products. Data for the title compound: brown

solid (5 mg, 70%); ¹H NMR (400 MHz) d ꢁ1.85 (br, 1H),

ꢁ1.52 (br, 1H), 1.93 (s, 3H), 2.08 (s, 3H), 2.76 (d, J = 8.3 Hz,

1H), 3.02 (s, 3H), 6.25 (d, J = 8.3 Hz, 1H), 7.76–7.79 (m, 3H),

8.08–8.15 (m, 2H), 8.74 (dd, J = 3.3 Hz, J = 1.7 Hz, 1H),

8.80–8.83 (m, 2H), 8.89 (dd, J = 3.3 Hz, J = 1.4 Hz, 1H),

9.02 (s, 1H), 9.15 (s, 1H), 9.27 (dd, J = 3.0 Hz, J = 1.7 Hz,

1H), 10.9 (s, 1H); LD-MS obsd 474.7; ESI-MS obsd 475.2123,

calcd 475.2129 [(M+H)⁺, M = C₃₀H₂₆N₄O₂]; l_abs(toluene)

419, 633 nm.

Data for FbC-He⁷P¹⁰HO¹⁷(diastereomer 1): LD-MS obsd

475.5; ESI-MS obsd 477.2286, calcd. 477.2285 [(M+H)⁺,

M = C₃₀H₂₈N₄O₂].

Data for FbC-He⁷P¹⁰HO¹⁷(diastereomer 2): LD-MS obsd

475.6; ESI-MS obsd 477.2288, calcd. 477.2285 [(M+H)⁺,

M = C₃₀H₂₈N₄O₂].

J = 1.6 Hz, 1H), 10.98 (s, 1H); LD-MS obsd 516.2; ESI-MS

obsd 517.2241, calcd 517.2234 [(M+H)⁺, M = C₃₂H₂₈N₄O₃];

l_abs(toluene) 403, 636 nm.

17-(1,3-Dioxolan-2-yl)-17,18-dihydro-3-(1-hydroxyethyl)-18,18-

dimethyl-10-phenylporphyrin (OxoFbC⁰-He⁷P¹⁰). A sample of

OxoFbC⁰-A⁷P¹⁰(15 mg, 0.029 mmol) in anhydrous THF/

methanol [11 mL (10 : 1)] was slowly treated with NaBH₄

(83 mg, 2.2 mmol) at 0 1C. The mixture was stirred for 1.5 h.

Water and ethyl acetate were added, and the resulting mixture

was stirred for 20 min. The organic layer was separated, dried

(Na₂SO₄), and concentrated. Column chromatography [silica,

CH₂Cl₂/ethyl acetate (20 : 1)] aﬀorded a green solid (11 mg,

71%): ¹H NMR (400 MHz) d ꢁ2.45 (br, 1H), ꢁ2.21 (br, 1H),

2.01 (s, 6H), 2.14 (d, J = 4.8 Hz, 3H), 2.42–2.44 (m, 1H),

4.53–4.61 (m, 2H), 4.79–4.87 (m, 2H), 6.44–6.46 (m, 1H),

7.71–7.75 (m, 3H), 8.12–8.14 (m, 2H), 8.55 (s, 1H), 8.80–8.82

(m, 1H), 8.94–8.95 (m, 1H), 9.00–9.03 (m, 2H), 9.19 (s, 1H),

9.27–9.29 (m, 1H), 10.14 (s, 1H); LD-MS obsd 518.3; ESI-MS

obsd 519.2384, calcd 519.2391 [(M+H)⁺, M = C₃₂H₃₀N₄O₃];

l_abs(toluene) 399, 636 nm.

7-Bromo-17-(1,3-dioxolan-2-yl)-17,18-dihydro-18,18-dimethyl-

10-phenylporphyrin

(OxoFbC⁰-Br⁷P¹⁰).

Following

a

17,18-Dihydro-7-(1-hydroxyethyl)-18,18-dimethyl-10-phenyl-

17-oxoporphyrin (OxoFbC-He⁷P¹⁰). Following a literature

procedure,⁵⁵a solution of OxoFbC⁰-He⁷P¹⁰(10 mg, 0.019 mmol)

in THF (15 mL) was treated with 10% aqueous HCl (5 mL). The

reaction mixture was stirred at room temperature for 36 h.

Ethyl acetate was added, and the organic layer was separated.

The organic layer was washed (aqueous NaHCO₃and brine),

dried (Na₂SO₄), and concentrated. Column chromatography

[silica, CH₂Cl₂/ethyl acetate (20 : 1)] aﬀorded a purple solid

literature procedure,⁵⁴a solution of OxoFbC-Br⁷P¹⁰(40 mg,

0.093 mmol) in toluene (25 mL) and ethylene glycol (140 mg,

2.26 mmol) was treated with TsOHꢀH₂O (9 mg, 0.05 mmol).

The resulting mixture was stirred under reﬂux for 20 h, cooled,

poured into ice-water, and extracted with ethyl acetate. The

organic extract was dried (MgSO₄) and concentrated under

reduced pressure. Column chromatography [silica, hexanes/

CH₂Cl₂(2 : 3)] aﬀorded a green solid (23 mg, 46%): ¹H NMR

(400 MHz) d ꢁ2.44 (br, 1H), ꢁ2.16 (br, 1H), 1.99 (s, 6H),

4.50–4.59 (m, 2H), 4.76–4.84 (m, 2H), 7.71–7.75 (m, 3H),

8.10–8.12 (m, 2H), 8.67 (s, 1H), 8.81–8.83 (m, 1H), 8.90–8.93

(m, 1H), 8.97–8.99 (m, 1H), 9.02 (s, 1H), 9.18 (s, 1H),

9.29–9.30 (m, 1H), 10.07 (s, 1H); ¹³C NMR d 24.1, 96.7,

126.3, 127.9, 129.1, 129.7, 130.5, 131.0, 134.8, 135.2, 135.8,

139.7, 142.9, 143.5 (other signals expected were not observed);

LD-MS obsd 552.1; ESI-MS obsd 553.1233, calcd 553.1234

[(M+H)⁺, M = C₃₀H₂₅BrN₄O₂]; l_abs(toluene) 405, 632 nm.

1

(5 mg, 50%): H NMR (400 MHz) d ꢁ2.71 (br, 1H), ꢁ2.33

(br, 1H), 2.09 (s, 6H), 2.19 (d, J = 5.1 Hz, 3H), 2.47–2.50

(m, 1H), 6.48–6.48 (m, 1H), 7.74–7.79 (m, 3H), 8.14–8.15 (m,

2H), 8.64 (s, 1H), 8.88 (d, J = 4.4 Hz, 1H), 9.16 (d, J = 4.4

Hz, 1H), 9.21 (d, J = 4.4 Hz, 1H), 9.24 (s, 1H), 9.38 (d, J =

4.4 Hz, 1H), 9.89 (s, 1H), 10.25 (s, 1H); LD-MS obsd 474.3;

ESI-MS obsd 475.2122, calcd. 475.2129 [(M+H)⁺, M =

C₃₀H₂₆N₄O₂]; l_abs(toluene) 407, 641 nm.

Zn(^II)-17,18-dihydro-7-(1-hydroxyethyl)-18,18-dimethyl-10-phenyl-

17-oxoporphyrin (OxoZnC-He⁷P¹⁰). A sample of OxoFbC-He⁷P¹⁰

(5 mg, 0.01 mmol) in CHCl₃(3 mL) was treated with

Zn(OAc)₂ꢀ2H₂O (35 mg, 0.16 mmol) in methanol (1 mL) for

12 h at room temperature. The reaction mixture was concentrated.

The crude solid was dissolved in CH₂Cl₂, and the organic phase

was washed (saturated aqueous NaHCO₃, water and brine), dried

(Na₂SO₄), and ﬁltered. The ﬁltrate was concentrated. The resulting

crude solid was treated with hexanes followed by sonication. The

resulting suspension was centrifuged, and supernatant was

removed leaving the title compound as a solid (4 mg, 70%):

¹H NMR (400 MHz, THF-d₈) d 2.03 (s, 6H), 2.05 (d, J =

5.6 Hz, 3H), 4.86 (d, J = 4.7 Hz, 1H), 6.33–6.35 (m, J =

4.7 Hz, 1H), 7.71–7.74 (m, 3H), 8.10–8.15 (m, 2H), 8.55

(s, 1H), 8.68 (d, J = 4.4 Hz, 1H), 8.98 (d, J = 4.4 Hz, 1H),

9.02 (d, J = 4.5 Hz, 1H), 9.07 (s, 1H), 9.23 (d, J = 4.5 Hz,

1H), 9.55 (s, 1H), 10.12 (s, 1H); LD-MS obsd 536.9; ESI-MS

7-Acetyl-17-(1,3-dioxolan-2-yl)-17,18-dihydro-18,18-dimethyl-

10-phenylporphyrin (OxoFbC⁰-A⁷P¹⁰). Following a procedure

for Stille coupling of chlorins,⁴⁰a mixture of OxoFbC⁰-Br⁷P¹⁰

(23 mg, 0.042 mmol), tributyl(1-ethoxyvinyl)tin (57 mL,

0.17 mmol), and (Ph₃P)₂PdCl₂(8 mg, 0.01 mmol) in anhydrous

CH₃CN/DMF [3.6 mL (3: 2)] under argon was stirred for 3 h at

85 1C in a Schlenk ﬂask. The reaction mixture was treated with

10% aqueous HCl at room temperature for 20 min. CH₂Cl₂

was added. The organic layer was washed (saturated aqueous

NaHCO₃, water, brine), dried (Na₂SO₄), and ﬁltered. The ﬁltrate

was concentrated. Column chromatography [silica, hexanes/

CH₂Cl₂/ethyl acetate (7:2:1)] aﬀorded a brownish-red solid

1

(18 mg, 84%): H NMR (400 MHz) d ꢁ1.91 (br, 1H), ꢁ1.63

(br, 1H), 2.05 (s, 6H), 3.03 (s, 3H), 4.50–4.58 (m, 2H),

4.75–4.83 (m, 2H), 7.76–7.79 (m, 3H), 8.12–8.15 (m, 2H),

8.76 (dd, J = 4.9 Hz, J = 2.0 Hz, 1H), 8.86 (dd, J = 4.9

Hz, J = 2.0 Hz, 1H), 8.88 (s, 1H), 8.93 (dd, J = 4.8 Hz, J =

1.6 Hz, 1H), 9.056 (s, 1H), 9.064 (s, 1H), 9.31(dd, J = 4.8 Hz,

obsd 537.1262, calcd 537.1263 [(M+H)⁺,

M

=

C₃₀H₂₄N₄O₂Zn]; l_abs(THF) 419, 609 nm.

c

2688 New J. Chem., 2011, 35, 2671–2690

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Static and time-resolved optical spectroscopy. Argon-purged

solutions of the samples in toluene (or THF for ZnOP-He³P¹⁰,

ZnOP-He³Pf¹⁰and ZnOP-A³P¹⁰) with an absorbance of r0.10

at the excitation wavelength were used for the ﬂuorescence

spectral, quantum yield, and lifetime measurements. Solutions

used for absorption studies in the same solvents contained

ambient O₂. Solutions of ZnOP-He³M¹⁰in n-hexane containing

a small percentage of THF (and ambient O₂) were prepared by

adding a small amount of a concentrated pigment solution in

THF to n-hexane followed by mixing. Static absorption

(Varian Cary 100 or Shimadzu UV-1800) and ﬂuorescence (Spex

Fluorolog Tau 2 or PTI Quantamaster 40) measurements were

performed at room temperature, as were all other studies. Static

emission measurements employed 2-4 nm excitation- and detection-

monochromator bandwidths and 0.2 nm data intervals. Emission

spectra were corrected for detection-system spectral response.

Fluorescence quantum yields were determined relative to

CH₂Cl₂; the sample cell was a sealed 4 mm i.d. NMR tube,

which was spun to mitigate photodecomposition. The ﬁlm

measurements were made on samples deposited on a copper

tip that was mounted in an evacuated chamber. All of the RR

spectra were acquired at ambient temperature.

The RR spectra were acquired with a triple spectrograph

(Spex 1877) equipped with a holographically etched 1200 or

2400 groove/mm grating in the ﬁrst or third stage. The

excitation wavelengths were provided by the discrete outputs of

a krypton ion (Coherent Innova 200-K3) laser using 415 nm

excitation. The scattered light was collected in a 901 conﬁguration

using a 50 mm f 1.4 Canon camera lens. A UV-enhanced

charge-coupled device (CCD) was used as the detector

(Princeton Instruments LNCCD equipped with an EEV

1152-UV chip). The data acquisition times were typically

0.5 h (180 ꢂ 10 s frames). Cosmic spikes were removed prior

to addition of the datasets. The laser power at the samples was

5–10 mW. The spectral resolution was B2.5 cm^ꢁ1. The

frequencies were calibrated using the known frequencies of

indene, fenchone, and 50 : 50 toluene/acetonitrile.

chlorophyll a in benzene⁶¹or toluene⁵⁶(both with F_f

=

0.325). Fluorescence lifetimes were obtained by two methods

(and typically averaged) as follows: (i) A phase-modulation

technique in which samples were excited at various wavelengths

in the Soret region and detected through appropriate colored

glass ﬁlters (Spex Fluorolog Tau 2). Modulation frequencies

from 4–250 MHz were utilized and both the ﬂuorescence phase

shift and modulation amplitude were analyzed. (ii) Decay

measurements using time-correlated-single-photon-counting

detection on an apparatus with an approximately Gaussian

instrument response function with a full-width-at-half-maximum

of B0.5 ns (Photon Technology International LaserStrobe

TM-3). Samples were excited in the Soret or Q regions using

excitation pulses at 337 nm from a nitrogen laser or in the blue to

green spectral regions from a dye laser pumped by the nitrogen

laser. The ﬂuorescence was detected via a monochromator with a

bandpass of r 2 nm.

Acknowledgements

This research was carried out as part of the Photosynthetic

Antenna Research Center (PARC), an Energy Frontier

Research Center funded by the U.S. Department of Energy,

Oﬃce of Science, Oﬃce of Basic Energy Sciences under Award

Number DE-SC0001035. We thank Ms Rachel Williams and

Ms Kaitlyn Faries for assistance with some of the studies.

Mass spectra were obtained at the Mass Spectrometry

Laboratory for Biotechnology at North Carolina State

University. Partial funding for the NCSU Facility was

obtained from the North Carolina Biotechnology Center and

the NSF.

FTIR Spectroscopy. The FTIR spectra of the solid compounds

in either pellet or ﬁlm forms were collected at room temperature

using a Bruker Tensor 27 spectrometer with a spectral resolution

of 4 cm^ꢁ1. The spectra of the solids in pellets were obtained in

KBr (B1–2 wt% compound). These spectra were collected in

transmission mode using a room-temperature DTGS detector,

averaging over 32 scans. The spectra of the solids as ﬁlms,

deposited on a commercially available Si(100) substrate that

was hydrogen-passivated, were obtained using a Harrick

Scientiﬁc Ge attenuated total reﬂection accessory (GATRt,

651 incidence angle relative to the surface normal). The Si(100)

substrates were placed in contact with the ﬂat surface of a

semispherical Ge crystal that serves as the optical element. The

IR spectra were collected with p-polarized light using a

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Article Doi

De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

DOI: 10.1039/c1nj20611g

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Article abstract of DOI:10.1039/c1nj20611g

Full text of DOI:10.1039/c1nj20611g

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