α-Oxymethyl ketone enolates for the asymmetric Mannich reaction. From acetylene and N-alkoxycarbonylimines to β-amino acids

Article abstract of DOI:10.1002/(SICI)1521-3773(20000317)39:6<1063::AID-ANIE1063>3.0.CO;2-Y

The insufficient diastereoselectivity and generality, which are the main problems of the 'acetate' aza - aldol reaction, can now be addressed through the reaction of the lithium enolate of endo-O-trimethylsilyl acetyl isoborneol with various N[(p-tolylsul

Full text of DOI:10.1002/(SICI)1521-3773(20000317)39:6<1063::AID-ANIE1063>3.0.CO;2-Y

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[8] A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys. Condens.
Matter 1992, 4, 1143.
[9] K. Kneipp, Y. Wang, R. R. Dasari, M. S. Feld, Appl. Spectrosc. 1995,
49, 780.
[10] C. Rodger, W. E. Smith, G. Dent, M. Edmondson, J. Chem. Soc.
Dalton Trans. 1996, 791.
[11] D. Graham, W. E. Smith, A. M. T. Linacre, C. H. Munro, N. D.
Watson, P. C. White, Anal. Chem. 1997, 69, 4703.
a-Oxymethyl Ketone Enolates for the
Asymmetric Mannich Reaction. From
Acetylene and N-Alkoxycarbonylimines
to b-Amino Acids**
Claudio Palomo,* Mikel Oiarbide, M.
Concepcion Gonzalez-Rego, Arun K. Sharma, Jesus
Â
Â
Â
Â
M. García, Alberto Gonzalez, Cristina Landa, and
Anthony Linden
The reactions of enolizable carbonyl compounds with
azomethine functions, usually referred to as Mannich-type
reactions (also termed aza ± aldol reactions), result in the
formation of b-amino acids, ketones, or aldehydes.[1] These
reactions are conceptually equivalent to the aldol reactions,
but, in sharp contrast, they have been substantially less
developed.[2, 3, 4] There are two reasons that can justify this
situation: firstly, the poorer electrophilicity of the azomethine
function relative to that of the carbonyl function, and
secondly, the preference of enolizable azomethines to under-
go a-deprotonation rather than addition.[5] To date, there are
two main ways to approach these problems. One strategy lies
in the use of activated forms of azomethines,[1, 2] and the other
in the use of trialkylsilylenol ethers or O-(trialkylsilyl)ketene
acetals as nucleophiles.[3, 6] The later strategy has led to
outstanding advances in enantioselective Mannich reactions[7]
catalyzed by chiral ruthenium,[8a±b] palladium,[8c±e] and cop-
per[8e±g] complexes. Some highly diastereoselective methods
involving chiral azomethines are also known,[6, 9] but in most
Figure 2. The signal corresponding to the quadrant aromatic stretches of
the labeled oligonucleotides, as two single samples and in a mixture.
separation and in varying proportions. In addition, these
initial experiments indicate that oligonucleotide probe design
is crucial and there is considerable potential for further
development particularly as both nonfluorescent and fluo-
rescent chromophores can be used as SERRS labels. The
modified oligonucleotide probes can also still be used as
substrates for polymerases, therefore demonstrating that they
remain biologically active. These results prove that, by
considering the chemistry involved, SERRS can feature in
the development of more effective DNA assays allowing the
development of assay formats not possible by conventional
methods.
Experimental Section
A solution of modified oligonucleotide (20 mL, 4 Â 10 8 m) was premixed on
ice with an aliquot of spermine tetrahydrochloride (20 mL, 8 Â 10 2 m).
Water (500 mL) and citrate-reduced silver colloid (500 mL) were added to
this solution. Analysis used a Renishaw 2000 Raman Microprobe with
excitation provided by a Spectra-Physics Model 2020 argon-ion laser
(100 mW, l ˆ 514.5 nm). The detector was a charge-coupled device (CCD).
Â
[*] Prof. Dr. C. Palomo, Dr. M. Oiarbide, M. C. Gonzalez-Rego,
Dr. A. K. Sharma
Â
Departamento de Química Organica
Universidad del País Vasco. Facultad de Química
Â
Apdo 1072, 20080 San Sebastian (Spain)
Samples were analysed over 1 s in a plastic micro-titre plate using a X10
1
Fax: (34)943-212236
objective with the grating centred at 1400 cm
.
E-mail: qoppanic@sc.ehu.es
Received: July 30, 1999 [Z13808]
Â
Dr. J. M. García, Dr. A. Gonzalez, C. Landa
Departamento de Química Aplicada
[1] L. J. McBride, M. D. Oneill, Am. Lab. (Boston) 1991, 23, 52, and
references therein.
[2] A. Castro, J. G. K. Williams, Anal. Chem. 1997, 69, 3915.
[3] B. K. Nunnally, H. He, L. C. Li, S. A. Tucker, L. B. McGown, Anal.
Chem. 1997, 69, 2392.
[4] U. Lieberwirth, J. Arden-Jacob, K. H. Drexhage, D. P. Herten, R.
Müller, M. Neumann, A. Schulz, S. Siebert, G. Sagner, S. Klingel, M.
Sauer, J. Wolfrum, Anal. Chem. 1998, 70, 4771.
[5] A. M. Stacy, R. P. Vanduyne, Chem. Phys. Lett. 1983, 102, 365.
[6] P. Hildebrandt, M. Stockburger, J. Phys. Chem. 1984, 88, 5935.
[7] D. A. Weitz, M. Moskovits, J. A. Creighton in Chemistry and
Structures at Interfaces, New Laser and Optical Techniques (Eds.:
R. B. Hall, A. B. Ellis), VCH, Deerfield Beach, FL, 1986, p. 197.
Â
Universidad Publica de Navarra
Campus de Arrosadía, 31006 Pamplona (Spain)
Dr. A. Linden
Organisch-Chemisches Institut der Universität Zürich
Winterthurerstrasse-190, CH-8057, Zürich (Switzerland)
[**] This work was supported by the University of the Basque Country, the
Basque Government (Projects UPV 170.215-G47/98, EX-1998-124)
and in part by M. E. C. (Project PB 98-0549). Grants to A.S. and
M.C.G. from the Basque Government, and to C.L. from the Govern-
ment of Navarra and M.E.C. are acknowledged.
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/angewandte/ or from the author.
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Table 1. Mannich-type synthesis of b-amino ketones 6 and 7.[a]
Product 6/7
instances, the chiral auxiliary is often difficult to recover.
Despite these achievements, however, there are very few
known diastereoselective methods involving chiral enolates,[1]
which are almost paramount in the parent aldol reactions.
Most importantly, the stereoselectivity attained with the
hitherto known a-unsubstituted chiral enolates is still rather
inadequate.[10, 11] We report here on a new diastereoselective
Mannich-type reaction that helps to solve the above problems.
The lithium enolate of the methyl ketone 3, the latter being
readily prepared from acetylene (1) and (1R)-(‡)-camphor
(2),[12] reacted, in the presence of an excess of LDA, with the
carbamates 4 or 5[13, 14] to afford the corresponding adducts 6/7
in good yields[15] and, in both cases, with diastereomeric ratios
greater than 96:4 (Scheme 1). In general carbamates 5
provided the adducts 7 essentially as single diastereomers
R
R1
d.r.[b]
yield[%][c]
m.p. [8C]
a
CH3CH2
Cbz
Boc
Cbz
Boc
Cbz
Cbz
Boc
Cbz
Cbz
Cbz
Cbz
Cbz
Boc
Cbz
98:2
94
71
66
60
80
82
60
90
65
55
81
54
72
60
98 ± 99
120
> 98:2[d]
98:2
b
CH3(CH2)2
82 ± 84
61 ± 62
104 ± 105
87 ± 88
77 ± 78
92 ± 94
86 ± 89
oil
103 ± 105
133 ± 136
105 ± 106
104 ± 105
> 99:1
> 98:2[d]
98:2
> 99:1
98:2
96:4
97:3
98:2
96:4
c
d
CH3(CH2)3
(CH3)2CH
e
f
g
h
i
PhCH2CH2
(CH3)2CHCH2
BnOCH2CH2
c-C6H11
c-C6H11CH2
Ph
j
k
> 99:1
> 98:2[d]
4-Cl-C6H4
[a] Reactions carried out on a 4 mmol scale. [b] Diastereomeric ratios
determined by HPLC analysis, using a LiChrospher 100 RP-18 column and
mixtures of CH3CN/H2O as eluant (Tˆ 408C). [c] Yields of 6 and 7 after
purification of the crude product by column chromatography and/or
crystallization from n-hexane. [d] Determined by 13C NMR spectroscopy.
O
NHR1
R
HC CH
HO
1
8 R1: Cbz
9 R1: Boc
a)
(1S)-( )-camphor isomer, which would lead to the corre-
sponding enantiomeric b-amino acids. Both (R)- and (S)-b-
amino acids are found in natural products[18b] and are also very
interesting as potential building blocks for b-lactam anti-
biotics[21] and for b-peptides.[22]
To gain further insight into the importance of the present
enolate model, three representative chiral a-oxymethyl
ketones were also examined. Scheme 2 illustrates that of
d)
O
2
O
NHR1
b)
Me3SiO
Xc
OSiMe3
R
HNR1
6 R1: Cbz
7 R1: Boc
O
c)
p-TolSO2
R
3
4 R1: Cbz
5 R1: Boc
Scheme 1. Asymmetric Mannich-type synthesis of b-amino acids.
a) ref. [12]; b) LDA (3.5 equiv), THF, 788C, 1 h then 4 or 5 (2 equiv),
NHCbz
NHCbz
O
O
THF,
788C, 15 min; c) H2 (1 atm), Pd/C, EtOAc, RT, (Boc)2O;
OSiMe3
d) (NH4)2Ce(NO3)6 (3 equiv), CH3CN/H2O, 08C, 16 h. Boc ˆ tert-butoxy-
carbonyl, Cbz ˆ benzyloxycarbonyl, LDA ˆ lithium diisopropylamide, Tol
ˆ tolyl.
OSiMe3
d.r. 98:2
d.r. 70:30
O
NHCbz
Ph
(Table 1), while the carbamates 4 led to slightly lower reaction
diastereoselectivities, typically 98:2 for carbamates with
linear side chains, and 96:4 for those bearing branched side
chains. For stereochemical correlation purposes the adducts 6
were converted into the N-Boc derivatives 7 in a one-pot
procedure.[16] The assigned configuration for the adducts was
established by a single-crystal X-ray structure analysis of the
b-amino ketones 7a and 7j.[17] The configuration for the other
adducts was assigned by analogy.
The excellent diastereoselectivity attained with N-[1-aryl-
sulfonyl)alkyl]carbamates derived from either nonenolizable
or enolizable aldehydes is of particular interest in that it
provides, through oxidative cleavage of the acyloin moiety, b-
alkyl-b-amino acids.[18] For example, the b-amino acid 8a, the
N-Cbz b-homovaline 8d, and the N-Boc-b-homophenylgly-
cine 9j were formed in yields of 81, 86, and 80%, respectively,
by prolonged exposure of the adducts 6a, 6d, and 7j to a
threefold excess of ammonium cerium nitrate (CAN) in
acetonitrile/water. Thus, b-amino acids appropriately protect-
ed at the amino function are obtained in an enantiomerically
pure form,[19] along with the recovery of the starting (1R)-(‡)-
camphor.[20] This result is also particularly important because
it makes it economically viable to use the more expensive
OSiMe3
d.r. 50:50
Scheme 2. Mannich-type adducts from some selected a-silyloxymethyl
ketones and carbamate 4a. The configuration at the newly created
stereogenic center was not determined. The diastereomeric ratios were
determined by HPLC.
these ketones only that derived from (1R)-( )-camphenilone
was comparable in terms of efficiency to 3. Therefore, owing
to the advantageous preparation and low cost of the methyl
ketone 3 it should be, from a practical standpoint, the reagent
of choice for highly diastereoselective ªacetateº aza ± aldol
reactions. In this respect, the example of double asymmetric
induction[23] depicted in Scheme 3 demonstrates the potential
scope of 3. Carbamate 10, which shows essentially no
diastereofacial selectivity against the lithium enolate of
methyl acetate, reacted with the lithium enolate of 3 to
produce 11, essentially as the sole isomer, in 70% yield. This
result was also consistently reproducible when mixtures of 10
with different epimeric compositions were employed.[24] In
addition, desilylation of 11 and subsequent oxidative cleavage
of the acyloin moiety in the resulting intermediate afforded
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Drury, III, T. Dudding, T. Lectka, J. Org. Chem. 1998, 63, 6090; g) D.
Ferraris, T. Dudding, B. Young, W. J. Drury, III, T. Lectka, J. Org.
Chem. 1999, 64, 2168.
[9] For recent examples on diastereoselective metal ester enolate ± chiral
azomethine additions, see a) F. A. Davis, P. Zhou, B. Chen, Chem. Soc.
Rev. 1998, 27, 13; b) T. Fujisawa, Y. Kooriyama, M. Shimizu,
Tetrahedron Lett. 1996, 37, 3881; c) T. P. Tang, J. A. Ellman, J. Org.
Chem. 1999, 64, 12; d) F. A. Davis, R. E. Reddy, J. M. Szewczyk, J.
Org. Chem. 1995, 60, 7037.
[10] a) K. Broadley, S. G. Davies, Tetrahedron Lett. 1984, 25, 1743; b) L. S.
Liebeskind, M. E. Welker, R. W. Fengl, J. Am. Chem. Soc. 1986, 108,
6328; c) T. Shono, N. Kise, F. Sanda, S. Ohi, K. Tsubata, Tetrahedron
Lett. 1988, 29, 231; d) M. Shimizu, Y. Kooriyama, T. Fujisawa, Chem.
Lett. 1994, 2419.
[11] For reviews dealing with the stereochemical problem of a-unsubsti-
tuted enolates, see a) M. Braun in ref. [2a], p. 133; b) M. Braun,
Angew. Chem. 1987, 99, 24, Angew. Chem. Int. Ed. Engl. 1987, 26, 24;
c) M. Braun, H. Sacha, J. Prakt. Chem. 1993, 335, 653.
Scheme 3. Diastereoselective synthesis of b-homo-allo-threonine 12.
a) CH3CO2Me, LDA (3.5 equiv), THF, 788C, 1 h then 10, 15 min; b) 3,
LDA (3.5 equiv), THF, 788C, 1 h then 10, 15 min; c) TBAF, THF, RT,
5 min; d) (NH4)2Ce(NO3)6 (3 equiv), CH3CN-H2O, 08C, 1 min; e) Me3-
SiCHN2, MeOH/C6H6. TBAF ˆ tetrabutylammonium fluoride.
[12] For the preparation and aldol reactions of 3, see a) C. Palomo, A.
Â
Gonzalez, J. M. García, C. Landa, M. Oiarbide, S. Rodríguez, A.
Linden, Angew. Chem. 1998, 110, 190; Angew. Chem. Int. Ed. 1998, 37,
the N-Cbz-O-benzyl-b-homo-allo-threonine 12 in 75% yield.
The diastereomeric purity of this compound was established
by both 13C NMR and HPLC analyses of the corresponding
methyl ester 13, and comparison with the chromatograms
from the mixture of 13/14. The determination of isomeric
ratios, the preparation procedures, and the analytical data for
compounds 6a ± j, 7a ± b, 7j, 8a, 8d, 9j, 11, and 12 can be
found in the Supporting Information.
Â
180; b) C. Palomo, M. Oiarbide, J. M. Aizpurua, A. Gonzalez, J. M.
García, C. Landa, I. Odriozola, A. Linden, J. Org. Chem. 1999, 64,
8193.
[13] For the preparation of a-sulfonylalkyl carbamates, see W. H. Pearson,
A. C. Lindbeck, J. W. Kampf, J. Am. Chem. Soc. 1993, 115, 2622, and
references therein.
[14] For leading references on reactions involving a-sulfonyl carbamates,
see a) ref. [13]; b) A. M. Kanazawa, J.-N. Denis, A. E. Greene, J. Org.
Chem. 1994, 59, 1238; c) R. Ballini, M. Petrini, Tetrahedron Lett. 1999,
40, 4449.
[15] In some instances, small amounts of the starting methyl ketone were
isolated from the reaction mixture.
[16] J. S. Bajwa, Tetrahedron Lett. 1992, 33, 2955.
In summary, an asymmetric Mannich-type reaction that
involves the use of acetylene and a-sulfonyl carbamates as the
consumable organic materials is described for the first time.
[17] Crystallographic data (excluding structure factors) for the structures
7a and 7j reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion nos. CCDC-130261 and 130262, respectively. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
[18] For reviews on b-amino acids, see a) D. C. Cole, Tetrahedron 1994, 50,
9517; b) G. Cardillo, C. Tomassini, Chem. Soc. Rev. 1996, 117; c) E.
Juaristi, D. Quintana, J. Escalante, Aldrichimica Acta 1994, 27, 3;
d) ref. [2c]; e) M. B. Smith, Methods of Non-a-Amino Acid Synthesis,
Dekker, New York, 1995.
[19] The enantiomeric purity of the final b-amino acids was determined by
HPLC analyses of their methyl ester derivatives, using a chiralcel OD
column and mixtures of n-hexane/isopropanol as the eluent. Compar-
ison of these chromatograms with those corresponding to the racemic
b-amino acid esters, the latter prepared by the reaction of the lithium
enolate of methyl acetate and the corresponding N-1[p-(tolylsulfony-
l)alkyl] carbamate, revealed an ee ꢀ 99% for the cases studied.
[20] (‡)-Camphor was typically recovered, after filtration of the crude
material through a Sep-pak cartridge and evaporation of the solvent,
in 75 ± 80% yield, and showed an optical rotation value of [a]D25
(EtOH, c ˆ 1.0) ˆ ‡41.5 ([a]2D5 (EtOH, c ˆ 1.0) ˆ ‡42.2 for the
starting camphor, purchased from Aldrich).
[21] R. Southgate, C. Branch, S. Coultton, E. Hunt in Recent Progress in the
Chemical Synthesis of Antibiotics and Related Microbial Products,
Vol. 2 (Ed.: G. Lukacs), Springer, Berlin, 1993, p. 621.
[22] For reviews, see a) D. Seebach, J. L. Matthews, Chem Commun. 1997,
2015; b) U. Koert, Angew. Chem. 1997, 109, 1922; Angew. Chem. Int.
Ed. Engl. 1997, 36, 1836; c) S. H. Gellman, Acc. Chem. Res. 1998, 31,
173.
[23] For an example of double asymmetric induction in aza ± aldol
reactions, see K. Ishihara, M. Miyata, K. Hattori, T. Tada, H.
Yamamoto, J. Am. Chem. Soc. 1994, 116, 10520.
[24] Apparently, these results indicate that the present Mannich-type
reaction proceeds through prior formation of the corresponding N-
alkoxycarbonylimine.
Received: July 23, 1999 [Z13773]
[1] For a recent review, see M. Arend, B. Westermann, N. Risch, Angew.
Chem. 1998, 110, 1096; Angew. Chem. Int. Ed. 1998, 37, 1044.
[2] a) Stereoselective Synthesis (Houben-Weyl), Vol. E21/3 (Eds.: G.
Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann), Thieme,
Stuttgart, 1996; b) Comprehensive Organic Synthesis, Vol. 2 (Eds.:
B. M. Trost, I. Fleming), 1st ed., Pergamon, Oxford, 1991; c) Enantio-
selective Synthesis of b-Amino Acids (Ed.: E. Juaristi), Wiley-VCH,
New York, 1997.
[3] a) N. Risch, M. Arend in ref. [2a], p. 1908; b) C. Gennari in ref. [2b],
p. 629; c) C. Gennari, A. Vulpetti in ref. [2c], p. 151; d) E. F. Kleinman
in ref. [2b], p. 893; e) M. R. Sardi, A. Heydari, J. Ipaktschi, Chem. Ber.
1994, 127, 1761.
[4] The exception is the reaction of metal ester enolates with non-
enolizable Schiff bases, which generally affords b-lactams. For
pertinent information, see a) D. J. Hart, D. C. Ha, Chem. Rev. 1989,
89, 1447; b) M. J. Brown, Heterocycles 1989, 29, 2225.
[5] For pertinent information on these problems, see N. Risch, M. Arend
in ref. [2a], p, 1833; b) S. E. Denmark, O. J. C. Nicaise, Chem.
Commun. 1996, 999.
[6] For recent examples on diastereoselective trialkylsilylketene acetal ±
chiral azomethine additions, see a) H. Kunz, A. Burgard, D. Schan-
zenbach, Angew. Chem. 1997, 109, 394; Angew. Chem. Int. Ed. Engl.
1997, 36, 386; b) F. Guenoun, T. Zair, F. Lamaty, M. Pierrot, R. Lazaro,
P. Viallefont, Tetrahedron Lett. 1997, 38, 1563; c) K. Higashiyama, H.
Kyo, H. Takahashi, Synlett 1998, 489; d) K. Ishihara, K. Hattori, H.
Yamamoto in ref. [2c], p. 159; e) H. Kunz, M. Weymann, A. Burgard
in ref. [2c], p. 407; f) R. Muller, H. Goesmann, H. Waldmann, Angew.
Chem. 1999, 111, 166; Angew. Chem. Int. Ed. 1999, 38, 184.
[7] S. Kobayashi, H. Ishitani, Chem. Rev. 1999, 99, 1069.
[8] a) H. Ishitani, M. Ueno, S. Kobayashi, J. Am. Chem. Soc. 1997, 119,
7153; b) S. Kobayashi, H. Ishitani, M. Ueno, J. Am. Chem. Soc. 1998,
120, 431; c) E. Hagiwara, A. Fujii, M. Sodeoka, J. Am. Chem. Soc.
1998, 120, 2474; d) A. Fujii, E. Hagiwara, M. Sodeoka, J. Am. Chem.
Soc. 1999, 121, 5450; e) D. Ferraris, B. Young, T. Dudding, T. Lectka, J.
Am. Chem. Soc. 1998, 120, 4548; f) D. Ferraris, B. Young, C. Cox, W. J.
Angew. Chem. Int. Ed. 2000, 39, No. 6
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Facile Synthesis of b-Derivatized PorphyrinsÐ
Structural Characterization of a b ± b-Bis-
Porphyrin**
Yongqi Deng, C. K. Chang,* and Daniel G. Nocera*
Linked porphyrin complexes have been a popular struc-
tural motif for studies of electron and energy transfer. The
ability to tune the electronic properties of porphyrins by their
catenation is central to controlling the efficiencies and rates of
these transfers,[1±6] engendering applications in areas ranging
from the design of ªmolecular wiresº and optoelectronic
materials to the development of biomimetic models of
photosynthetic reaction centers and light-harvesting antenna
complexes.[7±12] Modification of porphyrins is more common at
the meso-position than at the b-position, which confronts
more lengthy syntheses and lower product yields. This is
especially true for bis-porphyrins where several new emerging
methodologies for direct meso coupling[13±16] contrast a limited
set of approaches for the synthesis of b ± b linked bis-
porphyrins.[16b, 17] Porphyrin coupling chemistry has changed
in recent years with the emergence of metal-catalyzed cross-
coupling methods, which have been employed to link bis-
porphyrins via phenyl and ethynyl spacers at meso ± meso-,
meso ± b- and b ± b-positions.[7, 8] The direct coupling of
porphyrins by such methods, however, has not yet been
achieved. We now show the utility of metal-mediated cross-
coupling for the general synthesis of b-derivatized porphyrins,
including the preparation and first structural characterization
of a bis-porphyrin unit directly linked at the b-position.
Scheme 1 shows the strategy developed for the direct b ± b
coupling of the porphyrin rings. Air- and water-stable 2 was
isolated in 76% yield by applying the method of Miyaura
et al.[18] to couple the pinacol diboronate with bromoporphy-
rin 1.[19] Porphyrin 2 can also be prepared according to
Masudaꢁs method where the pinacol borane is the trans-
metalating reagent.[20, 21] The same boronate has recently been
applied to synthesize porphyrins derivatized at the meso-
position[22] in yields similar to that reported here for b-
derivatization. Cross-coupling 1 and 2 under typical Suzuki
reaction conditions affords the previously reported bis-
porphyrin 3a,[17, 23] but prepared here more conveniently and
in higher yield (62%).
Scheme 1. Synthesis of 3a ± e.
The crystal structure of the porphyrin dimer 3a[24] (Fig-
ure 1) is unique inasmuch as no porphyrin system directly
linked by a carbon ± carbon bond at the b-position of the rings
has been heretofore structurally characterized. Connection
Figure 1. ORTEP representation of 3a with hydrogen atoms omitted for
clarity. Thermal ellipsoids are drawn at the 30% probability level.
at the less sterically encumbered b-position results in a
dihedral angle (51.78) (defined by four Npyrrolyl atoms of each
porphyrin) between the two porphyrin macrocyles that is
considerably smaller than that observed between the phenyl
and porphyrin rings in tetraphenylporphyrin (TPP),[25] other
crystallographically characterized TPP type compounds[26, 27]
and, most pertinently, than two porphyrins directly linked by a
C C bond at their meso-positions (dihedral angle of 65 to
848).[14] The unique bridging C2 C2' bond (1.46 Š) is also
shorter than the C C bond of the meso ± meso coupled bis-
porphyrin (d ˆ 1.51 Š) and the sp3 ± sp3 distance of a recently
reported b ± b linked bis-chlorin (d ˆ 1.61 Š), which is typical
of a single bond.[28] These bond length comparisons suggest
greater electronic conjugation between two porphyrin macro-
cycles when linked at their b-positions.
[*] Prof. D. G. Nocera, Dr. Y. Deng
Department of Chemistry, 6-335
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (‡1)617-253-7670
E-mail: nocera@mit.edu
Prof. C. K. Chang
Department of Chemistry
Michigan State University
East Lansing, MI 48824 (USA)
Fax: (‡1)517-353-1793
E-mail: chang@msu.edu
[**] Financial support for this work was provided by the National Institutes
of Health (GM 47274). The authors would like to thank William
Cieslik from Hamamatsu Corporation for providing a C4334-0 Streak
Camera for luminescence lifetime measurements and Alan Heyduk
for assistance with the X-ray crystal structure determination.
Figure 2a compares the electronic absorption spectra of the
monomeric subunit (zinc(ii) etio-I porphyrin) and 3a. The
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Angew. Chem. Int. Ed. 2000, 39, No. 6