Conjugated macrocycles related to the porphyrins. Part 18: Synthesis and spectroscopic characterization of electron-rich benzi- and oxybenziporphyrins: Influence of steric and electronic factors on porphyrinoid aromaticity

Article abstract of DOI:10.1016/S0040-4020(01)00256-3

Although 4,6-dihydroxyisophthalaldehydes failed to condense with tripyrranes to produce porphyrinoid products, the related dimethoxydialdehydes reacted with tripyrrane 9 to give, following oxidation with aqueous ferric chloride solution, nonaromatic dimet

Full text of DOI:10.1016/S0040-4020(01)00256-3

TETRAHEDRON
Pergamon
Tetrahedron 57 <2001) 3657±3671
Conjugated macrocycles related to the porphyrins.
Part 18:q Synthesis and spectroscopic characterization of
electron-rich benzi- and oxybenziporphyrins: in¯uence of
steric and electronic factors on porphyrinoid aromaticity
Daniel T. Richter and Timothy D. Lashp
Department of Chemistry, Illinois State University, Normal, IL 61790-4160, USA
Received 2 October 2000; accepted 30 November 2000
AbstractÐAlthough 4,6-dihydroxyisophthalaldehydes failed to condense with tripyrranes to produce porphyrinoid products, the related
dimethoxydialdehydes reacted with tripyrrane 9 to give, following oxidation with aqueous ferric chloride solution, nonaromatic dimethoxy-
benziporphyrins 20 in excellent yields. In the presence of TFA, dications were generated that showed weakly diatropic properties by proton
NMR spectroscopy. This observation was attributed to the ability of the methoxy units to facilitate charge delocalization. Treatment of 20
with 50 equiv. of boron tribromide cleaved one of the methyl ethers to produce the related methoxyoxybenziporphyrins 24. These por-
phyrinoids showed strongdiamagnetic ringcurrents due to the presence of 18 p-electron delocalization pathways within these structures. On
treatment with TFA, the resulting dications show signi®cantly reduced ring currents although their diatropic character is retained to a far
greater extent than when the methoxy substituents are absent. For both 20 and 24, the methoxy groups are more effective at charge
delocalization, which results in increased diatropicity, when there is no adjacent alkyl substituent. Treatment of 20a with HBr in re¯uxing
acetic acid cleaved both methyl ethers to give the highly insoluble hydroxyoxybenziporphyrin 17. Although the spectroscopic properties of
the free base could not be assessed, this compound was soluble in TFA±chloroform and the resultingdication proved to be highly aromatic as
judged by NMR and UV±vis spectroscopy. Treatment of the 20b with HBr±acetic acid again cleaved both methyl ethers but also gave an
unexpected oxidation to give a diketone bearing a 38 alcohol unit. This novel porphyrinoid retains an 18 p-delocalization pathway, shows
porphyrin-like UV±vis spectra and is highly diatropic as the free base and in protonated form as judged by proton NMR spectroscopy.
q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
be discerned and for this reason porphyrins are often con-
sidered to be the bridged [18]annulenes of Nature.4,5 Proto-
nation of the inner nitrogens does not disrupt the aromatic
properties of the porphyrin macrocycle, and if anything
enhances its diatropicity, and metallation of the inner core
does not signi®cantly disrupt macrocyclic aromaticity.3
While the presence of closed 18p electron pathways
provides an explanation for the aromatic properties of
porphyrins, the nitrogen atoms clearly have a signi®cant
impact as well. In a recent paper,6 Schleyer has proposed
that the aromatic properties of porphyrins are best described
as 22p-electron delocalized systems that utilize two of the
nitrogen electron pairs <structure 2). Both descriptions can
equally well be applied to related systems such as the
porphyrin isomer Porphycene <3).7
The porphyrin macrocycle 1 has longbeen recognized as a
fully p-delocalized aromatic system. The proton NMR
spectra for porphyrins are particularly diagnostic showing
powerful diamagnetic ring currents with the internal NH's
shifted up®eld to, approximately, 24 ppm while the external
bridge methines <meso-protons) are generally down®eld
near 110 ppm.2 Porphyrins also have very characteristic
electronic absorption spectra dominated by a `Soret' band
near 400 nm <e<2£105), followed by four smaller `Q
bands' between 490 and 650 nm.3 Free base porphyrins
consist of two rapidly interconvertingtautomers 1a and
1b, with minor contributions from less favored `cis' tau-
tomers such as 1c <Chart 1)).3 In each of these structures,
an 18p electron delocalization pathway <shown in bold) can
In order to gain a better understanding of porphyrinoid
aromaticity, many examples of related macrocyclic systems
q For Part 17 see Ref. 1.
8
have been synthesized includingporphyrin isomers and
expanded porphyrins.9 Replacement of one or more of the
pyrrole subunits with other ringsystems potentially allows
the function of the nitrogen atoms to be rationally
assessed.10 Early studies were directed towards the synthesis
Keywords: carbaporphyrinoid; benziporphyrn; aromaticity; tautomeriza-
tion; macrocycles.
p
Correspondingauthor. Tel.: 11-309-438-8554; fax: 11-309-438-5538;
e-mail: tdlash@ilstu.edu
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020<01)00256-3

3658
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
OH
Me
O
Me
Et
Et
N
N
N
N
N
N
N
N
Et
H
H
N
N
N
N
H
H
H
H
H
N
N
Me
Me
Et
1a
1b
Et
Et
Et
Et
8
N
N
N
N
Scheme 1.
H
H
aromaticity greatly favors the former but other factors
can in¯uence the outcome of these competitive relation-
ships.12,17
1c
In an earlier study, we demonstrated that the addition of a
suitably placed hydroxy group to the benziporphyrin system
facilitates a `keto±enol' tautomerization to produce oxy-
benziporphyrin 8 <Scheme 1),18 at the time the ®rst example
of an aromatic porphyrin analogue with a carbocyclic ring in
place of a pyrrole subunit. The synthesis of 8 made use of
the so-called `311' methodology19±21 where a tripyrrane 9
was condensed with 5-formylsalicylaldehyde to generate,
followingan oxidation step with DDQ, the fully aromatic
macrocycle 8 <Scheme 2).18 Unlike benziporphyrin, which
shows the internal and external benzene protons between
7.7 and 8.0 ppm,17 oxybenziporphyrin shows a powerful
diamagnetic ring current by proton NMR spectroscopy.
The oxybenziporphyrin system may consist of three dif-
ferent tautomers <8a±c), although theoretical studies on
related carbaporphyrin structures indicate that tautomer 8a
should be favored.22 If more than one tautomer is present,
they must interconvert rapidly on the NMR timescale as
only one set of resonances is observed.17,18 The NH protons
show up as a broad signal at 24 ppm, while the internal CH
appears as a 4J coupled doublet near 27 ppm.18 The exterior
protons are similarly deshielded as a result of the porphyri-
noid ringcurrent. The porphyrinoid properties are also
obvious in the UV±vis spectrum where strongSoret-like
bands appear at 428 and 456 nm, while a series of Q
bands extend to 698 nm.17,18 Addition of trace amounts of
TFA results in the formation of a monocation that again
appears to favor the aromatic tautomer 10 over nonaromatic
structures such as 11. Further addition of acid produces a
dication that shows a drastically reduced diamagnetic ring
current by proton NMR spectroscopy and a UV±vis spec-
trum that showed a greatly diminished Soret band.17,18
These data indicate that the cross-conjugated phenolic can-
onical form 12b is favored over the aromatic contributor
12a, possibly due in part to crowdingwithin the central
cavity that may lead to considerable distortion of the macro-
cycle from planarity <Scheme 2). Similar properties were
observed for a series of oxybenziporphyrins with fused
aromatic rings <structures 13±15, Chart 2).17
N
N
N
N
N
H
H
H
H
N
N
N
3 (Porphycene)
2
Et
Me
Et
Et
N
N
H
H
H
N
H
N
N
Et
Me
Et
Et
Et
Et
4
5
Me
Me
N
Et
Et
Et
N
N
H
N
N
N
Me
Et
Me
Et
Et
Et
Et
6
7
Chart 1.
of porphyrin analogues with furan, thiophene, selenophene
or tellurophene rings,11 but the presence of the chalconide
elements provides a substitute for the nitrogen electron
pairs. The incorporation of carbocyclic rings into porphyrin
analogue structures has only been accomplished fairly
recently.12,13 For instance, carbaporphyrins <e.g. 4)14 and
dicarbaporphyrin 55 have been reported to retain macro-
cyclic aromaticity, while benziporphyrin 6 behaves as a
nonaromatic system overall.16,17 In the latter case, one can
imagine the existence of a fully aromatic tautomer 7 but
this arrangement can only exist at the expense of losing
the benzene subunit's aromatic properties. In this case,
the competition between 6-p arene and porphyrinoid
The addition of a hydroxyl unit onto the benziporphyrin
nucleus completely changes the properties of this macro-
cyclic system by `switchingon' the porphyrinoid aromat-
icity that would otherwise be disfavored. We anticipated
that the addition of a second hydroxy group <structure 16)
would further modify the properties of the porphyrin ana-
logues. While only one of the phenolic OH moieties would

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3659
OH
O
Me
O
CHO
HO2C
Me
Et
Me
N
N
Et
Et
R
H
N
CHO
1. H+
H
N
N
H
H
H
N
HO2C
Me
2. DDQ
Me
Me
N
H
N
H
N
R
Me
Et
Et
Et
13
R
Et
R
8a
9
O
O
Me
Et
N
N
O
O
N
N
Me
Me
H
H
H
H
Et
Et
Et
N
N
N
N
N
N
Me
Me
H
H
H
H
Et
Et
N
N
Me
Me
R
15
14
Et
R
Et
Et
8c
8b
OH
Me
O
+ H+
Me
- H+
Et
Et
Et
HO
HO
N
N
N
N
O
OH
Me
H
Me
H
H
N
N
Et
R
Et
Me
Me
Et
N
N
H
H
Et
H
N
Et
Et
Et
H
H
N
N
N
17
16
Me
Me
R
Chart 2.
Et
R
Et
R
10
+ H+
11
Monocations
- H+
required hydroxyoxybenziporphyrin 17 <Scheme 4).
However, this expectation proved to be elusive. While the
related monohydroxydialdehyde gave oxybenziporphyrin 8
in excellent yields, acid catalyzed condensation of 18a with
9, followed by oxidation with DDQ or FeCl3,25 gave only
trace amounts of porphyrinoid products. The presence of
two strongly electron-donating groups on one side of the
H
O
OH
Me
Me
Et
R
Et
N
N
H
H
H
N
H
H
H
N
N
N
Me
R
Me
OMe
Et
R
R
Et
OMe
12a
12b
R
Br
R
Dications
Br2-AcOH
Scheme 2.
MeO
MeO
Br
be expected to tautomerize to give the keto form <structure
17), the presence of the remainingelectron-donating
hydroxyl should signi®cantly alter the properties of these
carbaporphyrinoids.23
1. n-BuLi
Me
N
2.
a. R = H
O
b. R = Me
Ph
C
H
2. Results and discussion
OH
OMe
CHO
R
R
The synthesis of `dihydroxy' benziporphyrins requires the
availability of the related resorcinol derived dialdehydes 18.
These were synthesized in three steps from 1,3-dimethoxy-
benzene or 2,6-dimethoxytoluene by the established route
summarized in Scheme 3.24 Initially, it was anticipated that
dihydroxisophthalaldehyde 18a would smoothly undergo
`311' condensation with tripyrrane 9 to produce the
CHO
CHO
AlCl3
PhNO2
125 oC
HO
18
MeO
CHO
19
Scheme 3.

3660
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
OH
R
O
Me
CHO
HO2C
R
Me
HO
18
Et
HO
Et
N
N
CHO
HO2C
H+
N
N
H
H
H
H
N
H
N
Et
Me
R
Me
Et
Et
Et
a. R = H
Et
Et
17
12
b. R = Me
OMe
Me
OMe
1. H+
R
Et
Et
2. FeCl3
CHO
OMe
N
MeO
N
N
MeO
19
3. NaHCO3
H
H
H
N
CHO
R
Me
Et
Me
Et
H+
21
Et
MeO
N
H
N
Me
O
Me
R
Me
Et
Et
MeO
Et
Et
N
N
20
OMe
H
H
H
N
R
Et
Me
Et
Et
MeO
22
23
Scheme 4.
benzene precursor may be responsible for unwanted side
reactions, and we speculated that these problems might be
prevented by blockingoff the site between the OH units with
a methyl group. Unfortunately, attempts to condense 18b
with 9 also failed to produce isolatable quantities of any
macrocyclic products.
Despite these reservations, the condensation of dimethoxy-
dialdehydes 19a and 19b with tripyrrane 9 was carried out
under conventional acid catalyzed conditions, followed by
oxidation with aqueous ferric chloride solutions. Sur-
prisingly, this chemistry worked remarkably well. Conden-
sation of 19a with 9 gave the dimethoxybenziporphyrin 20a
in 73±80% yields, while 19b afforded the related macro-
cycle 20b in 55% yield <Scheme 4). The success with this
approach relied on minimal handling. Both compounds
could easily be recrystallized from chloroform±methanol
to give pure products without resorting to chromatography.
This unexpected setback forced us to reconsider our
synthetic strategy. The precursors to dihydroxyisophthalal-
dehydes 18a and 18b were the correspondingdimethoxy
compounds 19a and 19b, respectively <Scheme 3). The
lessened electron-donatingabilities of the methoxy groups
might reduce or circumvent the deleterious effects of the
hydroxyl moieties and allow macrocycle formation.
However, these condensations would result in the formation
of dimethoxybenziporphyrins 20 <Scheme 4) that could not
tautomerize to produce aromatic oxybenziporphyrins.
While the methoxy groups can be cleaved at a later stage,
this approach was initially considered to be far less desir-
able. Benziporphyrin is a relatively unstable compound and
proved to be somewhat dif®cult to isolate in pure form.17
The addition of two electron-donatingmethoxy rgoups
might further increase the reactivity of the ring system
makingthe isolation of these nonaromatic products
impractical.
The spectroscopic properties for dimethoxybenziporphyrins
20 con®rmed that they were non-aromatic compounds. For
the proton NMR spectrum of 20a in CDCl3, the meso-
protons appeared as two 2H singlets at 7.16 and 8.47 ppm,
while the inner benzene proton resonated at 5.07 ppm and
the external benzene CH gave a broad signal at 6.78 ppm
<Fig. 1). These values suggest that a small amount of
aromatic character may be a feature for 20a, although
these minor shifts are not present in the NMR spectra for
20b. However, addition of TFA to the NMR solution of 20a
resulted in a modest aromatic ringcurrent. Speci®cally, the
interior CH shifted up®eld to 20.68 ppm while the exterior
meso-resonances gave two deshielded singlets at 8.44 and

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3661
Figure 1. 400 MHz proton NMR spectrum of dimethoxybenziporphyrin 20a in CDCl3. The free base compound is only sparingly soluble in CDCl3 and this
results in the larger than usual solvent impurity peaks.
Figure 2. 400 MHz proton NMR spectrum of dimethoxybenziporphyrin 20a in TFA±CDCl3 showingthe emergence of some porphyrinoid aromatic character.

3662
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
O
Me
R
Et
MeO
N
N
BBr3
H
20
∆
H
N
a. R = H
Et
Me
b. R = Me
Et
Et
24
H+
H
OH
O
Me
Me
R
R
Et
Et
Et
MeO
MeO
N
N
N
H
H
H
Figure 3. UV±vis spectra of dimethoxybenziporphyrin 20a in 1% Et3N±
chloroform <dotted line) and 1% TFA±chloroform <solid line).
H
H
H
N
N
N
Et
Me
Me
Et
Et
Et
Et
9.60 ppm <Fig. 2). Further, the methyl substituents shift
from 2.63 ppm in the free base to 3.05 ppm in the diproto-
nated structure. The emergence of a diatropic ring current
for dication 21a is most likely due to favorable charge
delocalization. Resonance contributors 22a and 23a
<Scheme 4) can aid in the delocalization of positive charge
while at the same time providingthe 18 p-electron delocali-
zation pathways for porphyrinoid aromatic character. These
profound changes to the electronic characteristics for
dimethoxybenziporphyrin 20a upon protonation can also
be seen in the UV±vis spectra. In 1% triethylamine±chloro-
form, the free base 20a shows multiple absorptions between
300 and 700 nm, but in 1% TFA±chloroform a Soret-like
band emerges at 443 nm <log10 eˆ4.86) <Fig. 3). However,
this phenomenon is rather muted in the case of 20b. In the
proton NMR spectrum, relatively minor shifts are observed
<protonation naturally leads to some down®eld shifts)
although the internal CH still resonates up®eld at
12.8 ppm. The UV±vis spectrum in 1% TFA±chloroform
is also less porphyrin-like and fails to show a signi®cant
Soret-like absorption, although a broad band at 425 nm
<log10 eˆ4.79) is present <Fig. 4). The presence of a methyl
group between the two methoxy units leads to steric crowd-
ingand this undoubtedly leads to the observed differences.
In order for charge delocalization to favor partial porphyrin-
type aromatic character, the methoxy oxygens must be sp2
25
27
H
O
Me
R
Et
MeO
N
N
H
H
H
N
Et
Me
Et
Et
26
Scheme 5.
hybridized and the inability of the moieties to lie in the same
plane disfavors canonical forms such as 22b and 23b.
Treatment of 20b with 50 equiv. of BBr3 at room tempera-
ture for 16 h afforded the methoxyoxybenziporphyrin 24b in
77% yield <Scheme 5). However, prolonged reaction times
and/or elevated temperatures failed to cleave the second
methoxy unit. Dimethoxybenziporphyrin 20a failed to
react at all at room temperature but afforded the related
oxybenziporphyrin 24a in 62% yield on re¯uxingwith
BBr3 in 1,2-dichloroethane under nitrogen. As expected,
the new oxybenziporphyrins were fully aromatic
compounds showingpowerful diamagnetic ringcurrents in
their proton NMR spectra <Fig. 5). In both cases, the internal
CH was observed up®eld beyond 27 ppm while the meso-
protons appeared as a series of down®eld singlets between
9.0 and 10.5 ppm. The free base for 24a gave Soret bands at
422 and 453 nm <Fig. 6), while these absorptions were
observed at 428 and 458 nm for the methyl-substituted
version 24b <Fig. 7). Addition of TFA to oxybenziporphyrin
8 led to a considerable loss of aromatic character, and it was
of some interest to see whether this was also the case for the
related methoxy structures. Addition of TFA to 24a
produced a dication 25a <Scheme 5) that showed far more
aromatic character than had been the case for oxybenzi-
porphyrin 8. In the proton NMR spectrum, the interior CH
still resonated at 21.72 ppm while the meso-protons
appeared down®eld between 8.7 and 10 ppm <Fig. 8).
Although the aromatic ring current is substantially
reduced, resonance contributors such as 25a and 26a must
Figure 4. UV±vis spectra of dimethoxybenziporphyrin 20b in 1% Et3N±
chloroform <dotted line) and 1% TFA±chloroform <solid line).

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3663
Figure 5. 400 MHz proton NMR spectrum of methoxyoxybenziporphyrin 24a in CDCl3.
signi®cantly aid in charge delocalization and diminish the
contributions from nonaromatic canonical forms such as
27a <Scheme 5). The UV±vis spectrum for 24a in 5%
TFA±chloroform also retains a strongSoret band at
429 nm <log10 eˆ5.23), con®rmingthe retention of
porphyrinoid electronic characteristics <Fig. 6). As would
be expected, the methoxy unit is less effective for 24b.
In the proton NMR spectrum for 24b in TFA±CDCl3, the
interior CH shifts down®eld from 27.0 <for the free base) to
10.69 ppm. In addition, the meso-protons show up between
8.0 and 9.3 ppm. While there is clearly some residual
macrocyclic ringcurrent, the steric constraints due to the
extra methyl substituent reduces the contributions from
canonical forms such as 25b and 26b. In 5% TFA±chloro-
form, 24b still shows a Soret band at 437 nm <Fig. 7),
although the intensity of this electronic absorption is some-
what reduced <log10 eˆ5.03). The identity of these struc-
tures was further demonstrated by carbon-13 NMR
spectroscopy and mass spectrometry.
Although boron tribromide readily cleaves one of the meth-
oxy substituents, it failed to remove the second unit. The use
of trimethylsilyl iodide was also investigated but this
reagent produced extensive decomposition. However, HBr
was found to be effective in cleavingboth of the methyl
ethers from benziporphyrins 20. Dimethoxybenziporphyrin
20a was re¯uxed with HBr in acetic acid overnight; follow-
ingdilution with chloroform, the mixture was washed with
water and sodium bicarbonate to remove the acid. However,
at this point considerable quantities of a highly insoluble
material precipitated from these solutions. Initially, we
believed that this material resulted from the decomposition
of the startingmaterial but further analysis demonstrated
that it was in fact the desired hydroxyoxybenziporphyrin
Figure 6. UV±vis spectra of methoxyoxybenziporphyrin 24a: <A) in 1%
Et3N±chloroform <free base); <B) in 5% TFA±chloroform <dication).

3664
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
17 <Scheme 6). The new porphyrinoid was isolated as a
brown powder that could not be dissolved to any signi®cant
extent in organic solvents. However, addition of TFA
produced the soluble dication 28 and this allowed for full
spectroscopic characterization. In addition, accurate mass
data for 17 could be obtained by FAB mass spectrometry.
The diatropic ringcurrent for the protonated species was
larger than that exhibited for the methoxy analogue 25a.
The proton NMR spectrum of 17 in TFA±CDCl3 showed
the inner CH up®eld at 22.4 ppm while the external meso-
protons appeared down®eld as two 2H singlets at 8.8 and
9.9 ppm <Fig. 9). The symmetry of the protonated system
was also con®rmed by the carbon-13 NMR spectrum which
showed 5 alkyl carbons between 11 and 20 ppm, two meso-
carbons at 93.9 and 105.4 ppm, and 9 additional resonances
between 116 and 176 ppm. The UV±vis spectrum of 17 in
chloroform showed a broad Soret band at 406 nm. In 5%
TFA±chloroform the protonated system also showed an
intense Soret absorption at 429 nm <log10 eˆ5.15), providing
further evidence for the aromatic nature of dication 28 <Fig.
10). The OH would be expected to be more effective than
OMe in delocalizingthe positive charge for the protonated
species and this is borne out by the data for this species.
Dimethoxybenziporphyrin 20b was also treated with HBr in
re¯uxingacetic acid and a new reddish-brown product was
isolated in 60% yield. However, it was immediately obvious
from the spectroscopic data that the new compound was not
the expected hydroxyoxybenziporphyrin. Electron impact
mass spectrometry showed a molecular ion of m/z 523, a
value that is 16 units higher than for the expected structure.
Figure 7. UV±vis spectra of methoxyoxybenziporphyrin 24b: <A) in 1%
Et3N±chloroform <free base); <B) in 5% TFA±chloroform <dication).
Figure 8. 400 MHz proton NMR spectrum of methoxyoxybenziporphyrin 24a in TFA±CDCl3. Although the diamagnetic ring current for the protonated
porphyrinoid is reduced, the inner CH is still up®eld at 21.7 ppm).

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3665
OMe
O
Me
Me
Et
Et
Et
MeO
Me
HO
N
N
N
N
HBr
H
H
H
AcOH
N
N
∆
Et
Me
Et
Et
Et
Et
20a
17
OH
Me
H
O
Me
Et
O
Et
N
H+
HO
N
N
H
H
H
H
H
N
N
N
Et
Me
OH
Et
Me
Et
Et
Et
Et
28
HO
Scheme 6.
High resolution MS gave the formula C33H37N3O3 for this
compound, demonstratingthat an extra oxyegn atom is
present. The new structure shows a plane of symmetry by
proton and carbon-13 NMR spectroscopy. The aromatic
compound has a strongdiatropic ringcurrent by proton
NMR spectroscopy, with the internal CH proton producing
a resonance near 28.5 ppm while the external meso-protons
gave two 2H singlets at 9.4 and 10.2 ppm <Fig. 11).
Figure 9. 400 MHz proton NMR spectrum of hydroxyoxybenziporphyrin 17 in TFA±CDCl3. This diprotonated system shows a strongdiamagnetic ring
current due to the hydroxyl moieties enhanced ability to delocalize positive charge.

3666
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
and 8 sp2 carbon resonances between 119 and 155 ppm.
Together, these data demonstrate that the new compound
is the hydroxybenziporphyrin diketone 29 <Scheme 7).
The UV±vis spectrum for this novel aromatic structure
shows a broad Soret band region centered on 418 nm
together with three Q bands at 510, 544 and 593 nm <Fig.
12A). In the presence of TFA, the related cation 30 is
formed and this shows a split Soret band at 410 and
426 nm, together with weaker absorptions at 532, 568 and
584 nm <Fig. 12B). Proton NMR spectra of 29 in TFA±
CDCl3 show the interior CH strongly shifted up®eld to
26.45 ppm while the NH resonances are also present nearby
at 25.62 and 22.64 ppm <Fig. 13). The meso-protons appear
further down®eld than was the case for the free base at 10.1
and 10.9 ppm. In this case, the ringcurrent for the protonated
system appears to be similar to the free base although some
down®eld shifts are evident due to the presence of the positive
charge. The carbon-13 NMR for 30 also shows the presence of
the carbonyl units at 202.8 ppm, the tertiary alcohol carbon at
87.2 ppm and an overall plane of symmetry.
Figure 10. UV±vis spectra of hydroxyoxybenziporphyrin 17 in 5% TFA±
chloroform <dication).
The formation of the oxidized carbaporphyrinoid 29 is most
likely due to the presence of trace amounts of bromine in the
reaction mixtures, somethingthat is virtually impossible to
avoid when usinghydrobromic acid. This may induce the
formation of a hypobromite ester 31 from initially formed
hydroxyoxybenziporphyrin 32 and subsequent attack by
water would afford the observed product <Scheme 8). The
presence of a methyl substituent may increase the reactivity
of 32, although the low solubility of 17 could protect this
compound from further reaction.
However, the methyl group attached to the six-membered
carbocyclic ring gave a 3H singlet up®eld at 1.7 ppm,
suggesting the absence of an adjacent p-system. In addition,
the carbon-13 NMR spectrum showed several unusual
features. A resonance at 201 ppm was consistent with the
presence of a ketone moiety while a signal at 86.2 ppm
suggested the presence of a tertiary alcohol carbon.
However, two meso-carbons were present at 94.7 and 103.4
together with six alkyl absorptions between 12 and 31 ppm
Figure 11. 400 MHz proton NMR spectrum of porphyrinoid 29 in CDCl3.

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3667
OH
OMe
O
Me
Me
Me
MeO
Me
O
Et
Et
Et
Et
N
N
HBr
N
N
H
AcOH
H
H
N
N
∆
Me
Me
Et
29
Et
Et
Et
20b
OH
O
Me
OH
O
Me
Me
O
Me
Et
Et
Et
O
N
N
H
H
H
H
H
H
N
N
N
N
Me
Et
Me
Et
Et
Et
Et
30
Scheme 7.
3. Conclusions
Dimethoxybenziporphyrins were synthesized in excellent
yields by the `311' methodology. The methoxy units
were able to imbue the protonated structures with a degree
of aromatic character to aid in charge delocalization, par-
ticularly in the case of 20a where steric considerations were
favorable. Cleavage of one methyl ether unit with boron
tribromide afforded the related methoxyoxybenziporphyrins
24. The free base structures showed carbaporphyrin-type
aromatic characteristics and these were retained to a signi®-
cant extent in the related dications. Treatment of 20a with
HBr in re¯uxingacetic acid cleaved both of the methyl
ethers to give the hydroxyoxybenziporphyrin 17 while the
related 3-methylbenziporphyrin oxidized to give the
unusual carbaporphyrinoid diketone 29. Both 17 and 29
are strongly diatropic by proton NMR spectroscopy and
further extend the family of aromatic carbaporphyrinoid
systems.
4. Experimental
4.1. General
1,3-Dimethoxybenzene, 2,6-dimethoxytoluene, n-butyl-
lithium, BBr3, DDQ and TFA were purchased from Aldrich
Chemical and used without further puri®cation. Chroma-
tography was performed using Grade 3 neutral alumina or
70±230 mesh silica gel. Melting points were determined on
a Thomas Hoover capillary meltingpoint apparatus and
are uncorrected. UV spectra were obtained on a Cary 1
Bio UV±visible spectrophotometer. NMR spectra were
recorded on a Varian Gemini 400 MHz NMR spectrometer;
chemical shifts were relative to chloroform at 7.27 ppm. EI
and FAB mass spectral determinations were made at the
Mass Spectral Laboratory, School of Chemical Sciences,
University of Illinois at Urbana-Champaign, supported
in part by a grant from the National Institute of General
Medical Sciences <GM 27029). Elemental analyses were
Figure 12. UV±vis spectra of porphyrinoid 29: <A) in 1% Et3N±chloro-
form <free base); <B) in 5% TFA±chloroform <cation).

3668
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
Figure 13. 400 MHz proton NMR spectrum of porphyrinoid 29 in TFA±CDCl3. The protonated species retains a very strongdiamagnetic ringcurrent.
obtained from the School of Chemical Sciences Micro-
analysis Laboratory at the University of Illinois.
water, 0.1% aqueous ferric chloride solution,27 water and
saturated sodium bicarbonate solution, and the solvent
removed under reduced pressure <the aqueous solutions
were back extracted with chloroform at each stage in the
extractions). The resultingresidue was recrystallized from
chloroform±methanol to give the desired macrocycle
<112 mg; 80%) as a purple powder, mp .2608C; UV±vis
<CHCl3): lmax <log10 e) 328 <4.77), 423 <4.78), 560 <3.71),
598 <3.87), 640 <3.78), 695 nm <3.56); UV±vis <1% TFA±
CHCl3; dication): lmax <log10 e) 319 <4.34), 356 <4.65), 443
<4.86), 561 <3.71), 607 <4.29), 679 <3.73), 745 nm <3.82); 1H
NMR <CDCl3): d 1.37 <6H, t), 1.45 <6H, t), 2.63 <6H, s),
2.99 <4H, q), 3.09 <4H, q), 4.23 <6H, s), 5.07 <1H, br s), 6.78
<1H, s), 7.16 <2H, s), 8.47 <2H, s); 1H NMR <TFA±CDCl3):
d 20.68 <1H, s), 1.19 <1H, br s), 1.46 <6H, t), 1.58 <6H, t),
3.05 <6H, s), 3.39±3.50 <8H, m), 3.68 <2H, s), 4.49 <6H, s),
7.00 <1H, s), 8.44 <2H, s), 9.60 <2H, s); 13C NMR
<TFA±CDCl3): d 11.40, 15.39, 16.32, 19.05, 19.21, 58.39,
93.58, 97.42, 114.02, 116.74, 118.05, 140.12, 140.62,
145.32, 146.11, 147.63, 153.03, 174.35; fab hr ms: Calcd
for C34H39N3O21H: 522.31205. Found: 522.3122.
4.1.1. 9,13,14,18-Tetraethyl-2,4-dimethoxy-8,19-dimethyl-
benziporphyrin 020a). In a 100 mL pear shaped ¯ask,
tripyrrane dicarboxylic acid 920,26 <122 mg) was stirred
for 2 min in TFA <1 mL) under a nitrogen atmosphere, the
solution diluted with dichloromethane <99 mL) and 4,6-
dimethoxyisophthalaldehyde <53 mg) immediately added.
The resultingmixture was stirred in the dark at room
temperature overnight. The solution was then washed with
OH
OCH3
CH3
CH3
CH3O
O
32
20b
CH3
Br
O
O
CH3
H2O
HO
4.1.2. 9,13,14,18-Tetraethyl-4-methoxy-8,19-dimethyl-2-
oxybenziporphyrin 024a). Dimethoxybenziporphyrin 20a
<40 mg) was dissolved in 1,2-dichloroethane <50 mL),
BBr3 <0.5 mL) was added and the resultingmixture placed
under a nitrogen atmosphere and stirred under re¯ux over-
night. The solution was then cooled and washed with
O
O
29
31
Scheme 8.

D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
3669
water, followed by saturated sodium bicarbonate solution.
After removingthe solvent on a rotary evaporator, the
residue was chromatographed on silica eluting with 1%
methanol±chloroform. Subsequent recrystallization from
chloroform±methanol afforded the methoxyoxybenzi-
porphyrin <24 mg; 62%) as dark navy blue crystals, mp
2868C; UV±vis <1% Et3N±CHCl3): lmax <log10 e) 344
<4.54), 422 <5.25), 453 <4.81), 534 <3.94), 574 <4.39), 624
<3.80), 686 nm <3.64); UV±vis <5% TFA±CHCl3): lmax
<log10 e) 315 <4.43), 363 <4.58), 429 <5.23), 471 <4.57),
<99 mL), 5-methyl-4,6-dimethoxyisophthalaldehyde <48 mg)
was added and stirringunder nitrogen continued at room
temperature overnight. The solution was then washed with
water, 0.1% aqueous ferric chloride solution,27 water and
saturated sodium bicarbonate solution, and the solvent
removed under reduced pressure. The resultingbluish-
purple solid was recrystallized from chloroform±methanol
to give the dimethoxybenziporphyrin <63 mg; 55%) as
bluish-purple crystals, mp 2198C; UV±vis <CHCl3): lmax
<log10 e) 315 <4.63), 328 <4.64), 401 <4.87), 619 <3.70),
662 <3.78), 717 nm <3.62); UV±vis <1% TFA±CHCl3;
dication): lmax <log10 e) 319 <4.57), 377 <4.67), 425 <4.79),
1
554 <3.86), 602 <4.29), 687 nm <3.64); H NMR <CDCl3):
d 27.45 <1H, s), 1.65±1.75 <12H, m), 3.35 <3H, s), 3.55
<3H, s), 3.67±3.74 <4H, two overlappingquartets), 3.79 <2H,
q), 3.87 <2H, q), 4.43 <3H, s), 6.83 <1H, s), 9.07 <1H, s), 9.13
<1H, s), 9.92 <1H, s), 10.50 <1H, s); 1H NMR <TFA±CDCl3):
d 21.72 <1H, unresolved doublet), 20.09 <1H, br s), 1.45±
1.50 <6H, m), 1.65 <6H, t), 2.75 <1H, s), 2.82 <1H, s), 3.15
<3H, s), 3.15 <3H, s), 3.51±3.62 <8H, m), 4.44 <3H, s), 7.20
<1H, s), 8.73 <2H, s), 9.83 <1H, s), 9.96 <1H, s); 13C NMR
<CDCl3): d 11.70, 11.98, 17.22, 17.25, 18.36, 18.40, 19.46,
19.55, 19.81, 56.67, 93.58, 94.47, 103.77, 105.75, 105.80,
112.79, 112.83, 117.7, 123.87, 131.65, 133.29, 135.96,
136.21, 136.84, 137.59, 138.05, 139.31, 144.50, 144.77,
154.45, 155.39, 169.42, 188.73; 13C NMR <TFA±CDCl3):
d 11.44, 11.60, 15.63, 16.52, 19.21, 19.32, 58.50, 93.68,
93.70, 102.10, 115.44, 116.36, 117.20, 117.59, 118.06,
139.63, 139.74, 140.77, 140.81, 145.18, 145.67, 145.93,
145.96, 146.22, 151.27, 151.37, 174.79, 176.37; fab hr
ms: Calcd for C33H37N3O21H: 508.2964. Found:
508.2963. Anal. calcd for C33H37N3O2´ H2O: C, 75.40; H,
7.48; N, 7.99. Found: C, 75.68; H, 6.99; N, 7.90.
1
548 <3.73), 591 <3.88), 721 <3.81), 783 nm <3.85); H NMR
<CDCl3): d 1.30 <6H, t), 1.38 <6H, t), 2.46 <3H, s), 2.49 <6H, s),
2.82 <4H, q), 2.90 <4H, q), 4.03 <6H, s), 6.68 <2H, s), 7.03 <1H,
s), 7.84 <2H, s), 8.55 <2H, br s); H NMR <TFA±CDCl3): d
1
1.33±1.39 <12H, m), 2.39 <3H, s), 2.66 <6H, s), 2.95 <8H, q),
4.02 <6H, s), 4.25 <1H, br s), 7.04 <2H, s), 8.22 <2H, s), 9.29
<3H, vb); 13C NMR <CDCl3): d 10.08, 10.66, 15.61, 16.29,
18.30, 18.65, 64.56, 92.94, 114.97, 123.34, 124.05, 125.03,
140.28, 141.64, 147.34, 156.17, 164.42, 168.25; 13C NMR
<TFA±CDCl3): d 10.73, 11.03, 14.68, 15.64, 18.49, 18.75,
66.34, 93.78, 107.30, 121.44, 122.22, 128.63, 140.96,
142.27, 146.12, 147.63, 151.96, 159.01, 170.47; fab hr
ms: calcd for C35H41N3O21H: 536.3277. Found:
536.3276. Anal. calcd for C35H41N3O2´ 1/2H2O: C, 77.17;
H, 7.77; N, 7.71. Found: C, 77.37; H, 7.73; N, 7.60.
4.1.5. 9,13,14,18-Tetraethyl-4-methoxy-3,8,19-trimethyl-
2-oxybenziporphyrin 024b). In a 100 mL round bottom
¯ask, dimethoxy-methylbenziporphyrin 20b <30 mg) was
dissolved in dichloromethane <50 mL), 1 mL of a 1 M solu-
tion of BBr3 in dichloromethane <.50 equiv.) was added
under nitrogen and the solution stirred at room temperature
overnight. The solution was then washed with water,
followed by saturated sodium bicarbonate solution, after
which the solvent was removed under reduced pressure.
The residue was chromatographed on Grade III alumina
elutingwith 1% methanol±chloroform. Recrystallization
from chloroform±methanol afforded the oxybenziporphyrin
<29 mg; 77%) as ¯uffy brown needles, mp 236±2378C;
UV±vis <CHCl3): lmax <log10 e) 344 <4.52), 428 <5.20),
458 <4.69), 546 <3.87), 586 <4.36), 634 <3.85), 698 nm
<3.65); UV±vis <5% TFA±CHCl3; dication): lmax <log10 e)
321 <4.49), 378 <4.58), 437 <5.04), 559 <3.76), 605 <4.21),
704 <3.71), 760 nm <3.62); 1H NMR <CDCl3): d 27.03 <1H,
s), 1.70±1.75 <12H, m), 2.68 <3H, s), 3.45 <3H, s), 3.49 <3H,
s), 3.68±3.74 <4H, m), 3.81±3.89 <4H, m), 4.39 <3H, s), 9.09
4.1.3. 9,13,14,18-Tetraethyl-4-hydroxy-8,19-dimethyl-2-
oxybenziporphyrin 017). Dimethoxybenziporphyrin 24a
<40 mg) was dissolved in acetic acid <35 mL). Following
the addition of concentrated HBr <2 mL; 48%), the solution
was heated under re¯ux overnight. The solution was diluted
with chloroform and washed with water, followed by 10%
HCl in order to generate the hydrochloride salt, and evapo-
rated under reduced pressure. The residue was recrystallized
from chloroform±hexanes to give the hydrochloride salt
<24 mg; 70%) as brown crystals. The free base could be
obtained by dissolvingthe HCl salt in methanol, adding
1±2 drops of concentrated aqueous ammonia and then ®lter-
ingthe solution, followed by washingwith methanol to
remove traces of ammonia. The free base immediately
precipitates as a brown powder, mp.3008C; UV±vis <5%
TFA±CHCl3): lmax <log10 e) 315 <4.42), 368 <4.62), 429
<5.15), 471 <4.29), 554 <3.76), 601 <4.30), 648 <3.77),
1
<1H, s), 9.16 <1H, s), 9.87 <1H, s), 10.48 <1H, s); H NMR
1
711 nm <3.81); H NMR <TFA±CDCl3): d 22.38 <1H, s,
<TFA±CDCl3): d 0.69 <1H, s), 1.40±1.44 <6H, m),
1.51±1.56 <6H, m), 2.51 <3H, s), 2.94 <3H, s), 2.96 <3H, s),
3.25±3.37 <8H, m), 4.20 <3H, s), 5.21 <1H, br s), 5.24 <1H, br
s), 5.31 <1H, s), 8.05 <1H, s), 8.07 <1H, s), 9.13 <1H, s), 9.29
<1H, s); 13C NMR <CDCl3): d 10.77, 11.74, 11.84, 17.16,
17.31, 18.32, 18.39, 19.50, 19.82, 63.02, 93.38, 94.87,
105.26, 105.93, 111.28, 118.72, 124.39, 126.78, 132.07,
133.82, 135.86, 136.34, 136.96, 137.69, 138.14, 139.78,
144.47, 144.92, 154.52, 155.82, 167.15, 189.60; 13C NMR
<TFA±CDCl3): d 9.71, 11.19, 11.34, 15.16, 15.22, 16.11,
16.14, 18.85, 19.06, 66.44, 93.50, 93.79, 109.19, 117.76,
119.60, 119.82, 120.71, 122.11, 140.73, 140.86, 141.27,
145.61, 145.84, 146.63, 147.29, 149.04, 149.45, 154.96,
155.70, 170.24, 171.69; ei hr ms: Calcd for C34H39N3O2:
CH), 20.38 <1H, br s, NH), 1.47 <6H, t), 1.64 <6H, t), 2.41
<2H, s, NH), 3.16 <6H, s), 3.60 <8H, m), 6.93 <1H, s), 8.83
<2H, s), 9.91 <2H, s); 13C NMR <TFA±CDCl3): d 11.69,
15.86, 16.72, 19.31, 19.35, 93.90, 105.42, 116.81, 117.20,
139.25, 140.48, 144.77, 144.95, 145.27, 150.09, 175.78; fab
hr ms: Calcd for C32H35N3O21H: 494.28075. Found:
494.2808.
4.1.4.
9,13,14,18-Tetraethyl-2,4-dimethoxy-3,8,19-tri-
methylbenziporphyrin 020b). Tripyrrane dicarboxylic
acid 920,26 <100 mg) and TFA <1 mL) were combined in a
pear shaped ¯ask and stirred under a nitrogen atmosphere
for 2 min. The solution was diluted with dichloromethane

3670
D. T. Richter, T. D. Lash / Tetrahedron 57 )2001) 3657±3671
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic: San Diego, 2000; Vol. 2, pp 1±55.
521.3042. Found: 521.3049. Anal. calcd for C34H39N3O2´ 1/
2H2O: C, 76.95; H, 7.59; N, 7.92. Found: C, 77.33; H, 7.26;
N, 7.29.
9. Sessler, J. L.; Gebauer, A.; Vogel, E. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic: San Diego, 2000; Vol. 2, pp 55±124.
4.1.6. 9,13,14,18-Tetraethyl-3-hydroxy-2,8,19-trimethyl-
4-oxo-3,4-dihydro-2-oxybenziporphyrin 029). Dimethoxy-
methylbenziporphyrin 20b <41 mg) was dissolved in acetic
acid <40 mL), concentrated HBr <1 mL; 48%) was added
and the solution heated to re¯ux under nitrogen for 90 min.
The solution was cooled, diluted with chloroform and washed
with water, followed by saturated aqueous sodium bicarbo-
nate solution. The solvent removed on a rotary evaporator and
the residue chromatographed on silica eluting with 50%
dichloromethane±chloroform. Recrystallization from chloro-
form±methanol gave the hydroxy compound <22 mg; 60%) as
light brownish-red crystals, mp 2728C; UV±vis <1%
Et3N±CHCl3): lmax <log10 e) 418 <5.14), 510 <4.14), 544
<4.10), 593 nm <3.76); UV±vis <5% TFA±CHCl3): lmax
<log10 e) 410 <5.37), 426 <5.35), 532 <4.08), 568 <3.92),
10. Johnson, A. W. In Porphyrins and Metalloporphyrins; Smith,
K. M., Ed.; Elsevier: Amsterdam, 1975; pp 729±754.
11. Broadhurst, M. J.; Grigg, R.; Johnson, A. W. J. Chem. Soc. )C)
1971, 3681. Ulman, A.; Manassen, J. J. Am. Chem. Soc. 1975,
97, 6540. Ulman, A.; Manassen, J. J. Chem. Soc. Perkin
Trans. I 1979, 1066. Lash, T. D.; Motta, Y. G. Heterocycles
1983, 20, 2343. Haas, W.; Knipp, B.; Sicken, M.; Lex, J.;
Vogel, E. Angew. Chem., Int. Ed. Engl. 1988, 27, 409.
Vogel, E.; Rohrig, P.; Sicken, M.; Knipp, B.; Herrmann, A.;
Pohl, M.; Schmickler, H.; Lex, J. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1651. Lash, T. D.; Armiger, Y. L. S.-T.
J. Heterocycl. Chem. 1991, 28, 965. Vogel, E.; Pohl, M.;
È
Herrmann, A.; Wiss, T.; Konig, C.; Lex, J.; Gross, M.;
Gisselbrecht, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35,
1520. Chmielewski, P. J.; Latos-Grazynski, L.; Olmstead,
M. M.; Balch, A. L. Chem. Eur. J. 1997, 3, 268. Gross, Z.;
Saltsman, I.; Pandian, R. P.; Barzilay, C. M. Tetrahedron Lett.
1997, 38, 2383.
1
584 nm <4.08); H NMR <CDCl3): d 28.52 <1H, s, interior
CH), 1.69 <3H, s), 1.76 <12H, m), 3.58 <6H, s), 3.85 <4H, m),
3.97 <4H, m), 9.39 <2H, s), 10.19 <2H, s); 1H NMR
<TFA±CDCl3): d 26.45 <1H, s), 25.62 <1H, s), 22.64 <2H,
s), 1.55 <3H, s), 1.64 <6H, t), 1.89 <6H, t), 3.59 <6H, s),
4.06±4.17 <8H, m), 10.18 <2H, s), 10.91 <2H, s); 13C NMR
<CDCl3): d 11.96, 17.42, 18.56, 19.69, 19.99, 30.21, 86.17,
94.69, 103.37, 119.34, 119.92, 131.98, 136.47, 137.27,
137.40, 144.98, 154.08, 201.24; 13C NMR <TFA±CDCl3): d
12.02, 16.58, 17.54, 20.00, 28.76, 87.19, 93.78, 110.67,
120.99, 129.03, 137.88, 138.64, 139.91, 141.09, 141.13,
143.18, 202.85; ei hr ms: Calcd for C33H37N3O3: 523.2835.
Found: 523.2825. Anal. calcd for C33H37N3O3: C, 75.69; H,
7.12; N, 8.02. Found: C, 75.49; H, 6.68; N, 6.83.
12. Lash, T. D. Synlett 2000, 279. Lash, T. D. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic: San Diego, 2000; Vol. 2, pp 125±199.
13. For related studies on porphyrin isomers with inverted pyrrole
rings, see: Chmielewski, P. J.; Latos-Grazynski, L.;
Rachlewicz, K.; Glowiak, T. Angew. Chem., Int. Ed. Engl.
1994, 33, 779. Furuta, H.; Asano, T.; Ogawa, T. J. Am.
Chem. Soc. 1994, 116, 767. Chmielewski, P. J.;
Latos-Grazynski, L. J. Chem. Soc., Perkin Trans. 2 1995,
503. Chmielewski, P. J.; Latos-Grazynski, L.; Glowiak, T.
J. Am. Chem. Soc. 1996, 118, 5690. Ishikawa, Y.;
Yoshida, I.; Akaiwa, K.; Koguchi, E.; Sasaki, T.; Furuta, H.
Chem. Lett. 1997, 453. Furuta, H.; Ogawa, T.; Uwatoko, Y.;
Araki, K. Inorg. Chem. 1999, 38, 2676. Latos-Grazynski, L. In
The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic: San Diego, 2000; Vol. 2, pp
361±416.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grant No. CHE-9732054 and the
Petroleum Research Fund, administered by the American
Chemical Society.
14. Lash, T. D.; Hayes, M. J. Angew. Chem., Int. Ed. Engl. 1997,
36, 840. Lash, T. D. Chem. Commun. 1998, 1683. Hayes, M. J.;
Spence, J. D.; Lash, T. D. Chem. Commun. 1998, 2409. For
related studies, see: Lash, T. D.; Chaney, S. T. Tetrahedron
Lett. 1996, 37, 8825. Lash, T. D.; Chaney, S. T. Chem. Eur. J.
1996, 2, 944. Lash, T. D.; Chaney, S. T. Angew. Chem., Int.
Ed. Engl. 1997, 36, 839. Hayes, M. J.; Lash, T. D. Chem.
Eur. J. 1998, 4, 508. Lash, T. D.; Richter, D. T. J. Am.
Chem. Soc. 1998, 120, 9965.
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