An experimental and computational evaluation of the energetics of the isomeric methoxyphenylcarbenes generated in carbon atom reactions

Article abstract of DOI:10.1021/ja012079t

Carbon atom reactions with anisole and methoxybenzaldehyde demonstrate the reversible ring expansion of methoxyphenylcarbene (CH₃O-C₆H₄-C-H). Trapping with HBF₄ yields the methoxytropylium ion, analogous to the

Full text of DOI:10.1021/ja012079t

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An Experimental and Computational Evaluation of the
Energetics of the Isomeric Methoxyphenylcarbenes Generated
in Carbon Atom Reactions
C. Michael Geise,† Christopher M. Hadad,*,† Fengmei Zheng,‡ and
Philip B. Shevlin*,‡
Contribution from the Department of Chemistry, The Ohio State UniVersity,
100 West 18th AVenue, Columbus, Ohio 43210, and Department of Chemistry,
Auburn UniVersity, Alabama 36849-5312
Received August 30, 2001
Abstract: Carbon atom reactions with anisole and methoxybenzaldehyde demonstrate the reversible ring
expansion of methoxyphenylcarbene (CH3O-C6H4-C-H). Trapping with HBF4 yields the methoxytropylium
ion, analogous to the well-known reactions of phenylcarbene. For instance, in the reaction of carbon atom
with p-methoxybenzaldehyde, which proceeds by deoxygenation of the carbonyl group and formation of
the corresponding arylcarbene, the products formed are methoxytropylium fluoroborate, p-methoxytoluene
and m-methoxytoluene in yields of 69.4, 7.6, and 22.9%, respectively. Gas-phase density functional theory
calculations were also carried out. The observed product yields from the uniquely generated p- and
m-methoxyphenylcarbenes are in good agreement with the calculations. In the case of o-methoxyphenyl-
carbene, however, the calculations indicate that formation of dihydrobenzofuran is the most facile
rearrangement on the ground-state singlet surface. In contrast, ring expansion is observed to be the major
reaction path experimentally. The exothermicity of the deoxygenation step for carbon atom reaction with
methoxybenzaldehyde (∼100 kcal/mol) can allow for an excited singlet state to be formed initially. This
excited singlet state of the methoxyphenylcarbene will resemble open-shell phenylnitrene, which is known
to undergo ring expansion much more readily than phenylcarbene. On the basis of this analogy, we reconcile
the difference in reactivity of the arylcarbene predicted by density functional theory calculations with the
reactivity observed experimentally.
carbenes in carbon atom reactions.7 However, each of these
methods has its own potential drawbacks. As illustrated in eqs
1 and 2 for the formation of phenylcarbene, the reaction of
Introduction
Although carbenes are an extensively studied class of
energetic intermediates,1 investigations of their chemistry are
often complicated by the fact that one must ensure that it is the
reactions of the free carbene rather than those of an energetic
precursor that are being studied.2 This seems to be a particular
problem in the case of nitrogenous carbene precursors (diazo
compounds and diazirines) in which the precursor, either in its
ground or excited state, also yields carbene-type products.3 For
this reason, it is often prudent to avoid the use of nitrogenous
precursors and employ alternatives such as the cheleotropic
extrusion of carbenes from cyclopropanes,4 fragmentation
reactions,5 and gas-phase eliminations6 or the formation of
atomic carbon with an organic substrate can lead to carbenes
by several pathways including C-H insertion (eq 1) and
deoxygenation of a carbonyl compound (eq 2).8
* Corresponding authors. C.M.H.: (e-mail) hadad.1@osu.edu; (fax) (614)
292-1685. P.B.S.: (e-mail) shevlpb@mail.auburn.edu; (fax) (334) 844-6959.
† The Ohio State University.
(3) (a) Moss, R. A.; Liu, W. J. Chem. Soc., Chem. Commun. 1993, 1597. (b)
Modarelli, D. A.; Morgan, S.; Platz, M. S. J. Am. Chem. Soc. 1992, 114,
7034. (c) Chen, N.; Jones, M., Jr.; White, W. R.; Platz, M. S. J. Am. Chem.
Soc. 1991, 113, 4981. (d) Seburg, R. A.; McMahon, R. J. J. Am. Chem.
Soc. 1992, 114, 7183. (e) Tomioka, H.; Kitagawa, H.; Izawa, Y. J. Org.
Chem. 1979, 44, 3072. (f) Yamamoto, N.; Bernardi, F.; Bottoni, A.;
Olivucci, M.; Robb, M. A.; Wilsey, S. J. Am. Chem. Soc. 1994, 116, 2064.
(g) Liu, M. T. H. Acc. Chem. Res. 1994, 27, 287. (h) Buterbaugh, J. S.;
Toscano, J. T.; Weaver, W. L.; Gord, J. R.; Hadad, C. M.; Gustafson, T.
L.; Platz, M. S. J. Am. Chem. Soc. 1997, 119, 3580.
‡ Auburn University.
(1) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New York,
1971; p 224. (b) Carbenes; Jones, M., Jr., Moss, R. A., Eds. Wiley: New
York 1973. (c) Moss, R. A. In AdVances in Carbene Chemistry; Brinker,
U. H., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, p 59.
(2) (a) Bonneau, R.; Liu, M. T. H. In AdVances in Carbene Chemistry; Brinker,
U. H., Ed.; JAI Press: Greenwich, CT, 1998; Vol. 2, p 1. (b) Platz, M. S.
In AdVances in Carbene Chemistry; Brinker, U. H., Ed.; JAI Press:
Greenwich, CT, 1998; Vol. 2, p 133. (c) C¸ elebi, S.; Leyva, S.; Modarelli,
D.; Platz, M. S. J. Am. Chem. Soc. 1993, 115, 8613. (d) Platz, M. S.; White,
W. R., III; Modarelli, D. A.; C¸ elebi, S. Res. Chem. Intermed. 1994, 20,
175.
(4) (a) Glick, H. C.; Likhotvorik, I. R.; Jones, M., Jr. Tetrahedron Lett. 1995,
36, 5715. (b) Thamattoor, D. M., Jones, M., Jr.; Pan, W.; Shevlin, P. B.
Tetrahedron Lett. 1996, 37, 8333.
9
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J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 355

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A R T I C L E S
Geise et al.
Scheme 1. Proposed Mechanisms of Product Formation in the
Reaction of C + 4
with those predicted by computational techniques with density
functional theory (DFT).12
Experimental Section
General Information. All 1H and 13C NMR were recorded on Bruker
AM 250 or Bruker AM 400 spectrometers. Infrared spectra were
obtained with an IR-44 IBM FT-IR. The GC/MS spectra were recorded
on a Fisons Trios 2000 quadrupole spectrometer interfaced with a
Hewlett-Packard Series II 5890 gas chromatograph equipped with a
30 m × 0.25 mm o.d. DB-5 column. Most of the chemicals were used
as received from the Aldrich Chemical Co. The methoxybenzaldehydes
were purified by forming bisulfite addition products from which the
aldehydes were released by treatment with sodium carbonate solution.
13C amorphous carbon was used as received from Isotec Inc.
Reaction of Atomic Carbon with Anisole (4). The carbon arc
reactor was modeled after that described by Skell and co-workers.13
Anisole (15.0 mmol) was condensed with atomic carbon at 77 K. When
the reaction was over, the reactor was allowed to warm to room
temperature, the contents were dissolved in ether and the solution was
filtered and concentrated by rotary evaporation. The resulting products
were analyzed by GC and GC/MS and compared with the authentic
samples. Relative yields are shown in Table 1.
Reaction of Atomic Carbon with Anisole, Followed by the
Addition of HBF4. The reaction was carried out as described above
by cocondensing 4 (15.0 mmol) with atomic carbon at 77 K. Before
the reactor was allowed to warm, 1.0 mL of an ethereal solution of
HBF4 (7.3 mmol) was distilled into the reaction flask. The system was
kept at 77 K for 0.5 h and then warmed to room temperature. All of
the volatile substances were pumped to a U-trap and analyzed by GC
(Table 1). The residue was extracted with acetonitrile. The extract was
filtered, the solvent evaporated, and residue taken up in CD3CN, and
the resultant methoxytropylium fluoroborate (11, Figure 1) was analyzed
by NMR: 1H NMR (δ, ppm, CD3CN) 8.76 (m, 2H, H3), 8.59 (m, 2H,
H4), 8.44 (d, 2H, H2); 13C NMR (δ, ppm, CD3CN) 182.7 (C1), 152.7
(C3), 149.1 (C4), 139.8 (C2), 61.7 (OCH3).
Although these reactions are facile routes to carbenes, they
differ from most other methods in that they involve highly
exothermic reactions and would be expected to generate
carbenes with a great deal of excess energy and perhaps yield
chemistry different from carbenes with less internal energy. In
fact, we have reported that the tolylcarbenes, generated by the
C-atom deoxygenation of the corresponding tolylaldehydes, have
sufficient energy to undergo the phenylcarbene rearrangement
even when produced at 77 K.9 We have also presented evidence
for the formation of the first excited singlet state of methylene
(1B1) in the deoxygenation of formaldehyde10 and have recently
reported an unusual fragmentation of cyclopentylidene generated
in the deoxygenation of cyclopentanone.11 This fragmentation,
which proceeds in a stepwise manner, is certainly the result of
excess energy in the carbene and may be that of an electronically
excited singlet carbene.
Reaction of Atomic Carbon-13 with Anisole, Followed by the
Addition of HBF4. The reaction was carried out by cocondensing 4
(15.0 mmol) with 13C-enriched carbon vapor at 77 K.14 The resultant
methoxytropylium fluoroborate (11) was analyzed by NMR. A com-
parison of the intensities of the 13C NMR signals of methoxytropylium
with those of methoxytropylium from the reaction of unlabeled carbon
with 4 (taken under identical NMR conditions) revealed that the signals
for C-2, C-3, and C-4 had increased by 30.9, 6.1, and 63.0%,
respectively.
Reaction of Atomic Carbon with o-, m-, and p-Methoxybenz-
aldehyde (15-17), Followed by the Addition of HBF4. 2-, 3-, or
4-methoxybenzaldehyde (15-17, 14.5 mmol) was cocondensed with
atomic carbon at 77 K. Before the reactor was allowed to warm, 1.0
mL of an ethereal solution of HBF4 (7.3 mmol) was distilled into the
flask. The system was kept at 77 K for 0.5 h and then warmed to room
temperature. All of the volatile substances were pumped to a 77 K
U-trap and analyzed by GC. The residue was extracted with acetonitrile
and filtered. The solvent was evaporated by rotary evaporation. The
resulting products were analyzed by GC, GC/MS, and NMR (Table
1).
These considerations make it important to assess the effect
of excess energy on the chemistry of carbenes resulting
from C-atom reactions. In this paper, we shall do this by
generating the o-, m-, and p-methoxyphenylcarbenes
(CH3O-C6H4-C-H) 1, 2, and 3 (Scheme 1), by C-atom
insertions and deoxygenations and comparing their reactivities
Reaction of Atomic Carbon with 1:1 mixture of m- and
p-Methoxybenzaldehyde (16 and 17). A mixture of 16 and 17 (total
15.0 mmol) was condensed with atomic carbon at 77 K. When the
(5) Pezacki, J. P.; Pole, D. L.; Warkentin, J.; Chen, T.; Ford, F.; Toscano, J.;
Fell, J.; Platz, M. S. J. Am. Chem. Soc. 1997, 119, 3191.
(6) Brinker, U. H.; Ritzer, J. J. Am. Chem. Soc. 1981, 103, 2116.
(7) Armstrong, B. M.; McKee, M. L.; Shevlin, P. B. J. Am. Chem. Soc. 1995,
117, 3685.
(8) The enthalpies in eqs 1 and 2 were calculated using the experimental ∆Hf
for phenylcarbene of 103 kcal/mol: Poutsma, J. C.; Nash, J. J.; Paulino, J.
A.; Squires, R. R. J. Am. Chem. Soc. 1997, 119, 4686.
(9) Rahman, M. M.; Shevlin, P. B. Tetrahedron Lett. 1985, 26, 2959.
(10) Ahmed, S. N.; Shevlin, P. B. J. Am. Chem. Soc. 1983, 105, 6488.
(11) Xu, G.; Chang, T.-M.; Zhou, J.; McKee, M. L.; Shevlin, P. B. J. Am. Chem.
Soc. 1999, 121, 7150.
(12) (a) Ziegler, T. Chem. ReV. 1991, 91, 651. (b) Density Functional Methods
in Chemistry; Labanowski, J., Andzelm, J., Eds.; Springer-Verlag: New
York, 1991. (c) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms
and Molecules; Oxford University Press: New York, 1989.
(13) Skell, P. S.; Wescott, L. D., Jr.; Golstein, J. P.; Engel, R. R. J. Am. Chem.
Soc. 1965, 87, 2829.
(14) Armstrong, B. M.; Zheng, F.; Shevlin, P. B. J. Am. Chem. Soc. 1998, 120,
6007.
9
356 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002

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Energetics of Isomeric Methoxyphenylcarbenes
A R T I C L E S
Table 1. Products from Carbenes 1-3 Generated by C-Atom Reactions
relative yield of products (%)
reactants
5
6
7
8
9
11
15
16
17
C + 4
29.6
11.1
9.5
a
14.5
9.3
18.9
4.2
a
6.0
7.6
14.0
2.5
5.3
22.5
6.6
12.7
13.9
a
30.4
37.1
a
a
a
a
a
a
a
a
a
a
5.7
3.0
1.7
a
a
a
a
a
a
a
a
a
a
a
2.1
1.8
1.3
C + 4 + HBF4
C + 15 + HBF4
C + 16 + HBF4
C + 17 + HBF4
C + 16 + 17
25.5
61.5
33.2
69.4
a
a
10.0
a
15.2
60.8
22.9
77.9
27.1
28.1
30.8
a
a
2.1
7.7
6.1
13.9
6.0
2.8
2.9
6.6
a
a
C + 4 + O2
10.7
10.4
21.3
41.5
32.3
2.0
C + 4 + O2 + HBF4
b
C + 4 + O2
b
a Not detected. Oxygen added to the 77 K matrix after reaction.
Figure 1. Fate of 13C atoms undergoing reaction with 4 by C-H insertion to give carbenes 1-3. Activation energies and heats of reaction (B3LYP/6-
311+G**//B3LYP/6-31G*; kcal/mol) are given for singlets 1-3 from the lowest energy methoxy conformer of the carbene.
reaction was over, the reactor was allowed to warm to room temperature
and extracted with ether. The extract was filtered and the solvent was
concentrated by rotary evaporation. The resulting products were
analyzed by GC, GC/MS, and NMR.
Reaction of Atomic Carbon with Anisole in the Presence of
Oxygen. 4 (15.0 mmol) was distilled into the reactor together with O2
(excess) and cocondensed with atomic carbon at 77 K. When the
reaction was over, the reactor was allowed to warm to room temper-
ature. The reaction was extracted with ether. The extract was filtered,
and the solvent was concentrated by rotary evaporation. The resulting
products were analyzed by GC and GC/MS and compared with the
authentic samples (Table 1).
Reaction of Atomic Carbon with Anisole in the Presence of
Oxygen, Followed by the Addition of HBF4. 4 (15.0 mmol) was
distilled into the reactor together with O2 (excess) and cocondensed
with atomic carbon at 77 K. Before the reactor was allowed to warm,
1.0 mL of an ethereal solution of HBF4 (7.3 mmol) was distilled into
the flask. The system was kept at 77 K for 0.5 h and then warmed to
room temperature. All of the volatile substances were pumped to a 77
K U-trap and analyzed by GC. The residue was extracted with
acetonitrile and filtered. The solvent was evaporated by rotary evapora-
tion. The resulting products were analyzed by GC, GC/MS, and NMR
(Table 1).
Computational Methods. The geometry of each stationary point
was optimized using the three-parameter hybrid functional of Becke
for exchange15 and the gradient-corrected functional of Lee and co-
workers for electron correlation16 (B3LYP). The 6-31G* basis set17
was used for these optimizations, and analytical second derivatives were
computed to confirm each stationary point to be a minimum by yielding
zero imaginary vibrational frequencies for the intermediates and one
imaginary vibrational frequency for each transition state. These
(15) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 1372.
(16) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(17) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (b)
Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular
Orbital Theory; John Wiley & Sons: New York, 1986.
9
J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 357

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A R T I C L E S
Geise et al.
frequency analyses are known to overestimate the magnitude of the
vibrational frequencies. Therefore, we scaled the corresponding zero-
point vibrational energy (ZPE) corrections by 0.9806.18 Single-point
energy calculations were also carried with the B3LYP/6-311+G**
level17b with six Cartesian d functions using the B3LYP/6-31G*
geometries. The energies reported in the text will correspond to this
single-point energy calculation (B3LYP/6-311+G**//B3LYP/6-31G*).
All calculations were carried out with the Gaussian 98 suite19 of
programs. In this paper, the relative energy reported corresponds to
∆H0, which includes the electronic energy of the molecule and the
scaled zero-point vibrational energy correction obtained from the
vibrational frequency analysis.
C + 4 after reaction results in the formation of methoxy-
tropylium fluoroborate 11 (eq 3) along with the previously
mentioned products. When this reaction was carried out using
carbon vapor enriched in 13C, 11 with excess 13C in the 2-, 3-
and 4-positions (30.9, 6.1, and 63.0%, respectively) was formed
(see Figure 1). The formation of 11 can be rationalized by
proposing that carbenes 1-3 ring expand to the corresponding
methoxycycloheptatetraenes 12-14, which are then protonated
by HBF4 to give 11.
Although it is reasonable to assume that 5-7 result from H
abstraction by triplet 1-3, another possible route to these
isomeric methoxytoluenes would simply be insertion of CH2,
formed by H-atom abstraction by C, into the aromatic C-H
bonds of 4. To evaluate this possibility, we have added O2 to
the reactants as a trap for the triplet carbenes. This experiment,
which results in trapping triplet 1-3 as the corresponding
methoxybenzaldehydes 15-17 (eq 4, Table 1), indicates sub-
stantial formation of triplet 1-3 in this reaction.
Results and Discussion
Reaction of Atomic Carbon with Anisole. Since there is
evidence that singlet carbon atoms react with aromatic rings
by C-H insertion to give phenylcarbenes,14,20 the reaction of
C(1D) with 4 is expected to be a source of the o-, m-, and
p-methoxyphenylcarbenes 1-3 (Scheme 1). Accordingly, we
have reacted arc-generated carbon atoms with 4 at 77 K and
have obtained the o-, m-, and p-methoxytoluenes, 5-7, di-
hydrobenzofuran, 8, and phenyl vinyl ether, 9, in the ratios
shown in Table 1. In Scheme 1, we have outlined the most
reasonable routes to these products. Thus, insertion into a
methoxy C-H bond generates phenoxyethylidene 10, from
which subsequent hydrogen migration produces 9. An insertion
into the o-, m-, and p-C-H bonds of anisole generates the
isomeric methoxyphenylcarbenes 1-3. These three singlet
carbenes can undergo intersystem crossing (ISC) to their
corresponding triplet states which can then abstract hydrogen
from unreacted anisole to give the methoxytoluenes 5-7. The
o-methoxyphenylcarbene 1 has the additional ability to undergo
intramolecular insertion into a methoxy C-H bond producing
dihydrobenzofuran 8.
Table 1 demonstrates that 16, from trapping of triplet 3,
predominates in this reaction in both the presence and absence
of HBF4. The fact that total product yields do not decrease in
the presence of O2 indicates that C(3P), which would be rapidly
scavenged by oxygen, does not react with 4 to give 1-3. The
reaction must proceed instead from C(1D). Addition of oxygen
to the 77 K matrix after reaction between C and 4 does result
in some 15-17 from reaction with carbenes 1-3 respectively.
While the reaction of C with 4 provides evidence for the
intermediacy of carbenes 1-3, it does not evaluate the extent
of ring expansion to the corresponding methoxycycloheptatet-
raenes 12-14 (eq 3), a well-documented reaction of energetic
Generation of the o-, m-, and p-Methoxyphenylcarbenes
by Deoxygenation of the Corresponding Methoxybenzalde-
hydes. Although the trapping of 11 indicates that some of the
methoxyphenylcarbenes ring-expand to the corresponding meth-
oxycycloheptatetraenes 12-14, it is possible that these products
arise by C-atom addition to the π bonds of 4 followed by ring
expansion. To evaluate this possibility, we have generated
carbenes 1-3 by the deoxygenation of the corresponding o-,
m-, and p-methoxybenzaldehydes 15-17, a reaction that yields
a distinct methoxyphenylcarbene intermediate and is comparable
in exothermicity to the formation of 1-3 by initial C-H
phenylcarbenes.21,22 We have been successful in trapping
cycloheptatetraenes formed in the reaction of C with aromatic
hydrocarbons by adding HBF4 to the products at 77 K and
isolating the corresponding tropylium fluoroborates.14 In the
present investigation, addition of HBF4 to the 77 K matrix of
(18) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7;
Gaussian, Inc.: Pittsburgh, PA, 1998.
(21) For reviews with many references, see: (a) Jones, W. M. In Rearrangements
in Ground and Excited States; de Mayo, P., Ed.; Academic: New York,
1980; Vol. 1. (b) Wentrup, C. In Methoden der Organischen Chemie
(Houben-Weyl); Regitz, M., Ed.; G. Thieme: Stuttgart, 1989; Vol. E19b,
pp 824-1021. (c) Gaspar, P. P.; Hsu, J.-P.; Chari, S.; Jones, M., Jr.
Tetrahedron 1985, 41, 1479. (d) Platz, M. S. Acc. Chem. Res. 1995, 28,
487.
(22) Recent theoretical work includes: (a) Matzinger, S.; Bally, T.; Patterson,
E. V.; McMahon, R. J. J. Am. Chem. Soc. 1996, 118, 1535. (b) Wong. M.
W.; Wentrup, C. J. Org. Chem. 1996, 61, 7022. (c) Schreiner, P. R.; Karney,
W. L.; Schleyer, P. v. R.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F.,
III. J. Org. Chem. 1996, 61, 7030.
(20) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler,
M. B.; Ferrieri, R. A.; Wolf, A. P. J. Phys. Chem. 1986, 90, 4691.
9
358 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002