Rearrangement pathways of 2-hydroxy-2-methylpropylidene: An experimental and computational study

Article abstract of DOI:10.1021/jo0160827

Photolysis of exo-2-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)propan-2-ol in benzene-d₆ afforded phenanthrene and the β-hydroxycarbene intermediate 2-hydroxy-2-methylpropylidene. The carbene showed an overwhelming preference for 1,2-methyl migration as evident from the formation of 2-butanone as the major product via the enol 2-hydroxy-2-butene. Also produced, albeit in smaller amounts, were 1-methylcyclopropanol and 2,2-dimethyloxirane from intramolecular insertion into the C-H and O-H bonds, respectively. These results stand in sharp contrast to the intramolecular reactions of simple alkylcarbenes which usually prefer insertion into C-H bonds over 1,2-alkyl migrations. Calculations at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory give a lower activation barrier for 1,2-methyl migration leading to the eventual formation of 2-butanone than for the other two pathways. The lower activation energy for methyl migration, relative to C-H and O-H insertions, strongly supports the observed experimental product distribution of the carbene. The parent carbene exists in three distinct conformations, each with stabilizing interactions between the adjacent bonds and the empty p orbital and the filled sp² orbital of the carbene center. The most stable conformer is perfectly poised for a 1,2-methyl migration as the C-CH₃ group is involved in a hyperconjugative interaction with the empty p orbital and the O-H bond is simultaneously interacting with the sp² lone pair of the carbene.

Full text of DOI:10.1021/jo0160827

J . Org. Chem. 2002, 67, 3257-3265
3257
Rea r r a n gem en t P a th w a ys of 2-Hyd r oxy-2-m eth ylp r op ylid en e: An
Exp er im en ta l a n d Com p u ta tion a l Stu d y
Robin A. Farlow and Dasan M. Thamattoor*
Department of Chemistry, Colby College, Waterville, Maine 04901
R. B. Sunoj and Christopher M. Hadad*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
hadad.1@osu.edu
Received September 3, 2001
Photolysis of exo-2-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)propan-2-ol in benzene-d
6
afforded phenanthrene and the â-hydroxycarbene intermediate 2-hydroxy-2-methylpropylidene. The
carbene showed an overwhelming preference for 1,2-methyl migration as evident from the formation
of 2-butanone as the major product via the enol 2-hydroxy-2-butene. Also produced, albeit in smaller
amounts, were 1-methylcyclopropanol and 2,2-dimethyloxirane from intramolecular insertion into
the C-H and O-H bonds, respectively. These results stand in sharp contrast to the intramolecular
reactions of simple alkylcarbenes which usually prefer insertion into C-H bonds over 1,2-alkyl
migrations. Calculations at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory give a lower
activation barrier for 1,2-methyl migration leading to the eventual formation of 2-butanone than
for the other two pathways. The lower activation energy for methyl migration, relative to C-H
and O-H insertions, strongly supports the observed experimental product distribution of the
carbene. The parent carbene exists in three distinct conformations, each with stabilizing interactions
2
between the adjacent bonds and the empty p orbital and the filled sp orbital of the carbene center.
3
The most stable conformer is perfectly poised for a 1,2-methyl migration as the C-CH group is
involved in a hyperconjugative interaction with the empty p orbital and the O-H bond is
2
simultaneously interacting with the sp lone pair of the carbene.
I. In tr od u ction
Sch em e 1
Sch em e 2
Insertions into â-C-H bonds to form olefins and, where
possible, into γ-C-H bonds to form cyclopropanes are
among the most common intramolecular reactions of
1
many singlet alkylcarbenes (Scheme 1). Notably, how-
ever, alkyl migrations to the divalent carbon are much
less common in simple, acyclic alkylcarbenes, and do not
appear to compete effectively against hydrogen migra-
tions.
For instance, isopropylcarbene (1) may undergo a 1,2-H
migration to produce isobutylene, 1,3-C-H insertion to
produce 1-methylcyclopropane, or 1,2-methyl migration
to give cis- and trans-2-butenes (Scheme 2a). Experimen-
tally, however, the thermal and photochemical decom-
position of a number of different precursors to isopropyl-
carbene leads to 1-methylcyclopropane and isobutylene
chemistry is even more limited (Scheme 2b). tert-Butyl-
carbene can only undergo a 1,2-methyl migration to give
as the dominant products with very little, if any, of the
2
2
-butene isomers produced.
2
-methyl-2-butene and a 1,3-C-H insertion to give 1,1-
Replacing the â-H of isopropylcarbene with a methyl
group gives tert-butylcarbene (2), whose intramolecular
dimethylcyclopropane. However, the experimental results
are most interesting. Thermal decomposition of a variety
of precursors under several different conditions has
*
To whom correspondence should be addressed. (D.M.T.) E-mail:
dmthamat@colby.edu. Fax: (207) 872-3804. (C.M.H.) Fax: (614) 292-
685.
1) For an overview see: (a) Kirmse, W. Carbene Chemistry;
(2) (a) Friedman, L.; Shechter, H. J . Am. Chem. Soc. 1959, 81, 5512.
(b) Kirmse, W.; Doering, W. von E. Tetrahedron 1960, 11, 266. (c)
Friedman, L.; Berger, J . G. J . Am. Chem. Soc. 1961, 82, 492. (d)
Friedman, L.; Berger, J . G. J . Am. Chem. Soc. 1961, 82, 500. (e)
Franzen, V.; Schmidt, H. J .; Merz, C. Chem. Ber. 1961, 94, 2942. (f)
Kirmse, W.; von B u¨ low, B. Chem. Ber. 1963, 96, 3323. (g) Neuman, R.
C., J r. Tetrahedron Lett. 1964, 2541. (h) Taeger, E.; Fielder, C. J ustus
Liebigs Ann. Chem. 1966, 696, 42. (i) Mansoor, A. M.; Stevens, I. D.
R. Tetrahedron Lett. 1966, 1733. (j) Kirmse, W.; von Scholtz, H. D.;
Arold, H. J ustus Liebigs Ann. Chem. 1968, 711, 22.
1
(
Academic: New York, 1971. (b) Carbenes; J ones, M., J r., Moss, R., Eds.;
Wiley: New York, 1973; Vol. 1. (c) Moss, R. A. In Advances in Carbene
Chemistry; Brinker, U. H., Ed.; J AI Press: Stamford, CT, 1994; Vol. 1,
p 59. (d) Bonneau, R.; Liu, M. T. H. In Advances in Carbene Chemistry;
Brinker, U. H., Ed.; J AI Press: Stamford, CT, 1998; Vol. 2, p 1. (e)
Platz, M. S. In Advances in Carbene Chemistry; Brinker, U. H., Ed.;
J AI Press: Stamford, CT, 1998; Vol. 2, p 133.
1
0.1021/jo0160827 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/19/2002

3
258 J . Org. Chem., Vol. 67, No. 10, 2002
Farlow et al.
Sch em e 3
Sch em e 4
established a general preference for the formation of the
cyclopropane derivative (90%) over the olefin (10%).2d,3
Photolysis of tert-butyldiazomethane or tert-butyldiazir-
ine, on the other hand, gives the two products in a
significantly different ratio of 50:50.3e The latter results
are clearly anomalous and are thought to be contami-
Sch em e 5
nated by the chemistry of the excited states of the
nitrogenous precursors.1
d,e,3h
Considerable evidence has
been accumulated over the years to show that the excited
states of diazirines and diazo compounds can also give
rise to products that were originally thought to arise from
bona fide carbenes.1
d,e,4
Indeed, a more recent study of
tert-butylcarbene, photochemically generated from a reli-
able non-nitrogenous source (3), showed the same over-
whelming preference for 1,3-C-H insertion over 1,2-
methyl migration (Scheme 3) as observed in the thermally
generated carbene reactions.3h There is also a noteworthy
temperature effect wherein the cyclopropane derivative
is the sole product observed at -78 °C.3
carbenes have been reported to date.9,10 An acyclic
â-hydroxycarbene, HOCH , as generated ther-
mally from a diazirine precursor, proceeds to rearrange
g,h
2
-C-CH
3
Thus, for simple acyclic carbenes, insertions into 1,2-
and 1,3-C-H bonds are clearly preferred to alkyl migra-
exclusively via 1,2-H migration to generate propanal
a
(
Scheme 4a).9 Tomioka and Nunome reported the photo-
5
tions. However, substituents can have a profound influ-
chemical generation of a phenyl-substituted â-hydroxy-
carbene from the corresponding diazo precursor (Scheme
6
ence on the reactivity of carbenes. In particular, electron-
donating groups at the â-position are believed to accelerate
4
b), and they observed an experimental 4:1 preference
1
,2-migrations of hydrogen and a phenyl ring by stabiliz-
for alkyl (R) migration over O-H insertion and with no
7
ing the electron-deficient â-carbon in the transition state.
In this context, it is interesting that the introduction of
a weakly electron-donating methyl group in place of
hydrogen, as in going from isopropylcarbene to tert-
butylcarbene, does not increase the preference for a 1,2-
methyl migration. We, therefore, set out to examine the
effect of a stronger π-electron-donating functionality, such
as the OH group at the â-position, on the preference for
alkyl migration vs O-H and 1,3-C-H insertion reactions.
Currently there is limited information available about
the effect of â-heteroatoms on carbene reactivity.1
Specifically, only a few examples of (putative) â-hydroxy-
9
b
1
,3-C-H insertion.
Two reports10 of â-hydroxycarbenes utilized closely
related cyclic species as generated from diazirine precur-
sors 41 and 5
0a
10b
(Scheme 5). In both cases, 1,2-H
migrations were the dominant processes, and only small
amounts of 1,3-O-H insertions were observed. Alkyl
migrations, however, were conspicuously absent as evi-
dent from the lack of any ring-contracted products.
However, these observations could have been due to ring-
strain effects and improper orbital alignment to facilitate
the alkyl migration.
a,c,8
In each of these previous studies,9,10 nitrogenous
precursors were utilized, and the observed product
distribution may be due to species other than the desired
carbene (although this is less common in thermolysis
reactions). In this paper, we describe the chemistry of
(
3) (a) Kirmse, W.; Wedel, W. v. J ustus Liebigs Ann. Chem. 1963,
6
66, 1. (b) Frey, H. M. Adv. Photochem. 1964, 4, 225. (c) Frey, H. M.;
Stevens, I. D. R. J . Chem. Soc. 1965, 1301. (d) Goldstein, M. J .; Dolbier,
W. R., J r. J . Am. Chem. Soc. 1965, 87, 2293. (e) Chang, K.-T.; Shechter,
H. J . Am. Chem. Soc. 1979, 101, 5082. (f) Fukushima, M.; J ones, M.,
J r.; Brinker, U. H. Tetrahedron Lett. 1982, 23, 2212 (g) Armstrong, B.
M.; McKee, M. L.; Shevlin, P. B. J . Am. Chem. Soc. 1995, 117, 3689.
2
-hydroxy-2-methylpropylidene (6), an example of an
acyclic â-hydroxycarbene, which was generated from a
non-nitrogenous precursor.
(
3
h) Glick, H.; Likhotvorik, I. R.; J ones, M., J r. Tetrahedron Lett. 1995,
6, 5715.
(
4) (a) Tomioka, H.; Kitagawa, H.; Izawa, Y. J . Org. Chem. 1979,
4
1
4, 3072. (b) Seburg, R. A.; McMahon, R. J . J . Am. Chem. Soc. 1992,
14, 7183. (c) Modarelli, D. A.; Morgan, S. C.; Platz, M. S. J . Am. Chem.
Soc. 1992, 114, 7034. (d) White, W. R.; Platz, M. S. J . Org. Chem. 1992,
7, 2841. (e) Celebi, S.; Leyva, S.; Modarelli, D. A.; Platz, M. S. J . Am.
5
Chem. Soc. 1993, 115, 8613. (f) Yamamoto, N.; Bernardi, F.; Bottoni,
A.; Olivucci, M.; Robb, M. A.; Wilsey, S. J . Am. Chem. Soc. 1994, 116,
Carbene 6 is derived by formally replacing the â-hy-
drogen of 1 with a hydroxyl group. As in tert-butylcar-
bene, 6 does not have the option of a 1,2-H migration.
2
064. (g) Huang, H.; Platz, M. Tetrahedron Lett. 1996, 37, 8337. (h)
Huang, H.; Platz, M. S. J . Am. Chem. Soc. 1998, 120, 5990. (i) Kirmse,
W.; Buschhoff, M. Chem. Ber. 1967, 100, 1491.
(5) Certain strained cyclic carbenes do favor alkyl migrations over
hydrogen migrations and are not included in this discussion. For
examples, see ref 1a.
(9) (a) Schmitz, E.; H o¨ rig, C.; Gr u¨ ndemann, C. Chem. Ber. 1967,
100, 2093. (b) Tomioka, H.; Nunome, Y. J . Chem. Soc., Chem. Commun.
1990, 1243.
(
6) Nickon, A. Acc. Chem. Res. 1993, 26, 84.
7) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J . Phys.
(
Chem. A 1998, 102, 8467.
8) (a) Lambert, J . B.; Liu, X. Tetrahedron 1997, 53, 9989. (b) Creary,
X.; Butchko, M. A. J . Org. Chem. 2001, 66, 1115 and references therein.
(10) (a) Schmitz, E.; Stark, A.; H o¨ rig, C. Chem. Ber. 1965, 98, 2509.
(b) Morgan, K. M.; O’Connor, M. J .; Humphrey, J . L.; Buschman, K.
E. J . Org. Chem. 2001, 66, 1600.
(

Rearrangement of 2-Hydroxy-2-methylpropylidene
J . Org. Chem., Vol. 67, No. 10, 2002 3259
Sch em e 6a
Sch em e 7
a
Reagents and conditions: (a) N2CHCO2Et, CuSO4 (cat.); (b)
+
(
(
1) NaOH, (2) H3O ; (c) (1) MeLi, (2) aqueous NH4Cl; (d) (1) MeLi,
2) aqueous NH4Cl.
Its intramolecular reactions are limited to a possible 1,2-
methyl migration as well as insertions into the γ-C-H
and O-H bonds. However, in striking contrast to tert-
butylcarbene, â-hydroxycarbene 6 shows an overwhelm-
ing preference for a methyl migration, and insertions into
γ-C-H and O-H bonds are remarkably minor pathways.
This interesting behavior of 6 has also been examined
computationally, and the insights obtained are presented
below.
Sch em e 8
II. Resu lts a n d Discu ssion
A. Syn th esis of th e P r ecu r sor . Cyclopropanated
phenanthrene and many of its derivatives, such as 3,
have been used as reliable non-nitrogenous sources to
revealed the presence of 2-butanone (10), 1-methylcyclo-
propanol (11), and 2,2-dimethyloxirane (12) in a relative
ratio of 69:28:3 (Scheme 7) and a combined yield of 55%.
The identity of the products was confirmed by comparison
to authentic standards.14 Initial attempts with GC/MS
analysis were complicated by the instability of the
compounds on the GC column. NMR analyses circum-
vented these problems completely. In separate control
experiments, 10-12 were independently photolyzed, and
it was determined that these three compounds do not
interconvert under the conditions employed for the pho-
tolysis of 7.
photochemically generate several carbenes.3
h,11
Accord-
ingly, we undertook the synthesis of exo-2-(1a,9b-dihydro-
H-cyclopropa[l]phenanthren-1-yl)propan-2-ol (7) as a
1
precursor to the â-hydroxycarbene 6. The synthesis of 7
was accomplished as outlined in Scheme 6. Thus, phenan-
threne (8) was cyclopropanated with ethyl diazoacetate
using CuSO
4
as a catalyst, and the resulting ester was
1
2
hydrolyzed in situ to the corresponding carboxylic acid.
Treatment of the carboxylic acid with excess methyl-
lithium followed by quenching with aqueous ammonium
chloride led to the methyl ketone 911h after purification
by column chromatography. The ketone was subse-
quently converted into the tertiary alcohol 7 by the action
of methyllithium and aqueous ammonium chloride. The
The formation of 11 and 12 may be attributed to 1,3-
C-H and O-H insertions, respectively, from the carbene
6. As there are six available C-H bonds, as opposed to
only one O-H bond, insertion into the weaker C-H bond
is favored over that into the stronger O-H bond by a
factor of 1.6. The major product of the reaction, 2-bu-
tanone, likely arises from a 1,2-migration of the methyl
group in 6 to give enol 13, which subsequently tautomer-
izes to 10. It has been pointed out, however, that
â-hydroxycarbenes could, in principle, undergo a 1,3-
proton shift to afford a zwitterionic intermediate such
as 14 (Scheme 8).1 Carbocation rearrangement in 14
could then also lead to the ketone 10. Deuterium labeling
experiments would not distinguish between these two
pathways; however, a previous computational study of
the carbene derived from 5 indicates that the zwitterionic
1
13
structure of 7 was confirmed by means of H NMR,
C
NMR, and elemental analysis. In particular, the stereo-
chemistry of compounds 7 and 9 was established on the
basis of the low J values of approximately 4 Hz for
coupling between the two different types of protons on
the cyclopropyl ring. This value is consistent with the
exo substitution pattern.13
B. P h otoch em ica l Stu d ies. The alcohol 7 was pho-
tolyzed in benzene-d at room temperature using 1,3-
6
0b
benzodioxole as an internal standard. The photolyzate
1
was monitored periodically by H NMR spectroscopy until
the starting material had completely disappeared. Typi-
cally, the starting material was consumed in approxi-
1
10b
mately 12 h. Analysis by H NMR of the photolyzate
intermediate is unlikely to be involved. We should also
note that Kirmse and Buschhoff have experimentally
investigated the alkoxy analogue of 6, in which the
proton-shift issue was eliminated. However, their studies
utilized diazo or diazirine precursors, and significant
differences were observed under photolytic, thermal, and
Cu- or acid-catalyzed reactions.
The rearrangement of 6 is in sharp contrast to those
of both 1 and 2 in that neither of the latter two carbenes
(
11) (a) Richardson, D. B.; Durrett, L. R.; Martin, J . M., J r.; Putnam,
W. E.; Slaymaker, S. C.; Dvoretzky, I. J . Am. Chem. Soc. 1965, 87,
763. (b) Chateauneuf, J . E.; J ohnson, R. P.; Kirchhoff, M. M. J . Am.
2
Chem. Soc. 1990, 112, 3217. (c) Ruck, R. T.; J ones, M., J r. Tetrahedron
Lett. 1998, 39, 2277. (d) Ruck, R. T.; J ones, M., J r. Tetrahedron Lett.
1
998, 39, 4433. (e) Robert, M.; Likhotvorik, I.; Platz, M. S.; Abbot, S.
C.; Kirchhoff, M. M.; J ohnson, R. J . Phys. Chem. A 1998, 102, 1507.
f) Nigam, M.; Platz, M. S.; Showalter, B. M.; Toscano, J . P.; J ohnson,
R.; Abbot, S. C.; Kirchhoff, M. M. J . Am. Chem. Soc. 1998, 120, 8055.
g) Thamattoor, D. M.; Snoonian, J . R.; Sulzbach, H. M.; Hadad, C. M.
J . Org. Chem. 1999, 64, 5886.
12) (a) Drake, N. L.; Sweeney, T. R. J . Org. Chem. 1946, 11, 67. (b)
Ford, W. T.; Newcomb, M. J . Am. Chem. Soc. 1973, 95, 6277.
13) Wan, P.; Budac, D.; Earle, M.; Shukla, D. J . Am. Chem. Soc.
990, 112, 8048.
4i
(
(
(
(14) The ketone 10 and epoxide 12 were commercially available. The
alcohol 11 was prepared according to a literature procedure. See:
DePuy, C. H.; Klien, R. A.; Dappen, G. M. J . Org. Chem. 1962, 27,
3742.
(
1

3
260 J . Org. Chem., Vol. 67, No. 10, 2002
Farlow et al.
F igu r e 1. Interconversion pathways for 6. The free energies at 298 K (relative to 6) at the B3LYP/6-311+G**//B3LYP/6-31G*
level are given below each structure. Key bond distances are shown in angstroms.
shows much, if any, participation of a 1,2-methyl mi-
gration. Thus, it appears that the presence of the â-
hydroxy group strongly perturbs the rearrangement
pathways available to 6, thereby favoring 1,2-methyl
migration. Our computational investigation into the
properties and transformations of 6 is presented in the
next section.
focused our attention only on the singlet surface to
explore the possible rearrangement pathways.
The singlet carbene has been found to exist in three
major conformers (6, 6a , and 6b) with interesting struc-
tural features (Figure 1). The most stable conformer of 6
is found to have a syn H-O-C-Ccarbene arrangement. The
conformer with an anti H-O-C-Ccarbene arrangement
(6a ) is found to be 5.53 kcal/mol higher in free energy
(298 K) at the B3LYP/6-311+G**//B3LYP/6-31G* level.
All three conformers (6, 6a , and 6b ) are within 5.53
kcal/mol of each other. It can be seen from the optimized
C. Com p u ta tion a l Stu d ies. Theoretical studies on
the effect of bystander substituents on the reaction of
carbenes have gained considerable attention.7
,8b,15
Such
studies have often been employed to rationalize the
experimentally observed product distribution. Even though
the effect of substituents on the mode of rearrangement
in many cyclic carbenes has been studied, analogous
studies with acyclic carbenes are not widely available.
It is particularly interesting to understand the factors
responsible for the changes in rearrangement preferences
for a â-hydroxycarbene. From the previous discussions,
the experimentally observed product ratio for the rear-
rangement of 6 suggests that methyl migration is favored
over insertions into the γ-C-H and O-H bonds. To
rationalize the observed product distribution, we have
also undertaken a computational investigation of the
major rearrangement pathways available to 6.
3
geometry of 6 (and 6a ) that one of the C-CH bonds is
elongated compared to a typical C-C single bond. This
is due to a hyperconjugative interaction between the
C-CH
3
bond and the empty p orbital on the carbene
bond is of particular
carbon. An elongated C-CH
3
relevance for methyl migration in 6. Furthermore, there
is also a significant difference in the C-O bond length
of 6, 6a , and 6b. These differences are derived from a
hydrogen-bonding arrangement between the O-H group
2
and the carbene’s sp lone pair and donation of the oxygen
(16) (a) Sulzbach, H. M.; Platz, M. S.; Schaefer, H. F., III; Hadad,
C. M. J . Am. Chem. Soc. 1997, 119, 5682. (b) Xie, Y.; Schreiner, P. R.;
Schleyer, P. v. R.; Schaefer, H. F., III. J . Am. Chem. Soc. 1997, 119,
1
2
7
370. (c) Geise, C. M.; Hadad, C. M. J . Am. Chem. Soc. 2000, 122,
863. (d) Mendez, F.; Garcia-Garibay, M. A. J . Org. Chem. 1999, 64,
061. (e) Bettinger, H. F.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer,
Density functional theory methods have been success-
fully utilized to gain valuable insights for a number of
1
6
interesting carbenes. We have chosen to use the hybrid
H. F. In The Encyclopedia of Compuational Chemistry; Schleyer, P. v.
R., Allinger, N. L., Clark, T., Gasteiger, J ., Kollman, P. A., Schaefer,
H. F., Schreiner, P. R., Eds.; J ohn Wiley and Sons Inc.: Chichester,
U.K., 1998; pp 183-196. (f) Seburg, R. A.; Hill, B. T.; Squires, R. R. J .
Chem. Soc., Perkin Trans. 2 1999, 11, 2249. (g) Armstrong, B. M.;
McKee, M. L.; Shevlin, P. B. J . Org. Chem. 1998, 63, 7408. (h) Hu, J .;
Hill, B. T.; Squires, R. R. J . Am. Chem. Soc. 1997, 119, 11699. (i)
Poutsma, J . C.; Nash, J . J .; Paulino, J . A.; Squires, R. R. J . Am. Chem.
Soc. 1997, 119, 4686. (j) Gleichmann, M. M.; Doetz, K. H.; Hess, B. A.
J . Am. Chem. Soc. 1996, 118, 10551. (k) Bettinger, H. F.; Schleyer, P.
v. R.; Schreiner, P. R.; Schaefer, H. F. In Modern Electronic Structure
Theory and Applications to Organic Chemistry; Davidson, E. L., Ed.;
World Scientific Press Inc.: Singapore, 1997; pp 89-171. (l) Geise, C.
M.; Hadad, C. M.; Zheng, F.; Shevlin, P. B. J . Am. Chem. Soc. 2002,
B3LYP method17 for the present study. Full geometry
optimizations of 6 in both singlet and triplet electronic
states were performed using the B3LYP/6-31G* level of
theory. Using the most stable conformer of each multi-
plicity, the singlet state is found to be the ground state,
and the triplet state is higher in energy by 1.8 kcal/mol
at the B3LYP/6-311+G**//B3LYP/6-31G* level. At the
CBS-QB3 level of theory, the singlet state of 6 is similarly
favored by 3.3 kcal/mol over the triplet state. We have
1
24, 355. (m) Geise, C. M.; Hadad, C. M. J . Org. Chem. 2000, 65, 8348.
(17) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J .
(
15) (a) Evanseck, J . D.; Houk, K. N. J . Phys. Chem. 1990, 94, 5118.
b) Evanseck, J . D.; Houk, K. N. J . Am. Chem. Soc. 1990, 112, 9148-
156.
(
9
Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys.
Rev. B 1988, 37, 785.

Rearrangement of 2-Hydroxy-2-methylpropylidene
J . Org. Chem., Vol. 67, No. 10, 2002 3261
F igu r e 2. Rearrangement pathways for 6 (with a syn arrangement for H-O-C-Ccarbene). The free energies at 298 K (relative to
) at the B3LYP/6-311+G**//B3LYP/6-31G* level are given below each structure. Key bond distances are shown in angstroms.
6
Ta ble 1. Activa tion En er gies (k ca l/m ol) Com p u ted a t th e
give rise to the O-H insertion product. Instead, only 6b
can form the O-H insertion product because the migra-
tion occurs by interaction of the oxygen lone pair with
the empty p orbital of the carbene center. In the more
stable 6, the O-H hydrogen interacts with the sp lone
3
pair orbital of the carbene, and the C-CH bond interacts
a
B3LYP /6-31G* Level
reaction
∆EBW
∆H298
∆G298
6
6
6
6
6
6
6
f TS(6-10a ) f 10a
f TS(6-11a ) f 11a
a f TS(6a -10b) f 10b
a f TS(6a -11b) f 11b
b f TS(6b-12) f 12
f TS(6-6a ) f 6a
0.06
2.34
0.99
1.06
6.44
9.19
8.51
-0.42
1.12
0.64
0.01
4.48
8.23
7.46
0.09
2.02
0.86
0.89
5.05
8.58
7.80
2
with the empty p orbital of the carbene. As such, the
O-H group must rotate from its preferred conformation
in 6 (with a hydrogen bond between the O-H hydrogen
and the carbene lone pair) to the orientation in 6b to
insert into the O-H bond. In 6b, the O-H bond is
f TS(6-6b) f 6b
a
∆
EBW represents the energies evaluated at the bottom of the
well and do not include the ZPE corrections. Other parameters
2
pointed away from the carbene sp lone pair, and instead,
(
∆H298 and ∆G298) contain scaled ZPE as well as thermal and
entropic corrections as appropriate.
the oxygen lone pair is coordinated to the carbene’s empty
p orbital. Carbene 6b is, therefore, higher in energy than
lone pair into the empty p orbital of the carbene center.
Also, the barriers to interconversion among 6, 6a , and
6
and 6a . Furthermore, the free energy of activation to
form oxirane 12 from 6b (via TS(6b -12)) is ∼4.6
kcal/mol (Figure 4), and oxirane 12 formation will not
compete effectively with enol 10a formation.
We have examined the effect of basis set on the
energetics of these reactions, by single-point energy
calculations with the 6-31+G** and 6-311+G** basis sets
at the B3LYP level. These results are provided in Table
6
b are quite large (∼8 kcal/mol), and the conformation
in which the carbene is born may play a key role in
determining its reactions (Figure 1).
Rearrangement pathways for the carbene emanate
from the different conformations via unique transition
states (Figures 2-4). Calculated activation barriers at
the B3LYP/6-31G* level are given in Table 1. It can be
noted that the free energy of activation for the methyl
migration in 6, leading to the formation of (Z)-enol 10a ,
is ∼0.1 kcal/mol (Figure 2). A similar process from 6a
leading to 10b is found to have a slightly higher barrier
2
. Comparison of the computed free energy of activation
with the larger basis sets reveals trends similar to those
of the B3LYP/6-31G* results. Explicit inclusion of elec-
tron correlation has also been considered by evaluating
energies at the CCSD(T) level with the 6-31+G** and
(
∼0.8 kcal/mol, Figure 3). The very low free energy of
18
cc-pVDZ basis sets. The computed energies, without the
activation for methyl migration is due to the congenial
molecular geometry of the singlet carbene and the transi-
tion state for methyl migration. In particular, the elon-
inclusion of zero-point vibrational energy and thermal
corrections, are provided in Table 3. Interestingly, all of
the previously noted trends, such as the preferences for
3
gated C-CH bond in 6 (and 6a ) is oriented in a manner
1
,2-methyl migration to form the (Z)-enol 10a , remain
to stabilize the empty p orbital on the carbenic carbon and
the same with each theoretical level. These results are
in very good agreement with the experimental product
ratio of 69:28 for 10:11 as given in Scheme 7.
is perfectly poised to migrate the methyl group.1
6a
Formation of 11 by 1,3-C-H insertion is slightly higher
in energy (∼1.3 kcal/mol, Figure 2) compared to the
methyl migration process. The free energy of activation
Important geometrical details for the different pro-
cesses are highlighted in Figures 1-4. The C-CH
length in 6 (1.659 Å) is longer than those in 6a (1.614 Å)
and 6b (1.517 Å). Corresponding C-CH bond lengths
3
bond
(4.6 kcal/mol) leading to the formation of 12 from carbene
6
b is found to be the highest energy pathway among the
3
three major routes considered in this study (Figure 4).
It is also interesting to note that neither 6 nor 6a can
(18) Dunning, T. H., J r. J . Chem. Phys. 1989, 90, 1007.

3
262 J . Org. Chem., Vol. 67, No. 10, 2002
Farlow et al.
F igu r e 3. Rearrangement pathways for 6a (with an anti arrangement for H-O-C-Ccarbene). The free energies at 298 K (relative
to 6) at the B3LYP/6-311+G**//B3LYP/6-31G* level are given below each structure. Key bond distances are shown in angstroms.
F igu r e 4. Rearrangement pathways for 6b (with hydroxyl hydrogen in a pseudo-gauche arrangement with the carbene center).
The free energies at 298 K (relative to 6) at the B3LYP/6-311+G**//B3LYP/6-31G* level are given below each structure. Key
bond distances are shown in angstroms.
Ta ble 2. Activa tion En er gies (k ca l/m ol) Com p u ted Usin g
The distance between the carbenic carbon and the
carbon bearing the hydroxyl group shows an expected
variation on going from reactant to product. At the early
transition state TS(6-10a ), this bond is shortened and
eventually converts to a double bond in the product. The
1,3-C-H insertion transition states TS(6-11a ) and
TS(6a -11b), which form 11, possess a bridged hydrogen
as revealed by the structural features. Formation of the
cyclopropane ring takes place concomitantly as the
migration of the H atom from the methyl to the carbenic
carbon. A similar situation occurs in the transition state
for the O-H insertion, as evident from the structural
details of TS(6b-12). Migration of the H is accompanied
by the formation of the oxirane ring, resulting in 12.
a
th e B3LYP /6-31G* Geom etr ies
B3LYP/6-31+G**
B3LYP/6-311+G**
reaction
∆EBW ∆H298 ∆G298 ∆EBW ∆H298 ∆G298
6
6
6
6
6
6
6
f TS(6-10a ) f 10a
f TS(6-11a ) f 11a
0.17 -0.31 0.19 0.11 -0.37 0.14
1.47
0.25 1.15 1.61
0.74 0.97 0.93
0.39 1.29
0.53 0.79
a f TS(6a -10b) f 10b 1.11
a f TS(6a -11b) f 11b 0.51 -0.54 0.21 0.63 -0.44 0.33
b f TS(6b-12) f 12
f TS(6-6a ) f 6a
f TS(6-6b) f 6b
8.43
8.62
8.82
3.65 4.06 9.60
7.66 8.01 8.43
7.77 8.11 9.16
4.17 4.57
7.47 7.82
8.11 8.45
a
∆EBW represents the energies evaluated at the bottom of the
well and do not include ZPE corrections. Other parameters (∆H298
and ∆G298) include scaled ZPE as well as thermal and entropic
corrections as appropriate.
at the transition states TS(6-6a ) and TS(6-6b) are
shorter than that in 6 (Figure 1) due to the reduced
hyperconjugation between the empty p orbital on the
Involvement of a zwitterionic structure by a 1,3-proton
shift (Scheme 8) is addressed by a separate series of
calculations. All attempts to locate an optimized geometry
for the proposed zwitterionic intermediate 14 led to
spontaneous methyl migration to the carbocation center
to generate 10 or C-O bond formation to generate 12.
The preference for each process depended strongly on the
starting Ccarbene-C-O bond angle, but no intermediates
could be found, despite exhaustive attempts. The zwit-
terionic structure 14 would also be favored by polar
carbenic carbon and the C-CH
consequence of the geometrical features of the carbenes
and 6a is that the activation barrier for formation of
the (Z)-enol from 6 is slightly lower than that from 6a .
In the transition state, the C-CH bond has to undergo
3
bond. An important
6
3
further elongation compared to that in the parent carbene
so as to effect the methyl migration.

Rearrangement of 2-Hydroxy-2-methylpropylidene
J . Org. Chem., Vol. 67, No. 10, 2002 3263
Ta ble 3. Activa tion En er gies (k ca l/m ol) Com p u ted a t th e B3LYP a n d CCSD(T) Levels Usin g Va r iou s Ba sis Setsa
B3LYP
CCSD(T)
reaction
6-31G*
6-31+G**
6-311+G**
6-31+G**
cc-pVDZ
6
6
6
6
6
6
6
f TS(6-10a ) f 10a
f TS(6-11a ) f 11a
a f TS(6a -10b) f 10b
a f TS(6a -11b) f 11b
b f TS(6b-12) f 12
f TS(6-6a ) f 6a
0.06
2.34
0.99
1.06
6.44
9.19
8.51
0.17
1.47
1.11
0.51
8.43
8.62
8.82
0.11
1.61
0.93
0.63
9.60
8.43
9.16
0.66
1.37
2.32
0.89
6.68
7.54
6.58
0.55
0.82
2.34
0.28
5.30
7.58
7.02
f TS(6-6b) f 6b
a
The values are evaluated without ZPE and thermal corrections using the B3LYP/6-31G* optimized geometries in all cases.
F igu r e 5. Atomic charges at the AIM level (in electrons) for the rearrangement pathways for 6 using the B3LYP/6-311+G**//
B3LYP/6-31G* wave functions.
solvents, and such effects are not considered in these gas-
phase calculations. (We should also note that the current
experiments were performed in benzene.) Thus, any
involvement of a 1,3-proton shift in 6 will essentially be
accompanied by instantaneous formation of 10 or 12. It
was also conceivable that a transition state would exist
in which proton transfer and methyl migration occurred
simultaneously. Attempts to locate such a transition state
ultimately led to the same transition state (TS(6-10a ))
for 1,2-methyl migration from 6 to 10a , in which the O-H
bond participates to stabilize the sp2 lone pair in the
transition state.
sented here do reproduce the experimental observations
quite nicely.
The effect of the â-hydroxyl group on various migration
processes of the carbene can be further understood with
the help of charge density analyses for both the reactant
6
and the transition states. We have evaluated the atomic
charges on all of the systems using Bader’s theory of
20
atoms in molecules (AIM) and the natural population
21
analysis (NPA) method of Reed and Weinhold at the
B3LYP/6-311+G**//B3LYP/6-31G* level (the NPA results
are provided in the Supporting Information). Trends from
the AIM and NPA analyses were found to be in fairly
good agreement.
The potential energy surface for these carbene rear-
rangements is quite flat, and our calculated activation
barrier for the transformation of 6 f TS(6-10a ) f 10a
is less than 1 kcal/mol. Indeed, computational methods
have a certain uncertainty with regard to their “ac-
curacy”, and the B3LYP method can underestimate the
energies of carbenes.1 Furthermore, with such a flat
potential energy surface, dynamical and energetic effects
of the carbene and the carbene precursor may become
important as has been elegantly discussed in the litera-
ture in other systems.19 However, the calculations pre-
(19) (a) Carpenter, B. K. Acc. Chem. Res. 1992, 25, 520. (b)
Carpenter, B. K. Angew. Chem., Int. Ed. 1998, 37, 3340. (c) Doubleday,
C., J r.; Bolton, K.; Hase, W. L. J . Am. Chem. Soc. 1997, 119, 5251. (d)
Hrovat, D. A.; Fang, S.; Borden, W. T.; Carpenter, B. K. J . Am. Chem.
Soc. 1997, 119, 5253.
6m
(20) (a) Bader, R. F. W. Acc. Chem. Res. 1985, 18, 9. (b) Bader, R.
F. W. Chem. Rev. 1991, 91, 893. (c) Bader, R. F. W. Atoms in
Molecules: A Quantum Theory; Clarendon Press: Oxford, 1990.
(
21) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. A. J . Chem. Phys.
985, 83, 735. (b) Reed, A. E.; Weinhold, F. A.; Curtiss, L. A. Chem.
Rev. 1988, 88, 899.
1

3
264 J . Org. Chem., Vol. 67, No. 10, 2002
Farlow et al.
F igu r e 6. Atomic charges at the AIM level (in electrons) shown for the rearrangement pathways for 6a using the B3LYP/6-
11+G**//B3LYP/6-31G* wave functions.
3
F igu r e 7. Atomic charges at the AIM level (in electrons) shown for the rearrangement pathways for 6b using the B3LYP/6-
3
11+G**//B3LYP/6-31G* wave functions.
At the AIM level, the carbenic carbon in 6 is found to
1,2-methyl migration to form 2-butanone as the final
have slightly more negative charge than those in the 6a
and 6b conformers. As shown in Figures 5-7, there are
only minor differences in the charge distributions among
the various conformers of the reactant and the transition
states. (Due to numerical instabilities associated with the
numerical integration, charges on some of the ring atoms
could not be determined at the AIM level.) The OH group
polarizes the attached carbon atom to have a net positive
charge (+0.45 e in 6). The carbenic carbon becomes
slightly more negatively charged in the transition state
as compared to the reactant, and the adjacent positive
charge of the carbon bearing the OH group would help
to electrostatically stabilize the carbenic carbon. This
electrostatic effect would, therefore, lower the activation
barriers for methyl migration, as is observed.
On the basis of the free energies of activation as well
as the charge density distribution, it can be easily
understood that the major rearrangement pathway is the
methyl migration as compared to the C-H and O-H
insertion pathways. This prediction is in very good
agreement with the observed product distribution, where
the primary product happens to be the enol (which is then
converted to the tautomeric ketone).
product. The most stable conformer of 6 is perfectly
poised for a 1,2-methyl migration as the C-CH group
3
is involved in a hyperconjugative interaction with the
empty p orbital at the carbene center, and the O-H bond
2
is simultaneously interacting with the sp lone pair of
the carbene. The â-hydroxy substituent polarizes the
attached carbon to be positively charged, thereby favoring
the transition state for methyl migration by electrostatic
stabilization of the negative charge that is generated at
the carbene center in the migration transition state.
IV. Exp er im en ta l a n d Com p u ta tion a l Meth od s
Gen er a l P r oced u r es. Reactions requiring anhydrous con-
ditions were carried out under an argon atmosphere with oven-
dried equipment. Tetrahydrofuran was dried by passage
through two columns (2 ft × 4 in.) of activated alumina. All
other solvents and reagents were used as obtained from
commercial sources. Column chromatography was performed
on silica gel (70-230 mesh) or neutral alumina (80-100 mesh)
using the indicated solvent system. For preparative thin-layer
chromatography, 20 × 20 cm glass plates, coated with a
tapered layer of silica gel, were used. NMR spectra were
recorded at 400 MHz for 1H and 100 MHz for C.
13
exo-1-(1a ,9b-Dih yd r o-1H-cyclop r op a [l]p h en a n th r en -1-
yl)eth a n on e (9). As described in a previously published
procedure,12 phenanthrene (25.6 g, 143 mmol) and copper
sulfate (0.510 g, 3.18 mmol) were steadily heated to melting
III. Con clu sion s
The experimental and computational results for 6 are
in excellent agreement, and demonstrate a preference for

Rearrangement of 2-Hydroxy-2-methylpropylidene
J . Org. Chem., Vol. 67, No. 10, 2002 3265
whereupon ethyl diazoacetate (16 mL, 152 mmol) was added
dropwise to the mixture. Heating was discontinued upon
completion of addition, and the contents of the flask were
solidified within 1 h. Then, sodium hydroxide (10% in EtOH,
Each stationary point was characterized as either a true
minimum or a transition state by the corresponding Hessian
index of 0 and 1, respectively. Each transition state (TS(R-
P )) was then carefully confirmed (opt)calcfc) to connect to the
respective reactant (R) and product (P ), after displacement
(by ∼10%) along the normal mode for the imaginary vibra-
tional frequency.
1
00 mL) was added, and the reaction mixture was heated to
reflux for 12 h. Ethanol was removed under reduced pressure,
water (250 mL) was added, and the reaction mixture was
gently boiled for 15 min. The reaction mixture was then
filtered and the filtrate cooled and acidified to a pH of 5.5. A
tan, creamy precipitate was obtained that was recrystallized
from glacial acetic acid to afford white crystals of the carboxylic
acid in typical low yield (19% yield, 6.23 g). The carboxylic
acid (1.05 g, 4.45 mmol) was dissolved in THF (30 mL) and
The effect of electron correlation was evaluated by refining
the energies using single-point energy calculations at the
coupled cluster25 and B3LYP methods using basis sets of
increased flexibility. Single-point energy calculations on the
B3LYP/6-31G* optimized geometries were performed at the
CCSD(T)/6-31+G**, CCSD(T)/cc-pVDZ, B3LYP/6-31+G**, and
B3LYP/6-311+G** levels. All of these basis sets included six
Cartesian d functions.26 Single-point energy calculations were
performed using the “scf)tight” option. Zero-point vibrational
energies (at the B3LYP/6-31G* level) have been scaled by
0.980627 and are included in the reported energies, except for
17
the solution cooled to 0 °C. To this solution was added CH
1.60 M in diethyl ether, 5.85 mL, 9.35 mmol) in a dropwise
fashion, and the resulting solution was stirred for 25 h.
Aqueous NH Cl (25 mL) was then added, and the layers were
separated. The aqueous layer was extracted with CH Cl
(2 ×
5 mL). The combined organics were washed with water (50
mL) and brine (50 mL), dried over Na SO , and filtered, and
3
Li
(
4
2
2
2
8
2
the CCSD(T) relative energies. The standard CBS-QB3 method
was utilized as implemented in Gaussian98.
2
4
the solvent was removed under reduced pressure to give the
crude ketone. This material was recrystallized from EtOH (447
Atomic charges were evaluated at the AIM20 and NPA21
levels using the wave functions from the B3LYP/6-311+G**//
B3LYP/6-31G* calculations. The AIM charges were generated
1
1h
mg, 46% yield): mp 182-184 °C (lit. mp 185-186 °C); IR
-
1
1
29
(
(
1
1
KBr pellet) 1677 cm ; H NMR (CDCl
3
) δ 8.05 (m, 2 H), 7.48
with AIMALL.
m, 2 H), 7.34 (m, 4 H), 3.29 (d, J ) 3.7 Hz, 2 H), 2.31 (s, 3 H),
.40 (t, J ) 3.7 Hz, 1 H); 13C NMR (CDCl
) δ 208.5, 133.7,
3
Ack n ow led gm en t. D.M.T. acknowledges the Re-
search Corp. and the Division of Natural Sciences for
financial support, the National Science Foundation for
an NMR upgrade, and the Paul J . Schupf Scientific
Computing Center for molecular modeling facilities.
C.M.H. acknowledges support from the National Science
Foundation (Grant CHE-9733457) and computational
resources from the Ohio Supercomputer Center. C.M.H.
and R.B.S. thank Dr. C. Michael Geise (The Ohio State
University) for helpful discussions.
30.1, 130.0, 128.4, 127.5, 123.7, 34.6, 33.3, 31.9. Anal. Calcd
14O: C, 87.15; H, 6.02. Found: C, 86.77; H, 6.08.
for C17
H
exo-2-(1a ,9b-Dih yd r o-1H-cyclop r op a [l]p h en a n th r en -1-
yl)p r op a n -2-ol (7). The ketone 9 (720 mg, 3.10 mmol) was
dissolved in THF (15 mL) and the solution cooled to 0 °C. A
solution of CH
was added dropwise, and the resulting solution was stirred
for 1 h. Aqueous NH Cl (40 mL) was added, and the layers
were separated. The aqueous layer was extracted with ether
50 mL). The combined organic layers were dried over anhy-
drous Na SO , filtered, and concentrated at reduced pressure.
The residue was purified by flash chromatography on alumina
9:1 hexanes/ethyl acetate) followed by preparative thin-layer
3
Li (1.6 M in diethyl ether, 2.5 mL, 4.00 mmol)
4
(
2
4
1H
Su p p or tin g In for m a tion Ava ila ble: Spectral data (
(
13
NMR, C NMR, and IR) for 7 and computational results,
including absolute energies at all levels of theory, Cartesian
coordinates and vibrational frequencies for all stationary
points, and figures for the NPA atomic charges. This material
is available free of charge via the Internet at http://pubs.acs.org.
chromatography using hexanes/ethyl acetate (95:5), to give the
tertiary alcohol product (85.4% yield, 662 mg): mp 120-122
2
2
°
1
7
(
1
C (lit. mp 100-116 °C); IR (KBr pellet) 3545, 3475, 1238,
1 1
-
186 cm ; H NMR (DMSO-d
.23 (m, 4 H), 4.45 (br s, 1 H), 2.58 (d, J ) 4.5 Hz, 2 H), 0.17
6
) δ 8.03 (m, 2 H), 7.41 (m, 2 H),
1
3
J O0160827
t, J ) 4.5 Hz, 1 H); C NMR (CDCl
27.9, 126.1, 123.3, 69.3, 35.9, 29.5, 22.9. Anal. Calcd for
18O: C, 86.36; H, 7.25. Found: C, 86.21; H, 7.28.
Gen er a l P r oced u r e for P h otolysis. In a typical experi-
ment, a solution of the tertiary alcohol 7 (16.3 mg, 0.065 mmol)
in C containing a precisely weighed amount of 1,3-benzo-
3
) δ 135.9, 129.7, 128.9,
C
18
H
(23) (a) Labanowski, J . W.; Andzelm, J . Density Functional Methods
in Chemistry; Springer: New York, 1991. (b) Parr, R. G.; Wang, W.
Density Functional Theory in Atoms and Molecules; Oxford University
Press: New York, 1989.
6
D
6
(24) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
dioxole (6.8 mg, 0.056 mmol) as an internal standard was
taken in an NMR tube and deoxygenated. This solution was
then photolyzed at room temperature in a Rayonet photo-
chemical reactor to generate the carbene 6. The photolysis
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; 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.; J ohnson, 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.: Pitts-
burgh, PA, 1998.
1
reaction was monitored periodically by H NMR and continued
until the starting material was consumed. The products of
photolysis were compared to authentic samples of 2-butanone
and 2,2-dimethyloxirane, which were purchased, and 1-meth-
ylcyclopropanol, which was synthesized according to a litera-
1
4
ture procedure.
Com p u ta tion a l Meth od s. Full geometry optimizations of
minima as well as transition states have been performed using
(25) Stanton, J . F.; Gauss, J .; Watts, J . D.; Lauderdale, W. J .;
Bartlett, R. J . Int. J . Quantum Chem. 1992, S26, 879.
(26) Hehre, W. J .; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab initio
Molecular Orbital Theory; J ohn Wiley & Sons: New York, 1986.
1
7
the B3LYP hybrid Hartree-Fock density functional theory
2
3
method, using the 6-31G* basis set. All calculations were
2
4
performed with Gaussian 98.
(27) Scott, A. P.; Radom, L. J . Phys. Chem. 1996, 100, 16502.
(
28) Montgomery, J . A., J r.; Frisch, M. J .; Ochterski, J . W.; Peters-
(22) Thamattoor, D. M. Assorted Cyclopropylcarbenes. Ph.D. Dis-
son, G. A. J . Chem. Phys. 1999, 110, 2822.
sertation, Princeton University, Princeton, NJ , 1997.
(29) Keith, T. A. AIMALL; Yale University: New Haven, CT, 1996