Thermal Isomerizations of 1,1-Dimethyl-2,2-d2-cyclopropane

Article abstract of DOI:10.1021/jp9918712

The gas-phase thermal isomerization of 1,1-dimethylcyclopropane at 420 °C gives 3-methyl-1-butene and 2-methyl-2-butene. The mixture of products formed from 1, l-dimethyl-2,2-d₂-cyclopropane is consistent with conventional rationales; neither tert-butylcarbene nor isobutylcarbene is involved as a mechanistically significant intermediate. Both kinetic and deuterium labeling evidence suggests that heterogeneous wall-catalyzed processes afford secondary products, including 2-methyl-1-butene.

Full text of DOI:10.1021/jp9918712

J. Phys. Chem. A 1999, 103, 7821-7825
7821
Thermal Isomerizations of 1,1-Dimethyl-2,2-d2-cyclopropane
John E. Baldwin* and Rajesh Shukla
Department of Chemistry, Syracuse UniVersity, Syracuse, New York 13244
ReceiVed: June 8, 1999; In Final Form: August 6, 1999
The gas-phase thermal isomerization of 1,1-dimethylcyclopropane at 420 °C gives 3-methyl-1-butene and
-methyl-2-butene. The mixture of products formed from 1,1-dimethyl-2,2-d -cyclopropane is consistent with
2
2
conventional rationales; neither tert-butylcarbene nor isobutylcarbene is involved as a mechanistically significant
intermediate. Both kinetic and deuterium labeling evidence suggests that heterogeneous wall-catalyzed processes
afford secondary products, including 2-methyl-1-butene.
Introduction
The isomerization of 1 has been a veritable fixed point in
related hydrocarbon thermal rearrangement chemistry shown by
The gas-phase thermal isomerization of 1,1-dimethylcyclo-
propane (1) has been investigated in great detail. In a series of
classic papers Flowers and Frey characterized the major products
formed through homogeneous, unimolecular reactions in “aged”
other alkyl- and dialkyl-substituted cyclopropanes,6 so long
known, well studied, and thoroughly understood that it has not
been subjected to experimental or theoretical re-examination
in recent decades. One step in this direction is now reported,
based on the reactions shown by 1 and by 1,1-dimethyl-2,2-
d2-cyclopropane (1-d2).
-8
static Pyrex reaction vessels to be 3-methyl-1-butene (2) and
-3
2
-methyl-2-butene (3).1 Kinetic work over the temperature
range 447-511 °C with isomerizations of starting material
varied between 10% and 60% (to (2%) led to high-pressure
activation parameters for the isomerization, log A ) 15.05 and
Ea ) (62.6 ( 0.6) kcal/mol. The ratios of products were less
accurately determined (to (5%). The initial 2/3 ratio, 1:1.1
Several questions were considered as the present work was
initiated. Would substantial deuterium kinetic isotope effects
on the overall isomerization or on product distributions be seen?
Is 4 really a primary thermal product? Might 1 lead to the 2,2-
dimethyltrimethylene diradical (6) through cleavage of the C2-
(averaged over 300 runs), was pressure and temperature
9
C3 bond and then afford product 4 through a methyl migra-
independent. The isomerization rates at 442-481 °C were
shown to decrease at lower pressures in highly satisfactory
agreement with the requirements set by the Lindemann hypoth-
esis, and they did not go through a maximum at higher pressures.
1
0
tion? Could 1 give rise to isobutylcarbene (7) and tert-
butylcarbene (8) intermediates, which through hydrogen or
1
1
methyl shifts could lead to products 2 and 3?
Other products noted included approximately 1% of 2-methyl-
1
-butene (4) and traces (less than 1%) of some secondary
Experimental Section
“
cracking” compounds such as ethane, ethylene, propane, and
All reactions were carried out under a nitrogen or argon
atmosphere in flame- or oven-dried glassware. Diglyme was
dried over sodium and distilled under nitrogen. tert-Butyl alcohol
was stored over 4A molecular sieves. Varian Aerograph A90-
P3 and Hewlett-Packard 5890 and 5890 Series II instruments
and a 3393A integrator were used for preparative and analytical
gas chromatography. Bruker instruments (300 and 600 MHz
propene. Interestingly, 2-methyl-1-butene was not detected in
runs at pressures as low as 0.1 mm, and the rates of secondary
isomerizations of 2 and 3 proved to be highly pressure
dependent.
1
for H) were used to obtain NMR spectra.
1
,1-Dibromo-2,2-dimethylcyclopropane.12 A stirred mixture
prepared from potassium tert-butoxide (8.6 g, 77 mmol), tert-
butyl alcohol (50 mL), and pentane (50 mL) under argon was
cooled in a salt-ice bath; liquid isobutylene (Aldrich Chemical;
Products 2 and 3 presumably stem from cleavage of the C1-
C2 bond in 1 to afford the 1,1-dimethyltrimethylene diradical
1
2.5 mL, 7.42 g, 132 mmol) was injected, and then bromoform
(
5) followed by 1,2-hydrogen migration to either C1 or C3.
(4.5 mL, 13.0 g, 51 mmol) was added dropwise by syringe.
Formation of the minor product 4 might also be derived from
The reaction mixture was stirred and held at 0 °C for 14 h; it
was then warmed to room temperature and washed with water
this diradical, through transfer of a hydrogen from a methyl
group to the terminal methylene.1
(
12 × 50 mL). The organic phase was dried over anhydrous
Work on the chemically activated isomerization of 1 has
served as a testing ground for RRKM theory.4
,5
MgSO , filtered, and concentrated under reduced pressure to
4
give 8.4 g (71% based on CHBr3) of crude product as a pale-
1
*
Corresponding author. E-mail: jbaldwin@syr.edu.
yellow liquid: H NMR (CDCl3) δ 1.65 (s, 2 H), 1.62 (s, 6 H).
1
0.1021/jp9918712 CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/08/1999

7822 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Baldwin and Shukla
1
,1-Dimethylcyclopropane.13 To a stirred suspension of
TABLE 1: Mole Percent Compositions of Product Mixtures
from 1 at 420 °C
finely divided sodium (2 g) in dry diglyme (50 mL) was added
dropwise a mixture of 1,1-dibromo-2,2-dimethylcyclopropane
time (h)
1
2
3
4
(
1 g, 31 mmol) in ethanol (5 g) and water (1 mL). The reaction
was exothermic and the temperature was maintained below 70
C. The product which distilled from the flask was collected in
0
1
3.5
7
100
81.7
71.0
49.0
29.6
0
0
0
.5
7.6
11.9
20.2
24.0
10.8
16.7
29.3
42.0
0.0
0.0
1.5
3.0
°
.5
a trap cooled to -78 °C and then purified by distillation, 100
mg (40% yield): H NMR (CDCl3) δ 1.03 (s, 6H), 0.20 (s,
4
1
13
+
H); MS, m/z 70 (M , strong), no ion intensity at 68 or 69.
TABLE 2: Mole Percent Compositions of Product Mixtures
from 1-d at 420 °C
1
,1-Dimethyl-2,2-d2-cyclopropane. 1,1-Dibromo-2,2-dim-
2
ethylcyclopropane (1 g, 31 mmol) was reduced with sodium in
CH3CH2OD (5 g; Isotec, min 99 atom % D) and D2O (1 mL;
Aldrich, 99.9 atom % D) following the procedure given above.
time (h)
0
1-d
100
82.0
71.0
49.0
29.9
2
2-d
2
3-d
2
4-d
2
0
0
0
1
3
.5
.5
7.2
12.1
20.1
24.3
10.8
16.9
29.3
42.8
0.0
0.0
1.6
3.0
1
The deuterium-labeled product was obtained in 40% yield: H
2
NMR (CDCl3) δ 1.03 (s, 6 H), 0.20 (s, 2 H); H NMR (CHCl3,
7.5
13
+
5
% CDCl3) δ 0.20 (s); MS, m/z 72 (M , strong), no ion intensity
at 70 or 71.
Thermal Reactions. Thermal reactions of 1 and of 1-d2 at
4
20 °C were carried out in a 300 mL Pyrex bulb encased in an
14
aluminum block static reactor. A Bailey Instruments model
53 precision temperature controller and a Hewlett-Packard
2
model 2802A digital thermometer extending into a well in the
reactor were employed. The bulb was conditioned before kinetic
runs with 100 µL of cyclohexene at 420 °C for 24 h.
-
2
Samples of 1 or 1-d2 were injected into the evacuated (10
Torr) reactor through a septum with a gastight syringe; at the
end of a thermal reaction all products were transferred under
vacuum to a liquid nitrogen cooled U-tube attached to a vacuum
line and diluted with n-decane.
1,1-Dimethylcyclopropane (1) (100 µL) was injected into the
kinetic bulb at 420 °C; after 6 h, the product mixture was
collected, diluted with decane (100 µL), and separated by
preparative gas chromatography on an SE-30 column at 55 °C
into three fractions, (1 + 2), 4, and 3, in order of elution. The
identities of these hydrocarbons were established through
1
15
analytical GC and H NMR spectroscopic comparisons with
authentic samples.
1
,1-Dimethyl-2,2-d2-cyclopropane (1-d2) (100 µL) was kept
Figure 1. Mole percent concentrations of isomers versus reaction time
from thermal isomerizations of 1 (triangles) and 1-d (circles) at 420
C.
for 6 h at 420 °C; the product mixture was collected and diluted
with decane (100 µL). Three fractions were collected by
preparative GC; they were characterized by H and H NMR
spectroscopy.
2
°
1
2
Results and Discussion
1
2
1
-d2: H NMR (CDCl3) δ 1.03 (s, 6 H), 0.20 (s, 2 H); H
NMR δ 0.20 (s).
2
.80-5.00 (m, 1.0 H), 5.75-5.90 (m, 0.5 H); H NMR δ 2.20-
.40 (m, 0.49 D), 4.80-5.00 (m, 1.00 D), 5.75-5.90 (m, 0.51
D).
Isomerization Kinetics for 1. A series of gas-phase kinetic
runs at 420 °C for up to 13 h showed that 1 isomerized following
good first-order kinetics, with an isomerization rate constant k
1
-d2: H NMR δ 1.00-1.02 (m, 6H), 2.20-2.40 (m, 0.5 H),
2
4
2
-
5 -1
)
2.55 × 10 s , in fine agreement with the expectation k )
-5 -1
(
1.3-3.2) × 10
s
based on the reported activation param-
1
eters. The relative concentrations of 3 over time were consistent
with simple exponential growth, with 3(t) proportional to (1 -
exp(-kt)). The functions 2(t) and 4(t), however, were much
1
3
-d2: H NMR (CDCl3) δ 1.50-1.80 (m, 7.5 H), 5.16-5.30
2
(
0
m, 0.5 H); H NMR δ 1.50-1.80 (m, 1.55 D), 5.16-5.30 (m,
.45 D).
1
6
better fit with two exponential terms. The rate equations
appropriate to the minimal kinetic model adopted, 1 to 2 and 3
through parallel first-order reactions and 2 and 4 in equilibrium,
1
4
-d2: H NMR (CDCl3) δ 1.00-1.10 (m, 1.9 H), 1.70-1.80
2
(
m, 2.9 H), 1.98-2.12 (m, 1.2 H), 4.66-4.74 (m, 2.0 H); H
NMR δ 1.00-1.10 (m, 1.06 D), 1.70-1.80 (t, 0.14 D, JH-D )
17
led to the theoretical functions used to fit the experimental
2
.25 Hz), 1.98-2.12 (m, 0.75 D), 4.66-4.74 (m, 0.05 D).
-5 -1
-5 -1
points, with k ) 2.55 × 10
s
and k′ ) 6.92 × 10
s
(eq
Further aspects of the H NMR spectra, and of 13C spectra
2
1
-4 and Figure 1).
of deuterium-labeled products, are discussed below.
Isomerization Kinetics. Isomerization kinetics were followed
using 20 µL samples of neat 1 or 1-d2; product mixtures were
collected, diluted with decane (50 µL), and analyzed by GC on
a J&W GS-Alumina PLOT column at 140 °C with a head
pressure of 1.5 psi. Mole percent composition data are given in
Tables 1 and 2.
1
(t) ) 97.2 exp(-kt)
(t) ) 28.6 -12.9 exp(-kt) - 15.4 exp(-k′t)
(t) ) 60.5 - 60.5 exp(-kt)
(t) ) 6.3 -11.4 exp(-kt) + 5.1 exp(-k′t)
(1)
(2)
(3)
(4)
2
3
4

Isomerizations of 1,1-Dimethyl-2,2-d2-cyclopropane
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7823
The kinetically controlled product ratio 2/3 calculated from
the parameters in eqs 2 and 3 was 1:1.11, in agreement with
3
Flowers and Frey. A sample of 4 heated for 72 h at 420 °C
gave a 3:65:32 mixture of 2/3/4, proportions nearing the
equilibrium ratios one might expect (≈6:70:24) from the
1
8
standard heats of formation for 1 (-2.0 kcal/mol), 2 (-6.6
kcal/mol),19 3 (-9.9 kcal/mol), and 4 (-8.4 kcal/mol). The
20
20
kinetic behavior depicted in Figure 1 and eqs 1-4 shows that
(t) is always present at close to its equilibrium proportion, and
3
possible secondary reactions equilibrating 3 with 2 and 4 may
not be very evident in gas chromatographic analyses. For
reaction times up to 13 h, the equilibration between 2 and 4
has begun, but it has only begun; 2(t) remains larger than 4(t),
while at equilibrium 2/4 is about 1:4.
Isomerization Kinetics for 1-d2. Kinetic runs with 1-d2 at
20 °C for times up to 13 h gave very similar relative mole
2
Figure 2. Methyl H NMR absorptions for 3-d
2
.
4
percent versus time profiles. The data were fit using the same
theoretical functions used to model the reactions of 1, as
summarized in eq 5-8; the exponential constants were found
2
2
-d2. The H NMR spectrum of the labeled 3-methyl-1-butene
formed was consistent with 1-d2 giving a 1:1 mixture of 2-d2-a
and 2-d2-b. The cis and trans vicinal H-D coupling constants
1.5 and 2.6 Hz) apparent at δ 5.00 and 4.90 for 2-d2-a and at
δ 5.80 for 2-d2-b correspond with expectations; the vinyl J H-H
values for propene are 10.0 and 16.8 Hz, which when divided
by 6.51, the ratio of magnetogyric ratios γH/γD,
and 2.6 Hz. No signal intensity was detected at δ 1.03 for a
-
5
-1
-5 -1
2
to be m ) 2.53 × 10
s
and m′ ) 6.92 × 10 s . The R
(
regression coefficients for the data fits expressed in eq 1-8
3
ranged from 0.987 to 0.998.
2
3,24
give 1.5
1
-d (t) ) 97.2 exp(-mt)
(5)
2
deuterium in a methyl group.
2
-d (t) ) 29.4 -14.4 exp(-mt) - 14.8 exp(-m′t) (6)
2
3
-d (t) ) 61.4 - 61.4 exp(-mt)
(7)
2
4
-d (t) ) 6.3 - 11.2 exp(-mt) + 4.9 exp(-m′t) (8)
2
The data plots included in Figure 1 emphasize how little
impact the deuterium labels have on overall kinetic behavior.
The initial ratio of 2-d2/3-d2 products calculated from eqs 6 and
The formation of 2-d2-a and 2-d2-b from 1-d2, rather than
isotopomers 2-d2-c to 2-d2-f, rules out any significant role for
carbene intermediates 7-d2-a and 7-d2-b.
7
was 1:1.12, nearly identical with the 1:1.11 ratio found for
2
/3. The calculated rate constant ratios k/m and k′/m′ are 1.01
and 1.00. These ratios, related to some combination of equi-
librium (to diradicals or other intermediates) and primary and
secondary deuterium kinetic isotope effects provide no striking
mechanistic insights. Yet the comparative kinetic data (Figure
1
) do provide an important assurance; the 1-d2 sample contains
no trace of a catalytically significant impurity, and thus the
distributions of deuterium labels in products 2-d2, 3-d2, and 4-d2
will be indicative of whatever paths may be followed in
unlabeled 1,1-dimethylcyclopropane and should be consistent
with kinetically derivable inferences.
Deuterium Labeled Products. A thermal reaction of 1-d2
for 6 h at 420 °C gave a product mixture which was separated
and purified by preparative gas chromatography.
2
The H NMR results also suggest that 2-d2 is not being
formed to any substantial extent over the first 6 h of reaction
from other isomers; secondary reactions of 3-d or 4-d to form
2-d play no important role. This inference is of course consistent
with thermochemical considerations. A sample of 2-d obtained
1
-d2. The deuterium NMR spectrum of recovered starting
material showed only one absorption, for cyclopropyl CD2 at δ
.20. There was no detectable absorption intensity at δ 1.03
2
2
2
0
2
2
for deuterium in a methyl group, which effectively excludes
any mechanistically significant formation of tert-butylcarbene
as an intermediate leading to 3. Were tert-butylcarbene involved,
the deuterium-labeled analogues derived from 1-d2 would have
been 8-d2-a and 8-d2-b, both of which would have given some
from a 19 h thermal reaction had a similar H NMR spectrum,
but there were noticeable differences: the relative absorption
intensities for deuterium at C1, C2, and C3 were 1.71:1.00:
0.88. At such long reaction times there may be redispositions
of deuterium labels through secondary reactions and perhaps
some loss of deuterium.
21,22
1,1-dimethylcyclopropane with deuterium in a methyl group
2
as well as deuterium labeled versions of 3.
3-d . The H NMR spectrum of 3-d derived from 1-d
2
2
2
showed absorptions at δ 5.2 and in the 1.5-1.7 methyl region
with relative intensities of 0.45:1.55. The strongest methyl region
component was centered at δ 1.55, a four-line pattern for the
C4-CHD2 unit (Figure 2), with geminal and vicinal JH-D
couplings of 3.0 and 0.8 Hz. Thus, 3-d2-a is formed, and 3-d2-b
seemed the most plausible component responsible for the vinyl

7824 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Baldwin and Shukla
2H NMR signal. Closer examination of the spectra, however,
revealed a slightly more complex mixture.
Small amounts of deuterium label were found in the other
two methyl groups, as CH2D at δ 1.6 and 1.7 (Figure 2).
13
1
Through C { H} NMR spectroscopy, several deuterium labeled
versions of the C3-C4 moiety were detected. The 13C chemical
shifts for C3 and C4 are reported as δ 118.45 and 13.41,
respectively.1 Thanks to the upfield chemical shift perturbations
5
25
associated with deuterium substitution and distinctive spin-
spin coupling patterns for carbons bonded to zero, one, or two
deuterium atoms, H-C-CH3, H-C-CH2D, and H-C-CD2H
substructures were evident as singlets at δ 118.42, 118.38, and
1
18.35, and D-C-CH3 and D-C-CH2D were seen as triplets
at δ 118.085 and 118.049, with JC-D ) 23.2 ( 0.2 Hz. These
variations were confirmed in the absorptions for the C4 methyl
group alternatives (Figure 3), showing singlets for H-C-CH3
and D-C-CH3 at δ 13.38 and 13.26, a triplet for D-C-CH2D
at 12.99 (JC-D 19.2 ( 0.1 Hz), and a quintet for H-C-CD2H
at 12.84 (JC-D 19.2 ( 0.1 Hz). Although 3-d2-a and 3-d2-b may
be considered major components of the 2-methyl-2-butene
formed from 1-d2, the presence of other isotopomers suggests
that indirect, secondary reactions contribute as well.
Figure 3. Absorptions for C4-methyl for various 3-d isotopomers in
2
13
1
a
C { H} NMR spectrum at 150.856 MHz.
relative intensities 0.05 (C1), 0.14 (C2-methyl), 0.75 (C3), and
1.07 (C4). The intensity ratios for 4-d2 from a 19 h reaction
mixture were 0.09:0.17:0.60:1.14, quite similar and yet not
identical. The 4-d2 product is a complex mixture of isotopomers,
formed presumably from the several versions of 2-d2 and 3-d2
4-d2. Expectations for 2-methyl-1-butene-d2 products derived
directly from 1-d2 through the two most plausible standard
unimolecular thermal mechanisms are easily outlined. If reaction
were to take place through intermediates 5-d2-a and 5-d2-b and
migration of hydrogen from a methyl group, then the product
would be a mixture of 4-d2-a and 4-d2-b.
1
3
1
present in reaction mixtures. The C{ H} NMR spectrum of
the 6 h 4-d2 mixture provides further evidence for extensive
scrambling. For instance the C4 absorptions reflect contributions
from CH3CH2-, CH3CHD-, and CH3CD2- (singlets at δ
12.27, 12.19, and 12.11), CH2DCH2- and CH2DCHD- (triplets
at 11.98 and 11.90), and CD2HCH2- (quintet at 11.69), with
all triplets and the quintet having JC-D 19.4 Hz. The deuterium
1
3
isotope effects on C chemical shifts are very much in line
2
5
with expectations.
Homogeneous and Surfaced-Catalyzed Reactions. The
structural isomerizations leading to equilibration of alkenes 2,
If the C2-C3 of 1-d2 were to cleave to form 6-d2 followed
by methyl migration, the product would be a mixture of 4-d2-a
and 4-d2-c.
3, and 4, as well as the observed distributions of deuterium labels
in alkene products, point to surface-catalyzed reactions. Even
after thorough “aging” or “conditioning” of a kinetic bulb, there
may be Si-O-H groups on the surface that can trigger cationic
reactions. In the present case, one can well imagine proton
transfers from surface to alkenes, initiating equilibrations of
2
-methyl-2-butyl cations (tert-amyl cations or dimethylethyl-
carbonium ions) and 3-methyl-2-butyl cations. These isomer-
izations through both hydrogen and methyl shifts have been
But expectations based on the kinetic evidence would be of
a different nature; one would need to consider possible mech-
anisms for secondary isomerizations from the isotopomers of
2
6,27
documented by isotopic labeling experiments
and by NMR
2
8
spectroscopic studies.
2-d2 and 3-d2 in the product mixture. For instance, if products
were derived from 3-d2-a and 3-d2-b through protonation at C2,
followed by deprotonation, one would expect formation of
4
-d2-b and 4-d2-d.
Each of these cationic structures could be formed from, or
could lead to, two of the alkenes involved; the 3-methyl-2-butyl
cation is related to 2 and 3, and the 2-methyl-2-butyl cation to
3
and 4. Protonations of alkenes, carbocation isomerizations,
and deprotonations could lead to equilibrations among the three
alkenes and an approach to the thermodynamically defined
equilibrium mixture, to scramblings of deuterium labels, and
2
The experimental H NMR spectrum for 4-d2 from a 6 h
reaction mixture showed deuterium at every position, with

Isomerizations of 1,1-Dimethyl-2,2-d2-cyclopropane
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7825
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(
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1
(
Conclusions
4
The kinetic indications and the deuterium labeling results
provide persuasive grounds for considering the isomerization
of 1,1-dimethylcyclopropane (1) to the isomeric alkenes 3-meth-
yl-1-butene (2) and 2-methyl-2-butene (3) at 420 °C as reactions
that proceed through parallel first-order processes. There are
no significant net deuterium kinetic isotope effects distinguishing
the thermal isomerizations of 1 and 1-d2. Alkylcarbene inter-
mediates 7 and 8 are not involved. The traditional mechanistic
formulations based on hydrogen migrations from the 1,1-
dimethyltrimethylene diradical (5) appear quite adequate. 2-Meth-
yl-1-butene (4) is not a primary product; it is formed only
through secondary surface-catalyzed isomerizations that equili-
brate 2, 3, and 4. These secondary reactions introduce kinetic
and deuterium labeling complexities that might well have
interfered with the objectives of some labeling experiments, but
they did not impinge on the tests for the intervention of
alkylcarbene intermediates now being reported.
(
(
11) Bettinger, H. F.; Rienstra-Kiracofe, J. C.; Hoffman, B. C.; Schaefer,
H. F.; Baldwin, J. E.; Schleyer, P. v. R. Chem. Commun. 1999, 1515-
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(
12) Barfield, M.; Canada, E. D.; McDaniel, C. R.; Marshall, J. L.;
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14) Baldwin, J. E.; Carter, C. G. J. Am. Chem. Soc. 1982, 104, 1362-
(
1
(
1368.
(15) Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and H FT
1
NMR Spectra; Aldrich Chemical Co.: Milwaukee, 1993; pp 29, 30, 34.
(
(
16) DeltaGraph Pro 3; DeltaPoint: Monterey, CA 93940.
17) Szab o´ , Z. G. In Kinetic Characterization of Complex Reaction
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969; pp 1-80; see section 1.V.2 and references cited.
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1
(
(
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(
20) Wiberg, K. B.; Hao, S. J. Org. Chem. 1991, 56, 5108-5110.
Acknowledgment. Support for this work from the National
Science Foundation (CHE 9532016) is gratefully acknowledged.
We thank Deborah Kerwood, Syracuse University, and David
Kiemle, State University of New York, College of Environ-
mental Science and Forestry, for assistance with H NMR
spectroscopy.
(21) Banks, M. R.; Cadogan, J. I. G.; Gosney, I.; Hodgson, P. K. G.;
Jack, A. G. C.; Rodger, D. R. Chem. Commun. 1989, 1033-1034.
(22) Kirmse, W. Carbene Chemistry; Academic Press: New York, 1971;
pp 238-240 and references cited.
(
23) Bernstein, H. J.; Sheppard, N. J. Chem. Phys. 1962, 37, 3012-
014.
(24) Pople, J. A.; Schneider, W. G.; Bernstein, H. J. High-Resolution
Nuclear Magnetic Resonance; McGraw-Hill: New York, 1959; p 480.
25) Berger, S. Encyclopedia of Nuclear Magnetic Resonance; Grant,
2
3
(
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pp 1168-1172.
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