Experimental and computational study of the intramolecular reactivity of free tert-butylphenylmethylene. Modification of the chemistry of tert-butylmethylene by the introduction of a phenyl group

Article abstract of DOI:10.1021/jo981153w

The chemistry of ferf-butylphenylmethylene, 2, has been investigated experimentally and computationally. Free carbene 2 was generated by the thermal rearrangement of p-ter-butylphenyl-methylene and observed to rearrange by C-H insertion to give 1,1-dimethyl-2-phenylcyclopropane, 3, and by C-C insertion to yield 2-methyl-3-phenyl-2-butene, 4. An examination of the 3:4 ratio led to the conclusion that C-H insertion is favored over C-C insertion by 1.6 ± 0.1 kcal/mol in good agreement with a calculated (PMP2/6-31G(d)//MP2/6-31G(d)+ZPC/6-31(d)) value of 2.0 kcal/ mol. The S-T gap in 2 is estimated to be 5-6 kcal/mol.

Full text of DOI:10.1021/jo981153w

7408
J . Org. Chem. 1998, 63, 7408-7412
Exp er im en ta l a n d Com p u ta tion a l Stu d y of th e In tr a m olecu la r
Rea ctivity of F r ee ter t-Bu tylp h en ylm eth ylen e. Mod ifica tion of th e
Ch em istr y of ter t-Bu tylm eth ylen e by th e In tr od u ction of a
P h en yl Gr ou p
Brian M. Armstrong, Michael L. McKee,* and Philip B. Shevlin*
Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312
Received J une 16, 1998
The chemistry of tert-butylphenylmethylene, 2, has been investigated experimentally and compu-
tationally. Free carbene 2 was generated by the thermal rearrangement of p-tert-butylphenyl-
methylene and observed to rearrange by C-H insertion to give 1,1-dimethyl-2-phenylcyclopropane,
3, and by C-C insertion to yield 2-methyl-3-phenyl-2-butene, 4. An examination of the 3:4 ratio
led to the conclusion that C-H insertion is favored over C-C insertion by 1.6 ( 0.1 kcal/mol in
good agreement with a calculated (PMP2/6-31G(d)//MP2/6-31G(d)+ZPC/6-31(d)) value of 2.0 kcal/
mol. The S-T gap in 2 is estimated to be 5-6 kcal/mol.
Carbenes are among the most energetic of reactive
intermediates. However, they are often surprisingly
selective in their intramolecular reactivities, and there
is evidence that this selectivity is often imposed by the
geometry of the carbene. Common routes from carbenes
to kinetically stable products include insertion into bonds
R to the carbene center to give alkenes and insertion into
more remote bonds generating cyclic products.1 Although
carbenes with R hydrogens generally rearrange by a C-H
insertion, carbene geometries can force other pathways.
Examples include cyclopropylmethylene2 and cyclobutyl-
idene,3 which rearrange predominately by R C-C inser-
tion as a result of geometries in which orbital interactions
favor C-C insertion.1 In 2-norbornylidene a favorable
overlap between a â C-H bond and the carbene p orbital
leads to a â C-H insertion.3d,4
and those of Glick, Likhotvorik, and J ones6 confirm these
predictions. In this case, calculations reveal an unusual
geometry for the carbene in which an adjacent C-C bond
is strongly bent toward the empty p orbital. Geometry
optimization of 1 at the MP2/6-31G(d) level gives a bond
angle of 79.8°. A comparison of the geometry of the
carbene with that of the transition state for intramolecu-
lar C-H insertion reveals that they are quite similar.
This fact appears to be responsible for an extremely low
barrier to C-H insertion, strongly favoring it over C-C
insertion. In this case, it appears that the factors that
stabilize the carbene are similar to those that stabilize
the corresponding carbocation.
We have now extended these investigations from tert-
butylmethylene to tert-butylphenylmethylene, 2. It is
clear that going from an alkyl to a dialkyl or phenylalkyl
carbene will increase carbene lifetimes and consequently
the barrier to intramolecular reactions. For example, the
calculated barrier to H migration in methylcarbene is ∼1
kcal/mol,7 while it is ∼5 kcal/mol in dimethylcarbene7a,8,9
and in phenylmethylcarbene.10 However in the present
study, we are asking what the effect of the addition of a
phenyl group will have on two competing intramolecular
pathways. On going from 1 to 2, a change in carbene
geometry may alter the ratio of 1,3 C-H to 1,2 C-C
insertion. In this case, it seems reasonable that the
In some cases it appears that the high energy of
carbenes requires particular geometries for stabilization
and that these geometries lead, in turn, to preferred
reactivities. An example is tert-butylmethylene, 1, in
which intramolecular insertion into a â C-H bond is
strongly preferred over insertion into an R C-C bond.5
Calculations indicate that the ∆Hq
- ∆Hq
of 3.7
C-C
C-H
kcal/mol is such that C-C insertion is only possible at
elevated temperatures. Our own experimental results5
(1) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press:
New York, 1971. (b) Baron, W. J .; DeCamp, M. R.; Hendrick, M. E.;
J ones, M., J r.; Levin, R. H.; Sohn, M. B. In Carbenes; J ones, M., J r.,
Moss, R. A., Eds.; Wiley: New York, 1973; Vol. 1, p 1. (c) Moss, R. A.
In Advances in Carbene Chemistry; Brinker, U. H., Ed.; J AI Press:
Greenwich, 1994; Vol. 1, p 59.
(2) (a) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. W. J . Am. Chem.
Soc. 1968, 90, 1485. (b) Shevlin, P. B.; McKee, M. L. J . Am. Chem.
Soc. 1989, 111, 519. (c) Chou, J .-H.; McKee, M. L.; De Felippis, J .;
Squillacote. M.; Shevlin, P. B. J . Org. Chem. 1990, 55, 3291.
(3) (a) Friedman, L.; Shechter, H. J . Am. Chem. Soc. 1960, 82, 1002.
(b) Pezacki, J . P.; Pole, D. L.; Warkentin, J .; Ford, F.; Toscano, J .; Fell,
J .; Platz, M. S. J . Am. Chem. Soc. 1997, 119, 3191. (c) Schoeller, W.
W. J . Am. Chem. Soc. 1979, 101, 4811. (d) Sulzbach, H. M.; Platz, M.
S.; Schaefer, H. F., III; Hadad, C. M. J . Am. Chem. Soc. 1997, 119,
5682.
(6) Glick, HC.; Likhotvorik, I. R.; J ones, M., J r. Tetrahedron Lett.
1995, 36, 5715.
(7) (a) Evanseck, J . D.; Houk, K. N. J . Am. Chem. Soc. 1990, 112,
9148. (b) Ma, B.; Schaefer, H. F., III J . Am. Chem. Soc. 1994, 116,
3539.
(4) (a) Friedman, L.; Shechter, H. J . Am. Chem. Soc. 1961, 83, 3159.
(b) Freeman, P. K.; George, D. E.; Rao, V. N. M. J . Org. Chem. 1964,
29, 1682. (c) Kirmse, W.; Meinert, T.; Modarelli, D. A.; Platz, M. S. J .
Am. Chem. Soc. 1993, 115, 8918.
(5) Armstrong, B. M.; McKee, M. L.; Shevlin, P. B. J . Am. Chem.
Soc. 1995, 117, 3685.
(8) Ford, F.; Yuzawa, T.; Platz, M. S.; Matzinger, S.; Fu¨lscher, M.
J . Am. Chem. Soc. 1998, 120, 4430.
(9) The experimental value is lower than this presumably due to
quantum mechanical tunnelling.8
(10) Cramer, C. J .; Truhlar, D. G.; Falvey, D. E. J . Am. Chem. Soc.
1997, 119, 12338.
S0022-3263(98)01153-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/01/1998

Reactivity of Free-tert-Butylphenylmethylene
J . Org. Chem., Vol. 63, No. 21, 1998 7409
Ta ble 1. Rela tive En er gies (k ca l/m ol) of Sp ecies on th e P h en ylca r ben e a n d ter t-Bu tylp h en ylca r ben e P oten tia l
En er gy Su r fa ce
HF/6-31G(d)// B3LYP/6-31G(d)
PMP2/6-31G(d)//
MP2/6-31G(d)
b
HF/6-31G(d)
//MP2/6-31G(d)
+ZPE/6-31G(d)a
+ZPC/6-31G(d)a
Cp
entropyc
phenylcarbene singlet
phenylcarbene triplet
singlet 2 (2a )
triplet 2 (2d )
TS2-3
TS2-4
2b
2c
0.0
-34.1
0.0
-38.7
18.4
19.4
0.2
0.0
-5.7
0.0
-9.2
6.7
8.0
1.9
4.0
0.0
-7.5
0.0
0.0
-6.0
0.0
-6.5
3.0
4.4
1.8
3.7
0.0
-7.8
0.0
-8.0
1.8
3.8
1.8
3.0
3.81
3.98
6.67
7.32
6.06
6.34
6.17
5.75
73.20
76.22
100.85
102.95
95.99
98.44
96.02
92.47
-10.7
5.5
7.4
1.9
3.3
2.8
a
b
Zero-point corrections (scaled by 0.90 factor) are included in relative energies. Integrated heat capacities (kcal/mol) to 298 K from
unscaled HF/6-31G(d) vibrational frequencies. c Entropies in units of cal/(mol‚K) calculated using unscaled HF/6-31G(d) vibrational
frequencies.
Ta ble 2. Th er m od yn a m ic P r op er ties a n d F r ee En er gy Differ en cies (k ca l/m ol)
PMP2/6-31G(d)//
MP2//6-31G(d)
+ZPC/6-31G(d)
a
Cp
entropyb
∆G(298 K)
∆G(500 K)c
Ph-C-tBu singlet (C1)
0.0
1.8
3.8
1.8
3.0
6.67
6.34
6.06
6.17
5.75
100.85
98.44
95.99
96.02
92.47
0.0
2.2
4.6
2.7
4.6
0.0
2.7
5.6
3.7
6.3
TS2-3
TS2-4
2b
2c
a
b
Integrated heat capacities (kcal/mol) from HF/6-31G(d) vibrational frequencies. Entropies in units of cal/(mol‚K) calculated using
HF/6-31G(d) vibrational frequencies. c Free energy differences (kcal/mol) estimated from ∆G(500) ≈ ∆H(298 K) - 500∆S(298 K).
presence of the phenyl group, with its strong propensity
for stabilization of an adjacent empty p orbital, may
reduce the need for stabilization by adjacent σ bonds and
consequently raise the barrier to intramolecular C-H
relative to C-C insertion. We now report an experimen-
tal and computational investigation of 2 which provides
evidence to support this hypothesis.
Resu lts a n d Discu ssion
carbene p orbital and the aromatic π orbitals. Instead,
the minimum energy conformer, 2a , has its phenyl ring
twisted by 38.4° out of the plane formed by the carbene
carbon and its attached atoms. Conformer 2b, which
Com p u ta tion a l In vestiga tion s of th e En er gy a n d
Rea ctivity of 2. We have carried out a computational
study of the geometries and energies of singlet and triplet
2 and have investigated the rearrangement of 2 by C-H
allows maximum interaction between the carbene p
insertion to give 1,1-dimethyl-2-phenylcyclopropane, 3,
orbital and the aromatic π orbitals, is calculated to lie
and by C-C insertion to yield 2-methyl-3-phenyl-2-
1.8 kcal above 2a and has one imaginary frequency (HF/
butene, 4. Geometries were optimized at the HF/6-31G-
6-31G(d)). Steric interactions between the tert-butyl
(d) and MP2/6-31G(d) levels. Total energies relative to
group and the ortho hydrogens appear to bring about the
2, obtained at various computational levels, are listed in
calculated 38.4° degree rotation about the carbon phenyl
bond in 2a . These same steric interactions increase the
bond angles about the carbene carbon from 106° in PC
to 119° in 2. Similar factors appear to dictate the
Table 1.11 Table 2 lists thermodynamic properties and
free energies of relevant species. Single-point B3LYP/
6-31G(d) calculations on the MP2 geometries gave results
quite similar to those of the MP2 calculations (Table 1).
geometry of phenylmethylcarbene (PMC), which is cal-
The geometries and energies calculated for a number of
culated to have a bond angle of 116.7° and be twisted by
species on this energy surface are shown in Figures 1-3.
Unless otherwise noted, the energies in the following
paragraphs are those calculated at the MP2/6-31G(d)//
MP2/6-31G(d)+ZPC level.
The minimum energy geometry calculated for singlet
2 is shown in structure 2a in Figure 1. It is interesting
that 2, unlike phenylcarbene (PC),12 does not adopt a
conformation that allows maximum overlap between the
39.4° (BPW91/cc-pVDZ).10,13 It is interesting that the ylid
generated in the reaction of 2 with pyridine has an
abnormally short wavelength λmax, which has been at-
tributed to a large bond angle in the ylid.14 A twisting
of the phenyl group in the ylid analogous to that in the
carbene could also be a factor. An alternate carbene
conformation, 2c, in which the phenyl ring is rotated to
(12) (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. (d) Cramer, C. J .; Dulles, F. J .; Falvey, D. E. J . Am.
Chem. Soc. 1994, 116, 9787.
(13) In the present investigation, we calculate a dihedral angle of
31.9° for PMC and an angle about the carbene carbon of 114.6°.
(14) C¸ elebi, S.; Leyva, S.; Modarelli, D.; Platz, M. S. J . Am. Chem.
Soc. 1993, 115, 8613.
(11) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T.; Petersson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian94
(Rev. B.1); Gaussian, Inc.: Pittsburgh, PA, 1995.

7410 J . Org. Chem., Vol. 63, No. 21, 1998
Armstrong et al.
F igu r e 1. Energies and geometries calculated (MP2/6-31G(d)//MP2/6-31G(d)+ZPG/6-31(d)) for various conformations and electronic
states of carbene 2.
F igu r e 2. MP2/6-31G(d) geometries of the transition states
F igu r e 3. Comparison of the MP2/6-31G(d) geometries of
for C-H and C-C insertion by 2.
singlet 1 and 2.
allow an overlap between the aromatic π orbitals and the
filled carbene sp2 orbital, is 3.0 kcal/mol higher in energy
than 2a and has two imaginary frequencies. Since the
carbene p orbital in conformation 2c is not stabilized by
conjugation with the phenyl, one might expect stabiliza-
tion by the adjacent C-CH3 σ bond, as we observe in 1.
However, this interaction, with its abnormally small

Reactivity of Free-tert-Butylphenylmethylene
J . Org. Chem., Vol. 63, No. 21, 1998 7411
C-C-CH3 angle, would raise steric interactions between
ortho hydrogens and the methyl groups and is not
observed.
but the phenyl groups have rotated completely out of the
plane formed by the carbene carbon and its attached
atoms. In the present case, the energy required to reach
the transition state for C-C insertion (TS2-4 in Figure
2) of 3.8 kcal/mol is only 2.0 kcal/mol higher than that
required for C-H insertion. This is an energy difference
that can be evaluated experimentally. In the following
paragraphs we report our efforts to make this evaluation.
Exp er im en ta l In vestiga tion s of th e En er gy a n d
Rea ctivity of 2. A persistent problem in examining
carbene reactivity is the fact that one must take care to
ensure that it is the reactions of the free carbene rather
than those of a precursor that are being studied.14,17 This
seems to be a particular problem in the case of nitrog-
enous carbene precursors (diazo compounds and diazir-
ines) in which the precursor, either in its ground or
excited state, also yields carbene type products. This
problem has been circumvented in two general ways. The
first is simply to avoid the use of nitrogenous precursors
and employ alternatives such as the deoxygenation of
carbonyl compounds by atomic carbon5,18,19 and cheleo-
tropic extrusion of carbenes from cyclopropanes.6,19 The
second approach is to use a nitrogenous precursor fol-
lowed by rearrangement to a new carbene that is
distinctively separated from its precursor on the reactive
energy surface. This method has been used successfully
by generating a carbene that undergoes the phenylcar-
bene rearrangement20,21 to a new carbene that is now
capable of intramolecular rearrangement to stable prod-
ucts.18,22 In the present investigation, we have used the
second of these approaches to study 2.
Gen er a tion of 2 by th e P h en ylca r ben e Rea r -
r a n gem en t. In an attempt to avoid complications
associated with the chemistry of nitrogenous carbene
precursors, we have followed the lead of Chambers and
J ones22 and generated 2 by the phenylcarbene rearrange-
ment of p-tert-butylphenylcarbene, 5. The starting point
for the formation of 2 was p-tert-butylbenzaldehyde
tosylhydrazone lithium salt, 6. When 6 is mildly heated,
p-tert-butylphenyldiazomethane, 7, is evolved and may
then be passed into a tube furnace and pyrolyzed to yield
5. The phenylcarbene rearrangement then converts 5 to
the m- and o-tert-butylphenylcarbenes, 8 and 9, and then
to 2. When Chambers and J ones22 studied this reaction,
they found that some 9 was intramolecularly trapped as
dimethylindane while the remainder rearranged to 2,
which had been previously shown23 to yield both 3 and 4
As expected, 2 has a triplet ground state with a
singlet-triplet (S-T) splitting of 8.0 kcal/mol calculated
at the PMP2/6-31G(d)//MP2/6-31G(d)+ZPC/6-31(d) level.
To assess the singlet-triplet splitting accurately, we have
calculated the S-T in PC at this level and compared it
to our value for 2. Recent computational studies place
the S-T splitting in PC at 4-6 kcal/mol,12 in good
agreement with experiment.15 Our PMP2/6-31G(d)//MP2/
6-31G(d)+ZPC/6-31(d) value for S-T in PC is 7.8 kcal/
mol, indicating that our calculated singlet-triplet gap
in 2 is too high by ∼3 kcal/mol, leading to an actual value
of 5-6 kcal/mol. A comparison of this S-T gap to that
we have reported in 1 of 1-2 kcal/mol demonstrates that
the presence of the phenyl group increases the S-T gap
by 4-5 kcal/mol. In contrast to singlet 2, the triplet
adopts a geometry, 2d , that allows maximum overlap
between the carbene p orbital and the π orbitals of the
ring. In this case, the phenyl ring lies in the plane
formed by the carbene carbon and its attached atoms. It
is probable that the ability of the triplet to accommodate
a larger angle about the carbene carbon than the singlet
(138.5° vs 119.3°) brings about lowered steric interactions
between the tert-butyl group and the ring, allowing
maximum overlap between the carbene p orbital and the
aromatic π orbitals in the planar carbene conformer.
These considerations may be responsible for an abnor-
mally large S-T gap in 2. In the absence of steric
interactions, the addition of alkyl groups to carbene
centers lowers the S-T gap.16 Thus, we may have
expected a smaller S-T gap in 2 relative to PC. How-
ever, geometry constraints placed on singlet 2 by its
electronic structure increase steric interactions, raise the
energy of the singlet, and increase the S-T gap.
An inspection of the minimized geometry calculated for
singlet 2a in Figure 3 reveals a Me-CR-Ccarbene angle of
101.0°, in marked contrast to the angle of 79.8° calculated
for carbene 1.5 While the smaller than normal Me-CR-
Ccarbene angle in 2 indicates some stabilization by the
adjacent C-C σ bond, the effect is far less dramatic than
in 1. In the present case, it appears that, although the
stabilization of the carbene p orbital by the phenyl is not
optimal, it is sufficient to avoid the necessity of ap-
preciable bridging by neighboring σ bonds.
This lack of stabilization by bridging in 2 leads to a
ground-state geometry in 2a which is quite different from
the geometry calculated for the transition state for
intramolecular C-H insertion to give 3 (TS2-3 in Figure
2). Thus, unlike carbene 1, which is calculated to
undergo intramolecular C-H insertion with little or no
barrier, there is a higher barrier (1.8 kcal/mol) in 2. The
(17) (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.; J ones, M., J r.; 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.
(18) Fox, J . M.; Scacheri, J . E. G.; J ones, K. G. H.; J ones, M., J r.;
Shevlin, P. B.; Armstrong B.; Sztyrbicka, R. Tetrahedron Lett. 1992,
33, 5021.
geometries of 1 and 2 are compared in Figure 3.
A
comparison of the geometries of the transition states in
Figure 2 with that of 2a reveals that geometric factors
lower the differences in energies required for intramo-
lecular C-H and C-C insertion in 2 relative to 1. Not
only do TS2-3 and TS2-4 have substantially smaller Me-
Ca-Ccarbene angles than 2a (76.4° and 68.5°, respectively)
(19) Thamattoor, D. M., J ones, M., J r.; Pan, W.; Shevlin, P. B.
Tetrahedron Lett. 1996, 37, 8333.
(20) For reviews of the extensively studied phenylcarbene rear-
rangement see: (a) J ones, 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.; J ones, M., J r. Tetrahedron 1985,
41, 1479. (d) Platz, M. S. Acc. Chem. Res. 1995, 28, 487.
(21) For recent computational studies of the phenylcarbene rear-
rangement see ref 12.
(15) (a) Admasu, A.; Gudmundsdo´tter, A. D.; Platz, M. S. J . Phys.
Chem. A 1997, 101, 3832. (b) Poutsma, J . C. Nash, J . J .; Paulino, J .
A.; Squires, R. R. J . Am. Chem. Soc. 1997, 119, 4686.
(16) Sulzbach, H. M.; Bolton, E.; Lenoir, D.; Schleyer, P. v. R.;
Schaefer, H. F., III J . Am. Chem. Soc. 1996, 118, 9908, and references
therein.
(22) Chambers, G. R.; J ones, M., J r. Tetrahedron Lett. 1978, 52,
5193.

7412 J . Org. Chem., Vol. 63, No. 21, 1998
Armstrong et al.
in an 85:15 ratio. Since similar products are formed in
the present study, we have focused our attention on
measuring the ratio of 3 to 4 as a function of temperature
in order to obtain an experimental evaluation of the
differences in enthalpy and entropy between the in-
tramolecular C-C and C-H insertions.
When 2 is produced by the phenylcarbene rearrange-
ment of 5, the ratio of 3 to 4 decreases from 4.03 to 3.13
as the temperature is raised from 350 to 500 °C. Figure
4, which shows plot of ln(3:4) vs 1/T leads to a ∆∆Hq
F igu r e 4. ln(3:4) as a function of 1/T in the pyrolysis of p-tert-
butylphenyldiazomethane, 7.
P yr olysis of p-ter t-Bu tylben za ld eh yd e Tosylh yd r a -
zon e Lith iu m Sa lt, 6. All surfaces of the reaction vessel,
including the quartz chips packed into the pyrolysis tube, were
cleaned with a solution of ethanol and potassium hydroxide
(base bath), rinsed with tap water, and oven-dried prior to
initial use. The pyrolysis tube and packing were not cleaned
between uses, and the system was left under vacuum between
uses. p-tert-Butylbenzaldehyde tosylhydrazone lithium salt,
6 (0.5 mmol), was placed in a reservoir which could be heated
by means of an oil bath. This reservoir was connected directly
to the pyrolysis tube which extended into a tube furnace. The
reservoir containing the salt was heated to 125 °C over 1 h.
While the reservoir was heating, the pressure slowly increased
from 5 to 55 mTorr at 105 °C and then decreased from that
point. The diazo compound, 7, generated by heating the
lithium salt was pyrolyzed at 623, 673, and 773 K. Three
products, 1,1-dimethylindane, 1,1-dimethyl-2-phenylcyclopro-
pane, 3, and 2-methyl-3-phenyl-2-butene, 4, were generated.
Products were identified by a comparison of their 1H and 13C
NMR and GC/MS with those of authentic samples. The
following ratios of 3:4 were determined by GC and are the
result of 4-7 experiments at each temperature: 4.03 ( 0.11
(623 K); 3.67 ( 0.07 (673 K); 3.13 ( 0.15 (773 K).
(∆Hq
(∆Sq
- ∆HqC-H) of 1.6 ( 0.1 kcal/mol and a ∆∆Sq
C-C
- ∆SqC-H) of -0.2 ( 0.2 eu. Hence, C-H
C-C
insertion by 2 is more favorable than C-C insertion by
1.6 ( 0.1 kcal/mol. These values are in good agreement
with the calculated ∆∆Hq of 2.0 kcal/mol and in reason-
able agreement with a calculated ∆∆Sq of 2.45 eu and
confirm the prediction that C-C and C-H insertion are
competitive in 2.
Su m m a r y a n d Con clu sion s. The results of these
investigations indicate that the introduction of a phenyl
group on going from 1 to 2 lowers the difference in
barriers between intramolecular C-H and C-C insertion
from 3.6 to 2 kcal/mol. While part of this effect may
result from the added phenyl group stabilizing the
carbene and raising the barriers to intramolecular inser-
tions, we feel that it is the dramatic differences in
geometries between 1 and 2 that increase the barrier to
C-H insertion relative to C-C insertion in 2. The
geometry of 2 no longer resembles that of the transition
state for 1,3 C-H insertion, and the 1-2 C-C insertion
now becomes competitive. Quite good agreement be-
tween experimental and computational evaluations of the
difference in barrier heights for the two intramolecular
reactions of singlet 2 is observed with a modest expen-
diture of computational resources. These investigations
demonstrate that the intramolecular reactivity of free
singlet 2, like that of several other carbenes, may be
rationalized by an examination of its geometry.
Ack n ow led gm en t. B.M.A and P.B.S gratefully ac-
knowledge support of this work through National Sci-
ence Foundation Grant CHE-9508570. Computer time
was provided by the Alabama Supercomputer Network.
Su p p or tin g In for m a tion Ava ila ble: Total energies (har-
trees) at the HF/6-31G(d)//HF/6-31G(d), PMP2/6-31G(d)//MP2/
6-31G(d), and B3LYP/6-31G(d)//MP2/6-31G(d) levels and zero-
point energies (kcal/mol) at the HF/6-31G(d) level are given
in Table S1 for various species. Cartesian coordinates of
structures optimized at the MP2/6-31G(d) level are also
included (5 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
Exp er im en ta l Section .
Gen er a l In for m a tion . 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 were recorded on a Fisons Trio 2000 quadrupole spec-
trometer interfaced with a Hewlett-Packard Series II 5890 gas
chromatograph equipped with 30 m × 0.25 mm o.d. DB-5
column. p-tert-Butylbenzaldehyde was used as received from
Aldrich. 1,1-Dimethyl-2-phenylcyclopropane, 3,24 2-methyl-3-
phenyl-2-butene, 4,251,1-dimethylindane,26 and p-tert-butyl-
benzaldehyde tosylhydrazone23 were prepared by literature
methods.
J O981153W
(24) Shank, R. S.; Shechter, H. J . Org. Chem. 1959, 24, 1825.
(25) Abramovici, M.; Pines, H. J . Org. Chem. 1969, 34, 266.
(26) Bright, S. T.; Coxon, J . M.; Steel, P. J . J . Org. Chem. 1990, 55,
1338.
(23) Ando, W.; Sekiguchi, A.; Rothschild, A. J .; Gallucci, R. R.; J ones,
M., J r.; Barton, T. J .; Kilgour, J . A. J . Am. Chem. Soc. 1977, 99, 6995.