An unusual cleavage of an energetic carbene [1]

Full text of DOI:10.1021/ja990205b

7150
J. Am. Chem. Soc. 1999, 121, 7150-7151
Communications to the Editor
An Unusual Cleavage of an Energetic Carbene
Guopin Xu, Tsong-Ming Chang, Jinglan Zhou,
Michael L. McKee,* and Philip B. Shevlin*
excess energy in the C-atom reaction is responsible for the
observed cleavage.
Department of Chemistry, Auburn UniVersity
Auburn UniVersity, Alabama 36849-5310
ReceiVed January 21, 1999
ReVised Manuscript ReceiVed June 9, 1999
Transformation of a carbene from a divalent to a tetravalent
species is generally a highly exothermic process for which
1
numerous pathways have been observed. Particularly interesting
examples are two-bond â cleavages, such as that of cyclopropyl-
2
methylenes 1 to an alkene and an alkyne and the cleavage of
To investigate the energy surfaces connecting 2b with the
observed products, we have carried out a computational study in
which geometries were optimized and energies calculated at the
B3LYP/6-311+G(d)+ZPC level.7 Table 1 shows energies of
relevant species relative to the ground state of 2b. Since
deoxygenation of carbonyl compounds by carbon occurs along a
3
2
,5-dioxacyclopentylidene 2a to CO
2
and an alkene. In the latter
provides a thermodynamic driving
reaction, the stability of CO
2
,8
9
singlet energy surface, we have focused our calculations on
singlet species. Carbene 2b was found to have a singlet ground
state with an S-T splitting of 8.7 kcal/mol. Not surprisingly, the
most favorable reaction of 2b was H migration to 4 which has a
barrier of only 5.7 kcal/mol and is exothermic by 63.0 kcal/mol.
Several other intramolecular reactions including ring contraction
force. We now report that the parent cyclopentylidene, 2b, will
undergo an analogous cleavage when generated with sufficient
excess energy.
q
The deoxygenation of carbonyl compounds by atomic carbon,
which is generally exothermic by over 100 kcal/mol, is a
convenient route to carbenes which possess excess energy,2 and
the carbon atom deoxygenation of cyclopentanone, 3, is expected
to generate highly energetic 2b. Co-condensation of arc-generated
to methylenecyclobutane 9 (∆H ) 51.0 kcal/mol) and â C-H
q
insertion to give bicyclo[2.1.0]pentane 10 (∆H ) 27.5 kcal/mol)
c,4
were calculated to have high barriers and seem unlikely to play
a role in the chemistry of 2b.
In examining the energy surface leading from 2b to 5 and 6,
a reaction calculated to be exothermic by 23.2 kcal/mol, it is
immediately obvious that a concerted cleavage preserving the C2V
symmetry of 2b would lead to a planar allene and thus be a high-
energy process. Indeed, such a structure can be located lying 56.1
kcal/mol in energy above 2b with two negative eigenvectors. A
similar problem does not occur in the concerted cleavage of 2a
5
carbon with 3 at 77 K leads to cyclopentene, 4, allene, 5, and
ethylene, 6, in a 4:1:1 ratio (eq 1). These results raise the
possibility that the high exothermicity of the deoxygenation
generates 2b with enough energy to cleave to 5 and 6 in
competition with rearrangement to 4. While it is conceivable that
the cleavage products arise from chemically activated 4, none of
the reported thermal or photochemical decompositions of 4 show
this type of fragmentation.6
in which a calculated barrier of 10 ( 1 kcal/mol has been
3d,10
reported.
Since a careful search of the closed-shell surface
Since we observe that generation of 2b from diazo compound
by pyrolysis of tosylhydrazone lithium salt 8 at 180 °C gives
as the only detectable carbene product, it seems likely that the
connecting 2b with 5 and 6 fails to reveal a low-energy concerted
transition state, we have considered the possibility that the reaction
proceeds in a stepwise manner via biradical 11.
7
4
(1) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New
York, 1971. (b) Baron, W. J.; DeCamp, M. R.; Hendrick, M. E.; Jones, M.,
Jr.; Levin, R. H.; Sohn, M. B. Carbenes; Jones, M., Jr., Moss, R. A., Eds.;
Wiley & Sons: New York, 1973; Vol. 1, p 1. (c) Moss, R. A. In AdVances
in Carbene Chemistry; Brinker, U. H., Ed.; JAI Press: Greenwich, 1994; Vol.
1
, p 59.
2) (a) Friedman, L.: Shechter, H. J. Am. Chem. Soc. 1960, 82, 1002. (b)
(
Shevlin, P. B.; Wolf, A. P. J. Am. Chem. Soc. 1966, 88, 4735. (c) Skell, P.
S.; Plonka, J. H. Tetrahedron 1972, 28, 3571. (d) Shevlin, P. B.; McKee, M.
L. J. Am. Chem. Soc. 1989, 111, 519. (e) Chou, J.-H.; McKee, M. L.; De
Felippis, J.; Squillacote, M.; Shevlin, P. B. J. Org. Chem. 1990, 55, 3291.
(7) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
(8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.;
Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski,
J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J.
V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, (Revision A5);
Gaussian, Inc.: Pittsburgh, PA, 1998.
(
3) (a) Borden, W. T.; Hoo, L. H. J. Am. Chem. Soc. 1978, 100, 6274. (b)
Feller, D.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1981, 103,
2
558. (c) Feller, D.; Borden, W. T.; Davidson, E. R. J. Comput. Chem. 1980,
, 158. (d) Sauers, R. R. Tetrahedron Lett. 1994, 35, 7213 and references
1
therein. (e) See also Lawton, G.; Moody, C. J.; Pearson, C. J. J. Chem. Soc.,
Perkin Trans. 1, 1987, 877.
(
4) Rahman, M.; Shevlin, P. B. Tetrahedron Lett. 1985, 26, 2959.
(
5) The reactor is modeled after that described in Skell, P. S.; Wescott, L.
D., Jr.; Golstein, J. P.; Engel, R. R. J. Am. Chem. Soc. 1965, 87, 2829.
(
6) (a) Lewis, D. K.; Baldwin, J. E.; Cianciosi, S. J. J. Phys. Chem. 1990,
9
4, 7464. (b) Makulski, W.; Collin. G. J. J. Phys. Chem. 1987, 91, 708. (c)
(9) Ahmed, S. N.; Shevlin, P. B. J. Am. Chem. Soc. 1983, 105, 6488.
(10) We calculate a barrier of 8.1 kcal/mol at the B3LYP/6-311+G(d)+ZPC
level.
Shoemaker, J. O.; Carr, R. W., Jr. J. Phys. Chem. 1984, 88, 605. (d) Adam,
W.; Oppenl a¨ nder, T. J. Am. Chem. Soc. 1985, 107, 3924.
1
0.1021/ja990205b CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/14/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 30, 1999 7151
3,4-dideuteriocyclopentanone15 with C atoms at 77 K gives cis
and trans-1,2-dideuterioethylene in a 3:1 ratio as determined by
IR spectroscopy.16 When the reaction is carried out in the gas
phase with C atoms generated by the 80 °C thermolysis of
Table 1. Relative Energies (kcal/mol)
B3LYP/
6-311+G(d) +ZPC
CAS-PT2//B3LYP/
species
6-311+G(d)
+ZPC
2
4
b
0.0
-65.2
-18.6
-44.9
-33.9
62.5
0.0
-63.0
-23.2
-44.0
-32.1
56.1
0.0
0.0
18
diazotetrazole, a 1:1 ratio of cis and trans-1,2-dideuterioethylene
is produced. These results are consistent with the predicted two-
step cleavage of 2b in which rotation about the 3-4 bond
(
9
1
5 + 6)
competes with rupture of the second bond in biradical 11-d
further indication that excited species are involved in the formation
of 5 and 6 is the fact that adding 200 Torr of N to the gas-phase
reaction reduces the yield of 5 to only a trace.19
2
. A
0
concerted TS
TS2b/4
6.8
5.7
2
TS2b/9
52.0
28.2
7.7
51.0
27.5
8.7
TS2b/10
3
2
b
2
1
b*
1
18.5
22.4
38.6
19.4
18.9
36.0
36.6
23.6
40.8
37.5
20.1
38.2
TS2b/11
While these experimental and computational results are con-
sistent with the intermediacy of excited singlet 2b, they do not
require it. However, it is clear that the excess energy available to
the carbene generated by the C-atom deoxygenation brings about
the cleavage reaction. These data raise the interesting possibility
that carbenes generated by highly exothermic pathways may
undergo a set of reactions characteristic of the singlet excited
state. We are continuing to explore this possibility which may
open a new dimension in carbene chemistry.
An estimate of the energy of 11 from 1-pentene using the vinyl
and methyl BDEs places it 23 kcal/mol above 2b in energy. Single
point CAS-PT2 calculations1
1,12
using B3LYP/6-311+G(d)
geometries place 11 20.1 kcal/mol above 2b in energy. These
data indicate that when 2b is generated by conventional methods
of carbene synthesis its cleavage via biradical 11 is unfavorable
by at least 20-23 kcal/mol and will not be competitive with H
q
migration to give 4 (∆H ) 5.7 kcal/mol).
1
Since we have demonstrated that excited singlet ( B
1
) meth-
Acknowledgment. G. X. and P. B. S. gratefully acknowledge support
of this work through National Science Foundation Grant CHE-9508570.
Computer time was provided by the Alabama Supercomputer Network.
9
ylene is generated in the C-atom deoxygenation of formaldehyde,
we have investigated the possibility that the corresponding excited
state of 2b (2b*) is involved in the present deoxygenation. A
Supporting Information Available: Experimental procedures for the
deoxygenation of 3, preparation of cis-3,4-dideutro-cyclopentanone, and
analysis of cis and trans-1,2-dideuterioethylene. Tables of absolute
energies and geometries of relevant intermediates and transition states.
This material is available free of charge via the Internet at http://pubs.acs.org.
1
0,11
CAS-PT2 calculation
using B3LYP/6-311+G(d) geometries
gives an S -S
1
o
separation in 2b of 37.5 kcal/mol.12 These results
indicate that the exothermicity of formation of 2b by C-atom
deoxygenation (108 kcal/mol) could be channeled into the
production of 2b* in which cleavage to 11 is thermodynamically
feasible. A single point CAS-PT2 calculation places the transition
state connecting 2b and 11 0.7 kcal/mol higher in energy than
b*.
To test for the predicted lack of concert in the cleavage of 2b,
we have investigated the stereochemistry of the cleavage of cis-
,4-dideuteriocyclopentylidene, 2c. Thus, deoxygenation of cis-
JA990205B
(
2 5 8
13) The single point CAS-PT2 calculations on CH and C H used a (8,
8
) and (12, 12) complete active space, respectively. ANO basis sets of 4s3p1d
2
and 2s1p were employed for carbon and hydrogen. Geometries and zero point
corrections were obtained at the B3LYP/6-311+G(d) level.
(
14) Herzberg, G.; Johns, J. W. C. Proc. R. Soc. London, Ser. A 1966,
295, 107.
(
15) The cis-3,4-dideuteriocyclopentanone was synthesized by the diimide-
3
d
2
reduction of 4-(tert-butyldimenthylsiloxy)cyclopentene followed by oxida-
tion to the ketone.
(11) Andersson, K.; Blomberg, M. R. A.; F u¨ lsher, M. P.; Karlstr o¨ m, G.;
(16) Pure samples of cis and trans-1,2-dideuterioethylene were prepared17
and IR calibration curves constructed.
Lindh, R.; Malmqvist, P.- A° .; Neogr a´ dy, P.; Olsen, J.; Roos, B. O.; Sadlej, A.
J.; Sch u¨ ltz, M.; Seijo, L.; Serrano-Andr e´ s, L.; Siegbahn, P. E. M.; Widmark,
P.-O. MOLCAS, version 4; Lund University: Sweden, 1997.
(17) Nicholas, P. P.; Carroll, R. T. J. Org. Chem. 1968, 33, 2345.
(18) Shevlin, P. B.; Kammula, S. J. Am. Chem. Soc. 1977, 99, 2627.
(19) Since generation of C atoms from 5-diazotetrazole is always ac-
1
1
(
12) A CAS-PT2//B3LYP/6-311+G(d) calculation of the B
1
- A
methylene gave 23.7 kcal/mol as compared with an experimental value of
1.1 kcal/mol.14
1
gap in
1
3
18
companied by ethylene formation (from C + residual THF solvent), only
2
the fact that the yield of 5 is drastically reduced is relevant in this experiment.