C O M M U N I C A T I O N S
a
Table 1. Activation Parameters for Carbene/Alkene Additions
b
‡
∆H
‡
∆S
‡
‡
∆G
carbene
alkene
E
a
log A
-T∆S
c,d
PhCCl
PhCCl
PhCCl
TME
-1.7
1.24 (0.02)
1.1
-1.2 (0.02)
3.8 (0.2)
4.7 (0.3)
0.9 (0.02)
5.6 (0.3)
6.0 (0.06)
7.2
7.4 (0.01)
7.4
8.8 (0.06)
10.9 (0.3)
10.7 (0.2)
9.7 (0.02)
11.5 (0.3)
11.5 (0.04)
-2.3
0.68 (0.02)
0.5
-1.8 (0.02)
3.3 (0.2)
4.1 (0.3)
0.3 (0.02)
5.0 (0.3)
5.4 (0.06)
-28
-26 (0.01)
-27
-20 (0.2)
-10.5 (1.3)
-11.5 (1.1)
-16 (0.2)
-7.8 (1.1)
-7.8 (0.2)
8.3
7.9 (0.02)
8.0
6.0 (0.2)
3.1 (0.4)
3.4 (0.3)
4.7 (0.2)
2.3 (0.3)
2.3 (0.3)
6.0
8.6 (0.03)
8.5
4.2 (0.2)
6.4 (0.4)
7.5 (0.4)
5.0 (0.2)
7.3 (0.4)
7.7 (0.3)
c-C6H10
1-hexene
TME
c-C6H10
1-hexene
TME
c
d
CCl2
CCl2
CCl2
CClF
CClF
CClF
c-C6H10
1-hexene
a
Units are kcal/mol for Ea, ∆H , -T∆S , and ∆G ; M s-1 for log A; cal/(deg-mol) for ∆S . ∆H is calculated at 283 K; ∆G is calculated at 298
‡
‡
‡
-1
‡
‡
‡
b
c
K. Errors (in parentheses) are average deviations of two independent determinations. TME ) tetramethylethylene; c-C6H10 ) cyclohexene. From
reference.
5
b d
Negative activation energies refer to 263 < T < 300 for PhCCl or 273 < T < 304 for CCl2.
details). The calculated reaction parameters are functional
dependent and, unfortunately, do not provide a unified picture.
The B3LYP-based calculations identify distinct (gas phase)
potential energy minima representing 1:1 carbene-alkene
complexes, as well as TSs for cyclopropane formation, for all
the species presented in Table 1. However, no CCl2/TME
complex or TS could be located when the MPW1K functionals
were applied. When we employed the MPW1PW91 functionals,
CCl2/TME, CCl2/1-hexene, as well as PhCCl/TME complexes
and TSs were nonexistent.
Only the complexes of CCl2 and CClF with TME are bound
on the B3LYP enthalpy surface. Applying MPW1K functionals,
PhCCl/TME and CClF/TME as well as CCl2/cyclohexene
complexes are bound, but just the latter two carbene-alkene
pairs form enthalpy-bound complexes using MPW1PW91
the trends displayed in Table 1. More detailed calculations to
characterize the variational transition states for some of the
systems in Table 1 are planned.
In summary, we report the first measured activation param-
eters for the additions of CCl and CClF to simple alkenes and
2
demonstrate the existence of enthalpic barriers for CCl additions
2
to cyclohexene and 1-hexene. With these two alkenes, additions
‡
of PhCCl are “dominated” by entropic contributions to ∆G
‡
and additions of CCl display comparable contributions of ∆H
2
‡
‡
and ∆S , while CClF additions feature dominant ∆H contribu-
tions to ∆G . Entropic factors, however, control the additions
‡
of all three carbenes to the highly reactive alkene, tetrameth-
ylethylene.
Acknowledgment. We are grateful to the National Science
Foundation and the Petroleum Research Fund for financial support.
5
,7
functionals. Carbene-alkene complexes have been discussed,
7
and mostly dismissed, in the literature. We note, however, in
this context that we have recently detected a number of 1:1
Supporting Information Available: Figures S-1-S-85; com-
putational details, and Tables S-1-S-6. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
2
halocarbene-arene complexes by fast UV spectroscopy.
‡
The calculations uniformly predict ∆S ≈ -40 eu (P ) 1
atm, T ) 298 K) for elementary one-step carbene-alkene
addition reactions of the species investigated here (Tables S-1,
S-3, and S-5; Supporting Information). However, less negative
References
(
1) Doering, W. v. E.; Hoffmann, A. K. J. Am. Chem. Soc. 1954, 76, 6162.
2) Fedorynski, M. Chem. ReV. 2003, 103, 1099.
(
‡
4a
2
4b
values for ∆S are anticipated, if the reaction proceeds with
(3) CCl and CClF can be generated by LFP of their phenanthrene or indane
adducts, respectively, but the simultaneous production of aromatic side
the intermediate formation of carbene-alkene complexes. Such
products is a complication.
“
entropy-absorbing” complexes could also enjoy solvent cage
(
4) (a) Chateauneuf, J. E.; Johnson, R. P.; Kirchhof, M. M. J. Am. Chem. Soc.
5
b
1
990, 112, 3217. (b) Tippmann, E. M.; Platz, M. S. J. Phys. Chem. A 2003,
stabilization. Indeed, if the carbene-alkene complexes are
used as reference, ∆S values computed for the additions are
‡
107, 8547.
(
5) (a) Turro, N. J.; Lehr, G. F.; Butcher, J. A., Jr.; Moss, R. A.; Guo, W.
J. Am. Chem. Soc. 1982, 104, 1754. (b) Moss, R. A.; Lawrynowicz, W.;
Turro, N. J.; Gould, I. R.; Cha, Y. J. Am. Chem. Soc. 1986, 108, 7028. (c)
Gould, I. R.; Turro, N. J.; Butcher, J. A., Jr.; Doubleday, C. E., Jr.; Hacker,
N. P.; Lehr, G. F.; Moss, R. A.; Cox, D. P.; Guo, W.; Munjal, R. C.; Perez,
L. A.; Fedorynski, M. Tetrahedron 1985, 41, 1587.
dramatically increased (less negative) relative to the computed
bimolecular reaction values (Tables S-2, S-4, and S-6; Sup-
porting Information), but neither quantitative nor qualitative
‡
agreement with the experimental ∆S data is observed, even
(
6) (a) Skell, P. S.; Cholod, M. S. J. Am. Chem. Soc. 1969, 91, 7131. (b) Giese,
B.; Meister, J. Angew. Chem., Int. Ed. Engl. 1978, 17, 595. (c) Giese, B.;
Lee, W.-B. Angew. Chem., Int. Ed. Engl. 1980, 19, 835. (d) Giese, B.;
Lee, W.-B.; Meister, J. Ann. Chem. 1980, 725. (e) Giese, B.; Lee, W.-B.
Chem. Ber. 1981, 114, 3306.
for the reactant species where bound complexes have been
located. Notably, the variational transition-state model developed
by Houk does lead to activation entropies of reasonable
magnitude for carbene-alkene model additions, which are
(
7) (a) Houk, K. N.; Rondan, N. G.; Mareda, J. Tetrahedron 1985, 41, 1555.
(b) Houk, K. N.; Rondan, N. G.; Mareda, J. J. Am. Chem. Soc. 1984, 106,
7
a–c
analogous to the systems under investigation here.
4291. (c) Houk, K. N.; Rondan, N. G. J. Am. Chem. Soc. 1984, 106, 4293.
d) Blake, J. F.; Wierschke, S. G.; Jorgensen, W. L. J. Am. Chem. Soc.
989, 111, 1919.
(
1
Calculated activation enthalpies also agree rather poorly with
the experimental values and with expectations from variational
transition state theory. Houk’s model predicts that the activation
enthalpy calculated on the basis of a potential-surface maximum
must be larger than the activation enthalpy derived from the
variational transition state. Whereas we do calculate (B3LYP)
activation enthalpies 3-5 kcal/mol larger than the experimental
values for PhCCl, they are 1-2 kcal/mol less than the observed
values for CCl2 and CClF. Thus, potential energy surface
calculations based on some commonly employed DFT func-
tionals do not well reproduce either the observed parameters or
(
(
8) Moss, R. A.; Tian, J.; Sauers, R. R.; Ess, D. H.; Houk, K. N.; Krogh-
Jespersen, K. J. Am. Chem. Soc. 2007, 129, 5167.
9) Moss, R. A.; Tian, J.; Sauers, R. R.; Skalit, C.; Krogh-Jespersen, K. Org.
Lett. 2007, 9, 4053.
(10) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J. Am. Chem.
Soc. 1988, 110, 5595.
(
11) See: Moss, R. A. In Carbenes; Jones, M., Jr., Moss, R. A., Eds.; Wiley:
New York, 1973; Vol. 1, pp 216, 253, and references cited therein. As
expected, TME is 100 (CCl
cyclohexene.
2
) or 50 (CClF) times more reactive than
(12) Moss, R. A.; Tian, J.; Sauers, R. R.; Krogh-Jespersen, K. J. Am. Chem.
Soc. 2007, 129, 10019.
JA8005226
J. AM. CHEM. SOC. 9 VOL. 130, NO. 17, 2008 5635