C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism for the Rearrangement of 1
We have discovered a variety of conditions based on Ni/NHC
systems for the rearrangement of cyclopropylen-ynes to afford
cyclopentane- and cycloheptene-based heterocycles. However, the
use of ItBu led to the selective formation of the cyclopentane
products. Investigations focused on developing protocols for
selective cycloheptene formation and understanding the mechanistic
details of the rearrangements are currently underway.
Acknowledgment. We gratefully acknowledge the University
of Utah, ACS (PRF Type G), and the NSF (Career Award) for
support of this research.
Supporting Information Available: Detailed experimental pro-
cedures and compound characterization data are available (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(
1) (a) Nakamura, I.; Yamamoto, Y. Chem. ReV. 2004, 104, 2127. (b) L o´ pez,
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Table 3. Selective Formation of Cyclopentanesa
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(
2) (a) Zuo, G.; Louie, J. Angew. Chem., Int. Ed. 2004, 43, 2277. In contrast,
the Ir-catalyzed cyclization of cyclopropylen-ynes provides 1,3-dienes with
the cyclopropyl ring intact. See: (b) Chatani, N.; Inoue, H.; Morimoto,
T.; Muto, T.; Murai, S. J. Org. Chem. 2001, 66, 4433.
(
3) For Rh, see: (a) Wender, P. A.; Barzilay, C. M.; Dyckman, A. J. J. Am.
Chem. Soc. 2001, 123, 179. (b) Wender, P. A.; Sperandio, D. A. J. Org.
Chem. 1998, 63, 4164. (c) Binger, P.; Wedermeann, P.; Kozhushkov, S.
I.; de Meijere, A. Eur. J. Org. Chem. 1998, 113, 3. (d) Wender, P. A.;
Takahashi, H.; Witulski, B. J. Am. Chem. Soc. 1995, 117, 4720. For Ru,
see: (e) Trost, B. M.; Toste, F. D.; Shen, H. J. Am. Chem. Soc. 2000,
122, 2379.
(4) For leading reviews on NHCs, see: (a) Arduengo, A. J., III. Acc. Chem.
Res. 1999, 32, 913. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
4
1, 1290. (c) Bourissou, D.; Guerret, O.; Gabba ¨ı , F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39. [ICy ) 1,3-dicyclohexylimidazol-2-ylidene; IAd )
,3-diadamantylimidazol-2-ylidene; ItBu ) 1,3-di-tert-butylimidazol-2-
ylidene; IMes ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr )
1
1
,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; SIPr ) 1,3-bis(2,6-
diisopropylphenyl)-4,5-dihydroimidazolin-2-ylidene.]
(5) Pinke, P. A.; Stauffer, R. D.; Miller, R. G. J. Am. Chem. Soc. 1974, 96,
4229.
(
6) Rearrangements proceeded smoothly in the presence of a variety of radical
traps (e.g., 2,6-di-tert-butyl-4-methylphenol, 1,4-cyclohexadiene, phenyl
disulfide, glavinoxyl).
(
7) It is unclear at this time whether 6 would result from 5a or 5b. A variety
of Ni compounds are known to catalyze cycloaddition reactions of enynes
via initial oxidative coupling between an alkene and alkyne (see ref 8).
Thus, a similar pathway could led to the formation of 6 via intermediate
5a. Indeed, a similar mechanism has been proposed for analogous Rh-
and Ru-catalyzed chemistry (see ref 3). However, we recently discovered
that the Ni/NHC catalyst system mediates the isomerization of VCPs.
Consequently, 6 may be derived from the initial isomerization of the VCP
(
5b) and subsequent insertion of the alkyne (see ref 2a).
(
8) (a) Zhang, M.; Buchwald, S. L. J. Org. Chem. 1996, 61, 4498. (b) Wender,
P. A.; Smith, T. E. J. Org. Chem. 1995, 60, 2962. (c) Tamao, K.;
Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1988, 110, 1286.
a
Reaction conditions: 5 mol % Ni(COD)2, 5 mol % ItBu, toluene, room
(9) It is possible that isomerization to form 4 occurs via the generation of a
Ni-H complex (see ref 5). Indeed, the addition of catalytic amounts of
b
c
temperature. Isolated yields (average of two runs). SIPr was used as the
ligand instead of ItBu. Reaction was run at 40 °C.
d
4
HBF (10 mol %) to Ni/ItBu-catalyzed reactions of 1d led to complete
conversion and formation of 4d. We are currently investigating the
isomerization mechanism.
10) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff,
C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485.
(
functionality (entries 5 and 6). Furthermore, internal substitution
did not effect the rearrangement (entries 8 and 9). Interestingly,
rearrangement of 18 afforded a tetrahydrofuran product (19)
possessing a VCP moiety that resisted further isomerization to a
spirocyclopentane (entry 10).11
(
11) When Ni/SIPr was used as the catalyst, further isomerization occurred to
give a mixture of products. As shown in Table 1, Ni/ItBu is a less active
catalyst system than Ni/SIPr, which allows for the selective formation of
19.
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