from oxidative cyclization of nickel(0) to an enone 1a to form
oxa-nickelacycle 8 (Scheme 2).11 Subsequent coordination of
allene 2h takes place to give intermediate 9a; here, the steric
repulsive interaction is minimal between two cyclohexyl groups
of both 2h and 8. Insertion of an allene into the C–Ni bond leads
to the seven-membered oxa-nickelacycle 10, which undergoes
reductive elimination to give 3ah and a Ni(0) complex.
(d) E. Shirakawa, H. Yoshida and H. Takaya, Tetrahedron Lett.,
1997, 38, 3759; (e) E. Shirakawa, H. Yoshida, T. Kurahashi,
Y. Nakao and T. Hiyama, J. Am. Chem. Soc., 1998, 120, 2975;
(f) K. R. Reddy, C.-L. Chen, Y.-H. Liu, S.-M. Peng, J.-T. Chen
and S.-T. Liu, Organometallics, 1999, 18, 2574; (g) S. M. Nobre
and A. L. Monteiro, J. Mol. Catal. A: Chem., 2009, 313, 65.
6 The use of other bidentate ligands, such as dppf, dppp, binap, bpy,
did not afford any product.
7 Neither benzylidene-substituted pentane-2,4-dione nor diethyl
malonate participated in the cycloaddition with allene 2a even
for prolonged reaction times, and both were recovered unchanged
quantitatively. A stoichiometric reaction of such enones with
Ni(cod)2 and iminophosphine 7 did not afford oxa-nickelacycle
and resulted in no reaction. Thus, a-ester-substituted a,b-unsatu-
rated ketone will be a suitable substrate for oxa-nickelacycle
formation and cycloaddition with allenes.
In summary, we have developed a new nickel-catalyzed
[4+2] cycloaddition reaction of enones with allenes to provide
highly substituted dihydropyrans. Iminophosphine was found
to be a highly active ligand for the reaction; enones are
susceptible to oxidative cyclization of nickel(0) in the presence
of an iminophosphine ligand, and such reactions allow inter-
molecular cycloaddition with allenes. Further investigations
on the potential of the developed cycloaddition and detailed
mechanistic studies of the catalytic reaction mechanism are
currently underway in our laboratory.12
8 For details see ESIw.
9 For crystallographic evidence for the formation of oxa-nickelacycles,
see: (a) H. H. Karsch, A. W. Leithe, M. Reisky and E. Witt,
Organometallics, 1999, 18, 90; (b) K. K. D. Amarasinghe,
S. K. Chowdhury, M. J. Heeg and J. Montgomery, Organometallics,
2001, 20, 370; (c) J. Montgomery, K. K. D. Amarasinghe,
S. K. Chowdhury, E. Oblinger, J. Seo and A. V. Savchenko, Pure
Appl. Chem., 2002, 74, 129; (d) S. Ogoshi, M. Oka and
H. Kurosawa, J. Am. Chem. Soc., 2004, 126, 11802;
(e) H. P. Hratchian, S. K. Chowdhury, V. M. Gutierrez-Garcia,
K. K. D. Amarasinghe, M. J. Heeg, H. B. Schlegel and
J. Montgomery, Organometallics, 2004, 23, 4636; (f) S. Ogoshi,
M. Ueta, T. Arai and H. Kurosawa, J. Am. Chem. Soc., 2005,
127, 12810; (g) S. Ogoshi, M. Nagata and H. Kurosawa,
J. Am. Chem. Soc., 2006, 128, 5350; (h) S. Ogoshi, K. Tonomori,
M. Oka and H. Kurosawa, J. Am. Chem. Soc., 2006, 128, 7077;
(i) S. Ogoshi, T. Arai, M. Ohashi and H. Kurosawa, Chem.
Commun., 2008, 1347.
This work was supported by Grants-in-Aid from MEXT,
Japan. T.K. also acknowledges Asahi Glass Foundation,
Kansai Research Foundation, and Mizuho Foundation.
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12 As a result of preliminary experiments, we found that cyclo-
addition of 1a with 2a in the presence of Ni(cod)2/(S,S)-iPr-foxap
afforded 3aa with 54% ee in 18% yield. Further modifications of
chiral iminophosphine ligands to improve enantioselectivity and
yield are underway.
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6152 Chem. Commun., 2011, 47, 6150–6152
This journal is The Royal Society of Chemistry 2011