Rhodium(III)-Catalyzed Asymmetric Borylative Cyclization of Cyclohexadienone-Containing 1,6-Dienes: An Experimental and DFT Study

Because
of the inherent difficulty in differentiating two olefins,
the development of metal-catalyzed asymmetric cyclization of 1,6-dienes
remains challenging. Herein, we describe the first rhodium­(III)-catalyzed
asymmetric borylative cyclization of cyclohexadienone-tethered mono-,
1,1-di-, and (E)-1,2-disubstituted alkenes (1,6-dienes),
affording optically pure cis-bicyclic skeletons bearing
three or four contiguous stereocenters with high yields (25–93%),
and excellent diastereoselectivities (>20:1 dr) and enantioselectivities
(90–99% ee). This mild catalytic approach is generally compatible
with a wide range of functional groups, which allows several facile
conversions of the cyclization products. Furthermore, on the basis
of our SAESI-MS experiment and computational study, a Rh­(I)/(III)
catalytic cycle is proposed in this tandem reaction, and the Rh­(I)
active species catalyzes the overall transformation via sequential
oxidative addition of B2pin2, olefin insertion,
cyclizing conjugate addition, and reductive elimination. The irreversible
conjugate addition determines the overall regioselectivity of borylative
cyclization, and the ring strain favors the formation of 5,6-bicyclic
structure. This highlights the control of ring strain in diene cyclizations,
which provides a useful basis for future reaction designs.