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This account describes coupling reaction of 1,3-disubstituted secondary allylic carbonates with lithium aryl- and alkenylborates in the presence of a nickel catalyst. Borates examined are 4, 5, and 6, and reactivity and selectivity were investigated using the allylic carbonates 1a and 1b . Coupling of 1a, b with borates 4 was effected with the nickel catalyst, NiCl 2 (PPh 3 ) 2 or NiCl 2 (dppf), in THF at 45−65 °C to provide products 3 in good yields with almost 100% regio- and stereoselectivities. Trivalent organoboranes prepared from acetylenes by hydroboration with catecholborane also underwent coupling reaction with 1a, b after transformation to borates 5 with MeLi. Though coupling using 4 and 5 required elevated temperature (45−65 °C), cyclic borates 6 prepared in situ from boronates 8 and MeLi were found to couple with 1a, b at room temperature or below. Regio- and stereoselectivities were almost 100% as were observed in the cases of 4 and 5 . In these reactions, palladium complexes such as Pd(PPh 3 ) 4, Pd 2 (dba) 3 ·CHCl 3 + 2 PPh 3 showed no catalytic activity. Stereochemical aspect of the present reaction was studied using cyclohexenyl carbonate 24 with 2-furylborate 4e and phenylborate 6a, and was found to proceed with overall anti fashion. With additional experiments, the mechanism of the present reaction was discussed in terms of transient π-allylnickel intermediates.
Kobayashi et al. (Mon,) studied this question.