Nickel-catalyzed coupling reaction of 1,3-disubstituted secondary allylic carbonates and lithium aryl- and alkenylborates

This account describes coupling reaction of 1,3-disubstituted secondary allylic carbonates with lithium aryl- and alkenylborates in the presence of a nickel. catalyst. Berates examined are 4, 5, and 6, and reactivity and selectivity were investigated using the allylic carbonates la and Ib. Coupling of la,b with berates 4 was effected with the nickel catalyst, NiCl2(PPh(3))(2) or NiCl2(dppf), in THF at 45-65 degrees 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 la,b after transformation to berates 5 with MeLi. Though coupling using 4 and 5 required elevated temperature (45-65 degrees C), cyclic berates 6 prepared in situ from boronates 8 and MeLi were found to couple with la,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) . CHCl3 + 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 pi-allylnickel intermediates.