Y. Kobayashi et al., NICKEL-CATALYZED COUPLING REACTION OF 1,3-DISUBSTITUTED SECONDARY ALLYLIC CARBONATES AND LITHIUM ARYL- AND ALKENYLBORATES, Journal of organic chemistry, 61(16), 1996, pp. 5391-5399
This account describes coupling reaction of 1,3-disubstituted secondar
y allylic carbonates with lithium aryl- and alkenylborates in the pres
ence of a nickel. catalyst. Berates examined are 4, 5, and 6, and reac
tivity and selectivity were investigated using the allylic carbonates
la and Ib. Coupling of la,b with berates 4 was effected with the nicke
l 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 ster
eoselectivities. Trivalent organoboranes prepared from acetylenes by h
ydroboration with catecholborane also underwent coupling reaction with
la,b after transformation to berates 5 with MeLi. Though coupling usi
ng 4 and 5 required elevated temperature (45-65 degrees C), cyclic ber
ates 6 prepared in situ from boronates 8 and MeLi were found to couple
with la,b at room temperature or below. Regio- and stereoselectivitie
s 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) . C
HCl3 + 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 o
verall anti fashion. With additional experiments, the mechanism of the
present reaction was discussed in terms of transient pi-allylnickel i
ntermediates.