Clarification of a remarkable chelate effect leads to palladium-catalyzed base-free olefin arylation

Palladium-catalyzed, base-free olefin arylation is possible. Thus, reaction of chlorobenzene with styrene and zinc to yield cis-stilbene is catalyzed by Pd-(dippp)₂ (2) (or by Pd(OAc)₂ + 2dippp; dippp = 1,3-bis(diisopropylphosphino)propane). Other aryl chlorides are also reactive, whereas aryl bromides undergo substantial reduction and coupling. The reaction was discovered as a result of the observation that cis-(dippp)-Pd(Ph)Cl, formed upon oxidative addition of PhCl to 2, reacts with excess styrene (in the presence or absence of base) to generate H₂ and yield (dippp)Pd(styrene) and (dippp)PdCl₂(5). The latter complex is formed as a result of the instability of the putative cis-(dippp)Pd(H)Cl. 5 can be reduced to Pd(dippp)₂ with zinc in the presence of dippp. Pd(dippb)₂ (1) (dippb = 1,4-bis(diisopropylphosphino)butane), which yields a trans-oxidative-addition product with PhCl, is inactive in the base-free catalysis, and monophosphines are not effective. This novel reaction is complementary to the normal vinylation of aryl chlorides catalyzed by Pd(dippb)₂ under basic conditions, which leads to trans products.