Formation from a P,C-Palladacycle

In contrast to PPhs, P(c -Tol)3 cannot reduce Pd (OAc)2 to a Pd(0) complex, but a P,C-palladacycle, rran5-di(/r-acetato)-bis[o-(di-o-tolylphosphino)benzyl]dipaIladium (5) is formed via a cyclometallation [27, 55]. The palladacycle 5 is an efficient catalyst for Mizoroki-Heck reactions involving aryl bromides and activated aryl chlorides (i.e. substituted by EWGs) [lj,l,o,s-v, 27, 55]. When 5 is used as catalyst in C—N crosscoupling reactions, Louie and Hart wig [56] have established that the true catalyst is aPd(0) complex, Pd P(o-Tol)3 2 formed by reduction of the palladacycle by the nucleophile (a secondary amine as a hydride donor in the presence of a strong base). 

A catalytic cycle is proposed for Mizoroki-Heck reactions involving a P,C-palladacycle precursor based on the fact that a monoligated Pd(0) complex is formed from P,C-palladacycle precursors (see above). The structure of the Pd(0) complex 10 is close to thatof Pd° P(o-Tol)3 generated from Pd° P(o-Tol)3 2 as the minor but active species in oxidative additions of aryl bromides, as reported by Hartwig and Paul [62a]. The oxidative addition gives the dimeric complex [ArPd(/u.-Br) P(c -Tol)3 ]2 in equilibrium with the former T-shaped complex ArPdBr P(c -Tol)3 prone to react with a nucleophile [62b,c]. Such a mechanism must be vahd for the Pd(0) complexes 10 or 13 generated in situ from the 

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