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Allylic C-H alkylation of unactivated α-olefins: serial ligand catalysis resumed.

The palladium(0)-catalyzed alkylation reaction of allylic oxygenates has found extensive use in organic synthesis. Recent efforts, however, have focused on the development of catalytic methods to replace allylic C H bonds directly with C C bonds. The selective alkylation of normally inert C H bonds presents exciting opportunities for the development of novel methods and streamlined syntheses of complex molecules. Despite significant advances in C H alkylation, to date no method has been reported for the intermolecular allylic C H alkylation of unactivated a-olefins. Intermolecular palladium(II)-catalyzed allylic C H alkylation reactions using our Pd(OAc)2/bis(sulfoxide) catalysts 1 and 2 have previously been reported by our research group and others. Despite the use of various reaction conditions (DMSO versus no DMSO) and nucleophiles (methyl nitroacetate versus benzoylacetone 13) it was observed that both intermolecular reactions had substrate scopes limited to activated, allylarene structures. We were intrigued by the mechanistic underpinnings of this deficiency, particularly in light of the broad a-olefin scope demonstrated for intermolecular and intramolecular allylic C H esterification and amination reactions catalyzed by 1. Herein we report a mechanistic study that points to competitive ligand binding as the underlying cause for the limited substrate scope observed. Significantly, this mechanistic insight has led to the development of the first intermolecular allylic C H alkylation reaction of unactivated a-olefins. We postulated that the allylic C H alkylation of unactivated a-olefins may be achieved by a serial ligand catalysis mechanism (SLC). Under this scenario, multiple kinetically labile ligands with different electronic properties reversibly coordinate to the metal center and mediate individual steps of the cycle. Specifically, a SLC mechanism for palladiumcatalyzed allylic C H alkylation may proceed as follows: 1) catalytic palladium(II)/bis(sulfoxide)-promoted C H cleavage to furnish a p-allylPd intermediate, 2) stoichiometric DMSO-promoted functionalization through ionization of the p-allylPd intermediate, and 3) re-oxidation of Pd to Pd with a quinone. In the previously reported C H alkylation of allylarenes we found that the complex formed by Pd(OAc)2 and DMSO was sufficiently active to cleave the doubly activated allylic/benzylic C H bond in the absence of bis(sulfoxide) ligands, thus obviating the requirement for a SLC mechanism. In support of the hypothesis that unactivated a-olefins may be alkylated by SLC, stoichiometric studies showed that allylic C H cleavage of these substrates was effected by palladium(II)/phenyl bis(sulfoxide) catalyst 1 to afford pallylPd, trapped as chloride dimer 7, in good yields (78 %, Scheme 1). Significantly, although benzoylnitromethane