Allylic C-H functionalization via group 9 π-allyl intermediates.

Allylic C-H functionalization catalysed by group 9 Cp* transition-metal complexes has recently gained significant attention. These reactions have expanded allylic C-H functionalization to include di- and trisubstituted olefins, and a broad range of coupling partners. More specifically, several catalytic C-N, C-O, and C-C bond forming allylic C-H functionalization reactions have been reported, proceeding via MCp*-π-allyl intermediates. Herein we present an overview of these reactions by mechanistic paradigm. We also place this information in context of recent advances, as well as, limitations that remain for this class of reactions.

The Mechanism of Rhodium-Catalyzed Allylic C-H Amination.

Herein, the mechanism of catalytic allylic C-H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that the allylic C-H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidants in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry experiments concomitantly support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand proceeding through a Rh(IV) intermediate. Stoichiometric oxidation and bulk electrolysis of the proposed π-allyl intermediate are also reported to support these analyses. Lastly, evidence supporting the amination of an allylic acetate intermediate is presented. We show that Cp*Rh(III)2+ behaves as a Lewis acid catalyst to complete the allylic amination reaction.

Regioselective Cp*Ir(III)-Catalyzed Allylic C-H Sulfamidation of Allylbenzene Derivatives.

In this study we report the development of the regioselective Cp*Ir(III)-catalyzed allylic C-H sulfamidation of allylbenzene derivatives, using azides as the nitrogen source. The reaction putatively proceeds through a Cp*Ir(III)-π-allyl intermediate and demonstrates exclusive regioselectivity for the branched position of the π-allyl. The reaction performs well on electron-rich and electron-deficient allylbenzene derivatives and is tolerant of a wide range of functional groups, including carbamates, esters, and ketones. The proposed mechanism for this reaction proceeds via C-N reductive elimination from a Cp*Ir(V) nitrenoid complex at the branched position of the π-allyl.

Intermolecular Allylic C-H Etherification of Internal Olefins.

Herein we report on the development of an oxidative allylic C-H etherification reaction, utilizing internal olefins and alcohols as simple precursors. Key advances include the use of RhCp* complexes to promote the allylic C-H functionalization of internal olefins and the compatibility of the oxidative conditions with oxidatively sensitive alcohols, enabling the direct etherification reaction. Preliminary mechanistic studies, consistent with C-H functionalization as the rate determining step, are presented.