Migratory Decarboxylative Coupling of Coumarins: Synthetic and Mechanistic Aspects

A three-component coupling of vinyl triflates and boronic acids to alkenes catalyzed by palladium is reported. Using 1,3-dienes, selective 1,2-alkene difunctionalization is observed, whereas the use of terminal alkenes results in 1,1-alkene difunctionalization. The reaction outcome is attributed to the formation of stabilized, cationic Pdπ-allyl intermediates to regulate β-hydride elimination. S effort has been afforded to the development of high utility methods involving palladium-catalyzed alkene functionalization as highlighted by the Heck reaction and the Wacker oxidation. Inspired by the key mechanistic motifs of these fundamental reactions, a focus of our research program over the past several years has been the advancement of palladiumcatalyzed alkene difunctionalization reactions with the ultimate goal of selectively introducing two groups across the alkene. 7 As a specific example, we have reported the diarylation of conjugated and terminal alkenes using oxidative palladium catalysis (Scheme 1a). In this reaction, two aryl groups originating from an arylstannane are added across a 1,3-diene (or styrene) to yield the 1,2-diarylation product. Mechanistically, this reaction is thought to initiate by transmetalation to form Pdaryl species A, Heck insertion of a conjugated diene yielding B, and stabilization of the Pd-alkyl as aπ-allylC. Subsequent cross-coupling of a second equivalent of an aryl stannane results in product formation. The success of this reaction is partially attributed to stabilization of the electrophilic Pd-species by formation of either a π-allyl (for dienes) or a π-benzyl (for styrenes) intermediate suppressing β-hydride elimination. The obvious synthetic limitation of this 1,2-alkene difunctionalization reaction is the introduction of two identical aryl groups from the arylstannane as well as reasonably complex reaction conditions. To overcome this significant synthetic drawback, we considered an alternative and perhaps simpler approach. Specifically, the use of Pd(0) initiated catalysis is proposed wherein a vinyl triflate undergoes oxidative addition to yield Pd-vinyl species D. Heck insertion of a diene should produce intermediate B0 which is closely related to the proposed intermediate in the oxidative Pd-catalyzed diarylation reaction and should be stabilized by formation of a π-allyl C0 to suppress β-hydride elimination. It should be noted that vinyl triflates were specifically selected to initiate the catalysis since highly electrophilic Pd-species should result in alkene insertion in preference to the more common Suzuki coupling sequence. To complete the process, transmetalation of a boronic acid derivative and reductive elimination will introduce two different groups into the alkene framework to yield skipped diene products. Reports of related three-component coupling reactions of this type with Pd have been generally limited to substrates that are unable to undergo β-hydride elimination after Heck insertion such as norbornenes, alkynes, and carbene precursors. Allenes as the “alkene” coupling partner have been reported, where a Pd-πallyl species is directly formed viamigratory insertion to presumably avoid β-hydride elimination in a manner similar to our proposal. Herein, we present the successful development of a three-component coupling to achieve the formation of two sp sp carbon carbon bonds from the alkene framework using vinyl triflates as the organic electrophile and boronic acids as the organometallic reagent. Surprisingly, this complexity generating reaction occurs under very simple and mild reaction conditions, where alkene 1, 2-difunctionalization is achieved using conjugated dienes. Extension tomore challenging terminal alkenes, which has not previously been reported, is also presented, yielding the 1,1-alkene difunctionalization products under identical reaction conditions. For optimization, vinyl triflate 1a, diene 2a, and boronic acid 3a were selected (Table 1). Gratifyingly, the optimized conditions Scheme 1. Proposed Three-Component Coupling of Conjugated Dienes with Vinyl Triflates and Aryl Boronic Acids Received: February 16, 2011