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26.9.1.1.3.6 Variation 6: Transition-Metal-Catalyzed Coupling of Alkenylmetals with Carboxylic Acid Halides and Anhydrides

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261056.

Palladium(II)acyl complexes can be generated by the insertion of zerovalent palladium into the carbonhalogen bond of acid halides. In the presence of suitable organometallic donors, transmetalation and reductive elimination occurs, yielding enones. Alkenylstannanes are viable donors and the mild nature of the method is demonstrated by the successful formation of potentially base-sensitive products such as 54 (Scheme 13).[‌52‌] Alkenylzincs such as 55 are also good coupling partners,[‌53‌] and alkenylcoppers[‌41‌] and aluminums[‌54‌] can also be used. Decarbonylation of the intermediate palladium(II)acyl complexes can be a competing reaction, leading to alkene rather than enone products. Carrying out the reactions under an atmosphere of carbon monoxide can minimize this process.[‌55‌] Alkenylboronic acids such as 56 are popular choices as nucleophilic coupling partners in transition-metal-mediated reactions owing to their stability and ease of handling. They are converted into enones by coupling with thiol esters under palladium catalysis in the presence of copper(I) thiophene-2-carboxylate as an activating agent.[‌56‌] Rhodium(I) catalysts also promote the addition of alkenylboronic acids to acid anhydrides, to give products such as 57.[‌57‌] The mechanism of this latter process likely involves direct addition of an alkenylrhodium species to the anhydride.

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