Carbonylation reactions of alkyl iodides through the interplay of carbon radicals and Pd catalysts

Abstract

Numerous methods for transition metal catalyzed carbonylation reactions have been established. Examples that start from aryl, vinyl, allyl, and benzyl halides to give the corresponding carboxylic acid derivatives have all been well documented. In contrast, the corresponding alkyl halides often encounter difficulty. This is inherent to the relatively slow oxidative addition step onto the metal center and subsequent β-hydride elimination which causes isomerization of the alkyl metal species. Radical carbonylation reactions can override such problems of reactivity; however, carbonylation coupled to iodine atom transfer (atom transfer carbonylation), though useful, often suffers from a slow iodine atom transfer step that affects the outcome of the reaction. We found that atom transfer carbonylation of primary, secondary, and tertiary alkyl iodides was efficiently accelerated by the addition of a palladium catalyst under light irradiation. Stereochemical studies support a mechanistic pathway based on the synergic interplay of radical and Pd-catalyzed reaction steps which ultimately lead to an acylpalladium species. The radical/Pd-combined reaction system has a wide range of applications, including the synthesis of carboxylic acid esters, lactones, amides, lactams, and unsymmetrical ketones such as alkyl alkynyl and alkyl aryl ketones. The design of unique multicomponent carbonylation reactions involving vicinal C-functionalization of alkenes, double and triple carbonylation reactions, in tandem with radical cyclization reactions, has also been achieved. Thus, the radical/Pd-combined strategy provides a solution to a longstanding problem of reactivity involving the carbonylation of alkyl halides. This novel methodology expands the breadth and utility of carbonylation chemistry over either the original radical carbonylation reactions or metal-catalyzed carbonylation reactions.