Developing Organometallic Nucleophilic Reagents Via Photoredox Catalysis

Abstract The addition of organometallic reagents to the carbonyl group represents a key transformation, both in academia and industry. Most of these transformations rely on a mechanism in which accessible and reactive halides are transformed into the corresponding nucleophilic organometallic reactive compounds through a redox mechanism, using a metal (Cr, Mg, In, etc.) in low oxidation state, by electron transfer. With the advent of photoredox catalysis, the formation of radicals, through oxidation or reduction of suitable and tailored organic precursors, was merged with transition metal catalysis. By radical-to-polar crossover (RPCO­), a radical metal is combined with an organic radical to produce, via radical-radical trapping, a polar nucleophilic organometallic reagent. Using dual photoredox catalysis (metallaphotoredox catalysis), a reactive organometallic reagent can be prepared, avoiding the use of metals in low oxidation state. Herein, in addition to the description of the results obtained by our group and the contributions of others on the connection between carbonyl addition and radical-based photochemistry, we provide core guidance for further synthetic developments. We anticipate that extending the photoredox dual strategy beyond the Barbier reactions described here, taming less-activated carbonyls, studying other important electrophiles, will soon realize important breakthroughs. 1 Introduction 2 Photoredox Catalysis: A Survival Guide for the ‘Photo-Curious’ 3 Chromium Nucleophilic Organometallic Reagents 3.1 Allylation of Aldehydes 3.2 Allylation of Aldehydes via Dienes 3.3 Propargylation of Aldehydes via 1,3-Enynes 3.4 Alkenylation of Aldehydes 3.5 Alkylation of Aldehydes 3.6 Enantioselective Chromium-Mediated Photoredox Reactions 4 Titanium Nucleophilic Organometallic Reagents 4.1 Allylation Reactions 4.2 Propargylation Reactions 4.3 Allylation Reactions via Dienes 4.4 Benzylation Reactions 4.5 Alkylation Reactions 5. Cobalt Nucleophilic Organometallic Reagents 5.1 Allylation Reactions 6 Conclusion