Allyl-Zinc Reagent for Barbier Reaction: A Journey to Modern Aspects

Organozinc compounds are valuable reagents in organic chemistry for C-C bondformation reactions. Diallyl-Zn is considered a valuable nucleophilic reagent for C-C bondformation reactions as it is more tolerant toward functional groups, requires mild reaction conditions, and the allyl moiety can be further functionalised into many valuable motifs. This results in Barbier-type reactions with carbonyl molecules, with few exceptions such as azocarboxylates, ketimines, and carbon dioxide, which are closely related to Grignard reactions, to generate alcohol and alkene functionality in the product molecule, mostly in etheral media, including aqueous conditions at room temperature to reflux conditions. Unlike the Grignard reagent, formation of the allylzinc reagent generally proceeds through a two-electron oxidative addition mechanism or polar organometallic reaction pathway, particularly when starting from allyl halides and metallic zinc. Subsequently, nucleophilic addition of the allyl group to the carbonyl compound, followed by protonation, results in a homoallyl alcohol. The high importance of homoallyl alcohol in many naturalproduct and pharmaceutical syntheses, either in racemic or enantioselective approaches, makes allyl- zinc highly demanded. Many reports describe stoichiometric zinc metal and catalytic zinc, including waste-zinc sources, for preparing the allyl-zinc reagent, along with green-chemistry approaches that replace unhealthy ethereal solvents with aqueous or solvent-free conditions. Even the use of biomass-derived feedstock as the allylating substrate is motivating in terms of economy and eco-friendliness. This review summarises recent contributions from various authors working in this field. Over the last 15 years, several new synthetic methods involving diallylzinc have been developed, representing the most significant contributions over this period.