The chelation-control model explains the high diastereoselectivity obtained in additions of organometallic nucleophiles to α-alkoxy ketones but fails for reactions of allylmagnesium halides. Low diastereoselectivity in ethereal solvents results from no chelation-induced rate acceleration. Additions of allylmagnesium bromide to carbonyl compounds are diastereoselective using CHCl as the solvent even though rate acceleration is still absent. Stereoselectivity likely arises from the predominance of the chelated form in solution. Therefore, a revised chelation-control model is proposed.

Competition experiments demonstrate that additions of allylmagnesium halides to carbonyl compounds, unlike additions of other organomagnesium reagents, occur at rates approaching the diffusion rate limit. Whereas alkylmagnesium and alkyllithium reagents could differentiate between electronically or sterically different carbonyl compounds, allylmagnesium reagents reacted with most carbonyl compounds at similar rates. Even additions to esters occurred at rates competitive with additions to aldehydes. Only in the case of particularly sterically hindered substrates, such as those bearing tertiary alkyl groups, were additions slower.

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur. CONTENTS 4.2.5. Additions to β-Substituted Aldehydes 4.2.6. Additions to Aldehydes with Distant Chelating Groups 4.3. Additions to Cyclic Ketones 4.3.1. Alkoxy-Substituted Cyclic Ketones 4.3.2. Additions to Alkoxy-Substituted Five-Membered-Ring Ketones 4.3.3. Additions to Alkoxy-Substituted Four-Membered-Ring Ketones 4.4. Additions of Allylmagnesium Halides to Cyclic Hemiacetals 5. Diastereoselectivity of Reactions of Allylmagnesium Reagents with Carbonyl Compounds by Felkin−Anh Control 5.1. Felkin−Anh Stereoselectivity 5.2. Additions to α-Chiral Acyclic Ketones 5.3. Additions to Chiral Exocyclic Ketones and Aldehydes 5.4. Additions of Allylmagnesium Halides to Chiral Cyclic Ketones Controlled by Felkin−Anh Selectivity 5.5. Additions of Allylmagnesium Halides to Chiral Acyclic Aldehydes 6. Diastereoselectivity of Reactions with Carbonyl Compounds by Steric Approach Control 6.1. Steric Approach Control and Stereoselectivity

We report an iodoarene-catalyzed enantioselective synthesis of β,β-difluoroalkyl bromide building blocks. The transformation involves an oxidative rearrangement of α-bromostyrenes, utilizing HF–pyridine as the fluoride source and m-CPBA as the stoichiometric oxidant. A catalyst decomposition pathway was identified, which, in tandem with catalyst structure–activity relationship studies, facilitated the development of an improved catalyst providing higher enantioselectivity with lower catalyst loadings. The versatility of the difluoroalkyl bromide products was demonstrated via highly enantiospecific substitution reactions with suitably reactive nucleophiles. The origins of enantioselectivity were investigated using computed interaction energies of simplified catalyst and substrate structures, providing evidence for both CH−π and π–π transition state interactions as critical features.