Hydroboration. 81. Synthesis of 2-(Dialkylamino)boronic Esters and Acids via Hydroboration of Enamines. A Convenient Preparation of β-Dialkylamino Alcohols

Article abstract of DOI:10.1021/jo00227a014

Hydroboration of representative enamines with 1 equiv of borane-dimethyl sulfide (BMS) in tetrahydrofuran affords as the major product the corresponding <2-(dialkylamino)alkyl>boranes, which are charakterized by B NMR spectroscopy.These intermediates on methanolysis give the corresponding <2-(dialkylamino)alkyl>boronic acids in essentially quantitative yields.The boronic acids thus obtained can be reestrified with 1,3-propanediol to give the corresponding 2-<2-(dialkylamino)alkyl>-1,3,2-dioxaborinanes.Consequently, it is now possible to prepare dialkyloamino-substituted organoborane compounds with the potential to be elaborated to a wide variety of functionalized tiralkylamines.Alkaline hydrogen peroxide oxidation of these organoborane intermediates affords the corresponding β-dialkyloamino alcohols.The hydroboration of enamines, methanolysis of the intermediate borane derivatives, and oxidation of the boronate esters is accompanied by side reactions such as elimination and protonolysis.The magnitude of these side reactions varies considerably with the structures of the anamines.However, moderate to excellent yields of the desired organoboron compounds can frequently be achieved.The use of trimethylamine N-oxide dihydrate for the oxidation of the 2-(dialkylamino)boronate esters greatly suppresses the side reactions and vastly improves the yields of the β-dialkylamino alcohols.