Unusual Directive Effects in the Hydroboration of β,β-Disubstituted Enamines. Conversion of α-Substituted Aldehydes to the Corresponding Alkenes and β-Amino Alcohols

Article abstract of DOI:10.1021/jo00019a043

A comprehensive study of the conversion of β,β-disubstituted enamines into the corresponding alkenes and β-amino alcohols by hydroboration-elimination and hydroboration-oxidation, respectively, has been carried out.The amine moiety of β,β-disubstituted enamines was found to exert a decisive influence on the regioselectivity of the hydroboration reaction involving borane methyl sulfide (BMS).Thus, in the hydroboration of morpholino and piperidino enamines, the boron atom is initially placed predominantly in the α-position.Conversely, the pyrrolidino enamines direct the boron atom exclusively to the β-position.Three oxidizing agents, trimethylamine N-oxide, sodium perborate, and 30 percent hydrogen peroxide-solid sodium hydroxide, were tried in order to optimize the oxidation of the intermediate organoborane derivatives to the corresponding amino alcohols.Our results clearly indicated that 30 percent hydrogen peroxide-solid sodium hydroxide is best suited for this transformation.The yield of amino alcohol ranged from good to essentially quantitative.Enamines derived from β-aryl aldehydes, upon hydroboration with BMS followed by methanolysis and oxidation with neutral hydrogen peroxide, gave the corresponding 1,1-disubstituted alkenes.Contrary to the regioselectivity observed in the hydroboration reactions involving BMS, the hydroboration of β,β-disubstituted enamines using 9-borabicyclo<3.3.1>nonane (9-BBN) gave the trialkylborane intermediates in which the boron atom was placed exclusively at the β-position regardless of the amine moiety of the enamine.These trialkylborane derivatives were very stable and did not undergo the usual elimination reaction with either methanol or sodium hydroxide.However, on thermal decomposition, these afforded the corresponding 1,1-disubstituted alkenes in high yields.