NOVEL CONVERSION OF ALDEHYDES TO BORONIC ESTERS - SIMULTANEOUS IGOR2 COMPUTER-GENERATION AND EXPERIMENTAL-OBSERVATION OF AN UNUSUAL REARRANGEMENT OF ALPHA-AMINOBORANES

Utilization of the IGOR2 (Interactive Generation of Organic Reactions 2) program to study the addition of the boron-hydrogen bond to various carbon-carbon and carbon-heteroatom double bonds is reported. The IGOR2 program not only generated a number of hydroboration reactions but also indicated an unusual rearrangement of alpha-aminomonoalkylboranes to the corresponding B-(dialkylamino)monoalkylboranes. Independent and simultaneous experimental observation of these IGOR2-generated rearrangements was made during the hydroboration of beta,beta-disubstituted morpholino and piperidino enamines. Thus, the hydroboration of 2-ethyl-1-morpholino-1-butene using borane-methyl sulfide (BMS) gave B-morpholinoborane as a byproduct. Additionally, upon oxidation, 2-ethyl-1-butanol was isolated from the reaction mixture. These results are explained in terms of unusual rearrangements of the alpha-(dialkylamino)monoalkylborane intermediate. The use of BH3.THF-BF3.OEt2 or NaBH4-BF3.OEt2 for the hydroboration permits the utilization of any beta,beta-disubstituted enamine in this rearrangement reaction and vastly improves the yields of the rearrangement products. Nonoxidative workup of the rearrangement reaction mixture affords excellent yields of the corresponding boronic acids, which were isolated as 2-alkyl-1,3,2-dioxaborinanes.