VINYLIC ORGANOBORANES .6. A GENERAL-SYNTHESIS OF (E)-DISUBSTITUTED ALKENES OR KETONES VIA THE (E)-(1-SUBSTITUTED-1-ALKENYL)BORONIC ESTERS

Development of a general stereospecific synthesis of (E)-disubstituted alkenes utilizing a variety of hydroborating agents such as monohaloborane, thexylborane, thexylchloroborane, and dibromoborane is discussed. Hydroboration of 1-halo-1-alkynes with dialkylboranes (R2BH, 1), thexylmonoalkylborane (ThxBHR, 6), or alkylbromoborane (RBHBr.cntdot.SMe2, 10) provides the corresponding B-(cis-1-halo-1-alkenyl)alkylborane derivatives (2, 7, 11), respectively. Treatment of B-(cis-1-halo-1-alkenyl)dialkylborane (2) with sodium methoxide results in the intramolecular displacement of bromine by one of the alkyl groups on boron to produce B-(trans-1-alkyl-1-alkenyl)alkylborinate esters 3. Protonolysis of 3 provides trans-alkenes 4 in high yields and in > 99% isomeric purities. Similarly, the intermediates 8 and 12 afford the trans-disubstituted alkenes in excellent yields and in > 99% isomeric purities. Alternatively, oxidation of these vinylboron derivatives, 3, 8, and 12, with alkaline hydrogen peroxide provides the corresponding ketones in excellent yields.