10--R--Borabicyclodecanes: Asymmetric Hydroboration of Alkenes and Cyclic Dienes and the Asymmetric Synthesis of Homoallylic Amines and Erythro Homoallylic Cycloalkenols through Allylboration

Asymmetric conversions such as the allyboration of N-H aldimines, hydroboration of prochiral alkenes and cyclohexadienes and the cycloalkenylation of prochiral aldehydes and ketones through Soderquist's reagents are the main focus of the present work. The asymmetric allyboration of N-H aldimines, generated from either N-TMS or N-diisobutylalanyl precursors, with B-allyl-9-TMS-borabicyclo[3.3.1]decane provided homoallylic amines in good yields (60--90%) and good to high selectivity (60--89%.) The mechanistic details of the transformation were studied by molecular mechanics calculations.;The asymmetric hydroboration of prochiral alkenes and cyclohexadienes with B-H-10-TMS-9-BBD and B-H-10-Ph-9-BBD was explored. Hydroboration of 1,1-disubstituted, 1,2-disubstituted ( cis or trans) and trisubstituted olefins provided the corresponding alcohols in good yields and good to excellent enantioselectivity. The hydroboration of 1,1-disubstituted alkenes with B-H-10-Ph-BBD reagent provided the a-chiral secondary alcohols in excellent yields (83--97%) and good to excellent enantioselectivity (28--92%). Suzuki coupling with the enantioenriched trialkylboranes obtained from the hydroboration of 1,1-disubstituted olefin provided the corresponding hydrocarbons in good yields (50--84%). Hydroboration of 1,2-disubstitued olefins with B-H-10-TMS-BBD proved to be very selective with cis-2-butene (84% ee) and trans-2-butene (95% ee). However, B -H-10-Ph-BBD was selective for trans-2-butene (96% ee) while cis-2-butene gave 2-butanol in 32% ee.;The asymmetric hydroboration of 1,3-cyclohexadiene with B-H-10-TMS-9-BBD afforded a 93:7 mixture of regioisomers, (S)-2-cyclohexenol(>99% ee) and 3-cyclohexenol. Similarly, the hydroboration of 1,4-cyclohexadiene provided (S)-3-cyclohexenol in 90% ee and 80% yield. By contrast, the hydroboration of 1,3-cyclohexadiene with B-H-10-Ph-9-BBD afforded essentially a racemic mixture of 2-cyclohexenols (10% ee). Similar results were also observed for 1,4-cyclohexadiene. The corresponding trialkylborane B-2-cyclohexenyl-10-R-BBD obtained from the hydroboration of 1,3-cyclohexadiene was employed in the cycloalkenylation of aldehydes and ketones. Asymmetric synthesis of homoallylic cycloalkenols was achieved in moderate to good yields (35--75%) and excellent selectivity (90--99% ee) with the B-2-cyclohexenyl-10-TMS-BBD while from B-2-cyclohexenyl-10-Ph-BBD the corresponding cycloalkenols exhibited excellent selectivity (80--99% ee) but low yields (17--52%). Careful examination of the hydroboration and the alkylation steps revealed that only one of the isomeric cyclohexenylborane intermediates was undergoing the addition to ketones. The absolute configuration of the cycloalkenols obtained from either aldehydes or ketones was determined by optical rotation values comparison from the literature or by means of chemical transformations.