ORGANOBORANES FOR SYNTHESIS .14. CONVENIENT PROCEDURES FOR THE DIRECT OXIDATION OF ORGANOBORANES FROM TERMINAL ALKENES TO CARBOXYLIC-ACIDS

For the first time, a highly efficient and direct oxidation of a variety of organoborane intermediates into carboxylic acids has been demonstrated, without an increase or decrease of carbon atoms. Synthetically useful procedures have been developed for the ready conversion of representative terminal alkenes into carboxylic acids via oxidation of the organoboranes obtained by hydroboration of the terminal alkenes. The oxidation works well with a variety of organoboranes derived from reagents such as dibromoborane-methyl sulfide (HBBr2.SMe2), monobromoborane-methyl sulfide (H2BBr.SMe2), monochloroborane-methyl sulfide (H2BCl.SMe2), borane-methyl sulfide (H3B.SMe2), thexylborane (H2BThx), and dicyclohexylborane (HBChx2). The oxidation is achieved in a convenient manner with pyridinium dichromate (PDC), sodium dichromate in aqueous sulfuric acid (Na2Cr2O7-H2SO4) and chromium trioxide in 90% aqueous acetic acid (CrO3-HOAc-H2O). These oxidations afford carboxylic acids in very good yields with complete retention of the structure of the organic group attached to boron.