ORGANOBORANES FOR SYNTHESIS. 4. OXIDATION OF ORGANOBORANES WITH PYRIDINIUM CHLOROCHROMATE. A DIRECT SYNTHESIS OF ALDEHYDES FROM TERMINAL ALKENES VIA HYDROBORATION

Article abstract of DOI:10.1016/S0040-4020(01)88154-0

The oxidation of trialkylboranes containing primary alkyl groups with pyridinium chlorochromate (PCC) in methylene chloride provides the corresponding aldehydes in good yields.The stoichiometry for the oxidation of alcohols, borate esters and trialkylboranes with PCC has been examined.In view of the poor regioselectivity (only 94percent primary alkyl groups) and functional group tolerance observed in the hydroboration with borane (BH₃*THF or BH₃*SMe₂), a more selective hydroborating agent, bis(3-methyl-2-butyl)borane (disiamylborane), was utilized for the preparation of aldehydes from terminal alkenes.However, the formation of 3-methyl-2-butanone as a by-product, and the requirement of six moles of PCC per mole of aldehyde are major disadvantages in this method.This difficulty was circumvented by employing monochloroborane-dimethyl sulfide for hydroboration.This reagent exhibits high regioselectivity (> 99percent primary alkyl groups) in the hydroboration of terminal alkenes.Oxidation of the resulting dialkylchloroborane following hydrolysis affords the desired aldehydes in satisfactory yields.Consequently, the hydroboration of terminal alkenes, followed by PCC oxidation, represents a direct convenient method for the transformation of alkenes into the corresponding aldehydes.