C. Gundu Rao

Organoboranes for synthesis. 4 : Oxidation of organoboranes with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes via hydroboration

Abstract The oxidation of trialkylboranes containing primary a1kyl 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 94% primary alkyl groups) and functional group tolerance observed in the hydroboration with borane (BH 3 .THF or BH 3 .SMe 2 ), 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 (⪢ 99% 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.

Oxidation of organoboranes containing primary alkyl groups with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes

Abstract Organoboranes derived from terminal olefins are oxidized by pyridinium chlorochromate to aldehydes in good yields.

Neighbouring group participation and the tool of increasing electron demand : A review

Solvolytic reactions could be greatly influenced by the presence of neighbouring groups. The tool of increasing electron demand has been successfully used to detect such participation. For example, the tool has detected the presence of major π-participation inanti-7-norbornenyl and π-conjugation in cyclohexen-2-yl derivatives. The changes observed in the absolute values ofp+ with ring size in cyclic systems could be adequately explained by the I-strain concept. The tool has established major πδ-conjugation in cyclopropylcarbinyl and nortricyclyl derivatives. πδ-participation in the Coates’ cation has been unambiguously established. However, this sensitive probe has failed to provide evidence for δ-participation in the solvolysis of 2-norbornyl system. It, is concluded that the tool of increasing electron demand is quite sensitive to detect the presence or absence or neighbouring group participation.