Best Synthetic Methods: Oxidation and Reduction
Although oxidation and reduction are common organic synthesis transformations, there is always room for improvement. The ideal reaction might use a solid, easily-measured reagent, a minimal amount of solvent, need no work-up other than simple filtration, and proceed in > 95% yield. The zirconium borohydride piperazine complex reported (Tetrahedron Lett. 2004, 45, 3295. ) by M. Talbakksh and M.M. Lakouraj of Mazandaran University, Iran comes close to this ideal. The reagent is prepared by combining ZrCl4 and an ethereal solution of LiBH4 with piperazine at ice temperature. The resulting white powder is stable for months if stored dry ("vacuum dessicator"). The reagent reduces aldehydes in ether solution at room temperature. Ketones reduce in a few hours in refluxing ether, conditions that also reduce esters. Cyclohexenone 1 reduces cleanly to the allylic alcohol 2, without the conjugate reduction often seen with other reducing agents. In each case, the reaction mixture was filtered and the solvent evaporated to give the pure product(s).
Direct reduction of an aldehyde or ketone to the corresponding ether could
potentially telescope two reactions, reduction and protection, into one step. S.
Chandrasekhar of the Indian Institute of Chemical Technology, Hyderabad, reports
(Tetrahedron Lett. 2004, 45, 5497.
that in the present of polymethylhydrosiloxane (PMHS) and catalytic B(C6F5)3,
TMS ethers of alcohols will convert aldehydes to the corresponding dialkyl
ethers. The reaction works well for both saturated and benzylic alcohols. This
may prove to be a useful alternative to Williamson synthesis for the preparation
of complex ethers.
Hydrogen peroxide is an inexpensive oxidant, but it requires a catalyst to effect oxidation of an alcohol to the ketone. Removal of the catalyst then becomes an issue. Ronny Neumann of the Weizmann Institute of Science reports (J. Am. Chem. Soc. 2004, 126, 884. ) the development of a hybrid organic-tungsten polyoxometalate complex that is not soluble in organic solvents, but that nonetheless catalyzes the hydrogen peroxide oxidation of alcohols to ketones. The solid catalyst is removed by filtration after the completion of the reaction. The catalyst retained its activity after five recyles.
In the latest development of his elegant work with hydrazone derivatives, Andrew Myers of Harvard reports (J. Am. Chem. Soc. 2004, 126, 5436. ) that Sc(OTf)3 catalyzes the addition of 1,2-bis(t-butyldimethylsilyl)hydrazine, to aldehydes and ketones to form the t-butyldimethylsilylhydrazones. Addition of tBuOH/tBuOK in DMSO to the crude hydrazone effects low temperature Wolff-Kishner reduction. Alternatively, halogenation of ketone hydrazones can lead to vinyl halides such as 11, or the 1,1-dihalo derivatives, depending on conditions. Halogenation of aldehyde hydrazones provides the 1,1-dihalo derivatives such as 13.