No abstract

Cyclopropyl malonoyl peroxide (1), which can be prepared in a single step from the commercially available diacid, is an effective reagent for the dihydroxylation of alkenes. Reaction of 1 with an alkene in the presence of 1 equiv of water at 40 °C followed by alkaline hydrolysis leads to the corresponding diol (40-93%). With 1,2-disubstituted alkenes, the reaction proceeds with syn selectivity (3:1 to >50:1). A mechanism consistent with the experimental findings that is supported by oxygen-labeling studies is proposed.

Reactions employing inexpensive reagents from sustainable sources and with low toxicity are becoming increasingly desirable from an academic and industrial perspective. A fascinating example of a synthetic transformation that requires development of alternative procedures is the osmium catalysed dihydroxylation. Recently there has been considerable interest in achieving this reaction through metal-free procedures. This review describes the methods available for metal-free syn-dioxygenation of alkenes.

A detailed mechanistic understanding of the malonoyl peroxide mediated dihydroxylation of alkenes is presented. The reaction is first order in both alkene and peroxide with stoichiometric water playing a dual role. An ionic mechanism is proposed and supported by the use of 18O isotopically labelled peroxide, a radical clock probe and DFT calculations. Hammett analysis suggests the reaction proceeds via a discrete carbocation intermediate which is consistent with the stereochemical outcome of the transformation. A subsequent Woodward-type 1,3-dioxolan-2-yl cation has been trapped in situ and the mechanism of hydrolysis defined by isotopic labelling studies. Stable reaction intermediates have been isolated and characterised by X-ray crystallographic analysis and minor competing reaction pathways identified