Bishydroxylation of Alkenes

For much of this century, osmium tetroxide and potassium permanganate were alone in their ability to bishydroxylate alkenes. Such reactions are as 

Since Criegee s pioneering work in the late 1930s, the accepted mechanism for hydroxylation has been a concerted cycloaddition of alkene to the 0=0s=0 system.58 The development of the theory of pericyclic reactions in the 1960s and 1970s helped cement this proposal firmly in the minds of many workers.59 However, subsequent work has found few (if any) documented organometallic examples that followed the Woodward-Hoff-mann rules. 

In 1977, Sharpless proposed the intermediacy of a four-membered metal-lacycle in the oxidation of cyclododecene by Cr02Cl2.60 This was an attempt to explain a minor but primary product, in which an oxygen and a chlorine were added in a syn fashion to one face of the /3-bond. Later, this type of intermediate was incorporated into a new mechanism for osmylation,61 in which an initial [2 + 2] cycloaddition led to an osmaoxetane which in a second step underwent ring expansion to form the observed metal diolate product. At the time, the [2 + 2] process was viewed as a violation of orbital symmetry rules, and it was not until the extensive work in the 1970s and 1980s on cycloadditions to metal carbenes that a theoretical basis for such a process allowed its broader acceptance. 

However, the stepwise mechanism proposed a second step requiring the migration of an alkyl group to an oxo ligand. This has very little precedent. Prior to 1990, there were but a handful of possible examples, and in none of these were the oxo alkyl and the subsequent alkoxide directly observed 

One important example of this class of rearrangement was reported for a rhenium complex in 199367 and, in retrospect, took advantage of the lower barrier of hydrogen migrations as compared to alkyl migrations. Mayer and co-workers prepared the tris-acetylene hydroxide complex 11, as well as the oxo hydride 12. Upon standing at room temperature for 5 days in benzene solution, the hydroxide spontaneously forms the oxo hydride with loss of one equivalent of acetylene, as shown in Eq. (8). 

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