Hydrogen‐Bonding‐Mediated Directed Osmium Dihydroxylation

Abstract

This chapter focuses on the dihydroxylation of alkenes using osmium tetraoxide that is directed by alcohols and amine derivatives through hydrogen bonding between the substrate and the oxidant. Discussion focuses on the different types of directing groups that are viable. The outcome from directed dehydroxylation of all the major classes of alkenes, including cyclic and acyclic substrates and varied alkene substration patterns, is also addressed. The mechanism section outlines the different reactivity patterns that various ligands can impart onto the osmium oxides together with the importance of chosing a solvent that encourages hydrogen bonding. The influence that the directing group has on syn selectivity is also discussed, in both the context of its position in space and with respect to the alkane, and the relationship between the PKa of the acidic proton and syn selectivity. Osmium tetraoxide has established itself as the reagent of choice for the syn-dihydroxylation of olefins, primarily because of its inertness towards other functional groups and lack of over-oxidation products. Information on research in regard to dihydroxylation is given. Only a few other synthetic methods are known that accomplish the direct addition of a diol unit across an alkene while controlling the stereochemical course of the process. The Woodward modification of the Prevost reaction (adds two oxygen atoms in a syn fashion across an alkane) is discussed in detail in the comparison of methods section. Keywords: Dihydroxylation; Osmium tetroxide; Mechanisms; Site selectivity; Conformational factors; Hydrogen bonding; Directing group; Amines; Alcohols; Steric effects; Method comparisons; Sodium sulfite; Acidic methanol; Ethylenediamine; Experimental procedures