2.3- Epoxy alcohols with thiols

Mechanistic studies of the opening of the epoxide point to catalyst activation of both nucleophile and electrophile in a bimetallic array [79]. The Cr complex results in moderate ee only when used with thiol nucleophiles but enhancement is feasible by using a dithiol in a two-step selection process [80]. Enantioselective epoxide opening with carboxylic acids is more efficient with (salen)Co(III) complexes (often obtained via in situ oxidation of the Co(II) complex) than with the Cr analogs (Table 6, entries 4 and 5) [81]. This methodology was successfully extended to intramolecular desym-metrization of meso epoxy alcohols [82]. 

Under Payne rearrangement conditions, sodium /-butylthiolate provides 1 -/-butylthio-2,3-diols with very high regioselectivity. The selectivity is affected, however, by many factors including reaction temperature, base concentration, and the rate of addition of the thiol. These sulfides can then be converted to the l,2-epoxy-3-alcohols, which in turn react with a wide variety of nucleophiles specifically at the 1-position (Scheme 9.6). This methodology circumvents the problems associated with the instability of many nucleophiles under Payne conditions.85... 

Functionalized POSS a closed cage with a reactive organic group at each comer (e.g., epoxy, amine, acrylate, methacrylate, alcohol, isocyanate, sulfonate, carboxylic add, thiol, imides, silane, or nitrile.) It is also possible to have seven reactive groups and one unreactive or vice versa. 

Instead of matching the polarity of the POSS to the matrix, one can also imbue compatibility through reaction of POSS with the matrix by choosing appropriate chemistry. Virtually all chemistries are available to facilitate reactions such as alcohol, carboxylic acid, sulfonic acid, epoxy, chloroalkyl, acrylate, methacrylate, isocyanate, amine, thiol, silanol, and several others. 

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