A Heterogeneouscis-Dihydroxylation Catalyst with Stable, Site-Isolated Osmium-Diolate Reaction Centers

Abstract

Osmium tetroxide is by far the most versatile catalyst for cis-dihydroxylation (DH) of double bonds. [1, 2] When homogeneous catalysts are used, free OsO 4 is always present in some step of the catalytic cycle, and the high toxicity and volatility of OsO 4 have hitherto obstructed industrial application. Previous attempts to immobilize OsO 4 used polymers, for example, with coordination of OsO 4 on polyvinylpyridine. [3, 4] However, hydrolysis of the intermediate Os VI diolate complex requires that Os is detached from the polymeric Lewis base, [5] and this implies an inherent liability to Os leaching. Similarly, reports on immobilized alkaloids for asymmetric DH mention that Os leaching necessitates Os supplementation in subsequent runs. [6] In another attempt, OsO 4 was entrapped in polystyrene microspheres, but the mechanism by which OsO 4 is retained within the polymer is not understood. [7] Herein we report a solid with Os VIII type reactivity, and with a persistent bond between Os and the support. Rigorous heterogeneity tests and reactions with 12 olefins substantiate the value of the new Os catalyst.Our approach is rooted in the mechanism of the cisdihydroxylation, which comprises two stages: 1) attack of the Os VIII cis-dioxo complex on the olefin (osmylation), 2) reoxidation of Os VI to Os VIII and hydrolytic release of the diol. Two points are particularly relevant. First, if the hydrolytic conditions are not too drastic, tetrasubstituted olefins are not converted into cis-diols. [8, 9] These olefins are smoothly osmylated to an osmate(vi) ester, but the rate of subsequent hydrolysis is zero (0 % yield for a tetrasubstituted olefin vs. 83 % for a trisubstituted olefin, ref.[8]). Second, an Os VI monodiolate complex can be reoxidized to cis-dioxo Os VIII without release of the diol; subsequent addition of a second olefin results in an Os bisdiolate complex.[10] These two properties make it possible to immobilize a catalytically active Os compound by the addition of OsO 4 to a tetrasubstituted olefin that is covalently linked to a silica support (1 a, Scheme 1). The tetrasubstituted diolate ester (1 b) which is [*] Prof.

Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations

“…For immobilization of the Os itself, other specific strategies would be needed. 70 Finally, there are reports on immobilization of chiral Mo epoxidation catalysts by covalent attachment of ligands. 71,72 For instance, dioxoMo moieties were attached to a ligand derived from hydroxyproline, anchored in mesoporous US-Y zeolite; these catalysts were applied in the enantioselective and chemoselective β,γ-epoxidation of geraniol and nerol.…”

“…However, addition of an extra Os dose restores the activity. For immobilization of the Os itself, other specific strategies would be needed 17 …”

Development of a heterogeneous cis-dihydroxylation process with hydrogen peroxide as oxidant

“…5 We previously reported a new, truly heterogeneous cisdihydroxylation catalyst with Os immobilized on a modified silica support. 6 The catalyst is prepared through formation of stable Os-diolate esters in the reaction of OsO 4 with silicalinked tetrasubstituted olefins. The central idea is that these tertiary diolate esters are not hydrolyzed during reaction, keeping the catalyst fixed to the support, if the hydrolytic conditions are not too drastic.…”