A Heterogeneous cis-Dihydroxylation Catalyst with Stable, Site-Isolated Osmium ± Diolate Reaction Centers**

Osmium tetroxide is by far the most versatile catalyst for cis-dihydroxylation (DH) of double bonds.[1, 2] When homogeneous catalysts are used, free OsO4 is always present in some step of the catalytic cycle, and the high toxicity and volatility of OsO4 have hitherto obstructed industrial application. Previous attempts to immobilize OsO4 used polymers, for example, with coordination of OsO4 on polyvinylpyridine.[3, 4] However, hydrolysis of the intermediate OsVI 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, OsO4 was entrapped in polystyrene microspheres, but the mechanism by which OsO4 is retained within the polymer is not understood.[7] Herein we report a solid with OsVIII 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 OsVIII cis-dioxo complex on the olefin (osmylation), 2) reoxidation of OsVI to OsVIII 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 OsVI monodiolate complex can be reoxidized to cis-dioxo OsVIII 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 OsO4 to a tetrasubstituted olefin that is covalently linked to a silica support (1 a, Scheme 1). The tetrasubstituted diolate ester (1 b) which is (Table 1). The coordinates of Arg 62(n), Arg 62(l), Ala-Pro, Wa, Wb, Wc, and Wd were fixed during occupancy refinement. Arg 62(n) represents the Arg 62 conformation without Ala-Pro bound and Arg 62(l) represents the Arg 62 conformation with Ala-Pro bound to ceCyp3. The B-factors of Arg 62(n), Wa, Wb, Wc, and Wd were fixed to the same B-factor values as those of the native structure. Individual atomic B-factors for Arg 62(l) and Ala-Pro were refined together with occupancy (Figure 4). The restraints applied in the occupancy refinement are summarized below: QArg 62(n)‡QArg 62(l)ˆ 1 QArg 62(l)ˆQl QArg 62(n)ˆQWaˆQWbˆQWcˆQWd where