Chromium racemization catalyst

Chromium(salen) catalysts are excellent reagents for the desymmetrization of OT to-epoxides. Thus, tfr-stilbene oxide is converted to the (3, 3 )-aminoalcohol in the presence of catalytic quantities of chromium-salen complex in methylene chloride solution open to the atmosphere. The addition of small quantities of triethylamine was found to dramatically increase enantioselectivities (by almost 25%). This catalytic system also promotes an efficient aminolytic kinetic resolution (AKR) of racemic epoxides with 2-type symmetry (Equation 18) <20040L2173, 1999TL7303>. W fo-Epoxides can be opened with aromatic amines in water in the presence of 1 mol% of an Sc(ni) catalyst ligated to 1.2mol% of a chiral bipyridine ligand <2005OL4593>. 

A completely new type of ROP catalyst was recently reported by Rieger and coworkers. Chromium salphen complexes (Fig. 32) convert racemic (3-BL to slightly isotactic-enriched PHB (0.60 < Pm < 0. 70) with a molecular weight of up to 800,000 g/mol (PD up to 8.5). These catalysts combine high activity and high molecular weight products, featuring the desired stereocontrol at moderate reaction conditions [13]. 

Jacobsen s cobalt and chromium salen complexes 69 and 70 have proven extremely successful in the enantioselective ring opening of meso-epoxides (and kinetic resolution of racemic epoxides). Recent accounts of these most efficient and practical catalysts can be found elsewhere [71-73]. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

Jacobsen and co-workers have used similar catalysts in the enantioselective opening of epoxide rings. Stereospecific hydrolysis with a cobalt acetate chelate can be used to resolve racemic epoxides.97 Propylene oxide was opened with trimethylsilylazide in the presence of a 7 7 chromium azide chelate catalyst to produce (5)-l-azido-2-trimethyl-siloxypropane in quantitative yield with 97% ee.98 Cyclohexene oxide was opened with benzoic acid in the presence of 1 mol% cobalt chelate catalyst to give the hydroxyben-zoate in 98% yield with 77% ee.99... 

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