Highly stereoselective epoxidation of (−)-α-pinene over chiral transition metal (salen) complexes occluded in zeolitic hosts

Various transition metal complexes of seven different salen ligands have been incorporated in specially modified zeolitic host materials. The thus immobilized sterically demanding complexes have been tested in the diastereoselective epoxidation of (−)-α-pinene in the liquid phase in an autoclave at room temperature and elevated pressure using O2 as oxidant. In most cases conversions of 100% could be achieved. Best results so far – 100% conversion, 96% epoxide chemoselectivity and 91% diastereomeric excess – have been obtained in the presence of the entrapped [(R,R)-(N,N′)-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylenediamino]cobalt(II) = Co(salen-5) complex. Computer simulations were done in order to prove that the reaction can take place inside the pore system, i.e., (−)-α-pinene is able to diffuse through the microporous entrances of the carrier material.