Metal-complex catalysis during the reduction of functional groups by sodium borohydride in the synthesis of pyrrolidine derivatives

Abstract

The development of selective and universal methods for the reduction of functional groups is an important function of organic chemistry.There are many ways of using the mild and readily available reducing agent sodium borohydride in various catalytic systems with the aim of extending its reducing capabilities, the selectivity of its action, and its other properties. Reducing systems containing sodium borohydride and the salts of metals, such as CuCl 2 [1], CoCl 2 [2], NiCl 2 [3], ZrCl 3 [4], and others, are well known and have been well studied. It was demonstrated that the selectivity of reduction depends on the nature of the metal. The mechanism of reduction involving the formation of borides of the respective metals has also been studied [4]. However, the method has a series of important disadvantages, mainly associated with the quantities of the reagents employed. The ratios of the compound being reduced, the metal salt, and sodium borohydride amounted to 1:2:10.The use of metal complexes as catalysts with sodium borohydride significantly simplifies the process, making it suitable for extensive application. Complexes of CuCl 2 [5], CoCl 2 , and NiCl 2 [6] with triphenylphosphine and metal complexes of porphyrins such as iron meso-tetraphenylporphyrinate [ClFe(III)TPP] [7] and cobalt meso-tetra(p-sulfophenyl)porphyrinate [Co(III)TPPS] have been used successfully [8].As catalysts for reduction in a system containing sodium borohydride we used a series of familiar metal complexes, like the complexes of CoCl 2 and CuCl 2 with triethylbenzylammonium chloride (1) and (2) [9] and cobalt and copper meso-tetra [4-(2-hydroxyethyl)pyridyl]porphyrinates (CoTOEtPyP) (3) and (CuTOEtPyP) (4)