Thiazolium ions, reduction

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

A systematic study of the reduction of thiazolium salts by complex metal hydrides has been promised in a preliminary communication on the mechanism of the reduction with sodium borohydride. The formation of 3-benzyl-4-methylthiazolidine from the reaction of 3-benzyl-4-methylthiazolium bromide and sodium borohydride was shown to occur in a manner similar to the reduction of pyridinium ions.158... 

Many thiazolium salts are polarographically reducible [283, 284] 3-methylben-zothiazolium iodide [283] is reduced in a single wave a preparative reduction gave n between 1 and 2, and a mixture of dimer and monomer 2,3-dihydro-3-methylbenzothia-zole was found. Aryl groups in the 2-position hinder dimerization [283]. With TEAP as supporting electrolyte 2-methyl-3-ethylbenzothiazolium ion gives two, one-electron waves [284]. 

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