Triphase, solvent-free catalysis over the TS-1/H 2O 2 system in selective oxidation reactions

Abstract

Solvent-free, triphase conditions (solid-liquid-liquid) used in the oxidation of various organic substrates over the TS-1/H 2O 2 system (vis-à-vis conventionally used biphase conditions in the presence of a co-solvent) exhibit: (i) significant enhancement (3–10 times) in the reaction rates in the hydroxylation of aromatics (such as benzene, toluene, anisole and benzyl chloride), along with a reversal in regioselectivity of the products of substituted benzenes; and (ii) Baeyer-Villiger (BV) oxidation/rearrangement of cyclohexanone and acetophenone. In the case of benzyl chloride, triphase conditions resulted in the ring hydroxylation forming p-hydroxy and o-hydroxy benzyl chloride (75%) ( para:ortho ratio 85:15) along with side-chain oxyfunctionalization followed by HCl removal directly forming benzaldehyde (no formation of benzyl alcohol through hydrolysis). During BV oxidation, cyclohexanone and acetophenone gave caprolactone and phenyl acetate, respectively, as major products (70–90% selectivity). The other reaction was ring oxidation (30-10%) to hydroxy cyclohexanone and corresponding diketones (cyclohexanone) or hydroxy-acetophenones (acetophenone). Addition of a catalytic amount of mineral acid (H 2SO 4) significantly increased the conversion. Such reaction routes do not occur at all in the presence of co-solvent under biphase conditions. Factors responsible for enhancement in activity, change in regioselectivity and new routes are: (i) relative hydrophobic nature and restricted pore dimensions of titanium silicates; (ii) competitive diffusion of substrate with co-solvent; and (iii) Brönsted acidity of titanium silicates in the presence of H 2O 2 and water. Thus the present study opens up a new area where titanium silicate will be quite useful in various organic transformations, in an eco-safer way.