Tin triflate-catalyzed conversion of cellulose to valuable (α-hydroxy-)esters

The direct conversion of cellulose with metal-triflate catalysts in methanol is investigated. Sn-II-triflate remarkably catalyzes the formation of a mixture of useful alpha-hydroxy esters such as methyl lactate, methyl vinyl glycolate and methyl-4-methoxy-2-hydroxybutanoate, on top of methyl levulinate. Compared to other metal triflates or Sn salts, the catalytic features of Sn-II-triflate are distinct and linked to the interplay between its Bronsted and Lewis acidic component. A total ester yield (carbon-based, mol%) in the 60% range could be obtained from cellulose after 2 hat 200 degrees C for cellulose loadings up to 20 g L-1 with 4.8 mM of catalyst. The cascade reaction network, confirmed by feeding intermediates, highlights the importance of a fast retro-aldol of the hexose intermediates - opposed to their dehydration - when alpha-hydroxy esters are targeted. By manipulating the triflate-to-Sn ratio, nearly 40% of alpha-hydroxy esters can be produced in a one-pot approach. Such mixtures could help fuel the demand for functional biodegradable polyesters. (C) 2016 Elsevier B.V. All rights reserved.