Toward Functional Polyester Building Blocks from Renewable Glycolaldehyde with Sn Cascade Catalysis

Abstract

Having been inspired by formose-based hypotheses surrounding the origin of life, we report on a novel catalytic route toward a series of recently discovered four-carbon α-hydroxy acids (AHA) and their esters from accessible and renewable glycolaldehyde (GA) in various solvents. The synthesis route follows a cascade type reaction network, and its mechanism with identification of the rate-determining step was investigated with in situ 13 C NMR. The mechanistic understanding led to optimized reaction conditions with higher overall rates of AHA formation by balancing Brønsted and Lewis acid activity, both originating from the tin halide catalyst. An optimal H + /Sn ratio of 3 was identified, and this number was surprisingly irrespective of the Sn oxidation state. Further rate enhancement was accomplished by adding small amounts of water to the reaction mixture, boosting the rate by a factor of 4.5 compared with pure methanol solvent. The cascade reaction selectively yields near 60% methyl-4-methoxy-2-hydroxybutanoate (MMHB). In the optimized rate regime in methanol, an initial TOF of 7.4 mol GA mol Sn −1 h −1 was found. In sterically hindered alcohols (isopropyl alcohol), the rate of AHA formation was even higher, and the corresponding vinyl glycolate esters arose as the main product. Vinyl glycolic acid, 2,4dihydroxybutanoic acid, and its lactone were formed significantly in nonprotic solvent. The corresponding AHAs have serious potential as building blocks in novel biobased polymers with tunable functionality. The incorporation of vinyl glycolic acid in polylactic acid-based polyesters is illustrated, and postmodification at the vinyl side groups indeed allows access to a range of properties, such as tunable hydrophilicity, which is otherwise difficult to attain for pure poly(L-lactic acid).

Advances in the Conversion of Short-Chain Carbohydrates: A Mechanistic Insight

“…The incorporation of functional α-hydroxy compounds such as MVG or MMHB might offer an innovative solution to this problem. As a proof of concept, Dusselier et al have successfully incorporated up to 12 mol% of the corresponding acid of MVG in the PLA backbone [ 40 ]. The built-in vinyl groups were further functionalized via thiol-ene reactions, e.g., with thioglycerol or benzylmercaptan.…”

“…To study the Sn-catalyzed origin of these products from GA, Dusselier and coworkers used homogeneous Sn halide catalysts for the conversion of GA in alcoholic media [ 40 ]. Analogous to the conversion of trioses [ 54 ] and tetroses [ 39 ] in alcohols, Sn 2+ and Sn 4+ halides were the most active catalysts, yielding 55-58 % MMHB after 1 h at 90 °C in methanol.…”

“…Consequently, the reaction kinetics could be enhanced signifi cantly (up to 4.5 times, to a rate of 3.7 mol MMHB .mol Sn .h −1 ) by adding small aliquots of water to the reaction medium. Further investigation revealed that the ratio of Lewis acidic Sn centers over Brønsted acidity, coming from the [ 40 ] presence of (partially) dissociated Cl − ligands, also has a major impact on the reaction kinetics. This ratio can be described as H + :Sn and can be altered by carefully quenching H + or adding external HCl.…”

“…11 The strategy of tuning PLA properties by incorporating functional C 4 -α-hydroxy acids in the PLA backbone via copolymerization, as demonstrated by[ 40 ] Fig. 3.12 H-USY-catalyzed isomerization of tetroses in aqueous medium ( left ) and distribution ratio of the total tetrose amounts at equilibrium ( right )…”

“…16 Initial rate of MMHB formation in function of the H + :Sn ratio in the reaction mixture (Reprinted with permission from[ 40 ]. Copyright 2013, American Chemical Society)…”