Catalytic Dimerization of Crotonates

Abstract

A room-temperature dimerization of crotonates into 2-ethylidene-3-methylpentanedioates provides a sustainable route to difunctional monomers for step-growth polymerizations. We report two such dimerizations: (1) an organocatalytic dimerization using the N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (I i Pr 2 Me 2 ) and (2) a rapid dimerization (under 15 s to full conversion) using potassium t-butoxide in THF. In addition to unsaturated diesters, the resulting dimers can be easily converted to other step-growth monomers; namely, their corresponding diacids and saturated diesters. M ethacrylates are a versatile class of monomers and chemical intermediates generated on a vast industrial scale. In contrast, the chemistry of the isomeric crotonates is less well-developed; crotonates exhibit chemistry very different from their methacrylate congeners and are not readily polymerized by conventional radical or anionic methods. Both methacrylates and crotonates are currently generated from petrochemical feedstocks. Recently, a new, renewable source of crotonic acid has emerged through the catalytic pyrolysis of the biopolymer poly(3-hydroxybutyrate) (PHB). 1−3 PHB is produced by a variety of microorganisms, including methanotrophic bacteria that can be cultured from wastewater. 4,5 Thus, the generation of crotonate from PHB provides a potential strategy for generating C4 feedstocks from renewable resources, including wastewater streams. 1−3 These breakthroughs in biotechnology motivate the development of new catalytic transformations of crotonate as a potentially renewable C4 feedstock.The catalytic dimerization of crotonates and alkenoates provides an expedient synthesis of saturated or unsaturated substituted diesters. Although a variety of catalysts for the dimerization of crotonates have been reported, they generally afford poor to moderate yields and require relatively high catalyst loadings at elevated temperatures. 6−10 The dimerization of cyclohexyl crotonate with 25 mol % KO t Bu in a 3:7 DMSO/toluene mixture, proceeded to only 29% conversion after 27 h. 8 The dimerization of ethyl crotonate with 22 mol % [NBu 4 ][SiPh 3 F 2 ] in THF at room temperature afforded a 74% yield of ethyl 2-ethylidene-3-methylpentanedioate after 2 h, 11 but dimerization with P[N(CH 3 ) 2 ] 3 required more forcing conditions (300 MPa, 50°C, 24 h). 12 Fe catalysts, such as 1−5 mol % of FeH 2 (dmpe) 2 /hν or FeNpH(dmpe) 2 (dmpe = 1,2-bis(dimethylphosphino)ethane, Np = 2-naphthyl) generate (E)-2-ethylidene-3-methylpentanedioates selectively with yields of 73−95% in neat crotonate at room temperature. 13 Herein, we report two facile and selective dimerization reactions of crotonates and alkenoates to 2-alkylidene-3-alkylpentanedioates eq 1.Initial attempts to effect the group transfer polymerization (GTP) of isopropyl crotonate with N-heterocyclic carbenes (NHCs) 14−17 in the presence of silyl ketene acetals were unsuccessful; under conditions for the GTP of methyl methacrylate with 1-methoxy-2-methyl-1-(trimethylsilo...

Step-growth polymerisation of alkyl acrylates via concomitant oxa-Michael and transesterification reactions

“…However, for transesterification elevated temperatures are necessary. 22 The polymer's M n is not influenced by the initial reaction temperature (Table 1, entry 9). Finally, the principal possibility of obtaining higher molecular masses in this reaction was demonstrated by applying vacuum during the second reaction step (Table 1, entry 10).…”

Step-Growth Polymerisation of Alkyl Acrylates via Concomitant Oxa-Michael and Transesterification Reactions

“…However, for transesterification elevated temperatures are necessary. 22 The polymer's Mn is not influenced by the initial reaction temperature (Table 1, entry 9).…”

Production and Polymerization of Biobased Acrylates and Analogs

“…It should also be noted that crotonates can be converted into dimers by NHCs and thus yield unsaturated diesters (analogous to itaconates) that could potentially be polymerized with diols to produce biodegradable polyester resins. [ 277 ]…”