Minimal Hydrolases: Organocatalytic Ring-Opening Polymerizations Catalyzed by Naturally Occurring Carboxylic Acids

Organocatalysis has emerged during the last decade as a concept with a significant number of synthetic applications. Important driving forces for this growth are that organocatalytic strategies often operate at rather mild reaction conditions, with reduced waste formation, and that processes may lead to high selectivities. In this area, some organocatalytic examples have been reported in the field of ring-opening polymerizations of lactones, as well as for some (trans)esterifications. These reactions are catalyzed by organic molecules such as alpha-hydroxycarboxylic acids, amino acids, alpha-(dimethylamino)pyridine, N-heterocyclic carbenes, guanidines, or bifunctional thiourea-amines. With regard to a-hydroxycarboxylic acids, it is believed that these organocatalysts catalyze reactions in a similar fashion to hydrolases, such as lipases or esterases, thus leading to valuable biomimetic catalytic concepts. Importantly, advantages that are inherent to enzymatic catalysis can be combined with a broader range of potential reaction conditions (e.g., solvent-free), or exploited for the efficient and selective (tailored) modification of synthetically challenging substrates (e.g., ring-opening polymerizations with cellulose and other polysaccharides as initiators). Furthermore, alpha-hydroxycarboxylic acids are readily accessible by biotechnological routes such as fermentation, are nontoxic, and easy to apply. Herein, the uses of these "minimal hydrolases" in catalyzing ring-opening polymerizations and transesterifications are categorized and discussed. Albeit still at the level of proof-of-principle, many interesting practical applications can be conducted according to these biomimetic routes.