Allene as the Parent Substrate in Zinc-Mediated Biomimetic Hydration Reactions of Cumulenes

The aim of our present investigation is to unravel the general mode of biomimetic activation of a wide variety of cumulenes by carbonic anhydrase (CA) models. Carbonic anhydrases allow the specific recognition, activation and transfer not only of CO2 but also of heteroallenes XCY such as the polar or polarizable examples COS, CS2, H2CCO, and RNCS. Therefore, this enzyme class fulfils the requirements of excellent catalysts with a wide variety of important applications. Can this be extended to the isoelectronic but less reactive allene molecule, H2CCCH2 and extremely simplified models as mimetic concept for active center of the carbonic anhydrase? Allene is a waste product in the refinery, i.e. the C3-cut of the naphtha distillation; therefore, any addition product that can be obtained from allene in high yields will be of significant value. We investigated the complete catalytic cycle of a very simple model reaction, the hydration of allene, using density functional theory. Additionally, calculations were performed for the uncatalyzed reaction. There are two possible ways for the nucleophilic attack leading to different products. The zinc hydroxide complex and the water molecule can react at the central or the terminal carbon atoms (positional selecticity), the resulting products are 2-propen-1-ol and propen-2-ol, respectively, acetone. The calculations indicate a significant lower energy barrier for the rate determining step of the formation of propen-2-ol and therefore a well-expressed regioselectivity for the addition of such small molecules. The zinc complex has a pronounced catalytic effect and lowers the activation barrier from 262.5 to 123.9 kJ/mol compared with the uncatalyzed reaction. This work suggests the most probable paths for this reaction and discloses the necessity for the development of novel catalysts.