Chemoenzymatic synthesis of optically active 1,2-disubstituted ferrocenes with planar chirality

CAL-B-Catalyzed deacylation of benzylic acetates: Effect of amines addition. Comparison of several approaches

Herein, we report an efficient enantioselective cleavage of the acyl-moity of some secondary benzylic acetate derivatives catalyzed by lipase B from Candida antarctica (CAL-B) in the presence of triethylamine, as additive, in non aqueous media. The influence of the hydrophobicity of two solvent, the basicity of three amines and the amount of CAL-B were studied in the presence/absence of molecular sieves 4Å. The best results in term of selectivity are achieved using the triethylamine as basic additive and in that case, the reactivity is only best at low conversion. To establish the effect of the parallel and/or competitive hydrolysis and its impact on the reactivity and selectivity of the enzymatic resolution, the kinetic profiles of three CAL-B-deacylation approaches of phenylethylacetate have been compared, using different nucleophiles in competition with the internal water mediated by: Na₂CO₃, EtOH and by using the Et₃N as additive. Furthermore, a comparison between these deacylations with the acylation of 1-phenylethanol with isopropenylacetate, has been made. The appropriate modulation of some crucial parameters allows an optimal conversion and a high selectivity depending on the acetate structure and the introduced base. In the majority of cases, the (R)-alcohols are obtained with ee>99% and selectivities E>200 under mild conditions.

Enantiorecognition of planar "metallocenic" chirality by a nitrile hydratase/amidase bienzymatic system

For the first time, a bacterial strain expressing a nitrile hydratase/amidase activity was able to recognize a planar element of chirality: Rhodococcus rhodochrous PA-34 whole cells catalysed with a high level of enantioselectivity the biotransformation of a novel nitrile ferrocene derivative into its corresponding amide and/or acid. An important parameter in the enzymatic recognition is the choice of the inducer selected for the bacterial growth phase.

Kinetic resolution: a powerful tool for the synthesis of planar-chiral ferrocenes

Since the serendipitous discovery of ferrocene by Pauson and Kealy in 1951, it has become one of the most important structures in Organic Chemistry. Lately, kinetic resolution has emerged as a useful tool for the synthesis of planar chiral ferrocenes. This review aims to cover and discuss the development of this topic.