Mechanism and stereoselectivity in metal and enzyme catalyzed carbene insertion into X–H and C(sp2)–H bonds

Constructing highly proficient C–X (X = O, N, S, etc.) and C–C bonds by leveraging TMs (transition metals) (Fe, Cu, Pd, Rh, Au, etc.) and enzymes to catalyze carbene insertion into X–H/C(sp²)–H is a highly versatile strategy. This is primarily achieved through the in situ generation of metal carbenes from the interaction of TMs with diazo compounds. Over the last few decades, significant advancements have been made, encompassing a wide array of X–H bond insertions using various TMs. These reactions typically favor a stepwise ionic pathway where the nucleophilic attack on the metal carbene leads to the generation of a metal ylide species. This intermediate marks a critical juncture in the reaction cascade, presenting multiple avenues for proton transfer to yield the X–H inserted product. The mechanism of C(sp²)–H insertion reactions closely resembles those of X–H insertion reactions and thus have been included here. A major development in carbene insertion reactions has been the use of engineered enzymes as catalysts. Since the seminal report of a non-natural “carbene transferase” by Arnold in 2013, “P411”, several heme-based enzymes have been reported in the literature to catalyze various abiological carbene insertion reactions into C(sp²)–H, N–H and S–H bonds. These enzymes possess an extraordinary ability to regulate the orientation and conformations of reactive intermediates, facilitating stereoselective carbene transfers. However, the absence of a suitable stereochemical model has impeded the development of asymmetric reactions employing a lone chiral catalyst, including enzymes. There is a pressing need to investigate alternative mechanisms and models to enhance our comprehension of stereoselectivity in these processes, which will be crucial for advancing the fields of asymmetric synthesis and biocatalysis. The current review aims to provide details on the mechanistic aspects of the asymmetric X–H and C(sp²)–H insertion reactions catalyzed by Fe, Cu, Pd, Rh, Au, and enzymes, focusing on the detailed mechanism and stereochemical model. The review is divided into sections focusing on a specific X–H/C(sp²)–H bond type catalyzed by different TMs and enzymes.