Cobalt-catalysed selective synthesis of aldehydes and alcohols from esters

Direct conversion of esters to imines/enamines and applications to polyester waste upcycling

Semi-reductive transformations of esters remain an underdeveloped but valuable class of functional group interconversions. Here, we describe the development of a highly selective method for the interconversion of esters to imines, enamines, aldehydes or amines through an amine-intercepted zirconocene hydride (ZrH)-catalyzed reduction. This protocol employs an inexpensive zirconium catalyst in combination with hydrosilanes and simple unprotected amines. A variety of aryl, benzylic, and aliphatic esters are directly transformed to imines and enamines in up to 99% yield or aldehydes in up to 84% yield, with little-to-no reduction to the corresponding alcohols. The utility of this method for the direct catalytic chemical upcycling of polyester plastic waste is demonstrated through multiple unprecedented depolymerization transformations. Further, the efficient preparation of nitrogen-containing products is also presented, including single-flask multicomponent reactions and the reductive amination of esters.

Cobalt-Catalyzed Chemodivergent Synthesis of Cyclic Amines and Lactams from Ketoacids and Anilines Using Hydrosilylation

Here, commercially available Co2(CO)8 was utilized as an efficient catalyst for chemodivergent synthesis of pyrrolidines and pyrrolidones from levulinic acid and aromatic amines under slightly different hydrosilylation conditions. 1.5 and 3 equiv of phenylsilane selectively yielded pyrrolidone and pyrrolidine, respectively. Various ketoacids and amines were successfully tested. Plausible mechanism involves the condensation of levulinic acid and amine to form an imine, which cyclizes to 3-pyrrolidin-2-one followed by reduction to pyrrolidone. The final reduction of pyrrolidone gave pyrrolidine.

Cobalt-catalysed [1,2]-Wittig rearrangement of ethers to secondary alcohols

Stable ethers are successfully transformed into secondary alcohols via C-O bond activation using a simple cobalt pincer catalyst. Mechanistic studies indicate the involvement of radical pairs, and their sequential recombination and the subsequent hydrolysis results in the formation of secondary alcohols.

Partial reductions of carboxylic acids and their derivatives to aldehydes

Recent advances in partial reductions of inert carboxylic acids and their derivatives to active aldehydes are reviewed.