A Manganese Pre‐Catalyst: Mild Reduction of Amides, Ketones, Aldehydes, and Esters

Abstract

Anew (N-phosphinoamidinate)manganesecomplex is shown to be au seful pre-catalyst for the hydrosilative reduction of carbonyl compounds,a nd in most cases at room temperature.The Mn-catalyzed reduction of tertiary amides to tertiary amines,with auseful scope,isdemonstrated for the first time by use of this catalyst, and is competitive with the most effective transition-metal catalysts knownfor such transformations.K etones,a ldehydes,a nd esters were also successfully reduced under mild conditions by using this new Mn catalyst.The reduction of carbonyl compounds is an important class of transformations and it is employed widely in synthetic chemistry. [1] In moving beyond the use of harsh alkali-metal hydrides and related reductants,t ransition-metal catalysis offers an alternative for such transformations.W hile platinum-group-metal catalysts were examined early on, the high cost and low abundance of such metals has created motivation for the development of competitive late 3d transition-metal catalysts. [2] As such, ad iversity of effective base-metalcatalyzed carbonyl reduction protocols have emerged, including atom-economical transformations using hydrogen as the reductant. Complementary transformations such as hydrosilylation [3] are also of interest, in that they in some cases are successful when direct hydrogenation fails,and can be carried out conveniently without the need for pressurized reaction vessels.Whereas useful base-metal catalysts for the hydrosilative reduction of ketones,a ldehydes,a nd/or esters have been developed, [2,3b] selective reductions of amides to amines have proven to be significantly more challenging (Scheme 1). [4] The first reports of such transformations were disclosed simultaneously by the groups of Beller [5] and Nagashima, [6] whereby tertiary amides were reduced by the use of PMHS (PMHS = polymethylhydrosiloxane) in the presence of either [Fe(CO) 5 ] Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201709441. Scheme 1. Selective reduction of tertiary amides to tertiary amines by use of 3d transition-metal catalysts.

Reduction of Carboxylic Acids to Alcohols via Manganese(I) Catalyzed Hydrosilylation

“…Control experiments confirmed that no reaction took place under the screening conditions in the absence of a manganese complex. Even though there are reports of manganese(II) pincer complexes showing good activity in a range of other hydrosilylation reactions, 45 − 47 manganese(II) and manganese(III) salts like MnCl 2 , MnBr 2 , Mn(OAc) 2 ·(H 2 O) 4 , and Mn(OAc) 3 ·(H 2 O) 2 did not afford any product. The reaction could be carried out with good to excellent yields in a broad range of solvents, including even neat conditions ( Table 1 ).…”