Selective Reduction of Esters to Aldehydes under the Catalysis of Well‐Defined NHC–Iron Complexes

Abstract

On a direct course to the aldehyde: Hydrosilylation catalyzed by a well‐defined N‐heterocyclic‐carbene–iron complex under UV irradiation enabled the selective reduction of esters to aldehydes (see scheme; Bn=benzyl, Mes=mesityl). The low catalyst loading and very mild reaction conditions make this chemoselective transformation a promising alternative to the reduction of esters with diisobutylaluminum hydride.

Iron-catalyzed reduction of carboxylic and carbonic acid derivatives

“…Under these optimized conditions, various (hetero)aromatic and aliphatic aldehydes were isolated in high yields (55-95%) [25]. Again, some electron-donating or electron-withdrawing groups on the aromatic ring, and isolated alkenes are well tolerated in these conditions.…”

“…Scheme 17: Synthesis of (NHC)iron tetracarbonyl complexes [25]. Under these optimized conditions, various (hetero)aromatic and aliphatic aldehydes were isolated in high yields (55-95%) [25].…”

“…Oxidative addition of silane was observed and the corresponding hydride iron complexes were isolated in good yields, and completely characterized by NMR spectroscopy, X-Ray analyses. Based on these observations, the following mechanism was Scheme 19: Proposed catalytic cycle for the semi-reduction of esters [25].…”

“…Toward this goal, they developed a new class of tetracarbonyl iron(0) complexes bearing NHC ligands ([(NHC)Fe(CO) 4 ]). These complexes were prepared in one step from [Fe(CO)5 ] in moderate to good yields (36-63%), following a procedure described in the literature (Scheme 17)[25].…”

Ethers from esters; from exceptional transformation to synthetic method

“…Some recent papers report on progress in a controlled catalytic reduction of esters to aldehydes. [4][5][6] In this review methods are presented for reduction of esters to ethers, which have an obvious advantage of using esters as easily available precursors, and are compatible with most sensitive groups, either free or protected. For a long period reduction of esters to ethers was uninvestigated due to the undesired course of this reaction.…”