This Perspective highlights the recent developments in the reduction of unsaturated substrates catalysed by main group element compounds. Various activation modes are discussed and comparison with relevant examples from transition metal systems is made when possible. Main group element catalysis offers significant advantages through its lower cost and more benign environmental impact and has now reached the point where it can successfully compete with the more common catalysis based on precious transition metals.

Main group catalysed reduction of unsaturated bonds.

Described herein is that the catalytic construction of N-substituted five- and six-membered lactams from keto acids with primary amines by reductive amination, using an indium/silane combination. This relatively benign and safe catalyst/reductant system tolerates the use of a variety of functional groups, especially ones that are reduction-sensitive. A direct switch from synthesizing lactams to synthesizing cyclic amines is achieved by changing the catalyst from In(OAc)3 to InI3. This conversion occurs by further reduction of the lactam using the indium/silane pair.

Reductive Amination/Cyclization of Keto Acids Using a Hydrosilane for Selective Production of Lactams versus Cyclic Amines by Switching of the Indium Catalyst

The combination of a catalytic amount of InCl3 and acetic anhydride remarkably promotes the Knoevenagel condensation of a variety of aldehydes and activated methylene compounds. This catalytic system accommodates aromatic aldehydes containing a variety of electron-donating and -withdrawing groups, heteroaromatic aldehydes, conjugate aldehydes, and aliphatic aldehydes. Central to successfully driving the condensation series is the formation of a geminal diacetate intermediate, which was generated in situ from an aldehyde and an acid anhydride with the assistance of an indium catalyst.

Indium(III)-Catalyzed Knoevenagel Condensation of Aldehydes and Activated Methylenes Using Acetic Anhydride as a Promoter

A calcium-catalyzed direct reduction of propargylic alcohols and ethers has been accomplished by using triethylsilane as a nucleophilic hydride source. At room temperature a variety of secondary propargylic alcohols was deoxygenated to the corresponding hydrocarbons in excellent yields. Furthermore, for the first time, a catalytic deoxygenation of tertiary propargylic alcohols was generally applicable. The same protocol was suitable for an efficient reduction of secondary as well as tertiary propargylic methyl, benzyl and allyl ethers. Substrates containing an additional keto-, ester or secondary hydroxyl function were reduced with exceptional chemoselectivity at the propargylic position.

Highly chemoselective calcium-catalyzed propargylic deoxygenation.

Dehydrogenation of Alcohols by Bis(phosphinite) Benzene Based and Bis(phosphine) Ruthenocene Based Iridium Pincer Complexes

InBr3-catalyzed reduction of ketones with a hydrosilane: deoxygenation of aromatic ketones and selective synthesis of secondary alcohols and symmetrical ethers from aliphatic ketones

An InBr3—Et3SiH reducing system is used to selectively convert ketones (I) and (III) to a variety of secondary alcohols (II) and (IV) in moderate to good yields.

InBr3‐Catalyzed Reduction of Ketones with a Hydrosilane: Deoxygenation of Aromatic Ketones and Selective Synthesis of Secondary Alcohols and Symmetrical Ethers from Aliphatic Ketones.

Ken Nagasawa

Papers

InBr3-catalyzed reduction of ketones with a hydrosilane: deoxygenation of aromatic ketones and selective synthesis of secondary alcohols and symmetrical ethers from aliphatic ketones

InBr3‐Catalyzed Reduction of Ketones with a Hydrosilane: Deoxygenation of Aromatic Ketones and Selective Synthesis of Secondary Alcohols and Symmetrical Ethers from Aliphatic Ketones.