Metal‐Free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen

Abstract

The metal-free activation of hydrogen by frustrated Lewis pairs (FLPs) is a valuable method for the hydrogenation of polarized unsaturated molecules ranging from imines, enamines, and silyl enol ethers to heterocycles. However, one of the most important applications of hydrogenation technology is the conversion of unsaturated hydrocarbons into alkanes or alkenes. Despite the fast development of the FLP chemistry, such reactions proved as highly challenging. This Minireview provides an overview of the basic concepts of FLP chemistry, the challenge in the hydrogenation of unsaturated hydrocarbons, and first solutions to this central transformation.

Development of silica-supported frustrated Lewis pairs: highly active transition metal-free catalysts for the Z-selective reduction of alkynes

“…15 Only recently were highly selective metal-free catalysts for alkyne hydrogenation reported. 6,[16][17][18][19][20] These catalysts are composed of equimolar quantities of sterically hindered Lewis acids-bases, a.k.a. frustrated Lewis pairs (FLP), as introduced by Stephan.…”

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

“…[6] Metal-free acceptorless catalytic dehydrogenations are rare and have been reported for 1,4-cyclohexadienes [7] and ammoniaborane. [10] C(sp 3 )ÀHb ond activation through hydride abstraction by organoboranes has been observed as an undesired side reaction for selected amines [11] or was deliberately exploited for catalytic transfer hydrogenations using diisopropylamine [11a] or 1,4-cyclohexadienes [7] as dihydrogen surrogates.H owever,t he catalytic oxidation of organic substrates with simultaneous release of molecular hydrogen by FLPs as as ynthetic method has not been reported thus far. [10] C(sp 3 )ÀHb ond activation through hydride abstraction by organoboranes has been observed as an undesired side reaction for selected amines [11] or was deliberately exploited for catalytic transfer hydrogenations using diisopropylamine [11a] or 1,4-cyclohexadienes [7] as dihydrogen surrogates.H owever,t he catalytic oxidation of organic substrates with simultaneous release of molecular hydrogen by FLPs as as ynthetic method has not been reported thus far.…”

Experimental Insights into the Structure and Reactivity of Frustrated Lewis Pairs

“…[16] Despite the large interest in these processes, only few mechanistic details are known. Commonly accepted mechanistic schemes foresee: i) the formation of ionic intermediates due to a protonation by the onium salts; or ii) an activation by the LA species.…”