Frustrated Lewis pairs , catalytic hydrogenation

I 77 Frustrated Lewis Pairs A Metal-Free Strategy for Hydrogenation Catalysis Table 11.1 Catalytic hydrogenation of imines with phosphonium borate catalysts [45]. 

CATALYTIC HYDROGENATION WITH FRUSTRATED LEWIS PAIRS... 

A general approach to the hydrogenation of alkynes to cw-alkenes under mild conditions using the ansa-aminohydroborane (2-[bis(pentafluorophenyl)boryl]-A, A-dimethylaniline) (77) as a catalyst. The approach combines several reactions as the elementary steps of the catalytic cycle hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies." ... 

Abstract Acid-base catalysis with bifunctional catalysts is a very prominent catalytic strategy in both small-molecule organocatalysts as well as enzyme catalysis. In both worlds, small-molecule catalysts and enzymatic catalysis, a variety of different general acids or hydrogen bond donors are used. In this chapter, important parallels between small molecule catalysts and enzymes are discussed, and a comparison is also made to the emerging field of frustrated Lewis pair catalysis. 

The bifunctional acid-base catalysts discussed above are all Br0nsted acid/Br0nsted base catalysts. However, it is also possible to design related catalysts that are based on Lewis acid/Lewis base pairs. To prevent the self-association and irreversible intermolecular adduct formation, stericaUy encumbered Lewis acids and Lewis bases are typically used. Such hindered pairs are often called frustrated Lewis pairs (FLPs). This field has been reviewed very thoroughly recently [33]. Herein, we focus on the catalytic aspects of FLPs, especially their role as bifunctional hydrogen activators. 

A hydrogen-storage related and very interesting new use of boron in the catalytic oxidation of hydrogen was proposed by Wildgoose and co-workers. A frustrated Lewis acid-Lewis base pair is proposed leading to a boranate-based redox cycle (Figure 8.3). 

Fig ure 8.3 A frustrated Lewis acid-Lewis base pair can bind hydrogen and react at a catalytic electrode surface similar to a boranate redox system. (Reproduced from ref. 14 with the permission of Wiley-VCH Verlag GmbH.)... 

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