A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Abstract

Recently, a new approach to dihydrogen activation known as frustrated Lewis pairs (FLPs) concept has been introduced 1, 2, 3 . A combination of highly Lewis acidic boranes and sterically hindered bases can split hydrogen heterolytically generating onium (phosphonium, ammonium, etc.) borohydrides. These compounds show reduction activity resembling that of inorganic borohydrides like NaBH 4 , i. e. they are suitable mostly for reduction of polarized multiple bonds. Imines, enamines, silyl ethers 4, 5,6 , α,β-enones 7 , ynones 8 , Nalkylanilines 9 were hydrogenated using stoichiometric or catalytic amounts of FLPs. Due to heterolytic nature of FLP-H 2 adducts, hydrogenation of unactivated multiple C-C bonds using FLPs has some natural limitations, since during the respective step of the catalytic cycle a proton transfer from catalyst to substrate should take place (Fig. 1a). Although Greb et al. have implemented this approach to hydrogenation of alkenes under ambient conditions, this method is predictably restricted to the alkenes with high proton affinity 10 .

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…However, none of these C-H bond cleavage processes could be used for catalytic transformations. In our study of the many processes involving ambiphilic molecules, we found that the closest analogue of this type of transition state was reported by Repo and coworkers for describing the protodeborylation process that is at the core of his mechanism for the hydrogenation of alkynes ( Figure 7B) [78]. The protodeborylation reaction is, formally, the reverse reaction of C-H activation and is driven in this direction by thermodynamic factors.…”

“…It should be noted that an important side reaction in the activation of H2 using 1-NR2-2-B(C6F5)2-C6H4 is the protodeborylation reaction where the acidic proton on nitrogen once H2 is cleaved is transferred to one of the pentafluoroaryl groups to generate C6F5H (Scheme 13). Repo and coworkers cleverly exploited this transformation in the selective cis-hydrogenation of alkynes [78].…”

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation

“…This activity is remarkable considering the low reaction temperature and that hydrogenation is carried out under continuous flow. Hydrogenation of acetylene on frustrated Lewis acid-base pairs using a rigid amino-borane molecular catalyst has been reported in the literature 39 , and now the finding that Gr-based materials can also act as acetylene hydrogenation catalysts can open new opportunities in the area of metal-free hydrogenation catalysts. The conversion of acetylene in the presence of Gr as catalyst increases along the temperature, reaching an optimal value at temperatures between 110 and 120°C, in which 99% of acetylene was converted with only 21% conversion of ethylene ( Table 2, entry 2).…”

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…3,4 With regard to the latter class of "metal-free" catalysts, the primary challenge lies in coaxing main group element compounds into mediating the critical bond activation step necessary for catalytic turnover. 5 In this context, our observation in the mid 1990's that the strongly Lewis acidic organoborane tris-pentafluorophenyl borane, B(C6F5)3, 6 was a highly active catalyst for the hydrosilation of carbonyl 7 and imine 8 functions has proven to be a key discovery in the development of metalfree catalyst systems for the addition of Si-H 9-13 and subsequently H-H [14][15][16] across a number of unsaturated bonds. Although we initially surmised that B(C6F5)3 was serving to activate the carbonyl group via a classical Lewis acid-Lewis base interaction, kinetic and mechanistic studies strongly suggested that, although such adducts are quite thermodynamically favored, 17,18 the true role of the borane was to activate the silane, 7,19 as depicted in Fig.…”