Lewis pairs, frustrated, addition with

Treatment of ItBu or IPr with M-butyllithium led to the generation of a lithium salt of the NHC with the negative charge at the C5 position. Addition of tris (pentafluorophenyl)borane in an attempt to generate a frustrated Lewis pair [25,26] led instead to the formation of a crystallographically characterized abnormal NHC-borate adduct with Uthium coordinated to 2 equivalents of THF at the C2 position of the NHC, as shown in Figure 15.6 [27]. 

In 2006, Stephan introduced the concept of frustrated Lewis pairs , systems in which steric congestion precludes the formation of a Lewis acid-base adduct [25,26]. These combinations offer novel reactivity involving the activation of a variety of small molecules including amines, alkenes, alkynes, and molecular hydrogen [94]. While initially phosphines were the Lewis base of choice for FLP reactions, Al-heterocyclic carbenes were soon proven to be viable candidates as well. For instance, an FLP of ItBu and tris(pentafluorophenyl)borane was shown to react with molecular hydrogen in the usual FLP fashion, forming an imidazo-lium borate salt. This FLP was also used to activate ammonia, aniline, and diphe-nylamine, by addition of the NHC to the amine-borane adduct as shown in Scheme 15.5 [95]. 

Terminal alkynes Ar C=CH undergo a Markovnikov hydroamination with amines Ar NHR in the presence of (C6F5)3B as a catalyst to give enamines Ar N(R)C(Ar )=CFl2. The reaction is believed to involve a frustrated Lewis pair (FLP). ° An FLP has also been implicated in the addition P-, N-, C-, and H-based nucleophiles to olefin-tethered boranes. ... 

This chemistry resembles that of frustrated Lewis-pairs and it serves for redox-innocent metals as an alternative to oxidative addition reaetions for metals with accessible oxidation states. 

One of the more novel synthetic approaches to the synthesis of vinylphosphines entailed the addition of alkynes to a functionalized tertiary phosphine (Scheme 4.319) [433]. These compounds are part of a special class of compounds that contain an intramolecnlar frustrated Lewis acid-base pair (FLP). These FLPs have been used to react with small molecules such as hydrogen and carbon dioxide. The P—C bond-forming reaction was operationally trivial and consisted of mixing the aUcyne with the specialized precursor in a hydrocarbon solvent for 1 h. The product precipitated and was separated by simple filtration. Although the substrate scope for this reaction is limited at present, this approach does provide a pathway to an intriguing class of vinylphosphines. 

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