Frustrated Lewis pairs phosphine

In the framework of Frustrated Lewis Pairs, Erker also investigated the hydroboration of a-alkenyl-phosphines with HB(C6F5)2.52/53 Starting from dimesityl(vinyl)phosphine, the CH2CH2-bridged PB 40a was first... 

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

Does the presence of excessive steric bulk when a Lewis acid and a Lewis base attempt to form an adduct automatically render these species inert towards each other The unique behavior of sterically frustrated Lewis pairs (FLPs), pioneered by Stephan, is a vigorous research area with applications for small molecule activation and catalysis." The highly Lewis acidic and sterically bulky tris(pentafluorophenyl)borane" plays a role in many FLP reactions. The great promise of FLP chemistry was revealed by reactions between tris(pentafluorophenyl)borane and tertiary and secondary phosphines, where sterics preclude formation of classic adducts. A seminal example of a frustrated Lewis pair is that of the secondary phosphine di(2,4,6-trimethylphenyl)phosphine that is precluded from forming a classic adduct with tris(pentafluorophenyl)borane. The phosphine Lewis pair is frustrated since it cannot interact with boron to form the adduct. 

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]. 

In the field of frustrated Lewis pair chemistry, organoaluminum compounds are often employed as the Lewis acid of the pair to great effect Just as aluminum is a substitute for boron in this case, carbenes are often substituted for the standard phosphines as mentioned in Section 15.5.1. Zhang et al. [137] used several frustrated Lewis pair combinations (A1(C6F5)3 with a variety of phosphines and carbenes) to facilitate the polymerization of methyl methacrylate, a-methylene-y-butyrolactone, and y-methyl-a-methylene-y-butyrolactone. Even combinations that form classical Lewis acid/base adducts rather than FLPs could still exhibit excellent polymerization activity. 

The concept of frustrated Lewis pairs (FLP) in which Lewis acid-base couples formed by sterically crowded phosphines and pentafluoro-phenylboranes are sterically precluded from adduct formation, which was established and significantly developed by Stephan s group, has been reviewed by the latter. FLP open alternative reaction pathways and the discovery of their reactivity, even in its infancy, brings a new perspective to the area of small molecule activation and applications in catalysis. 

Reactions of phosphine-derived carbenes C3H2(NP-t-Bu2)2 and C3H2(NPN-i-Pr2)2 with 9-borabicyclo[3.3.1]nonane (9-BBN) carried out by Wang and Stephan have resulted in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). The H NMR spectrum of the t-Bu2 derivative has been found to be consistent with inequivalent carbene backbone C-H groups as two distinct resonances at = 5.67 f/HH = 6.3 Hz and /hp = 2.0 Hz) and = 6.01 /hh = 6 Hz and /hp = 1 Hz) have been observed for this compound. The analogous spectral data has been found for the N-i-Pr2 derivative. 

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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