Frustrated Lewis acid-base pairs

Very recently, such frustrated Lewis acid-base pairs (FLPs), as they are called, have been found to exhibit unique reactivity, including activation of molecular hydrogen (see Section 2.6 for additional details). The following example shows associative (A) reactions between a P/B-based FLP and CO2. 

SCHEME 4.319 Synthesis of frustrated Lewis acid-base pairs [433]. 

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. 

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 ... 

We recently initiated efforts within our own laboratory to synthesize terpy-based ligands that incorporate appended Lewis acid/base pairs [36]. Motivated by metal-free systems of frustrated Lewis pairs (FLPs) which bind and form adducts with small molecules, we sought to couple the reactivity of FLPs to a transition-metal fragment to afford a metal Lewis acid/base triad (LABT). The combination of both partners with a transition-metal center opens up the possibiUty to synergistically use these systems for further activation and/or redox transformations - currently a hmitation of metal-free FLP systems. 

Fig. 30 Reaction of the frustrated Lewis acid/Lewis base pair NHC/BlC Fsia with Hi...

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. 

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.)... 

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. 

Keywords Hydroboration Anti-Markovnikov rule Markovnikov rule 1 IB NMR Lewis acid-base Frustrated Lewis pair... 

The reaction between a Lewis acid group 13 species of the type R3M and a group 15 Lewis base of the type ER 3 typically yields the corresponding Lewis acid-base adduct R3M-ER 3. This reaction, of fundamental interest in main group chemistry, has recently received an increased attention due to the potential use of amine-borane adducts as a hydrogen storage material [39] and to the unusual reactivity of so-called Frustrated Lewis pairs [40-42]. 

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. 

Berke H, Jiang Y, Yang X, Jiang C, Chakraborty S, Landwehr A (2013) Coexistence of Lewis acid and base functions a generalized view of tbe frustrated lewis pair concept with novel implications for reactivity. Top Curr Cbem 334. doi 10.1007/128 2012 400... 

The seminal works [288-294] of Stephan and Erker uncovered the concept of Frustrated Lewis Pairs (FLPs) to describe sterically encumbered borane Lewis acid (most commonly B(C6p5)3) and base (e.g., BU3P) pairs that are sterically... 

Recall from Section 1.9 that the Lewis acid and base in a frustrated pair are too bulky or otherwise sterically constrained to bond with each other but the cleft between them acts like an enzyme active site and promotes remarkable chemical reactions, perhaps none more so than the activation of dihydrogen at or below room temperature. Two representative reactions mediated by P,B-based FLPs are as follows ... 

Knochel and collaborators developed and optimized regioselective metalations of pyrimidines and pyrazines, employing frustrated Lewis pairs consisting of boron trifluoride etherate and either a bulky, sterically hindered 2,2,6,6-tetramethylpiperidylzinc base or a 2,2,6,6-tetramethylpiperi-dylmagnesium base (Scheme 42) (13AG(I)6776).This method facilitates the metalation of positions, which without the presence of a Lewis acid would not be possible. 

A new Ru- 7 -arene complex (45) acts as a C-based Lewis acid catalyst for the hydrogenation of aldimines at ambient temperature via a frustrated Lewis pair mechanism with 102 atm H2 in DCM at 25 °C, 1 mol% catalyst gives up to 99% amine in 8 h. The catalyst and its mechanism have been extensively characterized by X-ray crystallography and NMR, including deuteration experiments with D2 which prove that exchange is occurring ortho- and para- to the boron. 

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