Heterolytic Cleavage of H₂by Frustrated B/N Lewis Pairs

Abstract

The concept of frustrated Lewis pairs (FLPs) was put forth by D. W. Stephan et al. after their remarkable discovery that H 2 can reversibly be activated by [(2,4,6-C 6 H 2 Me 3 ) 2 PC 6 F 4 B(C 6 F 5 ) 2 ], 1 which led to development of the first metal-free catalyst for hydrogenations of bulky imines as one of the fruitful applications of this concept. 2 The steric congestion of Lewis donors and acceptors precludes the formation of classical Lewis adducts, but provokes formation of FLPs with "unquenched" reactivity toward small molecules. 3À5 For instance, the mixture of appropriate phosphines and boranes can activate H 2 heterolytically under often mild conditions 6 and can undergo 1,2-addition reactions with olefins, as well. 7,8 After the pioneering work of Stephan et al., an increasing number of related FLP systems were found. The frustrated Lewis pairs were extended from the initial boron/phosphine species 9À20 to boron/carbene systems and borane/amine species. 21À24 Some of the resulting ionic products were shown to serve as active catalysts for the hydrogenation of imines, nitriles, and aziridines, as well as enamines and silyl enol ethers. 12,24,25 However, the mechanism of the H 2 activation by FLPs is still not fully understood. Theoretical studies proposed that the Lewis donor and acceptor initially form an "encounter complex" with long nonbonding distances between the Lewis centers frequently supported by multiple CH 3 3 3 F interactions. Such relatively weak specific forces, as well as the global electrostatic field of the Lewis pair, caused in sum too small interaction energies to allow proper identification by conventional analytical methodologies. H 2 can insert into this encounter complex, being thus activated by heterolytic HÀH splitting. 26,27 Recent sophisticated DFT studies by Grimme et al., which included dispersion forces, pointed out that the intermediate formed between H 2 and for instance a P/B FLP could show kinetic stabilization and would thus be spectroscopically detectable under the condition that the P 3 3 3 B nonbonding distance is over 4.5 Å. Otherwise, the H 2 heterolysis would be practically barrierless, once the H 2 molecule had "sneaked" into the FLP complex. 28 Herein, we explore several sterically hindered Lewis bases, mainly piperidine and pyridine derivatives, to modulate the B 3 3 3 N nonbonding distance of the FLP and to study the impact of varying the B 3 3 3 N distance on their ability to activate H 2 .

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…In related work, Berke et al [28] have used tetramethylpiperidine/RB(C 6 F 5 ) 2 pairs for dihydrogen activation. The parent system with R = C 6 F 5 seems to result in an irreversible H 2 activation; however, using the slightly less Lewis acidic (cyclohexyl)B(C 6 F 5 ) 2 results in reversible H 2 -splitting at elevated temperature.…”

“…The ammonium/hydrido borate contact ion pair 83 was obtained by heterolytic dihydrogen splitting by the tetramethylpiperidine/B(C 6 F 5 ) 2 (1) ), which is less pronounced as in 82. [28,72] Both compounds 82 and 83 were obtained from the respective N/B FLPs, posing the question of the Lewis acid/ Lewis base interaction in these reactive precursors. How do these systems circumvent termolecularity (i.e.…”

A Frustrated and Confused Lewis Pair

“…Such frustrated Lewis pairs (FLPs) undergo cooperative reactivity with aw ide variety of external substrates,d isplaying previously unprecedented metal-free pathways to small molecule capture and activation. Thef irst example of small molecule activation by FLPs, [1] and perhaps the most widely studied, is the heterolytic splitting and activation of H 2 , [2][3][4] which has resulted in the development of metal-free catalytic processes for the hydrogenation of imines,e namines,a nd silyl enol ethers. [5][6][7][8] FLPs have been developed that incorporate awide variety of combinations of Lewis acids and bases,b ut the majority consist of sterically hindered phosphorus-, nitrogen-, and carbene-centered Lewis bases in conjunction with fluorinated aryl borane Lewis acids.…”

A Strategy to Control the Reactivation of Frustrated Lewis Pairs from Shelf‐Stable Carbene Borane Complexes

“…Therefore, the development of a method to restore the FLP reactivity from a quenched CLA under accurately controlled reaction conditions rather than under ambient conditions should expand both the utility and versatility of Lewis pair systems. [7f]–[h]…”