Reactivity of “Frustrated Lewis Pairs”: Three‐Component Reactions of Phosphines, a Borane, and Olefins

Abstract

All or nothing: Sterically frustrated Lewis pairs of phosphines and the borane B(C6F5)3 exhibit unprecedented reactivity with olefins to afford both inter‐ and intramolecular alkanediyl‐linked phosphonium borates (see picture; black C, pink F, green B, orange P). These reactions are all the more remarkable given that any pair of these reagents do not react while the combination of all three reagents results in product formation.

New Insights into Frustrated Lewis Pairs: Structural Investigations of Intramolecular Phosphane–Borane Adducts by Using Modern Solid-State NMR Techniques and DFT Calculations

“…Signals were acquired following excitation with 30°pulses about 0.6 μs in length and repetition times ranging between 5 and 30 s. Proton decoupled 11 B{ 1 H} spectra were obtained at 9.4 T with 90°pulses of about 1.5 μs length and a 1 H decoupling pulse length of 7.1 μs applied in a TPPM-15 30 decoupling scheme.…”

“…Finally, the spectrum of the PPh 3 −B(C 6 F 5 ) 3 adduct 10 is characterized by an intermediate quadrupolar coupling constant (1.6 MHz) and an asymmetry parameter close to zero, consistent with the local C 3v symmetry of the central boron atom. The trimethylene-bridged phosphane−borane adduct 7 shows a characteristic 11 B MAS NMR line shape reflecting a quadrupolar coupling constant of 1.4 MHz and an asymmetry parameter of 0.55. In this case, the large asymmetry parameter can be attributed to the unsymmetric ligation pattern (C 6 F 5 groups and the trimethylene bridge) and the significant interaction with the phosphorus Lewis base center evident from the crystal structure.…”

“…Complementary 31 P{ 1 H}-CPMAS experiments reveal significant 31 P− 11 B scalar spin−spin interactions ( 1 J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, 11 …”

“…The present study introduces modern solid-state NMR techniques, for the first time, as an experimental probe of such interactions for a series of closely related P/B pairs (mainly substituted vinylene-bridged phosphane−borane adducts 16 ), in which the internuclear distance can be varied over a comparatively wide range by suitable choice of ligands. We explore compositional trends signifying weak 11 B··· 31 P covalent interactions by studying the NMR parameters (such as nuclear electric quadrupolar coupling constants, isotropic chemical shifts and indirect spin−spin coupling constants) with the goal of obtaining a deeper structural insight into the reactivity of those molecules. 11 B MAS NMR spectroscopy has developed into a wellestablished tool for the structural characterization of inorganic boron compounds, as well as boranes.…”

Reversible Complexation of Ethylene by a Silylene under Ambient Conditions

“…These include disilynes, 12 digermynes, 3 frustrated Lewis-pair borane-phosphine molecules, 13 silylenes, 14 silylene precursors, 7k and digallenes, 15 but reversibility in ethylene binding, which is key for the development of possible catalysts, remains rare. 3,4 Further studies of the electronic factors that govern the reversibility of ethylene complexation by silylenes and their heavier group 14 element congeners are in hand.…”