Geminal Frustrated Lewis Pair Research Articles

Reactivity of Oxygen-Bridged Geminal Al/P and Si/P Frustrated Lewis Pairs towards Heterocumulenes.

The two oxygen-bridged geminal frustrated Lewis pairs (FLP) tBu2 P-O-AlBis2 (Bis=CH(SiMe3 )2 ; 1) and tBu2 P-O-Si(C2 F5 )3 (2) were reacted with the heterocumulenes PhNCO, PhOCN, PhNCS, CS2 and PhNSO as well as SO2 . With isocyanate and cyanate, both 1 and 2, form addition products under formation of five-membered rings. With CS2 , isothiocyanate and sulfinylaniline, only 1 forms stable adducts, whereas 2 shows reactivity towards sulfinylaniline, but the product decomposed after a few minutes. The reaction of 1 with SO2 led to partial cleavage of the P-O-Al and Al-C units, as confirmed by X-ray diffraction studies of a complex aggregate. The reaction of 2 with SO2 affords the 1,2-addition product. All adducts were characterized by means of multinuclear NMR spectroscopy, X-ray crystallography and CHN analyses.

Aromaticity-enhanced reactivity of geminal frustrated Lewis pairs.

The presence of a cyclopropenimine moiety as the Lewis base partner in geminal frustrated Lewis pairs greatly enhances the reactivity of the system towards the activation of small molecules. This is mainly due to an increase of the aromaticity strength of this fragment during the activation reaction which results in a significant gain of stability ultimately leading to low barrier and high exergonic transformations.

Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer.

The reactivity of the geminal frustrated Lewis pair (FLP) (F5C2)3SnCH2P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3CC(O)CH=CH2, Ph[C(O)]2Ph, PhN=NPh and Me3SiCHN2), featuring polar or non‐polar multiple bonds and/or represent α,β‐unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV‐induced photoisomerization, which results in the highly selective complexation of cis‐azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2‐ or 1,4‐addition products, but to the non‐stereoselective (tBu)2PCH2‐transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X‐ray diffraction experiments.

An Adduct of Sulfur Monoxide to a Frustrated Sn/P Lewis Pair.

The geminal frustrated Lewis pair (F5C2)3SnCH2P(tBu)2 (1) reacted with N‐sulfinylaniline PhNSO to afford the first sulfur monoxide adduct of a main group metal, (F5C2)3SnCH2P(tBu)2⋅SO (2), which contains a SnCPSO ring. The second product is a phenylnitrene adduct of 1. The surprising stability of 2 was compared with the stabilities of the so far inaccessible O2 and S2 adducts of 1. Attempts to prepare these from 1 and the elemental chalcogens (O2, S8, Se∞, Te∞) led to four‐membered SnCPE ring systems. Quantum‐chemical investigations of 2 demonstrate the bond polarity of the SO unit to stabilize 2.

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

AbstractReaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH2⋅SMe2 gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]‐CH2‐O‐[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P‐benzamidine, and it couples two isonitriles at the FLP framework.

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles.

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH2 ⋅SMe2 gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH2 -O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework.

Activation of N2O and SO2 by the P-B Bond System. Reversible Binding of SO2 by the P-O-B Geminal Frustrated Lewis Pair.

Herein, we present the first transformation of borylphosphine into borylphosphinite using nitrous oxide. Borylphosphine reacts with N2O via insertion of a single oxygen atom into the P–B bond and formation of a P–O–B bond system. Borylphosphine and borylphosphinite capture SO2 and activate it in an irreversible and reversible manner, respectively.

Understanding the Reactivity of Neutral Geminal Group 14 Element/Phosphorus Frustrated Lewis Pairs.

The influence of the nature of the group 14 elements (E = Si, Ge, Sn) on the reactivity of (F5C2)3E-CH2-P(tBu)2 geminal frustrated Lewis pairs (FLPs) has been computationally explored by means of density functional theory calculations. To this end, the experimentally described activation reactions of CO2 and phenyl isocyanate have been investigated and compared to the analogous processes involving the corresponding B/P geminal FLP. It is found that the reactivity of these species is kinetically enhanced when going down the group 14 (Si < Ge < Sn). This trend of reactivity is quantitatively analyzed in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis method, which identify the interaction energy between the deformed reactants as the main factor controlling the reactivity of these group 14 containing geminal FLPs.

New Insights in Frustrated Lewis Pair Chemistry with Azides.

The geminal frustrated Lewis pair (FLP) tBu2PCH2BPh2 (1) reacts with phenyl‐, mesityl‐, and tert‐butyl azide affording, respectively, six, five, and four‐membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six‐membered ring structure, which is thermally stable with an N‐phenyl group, but rearranges when sterically more encumbered Mes−N3 and tBu−N3 are used. The reaction of 1 with Me3Si−N3 is believed to follow the same course, yet subsequent N2 elimination occurs to afford a four‐membered heterocycle (5), which can be considered as a formal FLP‐trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.

A Neutral Geminal Tin/Phosphorus Frustrated Lewis Pair.

The geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (2) was prepared by reacting (F5 C2 )3 SnCl with LiCH2 P(tBu)2 . It is neutral and contains an extremely electronegatively substituted, but relatively soft (hard-soft acid-base, HSAB) acidic tin function. Its FLP-type reactivity was proven by reaction with a variety of small molecules (CO2 , SO2 , CS2 , PhNCO, HCl, (Ph3 P)AuCl). However, it shows no reaction in H/D scrambling experiments with H2 /D2 mixtures and binds CO2 reversibly, as was observed by VT-NMR spectroscopy. Compound 2 and all its adducts were completely characterized by means of multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction experiments.

Influence of the Lewis Acid/Base Pairs on the Reactivity of Geminal E-CH2 -E' Frustrated Lewis Pairs.

The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me2 E-CH2 -E'Ph2 ) has been computationally explored within the density functional theory framework. To this end, the dihydrogen-activation reaction, one of the most representative processes in the chemistry of FLPs, has been selected. It is found that the activation barrier of this transformation as well as the geometry of the corresponding transition states strongly depend on the nature of the E/E' atoms (E=Group 15 element, E'=Group 13 element) in the sense that lower barriers are associated with earlier transition states. Our calculations identify the geminal N/Al FLP as the most active system for the activation of dihydrogen. Moreover, the barrier height can be further reduced by replacing the phenyl group attached to the acidic atom by C6 F5 or 3,5-(CF3 )2 C6 H3 (Fxyl) groups. The physical factors controlling the computed reactivity trends are quantitatively described in detail by means of the activation strain model of reactivity combined with the energy decomposition analysis method.

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C-X (X=Cl, Br) Bond Activation.

The geminal frustrated Lewis pair tBu2PCH2B(Fxyl)2 (1; Fxyl=3,5‐(CF3)2C6H3) is accessible in 65 % yield from tBu2PCH2Li and (Fxyl)2BF. According to NMR spectroscopy and X‐ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P⋅⋅⋅B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H2, EtMe2SiH, CO2/CS2, Ph2CO, and H3CCN. Terminal alkynes react with heterolysis of the C−H bond. Haloboranes give cyclic adducts with strong P−BX3 and weak R3B−X bonds. Unprecedented transformations leading to zwitterionic XP/BCX3 adducts occur on treatment of 1 with CCl4 or CBr4 in Et2O. In less polar solvents (C6H6, n‐pentane), XP/BCX3 adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph2CO with EtMe2SiH.

ChemInform Abstract: Intramolecular Frustrated Lewis Pairs: Formation and Chemical Features

AbstractReview: 148 refs.

Structural features and reactions of a geminal frustrated phosphane/borane Lewis pair

[Bis(pentafluorophenyl)phosphanyl]phenylacetylene (4) undergoes a regioselective hydroboration reaction with HB(C6F5)2 to yield the geminal frustrated Lewis pair 5 that bears pairs of strongly electron-withdrawing C6F5 substituents at both boron and phosphorus. The X-ray crystal structure analysis has revealed that there is no direct P–B interaction in 5. This has been confirmed by both solid state and solution NMR spectroscopy. The “non-interacting” geminal FLP 5 undergoes 1,2-P/B addition reactions to carbonyl compounds (benzaldehyde, trans-cinnamic aldehyde), to the CN bond of phenylisocyanate and to the CC triple bonds of several alkynes (1-pentyne, phenylacetylene, 4-octyne). The frustrated Lewis pair 5 and several reaction products were characterized by X-ray diffraction.

An Alkynylaminomethylaluminum Species with a Six-Membered Chair Conformation Al2C2N2 Framework: A New Path to Geminal N/Al Frustrated Lewis Pairs

The hydroalumination of the fused pentahydride aminocarbaaluminum species [(C6H3(iPr2-2,6)N(μ-AlH2)CH2)2(μ-AlH)NMe3] (1) with phenylacetylene resulted in the unexpected dimer [C6H3(iPr2-2,6)(*N)HCH2Al(CCPh)2]2 (2) with chiral centers at the nitrogen atoms. Compound 2 was characterized by X-ray structural analysis and contains geminal frustrated Lewis pairs.

Preorganized Frustrated Lewis Pairs

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu(2)PCH(2)BPh(2) with H(2), CO(2), and isocyanates and supported computationally.

ChemInform Abstract: Chemistry of a Geminal Frustrated Lewis Pair Featuring Electron Withdrawing C6F5 Substituents at Both Phosphorus and Boron.

AbstractThe phosphine borane (I) represents a new frustrated Lewis pair (FLP) with a decreased Lewis basicity on the phosphorus atom.

Chemistry of a geminal frustrated Lewis pair featuring electron withdrawing C6F5 substituents at both phosphorus and boron

Hydroboration of the electron poor phosphine (1-propenyl)P(C(6)F(5))(2) with Piers' borane [HB(C(6)F(5))(2)] gave the geminal frustrated Lewis pair (C(6)F(5))(2)P-CH(Et)-B(C(6)F(5))(2). It undergoes 1,2-addition reactions to an alkene and an alkyne and to the C=N bond of an isocyanate. With mesityl azide it undergoes a 1,3-addition reaction.