Oxy-Borylenes as Photoreductants: Synthesis and Application in Dehalogenation and Detosylation Reactions.

While the range of accessible borylenes has significantly broadened over the last decade, applications remain limited. Herein, we present tricoordinate oxy-borylenes as potent photoreductants that can be readily activated by visible light. Facile oxidation of CAAC stabilized oxy-borylenes (CAAC)(IPr Me )BOR (R=TMS, CH CH C H , CH CH (4-F)C H ) to their corresponding radical cations is achieved with mildly oxidizing ferrocenium ion.

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts.

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect Al- P spin-spin coupling constants, and Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences.

Sterically constrained tricyclic phosphine: redox behaviour, reductive and oxidative cleavage of P-C bonds, generation of a dilithium phosphaindole as a promising synthon in phosphine chemistry.

The redox behaviour of sterically constrained tricyclic phosphine was investigated by spectroelectrochemistry. The data suggested a highly negative reduction potential with the reversible formation of a dianionic species. Accordingly, reacted with two equivalents of Li/naphthalene by reductive cleavage of a P-C bond of one of the PC heterocycles.

New Reactivity Patterns in 3H-Phosphaallene Chemistry [Aryl-P=C=C(H)-tBu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation.

3H-Phosphaallenes, R-P=C=C(H)C-R' (3), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH (SMe ) and 3 a (R=Mes*, R'=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms.

Tridentate Ligand with Three Carbanions as Donor Atoms: Formation of Dinuclear, Heptacyclic Complexes of Boron, Aluminum, or Gallium with B-C-B, Al-C-Al, or Ga-C-Ga Three-Center-Two-Electron Bonds.

The trilithium compound 1,3-[PhMeSi-C(Li)═C(H)]CHLi () reacted with BCl, AlCl, or GaCl by salt elimination to yield dinuclear heptacyclic compounds (). Two tridentate tricarbanionic ligands coordinate two B, Al, or Ga atoms and adopt a helical arrangement of the ligands around a central E unit. The unusual structures comprise EC heterocycles with two 3c-2e (three-center-two-electron) E-C-E bonds and two C atoms of aromatic rings in the bridging positions.

Aspects of Phosphaallene Chemistry: Heat-Induced Formation of 1,2-Dihydrophosphetes by Intramolecular Nucleophilic Aromatic Substitution and Photochemical Generation of Tricyclic Phosphiranes.

3H-Phosphaallenes are accessible on a new and facile route and show a fascinating chemical behavior. The thermally induced rearrangement of Mes*P═C═C(H)R' (R' = Bu, Ad) afforded by C-H activation, isobutene elimination, and C-C and P-H bond formation bicyclic 1-benzo-dihydrophosphetes () with PC heterocycles. DFT calculations suggest a mechanism with intramolecular nucleophilic aromatic substitution and replacement of an alkyl group by the nucleophilic α-C atom of the phosphaallene.

Al/P- and Ga/P-Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C-C and C-N Bond Formation.

Al/P- and Ga/P-based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C-C bond yielded an unusual nitrile-ylide adduct in which a C-N moiety coordinated to the FLP backbone. Cleavage of a C-N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms.

3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β-Elimination, Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms.

A facile method for the efficient synthesis of 3H-phosphaallenes, R-P=C=C(H)-R', is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl-alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe ) groups). Only supermesityl compounds are persistent at room temperature in solution.

Sterically Constrained Bicyclic Phosphines: A Class of Fascinating Compounds Suitable for Application in Small Molecule Activation and Coordination Chemistry.

Bicyclic phosphines with two annulated, electronically unsaturated five-membered heterocycles are available through facile routes. In most cases, their phosphorus atoms are bound to heteroatoms such as oxygen or nitrogen (PN or PN O), whereas homoleptic coordination by three sp -hybridized carbon atoms has been reported only recently. Steric strain causes unique reactivity.

A P-H functionalized Al/P-based frustrated Lewis pair - hydrophosphination of nitriles, ring opening with cyclopropenones and evidence of P[double bond, length as m-dash]C double bond formation.

Hydroalumination of R-P(H)-C[triple bond, length as m-dash]C-tBu with bulky H-Al[CH(SiMe3)2]2 afforded the new P-H functionalized Al/P-based frustrated Lewis pair R-P(H)-C[[double bond, length as m-dash]C(H)-tBu]-AlR2 [R = CH(SiMe3)2; FLP 7]. A weak adduct of 7 with benzonitrile (8) was detected by NMR spectroscopy, but could not be isolated. tert-Butyl isocyanide afforded a similar, but isolable adduct (9), in which the isocyanide C atom was coordinated to aluminium.

Reactions of an Aluminium/Phosphorus Frustrated Lewis Pair (FLP) with α,β-Unsaturated Carbonyl Compounds: FLPs as Efficient Two-Electron Reductants with the Formation of Enolates, a cis-Enediolate, and an Allene.

The Al/P-based frustrated Lewis pair (FLP) Mes P-C(AltBu) =C(H)Ph (1; Mes=mesityl) reacted as an efficient two-electron reductant with benzil to afford a cis-enediolate that was coordinated to the FLP through P-O and Al-O bonds and the formation of a seven-membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide).

A Sterically Constrained Tricyclic PC Phosphine: Coordination Behavior and Insertion of Chalcogen Atoms into P-C Bonds.

A tricyclic phosphine has been generated that has a rigid molecular backbone with the P atoms exclusively bound to C(sp ) atoms as well as a very large Tolman angle and buried volume. It is an interesting new ligand in coordination chemistry (Au, Pd complexes) and shows unusual insertion reactions into its endocyclic P-C bonds facilitated by its inherent molecular strain.

Halogenated triphenylgallium and -indium in frustrated Lewis pair activations and hydrogenation catalysis.

The Lewis acids Ga(CF), In(CF) and Ga(CCl) are prepared and their Lewis acidity has been probed experimentally and computationally. The species Ga(CF) and In(CF) in conjunction with phosphine donors are shown to heterolytically split H and catalyse the hydrogenation of an imine. In addition, frustrated Lewis pairs (FLPs) derived from Ga(CF) and In(CF) and phosphines react with diphenyldisulfide to phosphoniumgallates or indates of the form [BuPSPh][PhSE(CF)] and [BuPSPh][(μ-SPh)(E(CF))] (E = Ga, In).

Structure and reactivity of an Al/P-based frustrated Lewis pair bearing relatively small substituents at aluminium.

Reaction of MesP─C≡C─Ph (Mes = mesityl) with dineopentylaluminium hydride afforded by hydroalumination a geminal Al/P-based frustrated Lewis pair (FLP; ). Its steric shielding is relatively low, and its reactivity in various secondary reactions is less hindered by steric repulsion than observed for related compounds having bulkier groups attached to aluminium. FLP yielded adducts with MeC─NCO or benzaldehyde via the formation of Al-O and P-C bonds.

Facile Modulation of FLP Properties: A Phosphinylvinyl Grignard Reagent and Ga/P- and In/P -Based Frustrated Lewis Pairs.

An Al/P-based frustrated Lewis pair (FLP) reacted with PhMgCl by an unexpected transmetalation and formation of a phosphinylvinyl Grignard reagent. This compound is well suited for the transfer of the basic FLP component to other Lewis acidic metal atoms and allowed the generation of a Ga/P and an In/P FLP. The Ga FLP showed a behavior different to that of the corresponding Al FLP, the In FLP allowed the chelating coordination of an Au atom by Au-Cl bond activation.

Cooperative Activation of Isocyanates by Al-N-Based Active Lewis Pairs and the Generation of a C Chain by Simultaneous Formation of Two C-C Bonds.

The active Al/N Lewis pair, (Me C) Al-C(SiMe )=C(H)-N(CHMe-CH ) CH (2), reacted with isocyanates to afford a fascinating variety of products. One equivalent of Ph-N=C=O yielded by the release of H-C≡C-SiMe an urea-type ligand which coordinated the Al atom in a chelating manner (4). Dipp-N=C=O gave a similar product, but the bulky substituent hindered the approach of the N-aryl group to Al.

A Significant but Constrained Geometry Pt→Al Interaction: Fixation of CO2 and CS2, Activation of H2 and PhCONH2.

Reaction of the geminal PAl ligand [Mes2PC(═CHPh)AltBu2] (1) with [Pt(PPh3)2(ethylene)] affords the T-shape Pt complex [(1)Pt(PPh3)] (2). X-ray diffraction analysis and DFT calculations reveal the presence of a significant Pt→Al interaction in 2, despite the strain associated with the four-membered cyclic structure. The Pt···Al distance is short [2.

A P-H Functionalized Al/P Frustrated Lewis Pair: Substrate Activation and Selective Hydrogen Transfer.

Hydroalumination of an alkynylphosphine gave an unprecedented P-H functionalized frustrated Lewis pair (FLP). The reactive P-H group does not influence the typical FLP properties, but the activation of substrates follows a new reaction pattern involving hydrogen transfer to yield unusual compounds with phosphaurea, iminophosphine, or phosphanyltriazene structural motifs.

Functionalized alkynyl-chlorogermanes: hydrometallation, Ge-Cl bond activation, Ge-H bond formation and chlorine-tert-butyl exchange via a transient germyl cation.

Treatment of alkynyl-arylchlorogermanes ArylnGe(Cl)(C[triple bond, length as m-dash]C-(t)Bu)3-n (n = 1, 2) with HM(t)Bu2 (M = Al, Ga) yielded mixed Al or Ga alkenyl-alkynylchlorogermanes via hydrometallation reactions. Intramolecular interactions between the Lewis-basic Cl atoms and the Lewis-acidic Al or Ga atoms afforded MCGeCl heterocycles. The endocyclic M-Cl distances were significantly lengthened compared to the starting compounds and indicated Ge-Cl bond activation.

Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P-N bond activation, a transient phosphaallene, a zwitterionic AlP2C2 heterocycle and a masked Al/P-based frustrated Lewis pair.

Treatment of the new alkynyl-chlorophosphine, Mes-P(Cl)-C ≡ C-CMe3, with LiNR2 afforded various unprecedented aminophosphines, Mes-P(NR2)-C ≡ C-CMe3, which showed a fascinating diversity in their reactivity towards H-Al(t)Bu2. NMe2 and NEt2 derivatives yielded the hydroalumination products Mes-P(NR2)-C(Al(t)Bu2) = C(H)-CMe3 which have an Al-N and an activated P-N bond. Elimination of aluminium amide yielded the transient 3H-phosphaallene, Mes-P = C = C(H)-CMe3, which finally afforded a five-membered AlP2C2 heterocycle with an Al-P bond and two exocyclic C = C bonds.

Alkynyl functionalized Al/P-based frustrated Lewis pairs - aluminium alkynide elimination and evidence for the formation of 3H-phosphaallenes [R-P=C=C(H)-(t)Bu].

Hydroalumination of dialkynylphosphines, aryl-P(C≡C-(t)Bu)2 (aryl = 2,4,6-Me3C6H2 (1), Ph (2)), with the bulky dialkylaluminium hydride H-Al[CH(SiMe3)2]2 afforded mixed alkenyl-alkynyl phosphines (3 and 4) with aluminium atoms in geminal positions to phosphorus. These compounds contain coordinatively unsaturated aluminium and phosphorus atoms and may be applicable as alkynyl functionalised frustrated Lewis pairs. Their unique constitution with dialkylaluminium and alkynyl groups in close proximity favours unusual secondary reactions with the elimination of an aluminium alkynide and formation of reactive 3H-phosphaallenes, aryl-P=C=C(H)-(t)Bu (5 and 6), which depending on the steric shielding by the aryl groups, oligomerize at room temperature over days or weeks.

Hydroalumination of Ketenimines and Subsequent Reactions with Heterocumulenes: Synthesis of Unsaturated Amide Derivatives and 1,3-Diimines.

The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom.

Germacyclobutenes: generation by 1,1-carbalumination or 1,1-carbagallation and their photophysical properties.

Aluminium- and gallium-functionalised alkenylalkynylgermanes, R(1) 2 Ge(C≡C-R(2) )[C{E(CMe3 )2 }=C(H)-R(2) ] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α-C atoms of the intact ethynyl groups. These interactions activate the Ge-C(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the E-C(vinyl) bonds by means of 1,1-carbalumination or 1,1-carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes.

Cooperative Ge-N Bond activation in aluminium-functionalised aminogermanes and spontaneous imine elimination via an intermediate germyl cation.

Hydrometallation of iPr2 N-Ge(CMe3 )(C≡C-CMe3 )2 with H-M(CMe3 )2 (M=Al, Ga) affords alkenyl-alkynylgermanes in which the Lewis-acidic metal atoms are not coordinated by the amino N atoms but by the α-C atoms of the ethynyl groups. These interactions result in a lengthening of the Ge-C bonds by approximately 10 pm and a comparably strong deviation of the Ge-CC angle from linearity (154.3(1)°).