Metal-Free B-B Dehydrocoupling Reaction of a Simple Borane Adduct: Convenient Access to a Nucleophilic Diborane(4).

The selective formation of homonuclear bonds is of key importance in synthetic chemistry. Especially, dehydrocoupling reactions are attractive as ecologically and economically friendly alternatives to established reductive bond forming reactions, since they do not require the use of stoichiometric amounts of a reducing reagent and produce only valuable dihydrogen as by-product. Here, we report on a metal-free B-B dehydrocoupling reaction that starts directly from a simple, easily accessible BH3 adduct, providing convenient access to a new nucleophilic dihydridodiborane in excellent yield. The dihydridodiborane in turn activates dihydrogen, allowing to obtain quantitatively the dideuteridodiborane from the dihydridodiborane by D2 activation. On the basis of detailed quantum-chemical calculations, the mechanism of this unprecedented reaction is elucidated. Some key points that are essential for metal-free dehydrocoupling are disclosed, paving the way for their systematic evaluation and application.

A Paramagnetic Nickel-Zinc Hydride Complex.

Reaction of a molecular zinc-hydride [{(ArNCMe)2CH}ZnH] (Ar = 2,6-di-isopropylphenyl) with 0.5 equiv. of [Ni(CO)Cp]2 led to the isolation of a nickel-zinc hydride complex containing a bridging 3-centre,2-electron Ni-H-Zn interaction. This species has been characterized in the solid-state by single crystal X-ray diffraction. DFT calculations are consistent with its formulation as a σ-complex derived from coordination of the zinc-hydride to a paramagnetic nickel(I) fragment. Continuous wave and pulse EPR experiments suggest that this species is not stable in solution and suggest that this species is labile in solution. Further experiments show that in the presence of catalytic quantities of nickel(I) precursors, zinc-hydride bonds can undergo either H/D-exchange with D2 or dehydrocoupling to form Zn-Zn bonds. In combination, the data support the activation and functionalisation of zinc-hydride bonds at nickel(I) centres.

Electrophilic Hydrosilylation of Electron-Rich Alkenes Derived from Enamines.

The low-electron count, air-stable, platinum complexes [Pt(ItBu')(ItBu)][BArF] (C1) (ItBu=1,3-di-tert-butylimidazol-2-ylidene), [Pt(SiPh)3(ItBuiPr)2][BArF] (C2) (ItBuiPr=1-tert-butyl-3-iso-propylimidazol-2-ylidene), [Pt(SiPh)3(ItBuMe)2][BArF] (C3), [Pt(GePh3)(ItBuiPr)2][BArF] (C4), [Pt(GePh)3(ItBuMe)2][BArF] (C5) and [Pt(GeEt)3(ItBuMe)2][BArF] (C6) (ItBuMe=1-tert-butyl-3-methylimidazol-2-ylidene) are efficient catalysts (particularly the germyl derivatives) in both the silylative dehydrocoupling and hydrosilylation of electron rich alkenes derived from enamines. The steric hindrance exerted by the NHC ligand plays an important role in the selectivity of the reaction. Thus, bulky ligands are selective towards the silylative dehydrocoupling process whereas less sterically hindered promote the selective hydrosilylation reaction. The latter is, in addition, regioselective towards the ß-carbon atom of both internal and terminal enamines, leading to ß-aminosilanes. Moreover, the syn stereochemistry of the amino and silyl groups implies an anti Si-H bond addition across the double bond. All these facts point to a mechanistic picture that, according to experimental and computational studies, involves a non-classical hydrosilylation process through an outer-sphere mechanism in which a formal nucleophilic addition of the enamine to the silicon atom of a platinum σ-SiH complex is the key step. This is in sharp contrast with the classical Chalk-Harrod mechanism prevalent in platinum chemistry.

Transition-Metal-Free Dehydropolymerization of Phosphine-Boranes at Ambient Temperature.

Stoichiometric reaction of phosphine-borane adducts RR'PH⋅BH3 (R=Ph, R'=H, Ph, Et, and R=R'=t Bu) with the strong acid HNTf2 (Tf=SO2 CF3 ) leads to H2 elimination and the formation of the triflimido derivatives, RR'PH⋅BH2 (NTf2 ). Subsequent deprotonation by using bases, such as diisopropylethylamine or the carbene IPr (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), led to the formation of P-mono- or -disubstituted polyphosphinoboranes [RR'P-BH2 ]n . Evidence for the intermediacy of transient phosphinoborane monomers, RR'PBH2 , was provided by trapping reactions.

Dehydrocoupling Polymerization: Poly(silylether) Synthesis by Using an Iron ß-Diketiminate Catalyst.

We describe the iron-catalyzed polymerizations of diol and silane monomers to obtain fourteen different poly(silylether) products with number average molecular weights (Mn ) up to 36.3 kDa. The polymerization reactions developed in this study are operationally simple and applicable to 1° and 2° silane monomer substrates and a range of benzylic and aliphatic diol substrates as well as one polyol example. The polymers were characterized by IR spectroscopy, DSC and TGA and, where solubility allowed, 1 H, 13 C{1 H}, 29 Si{1 H} NMR spectroscopies, GPC and MALDI-TOF were also employed. The materials obtained displayed low Tg values (-70.6 to 19.1 °C) and were stable upon heating up to T-5%,Ar 421.6 °C. A trend in T-5%,Ar was observed whereby use of a 2° silane leads to higher T-5%,Ar compared to those obtained using a 1° silane. Reaction monitoring was undertaken by in situ gas evolution studies coupled with GPC analysis to follow the progression of chain-length growth which confirmed a condensation polymerization-type mechanism.

Two Faces of the Bi-O Bond: Photochemically and Thermally Induced Dehydrocoupling for Si-O Bond Formation.

The diorgano(bismuth)alcoholate [Bi((C6H4CH2)2S)OPh] (1-OPh) has been synthesized and fully characterized. Stoichiometric reactions, UV/Vis spectroscopy, and (TD-)DFT calculations suggest its susceptibility to homolytic and heterolytic Bi-O bond cleavage under given reaction conditions. Using the dehydrocoupling of silanes with either TEMPO or phenol as model reactions, the catalytic competency of 1-OPh has been investigated (TEMPO=(tetramethyl-piperidin-1-yl)-oxyl). Different reaction pathways can deliberately be addressed by applying photochemical or thermal reaction conditions and by choosing radical or closed-shell substrates (TEMPO vs. phenol). Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single-crystal X-ray diffraction analysis, and (TD)-DFT calculations.

Selective Benzylic CH-Borylations by Tandem Cobalt Catalysis.

Metal-catalyzed C-H activations are environmentally and economically attractive synthetic strategies for the construction of functional molecules as they obviate the need for pre-functionalized substrates and minimize waste generation. Great challenges reside in the control of selectivities, the utilization of unbiased hydrocarbons, and the operation of atom-economical dehydrocoupling mechanisms. An especially mild borylation of benzylic CH bonds was developed with the ligand-free pre-catalyst Co[N(SiMe3 )2 ]2 and the bench-stable and inexpensive borylation reagent B2 pin2 that produces H2 as the only by-product. A full set of kinetic, spectroscopic, and preparative mechanistic studies are indicative of a tandem catalysis mechanism of CH-borylation and dehydrocoupling via molecular CoI catalysts.

Magnesium-Catalyzed Tandem Dehydrogenation-Dehydrocoupling: An Atom Economical Access to Alkynylboranes.

Owing to the unusual reactivity of sterically hindered amine-borane complexes, a catalytic system based on magnesium salts was designed to perform a tandem dehydrogenation-dehydrocoupling between terminal alkynes and boranes. The reaction is providing pure alkynylboranes within few minutes at room temperature, with only two molecules of hydrogen as a byproduct.

Well-Defined, Molecular Bismuth Compounds: Catalysts in Photochemically Induced Radical Dehydrocoupling Reactions.

A series of diorgano(bismuth)chalcogenides, [Bi(di-aryl)EPh], has been synthesised and fully characterised (E=S, Se, Te). These molecular bismuth complexes have been exploited in homogeneous photochemically-induced radical catalysis, using the coupling of silanes with TEMPO as a model reaction (TEMPO=(tetramethyl-piperidin-1-yl)-oxyl). Their catalytic properties are complementary or superior to those of known catalysts for these coupling reactions. Catalytically competent intermediates of the reaction have been identified. Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single-crystal X-ray diffraction analysis, and (TD)-DFT calculations.

Magnesocenophane-Catalyzed Amine Borane Dehydrocoupling.

The Lewis acidities of a series of [n]magnesocenophanes (1 a-d) have been investigated computationally and found to be a function of the tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1 a) has been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethylamine borane (2 a) and diisopropylamine borane (2 b) under ambient conditions. Furthermore, the mechanism of the reaction with 2 a has been investigated experimentally and computationally, and the results imply a ligand-assisted mechanism involving stepwise proton and hydride transfer, with dimethylaminoborane as the key intermediate.

Heavier Alkaline-Earth Catalyzed Dehydrocoupling of Silanes and Alcohols for the Synthesis of Metallo-Polysilylethers.

The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [Ae{N(SiMe3 )2 }2 ⋅(THF)2 ] (I-III) and [Ae{CH(SiMe3 )2 }2 ⋅(THF)2 ], (IV-VI) (Ae=Ca, Sr, Ba) is described. Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes are less tolerant towards bulky substrates. Some control over reaction selectivity towards mono-, di-, or tri-substituted silylether products was achieved through alteration of reaction stoichiometry, conditions, and catalyst. The ferrocenyl silylether, FeCp(C5 H4 SiPh(OBn)2 ) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C5 H4 SiPhH2 ) (1), and benzyl alcohol using barium catalysis. Stoichiometric experiments suggested a reaction manifold involving the formation of Ae-alkoxide and hydride species, and a series of dimeric Ae-alkoxides [(Ph3 CO)Ae(µ2 -OCPh3 )Ae(THF)] (3 a-c, Ae=Ca, Sr, Ba) were isolated and fully characterized. Mechanistic experiments suggested a complex reaction mechanism involving dimeric or polynuclear active species, whose kinetics are highly dependent on variables such as the identity and concentration of the precatalyst, silane, and alcohol. Turnover frequencies increase on descending Group 2 of the periodic table, with the barium precatalyst III displaying an apparent first-order dependence in both silane and alcohol, and an optimum catalyst loading of 3 mol % Ba, above which activity decreases. With precatalyst III in THF, ferrocene-containing poly- and oligosilylethers with ferrocene pendent to- (P1-P4) or as a constituent (P5, P6) of the main polymer chain were prepared from 1 or Fe(C5 H4 SiPhH2 )2 (4) with diols 1,4-(HOCH2 )2 -(C6 H4 ) and 1,4-(CH(CH3 )OH)2 -(C6 H4 ), respectively. The resultant materials were characterized by NMR spectroscopy, gel permeation chromatography (GPC) and DOSY NMR spectroscopy, with estimated molecular weights in excess of 20,000 Da for P1 and P4. The iron centers display reversible redox behavior and thermal analysis showed P1 and P5 to be promising precursors to magnetic ceramic materials.

Modification of Yttrium Silyl-Bridged Amide Alkyl Complexes through Si-H/C-H Cross-Dehydrocoupling of Silanes with a Silylamino Ligand: Synthesis, Reactivity, and Mechanism.

A new method for the modification of a silylamino ligand has been developed through mono and dual C(sp3 )-H/Si-H cross-dehydrocoupling with silanes. The reaction of [LY{η2 -(C,N)-CH2 Si(Me2 )NSiMe3 }] (L=bis(2,6-diisopropylphenyl)-ß-diketiminato, L' (1L '); L=tris(3,5-dimethylpyrazolyl)borate, TpMe2 (1TpMe2 )) with 2 equivalents of PhSiH3 in toluene gave the complexes [LY{η2 -(C,N)-C(SiH2 Ph)2 Si(Me2 )NSiMe3 }] (L=L' (2L' ); L=TpMe2 (2TpMe2 )). Moreover, 1TpMe2 reacted with the secondary silanes Ph2 SiH2 and Et2 SiH2 to afford the corresponding mono C-H activation products [TpMe2 Y{η2 -(C,N)-CH(SiHR2 )Si(Me2 )NSiMe3 }] (R=Ph (4 b); R=Et (4 c)). The equimolar reaction of 1TpMe2 with PhSiH3 also produced the mono C-H activation product 4 a ([TpMe2 Y{η2 -(C,N)-CH(SiH2 Ph)Si(Me2 )NSiMe3 }(thf)]). A study of their reactivity showed that4 a facilely reacted with 2 equivalents of benzothiazole by an unusual 1,1-addition of the C=N bond of the benzothiazolyl unit to the Si-H bond to give the C-H/Si-H cross-dehydrocoupling product [(TpMe2 )Y{η3 -(N,N,N)-N(SiMe3 )SiMe2 CH2 Si(Ph)(CSC6 H4 N)(CHSC6 H4 N)}] (5). These results indicate that this modification endows the silylamino ligand with novel reactivity.

Boron-Boron Dehydrocoupling of Boranes Initiated by Reaction with Iodine.

A new boron-boron dehydrocoupling strategy was established, providing convenient access to some diborane(4) compounds starting from simple borane adducts under mild conditions. In contrast to the traditional pathway using a reducing reagent, the reduction from BIII to BII was paradoxically initiated by the addition of the oxidation-reagent iodine. A reaction pathway for this unusual reaction was proposed based on quantum-chemical calculations.

A Highly Active Bidentate Magnesium Catalyst for Amine-Borane Dehydrocoupling: Kinetic and Mechanistic Studies.

A magnesium complex (1) featuring a bidentate aminopyridinato ligand is a remarkably selective catalyst for the dehydrocoupling of amine-boranes. This reaction proceeds to completion with low catalyst loadings (1 mol %) under mild conditions (60 °C), exceeding previously reported s-block systems in terms of selectivity, rate, and turnover number (TON). Mechanistic studies by in situ NMR analysis reveals the reaction to be first order in both catalyst and substrate. A reaction mechanism is proposed to account for these findings, with the high TON of the catalyst attributed to the bidentate nature of the ligand, which allows for reversible deprotonation of the substrate and regeneration of 1 as a stable resting state.

Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling.

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH2 ] (Dur=2,3,5,6-Me4 C6 H) and [(Me3 Si)2 NBH2 ] led to the formation of bridging borylene complexes of the form [(Cp*RuH)2 BR] (Cp*=C5 Me5 ; 1 a: R=Dur; 1 b: R=N(SiMe3 )2 ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF3 )2 -C6 H3 BH2 ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)4 [(Cp*Ru)2 B4 H5 (3,5-(CF3 )2 C6 H3 )4 ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)2 (µ-H)B4 H9 ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B4 chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.

Barium Siloxides and Catalysed Formation of Si-O-Si' Motifs.

The first unsupported barium siloxide, the homoleptic dimer [Ba2 {µ2 -OSi(SiMe3 )3 }3 {OSi(SiMe3 )3 }], is presented, and its structural features are discussed in the light of DFT computations. This complex, together with the related [Ba{µ2 -OSi(SiMe3 )3 }{N(SiMe3 )2 }]2 and their parent [Ba{N(SiMe3 )2 }2 ]2 , mediates the formation of asymmetric siloxanes R3 Si-O-SiR'3 through the first case of main group metal-mediated dehydrocoupling of silanols and hydrosilanes. Early kinetic analysis highlights an unusual catalytic manifold.

Spontaneous trans-Selective Transfer Hydrogenation of Apolar Boron-Boron Double Bonds.

The transfer hydrogenation of N-heterocyclic carbene (NHC)-supported diborenes with dimethylamine borane proceeds with high selectivity for the trans-1,2-dihydrodiboranes. DFT calculations, supported by kinetic studies and deuteration experiments, suggest a stepwise proton-first-hydride-second reaction mechanism via an intermediate µ-hydrodiboronium dimethylaminoborate ion pair.

Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane.

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp'FeI]2 (1) (Cp'=η5 -((1,2,4-tBu)3 C5 H2 )) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp'Fe(η6 -Tol)][Cp'FeI2 ] (2) (Tol=C6 H5 Me), [Cp'Fe(η6 -Tol)][BArF4 ] (3) (BArF4 =[B(C6 H3 (m-CF3 )2 )4 ]- ), [N(nBu)4 ][Cp'FeI2 ] (4), Cp'FeI2 (5), and [Cp'Fe(MeCN)3 ][BArF4 ] (6). The electronic structure of the [Cp'FeI2 ]- anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me2 NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2 N-BH2 ]2 (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me2 N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2 NH-BH2 -NMe2 -BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3 N⋅BH3 were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ2 -H2 BH⋅NMe2 H][BArF4 ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

Low-Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes.

The first soluble barium boryloxides [Ba]- OB{CH(SiMe3 )2 } are presented. These mono- or dinuclear complexes feature low coordination numbers, as low as two for [Ba(OB{CH(SiMe3 )2 }2 )2 ], which is further stabilized by intra- and intermolecular Ba⋅⋅⋅H3 C agostic interactions. Barium boryloxides and the parent [Ba{N(SiMe3 )2 }2 ⋅(thf)2 ] catalyze the dehydrocoupling of borinic acids with hydrosilanes, providing borasiloxanes under mild conditions.

1-Alkali-metal-2-alkyl-1,2-dihydropyridines: Soluble Hydride Surrogates for Catalytic Dehydrogenative Coupling and Hydroboration Applications.

Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-tert-butyl-1,2-dihydropyridine (1tLi) functions as a precatalyst to convert Me2 NH⋅BH3 to [NMe2 BH2 ]2 (89 % conversion) under competitive conditions (2.5 mol %, 60 h, 80 °C, toluene solvent) to that of previously reported LiN(SiMe3 )2 . Sodium and potassium dihydropyridine congeners produce similar high yields of [NMe2 BH2 ]2 but require longer times. Switching the solvent to pyridine induces a remarkable change in the dehydrocoupling product ratio, with (NMe2 )2 BH favoured over [NMe2 BH2 ]2 (e.g., 94 %:2 % for 1tLi). Demonstrating its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes, though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of Group 1 metal-catalysed processes are discussed.