Synthesis and Reactivity of the [NCCCO]- Cyanoketenate Anion.

Cyanoketene is a fundamental molecule that is actively being searched for in the interstellar medium. Its deprotonated form (cyanoketenate) is a heterocumulene that is isoelectronic to carbon suboxide whose structure has been the subject of debate. However, the investigation of cyanoketene and its derivatives is hampered by the lack of practical synthetic routes to these compounds. We report the first synthesis of the cyanoketenate anion in [K(18-crown-6)][NCCCO] (1) as a stable molecule on a multigram scale in excellent yields (>90 %). The structure of this molecule is probed crystallographically and computationally. We also explore the protonation of 1, and its reaction with triphenylsilylchloride and carbon dioxide. In all cases, anionic dimers are formed. The cyanoketene could be synthesized and crystallographically characterized when stabilized by a N-heterocyclic carbene. The cyanoketenate is a very useful unsaturated building block containing N, C and O atoms that can now be explored with relative ease and will undoubtedly unlock more interesting reactivity.

Frustrated Lewis pair catalyzed hydrodehalogenation of benzyl-halides.

10 mol% B(2,6-C6F2H3)3 in the presence of excess tetramethylpiperidine (TMP) and H2 (or D2) is shown to catalyze the hydrogenative dehalogenation of benzyl-halides to give corresponding toluene derivatives. These reactions proceed via an initial FLP activation of H2 yielding the ammonium hydridoborate [TMPH][HB(2,6-C6F2H3)3]. This species acts in analogy to a FLP to cooperatively activate C-X bond (X = Cl, Br, I) of benzyl-halides delivering hydride and generating the corresponding ammonium halide salts.

Selective Catalytic Frustrated Lewis Pair Hydrogenation of CO2 in the Presence of Silylhalides.

The frustrated Lewis pair (FLP) derived from 2,6-lutidine and B(C6 F5 )3 is shown to mediate the catalytic hydrogenation of CO2 using H2 as the reductant and a silylhalide as an oxophile. The nature of the products can be controlled with the judicious selection of the silylhalide and the solvent. In this fashion, this metal-free catalysis affords avenues to the selective formation of the disilylacetal (R3 SiOCH2 OSiR3 ), methoxysilane (R3 SiOCH3 ), methyliodide (CH3 I) and methane (CH4 ) under mild conditions. DFT studies illuminate the complexities of the mechanism and account for the observed selectivity.

Steric Influence on Reactions of Benzyl Potassium Species with CO.

Reactions of benzyl potassium species with CO are shown to proceed via transient carbene-like intermediates that can undergo either dimerization or further CO propagation. In a sterically unhindered case, formal dimerization of the carbene is the dominant reaction pathway, as evidenced by the isolation of ((Ph3 SiO)(PhCH2 )C)2 2 and PhCH2 C(O)CH(OH)CH2 Ph 3. Reactions with increasingly sterically encumbered reagents show competitive reaction pathways involving intermolecular dimerization leading to species analogous to 2 and 3 and those containing newly-formed five-membered rings tBu2 C6 H2 (C(OSiR3 )C(OSiR3 )CH2 ) (R=Me 6, Ph 7). Even further encumbered reagents proceed to either dimerize or react with additional CO to give a ketene-like intermediates, thus affording a 7-membered tropolone derivative 14 or the dione (3,5-tBu2 C6 H3 )3 C6 H2 CH2 C(O))2 15.

Reactions of a Dilithiomethane with CO and N2 O: An Avenue to an Anionic Ketene and a Hexafunctionalized Benzene.

Synthesis of value-added products from simple C1 feedstocks is an attractive alternative avenue to traditional fossil fuels. Hexa-substituted benzene derivatives are highly useful molecules but are often challenging to prepare. Herein, we report that the lithium complex [(Ph2 P(S))2 CLi2 (THF)]2 1 reacts with CO lead to C-C bond formation and migration of a Ph2 P(S)-fragment affording 2. Subsequent reaction with N2 O results in oxidative cleavage of a P-C bond affording [Ph2 P(S)OLi(THF)2 ]2 4 and the anionic ketene-derivative Ph2 P(S)CCOLi(THF)2 5. Heating 5 prompts cyclotrimerization giving the hexa-substituted benzene derivative [Ph2 P(S)CCOLi(THF)2 ]3 6 regioselectively. This transition metal-free protocol to a hexa-substituted benzene is viable on a gram scale and permits the incorporation of 13 C labels. The mechanisms of these reactions are detailed via extensive DFT computations.

Facile Synthesis of Cyanide and Isocyanides from CO.

The reaction of K[N(SiMe3 )2 ] with 13 CO proceeds in C6 D6 or THF affording K13 CN and O(SiMe3 )2 under mild conditions as confirmed by crystallographic characterization of K(18-crown-6)CN. Similarly reaction of the alkali metal amides, M[N(SiR3 )R'] (M=Li, K; R=Ph, Me; R'=alkyl, aryl) provides the corresponding 13 C labeled isocyanide RN13 C and MOSiR3 , generally in high yields. In some instances, the use of the sterically bulky Ph3 Si-substituent is required to preclude 1,2-silyl migration affording the silylcarbamoyl salt M[Me3 SiC(O)NR']. These reactions have been used to obtain 19 examples of 13 C labelled isocyanides, and several examples of gram scale reactions are reported. The mechanism of the reactions is probed via reliable DFT calculations.

Lithium Dicyclohexylamide in Transition-Metal-Free Fischer-Tropsch Chemistry.

Lithium dicyclohexylamide (Cy2NLi) reacts with syn-gas or CO to generate transient intermediates with carbene character, which are capable of reacting further with CO or H2, effecting sequential C-C and C-H bond formations from CO or H2, thus providing a transition-metal-free avenue to the fundamental reactions of the Fischer-Tropsch process. Further experimental and computational data indicate that reactions with CO and H2 are thermodynamically accessible, with a kinetic bias toward CO homologation.

Diphospha-Ureas from the Phosphaketene Ph3 GePCO.

The phosphaketene Ph3 GePCO is shown to react with the phosphide KP(tBu)2 to generate the anion [Ph3 GePC(O)P(tBu)2 ]- 1. This species reacts with CH3 I or ClGePh3 to give the dissymmetric diphospha-ureas (tBu)2 PC(O)P(GePh3 )(CH3 ) 2 and (Ph3 Ge)2 PC(O)P(tBu)2 3 respectively. Sequential treatment of 2 with a base and CH3 I affords a route to (tBu)2 PC(O)P(CH3 )2 5. These species are products of the first modular diphospha-urea synthesis. The subsequent thermal and photochemical reactivity of these species was also probed and described.

Synthesis of Urea Derivatives from CO2 and Silylamines.

A series of thirty-three N,N'-diaryl, dialkyl, and alkyl-aryl ureas have been prepared in pyridine or toluene by reaction of silylamines with CO2 . This protocol is shown to provide facile access to 13 C-labeled ureas, as well as chiral and macrocyclic ureas. These reactions proceed through initial generation of the corresponding silylcarbamates, which subsequently react with silylamine under thermal conditions to afford the thermodynamically favored urea and disilyl ether.

Acyl-Phosphide Anions via an Intermediate with Carbene Character: Reactions of K[PtBu2 ] and CO.

The analogy of the reactivity of group 1 phosphides to that of FLPs is further demonstrated by reactions with CO, affording a new synthetic route to acyl-phosphide anions. The reaction of [K(18-crown-6)][PtBu2 ] (1) with CO affords [(18-crown-6)K⋅THF2 ][Z-tBuP=C(tBu)O] (2⋅THF2 ) as the major product, and the minor product [K6 (18-crown-6)][(tBu2 PCO)2 ]3 (3). Species 2 reacts with either BPh3 or additional CO to give [K(18-crown-6)][(Ph3 B)tBuPC(tBu)O] (4) and [K(18-crown-6)][(OCtBu)2 P] (5), respectively. The acyl-phosphide anion 2 is thought to be formed by a photochemically induced radical process involving a transient species with triplet carbene character, prompting 1,2-tert-butyl group migration. A similar process is proposed for the subsequent reaction of 2 with CO to give 5.

Alkali Metal Species in the Reversible Activation of H2.

MP(tBu)2 (M=Li, Na, K), KH and KN(SiMe3 )2 are shown to activate HD reversibly. In the case of MP(tBu)2 this leads to isotopic scrambling and the formation of H2 , D2 , H(D)P(tBu)2 and MH(D) in C6 D6 . In toluene, KP(tBu)2 reacts with H2 but also leads to isotopic scrambling into the methyl groups of the solvent toluene. DFT calculations reveal that these systems effect H2 activation via cooperative interactions with the Lewis acidic alkali metal and the basic phosphorus, carbanion, or hydride centres, mimicking frustrated Lewis pair (FLP) behaviour.

Stoichiometric Reactions of CO2 and Indium-Silylamides and Catalytic Synthesis of Ureas.

The indium compounds In(N(SiMe3 )2 )2 Cl⋅THF (2) and In(N(SiMe3 )2 )Cl2 ⋅(THF)n (3) were shown to react with CO2 to give [(Me3 Si)2 N)InX(µ-OSiMe3 )]2 (X=N(SiMe3 )2 4, Cl 5). 0.05-2.0 mol % of the species 3 acts as a pre-catalyst for the conversion of aryl and alkyl silylamines under CO2 (2-3 atm) to give the corresponding ureas in 70-99 % yields. A proposed mechanism is supported by experimental and computational data.

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

The Lewis acids Ga(C6F5)3, In(C6F5)3 and Ga(C6Cl5)3 are prepared and their Lewis acidity has been probed experimentally and computationally. The species Ga(C6F5)3 and In(C6F5)3 in conjunction with phosphine donors are shown to heterolytically split H2 and catalyse the hydrogenation of an imine. In addition, frustrated Lewis pairs (FLPs) derived from Ga(C6F5)3 and In(C6F5)3 and phosphines react with diphenyldisulfide to phosphoniumgallates or indates of the form [tBu3PSPh][PhSE(C6F5)3] and [tBu3PSPh][(µ-SPh)(E(C6F5)3)2] (E = Ga, In). The potential of the FLPs based on Ga(C6F5)3, In(C6F5)3 and Ga(C6Cl5)3 and phosphines is also shown in reactions with phenylacetylene to give pure or mixtures of the products [tBu3PH][PhCCE(C6X5)3] and R3P(Ph)C=C(H)E(C6X5)3 A number of these species are crystallographically characterized. The implications for the use of these species in FLP chemistry are considered.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Synthesis and reactivity of Li and TaMe3 complexes supported by N,N'-bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands.

The dilithium complex of N,N'-bis(2,6-diisopropylphenyl)-o-phenylenediamide, [Li2L(thf)3], reacts with TaMe3Cl2 in THF/Et2O to yield [Li(Et2O)(thf)LTaMe3Cl] in which the phenylene backbone of (2-) is bound η(4) to the Ta centre. This dinuclear species reacts with MeLi to yield the tetramethyltantalum complex [Li(Et2O)(thf)LTaMe4]. Double deprotonation of N,N'-bis(2,6-diisopropylphenyl)(4,5-dimethyl)-o-phenylenediamine (H2L') in Et2O yielded the dilithium complex [Li2L'(OEt2)2]. The two additional methyl groups on L'(2-) change the observed reactivity towards TaMe3Cl2: rather than bridging between Ta and Li, ligand oxidation occurs to afford mononuclear [LiL'(OEt2)]. This monolithium radical species, which was characterized by EPR spectroscopy, can also be synthesized using the more conventional oxidant AgBF4. Double deprotonation of H2L with KCH2Ph in toluene followed by reaction with TaMe3Cl2 furnished [TaLMe3]. Preliminary reactivity studies show [TaLMe3] reacts with unsaturated substrates N,N'-dicyclohexylcarbodiimide and mesityl azide to undergo migratory insertion into one of the Ta-C bonds: the corresponding amidinate and triazenido ligands are generated. When subjected to UV irradiation, [TaLMe3] undergoes reduction accompanied by loss of a methyl group to yield the dimeric species [TaLMe2]2.

Organocatalysts with carbon-centered activity for CO2 reduction with boranes.

We report two organocatalysts for CO2 hydroboration to methylborylethers, which upon hydrolysis can produce methanol. These organocatalysts feature carbon-centered reversible CO2 binding, broad borane scopes, and high catalytic activities.