Catalytic HF Shuttling between Fluoroalkanes and Alkynes.

In this paper, we report BF3 ⋅ OEt2 as a catalyst to shuttle equivalents of HF from a fluoroalkane to an alkyne. Reactions of terminal and internal aliphatic alkynes led to formation of difluoroalkane products, while diarylalkynes can be selectively converted into fluoroalkenes. The method tolerates numerous sensitive functional groups including halogen, protected amine, ester and thiophene substituents. Mechanistic studies (DFT, probe experiments) suggest the catalyst is involved in both the defluorination and fluorination steps, with BF3 acting as a Lewis acid and OEt2 a weak Lewis base that mediates proton transfer. In certain cases, the interconversion of fluoroalkene and difluoroalkane products was found to be reversible. The new catalytic system was applied to demonstrate proof-of-concept recycling of poly(vinylidene difluoride).

Isolation of a Reactive Tricoordinate α-Oxo Gold Carbene Complex.

The [(P,P)Au=C(Ph)CO2 Et]+ complex 3 [where (P,P) is an o-carboranyl diphosphine ligand] was prepared by diazo decomposition at -40 °C. It is the first α-oxo gold carbene complex to be characterized. Its crystallographic structure was determined and DFT calculations have been performed, unraveling the key influence of the chelating (P,P) ligand. The gold center is tricoordinate and the electrophilicity of the carbene center is decreased. Complex 3 mimics transient α-oxo gold carbenes in a series of catalytic transformations, and provides support for the critical role of electrophilicity in the chemoselectivity of phenol functionalization (O-H vs. C-H insertion).

A Nucleophilic Gold(III) Carbene Complex.

The first AuIII carbene complex was prepared by reacting a geminal dianion with a (P,C) cyclometalated AuIII precursor. Its structure and bonding situation have been thoroughly investigated by experimental and computational means. The presence of a high-energy highest occupied molecular orbital (HOMO) centered at the carbene center suggests nucleophilic character for the AuIII carbene complex. This unprecedented feature was confirmed by reactions with two electrophiles (PhNCS and CS2 ), resulting in two types of C=C coupling reactions.

ß-Hydride Elimination at Low-Coordinate Gold(III) Centers.

This Article reports the first comprehensive study of ß-hydride elimination at gold(III). The stability/fate of gold(III) alkyl species have been investigated experimentally and computationally. A series of well-defined cationic cyclometalated gold(III) alkyl complexes [(P,C)gold(III)(R)][NTf2] [(P,C) = 8-diisopropylphosphino-naphthyl; R = Me, nPr, nBu] have been synthesized and spectroscopically characterized. While the cationic gold(III) methyl derivative 3c is stable for days at room temperature, the gold(III) n-propyl and n-butyl complexes 3a,b readily undergo ß-hydride elimination at low temperature to generate propylene and 2-butenes, respectively. The formation of internal olefins from the gold(III) n-butyl complex 3b shows that olefin isomerization takes place after ß-hydride elimination. Computational studies indicate that this isomerization proceeds through a chain-walking mechanism involving a highly reactive gold(III) hydride intermediate and a sequence of ß-hydride elimination/reinsertion into the Au-H bond. The reaction of the cationic gold(III) methyl complex 3c with ethylene was also explored. According to (1)H and (13)C NMR spectroscopy, a mixture of propylene, 1-butene, and 2-butenes is formed. DFT calculations provide detailed mechanistic insights and support the occurrence of migratory insertion of ethylene, ß-hydride elimination, and olefin exchange at gold(III).

Experimental and Theoretical Evidence for an Agostic Interaction in a Gold(III) Complex.

The first agostic interaction in a gold complex is described. The presence of a bonding C-H⋅⋅⋅Au interaction in a cationic "tricoordinate" gold(III) complex was suggested by DFT calculations and was subsequently confirmed by NMR spectroscopy at low temperature. The agostic interaction was analyzed computationally using NBO and QTAIM analyses (NBO=natural bond orbital; QTAIM=quantum theory of atoms in molecules).

Cationic gold(III) alkyl complexes: generation, trapping, and insertion of norbornene.

Migratory insertion of alkenes into gold-carbon bonds, a fundamental yet unprecedented organometallic transformation, has been investigated from a discrete (P,C) cyclometalated gold(III) dimethyl complex. Methide abstraction by B(C6F5)3 is shown to generate a highly reactive cationic Au(III) complex that evolves spontaneously by C6F5 transfer from boron. In the presence of norbornene, migratory insertion into the Au-C bond proceeds readily. The resulting norbornyl complex is efficiently trapped with pyridines or chloride to give stable four-coordinate adducts.

Catalytic version of enediyne cobalt-mediated cycloaddition and selective access to unusual bicyclic trienes.

Go cyclic! The use of [Co(H)(PMe3)4] as a cobalt catalyst allows the previously unattainable catalytic version of the cobalt-mediated cycloaddition of enediynes without the requirement of thermal or light activation (see scheme). The importance of a chelating group on the substrate that can selectively direct the reaction pathway toward the classical polycyclic 1,3-cyclohexadienes or a new family of bicyclic trienes is also demonstrated.

Stereoselective insertion of cyclopropenes into Mg-Mg bonds.

The reaction of cyclopropenes with compounds containing Mg-Mg bonds is reported. 1,2-Dimagnesiation occurs exclusively by syn-addition to the least hindered face of the alkene forming a single diastereomeric product. DFT calculations support a concerted and stereoselective mechanism. These findings shed new light on the stereochemistry of reactions involving magnesium reagents.

Alumination of aryl methyl ethers: switching between sp2 and sp3 C-O bond functionalisation with Pd-catalysis.

The reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) with aryl methyl ethers proceeded with alumination of the sp3 C-O bond. The selectivity of this reaction could be switched by inclusion of a catalyst. In the presence of [Pd(PCy3)2], chemoselective sp2 C-O bond functionalisation was observed. Kinetic isotope experiments and DFT calculations support a catalytic pathway involving the ligand-assisted oxidative addition of the sp2 C-O bond to a Pd-Al intermetallic complex.

Palladium-catalysed C-F alumination of fluorobenzenes: mechanistic diversity and origin of selectivity.

A palladium pre-catalyst, [Pd(PCy3)2] is reported for the efficient and selective C-F alumination of fluorobenzenes with the aluminium(i) reagent [{(ArNCMe)2CH}Al] (1, Ar = 2,6-di-iso-propylphenyl). The catalytic protocol results in the transformation of sp2 C-F bonds to sp2 C-Al bonds and provides a route to reactive organoaluminium complexes (2a-h) from fluorocarbons. The catalyst is highly active. Reactions proceed within 5 minutes at 25 °C (and at appreciable rates at even -50 °C) and the scope includes low-fluorine-content substrates such as fluorobenzene, difluorobenzenes and trifluorobenzenes. The reaction proceeds with complete chemoselectivity (C-F vs. C-H) and high regioselectivities (>90% for C-F bonds adjacent to the most acidic C-H sites). The heterometallic complex [Pd(PCy3)(1)2] was shown to be catalytically competent. Catalytic C-F alumination proceeds with a KIE of 1.1-1.3. DFT calculations have been used to model potential mechanisms for C-F bond activation. These calculations suggest that two competing mechanisms may be in operation. Pathway 1 involves a ligand-assisted oxidative addition to [Pd(1)2] and leads directly to the product. Pathway 2 involves a stepwise C-H → C-F functionalisation mechanism in which the C-H bond is broken and reformed along the reaction coordinate, guiding the catalyst to an adjacent C-F site. This second mechanism explains the experimentally observed regioselectivity. Experimental support for this C-H activation playing a key role in C-F alumination was obtained by employing [{(MesNCMe)2CH}AlH2] (3, Mes = 2,4,6-tri-methylphenyl) as a reagent in place of 1. In this instance, the kinetic C-H alumination intermediate could be isolated. Under catalytic conditions this intermediate converts to the thermodynamic C-F alumination product.

Catalyst control of selectivity in the C-O bond alumination of biomass derived furans.

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C-O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp2 C-O bond into a sp2 C-Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH ‡ = +19.7 (±2.7) kcal mol-1, ΔS ‡ = -18.8 (±7.8) cal K-1 mol-1 and ΔG ‡ 298 K = +25.3 (±0.5) kcal mol-1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C-O bond through population of the σ*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C-O bond that breaks is that adjacent to the sp2C-H bond. The aluminium(iii) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C-O bond functionalisation occurs via an initial C-H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C-H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C-H bond to the palladium catalyst. Isomerisation of the kinetic C-H aluminated product to the thermodynamic C-O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C-O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass.

Gold(iii)-arene complexes by insertion of olefins into gold-aryl bonds.

The synthesis and characterization of the first gold(iii)-arene complexes are described. Well-defined (P,C)-cyclometalated gold(iii)-aryl complexes were prepared and characterized by NMR spectroscopy. These complexes swiftly and cleanly reacted with norbornene and ethylene to provide cationic gold(iii)-alkyl complexes, in which the remote phenyl ring was η2-coordinated to gold. The interaction between the aromatic ring and the gold(iii) center was thoroughly analyzed by NMR spectroscopy, X-ray diffraction, and DFT calculations. The π-arene coordination was found to significantly influence the stability and reactivity of low coordinated gold(iii) alkyl species.