Facile Protocol for Catalytic Frustrated Lewis Pair Hydrogenation and Reductive Deoxygenation of Ketones and Aldehydes.

A series of ketones and aldehydes are reduced in toluene under H2 in the presence of 5 mol % B(C6F5)3 and either cyclodextrin or molecular sieves affording a facile metal-free protocol for reduction to alcohols. Similar treatment of aryl ketones resulted in metal-free deoxygenation yielding aromatic hydrocarbons.

Electrophilic phosphonium cations catalyze hydroarylation and hydrothiolation of olefins.

Electrophilic phosphonium cations (EPCs) are efficient main group catalysts for the hydroarylation of olefins under mild conditions, providing a facile route to substituted aniline, bis-arylamine, phenol, furan, thiophene, pyrrole, and indole derivatives. Similarly, EPCs catalyze the hydrothiolation of aryl olefins with thiophenol affording a series of alkyl aryl thioethers. Experimental data support a mechanism for these reactions that involves initial activation of the olefin.

Stoichiometric and catalytic inter- and intramolecular hydroamination of terminal alkynes by frustrated Lewis pairs.

Frustrated Lewis pairs (FLPs) based on sterically encumbered anilines and the Lewis acid B(C6 F5 )3 were found to react with terminal alkynes effecting intermolecular hydroamination affording iminium alkynylborate species of the form [RPhNC(R')Me][R'CCB(C6 F5 )3 ]. In these cases, the reagent ratio of borane, aniline, and alkyne is 1:1:2. These reactions could also be performed in an intramolecular fashion by using anilines with alkynyl substituents effecting cyclization reactions. The use of 10 mol % B(C6 F5 )3 under a H2 atmosphere provides a one-pot synthesis of the pyrrolidine 12, the piperidines 13-15, the azepane 16, the isoindoline 17, and the benzoxazine 18.

B(C6F5)3 mediated arene hydrogenation/transannulation of para-methoxyanilines.

The stoichiometric reaction of para-methoxyanilines and B(C6F5)3 under H2 results in reduction of the N-bound phenyl ring(s), and subsequent transannular ring closure with elimination of methanol, affording the respective 7-azabicyclo[2.2.1]heptane derivatives.

Enabling catalytic ketone hydrogenation by frustrated Lewis pairs.

Hydrogenation of alkyl and aryl ketones using H2 is catalytically achieved in 18 examples using 5 mol % B(C6F5)3 in an ethereal solvent. In these cases the borane and ether behave as a frustrated Lewis pair to activate H2 and effect the reduction.

Frustrated Lewis pair-mediated C-O or C-H bond activation of ethers.

Protocols for the FLP-mediated transformation of ethers are presented. Distinct reaction pathways involving either C-O or C-H bond activation occur depending on the application of oxophilic B(C6F5)3 or hydridophilic tritylium ions as the Lewis acid.

Frustrated Lewis pair catalyzed hydroamination of terminal alkynes.

Catalytic amounts of the Lewis acid B(C6 F5 )3 enable the hydroamination of terminal alkynes by aryl amines to the corresponding enamines. In accord with the results of stoichiometric reactions, the mechanism of this reaction involves a frustrated Lewis pair (FLP). The hydroamination can be followed by an FLP-catalyzed hydrogenation, resulting in a one-pot stepwise synthesis of amine derivatives.

Tautomerism and metal complexation of 2-acylmethyl-2-oxazolines: a combined synthetic, spectroscopic, crystallographic and theoretical treatment.

A synthetic, structural and theoretical investigation into the solid-state, solution and gas phase structure(s) of six 2-acylmethyl-4,4-dimethyl-2-oxazolines is reported. Four of these materials, viz.α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]benzenemethanol (3a), α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]-(4-nitrobenzene)methanol (3b), 1-(4,5-dihydro-4,4-dimethyl-2-oxazolyl)-3,3-dimethyl-1-buten-2-ol (3d) and (E)-1-phenyl-2-((3aR)-3,3a,8,8a-tetrahydro-2H-indeno[1,2-d]oxazol-2-ylidene)ethanone (3f) have been characterised in the solid-state by single crystal X-ray diffraction studies. These data represent the first solid-state structural studies of this class of compounds and details the first synthesis and full characterisation of chiral derivative 3f. All four of these materials are shown to exist in the solid phase in the enamine tautomeric form (e.g., 3a is best described as 2-[4,4-dimethyl-2-oxazolidinylidene]-1-phenylethanone) and it is suggested (NMR, IR) that this isomeric form is likely also retained in solution (e.g., CDCl3) as the more stable isomer. An investigation of the relative gas phase stabilities of the three possible (i.e., the (Z)-enol, keto and enamine) isomers of all five compounds by DFT at the B3LYP/6-311G(d) level of theory confirms the latter as the most stable form. The energy differences between the enamine and keto tautomers have been calculated to be the lowest for derivative 3d. These results are compared and contrasted with the previously reported NMR studies of such compounds which have identified the keto form as being a minor (albeit solution) tautomer. Equilibrium solution tautomer distributions for 3d are found to be solvent dependent. The protonated form of 3a, isolated as the HSO4(-) salt (i.e.4a), has been further characterised in the solid state by single crystal X-ray diffraction. These data represent the first example of a protonated oxazoline to be structurally elucidated and confirms that upon protonation, the keto (oxazoline) tautomer is the energetically favoured form in the solid-state. This observation is further supported by DFT studies for the gas phase protonated forms of such materials. Further DFT (B3LYP/6-311G(d)) calculations employing the SM8 or SMD solvation models were then applied to address the observed solution isomeric distribution for 3d; these results corroborate the gas phase theoretical treatment and also yield values that predict the higher solution stability of the enamine form as observed, although they fail to account for the existence of the keto form as a minor solution state tautomer. To access the availability of an enol-form, via hypothetical de-protonation to the enolate, compound 3a was treated with hydrated Cu(NO3)2 in EtOH solution. The resulting isolated green-coloured product (5), the first metal derivative of this entire class of ligands, is best described (IR, X-ray diffraction) as a coordinated enolate complex, i.e., Cu(3a-H)2. Complex 5 crystallizes in the P21/c space group with four molecules in the unit cell. The coordination geometry around the formal Cu(2+) metal centre is determined to be highly distorted square planar in nature (τ4 = 0.442). TD-DFT is used to give a reasonable explanation for the intensity of the absorbance band observed in the visible region for solutions of 5. These latter experiments strongly suggest that the title class of compounds may have considerable potential as ligands in coordination chemistry and/or metal-mediated catalysis.

Metal-free aromatic hydrogenation: aniline to cyclohexyl-amine derivatives.

Hydrogenation of the N-bound phenyl rings of amines, imines, and aziridine is achieved in the presence of H(2) and B(C(6)F(5))(3), affording the corresponding N-cyclohexylammonium hydridoborate salts.