Factors Dictating Carbene Formation at (PNP)Ir

The mechanistic subtleties involved with the interaction of an amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and cyclic ethers have been investigated using density functional theory. Our analysis has revealed the factors dictating reaction direction toward either an iridium-supported carbene or a vinyl ether adduct. The (PNP)Ir structure will allow carbene formation only from accessible carbons α to the ethereal oxygen, such that d electron back-donation from the metal to the carbene ligand is possible. Should these conditions be unavailable, the main competing pathway to form vinyl ether can occur, but only if the (PNP)Ir framework does not sterically interfere with the reacting ether. In situations where steric hindrance prevents unimpeded access to both pathways, the reaction may progress to the initial C−H activation but no further. Our mechanistic analysis is density functional independent and whenever possible confirmed experimentally by trapping intermediate species experimentally. We have also highlighted an interesting systematic error present in the DFT analysis of reactions where steric environment alters considerably within a reaction

Chiral Brønsted acid catalyzed enantioselective dehydrative Nazarov-type electrocyclization of aryl and 2-Thienyl vinyl alcohols

An efficient chiral Brønsted acid-catalyzed enantioselective dehydrative Nazarov-type electrocyclization (DNE) of electron-rich aryl- and 2-thienyl-β-amino-2-en-1-ols is described. The 4π conrotatory electrocyclization reaction affords access to a wide variety of the corresponding 1H-indenes and 4H-cyclopenta[b]thiophenes in excellent yields of up to 99% and enantiomeric excess (ee) values of up to 99%. Experimental and computational studies based on a proposed intimate contact ion-pair species that is further assisted by hydrogen bonding between the amino group of the substrate cation and chiral catalyst anion provide insight into the observed product enantioselectivities

Theoretical rationalisation for the mechanism of N-heterocyclic carbene-halide reductive elimination at CuIII, AgIII and AuIII

Reductive elimination of imidazolium salts from CuIII is extremely sensitive to the anionic ligand (X or Y) type on Cu (e.g. ΔG‡ ranges from 4.7 kcal mol-1 to 31.8 kcal mol-1, from chloride to benzyl). Weakly σ-donating ligands dramatically accelerate reductive elimination. Comparison with Ag/Au shows that the HOMO energy, strength of M-NHC and M-Y bonds and inherent stability of MIII with respect to MI are critical to governing reaction feasibility

Connecting Binuclear Pd(III) and Mononuclear Pd(IV) Chemistry by Pd–Pd Bond Cleavage

Oxidation of binuclear Pd(II) complexes with PhICl$_2$ or PhI(OAc)$_2$ has previously been shown to afford binuclear Pd(III) complexes featuring a Pd–Pd bond. In contrast, oxidation of binuclear Pd(II) complexes with electrophilic trifluoromethylating (“CF$_3^+$”) reagents has been reported to afford mononuclear Pd(IV) complexes. Herein, we report experimental and computational studies of the oxidation of a binuclear Pd(II) complex with “CF$_3^+$” reagents. These studies suggest that a mononuclear Pd(IV) complex is generated by an oxidation–fragmentation sequence proceeding via fragmentation of an initially formed, formally binuclear Pd(III), intermediate. The observation that binuclear Pd(III) and mononuclear Pd(IV) complexes are accessible in the same reactions offers an opportunity for understanding the role of nuclearity in both oxidation and subsequent C–X bond-forming reactions.Chemistry and Chemical Biolog

Promoting Difficult C–C Couplings: Which Ligand Does Best?

Producción CientíficaA Pd complex, cis-[Pd(C6 F5 )2 (THF)2 ] (1), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C-C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd0 L. Complex 1 ranks the coupling ability of some popular ligands in the order Pt Bu3 >o-TolPEWO-F≈tBuXPhos>P(C6 F5 )3 ≈PhPEWO-F>P(o-Tol)3 ≈THF≈tBuBrettPhos≫Xantphos≈PhPEWO-H≫PPh3 according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈Pt Bu3 ≈o-TolPEWO-F>PhPEWO-F>P(C6 F5 )3 ≫tBuBrettPhos>THF≈P(o-Tol)3 >Xantphos>PhPEWO-H≫PPh3 . This "meter" also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.Ministerio de Economía, Industria y Competitividad (CTQ2013-48406-P)Ministerio de Economía, Industria y Competitividad (CTQ2014-52796-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA256U13

Copper-Catalyzed Regioselective Ortho C-H Cyanation of Vinylarenes

A copper-based catalytic technique for the regioselective ortho C-H cyanation of vinylarenes has been developed. This method provides an effective means for the selective functionalization of vinylarene derivatives. A copper-catalyzed cyanative dearomatization mechanism is proposed to account for the regiochemical course of this reaction.National Institutes of Health (U.S.) (GM46059

Combining Sanford arylations on benzodiazepines with the nuisance effect

5-Phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-ones react under palladium- and visible light photoredox catalysis, in refluxing methanol, with aryldiazonium salts to afford the respective 5-(2- arylphenyl) analogues. With 2- or 4-fluorobenzenediazonium derivatives, both fluoroaryl- and methoxyaryl- products were obtained, the latter resulting from a SNAr on the fluorobenzenediazonium salt (“nuisance effect”). A computational DFT analysis of the palladium-catalysed and the palladium/ruthenium-photocalysed mechanism for the functionalization of benzodiazepines indicated that in the presence of the photocatalyst the reaction proceeds via a low-energy SET pathway avoiding the high-energy oxidative addition step in the palladium-only catalysed reaction pathway

Oxygen Reduction at Carbon Supported Lanthanides:The Role of the B-site

The kinetics of the oxygen reduction reaction (ORR) at carbon supported transition metal oxides in alkaline solutions is systematically investigated as a function of the nature of the B-site. The study is focused on LaBO3 (B = Cr, Co, Fe, Mn and Ni) nanoparticles synthesized by an ionic liquid route, offering fine control over phase purity and composition. Activity towards the ORR was compared with commercial Pt/Etek catalyst. Detailed electrochemical analysis employing a rotating ring-disc electrode provides conclusive evidences that the carbon support plays an important contribution in the faradaic responses. Decoupling the contribution of the carbon support uncovers that the reactivity of LaMnO3 towards the 4e- ORR pathway is orders of magnitude higher than for the other lanthanides. We rationalise these observations in terms of changes in the redox state at the B-site close to the formal oxygen reduction potential

Triarylborane catalysed n-alkylation of amines with aryl esters

The ability of halogenated triarylboranes to accept a lone pair of electrons from donor substrates renders them excellent Lewis acids which can be exploited as a powerful tool in organic synthesis. Tris(pentafluorophenyl)borane has successfully demonstrated its ability to act as a metal-free catalyst for an ever-increasing range of organic transformations. Herein we report the N-alkylation reactions of a wide variety of amine substrates including diarylamines, N-methylphenyl amines, and carbazoles with aryl esters using catalytic amounts of B(C6F5)3. This mild reaction protocol gives access to N-alkylated products (35 examples) in good to excellent yields (up to 95%). The construction of a C–N bond at the propargylic position has also been demonstrated to yield synthetically useful propargyl amines. On the other hand, unsubstituted 1H-indoles and 1H-pyrroles at the C3/C2 positions afforded exclusively C–C coupled products. Extensive DFT studies have been employed to understand the mechanism for this transformation