Terminal and Internal Alkyne Complexes and Azide-Alkyne Cycloaddition Chemistry of Copper(I) Supported by a Fluorinated Bis(pyrazolyl)borate

Copper plays an important role in alkyne coordination chemistry and transformations. This report describes the isolation and full characterization of a thermally stable, copper(I) acetylene complex using a highly fluorinated bis(pyrazolyl)borate ligand support. Details of the related copper(I) complex of HCºCSiMe3 are also reported. They are three-coordinate copper complexes featuring η2-bound alkynes. Raman data show significant red-shifts in CºC stretch of [H2B(3,5-(CF3)2Pz)2]Cu(HCºCH) and [H2B(3,5-(CF3)2Pz)2]Cu(HCºCSiMe3) relative to those of the corresponding alkynes. Computational analysis using DFT indicates that the Cu(I) alkyne interaction in these molecules is primarily of the electrostatic character. The π-backbonding is the larger component of the orbital contribution to the interaction. The dinuclear complexes such as Cu2(μ-[3,5-(CF3)2Pz])2(HCºCH)2 display similar Cu-alkyne bonding features. The mononuclear [H2B(3,5-(CF3)2Pz)2]Cu(NCMe) complex catalyzes [3+2] cycloadditions between tolyl azide and a variety of alkynes including acetylene. It is comparatively less effective than the related trinuclear copper catalyst {μ-[3,5-(CF3)2Pz]Cu}3 involving bridging pyrazolates.

Homogeneous Gold Catalysis: Mechanistic Investigation of Hydroalkoxylation of Various Alkynes

<p>The reaction mechanism of the gold(III)-catalysed hydroalkoxylation of alkynes is studied to provide a deeper understanding of homogeneous gold catalysis. The study is conducted computationally using Density Functional Theory (DFT), with the PBE0 and BP86 functionals and basis sets of triple-ζ quality (aug-cc-pVTZ and aug-cc-pVTZ-PP for the gold atom). It emphasises the mechanisms undergone by various alkynes when they are activated by gold(III) catalysts towards nucleophilic attack to first form an enol ether and followed by a second nucleophilic attack to form a ketal as the final product. Hydrogen bonding networks formed by the solvent methanols are found to play a crucial role in the mechanism especially in the hydrogen migration steps that follow after the nucleophilic attacks. The first nucleophilic attacks are predicted to have rather low activation energies and hence they are expected to proceed fast while the second additions vary in activation barriers, depending on the steric effects in the substrates. The activation barrier for the last hydrogen migration is highest for all of the three reactions investigated and is expected to be the rate determining step. Investigations of internal alkyne reactions reveal that each elementary step requires a higher activation energy compared to terminal alkynes, which explains the low experimental rate of such reactions. Due to the regioselectivity problem in internal alkyne reactions, this results in a mixture of products which is difficult to isolate due to the similarities in their reaction energies. The study also highlights the calculated thermodynamics and kinetics of the reactions, which can be useful in predicting experimental outcomes. Arrhenius plots of concentration of each intermediate species against time were produced to further help the understanding of these mechanisms, whether or not the reactions go to full completion or stop at the formation of enol ether.</p>

Homogeneous Gold Catalysis: Mechanistic Investigation of Hydroalkoxylation of Various Alkynes

<p>The reaction mechanism of the gold(III)-catalysed hydroalkoxylation of alkynes is studied to provide a deeper understanding of homogeneous gold catalysis. The study is conducted computationally using Density Functional Theory (DFT), with the PBE0 and BP86 functionals and basis sets of triple-ζ quality (aug-cc-pVTZ and aug-cc-pVTZ-PP for the gold atom). It emphasises the mechanisms undergone by various alkynes when they are activated by gold(III) catalysts towards nucleophilic attack to first form an enol ether and followed by a second nucleophilic attack to form a ketal as the final product. Hydrogen bonding networks formed by the solvent methanols are found to play a crucial role in the mechanism especially in the hydrogen migration steps that follow after the nucleophilic attacks. The first nucleophilic attacks are predicted to have rather low activation energies and hence they are expected to proceed fast while the second additions vary in activation barriers, depending on the steric effects in the substrates. The activation barrier for the last hydrogen migration is highest for all of the three reactions investigated and is expected to be the rate determining step. Investigations of internal alkyne reactions reveal that each elementary step requires a higher activation energy compared to terminal alkynes, which explains the low experimental rate of such reactions. Due to the regioselectivity problem in internal alkyne reactions, this results in a mixture of products which is difficult to isolate due to the similarities in their reaction energies. The study also highlights the calculated thermodynamics and kinetics of the reactions, which can be useful in predicting experimental outcomes. Arrhenius plots of concentration of each intermediate species against time were produced to further help the understanding of these mechanisms, whether or not the reactions go to full completion or stop at the formation of enol ether.</p>

Vinylidene, allenylidene, cyclic oxycarbene, and η2-alkyne complexes from reactions of (η5-C5Me5)OsCl(PPh3)2 with alkynes and alkynols

Reactions of Cp*OsCl(PPh3)2 (Cp* = pentamethylcyclopentadienyl) with alkynes and alkynols are described. Treatment of Cp*OsCl(PPh3)2 with phenylacetylene and trimethylsilylacetylene gave the vinylidene complexes Cp*OsCl(=C=CHPh)(PPh3) and Cp*OsCl(=C=CH2)(PPh3), respectively. Treatment of Cp*OsCl(PPh3)2 with the internal alkyne dimethyl acetylenedicarboxylate produced the η2-alkyne complex Cp*OsCl(η2-MeO2C≡CCO2Me)(PPh3). Treatment of Cp*OsCl(PPh3)2 with the propargylic alcohol HC≡CC(OH)Ph2 gave the osmium allenylidene complex Cp*OsCl(=C = C=CPh2)(PPh3). The outcomes of the reactions of Cp*OsCl(PPh3)2 with ω-alkynols HC≡C(CH2)nOH are dependent on the length of the -(CH2)n- linker. The reaction with 3-butyn-1-ol produced the cyclic oxycarbene complex Cp*OsCl{=C(CH2)3O}(PPh3) exclusively. The reactions with 4-pentyn-1-ol produced a mixture of the hydroxyalkyl vinylidene complex Cp*OsCl{=C=CH(CH2)3OH}(PPh3) and the cyclic oxycarbene complex Cp*OsCl{=C(CH2)4O}(PPh3) in about 10:1 molar ratio. The reaction with 5-hexyn-1-ol gave exclusively the hydroxyalkyl vinylidene complex Cp*OsCl{=C=CH(CH2)4OH}(PPh3).

DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes

Through DFT calculations, the equilibrium between Rh–alkyne and Rh–vinylidene species of PXNXP pincer ligated Rh cationic complexes is shown to be tuned by the P–Rh–P bite angle, which in turn is dictated by the nature of the X moiety of the pincer ligand.