Abstract:The ruthenium-hydride complex (PCy3)2(CO)RuHCl was found to be a highly effective catalyst for the alkyne-to-carboxylic acid coupling reaction to give synthetically useful enol ester products. Strong solvent effect was observed for the ruthenium catalyst in modulating the activity and selectivity; the coupling reaction in CH2Cl2 led to the regioselective formation of gem-enol ester products, while the stereoselective formation of (Z)-enol esters was obtained in THF. The coupling reaction was found to be strongly inhibited by PCy3. The coupling reaction of both PhCO2H/PhC≡CD and PhCO2D/PhC≡CH led to the extensive deuterium incorporation on the vinyl positions of the enol ester products. An opposite Hammett value was observed when the correlation of a series of para-substituted p-X-C6H4CO2H (X = OMe, CH3, H, CF3, CN) with phenylacetylene was examined in CDCl3 (ρ = +0.30) and THF (ρ = −0.68). Catalytically relevant Ru-carboxylate and -vinylidenecarboxylate complexes, (PCy3)2(CO)(Cl)Ru(κ 2 -O2CC6H4-p-OMe) and (PCy3)2(CO)(Cl)RuC(=CHPh)O2CC6H4-p-OMe, were isolated, and the structure of both complexes was completely established by X-ray crystallography. A detailed mechanism of the coupling reaction involving a rate-limiting C-O bond formation step was proposed on the basis of these kinetic and structural studies. The regioselective formation of the gem-enol ester products in CH2Cl2 was rationalized by a direct migratory insertion of the terminal alkyne via a Ru-carboxylate species, whereas the stereoselective formation of (Z)-enol ester products in THF was explained by invoking a Ru-vinylidene species.

The ruthenium hydride complex (PCy 3 ) 2 (CO)RuHCl was found to be a highly effective catalyst for the regio-and stereoselective hydrosilylation of alkynes to form vinylsilane products. (Z)-Vinylsilane products were selectively formed for sterically nondemanding terminal alkynes, while (E)-vinylsilane products resulted from sterically demanding terminal alkynes. Kinetic data were obtained from the hydrosilylation of phenylacetylene. The phosphine inhibition study showed an uncompetitive Michaelis−Menten type of inhibition kinetics. The empirical rate law rate = k obs [1] 1 [alkyne] 0 [silane] 0 was established from the reaction rate as a function of both [alkyne] and [silane]. DFT calculations were performed and found that Z/E isomerization is facile via a metallacyclopropene transition state and that the isomerization occurs prior to the silane substrate binding. A detailed mechanistic scheme on the hydrosilylation reaction has been delineated on the basis of both experimental and computational data.

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In this paper, the air-stable and readily available 1,3,5-triaza-7-phosphaadmantane (PTA) is reported as a practical and versatile nucleophilic phosphine organocatalyst. Under the mediation of 15-30 mol % of PTA, various electrophiles like aldehydes and imines readily undergo the MoritaBaylis-Hillman reactions with a variety of activated olefins, giving the corresponding adducts in high yields. In the phosphine-catalyzed [3 + 2] cycloaddition reaction of 4-substituted 2,3-butadienoates with N-tosylimines, PTA is also proven to be a comparable catalyst as tributylphosphine (PBu 3 ). By systematic comparison with other structurally similar N,P catalysts, it is concluded that the superiority of PTA in the above nucleophilic catalysis is attributable to its comparable nucleophilicity with that of trialkylphosphines. The feasibility to use PTA as an alternative catalyst in place of the air-sensitive trialkylphosphines is also discussed.