Publications

The first example of palladium-catalyzed stereoselective addition of diphenyl disulfide and diphenyl diselenide to the triple bond of terminal alkynes under microwave irradiation conditions is described. It was found that both the element—element (E-E) and carbon—element bonds can be activated in the catalytic system studied. The products of both reactions were isolated in quantitative yields. According to quantum-chemical calculations, the reaction mechanism involves the oxidative addition of the E-E bond to Pd⁰. Depending on the microwave power and reaction conditions, the next stage is either the reaction with alkyne or the carbon—element bond activation. The product of the oxidative addition of Ph₂Se₂ to Pd⁰, namely, dinuclear complex [Pd₂(SePh)₄(PPh₃₎₂], was detected by ³¹P{¹H}NMR spectroscopy directly in the Ph₂Se₂/PPh₃ melt formed under microwave irradiation conditions.

The mechanism and controlling factors of the C−C reductive elimination reactions of vinyl, phenyl, ethynyl, and methyl ligands from the Pd and Pt complexes RR'M(PH₃)₂ were studied with a density functional method. The barrier of C−C coupling from the symmetrical R₂M(PH₃)₂ (where M = Pd, Pt) complex decreases in the order R = methyl > ethynyl > phenyl > vinyl, and the exothermicity of the reaction increases in the same order. That is, the methyl−methyl coupling has the highest barrier and smallest exothermicity, while the vinyl−vinyl coupling has the smallest barrier and largest exothermicity. For the asymmetrical RR'M(PH₃)₂ complexes, the activation and reaction energies are found to be approximately the average of the corresponding parameters of symmetrical coupling reactions, and this simple rule is expected to be valid for other asymmetrical coupling reactions involving different substituted alkyl, vinyl, phenyl, and ethynyl groups as well as different transition-metal complexes. These C−C coupling reactions occur much more easily in Pd than in Pt complexes, because the Pd−R bonds are weaker than the Pt−R bonds. The major thermodynamic and kinetic factors determining the C−C coupling in these complexes have been discussed. For reactions with similar exothermicities, the kinetics of C−C bond formation is mainly determined by the orientation effect that includes the directionality of the M−C bond and the steric interaction between R and the other ligand (phosphine in the present case), which favors vinyl over phenyl over methyl. However the activation barrier is strongly dominated by exothermicity when it is very different between reactions.

A new catalytic system for the Ar₂E₂ (E = S, Se) addition to terminal alkynes (HC⋮C−R) has been developed to synthesize bis-element-substituted alkenes Z-H(ArE)CC(EAr)R with high stereoselectivity and yields. Utilizing phosphite ligand P(OiPr)₃ allowed solving two major problems of this catalytic reaction: (1) prevent catalyst polymerization and (2) simplify product purification procedures. Key intermediatestrans-[Pd(SPh)₂(P(OiPr)₃)₂] and trans-[Pd₂(SPh)₄(P(OiPr)₃)₂]were synthesized by S−S oxidative addition reaction to Pd(0) and studied by X-ray analysis. The equilibrium between the mononuclear and dinuclear complexes in solution was established by ³¹P NMR spectroscopy. In addition to the advantages in the synthetic procedure, the isolation of the stable palladium complexes with phosphite ligand made possible a detailed mechanistic study of the catalytic reaction.

The polymer-supported recyclable palladium catalyst was prepared for stereoselective diaryl disulfides addition to ­terminal alkynes with high yields. The 96-98% product purity was achieved after filtering the polymer-supported catalyst without ­special purification procedure.

The main concept behind the new procedure involves joint analysis of HMQC spectral data and theoretically calculated NMR chemical shifts. Using the combined experimental/ab-initio methodology, complete signal and stereochemical assignments were made for the isomers in HMQC spectrum. Chemical shifts of ⁷⁷Se were calculated with GIAO method at B3LYP/6-311G(d) level with good accuracy.

Regioselective Markovnikov-type addition of PhSH to alkynes (HC≡C-R) has been performed using easily available nickel complexes. The non-catalytic side reaction leading to anti-Markovnikov products was suppressed by addition of γ-terpinene to the catalytic system. The other side reaction leading to the bis(phenylthio)alkene was avoided by excluding phosphine and phosphite ligands from the catalytic system. It was found that catalytic amounts of Et₃N significantly increased the yield and selectivity of the catalytic reaction. Under optimized conditions high product yields of 60–85% were obtained for various alkynes [R=n-C₅H₁₁, CH₂NMe₂, CH₂OMe, CH₂SPh, C₆H₁₁(OH), (CH₂)₃CN]. The X-ray structure of one of the synthesized products is reported.

Combined density functional and ONIOM studies have been performed to investigate the mechanism of rhodium-catalyzed boration of imines. Catalytic imine boration has been found to proceed via the following stages: (1) oxidative addition of B−B to the Rh complex, (2) imine coordination, (3) migratory insertion of the imine into the rhodium−boron (Rh−B) bond, and (4) β-hydrogen elimination to give a monoboration product or carbon−boron (C−B) bond formation to yield a diboration product. The choice of the final stage depends on the structure of the imine and boration reagent. Bulky substrate molecules facilitate C−H bond activation and retard C−B bond formation, while in the absence of sterical hindrance C−B bond formation is preferred over C−H bond activation. The present study is the first that outlines the mechanistic differences in C C and CN bond boration and rationalizes the effect of bulky substituents on the mechanism of imine boration reaction. The expected difference in regioselectivity between imine and alkene boration is also discussed.