Theoretical Studies On Mechanism For Formation Or Activation Of Several Heterocyclic Compounds

Density functional calculations have been systematically performed to explore the mechanism of intramolecular cyclization of 2-benzyloxyphenyl trimethylsilyl ketone(acylsilane),cleavage of strong C-C bonds of quinoxaline and nucleophilic addition of acetylpyridinum and TMSCN catalyzed by tungsten complex.Our calculated results are in good agreement with experimental findings,and the error for optimized geometry parameters(bond length,bond angle and dihedral angle)of some key species is within 2%compared to crystallographic parameters(Cambridge Crystallographic Database),indicating that the calculated methods are credible.The intramolecular cyclization of 2-benzyloxyphenyl trimethylsilyl ketone involves a formation of siloxycarbene intermediate and a C-H bond insertion of siloxycarbene.The comparative studies on three possible insertion of siloxycarbene shows that the concerted insertion of siloxycarbene into C-H bond(pathway a),which needs overcoming an energy barriers of 45.1 kcal·mol-1,is the most unlikely pathway,and the stepwise insertion of siloxycarbene without spin multiplicity change(pathway c),which is energetically more favorable than the stepwise insertion of siloxycarbene with spin multiplicity change(pathway b).The formation of siloxycarbene is calculated to be endergonic by 22.9 kcal·mol-1 with an energy barrier of 30.2 kcal·mol-1,being the rate-determining step of the whole process.The activation of(PMe3)4W(?2-CH2PMe2)H on quinoxaline is divided into insertion and coordination activation process.The insertion process partically supports the results calculated by Miscione and his coworkers.The configuration adjust is not a direct transformation,but dissociation and re-coordination of trimethylphosphine.The insertion of tungsten complex into quinoxaline(rate-determining step)is not a stepwise process,but a concerted process.Three possible pathways for the coordination activation are comparatively investigated,and the rate-determining step is the transformation from ?-type W-N complex to ?-type complex.The comparison between the insertion and activation indicates that the insertion process is kinetically and thermodynamically more impossible than the coordination activation,and they can not transform each other.These are in good agreement with the experimental findings.For nucleophilic addition of acetylpyridinum with TMSCN to give 2-cyanoacetylpyridinum,the comparative investigations on four different reaction models(Model ?:in the existence of both TpW(NO)(PMe3)and 1,4-diazabicyclo[2.2.2]octane(DABCO);Model ?:in the existence of TpW(NO)(PMe3)only;Model ?:in the existence of DABCO only;and Model IV:in the inexistence of TpW(NO)(PMe3)and DABCO)show that TpW(NO)(PMe3)can accelerate the nucleophilic addition process and DABCO plays an important role in the dissociation of TMS+.