Morton Laurel A, Zhang Xin-Hao, Wang Ruitao, Lin Zhenyang, Wu Yun-Dong, Xue Zi-Ling
Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA.
J Am Chem Soc. 2004 Aug 25;126(33):10208-9. doi: 10.1021/ja0467263.
d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: DeltaH degrees = -1.8(0.5) kcal/mol and DeltaS degrees = -1.5(1.7) eu. Kinetic studies of the alpha-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: DeltaH1 = 16.2(1.2) kcal/mol and DeltaS1 = -22.3(4.0) eu for the forward reaction (4a --> 4b); DeltaH2 = 18.0(1.3) kcal/mol and DeltaS2 = -20.9(4.3) eu for the reverse reaction (4b --> 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer.
发现零价钨亚烷基炔基烷基配合物（Me3SiCH2）3W（CSiMe3）（PMe3）（4a）会与其双（亚烷基）互变异构体（Me3SiCH2）2W（=CHSiMe3）2（PMe3）（4b）发生罕见的、由PMe3促进的交换反应。该交换反应的热力学研究表明，4b更占优势，并得出了平衡常数Keq以及平衡的焓和熵：ΔH° = -1.8(0.5) kcal/mol，ΔS° = -1.5(1.7) eu。通过变温核磁共振对4a和4b之间α-H迁移的动力学研究得出了可逆反应的速率常数k1和k-1以及活化焓和熵：正向反应（4a→4b）的ΔH1 = 16.2(1.2) kcal/mol，ΔS1 = -22.3(4.0) eu；逆向反应（4b→4a）的ΔH2 = 18.0(1.3) kcal/mol，ΔS2 = -20.9(4.3) eu。在B3LYP水平上的从头算计算表明，PMe3与双（亚烷基）互变异构体的结合相对比与亚烷基炔基互变异构体的结合更强，因此稳定了双（亚烷基）互变异构体。
