C−H Bond Activation of Terminal Allenes: Formation of Hydride-Alkenylcarbyne-Osmium and Disubstituted Vinylidene-Ruthenium Derivatives

The reactivity of the dihydrides MH2Cl2(PiPr3)2 (M = Os (1), Ru (2)) toward allenes has been studied. Complex 1 reacts with 2 equiv of 3-methyl-1,2-butadiene and 1-methyl-1-(trimethylsilyl)allene
to give 1 equiv of olefin and the π-allene derivatives OsCl2(η2-CH2CCRMe)(PiPr3)2 (R = Me (3), Me3Si (4)). The X-ray structure of 4 proves the coordination to the metal center of the carbon−carbon
double bond of the allene with the lowest steric hindrance. In toluene,
complexes 3 and 4 are unstable and evolve
into the hydride-alkenylcarbyne derivatives OsHCl2(CCHCRMe)(PiPr3)2 (R = Me (5), Me3Si (6)). DFT calculations on the model compound
OsCl2(η2-CH2CCMe2)(PMe3)2 (3t) suggest that
the π-allene to hydride-alkenylcarbyne transformation involves
the migration of both hydrogen atoms of the CH2 group of
the allene. The first of them occurs between the terminal and central
carbon atoms and takes place throught the metal center. The second
one is a 1,2-hydrogen shift from the allene terminal carbon to osmium.
The reactions of the ruthenium complex 2 with the previously
mentioned allenes give olefins and RuCl2(η2-CH2CCRMe)(PiPr3)2 (R = Me (7), Me3Si (8)), which in dichloromethane and in the presence of allene afford
the disubstituted vinylidene complexes RuCl2(CCRMe)(PiPr3)2 (R = Me (9), Me3Si (10)). The structure of 10 in
the solid state has been determined by X-ray diffraction analysis.
DFT calculations show that the formation of 9 and 10 can be rationalized in terms of the initial isomerization
of 7 and 8 to alkenylcarbene species, which
subsequently undergo metathesis reactions with a second allene molecule.

本研究考察了二氢二氯双(三异丙基膦(PiPr₃))合金属配合物MH₂Cl₂(PiPr₃)₂（M分别为锇(Os，配合物1)、钌(Ru，配合物2)）与丙二烯(allene)的反应活性。配合物1分别与2当量的3-甲基-1,2-丁二烯和1-甲基-1-(三甲基硅基(trimethylsilyl))丙二烯反应，生成1当量的烯烃及π配位丙二烯衍生物OsCl₂(η²-CH₂=C=CRMe)(PiPr₃)₂（R分别为甲基(3)、三甲基硅基(4)）。配合物4的X射线晶体结构(X-ray structure)证实，丙二烯中空间位阻最低的碳碳双键与金属中心发生配位。在甲苯溶剂中，配合物3和4不稳定，会转化为氢化物-烯基卡拜(hydride-alkenylcarbyne)衍生物OsHCl₂(≡CCH=CRMe)(PiPr₃)₂（R分别为甲基(5)、三甲基硅基(6)）。针对模型配合物OsCl₂(η²-CH₂=C=CMe₂)(PMe₃)₂(3t)开展的密度泛函理论(DFT, Density Functional Theory)计算表明，从π配位丙二烯到氢化物-烯基卡拜的转化过程，涉及丙二烯CH₂基团的两个氢原子的迁移：第一步氢在丙二烯的端基碳与中心碳之间迁移，且该过程经由金属中心完成；第二步则为氢从丙二烯端基碳向锇中心的1,2-氢迁移。钌配合物2与前述丙二烯的反应则生成烯烃与RuCl₂(η²-CH₂=C=CRMe)(PiPr₃)₂（R分别为甲基(7)、三甲基硅基(8)）；该衍生物在二氯甲烷溶剂中且存在过量丙二烯时，可生成双取代亚乙烯基(vinylidene)配合物RuCl₂(=C=CRMe)(PiPr₃)₂（R分别为甲基(9)、三甲基硅基(10)）。配合物10的固态结构已通过X射线衍射分析(X-ray diffraction analysis)得以确定。密度泛函理论(DFT)计算结果显示，配合物9和10的生成过程可通过如下路径得到合理解释：首先7和8发生异构化生成烯基卡宾(alkenylcarbene)物种，随后该物种与另一分子丙二烯发生复分解(metathesis)反应。