Reactions of Ruthenium Cp Phosphine Complex with 4,4-Disubstituted-1,6-Enynes: Effect of Methyl Substituents in the Olefinic Fraction

We studied chemical reactions of Cp(PPh3)2RuCl with nine 1,6-enyne compounds (1−4, 8, 12, 19, 21, and 22) in which the triple bond is associated with propargylic alcohol and the olefinic group has various substituted methyl groups. For the enyne compounds 1−3 with no substituted methyl group, the reaction takes place at the propargylic alcohol first giving the allenylidene complex 6 which could undergo a skeletal rearrangement to yield the disubstituted vinylidene complex 7. By changing the propargylic alcohol to propargylic ether, the reaction gives the carbene complex 10 as the major product and the butadiene complex 9 by a cyclization reaction as the minor product. For enyne 12 with two methyl groups at the terminal carbon of the olefinic part, formation of either of the carbene complexes 15 and 16 with a substituted cyclopentenyl ring at Cα or the vinylidene complex 17 is controlled by the use of solvent. For the formation of 15 and 16, a C−C bond-forming cyclization reaction is proposed to occur at Cβ in an intermediate where the triple bond is π-coordinated. However, for the vinylidene intermediate, the reaction may proceed by the formation of the allenylidene, which undergoes a retro-ene reaction to bring about cleavage of the dimethyl substituted allyl group giving 17. For two enynes 21 and 22 where each olefinic portion is internally substituted with one methyl group, two vinylidene complexes 23 and 24 each with a five-membered ring bonded at Cβ are isolated. The reaction proceeds via formation of an allenylidene intermediate followed by a cyclization at Cγ. Stabilization of the cationic charge by the presence of methyl subsituents clearly controls the reaction pathway to give different products. These chemical reactions and their mechanisms are corroborated by structure determinations of five ruthenium complexes using single crystal X-ray diffraction analysis.

本研究考察了二(三苯基膦)氯化环戊二烯基钌(Cp(PPh3)2RuCl)与9种1,6-烯炔类化合物（编号1−4、8、12、19、21和22）的化学反应，此类烯炔的炔键连接炔丙醇，且烯基带有不同的甲基取代基。对于未在烯基上引入取代甲基的烯炔1−3，反应首先发生在炔丙醇位点，生成亚丙二烯基络合物6，该络合物可经骨架重排得到双取代亚乙烯基络合物7。当将炔丙醇替换为炔丙醚时，反应主要生成卡宾络合物10，同时经环化反应生成少量丁二烯络合物9。对于烯基末端碳带有两个甲基的烯炔12，其反应路径由溶剂调控，可生成两种卡宾络合物15、16（α位碳连接取代环戊烯环）或亚乙烯基络合物17。对于15和16的生成过程，研究提出反应经炔键π配位的中间体在β位碳发生C−C键形成的环化反应。而对于亚乙烯基中间体，反应则先形成亚丙二烯基中间体，再经逆烯反应断裂二甲基取代的烯丙基基团，最终得到产物17。对于两种烯基内部各带有一个甲基取代的烯炔21和22，可分离得到两种亚乙烯基络合物23和24，二者的β位碳均连接五元环。该类反应经亚丙二烯基中间体阶段，随后在γ位碳发生环化反应完成转化；甲基取代基对阳离子电荷的稳定作用显著调控了反应路径，从而得到不同产物。本研究通过单晶X射线衍射分析对5种钌络合物的结构进行表征，佐证了上述化学反应及其反应机理。