The Role of Group 14 Element Hydrides in the Activation of C鈥揌 Bonds in Cyclic Olefins

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• 作者：Owen T. Summerscales ; Christine A. Caputo ; Caroline E. Knapp ; James C. Fettinger ; Philip P. Power
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：September 5, 2012
• 年：2012
• 卷：134
• 期：35
• 页码：14595-14603
• 全文大小：400K
• 年卷期：v.134,no.35(September 5, 2012)
• ISSN：1520-5126

文摘

Formally, triple-bonded dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C₆H₃-2,6-(C₆H₃-2,6-ⁱPr₂)₂] have been previously shown to activate aliphatic, allylic C鈥揌 bonds in cyclic olefins, cyclopentadiene (CpH), cyclopentene (c-C₅H₈) and 1,4-cyclohexadiene, with intriguing selectivity. In the case of the five-membered carbocycles, cyclopentadienyl species ArECp [E = Ge (3), Sn (4)] are formed. In this study, we examine the mechanisms for activation of CpH and c-C₅H₈ using experimental methods and describe a new product found from the reaction between 1 and c-C₅H₈, an asymmetrically substituted digermene ArGe(H)Ge(c-C₅H₉)Ar (5), crystallized in 46% yield. This compound contains a hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with 3, explaining the fate of the liberated H atoms following triple C鈥揌 activation. We show that when these C鈥揌 activation reactions are carried out in the presence of tert-butyl ethylene (excess), compounds {ArE(CH₂CH₂tBu)}₂ [E = Ge(8), Sn(9)] are obtained in addition to ArECp; in the case of CpH, the neohexyl complexes replace the production of H₂ gas, and for c-C₅H₈ they displace cyclopentyl product 5 and account for all the hydrogen removed in the dehydroaromatization reactions. To confirm the source of 8 and 9, it was demonstrated that these molecules are formed cleanly between the reaction of (ArEH)₂ [E = Ge(6), Sn(7)] and tert-butyl ethylene, new examples of noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type 6 and 7 can be surmised to be reactive intermediates in the production of 3 and 4, along with H₂, from 1 and 2 and CpH (respectively), or the formation of 3 and 5 from 1. The reaction of 6 or 7 with CpH gave 3 or 4, respectively, with concomitant H₂ evolution, demonstrating the basic nature of these low-valent group 14 element hydrides and their key role in the 鈥榗ascade鈥?of C鈥揌 activation steps. Additionally, during the course of these studies a new polycyclic compound (ArGe)₂(C₇H₁₂) (10) was obtained in 60% yield from the reaction of 1,6-heptadiene and 1 via double [2 + 2] cycloaddition and gives evidence for a nonradical mechanism for these types of reactions.