Cobalt Catalysis in the Gas Phase: Experimental Characterization of Cobalt(I) Complexes as Intermediates in Regioselective Diels鈥揂lder Reactions

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• 作者：Lukas Fiebig ; Julian Kuttner ; Gerhard Hilt ; Martin C. Schwarzer ; Gernot Frenking ; Hans-G眉nther Schmalz ; Mathias Sch盲fer
• 刊名：Journal of Organic Chemistry
• 出版年：2013
• 出版时间：October 18, 2013
• 年：2013
• 卷：78
• 期：20
• 页码：10485-10493
• 全文大小：510K
• 年卷期：v.78,no.20(October 18, 2013)
• ISSN：1520-6904

文摘

In situ-formed cobalt(I) complexes are proposed to act as efficient catalysts in regioselective Diels鈥揂lder reactions of unactivated substrates such as 1,3-dienes and alkynes. We report the first experimental evidence for the in situ reduction of CoBr₂(dppe) [dppe = 1,2-bis(diphenylphosphino)ethane] by Zn/ZnI₂ to [Co(I)(dppe)]⁺ by means of electrospray MSⁿ experiments. Additionally, the reactivities of Co(II) and Co(I) dppe complexes toward the Diels鈥揂lder substrates isoprene and phenylacetylene were probed in gas-phase ion/molecule reactions (IMRs). Isoprene and phenylacetylene were introduced into the mass spectrometer via the buffer gas flow of a linear ion trap. The IMR experiments revealed a significantly higher substrate affinity of [Co(I)(dppe)]⁺ compared with [Co(II)Br(dppe)]⁺. Furthermore, the central intermediate of the solution-phase cobalt-catalyzed Diels鈥揂lder reaction, [Co(I)(dppe)(isoprene)(phenylacetylene)]⁺, could be generated via IMR and examined in the gas phase. Collision activation of this complex ion delivered evidence for the gas-phase reaction of isoprene with phenylacetylene in the coordination sphere of the cobalt ion. The experimental findings are consistent with the results of quantum-chemical calculations on all of the observed Co(I) dppe complex ions. The results constitute strong analytical evidence for the formation and importance of different cobalt(I) species in regioselective Diels鈥揂lder reactions of unactivated substrates and identify [Co(I)(dppe)]⁺ as the active Diels鈥揂lder catalyst.