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

In situ-formed cobalt(I) complexes are proposed to act as efficient catalysts in regioselective Diels–Alder 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 MS n experiments. Additionally, the reactivities of Co(II) and Co(I) dppe complexes toward the Diels–Alder 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–Alder 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–Alder reactions of unactivated substrates and identify [Co(I)(dppe)]+ as the active Diels–Alder catalyst.