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Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors

  • Author(s): Gordon, CP
  • Yamamoto, K
  • Liao, WC
  • Allouche, F
  • Andersen, RA
  • Copéret, C
  • Raynaud, C
  • Eisenstein, O
  • et al.

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© 2017 American Chemical Society. Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M-Cα/α′and M···Cβdistances, long Cα/α′-Cβbond length, and isotropic13C chemical shifts for both early d0and late d4transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have13C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d0systems, with upfield shifts to ca. -30 ppm for the α-carbon of metallacycles with higher dnelectron counts (n = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ11≥ δ22≥ δ33) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π∗(M-Cα/α′) character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates.

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