Cobalt-Promoted Tandem C(sp<sup>2</sup>)–H
Activation and C–C Coupling of Vinyldiphenylphosphine: σ-
vs π‑Coordination Modes within Cyclometalation
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Abstract
We
have studied the reactivity of a series of <i>terminal</i> alkenyldiphenylphosphines {Ph<sub>2</sub>P(CH<sub>2</sub>)<sub><i>n</i></sub>-CHCH<sub>2</sub>), <i>n</i> =
0, 1, 2, 3} under mild conditions (−70 °C) in reaction
with univalent Co(I). Methane and ethane elimination occurs when different
olefinic <i>P,C</i>-coordination modes are established.
Subsequent C–H activation and C–C coupling take place
in the reaction of Co(CH<sub>3</sub>)(PMe<sub>3</sub>)<sub>4</sub> with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic <i>P,C,P</i>-type ligand with asymmetric ring sizes attached to
the cobalt center of (PMe<sub>3</sub>)<sub>2</sub>Co{κ<sup>3</sup>-(<i>P,C,P</i>)-(Ph<sub>2</sub>)CH<sub>2</sub>CHCH-CH(PPh<sub>2</sub>)} (<b>1</b>). X-ray study of <b>1</b> revealed
that the complex has a unique cyclometalated structure of a three-membered
ring within the newly assembled bis-phosphine ligand. Cyclometalation
via selective vinyl-group C(sp<sup>2</sup>)–H activation of
stoichiometric amounts allyl-diphenylphosphine with Co(CH<sub>3</sub>)(PMe<sub>3</sub>)<sub>4</sub> afforded for the first time σ-allyl
coordination incorporated in a five-membered cobaltacycle of (PMe<sub>3</sub>)<sub>2</sub>Co{κ<sup>2</sup>(<i>P,C</i>)-HCCHCH<sub>2</sub>PPh<sub>2</sub>} (<b>2</b>); subsequent C–C coupling
is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine
and Co(CH<sub>3</sub>)(PMe<sub>3</sub>)<sub>4</sub> afforded a η<sup>3</sup>-coordinated π-allyl complex of composition (PMe<sub>3</sub>)<sub>2</sub>Co{η<sup>3</sup>-κ<sup>4</sup>-(<i>P,C,C,C</i>)-H<sub>2</sub>CCH-CH(CH<sub>2</sub>)<sub>2</sub>PPh<sub>2</sub>} (<b>3</b>). All unprecedented complexes
were characterized by multinuclear NMR spectroscopy and X-ray diffraction,
and their mechanisms of formation are discussed
