Palladacycle from Cyclometalation of the Unsubstituted
Cyclopentadienyl Ring in Ferrocene: Synthesis, Characterization, Theoretical
Studies, and Application to Suzuki–Miyaura Reaction
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Abstract
The
ferrocenylimines of general formula [(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)-CH<sub>2</sub>NCH-C(R)CH-C<sub>6</sub>H<sub>5</sub>] with
R = H (<b>2a</b>) and CH<sub>3</sub> (<b>2b</b>) were
conveniently prepared from ferrocenylmethylamine. Reaction of <b>2a</b>,<b>b</b> with lithium tetrachloropalladate in methanol
in the presence of anhydrous sodium acetate resulted in the formation
of the di-μ-chloro-bridged heteroannular cyclopalladated complexes <b>3a</b>,<b>b</b> via the unsubstituted ferrocenyl C–H
bond activation of the related ligands. Treatment of <b>3a</b>,<b>b</b> with triphenylphosphine gave Pd{[(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)CH<sub>2</sub>NCH-CHCH-C<sub>6</sub>H<sub>5</sub>]}ClPPh<sub>3</sub> (<b>4a</b>) and Pd{[(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>)-CH<sub>2</sub>NCH-C(CH<sub>3</sub>)CH-C<sub>6</sub>H<sub>5</sub>]}ClPPh<sub>3</sub> (<b>4b</b>), respectively.
The crystal structures of <b>4a</b>,<b>b</b> confirmed
the formation of a carbon–palladium bond by using a carbon
atom in the unsubstituted cyclopentadienyl ring. Additionally, theoretical
studies using density functional theory calculations were carried
out in order to account for the regioselectivity of cyclometalation.
As for the catalysts, using 0.1% of palladacycles <b>4a</b>,<b>b</b> in the presence of K<sub>3</sub>PO<sub>4</sub>·7H<sub>2</sub>O as base exhibited excellent yields in the Suzuki–Miyaura
coupling reaction of aryl bromides with phenylboronic acid
