Diverse C–C Bond-Forming
Reactions of Bis(carbene)platinum(II)
Complexes
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Abstract
The platinum(0) complex Pt(PPh<sub>3</sub>)<sub>4</sub> catalyzes
coupling of the carbene ligands of (CO)<sub>5</sub>Cr{C(OMe)(<i>p-</i>MeOC<sub>6</sub>H<sub>4</sub>)} (<b>1</b>). The
stable bis(carbene)platinum(II) complexes Cl<sub>2</sub>Pt{C(OMe)(Me)}<sub>2</sub> (<b>3</b>), Br<sub>2</sub>Pt{C(OMe)(Me)}<sub>2</sub> (<b>4</b>), and Cl<sub>2</sub>Pt{C(O<sup><i>i</i></sup>Pr)(Me)}<sub>2</sub> (<b>5</b>) can be induced to undergo
C–C coupling reactions by several means. Reduction of <b>3</b>–<b>5</b> to platinum(0) with cobaltocene results
in formation of internal olefins, (<i>E/Z</i>)-2,3-dimethoxybut-2-ene
(<b>6</b>) or (<i>E/Z</i>)-2,3-diisopropoxybut-2-ene
(<b>7</b>). Reaction of <b>3</b>–<b>5</b> with PPh<sub>3</sub> yields terminal olefins, 2,3-dimethoxybut-1-ene
(<b>13</b>) or 2,3-diisopropoxybut-1-ene (<b>15</b>),
along with Cl<sub>2</sub>Pt(PPh<sub>3</sub>)<sub>2</sub> (<b>12</b>) or Br<sub>2</sub>Pt(PPh<sub>3</sub>)<sub>2</sub> (<b>14</b>). In contrast, addition of pyridine to <b>3</b>–<b>5</b> does not effect C–C coupling; instead, the acyl complexes <i>cis</i>-Cl(py)Pt(COMe){C(OMe)(Me)} (<b>8</b>), <i>cis</i>-Br(py)Pt(COMe){C(OMe)(Me)} (<b>9</b>), and <i>cis</i>-Cl(py)Pt(COMe){C(O<sup><i>i</i></sup>Pr)(Me)}
(<b>10</b>) are obtained, with concomitant formation of alkyl
halide. Possible mechanistic pathways for C–C bond formation
are discussed, as well as explanations for the different reactivities
observed for pyridine and PPh<sub>3</sub>
