Imidazole Phosphines: Synthesis, Reaction Chemistry, and Their Use in Suzuki <i>C,C</i> Cross-Coupling Reactions

A straightforward consecutive synthesis methodology for the preparation of phosphino imidazoles 1-(4-PR<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>)-4,5-Me<sub>2</sub>-1<i>H</i>-C<sub>3</sub>HN<sub>2</sub> (<b>4a</b>, R = C<sub>6</sub>H<sub>5</sub>; <b>4b</b>, R = <sup><i>c</i></sup>C<sub>6</sub>H<sub>11</sub>) and 1-(4-PR<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>)-2-PR′<sub>2</sub>-4,5-Me<sub>2</sub>-1<i>H</i>-C<sub>3</sub>N<sub>2</sub> (R = C<sub>6</sub>H<sub>5</sub>: <b>6a</b>, R′ = C<sub>6</sub>H<sub>5</sub>; <b>6b</b>, R′ = <sup><i>c</i></sup>C<sub>6</sub>H<sub>11</sub>; <b>6c</b>, R′ = <sup><i>c</i></sup>C<sub>4</sub>H<sub>3</sub>O; R = <sup><i>c</i></sup>C<sub>6</sub>H<sub>11</sub>: <b>6d</b>, R′ = C<sub>6</sub>H<sub>5</sub>; <b>6e</b>, R′ = <sup><i>c</i></sup>C<sub>6</sub>H<sub>11</sub>; <b>6f</b>, R′ = <sup><i>c</i></sup>C<sub>4</sub>H<sub>3</sub>O) is presented. Phosphino imidazoles <b>6a</b>–<b>f</b> were reacted with [PdCl<sub>2</sub>(SEt<sub>2</sub>)<sub>2</sub>] (<b>7</b>), giving [Pd­(1-(4-PR<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>)-2-PR′<sub>2</sub>-4,5-Me<sub>2</sub>-1<i>H</i>-C<sub>3</sub>N<sub>2</sub>)­Cl<sub>2</sub>]<sub>2</sub>. Single crystals of [Pd­(1-(4-P­(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>)-2-P­(<sup><i>c</i></sup>C<sub>4</sub>H<sub>3</sub>O)<sub>2</sub>-4,5-Me<sub>2</sub>-1<i>H</i>-C<sub>3</sub>N<sub>2</sub>)­Cl<sub>2</sub>]<sub>2</sub> (<b>8</b>) suitable for single-crystal X-ray structure analysis could be obtained by using the synthesis-<i>cum</i>-diffusion strategy, confirming the formation of a neutral 18-membered Pd<sub>2</sub>P<sub>4</sub> cycle with two <i>trans</i>-configurated palladium centers. A Pt<sub>4</sub>P<sub>4</sub> cyclic compound, accessible by molecular recognition, was obtained via treatment of [Pt­(dppf)­(CC-C<sub>6</sub>H<sub>4</sub>-4-P­(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>)<sub>2</sub>] (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene) (<b>11</b>) with [PtCl<sub>2</sub>(SEt<sub>2</sub>)<sub>2</sub>] (<b>12</b>). The structure of <b>13</b> in the solid state was confirmed by crystal structure determination, proving the formation of a neutral molecular square composed of Pt­(dppf) and PtCl<sub>2</sub> corner units and 4-(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>P-C<sub>6</sub>H<sub>4</sub>-CC linkers. In addition, compounds <b>6a</b>–<b>f</b> were applied in the palladium-promoted Suzuki cross-coupling of 2-bromotoluene with phenylboronic acid using potassium carbonate as base. All <i>in situ</i> generated phosphino imidazole palladium species showed high catalytic activity at which the diphosphino systems featuring phenyl and cyclohexyl groups achieved the best results. Additionally, phosphine <b>6d</b> was applied in the coupling of 4-chlorotoluene with phenylboronic acid and in the synthesis of sterically hindered biaryls under mild reaction conditions, showing an excellent performance. In comparison with other catalytically active species, equal or higher productivities were obtained using lower catalyst loadings and lower temperatures.