Modular Phosphole-Methano-Bridged-Phosphine(Borane) Ligands. Application to Rhodium-Catalyzed Reactions

Abstract

The synthesis of the phospholyl­(phosphinoborane)­methane air- and moisture-stable hybrid ligands <b>4a</b>–<b>f</b>, starting from 1-phenylphospholes <b>1a</b>–<b>d</b>, was performed via P–C bond coupling on the methano bridge. Two strategies were investigated, according to the phospholyl moiety used as a nucleophilic or an electrophilic reagent. In the first pathway, the phospholyl anions react with the easily available (chloromethyl)­diphenylphosphine–borane <b>3</b> to afford ligands <b>4a</b>–<b>d</b> in 29–67% isolated yields. In the second pathway, the phospholyl­(dicyclohexylphosphinoborane)­methane ligands <b>4e</b>,<b>f</b> were synthesized in 18–23% yields, through a nucleophilic substitution on the cyanophosphole. Removal of the BH₃ moiety on <b>4a</b>–<b>c</b> assisted by DABCO leads to the hybrid phospholyl­(diphenylphosphino)­methanes <b>5a</b>–<b>c</b>. Compounds <b>4</b> and <b>5</b> were fully characterized by multinuclear NMR spectroscopy (¹H, ³¹P, ¹³C, ¹¹B), mass spectrometry, and elemental analysis, and the X-ray crystal structures of <b>4a</b>,<b>c</b> and <b>5a</b>,<b>b</b> have been established. Ligands <b>5a</b>,<b>b</b> were used to prepare the cationic rhodium complexes [Rh­(η⁴-COD)­(<b>5a</b>)]⁺ (<b>8a′</b>), [Rh­(η⁴-COD)­(<b>5b</b>)]⁺ (<b>8b</b>), [Rh­(<b>5a</b>)₂]⁺ (<b>9a′</b>), and [Rh­(<b>5b</b>)₂]⁺ (<b>9b</b>), containing four-membered chelate rings with BF₄^– or CF₃SO₃^– as counterions. Ligands <b>4a</b>–<b>f</b> were also used to synthesize the [Rh­(η⁴-COD)­(<b>4</b>)]⁺ chelate complexes <b>10a</b>–<b>f</b>, resulting from coordination of the phospholyl part and the BH₃ group via a η² mode with two bridging B–H–Rh 3c–2e bonds, as shown by the X-ray crystal structures of the complexes <b>10b</b>,<b>c</b>. Rhodium complexes <b>8</b> and <b>10</b> isolated or formed in situ with ligands <b>4</b> and <b>5</b> were studied for catalytic hydrogenation of methyl 2-(acetamidomethyl)­acrylate (<b>11</b>) and hydroboration of styrene (<b>13</b>) with catecholborane. In both reactions, the catalytic systems prepared either from the phospholyl­(phosphinoborane)­methane ligands <b>4</b> or the corresponding free ligands <b>5</b>, gave good to excellent conversions. In addition, the regioselectivity of the catalyzed hydroboration is slightly influenced using these ligands. Finally, the use of hybrid phospholyl­(phosphinoborane)­methanes as ligands offers a new, efficient way to improve catalytic processes, for designing both the structure and the electronic properties of the catalyst, or still to implement it without removing the borane protecting group