Oxapentadienyl−Rhodium−Phosphine Chemistry¹

Abstract

Treatment of [(PR3)2Rh(μ-Cl)]2 (R = Me or Et) with potassium oxapentadienide leads to the production of ((1−3-η)-5-oxapentadienyl)Rh(PR3)2 (1, R = Me; 2, R = Et) as equilibrium mixtures of anti and syn isomers. Similarly, treatment of [(PR3)2Rh(μ-Cl)]2 (R = Me, Et) with potassium 2,4-dimethyloxapentadienide generates ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PR3)2 (3, R = Me; 4, R = Et). Compounds 3 and 4 exist predominantly as the anti isomer but upon cooling exhibit two rotameric forms, the sickle-shaped and U-shaped rotamers. Treatment of 1 with an additional 1 equiv of PMe3 produces the 18e species ((1−3-η)-5-oxapentadienyl)Rh(PMe3)3 (5), which is stable at room temperature. In contrast, when 2 is treated with an additional 1 equiv of PEt3, no reaction can be detected by NMR at room temperature. However, upon cooling to −70 °C, the characteristic signals for the phosphine adduct ((1−3-η)-5-oxapentadienyl)Rh(PEt3)3 (6) are observed by NMR. Compound 3, like 1, reacts with an additional 1 equiv of PMe3 to produce ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PMe3)3 (7) at room temperature. Finally, treatment of 4 with PEt3 yields no phosphine adduct, even upon cooling to −70 °C. Treatment of 4 with methyl triflate (CH3O3SCF3) leads to methylation at the rhodium center and coordination of the oxapentadienyl CO group, producing the 18e species [(η5-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(Me)]+O3SCF3 - (8). Similarly, treatment of 4 with HBF4·OEt2 generates [(η5-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(H)]+BF4 - (9), together with a novel isomer (10). Isomer 10 results from hydride migration to C3 of the 2,4-dimethyloxapentadienyl ligand, followed by metal-mediated activation of a methyl (C5) C−H bond. Treatment of 8 with PPN+Cl- results in chloride attack at the rhodium center and production of ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(Me)(Cl) (11). In contrast, treatment of 9/10 with PPN+Cl- leads to hydride migration, dissociation of the resulting 2,4-dimethyloxapentadiene ligand, and production of [(PEt3)2Rh(μ-Cl)]2. Compounds 1, 5, and 8 have been characterized by single-crystal X-ray diffraction.

Versatile reactivity of a rhodium(i) boryl complex towards ketones and imines

“…The phosphorusphosphorus coupling constant of 36 Hz is in the typical range for cis-phosphines. [55][56][57] The phosphorus-rhodium coupling constants of 257 Hz and 160 Hz are fairly large and indicate the presence of a rhodium(I) species. [55][56][57] The signal at δ 28.1 ppm with the larger phosphorus-rhodium coupling constant is caused by the phosphorus atom in the trans position to the ring according to extended Hückel MO calculations.…”

“…[55][56][57] The phosphorus-rhodium coupling constants of 257 Hz and 160 Hz are fairly large and indicate the presence of a rhodium(I) species. [55][56][57] The signal at δ 28.1 ppm with the larger phosphorus-rhodium coupling constant is caused by the phosphorus atom in the trans position to the ring according to extended Hückel MO calculations. 58,59 The signal at δ 21.8 ppm exhibits an additional coupling of 8 Hz to the fluorine atoms and can be assigned to the phosphine which is located in the trans position to the carbon atom bearing the CF 3 group.…”

“…The phosphorus-rhodium coupling constant of 270 Hz or 168 Hz is comparable to those which were determined for 4 and 9 and again confirm the oxidation state of Rh(I). [55][56][57] The phosphorus-phosphorus coupling constant of 26 Hz is relatively small. Analogous to 4 the 1 H NMR spectrum displays three broad signals in the aromatic region.…”

“…Further decrease of the temperature to 188 K leads to a splitting into two signals at δ 23.7 and 13.1 ppm in a ratio of 1 : 2, which suggests the presence of a square planar rhodium complex bearing three PEt 3 ligands. The phosphorus-rhodium coupling constants of 119 Hz and 155 Hz indicate the presence of a Rh(I) species and the phosphorus-phosphorus coupling constant of 37 Hz is in a typical range for phosphine ligands in a mutual cis-position 50,51,[55][56][57]77. We suggest 9′ to be the σ-complex [Rh{η 1 -CPh 2 N(H)(Bpin)}(PEt 3 ) 3 ] with the anionic ligand bound to rhodium via the CPh 2 carbon atom due to the similarity of the 31 P{ 1 H} NMR data for organorhodium(I) complexes [Rh(R)-(PEt 3 ) 3 ]…”

2, 4‐Dimethylpenta‐1, 3‐dien‐ und 2, 4‐Dimethylpentadienyl‐Komplexe des Rhodiums und Iridiums

“…Me] ϩ[21,22] und bei [Ir(C 7 H 11 )(PR 3 ) 3 ] 2ϩ[24]. Gemeinsam ist diesen Verbindungen die Abfolge δ(C 1,5 ) << δ(C 3 ) < δ(C2,4 ).…”