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Oxapentadienyl−Rhodium−Phosphine Chemistry1

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journal contribution
posted on 24.08.2002, 00:00 by John R. Bleeke, Edward Donnay, Nigam P. Rath
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.