Insertions of Ketones and Nitriles into Organorhodium(I) Complexes and β-Hydrocarbyl Eliminations from Rhodium(I) Alkoxo and Iminyl Complexes

A series of tris(triethylphosphine)-ligated organorhodium(I) complexes were prepared, and their reactions with electron-poor arylnitriles and diarylketones were studied. [(PEt3)3Rh(Ar)] (Ar = phenyl (1a) or o-anisyl (1e)) reacted with an excess of electron-poor arylnitriles Ar′C≡N (Ar′ = p-CF3C6H4 or 3,5-bis(CF3)C6H3) to form Rh(I) iminyl complexes {(PEt3)3Rh[N═C(Ar)(Ar′)]} (2h−j). In contrast, 3,5-bis(CF3)C6H3CN did not insert into the M−C bond of the arylrhodium(I) complexes [(PEt3)3Rh(Ar)] (Ar = p-CF3C6H4 (1f) or 3,5-bis(CF3)C6H3 (1g)), containing more electron-poor aryl groups. The kinetic data for nitrile insertions were most consistent with a pathway involving initial ligand dissociation, followed by a classic migratory insertion. The iminyl complexes 2i−j decomposed at higher temperatures via β-aryl eliminations with selective migration of the more electron-poor aryl group 3,5-bis(CF3)C6H3 to form 1g and the corresponding nitriles. Migratory aptitudes of various aryl groups were assessed by studying β-aryl eliminations from a variety of iminyl complexes. Kinetic data for these β-aryl eliminations were most consistent with initial phosphine dissociation and carbon−carbon bond cleavage of the resulting 14-electron intermediate. Insertions of diarylketones Ar(Ar′)C═O (Ar = 3,5-bis(CF3)C6H3, Ar′ = Ph or 3,5-bis(CF3)C6H3)) into 1a also occurred, although the resulting Rh(I) alkoxides {(PEt3)2Rh[OC(Ph)(Ar)(Ar′)]} (3f−g) were not stable under the reaction conditions and could not be directly identified. Instead, a mixture of {(PEt3)3Rh[3,5-bis(CF3)C6H3]} (1g) and the ketone Ph(Ar′)C═O (Ar′ = Ph or 3,5-bis(CF3)C6H3)) were detected as the major products, indicating that decomposition of alkoxides 3f−g occurred by β-elimination of the more electron-poor aryl group. Independent preparation of 3f−g and studies on their thermal decomposition with added PEt3 confirmed that selective β-aryl elimination occurs to generate aryl complex 1g and the corresponding ketones. Analogous β-aryl eliminations from bis-phosphine rhodium(I) alkoxo complexes 3a−e and trisphosphine rhodium(I) alkoxo complexes 4b−e were also studied, and the kinetic results were most consistent with irreversible β-phenyl elimination from bis(phosphine) alkoxo complexes. Insertion of 3,5-bis(CF3)C6H3CN into an alkylrhodium(I) complex [(PEt3)3Rh(Me)] (1h) did not occur; however, the electron-poor ketone Ar2C═O (Ar = 3,5-bis(CF3)C6H3)) inserted into 1h, as judged by the detection of the corresponding alcohol HOC(Me)[3,5-bis(CF3)C6H3)]2 as the major organic product after quenching with Et3N·HCl. Vinylrhodium(I) complex [(PEt3)3Rh(CH═CH2)] (1i) also reacted with ketones of the type Ar2C═O (Ar = 3,5-bis(CF3)C6H3) to form a Rh(I) alkoxo complex (PEt3)2Rh{OC(CH═CH2)[3,5-bis(CF3)C6H3]2} (3h), which was stabilized by the intramolecular coordination of the vinyl moiety to the rhodium center. The alkynylrhodium(I) complex [(PEt3)3Rh(C≡CPh)] (1j) did not react with ketones or nitriles. Instead, the propargylic alkoxides {(PEt3)2Rh[OC(R)2(C≡CPh)]} (R = Me or Ph) that would have resulted from insertion were shown to react rapidly in the presence of added PEt3 to form the alkynyl complex 1j and the corresponding ketones via β-alkynyl eliminations.