(Diphenylphosphino)alkylaldehyde affords hydride- or alkyl-[(diphenylphosphino)alkylacyl]rhodium(iii) or (diphenylphosphino)alkylester complexes: theoretical and experimental diastereoselectivity

Abstract

The reaction of [RhCl(COD)]₂ (COD = 1,5-cyclooctadiene) with racemic PPh₂(CH(Ph)CH₂CHO) and pyridine (py) led to the oxidative addition of the aldehyde, and a single geometric isomer of [RhHCl(PPh₂(CH(Ph)CH₂CO))(py)₂] (1), with hydride trans to chloride, was obtained as a mixture of two diastereomers in a 95 : 5 ratio; this was in agreement with density functional theory (DFT) calculations. In a chloroform solution, the exchange of hydride by chloride yielded [RhCl₂(PPh₂(CH(Ph)CH₂CO))(py)₂] (2) as a mixture of a kinetically preferred species, trans-py-2a, and two diastereomers, cis-Cl-2b′ and cis-Cl-2b, with cis pyridines and a chloride trans to acyl; as predicted by the DFT calculations, the latter was the major species. Complex 1 reacted with racemic PPh₂(CH(Ph)CH₂CHO) or PPh₂(o-C₆H₄CHO) to afford [RhHCl(PPh₂(CH(Ph)CH₂CO))(κ¹-PPh₂(CH(Ph)CH₂CHO))(py)] (3) or [RhHCl(PPh₂(o-C₆H₄CO))(κ¹-PPh₂(CH(Ph)CH₂CHO))(py)] (4), respectively, both with a dangling alkylaldehyde. Diastereomeric mixtures with the ratios 3a/3a′ = 80 : 20 and 4a/4a′ = 50 : 50 were obtained. Complex 4 reacted with N-donors to afford cationic [RhH(NN)(PPh₂(o-C₆H₄CO))(κ¹-PPh₂(CH(Ph)CH₂CHO))]BPh₄ (NN = 1,10-phenanthroline, 5; 2,2′-bipyridine, 6) or with 8-aminoquinoline (aqui) or 2-(aminomethyl)pyridine to yield imination products with terdentate ligands: [RhH(PPh₂(o-C₆H₄CO))(κ³-PNN)]BF₄ (PNN = PPh₂(CH(Ph)CH₂CNC₉H₆N), 7 and PPh₂(CH(Ph)CH₂CNCH₂C₅H₄N), 8, respectively. Compounds 5–8 were obtained as equimolar a/a′ mixtures of diastereomers. Moreover, 5a and 5a′ could be separated. [RhCl(NBD)]₂ reacted with racemic PPh₂(CH(Ph)CH₂CHO) and N-donors to provide nortricyclyl (Ntyl) derivatives [RhCl(NN)(Ntyl)(PPh₂CH(Ph)CH₂CO)] (NN = phen, 9 and bipy, 10) as an a/a′ = 75 : 25 mixture of diastereomers. By reacting [RhCl(NBD)]₂ with PPh₂(CH(Ph)CH₂CHO) and quinoline-8-carbaldehyde in methanol, the phosphino-ester complex [RhCl(Ntyl)(C₉H₆NCO)(κ²-PPh₂CH(Ph)CH₂CO(OCH₃)] 11 was obtained. The initial equimolar mixture of two diastereomers readily transformed into a single diastereomer, which was found to be thermodynamically most stable by the DFT calculations. Furthermore, single crystal X-ray diffraction analysis of cis-Cl-2b, 5a, 7a, 10a and 11 is reported.