Interconversion of δ and λ Forms of Six-Membered Chelate Cycles − A Quantitative NMR Spectroscopic Study of (Diphosphane)rhodium Compounds

Abstract

The stereochemical flexibility of six-membered chelate cycles containing phosphorus donor groups has not yet been analysed in detail. With a number of compounds of the type [{κ-PR2CH2CH(OH)CH2-κ-PR′2}Rh(η4-COD)]+PF6− at hand, and since the solid state structures and the catalytic properties of these compounds are known, an ideal opportunity to study this topic was available. After assigning all of the important 1H, 13C, and 31P NMR spectroscopic resonances by a combination of one- and two-dimensional NMR spectroscopic methods quantitative interpretation of the 2D NOESY/EXSY spectra was achieved for four compounds [PR2 = PMes2; PR′2 = PPh2 (1), DBP (2), PEt2 (3), P(o-anisyl)2 (4)]. It is found that the conformations that these compounds adopt in solution are similar to the ones observed for them in the solid state. The conformational ensemble in solution comprises λ twist and δ twist forms; λ and δ isomers are clearly differentiated by quantitative NOE-based structure analysis (distance geometry). Since the sense of chirality of the ligand is known, an absolute assignment of these conformations is possible. The equilibrium constants at 298 K for the δ ⇄ λ isomerisation of compounds 1−4 are not very different from one another, with reaction enthalpies ranging from −8 to −2 kJ mol−1 and the reaction entropies in the range of −10 to 8 J K−1 mol−1. The activation barriers are again similar for 1−4 and lie at ΔH≠ ≈ 64 kJ mol−1. It is observed that the mesityl entities at the PMes2 donor groups behave as coupled rotors with two rotational pathways open to them during the δ ⇄ λ interconversion while they are rotationally fixed in the λ and δ conformations themselves. Comparing the above results with the enantioselective discrimination found for the same compounds in hydrogenation experiments it becomes clear that there is no direct correlation between the preference of one twist form over the other in the precatalyst and its chiral performance. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)