Synthesis and Stereoselective Interconversion of Chiral 1‐Aza‐3,6‐diphosphacycloheptanes

Cyclic seven‐membered bisphosphanes, namely 1‐aza‐3,6‐diphosphacycloheptanes 3–5 (3: 3,6‐diphenyl‐1‐(1‐phenylethyl)‐1‐aza‐3,6‐diphosphacycloheptane; 4: 1‐[(1R)‐1‐(4′‐methoxyphenyl)ethyl]‐3,6‐diphenyl‐1‐aza‐3,6‐diphosphacycloheptane; 5: 3,6‐diphenyl‐1‐[(1R)‐1‐phenylpropyl]‐1‐aza‐3,6‐diphosphacycloheptane), with chiral exocyclic substituents at the nitrogen positions have been synthesized, with the prevailing formation of meso stereoisomers (P_RP_S) as kinetically controlled products, by stereoselective condensation of 1,2‐bis(phenylphosphanyl)ethane, formaldehyde and primary optically pure amines, namely (S)‐(–)‐1‐phenylethylamine, (R)‐(+)‐1‐phenylethylamine, (R)‐(+)‐1‐phenylpropylamine, (R)‐(+)‐1‐(4′‐methoxyphenyl)ethylamine. The meso stereoisomers of bisphosphanes 3_RSS, 3–5_RSR readily form neutral and cationic P,P‐chelate complexes 6–9 with platinum(II) (6: cis‐dichloro‐(3R,6S)‐3,6‐diphenyl‐1‐[(1S)‐1‐phenylethyl]‐1‐aza‐3,6‐diphosphacycloheptaneplatinum(II); 7: cis‐dichloro‐(3R,6S)‐1‐[(1R)‐1‐(4′‐methoxyphenyl)ethyl]‐3,6‐diphenyl‐1‐aza‐3,6‐diphosphacycloheptaneplatinum(II); 8:bis{(3R,6S)‐[(1R)‐3,6‐diphenyl‐1‐(1‐phenylethyl)‐1‐aza‐3,6‐diphosphacycloheptane]}platinum(II) dichloride; 9: bis{(3R,6S)‐3,6‐diphenyl‐1‐[(1R)‐1‐phenylpropyl]‐1‐aza‐3,6‐diphosphacycloheptane}platinum(II) dichloride). The fast and selective formation of the cationic complex (9) of the meso isomer 5_RSR allowed for the separation of the 5_RRR and 5_SSR isomers of the bisphosphane 5. In solution, selective stereoconversion of the pure 3_RSS, 3_RSR and 4_RSR isomers into a mixture containing 3_RRS, 3_SSR and 4_SSR stereoisomers as the predominant products was observed. As a result, the stereoconversion is accompanied by a change in not only the value but also in the sign of the specific rotation, [α]_D²⁰. The ratio of the stereoisomers in an equilibrium mixture of 3_C(S) (3_RSS/3_RRS/3_SSS in a 24:48:28 ratio) is in accord with their relative energies that were calculated with the B3LYP/6‐31G* basis. Kinetic studies of the stereoconversion of bisphosphane 3_C(S) have been performed. The rate constants that were determined are indicative of a bimolecular interconversion mechanism, which is supposedly the reason for the relatively low activation energies for inversion at phosphorus. Molecular structures were obtained, by X‐ray crystal structure analysis, for the meso 3_RSS and 3_RRS stereoisomers of the N‐(S)‐1‐phenylethyl‐substituted bisphosphanes and for 4_RSR.