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

Abstract

Abstract 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‐[(1 R )‐1‐(4′‐methoxyphenyl)ethyl]‐3,6‐diphenyl‐1‐aza‐3,6‐diphosphacycloheptane; 5 : 3,6‐diphenyl‐1‐[(1 R )‐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 R P 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‐(3 R ,6 S )‐3,6‐diphenyl‐1‐[(1 S )‐1‐phenylethyl]‐1‐aza‐3,6‐diphosphacycloheptaneplatinum(II); 7 : cis ‐dichloro‐(3 R ,6 S )‐1‐[(1 R )‐1‐(4′‐methoxyphenyl)ethyl]‐3,6‐diphenyl‐1‐aza‐3,6‐diphosphacycloheptaneplatinum(II); 8 :bis{(3 R ,6 S )‐[(1 R )‐3,6‐diphenyl‐1‐(1‐phenylethyl)‐1‐aza‐3,6‐diphosphacycloheptane]}platinum(II) dichloride; 9 : bis{(3 R ,6 S )‐3,6‐diphenyl‐1‐[(1 R )‐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 ] D 20 . 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 .