Stericaliy Congested Molecules: 2,2′‐[(Biaryldiyl)bis(oxy)]bis[1,3,2‐oxazaphospholidines]

Abstract

Abstract The original suggestion that a through‐space mechanism was operative in the seven‐bond J (P, P) coupling constant of 30.3 Hz observed for 3.3′‐bis(1,1‐dimethylethyl)‐2,2′‐[3,3′,5,5′‐tetrakis(1,1‐dimethylethyl)‐1,1′‐biphenyl‐2,2′‐diyl]bis(oxy)}bis[1,3,2‐oxazaphospholidine] ( 1a )) was investigated. In the solid‐state CP‐MAS 31 PNMR spectrum of 1a , two nonequivalent P ‐atoms were observed; sufficient resolution could not be obtained to determine whether P, P coupling was present. The preparation and spectral data of the N ‐methyl analogue 1b and of the acyclic N ‐isopropyl analogue 6 ( Scheme 1 ) provided evidence that a ) the essentially exclusive formation ( R *, R *, S *)‐ 1a in the reaction of the disodium biphenyldiolate 3a with the phosphorochloridite 4a is the result of significant differences in the free energy of activation (Δ G *) for the formation of the various diastereoisomers due to the steric congestion within the molecule and that b ) the magnitude of the observed P,P coupling is dependent upon the degree of conformational freedom within the molecule. In the 31 P‐NMR spectrum of the P ‐sulfide 7 , which was prepared by the reaction of la with sulfur, 2 s resonances were observed that strongly suggested that the lone electrons pair on P are involved in the mechanism for the transmission of coupling data. The (4 S ,5 R ) ‐12 and (4 R , 5 S ) ‐12 of la were prepared in a three‐step reaction sequence starting from the corresponding enantiomerically pure norephredine 8 ( Scheme 2 ). Both (4 S , 5 R )‐ and (4 R , 5 S ) ‐12 were obtained as a diastereoisomer mixture that differ by the configuration of the axis of chirality, i.e. , ( R * R *, R *)‐ and ( R *, S *, R *) ‐12 were obtained. The major diastereoisomer was obtained upon recrystallization, and the atropisomers were observed to equilibrate in solution by monitoring the HC(5) resonance in the 1 H‐NMR with time (Δ G ° = 0.4 kcal/mol; Fig. 2 ). The process observed corresponds to the restricted rotation about the central single bond of the biphenyl system. The isolation of an atropisomer with only a single ortho substituent on each aryl ring is quite rare. In the 13 C‐NMR spectrum of both ( R *, R *, R *)‐ and ( R *, S *, R *) ‐12 , C(5) is two‐bond‐coupled to the oxazaphospholidine P‐atom ( 2 J (C(5), P ((2)) = 8.5 Hz) that is further virtually coupled to the P‐atom of the other oxazaphospholidine ring (7 J ( P (2), P (2′)) = 30 Hz; 9 J (C(5),P(2′)) = 0 Hz; δ(P(2)) = δ(P(2′)) = 136 ppm. In the 31 P‐NMR spectrum of ( R *, R *, S *) ‐12 , which was prepared from the racemic chloridite (mixture of three diastereoisomers was obtained), a 7 J(P(2),P(2′) of 36 Hz was observed. These observations provide strong evidence that seven‐bond P,P coupling occurs in all three diastereoisomers of 12 . The observed P,P coupling is both independent of the configuration of the chiral axis and the configuration of the asymmetric P‐centers . This independence of P,P coupling upon the configuration on P implies also the independence of the observed coupling upon the orientation of the lone‐pair of electrons on P provided that the conformations of the diastereoisomers are similar in solution . The X‐ray crystal structure of the complex formed from 1a and dichloro(cycloocta‐1,5‐diene)platinum(II) was obtained and the solid‐state structure discussed. The major diastereoisomer of (4 S ,5 R ) ‐12 was used as a chiral ligand in asymmetric hydrosilylation and hydrogenation reactions ( Scheme 3 ).