113500-40-6Relevant articles and documents
Sterically Congested Molecules: 2,2'-bis
Pastor, Stephen D.,Rodebaugh, R. K.,Odorisio, Paul A.,Pugin, Benoit,Rihs, Grety,Togni, Antonio
, p. 1175 - 1193 (2007/10/02)
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'-bis(oxy)>bis (1a) was investigated.In the solid-state CP-MAS 31P-NMR 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 biphenydiolate 3a with the phosphorochloridite 4a is the result of significant differences in the free energy of activation (ΔG*) 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 31P-NMR spectrum of the P-disulfide 7, which was prepared by the reaction of 1a with sulfur, 2s resonances were observed that strongly suggested that the lone electrons pair on P are involved in the mechanism for the transmission of the coupling data.The chiral analogues (4S,5R)-12 and (4R,5S)-12 of 1a were prepared in a three-step reaction sequence starting from the corresponding enatiomerically pure norephedrine 8 (Scheme 2).Both (4S,5R)- and (4R,5S)-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 recry stallization, and the atropisomers were observed to equilibrate in solution by monitoring the H-C(5) resonance in the 1H-NMR with time (ΔG0 = 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 13C-NMR spectrum of both (R*,R*,R*)- and (R*,S*,R*)-12, C(5) is two-bond-coupled to the oxazaphospholidine P-atom (2J(C(5),P((2)) = 8.5 Hz) that is further virtually coupled to the P-atom of the other oxazaphospholidine ring (7J(P(2),P(2')) = 30 Hz: 9J(C(5),P(2')) = 0 Hz; δ(P(2)) = δ(P(2')) = 136 ppm).In the 31P-NMR spectrum of (R*,R*,S*)-12, which was prepared from the racemic chloridite (mixture of three diastereoisomers was obtained) a 7J(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 .....
