The synthesis and characterization of 2‐{1‐{3,5‐bis(1,1‐dimethylethyl)‐2‐{[2,4,8,10‐tetrakis(1,1‐dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin‐6‐yl]oxy}phenyl}ethyl}‐4,6‐bis(1,1‐dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P‐NMR spectrum (1H‐decoupled) of 6, an unprecedented eight‐bond P,P coupling of J = 72.8 Hz is observed. In the X‐ray crystal structure of 6, an intramolecular P–P distance of 3.67 Å is found, which is within the sum of the van‐der‐Waals radii of the P‐atoms. The observed intramolecular P–P distance suggests that a through‐space coupling mechanism is operative. The solid‐state conformation of 6 is compared to the conformation obtained by semi‐empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid‐state structure is near a true energy minimum, but that crystal‐packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal‐packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P‐atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable‐temperature 31P‐NMR spectroscopy. Semi‐empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O‐atoms in a common plane, with the P‐atom lying above this plane.

A preparatively efficient method for the synthesis o f the substrate 10 i s described. T h i s undergoes a n intramolecular [4 t 21 cycloaddition t o give the lactam 11. Reduction of the amide carbonyl gives 6, t h e product of a n extremely facile [4 t 21 cycloreversion, a n d n o trace o f t h e Diels-Alder adduct 7 was found. A number o f transformations o f 11, including a skeletal rearrangement o f 14 t o 16, are described, and the structure a n d relative stereochemistry of t h e products is elaborated largely o n the basis o f their NMR data. T h e cis stereochemistry o f 20, a product obtained f r o m t h e cycloadduct 11 v i a hydrogenation, eliminative ether cleavage, a n d hydrogenation, is established by correlation w i t h a relay compound 27, independently synthesized via a bimolecular Diels-Alder reaction.

The conformation of the eight-membered membered 12H-dibenzo[d,g][1,3,2]dioxasilocin ring system was investigated both in the solid-state by X-ray crystallography and in solution by NOE experiments. Ab initio Hartree−Fock calculations were performed to locate all stationary points for the unsubstituted 12H-dibenzo[d,g][1,3,2]dioxasilocin ring system. The MM2* force field was parametrized to reproduce our ab initio results, and these data were compared to the experimental data. The transition states for conformational interchange were identified. The pseudoequatorial preference for a methyl substituent on either the C(12) carbon atom or silicon is greater than the difference in energy between a boat-chair and twist-boat conformation in 2,4,8,10-tetra-tert-butyl-substituted 12H-dibenzo[d,g][1,3,2]dioxasilocins. The conformations observed in the solid-state X-ray crystal structures of 12H-dibenzo[d,g][1,3,2]dioxasilocins are sensitive to crystal-packing forces and may be different from that in solution.

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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 31PNMR 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*)‐1ain the reaction of the disodium biphenyldiolate 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‐sulfide 7, which was prepared by the reaction of la 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 coupling data. The (4S,5R)‐12 and (4R, 5S)‐12 of la were prepared in a three‐step reaction sequence starting from the corresponding enantiomerically pure norephredine 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 recrystallization, and the atropisomers were observed to equilibrate in solution by monitoring the HC(5) resonance in the 1H‐NMR with time (ΔG° = 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 independence of the observed coupling upon the orientation of the lone‐pair of electrons on P provided that the conformations of the diastereoiso...