Stereodynamics of 9,11-Diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecanes. Highly Restricted Nitrogen Inversion and Isolated Phenyl Rotation. X-ray Crystallographic, Dynamic NMR, and Molecular Mechanics Studies.

The 1H NMR spectra of 10-benzyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.4,110.5,9]undecane (BnPh2) and 10-methyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.4,110.5,9]undecane (MePh2) decoalesce due to slowing inversion at nitrogen and to slowing isolated bridgehead phenyl rotation. The high nitrogen inversion barriers in MePh2 (ΔG⧧ = 12.2 ± 0.1 kcal/mol at 250 K) and BnPh2 (ΔG⧧ = 10.6 ± 0.1 kcal/mol at 215 K) are typical of tertiary amines in which at least one C−N−C bond angle is constrained to a small value. Compared to the minuscule rotation barriers about sp2−sp3 carbon−carbon bonds in simple molecular systems, the bridgehead phenyl rotation barriers in MePh2 (ΔG⧧ = 9.8 ± 0.1 kcal/mol at 210 K) and BnPh2 (ΔG⧧ = 9.8 ± 0.1 kcal/mol at 210 K) are unusually high. Molecular mechanics calculations (MMX force field) suggest that the origin of the high phenyl rotation barriers lies in the close passage of an o-phenyl proton and a methyl (or benzylmethylene) proton in the transition state. BnPh2 crystallized from h...