CONFORMATIONAL-ANALYSIS OF [3,3]CYCLOPHANES .5. A CONFORMATIONAL STUDY OF [3.3](2,6)PYRIDINOPHANE BY THE DYNAMIC NMR METHOD AND X-RAY STRUCTURAL-ANALYSIS
K. Sako et al., CONFORMATIONAL-ANALYSIS OF [3,3]CYCLOPHANES .5. A CONFORMATIONAL STUDY OF [3.3](2,6)PYRIDINOPHANE BY THE DYNAMIC NMR METHOD AND X-RAY STRUCTURAL-ANALYSIS, Liebigs Annalen, (10), 1996, pp. 1645-1649
A variable-temperature H-1-NMR study and X-ray structural analysis sho
w that the most stable conformational isomer of [3.3](2,6)pyridinophan
e 2 is the syn(boat-boat) conformer, and the relative stability order
of the three stable conformers is syn(boat-boat) > syn(chair-boat) > s
yn(chair-chair). This is in sharp contrast to the relative stability o
rder of the parent [3.3]metacyclophane 1: syn(chair-chair) > syn(chair
-boat) > syn(boat-boat). The high stability of the syn(boat-boat) conf
ormer 2c is primarily attributed to weak attractive interactions via i
ntramolecular hydrogen bonds between nitrogen lone pairs (N-1, N-2) an
d the axial hydrogen atoms (2A-H, 11A-H) on the central carbon atoms o
f the bridges, as suggested by their short transannular distances (2.5
0 Angstrom).