Solid-state formation of centrosymmetric cage dimeric 4-aryl-1,4-dihydropyridines via non-symmetric syn-dimers studied by C-13 cross-polarization magic angle spinning NMR spectroscopy

The solid-state formation of a centrosymmetric cage dimeric 4-aryl-1,4-dihy dropyridine was monitored by C-13 cross polarization magic angle spinning ( CP/MAS) NMR spectroscopy. The spectra of the photodimerizing derivatives do cument the changes in symmetry during the reaction and the reaction progres s. Furthermore, they clearly reflect characterizing conformational properti es of the molecules. Starting from centrosymmetric pairs of monomeric molec ules, dimerization proceeds to non-symmetrical syn-dimers, that subsequentl y cyclize to centrosymmetric cage dimers. Thus the application of C-13 CP/M AS NMR spectroscopy proves an unusual symmetry breaking in solid-state phot odimerization. Copyright (C) 1999 John Wiley & Sons, Ltd.