Solid-state photodimerization of 4-aryl-1,4-dihydropyridines studied by C-13 CPMAS NMR spectroscopy

C-13 CPMAS NMR spectroscopy has been applied to monitor the solid-state rea ction of two different photodimerizing 4-phenyl-1,4-dihydropyridines yieldi ng a cage dimer in one case and an anti-dimer in the other case. The spectr a of the reacting monomers exhibit a magnetical inequivalence of chemically equivalent CO and C2/4 carbon atoms caused by a rotation of the pseudoaxia lly oriented 4-phenyl substituent out off the plane through N1, C3, C8 whic h could be determined by X-ray crystal structure analyses of the centrosymm etrically arranged monomers. The C-13 CPMAS NMR monitoring of the cage dime r formation proves that the reaction takes place in two steps via a syn-dim er for which a non-symmetrical structure was derived from the spectrum. The non-symmetrical structure was confirmed by X-ray crystal structure analysi s of one structurally related derivative. A centrosymmetric structure for b oth the finally formed cage dimer and the anti-dimer of the other monitored photoreaction was proved by their spectra with one set of signals for each half of the dimers, respectively. Thus, conformational properties of the m olecules as well as the symmetry of the products can be directly derived fr om the C-13 CPMAS NMR spectra. (C) 1999 Elsevier Science B.V. All rights re served.