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

✍ Scribed by Andreas Hilgeroth; Günter Hempel; Ute Baumeister; Detlef Reichert

Publisher
Elsevier Science
Year
1999
Tongue
English
Weight
538 KB
Volume
13
Category
Article
ISSN
0926-2040

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✦ Synopsis

13 C CPMAS NMR spectroscopy has been applied to monitor the solid-state reaction of two different photodimerizing 4-phenyl-1,4-dihydropyridines yielding a cage dimer in one case and an anti-dimer in the other case. The spectra of the reacting monomers exhibit a magnetical inequivalence of chemically equivalent CO and C2r4 carbon atoms caused by a rotation of the pseudoaxially oriented 4-phenyl substituent out off the plane through N1, C3, C8 which could be determined by X-ray crystal structure analyses of the centrosymmetrically arranged monomers. The 13 C CPMAS NMR monitoring of the cage dimer formation proves that the reaction takes place in two steps via a syn-dimer for which a non-symmetrical structure was derived from the spectrum. The non-symmetrical structure was confirmed by X-ray crystal structure analysis of one structurally related derivative. A centrosymmetric structure for both 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 molecules as well as the symmetry of the products can be directly derived from the 13 C CPMAS NMR spectra.

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