Geometric isomerization solvent dependence

TABLE 13 Solvent Dependence of Quantum Yields for the Geometrical Isomerization of trans- and cis-l-(9-Anthryl)-2-nitroalkenes 80/81... 

Vielsmeier-Haack formylation of 9-(phenylaminomethylene)-6,7,8,9-tetrahydro-4//-pyrido[ 1,2-a ]pyrimidin-4-one 558 (R = COOEt) with a mixture of phosphoryl chloride-dimethylformamide at room temperature for 10 hours gave an E-Z mixture (578 and 579) of the 9-(A-phenyl-AL formylamino)methylene derivative (91H1455). While 9-phenylaminometh-ylene derivative 558 (R = COOEt) exhibited a solvent-dependent E-Z geometric isomerism, the presence of a formyl group on the amino moiety of 558 (R = COOEt) leads to an increase in the activation energy for isomerization around the C=C(9) double bond, thereby permitting the separation of the E and Z isomers (578 and 579) of the formylated product. 

Dioxo-6,7,8,9-tetrahydro-4//derivatives of (12) exhibit a solvent- and substituent-dependent oxo-enol tautomerism <85JHC1253>. 9-Amino-4-oxo-6,7-dihydro-4/f derivatives of (12) exist predominantly as the enamine tautomer and not as the 9-imino-6,7,8,9-tetrahydro tautomer. The latter tautomeric form predominates for 9-hydroxyimino-, 9-hydrazino-, and 9-aminomethylene-4-oxo-6,7,8,9-tetrahydro-4 f derivatives of (12) <83JCR(S)i6i, 83JCS(P2)165,83JCS(P2)1409,830MR(2l)687>, with a solvent-dependent E-Z geometric isomerism. 9-(Arylmethylenehydrazono)-4-oxo-6,7,8,9-tetra-... 

The variation of the dimer structure with reaction conditions is the same as in dimerization of styrene i.e., the dimer is exclusively linear and unsaturated with AcClO in benzene and exclusively cyclic with Bp30Et2 in a polar solvent, e.g., (CH2C1)2. The geometric isomerism in the linear dimers depends on that of the starting monomer and on reaction temperature... 

The concept of isomerism and isomer counts were used extensively in the development of Werner s theory. They are still important, but on a much more sophisticated level than Werner could have imagined some of the structures which he used are now known to be greatly distorted22 and others are not as simple as he believed. Geometric isomers may exist in one form or another, depending upon the solvent, temperature and other conditions, or they may change their coordination numbers. For example, it has been found that [Co(PEt3)2(NCS)2] is planar in the solid state,... 

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