Ethylenes, overcrowded

Class G. Examples of ethylenic overcrowding. Two molecules, difluorenylidene and dianthronylidene (dianthrone) are discussed. 

G. Examples of Ethylenic Overcrowding The planar centrosymmetric structure proposed for difluorenylidene (di-biphenylene-ethylene) (97) by Fenmore (1948) has been examined by Nyburg (1954). In a planar centrosymmetric molecule the overcrowded carbon atoms (asterisked in 97) would be only 2-5 A and their... 

In symmetric overcrowded or otherwise strained ethylenes, the strain may be partially released by rotation around the C=C bond or by other deformations, and the barrier to E-Z isomerization may be lowered compared to that of ethylene by ground state strain and by delocalization of the double-bond ir electrons into unsaturated substituents, forming a diradical transition state. The importance of the delocalization effect is illustrated by the low barrier (AC o = 23.2 kcal/ mol) in the diphenoquinone 125 (145), in which the ground state strain must be rather low. 

Among a variety of compounds whose molecular characteristics fall into this category, our current interests have further limited our discussion to the doublebond systems which may conveniently be classified as follows 1. chiral ( )-cyclo-alkenes, 2. chiral anti-Bredt rule compounds, 3. trans-doubly bridged ethylenes ( betweenanenes ), 4. bridged allenes, and 5. overcrowded olefins. 

I. Agranat, and Y. Tapuhi, Dynamic stereochemistry in overcrowded ethylenes. Conformational behavior of bianthrones, J. Org. Chem. 44, 1941-1948 (1979). 

G. Favini, M. Simmonetta, M. Sottocomola, and R. Todeschi, Geometry and energy of overcrowded ethylene. II. Bomanylidene, fenchylidene, and bifluorenylidene derivatives, J. Comput. Chem. 3(2), 178-184 (1982). 

B. L. Feringa, W. F. Jager, and B. De Lange, Resolution of sterically overcrowded ethylenes a remarkable correlation between bond lenghts and racemization barriers, Tetrahedron Lett. 33, 2887-2890 (1992). 

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