Transoid configurations

The comparison of rates of cycloaddition of maleic anhydride, tetracyanoethylene, and styrene to PPA shows that the latter, irrespective of the presence of electronegative groups, behaves in these reactions not as an electron-poor diene system. This fact, together with the composition of side products (giving evidence of PPA decarboxylation), allows the assumption to be made that the cycloaddition of dienophiles involves mainly decarboxylated polyene sections of cis-transoid structure213, 266. This is in agreement with the fact that PPA with predominant trans-transoid configuration interacts with these dienophiles at a substantially lower rate. The ultimate amounts of the dienophile combined with PPA of this structure is also considerably smaller. 

The success of the reaclion depends critically on experimental conditions. Ether -ligroin mixtures usually arc suitable as the solvent. The reaction is carried out at temperatures of — 15 to 25°. The reaction is restricted to ketones that can adopt a cisoid configuration. Ketones which must have a transoid configuration (e.g., 3-cyclo-hexenone) react to give products of 1,2-addition of the acetylenic unit. 

Structure III can exist in two geometrically diflerent (a cisoid and a transoid) configurations, however, the exact nature of the isomer has not been specified. 

Using a bipyridine-based template, Kelly and coworkers were able to stabilize a / -sheet like structure by the coordination of the peptide-modified bipyridine (bpy) to a Cu(II) center. [8] The 6,6 -bis-(acylamino)-2,2 -bipyridine-based peptide 3 is designed to promote a -sheet structure upon Cu(II) binding. This template was chosen because the distance between the chains in the cisoid conformation is ca. 5.2 A, which is similar to the distance between the two strands in )9-sheet structures. In the uncoordinated state, the peptide-modified bpy adopts a transoid configuration to minimize steric interactions. The cisoid conformation has to be adopted in order for efficient Cu(II) binding (4) and hence the peptide residues will be forced in a spacial arrangement favoring y9-sheet formation (Fig. 1). 

Calculated for the transoid configuration using the indicated angles and distances (S is a core atom). Clusters 40a-41 for further details (cf. Ref. 216). 

PolyaCGtyiGnes. Polyacetylene, one of the most studied of the EAPs, possesses the simplest EAP structure, consisting of alternating single and double carbon-carbon bonds, in either a cisoid or transoid configuration. (Fig. 1). The transoid configuration is thermodynamically favored, but cis-trans isomerization is a reversible process. 

N, N -diethyl benzidine. This stability was attributed to the 1,3-diene being predominantly in the unreactive transoid configuration. ... 

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