Cyclic transition states freedom

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

Some sort of cyclic aspect is usually a feature of good stereocontrol. This aspect might be in the form of a cyclic transition state or perhaps in the generation of a genuinely cyclic compound but in any case a cyclic aspect is associated with fewer degrees of freedom - or less Soppiness - in whatever system we are considering which usually means better control. Without trying to be exact, the cyclic aspect of reactions broadly decreases down the series -... 

The loss of rotational freedom when the cyclic transition state is formed from the acyclic starting material accounts for the observed negative entropies and volumes of activation. It has been established that the transition state of the closely related Cope rearrangement (see below) resembles the chair conformation of cyclohexane. An analogous transition state structure for the Claisen rearrangement would account for the fact that isomerization of vinyl a-methylallyl ether yields 97% trans 4-hexQm and 3% cw-4-hexenal, since there should be a preference for an equatorial orientation of the methyl group in a cyclohexane chair-like transition state . 

Cope rearrangements typically have entropies of activation of about — 10 eii. (see Table 5) and isomerization of ethyl allyl(l-ethyl-propenyl)cyanoacetate at 119°C has volumes of activation of about —7 cm. mole" in a variety of solvents. The negative entropies and volumes of activation are consistent with rate-limiting formation of a cyclic transition state which has fewer rotational degrees of freedom than the ground state. ... 

Electrocyclic ring closures of acyclic trienes and tetraenes have substantial negative entropies of activation, as anticipated for reactions in which rotational freedom is lost on assembling the cyclic transition states. Conversions of the more rigid cyclic trienes and tetraenes to bicyclic hydrocarbons usually have less negative entropies of activation. 

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