Factors Influencing the Rate of Cyclization

The influence of dienophile substitution on the rate of the intermolecular Diels-Alder reaction has been investigated (1, 2). From this data, it can be concluded that an electron-withdrawing group will activate a dienophile in the order shown [1]. The same order of reactivity obtains for the intramolecular cyclization [2-4]. 

In addition to these monosubstituted examples, an excellent comparison of dienophiles bearing more than one electron-withdrawing group has appeared (4). Still, this area is not yet entirely understood (8). It should be pointed out that some trienes cyclize so rapidly ([5]-in this case, probably under acid catalysis) that kinetic studies would be difficult. 

It is apparent from the Uterature (1, 2) that a-activated dienophiles (meth-acryloyl) are more reactive in the intermolecular cyclization that P-activated dienophiles (crotyl). While there may be other factors involved, this seems also to be true for the intramolecular cyclization [6]. It is clear that increasing the alkyl substitution on the dienophile retards the cyclization [7]. 

---

The preferential 1,5-ring closure of unsubsliluted 5-hexenyl radicals has been attributed to various factors these arc discussed in greater detail in Section 2.3.4. The mode and rate of cyclization is strongly influenced by substituents. The results may be summarized as follows (Scheme 4.13) ... 

The influence of ring strain on the reactivity of the Bergman reaction is very important. Nicolaou and Smith [107] suggested that the crucial factor for determining the reactivity is the interatomic distance between diyne termini (popularly known as the distance theory). The influence of stereoelec-tronic effects is also important, with the observation that chlorine substitution in the vinyl position retards the rate of the Bergman cyclization [108]. 

The kinetics of these reactions in comparison with those for methylenecyclo-propane analogs of compounds 160 have been studied by following the progress at pressures up to 3 kbar by on-line FT-IR spectroscopy [129]. The rate-enhancing influence of the additional strain in 160 overcompensates the expected retarding effect of the increased steric shielding by the second cyclopropane unit in 1 compared to methylenecyclopropane, and the cyclization rates for compounds 160 were faster by a factor of 6.8 to 8.1 in comparison with the corresponding methylenecyclopropane derivatives. 

Phenylsodium is formed initially in the reaction between benzonitrile and sodium (Scheme 79) (59JOC208). Trimerizations of aryl or heterocyclic nitriles catalyzed by amines or alkoxides resemble the reactions of the perfluoronitriles described above (Schemes 77 and 78 respectively). Kurabayashi et al. (71BCJ3413) studied the trimerization of benzonitrile under pressure in methanol, and have shown that the rate determining step is the reaction of the benzimino ether with benzonitrile. Steric factors exert an important influence in the trimerizations of aryl cyanides. The cyclizations of ort/zo-substituted aryl cyanides need more severe conditions than either the corresponding meta or para derivatives (Table 14). 

It may, therefore, be concluded that such parameters as intramolecular cyclization, intramolecular hydrogen abstraction, steric effects, and resonance effect can most satisfactorily explain the yields of monomers in thermal degradation of polyolefins. Unfortunately, excq)t for hydrogen abstraction, three otiier parameters have not been taken into consideration in deducing expressions of rate constants for diermal d adation of polyolefins. The resonance effect should influence the activation energy of the degradation reaction and the steric effects are more prominent in the frequency factors of the rate constants ... 

---