Ring sizes, favored

Strain energy of the bridgehead alkenes. A decrease in ring size favors the formation of the cyclopropylmethyl acetates. 

Ring sizes, favored. See Cyclizations mRNA ss messenger RNA gene cloning, 242 Robinson aneilation, 71-73,93 of enamines, 298... 

The temperature at which AG changes sign (i.e., where AG = 0) is given by the ratio AH/AS if these quantities are independent of T. The temperature at which rings become favored depends on the specific values of AH and AS, and, again, this is a matter of ring size. 

It is apparent that replacement names can be formed for rings of alt sizes, and the rules for forming them are simple. On the other hand, the names are usually quite long, and as a consequence, chemists more often than not avoid such names in favor of recognized trivial names or Hantzsch-Widman names for ring sizes for which they are available. Examples are given in (17)-(20). 

A systematic effort to correlate ease of ring closure with the stereoelectronic requirements of the transition state has been developed by Baldwin and co-workers. They classify ring closures with respect to three factors (a) ring size, (b) the hybridization of the carbon at the reaction site, and (c) the relationship (endocyclic or exocyclic) of the reacting bond to the forming ring. Certain types of ring closures are found to be favorable whereas others are unfavorable for stereoelectronic reasons. The relationships are summarized in Table 3.12. 

Intramolecular reactions usually dominate intermolecular addition for favorable ring sizes. Semiempirical (AMI) calculations found the intramolecular TS favorable to a comparable intermolecular reaction.68 (See Figure 4.1) The intramolecular TS, which is nearly 4 kcal/mol more stable, is quite productlike with a C—O bond distance of 1.6 A, and a bond order of 0.62. The bromonium ion bridging is unsymmetrical and fairly weak. The bond parameters for the intra- and intermolecular TSs are quite similar. 

Quantum mechanical/molecular mechanical study on the Favorskii rearrangement in aqueous media has been carried out.39 The results obtained by QM/MM methods show that, of the two accepted mechanisms for Favorskii rearrangement, the semibenzilic acid mechanism (a) is favored over the cyclopropanone mechanism (b) for the a-chlorocyclobutanone system (Scheme 6.2). However, the study of the ring-size effects reveals that the cyclopropanone mechanism is the energetically preferred reactive channel for the a-chlorocyclohexanone ring, probably due to the straining effects on bicycle cyclopropanone, an intermediate that does not appear on the semibenzilic acid pathway. These results provide new information on the key factors responsible for the behavior of reactant systems embedded in aqueous media. 

Alder et al. have studied various nonfused bicyclic ring systems with differing ring sizes with respect to the in,out-geometry displayed by these compounds <2001 JCS(P2)>. Compound 293 was synthesized and was found to favor... 

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