The Ring Size Effect

If the size of an annelated ring decreases, its angular strain increases due to a more pronounced bending of local hybrid orbitals. It is credible, therefore, to expect larger distortions of the benzene fragment. This turns out to be true as evidenced by benzocy-clobutene 11 and benzocyclopropene 12 (Fig. 12). Their theoretical structural parame- 

Selected structural parameters of benzocyclobutene 11 and benzocyclopropene 12 as obtained by several theoretical models and X-ray technique (IBDs in A and angles in degrees). 

Bond/Angle Theory Exp. Mol. Bond/Angle Theory Exp. 

It is interesting to mention that Boese et al. [49] gave BPL values estimated by X-ray measurements, which differed very little from the interatomic distances (IBDs). They are as follows f(C(l)-C(2)) = 1.41 A, f(C(2)-C(3)) = 1.40 A, /(C(3)-C(4)) = 1.41 A and Z(C(4)-C(5)) = 1.40 A implying that ortho and para bonds are shorter by 0.01 A as required by the MN localization pattern. Hence both IBDs and BPLs offer the same picture. 

Perusal of structural data of benzocyclopropene 12 presented in Table 3 shows that the shortest CC bond is C(l) C(2). It would be erroneous, however, to conclude that it has 

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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. 

The chromium-catalyzed coupling is extended to the intramolecular cycli-zation to the dials 43. The O-chelating silyl scavenger is used to control the stereoselectivity as shown in Scheme 25 [61,62]. The ring size effect on the stereoselectivity is also investigated. 

Partial rate constants for H-D exchange at the methyl and methine sites of cycloalkyl methyl ketones have been measured chiefly in order to examine the ring-size effect on exocyclic enol and enolate formation. Under base-catalysed... 

It has been reported that RCM of enamides affording five- and six-membered cyclic enamides readily proceeds when the enamides contain a protective group on the N atom. However, an attempt to create a seven-membered cyclic enamide through RCM of 84 resulted in exclusive formation of a six-membered ring 86 (Eq. 12.34) [43]. This reaction was thought to proceed by way of ruthenium-catalyzed isomerization to the intermediary olefin 85, followed by ring closure of the isomerized intermediate to the six-membered enamide 86, which is a typical example of the ring-size effect. 

The other pertinent information one might seek would be the effort on the rate of cyclization of including a heteroatom in the place of a methylene in the bridge. It is apparent that ethers are about the same as their all-carbon counterparts [25], while amides, with and without the carbonyl group in the bridge, are substantially slower [26]. Both this effect and the ring size effects noted above will probably vary with the position of the heteroatom in the bridge. 

Efficient photo-isomerization of water-soluble 1,4-diaryl-l,3-butadiene dendrimers (19) occurred in high quantum yield (Scheme 11). " E-Z photoisomerization of benzylidene-oxazolones (Z-20) and ( -20) was also reported as molecular photo-swiches. The ring size effects of aromatics of styrylbenzothiazol (21) were examined (Scheme 12). ... 

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