Strain energies carbocycles

In view of the strain energy incorporated in a carbocyclic three-membered ring (28.1 kcal/mol [65]),thebutadiyne-expanded [ ]rotanes must be true high-... 

Equivalently, the enthalpy of reaction 47 is equated to that of the analogous (unsubstituted, saturated, carbocyclic) reaction 48. The difference is 29 kJmol , taken as the strain energy of cyclopentane. 

It is now commonly accepted that most carbocycles are strained to some degree. One might anticipate that the absolute strain energies calculated for a given set of related molecules are directly proportional to their reactivities. Unfortunately, the... 

The synthesis of useful organic motifs via molecular rearrangement of strained molecules continues to be an area of profound interest to synthetic chemists. The strain energy of a variety of three-membered carbocycles has been utilized to develop new synthetic transformations. One of the important reactions that has been ejq)lored within this class of molecules is the vinylcyclopropane (VCP)-cyclopentene (CP) rearrangement, which involves the formation of a cyclopentene ring from the ring expansion of the VCP via C—C bond cleavage fScheme 11.11. 

A template contains some knowledge on the fragment conformation, i.e., symbolic description of the conformation, strain energy, flexibility, and coordinates. If no exact expression of a specific unit can be found in the library, similar templates are searched on several levels of generalization. If, for example, no template for a heterocycle can be found, the corresponding carbocycle is taken. The templates are obtained either from moleculai mechanics or X-ray crystallography. 

TABLE 13. Strain energies for the common carbocyclic compounds... 

Cyclobutanes are important building blocks, primarily resulting from their ring-expansion chemistry, which derives from substantial strain energy. In addition, the four-membered carbocycle is frequently found in numerous natural products and synthetically derived bioactive substances. Recently, asymmetric preparation of cyclobutanones 184 by [2 -I- 2] cycloaddition of dichloroketene 183 to chiral enol ethers 182 was reported (Scheme 16.27). The cycloaddition provides not only chiral cyclobutane... 

West (91), amplifying on these results, argued that since the solvolysis is bimolecular it must proceed either through a normal Sn2 bimolecular displacement or involve a rather stable pentacovalent intermediate. Both mechanisms. West believes, must involve a 5-coordinate transition state, and therefore may really be thought of as equivalent. West found that silacyclopentane was 13 times as reactive as diethylmethylsilane and 200 times as reactive as silacydohexane (which could be construed as evidence for I-strain in silacyclopentane). Since this order of reactivity is the same as that found in carbocyclic compounds, it was concluded that similar considerations of energy and entropy of reaction are encountered, a possibility that had also been advanced by Price. 

In comparison with the rich chemistry of a radical anions the synthetic utility of a radical cations is quite restricted, because pure a donors are mostly limited to Si-Si, C-Si, C-Ge, and C-Sn functionalities (Scheme 18). Only strained carbocyclic compounds [119] have low ionization energies (/ ) that make them readily accessible (t(C-C) and cr(C-H) donors, but they are of limited synthetic use. In contrast, Me3M-MMe3 (M = Sn, Ge, Si lEs ranging from 8.0 to 8.3 eV) [5] and Me4M (M = Sn, Ge, Si lEs ranging from 8.9 to 9.8 eV) [5] are, in general, better donors... 

C-H and C-C bond cleavage in a radical cations is feasible, but not a versatile option for synthetic use because of the very high oxidation potentials. Deprotonations in alkane radical cations have strongly negative Gibbs energies, with strong preference for tertiary over secondary or primary C-H bonds [118]. C-C bonds are selectively weakened in strained carbocycles [119]. 

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