Hyperconjugation ring strain

On the contrary, rearrangements of cyclobutylhalocarbenes to the corresponding halocyclopentenes could be anticipated to have lower energy barriers, because of greater relief of ring strain and less hyperconjugative stabilization in the carbenes... 

Further support for the rehybridization model was provided by analysis of the spin densities of annelated 1,4-naphthoquinones, n hthalenes, and cyclobutaben-zene radical anions by ESR. The hyperfine coupling constants at the methylene position decrease as ring strain increases. Calculation of spin densities using the rehybridization model leads to an excellent fit of experimental spin densities, while calculation of spin densities using either the Coulson-Crawford hyperconjugation model or INDO do not accurately correlate experimental values. Such results suggest that rehybridization can account for the observed changes in spin densities. 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

Product or reactant stabilizing factors that have been studied thus far include resonance/charge delocalization, solvation, hyperconjugation, intramolecular hydrogen bonding, aromaticity, inductive, jr-donor, polarizability, steric, anomeric, and electrostatic effects, as well as ring strain and soft-soft interactions. Product or reactant destabilization factors are mainly represented by anti-aromaticity, steric effects in some types of reactions, and, occasionally, electrostatic effects. What makes the PNS particularly useful is that it is completely general, mathematically provable,4 and knows no exception. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

We have also proposed a general definition of the concept of stabilization energy applying to any compound, as shown by the numerous applications described in this survey article. Using this concept, we have been able to carry out a critical analysis and to offer a unified definition of various former notions such as hyperconjugation, resonance, ring strain, and steric repulsion energies. 

Those results were confirmed by x-ray diffraction data <84AG(E)372,90AG(E)398> (cf. Section 1.10.3.1). Such a strong hyperconjugation in neutral (3) is due to an interaction of a strained three-membered ring and atoms of low negativity. It was shown that a carbene structure (8) is on the pathway of topomerization of (3) <85JA2773> (cf. Section 1.10.6.3). 

The efficiency of this hyperconjugation must depend essentially on the geometric configuration of the LEPs of the N(l ) and 0 atoms and the Cspi[O-0 and Cspin,-N(l ) bonds, on the one hand, and on the electronic state ofthe N and 0 atoms, on the other hand. In all of the SPs studied, the spiro center has a sterically strained structure. Because of the rigidity of the tetrahedral structure of the spiro center and the similar conformation of the indoline ring, the Cspiro—O bond makes an angle (t) of 165-171° with the LEP of the N(l) atom (Table 1). 

The authors have failed to establish whether hyperconjugation or steric strain relief is decisive for the effects observed with 6,7-dinitro brosylate 26S however, the aryl-group participation is the main reason for the low value of the isotopic effect upon solvolysis of the unsubstituted 2-exo-brosylate 264 (cf Contrary to the previous case in endo epimers the isotopic effects are rather large, sinre the latter solvolyzed without n-participation of the aromatic ring and without effective participation of the solvent in the transition state. Isotopic effects point to weak participation of the solvent in the transition state leading to a classical ion with... 

---