Geometric restriction

The attack by a reagent of a molecule might be hampered by the presence of other atoms near the reaction site. The larger these atoms and the more are there, the higher is the geometric restriction, the steric hindrance, on reactivity. Figure 3-6e illustrates this for the attack of a nucleophile on the substrate in a nucleophilic aliphatic substitution reaction. 

There have been other promising lines along which the theory of quenched-annealed systems has progressed recently. One of them, worth discussing in more detail, is the adsorption of fluids in inhomogeneous, i.e. geometrically restricted, quenched media [31,32]. In this area one encounters severe methodological and technical difficulties. At the moment, a set of results has been obtained at the level of a hard sphere type model adsorbed in sht-like pores with quenched distribution of hard sphere obstacles [33]. However, the problem of phase transitions has remained out of the question so far. 

These interactions are most commonly observed for divalent chalcogen atoms and the nitrogen atom (the electron donor D) lies within the X-E-Y (E = S, Se, Te) plane, preferably along the extension of one of the covalent bonds as in 15.3. This anisotropy is a clear indication that these short E N contacts have some bonding character, i.e., they are subject to the geometric restrictions of orbital overlap. Eor example, in the diselenide 15.4 the nitrogen lone pairs are clearly oriented towards the Se-Se linkage. ... 

Experimentally, there are two approaches to the elucidation of the structure of vinyl cations first, preparation and solvolysis of systems where because of geometric restrictions the intermediate vinyl cation by necessity is bent and second, by a careful examination of the stereochemistry of solvolysis of appropriate acyclic substrates. 

These are severe geometrical restrictions, but to come to an analytical solution we also have to apply severe thermal restrictions. 

In the packing model [50,62,68] the entanglement distance is interpreted by the gradual build-up of geometrical restrictions due to the existence of other chains in the environment or, more precisely, the entanglement distance is determined by a volume which must contain a defined number of different chains. This approach is based on the observation that, for many polymer chains, the product of the density of the chain sections between entanglements is... 

When the fact that ternary collisions are relatively rare occurrences is combined with the fact that there will probably be severe geometric restrictions on such reactions, one concludes that these reactions must have relatively low activation energies or else their reaction rates would be vanishingly small. This expectation is confirmed by experimental data on such reactions. 

In the [4 + 2] cycloadditions discussed so far, the enol ether double bond of alkoxyallenes is exclusively attacked by the heterodienes, resulting in products bearing the alkoxy group at C-6of the heterocycles. This regioselective behavior is expected for [4+2] cycloadditions with inverse electron demand considering the HOMO coefficients of methoxyallene 145 [100]. In contrast, all known intramolecular Diels-Alder reactions of allenyl ether intermediates occur at the terminal C=C bond [101], most probably because of geometric restrictions. 

Deviations from the Forster decay (Eq. 9.29) arise from the geometrical restrictions. In the case of spheres, the restricted space results in a crossover from a three-dimensional Forster-type behavior to a time-independent limit. In an infinite cylinder, the cylindrical geometry leads to a crossover from a three-dimensional to a one-dimensional behavior. In both cases, the geometrical restriction induces a slower relaxation of the donor. 

This selectivity is not achievable by simple chemical hydrolysis, since the strained P-lactam ring is much more susceptible to nucleophilic attack than the unstrained side-chain amide function. Normally, the electron-donating effect from the lone pair of the adjacent nitrogen stabilizes the carbonyl against nucleophilic attack (see Section 7.9.2) this is not possible with the P-lactam ring because of the geometric restrictions (see Box 3.20). 

Now the 1,3-cyclopentanediyl diradical is constrained to cychze in a disrotatory fashion while the trimethylene species might well close in both disrotatory and con-rotatory ways. Were all other factors constant one could infer that the geometrical restrictions imposed on the 1,3-cyclopentanediyl diradical entailed no significant deduction in rate of cyclization, and thus that conrotatory cyclization of the trimethylene diradical is not strongly preferred under the given reaction conditions and circumstances. 

All complexes should have the cis geometry, with the general formulas [cis-PtX2(Am)2] for Pt(II) and [cis-PtY2X2(Am)2] for Pt(IV) compounds (14). This geometric restriction is automatically answered for didentate amines such as ethylenediamine. 

Block copolymers of sufficiently large and incompatible sequences of repeating units undergo microphase separation. Because of the geometric restriction caused by the needed alignment of the junctions between the different blocks, the phases... 

Azo compound decomposition is much less susceptible to polar substituent effects, and so probably has less charge separation in the transition state,75 but is more sensitive to geometrical restrictions. Bridgehead azo compounds decompose at rates lower than expected on the basis of their tertiary nature, whereas peresters are much less strongly affected.70 The difference can be rationalized by the proposal that the transition state comes farther along the reaction coordinate in azo decomposition, so that the nonplanarity forced on the incipient radical by the ring system is felt more strongly there. 

Two other paramagnetic reactants which react with elq at close to diffusion-controlled rates are NO and O2. It has been already pointed out that whereas NO reacts with a diffusion-controlled specific rate of 3.14 =fc 0.2 X 1010 Af-1 sec.-1 (58), 02 reacts with a rate of 1.88 =fc 0.2 X 1010 M-1 sec.-1 (92) apd may therefore have some geometrical restrictions. The product of elQ + NO is most probably NO- which on reaction with NO may yield N20. This requires experimental verification. The formation of 02 from e aq + 02 has been demonstrated unambiguously by the fact that the rate of appearance of 02 - is identical with the rate of disappearance of elQ in its reaction with 02 (60). 02- disappears by... 

In general, the induced geometrical restrictions in compounds of type 38 modify the chemical behaviour to some extent in relation to the corresponding unbridged species. 

A series of [n,n]-ferrocenophanes is listed in Scheme 20 together with their AE1/2 values. The introduction of a second bridge enforces the c/.v-con figuration and a comparatively shorter Fe—Fe distance. Thus, the geometrical restrictions enhance the possibility... 

Generally, most radical cyclizations proceed kinetically in an exo fashion to provide the smaller of the two possible rings. Nonetheless, as an important exception, 1-endo and 8-endo cyclizations do occur when the (conformationally restrained) intermediate radicals possess geometrical restrictions, i.e. less degrees of freedom relative to normal heptenyl radicals. It also should be noted that the rate constants for 1-exo and 8-endo cyclization of 7-octenyl radical are almost identical—the preference for exo-closure decreases as the rings get bigger [86]. 

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