Steric difficulties

The steric difficulties in chains of mono-substituted units are minor compared with those which occur when alternate atoms of the chain bear two substituents. 

A number of larger cyclic conjugated systems have been prepared, many of them by Sondheimer and co-workers.16 [10]Annulene and [12]annulene are subject to considerable steric difficulties and probably are not planar, but the larger molecules are big enough to accommodate hydrogen atoms inside the rings and so can have trans double bonds and still be planar or nearly so. Likely conformations for some of these compounds are shown in Structures 39, 40, and 41. 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

It is also apparently possible, despite what would seem to be considerable steric difficulty, for a [1,3]-reaction to be antarafacial on the n component. Measurements of rates of racemization and of deuterium scrambling show that at least part of the rearrangement in Equation 12.78 proceeds by the path illustrated in Equation 12.79.130... 

Several explanations were proposed for this disappointing result. According to Bushby et al. (1997b), for the cross-linked polymers, a more likely interpretation is the steric difficulty of incorporating counterions into a relatively rigid polymer network. The authors consider the PF<7 anion as the counterion to each cation radical part of the cross-linked polymer. If this is so, it is unlikely to be insurmountable. A bulky anion can be replaced by an anion of small size. 

The experimental rate constants are in good agreement with those calculated from the additivity principle. Apparently, the steric difficulties mentioned above are not significant in these exchange reactions (cf. p. 192). 

The steric difficulty inherent in a hindered diene is partly solved by use of high pressure. Lewis acids can also accelerate the rate of high-pressure reactions, but more importantly, can improve stereo- and regioselectivity. Thus the reaction of 1 with the quinone 2 under high pressure results in both 3 and 4. The latter, undesired product is a result of pressure-promoted oxidation of 3. Use of ZnBr2 improves the yield of 3 and decreases formation of 4. Yb(fod)3 and Eu(fod)3 are useful catalysts in reactions of the enol ether 5. 

In general, the stabilization afforded to a radical by stabilizing substituents is not additive. The origin of this non-additivity is related to the steric difficulty for di- or trisubstituted radicals to reach a geometry where all substituents are able to fully interact with the single electron. 

As is seen from Table 4, the complexes containing an alternating copolymer as one of the interacting components are rich in the nonionic component (PVP or PEG). Probably part of the PVP(PEG) groups does not participate in the complex formation, due both to the increase of the distance between the active groups in the copolymer and the steric difficulties caused by the bulky anhydride groups. 

Yes, hexoses are too bulky base pairing in the Watson-Crick mode of hexose analogs of RNA runs into steric difficulties. This is a relatively solid result in this kind of work after all, research on molecular evolution cannot be expected to reach conclusions that are as definite as those in normal chemistry. This type of inquiry and experimental strategy will gradually allow us to approach a question that science will have to ask sooner or later, namely, whether the chemical uniqueness of life, as we know it, is intrinsic or accidental. For the time being, inquiring about the uniqueness of life is premature, but this may change some time in the future. 

The case of 2-chloro-2-methylcyclohexanone (2) presents no comparable steric difficulty, and here dehydrohalogenation by the Holysz method and with collidine both proceed rather poorly and in comparable yield. ... 

A more interesting field is that of the thermal 1,2-cycloadditions here concerted processes are forbidden, and experimental results can be compared with this prediction. A discrepancy arises in the case of keteiies, as there is considerable evidence in favour of a concerted mechanism for their thermal 1,2-cycloadditions (Section 6.1). However, it is possible to envisage the intervention of the perpendicular Tr-system of the C=0 bond of ketene, in such a way as to surmount the steric difficulties of an orthogonal approach of the reactants, required by a cis-trans (or supra,antara) cycloaddition the latter is symmetry-allowed as a thermal process when it is w + n = 4. 

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