Steric compression effect

Some other mechanisms will be discussed in later sections The Grant-Cheney formulation (88) of steric compression effects will appear in the context of y-gauche substituent effects (Section III-C), and hyperconjugative orbital interactions will be cited in interpretations of y-anti substituent (Section III-D) and intramolecular interaction effects (Section IV). 

In the intramolecular reactions studied by Bruice and Koshland and their co-workers, proximity effects (reduction in kinetic order and elimination of unfavourable ground state conformations) and orientation effects might give rate accelerations of 10 -10 . Hence, these effects can by themselves account for the enhancements seen in most intramolecular reactions. However, a factor of 10 -10 is less than the rate acceleration calculated for many enzyme reactions and certain intramolecular reactions, for example, hydrolysis of benzalde-hyde disalicyl acetal (3 X 10 ) (Anderson and Fife, 1973) and the lactonization reaction of[l] (10 ) where a trimethyl lock has been built into the system. If hydrolysis of tetramethylsuccinanilic acid (Higuchi et al., 1966) represents a steric compression effect (10 rate acceleration), then proximity, orientation, and steric compression... 

Besides proximity effect and orientation effect, steric compression effect could be a third factor to consider. Nonetheless there can still be an enhancement of 10 to 10" in rate constants not accounted for by these factors. Among the likely candidates, electrostatic stabilization of the transition state and release of ground state strain should be mentioned. The notion of freezing of substrate specificity by Bender is also a factor for consideration. An example is the aromatic hole in a-chymotrypsin (see below) which allows a favorable steric situation for the amino add side chain of the substrate. 

Shifts are also affected by steric compression of any kind on the atom nnder study. The effect on a C atom can... 

Na or Li and ammonia, excellent yields. " A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. ... 

The steeply increasing steric compression shown when passing from one to two adjacent methyl groups is dramatically illustrated in the case of peri-hydrogens. Annelation of abenzo ring has the usual rate-increasing effect (see Section IV, C, 4), i.e., on going from 4-chloro-... 

It was postulated that the rate decreased as the basicity of the halogen decreased and/or steric compression increased in f-BuX, and as the polarizability of halogen in MeX increased. The objective of the present research was to extend this model study to isobutylene polymerization systems, in particular to investigate the effect of reagent addition sequence and that of the nature of the halogen in f-BuX and MeX on the polymerization rate and PIB yield using Me3 Al coinitiator. 

Carbon-13. Steric compression by axial P substituents produces a so-called y-effect by their interaction with the two axial protons on the same side of the ring, and there is a resultant upfield shift of the carbon atoms bearing the axial protons. This effect has been noted for a number of dioxaphosphorinans (22)42>43 as well as for phos-phorinans.22 A small but significant shift of the C-4 resonance occurs when the... 

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

A notable deshielding jS-effect (10 ppm) has been observed in an aminophosphonium salt on the P+NMe 13C chemical shift when the NHMe group is further alkylated to NMe237. Steric compression has been used, with some caution, to explain the relatively shielded / -carbon atoms in triphenylphosphonio cycloalkyl salts when they are compared with their methylcycloalkyl analogues36. 

R is axial rather than equatorial, by the gauche effect (steric compression). If R is large (Bn) an (< ) disposition is preferred and 8C(3),q5) values are more downfield. 

Table 15 Steric Compression and Group Interaction Effects on 13C Chemical Shifts of Pyrrole Derivatives"...

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