Steric crowding, and

The dihalogen complexes with olefin donors were first identified spectroscopically in the mid-1960s [42-45] and extensive experimental and computational studies have been carried out by Chiappe, Lenoir and coworkers in recent years [46 - 48 ]. These systems are highly unstable, since the complexation of dihalogens with olefins is followed rapidly by the formation of ionic intermediates and further chemical transformations. Therefore, attention in the corresponding work has mostly focused on hindered olefins, although the spectral characteristics of complexes with less sterically crowded and alkyl- as well as chloro-substituted and cyclic olefins are also reported [44]. The absorption maxima for the dihalogen complexes with olefins (evaluated by the subtraction... 

In summary, the binding models first show a fraction of SB A or VML molecules that bind to Tn-PSM and jump between different aGalNAc residues of the mucin (Fig. 3A). As the number of bound lectin molecules increases, the affinity of the lectin decreases because of shorter diffusion distances on the mucin chain due to steric crowding and crosslinking by multiple bound lectin molecules (Fig. 3B and C). Finally, upon saturation binding, full lectin-mediated crosslinking of the complexes takes place (Fig. 3D). 

In the X-ray structures of both 3 and 4, the tetrahedral distortion is greater than that found in model A. Since there is an electronic preference for a square planar coordination, the pendant phenyl substituents in 3 and 4 likely result in further steric crowding and therefore in more distorted structures compared to model A. In the QM/MM model B an optimized 0 angle of 30° is found, close to the 34° angle of the X-ray structure. Since the phenyl and trimethyl phenyl groups are accounted for on a steric basis only in model B, the result supports the notion that the severely distorted coordination of the Pd center in 3 is due to a steric effect. 

The steric rather than the inductive origin of the secondary deuterium KIE is also suggested because kH/kD = 0.994 per deuterium found in the per-deuteropyridine-methyl iodide reaction is smaller (less inverse) than the kH/kn = 0.988 per deuterium found for the 4-deuteropyridine reaction. A secondary inductive KIE should be more inverse when a deuterium is substituted for a hydrogen nearer the reaction centre, i.e. at the meta- or ortho-rather than at the para-position of the pyridine ring. Thus, if the KIE were inductive in origin, the KIE in the perdeuteropyridine reaction should be more inverse than that observed for the 4-deuteropyridine reaction. If the observed KIE were the result of a steric KIE, on the other hand, a less inverse KIE per deuterium could be found in the perdeuteropyridine reaction, i.e. a less inverse KIE per deuterium would be expected if there were little or no increase in steric hindrance around the C—H(D) bonds as the substrate was converted into the SN2 transition state. Since the KIE per D for the perdeuteropyridine reaction is less than 1%, the transition state must not be sterically crowded and the KIE must be steric in origin. Finally, the secondary deuterium KIEs observed in the reactions between 2-methyl-d3-pyridine and methyl-, ethyl- and isopropyl iodides (entries 3, 7 and 9, Table 17) are not consistent with an inductive KIE. If an inductive KIE were important in these reactions, one would expect the same KIE for all three reactions because the deuteriums would increase the nucleophilicity of the pyridine by the same amount in each reaction. The different KIEs for these three reactions are consistent with a steric KIE because the most inverse KIE is observed in the isopropyl iodide reaction, which would be expected to have the most crowded transition state, and the least inverse KIE is found in the methyl iodide reaction, where the transition state is the least crowded. 

C -symmetric initiators have a pair of diastereotopic (nonequivalent) sites one site is sterically crowded and enantioselective, and the other site is less crowded and nonselective. The propagating polymer chain always prefers the less crowded site, but monomer coordination and migratory insertion occur at the more crowded enantioselective site. The polymer chain then back-flips to the less crowded site. This model offers a rationale for the back-flip of the polymer chain—the polymer chain is less stable at the more crowded site. 

When in compound 60 R = t-Bu, the s-trans form should be sterically crowded, and an increase of the s-cis form would be expected. The IR spectra (74SA(A)1471) seem to point out that relief of steric strain is better reached by distortion toward an s-gauche conformation than by changes in the relative amount of conformers. 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

The reactions of ammonia and primary amines with Tris and vinyl-Tris are much more rapid and complete (33). Reduced steric crowding and the discovery of much more effective catalysts such as trifluoroacetic acid and p-TSA were important contributing factors. 

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