Sterically crowded

Observed deviations from ideality are attributable to thiazole selfassociation. Such self-association is influenced by steric crowding as indicated by the behavior of methylthiazoles. The constants of selfassociation have been estimated for benzene solutions of thiazole (Kassoc = 3.2 at 5.5°C) and 5-methylthiazole at 6.5°C). 

Clearly the steric crowding that influences reaction rates in 8 2 processes plays no role in Stvfl reactions The order of alkyl halide reactivity in 8 1 reactions is the same as the order of carbocation stability the more stable the carbocation the more reactive the alkyl halide... 

There is Httie known chemistry of tetrakis-Cp thorium complexes. Pseudotetrahedral molecules, (rj-ring) Tb (15), where ring = Cp [1298-75-5] or Ind [11133-17-6], have a measurable dipole, resulting from deviation of symmetry (88). Difference of coordination environments, eg, v] in the indenyl system and in the Cp system, appears to indicate great steric crowding about the thorium center, which probably limits the reactivity and synthetic derivatization of these complexes. 

Epoxide formation from chlorohydrins is marked by an increase in rate with alkyl substitution (28) as shown in Figure 1. This phenomenon has been explained on the basis that steric crowding ia the chlorohydrin is somewhat reheved as the epoxide is formed, so that the greatest rehef of strain results from ring closure of the most crowded chlorohydrin (28). 

Other types of bonding include donation by Ligand TT-orbitals, as in the classical Zeiss s salt ion [Pt( 7 -CH2=CH2)Cl3] [12275-00-2] and sandwich compounds such as ferrocene. Another type is the delta (5) bond, as in the Re2Clg ion, which consists of two ReCl squares with the Re—Re bonding and echpsed chlorides. The Re—Re 5 bond makes the system quadmply bonded and holds the chlorides in sterically crowded conditions. Numerous other coordination compounds contain two or more metal atoms having metal—metal bonds (11). 

Steiic effects of another kind become important in highly branched substrates, in which ionization is facilitated by relief of steric crowding in going from the tetrahedral groimd state to the transition state for ionization. The ratio of the hydrolysis rates in 8OV0 aqueous acetone of t-butyl /F-nitrobenzoate and 2,3,3-trimethyl-2-butyl p-nitrobenzoate is 1 4.4. 

Aryl migrations are promoted by steric crowding in the initial radical site. This trend is illustrated by data from the thermal decomposition of a series of diacyl peroxides. The amount of product derived from rearrangement increases with the size and number of substituents ... 

Brominations and lodinations of tertiary perfluorinated carbanions can be reversible (equations 16 and 17) because of stability of the carbanions and steric crowding in the product [775J... 

Extensions of the enamine alkylation to a-tetralones have also been used (245-248), but product yields were lower, presumably due to steric crowding in a transition state where generation of an imonium salt gives rise to a repulsion between a methylene group on nitrogen and a peri aromatic proton. 

FIGURE 6.3 Many of the possible conformations about an -carbon between two peptide planes are forbidden because of steric crowding. Several noteworthy examples are shown here. 

FIGURE 16.15 Orientation effects in intramolecular reactions can be dramatic. Steric crowding by methyl groups provides a rate acceleration of 2.5 X 10 for the lower reaction compared to the upper reaction. (Adaptedfrom Milstien,. S., and Cohen, L. A., 1972. Stereopopnlation control I. Rate enhancements in the laetonization of o-hyelroxyhyeJroeinnamie acid. Journal of the American Chemical Society 94 9158-9165.)... 

This is also the case in the decamethylmetal-locenes of Ru and Os but not in the iron analogue which has a staggered conformation, presumably due to steric crowding around the smaller metal. 

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

Sterically crowded molecules, like neopentane, are too unstable. 

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

Adsorbability is influenced strongly hysteric hindrance, and because of this almost any function can be reduced in the presence of almost any other function in suitably constructed molecules. A case in point is the reduction of the aromatic ring in I in preference to reduction of the nitro function, producing 2. However, when R = CH3 (3) the nitro group was reduced instead, a fact attributed to a less sterically crowded environment 109). 

The above generalities apply particularly to palladium. Hydrogenation over platinum or rhodium are far less sensitive to the influence of steric crowding. Reduction of 1-t-butylnaphthalene over platinum, rhodium, and palladium resulted in values of /ci//c2 of 0.42, 0.71, and 0.024, respectively. Also, unlike mononuclear aromatics, palladium reduces substituted naphthalenes at substantially higher rates than does either platinum or rhodium. For example, the rate constants, k x 10 in mol sec" g catalyst", in acetic acid at 50 C and 1 atm, were (for 1,8-diisopropylnaphthalene) Pd (142), Pt(l8.4), and Rh(7.1)(25). 

Note that the otCH-pCH torsion angle is trans in all cases (D. W. Urry and N. G. Kumar, Biochemistry, 13, 1829-1831, (1974). A gauche orientation gives so much steric crowding that the complex will not form... 

A most attractive feature of radical reactions that recommends their use in the synthesis of complex molecules is that steric crowding, particularly on the radical center, is tolerated in many instances. Indeed, radical reactions are ideally suited for the con-... 

Unsubstituted imides, with a single stereogcnic center appended to nitrogen, are stereoselective-ly reduced with lithium triethylborohydride at — 78 C with diastereoselectivities up to 100% in the case of severe steric crowding (Ar = 2,6-dichlorophenyl) (see Appendix)46. 

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