Steric impedance

Presumably, at higher generations the increased bulk of the dendrons sterically impedes aggregation, resulting in smaller aggregate structures. 

The morpholine enamine of fra/i.s-2-decalone exists as a 1 4 mixture of A1- and A2-isomers DAD reacts with the former by equatorial attack and the latter by axial attack351 (Scheme 177). In contrast, less electrophilic alkenes, such as / -nitrostyrene and phenyl vinyl ketone, react only with the A2-isomer at C-3 by axial attack (see Section III.B), unless sterically impeded. 

Thus acetylacetonates of Zn, Ni, and Mn are trinuclear, Fig. 11-4, while Co(acac)2 is tetranuclear all have bridging j8-diketonate groups. The presence of bulky substituents on the j8-diketones such as Me3C sterically impedes oligomerization and monomers are formed. However, these are commonly solvated by H20, ROH, or py to give 5- or 6-coordinate complexes, trara-M(dike)2Ll 2. 

Obviously, the 5-methyl substituent sterically impedes the ethanol solvation of the NH form (25a), thus favouring reaction at the more exposed 4-amino group. In water, protonation of the ring nitrogen atom and solvation of the resulting NH form (25a) by the smaller water molecules can take place despite the methyl group [172],... 

Nucleophilic substitution at an allylic substrate under S 2 conditions may proceed via nucleophilic attack at the y-carbon, especially when substitution at the a-carbon sterically impedes the normal 8 2 reaction. These S 2 reactions with cyclohexenyl systems generally proceed via an anti addition of the nucleophile to the double bond, as depicted below (best overlap of participating... 

The nonheme diiron centers in proteins and model complexes with 0,N-donors can reach a number of oxidation states spanning from FenFen to FeIVFeIV The diiron(II) state is reactive with dioxygen yielding different products depending on the nature of ligands and reaction conditions (Figure 4.21). Outer-sphere electron transfer may occur for coordinatively saturated and sterically impeded complexes with sufficiently low redox potential.17... 

The only isolable complex is the tris(C2H4.)3Ni formulation. The D31, planar configuration sterically impedes addition of a fourth C2H4 ligand, which would yield an 18-electron complex. 

In the primary steric effect, steric impedance to protonation occurs when bulky groups surround the basic center. When this results in greater strain in the cation (the conjugate acid produced on protonation) than in the neutral molecule, the base is weaker.25... 

In most cases, Crams rule (sec. 4.7.B) predicts the major isomer when the reaction partner (or partners) contain a chiral center. To understand how this rule applies to orientational and facial selectivity, we must understand the transition state of the reaction (invoke the Zimmerman-Traxler model or one of the other models for predicting diastereoselectivity). The Zimmerman-Traxler model is used most often, and if it is applied to 423 and 424, the syn selectivity can be predicted. The facial selectivity shown in 427 and 428 arises from the methyl group. In 428, the enolate approaches from the face opposite the methyl, leading to diminished steric interactions and syn product (429). If the enolate approaches via 427, the steric impedance of the methyl group destabilizes that transition state relative to 428. In both 427 and 428, a Cram orientation is assumed (see above) although other rotamers are possible. The appropriate rotamer for reaction therefore is that where Rl is anti to the carbonyl oxygen. Since the phenyl group is Rl, 427 and 428 are assumed to be the appropriate orientation for the aldehyde. If an aldehyde or ketone follows anti-Cram selectivity, this aldehyde orientation must be adjusted. 

The bicyclic carbon skeleton of these agrochemicals sterically impedes nucleophilic attack by H2O and OH. As a result, the hydrolysis half-lives for both of these compounds are quite long. Because of their persistence in aquatic ecosystems, these agrochemicals have been banned for use in the U.S. however, they are still in use as soil insecticides for many crops outside of the U.S. 

The oxidation and thermal stability can be increased by various measures. For one, the individual chains can be suitably substituted so that chemical attack on the main chain is sterically impeded or made electronically more difficult. In ladder, parquet, and lattice polymers, on the other hand, a simultaneous attack on neighboring chains is statistically improbable and additionally prevented by the close packing of the chains. 

It should be noted that Ca and Sr have proved to be ineffective for condensation of dap with A-methylated H2N(CH2)3NH(CH2)3NH2. This is presumably due to steric impedance of attack by the 7V-methyl group on the azomethine bond in the L702, because of the presence of the CH3 group. As a result, synthesis of the corresponding L707 analogue cannot be achieved. 

---