Transition state/structure staggered

The marked diastereoselectivity observed with 9-BBN cannot be understood on the basis of a model that only takes account of minimization of allylic interactions in the ground-state conformation of the substrate (Scheme 7.4). The analysis is condensed by examination of the two limiting transition state structures in which the substituents on the allylic stereogenic center are staggered with respect to the partially formed bonds that arise in the course of the hydroboration (i.e., 28 and 31). As the results demonstrate, the use of a bulky borane reagent such as 9-BBN dramatically affects the dia-stereofacial preferences of the olefin. Careful examination reveals an important interaction, namely an unfavorable, non-bonding, steric interaction between the allylic substituent and reagent (R Q). For the reaction with bor-... 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

E2 eliminations as in Figure 4.26 usually proceed with anti- instead of. syn-selectivity. This is true although both eliminations benefit from transition states with 7t-like bonding interactions between the MOs of coplanar, breaking C-H- and C-X bonds (also cf. the discussion of Figure 4.20). The crucial factor is a steric effect that favors the anti- over the yyn-transition state, since the spectator substituents at C and adopt a nearly staggered structure in the anti-tran-... 

The poly(3-alkylthiophenes) exhibit a reversible thermochromic change that is due to a transition between low-temperature and high-temperature solid-state structures. The thermochromic mechanism involves the conformation of the alkyl group, which is dependent upon the temperature. At low temperatures the alkyl side chains adopt a fully extended, staggered conformation. As the temperature increases, the population of gauche conformations in the alkyl side chains increases... 

Tliere are a number of possible structures of the methonium ion (5), which have C Cz, or D h symmetries. G has an eclipsed structure C(e). whereas the staggered conformation [Cj(s)] was found to be the transition state for rotation of the H2 moiety of 3c-2e bond. These structures resemble a complex between Cl fu and a hydrogen molecule resulting in the formation of a 3c-2e bond. Of the possible structures, Olah et al. expressed a preference for the C, front side protonated configuration. 

The preference for the open-extended transition state and thus the anti product can be visualized as follows. If two prostereogenic centers with groups that are effectively large, medium, and small (Scheme 60) are combined, two possible diastereomeric products are possible (A and B). Assuming that no associative phenomenon exists, then the stereochemistry of the reaction should be determined solely by the steric demand of the substituents. If it is further assumed that the reaction proceeds only through staggered transition states, the elimination of detrimental eclipsing interactions results in only six possible transition structures (60.1-60.6, Scheme 60). 

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