Elimination reactions strain effects

At this point the mechanistic problem was considered in light of Scheme 4. Immediately, paths B and C were excluded on the basis of the stereospecificity of the reaction. Later work on substituent effects (see later discussion) also strongly argued against such intermediates based on their electron deficiency at carbon. Because the strain effects were inconsistent with the concerted fragmentation, path A was ruled out. This left path D, intermediacy of the metallaoxetane, by process of elimination. However, positive evidence for the structure of the transition state was at this point limited. 

The apparent nonreactivity of frflMs-l,2-dichloroethene (and of Dieldrin and Aldrin) is not immediately explained by this mechanism in view of the facile reactivity of cis-l,2-dichloroethene. Steric effects may account for the differential reactivities of cis- and trfl s-l,2-dichloroethene addition of superoxide ion would be expected to relieve steric strain between the two chlorines in the cis isomer. However, this reactivity pattern also is consistent with base-catalyzed elimination reactions of these systems.32... 

Brown s investigation of the addition compounds of trimethyl borane, diborane, and boron trifluoride with amines has provided a quantitative estimation for steric strain effects in chemical reactions. He also investigated the role of steric effects in solvolytic, displacement, and in elimination reactions. His results demonstrate that steric effects can assist, as well as hinder, the rate of a chemical reaction. 

Formation of trans isomers in overwhelming predominance in the ISOC reaction leading to five-membered rings (Entries a-d) has been ascribed to the orientation in which H% H , and R are on the exo face of TS 182b (this avoids a possible strain between R and NO or between H and [48b] that is presumably present in TS 182 a). Since elimination of silanol involving H in no way interferes with the orientation of H and R, a trans relationship between H and is abundantly clear. This fully accords with the widely accepted view that approach of the dipole and dipolarophile takes place in two parallel planes [49] and that the endo TS is preferred in the absence of obvious steric effects [50]. Formation of approximately 5% cis isomer when the dipolarophile terminus is disubstituted is accountable in terms of the cycloaddition taking place via TS 182a. 

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