Nucleophilic capture steric factors

Steric factors have been mentioned they are not expected to play a major role. In fact, several radical cations have been captured by attack on highly congested centers [154, 156]. The ring-opening substitution of tricyclane radical cation, 81 +, occurred exclusively at the tertiary carbon [215] whereas that of 82 occurred at the tertiary carbon further removed from the dimethyl-substituted bridge, i.e., at the less hindered of two tertiary carbons, not at the quaternary carbon [216]. These results clearly show that the nucleophilic substitution at the cyclopropane one-electron bond is subject to conventional steric hindrance and is not subject to inverse steric effects [154]. 

A study of several substituted alkenes in methanol developed some generalizations pertaining to the capture of bromonium ions by methanol For both E- and Z-disubstituted alkenes, the addition of both methanol and Br was completely anti stereospecific. The reactions were also completely regioselective, in accordance with Markovnikov s rule, for disubstituted alkenes, but not for mono substituted alkenes. The lack of high regioselectivity of the addition to monosubstituted alkenes can be interpreted as competitive addition of solvent at both the mono- and unsubstituted carbons of the bromonium ion. This competition reflects conflicting steric and electronic effects. Steric factors promote addition of the nucleophile at the unsubstituted position, whereas electronic factors have the opposite effect. 

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