Epoxide ring opening reactions carbocations

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

When the attacking nucleophile is carbon-centered, intramolecular epoxide ring-opening reactions provide an entry intocarbocyclic systems. For example, epoxy allylsilane 70 cyclizes in an overwhelmingly 5-exo fashion under Lewis acid catalysis to form a putative silyl-stabilized carbocation intermediate (71) which then undergoes... 

In some epoxide ring-opening reactions C—O bond cleavage is accompanied by the development of enough carbocation character at carbon —O) to allow rearrangement to occur. These reac-... 

Carbocation formation is initiated by epoxide ring opening in squalene oxide, giving a tertiary carbocation, and this is transformed into the four-ring system of the protosteryl cation by a series of electrophilic addition reactions (see Box 8.3). 

Rough guidelines for the prediction of regioselectivity in epoxide ring openings are summarized in Scheme 4.60. Under neutral or basic reaction conditions alkyl-or aryl-substituted epoxides react with most nucleophiles at the less substituted carbon atom [248-253]. Under acidic reaction conditions, however, product mixtures or preferential attack at the most substituted carbon atom can be observed. Acids can usually be used to enhance the reactivity of epoxides and to promote substitution at the site of an epoxide which forms a carbocation more readily. 

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