Cyclization electrophilic, regiochemistry

Presumably, the oxidative cyclization of 3 commences with direct palladation at the a position, forming o-arylpalladium(II) complex 5 in a fashion analogous to a typical electrophilic aromatic substitution (this statement will be useful in predicting the regiochemistry of oxidative additions). Subsequently, in a manner akin to an intramolecular Heck reaction, intermediate 5 undergoes an intramolecular insertion onto the other benzene ring, furnishing 6. (i-Hydride elimination of 6 then results in carbazole 4. 

While the regiochemistry of simple electrophilic additions to double bonds is controlled by a combination of electronic (Maikovnikov rule), stereoelectronic (trans diaxial addition to cyclohexenes) and steric factors,9 the intramolecular nature of electrophilic heteroatom cyclizations introduces additional conformational, stereoelectronic and entropic factors. The combination of these factors in cyclofunctionalization reactions results in a general preference for exo cyclization over endo cyclization (Scheme 4).310 However, endo closure may predominate in cases where electronic or ring strain factors strongly favor that mode of cyclization. The observed regiochemistry may differ under conditions of kinetic control from that observed under conditions of thermodynamic control. 

Regiochemistry of the electrophilic cyclization of 166 has been studied. Interestingly, 6(S)-endo-Trig cyclization seems to be more favoured than the expected 5(S)-exo-Trig... 

The second proposal by Speckamp and co-woikers used an allylsilane as an excellent nucleophile for an A -acyliminium cyclization reaction (Esch et al. 1987) (Scheme 7.13). The 3-effect of the silicon atom is a powerM determinant of the regiochemistry of allylsilane reactions with electrophiles, so the new carbon-carbon bond is formed at the vinyl caibon distal to silicon, that is, at the y-position (Hiemstra et al. 1984,1985). 

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