Intramolecular ring-closure reactions involving aromatic rings

Ring closure reactions taking place by intramolecular addition of an aromatic radical to a double bond have been widely studied on both their regio- and stereochemical aspects [93]. Aryl halides and diazonium salts substituted at ortho- position with a 0-allyl or TV-allyl chain were used for the preparation of 2,3-dihydrobenzofuranes and 2,3-dihydro-1/7-indoles under different reaction conditions. The reaction pattern involves the generation of an aryl radical 20, which reacts with the double bond in a 5-exo trig fashion to afford the exocyclic radical 21, plausible of reduction by a hydrogen donor to obtain the reduced-cyclized product 22 (Sch. 23) [93d,94]. 

The intramolecular Friedel-Crafts-type reaction of alkenyliodonium salts 23 proceeds efficiently at 40-60°C in various solvents including methanol, acetonitrile, and chloroform (Scheme 21). The Z isomer of 23c does not undergo ring closure, thus the reaction does not involve the formation of a vinyl cation. The mechanism of the reaction may well be in-plane SnVo- attack by the intramolecular aromatic nucleophile. 

The mechanism of the reaction involves intramolecular electrophilic substitution of aryl group on the acetal side as shown in the scheme. Based on that, electron-donating groups on the aromatic ring (especially para- to the position of ring closure) greatly enhance the ability of aromatic ring to cyclize. ... 

---