Rhodium-catalyzed intramolecular hydroarylation of 1-halo-1-alkynes: regioselective synthesis of semihydrogenated aromatic heterocycles.

The regioselective intramolecular hydroarylation of (3-halo-2-propynyl)anilines, (3-halo-2-propynyl) aryl ethers, or (4-halo-3-butynyl) aryl ethers was efficiently catalyzed by Rh2(OCOCF3)4 to give semihydrogenated aromatic heterocycles, such as 4-halo-1,2-dihydroquinolines, 4-halo-3-chromenes, or 4-(halomethylene)chromans, in good to excellent yields. Some synthetic applications taking advantage of the halo-substituents of the products are also illustrated.

Rh-catalyzed isomerization and intramolecular redox reaction of alkynyl ethers affording dihydropyrans and ketoolefins.

When arylmethyl 4-(sulfonyl)-3-butynyl ether and a catalytic amount of Rh(2)(tfa)(4) (tfa = CF(3)CO(2)-) were heated in toluene, 2-aryl-3-(sulfonyl)-5,6-dihydro-2H-pyran was produced in good yield. Ring closure proceeded in a highly regioselective manner, and no isomeric five-membered product was detected. Alternatively, 2-(5-sulfonyl-4-pentynyl)tetrahydrofuran lacking a benzyl substituent afforded a different product, 9-sulfonyl-1-hydroxy-8-nonen-4-one, upon reaction with the same rhodium catalyst at reflux in wet toluene.