The lithium-halogen exchange-initiated intramolecular conjugate addition reactions of some model unsaturated acylphosphoranes have been examined. The effects of halide type, chain length, and acceptor substitution pattern on the feasibility of ring construction were studied. The lithium-bromine exchange reactions in two 2-bromoaryl acceptors were found to be too slow, relative to competing side reactions, to allow practical carbocycle syntheses while 3-, 4-, 5-, and 6-membered carbocycles are formed in good to excellent yields from precursors that are vinyl m d saturated primary iodides. Highly efficient intramolecular conjugate addition reactions to @&disubstituted acceptor units are possible, and intermediate anions from intramolecular conjugate addition reactions are readily captured with electrophiles.Lithium-halogen exchange reactions provide an important method for the introduction of nucleophilic cent e r~.~ While the mechanistic aspects of these reactions are not entirely clear: many exchange reactions appear to be quite rapid, even a t low temperatures. We are currently studying cyclization reactions of the type shown in Scheme I where nucleophilic centers introduced through lithium-halogen exchange reactions subsequently undergo intramolecular bond-forming reactions with an internal electrophilic center. The successful execution of such schemes requires that both the exchange reaction and the cyclization reaction be faster than any irreversible reaction between the metalating reagent and the electrophilic center. Largely through the pioneering work of Parham and co-workers, lithium-halogen exchange reactions in aromatic systems have been shown to be possible in the presence of a variety of common electrophilic groups (COO-, COOR, CN, CONR2, CX, C=NR, N02h5 On the (1) Part 2: Cooke, M. P., Jr.; Houpis, I. N. Tetrahedron Lett. 1986, (2) Present address: College of Southern Idaho, Twin Falls, ID 83301. (Scheme I Scheme I1 I e basis of these studies, a number of exchange-initiated cyclization reactions have been described including cyclialkylation reactions of halide^,^,^ epoxides,' ketones,' and (5) Parham, W. E.; Bradsher, C. K.