Heck self-condensation of polycyclic haloalkenes: the case of (1R)-2-iodobornene

The unusual reactivity of a haloalkene undergoing Heck reaction behaving bo th as reagent and substrate (Heck donor and acceptor) is studied and discus sed in detail in the case of (1R)-2-iodo-1,7,7-trimethyl-bicyclo[2.2.1]hept -2-ene (2-iodobornene). This enantiopure substrate allows for a rational de scription of the mechanism which takes into account the nature of the produ cts and the substitution of one of the substituents (iodine and proton) at carbons 2 and 3 of the bornene skeleton. Under catalytic conditions the mai n products are the syn and anti-cyclotrimers, a head-to-tail dimer and an a rene derived from the latter. The products of the stoichiometric reaction b etween palladium(0) tetrakis(triphenylphosphine) and 2-iodobornene are prin cipally trans-Pd(PPh3)(2)PhI and various dimeric species. The results are r ationalized in terms of a catalytic reaction which takes into account the P d(H)-Pd(IV) oxidation state sequence. The major products in both the cataly tic and stoichiometric reactions are dimeric species with formal eliminatio n of an iodine molecule that are unprecedented in the literature for the He ck reaction. (C) 2001 Elsevier Science Ltd. All rights reserved.