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

The unusual reactivity of a haloalkene undergoing Heck reaction behaving both as reagent and substrate (Heck donor and acceptor) is studied and discussed 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 description of the mechanism which takes into account the nature of the products and the substitution of one of the substituents (iodine and proton) at carbons 2 and 3 of the bornene skeleton. Under catalytic conditions the main products are the syn and anti-cyclotrimers, a head-to-tail dimer and an arene derived from the latter. The products of the stoichiometric reaction between palladium(0) tetrakis(triphenylphosphine) and 2-iodobornene are principally trans-Pd(PPh3)2PhI and various dimeric species. The results are rationalized in terms of a catalytic reaction which takes into account the Pd(II)–Pd(IV) oxidation state sequence. The major products in both the catalytic and stoichiometric reactions are dimeric species with formal elimination of an iodine molecule that are unprecedented in the literature for the Heck reaction.