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Thermal and photochemical reactions of Bridgehead halogen compounds Perkins, Robert Ralph

Abstract

A method of halogen exchange at normally unreactive bridgehead positions was developed utilizing the in situ generation of aluminum trihalides from aluminum foil and iodine, liquid bromine and gaseous chlorine in various solvents. The resulting solutions of aluminum iodide, bromide and chloride promoted rapid exchange of primary, secondary and tertiary halides under mild conditions and in high yield. Aromatic halides proved to be inert to the reaction conditions. Halogen exchange was shown to proceed from Cl→Br→I as well as I→Br→C1. The rate of the reaction was found to follow the relative stability of the corresponding bridgehead carbenium ions i.e. l-tricyclo[3.3.1.1[sup 3,7]]decyl(adamantyl)>l-bicyclo[2.2.2]octyl >1-bicyclo-[2.2.1]heptyl>>-triptycenyl(9,10-0-Benzenoanthracene-9,10-dihydro). Photochemical reactions of bridgehead halides of the adamantyl and bicyclo[2.2.l]heptyl systems were examined and found to follow both free radical and ionic pathways, the relative amount of each depending upon the halogen atom and the solvent used. Iodides reacted via an almost exclusive ionic pathway, chlorides via an exclusive free radical pathway and bromides via both pathways. With polyhalogen compounds the loss of halogen atoms was stepwise, no evidence for any "dehydro" or "propellane" type intermediates resulting from concerted loss of two halogens was found. The irradiation of bridgehead iodides in alcohols produced bridgehead ethers with bridgehead amides resulting from reaction in alkyl nitriles. Reaction in alkyl amines did not lead to bridgehead amines and reduction products were formed instead. Halogen exchange was observed for the irradiation of adamantyl iodides in halogenated solvents while for adamantyl bromides, chlorides and fluorides halogenation was found. The nuclear magnetic resonance spectra (¹H, ¹³C and ¹⁹F) of the halogenated bridgehead derivatives of tricycle[3.3.1.1[sup 3,7]]decane (adamantane) showed anomalous chemical shifts upon successive addition of halogen atoms possibly due to interaction of the back lobes of the adamantane bridgehead carbon atoms.

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