ELECTROPHILIC ADDITIONS TO THE BICYCLO[1. 1.0]BUTANE SYSTEM OF TRICYCLO[4.1.0.0(2,7)]HEPTANE DERIVATIVES - HALOGEN ELECTROPHILES

The known reactions of 8,8-dibromotetracyclo[5.1.0. 0(2,4).0(3,5)]octa ne (3a) and homobenzvalene (7) with pyridinium bromide perbromide and iodine, respectively, were carried out in the presence of tetra-n-buty lammonium chloride. Th formation of the chloro-substituted norpinane d erivatives 6 and 9 is evidence for cationic intermediates. The same me chanism is operative in the reaction of pyridinium bromide perbromide with the dichlorotetracyclooctane 3b, which wa prepared from 7 and dic hlorocarbene. On exposure of tricyclo[4.1.0.0(2,7)]heptane (1) to N-br omosuccinimide in acetone/water/triethylamine, the bromonorpinanol 22, the bromonorcaranols 23, and cyclohex- 1 -ene- 1 -carboxaldehyde (24) were obtained. On the basis of the steric course and thermodynamic co nsiderations, the cationic intermediates generated in the above reacti ons by attack of the electrophiles at the bicyclobutane systems are as signed the halonium ion structure 38 and the nonclassical structures 3 4 and 35, respectively. Elemental bromine and iodine converted the phe nyltricycloheptane 10 into the respective diastereomeric norpinanes 11 and 12, which were transformed smoothly into the diastereomeric methy l ethers 13 and 14 by treatment with sodium methoxide in methanol. The reactions of 10 with pyridinium bromide perbromide in pyridine, cyano gen bromide in the presence of aluminium trichloride, and N-bromosucci nimide in acetone/water gave rise to norpinane derivatives, i.e. the p yridinium salt 15, the nitrile 16, and the alcohol 18, respectively. I n the case of cyanogen iodide in acetonitrile, the solvent participate d in the process to yield the 2-(norpinylimino) propionitriles 17. Cor responding to the configurations of the products, the attack of a halo gen electrophile at 10 leads to classical 6-phenyl-6-norpinyl cations 41, which may be approached by nucleophiles from the two possible face s. As origin for the low tendency of the cations 33-35 and 41 to rearr ange to norcaryl cations, the electronegativity of the halogen atoms i s suggested. The reduced migratory aptitude of a CHHal relative to a C H2 group results from its electron deficiency and from the decreased s tability of 7-halo-2-norcaryl relative to the parent 2-norcaryl cation s. The chlorophenyltricycloheptane 25 was prepared from 10 and treated with aqueous sulfuric acid to give the norpinanol 27. Formed by proto nation of the bicyclobutane system of 25, the cationic precursor of 27 shows a behaviour similar to that of cations 41.