Umwandlung von 6‐Methylen‐tricyclo[3.2.1.02,7]oct‐3‐en‐8‐onen in Polymethyltropyliumsalze

When dissolved in trifluoroacetic or fluorosulfonic acid, 6‐methylene‐tricyclo[3.2.1.02,7]oct‐3‐ene‐8‐one derivatives of type 2 (;scheme 1); give polymethyltropylium salts in moderate to good yields by CO‐extrusion. These tropylium salts can be isolated pure as hexachloroplatinates. Thus the tricyclic compound 6 gives 1,2,4‐trimethyltropylium trifluoroacetate 19 in trifluoroacetic acid (;scheme 3);. This salt in CDCl3 is in equilibrium with its covalent cycloheptatriene (;tropilidene); form 20, the ratio of the two forms being 1.5–2.1/1. The tropylium salt 19 is reduced by lithium aluminium hydride to a mixture of 1,2,4‐trimethylcycloheptatrienes, isomeric with respect to the double bonds, which on hydride abstraction with trityl‐tetrafluoroborate gives again the 1,2,4‐trimethyltropylium salt 19 (;scheme 3);. From the trimethyl‐substituted tricyclic compounds 7 and 8, in trifluoroacetic acid, are obtained respectively the 1,2,4,6‐ and 1,2,3,4‐tetramethyltropylium ions (;22 and 24); (;schemes 4 and 5);. In this way the 1,2,3,5,6‐pentamethyl‐tropylium ion (;26); was obtained from 9 (;scheme 6);. With the higher substituted tropylium trifluoroacetates in CDCl3 the equilibrium tropylium trifluoroacetate ⇄ trifluoroacetoxycycloheptatrienes lies well to the left. The hexamethylated tricyclic compound 10 gives a small quantity of heptamethyltropylium trifluoroacetate (;27); and as the main product the C(;3);‐protonated species 28 (;scheme 7);, which when treated with aqueous sodium hydrogencarbonate yielded unchanged educt 10. ‐ The heptamethyltropylium ion (;27); was, apart from polymeric species, the only product from the treatment of starting material 10 with fluorosulfonic acid (;50%);; its salts have as yet not been isolated in their pure form, however. The mechanism for the rearrangement of the tricyclic compounds of type 2 into tropylium salts is presented for compound 6 in scheme 8: The first step is the protonation at C(;9);. Ring opening of the cyclopropane of the tertiary carbenium ion 29 gives the allylic ion 30, which then yields the aromatic tropylium salt 19 by carbon monoxide extrusion in a linear cheletropic reaction. The smooth conversion with strong acids of the easily accessible tricyclic compounds of type 2 to the corresponding polymethylated tropylium salts, presents a new and useful method for the synthesis of the latter compounds.