Tropylium salts annelated

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

A number of furo-, pyrrolo- and thieno-annelated tropylium salts (XXVI, XXVII) have been prepared. 

As with cyclopentadienide salts, benzo-annelation leads to decreased stabilisation in the tropylium ring. This is illustrated by the pK + values for the tropylium, benzotropylium, 1,2,4,5-di-benzotropylium and tribenzotropylium cations which are respectively 4-75 [3,70], 1-7 [106], -3-7 [107] and --15 [108]. The latter value suggests that there is little if any special stabilisation in... 

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