Monodehydro annulenes

Formation of substituted products by electrophilic reactions is one of the characteristics of benzenoid compounds. It has been shown that a few dehydro-[4/j + 2]annulenes afford substituted annulenes under strictly limited reaction conditions. Monodehydro[14]annulene, on treatment at room temperature with copper(ii) nitrate-acetic anhydride, oleum-dioxane and subsequent methylation and acetic anhydride-borontrifluoride etherate, yielded monosubstituted products, 169a, 169b and 169c, respectively. The electrophilic reactions must have resulted in substitution of one of the outer protons, but the exact point of attack has not been determined . 

Oxidative coupling of /ra/w,rra/ij-4,10-tetradecadien-l,7,13-triyne (257) and subsequent prototropic rearrangement of the coupling products yielded mainly the geometrical isomers of monodehydro[14]annulene (258 and 259) and a byproduct, l,8-bisdehydro[14]annulene (260) . Formation of 260 indicates that dehydrogenation occurs under the basic conditions of prototropic rearrangement. 

It has been predicted theoretically that planar [4/i-t-2]annulcnes will be aromatic up to [22]annulene, but that [26]annulene will no longer be aromatic . The findings that monodehydro[22]annulene , [22]annulene and tetra-r-butylbisdehydro-[22]annulene (289) are diatropic are consistent with the theoretical prediction. Trisdehydro[26]annulene (292) and monodehydro[26]annulene (293) prepared... 

In order to get information on the effect of annelation of a 6tz ring onto macro-cyclic 4 starting material. The reference compound 337 was prepared by the reaction of 334 with maleic anhydride. It was found that the degree of paratropicity decreased in the sequence 337 > 336 > 335 > 333 ci 331. This is the same order as found for the reduction of diatropicity in the annelated bis-dehydro[14]annulenes, 324, 325, 327, 328 and 329. 

Monodehydro-[14]annulene (see Table 4) appears to be isolable in at least two forms, one being called the "stable conformer 66,86,91,92), 7, and the other the "unstable conformer 66,90), 8, which changes to 7 on standing.Two possible structures for7,12a and 726,have been considered,... 

Professor Sondheimer informs us that he and his colleagues no longer regard the two known forms of monodehydro-[14]annulene as conformational isomers but believe them to be geometric isomers >. 

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