Tetrakis radialene

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

When l,2,3,4-tetrakis(4-pyridinyl)cyclobutane is treated with ethyl chloroformate in the presence of ethyldiisopropylamine, radialene 74a is formed and can be isolated as red crystals. Addition of AgBEt to a red solution of 74a results in an immediate color change to deep blue caused by the formation of the dicationic species 75a (equation 5)43. Undoubtedly, the tetrapyridiniocyclobutane 73a is an intermediate in the formation of 74a. By way of contrast, the fourfold deprotonation of the analogous tetrakis(7V-methyl-4-pyridinyl)cyclobutane (73b) did not succeed in the presence of oxygen. Treatment with NaH/EtOH in the presence of oxygen produced the dication 75b, which could be reduced to the [4]radialene only electrochemically44. Deprotonation of 73b with sodium hydride... 

Reaction of equimolar amounts of cumulenes 8 and 10 in the presence of tetrakis(tri-phenylphosphane)nickcl(O) also results in formation of radialene 13.21,22... 

The dimerization of 19 also occurs in the presence of rhodium salts with stable crystalline rhodium complexes of 19 that can be isolated.26 it is interesting to note that the thermal dimerization of 19 gives only a 25% yield of the cyclodimer 20.27 The metal-catalyzed dimerization of cyclohepta-l,2,3-triene in the presence of tetrakis(triphenylphosphane)nickel(0) gives tricyclic 4-radialene 21.28 The structure of this cydobutane was confirmed by X-ray diffraction. 

By a related carbenoid approach, tetrakis(aryl, trifluoromethyl)[4]radialenes 62 were obtained as mixtures of four diastereoisomers besides the [3]cumulenes 85 (Scheme 4.17) [58, 77]. The thermal cyclodimerization of 85 to form diastere-omeric mixtures of [4]radialenes 62 [58, 77] has already been mentioned. 

Monocyclic [n]radialenes with w > 6 are unknown at present. On the other hand, naphtharadialene (148) would meet the radialene definition (see Section 4.2.1) it consists only of C(sp ) atoms and has as many exocyclic double bonds as possible. This compound has been postulated as an intermediate in flash pyrolysis reactions oftetramethyl-tetrakis(chloromethyl)naphthalenes 147 leading to polycycle 149, but was not detected directly (Scheme 4.33) [101]. 

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