Nitration benzo 6 -fused rings

Electrophilic aromatic substitution of other benzo-fused v-deficient systems generally follows predictable pathways. Thus, benzopyrylium salts are in general resistant to electrophilic substitution even in the benzo-fused ring. Chromones behave somewhat similarly, although substitution can be effected under forcing conditions. Coumarins, on the other hand, undergo nitration readily in the 6-position while bromination results in substitution at the 3-position as a consequence of addition-elimination. 

Fluoranthene (Called 1,2-benzo-acenaphthy-len or 1,8-o-phenylen-naphthylen in Ger), C16H10(Thought to be ClaH10in early literature), mw 202.24, colorless ndls, sp gr 1.158 at 20°, nD 1.739 at 20°, bp 393°, 217°(30mm Hg), mp 109.5-110.5°, sol In benz, chlf, eth, HAc hot ale. A tetracyclic fused ring aromatic hydrocarbon, this material is found in some coal tars. It forms definite complexes with many poly nitrated aromatics (Refs 1 to 6) Refs 1) Beil 5, 685, (340, 344), [609] 2276 2) J. vonBraun E. Anton, Ber... 

In the benzo fused series the oxygenated ring is by far the least stable to electrophilic reagents. This is exemplified by results of a study of 2-methyl-l,4-benzodioxin (49), which is polymerized by 1% sulfuric acid after one hour. Attempted nitration in glacial acetic acid followed by aqueous workup merely gave the hydrate (50) but in acetic anhydride a... 

The fact that the a > p reactivity order parallels that in thiophene rather than benzo[ ]thiophene has also been observed in nitration and bromination [77CS( 12)97], and was regarded as being explicable only by sophisticated MO methods. However this is not necessary. Reference to Scheme 8.1 shows that in benzo[ ]thiophene, 3-substitution is favored over 2-substitution because both canonicals for the former are benzenoid, whereas only one is for the latter. If, however, a second thiophene ring is fused to the benzene ring, then because of the bond fixation in thiophene, the nonbenzenoid canonicals for 2-substitution become much... 

Synthesis of the benzo[l,2-c 3,4-c ]bis[l,2,5]thiadiazole (509) required cyclization of diamine (508). Nitration of (505) led to 4-nitro-2,1,3-benzothiadiazole (506) in 95% yield. Amination of (506) with hydroxylamine in basic medium led to compound (507), immediately reduced to the diamino derivative (508). Ring closure of (508) with thionyl chloride and pyridine led to the benzo[l,2-c 3,4-c ]bis[l,2,5]thiadiazole (509) in 89% yield. Compound (509) has been successfully used in the synthesis of several fused tetracyclic compounds (Scheme 40) <75JHC829>. 

Nitration of benzo[Zi]thieno[3,2-Z)]benzo[Z)]thiophene 121 containing two fused benzene rings with concentrated nitric acid d— 1.51 gcm ) in acetic acid produced 2- (287) and 4-nitro derivatives (288), which were reduced to the corresponding amines (80ZOR438). 

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