The syntheses of 6-methylene-2,4-cyclohexadien-1-imine and related o-quinonoids by FVT of 1-hetero-1,2,3,4-tetrahydronaphthalenes

Aza-ortho-xylylenes in Organic Synthesis

“…Ripoll et al assigned the structure of aza-ortho-xylylene (9) to the yellow deposit, trapped at Ϫ196°C, arising from the flash vacuum thermolysis (FVT) of 1,2,3,4-tetrahydroquinoline (18). [23] This compound was very unstable and at Ϫ100°C it turned into a colorless polymeric material. The pyrolysates of 2-aminobenzyl alcohol (14) and 2-aminobenzylamine (15) behaved similarly.…”

“…The pyrolysates of 2-aminobenzyl alcohol (14) and 2-aminobenzylamine (15) behaved similarly. [23] [1,4]-Hydrogen shifts in nitrene 13, generated from ortho-tolyl azide (12), and in 2-aminophenylcarbene (17), generated during the photochemical decomposition of 2-aminophenyldiazomethane (16), both produced aza-ortho-xylylene (9), which was trapped in an argon matrix. [18,19] An analogous [1,4]-hydrogen shift in the nitrene generated from 3,5-dimethyl-2-(9-fluorenyl)phenyl azide afforded an aza-ortho-xylylene, which electrocyclized to form condensed dihydroacridine.…”

“…The FVT of 1,2,3,4-tetrahydroquinoline (18) with loss of ethylene [23] is shown in Scheme 3. N-Acyl-2,1-benzoxazines decompose thermally with a loss of formaldehyde.…”

“…[70] Scheme 48 Several examples of reactions between nucleophiles and aza-ortho-xylylenes generated from ortho-aminobenzyl alcohols under FVT conditions are known. [23,92] The intermediate 204 has been postulated in the synthesis of benzylamines such as 205 from 2-amino-6-methyl-3-nitrobenzyl alcohol (200) and secondary amines under Mitsunobu reaction conditions (triphenylphosphane/diethyl azadicarboxylate) (Scheme 49). [96] Under these conditions, no reactions between secondary amines and benzyl alcohol, 2-aminobenzyl alcohol, 3-nitrobenzyl alcohol, or 6-methyl-2-(dimethylamino)-3-nitrobenzyl alcohols took place, which supports the formation of intermediate 204 from 200.…”

“…Ripoll et al described the formation of aza-ortho-xylylene (9) from 2-aminobenzylamine (15) under FVT conditions (Scheme 3). [23] Saegusa and Ito [100Ϫ104] developed a mild method for the generation of aza-ortho-xylylenes by a fluoride-ion-induced 1,4-elimination of trimethylamine from the ortho-[N-alkyl-N-(trimethylsilyl)amino]benzyltrimethylammonium salts 211. This reaction proceeds satisfactorily in acetonitrile in the presence of cesium fluoride at 60°C.…”