Biradicals from Thermolysis of N-[2-(1-Alkynyl)phenyl]-N‘-phenylcarbodiimides and Their Subsequent Transformations to 6H-Indolo[2,3-b]quinolines

Abstract

Thermolysis of the carbodiimide 9a in gamma-terpinene at 138 degrees C produced 2-(phenylamino)quinoline (11a, 49%) and the parent 6H-indolo[2,3-b]quinoline (14a, 16%). Apparently, 11a was produced via the biradical 10a followed by hydrogen-atom abstraction from gamma-terpinene. A two-step biradical pathway through 12a or a one-step intramolecular Diels-Alder reaction could furnish 13a, which then underwent tautomerization to give 14a. With the carbodiimide 9b having a trimethylsilyl substituent at the acetylenic terminus, thermolysis in refluxing p-xylene at 138 degrees C produced the 6H-indolo[2,3-b]quinoline 14b (86%) exclusively. Treatment of 14b with 6 N NaOH in refluxing ethanol then furnished 14a in 92% yield. Similarly, the 6H-indolo[2,3-b]quinolines 14c-f were obtained from thermolysis of the carbodiimides 9c-f. The use of the aza-Wittig reaction between 4-methoxyphenyl isocyanate and the iminophosphoranes 2d and 2f to produce the corresponding carbodiimides followed by thermolysis furnished the 6H-indolo[2,3-b]quinolines 16d and 16f having a methoxy substituent at the C-2 position. Thermolysis of the carbodiimides 25a and 25b produced 26a and 26b having two indoloquinoline units connected at the 11 and 11' positions with either a three-carbon or a five-carbon tether. Using 1,4-phenylene diisocyanate for the aza-Wittig reaction with 2 equiv of the iminophosphorane 2g followed by thermolysis furnished 31 (66%) having two indoloquinoline units incorporated in the seven fused rings.

Synthesis and Reactions of 3-Ethynyl-2-(triphenylphosphoimino)-1-azaazulenes

“…In the 1 H NMR spectrum of 11, methylene protons were observed at δ 4.95 together with phenyl protons and seven-membered ring protons. In its 13 C NMR spectrum, a signal at δ 48.68 owing to a methylene carbon and a signal at δ 196.90 owing to a carbonyl carbon were observed. In its IR spectrum, carbonyl peak was seen at 1626 cm -1 .…”

“…Prolonged reaction resulted in increase of 12 and 13: the reaction for 48 h gave 12 (18%) and 13 (33%), and the reaction for 96 h gave 12 (8%) and 13 (44%). When the reaction was carried out in benzene for 8 h at 90 ℃ in a sealed tube, 11 (13), 12 (5%), and 13 (41%) were obtained. Treatment of 11 with Cu(OTf) 2 for 20 h gave 12 in 41% yield and formation of 13 was not observed.…”

“…We previously reported that reaction of 2-(triphenylphosphoimino)-1-azaazulene with aryl isocyanate proceeded via 2-aryl 1-azaazulen-2-yl carbodiimide intermediate. It is known that enyne-carbodiimides proceeded cyclization to give heteroaromatics such as carbolines and ellipticine analogues, [11][12][13][14][15][16][17] and underwent Pouson-Khand type reaction in the presence of Mo(CO) 6 . 26 Therefore, we next examined the reaction of 8 with aryl isocyanate.…”

“…10 In the recent studies about heteroaromatics such as carbolines and ellipticine analogues, benzannulated enyne-carbodiimides and o-ethynylaryliminophosphoranes were used as key intermediates. [11][12][13][14][15][16][17] In the line of our studies about 1-azaazulenes, [18][19][20] we investigated the synthesis and reaction of 3-ethynyl-2-(triphenylphosphoimino)-1-azaazulenes.…”

“…In its 13 C NMR spectrum, two signals at δ 192. 29 and 195.25 owing to carbonyl carbons were observed.…”