Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles

Abstract

SummaryDeprotonation of indazolium salts at low temperatures gives N-heterocyclic carbenes of indazoles (indazol-3-ylidenes) which can be trapped as rhodium complexes (X-ray analysis). In the absence of Rh, the indazol-3-ylidenes spontaneously dimerize under ring cleavage of one of the N,N-bonds and ring closure to an indazole–indole spiro compound which possesses an exocyclic imine group. The E/Z isomers of the imines can be separated by column chromatography when methanol is used as eluent. We present results of a single crystal X-ray analysis of one of the E-isomers, which equilibrate in solution as well as in the solid state. Heating of the indazole–indole spiro compounds results in the formation of quinazolines by a ring-cleavage/ring-closure sequence (X-ray analysis). Results of DFT calculations are presented.

Organic Synthesis with N‐Heterocyclic Carbenes of Indazole: Synthesis of Benzo(thio)imidates, Benzo[d][1,3]thiazines and Quinazoline‐4‐thiones

“…The same is true for the hydrolysis of phosphorylated thiobenzamides, [50] preparations starting from aldehydes, benzotriazole, and unsubstituted thiobenzamide, [51] and substitution reactions of primary amines with benzocarbodithioesters. [52] In support of our continued studies of N-heterocyclic carbenes in heterocycle synthesis [36][37][38][39][40]53] we report here on benzothioimidate as well as anthranilthioamide preparations that employ N-heterocyclic carbenes of indazole and the conversion of the latter into the title ring systems.…”

“…Melting points are uncorrected and were determined with an apparatus according to Dr. Tottoli (Büchi), yields are not optimized. The syntheses of starting materials 1a-c, [39] 1d, [36] 1e, [53] 1f [40] and 4a-f [40] have been described in our earlier publications.…”

“…[37,38] The N-heterocyclic carbenes of indazole are versatile starting materials for heterocycle synthesis. Thus, under certain conditions, they dimerize to spiro compounds [36] or undergo a ring-opening to the ketenimines III. These readily rearrange to acridines [39] or undergo trapping reactions with oxygen nucleophiles.…”

“…We used indazolium salts I as protonated precursors of the N-heterocyclic carbenes II (indazol-3-ylidenes) which can be trapped as thiones or rhodium complexes. [36] Alternatively, N-heterocyclic carbenes of indazoles are available by decarboxylation of indazolium-3-carboxylates. [37,38] The N-heterocyclic carbenes of indazole are versatile starting materials for heterocycle synthesis.…”

Synthesis of 2-anilinobenzimidates, anthranilamides, and 2,3-dihydroquinazolin-4(1H)-ones from N-heterocyclic carbenes of indazole

“…A mechanism was proposed which involves a [2+2]-cycloaddition of an in situ generated aryne C with the carbodiimides D to give the intermediary 7-azabicyclo[4.2.0]octa-1,3,5-trien-8-ylidene)methanamine B which undergoes a ring-cleavage to the intermediate A which we described earlier. 26,27 This intermediate, which can be represented by the neutral resonance form of an imino-ketenimine derivative or in a zwitterionic form reacts with alcohols to give the benzimidates II. However, this synthesis is restricted to the rather limited number of substituted carbodiimides and aryne precursors.…”