Nitrene C-H Bond Insertion Approach to Carbazolones and Indolones, and a Reactivity Departure for 7-Membered Analogues

A modular platform for facile access to 1,2,3,9-tetrahydro-4H-carbazol-4-ones (H4-carbazolones) and 3,4-dihydrocyclopenta[b]indol-1(2H)-ones (H2-indolones) is described. The requisite 6- and 5-membered 2-arylcycloalkane-1,3-dione precursors were readily obtained through a Cu-catalyzed arylation of 1,3-cyclohexanediones or by a ring expansion of aryl succinoin derivatives. Enolization of one carbonyl group in the diones, conversion to a leaving group, and subsequent azidation gave 2-aryl-3-azidocycloalk-2-en-1-ones. This two-step, one-pot azidation is highly regioselective with unsymmetrically substituted 2-arylcyclohexane-1,3-diones. The regioselectivity, which is important for access to single isomers of 3,3-disubstituted carbazolones, was analyzed mechanistically and computationally. Finally, a Rh-catalyzed nitrene/nitrenoid insertion into the ortho C-H bond of the aryl moiety gave the H4-carbazolones and H2-indolones. One carbazolone was elaborated to an intermediate reported in the total synthesis of N-decarbomethoxychanofruticosinate, (-)-aspidospermidine, (+)-kopsihainanine A. With 2-phenylcycloheptane-1,3-dione, prepared from cyclohexanone and benzaldehyde, the azidation reaction was readily accomplished. However, the Rh-catalyzed reaction unexpectedly led to a labile but characterizable azirine rather than the indole derivative. Computations were performed to understand the differences in reactivities of the 5- and 6-membered 2-aryl-3-azidocycloalk-2-en-1-ones in comparison to the 7-membered analogue, and to support the structural assignment of the azirine. 2-Arylcyclohexane-1,3-diones and 2-arylcyclopentane-1,3-diones can be converted in a facile 2-step, 1-pot manner to 2-aryl-3-azidocycloalk-2-en-1-ones. These azides can be smoothly cyclized to carbazolones and indolones with catalytic Rh2(O2CC7H15)4. Although 3-azido-2-phenylcyclohept-2-en-1-one could be readily prepared from 2-phenylcycloheptane-1,3-dione, by contrast the Rh-catalyzed cyclization gave an azirine in preference to the indole. Mechanistic and DFT studies complement the synthetic work.**image