Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

Abstract

The metalloradical activation of ortho‐benzallylaryl N‐tosyl hydrazones with [Co(TPP)] (TPP=tetraphenylporphyrin) as the catalyst enabled the controlled exploitation of the single‐electron reactivity of the redox non‐innocent carbene intermediate. This method offers a novel route to prepare eight‐membered rings, using base metal catalysis to construct a series of unique dibenzocyclooctenes through selective Ccarbene−Caryl cyclization. The desired eight‐membered‐ring products were obtained in good to excellent yields. A large variety of aromatic substituents are tolerated. The proposed reaction mechanism involves intramolecular hydrogen atom transfer (HAT) to CoIII–carbene radical intermediates followed by dissociation of an ortho‐quinodimethane that undergoes 8π cyclization. The mechanism is supported by DFT calculations, and the presence of radical‐type intermediates was confirmed by trapping experiments.

Transition Metal Catalyzed Insertion Reactions with Donor/Donor Carbenes

“…[23][24][25] A notable exception to this notion is in the field of metalloradical catalysis,w here triplet-type metal carbenes can undergo av ariety of single electron processes.W hile significant work has been completed in reactions with donorsubstituted triplet-type metal carbenes,s uch reports fall outside of the scope of this Minireview as we are primarily concerned with transition metal-catalyzed insertion reactions that involve singlet carbene,t wo-electron processes. [26][27][28][29][30] Some efforts have also been made to characterize the steric and electronic factors that influence donor/donor carbene stereoselectivity and reactivity,b ut these examples have primarily been within the context of intermolecular cyclopropanation reactions. [6][7][8]31] Metal-catalyzed insertions into atoms other than carbon are generally believed to proceed by astepwise mechanism.…”

Catalytic synthesis of 1H-2-benzoxocins; Cobalt(III)-carbene radical approach to 8-membered heterocyclic enol ethers

“…The isolated yields reported in Figure 3 are corrected for the E/Z ratio of these substrates, because only the E-isomer can convert to the final 8-membered ring. 22,23 The broad substrate scope demonstrated in Figure 3 reveals an exceptional functional group tolerance and high efficiency toward formation of medium-sized ring structures, which offers several possibilities to prepare 8-membered Oheterocycles containing biologically relevant substituents. Hence, to further explore the practicability of the protocol, we employed the cobalt-catalyzed cyclization reaction to synthesize the 1H-2-benzoxocins functionalized with different bioactive and druglike substituents shown in Figure 4A.…”

“…This metallo-radical strategy has been successfully employed in the synthesis of ring structures, such as cyclopropanes, 13 furans, 14 indenes, 15 indolines, 16 ketenes, 17 dihydronaphthalenes, 18 piperidines 19 and pyrrolidines. 20 Furthermore, an efficient synthetic protocol for the synthesis of 2H-chromenes synthesis has been developed (Figure 2A), 21 and we also successfully applied the intrinsic radical-type reactivity of cobalt(III)-carbene radicals to construct two types of new 8-membered carbocyclic compounds: dibenzocyclooctenes 22 and monobenzocyclooctadienes 23 (Figure 2A). Building on these prior results, and in view of our continued interest in cobalt(II)-based metalloradical catalysis, we envisioned that related open-shell catalysis could also uncover new possibilities for the synthesis of 8-membered O-heterocyclic compounds.…”

Efficient Copper Catalyzed Multi-component Synthesis of N-acyl amidines via Acyl Nitrenes

“…Since our group is focusing on CH activation using metallocarbenes and metallonitrenes for the catalytic formation of new CC and CN bonds, [54][55][56][57][58][59][60] we considered that a related acyl nitrene transfer strategy could be useful for the synthesis of N-acyl amidines. However, realizing new strategies to convert acyl nitrenes in a selective manner is rather troublesome.…”