Dehydrogenative C–H Phenochalcogenazination

Abstract

Heavy-atom-modified chalcogen-fused triarylamine organic materials are becoming increasingly important in the photochemical sciences. In this context, the general and direct dehydrogenative C–H phenochalcogenazination of phenols with the heavier chalcogens selenium and tellurium is herein described. The latter dehydrogenative C–N bond-forming processes operate under simple reaction conditions with highly sustainable O2 serving as the terminal oxidant.

O2-Mediated Dehydrogenative Phenoxazination of Phenols

“…Indeed, the latter tolerate only the most electron-rich phenols. 9 This finding is consistent with earlier initial rate investigations, which showed that phenoxazine (X = O) is the fastest azine substrate of the series compared to the other chalcogen congeners (X = S, Se, and Te). 8c The proposed reaction mechanism is depicted in Scheme 5 ( 8b ) and is based on the EPR characterization of the persistent and neutral N-centered radical, which is produced upon the exposure of phenoxazine to O 2 .…”

“…The concept is known to operate well with all four chalcogens: sulfur, selenium, tellurium, and oxygen. 9 However, considerably fewer examples are known for phenoxazines (X = O, Scheme 2 ) 10 compared to phenothiazines (X = S). 11 Impressively, MacMillan and coauthors recently demonstrated that a phenoxazine derivative could be utilized to oxidatively click on tyrosine units within peptides scaffolds in view of their late-stage functionalization ( Scheme 2 ).…”

O2-Mediated Te(II)-Redox Catalysis for the Cross-Dehydrogenative Coupling of Indoles

“…While the synthesis of Te-cat5 from I 2 -cat10 is a transition-metal-free process, the synthesis of the precursor, I 2 -cat10 , involves both copper and palladium salts in the two previous steps. 53 As expected, I 2 -cat10 did not show any conversion ( Scheme 3 ), highlighting the crucial importance of the tellurium atom for catalytic activity. Finally, delayed substrate addition experiments showed a severe decrease in product yield when Te-cat5 was prestirred beforehand under a O 2 atmosphere at 130 °C (eq 4, Scheme 3 ).…”

“…We were next intrigued by the role of the 4-Ph-phenol additive. One reasonable hypothesis was that the Te-cat5 (pre)catalyst might first click 53 on the 4-Ph-phenol additive ( add1 ) to produce the dehydrogenative phenochalcogenazination product Te-cat7 , which we speculated might be the true active catalyst. We therefore prepared and isolated Te-cat7 (see the Supporting Information ) and then engaged it as a control catalyst (10 mol %, Scheme 3 , eq 3), in the absence of any free 4-Ph-phenol add1 .…”

Te(II)-Catalyzed Cross-Dehydrogenative Phenothiazination of Anilines

“…This method should therefore contribute to the development of Te(II) redox catalysis in the context of cross-dehydrogenative couplings as well as to the specific field of oxidative click CDP reactions. 31 − 37…”