Visible-Light-Triggered Oxidative C–H Aryloxylation of Phenolic Amidines; Photocatalytic Preparation of 2-Aminobenzoxazoles

Abstract

Visible light efficiently mediates an intramolecular cyclization reaction of o-hydroxy-N-aryl-N,N-dialkylformamidines (phenolic amidines) leading to 2-aminobenzoxazole derivatives in the presence of only 1 mol% tris(2,2′-bipyridine)ruthenium(II) as a photoredox catalyst along with air as terminal oxidant. The protocol involves oxidative functionalization of an amidinic C–H bond to a C–O bond and affords excellent yields of products in a simple one-pot operation under mild conditions. The method represents the first example of an oxidative C–H aryloxylation reaction implementing visible-light-driven aerobic photoredox catalysis.

Key words

visible light - ruthenium - photoredox catalysis - intramolecular cyclization - oxidative ring-closing

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• 23 One-Pot Synthesis of 2-(Piperidin-1-yl)benzo[d]oxazole (2a) Directly from Benzoxazole 9 (Scheme 3); Typical Procedure: To an open flask charged with benzoxazole 9 (59.7 mg, 0.5 mmol), was added piperidine (98.8 μL, 1 mmol), and the reaction mixture was stirred under neat conditions for 10 min at r.t. under air. After confirming complete conversion by TLC, the reaction mixture was diluted with nitromethane (3 mL), followed by addition of [Ru(bpy)₃]Cl₂·6H₂O (1 mol%, 3.8 mg, 0.005 mmol). The mixture was irradiated through the flask’s bottom side at a distance of approximately 4 cm by using Luxeon Rebel high-power blue LEDs (ref. 20) [4.45 W, λ_max= 447.5 nm] with stirring under an air atmosphere (but no air bubbling) at r.t. for 26 h. The product 2a (87.0 mg, 86%) was isolated by following the same work up and purification procedure described in ref. 21.