Graphene oxide and Rose Bengal: oxidative C–H functionalisation of tertiary amines using visible light

Abstract

Visible light induced oxidative C-H functionalisation of tertiary amines catalysed by the combination of graphene oxide and Rose Bengal was developed. This reaction avoids the use of stoichiometric amounts of peroxy compounds as terminal oxidants. This reaction is useful for tri-alkyl amines including chiral tertiary amines. Both cyanide and trifluoromethyl nucleophiles were shown to participate in this reaction, providing a-cyano-and a-trifluoromethylated tertiary amines.Graphene oxide (GO), a two-dimensional carbon sheet, is traditionally used as a precursor to prepare graphene. Its unique physical and chemical properties have attracted the attention of chemists due to potential applications in plastic electronics, optical materials, solar cells and biosensors, 1 but its potential as a catalyst in organic transformation remains relatively unexplored. 2,3 The feasibility and potential of GO as catalyst were demonstrated by the seminal work of Bielawski and co-workers on the use of GO for the oxidation of alcohols and hydration of alkynes. 3a Subsequently, RGO was reported to catalyse the hydrogenation of nitrobenzene at room temperature. 3b The use of GO and reduced graphene oxide (RGO) as "carbocatalyst" in organic transformations is a nascent area and should lead to exciting discoveries.On a different note, the use of visible light in organic synthesis has attracted the attention of various synthetic organic chemists recently. 4 The groups of MacMillan, 5 Yoon, 6 and Stephenson 7 have showed the ability of metal-based photosensitizers, such as Ru(bpy) 3 Cl 2 (tris(2,2¢-bipyridine)-ruthenium(II) chloride) (Fig. 1), as photoredox catalysts for organic transformations under visible light irradiation. 8 Organic dyes, which are often Fig. 1 Ruthenium bipyridyl complex and Rose Bengal.used in dye-sensitized solar cells, are considered to be cheaper and easier to modify relative to metal-based photosensitizers, 9 thus they are attractive alternatives as photoredox catalysts. The viability of organic dyes as photoredox catalysts has been demonstrated by several groups. 10 For example, Zeitler et al. reported eosin Y catalysed dehalogenation and enantioselective a-alkylation using a combination of photoredox catalyst and organocatalyst. 10a Fukuzumi reported a selective aerobic bromination catalysed by 9-mesityl-10-methylacridinium perchlorate (Acr + -Mes). 10b We have also demonstrated that Rose Bengal (RB, Fig. 1) was able to photocatalyse a-oxyamination of 1,3-dicarbonyl compounds and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) using visible light. 10c Although GO has been reported as a photocatalyst for hydrogen production from water under UV irradiation, 11 the potential application of GO in synthetic photochemistry has not been explored. Therefore we wish to report the combination of GO and RB which works in synergy to efficiently catalyse the a-functionalisation of tertiary amines in the presence of visible light. The oxidative a-functionalisation of tertiary amines through highly reactive iminium intermedia...

Visible‐Light‐Promoted Stereoselective Alkylation by Combining Heterogeneous Photocatalysis with Organocatalysis

“…Recently, several cross-dehydrogenative couplings [31] of tetrahydroisoquinolines by homogeneous photocatalysis using Ir-or Ru-based complexes [5b,c, 7d, 8b,d, 15, 31] or organic dyes [33] such as Eosin Y [8a] have been reported. Here the photocatalytic key step is the reductive quenching of the excited chromophore leading to an amine radical cation, which is subsequently converted into an electrophilic iminium species.…”

Sunlight assisted synthesis of α-aminonitrile using Capillary flow microreactor: A new approach

“…The product was later confirmed using NMR ( 1 H and 13 C). The mechanism of the reaction was described by Rueping et al [5]; they suggested a different path to form the iminium intermediate which contrasts the previous mechanism described by Pan et al [13]. Once the RB is excited after absorption of a photon of light, the tertiary amine transfers a single electron to the dye which quenches it back to the ground state and becomes a radical.…”

Versatilities of graphene-based catalysts in organic transformations

“…Thus, the green light-emitting diodes irradiation of N-aryl-tetrahydroisoquinoline 17 and trimethylsilyl cyanide in the presence of GO and 16 afforded 18 with high conversion rate (Scheme 10). 40 The protocol was effective for both trialkyl and chiral tertiary amines as well as equally persuasive with trifluoromethyl nucleophiles.…”