Gold-catalyzed oxidation of arylallenes: Synthesis of quinoxalines and benzimidazoles

A gold-catalyzed oxidation of arylallenes to form α-diketones and aldehydes in good yields is presented. Further directed synthesis of quinoxalines and benzimidazoles, via the condensation of the resulting α-diketones and aldehydes with benzene-1,2-diamine, was achieved in high yields.


Results and Discussion
Our initial explorations focused on the reaction of 4-butylphenylallene (1a) (0.5 mmol) in the presence of a catalytic mixture of (Ph 3 P)AuCl (2 mol %), AgBF 4 (8 mol %), and H 2 SO 4 (0.5 mol %) in dioxane (1.0 mL) and water (10 mmol), at 60 °C for 24 h in air.This proceeded efficiently to form a 44:56 mixture of α-diketone 2a and aldehyde 3a in 70% combined yield (Scheme 1, Table 1, entry 1).The use of either the gold or silver pre-catalyst alone gave lower yields ( In order to assess the scope of this process, we examined the oxidation of several aryallenes under the optimized conditions indicated in entry 1 of Table 1.The results are summarized in Table 2. Phenylallene gave a good isolated yield of 1-phenylpropan-1,2-dione (2c) and benzaldehyde (3c) in a ratio of 43:57  a The ratio of 2 and 3 was determined by GC. b Isolated yield.
Having prepared a variety of α-diketones and aldehydes successfully, we then undertook the synthesis of quinoxalines and benzimidazoles (Scheme 2).Thus, the treatment of the corresponding mixture of α-diketone 2 and aldehyde 3 with benzene-1,2-diamine in the presence of 20 mol % oxalic acid afforded the desired quinoxalines 4 and benzimidazoles 5 in high yields (Table 3, entries 1-6).

Conclusion
We have developed a new gold-catalyzed oxidation of arylallenes to give α-diketones and aldehydes in good yields.In addition, the directed synthesis of quinoxalines and benzimidazoles via the condensation of the resulting α-diketones and aldehydes with benzene-1,2-diamine was achieved in high yields.This reaction appears to proceed via oxidation/hydration and oxidative cleavage of the allene, and investigations into the mechanism of this reaction are underway in our laboratory.

Experimental
General methods: Unless otherwise noted, materials were obtained from commercial suppliers and used without further purification.Thin layer chromatography (TLC) was performed on silica gel 60 F 254 and with UV light.Column chromatography was performed with silica gel (mesh 300-400). 1H NMR and 13 C NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer in CDCl 3 with Me 4 Si as internal standard.Data are reported as follows: Chemical shift in ppm (δ), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, br = broad and m = coupling constant in Hertz (Hz) and signal integration.Infrared spectra (IR) were obtained on 370 FT-IR spectrometer; absorptions are reported in cm −1 .Mass spectra were obtained under electron impact mode (EI) and high resolution mass spectra were measured on a high resolution mass spectrometer (GCT Premier).

Table 1 ,
[16][17]18 and 19).These results indicate that both the Au source and AgBF 4 play a crucial role in this oxidation.The superior efficiency of the tetrafluoroborate anion was demonstrated by a comparison with other weakly or non-coordinating counter anions.In addition, a change of the counter anion to OTf − , SbF 6 − , or NTf 2 − was also effective (Table1, entries 2-4).The use of other gold catalysts, e.g., (Ph 3 P)AuNO 3 and IMesAuCl, led to only to combined yields of 2a and 3a of 49% and 60%, respectively (Table1, entries[16][17].Decreasing the amount of the sulfuric acid also resulted in a lower yield, although the addition of a large amount of the acid did not affect the reaction (Table1, entries 8-9).Different acids were screened (Table1, entries 1, 5-7) and sulfuric acid was found to be the most effective.The use of solvents such as THF, toluene, DCE or ether resulted in a lower conversion (Table1, entries 10-13).Treatment of 1a in an atmosphere of O 2 (1 atm) afforded 2a and 3a in a combined yield of 47% (Table1, entry 20).When the reaction was conducted under a nitrogen atmosphere, only trace of products were observed (Table1, entry 21).
a Isolated yield.The ratio of 2 and 3 in the parentheses was determined by GC.