The Cycloaddition of the Benzimidazolium Ylides with Alkynes: New Mechanistic Insights

New insights concerning the reaction mechanism in the cycloaddition reaction of benzimidazolium ylides to activated alkynes are presented. The proposed pathway leading both to 2-(1H-pyrrol-1-yl)anilines and to pyrrolo[1,2-a]quinoxalin-4(5H)-ones involves an opening of the imidazole ring from the cycloaddition product, followed by a nucleophilic attack of the aminic nitrogen to a proximal carbonyl group and the elimination of a leaving group. The mechanistic considerations are fully supported by experimental data, including the XRD resolved structure of the key reaction intermediate.


Introduction
Pyrrolo[1,2-a]quinoxalinone derivatives are an important class of heterocyclic compounds due to their biological activities. Some carboxylic acid derivatives of pyrrolo[1,2-a]quinoxalin-4 (5H)-one show significant (about 100 times larger than disodium cromoglycate) antiallergic activity in the passive cutaneous anaphylactic (PCA) test following either, and in some cases both, intravenous or oral dosing [1]. Moreover, the quinoxaline system has been identified as a critical structural requirement for optimal interaction with the human immunodeficiency virus type 1 (HIV-1) non-nucleoside reverse transcriptase inhibitors (NNRTI) binding site [2]. 6-Fluoro-quinoxalinylethylpyridylthiourea (6-FQXTP, Fig 1) represent the prototype of this class of NNRTI [2].

Apparatus and analysis
All reagents and solvents were purchased from commercial sources and used without further purification. Melting points were recorded on a MEL-TEMP II apparatus in open capillary tubes and are uncorrected. Analytical thin-layer chromatography was performed with commercial silica gel plates 60 F 254 (Merck) and visualized under UV light. The NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer operating at 500 MHz for 1 H and 125 MHz for 13 C. Infrared (IR) data were recorded as films on potassium bromide (KBr) pellets on a FT-IR Shimadzu Prestige 8400s spectrophotometer. The X-Ray diffraction experiment was performed using a SuperNova Dual diffractometer equipped with a Cu (Kα radiation, λ = 0.684 Å) fine-focus sealed X-ray tube and a graphite monochromator. Detector resolution: 16.1593 pixels mm -1 . Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011), Tmin = 0.914, Tmax = 1.000. The reflections were recorded at room temperature on a small single crystal. Typical procedure for the cycloaddition reaction of the benzimidazolium ylides with DMAD A mixture of benzimidazolium salts 1a-i (3 mMol) and DMAD (0.852 g, 6 mMol) was suspended in 15 mL chloroform. Then, triethylamine (0.606 g, 6 mMol) dissolved in 10 mL chloroform was added drop wise under stirring in one hour. The stirring and refluxing were continued for 12 hours. After the reaction was finished (TLC), the obtained solution was cooled down at room temperature and then the reaction mixtures was washed with water (3 x 30 mL), dried over magnesium sulfate and evaporated under reduced pressure to give the crude product. The purification of the crude product was done by column chromatography on silica gel (eluted with CH 2 Cl 2 to 98/2 CH 2 Cl 2 / CH 3 OH) giving either two products in the cases of the 1a and 1b salts or a single product in the other cases.

Results and Discussion
The cycloaddition reaction of cycloimmonium ylides involves three stages: (i) generation of the ylide from the corresponding salt; (ii) a Huisgen 3+2 cycloaddition of ylide to dipolarophile, with the formation of a cycloadduct; (iii) total or partial dehydrogenation of the intermediary cycloadduct, with the final formation of a thermodynamically more stable aromatized adduct (Fig 2). The first two stages have been thoroughly investigated and described in literature [30,31], while the intermediate's dehydrogenation in the third stage leads to a large variety of products including total or partial hydrogenated [21,23], fully aromatized cycloadducts [18][19][20][21][22][23][24][25][26][27][28][29] or even with an altered structure of the cycloadduct [8,9,12,14]. It is the latter case we focus in the following.
In order to rationalize the literature data, to elucidate the reaction mechanism and to obtain new pyrrolo[1,2-a]quinoxalinone derivatives, we decide to study the reactions of benzimidazolium ylides (generated in situ, using triethylamine, from the corresponding salts 1a-i [27][28][29]) with the activated alkyne, DMAD, (Fig 3).
XRD resolved structure of an isolated reaction intermediate (4b , Fig 4) suggests that after the initial formation of the cycloaddition products with a dihydropyrrolo[1,2-a]benzimidazole structure (3a-i), the reaction mechanism ( Fig 5) involves a ring opening of the imidazole cycle [10,11,16,17] (and not of the pyrrole ring as proposed previously in the literature [12]) with the formation of a conformer of the 2-(1H-pyrrol-1-yl)anilines 4a-i.
The formation of compounds 4a,b may be assisted by the abstraction of the hydrogen atom from the acidic α-position of the ester/amide Y in the presence of excess triethylamine with concomitant fragmentation of the imidazole ring and aromatization of the pyrrole ring. Twisted conformations of the resulting 2-(1H-pyrrol-1-yl)anilines 4a-i (76°in the case of 4b) arise from a free rotation of the pyrrole ring around the N pyrrole -C aryl single bond. According to the nature of Z and Y substituents, the amines 4a-i are either stable (compounds 4a,b) or unstable (compounds 4c-i), a cyclization process to the six-membered ring of pyrrolo[1,2-a] quinoxalin-4(5H)-one structure (5-7) taking place in case of the latter, via elimination of an alkoxy group, Y (Fig 3, pathway ii). Given the tendency of spontaneous cyclization observed in the case of amines 4c-i, our next attempt was to convert the amines 4a,b into a corresponding pyrrolo[1,2-a]quinoxalin-4(5H)-one structure, expected to be thermodynamically more stable. Indeed, the desired pyrrolo[1,2-a]quinoxalin-4(5H)-one 5 was easily obtained from both 4a and 4b cases by reflux in solution (Fig 3). Conversion of the two amines to the corresponding quinoxalinone also occurs at room temperature, in solution. Results of 1 H NMR studies on 4b at room temperature (see S6 Fig) reveal that intramolecular cyclization is a slow process, spectra recorded after 11 days from sample preparation containing signals from both 4b and 5 in nearly equimolar ratio.
The isolation of the 2-(1H-pyrrol-1-yl)anilines 4a,b is the missing link that confirms the mechanism proposed by Georgescu [11,16,17] and in the same time infirms the concerted mechanism proposed by Zhang and Huang [9,10].
A summary of the products yielded through the cycloaddition of the benzimidazolium ylides to DMAD are listed in Table 1.
One may note from Table 1 that the isolated yields are low to moderate, in good agreement with values reported in the literature for this type of reactions. The lower yields in the case of the benzimidazolium ylides 2c, 2f and 2i should relate to a different behaviour of the NH 2 as leaving group compared to the alkoxy counterpart, that impacts in case of the former on cyclization to quinoxaline ring.

Conclusions
Our results reported herein complement previously reported literature data regarding the cycloaddition of benzimidazolium ylides to dipolarophiles, adding new insights into the reaction mechanism. A plausible explanation for obtaining both types of cycloaddition products is provided by the reaction mechanism and correlates to the literature data. The proposed pathway leading both to 2-(1H-pyrrol-1-yl)anilines and to pyrrolo[1,2-a]quinoxalin-4(5H)-ones, involving the opening of the imidazole ring, is supported by spectral analysis and X-ray diffraction experiments. We also shown that reaction selectivity toward pyrrolo[1,2-a]quinoxalin-4 (5H)-ones may be tuned by experimental conditions.  (TIFF) S1 File. Supporting Information document. Spectral characterization, NMR spectra ( 1 H and 13 C) of the obtained compounds, and 1 H-NMR studies on 4b at room temperature. (DOCX)