Ring-fused dimethoxybenzimidazole- benzimidazolequinone (DMBBQ): tunable halogenation and quinone formation using NaX/Oxone†

Ring-fused benzimidazolequinones are well-known anti-tumour agents, but dimeric ring-fused adducts are new. The alicyclic [1,2-a] ring-fused dimethoxybenzimidazole-benzimidazolequinone (DMBBQ) intermediate allows late-stage functionalization of bis-p-benzimidazolequinones. DMBBQs are chlorinated and brominated at the p-dimethoxybenzene site using nontoxic sodium halide and Oxone in HFIP/water. X-ray crystallography is used to rationalize site preference in terms of the discontinuity in conjugation in the DMBBQ system. Quinone formation occurs by increasing in situ halogen generation and water. Conversely, radical trifluoromethylation occurs at the quinone of the DMBBQ.


Introduction
Alicyclic ring-fused benzimidazolequinones are potent antitumour agents (Scheme 1a), studied as synthetic alternatives to mitomycin C. [1][2][3][4] Cerium(IV) ammonium nitrate oxidizes p-dimethoxybenzenes to p-benzoquinones, 5 and to dimeric adducts. 6 Bis-chloromethylbenzimidazolequinone 1 was reported in 38% yield (Scheme 1b) from the oxidation of 2-(chloromethyl)-4,7-dimethoxy-1-methyl-1H-benzimidazole using CAN (2.9 equiv.). 7 Bis-p-quinone 1 exhibited greater cytotoxicity against solid tumour cell lines than monomeric benzimidazolequinones. Recently, the CAN oxidation of 4,7dimethoxybenzimidazole alkoxyamine 2 was used to give the bis-benzimidazolequinone 3 (Scheme 1c). 8 Dimethoxybenzimidazole-benzimidazolequinone (DMBBQ) 4a was obtained in high yield (86%) when using less CAN, and in contrast to bis-p-quinone 3, DMBBQ 4a was insensitive to light. Radical trifluoromethylation of 4a using Langlois' reagent (NaSO 2 CF 3 ) occurred at the quinone to give 4b in 54% yield. 8 Elemental chlorine and bromine are corrosive, toxic, and ordinarily inconvenient for synthesis. The useful halogen is however generated in situ from the oxidation of a benign and inexpensive alkali metal halide. 9 Oxone (2KHSO 5 ·KHSO 4 ·K 2 SO 4 ) is a stable, safe and cheap external oxidant. 10 The active ingredient of Oxone is potassium peroxymonosulfate (KHSO 5 ) and when accompanied by stoichiometric equivalents of halide (NaX) can generate molecular halogen (X 2 ) for electrophilic chlorination and bromination (Scheme 1d). The halide with Oxone gives oxidative transformations with an accompanying halogenation of naphthols and phenols, 11 and with and without halogenation for indoles. 12,13 Other metal-free oxidations, such as N-bromosuccinimde (NBS) with a small amount of H 2 SO 4 , 14,15 and PhI(OCOCF 3 ) 2 (PIFA), 8,16,17 are reported to induce conversion of p-dimethoxybenzenes to p-quinones without halogenation. The disadvantage of using NBS and PIFA is the generation of organic waste products. Moreover, the combination of H 2 O 2 with HX gives chlorinated and brominated alicyclic [1,2-a] ringfused benzimidazolequinones from p-dimethoxybenzene precursors. 18 The accompanying aromatic halogenation is considered advantageous, since it offers further functionalization opportunities. However, unlike monomeric benzimidazoles, milder room-temperature conditions are required for Cl 2 and Br 2 generation when using DMBBQs as the substrates (see below), which led us to use NaX/Oxone. The use of Oxone is unprecedented in oxidative demethylations to give p-quinones. Herein, we disclose selective chlorination and bromination of novel CAN-derived alicyclic ring-fused DMBBQs at the aromatic site, despite possible activation at the quinone (Scheme 1e). Electrophilic aromatic halogenation of DMBBQs 5a-5d (Scheme 1f) is accompanied by one-pot quinone formation.

Results and discussion
Five to seven-membered alicyclic ring-fused dimethoxybenzimidazoles 6a-6c are readily available, 17 and used to prepare new DMBBQs 5a-5c. DMBBQ 5b was prepared in 73% yield by optimizing the conditions for CAN-oxidation of 6b in MeCN/ H 2 O (Scheme 2). 5-8 The latter solvent system also gave pyrido [1,2-a]benzimidazolequinone 7 in 15% yield. Replacing the MeCN with hexafluoroisopropanol (HFIP) improved the yield of the required DMBBQ 5b to 87% with no monomeric benzimidazolequinone 7 detected. Similar high yields (88 and 84%) using HFIP/H 2 O were obtained for the formation of pyrroloand azepino ring-fused DMBBQs 5a and 5c from 6a and 6c, respectively. The formation of 7 requires the intervention of a water molecule at the CAN generated radical cation to prevent dimerization. 6,18 The persistence of radical cation intermediates in HFIP, 19 and their shielding from water due to the formation of heterogeneous environments, 20 may account for the exclusive CAN-mediated dimerization.
DMBBQs 5a-5c were insufficiently crystalline for X-ray crystallography. However, the premise for the regioselective halogenation was supported by the preparation of bis(chloromethyl)-substituted DMBBQ 8 (76% yield), in an analogous fashion to 1 (see above), 7,8 but using less CAN (2.0 equiv.). The crystal structure of dimethylchloride 8 showed that the aromatic and quinone rings adopt an out-of-plane conformation with respect to each other (Scheme 3 and Fig. S1 †). The torsion angle (θ) of 67.6°describes the angle between the planes of the benzimidazole and benzimidazolequinone, which prevents effective orbital overlap over both rings, ruling out resonance activation of the 6-quinone position. Cyclic voltammetry and TD-DFT on DMBBQ 4a, provides further support for the site-selectivity for the electrophilic substitution with localized HOMO and LUMO at the respective dimethoxybenzimidazole and benzimidazolequinone motifs. 8 Chlorination of pyrrolo-fused dimer 5a was initially attempted using H 2 O 2 with HCl under conditions which mediated the regioselective chlorination of anilines (Table 1). 17 Reaction monitoring by TLC indicated the complete consumption of 5a after 1 h of reflux, but a low yield of 7′-chloride 9a (21%) was attributed to the formation of intractable baseline decomposition products. The requirement for milder conditions, led us to investigate the room temperature Oxone/NaCl reaction.
Addition of an aqueous solution of NaCl (1 equiv.) and Oxone (KHSO 5 , 1.2 equiv.) to 5a in MeCN gave chloride 9a in 39% yield, with column chromatography providing 52% recovery of 5a (Table 1). Polyfluorinated alcohols improve yields for NaCl/Oxone-mediated aliphatic and aromatic chlorinations, 21 and are known to accelerate aromatic chlorination in systems that generate in situ Cl 2 . 22 The reaction solvent was thus changed from MeCN to HFIP, with a greater consumption of Organic & Biomolecular Chemistry Paper substrate 5a observed, and 9a isolated in an improved yield of 51%. The highest yield of 9a (79%) after 5 h was obtained using a greater amount of NaCl (2 equiv.) with chlorination also accompanied by oxidation to give the bis-p-quinone 10a in 12% yield. Cl 2 and water are responsible for conversion of the p-dimethoxybenzimidazole to benzimidazolequinone. 18 Thus, optimized conditions for one-pot chlorination and quinone formation were achieved by increasing the amount of KHSO 5 (4 equiv.) and NaCl (4 equiv.), so increasing the likelihood of NaCl reaction with HOCl, over HOCl reaction with the substrate 5a (Scheme 1d). These conditions favoured Cl 2 formation and gave bis-benzimidazolequinone 10a in 71% yield, with an increase in water from 5 to 10% relative to HFIP giving exclusively 10a in 89% yield after 9 h. The conditions which favored selective chlorination over quinone formation (entry 4, Table 1) were applied to pyrido and azepino-ring fused DMBBQs 5b and 5c to respectively give 9b (75%) and 9c (72%) (Method A, Scheme 4). In these cases, column chromatography removed co-eluting mixtures of substrates 5b and 5c with respective bis-p-quinones 10b and 10c. The optimized conditions for quinone formation (entry 6, Table 1) were applied to 5b and 5c to respectively give 10b and 10c in isolated yields of 88 and 74%, with only the latter giving some dimethoxybenzene intermediate, 9c in 12% yield (Method B, Scheme 4).
NaBr replaced NaCl to facilitate regioselective bromination of 5a-5c (using entry 4, Table 1 conditions). There was a marginal decrease in the tendency for bromination compared to chlorination. Complete consumption of 5a occurred with the difference in R f values allowing separation of 7′-bromo-DMBBQ 11a from bis-p-quinone 12a by column chromatography in respective yields of 76% and 10% (Method A, Scheme 4). X-ray crystallographic structure determination for 11a supported the site for bromination (Scheme 4 and Fig. S2 †).
Under the same conditions, the pyrido-and azepino[1,2-a]fused dimers 5b and 5c gave selectively brominated DMBBQs 11b and 11c in respective yields of 68% and 71%. In a similar manner to the chlorinations, ring-expanded DMBBQs 5b and 5c were less reactive towards bromination than the pyrrolofused DMBBQ 5a with unreacted material separated by column chromatography as inseparable mixtures with respective bis-pquinones 12b and 12c. One-pot bromination and quinone formation (entry 6, Table 1 conditions) gave mostly the brominated dimeric quinones 12a-12c (69-75% yield), however column chromatography separated some dimethoxybenzimidazole intermediates 11a-11c (13-22% yield). The lower yields (with Method B) of brominated quinones 12a-12c are presumably due to the inferior oxidizing ability of Br 2 relative to Cl 2 . 23 The presence of a red-brown vapour in the NaBr/Oxonemediated oxidative brominations is evidence for the in situ formation of Br 2 (Fig. S3 †).
Functionalization at both available alicyclic ring-fused DMBBQ CH-positions was established. Trifluoromethylation at the quinone of 5b gave substrate 5d in a similar moderate yield (51%, Scheme 5) to that reported for DMBBQ 4b. 8 Using the Langlois' reagent, intractable baseline products were observed and Baran et al. have attributed peroxide reaction with sulfinate as a significant wastage reaction. 24 The • CF 3 radical is considered weakly nucleophilic, 25 and reported CHalkylations occur at the 'electron-poor' quinone. 26,27 Chlorination with quinone formation to give the fully functionalized bis-p-quinone 14 can be carried out in one or two steps with 5d depending on the desire to isolate more of the intermediate chloride 13. Using the optimized conditions for chlorination only (Table 1, entry 4 conditions) gave 13 in 73% yield, with the regioselectivity confirmed by the X-ray crystal structure (Scheme 5 and Fig. S4 †). The torsional angle is almost identical to that of 8, indicating the substituents of 13 have no influence on the adopted solid-state geometry. Oxidation of isolated DMBBQ 13 using NaCl and Oxone gave bis-p-quinone 14 in 85% yield. Increasing the amount of NaCl and Oxone to favour one pot oxidative chlorination (Table 1, entry 6 conditions), gave bis-p-quinone 14 from 5d in 68% yield, with some intermediate DMBBQ 13 (19%) separated by column chromatography. The smaller yields for chlorides 13 and 14 reflect the deactivation of the trifluoromethylated DMBBQs 5d and 13 towards the electrophilic oxidizing mixture. NaX/Oxone system is clearly an efficient system for ringfused DMBBQs, allowing one-pot halogenation and quinone formation. The reality is transformations developed for our specific heterocyclic system (ring-fused DMBBQs) are not readily transferable to other similar systems. According to the literature, 28 halogenation using NaX/Oxone would not be siteselective where there are other available heterocyclic CH-positions, e.g. the non-functionalised imidazole-2-position.

Paper
Organic & Biomolecular Chemistry Nevertheless, future work will aim to take advantage of the difference in reactivity between the two DMBBQ-CHs to increase scope to other site-selective functionalizations.

Measurements
All melting points were measured on a Stuart Scientific melting point apparatus SMP1. IR spectra were recorded using a PerkinElmer Spec 1 with ATR attached.
All NMR spectra were recorded using a Bruker Avance III 400 MHz spectrometer equipped with a 5 mm BBFO+, broadband autotune probe and controlled with TopSpin 3.5.7 acquisition software and IconNMR 5.0.7 automation software Copyright © 2017 Bruker BioSpin GmbH. The exception was 2,2′-bis(chloromethyl)-4′,7′-dimethoxy-1,1′-dimethyl-1H,1′H-[5,5′bibenzimidazole]-4,7-dione (8) recorded on a JEOL ECX 400 MHz NMR spectrometer equipped with a DEC AXP 300 computer workstation. The chemical shifts are in ppm relative to TMS. 13 C NMR spectra at 100 MHz are with complete proton decoupling. NMR assignments are supported by DEPT-135. 19 F NMR spectra were obtained at 376 MHz using automatic digital lock correction in the absence of an internal standard.
HRMS spectra were obtained at the National Mass Spectrometry Facility at Swansea University using a Waters Xevo G2-S or Thermo Scientific LTQ Orbitrap XL 1 mass spectrometer with an Atmospheric Solids Analysis Probe (ASAP), except for DMBBQ 8 obtained at NUI Galway using ESI time-of-flight mass spectrometer (TOFMS) in positive mode using a Waters LCT Mass Spectrometer. The precision of all accurate mass measurements was better than 5 ppm.
A single crystal of 8 was grown by slow evaporation from 10% MeOH/CH 2 Cl 2 at room temperature (Fig. S1 †). Single crystal data for 8 was collected at 299 K using an Oxford Diffraction Xcalibur (equipped with a Mo Kα X-ray source and a Sapphire detector) using the CrysAlis Pro software at NUI Galway.
The crystal structures of 8, 11a, and 13 were solved using SHELXT 31 and refined using SHELXL, 32 both of which were operated from within either the Oscail 33 or the OLEX2 34 software packages. For compound 11a, a split atom model for the disorder of the methoxy group containing atom O(3) was used to model the disorder, which refined to 72.5% for the methyl group of C(13A) compared to 27.5% for the methyl group of C(13B).

Reaction work-up for methods A and B
H 2 O (5 mL) was added, and the mixture extracted with CH 2 Cl 2 (3 × 5 mL). The organic extracts were dried (MgSO 4 ), evaporated to dryness, and purified by flash column chromatography using gradient elution of CH 2