Quaternary salts of 2H-imidazoles

摘要

J. CHEM. SOC. PERKIN TRANS. I 1983 Quaternary Salts of 2H-lmidazoles Alan R, Katritzky," Susana Bravo Borja, and Jorge Marquet Department of Chemistry, University of Florida, Gainesville,FI. 3261 1 U.S.A. Michael P.Sammes Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Treatment of the 4,5-diphenyl-2H-imidazoles (1a-e) with alkyl iodides gives novel 1Hf,2H-imidazolium salts (2) and (3). The metho-salt (2) N-methyl protons are readily exchanged for deuterium, and anions formed by the action of base give, with m-chlorobenzaldehyde, 2,3,5,7a-tetrahydroimidazo5,1-b) -oxazoles (4),hydrolysabfe in acid to 2- (3-chlorophenyl) -2-hydroxyethylamine (6). Borohydride reduc- tion of the metho-salt (2a) gave a dihydro-1 H-imidazole (7) which with acid yielded 2-methylamino- I ,2-diphenylethanone (8),and with methyl iodide a metho-salt (10).Direct (peracid) oxidation of 2H-imidatoles to A/,#'-dioxides is reported for the first time. 2H-Imidazoles have been relatively infrequently mentioned in the 1iterature.l Weiss first synthesised 2,2,4,5-tetrasubsti- tuted 2H-imidazoles from benzil, a ketone, and ammonium acetate and discovered their lability towards heat and acids. Further examples from benzil have been reported and this synthesis has given 4,5-bis(ethoxycarbonyl) derivative^.^ Other less convenient synthetic approaches have been also des- cribed.'-' 2H-Imidazole 1 -oxides result from (a) reactions of a-dioximes with certain aldehydes or ketones and subsequent alkali 8*9 or acid lo treatment, (b)photochemical dimerisation of a chloronitrosoalkane," (c) acid-catalysed condensation of diphenylmethanimine with a-hydroxyimino ketones," and (d) the action of bromine on 3-imidazolinet nitroxyl radi~a1s.l~ Apparently only a single 2H-imidazole 1-oxide has been prepared by N-oxidation of the corresponding 2H-imida~ole.~~ Ph cNxR' Ph N R (1) a;R = Me, R'=Me b; R = Me, R' = Et c; R = Me, R'= CH2CH,C02Et d; R = Me, R'=Ph e; R= Et, R'=Et 1 CH, "OiH-@ -a; R= Me, R=Me I b; R = Et , R'= Et However, several 4H-imidazoles have been converted into N,N'-dioxides by lead di0~ide.l~ Monoquaternisation of 4- amino-2H-imidazoles with methyl iodide at a ring nitrogen atom has been described.15 We now report the synthesis of a series of novel 2,2-di- substituted 4,5-diphenyl-2H-imidazoles,their quaternisation, and some additional reactions having potential synthetic applications.Results and Discussion Benzil, when treated with ketones and ammonium acetate afforded, by Weiss" procedure, 2,2-disubstituted 4,s di- phenyl-2H-imidazoles (la-) (75435) (Table 1). Quaternisations of the 2H-imidazoles with methyl iodide gave the corresponding 2,2-disubstituted l-methyl-4,5-di-phenyl-2H-imidazolium iodides (2a-e) (average 80) (Table 2). However, quaternisation with ethyl iodide was more difficult and the corresponding 2,2-disubstituted 1 -ethyl-4,5- diphenyl-2H-imidazolium iodides (3a and e) were obtained in only ca. 10 yield. 'H N.m.r., i.r.(Table 3), and elemental analyses (Tables 1 and 2) were consistent with the proposed structures. Few reported n.m.r. spectra of these compounds are avai1able.l We find absorptions at 6, 1.7-1.9 (2-CH3, Table 3) for the 2H-imidazoles (la-e), at the same position as reported for (1 a),j 2,2-dimethyl-4-pheny1-2H-imida~ole,~~4-substituted 2,2-dirnethyl-5-phenyl-2H-irnida~oles,~~and 4,5-diaryl-2,2-di- methyl-2H-imidazoles l8 (6, 1.6-1.9). Absorptions due to CH2 attached to C-2 appear at 6, 2.0-2.5 (Table 3), whilst that reported for the corresponding CH2 in (le) is at 6, 2.2. No n.m.r. data on the quaternised 2H-imidazolium salts t 2,5-Dihydro-lH-imidazole. (2a-e), (3a), and (3e) were previously available. Signals for methyl groups attached to the positively charged nitrogen appear at SH 4.0-4.2, whilst those for CH2 at the same position are in the range ijH 4.5-4.6.Substituents attached to C-2 are shifted 0.5p.p.m. downfield from their position in the corresponding 2H-imidazoles (1). 1.r. data of several aryl-2H-imidazoles have been re-ported; 16-19 a band near 1 610 cm-' (la-), Table 31 was assigned to the carbon-nitrogen double bonds. A band between 1490-1 500 cm-l, characteristic of these com-pounds," was also found in the spectra of compounds (1 a+). No literature i.r. data are available for the 2H-imidazolium salts (2a--e), (3a), and (3e) (Table 3). A characteristic band between 1 600 and 1 620 cm-' corresponds to v~=~found for the parent 2H-imidazoles.2066 J. CHEM. SOC. PERKIN TRANS. I 1983 Table 1. Preparation of 2,2-disubstituted 4,5-diphenyl-2H-imidazoles(1) Analysis () Found Compound Crystal form a M.p. ("C) Yield () I C (Required) A H I N Formula (I a) (1 b) (1c) (Id) (W Needles Prisms -Plates Needles 78-80 sublimes 92 Oil 80-82 104-106 75 85 75 80 85 82.5 (82.4 75.2 (75.585.0 (85.2 6.9 6.9 6.6 6.6 5.9 5.8 10.6 10.7) 8.2 8.4) 8.9 9.0) Cl8Hl8N2 C2IH22N2 C23H18N2 a Crystallisation solvent was light petroleum (b.p. 37- -52 "C).Lit.,' 79-80 "C. Lit.: 105-106 "C. Table 2. Preparation of 2,2-disubstituted l-alkyl-4,5-diphenyl-2H-imidazoliumiodides (2) and (3) Analysis (A)Found (Required)Crystal M.p. Yield Compound form ("0 PA) C H " (24 Needles 191-192 65 55.1 4.9 7.4 (55.4 4.9 7.1) (2b) Needles 169-171 70 56.4 5.3 6.9 (56.7 5.4 6.9) (24 Prisms 136-1 38 90 55.4 5.3 5.9 (55.5 5.2 5.8) (24 Prisms 173-1 75 55 61.1 4.7 6.1 (61.1 4.6 6.2) (2e) Plates 173-175 95 57.4 5.5 6.6 (57.4 5.5 6.7) (34 Prisms 180-181 10 56.5 5.3 6.9 (56.7 5.4 6.9) (3e) Needles 188-190 10 58.3 5.8 6.4 (58.4 5.7 6.5) a Crystallisation solvent was absolute EtOH.-~__-_ Table 3. Spectroscopic data for 2,2-disubstituted 4,5-diphenyl-2H-imidazoles (1) and 2,2-disubstituted l-alkyl-4,5-diphenyl-2H-imidazolium iodides (2) and (3) Chemical shift (6,) a r -~~ Aromatic R" R R; Vnl,Lb (cm-9 7.2-8.1 -1.7 1.7 1 600m, 1 550s, 1490s, 144Os, 1 355w, and 1260s 7.2-8.1 -1.68 0.8: 2.2 1 600m, 1 550s, 1490s, 1440s, 1 355w, and 1 260s 7.2-8.1 __.1.7 1.2: 2.0-2.7,' 4.1 1740s, 1600s, 1 550m, 1 490m, 1450~~ and 1 350s 7.4-7.9 -1.90 h 3 060m, 1 600s, 1 550s, 1 490s, and 1 250s 7.3-8.2 -0.75: 2.25 0.75," 2.25 1 600s, 1 550111,and 1 480m 7.3-8.2 4.0 2.0 2.0 1 600s, 1 560s, 1 450s, and 1 390s 7.3-8.2 4.1 2.2 0.8: 2.5 2 820s, 1620s, 1 600s, 1 490s, and 1 400s 7.3-8.2 4.0 2.3 1.2; 2.3-3.0,' 4.1 2920s, 1600m, 1 500m, and 1290s 7.2-8.1 4.0 1.9 h 2 920s, 1 600s, 1 300s, and 1 050w 7.3-8.2 4.2 0.85: 2.7 0.85; 2.7 2 910s, 1 600111, 1 550s,and 1 440s 7.3-8.0 1.3: 4.55 2.2 2.2 2 920s, 1 600s, 1 Om, and 1 450m 7.3-7.8 1.45: 4.65 1.0: 2.8 1.0; 2.8 2 920s, 1 600m, 1450m, and 1 290m a 60 MHz; CDC13 as solvent; SiMer as internal reference; singlets unless otherwise indicated.Mull in CHBr3. 10 H, multiplet. H, triplet. 2 H, quartet. f 4 H, multiplet. 9 15 H, multiplet. Included in aromatic region. 2067I. CHEM. SOC. PERKIN TRANS. I 1983 -PhrNxR Base PhCNxR Ph-N+ R' Ph-cN+ R' I I 11CH3 C" 2 (2) Ar 1Ar Scheme. The 1-methyl protons in the quaternised salts (2) are active: they were exchanged almost completely for deuterium at 25 "C after 48 h in deuterium oxide and E2H6acetone (concentration cu. 0.2~) in the presence of triethylamine or pyridine. These protons have an activity similar to that of the a-protons in pyridinium cations (50 exchange in 21 h at 20 "C; concentration 0.3~; piperidine as base),20*21 whilst they exchange faster than the corresponding N-methyl protons in I-methylpyridiniums (complete exchange in 8 d at room temperature; concentration 0.3~; piperidine as base).20*211,3-Dirnethylimidazolium iodides exchange only at the ring positions.22 The salts (2) are acidic enough to undergo aldol reactions with aldehydes using triethylamine as a catalyst.Thus, I ,2,2-trimethyl-4,5-diphenyl-2H-imidazolium(2a) and 2,2-diethy.I- 1-methyl-4,5-diphenyl-2H-imidazolium(2e) iodide reacted with m-chlorobenzaldehyde to give the bicyclic compounds (4a) (70) and (4b) (700/,), respectively. We consider the irrevers- ible final cyclization to be responsible for the success of this reaction (Scheme). The bicyclic compounds (4a) and (4b) were identified by their spectroscopic properties and elemental analysis.'H N.m.r. spectra showed in both cases a character- istic ABX system for the 2-H (6 4.45) and 3-H2 (6 3.65 and 2.95) protons. The I3C n.m.r. spectrum of compound (4b) confirmed the proposed structure, the characteristic absorptions being 6, 110.4 (C-7a), 95.3 (C-5),79.8 (C-2), and 54.6 p.p.m. (C-3). The imino carbon signal appeared at 6c 165.6 p.p.m. The signals were assigned by chemical shift considerations and off-resonance decoupled spectra. Treatment of compound (4a) with perchloric or fluoroboric acid did not cause ring-opening to the monocyclic structure (5a). However, hydrolysis with hydrochloric acid afforded the hydroxy amine hydrochloride (6) (20). The 'H n.m.r. spectrum of this compound showed a characteristic AA'X system (for protons at C-1 and C-2), and it was fully character-ized by 13Cn.m.r.and elemental analysis (see Experimental sect ion). This reaction between an aromatic aldehyde and 1,3,3-trimethyl-4,5-diphenyl-2H-imidazoliumiodide (2a) with sub- sequent hydrolysis of the bicyclic product (4a) offers an alternative method to the reaction with nitromethane, followed by reduction, for the preparation of compounds of type (6) in cases where a reduction step is undesirable. Treatment of 1,2,2-trimethyl-4,5-diphenyl-2H-imidazofium iodide (2a) with sodium borohydride afforded I72,2-trimethyl- 0 NHMe Ph /N PhUI PhLNPhPhC-CHPh Me 0-Me 0-(10) (11) a; R=Me b; RzEt 4,5-diphenyl-2,5-dihydro-lH-imidazole (7). No further reduc- tion occurred even with excess of borohydride.Some similar cases are found in the literature for related 4H-imida~oles,~~ and for 2H-imidazole 1,3-dio~ides.'~ With lithium aluminium hydride, 2H-imidazole 1 -oxides form 1 ,2-dihydro compounds only at low temperature.l2 Hydrolysis (heat; ~M-HC~; 1 h) of the 2,5-dihydro-lH- imidazole (7) afforded the a-amino ketone (8). Thus, succes- sive formation of the 2H-imidazole, quaternisation, reduction, and finally hydrolysis is a potentially useful way to convert a-diketones into a-(N-alkylamino) ketones. Asinger described a similar hydrolysis for the corresponding 5-met hyl-4-ethyl compound. Hydrolyses of 2,5-dihydro-1H-imidazoles un-substituted on nitrogen have been reported for the 5-ethyl-4- methyl and 4,5-diphenyl 24 compounds.Hydrolysis under milder conditions (25 "C)but for a longer time (one week) gave a product of high melting point (248-25OoC), with no aliphatic protons in the 'H n.m.r. spectrum, and which was characterized by elemental analysis and its mass spectrum as 2,3,5,6-tetraphenylpyrazine (9) (lit.,25 m.p. 249-250 "C). Following our study of the chemical properties of the imidazoline (7), it was quaternised with methyl iodide giving compound (10) in 60 yield. Only one oxidation of a 2H-imidazole to its N-oxide has been reported." We find that oxidation of compounds (la) and (le) with hydrogen peroxide in acetic acid gives the corresponding N,N'-dioxides (1 1a) and (1 I b). Elemental analysis, Lr., n.m.r., and mass spectra were consistent with the assigned structures.The mass spectra did not show the molecular ion, but the (M -I)+ peak was present. No intense (M -16)' or (M -17)+ peaks, frequently observed with heterocyclic N-oxides,26-28 were apparent, in accord with the findings in the 3,4-dihydro-2H-pyrrole 29 and 2,2-di-phenyl-2H-imidazolel2 N-monoxides series ; the latter com- pounds were, however, all reported to show a molecular ion. Experimental M .p.s were obtained in a Bristolscope hot-stage apparatus and are uncorrected. 1.r. spectra were run as CHBr, mulls on a Perkin-Elmer 283B spectrophotometer, using NaCi plates. 'H N.m.r. spectra were recorded on Varian A-60A and Varian EM 360L (60 MHz) instruments, and I3C n.m.r.2068 spectra on a JEOL FX 100 instrument; SiMe, was used as internal reference. 2,2- Disubstituted 4,5-D@henyl-2H-imidazoles(1 ).-General procedure (Tables 1 and3). Benzil(10.5 g, 0.05 rnol), ammonium acetate (40 g), and the appropriate ketone (0.05 mol) in glacial AcOH (100 ml) were boiled under reflux for 2 h. The solution was cooled, poured into ice-water (1 50 ml), and extracted with diethyl ether (3 x 50 ml). The combined extracts were washed with water until the washings were neutral, and were then dried (MgSO,) and evaporated (30°C; 30 mmHg). The product was recrystallised from light petroleum (b.p. 37-52 2,2- Disubstituted 1-Methyl- and 1-Ethyl-4,5-diphenyl-2H-imidazolium Iodides (2) and (3).-General procedure (Tables 2 and 3).Methyl (or ethyl) iodide (0.0080 mol) was added to the 2,2-disubstituted 4,5-diphenyl-2H-imidazole(0.0080 mol) in MeN02 (15 ml) and the solution was stirred under reflux until t.1.c. (silica gel; EtOAc) showed no starting material (5-6 h). The solution was cooled, the iodide salts were precipi- tated with diethyl ether (50 ml), dried under reduced pressure 40 "C; 0.5 (mmHg; 6 h) and recrystallised (absolute EtOH). Deuterium-Proton Exchange of the N-Methyl Protons of Compound (2a).-A solution of the salt (2a) (0.150 g) in ,H6acetone (2 ml) was stirred with a 10 solution of either Et3N or pyridine (as base) in D20 (0.1 ml). After 48 h ex- change was shown to be complete by the disappearance of the N+-Me protons (6 4.0) in the 'H n.m.r.spectrum. Cyclizution of the 2,2-Dialkyl-l-methyl-4,5-diphenyl-2H-imidazolium iodides (2a and e) with m-Chlorobenza1dehyde.- With (2e). Triethylamine (2.3 g, 0.0227 mol) and m-chloro- benzaldehyde (1.60 g, 0.0128 rnol) were added to a solution of the salt (2e) (3.00 g, 0.0072 moI) in CHZC12 (30 ml) and the reaction mixture was heated under reflux for 9 h, after which no starting material was detected (silica gel; EtOAc). The deposited triethylammonium iodide was separated, the remaining liquid was washed with water (3 x 20 nil), and the organic layer was dried (MgSO,) and evaporated (40°C; 30 mmHg) to give 2-(3-chlorophenyZ)-5,5-diethyl-7,7a-di-phenyl-2,3,5,7a-tetrahydroimidazo5,1- bloxazole (4 b) (70) as prisms from acetone, m.p.124-126 "C (Found: C, 75.0; H, 6.3 ;N, 6.5. C27H27C1Nz0 requires C, 75.25 ;H, 6.3 ;N, 6.5) ; 6" (CCI,) 0.9 (3 H, t), 1.3 (3 H, t), 2.0 (4 H, m), 2.9 (1 H, dd), 3.7 (1 H, dd), 4.6 (1 H, dd), and 7.0-7.9 (14 H); 6, (CDCI,) 8.6 (CH,), 9.9 (CH,), 27.1 (CH,), 32.1 (CH,), 54.6 (C-3), 79.8 (C-2), 95.3 (C-5), 110.4 (C-7a), 123.0-142.5 (aromatic), and 165.6 p.p.m. (C-7); vnBx.2 975m, 2 960m, 2 930m, 1 615m, 1 600m, 1 570m, 1 490m, and 1 355m cm-'. With (2a). Following the procedure for (2e), the salt (2a) (2.0 g, 0.0051 mol) was treated with nz-chlorobenzaldehyde (0.79 g, 0.0063 mol) and triethylamine (2.3 g, 0.0227 mol) to give 2-(3-chlorophenyl)-5,5-diniethyl-7,7a-diphenyl-2,3,5,7a-tetrahydroimidazo5,1 -boxazole (4a) (70) as prisms from acetone, m.p.146--148 "C (Found: C, 74.4; H, 5.8; N, 6.9. C25H23C1N20requires C, 74.5; H, 5.8; N, 7.0); 6, (CCI,) 1.6 (3 H, s), 1.7 (3 H, s), 2.9 (1 H, dd), 3.6 (1 H, dd), 4.7 (I H, dd), and 7.2-8.0 (14 H, m); vniaY.2 975m, 2 960m, 2 930m, 1 615m, 1 600m, 1 570m, 1 490m, and 1 355m cm-I. Acid Hydrolysis of 2-(3-Ch loropheny1)- 5,5-dimethyl-7,7a-diphenyl-2,3,5,7a-tetrahydroimidazo5,l -boxazole (4a).-To a solution of compound (4a) (1 g, 0.0025 mol) in CH2C1, (10 ml) was added concentrated HCI (4 ml) and the mixture was kept for 24 h. The precipitated solid was filtered off and character- J. CHEM. SOC. PERKIN TRANS. I 1983 ized as the hydroxy amine hydrochloride (6) (2073, which was crystallized as prisms from chloroform, m.p.141 "C(analysed as the dihydrate: Found: C, 39.2; H, 5.7; N, 5.3. C8HI1CI2- NO*lH20 requires C, 39.4; H, 6.1; N, 5.7); 6, (DzO) 3.1 (2 H, m), 4.9 (1 H, dd), and 7.3 (4 H, m); 6, (dioxane) 46.05 (CH), 69.78 (CH,), and 125.20, 126.76, 129.44, and 131.34 p.p.m. (aromatic); vnlax.3 400-2 500br and 1 160-1 OlObr cm-' . Reduction of 1,2,2-Trimethyl-4,5-dipheny1-2H-ir~lidazolium Iodide (2a).--NaBH, (0.37 g, 0.010 mol) was added (0.5 h) to a solution of the imidazolium iodide (2a) (2.0 g, 0.0051 mol) in methanol (10 ml). After 2 h the solvent was evap- orated off (40°C; 30 mmHg), the residue was washed with water (2 x 20 mI), filtered off, dried in uacuo (25 "C; 0.5 mmHg; 4 h), and characterized as 1,2,2-trimethyl-4,5-dipheny1-2,5-dihydro-l H-imidazole (7) (85) as prisms from acetonitrile, m.p.85-87 "C(Found: C, 81.5; H, 7.6; N, 10.5. CI8H20N2requires C, 81.8; H, 7.6, N, 10.6); 6H(CDC13) 1.6 (3 H, s), 1.8 (3 H, s), 2.6 (3 H, s), 4.9 (1 H, s), and 7.2-7.5 (10 H, m); vms. 3 030s, 2 980s, 2 240m, 1 620s, 1 500s, 1 450s, and 1 250s cm-'. Acid Hydrolysis of 1,2,2-Trimethyl-4,5-diphenyl-2,5-dihydro-1H-imidazole (7).-(a) A solution of compound (7) (1 g, 0.0038 mol) in ~M-HC~ (10 ml) was heated (steam-bath) for 1 h. After the solution had been cooled, CH2Clz (20 ml) was added and the organic layer was separated, dried (MgSO,), and filtered. The solvent was evaporated off (40°C; 30 mmHg) and the solid residue, obtained as prisms, m,p. 226 "C (from absolute ethanol), was characterized as the hydrochloride of the a-amino ketone (8) (90) (Found: C, 68.8; H, 6.1 ;N, 5.2.Calc. for C15H,INO: C, 68.8; H, 6.1 ;N, 5.2); 6, (CDC1,) 2.4 (3 H, s), 5.3 (1 H, s), and 7.2-8.2 (10 H, m); vmx. 1 670s, 1 600s, 1 540s, and 1 400br cm-l. (b) A solution of compound (7) (1 g, 0.0038 mol) in EtOH (10 ml), H20 (I .5ml), and concentrated HCl(2 ml) was kept at 25 "C for 8 d. A solid was formed which, after filtration and recrystallization from chloroform (prisms, m.p. 248-250 "C) (lit.,25 249-250 "C), was characterized as 2,3,5,6-tetraphenyl- pyrazine (9) (8) (Found: C, 87.2; H, 5.6; N, 7.2. Calc. for C28H~ON2:c, 87.5; H, 5.2; N, 7.3); SH(CDC13) aromatics only; vnux.3 200s, 1 700s, 1 400s, and 1 300s cm-'. Quaternisation of 1,2,2-Trimethyl-4,5-diphenyl-2,5-dihydro-1H-imidazole (7).--The dihydro-1H-imidazole (7) (1 g, 0.0038 mol) was stirred with Me1 (10 ml).A precipitate was immediately formed. After the mixture has been stirred for 10 h no starting material was detected by t.1.c. (silica gel; EtOAc). Evaporation of the solvent (30°C; 30 mmHg) and recrystallization of the residue (absolute EtOH) gave 1,2,2,3- tetramethyl-4,5-diphenyl-3,4-dihydro-2H-iniidazoliri~niodide (10) (60) as prisms, m.p. 188-190 "C (Found: C, 56.2; H, 5.7; N, 6.9. CI9Hz3LN2 requires C, 56.2; H, 5.7; N, 6.9); SH (CDClj) 1.8 (3 H, s), 2.2 (3 H, s), 2.9 (3 H, s), 3.7 (3 H, s), and 7.1-7.9 (10 H, m); vnlss. 2920s, 1630s, 1570m, 1470s, 1 450s, 1 380m, and 1 140br cm-'. 2,2-Dimethyl-4,5-diphenyl-2H-imiduzole1,3- Dioxide (1 1a).-A solution of the 2H-imidazole (la) (2.0 g, 0.0081 mol) in AcOH-30 H202 (20 ml; 5 : 1) was stirred at 25 "C for 2 h; water (100 ml) was then added and the N,N'-dioxide (1 la) separated out (88) and was recrystallized as prisms from acetone, m.p. 74-74.5 "C (Found: C, 73.0; H, 5.8; N, 9.9. CL7HZ6N2o2requires C, 72.8; H, 5.8; N, 10.0); 6, (CDC13) 1.6 (6 H, s) and 7.3-7.7 (10 H, m); vnlnX.1445s, 1 340s, 1 290s, 750s, and 655s cm-'. J. CHEM. SOC. PERKIN TRANS. I 1983 Acknowledgements We thank the Universidad de Guanajuata and Conacut (Mexico) for leave of absence and a grant to S. B. B. References 1 For a review see M. P. Sammes and A. R. Katritzky, Adv. Heterocycl.Chem., in the press. 2 M. Weiss, J. Am. Chem. 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