Transfer of alkoxycarbonyl from alkyl imidazolium-2-carboxylates to benzyl alcohol, a cyclohexanone enamine and diethylamine

摘要

J. CHEM. SOC. PERKIN TRANS. 1 1994 Transfer of Alkoxycarbonyl from Alkyl Imidazolium-2-carboxylates to Benzyl Alcohol, a Cyclohexanone Enamine and Diethylamine Cuross Bakhtiar and Edward H. Smith" Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AY Alkylimidazole-2-carboxylates may be alkylated with methyl triflate to give the corresponding N-methylimidazolium salts. These salts react with benzyl alcohol in the presence of 1.4-diaza- bicyclo2.2.2octane, with 1 -(pyrrolidin-1 -yl)cyclohexene and with diethylamine to give benzyl alkyl carbonates, an enamino ester and a urethane respectively; in one case a tetrahedral intermediate is observed. The corresponding phenyl ester was consumed without attack by benzyl alcohol at the carbonyl group.A 2-cyanoimidazolium salt underwent similar ill-defined consumption whereas a 2-d imet hylam inocarbonyl derivative remained unchanged. 2- Methylsu lfonyl imidazol iu m salts suffered attack by benzyl alcohol at the ring C-2. Attack by nucleophiles at the carbonyl group of 2-imidazolium ketones and the subsequent ejection of the imidazolium moiety with consequent acylation of the nucleophile (Scheme 1) is a Nuji,Scheme 1 well documented process and mimics the biological chemistry of the corresponding thiazolium species of thiamine (vitamin BJ. In principle, similar attack on other carbonyl-containing groups at the 2-position of the imidazolium salt should also result in group transfer to the nucleophile and we were interested in determining the range of groups susceptible to such a reaction.This paper describes work on ester- and amide-containing imidazolium salts. Results and Discussion Synthesis of the Imidazolium Salts (Scheme 2).-Starting imidazole 2-esters 1 for the dealkoxycarbonylation were all made in one pot by the sequential reaction of 2-lithio-N- methylimidazole with trimethylsilyl chloride and an alkyl chloroformate. The prior addition of the silylating agent was essential for success of the reaction which presumably proceeds through the 2-trimethylsilyl derivative.' Aminolysis of the ethyl ester lb, followed by dehydration gave the 2-cyano derivative 2. Treatment of lb with dimethylamine gave the amide 3.Trapping 2-lithioimidazoles with dimethyl disulfide produced the 2-methylthio derivatives which could be oxidized to the sulfones 4. N-Methylation of all these substrates with methyl triflate went smoothly and provided the desired imidazolium salts as stable, non-hygroscopic, white solids (esters 5, cyanide 6, amide 7and sulphones 8). Dealkoxycarbonylution Reactions.-Reaction of the salts 5a-f with benzyl alcohol in acetone in the presence of 1,4- diazabicyclo 2.2.2)octane (DABCO) at room temperature under argon resulted in clean dealkoxycarbonylation to give good yields of the benzyl alkyl carbonates easily separated I Me 4a R'=Me 2 b R'=Ph i. ii R2= Et v. vi 1 I R2 3 la Me b El Pi d Bu' e CH2Ph f CH2CC13 9 Ph / viii axTfo-5 R' = Me, X = C02R 6 R'=Me,X=CN 7 R' = Me, X = CONMQ 8a R' = Me, X = S0,Me 8b R' = Ph, X = S02Me Scheme 2 Reagents and conditions: i, BuLi, THF, -78 "C to -20 "C then MeSSMe, -78deg;C to room temp.; ii, mCPBA, CH,CI,, room temp.; iii, BuLi, THF, -78 "C to -20 "C then TMSC1, -78 "C to room temp.; iv, R'OCOCI, -78 "C to room temp.; v, NH,, MeOH; vi, POCl,, py, PhMe, reflux; vii, aq.Me,NH, MeOH; viii, MeOTf, CH,Cl,, room temp. from the other product, N,N'-dimethylimidazolium triflate, by solvent extraction with dry ether after removal of the acetone (Scheme 3). Both the alcohol and base are required for the success of the reaction; neither reacted with 5 separately. Formation of the benzyl carbonates occurs fairly rapidly.The half-lives for dealkoxycarbonylation of the methyl and isobutyl esters in 2H,acetone at 30 "C was 33 and 926 min, respectively, by 'H NMR spectroscopy. In the case of the ethyl 240 TfO -BnOH. DABCO, 0 I 5 R Yo Me 74 Et 71 Pi 76 Bu' 68 CH2Ph 73 CH2CC13 68 Scheme 3 ester, the 'H NMR spectrum of the reaction mixture after 4 min showed the presence of three ethyl Me triplets in the region 1-1.5 ppm. Assuming one triplet peak was due to starting material and one due to product, the third is either an intermediate or a side-product. In addition, a quartet for an ethyl CH, appeared at 3.5 ppm, upfield from the ester ethyl CH, quartets at 3.8-4.3 ppm of the starting material and carbonate. Thus, this third material appears to contain an ethereal OEt group.Since it has disappeared by the end of the reaction an intermediate rather than a side-product is indicated and tetrahedral intermediate structure 9 seems likely. Earlier, we 9 10 obtained stereochemical evidence consistent with the involve- ment of a tetrahedral intermediate in the cleavage of an acyl- imidazolium salt and the tetrahedral intermediate 10 has been isolated in the 0-acetylation of phenylhydroxylamine by 2- acetylthiazolium triflate.4 The nature of the alkyl group has little influence on the yields of these reactions although the rates of dealkoxy- carbonylation are slower the larger the alkyl group (the reaction of the isopropyl ester takes 48 h for completion).Looking at other nucleophiles, diethylamine readily effected demethoxycarbonylation of the ester 5a in acetone in the absence of DABCO to give N,N-diethyl 0-methyl carbamate (60), but aniline failed to react. Carbon nucleophiles such as pentane-2,4-dione and 5,5-dimethylcyclohexane-1,3-dione and the ambident phenol all failed to react with the imidazolium esters either in the presence of DABCO or as sodio derivatives, although in the latter case the insolubility of the nucleophiles may have been the cause of the lack of reaction. However, the pyrrolidinyl enamine 11 cleanly effected deethoxycarbonyl- ation of the ethyl ester 5b in dichloromethane in the absence of DABCO (Scheme 4). n I 11 5b Scheme 4 In contrast to its alkyl counterparts, the phenyl ester 5g reacted with benzyl alcohol and DABCO in a desultory manner and produced a complex mixture rather than any phenyl benzyl carbonate nor any imidazolium triflate.None of the products of this reaction could be isolated without decomposition. An alternative site of attack by nucleophiles J. CHEM. SOC. PERKIN TRANS. I 1994 for any of these substrates is C-2 of the imidazolium ring although this was never seen in the reactions of 5a-f. Evidence against attack at C-2 of the imidazolium ring in 5g too came from the results of reactions with the sulfones 8. The sulfones 8 certainly did appear to suffer attack at C-2 of the imidazolium ring. Thus, an olefinic singlet appeared in the 'H NMR spectrum of the crude product from 8a at 6.3 ppm simultaneously with the disappearance of the singlet at 7.95 ppm corresponding to the aromatic C-4(5) protons. This was even more clearly seen during the reaction of 8b where the corresponding signals undergoing interchange were AB quartets from the aromatic region (8.I ppm, J 1.5 Hz) in 8b to the olefinic region (6.7 ppm, J 3.6 Hz) in the product. These chemical shifts and coupling constants are consistent with a 2,3-dihydroimidazole structure 12 for the products; in the related compounds 13 the enamido protons appear as AB Me A 12 R=MeorPh quartets at 6.3-6.9 ppm with Jvalues 3 or 3.4 depending on the substituents. Since the methylsulfone group is readily displaced by nucleophiles from C-2 of imidazoles6 it is not unexpected that the imidazolium salts should suffer similar attack.Attempts to isolate the products of this attack led to their decomposition. The absence of olefinic peaks in the spectrum of the crude product mixture from 5g suggested that intermediates of the type 12 were not present. Neither cyano nor amide was cleaved from 6 or 7, respectively, after treatment with benzyl alcohol and DABCO in acetone for 48 h. The amide 7remained untouched whereas the cyano compound 6 was slowly consumed; again a complicated 'H NMR spectrum of a mixture of unstable products resulted. As far as we are aware the transfer of alkoxycarbonyl groups from 2-alkoxycarbonylimidazolium salts is the first demon- stration of imidazolium behaving as a leaving group from any carbonyl other than a ketone or aldehyde.Experimental M.p.s. were determined on a Kofler hot-stage or Gallenkamp apparatus and are uncorrected. IR spectra were recorded on a Perkin-Elmer 881 spectrophotometer as thin film (oils) or as Nujol mulls (solids) unless otherwise stated. 'H and '3C NMR spectra were recorded on a JEOL FX 90Q instrument, using tetramethylsilane as internal standard in CDCI, unless otherwise indicated. Coupling constants are given in Hz; signals are quoted as singlet (s), doublet (d), triplet (t), quartet (q), quintet (qn), septet (sep), multiplet (m) and broad (br). Mass spectra were recorded on a VG Micromass 7070B machine by EI or FAB (thiodiethanol) methods. Preparative gravity column chromatography was performed on Crosfield Sorbsil C60 silica gel.Petroleum refers to light petroleum of b.p. 40-60 "C. Ether refers to diethyl ether. Ether and tetrahydrofuran (THF) were distilled from sodium and potassium metal respectively under argon immediately prior to use. Dichloromethane was distilled from phosphorus pentoxide under argon just prior to use. Butyllithium was purchased from Aldrich Chemicals as solutions in hexanes. All other solvents and reagents were purified by standard methods. General Synthesis of 2-Alkoxycarbonyl-I-methylimidazoles 1 .-To a solution (0.5 mol dm- 3, of 1-methylimidazole in dry J. CHEM. SOC. PERKIN TRANS. 1 1994 THF at -78 "C under argon was added butyllithium (1.6 mol dm-3; 1.05 equiv.) dropwise. The reaction mixture was stirred for 30 min and then allowed to warm to -20 "C over 40 min when the yellow colour of the anion appeared.The mixture was cooled to -78 "C and chlorotrimethylsilane (1.05 equiv.) was added dropwise to it at such a rate that the temperature did not rise above -70 "C. The cold bath was removed and the mixture was allowed to warm to room temperature. After it had been stirred for 1 h, the mixture was cooled to -78 "C and the alkyl chlorofomate (1.05 equiv.) was added to it by syringe; the cold bath was then removed. After the temperature of the solution had reached ambient, the mixture was stirred for 12 h. Water was added to quench the reaction and the THF was removed on the rotary evaporator. The resultant slurry was taken up in dichloromethane (30 cm3) and the mixture washed with water (2 x 20 cm3) and the organic layer dried (MgSOJ.Removal of the drying agent and concentration gave the crude product which was purified on silica gel using petroleum-ether (3 : 1) as eluent. The following esters were made. Methyl 1 -methylimidazole-2-carboxylate73 la as a colour- less oil (71); v,,,(neat)/cm-' 1713 (CO); 6, 3.95 (3 H, S, OMe), 4.0 (3 H, s, NMe), 7.05 (1 H, fine d, J 1,4(5)-H) and 7.15 (1 H, fine d, J 1, 4(5)-H); 6, 36 (NMe), 52 (OMe), 126 C-4(5), 129 C-4(5), 136 (C-2) and 160 (CO); m/z 140 (M', 40), 110 (32), 109 (M' -OMe, 54) and 82 (M' + H -CO,Me, 100). Ethyl 1-methylimidazole-2-carboxylate7~8lb as a colourless oil (68); vmax/cm-l 171 1 (CO); 6, 1.4 (3 H, t, CH3CH20), 4.0 (3 H, s, NMe), 4.4 (2 H, q, CH3CH,0), 7.05 l H, fine d, J 1, 4(5)-H and 7.15 (1 H, fine d, J 1, 4(5)-H); 6, 14 (Me), 36 (NMe), 62 (CH,O), 126 C-4(5), 129 C-4(5), 137 (C-2) and 159 (CO); m/z 154 (M', llx), 110 (14), 109 (M' -OEt, 19) and 83 (M' + 2H -CO,Et, 100).Isopropyl 1-methylimiduzole-2-curboxylutelc as a colourless oil (63) (Found: M+, 168.0895. C,Hl,N20, requires M, 168.0898); v,,,(neat)/cm-l 1714 (CO); 6, 1.4 (6 H, d, J 7.2, CHMe,), 4.0 (3 H, s, NMe), 5.25 (1 H, sep, J7.2, CHMe,), 7.0 l H, br s, 4(5)-H and 7.15 l H, br s, 4(5)-H; 6,22 (Me), 36 (NMe), 69 (CHO), 126 C-4(5), 129 C-4(5), 137 (C-2) and 159 (CO); m/z 168 (M', 20), 110 (4373, 109 (M' -OPr', 50) and 82 (Mf + H -CO,Pr', 100).Isobutyl 1-methylimiduzole-2-curboxyluteId as a white crystalline solid, m.p. 45-46 "C (57) (Found: M+, 182.1055. C9HI4N2O2 requires M, 182.1055); v,,,,,(CHC~~)/C~-~ 1711 (CO); 6, 1.05 (6 H, d, J 7.2, CHMe,), 2.2 (1 H, sep, CHMe,), 4.0 (3 H, S, NMe), 4.15 (2 H, d, J 7.2, CH,O), 7.0 (1 H, fine d, J 1, 4(5)-H). and 7.15 l H, fine d, J 1, 4(5)-H; 6, 19 (Me), 27 (CH), 35 (NMe), 71 (CH,O), 126 C-4(5), 129 C- 4(5), 136 (C-2) and 159 (CO); m/z 182 (M', 17), 167 (M+ -Me, 473, 127 M+ + H -CH,=C(Me),, 25x1, 109 (M+ -OBu'. 98) and 82 (M+ + H -CO,Bu', 100). Benzyl 1 -methylimiduzole-2-curhoxylutele as a white solid, m.p. 66-67 "C (52) (Found: C, 66.4; H, 5.8; N, 12.8. Cl,Hl,N,02 requires C, 66.65; H, 5.59, N, 12.95); v,,,(CH,Cl,)/cm 1719 (CO); 6, 3.98 (3 H, S, NMe), 5.4 (2 H, s, CH,O), 7.0 l H, br s, 4(5)-H, 7.15 l H, br s, 4(5)-H and 7.2-7.5 (5 H.m, Ph); 6, 36 (NMe), 67 (CH,), 126-136 (six peaks) and 159 (CO); m/z 216 (M', 473, 110 (M' + H -OBn, 59),91 (PhCH,', 84) and 82 (M' + H -CO,OBn, 100). 2,2,2- Trichloroethyl 1-methylimiduzole-2-curboxyluteIf as a white solid, m.p. 38-39 "C (78) (Found: C, 32.5; H, 2.7; N, 10.5. C7H,C13N,0, requires 32.65; H, 2.74; N, 10.88); v,,,(CHCl,)/cm 1727 (CO); 6, 4.05 (3 H, S, NMe), 5.0 (2 H, s, CH,O), 7.1 l H, br s, 4(5)-H and 7.25 (1 H, br s, 4(5)-H); 6, 36 (NMe), 74 (CH,O), 95 (CCl,), 127 C-4(5), 130 C- 4(5), 135 (C-2) and 157 (CO); m/z 260 (1.5), 258 (4.2), 241 256 (4.6) (M'), 223 (llz), 221 (18) (M' -Cl), 109 (M' -OCH,CCl,, 100) and 82 (M' + H -C02CH,CC13, 21).Phenyl 1-methylimidazole-2-carboxylatelg as a white crystalline solid (4379, m.p.136-137 "C (lit.,8 142 "C); v,,,/cm-' 1732 (CO); 6,4.1 (3 H, s, NMe) and 7.1-7.5 7 H, m, 4(5)-H and Ph; 6, 36 (NMe), 1 15-1 30 (seven peaks) and 150 (CO); m/z 202 (M', 373, 158 (M' -CO,, 42), 109 (M' -OPh, 100) and 94 (38). 2-Cyano-1-methylimidazole9 2.-To an ice-cold solution of the ester lb (981 mg, 6.37 mmol) in methanol (20 cm3) was slowly added aqueous ammonia (35 by wt.; 5 cm3). The resultant solution was stirred at 0 "C for 2 h and then at room temperature overnight. The methanol was removed by rotary evaporation and the residue treated with water (10 cm3) and ex- tracted with ethyl acetate (2 x 5 cm3).The combined extracts were dried (MgSO,), filtered and rotary evaporated to give the crude amide as a solid. This was recrystallised from ethyl acetate to give 1-methylimidazole-2-carboxamideas a white solid (668 mg, 84), m.p. 167-168 "C (lit.," 165-167 "C or 170 "C); v,,,/cm-' 3331, 2924 (NH,) and 1666 (CO); 6, 1.8 (2 H, s, NH,), 4.05 (3 H, s, NMe) and 7.0 2 H, m, 4(5)-HI. This solid (663 mg, 5.3 mmol) was dissolved in dry toluene (20 cm3) containing dry pyridine (1.24 cm3, 15.44 mmol) and then phosphorus oxychloride (2.37 g, 15.5 mmol) was added dropwise to the solution over 5 min. The solution was then heated to reflux for 2 h. After cooling, the toluene was removed by rotary evaporation and the residue was cooled in an ice- bath.Water (25 cm3) was added very carefully to the cooled residue and the aqueous suspension was extracted with chloroform (3 x 10 cm3). The combined extracts were washed with aqueous copper sulfate (5 by wt.; 5 x 5 cm3) and water (5 cm3) and dried (Na,SO,). Removal of the drying agent and evaporation of the solvent gave a yellow oil which was chromatographed on silica gel using ether-petroleum (1 :1) as eluent to give the product as an oil (305 mg, 46); v,,,/cm-' 2236 (CN); 6, 3.9 (3 H, S, NMe), 7.1 l H, s, 4(5)-H and 7.2 l H, s, 4(5)-H; 6, 34 (NMe), 124 C-4(5), 132 (C-2); m/z 107 (M', loo), 80 (M+ -HCN, 9),66 (14), 53 (15) and 42 (62). 1,N,N-Trimethylimiduzole-2-curboxumide3.-To an ice-cold solution of the ester la (51 1 mg, 3.65 mmol) in methanol (10 cm3) was slowly added aqueous dimethylamine (40 by wt.; 10 cm3).The resultant solution was stirred at 0 "C for 5 h and then at room temperature overnight. Methanol was removed by rotary evaporation and water (25 cm3) was added to the residue. The aqueous suspension was extracted with ethyl acetate (3 x 8 cm3) and the combined extracts were washed with water (1 x 10 cm3) and dried (MgSO,). The drying agent was filtered off and the solvent was removed by rotary evaporation to give a colourless oil (469 mg, 84) (Found: M+, 153.0906. C7H1 1N30 requires M, 153.0902); v,,,/cm-' 1618 (C0);6, 3.1 (3 H, S, CONMe), 3.4 (3 H, S, CONMe), 3.85 (3 H, s, NMe), 6.95 l H, fine d, J 1, 4(5)-H and 7.0 l H, fine d, J 1,4(5)-H; 6,35 (NMe), 36 (NMe), 39 (NMe), 124 C- 4(5), 127 C-4(5), 140 (C-2) and 161 (CO); m/z 153 (M+, lo), 139 (M+ -CH,, 673, 109 (M' -NMe,, 3573, 96 (65)and82(Mf + H -CONMe,, 100).1-Methyl-2-methyfsu~ofonylimiduzole4a.--To a solution of 2-lithio-1 -methylimidazole generated from 1-methylimidazole (1.07 g, 13.05 mmol) in dry THF (20 cm3) at -78 "C under argon as above was added dimethyl disulfide (1.17 cm3, 13.02 mmol) dropwise. The cold-bath was removed and the mixture was allowed to come to room temperature. It was then stirred overnight. After this, water (20 cm3) was carefully added to the mixture which was then rotary evaporated. The aqueous residue was extracted with ether (3 x 8 cm3) and the com- bined extracts were washed with water (2 x 5 cm3) and dried (MgSO,).Removal of the drying agent and rotary evaporation of the solvent gave an oil which was chromatographed on silica gel using ether-petroleum (1 :2) as eluent. The product was eluted as an oil (1.02 g, 61) (Found: M', 128.0411. C5H8N2S requires M, 128.0408); v,,,/cm-' 974; 6, 1.55 (3 H, s, SMe), 3.6 (3 H, s, NMe), 6.85 l H, d, J2, 4(5)-H and 7.0 l H, d, J2, 4(5)-H; SC 16 (SMe), 33 (NMe), 122 C-4(5), 129 C-4(5) and 143 (C,); m/z 128 (M', loo), 113 (M' -Me, 20), 95 (77), 82 (M' + H -SMe, 32), 72 (76) and 42 (54). The above sulfide (205 mg, 1.6 mmol) was dissolved in dichloromethane (1 5 cm3) and treated dropwise at 0 "C under argon with a solution of m-chloroperoxybenzoic acid (50.40; 1.65 g, 4.8 mmol) in dichloromethane (10 cm3).After the addition, the solution was stirred at 0 "C for 1 h and then at room temperature for 10 h. After addition of water (25 cm3) to the mixture, the layers were separated and the aqueous layer was extracted with ether (5 x 10 cm3). The combined extracts were dried (MgSO,), filtered and rotary evaporated to give a solid which was chromatographed on silica gel using ether- petroleum (2 :1) as eluent. The product was obtained as a white crystalline solid (189 mg, 7473, m.p. 113-114deg;C (lit.," 117-118 "C); v,,,/cm-' 1322 and 1125 (SO,); dH 3.4 (3 H, s, SO,Me), 4.0 (3 H, s, NMe), 7.0 l H, br s, 4(5)-H and 7.1 l H, br s, 4(5)-H; dC 35 (NMe), 43 (SO,Me), 125 C-4(5), 129 C-4(5) and 143 (C-2); m/z 160 (M', loo), 97 (M+ + H -SO,, 80), 81 (M' -SO,Me, 3973, 56 (41), 54 (49) and 42 (78).2-Methylsulfonyl- 1 -phenylimidazole 4b.-A solution of 1-phenylimidazolel2 (630 mg, 4.38 mmol) in dry THF (20 cm3) was treated dropwise under argon at -78 "C with butyllithium (1.6 mol dm-3; 2.74 cm3, 4.38 mmol) to give an orange coloured solution. This was allowed to warm to -50 "C over 40 min after which dimethyl disulfide (0.394 cm3, 4.38 mmol) was added dropwise to it. After the addition, the cold-bath was removed and the mixture was stirred for 4 h. Water (30 cm3) was then added to the mixture and the THF was removed by rotary evaporation. The residue was extracted with dichloromethane (4 x 10 cm3) and the combined extracts were dried (Na,SO,), filtered and rotary evaporated.The residue was chromato- graphed on silica gel using dichloromethane as eluent to give the product as a solid (591 mg, 71), m.p. 105-106 "C (Found: C, 63.2; H, 5.35; N, 14.75. C,,H,,N,S requires C, 63.13; H, 5.30; N, 14.72); v,,,(CHC13)/cm-' 967; dH2.6 (3 H, S, NMe), 7.1 2 H, ABq, J 2,4(5)-H, 7.4 (5 H, m, Ph); dc 16 (SMe), 122, 125, 128 and 129 (2 peaks); m/z 190 (M', 20), 175 (M' -Me, 3),91 (2073, 81 (18), 77 (Ph', 1273, 32 (35) and 28 (100). The above sulfide (336 mg, 1.77 mmol) was oxidized to the sulfone using rn-chloroperoxybenzoic acid (50-60, 1.83 g, 5.32 mmol) as described for the 1-methyl analogue.The crude material was chromatographed on silica gel using ether- chloroform (1 : 1) as eluent to give the product as a white solid (247 mg, 63), m.p. 96-97 "C (Found: C, 53.7; H, 4.05; N, 12.5. C,,H,,N,O,S requires C. 54.05; H, 4.50; N, 12.61); v,,,(CHC13)/cm-1 1322 and 1134 (SO,); dH3.3 (3 H, s, SO,Me), 7.2 2 H, ABq, J 1.5, 4(5)-H and 7.45 (5 H, s, Ph); Sc 43 (SO,Me), 125.6,126 and 129 (3 peaks); m/z 222 (M', 75), 159 (M+ + H -SO,, 23), 157 (M' -H -SO,, 3573, 116 (63), 91 (39),77 (Ph', 54) and 28 (100). General Procedure for the N-Methylation of Imidazo1es.-To a solution (0.1 mol dm-3) of the imidazole in dichloromethane under argon at room temperature was added neat methyl triflate (1 equiv.) dropwise. The reaction was monitored by TLC and deemed finished with the disappearance of starting J.CHEM. SOC. PERKIN TRANS. 1 1994 material and the appearance of a base-line spot. The solvent was rotary evaporated and the product was recrystallised from the designated solvent. The following salts were prepared. 2-Methoxycarbonyl- 1,3-dimethylimidazolium triflate 5a as a white crystalline solid from acetone-ether (76), m.p. 96-97 "C (Found: C, 31.8; H, 3.6; N, 9.1. C8H,1F3N205S requires C, 31.58; H, 3.64; N, 9.21); V,,,/Cm-l 1744 (CO); d~(~H6- acetone) 4.0 (3 H, s, OMe), 4.15 (6 H, s, 2 x NMe) and 7.8 2 H, S, 4(5)-H. 2-Ethoxycarbonyl-1,3-dirnethylimidazoliumtriflate 5b as a white crystalline solid from acetone-ether (81), m.p. 115- 116deg;C (Found: C, 33.7; H, 4.0; N, 8.7.C9H13F3N205S re-quires C, 33.96; H, 4.12; N, 8.80); vrnax/cm-' 1745 (CO); dH(2H6acetone) 1.45 (3 H, t, J 8, MeCH,O), 4.2 (6 H, s, 2 x NMe), 4.6 (2 H, q, J 8, CH,O), 7.9 2 H, s, 4(5)-H. 2-Isopropoxycarbonyl- 1,3-dimethylimidazolium tripate 5c as a white crystalline solid from acetone-ether (89), m.p. 89- 90 "C (Found: C,36.1; H, 4.4; N, 8.4. C,oH,5F3N,05S requires C, 36.15; H, 4.55; N, 8.43); v,,Jcm-' 1742 (CO); dH(2H6acetone) 1.4 (6 H, d, J 9, CHMe,), 4.2 (6 H, s, 2 x NMe), 5.35(1 H,sep,J9, CHOC0)and 7.85 2 H,s,4(5)- HI; dc(2H6acetone) 21 (Me), 39 (NMe), 73 (CH) and 126 (C-4(5)).2-Isobutoxycarbonyl-1,3-dirnethylimidazoliumtripate 5d as a white crystalline solid from acetone-ether (85), m.p. 96-97 "C (Found: C, 36.2; H, 4.45; N, 8.4.C,,H17F3N,05S requires C, 38.15; H, 4.95; N, 8.09); V,,,/Cm-' 1742 (co); amp;('amp;- acetone) 1.0 (6 H, d, J 9, CHMe,), 2.15 (1 H, d, sep, J 3, J' 9, CHMe,), 4.2 (6 H, s, 2 x NMe), 4.6 (2 H, d, J 3, CH,OCO) and 7.75 2 H, s, 4(5)-H. 2-Benzyloxycarbonyl-1,3-dirnethylimidazoliumtriJlate 5e as a white crystalline solid from acetone-ether (7773, m.p. 85-86 "C(Found: C, 44.1: H, 3.9; N, 7.1. C14H~5F3N205Srequires c, 44.21; H, 3.98; N, 7.37); Vmax/Cm-' 1738 (co); d~('H6- acetone) 4.2 (6 H, s, 2 x NMe), 5.55 (2 H, s, CH,OCO), 7.3-7.6 (5 H, m, Ph) and 7.9 2 H, s, 4(5)-H. 1,3-Dimethyl-2-(2,2,2-trichloroethoxycarbonyl)imidazolium triJEate 5f as a white crystalline solid from acetone-ether (92), m.p.104-105 "C (Found: C, 25.9; H, 2.2; N, 6.3. C9H,,C13F3N,0,S requires C, 25.64; H, 2.39; N, 6.64); vmax/cm-l 1758 (CO); ~5,(~H~acetone) 4.35 (6 H, s, 2 x NMe), 5.3 (2 H, s, CH,) and 8.0 (2 H, s, 4(5)-H). 1,3-Dimethyl-2-phenoxycarbonylimidazoliurntriflate 5g as a white solid from acetone-ether (81), m.p. 102-103 "C (Found: C, 42.4; H, 3.35; N, 7.4. C,,H,,F,N,O,S requires C, 42.63; H, 3.58; N, 7.65); v,,,/cm-l 1744 (CO); dH(2H6acetone) 4.3 (6 H, s, 2 x NMe), 7.45 (5 H, br s, Ph) and 7.95 2 H, s, 4( 5)-H. 2-Cyuno-1,3-dirnethylimidazoliumtrijlate 6 as a crystalline solid from acetone-ether (8674, m.p. 158-159 "C (Found: C, 31.O; H, 2.7; N, 15.35. C,H8F3N303S requiresc, 31 .OO; H, 2.97; N, 15.49); ~3,(~H~acetone) 4.2 (6 H, s, 2 x NMe) and 8.05 2 H, S, 4(5)-HI.2-(N,N-Dimethy1uminocarbonyl)-1,3-dirnethylimidazolium trzflate 7 as a crystalline solid from acetone-ether (91), m.p. 126-127 "C (Found: C, 33.8; H, 4.35; N, 12.95. C9H,,F3N30,S requires C, 34.07; H, 4.45; N, 13.24); v,,,/cm-' 1681 (CO); ~5,(~H~acetone)3.1 (3 H, s, CONMe), 3.2 (3 H, s, CONMe), 4.0 (6 H, s, 2 x NMe), 7.8 2H, 4(5)-H; dc(2H6acetone) 35 (NMe), 36 (NMe), 37 (NMe) and 124 (C-4(5)). 1,3-Dimethyl-2-rnethylsulfonylimidazoliumtrijlate 8a as a crystalline solid from acetone-ether (92), m.p. 140-141 "C (Found: C, 25.9; H, 3.25; N, 8.3. C7HllF3N205S2 requires C, 25.93; H, 3.42; N, 8.64); v,,,(CH2C1,)/cm-' 1378 and 11 57 (SO,); dH(2H6acetone) 3.75 (3 H, s, SO,Me), 4.25 (6 H, s, 2 x NMe) and 8.0 2 H, s, 4(5)-H; 6c(2H6acetone) 38 (NMe), 44(SO,Me) and 127 (C4(5)).J. CHEM. SOC. PERKIN TRANS. 1 1994 1-Methyl-2-methylsu~onyl-3-phenylimidazoliumtrijlate 8b as a crystalline solid from acetone-ether (76), m.p. 133-1 34 "C (Found: C, 36.8; H, 3.3; N, 7.1. C12Hl,F3N20,S2 requires C, 37.30; H, 3.39; N, 7.25); vmax/cm-' 1358 and 1153 (SO,); ~5,(~H,acetone) 3.6 (3 H, s, SO,Me), 4.4 (3 H, s, NMe), 7.6-7.9 (5 H, m, Ph) and 8.1 (2 H, ABq, J 2,4(5)-H). Dealkoxycarbonylation: General Procedure.-To a solution of the 2-alkoxycarbonylimidazoliumsalt (1.5 mmol) in acetone (10 cm3) under argon was added the substrate (benzyl alcohol (1 equiv.) and DABCO (1 equiv.) or pyrrolidin-l-ylcyclohex-1-ene (1.1 equiv.) or diethylamine (1.1 equiv.) The mixture was stirred for the requisite time at room temperature after which the solvent was removed by rotary evaporation. The products were extracted from the residue by trituration with ether (3 x 5 cm3).The ether extracts were combined and rotary evaporated to give the crude products which were purified by chomatography on silica gel using ether-petroleum (2 :3) as eluent. The following were obtained. Benzyl methyl ~arbonate'~as a colourless oil (74); v,,,/cm-' 1752 (CO); 6, 3.8 (3 H, S, OMe), 5.2 (2 H, S, CH,) and 7.4 (5 H, s, Ph); 55 (OMe), 70 (CH,), 128 (2 peaks), 135 and 156 (CO); m/z 166 (M+, 38), 107 (M+ -MeOCO, 45) and 91 (PhCH,', 100). Benzyl ethyl carbonate as a colourless oil (7 1) (Found: M ,+ 180.0791.C1,H1203 requires M, 180.0786); v,,,/cm-' 1745 (CO); 6, 1.25 (3 H, t, OCH,Me), 4.15 (2 H, q, OCH,Me), 5.1 (2 H, s, OCH,Ph) and 7.35 (5 H, s, Ph); dC 14 (Me), 64 (CH,O), 69 (CH,O), 128 (3 peaks, Ph), 135 (Ph) and 155 (CO); m/z 180 (M', 4373, 108 (PhCH20H', 48), 107 (PhCH20+,51), 91 (PhCH,', 100) and 79 (65). Benzyl isopropyl carbonate as a colourless oil (76) (Found: M', 194.0939. C1 1H1403 requires M, 194.0943); vmax/cm-' 1738 (CO); 6, 1.3 (6 H, d, J7.2, Me,CHO), 4.9 (1 H, sep, J7.2, OCHMe,), 5.15 (2 H, s, OCH,) and 7.4 (5 H, s, Ph); 22 (Me), 69 (OCH), 72 (OCH,), 128 (2 peaks, Ph), 137 (Ph) and 156 (CO); m/z 194 (M', 18), 152 (M' + H -Pr', 24), 108 (PhCH,OH+, 4579, 107 (PhCH,O+, 50), 91 (PhCH,', loo), 79 (45) and 43 (Pr'+, 23).Benzyl isobutyl carbonate as a colourless oil (68) (Found: M', 208.1096. C12H1,0, requires M, 208.1099); v,,,/cm-' 1743 (CO); 6, 0.95 (6 H, d, J 7.2, Me,CH), 2.0 (1 H, apparent sep, J 7.2, Me,CH), 3.95 (2 H, d, J7.2,0CH2CH),5.2 (2 H, s, OCH,Ph) and 7.4 (5 H, s, Ph); dC 19 (Me), 28 (CH), 69 (OCH,), 74 (OCH,), 128 (2 peaks, Ph), 136 (Ph) and 155 (CO); m/z 208 (M', 16), 152 (M+ + H -Bu', 16), 108 (PhCH,OH+, 36), 107 (PhCH20+, 47), 91 (PhCH,', loo), 57 (Bu'+,72). Dibenzyl carbonateI4 as a colourless oil (73); v,aamp;m-l 1748 (CO); 6, 5.2 (4 H, s, OCH,) and 7.4 (1 0 H, s, Ph); m/z 242 (M', 0.1), 107 (PhCH,O+, 100) and 92 (51). Benzyl 2,2,2-trichloroethyl carbonate as a colourless oil (68) (Found: C, 42.2; H, 3.0.C1,H,Cl3O3 requires C, 42.36; H, 3.20); v,a,/cm-l 1765 (CO); 6, 4.75 (2 H, S, OCH,CCl3), 5.2 (2 H, s, OCH,Ph) and 7.4 (5 H, s, Ph); dC 71 (CH,), 77 (CH,), 129 (3 peaks, Ph), 135 (Ph) and 154 (CO); m/z 286 + 284 + 282 (M', 3, 9, 9), 107 (PhCH20+, 41), 91 (PhCH,', 100) and 79 (85). Ethyl 2-pyrrolidinylcyclohex- 1-enecarboxylate' as a yellow oil (68); v,,,/c~-' 1704 (CO); 6H 1.25 (3 H, t, MeCH,O), 1 .amp; 2.1 (8 H, m, CH,), 2.2-2.6 (4 H, m, allylic CH,), 3.35 (4 H, m, CH2N) and 4.1 (2 H, q, CH,O); m/z 223 (M', 279,210 (9), 194 (M' -Et, 779,113 (25) and 98 (100). N,N-Diethyl 0-methyl carbamate16 as a pale yellow oil; v,,/cm-' 171 1 (CO); dH1.O (6 H, t, 2 x MeCH,N), 3.2 (4 H, 9, 2 x CH,N) and 3.6 (3 H, s, OMe); dC 14 (Me), 41 (CH,), 52 (OMe) and 156 (CO).References 1 S. Ohta, S. Hayakawa and M. Okamoto, Tetrahedron Lett., 1984, 25, 5681; S. Ohta, S. Hayakawa, H. Moriwaki, S. Tsuboi and M. 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