1976 1257Generation and Reactions of a 3-Furylcarbene and of an Isoxazol-4-yl-carbeneBy Thomas L. Gilchrist and David P. J. Pearson, The Robert Robinson Laboratories, University of Liverpool,P.O. Box 147, Liverpool L69 3BX2,5-Diphenylfuran-3-carbaldehyde reacted with 1 -amino-trans-2,3-diphenylaziridine to give the hydrazone (6),which on photolysis or pyrolysis gave products derived from the 3-furylcarbene (5). In contrast to reactionsreported for 2-furylcarbenes there is no evidence of ring-opening in the 3-furylcarbene, and the reactions observed(insertion and cycloaddition) are those of a normal arylcarbene. A similar investigation of the correspondinghydrazone (8) from an isoxazole-4-carbaldehyde shows that ring cleavage does occur on pyrolysis. The relevanceof these observations to a possible mechanism for the thermal decomposition of 3-phenylprop-2-ynal is discussed.THE thermal tetramerisation of dimethyl acetylenedi-carboxylate is thought to involve a step in which onemolecule of the ester adds to another in 1,3-dipolarfashion, giving a 3-furylcarbene (Scheme 1).1 In view ofM e 0,C-/ bsol; c' ' C02MeSCHEME 1the unusually mild conditions under which some five-membered heteroaromatic azides and diazoalkanes areit seemed possible that a more general inter-conversion of acetylenes and 3-furylcarbenes might occur.Hoffman and Shechter have shown that certain 2-furylcarbenes are readily cleaved to give open-chainisomers (Scheme 2), this reaction occurring in preferenceto the more usual carbene reactions such as insertion andcy~loaddition.~ However, reactions of 3-furylcarbeneshave not been reported hitherto.E.LeGoff and R. B. LaCount, Tetrahedron Letters, 1967,2333; E. Winterfeldt and G. Giesler, Chem. Ber., 1968,101, 4022;R. Gericke and E. Winterfeldt, Tetrahedron, 1971, 27, 4109.2. P. A. S. Smith, L. 0. Krbechek, and W. Resemann, J . Amer.Chem. Soc., 1964, 86, 2026; P. A. S. Smith and J. G. Wirth, J.Org. Chern., 1968,33, 1146; P. A. S. Smith, G. J. W. Breen, M. K.Hajek, and D. V. C. Awang, ibid., 1970,35, 2216; P. A. S. Smithand H. Dounchis, ibid., 1973, 38, 2968; P. A. S. Smith and E. M.Bruckmann, ibid., 1974, 39, 1047.One reaction for which an extension of the mechanismshown in Scheme 1 could provide an explanation is thethermal decarbonylation of 3-phenylprop-2-pal.Whenthe aldehyde is heated above ca. 100 "C, carbonmonoxide is evolved and the dimeric aldehydes (1) and(2) are produced.4 By invoking an initial cycloadditionstep analogous to that of Scheme 1, a sequence can beconsidered (Scheme 3) which leads to the observedproducts.In order to test the feasibility of such a process we haveexplored the chemistry of a simple 3-furylcarbene andH H/C=yQER * H bsol; O c cCRSCHEME 2have also attempted to generate a 3-furylcarbene with aformyl group at the 4-position, as a model for the carbene(3). We have also explored a route to diradicalsanalogous to (4) to determine whether they can undergothe proposed ring opening.2,5-Diphenylfuran-3-carbaldehyde was used as start-ing material for generation of the 3-furylcarbene (6).R.V. Hoffmanand H. Shechter, J . Amer. Chew SOC., 1971,93, 6940; J . Org. Chem., 1974, 39, 2939.L. Claisen, Ber., 1898, 81. 1021; R. K. Bentley, U. Graf,E. R. H. Jones, R. A. M. Moss, V. Thaller, and R. A. Vere Hodge,J . Chem. SOC. ( C ) , 1969, 6831258 J.C.S. Perkin IEschenmoser and his co-workers have used aziridinyl-imines as masked diazo-compounds; these have theadvantage over other diazoalkane precursors, such astosylhydrazones, that they are cleaved thermally withoutthe introduction of an external base, and, being soluble inorganic solvents, they allow homogeneous reactions to2PhCEC-CHO + Ph? CHO -+ PhiH k0H o P h H o P hPhv U(31H(1 1SCHEME 3occur.The disadvantage of the reagents is that mildconditions are required to form the hydrazones from thecorresponding amino-aziridines, and less reactive carb-onyl compounds (such as diary1 ketones) do not react inthese conditions.Photolysis of the aziridinylimine (6) provided a goodroute to the carbene (5). In a variety of solvents,products were obtained which were consistent with theintermediacy of this carbene, as shown in Scheme 4. InPh(; Ph0 IEt2' 4 ( 5 8 '/o)P h d P h0 .IPh''0' .PhSCHENE 45yn + a n t i( 6 6)benzoylacetylene; the reactions are similar to those ofphen ylcarbene...* P h .- PhThe aziridinylimine (7) , prepared from 2,5-diphenyl-furan-3,4-dicarbaldehyde and l-amino-2,3-diphenylazir-idine, was readily decomposed thermally and photo-chemically, but gave a mixture of products from whichonly 2,5-diphenylfuran-3,4-dicarbaldehyde was isolated.The carbene was not intercepted by sulphur, by alkanes,or by olefins, and appears to be an efficient oxygenscavenger.Again, no evidence for ring cleavage wasobtained. In contrast, melt pyrolysis of the isoxazol-4-yl aziridinylimine (8) gave a mixture of the expectedring-cleavage products, benzoylacetylene and 4-toluo-nitrile.Pyrolysis of the lactone (9) in the vapour phase at675 "C and 0.02 mmHg gave the acetylene (10) in goodyield: a mechanism (Scheme 5) involving a diradicalPhSCHEME 5similar to the diradical (4) in Scheme 3 can account forthe formation of this product, and provides a possiblemodel for the fragmentation of Scheme 3.However, weconclude that the energy barrier to ring cleavage of3-furylcarbenes is higher than for 2-furylcarbenes, andthat there is no evidence for a general equilibration ofconjugated acetylenic carbonyl compounds and 3-furyl-carbenes. The known examples of cleavage of carbenesof the general formula ( l l ) , and that of the isoxazol-4-ylimine (euro;9, all involve the production of the fragmentW-X, either as molecular nitrogen or as a nitrile; frag-mentation to give an acetylene WZX has not been(111A similar difference in the stabilities of 2- andnone of these reactions was there any evidence for frag-mentation of the carbene into phenylacetylene and5 D. Felix, R. K. Miiller, U.Horn, R. Joos, J. Schreiber, andA. Eschenmoser, Helv. Chim. Acta, 1972, 55, 1276.C. D. Gutsche, G. L. Bachman, and R. S. Coffey, Tetra-hedron, 1962,18, 617; G. L. Closs and R. A. MOSS, J . Amey. Chern.SOG., 1964, 86, 4042.observed.3-azidofurans has been noted re~ently.~7 s. Gronowitz, C . Westerlund, and A.-B. Hornfeldt, A d aChem. Scand., 1975, B29, 2241976 1259EXPERIMENTAL$henylfuran (6) .-To a solution of 2,5-diphenylfuran-3-carbaldehyde (0.400 g, 1.61 mmol) in ether (5 ml) wasadded l-amino-trans-2,3-diphenylaziridine (0.400 g, 1.90mmol) in ether (5 ml). After the mixture had been kept a t5 "C for 16 h the crystalline precipitate was filtered off.Recrystallisation gave the aziridinylimine (6) (0.460 g,65), m.p. 115.5-117.5" (decomp.) (from ether-hexane)(Found: C, 84.5; H, 5.4; N, 6.4.C3,H2,N,0 requires C,84.5; H, 5.45; N, 6.4); lux. (cyclohexane) 314 nm(E 33 400); G(CDC1,) 3.70-3.85 (2 H, m), 7.00 (1 H),7.1-7.6 (18 H, m), 7.64-7.75 ( 2 H, m), and 8.07 (1 H);m/e 464 (C~,HZ~O,+), 412 ( M f -28), and 180 (C14H1zf3 base).Theimine (0.20 g, 0.45 mmol) dissolved in dry degassed ether(110 ml) was irradiated (500 W medium-pressure lamp,Pyrex filter) for 1 h. Layer chromatography (ether-pentane, 2 : 3) and distillation gave 3-(2-ethoxypropyl)-2,5-diphenylfuran (0.086 g, 62), b.p. 140-150" a t 3 xmmHg (Found: C, 82.2; H, 7.0. C2,HZ2O2 requires C, 82.3;H, 7.2); 6(CC1,) 1.19 (3 H, t, J 7 Hz), 1.23 (3 H, d, J 6.5Hz), 2.85 ( 2 H, dq, AB of ABX, JAB 15, Jax 6.5 Hz), 3.58(3 H, m, OCH and OCH,), 6.68 (1 H), 7 16-7 50 (6 H, m),and 7.63-7.80 (4 H, m) ; m/e 306 (M+), 262, and 233 (base).The imine (0.20 g, 0.45 mmol) wasirradiated in cyclohexane (110 nil) as described in the pre-ceding experiment, and gave 3-cyclohexylmethyl-2,5-diphen-ylfuran (0.082 g, 58), m.p.64-67" (Found: C, 87.0; H,7.4. CZ3H,,0 requires C, 87.3; H, 7.6); 6(CC14) 1.0-2.0(11 H, m), 2.53 (2 H, d, J 6.5 Hz), 6.47 (1 H), 7.05-7.45(6 H, rn), and 7.52-7.70 (4 H, m); m/e 316 (Mf), 233, and57 (basc).Irradiation of the imine (0.20 g, 0.45mniol) in cyclohexene gave a mixture of exo- and endo-3-bicyclo4.1.0heptan-7-yZ-2,5-diphenylfuran (0.094 g, 66),b.p. 210" a t 4 x mmHg (Found: C, 87.7; H, 7.0.C,,H,,O requires C, 87.9, H, 7.0); 6(CC14) 1.0-2.2 (11 H,m), 6.32 and 6.77 (together 1 H), and 7.0-8.0 (10 H, m);m/e 314 (M+), 233, and 105 (base).G.1.c. (5 OV1 onCelite; 250 "C) showed two isomers in the ratio 1 : 1.8, butthe isomers were not separated by preparative chromato-graphy.Irradiation of the imine (0.20 g, 0.45mmol) in benzene gave a mixture of isomers of 3-cyclohepta-trienyl-2,5-diphenylfuran (0.084 g, 60y0), which by catalyticreduction gave 3-cycloheptyl-2,5-diphenylfuran (0.070 g,490/,), b.p. 180-190" a t 10-1 mmHg (Found: C, 87.2; H,7.6. C2,H3,0 requires C, 87.3; H, 7.6); 6(CC14) 1.5-2.2(12H, ni), 2.8-3.1 (1 H, m), 6.58 (1 H), and 7.0-7.7 (lOH,m); wz/e 316 (M+) and 105 (base).Thevmolysis of the Irnine (6).-The imine (0.15 g, 0.34mniol) i n cyclohexane (30 ml) was heated under nitrogen at80 "C for 8 h ; no starting material then remained.Layerchromatography gave 3-(cyclohexylmethyl)-2,5-diphenyl-furan (0.055 g, 51), m.p. 64-67".2,5-Diphenylfuran-3,4-dicarbaldehyde.-A suspension of3,4-bishydroxymethyl-2,5-diphenylfuran 9 (1.28 g, 0.46mmol) in dry dichloromethane (50 ml) was added to asolution of chromium trioxide (6 g) in pyridine (9.5 g) anddichloromethane (150 ml) a t 20 "C. After 0.5 h the super-V. J. Traynelis, J. J. Miskel, and J. R. Sowa, J. Org. Chem.,1957, 22, 1269.S. Adjangba, D. Billet, and C. Mentzer, Bull. SOC. chim.Fmitce, 1962, 132.3-(trans-2, 3-Diphenylaziridin-l-yliminomethyZ)-2,5-di-Photolysis of the Aziridinylimine (6).-(a) I n ether.(b) I n cyclohexane.(c) I n cyclolzexene.(d) 172 benzene.natant liquid was decanted and the dark residue was dis-solved by shaking with dichloromethane (150 ml) andaqueous sodium hydroxide (5; 150 ml).The organicsolutions were combined and washed with aqueous sodiumhydroxide (5; 3 x 200 ml), hydrochloric acid ( 5 ; 150ml), aqueous sodium hydrogen carbonate (150 ml), andsaturated aqueous sodium chloride (100 ml) . Columnchromatography (silica ; chloroform) of the residue gave thedialdehyde (0.73 g, 57), m.p. 134-136" (from dichloro-methane-hexane) (Iit.,lo 140deg;), vmx. (KBr) 1682 and 1668cm-l; G(CDC1,) 7.40-7.60 (6 H, m), 7.85-8.02 (4 H, m),and 10.46 (2 H); m/e 276 (M+), 248, and 220.Reaction of 2,5-Diphenylfuran-3,4-dicarbaldehyde with 1-Amino-trans-2, 3-diphenyZaziridine.-The aldehyde (50 mg,0.18 mmol) was suspended in ether (5 ml) and the aziridine(38 mg, 0.18 mmol) was added.After 1 h a t room tempera-ture the mixture was subjected to column chromatographygiving the crude imine (7) as an oil; vmaX. 1 675 (GO) and1600 cm-l (C=N); 6(60 MHz; CCI,) 3.60-3.80 ( 2 H, m),7.0-7.9 (20 H, m), 8.50 (1 H, CH=N), and 10.35 (1 H,CH=O) .The imine(78 mg, 0.17 mmol) in benzene (110 ml) was irradiated underN, for 2 h (100 W rnedium-pressure immersion lamp, Pyrexfilter). Layer chromatography gave a mixture of cis- andtrans-stilbene (16 mg, 53) and 2,5-diphenylfuran-3,4-dicarbaldehyde (13 mg, 28).The imine (75 mg, 0.16 mniol) was heatedunder reflux in toluene (25 ml) under N, for 0.5 h. Layerchromatography gave trans-stilbene (18 mg, 63) and 2,5-diphenylfuran-3,4-dicarbaldehyde (10 mg, 23 yo).Pyrolysis of 3,3,4,6-Tetraphenyl-3H-furo3,4-cfuran- 1 -one(9).-Pyrolysis of the lactone (9) l1 (150 mg, 0.35 mmol) a t675 "C and 0.02 mmHg gave 2-diphenyZmethyZene-1,4-di-PhenyEbut-3-yn-l-one (10) (107 mg, 80), m.p. 142.5-143.5" (from dichloromethane-hexane) (Found : C, 90.3 ;H, 5.4.C2,H2,0 requires C, 90.6; H, 5.2); v,, (KBr)2 180w (EC), 1660s (GO), 1230, and 970 cm-l; A,,,.(cyclohexane) 251 (E 31 000) and 318 nm (19 000); G(CC1,)m), and 7.90-8.02 ( 2 H, m); +n/e 384 (iWf), 307, 279(M+ - PhCO), and 105 (base).5-Phenyl-3- (4-toly l) isoxazole-4-carbaldeJ~y de . 3-Phenyl-prop-2-ynal (1.3 g, 10 mmol) and 4-methylbenzohydrox-imidoyl chloride l2 (1.69 g, 10 mmol) were dissolved in di-chloromethane (20 ml), and triethylamine (1.1 g, l l mmol)in dichloromethane (10 ml) was added dropwise.After 1 11the mixture was evaporated to dryness and the residue waspartitioned between water and ether. The organic phasewas dried and evaporated ; crystallisation of the residuegave 5-~heny2-3-(4-toly2)isoxazoZe-4-carbaldehyde (0.80 g,30), m.p. 115-116" (from ether-hexane) (Found: C, 77.7;H, 5.3; N, 5.35. C1,H13N0, requires C, 77.5; H, 5.0; N,5.3); v, (KBr) 1 688s cm-l ( G O ) ; G(CDC1,) 2.43 (3 H),7.30 (2 H, J 8 Hz), 8.40-8.60 (3 H, m), 8.65 ( 2 H, d, J 8Hz), 7.98-8.12 (2 H, m), and 10.00 (1 H); wz/e 263 (iWf)and 234 ( M f - CHO).4-(trans-2,3-Diphenylaziridin- l-yliminonzetl~j~l) -5-phenyZ-3-(4-tolyZ)isoxazole (8) .-A solution of the foregoing aldehyde(0.30 g, 1.14 mmol) and l-amino-trans-2,3-diphenylaziridineReactions of the Imine (7).-(a) PhotoZjfsis.(b) Pyrolysis.7.05 (5 H), 7.14 (5 H), 7.26-7.43 (6 H, m), 7.55-7.70 (2 H,lo Y .Lepage and A. Verine, Com9.f. rend., 1973, 27'40, 1534.l1 D. V. Nightingale and B. Sukornick, J. 0r.g. Chem., 1959, 24.12 H. Rheinboldt, M. Dewald, F. Jansen, and 0. Schmitz-497.Dumont, Annalen, 1926, 451, 1611260 J.C.S. Perkin I(0.30 g, 1.4 mmol) in ether (10 ml) was kept a t 5 "C for 16 h.A crystalline solid was formed ; this was filtered off and re-crystallised to give the aziridinylimine (8) (0.35 g, 67y0),m.p. 138.5-140" (from ether-hexane) (Found: C, 81.5; H,5.6; N, 9.1. C31H2,N,0 requires C, 81.7; H, 5.5; N,9.2); vmX. (KBr) 1 620 and 1 598 cm-l; S(CDC1,) 2.40(3 H), 3.2-3.4 (2 H, m), 6.7-7.6 (19 H, m), and 7.95 (1 H).Photolysis of the Imine (8) in Ether.-A solution of theimine (205 mg, 0.45 mmol) in ether (125 ml) was irradiated(500 W medium-pressure lamp, Pyrex filter) for 1 h.Chromatography of the products gave cis- and trans-stilbene(67 mg, 94) and an oil, identified as 4-(2-ethoxypro~y2)-5-~henyZ-3-(4-tolyZ)isoxazoZe (70 mg, as), b.p. 160-170" a t0.06 mmHg (Found: C, 78.2; H, 7.0; N, 4.5. C21H,3N02requires C, 78.5; H, 7.2; N, 4.4); G(CDC1,) 0.98 (3 H. t,J 7 Hz), 1.00 (3 H, d, J 7 Hz), 2.41 (3 H), 2.88 (2 H, dq, J 10and 7 Hz), 3.25 (1 H, m), 3.40 (2 H, q, J 7 Hz), 7.26 (2 H, d,J 8 Hz), 7.35-7.48 (3 H, m), 7.56 (2 H, d, J 8 Hz), and7.77-7.92 (2 H, m); mje 321 (M+).Pyrolysis ofthe Imine (8).-The imine (280 mg, 0.61 mmol)was heated at 200 "C for 5 min and the residue was subjectedto layer chromatography. 4-Toluonitrile and benzoyl-acetylene were obtained as a 1 : 1 mixture, the i.r. spectrumof which was identical with that of a mixture of authenticspecimens: vmx. 3 290 (CZC-H), 2 220 (CN), 2 090 (ECH),and 1 650 cm-l ( G O ) .We thank the S.R.C. for a Research Studentship (to6/2397 Received, 9th December, 19751D, P. J. P.)
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