Pyrolyses of pentafluorophenyl prop-2-enyl and 2,3,3-2H3prop-2-enyl ethers. Formation of 1-fluorovinyl 2,3,4-trifluorophenyl ketonesviainternal Dielsndash;Alder reactions

摘要

1976 1463Pyrolyses of Pentaf luorophenyl Prop-2-enyl and [2,3,3-2H,] Prop-2-enylEthers. Formation of 1 -Fluorovinyl 2,3,4-Trif luorophenyl Ketones viaInternal Diels-Alder ReactionsBy Gerald M. Brooke" and David H. Hall, Chemistry Department, Science Laboratories, South Road, DurhamDH1 3LEThe formation of 1 -fluorovinyl 2,3,4-trifluorophenyI ketone (XIV) by pyrolysis of pentafluorophenyl prop-2-enylether (I) a t 440 "C in the vapour phase i s rationalised in terms of a reaction involving one of two possible internalD iel s-Al d er add u cts (V) of the i n ter med iat e 2,3,4,5,6 - pent af I uoro - 2 - ( prop - 2 -en y I ) cycl o h exa - 3,5 - d ien o n e ( 11).The same adduct is an intermediate in a reaction which enables an ortho-ortho rearrangement of the ally1 groupin (11) totake placebefore conversion of (11) into (XIV), a reaction which has been observed by using pentafluoro-phenyl [2,3,3-2H3]prop-2-enyl ether (VI).This gives not only 1 -fluor0[2,2-~H,]vinyl 2,3,4-trifluor0[5-~H~]-phenyl ketone (XVIII) (1 part), the product expected from 2- ([I ,I ,2-2H3] prop-2-enyl)cyclohexa-3,5-dienone(IX), but also 1 -fluorovinyl2,3.4-trifluor0[5,6-~H~]phenyl ketone (XIX) (1 part), from 2-( [2,3,3-2H,]p~op-2-enyl)cyclohexa-3.5-dienone (X), the relative proportions of the products indicating complete equilibration of(IX) and (X) before further reaction.IN a previous paper, the pyrolysis of pentafluorophenylprop-2-enyl ether (I) at 480 "C was described.l Onecomponent of the complex mixture of products was2,5(3,6,7,7ap-pentafluoro-3ap,4,5,7a-tetrahydroinden-l-one (IV), the formation of which was rationalised on thebasis of the isomerisation of one of two possible internalDiels-Alder adducts (111) from the initially formedClaisen rearrangement product (11) (Scheme 1).The implication of the alternative internal Diels-Alderadduct (V) was only revealed by using the deuterium-labelled material pentafluorophenyl [2,3,3-2HJprop-2-enyl ether (VI).Not only was the [3,3ap,5a-2H,]ketone(VII) formed, but also the [3ap,4,4-2HJketone (VIII), inthe ratio 9 : 10, respectively, and it was argued that while(VII) arose from the expected Claisen rearrangementmaterial (IX), the formation of (VIII) required the 2,4-dienone (X) (Scheme 2). It was proposed that com-pound (X) arose from (XI), the second possible internalDiels-Alder adduct from (IX), by a stepwise rearrange-G.M. Brooke, J.C.S. Perkin I, 1974, 233.ment, and moreover that the overall isomerisationrepresented by (IX) @ (X) (the ortho-ortho rearrange-C,F;O.CH,. CH = CH, ---+F L I-F FI IV1SCHEME 1 i, Ring cleavage and hydrogen transferment) attained equilibrium very rapidly in comparisonwith the subsequent formation of products via the interna1464 J.C.S. Perkin Ithe internal Diels-Alder adduct (V) by the mode alterna-tive to the one involved in the degenerate ortho-orthorearrangement, followed by loss of HF (Scheme 3). Inorder to determine the relative ease of cleavage of bondsDiels-Alder adducts (XII) and (XIII). The presence ofthe unlabelled bicyclic ketone (IV) obviously fails toreveal the degenerate or,tho-ortho rearrangement whichmust be proceeding via the internal Diels-Alder adduct0C 6 g OCH,C@ CD, --j @ F + F@ FD H FD ',D H&l D DH HF(V), but we have now isolated another material, from thepyrolysis of (I) at 440 OC, which can be related directlyPrior removal of the ketone (IV) from the pyrolysisproduct and analysis of the residue by coupled g.1.c.-mass spectrometry showed the presence of a materialwith M+ 204 (starting material -HF) present to theextent of ca.40%. This material (which readily poly-merised in air) was shown to be 1-fluorovinyl 2,3,4-tri-fluorophenyl ketone (XIV) on the basis of spectroscopicdata and further chemical reactions.The U.V. and i.r.spectra were consistent with the presence of a conjugatedketone; the lH n.m.r. spectrum showed two aromaticto (V).rSCHEME 2and two vinylic C-H absorptions; and the 19F n.m.r.spectrum showed one vinylic fluorine and three aromaticC-F absorptions. Oxidation of compound (XIV) withfuming nitric acid gave a carboxylic acid identified un-ambiguously as 2,3,4-trifluorobenzoic acid on the basisof elemental analysis and mass spectrum and the presenceof three fluorine absorptions in the 19F n.m.r. spectrum,one absorption having two coupling constants of 20.5 Hz,each characteristic of two adjacent ortho-fluorine atoms.Catalytic hydrogenation of the ketone (XIV) gave theexpected l-fluoroethyl2,3,4-trifluorophenyl ketone (XV).Both compounds (XIV) and (XV) showed large through-space F,F-coupling constants, J(FA,FB) 17.0 and 15.5Hz, respectively, which indicate rigid conformations inthese molecules.The formation of the ketone (XIV) can be explainedby invoking the opening of the four-membered ring ofa and b, we pyrolysed pentafluorophenyl [2,3,3-2HJprop-2-enyl ether (VI) under conditions identical with thosefor (I), isolated vinylic ketone material as before andimmediately reduced the olefinic bond with deuterium0.(Y)(XV) (XIWSCHEME 3in order to simplify the lH n.m.r.analysis of the product.Two compounds (XVI) and (XVII) were obtained in theratio 1 : 1.1, respectively (Scheme 4), which in turnshowed the pyrolysis products to be the unsaturatedketone (XVIII) [from (XI)] and the unsaturated ketone(XIX), which must have been produced from the internalDiels-Alder adduct (XX) (see Scheme 2).0 0D(XVIII 1D( XVI I0 0(XIXI (XVII 1SCHEME 4The isolation of the bicyclic ketones (VII) and (VIII)(1 : 1) in the same pyrolysis experiment, and the form1976 1465ation of essentially identical amounts of unsaturatedketones (XVIII) and (XIX) indicate common precursors,namely the 2,4-dienones (IX) and (X).Furthermore,the equilibrium between these compounds, (IX) and (X),must be established very rapidly in comparison withreactions arising from the two possible types of internalDiels-Alder adducts [(XI) and (XX) ; and (XII) and(XIII)]. We conclude therefore that cleavage of bondsa in (V) is much easier than cleavage of bonds b.EXPERIMENTALPyrolysis of PentaJuorophenyZ Prop-2-enyl Ether (I) .-The ether (I) (19.4 g) was distilled during 4 h from a vessela t 70 "C through a silica tube (20 cm x 1.5 cm diam.)packed with silica fibre and heated to 440 "C, into a trapcooled by liquid air connected to a high vacuum system(0.001 mmHg).The liquid product (18.8 g) was cooled to- 10 "C to precipitate 2,5~,6,7,7afbpentafluoro-3a~,4,5,7a-tetrahydroinden-l-one (IV), which was filtered off, and thefiltrate was washed with water to remove HF. Examin-ation of this product by coupled g.1.c.-mass spectrometry(V. G. Rlicromass 12B instrument) indicated a complexmixture with one component, M+ 204 (starting material-HF), present in significant amount (40%).Chromato-graphy of the mixture (6.0 g) on silica (70 cm x 3.5 cmdiam.) (carbon tetrachloride as eluant) gave an impuresaniple of this component, l-fluorovinyl 2,3,4-trifluorophen-yl ketone (XIV) (1.4 g), b.p. 42" a t 0.03 mmHg, as a slowmoving material which rapidly polymerised in the absenceof solvent and so was immediately hydrogenated or oxi-dised. The 19F n.m.r. spectrum [(CD,),COJ showed signalsa t 115.1 ( F A ) , 128.1 (FD), 134.2 (FB), and 160.3 (Fc) p.p.m.upfield from internal CFC1, [J(FA, anti-HA) 45.0, J(Fa, syn-J(FB,Hc) 6.0, J(FB,HD) 3.0, J(Fc,FD) 20.5, J(Fc,Ho) 6.0,J(Fc,HD) 3.0, J(F=,Hc) 9.0, J(FD,HD) 5.5 Hz]. The lHn.m.r. r(CD,),CO] showed T 2.3-3.3 (complex m, Hc andHD), 4.38 (anti-Ha), and 4.43 (syn-HB); vmx.1692 cm-l(GO) ; Lax* (cyclohexane) 252 nm (E 14, 700).Pyolyszs of Pentafluorophenyl [2,3, 3-2H,]P~op-2-enylEther (VI).-The ether (VI) (6.0 g) was pyrolysed as in theprevious experiment to give a product separated by chroma-tography on silica (CCl, as eluant) into l-fluor0[2,2-~H,]-vinyl 2, 3,4-trifluoro[5-2Hl]phenyl ketone (XVIII) and 1-fluorovinyl 2,3,4-trifluoro[5,6-2H2]phenyl ketone (XIX) (0.8g) ; this was not distilled, but was treated immediately withdeuterium. The slower moving components on the columnwere eluted with ether and finally separated by preparativet.1.c. on Kieselgel GF254 (chloroform as eluant) to give amixture of [3,3ap,5cc-2H,]-2,5,6,7,7a~-pentafluoro- (VII) and[3ap,4, 4-2H,]-2, 5p, 6,7, 7ap-pentafluoro-3aJ 4,5,7a-tetrahydro-inden-l-one (VIII) in the ratio 1 : 1 (from the intensities ofthe lH n.m.r.signals [(CD,),CO] at T 4.65 and 7.59 respec-tively 1.l-Fluoroethyl 2,3,4-Trifluoro$henyZ Ketone (XV) .-Thecrude vinyl ketone (XIV) (1.7 g) in ethyl acetate (30 ml) wasHB) 12.0, J(FA,FB) 17.0, J(FBFC) 20.8, J(FB,FD) 12.0,hydrogenated a t atmospheric pressure over palladium-charcoal (0.5 g; 10% w/w) at room temperature until ca.1 mol equiv. of gas had been absorbed. The catalyst wasfiltered off, the solution evaporated, and the residue purifiedby preparative g.1.c. (di-isodecyl phthalate, 178 "C) to give1-fluoroethyl 2,3,4-trifluorophenyZ ketone (XV), m.p. 46-47"C (from ethanol-water) (Found: C, 52.7; H, 2.6; F,36.1%; M+, 206; C,H,F40 requires C, 52.4; H, 2.6; F,36.8% ; M , 206).The leF n.m.r. spectrum (CFCI,) showedsignals at 128.7 (FD), 133.3 (FB), 162.3 (Fc), and 184.4 (FA)p.p.m. upfield from internal CFCl, CJ(FA,HE) 48.0, J(FA,v~c-J(Fc,FD) 19.0, J(Fc,Hc) 6.5, J(Fc,HD) 1.5, J(FD,Hc) 9.2,J(FD,HD) 6.2 Hz. The lH n.m.r. spectrum (CFC1,) showed72.52 (HJ, 2.90 (Hc), 4.54 (HE), and 8.56 (CH,) [J(CHE,CH,)7.0 Hz]; vma= 1 705 cm-l (GO).Reactions of 1-Fluorovinyl 2,3,4-TriJuoro[S, 6-zH,]phe~~yZKetoNe (XIX) and l-FZuor0[2,2-~H,]vinyZ 2,3,4-Trifluoro[5-2HJPhenyZ Ketone (XVIII) with Deuterium.-The crudemixture of ketones (XVIII) and (XIX) (see above), in ethylacetate, was treated a t atmospheric pressure with deuteriumover palladium-charcoal, and the product was isolated bypreparative t.1.c.as before. The lH n.m.r. spectrum[(CD,),CO] showed signals for ring protons "-6 and H-5 andside-chain protons H- 1 and H-2, with relative intensities1 : 0.23 : 1.71 : 2.19, which indicated the presence of both 1-fluoro[ 1,2,2, 2-2HJethyl 2,3,4-trifluor0[5-~H,]phenyl ketone(XVI) and l-fl~oro[l,2-~HJethyl 2,3,4-trifluoro[5,6-2H2]-phenyl ketone (XVII) in the ratio 1 : 1, respectively) inwhich 11% overall exchange of H for D at ring position 5and 82% overall exchange of H for D a t side-chain position1 had occurred. A separate experiment showed that theseexchange reactions could be induced on the silica bymoisture in the environment (the silica, the solvent, or theatmosphere).2,3,4-TriJuorobenzoic A cid.-The crude vinyl ketone(XIV) (0.14 g) was heated under reflux with nitric acid (10cms; 70% w/w) for 2 h. The mixture was diluted withwater and extracted with ether, and the dried (MgSO,)extracts were evaporated. Sublimation of the residue a t120 "C and 0.06 mmHg and recrystallisation from toluene-petroleum (b.p. 60-80 "C) gave 2,3,4-tri$uorobenzoic acid(0.043 g), m.p. 136.5-137 "C (Found: C, 48.0; H, 2.0; F,32.7%; M+, 176. C,H,F,O, requires C, 47.7; H, 1.7; F,32.3%; M , 176). The 19F n.m.r. spectrum (Et,O) showedsignals at 130.4 (F-4), 132.1 (F-2), and 163.0 (F-3) upfieldfrom external CFC1, [J(F-2,F-3) 19.0, J(F-Z,F-4) 12.5,H) 23-01 J(FA,FB) 15.5, J(FB,FC) 20.0, J(FB,FD) 12.0,J(F-3,F-4) 19.0, J(F-2,H-5) 2.5, J(F-2,H-6) 7.0, J(F-3,H-5)6.5, J(F-3,H-6) 2.5, J(F-4,H-5) 8.5, J(F-4,H-6) 6.0 Hz].The 1H n.m.r. spectrum (Et,O) showed 7 2.52 (H-6) and3.21 (H-5) (external Me,Si standard) (J5,6 9.0 Hz); vmx.1 690 cm-l (GO).We thank Professor W. I. R. Musgrave and Dr. R. R.Hughes for their interest in this work, Dr. R. S. Matthewsfor some n.m.r. measurements, and the S.R.C. for an award(to D. H. H.).[5/2491 Received, 22nd December, 1976