Overcrowded molecules. Part XI. A doubly lsquo;forbiddenrsquo; symmetry-allowed pericyclic reaction: the thermal rearrangement of (E)-2-benzylidene-(Z)-1-mesityl(phenyl)methyleneindane into (Z)-2-benzyl-1-mesityl(phenyl)methyleneindene

摘要

1976 1545Overcrowded Molecules. Part XIm1s2 A Doubly ' Forbidden ' Symmetry-allowed Pericyclic Reaction : the Thermal Rearrangement of (€)-2-Benrylidene-(2)-1 -mesityl( pheny1)methyleneindane into (Z)-E-Benzyl-I -mesityl( pheny1)methyleneindeneBy John S. Hastings, Harry G. Heller,' and Howard Tucker, Edward Davies Chemical Laboratories,University College of Wales, Aberystwyth SY23 1 NE(in part) Keith Smith, Chemistry Department, University College of Swansea, Swansea SA2 8PP(€)-2-Benzylidene-(Z)-l -mesityl(phenyl)methyleneindane cyclises by a disrotatory mode at 180 "C to givecis- 9 a, 1 0- d i hydro - 5,lO - d i p hen y I - 6,8,9 a - t r i methyl - 1 1 H- benzo [ 61 f I u oren e, which , at this tempera t u re, u n d erg oestwo ' forbidden ' processes in concert, namely conrotatory ring opening and a suprafacial E1.71 hydrogen shift, toyield (Z) -2-benzyl-I -mesityl(phenyl)methyleneindene.RING opening reactions of cyclohexadiene systemsproceeding by a conrotatory mode and suprafacial f1,3]not known.When it is possible for two 'forbidden'processes to proceed together, the pericyclic reactioncan occur with conservation of orbital symmetry,through an aromatic transition state, in accord with theselection rules. We report here the first examples ofthis type of reaction.Ph MesH/'ArCPh Mes'C/Mes Ph'C'CH R'Ar\R* MeMes Ph'C R'con r o t a t or yconcomitant[1,7 J H shitt;allowed15) 1PhCHMesH shift; forbiddenR'SCHEME 3Mesityl(pheny1)keten reacts with (E)-Z-benzylidene-indan-l-one (la) at 140 "C to give the dimethylene-indanes (2a) and (3a) and, from their thermal re-arrangement, the benzofulvenes (4a) and (5a). The di-methyleneindane-benzofulvene rearrangement (2a) _t(4a) is the first clear example of a symmetry-allowedPart X, H.G. Heller and M. Szewczyk, J.C.S. PevRin I,J. S. Hastings, H. G. Heller, H. Tucker, and K. Smith,9 R. B. Woodward and R. Hoffmann, Accounts Chem. Res.,4 H. E. Zimmerman, Accounts Chem. Res., 1971, 4, 272.1974, 1487.J.C.S. Chem. Comm., 1974, 348.1968, 1, 17.(6)Mes = 2,4,6 - Me,C,H2a ; R ' = R2= H , A r = Phb ; R'= Me,R2= H , A r = Phc ; R ' = R2= H , Ar=P-MeOaC,H4d ; R' = R2= D , Ar*p-MeO-C6H,e ; R1= D,R*= H , A r = p-Me0.C6H4f ; R'= H R2= D A r = P ' MeO-C6HLSCHEME 1hydrogen shifts are symetry-forbidden thermal pro-cesses involving anti-aromatic transition states 4 and ar1546 J.C.S.Perkin Ipericyclic reaction involving two ' forbidden ' processes,namely thermal conrotatory ring opening of a six-electron system and a concomitant suprafacial [1,7]hydrogen shift.The structures of the dienes (2a) and (3a) follow fromthe similarity of their U.V. and n.m.r. spectra to thoseof (E) -2-benzylidene-1 -diphen ylmethyleneindane 59 andfrom their photodehydrocyclisation in hexane to5-mesityl-10-phenyl-l l H-benzo [b] fluorene (8a). Thestereochemistry of the diene (2a) was assigned from theobservation that, on heating at 180 O C , it rearranges tothe benzofulvene (4a) rather than undergo cyclisationon to the phenyl group, followed by a [1,5] hydrogenshift, to give the cis-l0,1Oa-dihydrobenzo[b]fluorene (7a).The synthesis of the diene (2a) was undertaken in theexpectation that, on irradiation at 366 nm, it wouldundergo electrocyclic conrotatory ring closure to givethe more deeply coloured intermediate (9a), which wouldbe thermally stable because of steric interactions whichmust occur in the allowed disrotatory ring opening andthe unlikelihood of a methyl shift at room temperature(cf.' the stability of l,Sa-dihydro-1,1,5,7,8a-pentamethyl-naphthalene-2,3-dicarboxylic anhydride a t 160 "C).The intermediate (9a) could not be detected and it isbelieved that photochemical isomerisation of the diene(2a) to the diene (3a) is the preferred process, withsubsequent conrotatory ring closure onto the phenylgroup, followed by dehydrogenation.The structures of the benzofulvenes (4a) and (5a)were assigned from the close similarity of their U.V.andn.m.r. spectra to that of 2-benzyl-l-diphenylmethylene-indene and from the fact that they undergo no thermalor photochemical rearrangements other than intercon-version. On reduction with aluminium amalgam, theyyield 2-benzyl-3-mesityl(phenyl)methylindene (6), whoseU.V. and n.m.r. spectra resemble those of 2-benzyl-3-diphenylmethylindene.6 The stereochemistry of thebenzofulvenes is assigned on the basis that the mesitylgroup exerts a greater shielding effect on the methyleneprotons in the Z-isomer (4a) (7 6.96) than does thephenyl group in the E-isomer (5a) ('t 6.48).Mesityl(pheny1)keten did not react with (29-2-benzylidene-3-methylindan-l-one (lb) at 140 "C.At180 "C, r-l0,c- 1Oa-dihydrod-mesit yl-c-l l-me thyl- 10-phenyl-1 1H-benzo[b]fluorene (7b) was obtained, formedby ring closure of the diene (3b) by the sterically lesshindered disrotatory mode on to the phenyl group,followed by a [1,5] hydrogen shift (~f.~ the thermalrearrangement of 2-benzylidene-l-diphenylmethylene-3-methylindane). No benzofulvene was detected, norwould the benzofulvene (4b) be expected to be formedin a concerted process from the intermediate (lob),since only the ll-hydrogen atom anti to the 10-phenylsubstituent can migrate if orbital symmetry is to beconserved. The absence of benzofulvene derivatives inthis and the previously reported 6~8-lO dimethyleneindaneH.G. Heller and K. Salisbury, J . Ckem. SOC. ( C ) , 1970, 399.6 N. Campbell, P. S. Davison, and H. G. Heller, J . Chem.SOC., 1963, 993.7 H. G. Heller and R. M. Megit, J.C.S. Perkin I , 1974, 923.rearrangements precludes an acid- or a radical-catalysed[1,3] hydrogen shift in the diene-benzofulvene rearrange-ment (2a) _t (4a).The thermal rearrangement of the diene (2a) at180 "C to give the benzofulvene (4a) can only be ex-plained satisfactorily by two consecutive concertedreactions: (i) disrotatory ring closure to the dihydro-benzofluorene intermediate (lOa), followed by (ii) open-ing of the newly formed a-bond by the conrotatorymode, not prevented by steric constraints, with aconcomitant suprafacial [1,7] shift of the ll-hydrogenatom anti to the 10-phenyl group and retention ofconfiguration at the migration terminus, as shownbelow.This rV4, + ,2, + ,2,] process occurs withconservation of orbital symmetry [Figure (A)] and[ A 1 ( B )MO presentation showing conservation of orbital symmetry (A)and Mobius transition state (B) for thermal conrotatory ringopening and a concomitant [1,7] H shift in (10a)involves a Mobius transition state (B). The rearrange-ment (2a) + (4a) can be broken down into two' forbidden ' processes, a conrotatory electrocyclic ringopening in a six-electron system, and a suprafacial [1,3]sigmatropic change of order of hydrogen.In order to establish that hydrogen migration occursexclusively from the methylene group, S-deuterio-indanone derivatives were studied.In view of thepractical difficulties experienced in separating theproducts from the keten-ketone reactions, (E)-+-methoxybenzylideneindan-l-one (lc) and its deuterio-derivatives (Id and e) were used, in the expectation thatthe susceptibility of the carbonyl oxygen atom of theseketones to electrophilic attack by mesityl(pheny1)ketenwould be enhanced by the electronic effect of the p-methoxy-substituent, and that the sharp n.m.r. ab-sorption of the 9-methoxy-group would provide aconvenient probe for establishing product composition.The ketone (lc) with mesityl(pheny1)keten at 140 "C gavethe (2)-benzofulvene (4c), which on heating a t 180 "Cequilibrated to a 1 : 1 mixture of Z- and E-isomers, (4c)and (5c).At 140 "C, the ketone (Id) and mesityl(pheny1)ketengave a 1 : 1 mixture of (2)- and (E)-benzofulvenes (4d)and (5d).On repeating the reaction a t 180 OC, the onlyproducts isolated were the benzofluorene (8d) and itsdihydro-derivative (7d). Apparently a t the higher* H. G. Heller. D. Auld, and K. Salisbury, J . Chem. SOC. (C),0 H. G. Heller and K. Salisbury, J . Chem. SOC. ( C ) , 1970, 873.10 H. G. Heller and K. Salisbury, J . Chem. SOC. (C), 1970,1967, 1084.19971975 1547temperature isomerisation of the diene (2d) to the diene(3d) followed by electrocyclic ring closure on to thephenyl group occurs faster than deuterium migration inthe diene (2d) to give the benzofulvene (4d).Interaction of mesityl(pheny1) keten and the ketone(le) a t 140 'C gave a 5 : 1 mixture of (2)- and (E)-benzofulvenes (4e and f ) and (5e and f).The Z- andE-isomers were separated by fractional crystallisation.A ca. 1 : 4 ratio of intensities of olefinic and methyleneabsorptions (at T 3.75 and 7.00, and 3.83 and 6.51,respectively) indicated that 60% migration of hydrogenand 40% migration of deuterium from the methylenegroup had taken place in the rearrangement (2e) ---f(4e) and (4f), proving the exclusive involvement of themethylene group in the rearrangement.EXPERIMENTALU.V. spectra were measured for solutions in cyclohexanewith a Unicam SP 1800A spectrometer; n.m.r. spectrawere obtained for solutions in carbon tetrachloride with aPerkin-Elmer R12 (60 MHz) spectrometer (tetramethyl-silane as internal standard).Product distributions weredetermined by measuring the area under characteristicpeaks of spectra of crude products after removal of solvent.Solvents were dried and distilled before use. Petroleumrefers to the fraction of b.p. 60-80". For photoreactions,an assembly of four 125 W mercury discharge lamps (typeMBW/U, A.E.I. Lamp and Lighting Co., Leicester) withWood's glass filters was used. Products were separated bycolumn chromatography on alumina (Spence type H) withbenzene or benzene-petroleum as eluant.Reaction of Mesityl(phenyl)keten with the BenzylideneKetone (la) .-An intimate mixture of mesityl(pheny1)-keten 11 (5.3 g) and ketone (la) (5.0 g) was heated (4 h) a t140 "C.About half the resulting melt was removed andtriturated with petroleum, and the resulting solid (2.95 g)was chromatographed. The first yellow band gave amixture of E,E- and E,Z-dienes (0.35 g), separated byfractional crystallisation from benzene-petroleum (3 : 1).(E)-2-Benzylidene-(Z)- 1-mesityl(phenyZ)methyleneindane (2a)(0.21 g) separated as yellow crystals with an intense blue-green fluorescence, m.p. 199-204' (Found: C, 89.3; H,6.6. C32H28 requires C, 93.2; H, 6.8%); A,, 260 and355 nm (log c 4.38 and 4.17); z 2.46-3.25 (16H, complexm, ArH), 3.93 (lH, t, J 2 Hz, :CH*), 6.10 (2H, d, J 2 Hz,CH,), and 7.73, 7.76, and 7.80 (9H, s, o- and $-Me). Asecond crop of orange crystals with a green fluorescence(0.10 g ) , m.p. 180-181", was the E,E-isomer (3a) (Found:C, 93.0; H, 7.0%); A,, 260 and 355 nm (log E 4.54 and4.32); z 2.61-3.30 (16H, complex m, ArH), 4.05 (lH, t,J 2 Hz, :CH*), 6.17 (2H, d, J 2 Hz, CH,), 7.76 (3H, s,#-Me), and 7.79 (6H, s, o-Me) [cf.6 (E)-2-benzylidene-l-diphenylmethyleneindane] .The second band from chro-matography gave unchanged ketone (la) (2.51 g).After being heated for a further 10 h at 140 "C, the secondhalf of the reaction mixture was chromatographed. Thefirst yellow band gave a 1 : 1 mixture of the benzofulvenes(4a) and (5a). Nine fractional crystallisations frombenzene-petroleum (1 : 3) gave pure (Z)-2-benzyL1-mesityl-(phenyZ)methyZeneindene (4a) , m.p. 141-142" (Found : C,93.1; H, 6.7%), A,, 240, 278, and 342 nm (log E 4.38,4.07, and 4.04); T 2.40-3.36 (16H, complex m, ArH), 3.80(lH, t, J 1.8 Hz, XH-), 6.96 (2H, d, J 1.8 Hz, CH,), 7.74(3H, s, p-Me), and 7.95 (6H, s, o-Me).Repeated crystallis-ation of the combined residues from acetic acid-ethanolgave the E-isomer (5a), yellow needles, m.p. 139-140'(Found: C, 93.15; H, 6.85%); 268, 338, and 344 nm(log E 4.35, 4.22, and 4.22) ; z 2.70-3.40 (15H, complex m,ArH), 3.45br (lH, s, :CH*), 3.96 (IH, d, J 8 Hz, 7-H shieldedby mesityl group), 6.48br (2H, s, CH,), 7.74 (3H, s, p-Me),and 7.95 and 8.08 (6H, s, o-Me) (cf.5 2-benzyl-l-diphenyl-methyleneindene) ,When the ketone (la) (3.2 g) and mesityl(pheny1)keten(3 g) were heated (48 h) a t 140 "C, only a 1 : 1 mixture of(E)- and (2)-benzofulvenes (4a) and (5a) (3.43 g) wasobtained ; these were purified by chromatography andseparated as described above.2-BenzyZ-3-mesityZ(phenyl)methyZindene (6) .-Reduction ofa 1 : 1 mixture of (E)- and (2)-2-benzyl-l-mesityl(phenyl)-methyleneindene (0.3 g) (4a) and (5a) with aluminiumamalgam in moist ether (24 h) gave the indene (6) (0.11 g),m.p. 142-143" (from ethanol) (Found: C, 92.5; H, 7.35.C32H30 requires C, 92.75; H, 7.25%) ; Lx 265 (log E 4.20) ;T 2.6-3.3 (16H, complex m, aromatic), 4.00br (lH, s,methine), 6.58 (2H, s, CH,), 6.70br (2H, s, CH,), 7.62 (3H,s, p-Me), and 7.80 (6H, s, o-Me) [cf.5 2-benzyl-3-diphenyl-methylindene, A,,, 261 (log c 4.29); z 2.6-3.3 (19H,complex m, aromatic), 4.28br (lH, s, methine), 6.32 (2H,s, CH,), and 6.68br (2H, s, CH,)].Thermal Rearrangements of the Diene (2a) and the Benzo-fulvene (4a) .-The diene (2a) (60 mg) in o-dichlorobenzene(3 ml) was heated (30 h) at 180 'C.Solvent was removedand the residue chromatographed. The first band with ablue fluorescence gave 5-mesityl- 1 O-phenyl- 1 1H-benzo[b]-fluorene (2 mg) (sa), identified by its characteristic U.V.spectrum {Amx. 270, 311, and 324 nm (log E 4.59, 4.14, and4.13) ; cf.6 5,10-diphenyl-llH-benzo[b]fluorene, &= 270,310, and 323 nm (log E 4.82, 4.25, and 4.28)). The secondband gave a 4 : 1 mixture of (2)- and (E)-benzofulvenes(4a) and (5a) (55 mg).The (2)-benzofulvene (4a) (500 mg) was boiled (9 h) ino-dichlorobenzene. Thermal equilibration occurred and amixture of (2)- and (E)-benzofulvenes (4a) and (5a) wasobtained.Photochemical Reactions of the Dienes (2a) and (3a) and theBenzofulvenes (4a) and (5a) .-When ca.10-5~-solutions ofthe dienes (2a) and (3a) in hexane were irradiated, thedienes were quantitatively oxidised to the mesitylphenyl-benzofluorene (8a) within 1 h, whereas the benzofulvenes(4a) and (5a) under similar conditions showed no change intheir U.V. spectra on prolonged irradiation (3 days).Interaction of Mesitylphenylketen and (E)-2-BenzyZidene-3-methylindan-l-one (lb) .-Mesitylphenylketen (7.1 g) and(E)-2-benzylidene-3-methylindan-l-one (lb) (4.7 g) did notreact on heating a t 140 "C. The mixture was heated (16 h)at 180 "C under nitrogen and the product was chromato-graphed (elution with petroleum). The first band gaver- 10,c- 1Oa-dihydro-B-mesityl-c- 1 l-methyl- 1 O-Phenyl- 1 1H-benzo[b@uorene (7b) (3.6 g), m.p.200-201" (from aceticacid) (Found: C, 92.5; H, 7.15. C,,H,, requires C, 92.9;H, 7.1%), A,, 244, 313, 330, and 347 nm (log c 4.28, 4.26,4.42, and 4.31), z 2.4-3.5 (15H, complex m, aromatic),3.75 (lH, d, J 7 Hz, shielded 4-H), 5.4 (lH, d, Jeis-1o.loa7.0 (lH, quintet, Jlo,ll 8, J 1 l , ~ e 8 Hz, 11-H), 7.55, 7.85, and7.93 (9H, singlets, Me protons of mesityl), and 8.45 (3H, d,11 R. C. Fuson, L. J. Armstrong, J. W. Kreisley, and W. J.Schenck, jun., J . Amer. Chem. SOC., 1944, 66, 1464.8 Hz, 10-H), 6.58 (1H, t, Jcis-10.1oa 8, J 1 o . I l 8 Hz, 10a-H)1548 J.C.S. Perkin I:J 1 1 , ~ ~ 8 Hz, 11-Me) (GJ.~ r-10,c-10a-dihydro-5,10-dipheny1-c- 1 1-methyl- 1 1H-benzo[b]fluorene).Four deep red bandsfrom the column gave red intractable oils. The yellowbenzofulvenes could not be detected in this reaction, whichwas repeated several times with heating for differentperiods at temperatures ranging from 140 to 180'.*(E)-2-p-Methoxybenzylideneindan- 1-one ( lc) and its 3-Deuterio- and 3,3-Dideuterio-derivatives.-Methyl cinnamateor methyl 3-phenylpropynoate (10 g) in ethyl acetate(20 ml) was shaken with deuterium at atmospheric pressureover 10% palladium-charcoal (1 g). The fully deuteriatedester was hydrolysed with ethanolic 10% w/v potassiumhydroxide (25 ml) and the solution was acidified withhydrochloric acid. The deuteriated 3-phenylpropanoicacid (8 g) was treated with thionyl chloride (6 ml) and theresulting acid chloride, b.p.62-64' at 0.1 mmHg (5 g),was dissolved in petroleum (25 ml) and heated (1 h) in thepresence of anhydrous aluminium chloride (7 g ) to give thedeuteriated indan-1-one (3.8 g, 93%). A solution of theketone (1.7 g ) and p-methoxybenzaldehyde (1.7 g) inethanol (10 ml) was treated with ethanolic 10% w/vpotassium hydroxide (0.5 ml) . The resulting p-methoxy-benzylidene derivative l8 was crystallised from ethanol;m.p. 130-140' (3.3 g, 86% yield).Reaction of MesityE(phenyZ)keten with the p-Methoxy-benzylidene Ketone (lc) and its Deuterio-devivatives (Id and e) ,-An intimate mixture of mesityl(pheny1)keten (4.7 g)and the ketone (5.0 g) (lc) was heated (50 h) at 140 'C.Work-up as before gave (2)-1-mesityl(pheny1)methylene-2-p-methoxybenzylindene (4c) (2 g), m.p. 178-179' (fromtoluene-petroleum, 1 : 3) ; 7 2.70-3.46 (15H, complex m,ArH), 3.75 (lH, J 2 Hz, :CH*), 6.23 (3H, s, OMe), 7.00 (2H,d, J 2 Hz, CHs), 7.70 (3H, s, $-Me), aod 7.93 (6H, s, o-Me).Xi0 other hydrocarbons were detected. On heating ino-dichlorobenzene at 180 OC, a 1 : 1 mixture of (2)- and(E)-benzofulvenes (4c) and (5c) was obtained.When mesityl(pheny1)keten (2.4 g) and the ketone (Id)(2.6 g) were heated (60 h) at 140 'C, a low yield of a 1 : 1mixture of (2)- and (E)-benzofulvenes (4d) and (5d) (60mg) was obtained. Unchanged ketone (Id) (1.8 g) wasrecovered.When the reaction was repeated a t 180 'C,the only products isolated were 11,l I-dideuterio-5-mesityl-1O-~-methoxyphenyl-1lH-benzo[b]fluorene (8d) , its cis-10,lOa-dihydro-derivative (7d), and unchanged ketone (Id).Mesityl(pheny1)keten (0.65 g) and the ketone (le) (0.68 g )were heated (20 h) at 140 'C.The product was dissolvedin benzene and chromatographed. The yellow band gavea 5 : 1 mixture of (2)- and (E)-benzofulvenes (4e and f )and (5e and f) (0.72 g), from which the 2-isomer (4e and f)(0.60 g), m.p. 178-179', was obtained by crystallisationfrom toluene-petroleum. Its n.m.r. spectrum showedabsorptions a t T 3.75 and 7.00 in the ratio 1 : 4, indicatingthat a 40% migration of deuterium had taken place.The mother liquors were evaporated to dryness and theresidue was crystallised from ethanol, giving the E-isomer(5e and f), m.p. 159-160'; z 2.60-3.35 (14H, complex m,ArH), 3.83 (lH, t, J 2 Hz, :CH*), 3.95 (lH, d, J 8 Hz, 7-Hshielded by mesityl), 6.28 (3H, s, OMe), 6.51 (2H, d, J 2 Hz,CH,), 7.69 (3H, s, $-Me), and 8.05 (6H, s, o-Me) [olefinic tomethylene absorptions in the ratio 1 : 4, indicating a 40%migration of deuterium]. Unchanged ketone (4e) (0.17 g)was recovered.The benzofulvenes (4) and ( 5 ) were identified by theircharacteristic n.m.r. absorptions and the composition ofmixtures was determined from the intensity of the sharpmethoxy-absorptions at T 6.23 and 6.28.We thank the S.R.C. for a maintenance grant (to H. T.).[4/2702 Received, 30th December, 19741* We thank Dr. K. Salisbury for checking this experiment.l a P. Pfeiffer and E. Milz, Ber., 1938, 71, 272