476-484The deep blue-violet bis-o-quinonemethide 3 cyclizes thermally in a [4s+4a]cycloaddition reaction, yielding the colourless [3,2- b]furofurane 1a. The latter is re-opened by irradiation with UV light back to the methide in its Z,Z- configuration 2a. By metallation of 1a, the highly photoreactive acetylene 8a is obtained which again forms a methide under irradiation, but now in its E,E-configuration 3. The properties of this unique new photochromic system and related furofurans and alkyneshave been investigated

Haucke, Gunter

Heine, Niklas

Kral, Andreas

Laatsch, Hartmut

Schmidt, Andreas Johann

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Synthesis and properties of photochromic furofurans

Indian Journal of ChemistryVol. 36 A & B, June 1997, pp.476 - 484PapersSynthesis and properties of photochromic furofuransHartmut Laatsch ., Andreas Johann Schmidt, Andreas Kral, Niklas Heine & GUnter Haucke tInstitut fur Organische Chemie der Universitat Gottingen, TammannstraJ3e 2, D-37077 GottingenReceived 27 April 1997The deep blue-violet bis-o-quinonemethide 3 cyc1izes thermally in a [4s+4a]cyc1oaddition reaction,yielding the colourless [3,2- b]furofurane 1a. The latter is re-opened by irradiation with UV light back tothe methide in its Z,Z- configuration 2a. By metallation of la, the highly photoreactive acetylene 8a isobtained which again forms a methide under irradiation, but now in its E,E-configuration 3. The propertiesof this unique new photochromic system and related furofurans and alkynes have been investigated.The colourless, stable cis-6b, 13b-dihydro-5, 12-dimethoxynaphtho [2',1':4,5] furo [3,2-b] naphtho[2',1':4,5]furan (la) isomerizes in most solvents un-der UV light (<l>= 0.025 in cyciohexane) or on heatingin DMSO via 2a rapidly to the deep blue-violettrans,trans-bisquinone methide 3. The well charac-terized 3 formed in this way re-cyciizes thermallyunder formation of la 1. Both compounds form anovel photochromic system, which might gain tech-nical applicability for optical data storage or pho-tographic image processes, if the influence of thestructure on the photochromic behaviour and the1.: R = H; AJIIIX (1g e) = 347 (4.17), MeCNlb: R=MeOMe3: AJIIIX = 602 om, toluenes »,mechanism is predictable and tunable. It belongs tothe group of valence isomerizations and is related tothe spirobipyranes and the fulgides/, but photo-chromism had not been observed in L-type systemsearlier.As la is colourless and la/3 are blue-violet, thekinetic parameters of the cyclization can be deter-mined simply by measuring the decolourization rateof the methide at various temperatures: Both methideisomers 2a and 3 cyciize by a first order kinetics backto la.OMehv1.:R = H; AJIIIX = 626 om, toluenelb: R=MeA,hv4LAATSCH et al.: SYNTHESIS OF PHOTOCHROMIC FUROFURANSAs 3 has to isomerize to 2a (via 4) prior to cycli-sation, the activation parameters of the 2a~la reac-tion (AE = 60.2 kl/rnol in toluene) describe the realcyclisation, but those of the 3~ la reaction concernthe 3~4 or the 4~2a isomerisation (AE = 101.6kl/mol in toluene); in more polar solvents, the valuesare not lower by more than 1/3, thus excluding ionictransition states. The ground state energies of 2a11aand 3/1a differ by ~Go = -30 and -25 kJ/mol (toluene)respectively, la being the most stable isomer (seeFig. I); 4 was not determined due to its low concen-tration.The capability for photochromism is not easilypredictable, but depends at least in part on thennody-namic stabilities. As furofuran/methide rearrange-ments may proceed thermally in both directions, aco-existence of both forms under equilibrium condi-tions is only possible if the ground state energies donot differ too much. As AM I calculations and meas-urements show, this is fulfilled only in the naphtha-lene (~Go calc = ~28 kJ, expo value -30 kJ, see Fig. 1)and perhaps in the phenanthrene system C:'1Go calc =-67 kl). In the benzene series, the furofuran again isthe stable form, but the methide may be too unstableto exist C:'1Go calc = -109 kJ). Indeed, in solution thebenzofurofurans did not form co loured methides onirradiation, and most efforts to synthesize benzoqui-none methides of type 2a13 have been unsuccessful;only by photo-rearrangement of 29b, a transient bluecoloration was obtained perhaps due to the methide.Oppositely, in the anthracene series, the methide 5[Amax = 597 nm (log E 3.37, CCI4)]3 was the stableisomer (~Go calc = +70 kl), and on heating of 5 onlytraces of the corresponding furofuran were obtained.With furofurans of anthracenes and higher condensedsystems, only a single photo cycle should be possibletherefore. These furofuranes could be used only forirreversible photographic imaging processes.In spite of suitable ground state energy differ-ences, some dinaphthofurofurans (e.g. 17,18) did notshow photochromisms, and additional reasons mustbe responsible for this behaviour. The thermal cycli-sation of the corresponding bis-methide can betreated as a [4s+4a ]cycloaddition of two independentheterobutadienes, and HOMOILUMO considera-tions should be allowed. As a result of our experi-ments and PM3-calculations, thermal cyclisation ofmethides seems to be restricted to systems where thesum of coefficient squares of the reacting atom pairs47710060. l~-? =~~I?5Furofurari(la) E,E-Methide (3)Z,Z-Methide (2a)Fig. 1 -Experimental ground state free energy differencesand activation energies for the furan/methide rear-rangement in toluene; energy values in kJ/molOMeo5in the bis-methide is c2 (LUMOOJ) +c2(HOMOC4')~O.l I. This was fulfilled only with one of our ben-zene derivatives (from 2- tert.-butyl-4-methoxyphe-nol) and certain naphthalenes and phenanthrenes.Corresponding methides of all photostable furo-furans(e.g.lb, 1:=0.075; 17,1:=0.113; 18,1:=0.088)showed much lower sums.In the naphthalene system, the photochemical andacid-catalyzed formation of phenolic side products as7 reduced the number of photochromic cyclesgreatly. This unwanted side reaction may be ex-plained as [1,5]hydrogen shift yielding 6, and a sub-sequent [1,4 ]addition, starting from the dimericmethide in a cis- configuration 2a.A stabilisation against such a rearrangementseemed to be possible by bridgehead substitution offurofurans with alkyl groups. But metallation of la(or 7) and treatment with methyl iodide yielded solelythe dinaphthylalkyne 8b, and without the iodide, thedihydroxyalkyne 8a was obtained in high yield".This reaction is related to the behaviour of certainsimple furan derivatives5 and might have been pre-dictable; but it was very surprising that the alkynewas even much more photosensitive than the furo-furan la, giving exclusively the E,E-quinone methide3 [<1>=0.092 (MeCN), 0.27 (cHex)]. This reaction is,".:"';478 INDIAN J. CHEM. SEC A & B, JUNE 1997OMehv-281.-9 10extremely quick even at temperatures ofliquid nitro-gen and is complete within less than 20 ns whichexcludes the possibility that (excited) 2a or 4 areintermediates. Such a reaction was not known beforein the acetylene series, but proceeds in a similarmanner in salicyliden-anilides (9) which react photo-chernically" to anilino-o-quinone methides (10).SynthesesAlthough all the involved structures are very sim-ple, some syntheses have been remarkably difficultor gave unexpected results: In the naphthalene series,1,4-Add. •6 7OMeBuLi7-Sa8b: OMe instead of OHthermolysis of the spiroquinole ether 11(R = H) gavethe stilbene 12a or the parent furofuran Ia in highyield 1, but 11 (R = Me) delivered only 14, and 11(R = alkyl> Me) gave the monomeric methide 13, asalso did the corresponding benzene derivatives. Ofother aromatic systems, only the anthracene deriva-tive 5 was obtained in good yield by this way.A more general access to the photoreactive systemshould be possible via three independent ways, i.e.the synthesis of (a) furofurans, (b) of o,o'-dihy-droxyarylalkynes and (c) the generation of bis-methides by oxidation of stilbenes. The latter shouldbe accessible by catalytic reduction of to lane deriva-tives as 8a with stoichiometric amounts of hydrogen,by Wittig or McMurry reactions, etc.Synthesis of furofuransFurofurans have been claimed to be obtained al-ready in the last century by reductive coupling ofLAATSCH et al.: SYNTHESIS OF PHOTOCHROMIC FUROFURANSOMe15.: R=H15b: R=Me1617 18aromatic aldehydes with magnesium. Yet recent in-vestigations have shown that in these reactions apinacol rearrangement occurs forming predomi-nantly acetal-type [2,3-b [furofurans 15a. The ex-pected reaction proceeded, however, in some casesvery smoothly when titanium(III) salts were used inaqueous solution 7. Furo[3,2- b] furans have also beenobtained bl Claisen rearrangement of 1,4- diaryloxy-2-butynes .We obtained the furofurans 16, 17 and 18 byreduction of the corresponding aldehydes, but acid-sensitive furofurans like la underwent a further ring-opening with the formation of 7-type phenols andwere not accessible under these conditions. All ben-zaldehydes gave only benzofurans, and trials to ob-tain bridgehead-substituted benzene or naphthalene479OMeR=H ••12.: R=H11b:R=MeMe14derivatives by reduction of arylalkylketones failed asno (or different) reactions occurred.Only 16 showed photochromism and turned pur-ple red on irradiation; the furofurans 17 and 18 werestable in light and even did not rearrange formingring-opened naphthofurans of type 7, as it was pre-dicted by the values of their frontier orbital sum.Furo[l,3-b] furans by double Dotz-ReactionIn a variation of the Dotz reaction developed bySemmelhack et ai.9, alkynes react with chromiumcarbene complexes leading to formation of annel-lated naphthalenes. Using a corresponding procedurestarting from rac-diyn-dioles it should be possible tobuild also acid-sensitive furofurans in a one-pot re-action. In this way, a wide range of substitutionpatterns should be accessible; such a double Dotzreaction has not been reported before.The sensitive rac-diyn-diols 19 were accessible byosmium- catalysed bishydroxylation of(E)-endiynesin moderate yield, but we were not able to performthis reaction with the chromium carbene.Syntheses via stilbenesBridge-head substituted furofurans are of specialinterest, as substituents at the exocyclic double bondsof the isomeric quinone methide (type lb) shouldhave a substantial influence on the cis/trans isomer-isation and hence on its thermal cyclisation. Addi-tionally, they should stabilize the furofuran againstprotic ring opening.480 INDIAN 1. CHEM. SEC A & B, JUNE 1997•R19 10OR 0 OBzlOAc11Mella: R=Bzllb: R=p-Ts13For their synthesis, we have used the McMurryreaction which is known to be very insensitive tosteric hindrance. Reductive coupling of salicylalde-hyde or o-hydroxyacetophenones to stilbenes waseasy. But when we treated various naphthaldehydesand naphthophenones with low-valent titanium re-agents, predominantly deoxygenated products wereobtained 10. On reductive coupling of llb, after hy-drolytic work-up the desired dihydroxystilbene llbwas not observed, but upon prolonged standing of thereaction mixture, the furofuran 1b was formed, pre-sumably via oxidation of llb and cyclization of theintermediate lb. The cis-fusion ofthe rings had beendemonstrated for la by NMR measurements pre-viously, but now a direct proof by crystal structureanalysis was possible (Fig.2).All attempts to open the furofuran 1b thermally orphotochemically to the methide lb in analogy to theparent compound la, were unsuccessful. This is dif-ficult to explain on the basis of an antarafacial[4+4 ]cycloreversion, because PM3 calculations gavenearly identical results for the ground state differ-ences of the thermochromic reactions la-+la andIb-+lb. It is explained easily, however, by the lowfrontier orbital sum oflb (!: = 0.075).As the formation of a phenol of type 7 is notpossible, Ib shows indeed a much higher stabilityagainst proto lytic ring opening than la. But on dis-24..0C(12)0(2)»:0C(4)C(3)"0Fie. 2......crystal structure of cis-5, l2-dimethoxy-6b, 13b- di-methyldinaphtho(3,2-b ]furofuran (1 b).solving in trifluoroacetic acid, a slow but irreversibletransformation into 15b was observed. A similarbehaviour of other furofurans could explain someirreproducible and contradictory results in the earlyliterature which may reflect changing isomer ratiosdue to varying acidities during reductive coupling ofe.g. salicylic aldehydes.Similarly, stilbene syntheses using Wittig reac-tions have been easy in the benzene series. But againin the naphthalene series, the Wittig reaction failedLAATSCH et al.: SYNTHESIS OF PHOTOCHROMIC FUROFURANS 481due to a lack of reactivity (22a +23) or the instabilityof the phosphonium salts (e.g. 24).In the benzene series, the syntheses were success-ful, but none of the stilbene derivatives gave a col-OUTed methide or a furofuran on oxidation; solelyphenyl-benzofurans (type 7) were obtained.Diarylalkynes by Friedel-Crafts reactionsWe have found it most convenient to synthesizeo,o'-dihydroxyarylalkynes by reaction of phenol de-OH~~Me25MeMe27rivatives with tetrachlorocyclopropene (26), andphotolysis of the cyclopropenone (e.g. 28) obtainedthereb/1.Using this sequence, a considerable number ofbenzene and naphthalene derivatives was accessible.Only one of the benzene derivatives tested (2,2'-(ethindiyl)bis(3-tert.- butyl-2-hydroxy- 5-methoxy-benzene) turned blue on irradiation in solution:iprobably due to methide formation [c2 (L UMOo 1)+c(HOMOC4') = 0.135]. Phenyl-benzofurans were iso-I. TBDMS-trillate2. hv3. F-28OHRUY-2RI29a: RI, R2 = Alk, OAlk29b*:RI = OMe, R2 = I-Bu31*~~I ~ 2.& HR323036482 INDIAN 1. CHEM. SEC A & B, JUNE 1997lated with only one ring closed in all other cases. Wegot a colour reaction in the naphthalene system andco-existence of methide and furofuran form in some(indicated by an asterix), but no reaction in otherscases. The same was true in the phenanthrene systemwhere both 34 isomers were photoreactive (Amax ofthe methides = 627,653 mm), but 36 was not.Donor substituents inpara-position to the methidecarbonyl enhanced the stability of the methides andgave rise to a bathochromic shift, and correspond-ingly the 4-methyl derivative 27 gave a less stablered-violet methide than alkyne 8a, as also the 4-chlo-ronaphthyl alkyne 27 (CI instead of Me) did onirradiation.Alkyne-methide rearrangementWe have interpreted this photoreaction as a doublethermally allowed [1,5]hydrogen shift which occursin the following way: On irradiation, the stiff rod-I ikeacetylene reaches the excited state SI which shouldhave a trans-alkene structure (calcd. naphthalene-al-kyne-naphthalene bond angle = 129.8° for 29b, R 1 =R2 = Me) if diarylalkynes behave similar to unsub-stituted acetylenel2 itself. But contrary to the experi-OMe OMe~trans-8a*hv OMeOMeOHrO~<,00000000 oi°8a 000000I IMecis-Sa* 3HHO32 37 38LAATSCH et al.: SYNTHESIS OF PHOTOCHROMIC FUROFURANS 483mental result, in this case the Z,Z-methide 2a shouldbe expected.Only one of the benzene derivatives (29b) showedmethide formation in solution, and the alkynes 32, 33and 36 gave only phenols of type 7. Besides elec-tronic factors, steric interactions also seem to influ-ence the photochemical behaviour of the0,0 '-dihydroxydiarylalkynes and therefore the stabil-ity of their photo products. As the crystal structureshows, hydrogen bridges are fixing the acetylene 32(R = H) even in the ground state in a bent "cis"-con-,;D' formation. This seems to be the case in solution also,as 32 and all other 0,0' - dihydroxydiarylalkynes showsignals of chelated OH groups in the NMR spectra at():::::6.3.Fig. 3-'--PM3-Hyper space of the thermal isomerisation ofZ,Z- hexa-2,4-diendial to 3a,6a-dihydrofuro[3,2-b]furan as a model system of la/2aStarting with such a geometry, a concerted double[I ,S]hydrogen shift cannot yield the Z,Z-methide, butmust end up with the E,E- configuration, as it wasE,,,,," ,,,,,,,SlSo4,,,,,,,,,,,,W2818qFig. 4-Jablonski-diagram of the 3 -+ la cyclization for ground state and excited state; arrowsupwards = absorption, downwards = fluorescence.484 INDIAN 1. CHEM. SEC A & B, JUNE 1997found experimentally. But a unsymmetrically dis-torted triple bond as in 32 may give rise to a two-stepmechanism: After the first [1,5]-H shift to 37, asubsequent 1,4-addition reaction yielding 38 presum-ably is more effective than the formation of themethide by a second [1,5]-H shift. Hydrogen bridgesbetween l-OH and 8-0R prevented also the photoreaction of 31 (R2 = OH, OAlk), whereas the S-ORisomer gave the methide.Hyper spaces and transition statesRadicals as intermediates of the photochemicalmethide rearrangement have been excluded on thebasis ofESR experiments and ionic states on the basisof the low solvent dependency of the kinetic parame-ters. The identification of the short-living Z,Z-methide 2a and also its E,Z-isomer in therearrangement of 1a supports the formulation of thethermal cyclisation of2a to 1a as a thermally allowedantarafacial [4s+4a] reaction. Corresponding to AM 1and PM3 calculations on model systems and on1al2a, a concerted cyclisation should predominate,but the energy difference to a non-concerted reactionis only 1 kcal/rnol (Fig. 3).Results of experiments and semiempirical calcu-lations of ground states and the excited state have ledto an understanding of the la ~ 3 isomerization asdepicted in Fig. 4. The ground state profile wasdetermined experimentally by thermodynamic andkinetic measurements (see Fig. 1). As la (at -196°C)and 3 show fluorescence, in the S Istate, their minimashould be opposite to the ground state minima withenergy differences corresponding to their fluores-cence wavelength. For 2a*, the minimum was calcu-lated. Also other results of semiempirical calcula-tions are in good agreement with this model butindicate that higher excited states may also be in-volved in the photo processes.AcknowledgementA generous financial support of the Deutsche For-schungsgemeinschaft and the Fonds der ChemischenIndustrie is greatly acknowledged.ReferencesI Laatsch H, Liebigs Ann Chem, 1982, 1808.2 DUrH, Angew Chem, 101, 1989,427.3 Laatsch H & SchmidtAJ, ZNaturJorsch, 48b, 1993,1291.4 Schmidt A J & Laatsch H, Angew Chem lnt Ed. Engl, 30,1991,866.5 Wakefield B J, Organolithium methods, (Academic Press,London) 1988,p. 177.6 Senier A & Shepheard F G, J chem Soc, 95,1909, 1943;101,1912,1950; de Gaouck V & Le Fevre R J W, JchemSoc, 1939, 1457; Hadjoudis E & Hayon E, Jchem. Phys,741970,3184.7 Clerici A, Porta 0 & Arnone A, J org Chem, 55, 1990,1240.8 Ramakanth S, Narayanan K & Balasubramanian K K,Tetrahedron, 40,19844473; 42, 1986,863.9 Semmelhack M F, Bozell J J, Keller L, Sato T, Spiess E,WulffW & Zask A, Tetrahedron, 41,1985,5803.10 Laatsch H, Talvitie A, Kral A & Ernst B-P, Jprakt Chem,338,1996, 140.11 Schmidt A J, Dissertation, University of Gottingen, 1992.12 Ingold C K & King G W, Jchem Soc. 1953,2702.

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Synthesis and properties of photochromic furofurans

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