Angewandte Photoisomerization D01:10.1002/anie.20070446 Photoisomerization of cis,cis-and cis,trans-1,4-Di-o-tolyl-1,3- butadiene in Glassy Media at 77 K:One-Bond-Twist and Bicycle-Pedal Mechanisms* Jack Saltiel,*Michael A.Bremer,Somchoke Laohhasurayotin,and Tallapragada S.R.Krishna lo BT) ciS-rans out two hon nretinyl photois applied ge for ph stable (#-DTBA)con (BP) e-confining medi in s failed t polyene dou le bondsandthehu-twist mec (HT hanisms,because a mixture of ct-DTB.and c scheme 1.both mchanis nt for the fo mation of a CH are GquibammitureofpTE,andDg,omo h 2)anis tran toproduct mixtures that difer from thei ct-DTB, H on formation of T prod Scheme 1.Illustration of OBT and HT-1 mechanisms for ct-DTB. The alter ative in Our vation of a majo s Ic (P)slass at 77K (DPB)in o organic glasses at 77K a of forthe two-bond photoisomeriz of co in the rig DPB product forms as thesr ns confor consisten omers are T proces()abou the benzy cribe herein that show unequivocally that c-TB CH group of the diene moiety.despite the fact that HT-1 and on in IP glass at7 compeonmihhcdreotondBPpOe M.ABe and Biochm a Sta 32306 390U5A y of a Hitac Aconfirm the report onc-TBandextentoDTB Chem.Int.Ed 2008 These are not the final page numbers!
Photoisomerization DOI: 10.1002/anie.200704465 Photoisomerization of cis,cis- and cis,trans-1,4-Di-o-tolyl-1,3- butadiene in Glassy Media at 77 K: One-Bond-Twist and Bicycle-Pedal Mechanisms** Jack Saltiel,* Michael A. Bremer, Somchoke Laohhasurayotin, and Tallapragada S. R. Krishna Viscous media enhance torsional barriers of olefins in the lowest excited singlet state[1] and inhibit one-bond-twist (OBT) cis–trans photoisomerization.[2] Two mechanisms involving concerted rotation about two bonds in S1, initially postulated to explain retinyl photoisomerization within the protein environment in rhodopsin and bacteriorhodopsin, have been applied generally to account for photoisomerization in volume-confining media. The bicycle-pedal mechanism (BP) involves simultaneous rotation in S1 about two polyene double bonds,[3] and the hula-twist mechanism (HT) involves simultaneous rotation about a double bond and an adjacent single bond (equivalent to a 1808 translocation of one CH unit).[4] These mechanisms are assumed to reduce free-volume requirements by confining motion to the vicinity of the isomerizing double bonds while minimizing the motion of bulky substituents. The claim that the photoisomerization of previtamin D to tachysterol at 92 K in an EPA glass (ether/ isopentane/ethylalcohol = 5:5:2) gives HT products[5] stimulated the revival of the HT mechanism.[6] Observations of trans photoproduct conformer mixtures that differ from their equilibrium compositions on irradiation of cis isomers in organic glasses at low tempertaure were assumed to support the formation of HT products.[6, 7] The alternative interpretation, advanced earlier by the Alfimov[8] and Fischer[9] groups, that such results reflect different equilibrium conformer compositions for the two isomers, was ignored. A case in point is the photoisomerization of 1,4-diphenyl- 1,3-butadiene (DPB) in organic glasses at 77 K[7, 10] and in the solid state.[11] Irradiation of the cis,cis isomer cc-DPB in glassy media of relatively high viscosity, EPA[7] or methylcyclohexane[10] at 77 K,[12] gives only the cis,trans isomer ct-DPB. Spectroscopic observations show that in the rigid glasses the ct-DPB product forms as the s-trans conformer, consistent with OBT in fluid solution.[13] Nevertheless, ct-DPB formation was attributed to the HT process (HT-1) about the benzylic CH group of the diene moiety, despite the fact that HT-1 and OBT products from the cis DPB isomers are indistinguishable.[7] The photoisomerization of cis,trans-1,4-di-o-tolyl-1,3- butadiene (ct-DTB) in EPA at 77 K was studied in an attempt to avoid that ambiguity. It was reasoned that starting from the more stable conformer ct-DTBA, HT-1 and OBT mechanisms would give unstable (tt-DTBB) and stable (tt-DTBA) conformers, respectively. Claims to the contrary notwithstanding,[7] that study also failed to distinguish between OBT and HT-1 mechanisms, because a mixture of ct-DTBA and ctDTBB conformers was present at the outset. As shown in Scheme 1, both mechanisms account for the formation of a non-equilibrium mixture of tt-DTBA and tt-DTBB conformers.[10] Our observation of a major direct cis,cis- to trans,trans-1,4- diphenyl-1,3-butadiene (cc-DPB!tt-DPB) reaction channel on irradiation of cc-DPB in isopentane (IP) glass at 77 K provided the first experimental evidence for BP photoisomerization under such conditions.[10] The BP process also accounts for the two-bond photoisomerizations of ccDPB,[11] muconate salts,[14a] and cis,trans,cis-1,6-diphenyl- 1,3,5-hexatriene (ctc-DPH) derivatives in the solid state.[14b] The interconversion of ctt-DPH and tct-DPH isomers are examples of BP photoisomerization in solution.[15] We describe herein results that show unequivocally that cc-DTB! tt-DTB photoisomerization in IP glass at 77 K proceeds by sequential two-step OBT processes via the ct-DTB isomer in competition with the direct two-bond BP process. The preparation of the DTB isomers has been described.[7, 16] Irradiations of approximately 4 and 9 ? 105m ctand cc-DTB solutions, respectively, in EPA and IP glasses at 77 K were carried out in the dewar flask of the phosphorescence accessory of a Hitachi F-4500 fluorimeter using a 450- W Hanovia Hg lamp (Pyrex filter).[7, 17] Reaction progress was monitored in situ by fluorescence spectroscopy. Results in EPA confirm the report on ct-DTB[7] and extend it to cc-DTB. Starting from either ct- or cc-DTB, similar mixtures of ttDTBA and tt-DTBB conformers form. Principal component Scheme 1. Illustration of OBT and HT-1 mechanisms for ct-DTB. [*] Prof. J. Saltiel, M. A. Bremer, Dr. S. Laohhasurayotin, Dr. T. S. R. Krishna Department of Chemistry and Biochemistry Florida State University Tallahassee, FL 32306-4390 (USA) Fax: (+1) 850-644-8281 E-mail: saltiel@chem.fsu.edu [**] J.S. thanks the National Science Foundation (Grant No. CHE- 0314784) for partial support of this research Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie Angew. Chem. Int. Ed. 2008, 47, 1 – 5 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 These are not the final page numbers!
Communications analysis (PCA)t component syste n.which sts of the three pureis 002 that th A and B conform of -DTB and c-DTB have y -DTB 002002 at 77 k >300 nm) s可obt ed on irradiat of c-DTEP and 4) 400 5060 hree p in eigenvector combination DTB C-DTB→DTB tion coefficient points of th (Figure 2).Begi TB corner of the tetrah C-PTB-DTB edge and 400454 s(300 nm,77K in the fluc pectra bration ing a range of Figure 2)move to the c DTB -DTB.relaxation and the pres ence of residual cr of cl the same tetra dron. tion in the in Ep -DIB ormation by pathway in P(Figure).The structuredTB www.angewandte.org These are not the final page numbers
analysis (PCA) treatment[10, 18] of a matrix of spectra obtained on irradiation of both cis DTB isomers reveals a fourcomponent system, which consists of the three pure isomer spectra (insensitivity to changes in the excitation wavelength lexc shows that tt-DTB exists as the tt-DTBA conformer and that the A and B conformers of ct-DTB and cc-DTB have very similar spectra) and a fourth spectrum, obtained by selfmodeling,[18] corresponding to the higher-energy tt-DTBB conformer (Figure 1). The pure spectra define a tetrahedron in eigenvector combination coefficient space. The ccDTB!ct-DTB!tt-DTB sequence is revealed by the time evolution of the combination coefficient points of the spectra starting from cc-DTB (Figure 2). Beginning at the ccDTB corner of the tetrahedron, the spectra move on the cc-DTB/ct-DTB edge and eventually proceed to a mixture containing the two conformers, tt-DTBA and tt-DTBB. The broad features of the final fluorescence spectra narrow significantly on warming and recooling. Points for spectra obtained after thermal equilibration using a range of lexc (^ in Figure 2) move to the ctDTB/tt-DTBA edge of the tetrahedron, consistent with ttDTBB!tt-DTBA relaxation and the presence of residual ctDTB. Points for spectra starting from ct-DTB, omitted for the sake of clarity, lie on the ct-DTB/tt-DTBA/tt-DTBB plane of the same tetrahedron. Results in IP are strikingly different. Comparison of the initial spectral evolution of cc-DTB photoisomerization in the two media allows visual confirmation of tt-DTB formation by a sequential two-step pathway in EPA and by a direct pathway in IP (Figure 3). The structured tt-DTB fluorescence is clearly evident after 20 s of 300-nm irradiation in IP whereas the early spectra in EPA resemble the fluorescence spectrum of ct-DTB (see also Figures 1 and 4). These results mirror the behavior of cc-DPB in the two media.[7, 10] Analogous PCA treatment of the combined matrix of spectra starting from both cc- and ct-DTB in IP reveals a three-component system that is accounted for exactly by the three pure isomer spectra (Figure 4). Beginning from cc-DTB, spectral points in the combination coefficient plot (Figure 5) deviate from the cc-DTB/ct-DTB edge of the triangle from Figure 1. Normalized fluorescence spectra of tt-DTBB (ttB), tt-DTBA (ttA), ct-DTB (ct), and cc-DTB (cc) in EPA at 77 K. Figure 2. Combination coefficients (a, b, g) for the spectral matrix (lexc=300 *, 315 ~, 345 *, before +, and after thawing ^, 330– 350 nm, pure components &) obtained on irradiation of cc-DTB in EPA at 77 K. (lrad>300 nm). Figure 3. Evolution of fluorescence spectra (normalized) during the first 90 min of irradiation under similar conditions (300 nm, 77 K in the fluorimeter) in EPA (a) and in IP (b). Figure 4. Fluorescence spectra of pure tt-, ct-, and cc-DTB isomers in IP at 77 K. Communications 2 www.angewandte.org 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 1–5 These are not the final page numbers!
Angewandte 0.01 -0.01 C-DTB -88 0.02 00 0.02 004 -0020 Tothe extent that this is maintainedin ct-DTB (s)in (300 nm EPA might appear emark le if it were not for a conta ations in ct-DTE that the m H the star,consistent with direct cDTB-DTB two-bond -with the n-bondDTB st to t situation in EPA. Pubished colmer either c ing from eithe stru redDTB pec rum.The medium change to IP hi n dominant conformer a1993.97.251 663967891a6-1037b15aie and OBT pathways (Scheme 2).H pathways are eliminated 马风风起 to The ompeting photoomef :e)R.S H.Liu G.S. e formation of unstable confo The-raerystastructure of c-TB reveals a single (B3LYPith the 6-31G(d.p)basis set) .A.M.Turek..Chem 四8KmR1ak1咖aamA2m 08 Wiley-VCH Verlag CmbH Co.KGaA.Weinhein ndte.org These are not the final page numbers
the start, consistent with direct cc-DTBA!tt-DTBA two-bond isomerization in competition with the one-bond cc-DTBA! ct-DTBA process. In contrast to the situation in EPA, there is no significant spectral change on thawing and recooling the solution after irradiation of either cc- or ct-DTB. Starting from either cis isomer, final photoproduct spectra coincide with the highly structured tt-DTBA spectrum. The medium change from EPA to IP has a dramatic effect on the equilibrium conformer distributions of cc- and ct-DTB, shifting each to a single dominant conformer. This serendipitous medium effect allows us to establish that in IP the photoisomerizations of cc- and ct-DTB involve no unstable conformer products. Cis– trans photoisomerizations that convert stable reactant conformers to stable product conformers are accounted for by BP and OBT pathways (Scheme 2). HT pathways are eliminated in IP, because they require formation of unstable conformers. The inhibition of the BP process aside, we see no reason to expect a mechanism change in EPA.[10] The X-ray crystal structure of cc-DTB reveals a single conformer, cc-DTBA, with the o-tolyl groups in parallel planes oriented at 568 to the plane of the diene moiety.[16] Calculated conformer structures (B3LYP with the 6-31G(d,p) basis set) for ct-DTB similarly show large o-tolyl–diene dihedral angles (42.1 and 19.78 for ct-DTBA and 62.0 and 20.88 for ct-DTBB on the cis and trans sides of the molecule, respectively, Figure 6).[16] ct-DTBB is predicted to be 1.95 kcalmol1 higher in energy in the gas phase.[16] To the extent that this energy difference is maintained in IP, the predominance of the ct-DTBA conformer at 77 K in that medium is not surprising. The presence of both conformers in EPA might appear remarkable if it were not for a highly significant precedent. Strong CD spectral evidence shows that the previtamin D and tachysterol conformer distributions change significantly on changing the medium from a hydrocarbon glass to EPA at 92 K.[19] The claim of HT products from previtamin D in EPA[5] is questionable, because it did not consider that medium effect. Received: September 27, 2007 Published online: && &&, &&&& .Keywords: dienes · isomerization · one-bond twist · photochemistry [1] a) J. Saltiel, A. S. Waller, D. F. Sears, Jr., J. Am. Chem. Soc. 1993, 115, 2453 – 2465; b) J. Saltiel, A. S. Waller, D. F. Sears, Jr., C. Z. Garrett, J. Phys. Chem. 1993, 97, 2516 – 2522. [2] a) J. Saltiel, J. Am. Chem. Soc. 1967, 89, 1036 – 1037; b) J. Saltiel, J. Am. Chem. Soc. 1968, 90, 6394 – 6400. [3] A. Warshel, Nature 1976, 260, 679 – 683. [4] a) R. S. H. Liu, A. E. Asato, Proc. Natl. Acad. Sci. USA 1985, 82, 259 – 263; b) R. S. H. Liu, D. Mead, A. E. Asato, J. Am. Chem. Soc. 1985, 107, 6609 – 6614. [5] A. M. MIller, S. Lochbrunner, W. E. Schmid, W. Fuß, Angew. Chem. 1998, 110, 520 – 522; Angew. Chem. Int. Ed. 1998, 37, 505 – 507. [6] a) R. S. H. Liu, G. S. Hammond, Proc. Natl. Acad. Sci. USA 2000, 97, 11153 – 11158; b) R. S. H. Liu, Acc. Chem. Res. 2001, 34, 555 – 562; c) R. S. H. Liu, G. S. Hammond in Handbook of Organic Photochemistry and Photobiology, 2nd ed. (Eds.: W. M. Horspool, F. Lenci), CRC, London, 2004, pp. 26/1 – 26/11; d) R. S. H. Liu, G. S. Hammond, Acc. Chem. Res. 2005, 38, 396 – 403; e) L.-Y. Yang, M. Harigai, Y. Imamoto, M. Kataoka, T.-I. Ho, E. Andrioukhina, O. Federova, S, Shevyakov, R. S. H. Liu, Photochem. Photobiol. Sci. 2006, 5, 874 – 882; f) R. S. H. Liu, L. Y. Yang, J. Liu, Photochem. Photobiol. 2007, 83, 2 – 10. [7] L. Yang, R. S. H. Liu, K. J. Boarman, N. L. Wendt, J. Liu, J. Am. Chem. Soc. 2005, 127, 2404 – 2405. [8] M. V. Alfimov, V. F. Razumov, A. G. Rachinski, V. N. Listvan, Yu. B. Scheck, Chem. Phys. Lett. 1983, 101, 593 – 597. [9] N. Castel, E. Fischer, J. Mol. Struct. 1985, 127, 159 – 166. [10] J. Saltiel, T. S. R. Krishna, A. M. Turek, R. J. Clark, Chem. Commun. 2006, 1506 – 1508. [11] J. Saltiel, T. S. R. Krishna, R. J. Clark, J. Phys. Chem. A 2006, 110, 1694 – 1697. [12] For organic glass viscosities, see: a) J. R. Lombardi, J. W. Raymonda, A. C. Albrecht, J. Chem. Phys. 1964, 40, 1148; Figure 5. Combination coefficients for the spectral matrix obtained on irradiation of cc-DTB (*) and ct-DTB (~) in IP at 77 K (lrad>300 nm). Points on the ct-DTB/tt-DTB side are for lexc=345 nm. Spectra for lexc=300 and 315 nm define the lower line, revealing some cc-DTB contaminations in ct-DTB. Scheme 2. The competing photoisomerization pathways of cc-DTB in IP at 77 K. Figure 6. Stationary-point geometries on the ct-DTB So surface; ctDTBA on the left corresponds to the global energy minimum.[16] Angewandte Chemie Angew. Chem. Int. Ed. 2008, 47, 1 – 5 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 3 These are not the final page numbers!
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b) H. Greenspan, E. Fischer, J. Phys. Chem. 1965, 69, 2466 – 2469; c) G. A. von Salis, H. Labhart, J. Phys. Chem. 1968, 72, 752 – 754; d) A. C. Ling, J. E. Willard, J. Phys. Chem. 1968, 72, 1918 – 1923. [13] a) J. H. Pinckard, B. Wille, L. Zechmeister, J. Am. Chem. Soc. 1948, 70, 1938 – 1944; b) L. R. Eastman, Jr., B. M. Zarnegar, J. M. Butler, D. G. Whitten, J. Am. Chem. Soc. 1974, 96, 2281 – 2283; c) W. A. Yee, S. J. Hug, D. S. Kliger, J. Am. Chem. Soc. 1988, 110, 2164 – 2169. [14] a) T. Odani, A. Matsumoto, K. Sada, M. Miyata, Chem. Commun. 2001, 2004 – 2005, and references therein; b) Y. Sonoda, Y. Kawanishi, S. Tsuzuki, M. Goto, J. Org. Chem. 2005, 70, 9755 – 9763. [15] a) J. Saltiel, D.-H. Ko, S. A. Fleming, J. Am. Chem. Soc. 1994, 116, 4099 – 4100; b) J. Saltiel, S. Wang, L. P. Watkins, D.-H. Ko, J. Phys. Chem. A 2000, 104, 11443 – 11450. [16] J. Saltiel, T. S. R. Krishna, L. Laohhasurayotin, K. M. Fort, R. J. Clark, J. Phys. Chem. A, DOI: 10.1021/jp077342c. [17] J. Saltiel, T. S. R. Krishna, A. M. Turek, J. Am. Chem. Soc. 2005, 127, 6938 – 6939. [18] J. Saltiel, D. F. Sears, Jr., J.-O. Choi, Y.-P. Sun, D. W. Eaker, J. Phys. Chem. 1994, 98, 35 – 46. [19] a) P. A. Maessen, H. J. C. Jacobs, J. Cornelisse, E. Havinga, Angew. Chem. 1983, 95(Suppl.), 994 – 1004, Angew. Chem. Int. Ed. Engl. 1983, 22(Suppl.), 994 – 1004; b) P. A. Maessen, Ph.D. Thesis, Leiden, 1983. Communications 4 www.angewandte.org 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 1–5 These are not the final page numbers!
Communications ◇ Photoisomerization o J.Saltiel,*M.A.Bremer m-m 60b Photoisomerization of cis,cisand Let's twist again:In the isomerization of Instead,the isomerization is found to o.ito7tadcne050 (e and Bicycle-Pedal Mechanisms scheme). photoproduct conformers do not form. Angew.Chem.Int.Ed.2008.47.1-5 2008 Wiley-VCH Verlag GmbH Co.KGaA,Weinheim These are not the inl pagem www.angewandte.org
Communications Photoisomerization J. Saltiel,* M. A. Bremer, S. Laohhasurayotin, T. S. R. Krishna &&& — &&& Photoisomerization of cis,cis- and cis,trans-1,4-Di-o-tolyl-1,3-butadiene in Glassy Media at 77 K: One-Bond-Twist and Bicycle-Pedal Mechanisms Let’s twist again: In the isomerization of cis,cis-1,4-di-o-tolyl-1,3-butadiene in isopentane glass at 77 K, hula-twist pathways are eliminated, because unstable photoproduct conformers do not form. Instead, the isomerization is found to proceed by bicycle-pedal (BP) and onebond-twist (OBT) mechanisms (see scheme). Angewandte Chemie Angew. Chem. Int. Ed. 2008, 47, 1 – 5 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 5 These are not the final page numbers!