মুখ্য Journal of Organic Chemistry Application of photoelectron spectroscopy to molecular properties. 45. Reactivity of...
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J. Org. Chem. 1991,56,3445-3447 revealed the existence of the equilibrium mixture consisting of 1 and 2 in a ratio of 1486. The same equilibrium mixture was also obtained starting from 2 under the same reaction conditions. Reaction of the Endo Dibromide 1 with Bromine in the Presence of Radical Inhibitors. To a solution of 40 mg (0.12 mmol) of endo dibromide 1 in 0.6 mL of CDCls in a NMR tube was added 16 mg of 2,4,6-tri-tert-butylphenol(0.06 mmol) (or 2,4,6-tri-tert-butylnitrosobenzene)and followed by addition of 20 mg (0.12 mmol) of bromine. After 10 h no configuration isomerization was observed. 3445 Scheme I - 0 R X & 7' c\ -co,;cc~l,co /!-Ox c- 0 R\ /c=c=c=O X II 0 Acknowledgment. We are indebted to Atatiirk University (Grant 1987/40) and Department of Chemistry for financial support of this work and to wish the express their appreciation to Prof. Dr. M. Schlosser (University of Lausanne) for 13C NMR spectra and to both the referees for their very helpful suggestions concerning the radical mechanism. w e t r y NO. 1, 132748-50-6; 2,117404-71-4;3, 117269-29-1; 4, 117269-30-4;5, 117404-74-7;11,132699-64-0;Br2,7726-95-6. Reactivity of [ (Alky1thio)methylenelketenesin the Gas Phase and Photoelectron Spectra of Thiophen3(2H)-ones1 F. Chuburu, S. Lacombe, and G. Pfister-Guillouzo* Laboratoire de Physico-Chimie Mol6culaire, URA CNRS 474, Avenue de l'Universit6, MOO0 Pau, France A. Ben Cheik, J. Chuche, and J. C. Pommelet t 1400 Laborotoire de Chimie Organique Physique, URA CNRS 459, Universitl de Reims, Facult6 des Sciences, Moulin de la Housse, BP 347, 51062 Reims, France Received October 11, 1990 Flash vacuum pyrolysis of substituted Meldrum's acid derivatives is known to give methyleneketenes (alkyl2and alkox?' compounds) (Scheme I). However, alkylthio3ps and alkylaminoS derivatives further react to the corresponding five-membered heterocycles, and thiophen-3(W)-ones or pyrrol-3(W)-ones are generally characterized in solution as the reaction products. The only exception concerns th; e sterically strained bis(alky1thio) Meldrum's acid derivative 1 [isopropylidene (1,3-dithiolan-2-ylidenelmdonate], for which the gas-phase characterization of [bis(alkylthio)methylene]ketene2 could be performed.' As photoelectron spectroscopy (PES) has proved to be a highly efficient tool for the gas-phase characterization of elusive compounds,' the PES detection of other [(al(1) Application of PhotoelectronSpectroscopy to Molecular Properties. 45. Part 44: Lacombe, s.; Pfieter-Guillouzo, G.;Guillemin, J. C.; Denis, J. M. J. Chem. SOC.,Chem. Commun., in prees. (2) Mohmand, 5.;Hirabayaechi, T.; Bock,H. Chem. Ber. 1981, 114, 2609. (3)Ben Cheikh, A.; Dhimane, H.; Pommelet,J. C.; Chuche, J. Tetrahedron Lett. 1988,29, 5919. (4) Chuburu, F.; Lacombe, 5.;Pfieter-Guillouzo, G.; Ben Cheikh, A.; Chuche, J.; Pommelet, J. C. J. Am. Chem. Soc., submitted for publication. (5) Hunter, G.A.; McNab, H. J. Chem. SOC.,Chem. Commun. 1990, 375. (6) McNab, H.; Monahan, L. C. J. Chem. SOC.,Perkin Tiam. 1 1988, 863. 869. ---, --(7) (a) Bock, H.; Solouki, B. Angew. Chem., Znt. Ed. Engl. 1981,20, 427. (b) Bock,H. Phosphorus,Sulfur Silicon Relat. Elem. 1990,49-50, 3. (c) Weatwood, N. P. C. Chem. Soc. Reo. 1989,18,317-343. (d) Schulz, R.; Schweig, A. In Structure and Reactioity; Libman, J. F., Ed.;VCH: New York, 1988; Chapter 8. in i4 io e in .1 4 . io . ie i4 io e ie 14 io . b iw-1 (I Figure 1. Photoelectron spectra of (a) isopropylidene [l(methy1thio)ethylidenelmalonate(3), (b) the pyrolysis of 3 at 893 K, (c) the difference obtained by digitally subtracting acetone (7). from spectrum b, and (d) pure 5-methylthiophen-3(2H)H)-one ky1thio)methylenelketenes has been attempted and their gas-phase reactivity is described. The flash vacuum pyrolysis of the alkylthio Meldrum's acid derivatives is monitored 'in situ" by photoelectron spectroscopy: the compounds are pyrolyzed in the ionization chamber of the spectrometer (short path pyrolysis: SPP). See ref 8 for a detailed description of the apparatus. When submitted to SPP, alkylthio compounds 3 and 5 begin to split off acetone and C02 at 673 K and the re(8) Vallee, Y.; Ripoll, J. L.; Lacombe, s.; Pfister-Guillouzo,G. J. Chem. Res. Synop. 1990,40, J. Chem. Res., Miniprint 401. 0022-3263/91/1956-3~5$02.50/0 0 1991 American Chemical Society Notes 3446 J. Org. Chem., Vol. 56, No. 10, 1991 11.72 L . . 18 . 16 . . 14 & . 8 10 12 6 rp,w, Figure 3. Photoelectron spectrum of 3-methoxythiophene (13) (prepared by Williamson etherification of 3-bromothiopheneunder I * ;e : , 14 ; ; ; LO ; 6 . Ihrl I , la , 1 14 , J 10 1 - 6 1h.J Cu oxides catalysislB). Figure 2. Photoelectron spectra of (a) isopropylidene [bis- (methy1thio)methylenelmalonate(S),(b) the pyrolysis of 5 at 793 K, (c) the difference obtained by digitally subtracting acetone from spectrum b, and (d) pure 5-(methylthio)thiophen-3(2H)-one (8). action is led to completion at 893 K for 3 and 793 K for 5 (disappearanceof the bands corresponding to the starting compounds in Figures l b and 2b). By digitally subtraction of acetone from these spectra, the difference spectra displayed in Figures ICand 2c are obtained. The thiophen3(2H)-ones 7 and 8 were independently prepared under the previously reported pyrolysis conditions (843 K for 3 and 813 K for 5) and their structures confirmed by NMR analysise3 The PES spectra of 7 and 8 are presented in Figures Id and 2d, respectively, and it is observed that they are identical with the difference spectra recorded under SPP conditions (Figures ICand 2c) in spite of the presence of C02in these latter. It is thus concluded that, even under SPP conditions, the methyleneketenes 4 and 6 are not observed and give rise to the heterocycles 7 and 8 between 673 and 793 or 893 K, the evolution of the SPP photoelectron spectra is described as the disappearance of the starting compounds with simultaneous formation of the thiophen-3(2H)-ones, and the possible presence of the methyleneketenes 4 and 6 is not straightforward. We have checked that in the gas phase no keto-enol equilibrium was observed (no evolution of the photoelectron spectra even at high temperature). These results indicate that a sole isomer is present in the gas phase. The identification of this isomer could be inferred from the experimental IPSof the species under study. However, a comparison between calculated and observed IPSdoes not unambiguously solve this question (Figure 4). Actually, different polarization effects (not included in Koopmans's approximation) are expected for each isomer. Moreover, some discrepancies have been observed for molecules bearing third-row a t o m ~ . ~ J ~ Thus our analysis relies on the evolution of the spectra with excitation energy 21.21 eV (He I) and 40.81 eV (He 11)combined with the localization properties of the wave function inferred from MNDO calculations." F. (9) Von Niessen, W.;CI t , I131 I 1101 Exp IF 171 I Exp I - €1 IP I 8.40 9.13 I I 1133 11.72 4 1 2 4 4 4 I 1 2 5 4 /llS I 1281 1234 I I Figure 4. MNDO orbital localizations and eigenvalues of 5methyl- and 5-(methylthio)thiophen-3(2H)-ones(7 and 8) and of 5-methyl-3-hydroxythiophene (10) with the experimental IP observed for 7, 8, and 3-methoxythiophene(13). As a matter of fact, it has been demonstrated that relative band intensities of MOs mainly localized on 3s and 3p AOs of sulfur diminish significantly in the transition from the He I to the He I1 spectrum. On the contrary, the relative band intensities of MOs with high coefficients at oxygen increase on going from He I to He I1 spectra." It follows from the localization properties of the wave functions computed for compounds 7 (keto form) and 10 (enol form) that a different behavior is expected for 7 and 10 on going from He I to He I1 excitation energy. For the enol compound 10, the three first ?r orbitals are described as mixtures of sulfur, oxygen, and carbon AOs (first and J. Org. Chem. 1991,56,3447-3449 third ones) or of sulfur and carbon AOs (second one). It is thus expected that for the first and third MOs the relative band intensities will not vary to a large extent on going from He1 to He11 excitation energy, as the decrease due to the contribution of sulfur AOs should be moderated by the increase arising from oxygen AOs. For the band associated with the ionization of the second MO, the important contribution of carbon AOs should temper the expected decrease arising from the participation of the sulfur AO. These assumptions are in complete agreement with the experimental He I and He I1 spectra of the model methoxy compound 13 featuring the enol form: for this product, as expected, no dramatic change of the relative band intensities is observed on going from He I to He I1 excitation energy, except a very slight decrease of the fourth one at 11.72 eV (ionization of a sulfur lone pair) (Figure 3). On the contrary, for the keto isomer 7, the decrease of the first band intensity and the invariance of the third one (both associated with T ionizations) may be anticipated. For the second band, attributed to the ionization of the oxygen lone pair, an important intensity increase is forecast. The relative intensity changes observed for the studied compound on going from He I to He I1 excitation energy are different from the evolution demonstrated for 13 and support our latter expectations in favor of the keto compound 7 (Figure 1): a strong decrease of the first, 8.78-eV band; a weaker decrease of the third, 11.02-eV band; a significant increase of the second, 9.50-eV band (the strong decrease of the fourth, 11.96-eV band, as for the model methoxy isomer 13, arises from the important contribution of sulfur in a lone pair type MO). We thus conclude the existence of the keto isomer 7 in the gas phase. The attribution of the spectrum of 8 (for which no He I1 spectrum could be recorded due to too weak intensity signals) follows from the description of the spectnun of 7 (Figure 2). The alkylthio substitution should bring about a new ionization associated with the antisymmetric combination of the sulfur lone pairs (ns, - nsJ. In agreement with the stabilization expected from the carbonyl lone pair12on the experimental IPS(8.2 and 8.8 eV) reported for the l,l-bis(methylthio)ethylene,13the bands associated with the ionizations of the symmetric (ns, + nsz) and antisymmetric (ns, - ns,) combinations are thus observed at 8.60 and 9.35 eV, respectively (the first one not very far from the corresponding one of 7 at 8.78 eV). On the other hand, the no lone pair ionization should be observed at the same energetic level in 7 and 8: 9.50 eV. The greater intensity of the 9.35-9.50-eV band of 8 is then accounted for by the attribution of two ionizations arising from the (ns, - nsJ orbital and from the no orbital. Moreover, from the results on the 1,l-bis(methy1thio)-and (methylthio)ethylene,13 it may be anticipated that the ionization related to the (T + ns, + nsJ orbital will be found at a deeper energetic position than for 7 (11.02 eV): indeed for 8 this ionization is hidden in the broad band at 12.34 eV. The observation of the sole keto isomers of these derivatives of thiophen-3(2H)-ones in the gas phase is to be related with the results of their keto-enol equilibrium in solution. It has been shown that the parent thiophen-3(2H)-one (9) and its 2,bdimethyl derivative existed in 75/25 and 90/ 10 keto/enol mixtures, respectively, in CHC13 solu(12) Sandorfy, C.; Lewis, J. W.; Wladislaw, B.; Calegao, J. C. Phosphorus Sulfur Relat. Elem. 1989.6.287. (13) Bock, H.; Wagner, G.; WitGl, K.; Sauer, J.; Seebach, D. Chem. Ber. 1974, 107, 1869. 0022-3263/91/1956-3447$02.50/0 3447 tions6J4and that the percentage of the enol form increased with solvent ability to form hydrogen bonds (100% enol form of 9 in DMSO"). On the contrary, compounds 7 and 8 were reported to exist only in the keto form in CDC13 ~olutions.~ We have checked that, for these two latter compounds 7 and 8, the keto-enol equilibrium was less shifted than for the other previously mentioned thiophen-3(2H)-ones: the keto/enol percentages were found to be 66/34 for 7 and 83/20 for 8 in CD3COCD3and 34/66 and 30/70, respectively, in DMSO-dGfrom NMR spectra. The observation of the keto as the only tautomer for 7 and 8 in CDC13bolsters the argument in favor of the identification of the gas-phase species as the keto tautomer since CDC13,of all the solvents mentioned, is the one most approximating the gas phase (Le., most lacking in H-bonding ability). I t could be pointed out at this stage that, in the MNDO approximation, neglecting correlation effects, the relative energies of the two keto or enol isomers are calculated to be very close (within l kcalmmol-l). In conclusion, compounds 7 and 8 are observed in the keto form both in the gas phase and in CDC13 solution^.^ However, the exclusive formation of the cyclic thiophen3(2H)-ones instead oE the [ (alky1thio)methylenelketenes from the Meldrum's acid derivatives is a different result from that found in the case of the related methoxy compound in this latter case, the (methoxymethy1ene)ketenes were characterized in the gas phase and there was no evidence of furan-3(2H)-one formation. Further investigations dealing with these different behaviors are in progreas. Experimental Section Photoelectron spectra were recorded on an Helectros 0078 photoelectron spectrometer equipped with a 1 2 7 O cylindrical analyzer and monitored by a microcomputer supplemented with a digital analog converter. The spectra are calibrated on the known ionizations of xenon (12.13 and 13.43eV) and argon (15.76 and 15.93 eV). The IPSare accurate within 0.02 eV. The short path pyrolysis system has been described elsewhere? The starting Meldrum's acids were synthesized according to the reported methods.lS The preparations of the thiophen-3(2H)-onesssand 3-metho~ythiophene'~ have been described. Calculations were performed with the AMPAC program" on a Vax computer on fully optimized geometries. (14) Capon, B.; Kwok, F. C. J . Am. Chem. SOC.1989,111,5346. (15) Huang, X.; Chen, B. C. Synthesis 1986, 967; 1987, 481. Bihlmayer, G.A.; Derflimberg, G.;Derkosch, J.; Polansky, 0. E. Monatsh. Chem. 1967,98,564. (16) Kato, S.;Ishizaki, M. Jpn. Kokai Tokkyo Koho, Jap. P. 62108029, 1987; Appl. 245294, 1985; Chem. Abstr. 1988,108(11), 93811b. (17) Dewar, M. J. S.; Stewart, J. J. P. QCPE Bull. 1986, 6, 24. Simple and Convenient Synthesis of tert-Butyl Ethers of Fmoc-serine, Fmoc-threonine, a n d Fmoc-tyrosine J. Gordon Adamson, Mark A. Blaskovich, Hester Groenevelt, and Gilles A. Lajoie* Guelph- Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Receioed August 23, 1990 The protection of the hydroxyl function of serine, threonine, and tyrosine as an acid-labile tert-butyl ether is a well-established strategy for the synthesis of polypeptides when using the base-labile g-fluorenylmethoxycarbonyl (Fmoc) mode of protection for the a-amino 0 1991 American Chemical Society