Die Kinetik der entartetem CopUmlagerungen der Titelverbindungen wurde mittels dynamischer "C-NMR-Spektroskopie gemessen. Die aus den NMR-Linienformiinderngen abgeleiteten Reaktionsgeschwindigkeitskonstanten k lieferten auf der Basis der Eyring-Gleichung folgende Aktivierungsparameter. Semibullvalen (1) A H * = 5.2 kcal mol-', AS* = -3.2 cal K-I mol-I, AG& = 6.2 kcal mol-I; 1,5-Dimethylsemibullvalen (2) AH * = 4.5 kcal mol-I, AS* = -1.6 cal K-' mol-I, AG& = 5.0 kcal mol-I; 2,6-Dibrom-l,5-dimethylsemibullvalen (3) A H * = 7.5 kcal mol-I, AS' z 0 cal K-' mol-I, AG& = 7.4 kcal mol-I.Die Substituenteneffekte werden mit denen anderer substituierter Semibullvalene verglichen und im Hinblick auf die Natur des ubergangszustandes der Cope-Umlagerung in iiberbriickten Homotropilidenen diskutiert. Die f i i 2 bestimmte Barriere ist die niedrigste, die bisher mit der dynamischen NMR-Spektroskopie gemessen wurde.Der Mechanismus der Cope-Umlagerung wird nach wie vor intensiv studiert '), und insbesondere der SubstituenteneinfluB auf die Valenztautomerie iiberbriickter Homotropilidene vom Barbaralanund Semib~llvalentyp~." ist im Zusammenhang mit synthetischen Arbeiten4) von Interesse. Fur die drei mechanistischen Alternativen der Cope-Umlagerung - Wir haben fruher uber Ergebnisse an 3,7-disubstituierten Barbaralanen berichtet", nach denen Unterschiede fur die Struktur des ubergangszustandes der Barbaralan-und der Semibullvalen-Umlagerung erwartet werden konnen. So ergab sich, da13 Cyangruppen in 3,7-Position die Aktivierungsbarriere im Barbaralan kaum beeinflussen, im Semibullvalen dagegen den Grundzustand starker stabilisieren als den ubergangszustand. Dies konnte bedeuten, daD der ubergangszustand der Semibullvalen-Valenztautomerie einen starkeren Diallylcharakter aufweist. Da das H O M O des Allylsystems am mittleren C-Atom einen Knoten besitzt, sollte der SubStituenteneinfluD auf den 'Ubergangszustand der Cope-Umlagerung dann weniger stark zur Geltung kommen.Die NMR-Spektroskopie ist nach wie vor die Methode der Wahl fur die Untersuchung entarteter Gleichgewichtsreaktionen der hier interessierenden Art, insbesondcre da neuerdings auch Messungen rnit Festkorpern durchgefiihrt werden konnen *I. In der vorliegenden Arbeit berichten wir uber "C-NMR-Messungen an den Semibullvalenen 1-3, die zum Ziel hatten, den EinfluB der 1,5-Dimethylsubstitution sowie den zusatzlichen Effekt zweier Bromatome in Chem. Ber. 122 (1989) 925-931 0 VCH Verlagsgesellschaft mbH, D
873 rate ( Table I, entries 1-3, 5, and 6, and Figure 1): indicative of Newtonian behavior. (14) Cohen, Y.; Metzner, A. B. J. Rheol. 1985, 29, 67. (15) Brackman, J. C.; van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988, 4, 1266. Binding of PVME or PPO to spherical CTAB micelles'0 is driven both by a reduction of the unfavorable core-water contact and by a favorable Gibbs energy for transfer of polymer segments from the aqucous to the micellar environment. See also: Ruckenstein, E.; Huber, G.; Hoffmann, H.
In order to examine the possible preference for anti or syn attack in bicyclic cyclobutenes, a series of heterocyclic bicyclo-[2.2.O]hcxcncs, 3 -8, has bccn subjcctcd to thc reaction towards 1.3-dipolcs, particularly diazoalkanes. All dipolar cycloadditions exclusively lead to unri-configurated tricyclic products with varying regiochemistry in the cases of 4-8. Similarly, the Pd-catalyzed methylene transfer reactions only yield unri products. These results are in remarkable contrast to the known cycloaddition of, e. g., dichlorocyclobutene (1) or the carbonate 24.The syn addition of 1,3-dipoles to cis-3,4-disubstituted cyclobutenes (e.g. 1 to 2) represents a remarkable phenomenon because of the contra-sterical cffects coming into force in this reaction". 6, R=CH3 7, R = H As possible explanations for that effcct intramolccular interactions in the cyclobutene, pyramidalization at the olefinic C atoms and intermolecular LUMO(dipo1e)-n(cyc1obutene) interactions have been postulated. In order to investigate the influence of hetero substituents in 3,4-poition in a clearly defined sterical arrangement the reactivity of the bicycles 3-B3' with diazoalkanes 9a-e has been studied.In addition to that the bridging of the substituents in 3,4-position facilitates structural assignments to the tricyclic adducts, since in these adducts the central four-membered ring should be nearly planar. Thus, it should be. possible to differentiate syn and anti adducts with the help of the dependence of the vicinal coupling on dihedral angles. Extraordinarily large trans couplings, as can be observed in bicyclic adducts of 3,edichloro-or 3,4-dibromocyclobutenes, are therefore not to be expected. Part of the work (regarding compound 3) has been published in preliminary form4).The reaction of diazomethane with 3 yields 50% of the tricyclic pyrazoline 10a. Apart from 55% of a photoproduct, on which we will report separately, photolysis of the pyrazoline 10a yields 45% of the housane l l a . Both, NMR spectroscopical data of 10a and l l a , prove the anti structure and therefore the anti cycloaddition. Hz) appears at 6 = 4.30. Since in a syn structure each proton of the four-membered ring should show two large cis couplings (dihedral angle ca. 00) the single doublet split proves the anti structure. This doublet can be assigned to 2-H by means of I3C-NMR spectroscopy. Thus, in the 'Hcoupled "C-NMR spectrum the C-2 signal shows a more complex fine structure than that of C-5 and can be correlated with the doublet proton resonance at 6 = 4.30 by means of selective 'H decoupling. The proton 5-H, vicinal to the azo group, absorbs at lower field (6 = 4.75).In the case of the housane l l a the anti geometry can be proved by the missing W coupling. The signal of the ex0 proton 3-H, (Rz = H) appears as a distinct quartet caused Chem.
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