Der Mann hinter der Base: Hugo Schiff (1834–1915) war ein Schüler Friedrich Wöhlers in Deutschland, verbrachte jedoch seiner freidenkerischen Ansichten wegen fast seine gesamte akademische Laufbahn in Italien, wo er, neben zahlreichen anderen Errungenschaften, die Schiff-Basen (Imine) entdeckte und charakterisierte. Sein wissenschaftliches Wirken erstreckte sich über einen Zeitraum von über 60 Jahren, und es war ihm vergönnt, die Anwendung der Schiff-Basen in [2+2]-Cycloadditionen mit Ketenen zur Bildung von β-Lactamen zu erleben.
Cyclobutenediones 5 disubstituted with HO (a), MeO (b), EtO (c), i-PrO (d), t-BuO (e), PhO (f), 4-MeOC6H4O (g), 4-O2NC6H4O (h), and 3,4-bridging OCH2CH2O (i) substituents upon laser flash photolysis gave the corresponding bisketenes 6a-i, as detected by their distinctive doublet IR absorptions between 2075 and 2106 and 2116 and 2140 cm-1. The reactivities in ring closure back to the cyclobutenediones were greatest for the group 6b-e, with the highest rate constant of 2.95 x 10(7) s-1 at 25 degrees C for 6e (RO = t-BuO) in isooctane, were less for 6a (RO = OH, k = 2.57 x 10(6) s-1 in CH3CN), while 6f-i were the least reactive, with the lowest rate constant of 3.8 x 10(4) s-1 in CH3CN for 6h (RO = 4-O2NC6H4O). The significantly reduced rate constants for 6f-i are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5f-h by the ArO substituents compared to alkoxy groups and to angle strain in the bridged product cyclobutenedione 5i. The reactivities of the ArO-substituted bisketenes 6f-h in CH3CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the sigma p constants of the aryl substituents. Computational studies at the B3LYP/6-31G(d) level of ring-closure barriers are consistent with the measured reactivities. Photolysis of squaric acid (5a) in solution provides a convenient preparation of deltic acid (7).
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ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAcid-catalyzed hydration of di-tert-butylketeneShaikh Habibul Kabir, Hani R. Seikaly, and Thomas T. TidwellCite this: J. Am. Chem. Soc. 1979, 101, 4, 1059–1060Publication Date (Print):February 1, 1979Publication History Published online1 May 2002Published inissue 1 February 1979https://pubs.acs.org/doi/10.1021/ja00498a059https://doi.org/10.1021/ja00498a059research-articleACS PublicationsRequest reuse permissionsArticle Views109Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTNew Tricks from an Old Dog: Bisketenes after 90 YearsAnnette D. Allen, Jihai Ma, Michael A. McAllister, Thomas T. Tidwell, and Da-chuan ZhaoCite this: Acc. Chem. Res. 1995, 28, 6, 265–271Publication Date (Print):June 1, 1995Publication History Published online1 May 2002Published inissue 1 June 1995https://doi.org/10.1021/ar00054a004RIGHTS & PERMISSIONSArticle Views265Altmetric-Citations46LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts
The hydrolysis of triethyl phosphate in water and in 35% dioxane – 65% water has been examined. Hydrolysis in neutral water proceeds with a rate constant of 8.35 × 10 −6 s −1 at 101°, ΔH* = 23.4 kcal/mol, ΔS* = −20 e.u., a solvent isotope effect [Formula: see text] of 1.3, C—O bond cleavage as shown by 18 O labeling, and no catalysis by 0.5 M sulfuric acid. These results are consistent with the B AL 2 mechanism of hydrolysis and the same pathway is indicated for the reaction in neutral 35% dioxane –65% water. Perchloric acid catalyzes the reaction in dioxane–water with C—O bond cleavage in 0.904 M acid, ΔH* = 24.1 kcal/mol, ΔS* = −17 e.u., and the solvent isotope effect [Formula: see text] in 0.556 M acid. These results indicate that the A AL 2 pathway of hydrolysis is followed under these conditions. The reactivity of triethyl phosphate is compared with that of ethyl acetate.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTKinetic and Theoretical Studies of Ring Closure of Unstabilized Bisketenes to CyclobutenedionesAnnette D. Allen, Jim D. Colomvakos, Ian Egle, Janusz Lusztyk, Michael A. McAllister, Thomas T. Tidwell, Brian D. Wagner, and Da-chuan ZhaoCite this: J. Am. Chem. Soc. 1995, 117, 28, 7552–7553Publication Date (Print):July 1, 1995Publication History Published online1 May 2002Published inissue 1 July 1995https://pubs.acs.org/doi/10.1021/ja00133a032https://doi.org/10.1021/ja00133a032research-articleACS PublicationsRequest reuse permissionsArticle Views147Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
Eleven stationary points on the singlet C 2 H 2 F 3 + potential energy surface have been calculated using the 3-21G basis set, and characterized as minima (four structures) or first-order saddle points (seven structures) by vibrational analysis. To check the reliability of this basis set, three of the structures have also been optimized at the 6-31G* level; although the geometries change somewhat, the relative energies and nature (maxima, minima) of the structures remain the same. For CF 3 CH 2 + the minimum energy structure has one C—F bond coplanar with the vacant p-atomic orbital at the cationic centre. The structure is 16.4 kcal/mol less stable than the lowest energy conformation of FCH 2 CF 2 + , and the barrier for the 1,2 fluorine migration which connects the two structures is low. The cation F 2 CHCHF + has a conformation that is a minimum on the potential energy surface that is 16.9 kcal/mol higher in energy than FCH 2 CF 2 + ; the two structures are separated by a barrier for 1,2 hydrogen migration of 23.5 kcal/mol. The electronic effects in the various structures have been studied using a quantitative PMO analysis of the interactions between the two carbon fragments of the ions. For CF 3 CH 2 + the net effect of the fluorine is highly destabilizing; the principal stabilizing interactions between CF 3 + and CH 2 consist of π donation from CF 3 + to CH 2 and homoconjugation of a fluorine lone pair with the cationic centre. No net stabilization attributable to fluorine bridging could be found.
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