Synergetische Destabilisierung durch geminale Estergruppen

Abstract

Dem anomeren Effekt geminaler Donorsubstituenten sollte ein inverser Effekt geminaler Acceptorsubstituenten entsprechen. Dies konnte für die Ester 1 und 2 durch Ableitung von Gruppeninkrementen bestätigt werden: Die Diester werden um ca. 3 kcal mol‐1 gegenüber Monoestern und der Triester wird um ca. 6 kcal mol‐1 gegenüber Diestern destabilisiert. Die Reaktivität dieser Verbindungen (z. B. ihre Acidität) erscheint mit diesem Befund in einem neuen Licht (R = H, Me).

Thermochemical study of aliphatic and phenyl-substituted geminal diacetates

“…"Taken from the specific enthalpies of combustion in Table I11. J'From the measurements of vapor pressures at different temperatures from The procedure of the calculation and the definition of strain-free increments for esters (alkoxycarbonyl group), ethers (alkoxy group), and acetates (acyloxy group) is known from previous work [1][2][3]23]. A summary of strain-free increments used is recorded in Table VI.…”

“…Using these increments we estimated the geminal interaction AAHgem of two acetate substituents in the manner described previously [1][2][3]. They are listed in Table VII.…”

“…As a result it was shown that electron donating substituents like alkoxy groups (in acetals [2,11] and ortoesters [7]) or amino groups (in aminales [8]) experience synergetic stabilization, generally called "anomeric stabilization," which is mainly caused by 7r-a* conjugation (a through bond effect). On the contrary, combinations of electron-withdrawing substituents (CN, COOR, COR) show a destabilizing synergetic effect [1,5,9,13] mainly caused by repulsive interaction through space. We turn our attention in this publication to the geminaI interaction of .…”

“…1. We analyzed the geminal interaction from the standard enthalpies of formation in the gas state AfH~ of 1 and 2 in the previously described manner [1][2][3]. They are obtained from the calorimetrically measured enthalpies of combustion AcH ~ and enthalpies of sublimation A~ubH ~ or enthalpies of vaporization Avapn o by a transpiration method [17a].…”

“…The systematic quantitative investigation of the dependence between the energetics of geminal substituted molecules and their structures has received special interest in this laboratory [1][2][3][4][5][6][7][8][9][10][11] recently. We provided a systematic definition of the geminal interactions of different combinations of substituents, donors (OR, NR2, F) or acceptors (CN, CO2R, COR) and we investigated the interaction of two or three different or identical substituents on the central C-atom.…”