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Preparation and Chemistry of an Unexpectedly Stable α‐Oxoketene−Pyridine Zwitterion, 2,2‐Bis( tert ‐butylcarbonyl)‐1‐[4‐(dimethylamino)pyridinio]ethen‐1‐olate
Author(s) -
Kollenz Gert,
Holzer Susanne,
Kappe C. Oliver,
Dalvi Turkaram S.,
Fabian Walter M. F.,
Sterk Heinz,
Wong Ming Wah,
Wentrup Curt
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200104)2001:7<1315::aid-ejoc1315>3.0.co;2-2
Subject(s) - zwitterion , chemistry , pyridine , medicinal chemistry , nucleophile , cycloaddition , acetonitrile , aniline , polymer chemistry , organic chemistry , molecule , catalysis
Treatment of dipivaloylketene 1 and its dimers 2 and 3 with 4‐(dimethylamino)pyridine (DMAP) in acetonitrile affords the α‐oxoketene‐pyridine zwitterion 4 as a pale yellow solid. This is the first example of a stable zwitterion obtained from a true α‐oxoketene. In anhydrous solution at room temp., compound 4 largely cleaves into the reactants 1 and DMAP. At −60 °C the equilibrium is shifted entirely towards the zwitterion 4 , as shown by 13 C NMR measurements. Calculations using density functional theory (B3LYP/6−311+G**) are in excellent agreement with the formation and relative stability of 4 . 15 N labelling experiments demonstrate that the ring nitrogen atom of DMAP is involved in generating the new zwitterionic C−N bond. Reactions of 4 with NH and OH nucleophiles in solution afforded the corresponding dipivaloyl acetic acid derivatives 6 and 7 , whereas acetone or benzylidene aniline undergo cycloaddition reactions of the hetero‐Diels−Alder type.