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Reactions of Sterically Hindered ‘Thiocarbonyl Ylides' with 1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Isolation of a Cyclic Seven‐Membered Ketene Imine
Author(s) -
Huisgen Rolf,
Mlostoń Grzegorz,
Langhals Elke,
Oshima Takumi
Publication year - 2002
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(200209)85:9<2668::aid-hlca2668>3.0.co;2-3
Subject(s) - ketene , chemistry , imine , stereocenter , ylide , steric effects , diastereomer , trifluoromethyl , intramolecular force , medicinal chemistry , stereochemistry , cycloaddition , organic chemistry , alkyl , enantioselective synthesis , catalysis
When ‘thiocarbonyl ylide' 1A (=(2,2,4,4‐tetramethyl‐3‐oxocyclobutylidenesulfonio)methanide) is generated from the dihydrothiadiazole 5A by N 2 extrusion at 40° in the presence of 2,3‐bis(trifluoromethyl)fumaronitrile (( E )‐ 10 ), a cyclic seven‐membered ketene imine 11 and trans ‐thiolane 12 are formed (81 : 19). The reaction of 1A with ( Z )‐ 10 furnishes 11, 12 , and cis ‐thiolane 25 in the ratio of 82 : 12 : 6. The strained ketene imine 11 is crystalline and storable as a consequence of the stabilizing ‘perfluoroalkyl effect'. The ketene imine group is stereogenic; 11 has a transoid structure with respect to the CF 3 groups, and there is no evidence for the cisoid diastereoisomer. Ketene imine 11 adds H 2 O, MeOH, and PhNH 2 . In solution at 60°, 11 undergoes an irreversible ring contraction, furnishing the thiolanes 12 / 25 98 : 2. The rate constant of this first‐order rearrangement increases 850‐fold, as the solvent polarity rises from cyclohexane to CD 3 CN, in accordance with a zwitterionic intermediate. It is the same intermediate that is initially formed from 1A and 10 , and its intramolecular N ‐ and C ‐alkylation give rise to 11 and 12 + 25 , respectively. In contrast to 1A , thiocarbonyl ylide 27 , which harbors the sterically less‐demanding adamantylidene group, reacts with ( E )‐ 10 to give trans ‐thiolane 29 , but no ketene imine. The precursor 26 catalyzes the ( Z )/( E ) isomerization of 10 (( E )/( Z ) ca. 95 : 5 at equilibrium), thus obviating conclusions on steric course and mechanism of this cycloaddition.