2,2,6,6-Tetramethylcyclohexanethione S-methylide, a highly hindered thiocarbonyl ylide: two-step cycloadditions
The switching from the concerted 1,3-dipolar cycloaddition to a two-step pathway via zwitterionic intermediates requires a major energy difference between HOMO–LUMO energies of 1,3-dipole and dipolarophile, as well as sterically demanding reactants. In contrast to previously studied models, the titl...
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Published in | Tetrahedron Vol. 61; no. 25; pp. 6143 - 6153 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
OXFORD
Elsevier Ltd
20.06.2005
Elsevier |
Subjects | |
Online Access | Get full text |
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Summary: | The switching from the concerted 1,3-dipolar cycloaddition to a two-step pathway via zwitterionic intermediates requires a major energy difference between HOMO–LUMO energies of 1,3-dipole and dipolarophile, as well as sterically demanding reactants. In contrast to previously studied models, the title compound
1C
, a thiocarbonyl ylide prepared by N
2 extrusion from dihydrothiadiazole
7C
at 80
°C, combined with 2,3-bis(trifluoromethyl)fumaronitrile (
11
) to give a zwitterion (
gauche-
10
); the latter failed to close the thiolane ring by 1,5-cyclization, but formed the seven-membered ketene imine
9C
by 1,7-cyclization. X-ray analysis of
9C
revealed an angle-deformed cumulated bond system and a
transoid relation of the CF
3 groups. The relatively stable
9C
allowed
19F NMR recordings from −90 to +90
°C; temperature-dependent line broadening resulted from equilibration with ≤1% of an unknown isomer. Among various possible angle-strained rate processes, an inversion
transoid
19
⇌
cisoid
20
is preferred which involves a topomerization at the C
N bond; lateral inversion and rotation are discussed. At 80
°C in solution, ketene imine
9C
slowly suffered fragmentation to give
trans- and
cis-1,2-bis(trifluoromethyl)cyclopropane-1,2-dicarbonitrile (
13
)+thioketone
6C
by intramolecular substitution. The reaction of
1C
with ethenetetracarbonitrile furnished a tetracyanothiolane
3C
, whereas
1C
and dimethyl 2,3-dicyanofumarate (
(
E)-
26
) afforded thiolanes of the same
trans,
cis-ratio as
1C
with dimethyl 2,3-dicyanomaleate (
(
Z)-
26
); a preceding (
E,
Z)-equilibration of
26
thwarts mechanistic conclusions. When the solvent contained water or methanol, short-lived ketene imines
4C
and
31
were intercepted.
Graphical Abstract |
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ISSN: | 0040-4020 1464-5416 |
DOI: | 10.1016/j.tet.2005.02.062 |