Communications
temperature, it does not only induce the cyclization
À
of 1, but a B H bond cleavage also occurs. The
formation of adduct 7 (94% yield) pointed towards
the electrophilic character of the 2 position of the
putative, first formed, dithiolium 6. To evaluate the
scope of this s–s bond cleavage, we then reacted 1
with the weakly Brønsted acidic 2,6-dimethylthio-
phenol, and observed the clean formation of adduct 8
(94% yield). Further decreasing the acidity of the
substrate, methanol was added to ethynyl dithiocar-
bamate 1, and adduct 9 was quantitatively formed
after 5 min at room temperature. The same com-
pound 9 was also readily obtained in 97% yield by
addition of sodium methoxide to dithiolium trifluor-
omethanesulfonate 2.
It is difficult to believe that methanol is acidic
enough to protonate the carbon–carbon triple bond
of 1, but mesoionic carbenes are strongly basic.
Therefore, we envisaged the existence of an equilib-
rium between the ethynyl dithiocarbamate 1 and its
MIC isomer A. Calculations at the RI-BP86/TZVPP
level of theory[14] predict that A is 8.3 kcalmolÀ1
higher in energy than its acyclic form 1, with a
small energy barrier (0.8 kcalmolÀ1) for the ring-
opening process. To test experimentally the possible
existence of such equilibrium, we first reacted 1 with
Scheme 4. The reactivity of 1 does not parallel the acidity of the substrates,
indicating the key role played by the heteroatom lone pair of the substrate. The
À
N H bond cleavage of methyl hydrazine occurs at the more basic nitrogen.
À
Bond activation by 1 is not limited to X H bonds, as shown by the cleavage of
diphenyldisulfide.
carbon dioxide and elemental sulfur, which are known to
readily react with singlet carbenes[15] and MICs,[12] and very
unlikely to interact with the carbon–carbon triple bond of 1,
at least at room temperature.[16] After 30 min, the corre-
sponding MIC adducts 10 and 11 were isolated in 98 and 97%
yield, respectively (Scheme 3), and fully characterized by
single-crystal X-ray diffraction studies (see the Supporting
Information).
not electrophilic enough to react with the carbon–carbon
triple bond of 1, or the carbene center of MIC A, there is a
collaborative effect between the nucleophilic and electro-
philic centers of 1 as schematically represented by I. Indeed,
we found that N-(2,6-diisopropylphenyl)-N-methylamine,
which is obviously more acidic than diethylamine but more
hindered, does not react with 1, suggesting that a heteroatom
lone pair of the substrate has to interact with the electrophilic
center of 1 to allow for the reaction. A confirmation of this
hypothesis was brought by the reaction of 1 with methyl
hydrazine, which cleanly led to the formation of 15 (82%
yield). This adduct, which has been characterized by single-
crystal X-ray diffraction studies, features the more basic
nitrogen atom bonded to the electrophilic C-2 center. Clearly,
a simple deprotonation of hydrazine would occur at the
nonsubstituted nitrogen, which would have led to
adduct 16.[18]
To show that the scope of the bond activation with ethynyl
dithiocarbamate 1 is broad, 1 was reacted with diphenyldi-
sulfide, a substrate that has been used to demonstrate the
efficiency of frustrated Lewis pairs.[4,19] The reaction was
complete in 15 min at room temperature and the expected
adduct 17 was isolated in 91% yield.
These results as a whole demonstrate that a simple ethynyl
dithiocarbamate can readily activate at room temperature a
variety of bonds. This is partly due to the existence of an
equilibrium with the nonobservable mesoionic carbene
isomer, but also to the cooperative effect of the nucleophilic
and electrophilic centers of the ethynyl dithiocarbamate.
Interestingly, in the resulting adduct, the fragment of the
substrate that binds to the C-2 carbon should present a
nucleophilic character because of the presence of the other
heteroatoms. On the other hand, the bond between the C-5
Scheme 3. Reactivity of 1 with carbon dioxide and elemental sulfur,
suggesting the existence of an equilibrium between 1 and A.
Continuing our investigations, we observed that the very
weakly acidic diethylamine and diethylphosphine are cleaved
at room temperature, affording adducts 12 and 13, in 78 and
56% yield, respectively[17] (Scheme 4). In contrast, phenyl-
acetylene, which is more acidic, does not react with 1. We
verified that lithium phenylacetylide reacts with dithiolium
salt 2 to give 14 (90% yield), demonstrating that 1 and even
the putative mesoionic carbene A were not able to deproto-
nate phenylacetylene.
Since the observed reactivity of 1 does not parallel the
acidity of the substrates, we postulate that, when the latter is
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9923 –9925