structure of the products makes them interesting from the
synthetic, theoretical, and physical viewpoints alike. Ad-
ditionally this compound class might also gain application
in materials science in the future.
the C-O bond length is 1.255 Å.11 The N-N bonds at 1.301
and 1.314 Å are also considerably shorter than the typical
data for tetrazines (e.g., 1.330 Å for di(2-pyrimidyl)-
tetrazine12). These data point to a quinoidal-like structure,
although the C1-C11 bond, which was expected to have a
double bond character, is quite elongated at 1.46 Å. This
might be attributed to the fact that the two heterocyclic rings
are tilted by ca. 43°, which should result in a decreased p-p
overlap. Such a distortion, although quite rare among organic
compounds, might still correspond to a singlet state of a
sterically crowded double bond.13 To rule out the mesomeric
zwitterionic form of 3a, we compared the UV spectra of 3a,
1, and di(3-pyridyl)-tetrazine in solvents of different polar-
ity.14 The obtained ∆λmax values (12, 14, and 12 nm,
respectively) do not support the zwitterionic nature of 3a.
This is ruled out both by the X-ray data and by UV
measurements.15
The reaction of the nucleophilic carbene derived from 1,3-
dimesityl-imidazolium tetrafluoroborate (2a) and dipyrazolyl-
tetrazine (1) gave a new compound (3a) in excellent yield
(Table 1). Spectroscopic investigation of 3a revealed a so
Table 1. Influence of the Base and Solvent on the Reaction of
3,6-Bis(3′,5′-dimethylpyrazolyl)-tetrazine (1) and
1,3-Bis(mesityl)-imidazolium Tetrafluoroborate (2a)
The presence of a carbonate base was found to be crucial
for the formation of 3a (Table 1, entries 1-3 vs 4-7). These
carbonates probably have a dual role in the process: besides
generating the carbene, the oxygen atom in the product also
originates from this species. The nature of the carbonate and
the solvent has only a limited influence on the process
because sodium, potassium, and cesium salts were equally
efficient (entry 4). The nucleophilicity of the carbonate ion
is important in the transformation because using an excess
of sodium hydrogencarbonate and prolonged heating (entry
3) we obtained 3a in poor yield. Of the nonnucleophilic
solvents tested, acetonitrile and tetrahydrofurane were chosen
for further studies because they combined acceptable solubil-
ity and a facile workup. A priori carbene generation was
found beneficial (entry 7).
entry
base
solvent
yield
1
2
3
4
5
6
7
NaH
MeCN or THF
MeCN or THF
MeCN
DMA
PhMe
-
EDIPA or Et3N
NaHCO3
K2CO3
K2CO3
K2CO3
traces
40%a
80%b
92%
95%
96% (83%c)
THF
MeCN
K2CO3
a
b
5 equiv of NaHCO3, 41 h heating. Na2CO3 and Cs2CO3 gave 3a in
c
80% and 83% yield, respectively. All reagents were mixed at the same
time.
To establish the scope of this transformation, a series of
NHC precursors (2a-l)16 were reacted with 1 (Table 2).
The experiments17 revealed that the nature of the carbene
backbone has a substantial influence on the efficiency of the
process. Reactants based on the imidazole framework were
all very efficient, and the products 3a-g were all isolated
in good yield (entries 1-7). N-Aryl18 (entries 1 and 2) and
N-alkyl (entries 3-7) derivatives worked equally well; the
far unprecedented structure, which was confirmed by X-ray
crystallography (Figure 1). The C-O bond in 3a has a strong
double bond character (1.235 Å) as compared to the
analogous 1,5-dimethyl-3-(3′-pyrazolyl)-6-oxotetrazane, where
(11) Wu, J.-Z.; Bouwman, E.; Reedijk, J.; Mills, A. M.; Spek, A. L.
Inorg. Chim. Acta 2003, 351, 326.
(12) Glockle, M.; Hubler, K.; Kummerer, H.-J.; Denninger, G.; Kaim,
W. Inorg. Chem. 2001, 40, 2263.
(13) Sulzbach, H. M.; Bolton, E.; Lenoir, D.; Schleyer, P. v. R.; Schaefer,
H. F., III. J. Am. Chem. Soc. 1996, 118, 9908.
(14) For details see the Supporting Information.
(15) ESR studies on 3a, 3c, and 3e ruled out the presence of any radical
character both in solution and in the solid state.
(16) (a) Paczal, A.; Be´nyei, A. C.; Kotschy, A. J. Org. Chem. 2006, 71,
5969. (b) Guillen, F.; Winn, C. L.; Alexakis, A. Tetrahedron: Asymmetry
2001, 12, 2083. (c) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles,
J. H.; Melder, J.-P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. 1995, 34,
1021. (d) Herrmann, W. A.; Koecher, C.; Goossen, L. J.; Artus, G. R. J.
Chem.-Eur. J. 1996, 2, 1627.
(17) General procedure: 1 equiv of the carbene precursor (2a-l) and 1
equiv of K2CO3 were stirred in the degassed solvent in a capped vial at
75-80 °C for 40 min under an argon atmosphere. The preheated solution
of 1 (1 equiv) in the appropriate solvent was added via cannula, and the
mixture was stirred at the same temperature until all starting material was
consumed as judged by TLC. After removal of the volatiles in a vacuum,
the product was purified by column chromatography on silica gel, using
methanol/ethylacetate (1:4) as eluent.
Figure 1. ORTEP diagram of compound 3a (thermal ellipsoids
set at 50% probability) with partial numbering scheme. Selected
bond lengths (Å): C1-N2 1.341(4), C1-N5 1.344(4), C3-C4
1.337(4), C14-O11 1.235(4), C14-N13 1.384(5), C14-N15
1.386(4), N12-N13 1.314(4), N15-N16 1.301(4), C11-N12
1.332(4), C11-N16 1.333(4), C1-C11 1.464(4). Torsion angle
(deg): N2-C1-C11-N12 43.4.
(18) N,N′-Bis(2,6-diisopropyl-phenyl)-imidazolium salts (IPr) and their
dihydro analogue (SIPr) led to the decomposition of 1 without traces of 3.
3438
Org. Lett., Vol. 9, No. 17, 2007