Notes and references
z Using a similar protocol, 1ꢀHCl was prepared in 96% yield from
N,N0-bis(2,6-di-isopropylphenyl)formamidine. DAC
1
was also
generated via deprotonation of 1ꢀHCl with NaHMDS in aromatic
hydrocarbons.
y Under an inert atmosphere, 2 was found to be indefinitely stable in
the solid state (mp = 166–168 1C; dec.) and for several days in
solution (C6D6, C7D8 and 1,4-dioxane).
z A triazolylidene was reported to insert into the NH bond of
morpholine, see: D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H.
Teles, J.-P. Melder, K. Ebel and S. Brode, Angew. Chem., Int. Ed.
Engl., 1995, 34, 1021.
8 If the pKas of the conjugate acids of the DACs described herein are
significantly lower than those of typical NHCs (16–24),12 carbene
generation via NH3-facilitated deprotonation (followed by NH3
activation) may be possible. However, Et3N was found to be
insufficiently basic to generate 2 from 2ꢀHCl or 1 from [1H][OTf].
** At temperatures greater than 200 1C, DAC decomposition was
observed and independently confirmed by performing analogous
high temperature experiments under an inert atmosphere in sealed
vessels.
Scheme 5 Proposed mechanism for the formation of NH3 adducts 5
and 6 from their respective protonated precursors.
(generated in situ from 1ꢀHCl and NaHMDS) at ꢁ78 1C,
which resulted in the formation of the expected product (6) in
high yield (Scheme 4). Similarly, condensing NH3 onto solid 2ꢀ
HCl, 1ꢀHCl or [1H][OTf] (OTf = triflate) followed by warming
to ambient temperature also resulted in the formation of 5 and
6, respectively, in excellent yields. The results of the reactions
involving the HCl adducts or [1H][OTf] were in accord with
previous observations7,8 that [1H][OTf] and 2ꢀHCl readily
react with alcohols and water to form the corresponding
alkoxy or hydroxy adducts, respectively.w As such, we suspect
that NH3 added directly to the C1 position of [1H][OTf]
(Scheme 5) or displaced the chloride atoms in the afore-
mentioned HCl adducts via a nucleophilic type substitution
reaction.8 The latter is supported by the relatively long C–Cl
bond length observed in the solid state structure of 2ꢀHCl
(Fig. 2), an indicator of a weakened chloride bond.
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While it is tempting to surmise that the free DACs 1 and 2
reacted with NH3 in a similar fashion as their protonated
adducts (i.e., via the formation of a Werner-type complex4),
this requires the buildup of negative charge on the carbene
nucleus. Alternatively, these DACs may be activating the N–H
bond of NH3 in a concerted fashion and in a manner
analogous to that proposed for the CAACs.5 For example, 5
was observed to form rapidly upon bubbling NH3 through a
dilute solution of 2 in C6D6 ([2]0 = 8.9 ꢃ 10ꢁ2 M, 25 1C).
Unfortunately, no reaction was observed between 2 and H2,
even under forcing conditions (1000 psi H2, 200 1C) in C6H6 or
1,4-dioxane.** This lack of reactivity was surprising considering
the similar bond-dissociation energies of NH3 and H2 (107 and
104 kcal molꢁ1, respectively)10,11 and given that Bertrand’s
alkyl amino carbenes, a class of substrates that shares many
similar reactivities as the DACs, were reported to activate both
NH3 and H2.5 Further investigation is underway.
S. Yao, R. Muller, M. Kaupp and M. Driess, J. Am. Chem. Soc.,
¨
2010, DOI: 10.1021/ja1031024.
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In conclusion, we report the synthesis and first solid state
characterization of a free DAC, and demonstrate the ability of
this carbene to split ammonia. These results expand the few
known organic5 systems capable of activating an industrially
important small molecule. An attractive feature of the substrates
reported herein is that they can be obtained in high-yield over
two steps starting from inexpensive, commercially-available
materials and are readily-amenable to further modification.
As a result, we believe that DACs offer promise in the
development of new synthetic strategies for transforming NH3
into structurally- and functionally-diverse nitrogen-containing
compounds.
6 For reviews on NHCs and related compounds, see: D. Bourissou,
O. Guerret, F. P. Gabbaı and G. Bertrand, Chem. Rev., 2000, 100,
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39; K. Ofele, E. Tosh, C. Taubmann and W. A. Herrmann, Chem.
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We are grateful to the National Science Foundation
(CHE-0645563 and 0741973), the Robert A. Welch Foundation
(F-1621), the Alfred P. Sloan Foundation, and the Arnold and
Mabel Beckman Foundation for their generous financial
support.
ꢂc
This journal is The Royal Society of Chemistry 2010
4290 | Chem. Commun., 2010, 46, 4288–4290