Stable planar six-π-electron six-membered N-heterocyclic carbenes with tunable electronic properties

Article abstract of DOI:10.1021/ja052987g

Carbene analogues of borazines are highly thermally stable. Keeping quasi-identical steric demands, the electronic properties of the carbene can be precisely tuned by varying the nature of the substituents at the boron centers. Copyright

Full text of DOI:10.1021/ja052987g

Published on Web 06/30/2005
Stable Planar Six-π-Electron Six-Membered N-Heterocyclic Carbenes with
Tunable Electronic Properties
Carsten Pra¨sang, Bruno Donnadieu, and Guy Bertrand*
UCR-CNRS Joint Research Chemistry Laboratory (UMR 2282), Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403
Received May 6, 2005; E-mail: gbertran@mail.ucr.edu.
Scheme 1
The last 15 years have seen tremendous advances in carbene
chemistry that transformed these electron-deficient species1 into the
realm of isolable compounds2 and powerful tools for synthetic
chemists. Despite the existence of several families of stable
carbenes, it has to be recognized that, so far, only the cyclic diamino
carbenes (NHCs) have found numerous applications. NHCs are
excellent catalysts on their own,3 and there is increasing experi-
mental evidence that NHC-metal catalysts surpass their phosphine-
based counterparts in both activity and scope of application.4 Having
said that, it is clear that the success of phosphorus- and nitrogen-
based ligands was, and is still, due to their enormous structural
Scheme 2
diversity. So far, the structures of stable NHCs available have been
largely restricted to saturated and unsaturated five-membered
heterocycles A5 and B,6 the only exceptions being carbenes C-F7
(Scheme 1). Moreover, it has been shown that free acyclic carbenes,
including diamino carbenes,8 are far more fragile than NHCs and
are poor ligands for transition metal complexes.9 Therefore, it is
of interest to design new stable cyclic diamino carbenes possessing
different scaffolds and electronic structures.
Borazines G, discovered by Stock10 in 1926, and currently used
for the fabrication of advanced material based on boron nitride,11
constitute a family of extremely stable heterocycles. Because they
are isoelectronic to benzene, they are often regarded as the
archetypical example of an inorganic aromatic compound, although
the extent of aromaticity of G in relation to that of benzene is still
debated.12 Since boranes are isoelectronic with both carbocations
and carbenes, it was reasonable to believe that the mixed organic/
inorganic heterocycles H and I would be quite stable. Importantly,
the lone pairs of the nitrogen atoms, adjacent to the carbene center
of I, can interact both with the carbene and with the boron vacant
c, which were isolated after recrystallization from hexanes as light-
orbitals. Varying the nature of the substituents at the boron centers,
yellow crystals13 (Scheme 2).
and thus modifying their Lewis acidity, should allow for the
Both types of heterocycles 4 and 5 are perfectly stable at room
preparation of carbenes with quasi-identical steric demands, but
temperature, in solution, and in the solid state. The single-crystal
different electronic properties; this would permit a better under-
standing of the factors governing the catalytic activity of the ensuing
X-ray diffraction study15 of 4c and 5c (Figure 1) reveals, in both
cases, the expected planar six-membered ring skeleton with a
propeller-like arrangement of substituents. The cyclohexyl rings
feature the most protecting conformation for the electron-deficient
complexes.
Derivative 4a13 was obtained in one step (83% yield) by treatment
of N-silylated formamidine 1 with compound 2. To replace the
carbon center. As always observed, when comparing the geometric
amino group at the boron centers by a weaker π-donor substituent,
parameters of NHCs and those of their NHC(H+) precursors, the
a different synthetic approach has been designed; indeed, 1,3-
N2-C1-N3 angle for carbene 5c (114.5°) is more acute than that
dibromo-1,3-diboranes of type 2 are only readily available with
amino substituents. Derivative 3 was prepared according to a known
procedure.14 Then, deprotonation with n-butyllithium and subse-
quent treatment with the desired dibromoborane afforded derivatives
4b,c13 in 74 and 71% yield, respectively. The influence of the nature
of the boron substituents is clearly apparent in the 1H NMR
chemical shift of the iminium proton: 8.8 (4a), 9.4 (4b), 9.7 (4c)
ppm. Addition at -78 °C of lithium tetramethyl piperidine to a
THF solution of derivatives 4a-c cleanly afforded carbenes 5a-
for cation 4c (122.8°). For the cation 4c, the rather short endocyclic
N-C distances (1.33 and 1.32 Å) and the somewhat longer N2-
B1 and N3-B2 bond distances (1.46 and 1.47 Å) compared to those
of B1-N1 and B2-N1 (1.42 and 1.43 Å) suggest that the nitrogen
lone pairs interact primarily with the carbon center. Indeed, in
borazines, B-N distances are in the range of 1.42-1.44 Å.16 In
other words, the six π-electrons are divided into two separated allyl
moieties: a cationic four π-electron NCN and a neutral four
π-electron BNB system. In contrast, carbene 5c exhibits five B-N
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10182
J. AM. CHEM. SOC. 2005, 127, 10182-10183
10.1021/ja052987g CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
and 6c (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 1. Thermal ellipsoid diagram (50% probability) of 4c (left) and 5c
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(13) Selected spectroscopic data: 13C{1H} δ 165.0 (4a), 165.0 (4b), 163.6 (4c),
282.9 (5a), 281.5 (5b), 281.6 (5c), 242.1 (J ) 47.4 Hz) (6c), 242.1 (J )
47.4 Hz) (7a), 223.1 (J ) 38.2 Hz) (7b), 224.3 (J ) 38.6 Hz) (7c), [NCN];
11B{1H} δ 25 (4a), 26 and 33 (4b), 35 (4c), 24 (5a), 24 and 31 (5b), 31
(5c), 32 (6c), 24 (7a), 25 and 32 (7b), 30 (7c); IR (CH2Cl2) ν 2069 and
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In conclusion, carbene analogues of borazines are readily
available as highly thermally stable species [mp 176 °C without
decomposition (7c)]. They give rise to stable transition metal
complexes. Importantly, their electronic properties can readily be
tuned by varying the nature of the boron substituents. Their
efficiency as organic catalysts and as ligands for transition metal
catalysts is currently under investigation.
Acknowledgment. We are grateful to RHODIA and the NIH
(R01 GM 68825) for financial support of this work, and to the
Alexander von Humboldt Foundation (C.P.).
Acknowledgment. Full experimental and spectroscopic data for
all new compounds, and X-ray crystallographic data for 4c, 5c,
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