From anionic N-heterocyclic dicarbenes to abnormal carbene-borane complexes: A logical synthetic route

Article abstract of DOI:10.1021/om201259d

The anionic N-heterocyclic dicarbenes (NHDCs) I are demonstrated to be a convenient platform from which abnormal NHC (aNHC)-borane complexes V-VII may be prepared. The anionic NHDC binuclear complex II and the aNHC mononuclear adduct VII can be interconverted through lithiation and protonation, respectively, of the C2 carbon atom of the imidazole ring.

Full text of DOI:10.1021/om201259d

Communication
pubs.acs.org/Organometallics
From Anionic N-Heterocyclic Dicarbenes to Abnormal
Carbene−Borane Complexes: A Logical Synthetic Route
Yuzhong Wang, Mariham Y. Abraham, Robert J. Gilliard, Jr., Pingrong Wei, Jared C. Smith,
and Gregory H. Robinson*
Department of Chemistry, The University of Georgia, Athens, Georgia 30602-2556, United States
*
S Supporting Information
ABSTRACT: The anionic N-heterocyclic dicarbenes (NHDCs) I are demon-
strated to be a convenient platform from which abnormal NHC (aNHC)−borane
complexes V−VII may be prepared. The anionic NHDC binuclear complex II and
the aNHC mononuclear adduct VII can be interconverted through lithiation and
protonation, respectively, of the C2 carbon atom of the imidazole ring.
he considerable impact of carbenes on synthetic and catalytic
chemistry is all the more impressive considering that the first
T
crystalline N-heterocyclic carbene (NHC), 1,3-di-1-adamantylimi-
1
dazol-2-ylidene, was reported merely two decades ago. While
2
their utility in transition-metal catalysis is well documented, these
ubiquitous ligands are attracting increasing attention in main-
3−5
group chemistry.
Indeed, recent reports detailing the role of
NHCs in the stabilization of highly reactive main-group
6−11
molecules
are noteworthy.
Imidazole-based abnormal N-heterocyclic carbenes (aNHCs),
moieties in which the carbene center resides at the C4 (or C5)
position, have begun to play a role in the synthesis of transition-
1
2−14
metal complexes.
This is attributed to the fact that aNHCs
possess stronger electron-donating capabilities than (normal)
1
5
NHCs. Bertrand’s significant isolation of a metal-free
1
6−18
aNHC
followed Crabtree’s observation of transition-
19
metal−aNHC complexes a decade earlier. Although synthetic
and catalytic applications of transition-metal−aNHC complexes
1
2−14
are increasing,
the literature reveals a paucity of studies
concerning main-group-element−aNHC complexes. While
II that was synthesized by combining I with BEt (eq 1):
3
20
23
Carty synthesized aNHC-stabilized phosphinidenes and
silylation for V, alkylation for VI, and protonation for VII.
Moreover, VII may readily be converted back to the starting
material, II, via lithiation. This interconversion between II and
VII is particularly interesting, since it provides new synthetic
routes for both anionic NHDC binuclear complexes and aNHC
mononuclear adducts.
Tamm prepared an aNHC−borane and aNHC-P −borane
4
2
1,22
complexes,
it would be desirable to prepare and study a
variety of aNHC−main-group complexes. To this end, the
development of facile synthetic routes in this area is necessary.
This laboratory recently reported that lithiation of NHCs
afforded the first examples of anionic N-heterocyclic dicarbenes
In notable contrast to the reaction of the C4-bound
i
23
yttrium(III)−anionic NHDC adduct (III) with Me SiCl, which
(
NHDC; I, R = 2,6-Pr C H ). Corresponding NHDC−borane
3
2
6
3
26
24
resulted in the “normal” NHC−Y(III) complex IV (eq 2),
reaction of II with Me SiCl in THF affords the C2-silylated
binuclear complexes were subsequently prepared (eq 1).
Herein, we report that I may also be conveniently utilized to
prepare various aNHC−borane complexes (V−VII) through
simple transformations of the (normal) carbene center (C2) of
3
25
aNHC−BEt adduct V in high yield (93.0%) (eq 3). In addition,
3
reaction of II with highly electrophilic TfOMe (1:1) in toluene
gives the C2-methylated aNHC-BEt complex VI (89.8% yield)
3
(
eq 4). However, reaction of II with excess TfOMe (II:TfOMe =
1
:5) yields a mixture of VI and the corresponding aNHC−boron
25
monotriflate adduct (Figure S1, Supporting Information).
Received: December 20, 2011
Published: January 18, 2012
©
2012 American Chemical Society
791
dx.doi.org/10.1021/om201259d | Organometallics 2012, 31, 791−793

Organometallics
Communication
The aNHC−BEt adduct (VII) was prepared by protonation
3
of the C2 carbon of II using HCl·NEt (81.6% yield) (Scheme 1)
3
Scheme 1. Conversion between the Anionic NHDC−Borane
II and aNHC−Borane Adduct VII
+
−
or imidazolium chloride (NHC-H ·Cl , where NHC = :C[N(2,6-
i
2
Pr C H )CH] ) as the protonation reagents.
6
3
2
Unlike the normal carbene−borane complexes NHC−BEt
3
i
(
NHC = :C[N(Mes)CH] , :C[N( Pr)CH] ), which undergo a
2 2
27
BH -substitution reaction to afford NHC−BH adducts,
aNHC−BEt does not exhibit this reactivity (Scheme 2).
3
3
3
Scheme 2. Reactivity Comparison between NHC−Borane
and aNHC−Borane Adducts
This may be attributed to the donating ability of the C4 carbon
being stronger than that of the C2 carbon of the imidazole
2
8,29
ring,
which is also consistent with the reported irreversible
21
formation of a aNHC−B(C F ) adduct. When it is combined
with nBuLi in hexane, VII is quantitatively converted back to II
Scheme 1). This VII-to-II synthetic strategy may be extended to
6
5 3
(
prepare imidazole-based transition-metal binuclear complexes that
may play an important role in the development of new catalysts
30
and materials.
25
Single-crystal X-ray structural analyses (Figure 1) show
that in compounds V−VII, while C(1) is functionalized by tri-
methylsilyl, methyl, and proton, respectively, the abnormal
carbene center (C(3)) binds to a triethylborane unit, achieving
a tetrahedral geometry around the boron center. The B−C
NHC
bond distances (1.657−1.661 Å) in V−VII are similar to that
2
3
(
1.656(3) Å) in II and those (1.678(6)−1.683(5) Å) of NHC−
2
7
11
BEt adducts.
A
B NMR singlet resonance was observed
3
Figure 1. Molecular structures of (a) V, (b) VI, and (c) VII. Thermal
ellipsoids represent 30% probability; hydrogen atoms on carbon are omitted
for clarity, except for that at C(1) of VII. Selected bond distances (Å) and
angles (deg): for V, B(1)−C(3) = 1.661(5), C(1)−Si(1) = 1.926(2),
N(1)−C(1)−Si(1) = 124.82(18), N(2)−C(1)−Si(1) = 131.07(18),
C(2)−C(3)−B(1) = 125.3(2), N(2)−C(3)−B(1) = 131.8(3); for VI,
B(1)−C(3) = 1.657(3), C(1)−C(28) = 1.483(3), N(1)−C(1)−C(28) =
at −14.4 ppm for V, −15.0 ppm for VI, and −15.2 ppm for
23
VII, comparing well with the BEt resonance (−15.6 ppm) of
3
II and those (−12.6 and −13.3 ppm) of C2-bound BEt carbene
3
27
complexes.
A logical preparative route to aNHC−borane complexes
V−VII has been developed, which involves convenient func-
ionalization of the “normal” carbene center (C2) of a C4-bound
anionic NHDC−BEt adduct (II). Moreover, aNHC−BEt adduct
1
25.09(19), N(2)−C(1)−C(28) = 127.89(19), C(2)−C(3)−B(1) =
126.51(19), N(2)−C(3)−B(1) = 131.33(18); for VII, B(1)−C(3) =
1.658(3), C(2)−C(3)−B(1) = 128.1(2), N(2)−C(3)−B(1) = 129.67(19).
3
3
7
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Organometallics
VII) may be quantitatively converted back to II through a simple
Communication
(
(28) Gruendemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.;
Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473.
29) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.;
lithiation.
(
Crabtree, R. H. Organometallics 2004, 23, 2461.
ASSOCIATED CONTENT
■
* Supporting Information
(30) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677.
S
Text, figures, and CIF files giving full details of the syntheses
and X-ray crystal determination. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Tel: 706-548-8432. E-mail: robinson@uga.edu.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Science Foundation for support
of this work: CHE-0953484 (G.H.R., Y.W.).
■
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