Reluctance of 4-chloro-5-metalla-1,3,2-diazaborolines to undergo metal halide β-elimination: An opportunity for C-functionalization of 1,3,2-diazaborolines

Article abstract of DOI:10.1021/ic801424k

N,N′-Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene reacts with dichlorophenylborane, affording the N,N′-bis(2,6-diisopropylphenyl)-2- phenyl-4-chloro-1,3,2-diazaboroline in a one-step process. The addition of lithium diisopropylamide gives rise to the 4-chloro-5-lithio-1,3,2-diazaboroline derivative, which cleanly undergoes a transmetalation reaction with TiCl ₄·2THF. Both the lithium and titanium complexes are stable with respect to metal chloride elimination and have been characterized by multinuclear NMR spectroscopy and by single-crystal X-ray diffraction studies. These findings open an avenue for the C-functionalization of 1,3,2-diazaborolines.

Full text of DOI:10.1021/ic801424k

Inorg. Chem. 2008, 47, 9751-9753
Reluctance of 4-Chloro-5-metalla-1,3,2-diazaborolines To Undergo Metal
Halide ꢀ-Elimination: An Opportunity for C-Functionalization of
1,3,2-Diazaborolines
Emrah Giziroglu,† Bruno Donnadieu, and Guy Bertrand*
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry,
UniVersity of California at RiVerside, RiVerside, California 92521-0403
Received July 29, 2008
N,N′-Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene reacts with
dichlorophenylborane, affording the N,N′-bis(2,6-diisopropylphenyl)-
2-phenyl-4-chloro-1,3,2-diazaboroline in a one-step process. The
addition of lithium diisopropylamide gives rise to the 4-chloro-5-
lithio-1,3,2-diazaboroline derivative, which cleanly undergoes a
transmetalation reaction with TiCl4 · 2THF. Both the lithium and
titanium complexes are stable with respect to metal chloride
elimination and have been characterized by multinuclear NMR
spectroscopy and by single-crystal X-ray diffraction studies. These
findings open an avenue for the C-functionalization of
1,3,2-diazaborolines.
Figure 1
shown in B. Bent allenes A can easily be synthesized by
deprotonation of the conjugate acids. For preparing alkynes,
one of the simplest approaches is to perform a 1,2-elimination
from a suitable unsaturated precursor. It is well-known that
acyclic alkenes with an alkali metal and a halogen on
neighboring carbon atoms are unstable with respect to metal
halogen salt elimination and readily give rise to alkynes. In
contrast, their cyclic versions are often more stable,5 except
when aromatic compounds are involved;6,7 for instance,
1-fluoro-2-lithiobenzene decomposes at -60 °C.6d
On the basis of this analysis, we chose to target 4,5-
dehydro-1,3,2-diazaborolines 3, which feature, as desired,
two nitrogen atoms bonded to the unsaturation and a 6π-
electron system. Here we report the synthesis, spectroscopic
investigation, and single-crystal X-ray diffraction studies of
We have recently shown that it is possible to polarize the
p system of allenes up to the breaking point, which led to
the discovery of stable bent allenes A,1,2 also called
carbodicarbenes after Frenking’s computational studies3
(Figure 1). An obvious extension of this work is to attempt
to polarize a Ct C triple bond, in the hope of bending the
usually very rigid linear skeleton of alkynes, without
significant destabilization.4 By analogy with the approach
used in the case of allenes, one option is to attach amino
groups at both ends of the Ct C triple bond and to
incorporate the alkyne moiety into a five-membered ring, as
* To whom correspondence should be addressed. E-mail: guy.bertrand@
ucr.edu. Tel: 1 951 827 2719. Fax: 1 951 827 2725.
† Present address: Department of Chemistry, Adnan Menderes University,
09100 Aydin, Turkey.
(1) (a) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 3206–3209. (b) Lavallo, V.; Dyker, C. A.;
Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5411–
5414.
(2) See also: (a) Fu¨rstner, A.; Alcarazo, M.; Goddard, R.; Lehmann, C. W.
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F. E. Angew. Chem., Int. Ed. 2008, 47, 4057–4061.
(3) (a) Tonner, R.; Frenking, G. Angew. Chem., Int. Ed. 2007, 46, 8695–
8698. (b) Tonner, R.; Frenking, G. Chem.sEur. J. 2008, 14, 3260–
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126, 14700–14701. (b) Riggs, J. C.; Ramirez, A.; Cremeens, M. E.;
Bashmore, C. G.; Candler, J.; Wirtz, M. C.; Coe, J. W. D.; Collum,
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K.; Lohrenz, J. C. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 1455–
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(4) Five-membered cyclic alkynes have recently been isolated:(a) Suzuki,
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10.1021/ic801424k CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 21, 2008 9751
Published on Web 10/09/2008

COMMUNICATION
Scheme 1
a novel 4-chloro-1,3,2-diazaboroline, as well as its 5-lithio
derivative. We also describe the transmetalation reaction of
the latter with titanium.
Heterocycles containing boron and nitrogen are an im-
portant class of compounds owing to their wide applications
in industry and academia.8 Among them, 1,3,2-diazaborolines
have gained increasing attention9 because recent physico-
chemical studies point to possible applications, especially
for optoelectronic devices.10 Therefore, several synthetic
routes are available.8 Of particular interest, it has recently
been shown that N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-
1,3-butadiene (1)11 reacts with boron trichloride in methylene
chloride to afford the corresponding N,N′-bis(2,6-diisopro-
pylphenyl)-2,4-dichloro-1,3,2-diazaboroline.12 Although this
compound features a proton next to a halogen-substituted
carbon, the presence of the chlorine on the boron atom was
not desirable for our purpose. Thus, we extended this
synthetic methodology to dichlorophenylborane. Indeed, the
reaction of 1 with 1 equiv of dichlorophenylborane in CH2Cl2
cleanly afforded N,N′-bis(2,6-diisopropylphenyl)-2-phenyl-
4-chloro-1,3,2-diazaboroline (2; Scheme 1), which was
isolated after crystallization from hexanes at -30 °C in 78%
yield, as air-stable colorless crystals.12
Figure 2. Molecular structure of 4 in the solid state (hydrogen atoms are
omitted for clarity; ellipsoids are drawn at 30% probability). Selected bond
lengths [Å] and angles [deg]: B1A-N1A 1.432(13), B1A-N2A 1.430(13),
N1A-C1A 1.436(12), N2A-C2A 1.411(12), C1A-C2A 1.338(13),
Cl1A-C2A 1.721(9), Li1A-C1A 2.226(18), Li2A-C1A 2.195(19),
Li1A-N3A 1.974(19), Li2A-N3A 2.005(19), Li1A-O2A 1.914(19);
N2A-B1A-N1A 103.3(8), B1A-N1A-C1A 113.0(7), C2A-N2A-B1A
106.3(7),C2A-C1A-N1A102.0(7),C1A-C2A-N2A115.3(8),Li2A-C1A-Li1A
65.9(7), Li1A-N3A-Li2A 74.4(7).
Surprisingly, according to NMR spectroscopy, the reaction
of 2 with 1 equiv of lithium diisopropylamide (LDA) in a
tetrahydrofuran (THF) solution at -78 °C gave rise to a 1:1
mixture of the starting material 2 and a new compound 4.
After the addition of a second 1 equiv of LDA, the
conversion of 2 to 4 was completed, and the latter compound
was isolated after crystallization from a hexane solution at
-30 °C as air-sensitive pink crystals (87% yield). Interest-
ingly, the 1H and 13C NMR spectra showed the presence of
a i-Pr2N group and two THF molecules, ruling out the
formation of the desired 4,5-dehydro-1,3,2-diazaboroline 3.
Figure 3. Molecular structure of 5 in the solid state (hydrogen atoms are
omitted for clarity; ellipsoids are drawn at 30% probability). Selected bond
lengths [Å] and angles [deg]: B1-N1 1.459(3), B1-N2 1.434(2), N1-C2
1.380(2), N2-C1 1.419(2), C1-C2 1.366(3), Cl3-C2 1.7178(18), Ti1-C1
2.0716(19), Ti1-N3 1.8411(17); N2-B1-N1 103.43(16), C1-N2-B1
111.30(15), C2-N1-B1 107.44(15), C2-C1-N2104.79(15), C1-C2-N1
113.03(16).
A single-crystal X-ray diffraction study13 unambiguously
demonstrated that 4 was 4-chloro-5,5-dilithio-1,3,2-diaz-
aboroline, with the two lithium atoms sharing a disopropy-
lamino moiety and each being further coordinated by a THF
molecule (Figure 2). Complex 4 is reminiscent of 3-bromo-
2-lithiobenzofuran and 3-iodo-2-lithioindole, prepared and
crystallographically characterized by Boche et al.7
(8) (a) Kawaguchi, M. AdV. Mater. 1997, 9, 615–625. (b) Engelhardt, U.
Coord. Chem. ReV. 2002, 215, 53–91. (c) Pra¨sang, C.; Donnadieu,
B.; Bertrand, G. J. Am. Chem. Soc. 2005, 127, 10182–10183.
(9) (a) Weber, L. Coord. Chem. ReV. 2001, 215, 39–77. (b) Weber, L.
Coord. Chem. ReV. 2008, 252, 1–31.
(10) (a) Weber, L.; Domke, I.; Schmidt, C.; Braun, T.; Stammler, H. G.;
Neumann, B. Dalton Trans. 2006, 2127–2132. (b) Weber, L.; Werner,
V.; Domke, I.; Stammler, H. G.; Neumann, B. Dalton Trans. 2006,
3777–3784. (c) Weber, L.; Domke, I.; Kahlert, J.; Stammler, H. G.
Eur. J. Inorg. Chem. 2006, 341, 9–3424. (d) Maruyama, M.;
Kawanishi, Y. J. Mater. Chem. 2002, 12, 2245–2249.
(11) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999,
55, 14523–14534.
(12) Hinchliffe, A.; Mair, S. F.; McInnes, E. J. L.; Pritchard, R. G.; Warren,
J. E. Dalton Trans. 2008, 222–233.
(13) CCDC 696763 (2), 696764 (4), and 696765 (5) contain the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
9752 Inorganic Chemistry, Vol. 47, No. 21, 2008

COMMUNICATION
functionalized, opening up a new avenue in 1,3,2-diazaboro-
line chemistry. Moreover, the boron heterocycle of type 6,
which is present in complexes 4 and 5, can be regarded as
an anionic version of the so-called abnormal N-heterocyclic
carbenes 7,14 which have found numerous applications as
strong donor ligands for transition-metal-based catalysts
(Figure 4). The use of these anionic boron heterocycles 6 in
organometallic catalysis is under active investigation.
Figure 4
Derivative 4 appeared to be stable at room temperature,
in solution, and in the solid state under an argon atmosphere,
for at least 2 weeks. All attempts to induce the ꢀ-elimination
of lithium chloride failed. Therefore, in the hope of favoring
the elimination reaction, transmetalation of 4 was attempted.
When the lithium complex 4 was treated at room temper-
ature with 1 equiv of TiCl4 · 2THF in benzene, a clean
reaction occurred. After workup, the resulting solid was
recrystallized from a hexanes solution at -30 °C. Complex
5 was isolated in 28% yield, as extremely air-sensitive red
crystals, and its structure ascertained by a single-crystal X-ray
diffraction study (Figure 3).13 All of our attempts to induce
the ꢀ-elimination reaction failed again.
Acknowledgment. We are grateful to the NSF (Grant
CHE 0808825) for financial support of this work and the
Scientific and Technological Research Council of Turkey
(TUBITAK) for a Graduate Fellowship (E.G.).
Supporting Information Available: Crystallographic data in-
cluding CIF files as well as synthetic and spectroscopic information.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC801424K
The reactivity and functionalization of 1,3,2-diazaborolines
typically occur at boron, less frequently at nitrogen, but not
at carbon.9 Therefore, although we have not as yet been able
to prepare our targeted bent alkyne, this work shows that
the unsaturated carbon atoms of the ring can also be
(14) For examples, see: (a) Arnold, P. L.; Pearson, S. Coord. Chem. ReV.
2007, 251, 596–609. (b) Peris, E.; Crabtree, R. H. Coord. Chem. ReV.
2004, 248, 2239–2246. (c) Chianese, A. R.; Kovacevic, A.; Zeglis,
B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461–
2468.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9753