An efficient synthetic route to stable bis(carbene)borylenes [(L 1)(L2)BH]

Article abstract of DOI:10.1039/c4cc03497j

Two-electron reduction of bis(carbene) boronium salts allows for the preparation of unsymmetrically substituted nucleophilic boron derivatives of type (L₁)(L₂)BH, which are characterized by X-ray crystallography. A single electron reduction of the same starting materials leads to the corresponding boron-centered radical cations (L₁)(L ₂)BH⁺, X^-. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03497j

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An efficient synthetic route to stable
bis(carbene)borylenes [(L1)(L2)BH]†
Cite this: Chem. Commun., 2014,
50, 7837
David A. Ruiz, Mohand Melaimi and Guy Bertrand*
Received 8th May 2014,
Accepted 3rd June 2014
DOI: 10.1039/c4cc03497j
www.rsc.org/chemcomm
Two-electron reduction of bis(carbene) boronium salts allows for
the preparation of unsymmetrically substituted nucleophilic boron
derivatives of type (L1)(L2)BH, which are characterized by X-ray
crystallography. A single electron reduction of the same starting
materials leads to the corresponding boron-centered radical
cations (L1)(L2)BHꢀ+, XÀ.
Since the isolation of the heterocyclic boryl anion A by Yamashita,
Nozaki and co-workers1 in 2006 (Chart 1), the number of boron
centered nucleophiles has grown gradually.2,3 However, up to now,
compound C is the only example of a stable neutral, nucleophilic,
tricoordinate boron derivative.4 This compound, which is
isoelectronic with amines and phosphines, was prepared by
Chart 1
reduction of the CAAC-BBr3 adduct B [CAAC = cyclic (alkyl)(amino)- Therefore, we turned our attention to the less bulky but unknown
carbene]5,6 with KC8. This synthetic route, which leads to C in only CAAC monotriflate borane 2. As already observed with NHC
33% yield, is far from being understood, and certainly does not adducts,11 we found that although (CAAC)BH(OTf)2 is readily avail-
have a broad scope. For example, Robinson and co-workers have able by treatment of (CAAC)BH3 112 with excess trifluoromethane
reported that the reduction of the analogous (NHC)-BBr3 adduct D sulfonic acid, the desired monotriflate derivative (CAAC)BH2(OTf)
(NHC = N-heterocyclic carbene)7 with KC8 affords dimers of type E.8 could not be selectively prepared. In contrast, when the CAAC borane
Moreover, the preparation of derivatives, featuring two different complex 1 is instead reacted with methyl trifluoromethanesulfonate,
carbene ligands, which would allow for a fine tuning of the the desired CAAC monotriflate borane 2 is formed, and can be
electronic properties of the boron center, could not be achieved. isolated in 95% yield as a white powder (Scheme 1). The 11B NMR
Herein, we report a stepwise and more versatile synthesis that spectrum displays a broad signal at À6.1 ppm, which is shifted
allows for the isolation of different unsymmetrically substituted downfield from 1 (À28.5 ppm), and the 19F NMR spectrum shows a
derivatives of type (L1)(L2)BH, as well as for the EPR characteriza- singlet at À76.2 ppm, which is indicative of a triflate group covalently
+
tion of one of the corresponding radical cations (L1)(L2)BHꢀ , XÀ.
bound to boron. The structure of 2 was ascertained by a single crystal
We first envisaged a synthetic route, in which a second carbene X-ray diffraction study (Fig. S31, ESI†).
would be introduced by simple displacement of a triflate group from
We were pleased to observe that 2 readily reacts with carbenes
the known (CAAC)BH(OTf)2.3g However, no reaction was observed La and Lb, affording the desired bis(carbene) boronium salts
9
with the benzimidazolylidene La and cyclopropenylidene Lb,10 3a,b, which were isolated as white solids in 95 and 80% yields,
probably due the excessive steric hindrance around boron. respectively. The 11B NMR spectrum of these derivatives shows
an upfield well-defined triplet (3a = À28.6 ppm, JBH = 82.9 Hz;
UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla, CA 92093-0343, USA.
E-mail: guybertrand@ucsd.edu; Tel: +1 858 534 5412
3b = À27.7 ppm, JBH = 87.9 Hz), whereas the 19F NMR spectrum
indicates that the triflate group is no longer covalently bound
(3a = À78.0 ppm; 3b = À79.3 ppm). Interestingly, these species
are not sensitive to air and moisture, but all attempts to obtain
the desired neutral tricoordinate boron derivatives 5a,b by
deprotonation or reduction of 3a,b failed.
† Electronic supplementary information (ESI) available: Synthetic procedures,
analytical data, and CIF files for single crystal X-ray structural analysis. CCDC
1001683–1001687. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc03497j
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Scheme 1
In order to further increase the acidity of the proton bonded
to boron, we chose to replace one of the hydrogen atoms by a
triflate group. This is readily achieved by simple treatment of
3a,b with triflic acid. Compounds 4a,b were isolated as white
solids in 70 and 67% yield, respectively. The 19F NMR spectrum
displays two peaks indicative of both a bound and a free triflate
group (4a = À75.4, À78.1 ppm; 4b = À76.2, À78.0 ppm), and the
11B NMR signal shifts downfield compared to 3a,b and becomes
broad (4a = À7.5 ppm; 4b = À5.1 ppm). The 1H NMR spectra are
quite convoluted, probably because of the steric hindrance,
which prevents rotation around the boron–carbon bonds. This
is confirmed by the solid-state structure of the corresponding
BPh4 salts, obtained by anion exchange. These compounds are
extremely robust, as during work-up a water wash is performed.
Attempted deprotonation of 4a,b failed again. However,
mixing 4a and 4b with two equivalents of KC8 in THF leads to
an immediate and intense blue and red colored solutions,
respectively. After workup, the reduced products 5a,b were
isolated in 87 and 82% yield, respectively. Although extremely
Fig. 1 Molecular structure of 5a (left) and 5b (right) in the solid state.
Hydrogen atoms, except the B–H, and solvent molecules are omitted
for clarity. Selected bond lengths [Å] and angles [1]: 5a B1–C1 1.462(3),
B1–C2 1.572(2), B1–H1 0.930, C1–N1 1.418(2), C2–N2 1.371(2),
C2–N3 1.366(2); C1–B1–C2 127.84(15), C1–B1–H1 116.07, C2–B1–H1
116.08. 5b B1–C1 1.4692(16), B1–C2 1.5521(17), B1–H1 1.107, C1–
N1 1.4262(15); C1–B1–C2 128.50(10), C1–B1–H1 121.58, C2–B1–H1 111.86.
sensitive to air and moisture, derivatives 5a,b can be stored for Concomitantly there is an elongation of the C1–N1 bond [5a:
months, under an inert atmosphere, with no signs of decom- 1.418(2); 5b: 1.4262(15) Å] compared to that of the salt precursors
position. The 1H NMR spectra are simplified compared to those [4a: 1.307(2); 4b: 1.297(5) Å]. These geometric parameters clearly
of 4a,b as the boron center is no longer a chirality center. The indicate that the formal boron lone pair is mainly delocalized on
11B NMR spectrum appears as a doublet at À1.3 (JBH = 82.4 Hz) the CAAC ligand. This is confirmed by DFT calculations, as can
and 0.8 ppm (JBH = 89.7 Hz) for 5a and 5b, respectively. It is be seen from the HOMO diagrams (Fig. 2).
interesting to note that these signals are high-field shifted
Despite the delocalization of the lone pair, the boron center
compared to that observed for C (+12.5 ppm), a trend which of 5a,b is electron rich. Indeed, both compounds react with
is in agreement with the inferior p-acceptor properties of trifluoromethanesulfonic acid, leading to the conjugate acids
benzimidazolylidene La and cyclopropenylidene Lb compared 3a,b. Moreover, we found that the radical cation 6b is persistent
to CAACs.13
for several hours at room temperature. It can be prepared by
Single crystals of 5a and 5b, suitable for X-ray diffraction adding one equivalent of KC8 to a DME solution of 4b. The
studies, were obtained from a concentrated pentane solution room temperature EPR spectrum of 6b displays couplings with
(Fig. 1). The B1–C2 bond lengths [5a: 1.572(2); 5b: 1.5521(17) Å] boron [a(11B) = 4.994 G], hydrogen [a(1H) = 10.065 G], and only
are typical for B–C single bonds. In contrast, for both com- one nitrogen nuclei [a(14N) = 6.627 G] (Fig. 3). This suggests
pounds, the B1–C1 bond [5a: 1.462(3); 5b: 1.4692(16) Å] is short, again that the unpaired electron is mainly delocalized over the
and falls into the range of known boron–carbon double bonds.14 CAAC and BH fragments, with very little contribution by Lb.
7838 | Chem. Commun., 2014, 50, 7837--7839
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The synthetic route described herein paves the way for the
preparation of a variety of bis(carbene)borylene adducts, which
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This work was supported by the DOE (DE-FG02-13ER16370) and
the NSF (CHE-1316956). D. A. R. gratefully acknowledges the U.S.
Department of Education for a GAANN fellowship. Thanks are
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This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7837--7839 | 7839