Boryltrihydroborate: Synthesis, structure, and reactivity as a reductant in ionic, organometallic, and radical reactions

Article abstract of DOI:10.1021/ja105277u

Reaction of lithium 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2- diazaborol-2-ide with borane·THF provides the first boryl-substituted borohydride: lithium [1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2- diazaborol-2-yl]trihydroborate. The c

Full text of DOI:10.1021/ja105277u

Published on Web 08/02/2010
Boryltrihydroborate: Synthesis, Structure, and Reactivity as a Reductant in
Ionic, Organometallic, and Radical Reactions
Kyoko Nozaki,*,† Yoshitaka Aramaki,† Makoto Yamashita,*,† Shau-Hua Ueng,‡ Max Malacria,§
Emmanuel Lacoˆte,§ and Dennis P. Curran*,‡
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo,
7-3-1 Bunkyo-ku, Tokyo 113-8656, Japan, Department of Chemistry, UniVersity of Pittsburgh,
Pittsburgh, PennsylVania 15260, USA, and UPMC UniV Paris 06, Institut Parisien De Chimie Mole´culaire (UMR
CNRS 7201), C 229, 4 place Jussieu, 75005 Paris, France
Received June 16, 2010; E-mail: nozaki@chembio.t.u-tokyo.ac.jp; makotoy@chembio.t.u-tokyo.ac.jp; curran@pitt.edu
Abstract: Reaction of lithium 1,3-bis(2,6-diisopropylphenyl)-2,3-
dihydro-1H-1,3,2-diazaborol-2-ide with borane ·THF provides the first
boryl-substituted borohydride: lithium [1,3-bis(2,6-diisopropylphenyl)-
2,3-dihydro-1H-1,3,2-diazaborol-2-yl]trihydroborate. The compound
is fully characterized by 11B, H, and Li NMR spectra and other
means, and these data are compared to neutral and anionic
benchmark compounds. The compound crystallizes as a dimer
complexed to four THF molecules. The dimer lacks the bridging B-H
bonds seen in neutral boranes and is instead held together by ionic
Li---HB interactions. A preliminary scan of reactions with several
iodides shows that the compound participates in an ionic reduction
(with a primary-alkyl iodide), an organometallic reduction (Pd-
catalyzed with an aryl iodide), and a radical reduction (AIBN-initiated
with a sugar-derived iodide). Accordingly the new borylborohydride
class may share properties of both traditional borohydrides and
isoelectronic N-heterocyclic carbene boranes.
1
7
Figure 1. Neutral N-heterocyclic carbenes 1 and NHC-boranes 2 are
isoelectronic with boryl 3 and borylborohydride 4 anions, respectively.
Scheme 1. Synthesis of Boryltrihydroborate 6
Tetrahydroborates (BH4-) are parents of an exceptionally
important class of reagents in both organic and inorganic chemistry.1
Often called borohydrides, these reagents are used as reductants,
as precursors for all kinds of interesting main group and organo-
metallic compounds, and as hydrogen storage reagents, among other
applications. Countless substituted borohydrides are known, with
one or more hydrogen atoms replaced by carbon, oxygen, nitrogen,
or other atoms.1 However, simple boron-substituted borohydrides
appear to be unknown. Their absence might be due to a gap in
methodologysthe boron Lewis bases (boryl anions) needed to make
borylborohydrides have been unknown until recently.2
Table 1. Comparison of Selected NMR Spectroscopic Values of 6
with Benchmarksa
Compd
1H NMR
11B NMR
1J(11B1H)
7Li NMR
7Li ν1/2
6
-0.98, br
-0.52,
quadruplet
0.52, br
37.1, -44.7
-42.0,
broad
81
-0.73
-0.53
11
18
LiBH4
We set out to explore the prospect of using new methods to make
boryl anions a stepping stone to make borylborohydrides. In
particular, boryl anions 3 are isoelectronic with well-known
N-heterocyclic carbenes 1 (Figure 1).3 Addition of a boryl anion 3
to borane might give borylborohydride 4, which is isoelectronic
with the N-heterocyclic carbene borane (NHC-borane) 2. NHC-
boranes 2 are an emerging class of reagents with applications in
ionic, organometallic, and radical reactions.4 Thus, we felt that
compounds like 4 were doubly interesting as boryl-substituted
borohydrides and as anionic analogs of neutral NHC-boranes.5
According to the latter analogy, anions like 4 can also be viewed
as unusual examples of complexes between a boron-derived Lewis
acid (BH3) and a boron-derived Lewis base (3).
quintet
dipp-Imd-
BH3
-36.2
89
s
s
b
a Solvent is THF-d8, chemical shifts are in ppm, J and ν1/2 are in Hz.
b Compound 2 with R ) 2,6-diisopropylphenyl.
yield after two recrystallizations (Scheme 1). Unlike NHC-boranes
but like many borohydrides, 6 is sensitive to ambient laboratory
conditions, and it was synthesized and stored in a glovebox for
this preliminary study.
Comparisons of representative spectroscopic data of 6 with data
of benchmarks lithium borohydride (LiBH4) and 2,6-diisopropy-
lphenylimidazol-2-ylidene borane (dipp-Imd-BH3, 2: R ) 2,6-
diisopropylphenyl)7 are summarized in Table 1. The 1H NMR
spectrum of 6 in THF-d8 shows a broad signal assignable to three
hydrogen atoms on the borate at a relatively high field of δH -0.98.
The signal was sharper in the 1H{11B} decoupled NMR spectrum.
Reaction of boryllithium 56 with BH3 ·THF gave a colorless,
crystalline lithium boryltrihydroborate 6, which was isolated in 19%
† The University of Tokyo.
‡ University of Pittsburgh.
§ Université Pierre et Marie Curie.
9
10.1021/ja105277u  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 11449–11451 11449

C O M M U N I C A T I O N S
Table 2. Preliminary Ionic, Organometallic and Radical Reactions
of 6, and Benchmarks LiBH4 and dipp-Imd-BH3
Figure 2. ORTEP drawing of 6. (50% thermal ellipsoids. One half of the
whole structure is the asymmetric unit. Asterisks on atomic labels show
the other half. Disordered THF molecules and all hydrogen atoms except
the hydroborate hydrogens are omitted for clarity.)
A similar broadened signal was also observed in the spectrum of
dipp-Imd-BH3 (δH 0.52). In contrast, LiBH4 showed a sharp
quadruplet due to coupling with 11B (I ) 3/2, J ) 81 Hz) paired
with a satellite septuplet due to coupling with 10B (I ) 3, J ) 27
Hz) at δH -0.52. The difference in sharpness between the signals
of 6 and LiBH4 probably comes from the difference in the strength
of the quadrupolar interaction because of the symmetry of the
molecules.8 Compared to dipp-Imd-BH3, the upfield-shifted BH3
signal of 6 may reflect more partial negative charge on the borate
center.
The 11B NMR spectrum of 6 in THF-d8 showed resonances at
δB 37.1 and -44.7. The downfield resonance is assigned to the
boryl substituent and the upfield one to borate anion (compare δB
-42.0 for LiBH4 and -36.2 for dipp-Imd-BH3).9 The shape of the
BH3 signal seemed to be a broad quartet due to a coupling with
three equivalent protons with a very small shoulder. In comparison,
LiBH4 and dipp-Imd-BH3 exhibit sharp quintet [1J(11B1H) ) 81
Hz] and quartet [1J(11B1H) ) 89 Hz] signals, respectively. The
significant broadening of the BH3 signal in 6 may come from the
direct connection to the quadrupolar boron nucleus in the boryl
substituent. A sharp (ν1/2 ) 11 Hz) 7Li NMR signal of 6 appeared
at δLi -0.73. Quadrupolar coupling between boron and lithium
atoms was observed in boryllithium 5 to afford a broadened signal
(ν1/2 ) 36 Hz) but not in 6. The aforementioned 7Li NMR chemical
shift is close to that of LiBH4 (δLi -0.53), indicating a similar
situation in anion-cation interactions.
Colorless single crystals of 6 suitable for X-ray analysis were
obtained from ether solution with a slow vapor diffusion of pentane.
Compound 6 crystallized as a contact ion pair with two THF
molecules coordinating to the lithium atom (Figure 2). The
boryltrihydroborate moiety simultaneously coordinated to two
lithium atoms to assemble a dimeric structure with no other
interaction between the two units. The Fourier map showed the
existence of three hydrogen atoms on B2. Unlike B2H6, there is no
bridging hydrogen atom between the two boron atoms of 6. The
B1-B2 length of 1.699(4) Å is comparable to previously reported
B(sp2)-B(sp3) bonds.10 Interatomic distances, B2-Li1 of 2.441(5)
Å and B2-Li1* of 2.542(5) Å, are in the range of µ1 and µ2
coordination modes. The N1-B1-N2 bond angle of 101.52° is
close to those of other boryllithium species [98.7(2)°-101.89(16)°].
The two different B1-B2-Li angles [129.3(2)°, 158.6(2)°] support
an unsymmetrical coordination of the boryltrihydroborate moiety
to the two lithium atoms.
With this understanding of the structural features, we scaled up
the reaction in Scheme 1 to produce about 1 g of 6 for a preliminary
study of its reactivity. Typical substrates for ionic, organometallic,
and radical reactions were reacted under identical conditions with
boryltrihydroborate 6, LiBH4, and dipp-Imd-BH3. The results of
this study are summarized in Table 2 with a focus on products and
yields. Many of the reactions were followed by 11B NMR
spectroscopy. This and other additional information such as stability
tests are found in the Supporting Information.
To compare the hydridic character, we examined three reactions
with reactive primary iodide 7, all at 60 °C in THF-d8. The reaction
with LiBH4 (entry 2) was completed in 5 h and gave 8 in 72%
isolated yield, while the reaction with 6 (entry 1) took 28 h to
complete, giving 8 in comparable yield. The reaction of 7 with
dipp-Imd-BH3 (entry 3) was stopped after 28 h, at which point the
9
11450 J. AM. CHEM. SOC. VOL. 132, NO. 33, 2010

C O M M U N I C A T I O N S
hydro-1H-1,3,2-diazaborol-2-yl]trihydroborate 6. A preliminary scan
of ionic, organometallic, and radical reactions suggests that 6 and
presumably other members of this new class of compounds will
have interesting properties. For example, 6 exhibits ionic reactivity
similar to that of LiBH4 and very different from that of NHC-
benchmark dipp-Imd-BH3, but it exhibits radical reactivity similar
to that of dipp-Imd-BH3 and very different from that of LiBH4.
With the advent of methods to make boryl anions, it should now
be possible to make and study a range of borylborohydrides.
Acknowledgment. This work was supported by grants from
MEXT, Japan (KAKENHI 21245023 and 21685006) and from the
US National Science Foundation (CHE-0645998). We thank Prof.
Louis Fensterbank (UPMC) and Mr. Andrey Solovyev (Univ. of
Pittsburgh) for helpful discussions.
Figure 3. 11B NMR spectrum of the reaction of 6 with 7 at the 5 h time
point showing the presense of 6 (decreasing), 14 (increasing, then
decreasing), and 13 (final boron product).
Supporting Information Available: Contains details of synthesis
and characterization of 6, details of experiments in Table 2, copies of
spectra, and a cif file of the crystal structure. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 4. Structures of boron-containing reaction products 13 and 14.
References
conversion was 29%. The reduction of 7 by dipp-Imd-BH3 is a
high yielding reaction but needs much higher temperatures for
speedy conversion.4d These results suggest that 6 is less reactive
than LiBH4 but more reactive than dipp-Imd-BH3 in ionic reductions.
We selected aryl iodide 9 as the substrate for preliminary
organometallic reductions because it has been used previously in
analogous NHC-borane reactions.4d Again reactions were conducted
at 60 °C in THF-d8, now with Pd(dppf)Cl2 (12%) as the catalyst.
Boryltrihydroborate 6 and LiBH4 reactions were completed quickly
(3 h and 15 min) and gave product 10 in comparable isolated yields
(65% and 66%, entries 4,5). Dipp-Imd-BH3 did not react under
these conditions (entry 7).
For the radical reduction, we selected hindered, electronically
deactivated iodide 11, which we assume will resist SN2 substitution.
These reactions were conducted in C6D6 with AIBN for a fixed
time of 2 h. The precursor 11 and the product 12 were inseparable,
but their combined isolated yields were good in each case. In this
sequence of reactions, 6 provided the best result, giving 78%
conversion to 12 (entry 8). In contrast dipp-Imd-BH3 gave 37%
conversion (entry 10) and LiBH4 gave no conversion, possibly due
to its low solubility (entry 9).
(1) (a) Chen, G. In Organometallics: Boron Compounds; Kaufmann, D. E.,
Matteson, D. S., Eds.; Georg Thieme Verlag: Stuttgart-New York, 2004;
Vol. 6, pp 81-115. (b) Rittmeyer, P.; Wietelmann, U. Hydrides. Ullmann’s
Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2002. (c)
For a silyl borohydride : Lippert, W.; No¨th, H.; Ponikwar, W.; Seifert, T.
Eur. J. Inorg. Chem. 1999, 817–823.
(2) (a) Yamashita, M.; Nozaki, K. Bull. Chem. Soc. Jpn. 2008, 81, 1377–1391.
(b) Yamashita, M.; Nozaki, K. Pure Appl. Chem. 2008, 80, 1187–1194. (c)
Yamashita, M.; Nozaki, K. J. Synth. Org. Chem. Jpn. 2010, 68, 359–369.
(3) (a) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348–1352. (b) Arduengo,
A. J.; Bertrand, G. Chem. ReV. 2009, 109, 3209–3210, and subsequent
reviews in this special issue.
(4) (a) Ueng, S.-H.; Makhlouf Brahmi, M.; Derat, E.; Fensterbank, L.; Lacôte,
E.; Malacria, M.; Curran, D. P. J. Am. Chem. Soc. 2008, 130, 10082–
10083. (b) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank,
L.; Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P.
J. Am. Chem. Soc. 2009, 131, 11256–11262. (c) Monot, J.; Makhlouf
Brahmi, M.; Ueng, S.-H.; Robert, C.; Murr, M. D.-E.; Curran, D. P.;
Malacria, M.; Fensterbank, L.; Lacôte, E. Org. Lett. 2009, 11, 4914–4917.
(d) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.; Curran,
D. P.; Malacria, M.; Fensterbank, L.; Lacôte, E. Chem.sEur. J. 2009, 15,
12937–12940. (e) Lindsay, D. M.; McArthur, D. Chem. Commun. 2010,
46, 2474–2476. (f) Walton, J. C.; Makhlouf Brahmi, M.; Fensterbank, L.;
Lacôte, E.; Malacria, M.; Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P.
J. Am. Chem. Soc. 2010, 132, 2350–2358. (g) Tehfe, M.-A.; Makhlouf
Brahmi, M.; Fouassier, J.-P.; Curran, D. P.; Malacria, M.; Fensterbank,
L.; Lacoˆte, E.; Laleve´e, J. Macromolecules 2010, 43, 2261–2267. (h)
Matsumoto, T.; Gabbaï, F. P. Organometallics 2009, 28, 4252–4253.
(5) Indeed, as the paper was in preparation, a theoretical study appeared
suggesting that, like NHC-boranes, borylborohydrides will have relatively
weak B-H bonds. Lai, C.-H.; Chou, P.-T. J. Comput. Chem. 2010, 31,
2258–2262.
(6) (a) Segawa, Y.; Yamashita, M.; Nozaki, K. Science 2006, 314, 113–115.
(b) Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2007,
46, 6710–6713. (c) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K.
J. Am. Chem. Soc. 2007, 129, 9570–9571. (d) Kajiwara, T.; Terabayashi,
T.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2008, 47, 6606–
6610. (e) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. Chem. Lett.
2008, 37, 802–803. (f) Segawa, Y.; Suzuki, Y.; Yamashita, M.; Nozaki,
K. J. Am. Chem. Soc. 2008, 130, 16069–16079. (g) Terabayashi, T.;
Kajiwara, T.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131,
14162–14163.
We also obtained significant information about the boron-containing
products in both the ionic and organometallic reactions of 6. The time
course 11B NMR spectra for both reactions showed the growth of a
new broad signal at δB -35.5. This later declined, being replaced by
a signal at δB 20. Parallel changes were seen in the series of 1H NMR
spectra. The complete set of spectra showing reaction progress are
provided in the Supporting Information. Figure 3 shows a typical 11
B
NMR spectrum at the 5 h point during the reduction of 7 (Table 2,
entry 1) by 6. At this point, resonances for the starting borate 6 (δB
37.1 and -44.7) still accompany those of the two new products.
The known borane 136a (Figure 4) is responsible for the signal
at δB 20, and indeed this product was isolated in excellent yield
from both the ionic (97%) and organometallic (81%) reductions
(entries 1 and 4). We attribute the intermediate signal at δB -35.5
to the sp3 boron atom in boryldihydroiodoborate 14,11 whose other
boron signal presumably overlaps with that of the starting material
6. The instability of 14 resembles the analogous carbene borane
(dipp-Imd-BH2I), which can also be readily made and characterized
in situ4d but reverts to the corresponding imidazolium salt on
workup and chromatography.
(7) Ramnial, T.; Jong, H.; McKenzie, I. D.; Jennings, M.; Clyburne, J. A. C.
Chem. Commun. 2003, 1722–1723.
(8) Beall, H.; Bushweller, C. H. Chem. ReV. 1973, 73, 465–486.
(9) Ehemann, M.; No¨th, H.; Schmidt-Sudhoff, G. Z. Anorg. Allg. Chem. 1972,
394, 33–52.
(10) (a) Haubold, W.; Hrebicek, J.; Sawitzki, G. Z. Naturforsch., B: Chem. Sci.
1984, 39, 1027–1031. (b) Nguyen, P.; Dai, C.; Taylor, N. J.; Power, W. P.;
Marder, T. B.; Pickett, N. L.; Norman, N. C. Inorg. Chem. 1995, 34, 4290–
4291. (c) Clegg, W.; Dai, C. Y.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.;
Norman, N. C.; Pickett, N. L.; Power, W. P.; Scott, A. J. J. Chem. Soc.,
Dalton Trans. 1997, 839–846. (d) Grigsby, W. J.; Power, P. P. Chem.sEur.
J. 1997, 3, 368–375. (e) Neu, A.; Mennekes, T.; Paetzold, P.; Englert, U.;
Hofmann, M.; von Rague´ Schleyer, P. Inorg. Chim. Acta 1999, 289, 58–
69. (f) Clegg, W.; Marder, T. B.; Nlate, S.; Scott, A. J. Act. Crystallogr.,
Sect. C 2007, 63, 603–605.
(11) Likewise, in the reduction of 7 with LiBH4 (Table 2, entry 2), we observed
the formation of LiBH3I. See the Supporting Information.
In summary, we have prepared and characterized the first
borylborohydride: lithium [1,3-bis(2,6-diisopropylphenyl)-2,3-di-
JA105277U
9
J. AM. CHEM. SOC. VOL. 132, NO. 33, 2010 11451