Transition-metal-catalyzed synthesis of diboranes(4)

Article abstract of DOI:10.1002/anie.201104854

The diboranes(4) bis(catecholato)diborane (B₂Cat₂) and bis(pinacolato)diborane (B₂Pin₂) are important precursors for organoboronic esters, which are versatile reagents for the formation of carbon-carbon bonds. A new catalytic synthesis for these compounds starts from catecholborane or pinacolborane and gives the dehydrocoupling products B₂Cat₂ and B₂Pin₂ with turnover numbers of up to 11 600 (see scheme). Copyright

Full text of DOI:10.1002/anie.201104854

DOI: 10.1002/anie.201104854
Dehydrocoupling of Boranes
Transition-Metal-Catalyzed Synthesis of Diboranes(4)
Holger Braunschweig* and Frank Guethlein
Organoboranes are organic molecules featuring an sp2-
hybridised BR2 group. This BR2 fragment, with its well-
À
documented s basicity and p acidity, lends the C B bond a
unique reactivity that has been impressively exploited by
organic chemists. In the early 20th century, organoboranes
featured in organic chemistry merely as placeholders for
À
À
hydroxy groups, the oxidation of a C BR2 bond to C OH
being a reliable method for the synthesis of alcohols.
However, it was only after the discovery of the Suzuki–
Miyaura cross-coupling of organoboranes with aryl halides
that this functional group became widely used in organic
chemistry.[1] This method has since developed into one of the
Scheme 1. Typical synthesis of bis(pinacolato)diborane.
À
most versatile synthetic methods available for C C bond
directly obtained from reductive coupling of chlorocatechol-
borane with sodium amalgam, while the pinacol derivative 2 is
not accessible by this route.[11] However, owing to the harsh
conditions and toxicity of the reagents, reductive coupling
procedures have distinct disadvantages. Furthermore, these
multistep methods are tedious, and owing to the loss of the
dimethylamino groups, the overall process leads to large
amounts of waste material, making the diboranes(4) rela-
tively expensive.
Starting from catecholborane (HBCat) and pinacolborane
(HBPin), which can be prepared and handled with relative
ease,[12] we sought to develop catalytic syntheses for B2Cat2
and B2Pin2. Homonuclear dehydrocoupling reactions of this
kind have been established for the formation of several
element–element bonds, such as silicon,[13] phosphorus,[14] and
others.[15] The formation of a boron–boron bond was reported
by Sneddon and Corcoran, Jr. in the reaction of boranes and
carboranes with PtBr2, while Himmel et al. recently obtained
a doubly base-stabilized diborane(4) from a corresponding
diborane(6) by a rhodium-catalyzed dehydrogenation.[16]
formation and it was the subject of the Nobel Prize in
Chemistry in 2010.[2] Consequently, the need for convenient
and economical methods for the installation of the BR2 group
in organic molecules has fuelled research into organoborane
chemistry for some time.
The hydroboration of unsaturated organic substrates
developed by Brown and co-workers in the 1950s brought
organoborane chemistry out of obscurity and added boron to
the organic chemistꢀs repertoire.[3] For a long time, simple
hydroboration remained the method of choice for the syn-
thesis of organoboronates. In the 1980s, the groups of
Sneddon, Marder, and Nçth reported the transition-metal-
catalyzed hydroboration reaction, thus providing access to
borane products with alternative chemo- and regioselectivity
to complement those prepared by classical hydroboration.[4]
Furthermore, catalytic diboration of unsaturated com-
pounds[5] as well as the borylation of arenes[6] and alkanes[7]
gave access to a much broader variety of organoboronates.
An indispensable starting material in many of these
reactions is a tetraalkoxydiborane(4) of the general formula
À
During the catalytic borylation of C H bonds with HBPin,
À
(RO)2B B(OR)2. The two most commonly used reagents of
Marder et al. observed the formation of small amounts of 2 as
a byproduct, whereas an equilibrium between HBPin and 2 +
H2 is assumed under these reaction conditions (Scheme 2).[17]
Although there is no significant thermodynamic driving force
for this dehydrocoupling,[18] removal of H2 from the reaction
mixture is expected to shift the equilibrium to the side of the
diboranes(4), thus facilitating their accumulation. Herein we
present our successful studies on the dehydrocoupling of
HBCat and HBPin to give the corresponding diboranes(4) by
homogeneous and heterogeneous catalysis.[19]
this class of compounds are bis(catecholato)diborane (B2Cat2;
1) and bis(pinacolato)diborane (B2Pin2; 2), which are cur-
rently synthesized by a method established by Brotherton in
1960,[8] which has been modified several times over the
intervening decades.[9] Starting from boron tribromide,
bromobis(dimethylamino)borane is synthesized in two steps.
Subsequently, formation of the boron–boron bond is achieved
by reductive coupling with sodium to yield B2(NMe2)4 (3),
which is further converted into 1 and 2 upon reaction with the
corresponding diols (Scheme 1).[9a,d,10] Alternatively, 1 can be
[*] Prof. Dr. H. Braunschweig, F. Guethlein
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
Scheme 2. Transition-metal-catalyzed synthesis of diboranes(4).
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To obtain a proof-of-principle for the catalytic homocou-
platinum on charcoal and platinum on alumina afforded
TONs of 190 and 220, respectively. Reducing the catalyst
loading even further to only 0.006 mol%, we showed that the
TON can be again significantly increased, giving a TON of
350 (based on isolated 1).
pling, HBCat and HBPin were combined with various late
transition-metal complexes in a variety of solvents. We
detected the diboranes(4) by GC/MS, although at best only
in trace amounts. We reasoned that a higher concentration of
monoborane in the reaction solution may promote the
formation of B2Cat2 and B2Pin2, in accordance with the
observations of Marder et al.[17] This hypothesis was key to the
development of the coupling procedure: conducting the
reactions in neat borane led to a dramatic improvement in
diborane(4) formation (Table 1).
Given the surprising success of the B2Cat2 synthesis, we
sought to apply this catalytic dehydrocoupling method to
pinacolborane (HBPin). However, this reaction is much less
straightforward than the synthesis of B2Cat2, especially under
homogeneous catalysis conditions. The aforementioned cata-
lysts suitable for the formation of B2Cat2 afforded TONs of no
more than 10 in the case of B2Pin2. Furthermore, the
formation of the latter is accompanied by various side
products, of which PinBOH and (PinB)2O were identified
by mass spectrometry and NMR spectroscopy.[20] Neverthe-
less, we managed to synthesize B2Pin2 under heterogeneous
catalytic conditions using palladium on charcoal and platinum
on charcoal (0.025 mol%), both catalysts affording TONs of
60. As in the case of 1, the output of 2 could be improved by
further reducing the catalyst loading. In the reaction of HBpin
with 0.005 mol% of platinum on alumina, a TON of 93 (based
on isolated 2) was achieved after 8 h.
Table 1: Synthesis of B2Cat2 using homogeneous catalysts.[a]
Entry
Catalyst (loading [mol%])
TON
1
2
3
4
5
6
7
8
[(dppm)PtCl2] (0.05%)
[(dcpe)PtCl2] (0.05%)
[(dcpe)PdCl2] (0.05%)
[(dcpe)NiCl2] (0.05%)
[(dppm)PtCl2] (0.025%)
[(dcpe)PtCl2] (0.025%)
[(dcpe)PdCl2] (0.025%)
[(dcpe)NiCl2] (0.025%)
90
65
75
35
160
90
90
50
For the dehydrocoupling reactions described herein, the
reaction temperature must be chosen carefully. In general,
formation of the diboranes(4) can be observed at temper-
atures as low as 758C, but higher temperatures enhance their
formation significantly. However, boranes such as HBCat are
known to decompose under prolonged heating,[12a] and HBPin
is no exception. Our experiments showed that the best results
could be achieved at temperatures between 1058C and 1108C,
which is only marginally below the boiling points of HBCat
and HBPin. Although the rate of the dehydrocoupling
decreases as the reaction progresses, the yield of diborane(4)
can still be increased slightly by prolonged heating. However,
reaction times exceeding 24 h are best avoided, as degrada-
tion of HBCat and HBPin increases and starts to predominate
over the formation of B2Cat2 and B2Pin2.
[a] Reaction conditions: neat borane is heated with the catalyst to 1108C
for 20 h.
Initially, synthesis of B2Cat2 was attempted under homo-
geneous catalytic conditions. When employing a typical
catalyst loading of 0.05 mol% (Table 1, entries 1–4),
[(dppm)PtCl2]
(dppm = bis(diphenylphosphino)methane)
appears to be the most efficient catalyst, yielding a turnover
number (TON) of 90. With [(dcpe)PdCl2] (dcpe = bis(dicy-
clohexylphosphino)ethane) and [(dcpe)PtCl2], slightly lower
TONs of 75 and 65 were observed. A larger excess of HBCat
(0.025 mol% of catalyst; Table 1, entries 5–8) significantly
improved the TONs to a maximum of 160 for [(dppm)PtCl2].
The next logical step was to investigate the reactivity of
HBCat with common heterogeneous catalysts. When using
0.05 mol% of catalyst (Table 2, entries 1–3), the best results
were achieved with platinum on charcoal (10 wt%) and
platinum on alumina (0.5 wt%), affording 105 and 95 turn-
overs, respectively. In contrast to the homogeneous catalysts,
palladium appears to be far less efficient than platinum, as
palladium on charcoal (10 wt%) yielded a TON of only 35.
Again, by decreasing the catalyst loading, the formation of
B2Cat2 was improved significantly (Table 2, entries 4–6), since
Complete conversion of borane to diborane(4) was not
observed, however. Although strong evolution of hydrogen
occurs initially, a significant decrease is apparent after several
hours. Investigation by GC-MS showed that the rate of
formation of 1 and 2 decreases with increasing reaction time,
and the overall yield does not exceed 5%.
Therefore, we carried out additional experiments to
establish the reason for the decreasing rate of formation of
diboranes(4). The most apparent reason, that is, deactivation
of the catalyst, could be ruled out, as reactions of used
heterogeneous catalyst with fresh borane resulted in almost
identical TONs. Given that a large excess of borane yielded
the best results to date, we reasoned that an increasing
concentration of diborane(4) accumulating in the reaction
mixture might inhibit the reaction. In this case, continuous
removal of the diboranes 1 and 2 from the reaction mixture
should drive the dehydrocoupling forward. We achieved this
by using a Soxhlet-type reaction vessel, in which freshly
distilled, neat borane is continuously reacted with the hot
heterogeneous catalyst, whereas 1 and 2 are purged out of the
reaction zone immediately after their formation (for details
and experimental setup, see the Supporting Information).
Table 2: Synthesis of B2Cat2 using heterogeneous catalysts.[a]
Entry
Catalyst (loading [mol%])
TON
1
2
3
4
5
6
7
Pd on charcoal (0.05%)
Pt on charcoal (0.05%)
Pt on alumina (0.05%)
Pd on charcoal (0.025%)
Pt on charcoal (0.025%)
Pt on alumina (0.025%)
Pt on alumina (0.006%)
35
105
95
75
190
220
350
[a] Reaction conditions: neat borane is heated with the catalyst to 1108C
for 20 h.
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c) H. C. Brown, Hydroboration, Wiley-Interscience, New York,
1962.
The use of this alternative procedure resulted in dramatic
increases in the TONs. The catalysts of choice were Rh, Pd,
and Pt supported on alumina (0.5 wt%), as they are
commercially available in pellet form and can thus be kept
within the reaction chamber with ease. For the formation of
B2Pin2, TONs of 460, 670, and 1850 were determined by GC.
In the case of B2Cat2, the TONs could be raised even further
to 11600 for Pt and 6500 and 3920 for Rh and Pd, respectively
(Table 3). A typical experiment for the synthesis of 1 starting
from 30.0 g of HBCat yielded 8.05 g (27%; 8854 TONs) of
analytically pure B2Cat2 after workup, and about 10 g of pure
HBCat could be recovered by distillation for further use (see
the Supporting Information for full details).
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Table 3: Heterogeneous catalysis with continuous removal of 1 and 2.[a]
Catalyst (reaction time)
Product
TON
[GC]
TON
(isolated)
Rh on alumina (20 h)
Pd on alumina (40 h)
Pt on alumina (20 h)
Rh on alumina (20 h)
Pd on alumina (40 h)
Pt on alumina (48 h)
1
1
1
2
2
2
6500
3920
11600
460
670
1850
3707
2157
8854
284
513
1050
[a] Reaction conditions: neat borane is continuously distilled into a
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In summary, we have reported a new one-step procedure
for the synthesis of two of the most widely used and
synthetically relevant diboranes(4), B2Cat2 and B2Pin2, by
metal-mediated, catalytic dehydrocoupling of the commer-
cially available boranes HBCat and HBPin. Although the
homocoupling of both boranes using both homogeneous and
heterogeneous catalysts was successful, the practice of con-
tinuous removal of B2Cat2 and B2Pin2 provided a marked
increase in efficiency. This effect was particularly salient in the
synthesis of B2Cat2 with platinum on alumina, which pro-
ceeded with TONs of up to 11600. Further studies, including
the use of different catalysts and boranes, are ongoing in our
laboratory.
Received: July 12, 2011
Revised: September 8, 2011
Published online: November 4, 2011
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Keywords: dehydrocoupling · diborane(4) ·
heterogeneous catalysis · homogeneous catalysis
.
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