Alkali Metal Hydridotriphenylborates [(L)M][HBPh3] (M = Li, Na, K): Chemoselective Catalysts for Carbonyl and CO2 Hydroboration

Article abstract of DOI:10.1021/jacs.6b06319

Light alkali metal hydridotriphenylborates M[HBPh₃] (M = Li, Na, K), characterized as tris{2-(dimethylamino)ethyl}amine (L) complexes [(L)M][HBPh₃], act as efficient catalysts for the chemoselective hydroboration of a wide range of a

Full text of DOI:10.1021/jacs.6b06319

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Alkali Metal Hydridotriphenylborates [(L)M][HBPh3] (M = Li, Na, K):
Chemoselective Catalysts for Carbonyl and CO2 Hydroboration
Debabrata Mukherjee, Hassan Osseili, Thomas Paul Spaniol, and Jun Okuda
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06319 • Publication Date (Web): 11 Aug 2016
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Alkali Metal Hydridotriphenylborates [(L)M][HBPh ] (M = Li,
7
8
9
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1
1
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4
5
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5
5
6
3
Na, K): Chemoselective Catalysts for Carbonyl and CO Hy-
2
0
1
2
3
4
5
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7
8
9
0
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0
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0
droboration
†
†
Debabrata Mukherjee, Hassan Osseili, Thomas P. Spaniol, and Jun Okuda*
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany.
Supporting Information Placeholder
—•
12
taminated with the BPh3 radical anion. Presumably, the
ABSTRACT: Light alkali metal hydridotriphenylborates
ancillary Me TREN renders a stronger hydridic character
M[HBPh ] (M = Li, Na, K), characterized as tris{2-(dimethyl-
6
3
onto the Si−H bonds in 1-3 to facilitate the process.
amino)ethyl}amine (L) complexes [(L)M][HBPh3], act as
efficient catalysts for the chemoselective hydroboration of a
wide range of aldehydes and ketones using pinacol borane
HBpin. The lithium derivative showed a remarkably high
Scheme 1. Synthesis of [(L)M][HBPh ] (4-6) from
3
[
(L)M][N(SiHMe ) ] (1-3).
2 2
−1
TOF of ≥ 17 s . These compounds also catalyze the hydrobo-
L
M{N(SiHMe
2
)
2
}
2 2
[(L)M][N(SiHMe ) ]
rative reduction of CO2 to give formylborane HCO Bpin
solvent
2
30 min, 25 °C
M = Li, 1; solvent = pentane
M = Na, 2; K, 3; solvent = THF
without any over-reduction.
2
Me N
NMe
2
N
BPh
3
Me N
THF
2
In contrast to saline light alkali metal hydrides MH (M = Li,
2
h, 25 °C
6
Me TREN = L
Na, K), hydridoborates M[HBR ] (R = alkyl, aryl, alkoxy, etc.)
3
L
are widely used as stoichiometric reducing agents in organic
M[HBPh3]
[(L)M][HBPh3]
1
THF
0 min, 25 °C
synthesis. The nature of the metal M and the boron substit-
M = Li, 4; Na, 5; K, 6
3
2
uents R influence the selectivity. For instance, Li[HBEt ] is
3
3
strongly reducing and less selective, while K[HBPh ] is mild
and chemoselective for carbonyl reduction. Although hydri-
3
The borohydrides 4-6 are colorless crystalline solids and
insoluble in aliphatic and aromatic hydrocarbons but highly
soluble in THF. Their H NMR spectra in THF-d suggest a
4
dotriphenylborates M[HBPh ] (M = Li, Na, K) could be syn-
1
3
4
a,5
8
thesized from the reaction of saline MH with BPh , only a
4
3
C3v symmetric κ -coordination of Me TREN. Characteristic
doublets at around δ −8.2 ppm ( J = 78 Hz) in the B NMR
spectra are attributed to [HBPh ] . Complexes 4 and 6 were
6
−
few compounds containing the [HBPh ] anion have been
reported and they are rarely applied. Recently, nucleophilic
main group metal hydrides and 1,3,2-diazaphospholene
1
11
3
4
,6
BH
−
7
8
3
also characterized by single crystal X-ray crystallography
received attention as catalysts for carbonyl and CO hydrosi-
13
2
(
Figure 1).
lylation or hydroboration. Provided the hydridic B-H bond in
−
9
[HBPh ] would allow insertion followed by σ-bond metath-
3
7
−
10
esis, catalysis by [HBR ] would be conceivable.
3
Herein, we report that a series of well-defined, soluble group
1 metal hydridotriphenylborates [(L)M][HBPh ] (M = Li, 4;
3
Na, 5; K, 6) show catalytic activity towards the hydroboration
of carbonyls and CO . Coordination by the tetradentate
2
tris{2-(dimethylamino)ethyl}amine (Me TREN = L), known
6
to induce deaggregation of alkali metal compounds, was
11
crucial.
Figure 1. Molecular structures of 4 and 6. Displacement
parameters are shown at 50% probability. Hydrogen atoms
except the B-H's are omitted for clarity.
Complexes 4-6 were prepared by treating M[HBPh ] with
3
Me TREN in THF, but can also be synthesized in high yields
6
following a BPh mediated β-SiH abstraction from the easily
3
accessible tetramethyldisilazides 1-3 in THF (Scheme 1).
While the lithium compound 4 is a separate ion pair, the
potassium 6 is zwitterionic. The sodium analogue 5 is also a
separate ion pair, but the poor quality of the X-ray data pre-
vents further discussion. The THF molecule in 4 presumably
binds during the crystallization process. For comparison,
Cyclodisilazane (Me HSiN−SiMe ) , the head-to-tail dimer of
2
2 2
silaimine Me HSiN=SiMe , was identified as the major by-
2
2
product. This synthetic route is clean, unlike Brown's earlier
attempts from tBuLi and BPh , which gave Li[HBPh ] con-
3
3
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e
both [(L)Li][HBEt ] and [(L)Li][BH ] are neutral compounds
with bridging hydrides and soluble in benzene. Complex 6
11
p-X-C H
4
H
H
H
H
4 (0.01)
4 (0.01)
4 (0.01)
4 (0.01)
<0.17
<0.17
<0.17
48
≥60
3
4
6
1
1a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
2
CH CH CH
2
≥60
≥60
0.21
3
2
is structurally related to that of [(L)KCH Ar] (Ar = C H ,
2
6
5
1
1c,d
11f
1
3
Cyclohexyl
C H Me )
and [(L)KSiPh ]. One phenyl ring of the
3
6
3
2
−
3
f
[HBPh3] is η -coordinated to the potassium via the ipso-
/ortho- carbons and is significantly distorted. The K∙∙∙H dis-
tance (3.21 Å) is too long to be a bonding interaction. The
boron centers in both 4 and 6 have nearly perfect tetrahedral
geometry.
14
PhCH=CH
f
Li[HBPh
0.01)
3
]
g
48
d
15
PhCH=CH
H
n.c.
(
f
d
16
2-Cyclohexen-1-one
4 (1)
<0.17
n.c.
a
Carbonyl compounds such as ketone Ph CO and aldehyde
PhCHO readily inserted into the B-H bonds of 4-6 (Scheme
2
HBpin = 0.27 mmol, substrate = 0.27 mmol, 0.5 mL of sol-
b
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
vent. Time for complete substrate consumption, detected
by NMR spectroscopy. 23% conversion. n.c. = not calculat-
ed. X = H, Me, OMe, NO , CN, Br, and F. regioselective 1,2-
reduction. 64% conversion.
c
d
2) to give alkoxyborates, which were characterized in situ by
e
f
solution NMR spectroscopy. Several other unsaturated func-
tional groups did not react.
2
g
Scheme 2. Chemoselective carbonyl insertion in 4-6.
Benzophenone was chosen as the model substrate to com-
pare the catalysts. Lithium 4 exhibited supreme efficiency
and a catalyst loading as low as 0.001 mol% led to complete
reduction within 1.5 h, providing a remarkably high TOF of
3
−1
−1
6
6.6×10 h or 17 s (Table 1, entry 2). Complexes 5 and 6 are
similar in activity but much poorer compared to 4 (entries 3
and 4). Ligand-free Li[HBPh ] was also active but not as
much as 4 (entry 5). Thus the coordination of Me TREN is
critical for high activity. The activity of commercially availa-
ble Li[HBEt ] was almost the same as that of Li[HBPh ] (en-
try 6), but unlike 4, [(L)LiHBEt ] was much less active (entry
7
[nBu N][HBPh ] was close to that of 5 and 6 (entry 8). BPh
4 3 3
was only marginally active compared to the other investigat-
ed catalysts (entry 9), which emphasizes the importance of
3
6
3
3
3
). The TOF of the metal-free alkylammonium salt
1
9
Selective reduction of carbonyls in the presence of other
reducible functional groups is challenging. Catalytic process-
1
4
7b,c,8,15
es such as hydrosilylation, hydroboration,
hydrogena-
1
6
17
tion, and transfer hydrogenation have been investigated to
achieve this chemoselective transformation, including enan-
tioselective reduction. As summarized in Table 1, complexes
-6 were found to catalyze carbonyl hydroboration with
the Ph
3
B−H bond for fast catalysis. The high activity of the Li
catalyst is probably due to its higher degree of polarization in
1
8
−
the [RR'CHOBPh
] intermediate as compared with Na and
The reduction of other aromatic and aliphatic ketones
and aldehydes was accomplished by using 0.01 mol% of 4
entries 7-11), showing the applicability to a wide range of
functional groups. In comparison, a copper carbene catalyst
3
20
4
K.
pinacolborane (HBpin) under mild condition.
(
Table 1. Carbonyl hydroboration catalyzed by hydri-
a
dotriphenylborates.
3
−1
had the previously highest TOF of 1×10 h for Ph CO hy-
2
3
15f
−1
drob0ration. For aldehydes, a TOF of above 13.3×10 h was
reported for a molecular tin(II) hydride catalyst. Among the
main group catalysts, the most active molecular magnesium
7c
3
−1
3
−1
catalyst recorded a TOF of 0.5×10 h for Ph CO and 8×10 h
2
1
5a
TOF
for PhCHO. Reduction of α,β-unsaturated cinnamaldehyde
took place exclusively at the 1,2-position, but at a much slow-
er rate (entry 14). The reaction time of 48 h showed the lon-
gevity of the catalyst, though. Catalyst 4 was again a better
Catalyst
mol%)
Time
(h)
3
Entry
R
R'
b
(10
(
−
1
h )
≥60
66.6
0.2
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4 (0.01)
4 (0.001)
5 (0.1)
<0.17
choice than the parent Li[HBPh ] (entry 15). 2-Cyclohexen-1-
3
2
3
4
1.5
5
one was also rapidly reduced in 1,2-fashion with a higher
catalyst loading (entry 16). Benzophenone was reduced on a
mmol scale to show practical applicability. Notably, sub-
strates such as esters, amides, and pyridine were not re-
duced, even at a higher loading of 4 and under forcing condi-
tions.
6 (0.1)
5
0.2
Li[HBPh3]
5
6
7
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
0.75
13.3
10
(
0.01)
LiHBEt3
To verify the chemoselectivity, a series of competitive hy-
droboration experiments were carried out by subjecting
stoichiometric mixtures of benzophenone with all the other
non-carbonyl substrates listed in Scheme 2 in the presence of
1
(
0.01)
[
(L)LiHBEt3]
6
6
1.67
0.16
(
0.01)
0
.1 mol% of 4. In all cases, the benzophenone was selectively
[nBu N][HB
4
8
9
reduced immediately. This also proves that the competing
groups do not inhibit the catalysis. Furthermore, catalyst 4
exhibited “living” behavior: once the first loading was con-
sumed, the same reaction mixture successfully completed
Ph ] (0.1)
21
3
c
d
Ph
Ph
Ph
BPh (0.1)
5
n.c.
3
10
Me
4 (0.01)
<0.17
≥60
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Table 2. Hydroboration of CO with HBpin catalyzed
two more runs with equal activity. Additionally, catalyst 4
reacts with air or moisture very slowly, leading to only partial
decomposition at ambient temperature after 24 h.
2
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
by hydridotriphenylborates.
O
O
Catalyst (1 mol%)
THF, 25 °C
O
O
Selective reduction of aldehyde over ketone is another syn-
CO
2
+
B H
B
O
O
1
5g,22
thetically important transformation.
Quite remarkably,
H
(
1 atm)
0.1 mol% of 4 preferentially reduced benzaldehyde over ben-
zophenone with 95% selectivity.
−1
Entry
Catalyst
Time
TOF h
10
Scheme 3. CO insertion into the B−H bonds of 4-6.
2
1
4
5
10 h
16 h
16 h
50 h
CO
2
(1 atm)
2
3
4
6.25
6.25
2
[
(Me
6
TREN)M][(HBPh
3
)]
[(Me
6 2 3
TREN)M(O CHBPh )]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
THF, 25 °C
6
M = Li (4), Na (5), K (6)
M = Li (7), Na (8), K (9)
[(nBu )N][HBPh ]
4
3
a
HBpin = 0.14 mmol, 0.5 mL of solvent.
Rapid CO (1 atm) insertion took place into the B-H of
2
[
HBPh3]− in 4-6 under ambient conditions (Scheme 3) to
afford the formyltriphenylborate complexes
[(L)M{(HCO )BPh }] (M = Li, 7; Na, 8; K, 9), which were fully
Noticeably, the present system selectively provided the pri-
marily reduced formoxyborane (HCO Bpin). Adding an extra
2
2
3
equiv of HBpin after completion did not result in further
reduction. This also explains the inactivity of this system
towards esters. A Cu and a Pd catalyst are the only two
other examples known for this selectivity. The lithium cata-
lyst was again better than the others (Table 2, entries 1-4).
Overall, the activities are much lower compared to the car-
characterized. This has been previously reported only on a
single occasion in a Pd/Pt system, and that too without
29
30
23
structural characterization. In contrast, CO2 insertion in
−
[HB(C F ) ] is better studied, but requires harsher condi-
6
5 3
24
tions (t ≥100 ˚C or PCO2 ≥1 atm) and longer reaction times.
2
9
31
bonyl hydroboration, but superior to the Cu and Ru cata-
lysts, and also the two other main group metals.
3
2
Currently we propose simplified mechanistic
scheme that takes into account the experimental
finding that stoichiometric reactions of 4 with Ph CO
as well as the intermediate with HBpin are complete
instantaneously. Although rapid hydride exchange
between [BH ] and BR (R = alkyl) was postulated
before, we failed to observe an exchange between
HBPh ] and HBpin, as H and B NMR spectra of a
3
:3.5 mixture of 4 and HBpin in THF-d are identical
a
7
2

4
3
1
0

1
11
[
1
8
to the individual spectra (see SI). The actual mecha-
nism could be more complex since alkali sodium
alkoxides were reported to catalyze the addition of HBpin
1
5h
33
to carbonyls. Further mechanistic and computational stud-
Figure 2. Molecular structures of 7-9. Displacement parame-
ters are shown at 50% probability. Hydrogen atoms are omit-
ted for clarity.
ies are currently underway.
Scheme 4. Proposed catalytic cycle for the carbonyl
and CO hydroboration mediated by 4.
2
X-ray analysis showed that the formyl moieties are bridging
between the metal and the boron with a varying coordina-
[(L)Li][HBPh3]
25
tion pattern (Figure 2). The M∙∙∙O1 distance systematically
RR'HCOBpin
O
changes following the trend Li > Na > K. In 7 and 9, the for-
or
1
2
HCO Bpin
2
R
R'
mate has η and η coordination to Li and K, respectively,
while 8 has an intermediate situation. Variation in anion
coordination was also observed in their benzyl and mesityl
or
CO2
HBpin
1
1c,d
complexes.
[(L)Li][XOBPh3]
The hydridotriphenylborates 4-6 catalyzed the hydrobora-
X = CHRR', C(O)H
tion of CO as shown in Table 2. Boron-mediated CO se-
2
2
The perfluorinated analogues [(L)M][HB(C F ) ] (M = Li, 10;
7b,24
6 5 3
questration operates under mild conditions.
CO reduc-
2
Na, 11; K, 12) were synthesized by treating 1-3 with B(C F ) .
Their physical and spectroscopic properties are similar to
those of 4-6, but they were totally inert towards PhCHO or
CO (1 atm). This can be explained by the decreased hy-
6
5 3
tion by B-H bonds traces back to as early as the 1950s, when
26
the reaction of CO with NaBH was reported. Commercial
2
4
BH (THF) solution also reacts with CO (1 atm) at room
3
2
2
temperature, with the reaction actually promoted by the 0.5
9,24
dridicity of the [HB(C F ) ] anion.
2
7
6 5 3
mol% of NaBH4 present as a stabilizer. Some metal and
In conclusion, we have described the exceptional perfor-
mance of compounds 4-6 as a chemoselective hydroboration
catalysts for carbonyl and CO that outcompetes many other
metal catalysts. Apparently the combination of the Lewis
acidic alkali metal and the hydridic borate results in efficient
organic catalysts have emerged for CO hydroboration that
2
use catecholborane (HBcat) or HBpin to produce formate,
24
2
acetal, and methoxide derivatives. A few hydroborates were
28
shown to catalyze this process.
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2
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4
5
6
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8
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1
1
1
1
1
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3
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
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5
5
5
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(
ASSOCIATED CONTENT
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Corresponding Author
*jun.okuda@ac.rwth-aachen.de
Author Contributions
†
These authors contributed equally.
(
i) King, A. E.; Stieber, S. C. E.; Henson, N. J.; Kozimor, S. A.; Scott, B.
L.; Smythe, N. C.; Sutton, A. D.; Gordon, J. C. Eur. J. Inorg. Chem.
016, 1635.
16) Scott, D. J.; Fuchter, M. J.; Ashley, A. E. J. Am. Chem. Soc. 2014,
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Notes
The authors declare no competing financial interests.
2
(
1
ACKNOWLEDGMENT
(
17) (a) Johnstone, R. A. W.; Wilby, A. H.; Entwistle, I. D. Chem. Rev.
We thank the Deutsche Forschungsgemeinschaft through
the International Research Training Group “Selectivity in
Chemo- and Biocatalysis” for financial support and the Alex-
ander von Humboldt Foundation for a fellowship to D.M.
We also thank K.-N. Truong and Prof. U. Englert for collect-
ing X-ray data.
1985, 85, 129. (b) Su, F.-Z.; He, L.; Ni, J.; Cao, Y.; He, H.-Y.; Fan, K.-N.
Chem. Commun. 2008, 3531.
18) (a)
(
Hirao, A.; Itsuno, S.; Nakahama, S.; Yamazaki, N., J. Chem. Soc., Chem.
Commun.1981, 315. (b) Corey, E. J.; Bakshi, B. K.; Shibata, S.; J. Am.
Chem. Soc. 1987, 109, 5551.
n
(19) Synthesized from the reaction between K[HBPh
3
] and ( Bu)
4
NI.
REFERENCES
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20) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry: A
(
1) Brown, H. C.; Ramachandran, P. V. In Reductions in Organic
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(21) Catalysis in the presence of PhCH OH required 2 equiv of HBpin
2
(
as the uncatalyzed -OH/-BH dehydrogenative coupling was also in
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M[(RO)2Bpin] that undergoes reaction with HBpin to form the
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conclusive NMR spectroscopic evidence for these species or any
effect by free BPh3.
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