Grignard reagents-catalyzed hydroboration of aldehydes and ketones

Article abstract of DOI:10.1016/j.tet.2020.131145

Simple, commercially available Grignard reagents have been used as highly efficient precatalysts for the hydroboration of a wide range of aldehydes and ketones. The reaction employs very low catalyst loadings (aldehydes: 0.05 mol%, ketones: 0.5 mol%), and

Full text of DOI:10.1016/j.tet.2020.131145

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Grignard reagents-catalyzed hydroboration of aldehydes and ketones
a
a
a
b
b
c
Weifan Wang , Kai Lu , Yi Qin , Weiwei Yao , Dandan Yuan , Sumod A. Pullarkat ,
a
a, *
Li Xu , Mengtao Ma
College of Science, Nanjing Forestry University, Nanjing, 210037, People’s Republic of China
College of Pharmacy, Nanjing University of Chinese Medicine, Nanjing, 210023, People’s Republic of China
Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371,
a
b
c
Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 February 2020
Received in revised form
Simple, commercially available Grignard reagents have been used as highly efficient precatalysts for the
hydroboration of a wide range of aldehydes and ketones. The reaction employs very low catalyst loadings
(
aldehydes: 0.05 mol%, ketones: 0.5 mol%), and proceeds rapidly (aldehydes: 10 min, ketones: 20 min)
15 March 2020
under neat condition at room temperature. The Grignard reagent catalyst demonstrated good substrate
scope, functional group tolerance, and high chemoselectivity in the carbonyl hydroboration. DFT calcu-
lations were performed to investigate the possible reaction mechanism. In contrast to the traditional
stoichiometric use of Grignard reagents, this newly developed protocol provides a catalytic application of
these reagents for molecular transformations.
Accepted 16 March 2020
Available online xxx
Keywords:
Grignard reagents
Hydroboration
Solvent-free
Aldehyde
©
2020 Elsevier Ltd. All rights reserved.
Ketone
1
. Introduction
with aldehydes and ketones to form alcohols. When reacted with
another halogenated compound in the presence of a suitable
catalyst, they can also be used as transfer reagents for alkyl and aryl
moieties in the CeC cross-coupling reaction to increase the carbon
chain length of products [13ꢀ16], and even reacted with carbon
dioxide to generate the corresponding carboxylic acid containing
one more carbon atom than that of the starting material [17].
However, to our surprise, the amount of Grignard reagents
employed in these protocols are stoichiometric and very few cat-
alytic applications of Grignard reagents have been reported to date
[18,19].
Hydroboration of carbonyl compounds is an attractive organic
transformation among the various carbonyl reduction methodolo-
gies available because boron hydrides are relatively stable and
helps to circumvent the use of highly flammable and pressurized
hydrogen gas and stoichiometric amounts of metal hydride re-
agents. In addition, the resultant borate esters are versatile syn-
thetic intermediates which can be further hydrolyzed to various
functional alcohols [20ꢀ23]. Until now, the catalytic hydroboration
of aldehydes and ketones is achieved with numerous catalysts
based on transition metals and main group elements [24]. Among
main group elements, literature reports about the earth-abundant
and inexpensive alkaline earth metal catalyzed hydroboration of
carbonyls are relatively few [25e33]. In most instances, these
Since their discovery by Victor Grignard in 1900, Grignard re-
agents have proved to be an extremely powerful and ubiquitous
synthetic tool in synthetic chemistry due to their ease of prepara-
tion and broad application in organic and organometallic synthesis
[
1,2]. Over the past 100 years and more since their discovery,
considerable efforts have been made to investigate the formation,
structure, reactivity, application, and mechanism of Grignard re-
agents [3ꢀ6]. Grignard reagents, especially the classical
magnesium-based Grignard reagents (RMgX), are probably the
most widely used organometallic reagents in inorganic, organic
and organometallic chemistry to date. Nevertheless, the develop-
ment of organomagnesium chemistry is still vital and full of sur-
prises for their easy accessibility, high reactivity, improvement of
atom economy and widespread application. Not only do they find
extensive use on a small scale in many research laboratories
worldwide but they also have been prepared and utilized on a
larger scale in diverse industrial processe [7ꢀ12]. For example, one
of the most important uses of the Grignard reagent is the reaction
*
Corresponding author.
E-mail address: mengtao@njfu.edu.cn (M. Ma).
https://doi.org/10.1016/j.tet.2020.131145
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2
W. Wang et al. / Tetrahedron xxx (xxxx) xxx
catalysts stabilized by sterically hindered ligands are expensive and
relatively difficult to synthesize. Recently, Hreczycho and co-
workers reported catalyst-free and solvent-free hydroboration of
aldehydes. However, this protocol requires higher temperature and
longer reaction time to get better conversions. More importantly,
this method is completely ineffective for ketones and gave only
trace amounts of the desired boronic esters even at elevated tem-
perature [34]. Quite recently, Ma et al. reported hydroboration of
ketones in the absence of a catalyst, however, it also requires
excessive HBpin and higher temperature [35]. Therefore, it is
essential to develop a convenient and easily attainable alkaline
earth metal-based catalyst for hydroboration of both aldehydes and
ketones at ambient temperature which will offer a powerful and
sustainable alternative.
better than that of most reported alkaline earth metal-based
complexes [25e33](e.g. MeMgI: 0.05 mol%, 10 min, 99% yield vs
n
magnesium
alkyl
): 0.05 mol%, 15 min, in C
0.01 mol%, 12 h, in THF-d
complex
[CH{C(Me)NAr}
2
-Mg Bu]
i
(Ar ¼ 2,6- Pr
2
C
6
H
3
:
6
D
6
, 95% yield;
78% yield;
[Mg(thf) ][HBPh
6
3
]
2
8
,
i
[PhC(N Pr)
2
CaI]: 0.5 mol%, 40 min, in benzene, 83% yield). When
, C and CD CN were added into
organic solvents such as CDCl
3
6
D
6
3
the above reaction under the same conditions, only trace conver-
sion was observed (Table S1, entries 6e8).
With the optimized reaction conditions in hand, namely
0.05 mol% of MeMgI under neat condition at room temperature for
10 min, we explored the scope of the Grignard reagent catalyzed
hydroboration of aldehydes. The position of halogen on the phenyl
ring has no obvious effect on the reactivity as demonstrated by the
clean hydroboration of 2/3/4-chlorobenzaldehyde and 2/3/4-
fluorobenzaldehyde (Table 1, 1bꢀ1g). This result is different from
the rare-earth ytterbium catalyst reported previously by Ma group,
wherein the substituent position (o, m, p) on the benzene ring has
significant impact on the reactivity [38]. Both reactions employing
benzaldehydes bearing electron-withdrawing as well as those with
Herein, we report the highly efficient hydroboration of alde-
hydes and ketones with HBpin catalyzed by simple, commercially
available and inexpensive Grignard reagents in truly catalytic
amounts.
2
. Results and discussion
electron-donating substituents such as O
MeCOO- proceeded rapidly and gave quantitative yields (Table 1,
hꢀ1l). Even the sterically bulky mesitaldehyde gave an excellent
2
N-, Me-, MeO- and
We began our investigations into a plausible catalytic applica-
1
tion of Grignard reagents with the addition of only 0.05 mol% of
MeMgI as a catalyst to the mixture of benzaldehyde and HBpin
under neat condition. To our surprise, the corresponding borate
ester product was observed in a quantitative yield after 10 min at
room temperature (Table S1, entry 1). Other commercially available
Grignard reagents have been screened as well. All tested Grignard
reagents showed excellent catalytic activity that gave almost full
conversions under the same conditions (Table S1, entries 2e5). It
needs to be highlighted that the catalytic performance of these
Grignard reagents is better than most of transition metal catalysts,
and could be comparable to that of rare-earth metal-based catalysts
yield (Table 1,1j). When 4-acetyloxy substituted benzaldehyde was
used, the ester moiety was retained intact during the hydro-
boration process (Table 1, 1l). This trend is consistent with the fact
that hydroboration of esters is more difficult than that of aldehydes
due to their steric and electronic factors [41]. Aliphatic aldehydes
such as cinnamaldehyde and 9-anthraldehyde as well as hetero-
cyclic aldehydes such as 2-thiophenecarboxaldehyde and 3-
pyridinecarboxaldehyde all afforded the corresponding borate
ester products in very high yields (Table 1,1mꢀ1q). To our delight,
cinnamaldehyde underwent hydroboration exclusively at the
carbonyl group leaving the olefinic functionality intact when
[
36ꢀ40]. In addition, the reactivity of Grignard reagent is indeed
Table 1
Hydroboration of Aldehydes Catalyzed by MeMgI .
a
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1

W. Wang et al. / Tetrahedron xxx (xxxx) xxx
3
catalyzed by 0.05 mol% of MeMgI under neat condition at room
temperature for 10 min (Table 1,1n).
Subsequently, we also examined the chemoselective hydro-
boration of aldehydes over ketones. Equimolar amounts of benz-
aldehyde, acetophenone and pinacolborane were treated with
0.5 mol% MeMgI under neat condition at room temperature. It
resulted in 97% conversion of benzaldehyde in 20 min while ace-
tophenone remained almost unreacted (>98%). Similar chemo-
selectivity was also observed in the competitive catalytic
hydroboration reactions of 4-fluorobenzaldehyde over 4-
fluoroacetophenone and 4-methylbenzaldehyde over 4-
methylacetophenone (Scheme 1). Lastly, a large-scale hydro-
boration of benzaldehyde/acetophenone (15 mmol) with HBpin
(15.2 mmol) was performed with 0.05/0.5 mol% of MeMgI under
neat condition at room temperature for 30 min/1 h respectively.
Pleasingly, the conversions in both cases were successfully ach-
ieved with 99% yields (Scheme 2).
Encouraged by the above results from the aldehyde hydro-
boration reaction, we extended the substrate scope to a wide range
of ketones. As expected, a relatively higher catalyst loading and
slightly longer reaction time was required for hydroboration of the
more sterically bulky ketonic carbonyl functionality when
compared with aldehydes in order to achieve similar results. This
trend is in line with reported literatures [24]. The preliminary
investigation was carried out on the hydroboration of acetophe-
none with HBpin catalyzed by different catalyst loadings of MeMgI
with/without solvent at room temperature for 20 min. Upon
increasing catalyst loading, the hydroboration yield was gradually
increased. Acetophenone was cleanly converted into the corre-
sponding borate ester at 0.5 mol% catalyst loading within 20 min
under neat condition (Table S2, entries 1e4). Similar to what was
In addition, we used DFT calculations (M06e2X [42,43]) to
investigate the possible mechanistic pathway of the hydroboration
of PhCHO with HBpin in the presence of MeMgI (see SI for
computational details). The computed free energy profile and
transition states are displayed in Scheme 3 respectively (the opti-
mized structures of minimum species are shown in Fig. S2, the
cartesian coordinates of all optimized structures are also provided
in SI). The whole pathway consists of the following six stages: (1)
The magnesium atom of the catalyst (MeMgI) associates with one
oxygen atom of HBpin to form an encounter complex (Int1), which
is exergonic by 1.6 kcal/mol. Then, the methyl group is likely to
attack the boron center to generate the zwitterionic intermediate A
via TS1 (being endothermic of 4.5 kcal/mol and the barrier is
12.6 kcal/mol, relative to Int1). (2) A binds with a benzaldehyde
molecule driven by coordination interaction between the magne-
sium and oxygen atoms to form Int2, which is exergonic of
11.7 kcal/mol. Subsequently, the hydride transfers from boron to
benzaldehyde to generate the intermediate B via TS2. This process
involves a barrier of 4.3 kcal/mol and is exergonic by 29.2 kcal/mol
observed for aldehyde when an organic solvent such as CDCl
3 6 6
, C D
and CD CN was employed, only trace yield was obtained under the
3
same conditions as that employed for the MeMgI-catalyzed alde-
hyde hydroboration (Table S2, entries 5e7).
Similarly, various aromatic ketones and aliphatic ketones all
underwent the hydroboration process smoothly with 0.5 mol% of
MeMgI under neat condition at room temperature for 20 min. It
should be noted that the catalytic activity of MeMgI was indeed
more efficient than that of most of previous reported cases [24]. It
was also demonstrated that acetophenone substrates with
electron-withdrawing or electron-donating groups such as F-, Cl-,
2
Br-, O N-, NC-, MeO- and Me-underwent full conversions (Table 2,
2
bꢀ2l). Once again, no obvious substituent effect was found in the
hydroboration of (o, m, p)-F-substitutes and (o, m, p)-Me-
substituted analogues. Changing the methyl moiety of acetophe-
none to isopropyl also produced the corresponding borate ethers in
excellent yield (Table 2, 2m). The hydroboration of alkyl ketones
could also be completed in high yield (Table 2, 2n, 2).
Table 2
a
Hydroboration of Ketones Catalyzed by MeMgI .
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1

4
W. Wang et al. / Tetrahedron xxx (xxxx) xxx
Scheme 1. Chemoselective Hydroboration of Aldehyde vs Ketone Catalyzed by MeMgI.
(relative to Int2). (3) Ligand exchange occurs between B and
another HBpin molecule to generate the intermediate C, at the
same time, release a Bpin-CH molecule. This process is exergonic
3
by 1.2 kcal/mol (4) The alkoxy group of C could nucleophilically
attack the boron center to generate the zwitterionic intermediate D
via the transition state TS3. This process involves a barrier of
2.4 kcal/mol and is exergonic by 12.0 kcal/mol (relative to C). (5)
Similar to step (2), another molecule of benzaldehyde could bind
with D to give the intermediate E (being exergonic by 4.5 kcal/mol),
and a hydride migrate to the carbonyl carbon could follow to form
the intermediate F (being exergonic by 16.6 kcal/mol and the bar-
rier is 9.4 kcal/mol). (6) Starting from F, the final product could be
Scheme 2. Large-Scale Reaction of Benzaldehyde/Acetophenone with HBpin.
Scheme 3. Computed Gibbs Free Energy Profile of the MeMgI Catalyzed Hydroboration of PhCHO.
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W. Wang et al. / Tetrahedron xxx (xxxx) xxx
5
{1
H}, and 19F{ H} NMR spectra were recorded at 25 C on Bruker
ꢁ
1
Avance III 600 MHz spectrometer in deuterated solvents and were
referenced to the resonances of the solvent used. Chemicals were
purchased from Sigma-Aldrich, Alfa Aesar, and Acros and used
without further purification.
Experimental procedure. General procedure for catalytic
hydroboration of aldehydes. To the mixture of aldehydes
(0.2 mmol) and HBpin (0.22 mmol) in a 10 mL Schlenk flask
equipped with a magnetic stir bar, a stock solution containing
MeMgI (0.05 mol%, 0.04 mL) was added. Then the reaction mixture
ꢁ
was stirred at 25 C for 10 min. The progress of the reaction was
monitored by H, 13C, B, and F NMR spectroscopy which indi-
1
11
19
cated the completion of the reaction by the disappearance of
aldehyde (RCHO) proton and appearance of a new CH resonance.
2
Two examples (1a, 1 m) were selected to purify to get pure prod-
ucts: upon completion of the reaction, the combined organic layers
were purified by silica gel column chromatography using ethyl
acetate/hexane (1:9) mixture as eluents to obtain the correspond-
ing pure alcohol products (96% and 94% yields respectively).
General procedure for catalytic hydroboration of ketones. To
the mixture of ketones (0.2 mmol) and HBpin (0.22 mmol) in a
10 mL Schlenk flask equipped with a magnetic stir bar, a stock
solution containing MeMgI (0.5 mol%, 0.04 mL) was added. Then
ꢁ
the reaction mixture was stirred at 25 C for 20 min. The progress of
1
13 11
19
the reaction was monitored by H, C, B, and F NMR spectros-
copy which indicated the completion of the reaction by the
appearance of a new CH resonance. Two examples (2d, 2l) were
selected to purify to get pure products: upon completion of the
reaction, the combined organic layers were purified by silica gel
column chromatography using ethyl acetate/hexane (1:9) mixture
as eluents to obtain the corresponding pure alcohol products (94
and 95% yields respectively).
Scheme 4. Proposed Mechanistic Pathway.
obtained through ligand exchange with another HBpin molecule, at
the same time, C is regenerated which is ready for the next catalytic
cycle. This process is endothermic by 2.1 kcal/mol. The whole
hydroboration of PhCHO with HBpin catalyzed with MeMgI is
therefore exergonic by 70.2 kcal/mol (relative to the reactants
HBpin, PhCHO and the catalyst MeMgI). This is consistent with the
experimental phenomenon which the carbonyl hydroboration
catalyzed by MeMgI is an exothermic reaction. These computa-
tional results suggest that this reaction is thermodynamically and
kinetically feasible under the experimental conditions. According
to the stoichiometric reaction and the DFT study, a possible
mechanistic pathway is proposed in Scheme 4.
Notes
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
3
. Summary
We are thankful for financial support from the National Natural
Science Foundation of China (21772093) and the Natural Science
Foundation of Jiangsu Province, China (BK20181421).
In summary, we have demonstrated that a series of simple,
inexpensive, commercially available Grignard reagents can be
employed in truly catalytic scale as efficient precatalysts for the
hydroboration of a wide range of aldehydes and ketones with
HBpin. The reaction proceeded rapidly with very low catalyst
loading under neat condition at room temperature. It demon-
strated high functional group tolerance for pyridine, ester, halides
and nitro group etc. High chemoselectivity for aldehydes over ke-
tones were also observed for the hydroboration reaction. Notably, a
large-scale hydroboration was also carried out in very high yield
under the optimized conditions and proceeded smoothly. In addi-
tion, DFT calculations were performed to investigate the possible
reaction mechanism.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2020.131145.
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Please cite this article as: W. Wang et al., Grignard reagents-catalyzed hydroboration of aldehydes and ketones, Tetrahedron, https://doi.org/
0.1016/j.tet.2020.131145
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