Nickel-Catalyzed Chemoselective Acetalization of Aldehydes With Alcohols under Neutral Conditions

Article abstract of DOI:10.1002/asia.201900908

A molecularly defined Ni^II-complex catalyzing the chemoselective acetalization of aldehydes with alcohols under neutral conditions is reported. The reaction is general, efficient and showed a wide substrate scope (including aliphatic aldehydes) as well as excellent functional group tolerance. Reusability of the present nickel catalyst is also demonstrated.

Full text of DOI:10.1002/asia.201900908

CHEMISTRY
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Accepted Article
Title: Nickel catalyzed chemoselective acetalization of aldehydes with
alcohols under neutral conditions
Authors: Balaraman Ekambaram, Murugan Subaramanian, Vinod G
Landge, Akash Mondal, and Virendrakumar Gupta
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To be cited as: Chem. Asian J. 10.1002/asia.201900908
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Nickel catalyzed chemoselective acetalization of aldehydes with
alcohols under neutral conditions
Murugan Subaramanian,[a] Vinod G. Landge,[a] Akash Mondal,[a] Virendrakumar Gupta*[b] and
Ekambaram Balaraman*[a]
Abstract:
Molecularly
defined
Ni(II)-complex
catalyzed
Na2SO4 as the dehydrating agent for the cyclic acetalization.[10]
Hudnall and co-workers reported antimony(V) catalyzed
acetalization of aldehydes in the presence of a stoichiometric
amount of either triethoxymethane or triethoxysilane, and the
products can be purified by distillation process which limits the
scope of this protocol for thermally stable substrates.[11] However,
both of these methods require a stoichiometric amount of toxic
triethyl orthoformates and triethyl orthoformate/silicate,
respectively. Thus, it would be of interest to develop a simple and
eco-benign catalyst that works under neutral and mild reaction
conditions and avoid the use of toxic orthoformates. Here, we
chemoselective acetalization of aldehydes with alcohols under neutral
conditions is reported. The reaction is general, efficient and showed a
wide substrate scope (including aliphatic aldehydes) as well as
excellent functional group tolerance. Interestingly, a reusability of the
present nickel catalysis is also demonstrated
Development of new catalytic approaches, in particular, catalysts
based on cheap, earth-abundant, non-precious metals for
sustainable chemical synthesis is the key motivation in
contemporary science. The carbonyl compounds such as
aldehydes and ketones are the most significant functional groups
in organic synthesis owing to their ubiquitous, and versatile
reactivity. In the multi-step organic transformations, a green
protocol for the protection or masking of carbonyl compounds that
operate under neutral conditions is highly required. In this regard,
acetalization reaction using widespread alcohols has attracted
much interest. Acetals serve as an excellent protective group of
carbonyl compounds[1] as well as they found various applications
in the synthesis of chiral auxiliaries, steroids, flavoring agents, and
pharmaceutical moieties.[2-4]
report
a molecularly defined nickel(II) complex catalyzed
acetalization of aldehydes using widespread alcohols. The
reaction proceeds rapidly under mild and neutral conditions and
operates at ambient temperature (Scheme.1). Interestingly,
reusability of the present nickel catalysis is successfully
demonstrated.
In general, acetalization involves the condensation of carbonyl
compounds with alcoholic substrates in the presence of
Brönsted/Lewis acid, mineral acid, or base catalysts.[5] However,
these protocols often require a toxic orthoformate as an alcohol
source (for acetalization). Indeed, these approaches have
inherent limitations such as handling of corrosive acid catalyst,
high temperature, longer reaction time, and limited substrates
scope. In recent times, various catalytic methods including
organocatalysts,[6a-f]
photo-redox
catalysts,[7a-c]
and
heterogeneous catalysts[8a-c] and transition-metal catalysis[9] have
been reported in replacement of highly acidic conditions.
Nevertheless, these methods require high catalyst loading and
harsh reaction conditions.
Recently, Lyons et al. reported the acetalization of aldehydes in
the presence of tropylium salt as organo-Lewis acid catalyst using
triethyl orthoformate as alcohol source. This method requires
heating condition, longer reaction time, and an excess amount of
Scheme 1. Previous reports vs our approach for the acetalization of
carbonyl compounds.
We began our initial investigation using benzaldehyde and
methanol as the model substrates for the acetalization process.
At the beginning of our study, Ni-NNN pincer[12] cat.1 and cat.2
(1mol %) were used as a catalyst for the acetalization of
[a]
Mr. M. Subaramanian, Mr. Vinod G. Landge, Mr. A. Mondal, Prof.
Ekambaram Balaraman
Department of Chemistry
Indian Institute of Science Education and Research Tirupati (IISER-
Tirupati), Tirupati – 517507, India.
o
aldehydes with methanol at 32 C for an hour. Interestingly, the
conversion of the aldehyde 1a into acetal 2a was observed in
good yields using cat.1 (Table 1, entry 1) and cat.2 (Table 1, entry
2). Under similar conditions, other Ni-sources were applied for the
acetalization reaction and the results are summarized in table 1.
Notably, Ni(II) chloride diglyme (Cat.3) and Ni(II) chloride
bis(diphenylphosphino)propane (Cat.4) complexes gave the
E-mail: eb.raman@iisertirupati.ac.in
[b]
Dr. Virendrakumar Gupta
Polymer Synthesis & Catalysis, Reliance Research & Development
Centre, Reliance Industries Limited,
Ghansoli - 400701, Navi Mumbai, India.
Email: virendrakumar.gupta@ril.com
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excellent conversion and afforded the corresponding acetal 2a in
With the optimized conditions in hand, the present Ni-catalyzed
acetalization strategy was applied to various aldehydes using 1
mol% of cat.3 under neutral conditions. A broad range of
substrates including aromatic, hetero-aromatic, alicyclic, and
aliphatic aldehydes underwent acetalization and gave the
corresponding products in good to excellent yields (Table 2).
Notably, various electron-donating groups (-OMe and -SMe), as
well as electron-withdrawing groups (-F, -Cl, -Br, -CN, and -NO2)
in the aromatic ring, gave the corresponding acetals in moderate
to excellent yields (Table 2, products 2a-2i). The strong electron-
withdrawing groups at the para position of the aromatic aldehyde
(p-CN and p-NO2) gave the desired acetals in moderate yields
under standard conditions (products 2i in 55%, and 2j in 61%
yield). However, the acetalization of salicylaldehyde afforded a low
yield of (15%) expected 2n and the reason might be strong
coordination of phenolic hydroxyl group to the nickel center (Table
2, entry 2n). Interestingly, heteroaromatic aldehydes such as
quinoline 2-carboxaldehyde (1t), 2-furaldehyde (1u), and 2-
thiophenaldehyde (1v) were successfully converted into the
corresponding derivatives acetal in moderate to good yields under
optimized reaction conditions. Gratifyingly, alicyclic and aliphatic
aldehydes also provided the acetalization products 2y in 44%,
and 2z in 39% of isolated yields, respectively under the present
Ni-catalyzed conditions.
96% and 95% yields, respectively (Table 1, entry 3,4).
Table 1. Optimization of the reaction conditions.[a]
Entry
Catalyst
Mol%
T (oC)
Yield[b]
1
2
3
4
5
6
7
8
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Ni(OTf)2
Ni(PPh3)4
NiCl2
NiBr2
NiCp2
Ni(OAc)2. 4H2O
Cat. 3
Cat. 3
No catalyst
Cat. 3
Cat. 3 / 2-me-2-
butene
1
1
1
1
1
1
1
1
1
1
0.1
0.01
-
32
32
32
32
32
32
32
32
32
32
32
32
32
50
32
75
72
96 (84)[c]
95
67
77
76
74
n.d
n.d
92
90
n.d
92
9
10
11[d]
12[d]
13[e]
14
15
1
1/10
94
[a]Reaction conditions: benzaldehyde 1a (0.5mmol), Cat. [Ni], and
MeOH (0.5 mL) stirred at 32 oC for an hour. [b]Yields determined by 1H
NMR using toluene as internal standard. [c]Isolated yields. [d] 2 hours.
[e]3 hours. n.d- not detected.
Table 2. Nickel-catalyzed acetalization of aldehydes: Scope of
aldehydes.[a][b]
Other commercially nickel salts such as Ni(OTf)2, Ni(PPh3)4, NiCl2
and NiBr2 under similar reaction conditions resulted in moderate
yields (Table 1, entry 5-8). However, NiCp2 and Ni(OAc)2.4H2O
did not give the desired product 2a (Table 1, entry 9,10). The
nickel catalyst is essential for the effective and rapid conversion
of an aldehyde into acetal as it was evident by the reaction in the
absence of the catalyst (Table 1, entry 13). Besides, the nickel
catalyst shows remarkable activity on the acetalization process
with very low catalytic loading. Notably, when we reduce the
catalyst loading from 1 mol% to 0.01 mol% the reaction preceded
smoothly and afforded the desired product 2a in excellent yield
(Table 1, entries 11-12). A control experiment with 2-methyl 2-
butene (radical and chloride quencher) showed that this reaction
does not follow the classical mineral acid catalytic pathway[5m]
(Table 1, entry 15). Indeed, increasing the temperature does not
have any influence on product formation (Table 1, entry 14). Next,
we have performed the Ni-catalyzed acetalization reaction in the
presence of bases such as K2CO3 and NaHCO3 under standard
reaction conditions. Indeed, the reaction yielded the
corresponding product 2a in ~32% yield. This result clearly
showed that the present acetalization does not follow the classical
(mineral) acid catalysed pathway. However, in the presence of a
base the reaction is slow as well as incomplete.
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[a]Reaction conditions: aldehyde 1a (0.5 mmol), Cat. 3 (1 mol%), and
Hence, the current catalytic is applicable in organic synthesis for
selective protection/deprotection of carbonyl compounds.[1,13]
MeOH (0.5 mL) stirred at 32 oC for an hour. [b]Isolated yields.
Next, we have examined the scope of alcohols under optimized
Ni-catalysis. Thus, 3-chlorobenzaldehyde (1k) was selected as a
benchmark substrate, and the acetalization reaction of 1k with
various (aliphatic and aromatic) alcohols, and diols were
investigated under standard reaction conditions (Table 3). In
particular, 1,2-, 1,3-, and 1,4-diols were smoothly undergone the
acetalization reaction to afford the cyclic acetals 3a-c (5-7
membered cyclic acetals) with excellent isolated yields. On the
other hand, the reaction of 1,5 pentane diol gave a trace amount
of the expected product under standard conditions and this might
be the formation of less-stable 8-membered cyclic acetal (Table
3, product 3d). Interestingly, the present protocol was tolerated
with various chain lengths of alcohols from methanol to decanol
and afforded the corresponding acetals 3e-j in good to excellent
yields (61%-75% yields). In addition to that, primary aromatic
alcohol such as benzyl alcohol also converted into the
corresponding acetal 3m in 74%, and 4-nitrobenzaldehyde with
ethanol resulted the product 3n in 82% isolated yields. However,
our Ni-catalyst failed to yield the acetalization of aldehydic product
using secondary and tertiary alcohols under optimal conditions.
Scheme.2 Chemoselective acetalization of aldehydes.
The time-dependent experiments of acetalization of aldehyde (1a)
with methanol were conducted using cat.3 to study the reaction
kinetics. Thus, continuous sampling was undertaken with different
time intervals, and the conversion of aldehyde (1a) and yield of
acetal (2a) were determined by gas chromatography. As shown
in Figure 1, the starting material aldehyde was completely
consumed within 20 min. Notably, no intermediates were detected
on gas chromatography as well as NMR experiments. The
acetalization of benzaldehyde in the presence of methanol-D4
Table 3. Nickel-catalyzed acetalization of aldehydes: Scope of
alcohols.[a][b]
1
was monitored using H NMR experiments, and the majority of
aldehyde substrates were converted into acetal within 5 minutes
(See ESI). This result demonstrated that the acetalization reaction
proceeds very rapidly in the presence of a nickel catalyst under
mild and neutral reaction conditions.
Figure.1 Kinetic profile for the acetalization of aldehydes (1a).
Interestingly, the reusability of the present Ni-catalyzed
acetalization reaction of aldehyde was successfully demonstrated
(Fig. 2). Thus, the reaction of 1a under standard reaction
conditions (using 1 mol% of cat.3) was carried out by externally
adding starting material 1a into the reaction mixture (without
additional catalyst) in a time interval of 60 minutes and monitored
the catalytic efficiency of the nickel catalyst. Notably, the desired
product 2a was obtained in excellent yields up to the fifth run of
the catalytic cycle (Fig.2a). Gratifyingly, the same reusable
strategy is also applicable to various aldehydes (Fig. 2b). Thus,
as expected, the corresponding acetal products 2 were obtained
in excellent yields up to the fifth cycle at room temperature. It is
noteworthy that the same reaction under mineral acid catalysis
was failed to resue.[5m] These results suggest that the present
[a]Reaction conditions: aldehyde 1k or 1j (0.5 mmol), Cat.3 (1 mol%),
and alcohol (0.5 mL) stirred at 32 oC for an hour. [b]Isolated yields.
The chemoselectivity of the present strategy was performed using
inter-, and intra-molecular competitive experiments under
standard reaction conditions (Scheme. 2). Thus, in the
intermolecular competitive reaction, the formation of acetal (2a)
was obtained as the primary product (94% yield along with 4%
ketal (2a')). In the case of an intramolecular competitive reaction,
an excellent yield of acetal with a trace amount of ketal was
obtained (Scheme 2b). Thus, both these results revealed that the
present Ni-catalyzed acetalization strategy is chemoselective.
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acetalization does not follow the classical (mineral) acid catalysed
pathway.
Scheme.5 Homogeneity test for Ni-catalyzed acetalization of
aldehyde (1a).
Thus, initially the nickel species coordinates with aldehyde (1) and
makes the carbonyl carbon of aldehyde more electropositive.
Then, the addition of alcohol with the aldehyde-coordinated Ni-
complex led to the formation of hemiacetal and further addition of
alcohol to give the product acetal (2) and regenerates the catalyst.
However, the protonation of the nickel alkoxide formed as
intermediate after the nucleophilic attack of alcohol is not well
understood at present, and is probably occurring via an
intermolecular mechanism in which alcohol (or solvent) molecules
are involved. Indeed, further studies are under progress in our
laboratory and the detailed studies will be elaborated and will be
communicated in due course.
Figure 2. Recyclability test for (a) 1a, and (b) with different aldehydes.
We have performed the Ni-catalyzed acetalization reaction of 1a
with a combination MeOH and EtOH (1:1). Indeed, the reaction
affords the products 2a, 2a’ and 2a’’ in 49%, 31% and 10% yields,
respectively (Scheme 3). The formation of 2a’ confirms the
formation nickel hemiacetal II intermediate, and followed by the
addition of ethanol leads to the product formation. Significantly,
the higher yield of product 2a suggested that, the acetalization
reaction of aldehydes with MeOH is much faster than the EtOH.
In conclusion, a facile, simple, and efficient method for the
acetalization of aldehydes with alcohols using a well-defined
nickel(II) complexas a catalyst is reported. The reaction operates
under mild, neutral conditions at room temperature. The present
strategy showed a wide range of substrate scope including
aromatic, heteroaromatic, alicyclic and aliphatic groups as well as
various alcohols such as long-chain alcohols, benzyl alcohols,
and diols are well tolerated. The present strategy provides an
elegant, cost-effective, and eco-benign method for the protection
of various aldehydes. A chemoselective acetalization of aldehyde
is also successfully performed. Gratifyingly, reusability of the
present nickel catalysis is also sucessfully demonstrated.
Scheme.3 Ni-catalyzed acetalization of aldehyde (1a) in the
presence of 1:1 ratio of MeOH and EtOH.
Performing acetalization of benzaldehyde (1a) with methanol
under dark conditions (covered the reaction vessel with
aluminium foil and in the absence of light), no significant change
in the product (2a) yield was observed. Indeed, in the presence of
free radical quenchers such as TEMPO, BHT and 1,1-
Diphenylethylene, the reaction proceeds successfully and yielded
the acetal products 2a in 85%, 92%, and 94% yields, respectively
(Scheme 4). Hence, it was conformed that the present reaction
does not follow the radical type mechanism.
Experimental Section
General procedure for the acetalization of aldehydes with
alcohols
In an oven dried screw cap reaction tube (15 mL), aldehyde
(0.5 mmol), Cat.3 (1 mol %), and alcohol (0.5 mL) were added
under an argon atmosphere. Then the reaction mixture was
o
stirred at 32 C for 1 h. After completion of the reaction, ethyl
acetate (1 mL) was added and the reaction mixture was filtered
through the celite filter. Subsequently, the organic layer was
evaporated under reduced pressure followed purification by silica
gel chromatography using pet.ether and ethyl acetate as an
eluent to give the spectroscopically pure product. All the
compounds were completely characterized, and the
spectroscopic data with a copy of spectra are provided in ESI.
Scheme.4 Acetalization of 1a in the presence of free radical
quenchers.
Conflict of interest
To determine if any heterogeneous Ni-nanoparticles were formed
during the reaction, we performed the reaction in the presence of
Hg indicating the homogeneous character of the Ni-catalyst
(Scheme 5).
The authors declare no conflict of interest
Acknowledgements
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This research is supported by the SERB, India (Grant No:
CRG/2018/002480/OC). EB thanks Director, CSIR-NCL, Pune
and Director, IISER-Tirupati for their support. MS thanks the UGC,
India for the research fellowship.
Keywords: Acetalization • nickel • aldehydes • alcohols •
homogeneous catalysis
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Table of Contents
COMMUNICATION
Murugan Subaramanian, Vinod G.
Landge, Akash Mondal, Virendrakumar
Gupta* and Ekambaram Balaraman*
Page No. – Page No.
Nickel catalyzed chemoselective
acetalization of aldehydes with
alcohols under neutral conditions
Text for Table of Contents
Molecularly defined Ni(II)-complex catalyzed chemoselective acetalization of aldehydes with alcohols under neutral conditions is
reported. The reaction is general, efficient and showed a wide substrate scope (including aliphatic aldehydes) as well as excellent
functional group tolerance. Interestingly, a reusability of the present nickel catalysis is also demonstrated.
This article is protected by copyright. All rights reserved.