Catalyst-free dehydrative α-alkylation of ketones with alcohols: Green and selective autocatalyzed synthesis of alcohols and ketones

Article abstract of DOI:10.1002/anie.201308642

Direct dehydrative α-alkylation reactions of ketones with alcohols are now realized under simple, practical, and green conditions without using external catalysts. These catalyst-free autocatalyzed alkylation methods can efficiently afford useful alkylated ketone or alcohol products in a one-pot manner and on a large scale by Ci￡C bond formation of the in situ generated intermediates with subsequent controllable and selective Meerwein-Pondorf-Verley-Oppenauer-type redox processes. Plain and simple: The title reaction has been realized under simple and practical conditions without using external catalysts, and can afford alkylated ketone or alcohol products in a one-pot manner and on a large scale. The reaction proceeds by Ci￡C bond formation of the in situ generated intermediates with subsequent controllable and selective Meerwein-Pondorf-Verley-Oppenauer-type redox processes. Copyright

Full text of DOI:10.1002/anie.201308642

Angewandte
Chemie
DOI: 10.1002/anie.201308642
Synthetic Methods
Catalyst-Free Dehydrative a-Alkylation of Ketones with Alcohols:
Green and Selective Autocatalyzed Synthesis of Alcohols and
Ketones**
Qing Xu,* Jianhui Chen, Haiwen Tian, Xueqin Yuan, Shuangyan Li, Chongkuan Zhou, and
Jianping Liu
Abstract: Direct dehydrative a-alkylation reactions of ketones
with alcohols are now realized under simple, practical, and
green conditions without using external catalysts. These
catalyst-free autocatalyzed alkylation methods can efficiently
afford useful alkylated ketone or alcohol products in a one-pot
=
manner and on a large scale by C C bond formation of the in
situ generated intermediates with subsequent controllable and
selective Meerwein–Pondorf–Verley–Oppenauer-type redox
processes.
À
A
mong C C bond construction methods, a-functionaliza-
tion of carbonyl compounds is one of the most fundamental
strategies to synthesize heterocycle-, pharmaceutical-, and
natural-product-directed molecules.[1,2] Conventionally, a-
functionalization of carbonyl compounds was achieved by
reactions of carbonyl compounds and organohalides (Scheme
1a).[1,2] However, these methods require reactive and toxic
organohalides and large amounts of bases, and the concurrent
formation of undesired waste salts is inevitable. Since alcohols
are comparatively greener chemicals, and organohalides are
mostly obtained from alcohols by halogenation, alcohols have
recently been directly used as greener alkylating reagents.[3,4]
Regarding the a-alkylation of ketones to generate alkylated
ketone or alcohol products,[2] several transition-metal (TM)-
catalyzed methods have been achieved by anaerobic dehy-
drogenative alcohol activation (Scheme 1b).[5–7] Despite their
advantages, these TM-catalyzed reactions still have obvious
drawbacks, such as the use of expensive and toxic rare metal
complexes (Ru,[5] Ir,[6] Pd,[7] etc.[7]) or capricious ligands, large
amounts of hydrogen acceptors or bases, and metal contam-
ination in products. Herein we report the catalyst-free
autocatalyzed dehydrative a-alkylation reactions of ketones
with alcohols (Scheme 1c), which selectively afforded useful
alkylated ketone or alcohol products under simple and
practical one-pot conditions. As to known TM-free reactions
of carbonyl compounds and alcohols, they were limited to
simple Meerwein–Pondorf–Verley–Oppenauer (MPV-O)
Scheme 1. Background of the research.
redox reactions (Scheme 1d)[8] without the formation of
À
new C C bonds.
In our studies on alcohol-based TM-catalyzed aerobic
reactions,[4a,9,10] we accidentally observed that aldehydes
efficiently catalyzed dehydrative alkylation reactions of
amides, amines, and secondary alcohols with alcohols by
a TM-free mechanism involving MPV-O-type redox process-
es.[11,12] We thus deduced that ketones may similarly be good
catalysts[13,14] and can lead to new TM-free alkylation
methods. As shown in Scheme 2A, we hypothesized that a-
alkylation of ketones with alcohols may proceed successively
by an MPV-O-type aldehyde formation (step a; Scheme 2B,
X = O),[8] dehydrative aldol condensation (step b), and MPV-
O-type reduction of chalcone intermediates by alcohols to
give alkylated ketones or alcohol products (steps c and d;
Scheme 2B, X = C, O), thus regenerating aldehydes for the
catalytic cycle.[11b] If this process can be realized, it may lead to
advantageous and green catalyst-free autocatalyzed a-alky-
lation reactions because no additional ketones are required.
Consequently, the use of external catalysts can be avoided.
We began our study by investigating the reaction of benzyl
alcohol (1a) and acetophenone (2a) without adding any
external catalysts (Table 1).[15] Thus, directly heating solvent-
free mixtures of 1a and 2a with different bases[16] at 1108C
under air led to the formation of the alkylated ketone 3a and
alcohol 4a in varied yields and ratios. The results showed that
[*] Prof. Dr. Q. Xu, J. Chen, H. Tian, X. Yuan, S. Li, C. Zhou, Dr. J. Liu
College of Chemistry and Materials Engineering
Wenzhou University
Wenzhou, Zhejiang 325035 (China)
E-mail: qing-xu@wzu.edu.cn
[**] This research was supported by NNSFC (20902070), SRF for ROCS
of SEM, ZJNSF (Y4100579), and ZJQJTP (QJD0902004).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308642.
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Table 2: Synthesis of alkylated alcohols.[a]
Scheme 2. A) Postulated route for catalyst-free autocatalyzed a-alkyla-
tion of ketones with alcohols. B) TM-free MPV-O-type redox processes.
Table 1: Optimization of reaction conditions for the synthesis of
alkylated alcohols.[a]
Entry 1a (equiv) NaOH (mol%) Atm. T [8C] 4a+3a 4a/3a[b]
Yield [%]
[a] Unless otherwise noted, see entry 6 of Table 1 for reaction conditions.
Yields of isolated 4 were based on 2. The 4/3 ratios were determined by
1H NMR spectroscopy. [b] 100 mol% KOH, 1608C, 3 days. [c] 100 mol%
KOH, 1608C, 24 h. 4/3 ratio determined by GC-MS.
1
2
3
4
5
6
1.5
1.5
2.0
2.0
3.0
3.0
30
30
30
60
60
100
air
N2
N2
N2
N2
N2
110
110
120
120
120
130
95
96
98
97
98
98 (76)
70:30
75:25
87:13
90:10
93:7
97:3
alcohols 4 in moderate to good yields and high selectivities.
By bearing the versatile halide groups, 4 f–j are potentially
useful substrates in synthesis. Similarly, 2-thienylmethanol
could also be used as the alkylating reagent, thus giving
a moderate yield of 4m in high selectivity (> 99:1). Moreover,
electron-rich (4n and 4q) and electron-deficient acetophe-
nones (4o and 4p), as well as a heteroaryl methyl ketone (4r),
also reacted smoothly with 1a to afford the target alcohols 4
in moderate to good yields and good to high selectivities. In
addition, aliphatic alcohols and ketones such as n-hexanol and
2-octanone were also be employed, but they require harsher
reaction conditions with KOH as the base. The reactions
afforded low yields of the products (4s and 4t), and is most
possibly due to the much lower reactivities of the aliphatic
substrates. As for the substituted ketone 3,4-dihydronaph-
thalen-1(2H)-one, selectivity of the reaction was not high and
only a low yield of 4u was obtained. This result may be
attributed to the steric hindrance of the substituents in the
reduction step, and further implies that it may be more
suitable for ketone-product preparation. In comparison with
our previous TM-free dehydrative b-alkylation of secondary
alcohols,[11b] the present method requires no external catalysts
at all, thus greatly simplifying the reaction conditions.
[a] Solvent-free mixture of 1a, 2a (2 mmol), and NaOH sealed in a 10 mL
Schlenk tube in air or N2 was heated for 24 h and then monitored by
1H NMR spectroscopy or GC-MS. [b] Conversions of 2a and 4a/3a ratios
were determined by 1H NMR analysis. Yields of isolated 4a are shown in
parenthesis.
NaOH is a better base.[16,17] Thus, with 1.5 equivalents of 1a,
the reaction gave 4a as the major product in a good yield and
selectivity (entry 1). In contrast, other bases (LiOH, KOH,
CsOH) either gave lower yields or lower selectivities of 4a, or
led to the formation of unknown by-products.[15] In contrast to
using a solvent, which led to decreased selectivity of 4a,[15]
a nitrogen atmosphere, higher loadings of 1a and NaOH, and
higher temperatures were all capable of enhancing the
selectivity of 4a. Thus, by using 3 equivalents of 1a and
100 mol% of NaOH, the reaction at 1308C could afford
a good yield of 4a in high selectivity (entry 6).
The optimized reaction conditions were then applied to
various primary alcohols and ketones (Table 2). Thus, both
electron-rich (4b–e, 4k, and 4l) and electron-deficient
benzylic alcohols (4 f–j), including an ortho-substituted one
(4e), those bearing bulky groups (4k and 4l), and those with
the reactive F, Cl, and Br groups, all afforded the target
2
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Table 3: Optimization of the reaction conditions for synthesis of
We then considered the a-alkylation reaction leading to
the alkylated ketones 3. Since solvent-free conditions, inert
atmosphere, higher loadings of 1, and higher temperatures
favor the production of 4, we deduced that solvent, aerobic
conditions, lower loadings of 1, and lower temperatures
should favor the formation of 3. However, initial attempts
with 1a and 2a, employing the above-mentioned protocols,
were not successful.[15] In contrast, the reaction of the
substituted 1b and 2b gave a rather good yield and selectivity
for 3b and 4b when they were directly heated with NaOH
under air in toluene at 1108C (Table 3, entry 1). Consistent
with preceding observations, more air enhanced the selectiv-
ity for 3b (entry 2). The same was observed when KOH was
used (entries 3 and 4), thus implying that air can oxidize 4 into
3 under basic conditions. As a result, KOH exhibited higher
yield and selectivity to give a good isolated yield of 3b.
Addition of the substrates in reverse order was also tested to
improve the yield and selectivity of 3b, however, only a lower
yield of 3b was obtained because of the formation of by-
products (entry 5). Additional screening of the reaction
conditions using higher or lower amounts of KOH
alkylated ketones.[a]
Entry Base (mol%) Sol. (mL) Air (mL)[b] 3b+4b
3b/4b[c]
Yield [%]
1
2
3
4
NaOH (50)
NaOH (50)
KOH (50)
KOH (50)
KOH (50)
KOH (100)
2
2
2
2
2
4
10
100
10
100
100
100
93
80
99
95 (68)
(43)
97 (71)
82:18
93:7
89:11
95:5
94:6
>99:1
5[d]
6
[a] Unless otherwise noted, 1b (1.2 equiv), 2b (2 mmol), and base in
toluene were sealed under air in a Schlenk tube. The mixture was then
heated at 1108C for 24 h and monitored by GC-MS. [b] 10 or 100 mL
Schlenk tubes were used. [c] Conversions of 2b and 3b/4b ratios were
determined by GC-MS. Yields of isolated 3b are shown within
parentheses. [d] 1.2 equiv of 2b.
and solvent[15] showed that by using double the
Table 4: Synthesis of alkylated ketones.[a]
loading of both KOH and toluene, yet still main-
taining the same base concentration, the reaction
afforded a higher yield of 3b in the highest (> 99%)
selectivity (entry 6).
The optimized reaction conditions were then
applied to other benzylic alcohols and ketones
(Table 4). Thus, various electron-rich (3b–e and
3m) and electron-deficient (3g–l) benzylic alcohols
and the unsubstituted 1a (3 f), all reacted efficiently
with 2b to give a greater than 99% selectivity of
alkylated ketones 3 in moderate to good yields.
Similarly, by using substituted benzylic alcohols, 2a
and substituted acetophenones reacted efficiently to
give 3 in good yields and greater than 99% selectivity
(3r–t). Additionally, heteroaryl methanols such as
pyridyl methanols and 2-thienylmethanol, were used
as the alkylating reagents to afford 3p and 3q,
respectively, in greater than 99% selectivity, albeit in
lower yields resulting from formation of unknown
by-products. In addition to the above methyl
ketones, this method could also be applied to
substituted 3,4-dihydronaphthalen-1(2H)-ones to
give good yields and greater than 99% selectivity
of the target 3u and 3v. In contrast, it seemed 1a/2a
is the only unsuitable substrate combination in the
present method, for under similar reaction condi-
tions only a low yield of 3a was obtained in a low
selectivity.[15] NaOH was a better base, but the yield
and selectivity of 3a were not high either. Thus, the
best result was obtained by using NaOH as the base
in the presence of more air by adapting the reaction
tube with an air balloon. The low selectivity of 3a
may be attributed to easy reduction of the unsub-
stituted 3a into 4a by 1a (e.g., Table 1, entry 1).
The synthetic potential of the above a-alkylation
reactions was also tested by running the model
[a] Unless otherwise noted, see entry 6 of Table 3 for reaction conditions. Yields of
the isolated 3 were based on 2. The 3/4 ratios were determined by GC-MS. [b] 1208C.
[c] 2 equiv of 1a. [d] 50 mol% KOH, 2 mL toluene. [e] 50 mol% NaOH, 4 mL
reactions on a larger scale. For example, 20 mmol toluene, air balloon.
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=
scale reactions of 1a with 2a and 1b with 2b could effectively
afford the target alcohol 4a and ketone 3b, respectively, in
acceptable yields and high selectivities [Eqs. (1) and (2)].[15]
These results imply that the present autocatalyzed alkylation
methods should be more suitable for preparative purposes
than other methods since no external catalysts are required.
C C bond formation and controllable and selective MPV-O-
type redox processes under simple, practical, green, and one-
pot conditions. These results not only represent a technical
and conceptual advance in the field, but also reveal that these
potentially useful methods may provide new possibilities in
synthesis. Extension of the autocatalyzed alkylation methods,
synthetic applications, and additional mechanistic studies are
underway.
Received: October 4, 2013
Published online: && &&, &&&&
Keywords: alcohols · aldehydes · autocatalysis · reduction ·
.
synthetic methods
Clearly, the above satisfactory results (Tables 1–4) suggest
that the proposed autocatalyzed route in Scheme 2A has
been realized, and was further investigated as follows. Firstly,
for MPV-O reduction of 2 by 1 under basic conditions to give
the aldehydes 5 and secondary alcohols 6 (step a), the key
step for initiating the reaction, is known in the literature[8] and
can be easily confirmed. Thus, the ketones 2 could be reduced
by 1a to give the corresponding alcohols 6 and benzaldehyde
(5a) in the presence of KOH [Eq. (3)].[11b,15] Then, base-
mediated aldol condensation of 2 and 5, which is well
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7
[Eq. (4)].[11b] As shown in Scheme 2A, 7 underwent a TM-
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In conclusion, we have developed catalyst-free autocata-
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4
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[15] See the Supporting Information for details.
[16] Although bases were essential additives in TM-catalyzed
dehydrative alkylation reactions, they were not considered as
the catalyst of the reactions (Ref. [5]–[7]). We observe the point
in this work.
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Communications
Synthetic Methods
Q. Xu,* J. Chen, H. Tian, X. Yuan, S. Li,
C. Zhou, J. Liu
&&&& — &&&&
Catalyst-Free Dehydrative a-Alkylation of
Ketones with Alcohols: Green and
Selective Autocatalyzed Synthesis of
Alcohols and Ketones
=
Plain and simple: The title reaction has
been realized under simple and practical
conditions without using external cata-
lysts, and can afford alkylated ketone or
alcohol products in a one-pot manner
and on a large scale. The reaction pro-
ceeds by C C bond formation of the in
situ generated intermediates with subse-
quent controllable and selective Meer-
wein–Pondorf–Verley–Oppenauer-type
redox processes.
6
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