Direct organocatalytic hydroalkoxylation of α,β-unsaturated ketones

Article abstract of DOI:10.1016/j.tetlet.2006.08.134

The direct addition of a variety of alcohols to in situ activated olefins was observed in the presence of mild bifunctional amine/acid catalysts. Unlike existing methods, the reactions proceed at room temperature and in the absence of transition metals. The use of simple commercially available catalysts, amines and acids makes this an attractive method for the preparation of β-alkoxy ketones, which are prevalent targets and intermediates in organic synthesis.

Full text of DOI:10.1016/j.tetlet.2006.08.134

Tetrahedron Letters 47 (2006) 7689–7693
Direct organocatalytic hydroalkoxylation of
a,b-unsaturated ketones
Dhevalapally B. Ramachary* and Rumpa Mondal
School of Chemistry, University of Hyderabad, Central University (P.O.), Hyderabad 500 046, India
Received 28 June 2006; revised 22 August 2006; accepted 31 August 2006
Abstract—The direct addition of a variety of alcohols to in situ activated olefins was observed in the presence of mild bifunctional
amine/acid catalysts. Unlike existing methods, the reactions proceed at room temperature and in the absence of transition metals.
The use of simple commercially available catalysts, amines and acids makes this an attractive method for the preparation of b-alk-
oxy ketones, which are prevalent targets and intermediates in organic synthesis.
ꢀ 2006 Elsevier Ltd. All rights reserved.
b-Hydroxy ketones and their alkoxy analogues are very
important as valuable building blocks and structural
motifs in a variety of natural products and in organic
synthesis.1 They are typically prepared either via aldol
chemistry or sequential epoxidation and reduction of
enones.2 Direct preparation by addition of water to an
enone is an attractive alternative, but only one method
exists for this transformation.3 Similarly, preparation
of b-alkoxy ketones also represents a challenging prob-
lem in synthetic organic chemistry. Such hydroalkoxyla-
tion additions of alcohols to a,b-unsaturated ketones,
aldehydes or esters have recently been reported to be
promoted by several catalysts such as PMe3,3 DBU,4
Tf2NH,5 P(RNCH2CH2)3N6 and transition metal com-
plexes;7 however, the hydroalkoxylation addition of
alcohols to a,b-unsaturated ketones remains a challenge,
mainly because of the lower reactivity of alcohols and
also due to the greater number of applications of the
resulting products. On the other hand, hydroalkoxyla-
tion reactions of other highly reactive heteroatom nucleo-
philes such as thiols8 and amides9 to more reactive a,b-
unsaturated aldehydes via iminium ion intermediates
have recently been realized by amine/acid bifunctional
catalysts. In this context, we are interested in developing
a novel and ‘green’ amine/acid bifunctional catalyst for
the oxy-Michael addition reaction of less reactive alco-
hols to a,b-unsaturated ketones via iminium ion catalysis
(Scheme 1). Herein, we report a general and green
Keywords: Amino acids; Amine/acid catalysts; Enones; Hydroalkoxyl-
ation; Organocatalysis.
*
Scheme 1. Direct organocatalytic hydroalkoxylation of a,b-unsatu-
rated ketones.
Corresponding author. Tel.: +91 40 23134816; fax: +91 40
23012460; e-mail: ramsc@uohyd.ernet.in
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.134

7690
D. B. Ramachary, R. Mondal / Tetrahedron Letters 47 (2006) 7689–7693
Table 1. Optimization of the organocatalytic hydromethoxylation of
non-3-en-2-one 1a with MeOH 4aa
metal-free synthetic method for the hydroalkoxylation of
enones and other a,b-unsaturated substrates 1 by use of
an amine/acid 2/3 as a bifunctional catalyst (Scheme 1).
Recently, bifunctional amine/acid-catalysis has emerged
as a powerful synthetic tool for the development of both
achiral and chiral catalyzed condensations, cycloaddi-
tions, 1,2- and 1,4-additions of enals, enones and
ketones with many electrophiles.10 We reasoned that
this catalysis strategy might be applicable to the in situ
generation and conjugate addition of highly activated
olefins if a suitable nucleophilic alcohol was present.
Such a process would constitute a metal-free, green
hydroalkoxylation of enones.
Entry Amine 2
Additive 3
Time Yieldb
(h)
(%)
1
Proline 2a
—
DBU 2f
DABCO 2g
Et3N 2h
—
—
—
—
HCl
CH3CO2H 3a 12
CF3CO2H 3b 12
p-TSA 3d
CH3SO3H 3c 17
CH3SO3H 3c 16
CH3SO3H 3c 12
CH3SO3H 3c 11
DBU 2f
CH3SO3H 3c 17
CH3SO3H 3c 28
12
5
5
5
6
16
12
11
12
57
32
20
20
23
<5
<5
40
20
25
20
50
73
70
45
45
64
58
64
—
55
2c
3c
4c
5
Proline 2a
Proline 2a
Proline 2a
Pyrrolidine 2b
Piperidine 2c
Morpholine 2d
Diamine 2e
Pyrrolidine 2b
Pyrrolidine 2b
Pyrrolidine 2b
Pyrrolidine 2b
Pyrrolidine 2b
Piperidine 2c
Morpholine 2d
Diamine 2e
—
6d
7d
8
During our investigations on organo-catalyzed Diels–
Alder11 reactions of enones 1 in MeOH, we observed
that MeOH was itself undergoing Michael addition to
enones 1 under proline catalysis. In the presence of cat-
alytic amounts of proline, we observed the direct addi-
tion of MeOH to non-3-en-2-one 1a with moderate
conversion (Table 1, entry 1). A number of organic
amines, amine/base and amine/acid were tested as cata-
lysts using the hydromethoxylation of non-3-en-2-one
1a as a benchmark, at room temperature (Table 1). Pyr-
rolidine/CH3SO3H 2b/3c (Table 1, entries 13 and 19)
were the best catalysts for hydromethoxylation of 1a
compared to other catalysts such as amines 2a–h (Table
1, entries 5–8), acid 3c (Table 1, entry 18) and other
amine/acid bifunctional catalysts 2/3 (Table 1, entries
9–16 and entry 21). When the catalyst 2b/3c loading
was changed from 30 mol % to 10 mol % for the hydro-
methoxylation of enone 1a, the product yield decreased
even after longer reaction times (Table 1, entry 19).
Pyroglutamic acid 2i did not catalyze the oxy-Michael
reaction of 1a with 4a (Table 1, entry 20) and this is a
good support for iminium ion catalysis rather than for
acid/base catalysis.
9
10
11
12
13
14
15
16
17
18
19e
20d
21
22
12
—
Pyrrolidine 2b
Pyroglutamic acid 2i
N-Methylpyrrolidine 2j CH3SO3H 3c
—
17
8
a 30 mol % each of amine 2 and additive 3 were mixed at the same time,
to this MeOH 4a and enone 1a were added at room temperature.
b Yield refers to the column purified product.
c Longer reaction times led to decomposition.
d Enone 1a was recovered.
e 10 mol % each of Pyrrolidine 2b and CH3SO3H 3c was used as
catalyst.
Table 2. Optimization of the organocatalytic hydroalkoxylation of
non-3-en-2-one 1a with various alcohols 4a–h and benzyl thiol 40aa
Next, we proceeded to investigate the scope and limita-
tions of the hydroalkoxylation reaction of non-3-en-2-
one 1a with a range of alcohols 4a–h and benzyl thiol
40a under pyrrolidine/CH3SO3H-catalysis at room tem-
perature (Table 2). As shown in Table 2, larger alkyl
alcohols 4c–e furnished hydroalkoxylation products
5ac–ae in lower yields compared to smaller alkyl alco-
hols 4a–b in oxy-Michael reactions. The reason may
Entry RXH
Time (h) Product Yieldb (%)
1
2
3
4
5
6
7
8
9
MeOH, 4a
EtOH, 4b
17
21
37
66
37
12
49
5aa
5ab
5ac
5ad
5ae
5af
73
51
48
21
<3
45
<3
50
77
nPrOH, 4c
iPrOH, 4d
BuOH, 4e
PhCH2OH, 4f
PhOH, 4g
Representative experimental procedure. Pyrrolidine/methanesulphonic
acid-catalyzed hydroalkoxylation reactions of enones: Pyrrolidine 2b
(0.15 mmol) and methanesulfonic acid 3c (0.15 mmol) were stirred at
25 ꢁC for 10 min, then alcoholic solvent 4 (0.5 mL) and enone 1a–i
(0.5 mmol) were added and stirring was continued at the same
temperature for the time indicated in Tables 1–4. The crude reaction
mixture was worked-up with aqueous NH4Cl or NaHCO3 solution
and the aqueous layer was extracted with dichloromethane
(2 · 20 mL). The combined organic layers were dried (Na2SO4),
filtered and concentrated. Pure hydroalkoxylated products 5 were
obtained by column chromatography (silica gel, mixture of hexane/
ethyl acetate as eluent).Many of the hydroalkoxylated products 5 are
commercially available or have been described previously, and their
analytical data matched with literature values. New compounds were
characterized on the basis of IR, 1H and 13C NMR and analytical
data (see Supplementary data).
5ag
5ah
50aa
CH2@CHCH2OH, 4h 96
PhCH2SH, 40a
25
a 30 mol % each of amine 2b and additive 3c were mixed at the same
time, to this alcohol 4a–h or thiol 40a and enone 1a were added at
room temperature.
b Yield refers to the column purified product.
be due to moderate steric hindrance with larger alkyl
groups. Benzyl alcohol 4f furnished hydrobenzyloxy

D. B. Ramachary, R. Mondal / Tetrahedron Letters 47 (2006) 7689–7693
Table 3. Chemically diverse hydroalkoxylated products 5a
7691
product 5af in moderate yield but phenol 4g gave a poor
conversion under identical conditions (Table 2, entry 7).
Benzyl thiol 40a furnished the expected Michael product
50aa in a good yield (Table 2, entry 9) and allyl alcohol
4h furnished the expected hydroallyloxylation product
5ah in moderate yield (Table 2, entry 8). To access the
asymmetric induction in these reactions, hydrobenzyl-
oxylation of enone 1a with benzyl alcohol 4f under
L-proline 2a catalysis was carried out furnishing the
expected product 5af in 40% yield, but with only 11%
ee as shown in Eq. 1
Entry Enone R3–OH
Time (h) Product
Yieldb (%)
1
2
3
4
5
6
7
8c
9d
10
11
12e
13
14
15e
1b
1b
1c
1c
1d
1d
1e
1e
1e
1f
MeOH, 4a 23
EtOH, 4b 22
MeOH, 4a 15
EtOH, 4b 14
MeOH, 4a 17
EtOH, 4b 18
MeOH, 4a 28
EtOH, 4b 30
nPrOH, 4c 25
MeOH, 4a
EtOH, 4b
MeOH, 4a 96
MeOH, 4a 17
5ba
5bb
5ca
5cb
5da
5db
5ea
65
60
75
60
75
75
99
5eb, 5eb0 >95
5ec, 5ec0 >95
ð1Þ
3
3
6a
6b
50
50
<5
55
55
—
1f
1g
1h
1h
1i
5ga
5ha
5hb
5ia
We synthesized several hydroalkoxylation products 5
from different enones 1b–i and alcohols 4a–c under pyr-
rolidine/CH3SO3H 2b/3c-catalysis. The results in Table
3 indicate the broad scope of this novel methodology
covering a structurally diverse group of enones 1b–i
and alcohols 4a–c with many of the yields obtained
being very good, or indeed better, than previously
published reactions starting from the corresponding
enones 1.
EtOH, 4b
MeOH, 4a 40
22
a See experimental section.
b Yield refers to the column purified product.
c Ratio 5eb:5eb0 = 1:5.6 as determined by 1H and 13C NMR analysis.
d Ratio 5ec:5ec0 = 1:5 as determined by 1H and 13C NMR analysis.
e Enones 1g and 1i were recovered.
As shown in Table 3, the enones oct-3-en-2-one 1b, hept-
3-en-2-one 1c and pent-3-en-2-one 1d furnished hydro-
methoxylated 5ba–da and hydroethoxylated 5bb–db
products from methanol and ethanol, respectively, in
good yields (Table 3, entries 1–6). 4-Methoxy-but-3-
en-2-one 1e gave the useful organic intermediates, di-
methyl acetal 5ea, diethyl acetal 5eb0 and dipropyl acetal
5ec0 in very good yields by reaction with MeOH, EtOH
and nPrOH, respectively, as shown in Table 3, entries 7–
9. Interestingly, hydroalkoxylation of 1e with EtOH and
nPrOH furnished the unexpected acetals 5eb0 and 5ec0 as
the major products rather than the expected acetals 5eb
and 5ec which can be explained by amine/acid-catalyzed
retro-Michael/Michael reactions of acetals 5eb and 5ec
with EtOH and nPrOH, respectively. Reaction of methyl
vinyl ketone 1f with MeOH and EtOH under 2b/3c-
catalysis furnished the unexpected tandem hetero-
Diels–Alder/acetalization products 6a and 6b in good
yields rather than the expected oxy-Michael products
5fa and 5fb (Table 3, entries 10–11). The stereochemist-
ries of 6a and 6b were established based on 2D NMR
analysis. To our surprise, benzylidene acetone 1g gave
the expected oxy-Michael product 5ga with very poor
conversion and did not furnish the self-Diels–Alder11
product 7 even after four days (Table 3, entry 12).
Hydromethoxylation and hydroethoxylation of cyclo-
hexenone 1h furnished the expected products 5ha and
5hb in moderate yields (Table 3, entries 13–14), however,
the reaction of 3-methylcyclohexenone 1i did not give
5ia or 5ib.
Acetals 5ea, 5eb0 and 5ec0 are useful intermediates in
organic synthesis emphasizing the value of this green
approach.
A possible reaction mechanism for the hydroalkoxyl-
ation of 1, 4 and 2a or 2b/3c is illustrated in Scheme
2.10 First, reaction of the amino acid 2a or amine/acid-
catalyst 2b/3c with enone 1 generates the iminium cation
9 (an excellent electrophile) which undergoes a Michael
type reaction with in situ activated alcohol nucleophiles
4 to generate Michael adduct 10, which is in equilibrium
with 9. Hydrolysis of 10 furnishes the expected oxy-
Michael product 5 and free catalyst amino acid 2a or
4-Ethoxypentan-2-one 5db has been observed in the vol-
atile components of Indian long pepper, Piper longum
Linn., and also as a volatile metabolite in human urine.12

7692
D. B. Ramachary, R. Mondal / Tetrahedron Letters 47 (2006) 7689–7693
and MeOH at rt (Table 4, entries 2–3).13 By changing
the solvent MeOH to THF in the diamine 2e-catalyzed
reaction of enone 1h, we did not observe any reaction
and recovered only the starting material even after four
days (Table 4, entry 4). The bifunctional catalyst, 2e/3c-
catalyzed the formation of the hydromethoxylated prod-
uct 5ha in good yield from enone 1h with MeOH (Table
4, entry 5). Based on the above results, we propose that
11 is formed via a solvent induced amine-catalyzed
Basavaiah–Baylis–Hillman reaction as shown (see Table
4).13 Product 11 was shown to possess synergistic herbi-
cidal effects and has been used along with 2,4,5-T and
atrazine against Chenopodium album.14
Scheme 2. Proposed reaction mechanism for the organocatalytic
hydroalkoxylation of a,b-unsaturated ketones.
In conclusion, we have shown that the mild bifunctional
amine/acid 2b/3c catalyst, in the presence of a,b-unsatu-
rated systems, can be used to catalyze the hydroalkoxyl-
ation of in situ activated olefins in the absence of added
transition metals. Our proposed catalytic cycle suggests
that this is a very practical system that could be
extended to other classes of bifunctional catalysts to
generate active olefins with LUMOs matched to the
HOMOs of alcohols. Additionally, the possibility of
using other chiral bifunctional amine/acid catalysts
would further extend the applicability and practicality
of this novel reactivity.
amine/acid 2b/3c, which is returned to the catalytic
cycle. The proposed reaction mechanism was supported
by the results presented in Eq. 1 and Table 1, entries 1
and 20.
To understand the role of amine-catalysts 2 on the
hydromethoxylation of cyclic enones, we screened cata-
lysts 2a, 2b, 2e and 2e/3c for hydromethoxylation of en-
one 1h at room temperature as shown in Table 4. Under
proline-catalysis, enone 1h furnished the expected oxy-
Michael product 5ha in 50% yield. Interestingly, amines
2b and (S)-2e catalyzed the formation of the unexpected
product 11 in good to moderate yields from the enone 1h
Acknowledgements
We thank DST (New Delhi) for funding this project. We
thank UGC (New Delhi) for recognizing our University
of Hyderabad as a ‘University with potential for excel-
lence’ (UPE) and also recognizing our School of Chem-
istry as a Center for Advanced Studies in Chemistry and
for providing some instrumental facilities. R.M. thanks
CSIR (New Delhi) for her research fellowship. We
thank Dr. Abani K. Bhuyan, Mr. D. Krishna Rao and
Mr. S. Satyanarayana for their help in recording 2D
NMR spectra.
Table 4. Direct organocatalytic solvent induced Basavaiah–Baylis–
Hillman reaction
Entry
Catalyst
Time (h)
Product
Yielda
1
2
3
4b
5
2a
2b
40
11
40
96
40
5ha
11
50
50
77
—
65
Supplementary data
2e
2e
11
11
Experimental procedures and analytical data for all new
compounds. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.tetlet.2006.08.134.
2e/3c
5ha
a Yield refers to the column purified product.
b THF used as solvent.
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