Et3B-mediated and palladium-catalyzed direct allylation of β-dicarbonyl compounds with Morita-Baylis-Hillman alcohols

Article abstract of DOI:10.3762/bjoc.12.234

A practical and efficient palladium-catalyzed direct allylation of β-dicarbonyl compounds with both cyclic and acyclic Morita-Baylis-Hillman (MBH) alcohols, using Et3B as a Lewis acid promoter, is described herein. A wide range of the corresponding functionalized allylated derivatives have been obtained in good yields and with high selectivity.

Full text of DOI:10.3762/bjoc.12.234

Et3B-mediated and palladium-catalyzed direct allylation of
β-dicarbonyl compounds with Morita–Baylis–Hillman alcohols
Ahlem Abidi, Yosra Oueslati and Farhat Rezgui*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2402–2409.
Université de Tunis EL Manar, Laboratoire de Chimie Organique
Structurale et Macromoléculaire, Faculté des Sciences Campus
Universitaire, 2092 Tunis, Tunisia
doi:10.3762/bjoc.12.234
Received: 29 July 2016
Accepted: 30 October 2016
Published: 15 November 2016
Email:
Farhat Rezgui* - rez_far@yahoo.fr
Associate Editor: I. Marek
* Corresponding author
© 2016 Abidi et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
allylic substitution; Morita–Baylis–Hillman; palladium; triethylborane
Abstract
A practical and efficient palladium-catalyzed direct allylation of β-dicarbonyl compounds with both cyclic and acyclic
Morita–Baylis–Hillman (MBH) alcohols, using Et3B as a Lewis acid promoter, is described herein. A wide range of the corre-
sponding functionalized allylated derivatives have been obtained in good yields and with high selectivity.
Introduction
In nucleophilic allylic substitutions, π-allylpalladium com- inorganic acids, e.g., As2O3 [11], B2O3 [12], CO2 [13,14].
plexes are useful intermediates for the construction of More recently the Lewis acids, such as, Ti(OiPr )4 [15], BEt3
carbon–carbon and carbon–heteroatom bonds in organic synthe- [16-19], BPh3 [20], SnCl2 [21], and FeCl3 [22], have also been
sis [1]. Usually, palladium species are used as catalysts in the reported to catalyze these reactions by coordination with the
Tsuji–Trost reaction involving, as substrates, allyl carboxylates hydroxy moiety, thereby increasing its leaving group ability
[2], carbonates [3], and phosphates [4]. Obviously, the direct [23-28].
nucleophilic allylic substitution of allyl alcohols is a more
attractive process especially from an economical and environ- Recently, Tamaru and co-workers have intensively investigated
mental point of view [5], as water, generated by this reaction, is the use of triethylborane as an additive with either Pd(PPh3)4 or
a non-toxic byproduct. However, the poor ability of the hydroxy Pd(OAc)2 as catalysts for the allylation of a variety of active
moiety, as a leaving group, has limited the use of the allyl alco- methylene compounds [29], aldehydes [30], ketones [31], and
hols as substrates. Correlatively, some efforts have been made imines [32] with only common allylic alcohols.
in this direction by the use of transition metals such as copper
[6], nickel [7], ruthenium (I, II) [8], and palladium (0, II) [9,10] As part of an ongoing program studying the behavior of MBH
as the catalyst or by converting the allylic alcohols into esters of derivatives [33] towards β-dicarbonyl compounds, our research
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group [34-37] has reported an interesting synthesis of bicyclic Results and Discussion
dienones in a one-pot process involving the reaction of 2-(acet- We first investigated the allylation of diethyl malonate (2a)
oxymethyl)cyclohex-2-enone with 1,3-dicarbonyl compounds with the allylic alcohol 1a in DMF in the presence of Pd(OAc)2
using K2CO3 as a weak base.
(10 mol %), PPh3 (20 mol %). Under these conditions, no reac-
tion took place at 80 °C for 3 days (Table 1, entry 1).
Later, Chamakh and Amri [38] have described a one-pot syn-
thesis of (E)-4-alkylidene-2-cyclohexen-1-ones through a cross On the other hand, the allylation slowly proceeded in the pres-
coupling of the MBH carboxylates with aliphatic 1,3-diketones ence of NaH (0.5 equiv) but only a trace of the expected prod-
in the presence of K2CO3. A drawback of these synthetic ap- uct 3a was obtained (Table 1, entry 2). Interestingly, the allyla-
proaches is the need to first perform the acylation step of the tion reaction, carried out in DMF at 80 °C, gave a better result
corresponding allyl MBH alcohols. For this reason, we herein (30% yield in 12 h), using, in addition to NaH (0.5 equiv),
report an efficient direct method for the allylation of β-dicar- 1 equiv of Et3B (Table 1, entry 3). Moreover, a remarkable
bonyl compounds with MBH alcohols [39,40] 1a and 1b improvement in yield (60% in 6 h) was also observed for the
(Figure 1) considered as multi-functionalized starting materials allylation of diethyl malonate (2a) with the MBH alcohol 1a
bearing both allyl alcohol and Michael acceptor moieties.
using an excess of Et3B (3 equiv) and 1 equiv of NaH (Table 1,
entry 4).
Mechanistic considerations
Scheme 1 illustrates the most probable catalytic cycle for the
allylation of diethyl malonate (2a) with allyl alcohol 1a. We
assume that there is first an activation of 1a through its conver-
sion into I1 using Et3B [29], which further gives, in the pres-
ence of Pd(0), the π-allylpalladium complex I2. This intermedi-
ate reacts then with the diethyl malonate carbanion I3, in situ
formed, to generate the promoter Et3B of this nucleophilic
allylic substitution and the desired allylated product 3a.
Figure 1: Cyclic and acyclic MBH alcohols.
The interest of this reaction is the fact that it would be possible
to directly convert, both cyclic 2-(hydroxymethyl)cyclohex-2- Next, under the previously optimized conditions (Table 1, entry
enone (1a) and acyclic ethyl 2-(hydroxymethyl)acrylate (1b), 4), we examined the scope of the catalytic system, Et3B/
under the action of 1,3-dicarbonyl compounds 2, in the pres- Pd(OAc)2/PPh3, for a wide range of 1,3-dicarbonyl compounds
ence of an appropriate palladium catalyst and Et3B as a Lewis and related derivatives (pKa = 9–14) [29,31] using two typical
acid promoter, into the allylation compounds 3–8 with the for- MBH alcohols 1a and 1b. The results of this study are summa-
mation of only water as a byproduct. These derivatives can be rized in Table 2.
further used as synthetic intermediates in numerous synthetic
routes to heterocyclic compounds and molecules of biological Like diethyl malonate (2a), the malonate derivative 2b
interest [41,42].
(pKa = 13) [29] similarly reacted with the allylic alcohol 1a in
Table 1: Palladium-catalyzed allylation of diethyl malonate (2a) with the MBH alcohol 1a.
Entry
NaH (equiv)
Et3B (equiv)
T (°C), Time (h)
Yield 3a (%)
1
2
3
4
none
0.5
0.5
1
none
none
1
80/72
80/24
80/12
80/6
N.R
trace
30
3
60
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Beilstein J. Org. Chem. 2016, 12, 2402–2409.
Scheme 1: Proposed catalytic cycle involving palladium catalysis for Et3B-promoted allylation of diethyl malonate with MBH alcohol 1a.
Table 2: Palladium-catalyzed allylation of 1,3-dicarbonyl compounds with MBH alcohol 1aa.
Entry
Pronucleophile 2
Time (h)
Yield 3 (%)a
Yield 4 (%)b
1
2
3
4
5
6
7
8
9
CH2(CO2Et)2, 2a
6
6
3
4
6
3
3
3
3
3a (60) [48]
–
CH2(CO2Me)2, 2b
NCCH2CO2Et, 2c
3b (65) [48]
–
3c (45) [48]
4c (23)
MeCOCH2CO2Me, 2d
MeCOCH2CO2Et, 2e
PhCOCH2CO2Et, 2f
MeCOCH2COMe, 2g
PhCOCH2COPh, 2h
PhCOCH2COMe, 2i
3d (62) [34,44]
3e (72) [34,44]
3f (76) [34,44]
3g (57) [34,44]c
3h (62) [34,44]
3i (60) [34,44]
–
–
–
aReaction conditions: dicarbonyl compounds 2 (1.1 mmol), allylic alcohol 1a (1.0 mmol), Et3B (3 mmol), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), NaH
(1 mmol) in DMF (5 mL) under N2. bisolated yield; ccontaining the enolic form 5g.
the presence of the catalytic system Pd(0)/Et3B to exclusively selectively gave the monoallylation products 3d–i in moderate
give the mono-allylated product 3a in 65% yield, whereas the to good yields [44].
same reaction with ethyl 3-cyano-3-oxopropanoate (2c, pKa =
10.7) [43] whose acidity is relatively higher, led to a mixture of The analysis of 1H NMR spectra of the β-dicarbonyl deriva-
the mono- and bis-alkylation products 3c and 4c in 45 and 23% tives 3a–i in CDCl3 revealed that a keto–enol tautomerism
yields, respectively (Table 2, entries 1–3).
exists only for the acetylacetone derivative 3g and its enolic
form 5g in a 54:46 ratio, respectively (Table 2, entry 7), where-
Under the same conditions, the allylation of a variety of β-keto as the other compounds 3a–f, 3h and 3i are exclusively in the
esters and β-diketones (Table 2, entries 4–9), in DMF at 80 °C, β-dicarbonyl form [45-47].
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Beilstein J. Org. Chem. 2016, 12, 2402–2409.
Moreover, the allylation of ethyl cyclopentanone-2-carboxylate substitution reaction with the β-keto ester carbanion derived
(2j) [49] as a cyclic β-keto ester, with alcohol 1a, catalyzed by from 2j, affording the monoallylated compound 3j. The conver-
the same system, smoothly proceeded at 80 °C in DMF, provid- sion of the keto ester 3j into the tricyclic product 6j was further
ing, after 2 h, the mono-allylated product 3j in 12% yield, along performed through an intramolecular conjugate addition of the
with the tricyclic compound 6j, in good yield, which is result- β-keto ester carbanion onto the enone moiety (Scheme 2)
ing from the intramolecular conjugate addition of 3j carbanion [50,51].
on the enone moiety (Table 3, entry 2). Under the same condi-
tions, the keto ester 2j reacted with alcohol 1a, in DMF at 80 °C The structure of the tricyclic compound 6j was elucidated on
for 6 h longer reaction time, to selectively afford the compound the basis of X-ray diffraction analysis (Figure 2).
6j in 76% yield (Table 3, entry 3).
In a previous report, Alexakis and co-workers have demon-
strated that copper-catalyzed conjugate addition of Grignard
Table 3: Palladium-catalyzed allylation of β-keto ester 2j with the MBH
alcohol 1aa.
reagents onto α-methyl cyclic enones, affording mainly the
trans-2,3-disubstituted cyclohexanones as being the thermo-
dynamic products [52]. Similarly, we believe that the intramo-
lecular conjugate addition of carbanion 3j onto α-substituted
enone moiety is under thermodynamic control, leading to the
tricyclic compound 6j, in which the two hydrogens H2 and H3,
are on a trans-ring junction whereas the second ring junction is
cis (Figure 2).
Entry
Nucleophile T (°C)/Time (h)
Yield (%)
1
2
3
2j
0 to rt/24
80/2
N.R.
3j: 12% 6j: 75%
6j: 76%
80/6
aReaction conditions: dicarbonyl compounds 2j (1.1 mmol), allylic MBH
alcohol 1a (1.0 mmol), Et3B (3 mmol), Pd(OAc)2 (10 mol %), PPh3
(20 mol %), NaH (1 mmol) in DMF (5 mL) under N2.
Encouraged by these successful results on the allylation of
β-dicarbonyl compounds with the alcohol 1a in the presence of
Pd/Et3B, we attempted to extend this methodology to the
A plausible reaction mechanism for the formation of the acyclic MBH alcohol 1b (Table 4). We initially reacted this
tricyclic compound 6j from the MBH alcohol 1a is presented in substrate with ethyl acetoacetate (2e, 1.1 equiv) in the presence
Scheme 2. We believe that the treatment of the MBH alcohol 1a of the catalyst Pd(OAc)2 without any additive. After stirring the
with Et3B in the presence of Pd(OAc)2 may generate a π-allyl- reaction mixture for 24 h, either at room temperature or in
palladium intermediate I that further undergoes a nucleophilic refluxing THF, the starting alcohol 1b was recovered (Table 4,
Scheme 2: Mechanistic pathway leading to the tricyclic compound 6j.
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Beilstein J. Org. Chem. 2016, 12, 2402–2409.
Figure 2: X-ray crystal structure of tricyclic compound 6j.
entry 1). Next, the use of NaH as a strong base with ethyl aceto- Encouraged by these results and those of cyclic MBH alcohol
acetate (2e) allowed the carbanion formation that then reacted 1a, we selected the acyclic alcohol 1b to react with stabilized
with alcohol 1b, affording a trace of the allylation product 7e carbanions derived from the β-keto esters 2e–f and β-diketones
with only a 60% conversion of the starting material (Table 4, 2g–i in anhydrous THF using 1 equiv of NaH and 2 equiv of
entry 2).
Et3B. Under these conditions, all these reactions worked well in
refluxing THF, affording in 2 h the corresponding monoallyla-
The addition of a Lewis acid (1 equiv of Et3B) started the reac- tion products 7e–i [41,44] in 60–70% yields (Table 5, entries
tion by activating the hydroxy group, affording the allylation 1–4).
product 7e in 30% isolated yield (Table 4, entry 3). To further
improve the performance of this allylation reaction, we em- Conclusion
ployed 2 equiv of Et3B (Table 4, entry 4). The reaction afforded In summary, we have developed a mild and direct process for
within 20 h, at room temperature, the compound 7e in 60% the C–C bond formation from the reaction of the MBH alco-
yield. Interestingly, in refluxing THF, this reaction was hols 1a and 1b with β-dicarbonyl compounds 2 in the presence
achieved in shorter reaction time (2 h), leading to the same of a palladium catalyst and Et3B (promoter) with the formation
compound in the same yield (Table 4, entry 5).
of only water as a side product. This method provides a straight-
Table 4: Palladium-catalyzed allylation of ethyl acetoacetate 2e with the MBH alcohol 1b.
Entry
NaH (equiv)
Et3B (equiv)
T (°C)/Time (h)
Conv. 1b (%)/yield 7e (%)
1
2
3
4
5
none
none
rt to reflux/24
rt to reflux/24
reflux
N.R.
1
1
1
1
none
60/trace
100/30
100/60
100/60
1
2
2
rt/20
reflux/2
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Beilstein J. Org. Chem. 2016, 12, 2402–2409.
Table 5: Palladium-catalyzed allylation of a variety of β-dicarbonyl compounds with the MBH alcohol 1b.
Entry
β-Dicarbonyl compound 2
Time (h)
Compound 7
Yield 7 (%)
1
2
3
4
MeCOCH2CO2Et, 2e
Ph COCH2CO2Et, 2f
MeCOCH2COMe, 2g
Ph COCH2COMe, 2i
2
2
2
2
7e [41,44]
7f [41,44]
7g [41,44]
7i [41,44]
60
68
65a
70
aContaining 30% of the enolic form 8g.
forward and practical route to a range of allylated compounds. ture was colored black. When the reaction, followed by TLC,
Further work is in progress in our laboratory to investigate the was finished, the mixture was diluted with CH2Cl2 (20 mL) and
Pd catalysis of the reaction of the MBH adducts with various washed with 2 M HCl, sat. NaHCO3 and then brine. The
pronucleophiles including nitroalkanes, amines and thiols.
organic extracts were dried over MgSO4 and concentrated in
vacuo and the residual oil was subjected to column chromatog-
raphy over silica gel (gradient: petroleum ether/ether = 4:1) to
give the pure allylated products 3–8 in moderate to good yields.
Experimental
General
IR spectra were recorded on a Bruker IFS 66v/S spectrometer.
1H NMR and 13C NMR spectra were recorded either on a
Bruker AC-300 spectrometer (300 MHz for 1H and 75 MHz for
13C) in CDCl3, using TMS as an internal standard (chemical
shifts in δ values, J in Hz). Mass spectra (EI) were recorded on
an Hewlett-Packard (70 eV) apparatus. Analytical thin-layer
chromatography (TLC) was performed using Fluka Kieselgel
60 F254 precoated silica gel plates. Visualization was achieved
by UV light (254 nm). Flash chromatography was performed
using Merck silica gel 60 and a gradient solvent system (petro-
leum ether/ether as eluents).
Supporting Information
Supporting Information File 1
Experimental procedures, characterization and spectral data
for synthesized compounds and X-ray data for compound
6j.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-234-S1.pdf]
Acknowledgements
General procedure for the allylation of
β-dicarbonyl compounds with the MBH alco-
hols 1a and 1b
The authors thank the Ministry of Higher Education, Tunisia for
financial support.
Into a nitrogen-purged two-necked flask, equipped with a reflux References
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Et3B solution 1.0 M (2 to 3 mmol) in THF. The mixture was
stirred for 5 to 10 min at room temperature, then the MBH
allylic alcohol 1a or 1b (1 mmol) was added. The mixture was
stirred and heated at 80 °C for 3 to 6 h during which the mix-
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(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.12.234
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