PS-BEMP as a basic catalyst for the phospha-Michael addition to electron-poor alkenes

Article abstract of DOI:10.1039/c6ob00242k

PS-BEMP was used as a heterogeneous catalyst for the phospha-Michael addition of phosphorus nucleophiles to a variety of electron-poor alkenes. The addition reactions were generally performed with equimolar amounts of reagents under solvent free conditions. The protocol proved to be very efficient for the addition to aromatic, non-aromatic and cyclic ketones, giving good yields (78-85%) in all cases. The protocol was also extended with good results to α,β-unsaturated esters and nitriles. This demonstrates that PS-BEMP is a good catalyst for the phospha-Michael addition to electron-poor alkenes.

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ARTICLE
PS-BEMP as a Basic Catalyst for the Phospha-
Michael Addition to Electron-poor Alkenes
Giacomo Strappaveccia,a Luca Bianchi,a Simone Ziarelli,a Stefano Santoro*,a
Daniela Lanari,b Ferdinando Pizzoa and Luigi Vaccaro*a
PS-BEMP was used as a heterogeneous catalyst for the phospha-Michael addition of
phosphorus nucleophiles to a variety of electron-poor alkenes. The addition reactions were
generally performed with equimolar amounts of reagents in solvent free conditions. The
protocol proved to be very efficient for the addition to aromatic, non-aromatic and cyclic
ketones, giving in all cases good yields (78-85%). The protocol was also extended with good
results to α,β-unsaturated esters and nitriles. This demonstrates that PS-BEMP is a good
catalyst for the phospha-Michael addition to electron-poor alkenes.
polystyrene (PS-BEMP) has been extensively used for several
base-catalyzed reactions such as the conjugative addition of C-
S- and N- nucleophiles,14 the hydrophosphonylation of
Introduction
Organophosphorus compounds are particularly important for
their applications in agriculture, where they are mainly used as
insecticides and herbicides.1 In organic chemistry
organophosphorus compounds are widely used as intermediates
is synthesis and as ligands in transition metal catalysed
reactions.2 For these reasons, the development of new efficient
protocols for the formation of carbon-phosphorus bond is of
aldehydes15 and the phenolysis of epoxides.16
Based on our previous experiences, in this contribution we
report the use of PS-BEMP to promote the phospha-Michael
addition to a variety of α,β-unsaturated ketones
1 under solvent
free conditions (SolFC). Moreover, preliminary results about
the applicability of this procedure to α,β-unsaturated esters 10
and nitriles 11 are also reported.
significant interest.
A particularly attractive strategy is
represented by the conjugate addition of a phosphorus
nucleophile to an electron-deficient species, namely the Results and discussion
phospha-Michael addition.3 The interest for such a process is
also justified by its high atom economy which classifies it as a Initially, we compared the efficiency of different types of
potentially valuable tool for achieving a sustainable access to heterogeneous bases as catalysts for a representative reaction,
target molecules containing P-C carbon bonds. The typical namely the addition of diethyl phosphite (2) to (E)-4-phenylbut-
substrates for phospha-Michael additions are α,β-unsaturated 3-en-2-one (1a) (see Table 1 for results and Scheme 1 for the
esters,4 malonates,4a,5 nitroalkenes6 and ketones.4a,7 This structures of the bases used in this study).
reaction has been promoted by Lewis acids,4c,5d-e,6b,7a-c,
transition metals,8 microwaves9,4a and bases.10,5a-c,6a,7e-f,7h
N
Regarding the phospha-Michael addition to α,β-unsaturated
N
N
N
N
N
ketones, this process remains challenging in terms of substrate
scope and product selectivity. In fact, a strong limit for the
efficiency of this reaction is represented by the need for a large
amount of a strong base to sufficiently activate the poor P-
nucleophiles.
PS
PS
PS
PS-DMA
PS-DMAP
PS-TBD
For this class of substrates a base is generally required7e-f,7h but,
to the best of our knowledge, there is no report on the use of a
heterogeneous base in such a process.11 Heterogeneous
catalysis is of obvious interest in chemistry and industry
because it offers many advantages such as simplicity in
handling, more environmentally safe disposal and easier work-
up compared to the related homogeneous processes.12
N
N
N
P
N
N
N
N
N
JJ
PS
PS
N
JJ-TBD
PS-BEMP
PS-DBU
In recent years our research group has focused on the use of
heterogeneous bases, particularly phosphazene bases, as
catalysts.13 In fact, 2-tert-butylimino-2-diethylamino-1,3-
Scheme 1 Supported bases used in the present study.
All the reactions were performed under SolFC since, as
experienced in other cases by our research group,14-16 the
dimethylperhydro-1,3,2-diazaphosphorine
supported
on
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addition of a solvent completely hampered the reactivity,
resulting in the complete recovery of reactants. The
combination of a heterogeneous catalyst and SolFC are
therefore crucial for such kind of transformation to reach the
needed reactivity. Indeed, immobilized catalysts are generally
The results with these last three resins (JJ-TBD, PS-DBU and
PS-BEMP) were almost identical and we decided for practical
reasons to continue the optimization using PS-BEMP. In fact,
JJ-TBD and PS-DBU feature low loadings of the supported
bases (0.85 mmol/g and 0.91 mmol/g, respectively), which
results in the need of performing the reaction using a relatively
large mass of resin, with consequent problems for the stirring in
SolFC. The higher loading of PS-BEMP (2.13 mmol/g), on the
other hand, allows the use of a smaller mass of solid catalyst
with positive effects on the manageability of the reaction.
In an attempt to increase the conversion, we ran the reaction
with PS-BEMP for longer time (48 h, Table 1, entry 8). While
the conversion remained unchanged, we observed the formation
of 20% of the desired product 3a. This can be explained with
the higher thermodynamic stability of product 3a compared to
3'a, with the formation of the latter being faster (kinetic
product).7j Therefore, we decided to increase the temperature to
60 °C and the catalyst loading to 10 mol%, obtaining 80%
affected by
a
moderate reactivity compared to their
homogeneous counterpart, and this drawback can often be
overcome by performing the reaction without solvent.14-16
For the solid bases used in this study, the polymeric support
used was a 200-400 mesh polystyrene in all the cases co-
polymerized with 2% of divinylbenzene (cross-linker), except
in the case of JJ-TBD where a (1,4-bis(vinylphenoxy)-butane-
is used as cross-linker).
First we tested few polystyrene immobilized bases, such as 4-
(dimethylamino)pyridine (PS-DMAP), dimethylamine (PS-
DMA) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (PS-TBD)
(Scheme 1). Performing the reactions at 30 °C with 5 mol% of
catalyst for 24 hours resulted in no conversion for all the three
bases (Table 1, entry 2-4) with complete recovery of reactants
1a and
2.
We also tested the influence of the support by performing the
reaction with 1,5,7-triazabicyclo[4.4.0]dec-5-ene immobilized conversion and a 60:40 mixture of 3a
:3'a after 24 h (Table 1,
on JandaJel (JJ) resin.17 This considerably improved the
entry 9). Increasing the catalyst loading to 20 mol% lead to
complete conversion with full selectivity towards the Michael
product 3a (Table 1, entry 10). Attempts to reduce the catalyst
loading to 10 mol% while increasing either the amount of
reactivity,13c resulting in a 78% conversion, but with the
exclusive formation of product 3'a coming from the
nucleophilic attack at the carbonyl (Table 1, entry 5). Using
polystyrene supported 1,8-diazabicyclo[5.4.0]undec-7-ene (PS-
DBU), a 77% conversion was observed but still with the
exclusive formation of the 1,2-addition product 3'a (Table 1,
entry 6). Very similar results were obtained using PS-BEMP
(Table 1, entry 7).
phosphite
2 or the temperature did not lead to satisfactory
results (Table 1, entries 11 and 12).
Table 2 Screening of different phosphorus nucleophiles using 1a
as electrophile and PS-BEMP as catalyst.
Table 1 Catalyst screening and reaction conditions optimization for the
O
phospha-Michael reaction between (E)-4-phenylbut-3-en-2-one (1a
)
O
R
R
R
R
O
P
PS-BEMP
(20 mol%)
O
and diethyl phosphite (
2
).
P
OH
O
P
H
R
R
+
+
O
P
O
P
SolFC
EtO
OEt
O
OEt
EtO
O
OH
O
P
H
Cat.
1a
4-6
1 equiv.
7a-9a
7'a-9'a
EtO
OEt
4,7 R = OMe
5,8 R = OPh
6,9 R = Ph
+
+
SolFC
3a
1a
2
3'a
T
(°C)
60
Entry
Nucleophile
t (h) Conv. (%)a
1,4:1,2a
T
(°C)
30
30
30
30
30
30
30
30
60
60
60
80
Conversion
1,4:1,2
Entrya
t (h)
Cat. (mol%)
-
24
24
24
72
24
48
72
70
100
37
>99:0
>99:0
70:30
70:30
70:30
>99:0
>99:0
4
4
5
5
5
5
6
(%)b
(
3a
:
3'a)
b
1
2
3
4
5
6
7
80
1
2
24
24
24
24
24
24
24
48
24
24
24
24
0
/
/
/
/
60
PS-DMAP (5)
PS-DMA (5)
PS-TBD (5)
0
60
45
3
0
100
130
80
45
4
0
55
5
JJ-TBDc (5)
78
77
76
76
80
100
83
87
0:>99
0:>99
0:>99
20:80
60:40
>99:0
70:30
>99:0
45
6
PS-DBUd (5)
PS-BEMPe (5)
PS-BEMPe (5)
PS-BEMPe (10)
PS-BEMPe (20)
PS-BEMPe (10)
PS-BEMPe (10)
a Conversion of 1a to 3 determined by GLC analyses, the remaining
material was unreacted 1 and 2 mixture.
7
8
9
Having identified the best reaction conditions, we explored the
possibility of extending the protocol to other phosphorus
nucleophiles, using α,β-unsaturated ketone 1a as the
10
11f
12
electrophile (Table 2). Dimethyl phosphite (4) showed good
a 1 eq. of
2
was used. b Conversion of 1a to
3
determined by GLC
reactivity. Indeed, in the optimized reaction conditions (1 eq. of
nucleophile, 20 mol% of PS-BEMP at 60 °C), we observed the
selective formation of product 7a with a 70% conversion (Table
2, entry 1). Increasing the temperature to 80 °C allowed to
obtain full conversion while retaining complete selectivity
analyses, the remaining material was unreacted and mixture. GC-
1
2
MS as well as NMR analyses, were used in the assignments of the
peaks to the proper products. c Loading: 0.85 mmol/g. d Loading: 0.91
mmol/g. e Loading: 2.13 mmol/g. f 1.2 eq. of
2 were used.
2 | J. Name., 2012, 00, 1-3
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ARTICLE
O
(Table 2, entry 2). Diphenyl phosphite (5) showed lower
EtO OEt
P
O
reactivity, with a 37% conversion in the optimized reaction
conditions. Moreover, the product was obtained as a mixture
with a 70:30 ratio between the 1,4- and 1,2-addition products
(Table 2, entry 3). Increasing the reaction time (Table 2, entry
4) or the temperature (Table 2, entry 5) did not lead to
significant improvements. Prolonged reaction time at 130 °C
(Table 2, entry 6) only led to 55% conversion, but with
complete selectivity towards product 8a. Diphenyl phosphine
80
24
82
2
3
3b
O
EtO OEt
P
O
80
80
80
80
80
36
24
48
48
24
78
81
80
82
82
3c
Cl
O
EtO OEt
oxide (6) gave unsatisfactory results, with a 45% conversion of
1a to 9a after 72 h at 80 °C (Table 2, entry 7).
Next, we explored the scope of the reaction using nucleophiles
P
O
4
5
6
7
3d
2
and
4
and a variety of α,β-unsaturated ketones (1a-g, Table
MeO
O
3). Very satisfactory results were obtained in the reaction of
2
EtO OEt
P
O
with aromatic substrates such as 1a (Table 3, entry 1), and
several types of chalcones, such as (E)-chalcone (1b), (E)-3-(4-
chlorophenyl)-1-phenylprop-2-en-1-one
(
1c
)
and (E)-3-(4-
3e
methoxyphenyl)-1-phenylprop-2-en-1-one (1d), with yields
always ranging between 78 and 82%, regardless of the
substituents present on the substrates (Table 3, entries 2-4). The
O
EtO OEt
P
O
representative reaction of 1a with
2 was also performed on a
3f
larger scale giving almost identical results with the smaller
scale (Table 3, footnote b). Good results were also obtained
with non-aromatic compounds such as (E)-1-(2,6,6-
trimethylcyclohex-1-en-1-yl)but-2-en-1-one (1e) and its isomer
O
MeO OMe
P
O
7a
(E)-1-(2,6,6-trimethylcyclohex-2-en-1-yl)but-2-en-1-one
obtaining 80% and 82% yields, respectively (Table 2, entries 5
and 6). Similar results were obtained using as nucleophile,
(1f),
O
MeO OMe
P
O
4
80
80
80
80
24
24
48
48
85
83
80
82
obtaining yield between 82% and 85% (Table 3, entries 7-11).
It is worth noting that in our reaction conditions products 3f and
7f were obtained without noticeable formation of the related
isomerization products 3e and 7e. Finally, also a cyclic enone
8
9
7b
O
MeO OMe
P
O
such as cyclohex-2-enone (1g) in the reaction with phosphite
4
furnished the corresponding product in 80% yield (Table 3,
7d
MeO
entry 12). All the compounds 3a-f and 7a-g have been fully
O
MeO OMe
1
characterized and copies of the H, 13C and 31P NMR spectra
P
O
are reported as supplementary materials.
10
11
These results confirmed that PS-BEMP can be considered as an
efficient heterogeneous catalyst for the phospha-Michael
addition to different kinds of α,β-unsaturated ketones.
7e
O
MeO OMe
P
O
Table 3 Substrate scope of the phospha-Michael addition between
phosphorus nucleophiles
2
and
4
and α,β-unsaturated ketones 1a-g.
7f
O
O
R2
R
R2
PS-BEMP
O
O
P
(20 mol%)
O
R2 P R2
OMe
O
+
R
R1
R1
12
60
24
80
SolFC
P
MeO
H
3a-f or 7a-g
7g
1a-g
2, R2 = OEt
4, R2 = OMe
a
Isolated yields of the pure product; reactions performed using
equimolar amounts of reactants. Reaction performed on a 5 mmol
scale gave 79% yield.
b
Entry
1
Product
T (°C)
60
t (h) Yield (%)a
O
EtO OEt
P
O
Next, we tested whether this methodology is also applicable to
other electron-poor alkenes, such as α,β-unsaturated esters and
24
80b
nitriles (Table 4), using ethyl (2) and methyl phosphite (4) as
phosphorus nucleophiles. Methyl acrylate 10a showed good
3a
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reactivity, affording the expected 1,4-addition products with
methyl ( ) and ethyl phosphite ( ) in 85% and 79% yield,
respectively (Table 4, entries 1 and 2). Methyl crotonate (10b
gave less satisfactory results, affording the addition products in
34% and 52% yield when reacting with phosphite and 2,
respectively (Table 4, entries 3 and 4). α,β-Unsaturated nitriles
such as acrylonitrile (11a) and cinnamonitrile (11b) gave
excellent results with both nucleophilic phosphites, with yields
ranging from 76% to 90% (Table 4, entries 5-8).
Experimental
4
2
Compounds 3a 7j
12c 20 12d 20
13a
7d 7f 13c and 13d are new. Characterization data and copies
of the 1H, 13C and 31P NMR spectra for all compounds 3a
7a
12a and 13a are presented in the ESI.
,
3b
21 and 13b7j are known, while compounds 3e
-
- , , , , , , ,
d 7b 7a 18 7b 7d 7e 7h 7g 7i 12a 19 12b 4b
)
,
,
,
f
,
,
,
4
-f
,
-
g
,
-d
-d
Typical experimental procedure for phospha-Michael
addition to α,β-unsaturated ketones.
Table 4 Substrate scope of the phospha-Michael addition between
In a screw capped vial equipped with a magnetic stirrer PS-
BEMP (43 mg, 0.1 mmol, 2.13 mmol/g), (E)-4-phenylbut-3-en-
phosphorus nucleophiles
2
and
4
and α,β-unsaturated esters (10) or
nitriles (11).
2-one (1a) (75 mg, 0.5 mmol), and diethyl phosphite (2) (70
O
R2
R
R2
PS-BEMP
O
mg, 0.065 mL, 0.5 mmol) were consecutively added and the
resulting mixture was left under stirring at 60 °C. After 24
hours EtOAc (1 mL) was added and the catalyst was filtered off
and the solvent was removed under vacuum. The obtained oil
was purified by flash column chromatography on silica gel
(petroleum ether/EtOAc 95/5). 3a was obtained as an oil (114
mg, 80% yield).
P
(20 mol%)
R1
R2 P R2
R1
+
R
SolFC
H
80 ºC, 24h
10, R1 = CO2Me
11, R1 = CN
2, R2 = OEt
4, R2 = OMe
12, R1 = CO2Me
13, R1 = CN
Entry
Substrate
R
H
R1
CO2Me
CO2Me
CO2Me
CO2Me
CN
R2
Yield (%)a
85 (12a)
79 (12b)
34 (12c)
52 (12d)
90 (13a)
89 (13b)
76 (13c)
86 (13d)
MeO
EtO
10a
10a
10b
10b
11a
11a
11b
11b
1
2
3
4
5
6
7
8
H
Acknowledgements
Me
Me
H
MeO
EtO
We gratefully acknowledge the Università degli Studi di
Perugia, and the Ministero dell’Istruzione, dell’Università e
della Ricerca (MIUR) for financial support. S.S. gratefully
acknowledges the MIUR for financial support through the
fellowship PGR123GHQY, funded within “Programma
Giovani Ricercatori, Rita Levi Montalcini".
MeO
EtO
H
CN
Ph
Ph
CN
MeO
EtO
CN
a Isolated yields of the pure product; reactions performed using 2 equivalents
of electrophile relative to the phosphite.
Notes and references
a
Laboratory of Green Synthetic Organic Chemistry, CEMIN
–
Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto 8,
Perugia, Italia.
Dipartimento di Scienze Farmaceutiche, Università di Perugia, Via del
Liceo, 06123 Perugia, Italia.
E-mail: luigi.vaccaro@unipg.it; Fax: +39 075 5855560; Tel: +39 075
5855541
These preliminary results show that PS-BEMP can also
efficiently catalyze the conjugate addition of phosphites to
other Michael acceptors other than α,β-unsaturated ketones,
such as α,β-unsaturated esters and nitriles.
b
We should point out, as a possible limitation of our
methodology, that all our attempts to date to reuse the
recovered catalyst in consecutive reaction runs failed.
†
Electronic
Characterization data and copies of the H, 13C and 31P NMR spectra for
compounds 3a 7a 12a and 13a are reported in ESI. See
DOI: 10.1039/b000000x/
Supplementary
Information
(ESI)
available:
1
-
f
,
-
g
,
-
d
-d
1
2
3
4
(a) D. E. C. Corbridge, Phosphorus: chemistry, biochemistry and
technology, CRC Press, Boca Raton, 6th edn., 2013; (b) A. D. F. Toy,
E. N. Walsh, Phosphorus Chemistry in Everyday Living, American
Chemical Society, Washington DC, 1987; (c) L. D. Quin, A Guide to
Organophosphorus Chemistry, Wiley, New York, 2000.
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Wiley-VCH, Weinheim, 2008; (b) R. Martin and S. L. Buchwald,
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2004, 104, 1629-1662.
Conclusions
In conclusion, we reported a simple protocol for the synthesis
of different types of diethyl and dimethyl phosphonates under
solvent free conditions. Generally equimolar amounts of
reactants were used, with 10-20 mol% of PS-BEMP as a
heterogeneous basic catalyst. To the best of our knowledge this
is the first report on the use of a supported base for catalysing
the phospha-Michael reaction on α,β-unsaturated ketones. We
obtained in all cases good yields (78-85%) using two
phosphorus nucleophiles such as diethyl phosphite (
dimethyl phosphite ( ) and a wide array of aromatic and non-
aromatic α,β-unsaturated ketones as electrophiles. Moreover,
we demonstrated that the methodology is also applicable to
other electron-poor alkenes, such as α,β-unsaturated esters and
nitriles.
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