Synthesis and properties of trialkyl(2,3-dihydroxypropyl)phosphonium salts, a new class of hydrophilic and hydrophobic glyceryl-functionalized ILs

Article abstract of DOI:10.1039/c1gc16035d

A series of new ionic liquids (ILs) based on trialkylglycerylphosphonium cations have been prepared starting from 3-chloropropane-1,2-diol or (2,2-dimethyl-1,3-dioxolan-4-yl)methanol, two compounds readily obtainable from glycerol (a widely available and

Full text of DOI:10.1039/c1gc16035d

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PAPER
Synthesis and properties of trialkyl(2,3-dihydroxypropyl)phosphonium
salts, a new class of hydrophilic and hydrophobic glyceryl-functionalized ILs
Fabio Bellina,* Cinzia Chiappe* and Marco Lessi
Received 22nd August 2011, Accepted 22nd September 2011
DOI: 10.1039/c1gc16035d
A series of new ionic liquids (ILs) based on trialkylglycerylphosphonium cations have been
prepared starting from 3-chloropropane-1,2-diol or (2,2-dimethyl-1,3-dioxolan-4-yl)methanol,
two compounds readily obtainable from glycerol (a widely available and cheap waste product). All
the new phosphonium salts were characterized by NMR, IR and DSC, and their efficacy as polar
additives in a typical base-promoted Baylis–Hillman reaction was also evaluated.
Recently, in continuation of our studies on the synthesis
Introduction
and applications of new ILs starting from renewable sources,11
According to current convention, Ionic Liquids (ILs) are
we became interested in preparing phosphonium-based ILs
compounds entirely composed of ions (like inorganic salts) that
containing two vicinal hydroxyl groups. In particular, in this
have a low melting point, usually below 100 ◦C. This class of
substances have received great attention since the discovery, by
Wilkes and Zawarotko in 1992,1 of air and water stable ILs
based on the imidazolium cation. Nowadays, there is no doubt
that imidazole-based ILs are a focal point both for academic
paper we report the preparation and the characterization of a
new family of n-butyl- or n-octylphosphonium salts 1 and 2,
respectively, starting from commercially avaliable trialkylphos-
phines (3) and 1-chloro-2,3-propandiol (4) or (2,2-dimethyl-
1,3-dioxolan-4-yl)methanol (5), two derivatives of glycerol
and industrial research and, thanks to their peculiar properties
(Fig. 1).12 These new ILs have also been tested as additives in
(i.e. thermal and electrochemical stability, low vapour pressure,
etc.), they have been applied in different areas of chemistry,
a typical Baylis–Hillman reaction, due to their stability in the
basic reaction conditions.
ranging from process technologies to fine chemicals’ synthesis.2
Moreover, the synthetic versatility of the imidazole core allowed
the preparation of more specialized ILs, the so called Task-
Specific Ionic Liquids (TSILs),3 whose structures have been
tailored for well-defined applications, such as CO2 capture,4
metal dissolution,5 and catalysts’ stabilization.6 However, the
presence of an acidic C–H at position 2 of several imidazolium
salts and a recently reported thermal instability of the same
imidazolium salts limit the fields of application of this interesting
class of compounds.7
In order to overcome these difficulties, organic cations
other than imidazolium have been suggested. Among them,
phosphonium salts emerged as a very promising alternative
to imidazolium analogues due to a superior thermal stability
and an inertness in basic reaction media.8 Phosphonium based
ILs have been known since the 1980s,9 but they were scarcely
used until 1990, when phosphines used as precursors became
available on a large scale.10 Moreover, phosphonium-based ILs
Fig. 1 Structures of new phosphonium-based ILs 1 and 2, and of their
precursors 3, 4 and 5.
are generally insoluble and less dense than water, properties that
may be useful during the work-up of the reaction mixtures.
Experimental
NMR spectra were recorded at room temperature using a Varian
300 instrument at 300 MHz (1H), 75.7 MHz (13C), 121.4 MHz
Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35,
56126, Pisa, Italy. E-mail: bellina@dcci.unipi.it, cinziac@farm.unipi.it
148 | Green Chem., 2012, 14, 148–155
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(31P) and 282.2 (19F) in DMSO-d6, CDCl3 or D2O. FT-IR
spectra were recorded using a Perkin Elmer Spectrum One
FTIR-ATR (400–4000 cm-1). Differential scanning calorimetry
(DSC) measurements were performed on a Mettler Toledo
822e instrument according to the following procedure: the
samples (5–10 mg) were sealed in aluminum pans under an
inert atmosphere, and the temperature was calibrated using
indium and tin as standards; the samples were then heated
under high vacuum to give 7.71 g (78%) of 1b as a colorless
liquid.
FT-IR (neat, cm-1): 3579;3368; 2961; 2876; 1458; 1415; 1095;
1
1067; 824; H-NMR (300 MHz; DMSO; TMS): 5.47 (d, J =
5.1 Hz, 1H, CHOH), 4.95 (t, J = 5.1 Hz, 1H, CH2OH), 3.84
(m, 1H, CHOH), 3.36 (m, 2H, CH2OH), 2.32 (m, 8H, P+CH2),
1.51 (m, 12H, CH2CH2), 0.92 (t, J = 7.1 Hz, 9H); 13C-NMR
(75.4 MHz; DMSO; TMS): 67.0 (d, J = 6 Hz, CH2OH); 66.8 (d,
J = 14 Hz, CHOH); 24.0 (d, J = 16 Hz, P+CH2CH2CH2CH3);
23.31 (d, J = 4, P+CH2CH2CH2CH3); 23.27 (d, J = 50 Hz,
P+CH2CH(OH)CH2OH); 19.0 (d, J = 48, P+CH2CH2CH2CH3);
13.5; 31P-NMR (121.4 MHz, DMSO, H3PO4(aq, ext)): 34.3; -143.2
(ept, J = 710 Hz); 19F-NMR (282.2 MHz, DMSO, C6F6): -70.2
(d, J = 711 Hz). C15H34F6O2P2 (422.37); calcd C 42.65, H 8.11;
found C 42.81, H 8.08.
◦
from 30 ◦C to 100 ◦C, at 20 C min-1 (1st heating), cooled
to the set low temperature (-50 ◦C) at the same scan rate
(1st ◦cooling), and heated again to the set high temperature,
200 C at 20 ◦C min-1 (2nd heating). Crystallization and melting
enthalpies were evaluated from the integrated areas of melting
peaks. The MS and MS-MS analyses were carried out using a
Perkin Elmer/Sciex API365 instrument with Turbo Ion Spray as
Ion source, in flow injection analysis positive ions. GC-analyses
were carried out using an Alltech AT-35 bonded FSOT column
(30 m ¥ 0.25 mm i.d.) and an Alltech AT-1 bonded FSOT column
(30 m ¥ 0.25 mm i.d.).
Tributyl(2,3-dihydroxypropyl)phosphonium tetrafluoroborate
(1c)
A round bottomed flask was charged with 1a (7.35 g, 23.5 mmol)
and H2O (15.5 mL). The resulting suspension was gently warmed
until a solution was obtained; then, NaBF4 (2.90 g, 26.4 mmol)
was added in one portion. The reaction mixture was warmed at
40 ◦C for 18 h, then cooled to room temperature and diluted
with CH2Cl2 and water. The two phases were separated, and the
organic phase was washed with water to negative AgNO3 test,
concentrated at reduced pressure and dried at 60 ◦C for 18 h
under high vacuum to give 5.83 g (68%) of 1c as a pale orange
liquid.
FT-IR (neat, cm-1): 3521; 2961; 2935; 2875; 1466; 1384; 1046;
1029; 1H-NMR (300 MHz; DMSO; TMS): 5.42 (d, J = 5.2 Hz,
1H, CHOH), 4.92 (t, J = 5.4 Hz, 1H, CH2OH), 3.83 (m, 1H,
CHOH), 3.36 (m, 2H, CH2OH), 2.26 (m, 8H, P+CH2), 1.45 (m,
12H, CH2CH2), 0.92 (t, J = 7.1 Hz, 9H, CH3); 13C-NMR (75.4
MHz; DMSO; TMS): 66.8 (m, CHOH, CH2OH); 24.0 (d, J =
16 Hz; P+CH2CH2CH2CH3); 23.3 (m, P+CH2CH2CH2CH3);23.2
(d, J = 50 Hz, P+CH2CH(OH)CH2OH); 19.0 (d, J = 48,
P+CH2CH2CH2CH3); 13.7; 31P-NMR (121.4 MHz, DMSO,
H3PO4(aq, ext)): 34.4; 19F-NMR (282.2 MHz, DMSO, C6F6):
-147.9; -148.0. C15H34BF4O2P (364.21); calcd C 49.47, H 9.41;
found C 49.53, H 9.44.
Tributylphosphine (3a) was distilled and stored under Ar.
Compounds 7a and 7b were prepared as reported in the
literature.13 All the other commercially available precursors were
used as received.
Tributyl(2,3-dihydroxypropyl)phosphonium chloride (1a)
A round bottomed flask was charged with deareated 1-chloro-
2,3-dihydroxypropane (4, 5 g, 45.2 mmol) and tributylphosphine
(3a, 14 mL, 11.31 g, 56 mmol). The resulting reaction mixture
was vigorously stirred at 120 ◦C under argon, and the progress of
the reaction was monitored by 31P NMR. After 72 h, the reaction
was cooled to room temperature, then water and petroleum ether
were added. The aqueous phase was extracted three times with
petroleum ether, concentrated at reduced pressure and dried at
70 ◦C for 18 h under high vacuum to give 13.50 g (96%) of 1a as
a colorless liquid.
FT-IR (neat, cm-1): 3229;2958; 2931;2872; 1464;1382; 1092;
1046; 1H-NMR (300 MHz; DMSO; TMS):5.97 (s, 1H,OH), 5.24
(s, 1H,OH), 3.90 (m, 1H, HOCH), 3.38 (m, 2H,HOCH2), 2.35
(m, 8H, P+CH2), 1.46 (m, 12H, CH2CH2CH3), 0.92 (t, J = 7.0
Hz, 9H, CH3).; 13C-NMR (75.4 MHz; DMSO; TMS): 66.6 (m,
CHOH, CH2OH); 23.8 (d, J = 16 Hz; P+CH2CH2CH2CH3);
23.4 (d, J = 48 Hz, P+CH2CH(OH)CH2OH); 23.2 (d, J = 4 Hz;
P+CH2CH2CH2CH3); 18.9 (d, J = 48 Hz, P+CH2CH2CH2CH3);
13,7; 31P-NMR (121.4 MHz, DMSO; H3PO4(aq, ext)): 34.2.
C15H34ClO2P (312.86); calcd. C 57.59, H 10.95; found C 57.74,
H 11.00.
Tributyl(2,3-dihydroxypropyl)phosphonium
bis(trifluoromethane)sulfonimide (1d)
A round bottomed flask was charged with 1a (3.86 g, 12.3 mmol)
and H2O (8.5 mL). The resulting suspension was gently warmed
until a solution was obtained; then Li(NTf)2 (3.90 g, 13.6 mmol)
was added in one portion. The reaction mixture was warmed at
60 ◦C for 18 h, then cooled to room temperature and diluted
with CH2Cl2 and water. The two phases were separated, and the
organic phase was washed with water to negative AgNO3 test,
concentrated at reduced pressure and dried at 60 ◦C for 18 h
under high vacuum to give 6.80 g (99%) of 1d as a colorless
liquid.
Tributyl(2,3-dihydroxypropyl)phosphonium hexafluorophosphate
(1b)
A round bottomed flask was charged with 1a (7.00 g, 22.4 mmol)
and H2O (15.0 mL). The resulting suspension was gently warmed
until a solution was obtained; then, KPF6 (4.63 g, 25.1 mmol)
was added in one portion. The reaction mixture was warmed at
45 ◦C for 18 h, then cooled to room temperature and diluted
with CH2Cl2 and water. The two phases were separated, and the
organic phase was washed with water to negative AgNO3 test,
concentrated at reduced pressure and dried at 60 ◦C for 18 h
FT-IR (cm-1): 3525; 2965; 2938; 2877; 1467; 1347; 1178; 1133;
1
1097; 1052; H-NMR (300 MHz; DMSO; TMS): 5.44 (s, 1H,
OH), 4.91 (s, 1H, OH), 3.84 (m, 1H, CHOH), 3.37 (m, 2H,
CH2OH), 2.31 (m, 8H, P+CH2), 1.46 (m, 12H, CH2CH2), 0.92
(t, J = 7.1 Hz, 9H, CH3); 13C-NMR (75.4 MHz; DMSO; TMS):
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120.1 (q, J = 322 Hz; CF3S(O2)); 66.9 (d, J = 6 Hz, CH2OH); 66.8
(d, J = 14 Hz, CHOH); 23.4 (d, J = 16 Hz, P+CH2CH2CH2CH3);
23.31 (d, J = 4, P+CH2CH2CH2CH3); 23.28 (d, J = 40 Hz,
P+CH2CH(OH)CH2OH); 19.0 (d, J = 48, P+CH2CH2CH2CH3);
13.5; 31P-NMR (121.4 MHz, DMSO, H3PO4(aq, ext)): 34.4; 19F-
NMR (282.2 MHz, DMSO, C6F6): -78.8. C17H34F6NO6PS2
(557.55); calcd C 36.62, H 6.15; found C 36.79, H 6.13.
(1.71 g, 15.6 mmol) was added in one portion under stirring. The
resulting reaction mixture was stirred at room temperature for
24 h and diluted with CH2Cl2 and water. The organic extract was
repeatedly washed with water, concentrated at reduced pressure
◦
and dried at 60 C for 18 h under high vacuum to give 4.24 g
(77%) of 2c as a pale yellow liquid.
FT-IR (neat, cm-1):3514; 2925; 2855; 1465; 1411; 1378; 1051;
1
1031; H-MNR (300 MHz; DMSO; TMS): 5.46 (s, 1H, OH),
Trioctyl(2,3-dihydroxypropyl)phosphonium chloride (2a)
4.98 (s, 1H, OH), 3.84 (m, 1H, CHOH), 3.38 (m, 2H, CH2OH),
2.33 (m, 8H, P+CH2), 1.48 (m, 36H, CH2-alk), 0.88 (t, J =
6.5Hz, 9H, CH3); 13C-NMR(75.4MHz;DMSO;TMS): 66.7(m,
CH2OH, CHOH); 31.8; 30.7 (d, J = 15 Hz, P+CH2CH2-
CH2CH2–); 29.0; 28.8; 23.3 (d, J = 50 Hz; CH2CH(OH)CH2OH);
22.6; 21.2 (d, J = 4 Hz, P+CH2CH2CH2CH2-); 19.1 (d, J = 47
Hz, P+CH2CH2CH2CH2-); 14.3; 31P-NMR (121.4 MHz; DMSO;
H3PO4(aq, ext)): 35.9; 19F d (282.2 MHz; DMSO; C6F6): -148.36;
-148.41. C27H58BF4O2P (532.53); calcd C 60.90, H 10.98; found
C 61.02, H 11.01.
A round bottomed flask was charged with deareated 4 (4.00 g,
36.2 mmol) and trioctylphosphine (3b, 14.7 mL, 12.19 g, 32.9
mmol). The resulting reaction mixture was vigorously stirred
at 120 ◦C under argon, and the progress of the reaction was
monitored by 31P NMR. After 72 h the reaction was cooled to
room temperature, then water and petroleum ether were added.
The organic phase was then washed three times with water,
concentrated at reduced pressure and dried at 70 ◦C for 18 h
under high vacuum to give 15.00 g (95%) of 2a as a colorless
liquid.
Trioctyl(2,3-dihydroxypropyl)phosphonium
bis(trifluoromethane)sulfonimide (2d)
FT-IR (neat, cm-1):3238; 2955; 2915; 2855; 1465; 1377; 1280;
1
1069; 1050; H-NMR (300 MHz; DMSO; TMS): 6.00 (s, 1H,
OH), 5.23 (s, 1H, OH), 3.90 (m, 1H, CHOH), 3.38 (m, 2H,
CH2OH), 2.41 (m, 8H, P+CH2), 1.49 (m, 36H, CH2-alk), 0.87
(m, 9H., CH3); 13C-NMR (75.4 MHz; DMSO; TMS): 66.6 (m,
CH2OH, CHOH); 31.7; 30.6 (d, J = 16 Hz, P+CH2CH2CH2CH2-
); 28.9; 28.7; 23.5 (d, J = 50 Hz, CH2CH(OH)CH2OH);
22.5; 21.2(d, J = 4.5 Hz, P+CH2CH2CH2CH2–); 19.1 (d, 47
Hz, P+CH2CH2CH2CH2-); 14.2; 31P d (121.4 MHz; DMSO;
H3PO4(aq, ext)): 34.2. C27H58ClO2P (481.17); calcd C 67.40, H 12.15;
found C 67.62, H 12.19.
A round bottomed flask was charged with 2a (4.00 g, 8.3 mmol)
and a mixture of ethanol and water (2 : 1) (6.0 mL) then Li(NTf)2
(2.65 g, 15.6 mmol) was added in one portion under stirring.
The resulting reaction mixture was stirred at 60 ◦C for 18 h
and diluted with CH2Cl2 and water. The organic extract was
repeatedly washed with water, concentrated at reduced pressure
◦
and dried at 60 C for 18 h under high vacuum to give 5.67 g
(94%) of 2d as a colorless liquid.
FT-IR (neat, cm-1): 3509; 2927; 2858; 1465; 1409; 1348;
1179; 1134 1055; 1H-NMR (300 MHz; DMSO; TMS): 5.53
(s, 1H, OH), 5.05 (s, 1H OH), 3.83 (m, 1H, CHOH), 3.38
(m, 2H, CH2OH), 2.36 (m, 8H, P+CH2), 1.47 (m, 36H, CH2-
alk), 0.88 (t, J = 6.6 Hz, 9H, CH3); 13C-NMR (75.4 MHz;
DMSO; TMS): 119.5 (q, J = 320 Hz, CF3S(O2)); 66.4 (d,
J = 6 Hz, CH2OH); 66.2 (d, J = 14 Hz, CHOH); 31.3; 30.2
(d, J = 15 Hz, P+CH2CH2CH2CH2–); 28.5; 28.3; 22.8 (d,
J = 50 Hz, CH2CH(OH)CH2OH); 22.1; 20.7 (d, J = 4.5 Hz,
P+CH2CH2CH2CH2–); 18.6 (d, J = 47 Hz, P+CH2CH2CH2CH2-
); 13.8; 31P-NMR (121.4 MHz; DMSO; H3PO4(aq, ext)): 34.3; 19F-
NMR (282.2 MHz; DMSO; C6F6): -79.1. C29H58F6NO6PS2
(725.87); calcd C 47.99, H 8.05; found C 47.77, H 8.01.
Trioctyl(2,3-dihydroxypropyl)phosphonium
hexafluoroPhosphate (2b)
A round bottomed flask was charged with 2a (5.0 g, 10.4 mmol)
and acetone (10.0 mL), then KPF6 (2.93 g, 16.0 mmol) was added
in one portion under stirring. The resulting reaction mixture was
stirred at room temperature for 24 h, filtered and concentrated at
reduced pressure. The resulting crude product was diluted with
CH2Cl2 to remove the excess of KPF6, filtered and concentrated
at reduced pr◦essure to give 5.82 g (92%) of 2b as a colorless solid.
Mp = 48 C; FT-IR (neat, cm-1): 3572; 2926; 2856; 1464;
1
1408; 1378; 1096; 1035; 832; H d (300 MHz; DMSO; TMS):
5.33 (s, 1H, OH), 4.77 (s, 1H, OH), 3.85 (m, 1H, CHOH), 3.38
(m, 2H, CH2OH), 2.29 (m, 8H), 1.5 (m, 36H, CH2-alk), 0.88 (t,
J = 6.5 Hz, 9H); 13C-NMR (75.4 MHz; DMSO; TMS): 66.9
(d, J = 6 Hz, CH2OH); 66.7 (d, J = 14 Hz, CHOH); 31.9;
30.8 (d, J = 15 Hz, P+CH2CH2CH2CH2-); 29.1; 28.8; 23.3 (d,
J = 50 Hz, CH2CH(OH)CH2OH); 22.7; 21.3 (d, J = 4 Hz,
P+CH2CH2CH2CH2-); 19.2 (d, J = 47 Hz, P+CH2CH2CH2CH2-);
14.4; 31P-NMR (121.4 MHz, DMSO, H3PO4(aq, ext)): 35.8; -141.6
(hept, J = 709 Hz); 19F-NMR (282.2 MHz; DMSO; C6F6): -70.4
(d, J = 710 Hz). C27H58F6O2P2 (590.69); calcd C 54.90, H 9.90;
found C 55.02, H 9.96.
Tributyl(2,3-dihydroxypropyl)phosphonium mesylate (1e)
A round bottomed flask was charged with deareated 7a (5.00 g,
25 mmol) and 3a (7.1 mL, 5.76 g, 27 mmol). The reaction
◦
mixture was vigorously stirred at 115 C under argon, and the
conversion of the reactants was monitored by 31P-NMR. After
72 h the reaction was cooled at room temperature, and the crude
product was washed three times with petroleum ether at 50 ◦C,
concentrated at reduced pressure and dried at 50 ◦C for 18 h
under high vacuum to give 9.07 g (88%) of 8a as a pale yellow
liquid [1H-NMR (300 MHz; DMSO; TMS): 4.38 (m, 1H), 4.17
(m, 1H), 3.63 (m, 1H), 2.6 (m, 2H), 2.24 (m, 9H), 1.43 (m,
18H), 0.92 (t, J = 7.2 Hz, 9H). 31P-NMR (121.4 MHz, DMSO,
H3PO4(aq, ext)): 35.6]. A solution of compound 8a in water (90 mL)
was warmed at 40 ◦C and Amberlyst 15 (0.47 g, 10% mol of H+)
was then added. The reaction mixture was vigorously stirred
TriOctyl(2,3-dihydroxypropyl)phosphonium tetrafluoroborate
(2c)
A round bottomed flask was charged with 2a (5.00 g, 10.4 mmol)
and a mixture of ethanol and water (2 : 1) (8.0 mL), then NaBF4
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at the same temperature and the evolution of the reaction was
monitored by NMR. After 5 h the mixture was filtered on celite,
concentrated at reduced pressure and dried at 70 ◦C for 18 h
under high vacuum to give 8.18 g (100%) of 1e as a pale yellow
liquid.
8c as a pale yellow liquid [1H-NMR (300 MHz; DMSO; TMS):
4.36 (m, 1H); 4.16 (m, 1H); 3.64 (m, 1H); 2.59 (m, 2H); 2.25 (m,
9H); 1.38 (m, 42H); 0.87 (t, J = 7.2 Hz, 3H)].
A solution of compound 8c in a mixture of ethanol and water
◦
(2 : 1) (100 mL) was warmed at 40 C and 0.51 g (10% mol of
FT-IR (cm-1): 3326; 2959, 2933; 2873; 1465; 1417; 1214; 1170;
H+) of Amberlyst 15 were then added. The reaction mixture was
vigorously stirred at the same temperature and the evolution of
the reaction was monitored by NMR. After 8 h the mixture was
filtered on celite, concentrated at reduced pressure and dried at
70 ◦C for 18 h under high vacuum to give 12.10 g (100%) of 2e
as a pale yellow liquid.
1
1094; 1036;. H-NMR (300 MHz; DMSO; TMS): 5.43 (s, 2H,
OH), 3.87 (s, 1H, CHOH), 3.44 (m, 1H), 3.30 (m, 1H), 2.08
(m, 8H, P+CH2), 1.44 (m, 12H, CH2CH2), 0.92 (t, J = 7.0
Hz, 9H, CH3);13C-NMR (75.4 MHz; DMSO; TMS): 66.7 (m,
CH2OH, CHOH); 40.1; 23.9 (d, J = 16 Hz, P+CH2CH2CH2CH3);
23.32 (d, J = 50 Hz, P+CH2CH(OH)CH2OH); 23.30 (d, J =
4, P+CH2CH2CH2CH3); 18.9 (d, J = 48, P+CH2CH2CH2CH3);
13.8; 31P d (121.4 MHz; DMSO; H3PO4(aq, ext)): 34.4. C16H37O5PS
(372.50); calcd C 51.59, H 10.01; found C 51.51, H 9.97.
FT-IR (cm-1):3326; 2924; 2854; 1464; 1378; 1215; 1175; 1097;
1036;1H-NMR (300 MHz; DMSO; TMS): 5.24 (s, 2H), 3.87 (m,
1H), 3.42 (m, 2H), 2.51 (m, 11H), 1.43 (m, 36H), 0.87 (t, J =
7.1,9H); 13C d (75.4 MHz; DMSO; TMS): 66.19 (m; CH2OH,
CHOH); 31.3; 30.2 (d, J = 15 Hz, P+CH2CH2CH2CH2–);
28.5; 28.3; 22.9 (d, J = 50, CH2CH(OH)CH2OH); 22.1;
20.8 (d, J = 4 Hz,P+CH2CH2CH2CH2–); 18.6 (d, J = 47
Hz, P+CH2CH2CH2CH2–); 13.8; 31P d (121.4 MHz; DMSO;
H3PO4(aq, ext)): 34.3. C28H61O5PS (540.82); calcd C 62.18, H 11.37;
found C 62.29, H 11.34.
Tributyl(2,3-dihydroxypropyl)phosphonium tosylate (1f)
A round bottomed flask was charged with deareated 7b (5.73 g,
20 mmol) and 3a (6.0 mL, 4.86 g, 24 mmol). The reaction mixture
was vigorously stirred at 115 ◦C under argon, and the conversion
of the reactants was monitored by 31P-NMR. After 48 h the
reaction was cooled at room temperature, and the crude product
was treated with water and petroleum ether. The aqueous phase
was concentrated at reduced pressure and dried at 60 ◦C for 18 h
under high vacuum to give 9.42 g (94%) of 8b as a pale yellow
liquid [1H-NMR (300 MHz; DMSO; TMS): 7.47 (d, J = 8.1 Hz,
2H), 7.11 (d, J = 8.1 Hz, 2H), 4.36 (m, 1H), 4.18 (m, 1H), 3.63
(m, 2H), 2.43 (m, 9H), 1.43 (m, 18H), 0.906 (t, J = 7.2, 9H);
31P d (121.4 MHz, DMSO, H3PO4(aq, ext)): 34.09.]. A solution of
compound 8b in water (65 mL) was warmed at 40 ◦C and 0.41 g
(10% mol of H+) of Amberlyst 15 were then added. The reaction
mixture was vigorously stirred at the same temperature and the
evolution of the reaction was monitored by NMR. After 24 h the
mixture was filte◦red on celite, concentrated at reduced pressure
and dried at 70 C for 18 h under high vacuum to give 8.60 g
(100%) of 1f as a pale yellow liquid.
Trioctyl(2,3-dihydroxypropyl)phosphonium tosylate (2f)
A round bottomed flask was charged with deareated 7b (4.37 g,
15 mmol) and 3b (9.0 mL, 7.48 g, 20 mmol). The reaction mixture
was vigorously stirred at 120 ◦C under argon, and the conversion
of the reactants was monitored by 31P-NMR. After 48 h the
reaction was cooled at room temperature, diluted with petroleum
ether and washed repeatedly with water. The organic phase was
concentrated at reduced pressure to give 9.00 g (91%) of 8d as
a pale yellow liquid [1H-NMR (300 MHz; DMSO; TMS): 7.50
(d, J = 7.8 Hz, 2H), 7.09 (d, J = 7.8, 2H), 4.34 (m, 1H), 4.14 (m,
1H), 3.61 (m, 1H), 2.58 (m, 2H), 2.22 (m, 9H), 1.36 (m, 42H),
0.87 (t, J = 6.9 Hz]. A solution of compound 8d in a mixture
of ethanol and water (2 : 1) (100 mL) was warmed at 40 ◦C and
Amberlyst 15 (0.51 g, 10% mol of H+) was then added. The
reaction mixture was vigorously stirred at the same temperature
and the evolution of the reaction was monitored by NMR. After
8 h the mixture was filtered on celite, concentrated at reduced
pressure and dried at 70 ◦C for 18 h under high vacuum to give
8.39 g (100%) of 2f as a colorless solid.
FT-IR (cm-1): 3339; 2959; 2932; 2872; 1599;1495; 1465;
1
1404;1383;1339;1217;1174; 1120; 1100; 1032; 1009; H-NMR
(300 MHz; DMSO; TMS): 7.49 (d, J = 8.1 Hz, 2H, CH-
Ar), 7.16 (d, J = 8.1, 2H, CH-Ar), 4.83 (s, 2H, OH),
3.83 (m, 1H, CHOH), 3.29 (m, 2H, CH2OH), 2.33 (m,
8H, P+CH2), 1.49 (m, 12H, CH2CH2), 0.90 (t, J = 7.1
Hz, 9H, CH3); 13C-NMR (75.4 MHz; DMSO; TMS): 145.4;
137.7; 128.0; 125.4; 66.1 (m, CH2OH, CHOH); 23.3 (d,
Mp: 39 ◦C; FT-IR (cm-1): 3294; 2954; 2924; 2854; 1660; 1597;
1
1467; 1434; 1181; 1121; 1033: 1010; H d (300 MHz; DMSO;
TMS):7.50 (d, J = 8.0 Hz, 2H), 7.11 (d, J = 8.0 Hz, 2H),
4.57 (sb, 1H), 3.81 (m, 1H), 3.35 (m, 1H), 2.38 (m, 9H), 1.4
(m, 36H), 0.86 (t, J = 5.9 Hz, 9H); 13C d (75.4 MHz; DMSO;
TMS): 146.1; 138.3; 128.6; 126.2; 66.9 (m, CH2OH, CHOH);
32.0; 38.9 (d, J = 15 Hz, P+CH2CH2CH2CH2–); 29.2; 28.9; 23.5
(d, J = 50 Hz, CH2CH(OH)CH2OH); 22.8; 21.4 (2C); 19.7 (d, J =
47 Hz, P+CH2CH2CH2CH2–); 14.6.31P d (121.4 MHz, DMSO,
H3PO4(aq, ext)): 34.4. C34H65O5PS (616.92); calcd C 66.19, H 10.62;
found C 66.11, H 10.66.
J
= 16 Hz, P+CH2CH2CH2CH3); 22.7 (d, J = 50 Hz,
P+CH2CH(OH)CH2OH); 22.6 (d, J = 4; P+CH2CH2CH2CH3);
20.7; 18.3 (d, J = 47, P+CH2CH2CH2CH3); 13.2; 31P d (121.4
MHz; DMSO; H3PO4(aq, ext)): 34.4. C22H41O5PS (448.60); calcd C
58.90, H 9.21; found C 59.02, H 9.24.
Trioctyl(2,3-dihydroxypropyl)phosphonium mesylate (2e)
A round bottomed flask was charged with deareated 7a (5.10 g,
24 mmol) and 3b (14.6 mL, 10.70 g, ◦29 mmol). The reaction
mixture was vigorously stirred at 120 C under argon, and the
conversion of the reactants was monitored by 31P-NMR. After
72 h the reaction was cooled at room temperature, diluted with
petroleum ether and washed repeatedly with water. The organic
phase was concentrated at reduced pressure to give 13 g (93%) of
General procedure for Baylis–Hillman reaction (Table 2)
A flask was charge with 9 (125 mL, 130.5 mg, 1.23 mmol), 10
(150 mL, 137.7 mg, 1.37 mmol), DABCO (70 mg, 0.61 mmol, 50
mol%) and the phosphonium-based ionic liquid (0.123 mmol,
10 mol%), see Table 2. The reaction mixture was stirred for 24 h
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Table 1 Physico-chemical properties of trialkyl(2,3-dihydroxypropyl)-phosphonium salts 1 and 2
Phosphonium salt
OH signals
Y-
1H-NMR (d)a
FT-IR (cm-1)b
Tm(◦C)/DH(J g-1)c
Water Solubilityd
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
Cl
5.97; 5.24
5.47; 4.92
5.42; 4.92
5.44; 4.91
5.43 (2H)
4.83 (2H)
6.00; 5.23
5.33; 4.77
5.46; 4.98
5.53; 5.05
5.24 (2H)
4.57 (2H)
3229
3579
3521
3525
3326
3339
3238
3572
3514
3509
3326
3294
n.d
52/19.4
n.d.
n.d.
n.d.
y
l
l
PF6
BF4
Tf2N
MsO
TsO
Cl
PF6
BF4
Tf2N
MsO
TsO
n
y
y
n
n
n
n
n
n
n.d.
40/18
48/46.2
n.d.
n.d.
n.d.
39/19.9
a DMSO-d6 used as solvent. The concentration of samples was 0.5 M for tributylphosphonium derivatives, 0.7 M for trioctylphosphonium derivatives;
b The analysis was recorded neat; c Conditions: see experimental; d Solubility test are carried out at RT, using 20 mg of Il and 1 mL of water: y =
soluble; l = low soluble; n = not soluble.
Table 2 Baylis–Hillman reaction involving acetaldehyde (9) and ethy-
lacrylate (10) in presence of 1a–f and 2a–f ILs
Entrya
IL
Yield(%)b
1
2
3
4
5
6
7
8
—
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
41
64
60
66
63
60
57
54
74
66
53
44
49
Scheme
1
Preparation of trialkyl(2,3-dihydropropyl)phosphonium
chloride 1a and 2a.
Phosphonium chlorides 1a and 2a were then converted into
the corresponding hexafluorophosphates 1b and 2b, tetrafluo-
roborates 1c and 2c, and bistriflimides 1d and 2d using metathesis
reactions (Scheme 2).
9
10
11
12
13
a The reactions were run using 1.23 mmol of 9, 1.37 mmol of 10, 0.61
mol of DABCO and 0.123 mmol of a phosphonium salt at RT for 24 h.
b Determined by 1H-NMR analysis using DABCO as internal standard.
at room temperature, thus a small portion of the crude reaction
mixture was diluted with CDCl3 and the yield of the reaction was
calculated using DABCO as an internal standard. The yields are
shown in Table 2.
Scheme 2 Metathesis reaction involving trialkyl(2,3-dihydropropyl)-
phosphonium chloride 1a and 2a.
The crude reaction mixture of entry 9 in Table 2 was
purified by flash chromatography and the characterization of
the product, obtained with 75% yield as a colorless liquid, is in
accordance with the lxiterature.14
The experiments of Table 3 were carried out according to the
protocol used for the reactions reported in Table 2.
It is noteworthy to observe that the metathesis reactions
involving 1a could be performed using water as the solvent,
but the synthesis of trioctylphosphonium salts 2b–d required
a solvent optimization due to the insolubility of 2a in H2O.
Initially, the metathesis reactions involving 2a and NaBF4 or
LiNTf2 were carried out in acetone, a solvent able to dissolve
2a but not the inorganic by-products (MCl).16 However, these
Results and Discussion
1
reactions gave a mixture of products, as shown by 31P and H-
The synthesis of tributyl(2,3-dihydroxypropyl)phosphonium
chloride (1a) and trioctyl(2,3-dihydroxypropyl)phosphonium
chloride (2a) was performed by reacting commercially avail-
able tributylphosphine (3a) or trioctylphosphine (3b) with 3-
chloropropane-1,2-diol (4), as depicted in Scheme 1. Due to
the easy oxidation of trialkylphosphines in the open air,15 the
preparation of chlorides 1a and 2a was carried out under an
inert atmosphere (see experimental).
NMR spectra of the crude reaction mixtures (Fig. 2). Besides
the signals of the expected products 1c and 1d at 3.81 (CH), 3.42
and 3.29 ppm (CH2) in 1H spectrum and of a signal at 36.0 ppm
in 31P-NMR (Fig. 2) groups of signals shifted downfield at 4.36
ppm (CH), 4.16 and 3.63 ppm (CH2) in 1H-NMR and a signal
at 35.5 ppm in the 31P spectrum can be seen. In contrast, a single
product (2b) was obtained when KPF6 was used as the reaction
partner.
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Table 3 Screening of reaction condition using 2b in 24h
Entrya
9 (Eq.)
10(Eq.)
DABCO (%mol)
IL (%mol)
T/◦C
Yield (%)b
1
2
3
4
5
6
7
8
9
1
1
1
3
1
1
3
1
3
1.1
1.1
1.1
1
50
50
100
50
50
50
50
50
50
10
50
10
10
10
10
10
10
10
rt
rt
rt
rt
rt
50
50
0
74
51
63
79
57
44
65
47
62
3
1.1
11
1.1
1
0
a The reactions were run for 24 h. b Calculated by 1H-NMR, using DABCO as internal standard.
Fig. 2 NMR spectra of the products arising from metathesis reactions
of 2a in acetone.
The electrospray mass (ESI-MS) analysis of the crude reaction
mixure of 2c and 2d showed that the co-products of the reaction
were the corresponding ketals 6a and 6b (Fig. 3), respectively
arising from the reactions, probably catalyzed by Li+ or Na+
ions, of 2c and 2d with the solvent.17
Scheme
3
Preparation of trialkyl(2,3-dihydropropyl)phospho-
niummesilates and tosylates1e–f and 2e–f, respectively.
salts 1e–f and 2e–f resulted from the deprotection with
R
Amberlyst 15ꢀ using a solvent mixture of EtOH and H2O
(Scheme 3).18
Fig. 3 Structures of ketals 6a and 6b.
It notable that phosphonium salts 8c and 8d gave 1H-NMR
spectra very similar to that obtained for 6a and 6b
(Fig. 3), which definitively confirmed the structures of these last
compounds.
However, when the metathesis reaction of 2a was carried out
in a 2 : 1 mixture of ethanol and water the required product 2c
and 2d were obtained with high chemical purity (Scheme 2).
The syntheses of mesylates 1e and 2e, and of the tosylates
1f and 2f were performed according to the reaction sequence
depicted in Scheme 3, starting from commercially available 2,2-
dimethyl-1,3-dioxolan-4-yl)methanol (5).
In particular, the protected glycerol derivative 5 was converted
into the mesylate (7a) or tosylate (7b), according to a known
procedure.13 Crude 7a and 7b so obtained were then reacted
with phosphines 3a or 3b, obtaining the phosphonium salts
8a–d. Finally, the required (2,3-dihydroxypropyl)phosphonium
Characterization of the novel ILs
The novel phosphonium-based ILs had been fully characterized
by spectroscopic analysis (NMR and IR) and by DSC analysis.
Of particular interest is the analysis of the H-NMR and IR
signals of the OH groups, which can give information on the
interaction existing between the cation and the anion of any
phosphonium salt. From our experimental results, summarized
1
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in Table 1, the values of the chemical shifts and of the stretching
vibrations of the OH groups were influenced by the nature of
the anion.
(compare entry 1 with entries 2–13, Table 2). In particular,
when tributylphosphonium salts are used as additives, invariably
higher chemical yields resulted than those observed in the
absence of the ILs (entries 2–7, Table 2). In contrast, the yields
of the reactions involving trioctyl phosphonium salts seem to
depend on the nature of the counteranion (entries 8–13, Table 2).
Specifically, high yields were obtained with octylphosphonium
salts containing non-coordinating anions (PF6, BF4) (entries 9
and 10, Table 2), while when ILs containing a coordinating anion
(such as Cl, OMs, or OTs) were used the yields resulting were
only slightly higher than those observed when the reaction was
performed in the absence of any additive (compare entries 8, 12
and 13 with entry 1, Table 2).
In particular, the values of the OH IR stretching is between
3220 and 3340 cm-1 for anions able to form hydrogen bonds
(Cl, OTs, OMs), while with poor-coordinating anions (BF4,
PF6, Tf2N) the corresponding OH stretching frequencies were
blue-shifted due to the absence of any interaction with these
anions. A similar trend was also observed by examining the 1H-
NMR spectra, where chlorides 1a and 2a displayed chemical
shifts at a lower field than those observed for the corresponding
phosphonium salts containing less coordinating anions (such as
1b, c and 2b, c)
Finally, DSC data showed that all the newly prepared
phosphonium salts have melting points significantly lower than
100 ◦C, and several of them do not display any solid phase
transition until temperatures as low as -50 ◦C
The best result was observed when trioctyl(2,3-dihydroxy-
propyl)phosphonium hexafluorophosphate 2b was used as
the additive; in fact, the required B–H adduct ethyl 2-
(hydroxy(phenyl)methyl)acrylate (11) was isolated in 74% yield
after 24 h at room temperature.
In order to optimize the reaction conditions, we then set up a
study on the influence of the reaction temperature and the molar
ratio of the reagents on the efficiency of the B–H reaction, using
2b as the additive (Table 3).
From this study it emerged that the efficiency of the reaction
may be improved when a molar excess of aldehyde (3 equiv.) was
employed (entry 4, Table 3), while an increase in the amount
of DABCO (entry 3, Table 3) or of 2b (entry 2, Table 3) led
to lower yields. Similarly, a negative effect◦was observed when
the reaction temperature was raised to 50 C (entries 6 and 7,
Table 3).
Baylis–Hillman reaction
In general, the base-promoted B–H reaction has great impor-
tance from a synthetic viewpoint, either for its high atom-
economy or due to the high grade of functionalization present in
the products, useful building blocks for more complex molecules.
The principal drawback of the synthetic protocols involving
the B–H reaction is the long reaction time generally required to
drive the reaction to completion. In fact, in order to obtain good
yields days or weeks are required. For this reason several studies
have been carried out using small amounts of polar solvents, and
in particular the use of ionic liquids evidenced a general decrease
of the reaction time.19,20 This positive effect has been attributed
to a stabilization of zwitterionic intermediates exerted by polar
additives. The presence of H-donors, indeed, has been postulated
to be beneficial for the reaction speed, as demonstrated by
computational studies.21 However, it is noteworthy that the use
of imidazolium-based ionic liquids as additives for the B–H
reaction generally gave rise to a low isolated yield of the B–H
adduct, despite the increase observed in reaction speed, probably
due to the low stability of this class of ionic liquids in the reaction
conditions.22
Conclusions
In conclusion, this work has demonstrated that a variety of
trialkyl(2,3-dihydroxypropyl)phosphonium salts may be effi-
ciently prepared starting from cheap and commercially avail-
able glycerol derivatives. The 2,3-dihydroxypropyl group on
the phosphonium cation is able to hydrogen bond the most
coordinating anions, giving salts that are generally liquid at
room temperature. The interactions between ions are evidenced
by NMR and IR data. The presence of long alkyl chains
counterbalances the presence of the hydroxyl groups, limiting
the solubility of these salts in water. All of the new com-
pounds revealed to be efficient additives in a typical Baylis–
Hillman reactions. In particular, the hydrophobic trioctyl(2,3-
dihydroxypropyl)phosphonium hexafluoro-phosphate (2b) was
the most efficient, giving the required Baylis–Hillman adduct in
a chemical yield almost double than that obtained in the absence
of any phosphonium salt.
Taking into account these considerations, it seems interesting
to us to test the efficiency of our new phosphonium salts as
additives in a typical B–H reaction. In particular, we studied the
reaction between benzaldehyde (9) and ethyl acrylate (10) in the
presence of DABCO as the catalyst (Scheme 4).
Notes and references
Scheme 4 Baylis–Hillman reaction involving acetaldehyde (9) and
ethylacrylate (10).
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