Aqueous-phase synthesis of α-hydroxyphosphonates catalyzed by β-cyclodextrin

Article abstract of DOI:10.1080/00397911.2010.523492

α-Hydroxyphosphonates were synthesized from aromatic/heteroaromatic aldehydes with triethyl phosphite in the presence of β-Cyclodextrin in an aqueous medium. The β-Cyclodextrin can be recovered and reused without loss of catalytic activity. Taylor & Franc

Full text of DOI:10.1080/00397911.2010.523492

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Aqueous-Phase Synthesis of α-
Hydroxyphosphonates Catalyzed by β-
Cyclodextrin
Katla Ramesh a , Bandaru Madhav a , Sabbavarapu Narayana Murthy a
& Yadavalli Venkata Durga Nageswar a
a Organic Chemistry Division I, Indian Institute of Chemical
Technology , Hyderabad , India
Accepted author version posted online: 18 Jul 2011.Published
online: 14 Sep 2011.
To cite this article: Katla Ramesh , Bandaru Madhav , Sabbavarapu Narayana Murthy & Yadavalli
Venkata Durga Nageswar (2012) Aqueous-Phase Synthesis of α-Hydroxyphosphonates Catalyzed by
β-Cyclodextrin, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 42:2, 258-265, DOI: 10.1080/00397911.2010.523492
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Synthetic Communications1, 42: 258–265, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.523492
AQUEOUS-PHASE SYNTHESIS OF
a-HYDROXYPHOSPHONATES CATALYZED
BY b-CYCLODEXTRIN
Katla Ramesh, Bandaru Madhav, Sabbavarapu Narayana
Murthy, and Yadavalli Venkata Durga Nageswar
Organic Chemistry Division I, Indian Institute of Chemical Technology,
Hyderabad, India
GRAPHICAL ABSTRACT
Abstract a-Hydroxyphosphonates were synthesized from aromatic/heteroaromatic
aldehydes with triethyl phosphite in the presence of b-cyclodextrin in an aqueous medium.
The b-cyclodextrin can be recovered and reused without loss of catalytic activity.
Keywords Aromatic=heteroaromatic aldehydes; b-cyclodextrin; a-hydroxyphosphonate;
triethyl phosphate; water
INTRODUCTION
a-Hydroxyphosphonate derivatives are important organophosphorus
compounds associated with a wide variety of biological and pharmaceutical
activities.[1] These derivatives are extensively used as synthetic intermediates[2]
with iterative manipulation of functional groups to 1,2-diketones[3] and
a-ketophosphonates.[4] The hydrolyzed products of a-hydroxyphosphonates exhibit
a wide range of medicinal properties such as antiviral,[5] antibacterial,[6] and
anticancer activities.[7]
In the past, there have been reports on the synthesis of a-hydroxyphosphonates
from the corresponding aldehydes by the reaction of dialkyl (or) trialkyl phosphates,
catalyzed by bases such as ethyl magnesium bromide,[8] potassium fluoride on
alumina,[9] quinine,[10] lithium diisopropylamine (LDA),[11] MgO,[12] NH4VO3,[13]
and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),[14] as well as acids such as
Received August 20, 2010.
Address correspondence to Yadavalli Venkata Durga Nageswar, Organic Chemistry Division I,
Indian Institute of Chemical Technology, Hyderabad 500607, India. E-mail: dryvdnageswar@gmail.com
258

a-HYDROXYPHOSPHONATES
259
alumina,[15] Ti(OPri)2,[16] LiClO4 ꢀ Et2O, TMSCl,[17] and guanidine hydrochloride.[18]
Most of these catalysts are very expensive and moisture sensitive, and all the
reported methods involve usage of volatile and flammable organic solvents and
extensive reaction times, resulting in poor yields. However, with increasing environ-
mental concerns, an environmentally benign method under neutral conditions is
highly desirable.
Presently, organic reactions in the aqueous phase have attracted attention
because of the advantages of water, an environmentally benign and economically
affordable solvent. However, the fundamental problem in performing the reaction
in water is that many organic substrates are hydrophobic and insoluble in water.
In continuation of our efforts to develop greener chemical approaches for the syn-
thesis of novel reaction intermediates and heterocyclic moieties,[19] herein we disclose
the synthesis of a-hydroxy phosphonates using b-cyclodextrin as a reusable catalyst
under supramolecular catalysis.
RESULTS AND DISCUSSION
Cyclodextrins (CDs) are cyclic oligosaccharides possessing hydrophobic
cavities, which bind substrates selectively and catalyze chemical reactions with high
selectivity. They catalyze the reactions by supramolecular catalysis involving revers-
ible formation of host–guest complexes by noncovalent bonding as seen in enzymes.
We describe herein the remarkable catalytic activity of b-cyclodextrin in the reaction
of aldehydes toward a variety of triethyl phosphites to give exclusively substituted
a-hydroxy phosphonates.
In this study, a model reaction was conducted by reacting benzaldehyde with
triethyl phosphite in H2O to obtain the corresponding a-hydroxy phosphonate in
poor yield due to poor solubility of the aldehyde in H2O. When the same reaction
was conducted using b-CD at room temperature, the product was obtained in
moderate yield (65%). In a controlled reaction using b-CD as a supramolecular
catalyst, at 70 ꢁC the product was obtained in excellent yield (Scheme 1). This
encouraging result made us expand the scope of the reaction by reacting various
aromatic and heteroaromatic aldehydes with triethyl phosphite to obtain the
corresponding a-hydroxyphosphonates in quantitative yields. In general, all the
reactions were clean and resulted in good yields (80–93%). Substitution on aro-
matic aldehyde played a crucial role in governing the product yield as it can be
seen from the Table 1. Electron-withdrawing group on benzaldehyde gave a good
yield (Table 1, entry 2), whereas electron-donating group on benzaldehyde gave a
lower yield (Table 1, entry 8) in comparison with benzaldehyde (Table 1, entry 1).
Scheme 1.

260
K. RAMESH ET AL.
Table 1. b-CD-II catalyzed synthesis of a-hydroxyphosphonates
Entry
1
Aldehyde
Phosphate
Product
Time
Yield (%)a
10 h
89
2
8 h
93
3
4
9 h
90
88
10 h
5
6
7
8
9.5 h
10 h
12 h
11 h
86
87
90
81
9
10.5 h
80
89
10
11
10 h
10.5 h
85
(Continued )

a-HYDROXYPHOSPHONATES
Table 1. Continued
261
Entry
Aldehyde
Phosphate
Product
Time
Yield (%)a
12
13
10.5 h
10 h
81
83
No reaction
No reaction
14
15
12 h
14 h
No reaction
16
16 h
Notes. Reaction conditions: b-cyclodextrin (1.0 equiv), aldehyde (1 equiv), and triethyl phosphite (1
equiv) at 60–70 ꢁC for 8–12 h. All products were confirmed by 1H and 13C NMR and direct comparison
with authentic samples.
aYields of the isolated product.
When we reacted salicylaldehyde with triethyl phosphite, an enhancement of
product yield was observed. With optimized reaction conditions on hand, we
attempted to synthesize a0-methyl-a -hydroxy phosphonates by reacting acetophe-
none with triethyl phosphate, but no reaction was observed even after prolonged
1
reaction times. All the products were characterized by H and 13C NMR, infrared
(IR) spectrometry, and mass spectrometry and compared with literature reports.
The catalytic activity of b-cyclodextrin for these reactions was established by
the fact that no reaction was observed in the absence of cyclodextrin. Evidence for
1
complexation between the aldehyde and cyclodextrin is supported by H NMR stu-
1
dies. H NMR spectra (dimethylsulfoxide, DMSO) of b-CD, b-CD-benzaldehyde
complex were examined, and it was observed that there was an upfield shift of H-C
(3) (0.02 ppm) and H-C (5) (0.02 ppm) protons of cyclodextrin in the b-CD-
benzaldehyde complex when compared to b-CD, indicating the formation of an
inclusion complex of aldehyde with b-CD. The complexation with b-CD increases
the reactivity of aldehyde carbonyl functionality because of intermolecular hydrogen
bonding with the CD-hydroxyl groups, facilitating the addition of phosphite.
Here, b-CD not only forms the inclusion complex with aldehyde but is also involved
in the intermolecular hydrogen bonding with the guest molecule to promote the
reaction.

262
K. RAMESH ET AL.
The generality of this reaction was investigated for the synthesis of various
a-hydroxyphosphonate derivatives by using different substituted aldehydes with
triethyl phosphite at optimal conditions. The reaction proceeded smoothly to afford
the corresponding a-hydroxyphosphonates.
EXPERIMENTAL
Thin-layer chromatography (TLC) was carried out using silica-gel 60 F254
precoated plates. Visualization was accomplished with an ultraviolet (UV) lamp
1
or I2 stain. All products were characterized by their NMR and MS spectra. H
and 13C NMR were recorded on 200 MHz in CDCl3 using tetramethylsilane
(TMS) as an internal standard; chemical shifts were reported in parts per million
(ppm, d).
General Procedure
b-Cyclodextrin (1 mmol) was dissolved in water (20 mL). Aldehyde (1.0 mmol)
was added to this clear solution and stirred for 10 min, and then triethylphosphite
(2.0 mmol) was added, after which the reaction was heated at 60–70 ꢁC until com-
pletion of the reaction as indicated by TLC. The reaction mixture was extracted with
ethyl acetate (3 ꢂ 10 mL). The organic layers were washed with water and a saturated
brine solution and dried over anhydrous Na2SO4. The combined organic layers were
evaporated under reduced pressure, and the resulting crude product was purified by
column chromatography by using ethyl acetate and hexane (3:7) as eluent. The aque-
ous layer was cooled to 5 ꢁC to recover b-CD (Table 1). The products were confirmed
by comparison with authentic samples.
Spectral Data of Principal Compounds
Diethyl hydroxy(phenyl)methylphosphonate (Table 1, Entry 1).[20]
Solid, mp, 76–78 ꢁC; IR: 3251, 2987, 1730, 1621, 1473; 1H NMR (200 MHz) d
1.10–1.54 (m, 6H), 3.0 (s, OH), 3.85 (s, 1H), 3.99–4.49 (m, 4H), 6.89–6.94 (m,
2H), 7.12 (s, 1H), 7.24–7.34 (m, 2H); 13C NMR (75 MHz, CDCl3) d 14.1, 16.07,
63.18, 64.81, 112.44, 123.92, 125.72, 130.69, 138.61; MS m=z (ESI); 267 (M þ Na)þ.
Anal. calcd. for (C11H17O4P): C, 54.10; H, 7.02. Found: C, 54.02; H, 6.95.
Diethyl hydroxy(4-nitrophenyl)methylphosphonate (Table 1, Entry
1
2).[20] IR: 3179, 2991, 2114, 1751, 1627, 1523, 1342 cmꢃ1; H NMR (200 MHz,
CDCl3) d 1.02–1.75 (m, 6H), 3.2 (s, OH), 3.85–4.58 (m, 4H), 6.88–6.97 (m, 2H),
8.18–8.44 (m, 2H), 4.74 (s, 1H);13C NMR (75 MHz, CDCl3) d 15.72, 29.68, 64.99,
71.79, 110.43, 122.00, 127.46, 136.29. 159.25, 170.25; MS m=z (ESI); 312 (M þ Na).þ
Anal. calcd. for (C11H16NO6P): C, 45.68; H, 5.58; N, 4.84. Found: C, 45.62, H,
5.51; N, 4.79.
Diethyl (4-bromophenyl)hydroxy methylphosphonate (Table 1, Entry
1
4).[20] IR: 3250, 2990, 2854, 1736, 1631, 1484, 1233 cmꢃ1, H NMR (200 MHz,
CDCl3) d 1.05–1.44 (m, 6H), 3.25 (s, OH), 3.87 (s, 1H), 4.06–4.32 (m, 4H),

a-HYDROXYPHOSPHONATES
263
6.51–6.59 (m, 2H), 6.66–7.03 (m, 2H); 13C NMR (75 MHz, CDCl3) d 14.5, 62.3, 71.1,
124.5, 125.9, 130.4, 132.0, 134.2; MS m=z (ESI); 345 (M þ Na)þ. Anal. calcd. for
(C11H16BrO4P): C, 40.89; H, 4.99. Found: C, 40.83; H: 4.92.
Diethyl (4-flurophenyl)(hydroxy)methylphosphonate (Table 1, Entry
5).[20] IR: 3341, 2924, 2854, 1735, 1631, 1485; 1H NMR (200 MHz, CDCl3) d
1.07–1.4 (m, 6H), 3.7 (s, OH), 3.89 (s, 1H), 4.00–4.32 (m, 4H), 6.75 (m, 2H),
7.19–7.25 (m, 2H); 13C NMR (75 MHz, CDCl3) d 14.6, 61.8, 69.9, 114.9, 127.8,
130.0, 131.8, 156.2; MS m=z (ESI); 263 (M þ H)þ. Anal. calcd. for (C11H16FO4P):
C, 50.39; H, 6.15. Found C, 50.35; H, 6.11.
Diethyl (4-chlorophenyl)(hydroxy)methylphosphonate (Table 1, Entry
1
6)[20]. Solid, mp, 67–68 ꢁC; IR: 3245, 2923, 2854, 1738, 1620, 1036 cmꢃ1; H NMR
(200 MHz, CDCl3) d 0.99–1.52 (m, 6H), 3.9 (s, OH), 3.86 (s, 1H), 4.12–4.24 (m, 4H),
6.74–6.79 (m, 2H), 7.17–7.26 (m, 2H); MS m=z (ESI); 301 (M þ Na)þ. Anal. calcd.
for (C11H16ClO4P): C, 47.41; H, 5.79. Found: C, 47.32; H, 5.74.
Diethyl hydroxy(2-hydroxy phenyl)methylphosphonate (Table 1, Entry
1
7). White solid, mp 96–97 ꢁC; H NMR (200 MHz, CDCl3): d 1.41 (m, 6H), 3.5 (s,
OH), 3.88 (s, 1H), 4.12–4.36 (m, 4H), 5.96–6.20 (m, 2H), 6.89 (m, 2H), 8.01 (s, OH);
13C-NMR (75 MHz, CDCl3) d 14.2, 61.5, 65.4, 115.3, 120.8, 128.1, 128.8, 129.1,
140.5; MS m=z: 283 (M þ Na)þ. Anal. calcd. for (C11H17O5P): C, 50.77; H, 6.58.
Found: C, 5.71; H, 6.52.
Diethyl hydroxy(4-methoxyphenyl)methylphosphonate (Table 1, Entry
1
8).[20] White solid, mp 119–120 ꢁC; IR (KBr): 3268, 1225, 1051 cmꢃ1; H NMR
(200 MHz, CDCl3): d 7.39 (dd, 2H, J ¼ 8.7, 2.8 Hz, Ar-H), 6.87 (d, 2H, J ¼ 8.5 Hz,
Ar-H), 4.97 (d, 1H, J ¼ 10 Hz, P-CH), 3.91 (m, 4H, O–CH2), 3.82 (s, 3H, Ar-O–
CH3), 1.26 (t, 3H, J ¼ 7.8 Hz, CH3), 1.21 (t, 3H, J ¼ 7.8 Hz, CH3); MS m=z 275
(M þ H)þ. Anal. calcd. for (C12H19O5P): C, 52.55; H, 6.98. Found: C, 52.49; H, 6.93.
Diethyl hydroxy(p-tolyl)methylphosphonate (Table 1, Entry 9).[20]
1
Solid, mp 94–95 ꢁC; IR: 3253, 2985, 2858, 1733, 1627, 1492, 1247, 1035 cmꢃ1, H
NMR (200 MHz, CDCl3) d 0.88–1.51 (m, 6H), 2.07 (s, 3H), 4.73 (d,1H), 3.93–4.28
(m, 4H), 6.56–6.99 (m, 2H), 7.07–7.49 (m, 2H); 13CNMR (75 MHz, CDCl3) d
15.96, 20.98, 29.67, 64.61, 71.65, 110.03, 126.97, 130.90, 132.80, 138.73, 169.16;
MS m=z (ESI); 281 (M þ Na)þ. Anal. calcd. for (C12H19O4P): C, 55.81; H, 7.42.
Found: C, 55.78; H, 7.38.
Diethyl hydroxyl(naphthalen-2-yl)methylphosphonate (Table 1, Entry
1
10).[20] IR: 3351, 2974, 2844, 1735, 1631, 1485; H NMR (200 MHz, CDCl3) d
1.05–1.2 (m, 6H), 3.82 (s, OH), 4.01 (s, 1H), 4.10–4.32 (m, 4H), 7.15–7.29 (m,
2H), 7.58–7.75 (m, 3H), 7.90–8.12 (m, 2H); 13C NMR (75 MHz, CDCl3) d 14.2,
61.9, 71.0, 124.1, 125.8, 126.5, 127.1, 127.4, 127.9, 131.0, 133.9; MS m=z (ESI):
295 (M þ H)þ. Anal. calcd. for (C15H19O4P): C, 61.22; H, 6.51. Found: C, 61.18;
H, 6.48.
Diethyl hydroxyl(pyridyin-2-yl)methylphosphonate (Table 1, Entry
1
11). Colorless oil; IR: 3351, 2957, 1630, 1621, 1373; H NMR (200 MHz, CDCl3)
d 1.09–1.24 (m, 6H), 3.12 (s, OH), 3.89–4.39 (m, 4H), 4.78 (s, 1H), 6.79–6.84 (m,

264
K. RAMESH ET AL.
2H), 7.12–7.24 (t, 2H); 13C NMR (75 MHz, CDCl3) d 14.6, 63.28, 69.81, 120.44,
124.91, 135.21, 145.69, 156.61; MS m=z (ESI); 268 (M þ Na)þ. Anal. calcd. for
(C10H16O4P): C, 48.98; H, 6.58; N, 5.71. Found: C, 48.90; H, 6.51; N, 5.68.
Diethyl hydroxy(thiophene-2-yl)methylphosphonate (Table 1, Entry
1
12).[20] IR: 3350, 2890, 2848, 1726, 1611, 1484, 1230 cmꢃ1; H NMR (200 MHz,
CDCl3) d 1.03–1.34 (m, 6H), 3.84 (s, OH), 3.89 (s, 1H), 4.16–4.32 (m, 4H),
6.51–6.69 (m, 2H), 7.03 (m, 1H); 13C NMR (75 MHz, CDCl3) d 14.6, 61.5, 72.3,
124.5, 126.8, 127.5, 135.8; MS m=z (ESI): 273 (M þ Na)þ. Anal. calcd. for
(C9H15O4PS): C, 43.20; H, 6.04. Found: C, 43.16; H, 6.02.
Diethyl furan-2-yl(hydroxyl)methylphosphonate (Table 1, Entry
13).[20] IR: 3265, 2973, 2884, 1738, 1620 cmꢃ1 1H NMR (200 MHz, CDCl3) d
,
1.2–1.42 (m, 6H), 3.85 (s, OH), 4.12 (s,1H), 4.22–4.44 (m, 4H), 6.04–6.20 (m, 2H),
7.26 (m, 1H); 13C NMR (75 MHz, CDCl3) d 14.6, 62.0, 71.9, 110.1, 112.5, 138.9,
153.4; MS m=z (ESI): 257 (M þ Na)þ. Anal. calcd. for (C9H15O5P): C, 46.16; H,
6.46. Found: C, 46.10; H, 6.39.
CONCLUSION
In conclusion, we have developed an environmentally friendly method to syn-
thesize a-hydroxy phosphonates in good yields under neutral conditions involving
supramolecular catalysis in H2O. This method has the additional advantage that
the reaction excludes highly flammable organic solvents, moisture-sensitive catalysts,
and elevated reaction temperatures.
ACKNOWLEDGMENTS
B. M. and S. N. M. thank the Council of Scientific and Industrial Research and
K. R. thanks the University Grants Commission, New Delhi, for the fellowships.
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