Uncatalyzed addition of TMSCN to acylphosphonates

Article abstract of DOI:10.1055/s-2006-951558

The cyanosilylations of various alkyl- and arylphosphonates under comparatively mild conditions furnished the trimethylsilyloxycyanophosphonates in high yield. The addition to ketophosphonate functions works without the influence of a catalyst. Georg Thie

Full text of DOI:10.1055/s-2006-951558

LETTER
3329
Uncatalyzed Addition of TMSCN to Acylphosphonates
Uncatalyzed
A
ddit
y
ion of
T
MSC
h
N
to
A
cylph
a
osphonatesn S. Demir,* Ömer Reis, Metin Kayalar, Serkan Eymur, Barbaros Reis
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Fax +90(312)2103200; E-mail: asdemir@metu.edu.tr
Received 28 August 2006
In our previous work we have shown that acylphospho-
nates are potent acyl anion precursors and that they under-
Abstract: The cyanosilylations of various alkyl- and arylphospho-
nates under comparatively mild conditions furnished the trimethyl-
silyloxycyanophosphonates in high yield. The addition to
ketophosphonate functions works without the influence of a cata-
lyst.
go
nucleophile-promoted
phosphonate–phosphate
rearrangement in order to provide corresponding acyl an-
ion equivalents as reactive intermediates.8 During our in-
vestigations regarding the nature of carbanionic
intermediates that are generated via phosphonate–phos-
phate rearrangement we needed to synthesize compound
2a (Scheme 1). In an initial reaction, acylphosphonate 1a
was dissolved in 1 mL toluene, and 1.2 mmol (1.2 equiv)
of TMSCN was slowly added via a syringe. After the
completion of the reaction (15–30 min), a reaction mix-
ture was concentrated under vacuum and purified by col-
umn chromatography (ether eluent) to obtain 2a in 98%
yield.
Key words: cyanohydrins, addition reactions, phosphorus, nucleo-
philic additions
The addition of trimethylsilyl cyanide (TMSCN) to carbo-
nyl compounds along with the subsequent hydrolysis in
turn produces cyanohydrins.1 Cyanohydrins are useful in-
termediates for the synthesis of many interesting com-
pounds with biological properties. The two functional
groups (–OH and –CN) can be easily transformed into
various compounds.2 The general route to O-TMS cyano-
hydrins is the addition of TMSCN to the aldehydes (or ke-
tones), which only occurs in the presence of Lewis acid3a
and nucleophilic catalysis.3b,c When considering the value
of O-TMS cyanohydrins, it is not surprising that catalysis
was reported hundreds of times so far for this particular
transformation, and continue to be reported at the same
pace.4
O
Me3SiO
Ph
CN
TMSCN
toluene
Ph
PO(OEt)2
PO(OEt)2
2a
1a
Scheme 1
After rapid screening with 1a and 1b, we found that the re-
action indeed proceeds smoothly in various solvents. For
example, we observed similar purities in DMF, toluene,
CH2Cl2, and MeCN. Moreover, neat 1b reacted with TM-
SCN providing a quantitative conversion. Although the
reaction in DMF provided faster conversions, we chose
the toluene as the reaction medium for practical reasons.
We initially attributed the enhanced reactivity of acyl-
phosphonates to the presence of a phosphonate moiety
that could interact with TMSCN through P=O and acti-
vates it to execute an intramolecular attack. Interestingly,
Beletskaya7 reported that the reaction of a-, b-, and g-ke-
tophosphonates with TMSCN only occurs with catalytic
Bu3SnCN at elevated temperatures (50–60 °C, 6–8 h).
This report was in good agreement with our observations.
Therefore, we carried out some control experiments prior
to proceeding. In two separate experiments, benzaldehyde
(3) along with a one-to-one mixture of benzaldehyde (3)
and benzoylphosphonate (1a) were reacted with one
equivalent of TMSCN. Crude reaction mixtures were in-
spected for two possible products, namely 2a and 4. We
observed that there were no traces of the product 4 in both
reaction mixtures, whereas 2a was the single product in
the latter reaction (Scheme 2). Although we have no ex-
planation for Belatskaya’s report, we did observe 1a to re-
act with TMSCN without needing a catalyst in 15–30
minutes (<5 min in DMF).
Phosphorus analogues of hydroxycarboxylic acids as well
as phosphorylated a-hydroxynitriles are interesting as po-
tential biologically active substances, mild water-soluble
phosphorylating agents for biological systems, and water-
soluble ligands for metal complex catalysis. Only a few
methods have been described for the addition of carbon
nucleophiles to acylphosphonates,5 and as far as we know,
only two methods describe the synthesis of phosphorylat-
ed a-hydroxynitriles. Konovalova et al.6 obtained these
compounds by the reaction of trimethylsilyl diethyl phos-
phite with esters and nitriles of a-oxocarboxylic acids.
Beletskaya7 reported the reaction of a-, b-, and g-keto-
phosphonates with TMSCN to only occur with catalytic
Bu3SnCN at elevated temperatures (50–60 °C, 6–8 h) and
acetylphosphonate was only example for a-ketophos-
phonate. In this present paper, we investigated the scope
of the addition reaction of TMSCN to ketophosphonate
for the synthesis of a-trimethylsilyloxynitrile, which are
interesting precursors in synthetic organic chemistry, in
which we found that TMSCN adds to ketophosphonate in
high yield without any influence from a catalyst.
SYNLETT 2006, No. 19, pp 3329–3333
0
1
.1
2
.2
0
0
6
Advanced online publication: 23.11.2006
DOI: 10.1055/s-2006-951558; Art ID: G28406ST
© Georg Thieme Verlag Stuttgart · New York

3330
A. S. Demir et al.
LETTER
HO CN
O
O
Me3SiO CN
OEt
OEt
+
TMSCN
toluene
1 N HCl, THF
P
P
4
Ph
PO(OEt)2
Ph
H
+
2a
OEt
OEt
O
not
observed
O
3
1a
2a
7
TMSCN
toluene
Scheme 5
From a synthetic point of view, this reaction provides not
only biologically active a-hydroxyphosphonates with
quaternary carbon but also in a highly functionalized
form. These products can be used for the generation of
valuable acyl anion equivalents via phosphonate–phos-
phate rearrangement.8 As a general extension of this
work, we are currently investigating the asymmetric
version of this reaction as well as addition reactions with
TMS derivatives other than TMSCN.
Me3SiO
Ph
CN
4
not observed
Scheme 2
Shibasaki and co-workers showed that enantioselectivies
in chiral Lewis acid, the catalyzed addition of TMSCN to
aldehydes was sometimes enhanced by the addition of
‘promoters’ such as Bu3PO, CH3P(O)Ph2, and Ph3PO.9
This phosphine oxide activation effect led to the incorpo-
ration of phosphine oxide moieties into the catalytic
Lewis acid structures as a Lewis acid–Lewid base pair for
what has been described as a ‘two-center’ catalysis. Later,
Corey and Ryu10 investigated the effect of phosphine ox-
ide, and proposed that a reaction between phosphine oxide
and TMSCN occurred as follows (Scheme 3).
A highly efficient synthesis of trimethylsilyloxycyano-
phosphonates was described via the cyanosilylations of
various alkyl and arylphosphonates. The cyanosilylations
take place under comparatively mild conditions in terms
of temperature and reaction time. The high-yield addition
of TMSCN to ketophosphonate works without the influ-
ence of a catalyst. Acid hydrolysis of the trimethylsilyl-
oxycyanophosphonates can be derived from a-hydroxy-a-
cyanophosphonates in high yield.
General Procedure for TMSCN Addition
Ph3P(OTMS)(N=C:)
Ph3PO + TMSCN
Acylphosphonate (1 mmol) was dissolved in toluene (1 mL) and
TMSCN (1.2 mmol, 1.2 equiv) and slowly added via a syringe (5
min). After the completion of the reaction (15–30 min), the reaction
mixture was concentrated under vacuum and (if needed) purified by
column chromatography (Et2O eluent) to obtain 2.
5
Scheme 3
Diethyl Cyano(phenyl)(trimethylsilyloxy)methylphosphonate
(2a)
They supported their proposal with several NMR and IR
experiments and identified 5 as a more reactive cyano-
silylating reagent than isomeric cyanide. This seems to be
in agreement with our initial proposal where a phos-
phonate moiety activates the TMSCN through an inter-
action via P=O bond as in 6 to afford 2a (Scheme 4).
Yield 321 mg (98%); white semisolid. 1H NMR (CDCl3): d = 0.19
(9 H, s), 1.22 (3 H, dt, J = 0.6, 7.1 Hz), 1.35 (3 H, t, J = 7.1, 0.5 Hz),
3.90–4.00 (1 H, m), 4.02–4.13 (1 H, m), 4.17–4.28 (2 H, m), 7.37–
7.45 (3 H, m), 7.66–7.71 (2 H, m). 13C NMR (CDCl3–CCl4): d =
0.98, 16.2 (d, J = 5.2 Hz), 16.4 (d, J = 5.9 Hz), 64.8 (d, J = 7.4 Hz),
65.4 (d, J = 7.2 Hz), 73.1 (d, J = 176.5 Hz), 117.9, 126.9 (d, J = 4.5
Hz), 128.4 (d, J = 2.2 Hz), 129.5 (d, J = 3.3 Hz), 134.5 (d, J = 3.6
Hz). 31P NMR (CDCl3): d = 11.3. Anal. Calcd for C15H24NO4PSi
(341.41): C, 52.77; H, 7.09; N, 4.10. Found: C, 52.61; H, 7.12; N,
4.28.
Me
Me
Si
NC
Me
O
O
OEt
OEt
Me3SiO
P
CN
2a
OEt
OEt
O
P
Diethyl Cyano(4-fluorophenyl)(trimethylsilyloxy)methylphos-
phonate (2b)
Yield 351 mg (98%); colorless liquid. 1H NMR (CDCl3): d = 0.19
(9 H, s), 1.23 (3 H, t, J = 7.1 Hz), 1.35 (3 H, t, J = 7.39 Hz), 3.95–
4.03 (1 H, m), 4.05–4.14 (1 H, m), 4.17–4.28 (2 H, m), 7.08–7.13 (2
H, m), 7.64–7.68 (2 H, m). 13C NMR (CDCl3–CCl4): d = 0.96, 16.2
(d, J = 5.3 Hz), 16.4 (d, J = 5.3 Hz), 64.8 (d, J = 7.4 Hz), 65.5 (d,
J = 7.4 Hz), 72.4 (d, J = 178 Hz), 115.4 (dd, J = 1.7, 21.1 Hz),
117.7, 128.8 (dd, J = 4.0, 8.5 Hz), 130.5 (m), 163.3 (dd, J = 2.8, 250
Hz). 31P NMR (CDCl3): d = 11.1. Anal. Calcd for C15H23FNO4PSi
(359.41): C, 50.13; H, 6.45; N, 3.90. Found: C, 50.22; H, 6.21; N,
4.13.
6
Scheme 4
Several aromatic and aliphatic acylphophonates were re-
acted with TMSCN in toluene, the results of which are
summarized in Table 1. The compound 2i that was de-
rived from proloylphosphonate 1i showed a complex
spectrum at room temperature due to the restricted rota-
tion.
Synlett 2006, No. 19, 3329–3333 © Thieme Stuttgart · New York

LETTER
Uncatalyzed Addition of TMSCN to Acylphosphonates
3331
Table 1 Uncatalyzed Addition of TMSCN to Acylphosphonates
Entry
Acylphosphonate 1a
Product 2
Yield (%)b
98
CN
O
Me3SiO
OEt
OEt
OEt
P
OEt
P
1
O
O
2a
1a
O
CN
Me3SiO
OEt
OEt
OEt
OEt
P
P
2
3
98
97
O
O
F
F
1b
2b
CN
O
Me3SiO
OEt
OEt
OEt
OEt
P
P
O
O
MeO
MeO
2c
1c
O
CN
Me3SiO
OEt
OEt
OEt
OEt
P
P
4
5
6
97
98
97
O
O
Cl
Cl
1d
2d
O
O
CN
Me3SiO
OEt
OEt
OEt
OEt
P
P
O
O
1e
1f
2e
CN
Me3SiO
OEt
OEt
OEt
OEt
P
P
O
O
2f
O
CN
Me3SiO
OEt
OEt
OEt
OEt
7
8
94
87
P
P
O
O
1g
2g
CN
O
Me3SiO
OEt
OEt
OEt
OEt
P
P
O
O
2h
1h
O
NC
OSiMe3
OEt
OEt
OEt
P
P
9
93
83
OEt
N
N
CBz
2i
O
O
CBz
1i
Ph
Ph
O
O
O
O
P
OEt
OEt
OEt
OEt
P
N
N
10
NC OSiMe3
O
O
O
1j
2j
a All phosphonates except 1i,j are known.8,
b Isolated yields. Products of type 2a–j can be quantitatively hydrolyzed to the corresponding alcohols by acidic hydrolysis. For example,
compound 2a was readily hydrolyzed in a mixture of 1 N HCl and THF, in turn affording 7 (Scheme 5).
Synlett 2006, No. 19, 3329–3333 © Thieme Stuttgart · New York

3332
A. S. Demir et al.
LETTER
Diethyl Cyano(4-methoxyphenyl)(trimethylsilyloxy)meth-
ylphosphonate (2c)
(279.35): C, 43.00; H, 7.94; N, 5.01. Found: C, 42.83; H, 7.77; N,
4.78.
Yield 360 mg (97%); colorless liquid. 1H NMR (CDCl3): d = 0.18
(9 H, s), 1.21 (3 H, t, J = 7.1 Hz), 1.37 (3 H, t, J = 7.1 Hz), 3.83 (3
H, s), 3.86–3.97 (1 H, m), 3.90–4.11 (1 H, m), 4.17–4.31 (2 H, m),
6.91 (2 H, m), 7.58 (2 H, m). 13C NMR (CDCl3–CCl4): d = 1.0, 16.2
(d, J = 5.2 Hz), 16.4 (d, J = 5.6 Hz), 55.3, 64.7 (d, J = 7.4 Hz), 65.3
(d, J = 7.2 Hz), 72.7 (d, J = 179.0 Hz), 113.7 (d, J = 1.9 Hz), 118.0,
126.2 (d, J = 3.7 Hz), 128.3 (d, J = 4.7 Hz), 160.5 (d, J = 2.3 Hz).
31P NMR (CDCl3): d = 11.4. Anal. Calcd for C16H26NO5PSi
(371.44): C, 51.74; H, 7.06; N, 3.77. Found: C, 51.58; H, 7.23; N,
3.88.
(2S)-Diethyl {1-[(Benzyloxy)carbonyl]pyrrolidin-2-yl}(cy-
ano)(trimethylsilyloxy)methylphosphonate (2i)
Yield 407 mg (87%); yellow oil; [a]D25 –23 (c 1.8, CHCl3). 1H NMR
(DMSO, 90 °C): d = 0.25 and 0.32 (9 H, s), 1.30–1.36 (6 H, m),
1.82–2.23 (4 H, m), 3.34–3.81 (2 H, m), 4.06–4.30 (4 H, m), 4.64–
4.67 (1 H, m), 5.05–5.25 (2 H, m), 7.31–7.41 (5 H, m). 13C NMR
(DMSO, 90 °C): d = .074, 0.89, 1.71, 15.6 (d, J = 6.2 Hz), 15.9 (d,
J = 4.8 Hz), 22.9, 26.3, 26.9, 47.0, 47.6, 59.5, 64.4 (d, J = 6.9 Hz),
64.7 (d, J = 8.1 Hz), 66.6, 72.0, 73.7, 117.5, 127.7, 127.8, 128.3,
136.7, 155.2, 155.5, 169.8. 31P NMR (DMSO): d = 11.8, 12.1, 13.0.
HRMS: m/z calcd for C21H33N2O6PSi: 468.1845. Found: 468.1843.
Diethyl Cyano(trimethylsilyloxy)(4-chlorophenyl)methylphos-
phonate (2d)
Yield 363 mg (97%); colorless liquid. 1H NMR (CDCl3): d = 0.20
(9 H, s), 1.04 (3 H, t, J = 7.0 Hz), 1.17 (3 H, t, J = 7.0 Hz), 3.73–
3.81 (1 H, m), 3.86–3.95 (1 H, m), 3.97–4.09 (2 H, m), 7.19 (2 H, d,
J = 8.5 Hz), 7.39 (2 H, dd, J = 8.6, 2.0 Hz). 13C NMR (CDCl3–
CCl4): d = 0.8, 15.2, (d, J = 5.7 Hz), 15.4, (d, J = 5.8 Hz), 63.6 (d,
J = 6.9 Hz), 64.2 (d, J = 7.6 Hz), 70.4. 116.1, 127.1, 127.2, 127.2,
127.5, 132.2, 134.6. 31P NMR (CDCl3): d = 10.9. Anal. Calcd for
C15H23ClNO4PSi (375.86): C, 47.93; H, 6.17; N, 3.73. Found: C,
47.71; H, 6.33; N, 3.51.
(2S)-Diethyl 1-Cyano-1-trimethylsilyloxy-2-(1,3-dioxoisoindo-
lin-2-yl)-3-phenylphosphonate (2j)
Yield 426 mg (73%); yellow liquid; [a]D25 –136 (c 0.7, CHCl3). 1H
NMR (CDCl3): d = 0.16 (9 H, s), 1.37–1.46 (6 H, m), 3.48 (1 H, dd,
J = 14.1, 3.4 Hz), 3.99 (1 H, t, J = 13.0 Hz), 4.31–4.43 (4 H, m),
5.07–5.18 (1 H, m), 7.16–7.22 (5 H, m), 7.65–7.86 (4 H, m). 13C
NMR (CDCl3–CCl4): d = 1.0, 14.1, 16.3, 16.4 (d, J = 5.5 Hz), 22.7,
29.7, 31.9, 32.9, 56.4 (d, J = 3.1 Hz), 65.4 (d, J = 7.7 Hz), 65.6 (d,
J = 7.1 Hz), 70.4, 72.1, 116.1, 123.2, 123.3, 126.7, 128.5, 128.7,
131.0, 131.9, 133.9, 134.2, 136.8, 167.6, 168.0. 31P NMR (CDCl3):
d = 11.9, 12.4. HRMS: m/z calcd for C25H31N2O6PSi: 514.1689.
Found: 514.1687.
Diethyl Cyano(trimethylsilyloxy)(2-methylphenyl)methylphos-
phonate (2e)
Yield 348 mg (98%); colorless liquid. 1H NMR (CDCl3): d = 0.26
(9 H, s), 1.26 (3 H, t, J = 6.2 Hz), 1.37 (3 H, t, J = 7.0 Hz), 2.69 (3
H, s), 3.91–4.30 (4 H, m), 7.19–7.29 (3 H, m), 7.26 (1 H, d J = 7.0
Hz). 13C NMR (CDCl3–CCl4): d = 0.863, 15.3 (d, J = 5.7 Hz), 15.5
(d, J = 5.0 Hz), 20.6, 63.5 (d, J = 7.2 Hz), 64.0 (d, J = 7.3 Hz), 72.0
General Procedure for the Hydrolysis of 2
Compound 2a (1 mmol) was dissolved in THF (1 mL), and 1 N HCl
(1 mL) was slowly added into the mixture. After the completion of
the reaction (TLC, ca. 15 min), the reaction was diluted with Et2O
and washed with brine in order to quantitatively obtain a pure
hydrolysis product 7.
(d, J = 176 Hz), 116.8, 124.7, 127.0, 127.1, 128.1, 130.9, 131.9. 31
P
NMR (CDCl3): d = 11.9. Anal. Calcd for C16H26NO4PSi (355.44):
C, 54.07; H, 7.37; N, 3.94. Found: C, 53.81; H, 7.23; N, 3.68.
Diethyl Cyano(hydroxy)(phenyl)methylphosphonate (7)
1
Diethyl Cyano(cyclohexyl)methylphosphonate (2f)
White solid; mp 71–73 °C. H NMR (CDCl3): d = 1.16 (3 H, t,
Yield 336 mg (97%); colorless liquid. 1H NMR (CDCl3): d = 0.30
(9 H, s), 1.16–1.30 (5 H, m), 1.40 (6 H, m), 1.68–1.71 (1 H, m),
1.84–2.07 (5 H, m), 4.15–4.33 (4 H, m). 13C NMR (CDCl3): d = 1.3,
16.4 (2 C, m), 25.8, 26.0, 27.1 (d, J = 5.8 Hz), 27.7 (d, J = 3.7 Hz),
30.8, 45.2, 65.3 (d, J = 7.4 Hz), 64.5 (d, J = 7.5 Hz), 74.4 (d,
J = 175.5 Hz), 117.4 (d, J = 2.4 Hz). 31P NMR (CDCl3–CCl4): d =
14.0. Anal. Calcd for C15H30NO4PSi (347.46): C, 51.85; H, 8.70; N,
4.03. Found: C, 51.61; H, 8.62; N, 4.28.
J = 7.1 Hz), 1.28 (3 H, t, J = 7.1 Hz), 3.96–4.20 (4 H, m), 4.90 (1 H,
br s), 7.32–7.42 (3 H, m), 7.61–7.64 (2 H, m). 13C NMR (CDCl3):
d = 16.2 (2 C, m), 65.6 (d, J = 7.8 Hz), 65.8 (d, J = 7.3 Hz), 71.0 (d,
J = 166.6 Hz), 117.8 (d, J = 3.1 Hz), 126.3 (d, J = 3.8 Hz), 128.4 (d,
J = 2.3 Hz), 129.4 (d, J = 3.1 Hz), 133.6 (d, J = 4.7 Hz). 31P NMR
(CDCl3): d = 12.04. Anal. Calcd for C12H16NO4P (269.23): C,
53.53; H, 5.99; N, 5.20. Found: C, 53.34; H, 6.22; N, 4.88.
Acknowledgment
Diethyl 1-Cyano-1-trimethylsilyloxy-2-methylpropylphospho-
nate (2g)
The financial support from the Middle East Technical University,
Scientific and Technical Research Council of Turkey (TÜBITAK),
the Turkish Academy of Sciences (TÜBA), and the Turkish State
Planning Organization is gratefully acknowledged.
Yield 288 mg (94%); colorless oil. 1H NMR (CDCl3): d = 0.29 (9 H,
s), 1.10 (3 H, d, J = 6.7 Hz), 1.15 (3 H, d, J = 6.8 Hz), 1.35–1.41 (6
H, m), 2.14–2.29 (1 H, m), 4.19–4.30 (4 H, m). 13C NMR (CDCl3–
CCl4): d = 1.4, 16.5 (2 C, m), 17.6 (d, J = 6.3 Hz), 18.0 (d, J = 3.7
Hz), 36.1, 64.2 (d, J = 7.2 Hz), 64.3 (d, J = 7.5 Hz), 74.6 (d, J = 176
Hz), 117.0 (d, J = 3.4 Hz). 31P NMR (CDCl3): d = 14.20. Anal.
Calcd for C12H26NO4PSi (307.4): C, 46.89; H, 8.53; N, 4.56. Found:
C, 46.71; H, 8.37; N, 4.78.
References
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Diethyl Cyano(methyl)(trimethylsilyloxy)methylmhosphonate
(2h)
Yield 242 mg (87%); colorless liquid. 1H NMR (CDCl3): d = 0.27
(9 H, s), 1.31–1.40 (6 H, m), 1.74 (3 H, d, J = 14.9 Hz), 4.18–4.29
(4 H, m). 13C NMR (CDCl3–CCl4): d = 1.30, 16.4 (2 C, m), 24.8,
64.3 (d, J = 7.2 Hz), 64.8 (d, J = 7.3 Hz), 66.6 (d, J = 182.5 Hz),
118.4. 31P NMR (CDCl3): d = 13.71. Anal. Calcd for C10H22NO4PSi
Synlett 2006, No. 19, 3329–3333 © Thieme Stuttgart · New York

LETTER
Uncatalyzed Addition of TMSCN to Acylphosphonates
3333
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Synlett 2006, No. 19, 3329–3333 © Thieme Stuttgart · New York