A new efficient procedure for asymmetric synthesis of α-aminophosphonic acids via addition of lithiated bis(diethylamino)phosphine borane complex to enantiopure sulfinimines

Article abstract of DOI:10.1016/S0957-4166(02)00684-5

The addition of lithiated bis(diethylamino)phosphine borane complex to enantiopure p-toluenesulfinimines is highly diastereoselective, affording the corresponding addition products with high efficiency (yields from 72 to 100%). The addition products were readily converted into α-amino-α-arylmethylphosphonic acids with high enantiomeric purities (from 72 to >98%).

Full text of DOI:10.1016/S0957-4166(02)00684-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 2571–2576
A new efficient procedure for asymmetric synthesis of
a-aminophosphonic acids via addition of lithiated
bis(diethylamino)phosphine borane complex to enantiopure
sulfinimines
Marian Mikołajczyk,* Piotr Łyz; wa and Jo´zef Drabowicz
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Department of Heteroorganic Chemistry,
90-363 Ło´dz´, Sienkiewicza 112, Poland
Received 7 October 2002; accepted 22 October 2002
Abstract—The addition of lithiated bis(diethylamino)phosphine borane complex to enantiopure p-toluenesulfinimines is highly
diastereoselective, affording the corresponding addition products with high efficiency (yields from 72 to 100%). The addition
products were readily converted into a-amino-a-arylmethylphosphonic acids with high enantiomeric purities (from 72 to >98%).
© 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
aminophosphonic acids during the past decades.1,3 In
the course of our studies on the application of the
p-toluenesulfinyl group as a chiral auxiliary in asym-
metric synthesis,4–6 we have developed a novel method
for the asymmetric synthesis of a- and b-aminophos-
phonic acids based on the highly diastereoselective
addition of phosphite anions and a-phosphonate car-
banions, respectively, to enantiomeric sulfinimines 1.7,8
However, in contrast to the very efficient addition of
a-phosphonate carbanions to 1, the reaction of 1 with
dialkyl phosphite or diamido phosphite anions was
found to be reversible, most probably due to the com-
parable stability of both anions in Eq. (1), i.e. the
starting phosphite anion and the amide ion of the
adduct formed. The reversibility of the above reaction
exerted an influence on the formation of addition prod-
ucts, which were obtained in low yields in the reactions
with diamido phosphite anions.
Aminophosphonic and aminophosphinic acids are
phosphorus analogues of amino acids in which the
planar carboxylic group is replaced by a phosphonic
acid, P(O)(OH)2, or phosphinic acid, P(O)(OH)R, moi-
ety.1 Because of the tetrahedral configuration of phos-
phorus, these compounds serve as biologically stable
analogues of unstable tetrahedral carbon intermediates
formed in enzymatic processes and, therefore, act as
enzyme inhibitors. In addition to this biological func-
tion, aminophosphonic acids exhibit a wide spectrum of
biological activities. Thus, many natural and synthetic
aminophosphonic acids show antibacterial, anticancer
and antiviral properties as well as exhibiting pesticidal,
insecticidal and herbicidal activity.1,2 As such, some of
these compounds have found commercial applications
in agriculture and medicine.
As in the case of other classes of chiral bioactive
compounds, the biological activity of aminophosphonic
acids depends strongly on the absolute configuration of
the stereogenic carbon atom bearing the amino group.
As a consequence, considerable research has been
devoted to the asymmetric synthesis of a- and b-
(1)
It was reasonable to expect that the use of more
nucleophilic and less stabilized phosphorus anions
would suppress the reversibility of this reaction and
* Corresponding author. Tel.: (+48-42)6815832; fax: (+48-42)6847126;
e-mail: marmikol@bilbo.cbmm.lodz.pl
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00684-5

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M. Mikołajczyk et al. / Tetrahedron: Asymmetry 13 (2002) 2571–2576
shift it towards the addition products. With this in
mind, we turned our attention to the lithiated
diaminophosphine borane complex 2 introduced to
organic synthesis by Konchel9 as a useful nucleophilic
phosphorus reagent. It is easily prepared from bis(di-
ethylamino)chlorophosphine borane complex and
lithium naphthalenide (Eq. (2)) and its high nucleo-
philicity was demonstrated nicely by the cross-coupling
reaction with aryl iodides and bromides which occurs in
the absence of palladium or nickel catalyst.
(2)
products 4 were obtained in very high yields (70–100%)
as white solids (see Scheme 1 and Table 1).
To further characterize the adducts 4 obtained and to
determine the diastereoselectivity of the addition, the
1H and 31P NMR spectra of 4 were recorded. It turned
out, however, that they were not indicative of the
presence of the diastereomers of 4, which could be
Preliminary experiments revealed that the addition of 2
to sulfinimines 1 occurs efficiently, thus confirming that
our reasoning was correct. We wish to describe herein a
new procedure for the asymmetric synthesis of a-
aminophosphonic acids 3 involving a highly efficient
and stereoselective addition of the borane complex 2 to
enantiomeric S- and R-sulfinimines 1 as a key step.10
1
formed in the addition reaction. Thus, the H NMR
spectrum of each adduct showed only one singlet of the
p-toluenesulfinyl methyl protons at 2.20 ppm, and a
multiplet due to the CH and NH protons at 5.13–5.30
ppm. In the phosphorus spectra only one multiplet at
91.4–93.5 ppm characteristic of the P-BH3 group was
observed. Therefore, the isolated addition products
were converted into the corresponding free a-
aminophosphonic acids 3 by heating for 4 h in a
refluxing mixture of glacial acetic acid and hydrochloric
acid (36% aq). The yields, specific rotation values and
enantiomeric purities of 3 prepared in this way are
collected in Table 1. The latter values were determined
2. Results and discussion
To a freshly prepared solution of the borane complex 2
in THF S- and R-sulfinimines 1 (dissolved in THF)
were added at −78°C. The additions were completed at
−78°C and the reactions were allowed to warm to room
temperature. After standard work-up and purification
by flash chromatography, the corresponding addition
Scheme 1.

M. Mikołajczyk et al. / Tetrahedron: Asymmetry 13 (2002) 2571–2576
2573
Table 1. Asymmetric addition of the aminophosphine borane complex 2 to (S)- and (R)-sulfinimines 1 and hydrolysis of the
adducts 4 to a-aminophosphonic acids 3
Sulfinimine 1a
Adduct 4
a-Aminophosphonic acid (S)-(−)- and (R)-(+)-3
Yield (%)b
[h]D (c)c
Yield (%)b
[h]D (c)d
ee (%)
(S)-(+)-1a
(R)-(−)-1a
(S)-(−)-1b
(R)-(+)-1b
(S)-(+)-1c
(S)-(+)-1d
(S)-(−)-1e
4a
4a
4b
4b
4c
4d
4e
100
100
75
72
96
+78.4 (1.0)
−78.0 (1.1)
+96.4 (1.12)
−97.9 (1.05)
+83.2 (1.2)
+30.6 (1.41)
+25.2 (1.12)
3a
3a
3b
3b
3c
3d
3e
92
95
70
75
93
75
89
−19.3 (1.29)
+19.9 (1.16)
−4.2 (0.85)
+4.8 (0.35)
−19.3 (0.54)
−20.5 (0.81)
−17.1 (0.23)
98
\98
77
89
\98
\98
76
86
72
a Sulfinimines 1 were prepared from (S)-(−)- or (R)-(+)-menthyl p-toluenesulfinate according to the procedure described by Davis at al.11
b Isolated yields.
c Measured in CHCl3.
d Measured in 1N NaOH.
Scheme 2.
by the method elaborated by Glowacki et al.12 and
based on the 31P NMR nonequivalence of
diastereomeric salts of N-phthaloyl protected a-
aminophosphonic acids with optically active amines.
Hence, all the a-aminophosphonic acids 3 synthesized
in this work were converted into N-phthaloyl deriva-
tives 5 and then treated with equimolar amounts of
(−)-ephedrine (Ephe) to form the corresponding salts 6
(Scheme 2). As expected, the diastereomeric salts 6
showed the nonequivalent 31P NMR signals integration
of which allowed determination of the enantiomeric
purity of the acids 3. The 31P NMR chemical shifts of
the acids 5 and their salts 6 are collected in Table 2.
tions is analogous to that of the addition of lithium
diamido phosphites. However, it is opposite to that
observed with lithium dialkyl phosphites.8 In the latter
case the stereoselectivity of additions to (S)-(+)-1a was
rationalized by Davis14 by proposing a six-membered
chelating transition state model in which the lithium
cation is coordinated to the sulfinyl and phosphite
oxygens. A reversal of diastereoselectivity observed by
us in the addition of 2 may reasonably be explained in
terms of the transition state model shown below where
Table 2. 31P NMR chemical shifts and chemical shift dif-
ferences of N-phthaloyl-1-aminophosphonic acids 5 and
their salts 6 with (−)-ephedrine
It was gratifying to find that the a-aminophosphonic
acids (−)- and (+)-3a, (−)-3c and (−)-3d were obtained
as practically pure enantiomers (ee]98%). For the
acids (+)- and (−)-3b as well as for (−)-3e the ee values
were in the range between 76 and 89%.
Acid 3
Acid 5
Salt 6
l (ppm)a
l (ppm)b (Dl)
(−)-3a
(+)-3a
(−)-3b
(+)-3b
(−)-3c
(−)-3d
(−)-3e
15.9
12.43d, 10.65 (1.78)
10.71c
15.10
18.22
17.00
16.80
19.57
18.61
Taking into account the fact that the absolute configu-
ration of a-amino-a-phenylmethylphosphonic acid 3a is
known to be (S)-(−) and (R)-(+)13 as well as the fact
that the bonds around the stereogenic a-carbon atom
are not broken during the conversion of 4a“3a, it is
possible to assign SS,SC stereochemistry for the adduct
(+)-4a. The reasonable assumption is that the dextroro-
tatory adducts 4b–e formed by the addition of 2 to
(S)-1 all have the same stereochemistry. It is interesting
to point out that the steric course of the above addi-
11.33d, 11.81 (0.48)
11.98, 12.43d (0.45)
12.76c
12.09c
11.98d, 12.41 (0.43)
a In CDCl3 and DMSO-d6.
b In CDCl3.
c Only one signal visible.
d Chemical shift of major diastereomer is underlined.

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M. Mikołajczyk et al. / Tetrahedron: Asymmetry 13 (2002) 2571–2576
rated. The aqueous layer was extracted with ethyl ether
and combined organic layers were dried over MgSO4
and evaporated. Purification by flash chromatography
(silica gel, hexane:ether 2:1 or 1:1) afforded the desired
N - p - toluenesulfinyl - 1 - amino - 1 - arylmethylphosphine
borane complex 4.
4.2.1. Data for (+)-N-p-toluenesulfinyl-1-amino-1-
phenylmethyl-bis(diethylamino)phosphine borane, 4a
(prepared from (+)-1a). Obtained in ꢀ100% yield as a
white solid, mp=144–145°C, [h]D=+78.4 (c 1.0;
Scheme 3.
1
CHCl3). H NMR (CDCl3) l 0.612 (t, 6H, J=7.03 Hz),
steric hindrance factors determine the stereoselectivity.
It is not excluded that the lithium atom of 2 is coordi-
nated to the nitrogen lone pair, facilitating the delivery
of the phosphorus atom to the prochiral trigonal car-
bon center from the less hindered face occupied by the
lone pair of electrons on sulfur (Scheme 3).
1.19 (t, 6H, J=7.02 Hz), 2.20 (s, 3H), 2.67–2.83 (m,
4H), 2.86–3.45 (m, 4H), 5.13–5.30 (m, 2H), 6.85–7.30
(m, 9H). 31P NMR (CDCl3) l 92.32–93.59. HRMS
calcd for C22H37BN3OPS (M+H), 434.2570; found
434.2565.
4.2.2. Data for (−)-N-p-toluenesulfinyl-1-amino-1-
phenylmethyl-bis(diethylamino)phosphine borane, 4a
(prepared from (−)-1a). Obtained in ca. 100% yield as a
white solid, mp=145–146°C, [h]D=−78.0 (c 1.1;
3. Conclusion
We have described herein a new procedure for the
asymmetric synthesis of a-aminophosphonic acids
which involves a very effective and highly diastereo-
selective addition of the lithiated bis(diethyl-
amino)phosphine borane complex to enantiopure p-
toluenesulfinimines derived from aldehydes. The stereo-
selectivity of the addition was explained by proposing
a transition state model where steric approach control
is a decisive factor.
1
CHCl3). H NMR (CDCl3) l 0.611 (t, 6H, J=7.05 Hz),
1.19 (t, 6H, J=7.02 Hz), 2.20 (s, 3H), 2.67–2.80 (m,
4H), 2.86–3.44 (m, 4H), 5.13–5.30 (m, 2H), 6.85–7.20
(m, 9H). 31P NMR (CDCl3) l 92.13–93.37. HRMS
calcd for C22H37BN3OPS (M+H), 434.2570; found
434.2562.
4.2.3. Data for (+)-N-p-toluenesulfinyl-1-amino-1-(2-
thienyl)methyl-bis(diethylamino)phosphine borane, 4b
(prepared from (−)-1b). Obtained in 75% yield as a
white solid, mp=126–127°C, [h]D=+96.4 (c 1.12;
1
4. Experimental
4.1. General
CHCl3). H NMR (CDCl3) l 0.73 (t, 6H, J=7.04 Hz),
1.18 (t, 6H, J=7.05 Hz), 2.29 (s, 3H), 2.83–3.11 (m,
4H), 3.11–3.42 (m, 4H), 5.22 (t, 1H, J=6.22 Hz), 5.51
(dd, 1H, J=5.86 Hz, J=10.64 Hz), 6.58–6.69 (m, 2H),
6.69–7.00 (m, 1H), 7.01–7.41 (like AB system, 4H,
J=7.99 Hz). 31P NMR (CDCl3) l 91.37–92.61. Anal.
calcd for C19H35BN3OPS2: C, 54.66; H, 8.03; N, 9.56;
P, 7.05; S, 14.59; found: C, 54.59; H, 8.20; N, 9.32; P,
7.33; S, 14.49%.
NMR spectra were recorded on a Bruker 1C 200
spectrometer at 200 MHz. All optical rotation measure-
ments were carried out on a Perkin–Elmer MC 241
photopolarimeter at room temperature. Reactions were
monitored by TLC chromatography (Merck Kieselgel
60254). Column chromatography was conducted on
Merck silica gel (70–230 mesh).
4.2.4. Data for (−)-N-p-toluenesulfinyl-1-amino-1-(2-
thienyl)methyl-bis(diethylamino)phosphine borane, 4b
(prepared from (+)-1b). Obtained in 72% yield as a
white solid, mp=132–133°C, [h]D=−97.9 (c 1.05;
4.2. General procedure for the addition of lithiated bis-
(diethylamino)phosphine borane complex 2 to sulfin-
imines 1
1
CHCl3). H NMR (CDCl3) l 0.727 (t, 6H, J=7.06 Hz),
1.18 (t, 6H, J=7.06 Hz), 2.29 (s, 3H), 2.61–3.03 (m,
4H), 3.03–3.46 (m, 4H), 5.23 (t, 1H, J=6.30 Hz), 5.51
(dd, 1H, J=5.9 Hz, J=10.62 Hz), 6.54–6.76 (m, 2H),
6.98–7.00 (m, 1H), 7.01–7.40 (AB, 4H, J=7.9 Hz). 31P
Bis(diethylamino)chlorophosphine (0.01 mol) in THF
(10 mL) was cooled to 0°C and borane·methyl sulfide
complex (5 mL) was added. The reaction mixture was
stirred for 4 h at rt and the solvent was removed under
vacuum affording the pure bis(diethylamino)-
chlorophosphine borane complex. This complex was
dissolved in THF (10 mL) and added slowly to lithium
naphthalenide prepared from naphthalene (0.02 mol)
and cut lithium wire (0.02 mol) at −78°C. After stirring
for 30 min, the solution of appropriate sulfinimine 1a–e
(0.023 mol) in THF (5 mL) was dropped. The reaction
mixture was stirred for 4 h at −78°C. Then, after
warming to room temperature and quenching with a
water solution of NH4Cl, the organic layer was sepa-
NMR (CDCl3)
l
91.38–92.59. Anal. calcd for
C19H35BN3OPS2: C, 54.66; H, 8.03; N, 9.56; P, 730; S,
14.95; found: C, 54.63; H, 8.19; N, 9.49, P, 7.30; S,
14.42%.
4.2.5. Data for (+)-N-p-toluenesulfinyl-1-amino-1-(p-
fluorophenyl)methyl-bis(diethylamino)phosphine borane,
4c (prepared from (+)-1c). Obtained in 96% yield as a
white solid, mp=138–139°C, [h]D=+83.2 (c 1.12;
1
CHCl3). H NMR (CDCl3) l 0.65 (t, 6H, J=7.04 Hz),
1.19 (t, 6H, J=7.04 Hz), 2.23 (s, 3H), 2.64–2.88 (m,

M. Mikołajczyk et al. / Tetrahedron: Asymmetry 13 (2002) 2571–2576
2575
4H), 2.88–3.44 (m, 4H), 5.16–5.27 (m, 2H), 7.01–7.28
(m, 8H). 31P NMR (CDCl3) l 91.90–93.05. HRMS
calcd for C22H36BFN3OPS (M+H), 452.2471; found
452.2463.
4.3.5. Data for (−)-1-amino-1-(p-fluorophenyl)methyl-
phosphonic acid, 3c (prepared from (+)-4c). Obtained in
93% yield, mp=284–287°C, [h]D=−19.3 (c 0.54; 1N
NaOH). 31P NMR (D2O) l 10.72. HRMS calcd for
C7H9NO3FP (M+H) 206.0382; found 206.0388.
4.2.6. Data for (+)-N-p-toluenesulfinyl-1-amino-1-(p-bro-
mophenyl)methyl-bis(diethylamino)phosphine borane, 4d
(prepared from (+)-1d). Obtained in 86% yield as white
4.3.6. (−)-1-Amino-1-(p-bromophenyl)methylphosphonic
acid, 3d (prepared from (+)-4d). Obtained in 75% yield,
mp=281–283°C, [h]D=−20.5 (c 0.81; 1N NaOH). 31P
NMR (D2O) l 10.7. HRMS calcd for C7H9NO3BrP
(M+H) 265.9581; found 265.9586.
1
solid, mp=154–155°C, [h]D=+30.6 (c 1.41; CHCl3). H
NMR (CDCl3) l 0.67 (t, 6H, J=6.93 Hz), 1.18 (t, 6H,
J=6.93 Hz), 2.27 (s, 3H), 2.72–2.81 (m, 2H), 2.88–2.97
(m, 2H), 3.15–3.24 (m, 2H), 3.31–3.40 (m, 2H), 5.18–
5.21 (m, 2H), 6.89–6.91 (m, 4H), 7.04–7.06 (m, 2H),
7.22–7.25 (m, 2H). 31P NMR (CDCl3) l 92.81–93.33.
HRMS calcd for C22H36BBrN3OPS (M+H), 512.1676;
found 512.1640.
4.3.7. (−)-1-Amino-1-(p-N,N-dimethylphenyl)methylphos-
phonic acid, 3e (prepared from (+)-4e). Obtained in 89%
yield, mp=219–222°C, [h]D=−17.1 (c 0.23; 1N
NaOH). 31P NMR (D2O) l 10.91. HRMS calcd for
C9H15N2O3P (M+H) 231.0898; found 321.0901.
4.2.7. Data for (+)-N-p-toluenesulfinyl-1-amino-1-(p-
N,N - dimethylaminophenyl)methyl - bis(diethylamino)-
phosphine borane, 4e (prepared from (−)-1e). Obtained in
72% yield as a white solid, mp=129–130°C, [h]D=
Acknowledgements
1
+25.2 (c 1.12; CHCl3). H NMR (CDCl3) l 0.66 (t, 6H,
J=7.05 Hz), 1.18 (t, 6H, J=7.04 Hz), 2.24 (s, 3H), 2.83
(s, 6H), 2.71–2.99 (m, 4H), 2.99–3.42 (m, 4H), 5.03–5.15
(m, 2H), 6.34–7.34 (AB, 4H, J=8.2 Hz), 6.92–6.98 (m,
4H). 31P NMR (CDCl3) l 91.44–92.77. HRMS calcd
for C24H42BHN4OPS (M+H), 477.2992; found
477.3000.
Financial support by the State Committee for Scientific
Research (Grant No Z-005/T09/99) is gratefully
acknowledged. We thank also Dr. M. Sochacki for his
assistance in accurate mass determinations.
4.3. General procedure for hydrolysis of N-p-toluene-
sulfinyl-1-amino-1-arylmethyl-bis(diethylamino)phosphine
borane 4
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