Diastereoselective addition of Di-(trimethylsilyl)phosphite to chiral N-(R)-α-methylbenzyl and N-(1-methoxycarbonyl-iso-pentyl) Schiff bases of various aldehydes

Article abstract of DOI:10.1002/hc.20624

The addition of bis-(trimethylsilyl) phosphite to chiral imines of several aldehydes was diastereoselective. The separation of predominant diastereoisomers of a majority of formed aminophosphonic acids has been observed. Moreover, the ferrocene-derived ac

Full text of DOI:10.1002/hc.20624

Heteroatom Chemistry
Volume 21, Number 5, 2010
Diastereoselective Addition of
Di-(Trimethylsilyl)Phosphite to Chiral
N-(R)-α-Methylbenzyl and
N-(1-methoxycarbonyl-iso-pentyl) Schiff
Bases of Various Aldehydes
Jarosław Lewkowski and Rafał Karpowicz
Department of Organic Chemistry, Faculty of Chemistry, University of Ło´dz´, Tamka 12,
91-403 Ło´dz´, Poland
Received 13 April 2010; revised 10 June 2010
conditions are suitable neither for a furan ring
ABSTRACT: The addition of bis-(trimethylsilyl)
[6–11] nor for a ferrocene moiety [12,13]. To avoid
phosphite to chiral imines of several aldehydes was
this problem, 100% diastereoselective addition of hy-
diastereoselective. The separation of predominant
pophosphorous acid to chiral Schiff bases has been
diastereoisomers of a majority of formed aminophos-
applied [14–18] and formed aminophosphonous
phonic acids has been observed. Moreover, the
acids were subsequently oxidized to give aminophos-
ferrocene-derived acid 5d occurred in diastereose-
phonic systems. Unfortunately, known oxidation
ꢀ
C
lectivity up to 100%.
2010 Wiley Periodicals, Inc.
methods turned out to be too rigid for both fu-
ranic and ferrocene systems. Thus, Zon´ [19] and
Boduszek [20] presented the method for synthe-
sis of aminophosphonic acids through addition of
bis-(trimethylsilyl) phosphite to azomethine bond of
Schiff bases, where silyl esters formed decomposed
to free acids in mild methanolysis conditions. They
did not study the stereochemistry of addition of silyl
phosphite to chiral imines.
Mikołajczyk and co-workers invented an inter-
esting method for highly stereoselective synthesis of
aminophosphonic acids using the addition of dialkyl
phosphite anions to chiral sulfimines [21], which
achieved promising results.
Heteroatom Chem 21:326–331, 2010; Published online in
Wiley InterScience (www.interscience.wiley.com). DOI
10.1002/hc.20624
INTRODUCTION
Addition of dialkyl and diaryl phosphites to azome-
thine bond of chiral Schiff bases, such as N-α-
methylbenzylimines, has been described in sev-
eral papers [1–5]. Diastereoselectivity of reported
reactions varied from 2:1 to 9:1 of diastereoiso-
meric ratio. Obtained diastereoisomeric aminophos-
phonates should be separated by chromatographic
methods and then undergo hydrolysis to obtain free
acids, which influenced badly substituents—such
RESULTS AND DISCUSSION
Considering the above, we were prompted to study
this problem, especially because preliminary re-
sults demonstrated interesting phenomena accom-
panying this process [22]. That is why we per-
formed the addition of bis-(trimethylsilyl) phosphite
Correspondence to: Jarosław Lewkowski; e-mail: jlewkow@
uni.lodz.pl.
Contract grant sponsor: University of Ło´dz´.
Contract grant number: 505/0683.
2010 Wiley Periodicals, Inc.
c
ꢀ
326

Addition of Di-(Trimethylsilyl)Phosphite to Chiral N-(R)-α-Methylbenzyl and N-(1-methoxycarbonyl-iso-pentyl) 327
SCHEME 1
to two groups of chiral imines of various aldehydes,
i.e., N-(R)-α-methylbenzylimines 2a-f and N-(1-
methoxycarbonyl-3-methylbutylimines) 3b-e to in-
vestigate the stereochemistry of these reactions.
Chiral Schiff bases 2a-f were synthesized fol-
lowing the classical methodology [22] by mixing
(R)-α-methylbenzylamine with an appropriate alde-
hyde 1a-f in methanol and were obtained in 95–99%
yields.
Obtained imines have undergone the addition
of bis-(trimethylsilyl) phosphite to their azomethine
bond. Bis-(trimethylsilyl) phosphite has been pre-
pared in situ by the reaction of diethyl phosphite
with bromotrimethylsilane in dichloromethane [20].
In situ generated bis-(trimethylsilyl) phosphite un-
derwent the reaction with chiral Schiff bases 2a-f in
dichloromethane. The conversion was monitored by
means of 31P NMR, and when it was complete the
reaction was quenched by methanolysis. After ad-
dition of propylene oxide, resulting acids 4a-f were
obtained in 35–70% yields (Scheme 1).
Imines 3b-e have been synthesized by the action
of triethylamine on leucine methyl ester hydrochlo-
ride and subsequent reaction of newly formed
leucine methyl ester with an appropriate aldehyde
1a-f in boiling dichloromethane for 3 days. The usual
workup gave desired imines 3a-f in 96–98% yields.
Obtained imines 3b-e have undergone the addi-
tion of bis-(trimethylsilyl) phosphite to their azome-
thine bond as it took place in the case of imines 2a-f.
Bis-(trimethylsilyl) phosphite [20] generated in the
same way underwent the reaction with chiral Schiff
bases 3a-f in dichloromethane. The conversion was
monitored by means of 31P NMR, and when it was
complete the reaction was quenched by methanoly-
sis. After addition of propylene oxide, resulting acids
5b-e were obtained in 20–60% yields (Scheme 1).
The diastereoselectivity of the formation
of N-((R)-α-methylbenzyl)aminomethyl-phosphonic
acids 4a-f varied from de = 9% in the case of 4-
pyridyl derivative 4e up to de = 82% in the case
of N-((R)-α-methylbenzyl)-amino(ferrocenyl)methyl
phosphonic acid 4d (Table 1). What is impor-
tant is that the predominant diastereoisomers
of aminophosphonic acids 4a, 4c, 4d, and 4f
crystallized from the postreaction mixture alone,
not contaminated by the second isomer; thus
some kind of self-separation of diastereoisomers
took place. In the case of 4-pyridyl derivative
4e such a self-separation did not occur and
the product was obtained as the mixture of
diastereoisomers.
Compounds 4a and 4c crystallized as monohy-
drates, which was demonstrated by the elemental
analysis. It was probably due to the workup; as in
the case of 4c, propylene oxide was not added to the
postreaction mixture, so removal of water was not
executed. As for the isolation of 4a, water was added
instead of methanol and therefore, the product could
crystallize as a monohydrate.
Heteroatom Chemistry DOI 10.1002/hc

328 Lewkowski and Karpowicz
TABLE 1 Results of Addition of Bis-(trimethylsilyl) Phosphite to N-(R)-α-Methylbenzylimines) 2a-f
A Single Isomera
No.
R
Y [%]
mp [◦C]
31P NMR
α2D0
31P NMR
dr (de[%])
4a
4b
4c
4d
4e
4f
Ph
2-furyl
2-thienyl
ferrocenyl
4-pirydyl
cyclohexyl
59
66
51
32
58
68
174–177
–
156–158
200–201
215–217
212–214
15.02
–
15.60
15.64
–
+ 63.67 (c = 2.96 in 1M NaOH)
15.84 and 15.02
13.69 and 12.99
15.60 and 14.48
15.64 and 15.25
13.74 and 13.00
19.77 and 19.01
1:5 (67)
5:4 (11)
5:1 (67)
9:1 (80)
5:6 (9)
–
+ 16.93 (c = 1.75 in 1M NaOH)
+ 7.38 (c = 2.84 in 1M NaOH)
–
+ 23.02 (c = 1.04 in 1M NaOH)
19.77
4:1 (60)
aSpectral data of a self-separated diastereoisomer.
Unfortunately, the reaction with N-(2-
furfurylidene)-(R)-α-methylbenzylamine 2b was
not successful, as the corresponding acid 4b was
not isolated and only the NMR investigation of the
postreaction mixture demonstrated the occurrence
of the reaction with a 5:4 dr.
Summarizing, we have observed the separation
of predominant diastereoisomers of aminophospho-
nic acids 4a, 4c, 4d, and 4f, as well as 5b, which
easily crystallized from the postreaction mixture.
Moreover, the ferrocene-derived acid 5d occurred
in diastereoselectivity up to 100%.
As for the formation of N-(1-methoxycarbonyl-
3-methylbutyl)aminomethylphosphonic acids 5b-e,
the diastereoselectivity varied from de = 14% in
the case of N-(1-methoxycarbonyl-3-methylbutyl)
amino(thienyl)-methylphosphonic acid 5c, up to
de = 100% in the case of N-(1-methoxycarbonyl-3-
methylbutyl)amino(ferrocenyl) - methylphosphonic
acid 5d (Table 2). As in the previous case,
the self-separation of predominant diastereoiso-
mers of aminophosphonic acid 5b and 5c
was observed. Moreover, N-(1-methoxycarbonyl-3-
methylbutyl) amino(ferrocenyl)-methylphosphonic
acid 5d occurred as the one exclusive diastereoiso-
mer. Although its proper purity was confirmed by the
NMR spectroscopy, elemental analysis of the acid 5d
showed some discrepancies in C%. This is probably
due to masking carbon by iron in burning analysis.
Unfortunately, the X-ray measurements could
not be performed as the product 5d crystallized as
powder and our attempts to grow the proper crystal
failed.
It is very difficult to state the reason why the
addition of bis-(trimethylsilyl) phosphite to both
ferrocene-containing imines 2d and 3d turned out
to be much more diastereoselective than other cases.
This may be explained by the Houk model, which ac-
cording to Houk is “the same as the Felkin–Anh ver-
sion of Cram’s rule for nucleophilic additions” [23].
According to this model, the hydrogen bond forms
between the hydrogen of bis-(trimethylsilyl) phos-
phite and the nitrogen of the azomethine bond from
the side opposite to the largest substituent. And in
the case of ferrocenyl derivatives 2d and 3d, di-
astereoselection is reinforced due to the stabilizing
interaction between the phosphorus atom orbital
and the ferrocenyl substituent as it is presented in
Scheme 2.
EXPERIMENTAL
Aminophosphonic Acids 4a, 4c–f, and 5b–e;
General Procedure
Also unfortunately, the reaction with N-(2-ben-
zylidene)-(1-methoxycarbonyl-3-methylbutylamine)
3a and 3f did not occur; that is why these cases are
not mentioned in Table 2.
Diethyl phosphite (2.5 mmol) was dissolved in
dry dichloromethane and to this solution bro-
motrimethylsilane (6.8 mmol, 0.9 mL) was added
TABLE 2 Results of Addition of Bis-(trimethylsilyl) Phosphite to N-((S)-1-Methoxycarbonyl-3-Methylbutylimines) 3b-e
A Single Isomera
No.
R
Y [%]
mp [◦C]
31P NMR
α2D0
31P NMR
dr (de[%])
5b
5c
5d
5e
2-furyl
2-thienyl
ferrocenyl
4-pirydyl
28
60
17
38
185–186
142–145
205–206
187–189
13.14
–
16.27
–
+ 33.48 (c = 1.93 in 1M NaOH)
13.14 and 12.74
14.15 and 13.32
16.27b
5:1 (67)
4:3 (14)
100
–
−19.89(c = 1.41 in 1M NaOH)
–
13.41 and 12.64
3:8 (46)
aSpectral data of a self-separated diastereoisomer.
bThe exclusive formation of the only one diastereoisomer was observed.
Heteroatom Chemistry DOI 10.1002/hc

Addition of Di-(Trimethylsilyl)Phosphite to Chiral N-(R)-α-Methylbenzyl and N-(1-methoxycarbonyl-iso-pentyl) 329
SCHEME 2
(d, JPH = 18.6 Hz, CHP, 1H); 6.72 (m, CH2tioph, 1H);
6.78 (dd, J = 3.5 and 5.0 Hz, CH3tioph, 1H); 7.10 (d,
J = 5.0 Hz, CH5tioph, 1H); 7.16 (m, PhH, 5H).
2
dropwise for 10–15 min. The mixture was stirred for
1 h at room temperature. Then, a solution of an ap-
propiate imine (2.5 mmol) in dry dichloromethane
was added and the mixture was refluxed for 4 h.
Then, the solution was evaporated in vacuo and the
residue was dissolved in dry methanol and stirred for
30–45 min until precipitation of a solid, which was
filtered off and collected. For acids 4a and 4f, wa-
ter was added instead of methanol and the mixture
was refrigerated for 7 days until the product precip-
itated. In the case of 4d, 5b, and 5e the solid did not
precipitate after methanolysis; therefore 10–20 mL
of propylene oxide was added and the mixture was
refrigerated for 3–7 days. Then the solid was filtered
off and collected.
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 1.16 (d, J = 6.8 Hz, CHCH3, 3H); 3.51 (q, J = 6.8 Hz,
CHCH3, 1H); 3.60 (d, 2 JPH = 18.0 Hz, CHP, 1H); 6.68
(m, CH2tioph, 1H); 6.86 (dd, J = 3.5 and 5.0 Hz, CH3tioph
,
1H); 7.11 (d, J = 5.0 Hz, CH5tioph, 1H); 7.17 (m, PhH,
5H).
N-((R)-α-methylbenzyl)-amino(ferrocenyl)methyl
Phosphonic Acid 4d
Predominant diastereoisomer: Calcd for C19H22
FeNO3P: C, 57.16; H, 5.55; N, 3.51. Found: C, 57.08;
H, 5.50; N, 3.50.
1H NMR (200 MHz, D2O): δ 1.35 (d, J = 5,9 Hz,
CH3); 3.45 (d, 2 JPH = 15.0 Hz, 1H, CHP); 3.89 (s, 5H,
C5Hf5er); 3.96 (m, 3H, CHPh, (CH)f2er); 4.15 (m, 2H,
(CH)f2er); 7.17 (d, J = 6.8 Hz, 1H, PhH); 7.31 (t, J =
6.8 Hz, 2H, PhH); 7.43 (d, J = 6.8 Hz, 2H, PhH).
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
N - ((R) - α - methylbenzyl) - amino(phenyl)methyl
Phosphonic Acid 4a.
Predominant diastereoisomer: Calcd for C15H18NO3
P.H2O: C, 58.25; H, 6.52; N, 4.53. Found: C, 58.66; H,
6.52; N, 4.47.
1H NMR (200 MHz, D2O): δ 0.94 (d, J = 6.4 Hz,
CHCH3, 3H); 3.15 (q, J = 6.4 Hz, CHCH3, 1H); 3.12
(d, 2 JPH = 17.0 Hz, CHP, 1H); 6.81 (m, PhH, 4H); 6.99
(m, PhH, 6H).
2
δ 1.15 (d, J = 5,9 Hz, CH3); 3.14 (d, JPH = 14.9 Hz,
1H, CHP); 3.89 (s, 5H, C5Hf5er); 3.86 (m, 3H, CHPh,
(CH)2fer); 4.15 (m, 2H, (CH)f2er); 7.17 (d, J = 6.8 Hz,
1H, PhH); 7.31 (t, J = 6.8 Hz, 2H, PhH); 7.43 (d, J =
6.8 Hz, 2H, PhH).
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 0.98 (d, J = 6.4 Hz, CHCH3, 3H); 3.45 (q, J = 6.4 Hz,
CHCH3, 1H); 3.51 (d, 2 JPH = 20.0 Hz, CHP, 1H); 6.81
– 6.99 (m, PhH, 10H).
N-((R)-α-methylbenzyl)-amino(4-pyridyl)methyl
Phosphonic Acid 4e.
Predominant diastereoisomer: Calcd for C14H17
N2O3P: C, 57.53; H, 5.86; N, 9.58. Found: C, 57.32;
H, 6.05; N, 9.46.
N - ((R) - α - methylbenzyl) - amino(thienyl)methyl
Phosphonic Acid 4c.
Predominant diastereoisomer: Calcd for C13H16NO3
1H NMR (200 MHz, D2O): δ 1.20 (d, J = 6.3 Hz,
CHCH3, 3H); 3.74 (q, J = 6.3 Hz, CHCH3, 1H); 3.56
(d, 2 JPH = 19.5 Hz, CHP, 1H); 7.01 (m, PhH, 3H); 7.18
(m, PhH, 2H); 8.00 and 8.22 (2d as the A2B2 system,
J = 5.5 Hz, Hα and Hβ, 2 × 2H).
.
PS H2O: C, 49.52; H, 5.75; N, 4.44. Found: C, 49.74;
H, 5.79; N, 4.32.
1H NMR (200 MHz, D2O): δ 1.21 (d, J = 6.6 Hz,
CHCH3, 3H); 3.75 (q, J = 6.6 Hz, CHCH3, 1H); 3.95
Heteroatom Chemistry DOI 10.1002/hc

330 Lewkowski and Karpowicz
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 1.15 (d, J = 6.9 Hz, CHCH3, 3H); 3.38 (q, J = 6.9 Hz,
CHCH3, 1H); 3.32 (d, 2 JPH = 19.4 Hz, CHP, 1H); 7.01
(m, PhH, 3H); 7.18 (m, PhH, 2H); 8.00 and 8.22 (2d
as the A2B2 system, J = 5.5 Hz, Hα and Hβ, 2 × 2H).
CHCH2, 1H); 2.94 (t, J = 6.6 Hz, CHCOOMe, 1H);
2
3.25 (s, OCH3, 3H); 3.89 (d, JPH = 17.3 Hz, CHP,
1H); 6.89 (m, CH3thioph, CH4thioph, 2H); 7.20 (m, CHt5hioph
1H).
,
N - (1 - methoxycarbonyl - 3 - methylbutyl)amino
(ferrocenyl)-methylphosphonic Acid 5d.
N-((R)-α-methylbenzyl)-amino(cyclohexyl)methyl
Phosphonic Acid 4f.
Exclusive diastereoisomer: Calcd for C18H26FeNO5
P: C, 51.08; H, 6.19; N, 3.31. Found: C, 51.43; H, 6.45;
N, 3.89.
Predominant diastereoisomer: Calcd for C15H24
NO3P: C, 60.59; H, 8.14; N, 4.71. Found: C, 60.28; H,
8.01; N, 4.60.
1H NMR (200 MHz, D2O): δ 0.83 and 0.85 (2d,
J = 5.0 Hz, CH(CH3)2, 6H); 1.45 (m, CHCH2, CHCH2,
1H NMR (200 MHz, D2O): δ 0.91 (m, CHc2hex
,
2
3H); 3.21 (s, OCH3, 3H); 3.26 (d, JPH = 12.5 Hz,
6H); 1.19 (d, J = 6.6 Hz, CHCH3, 3H); 1.38 (m,
CHc2hex and CHchex, 5H); 2.06 (dd, 2 JPH = 15.9 Hz and
3 JHH = 2.7 Hz, CHP, 1H); 4.10 (q, J = 6.3 Hz, CHCH3,
1H); 7.11–7.32 (m, PhH, 5H).
1H, CHP); 3.93 (m, CHCOOMe, 1H); 4.05 (m, 3H,
(CH)f2er); 4.18 (m, 2H, (CH)2fer); 4.21 (s, 5H, C5Hf5er).
N-(1-methoxycarbonyl-3-methylbutyl)amino(4-
pyridyl)-methylphosphonic Acid 5e.
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 0.91 (m, CHc2hex, 6H); 1.19 (d, J = 6.6 Hz, CHCH3,
Predominant diastereoisomer: Calcd for C13H21
N2O5P: C, 49.37; H, 6.69; N, 8.86. Found: C, 48.89;
H, 6.91; N, 8.63.
2
3H); 1.38 (m, CHc2hex and CHchex, 5H); 2.15 (dd, JPH
= 11.5 Hz, and 3 JHH = 3.8 Hz, CHP, 1H); 3.95 (q, J =
6.6 Hz, CHCH3, 1H); 7.11–7.32 (m, PhH, 5H).
1H NMR (200 MHz, D2O): δ 0.67 and 0.69 (2d,
J = 4.2 Hz, CH(CH3)2, 6H); 1.28 (dd, J = 7.3 and
6.9 Hz, CHCH2, 2H); 1.36 (m, CHCH2, 1H); 2.93 (t,
J = 7.3 Hz, CHCOOMe, 1H); 3.23 (s, OCH3, 3H); 3.61
N-(1-methoxycarbonyl-3-methylbutyl)amino(2-
furyl)-methylphosphonic Acid 5b.
Predominant diastereoisomer: Calcd for C12H20NO6
P.H2O: C, 44.58; H, 6.86; N, 4.33. Found: C, 44.51; H,
7.11; N, 4.12.
2
(d, JPH = 18.7 Hz, CHP, 1H); 7.36 and 8.29 (2d as
the AA’BB’ system, J = 4.9 Hz, Hα and Hβ, 2 × 2H).
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 0.67 and 0.69 (2d, J = 4.2 Hz, CH(CH3)2, 6H);
1.28 (dd, J = 7.3 and 6.9 Hz, CHCH2, 2H); 1.36 (m,
CHCH2, 1H); 2.93 (t, J = 7.3 Hz, CHCOOMe, 1H);
3.23 (s, OCH3, 3H); 3.78 (d, 2 JPH = 18.0 Hz, CHP, 1H);
7.36 and 8.26 (2d as the AA’BB’ system, J = 4.9 Hz,
Hα and Hβ, 2 × 2H).
1H NMR (200 MHz, D2O): δ 0.76 and 0.79 (2d,
J = 6.0 Hz, CH(CH3)2, 6H); 1.34 (dd, J = 6.2 and
6.7 Hz, CHCH2, 2H); 1.40 (m, CHCH2, 1H); 2.89 (t,
J = 6.2 Hz, CHCOOMe, 1H); 3.24 (s, OCH3, 3H); 3.75
2
(d, JPH = 18.6 Hz, CHP, 1H); 6.14 (t, J = 2.6 Hz,
CH3fur, 1H); 6.27 (dd, J = 2.6 and 2.3 Hz, CH4fur, 1H);
7.33 (m, CH5fur, 1H).
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 0.95 (d, J = 6.8 Hz, CH(CH3)2, 6H); 1.15 (dd, J =
6.2 and 6.7 Hz, CHCH2, 2H); 1.25 (m, CHCH2, 1H);
3.16 (s, OCH3, 3H); 3.27 (t, J = 6.2 Hz, CHCOOMe,
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1H); 3.81 (d, JPH = 18.9 Hz, CHP, 1H); 6.06 (m,
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N-(1-methoxycarbonyl-3-methylbutyl)amino(2-
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Predominant diastereoisomer: Calcd for C12H20NO5
PS: C, 44.85; H, 6.27; N, 4.36. Found: C, 44.56; H,
6.11; N, 4.13.
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4.06 (d, 2 JPH = 18.0 Hz, CHP, 1H); 6.91 (m, CH3thioph
,
CH4thioph, 2H); 7.22 (m, CHt5hioph, 1H).
Minor diastereoisomer: 1H NMR (200 MHz, D2O):
δ 0.70 and 0.80 (2d, J = 5.6 Hz, CH(CH3)2, 6H);
1.45 (dd, J = 5.6 and 6.6 Hz, CHCH2, 2H); 1.42 (m,
Heteroatom Chemistry DOI 10.1002/hc

Addition of Di-(Trimethylsilyl)Phosphite to Chiral N-(R)-α-Methylbenzyl and N-(1-methoxycarbonyl-iso-pentyl) 331
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