Base controlled (1,1)- and (1,2)-hydrophosphination of functionalized alkynes

Article abstract of DOI:10.1016/j.tetlet.2008.01.066

The (1,1)- and (1,2)-addition of diphenylphosphine to 3-butyn-2-one and ethyl propiolate can be controlled chemoselectively by regulating the amount of triethylamine as the external base and in the presence of the chiral organopalladium(II) template deriv

Full text of DOI:10.1016/j.tetlet.2008.01.066

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1762–1767
Base controlled (1,1)- and (1,2)-hydrophosphination
of functionalized alkynes
Yi Zhang, Lulu Tang, Yi Ding, Jia-Hui Chua, Yongxin Li, Mingjun Yuan, Pak-Hing Leung *
Division of Chemistry and Biological Chemistry, Nanyang Technological University, Singapore 637371, Singapore
Received 13 November 2007; revised 22 December 2007; accepted 14 January 2008
Available online 19 January 2008
Abstract
The (1,1)- and (1,2)-addition of diphenylphosphine to 3-butyn-2-one and ethyl propiolate can be controlled chemoselectively by
regulating the amount of triethylamine as the external base and in the presence of the chiral organopalladium(II) template derived
from (R)-N,N-dimethyl-1-(1-naphthyl)ethylamine.
Ó 2008 Elsevier Ltd. All rights reserved.
The synthesis of functionalized chiral phosphines is con-
sidered important since these ligands play vital roles in
transition metal catalyzed reactions.1 The addition of sec-
ondary phosphines to carbon–carbon multiple bonds is
generally considered a straightforward and efficient organic
synthetic process.2 However, this approach continues to
pose considerable challenges in the preparation of oxy-
gen-sensitive and highly reactive phosphines. Transition
metal complex-assisted hydrophosphination reactions have
been reported.3 In general, metal complexes offer better
stabilization and selectivity in hydrophosphination reac-
tions than other reaction promoters such as strong bases,4
acids5 and free radicals.6 We have previously reported a
series of chiral complex-promoted asymmetric 1,2-addition
reactions with alkynes in the absence of an external base
promoter.7 Herein we present the preparation of function-
alized diphosphines from substituted alkynes via a simple
chiral palladium template-promoted hydrophosphination
reaction. By regulating the amount of triethylamine, as a
mild external base, the (1,1)- and (1,2)-addition pathways
could be controlled chemoselectively. It is important to
note that the functionalized 1,1-diphosphine products
could not be produced by the traditional addition reaction
using 1,1-bis(diphenylphosphino)-ethylene as the starting
material.8
In the absence of triethylamine, diphenylphosphine
shows no reactivity towards the functionalized alkynes 2a
and 2b under ambient conditions, even in the presence of
the standard chiral reaction promoter (R)-3.7 In the pres-
ence of 2 mol % of triethylamine and the chiral palladium
complex (R)-3, the reaction between diphenylphosphine
and 3-butyn-2-one 2a proceeded smoothly at ꢀ78 °C gener-
ating a 1:2 mixture of the stereoisomeric 1,1-addition prod-
ucts 4a and 5a regiospecifically (Scheme 1). The keto groups
in these two isomeric product complexes are located on the
opposite side of the square-plane. Prior to purification, the
31P NMR spectrum of the crude reaction product in CDCl3
exhibited two pairs of doublets at d ꢀ18.8, 9.6 (JPP = 52 Hz,
minor product) and ꢀ18.5, 8.2 (JPP = 54 Hz, major prod-
uct), respectively. The 202 MHz 31P NMR spectrum did
not detect the presence of any sterically favourable (1,2)-
addition products in the crude mixture. The isomeric mix-
ture was then treated with concentrated hydrochloric acid
to remove the chiral naphthylamine ligand to afford a single
dichloro complex 6a in 85% yield. The 31P NMR spectrum
of 6a in CD2Cl2 exhibited a sharp singlet at d ꢀ37.5. The
X-ray structure of 6a is shown in Figure 1. Interestingly,
both the P(1)–C(13)–P(2) [92.8(1)°] and the P(1)–Pd(1)–
P(2) [74.5(1)°] angles within the 4-membered ring are both
rather small (see Tables 1 and 2).
*
Corresponding author. Tel.: +65 6316 8905; fax: +65 6791 1961.
E-mail address: pakhing@ntu.edu.sg (P.-H. Leung).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.066

Y. Zhang et al. / Tetrahedron Letters 49 (2008) 1762–1767
1763
Me2 Ph2
Me2
N
Me
NCMe
NCMe
Me
N
P
R
Pd
Pd
Ph
Ph
P
Ph2
2
P H
-
-
ClO4
(R)-3
ClO4
Ph2
P
Ph2
P
1
R
conc. HCl
aq. KCN
R
Cl
Cl
4
+
+
Pd
Me2 Ph2
P
Ph2
P
trace NEt3
H C C R
2a, b
Me
N
P
R
Ph2
Pd
6
7
P
a
=COMe
b=COOEt
Ph2
Yield: 95%
Yield: 85%
-
ClO4
5
Scheme 1.
Table 1
˚
Selected bond lengths (A) and bond angles (°) of 6a
Pd(1)–P(2)
Pd(1)–P(1)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
P(1)–C(13)
P(2)–C(13)
C(13)–C(14)
2.2289(5)
2.2322(6)
2.3494(7)
2.3544(8)
1.8654(19)
1.8639(19)
1.528(3)
C(14)–C(15)
1.520(3)
P(2)–Pd(1)–P(1)
P(2)–C(13)–P(1)
P(2)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(1)
P(2)–Pd(1)–Cl(2)
P(1)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–Cl(2)
74.534(19)
92.83(9)
171.79(3)
97.28(3)
94.31(3)
168.52(3)
93.84(3)
Interestingly, when the above hydrophosphination of
3-butyn-2-one 2a was performed under similar reaction con-
ditions but in the presence of excess triethylamine (20 equiv),
the reaction proceeded exclusively via the (1,2)-addition
pathway (Scheme 2). In CDCl3, the 202 MHz 31P NMR
spectrum of the crude mixture indicated the four diastereo-
meric (1,2)-addition products only: four pairs of doublets
were recorded with the intensity ratio of ca. 7:1:1:1 at d
Fig. 1. Molecular structure of dichloro complex 6a.
Treatment of a CH2Cl2 solution of 6a with aqueous
potassium cyanide liberated the keto-substituted diphos-
phine 7a as a white solid in 95% yield. The 31P NMR spec-
trum of the liberated ligand in CDCl3 exhibited a sharp
singlet at d ꢀ5.3.
Table 2
Crystallographic data for complexes 6a, 6b, 8a and 8b
Formula
6a
C
6b
C29H28Cl2O2P2Pd
8a
C44H47ClNO5.50P2Pd
8b
C43H44ClNO6P2Pdꢁ0.5CH2Cl2
28H26Cl2OP2Pd
M
Space group
Crystal system
617.73
Cc
647.75
881.62
917.05
P2(1)
P2(1)/c
Monoclinic
20.6239(7)
16.0221(5)
18.0126(5)
5586.9(3)
8
P2(1)2(1)2
Orthorhombic
29.8157(17)
10.9192(6)
13.2866(7)
4325.6(4)
4
Monoclinic
17.3953(7)
8.7688(3)
18.2530(6)
2699.2(2)
4
Monoclinic
19.6133(8)
9.4188(3)
23.1706(9)
4078.0(3)
4
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
V (A )
Z
T (K)
298(2)
0.71073
1.024
0.0244
0.0575
173(2)
0.71073
0.995
0.0408
0.0944
298(2)
0.71073
0.610
0.0661
0.1167
173(2)
0.71073
0.714
0.0540
0.1564
˚
k (A)
l (mmꢀ1
)
R1 (obs. data)a
wR2 (obs. data)b
Flack parameter
ꢀ0.02(6)
0.01(3)
a
R1 = RkF0j ꢀ jFck/RjF0j.
2
2
1=2
2
2
b
wR2 ¼ fR½wðF 20 ꢀ F 2cÞ ꢂ=R½wðF 02Þ ꢂg ; wꢀ1 ¼ r2ðF 0Þ þ ðaPÞ þ bP:

1764
Y. Zhang et al. / Tetrahedron Letters 49 (2008) 1762–1767
Me2
Ph2
P
Ph
Ph
Ph2
P
2
P H
R
R
+
Me
N
Cl
Cl
H C C R
conc. HCl
Pd
Pd
1
2
P
P
2a: R = COMe
2b: R = COOEt
Ph2
Ph2
12
-
Me2
N
ClO4
8a, b
Me
NCMe
Yield: 90%
Pd
a
excess NEt3
, R=COMe
b, R=COOEt
NCMe
aq. KCN
R
-
ClO4
(R)-3
Ph2
P
Me2 Ph2
N
Me2 Ph2
R
R
Me
Me
P
Me
Me
N
P
P
Ph2
Pd
Pd
+
P
P
Ph2
Ph2
13
Yield: 95%
-
-
ClO4
8
ClO4
9
Scheme 3.
Me2 Ph2
N
Me2
N
Ph2
P
P
Pd
Pd
+
R
P
P
R
Ph2
Ph2
-
-
ClO4
ClO4
10
11
Scheme 2.
45.1, 48.9 (JPP = 32 Hz, major signals); 45.4, 48.3 (JPP
32 Hz); 29.2, 64.2 (JPP = 32 Hz) and 41.0, 70.5 (JPP
=
=
22 Hz), respectively. Thus the spectroscopic study revealed
that all the four possible diastereomeric 1,2-addition prod-
ucts 8a–11a were generated in the hydrophosphination reac-
tion. The major isomer 8a was subsequently crystallized
from acetone–diethyl ether as pale yellow prisms in 40%
yield. The molecular structures and the absolute stereochem-
istry of 8a were determined by X-ray crystallography
(Fig. 2). The absolute chirality at C(16) is S. The P(2)–
Pd(1)–P(1) bond angle [84.8(1)°] in 8a is significantly larger
than its counterpart in complex 6a (Table 3). Unfortunately,
the treatment of 8a with concentrated hydrochloric acid
generated the racemic dichloro complex 12a in 90% yield
(Scheme 3). The 202 MHz 31P NMR spectrum of 12a in
CDCl3 exhibited a pair of doublets at d 47.4, 70.6
(JPP = 9 Hz).
Fig. 3. Molecular structure of the cationic complex 8b.
merism of the carbonyl functionality during the acid
treatment. This stereo-dynamic process readily disturbed
the chirality of the stereogenic carbon centre in the chiral
diphosphine ligand. To avoid this intrinsic problem, the
keto group in the original alkyne was replaced by its ester
analogue. Thus, in the presence of excess triethylamine
(2 equiv), the hydrophosphination of ethyl propiolate 2b
The racemization of the chiral keto-substituted diphos-
phine ligand is attributed to the classic keto–enol tauto-
Table 3
˚
Selected bond lengths (A) and bond angles (°) of 8a
Pd(1)–C(1)
Pd(1)–N(1)
Pd(1)–P(1)
Pd(1)–P(2)
2.042(11)
2.141(6)
2.246(2)
2.345(3)
1.860(9)
1.534(13)
1.822(10)
1.496(15)
80.8(4)
C(16)–P(2)
C(15)–C(16)
C(15)–P(1)
C(16)–C(17)
C(1)–Pd(1)–N(1)
C(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
C(1)–Pd(1)–P(2)
N(1)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
96.0(3)
176.0(3)
177.8(3)
98.3(2)
84.82(9)
Fig. 2. Molecular structure of the cationic complex 8a.

Y. Zhang et al. / Tetrahedron Letters 49 (2008) 1762–1767
1765
Table 4
(JPP = 33 Hz). The spectroscopic signals confirmed the
formation of 8b and its regio-isomer 10b. In a further test
of optical purity, the diastereomers 14 and 15 were
prepared from 8b and the equally accessible (S)-3 (Scheme
4). The 31P NMR spectrum of the crude product showed
two clearly different pairs of doublets with similar intensity
at d 35.4, 69.9 (JPP = 26 Hz) and 47.5, 47.9 (JPP = 31 Hz).
Interestingly, the reaction rate and the (1,1)- and (1,2)-
chemoselectivity of the hydrophosphination reaction are
sensitive to the functionality present on the reacting alkyne.
The ester-substituted alkyne 2b appeared to favour the
(1,2)-addition pathway. Thus when the amount of triethyl-
amine was reduced to 2 mol %, the hydrophosphination
reaction between diphenylphosphine and 2b at ꢀ78 °C
continued to give mostly the (1,2)-addition products. In
contrast to the similar reaction involving keto-alkyne 2a,
only a small quantity of (1,1)-addition ester products,
together with some as yet unidentified materials were gen-
erated under these conditions. The optimum condition for
the synthesis of the (1,1)-addition products required the
presence of 20 mol % of triethylamine and the reaction
was conducted at room temperature. Under these condi-
tions, a 2:3 mixture of the two stereoisomeric (1,1)-addition
products 4b and 5b could be obtained in 50% isolated yield
(Scheme 1). Treatment of the mixture with concentrated
hydrochloric acid generated a single dichloro complex 6b,
quantitatively. As shown in Figure 4, the X-ray structural
analysis of 6b revealed that it has a similar molecular orien-
tation to its keto counterpart 6a (Table 5). There are no
major steric repulsions around the ester functional group
in 6b. Thus, apart from the quantity of the external base
used, the electronic properties of the functional group in
the substituted-alkynes also play a major role in reaction
mechanism of the current hydrophosphination reaction
for the chemoselective formation of the (1,1)- and (1,2)-
addition products. We are currently investigating the mod-
˚
Selected bond lengths (A) and bond angles (°) of 8b
Pd(1)–C(1)
Pd(1)–N(1)
Pd(1)–P(2)
Pd(1)–P(1)
P(1)–C(27)
P(2)–C(31)
C(27)–C(28)
C(27)–C(31)
C(1)–Pd(1)–N(1)
C(1)–Pd(1)–P(2)
N(1)–Pd(1)–P(2)
C(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
P(2)–Pd(1)–P(1)
2.063(5)
2.162(5)
2.2440(14)
2.3814(13)
1.871(6)
1.833(5)
1.528(8)
1.532(8)
79.4(2)
97.44(16)
176.62(13)
179.16(16)
99.80(13)
83.33(5)
at ꢀ78 °C gave only three of the four possible (1,2)-addi-
tion diastereomeric products (Scheme 2).
In CDCl3, the 202 MHz 31P NMR spectrum of the crude
product showed three pairs of doublets with the intensity
ratio of ca. 18:3:1 at d 47.6, 48.9 (JPP = 32 Hz); 28.5, 66.4
(JPP = 33 Hz) and 35.4, 69.9 (JPP = 26 Hz), respectively.
The major product 8b was isolated from dichloro-
methane–diethyl ether as pale yellow prisms in 55% yield.
The molecular structures and the absolute stereochemistry
of 8b were determined by X-ray crystallography (Fig. 3 and
Table 4). The treatment of complex 8b with concentrated
hydrochloric acid gave the enantiomerically pure dichloro
complex 12b in 90% yield, with [a]436 ꢀ78 (c 0.5, CH2Cl2,
24 °C). In contrast to its keto-analogue, the absolute ste-
reochemistry of the chiral diphosphine ligand in 12b
remained unchanged during the acidic treatment, as the
ester group was not involved in a similar tautomerism pro-
cess. As shown in Scheme 3, the treatment of a CH2Cl2
solution of 12b with aqueous potassium cyanide liberated
the optically pure diphosphine 13b as a white solid in
95% yield, with [a]436 ꢀ33 (c 1.0, CH3Cl, 24 °C). The 31P
NMR spectrum of the free diphosphine in CDCl3 exhibited
a pair of doublets at d ꢀ16.6, 2.4 (JPP = 24 Hz).
The optical purity of the liberated chiral diphosphine
was confirmed by the quantitative recoordination of 13b
to (R)-3: the 202 MHz 31P NMR spectrum of the crude
product showed only two pairs of doublets with similar
intensity at d 47.6, 48.9 (JPP = 32 Hz) and 28.5, 66.4
Me2
N
Me2
N
Ph2
P
Me
NCMe
NCMe
COOEt
Me
Pd
Pd
P
Ph2
-
-
ClO4
(S)-3
ClO4
14
+
+
Me2 Ph2
Ph2
P
Me
N
P
COOEt
Pd
P
COOEt
P
Ph2
Ph2
-
ClO4
15
13b
Scheme 4.
Fig. 4. Molecular structure of dichloro complex 6b.

1766
Y. Zhang et al. / Tetrahedron Letters 49 (2008) 1762–1767
Table 5
Selected physical and spectroscopic data for 8a: Mp 203–
204 °C; [a]D ꢀ58 (c 0.5, CH2Cl2, 24 °C). Anal. Calcd for
C42H42ClNO5P2Pd: C, 59.7; H, 5.0; N, 1.7. Found: C,
59.2; H, 5.4; N, 1.6. 31P NMR (CDCl3): d 45.1, 48.9
˚
Selected bond lengths (A) and bond angles (°) of 6b
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
P(1)–C(13)
P(2)–C(13)
C(13)–C(14)
C(14)–C(15)
P(1)–Pd(1)–P(2)
P(1)–C(13)–P(2)
P(1)–Pd(1)–Cl(1)
P(2)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(2)
P(2)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–Cl(2)
2.2207(8)
2.2382(9)
2.3607(10)
2.3622(9)
1.866(3)
1.871(3)
1.536(5)
1.528(5)
74.23(3)
92.10(15)
93.58(4)
167.73(4)
172.76(4)
98.54(4)
93.65(4)
1
(JPP = 33 Hz); H NMR (CDCl3): d 1.88 (s, 3H, COMe),
3
1.95 (d, 3H, JHH = 6.1 Hz, CHMe), 2.43 (d, 3H, JPH
=
0
1.2 Hz, NMeeq), 2.51 (dd, 3H, JPH ¼ JP H ¼ 3:7 Hz,
NMeax), 2.76–2.88 (m, 1H, P2CHH0), 2.95–3.06 (m, 1H,
P2CHH0), 3.77–3.88 (m, 1H, P1CHCOMe), 4.48 (qn, 1H,
3JHH = 4JPH = 6.1 Hz, CHMe), 6.87–8.19 (m, 26H,
aromatics).
Selected physical and spectroscopic data for 8b: Mp 182–
184 °C; [a]D ꢀ110 (c 0.5, CH2Cl2, 24 °C). Anal. Calcd for
C43H44ClNO6P2Pd: C, 59.0; H, 5.1; N, 1.6. Found: C,
58.6; H, 5.2; N, 1.6. 31P NMR (CDCl3): d 47.6, 48.9
1
3
(JPP = 32 Hz); H NMR (CDCl3): d 0.87 (t, 3H, JHH
=
3
7.1 Hz, COOCH2CH3), 1.96 (d, 3H, JHH = 6.2 Hz,
CHMe), 2.48 (d, 3H, JPH = 1.2 Hz, NMeeq), 2.61 (dd,
ifications of the (1,1)-hydrophosphination reaction to gen-
erate a family of asymmetric 4-membered metal chelates .
Reactions involving air-sensitive compounds were per-
formed under a positive pressure of purified nitrogen. Pro-
ton NMR spectra were recorded at 300 MHz on a Bruker
ACF300 NMR spectrometer. All the 31P NMR spectra
were recorded at 202 MHz on a Bruker ACF500 NMR
spectrometer.
CCDC-667577 [for complex 6a], -667578 [for complex
8a], -667579 [for complex 8b] and -667580 [for complex
6b] contain the supplementary crystallographic data for
this letter. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.
cdcc.cam.ac.uk/data_request/cif.
0
3H, JPH ¼ JP H ¼ 3:7 Hz, NMeax), 2.89–3.00 (m, 1H,
P2CHH0), 3.06–3.16 (m, 1H, P2CHH0), 3.17–3.29 (m, 1H,
P1CHCOOEt), 3.78–3.94 (m, 2H, COOCH2CH3), 4.55
3
(qn, 1H, JHH = 4JPH = 6.2 Hz, CHMe), 6.87–8.26 (m,
26H, aromatics).
Selected physical and spectroscopic data for 12a: Mp
188–189 °C. Anal. Calcd for C28H26Cl2OP2Pd: C, 54.4;
H, 4.2. Found: C, 54.1; H, 4.0. 31P NMR (CDCl3): d
1
47.4, 70.6 (JPP = 9 Hz); H NMR (CDCl3): d 1.80 (s, 3H,
COMe), 2.57–2.68 (m, 1H, P2CHH0), 2.75–2.87 (m, 1H,
P2CHH0), 3.66–3.78 (m, 1H, P1CHCOMe), 7.46–8.15 (m,
20H, aromatics).
Selected physical and spectroscopic data for 12b: Mp
257–258 °C; [a]436 ꢀ78 (c 0.5, CH2Cl2, 24 °C). Anal. Calcd
for C29H28Cl2O2P2Pd: C, 53.8; H, 4.4. Found: C, 53.4; H,
4.4. 31P NMR (CDCl3): d 49.0, 73.6 (JPP = 9 Hz); 1H NMR
Selected physical and spectroscopic data for 6a: Mp 212–
214 °C. Anal. Calcd for C28H26Cl2OP2Pd: C, 54.4; H, 4.2.
Found: C, 54.1; H, 4.0. 31P NMR (CD2Cl2): d ꢀ37.5; H
1
3
NMR (CD2Cl2): d 1.42 (s, 3H, COMe), 2.23 (dt, 2H,
3JHH = 7.6 Hz, JPH = 15.1 Hz, PCCH2), 5.23 (tt, 1H,
3JHH = 7.6 Hz, JPH = 10.6 Hz, PCH), 7.44–8.17 (m, 20H,
aromatics).
(CDCl3): d 0.90 (t, 3H, JHH = 7.1 Hz, COOCH2CH3),
2.72–3.01 (m, 2H, PCH2CHCOOEt), 3.47–3.61 (m, 1H,
PCHCOOEt), 3.82–3.85 (m, 2H, COOCH2CH3), 7.43–
8.13 (m, 20H, aromatics).
Selected physical and spectroscopic data for 6b: Mp 197–
199 °C. Anal. Calcd for C29H28Cl2O2P2Pd: C, 53.8; H, 4.4.
Selected physical and spectroscopic data for 13b: [a]436
ꢀ33 (c 1.0, CH3Cl, 24 °C). 31P NMR (CDCl3): d ꢀ16.6,
2.4 (JPP = 24 Hz); 1H NMR(CDCl3): d 0.93 (t, 3H,
3JHH = 7.1 Hz, COOCH2CH3), 2.16–2.26 (m, 1H,
PCH0HCHCOOEt), 2.60–2.73 (m, 1H, PCH0HCHCOOEt),
3.19–3.28 (m, 1H, PCHCOOEt), 3.68–3.89 (m, 2H,
COOCH2CH3), 7.26–7.38 (m, 20H, aromatics).
1
Found: C, 53.5; H, 4.6. 31P NMR (CDCl3): d ꢀ37.3; H
3
NMR (CDCl3): d 0.98 (t, 3H, JHH = 7.1 Hz, COOCH2
3
CH3), 2.20 (dt, 2H, JHH = 8.0 Hz, JPH
= 15.1 Hz,
3
PCCH2), 3.81 (q, 2H, JHH = 7.1 Hz, COOCH2 CH3),
3
5.19 (tt, 1H, JHH = 8.0 Hz, JPH = 10.6 Hz, PCH), 7.48–
8.24 (m, 20H, aromatics).
Selected physical and spectroscopic data for 7a: 31P
NMR (CDCl3): d ꢀ5.3; 1H NMR (CD2Cl2): d 1.36 (s,
Acknowledgements
3
3H, COMe), 2.53 (dt, 2H, JHH = 5.4 Hz, JPH = 9.1 Hz,
We are grateful to Nanyang Technological University
for supporting this research and the research scholarships
to Y.Z., L.L.T., Y.D. and M.Y.
3
PCCH2), 4.19 (tt, 1H, JHH = JPH = 5.4 Hz, PCH), 7.25–
7.56 (m, 20H, aromatics).
Selected physical and spectroscopic data for 7b: 31P
1
NMR (CDCl3): d ꢀ5.9; H NMR (CDCl3): d 0.96 (t, 3H,
References and notes
3
3JHH = 7.2 Hz, COOCH2CH3), 2.41 (dt, 2H, JHH
=
5.9 Hz, JPH = 9.2 Hz, PCCH2), 3.56 (q, 2H, 3JHH = 7.2 Hz,
COOCH2CH3), 3.91 (tt, 1H, JHH = JPH = 5.9 Hz, PCH),
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3
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