Kinetics and mechanism of the pyridinolysis of N-aryl-P,P-diphenyl phosphinic amides in dimethyl sulfoxide

Article abstract of DOI:10.1002/poc.1788

Kinetic studies for the reactions of Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines have been carried out in dimethyl sulfoxide at 85.0 °C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridine, exhibit positive deviations

Full text of DOI:10.1002/poc.1788

Research Article
Received: 19 May 2010,
Revised: 21 July 2010,
Accepted: 28 July 2010,
Published online in Wiley Online Library: 5 October 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1788
Kinetics and mechanism of the pyridinolysis of
N-aryl-P,P-diphenyl phosphinic amides in
dimethyl sulfoxide
a
a
a
*
*
Arun Kanti Guha , Chan Kyung Kim and Hai Whang Lee
Kinetic studies for the reactions of Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines have been carried out in
dimethyl sulfoxide at 85.0 -C. The two strong p-acceptor substituents, X ¼ 4-Ac and 4-CN in the X-pyridine, exhibit
¨
positive deviations from both the Hammett and Bronsted plots. The Hammett plots for substituent Z variations are
anomalously biphasic concave upwards with a break point at Z ¼ H, and the sign of rZ changes from negative for
electron-donating to positive for electron-withdrawing substituents Z in the leaving group. The negative sign of the
cross-interaction constants (rXZ) for both electron-donating (large magnitude of rXZ ¼ S1.54) and -withdrawing
(rXZ ¼ S0.27) substituents Z indicates that the reaction proceeds through a concerted mechanism. These results are
indicative of frontside and backside nucleophilic attacks at the P reaction center for electron-donating
and -withdrawing substituents Z, respectively. The poor leaving group mobility of NHPhZ yields the change of
the nucleophilic attacking direction, resulting in biphasic concave upwards Hammett plots for substituent Z
variations. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: biphasic concave upward free energy relationship; N-aryl-P,P-diphenyl phosphinic amides; negative sign of rlg;
phosphoryl transfer reaction; pyridinolysis
of the nucleophiles and leaving groups on the reaction
mechanism.
INTRODUCTION
—
O center is important
—
Nucleophilic substitution at
a
P
in chemistry since the phosphorus chemistry is interesting
because of its relevance to biological chemistry and usefulness
in synthesis. Two main types of displacement processes are
reported regarding the phosphoryl species; either stepwise
(AN þ DN) through a trigonal bipyramidal pentacoordinate (TBP-5C)
intermediate or a concerted process (ANDN).[1,2]
RESULTS AND DISCUSSION
The reactions were carried out under pseudo-first-order con-
ditions with a large excess of pyridine. The observed pseudo-
first-order rate constants (kobsd) for all reactions obeyed Eqn (1)
with negligible k0 (¼ 0) in DMSO.
Nonlinear free energy relationships for variation of the
nucleophile (X) and/or substrate (Y) and/or leaving group (Z)
were observed for the pyridinolyses of Z-aryl bis(4-methoxy-
phenyl) phosphates,[3] Y-aryl phenyl isothiocyanophosphates,[4]
diphenyl thiophosphinic chloride,[5] O,O-diphenyl Z-S-aryl phos-
phorothiolates,[6] dimethyl chlorothionophosphate,[7] and diethyl
chlorothionophosphate.[7] In contrast, the pyridinolyses of Y-aryl
phenyl chlorophosphates,[8] diphenyl phosphinic chloride,[5]
Y-O-aryl phenyl phosphonochloridothiates,[9] dimethyl chloro-
phosphate,[7] and diethyl chlorophosphate[7] showed linear free
energy correlations. However, the anilinolyses of phosphates
and derivatives showed linear free energy correlations[10–19]
except Y-O-aryl methyl phosphonochloridothioates,[20] As seen,
the substituent effects of the nucleophiles and/or substrates and/
or leaving groups upon the pyridinolysis mechanism were more
surprising than those upon the anilinolysis mechanism.
kobsd ¼ k0 þ k2½XC5H4Nꢀ
(1)
The second-order rate constants were determined with at least
five pyridine concentrations. The linear plots of Eqn (1) suggest a
lack of any base-catalysis or side reactions; the overall reaction is
described by Scheme 1.
The second-order rate constants [k2 (Mꢁ1 sꢁ1)] and selectivity
parameters (rX, bX, rZ, and rXZ) are summarized in Table 1. The bX
(¼ bnuc) values were determined using pKa values in water;
the slopes from the plots of log k2(DMSO) against pKa(H2O).
Justification of this procedure has been experimentally and
theoretically provided.[21]
*
Correspondence to: C. K. Kim and H. W. Lee, Department of Chemistry, Inha
University, 253 Yonghyundong, Namgu, Incheon 402-751, Korea.
E-mail: kckyung@inha.ac.kr; E-mail: hwlee@inha.ac.kr
In this paper, kinetic studies of the reactions of Z-N-
aryl-P,P-diphenyl phosphinic amides with X-pyridines in DMSO
at 85.0 8C (Scheme 1) are reported to gain further information
into the phosphoryl transfer reactions and the substituent effects
a
A. K. Guha, C. K. Kim, H. W. Lee
Department of Chemistry, Inha University, Incheon 402-751, Korea
J. Phys. Org. Chem. 2011, 24 474–479
Copyright ß 2010 John Wiley & Sons, Ltd.

KINETICS AND MECHANISM OF THE PYRIDINOLYSIS
O
P
O
P
1.40
1.05
0.70
0.35
0.00
-0.35
DMSO
85.0 o
+
NHPhZ
+
XC5H4N
Ph
Ph
C
NC5H4X
NHPhZ
Ph
Z=
Ph
4-MeO
4-Me
3-Cl
4-Cl
H
X = 4-MeO, 4-Bn, H, 3-Ac, 4-Ac, 3-CN, 4-CN
Z = 4-MeO, 4-Me, H, 4-Cl, 3-Cl
Scheme 1. The reaction system
X = 4-Ac
X = 4-CN
¨
The Hammett and Bronsted plots for substituent X variations in
the nucleophiles show typical trends of nucleophilic substitution
reactions, rX < 0 and bX > 0 (Figs 1 and 2). The two strong
p-acceptor substituents, X ¼ 4-Ac and 4-CN in the X-pyridine,
¨
exhibit positive deviations from both the Hammett and Bronsted
-0.4
-0.2
0.0
0.2
0.4
0.6
plots (vide infra). However, the Hammett plots (Fig. 3) for
substituent Z variations in the leaving groups are anomalously
biphasic concave upwards with a break point at Z ¼ H (sZ ¼ 0),
and as a result, the sign of rZ (¼ rlg) changes from negative for
electron-donating substituents (Z ¼ 4-MeO, 4-Me, H) to positive
for electron-withdrawing substituents (Z ¼ H, 4-Cl, 3-Cl). As
seen in Fig. 3, the sign of rZ for electron-donating substituents
Z is negative, regardless of the nature of the substituent X,
electron-donating or -withdrawing.
The anomalous negative sign of rZ implies that the N atom
in the leaving group becomes more positively charged in the
transition state (TS) than in the ground state (GS: reactants). To
rationalize the unusual negative rZ value for electron-donating
substituents Z, the TS I involving frontside nucleophilic attack
(Scheme 2) is proposed.
σ
X
Figure 1. The Hammett plots (log k2 vs. sX) of the reactions of
Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines in DMSO at
85.0 8C
through-space interaction. Thus, as a result, the charge of the N
atom in the leaving group becomes more positive in the TS than
the GS. Considerably large magnitudes of rZ (¼ ꢁ2.60 to ꢁ4.10)
indicate that the electron transfer by through-space interaction
from the leaving group to the nucleophile is efficient. As seen in
Table 1, the magnitude of the rZ value gradually increases as
substituent X of the nucleophile changes from electron-donating
(rZ ¼ ꢁ2.60: weakest through-space interaction when X ¼ 4-MeO)
to electron-withdrawing (rZ ¼ ꢁ4.10: strongest through-space
interaction when X ¼ 3-CN). This could be supporting evidence
of the proposed TS I. After TS I, the fast intramolecular exchange
When both the leaving group and nucleophile occupy the
equatorial positions in a TBP-5C TS I, the p-cloud of the phenyl
ring in the ‘electron-rich’ leaving group interacts strongly with the
p-cloud of the pyridine ring in the adjacent equatorial position by
Table 1. Second-order rate constants (k2 ꢂ 104, Mꢁ1 sꢁ1) and selectivity parametersa for the reactions of Z-N-aryl-P,P-diphenyl
phosphinic amides with XC5H4N in DMSO at 85.0 8C
b c
,
d e
X\Z
4-MeO
4-Me
H
4-Cl
3-Cl
rZ
rZ ,
4-MeO
4-Bn
H
27.5
20.0
17.2
9.02
20.0
16.6
6.90
0.73
0.12
15.7
10.9
9.02
3.67
7.94
5.53
2.32
1.01
0.17
5.49
2.98
2.37
1.02
2.73
1.79
0.526
1.16
0.20
12.1
7.23
4.88
1.79
5.03
3.98
1.02
1.28
0.22
14.9
9.31
6.81
2.31
6.51
4.81
1.38
1.25
0.21
ꢁ2.60
ꢁ3.09
ꢁ3.21
ꢁ3.48
ꢁ3.16
ꢁ3.50
ꢁ4.10
1.20
1.37
1.25
1.00
1.03
1.20
1.15
3-Ac
4-Ac
4-CN
3-CN
f g
rX ,
f h
bX ,
rXZ ¼ ꢁ1.54f,i
rXZ ¼ ꢁ0.27f,j
a The s values and the pKa values were taken from Refs [22–27], respectively.
b For Z ¼ 4-MeO, 4-Me and H.
c Correlation coefficients, r, are better than 0.990.
d For Z ¼ H, 4-Cl, and 3-Cl.
e r ꢃ 0.979.
f The two p-acceptors, 4-Ac and 4-CN, are excluded.
g r ꢃ 0.996.
h r ꢃ 0.986.
i r ¼ 0.998.
j r ¼ 0.997.
J. Phys. Org. Chem. 2011, 24 474–479
Copyright ß 2010 John Wiley & Sons, Ltd.
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A. K. GUHA, C. K. KIM AND H. W. LEE
X
1.40
1.05
0.70
0.35
0.00
-0.35
O
P
O
N
Ph
Z=
XC5H4N
Ph
P
NHPhZ
Ph
4-MeO
4-Me
3-Cl
4-Cl
H
N
Ph
Z
H
X = 4-Ac
X = 4-CN
TS III
TS II
Scheme 3. Two plausible TSs for electron-withdrawing substituents Z
process of the leaving group from the equatorial to the apical
position would follow by a Berry-type pseudorotation (or turnstile
rotation).[28,29]
When substituent Z of the leaving group is electron-
withdrawing, the sign of rZ is positive and in accordance with
those expected from a typical nucleophilic substitution reaction.
Two plausible TSs, frontside (TS II) and backside nucleophilic
attack (TS III), can be proposed as shown in Scheme 3. In the TS II,
electron transfer by through-space interaction would occur from
the nucleophile to the leaving group, in an opposite direction
to that in the TS I. In the TS III, both the nucleophile and
leaving group occupy the apical positions in a TBP-5C TS, where
substituents X and Z are far apart.
1.1
2.2
3.3
4.4
5.5
6.6
pKa(X)
¨
Figure 2. The Bronsted plots [log k2 vs. pKa(X)] of the reactions of
Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines in DMSO at
85.0 8C
1.5
1.2
0.9
0.6
The cross-interaction constants (CICs), rXZ, Eqns (2) and (3), are
determined, where X and Z represent the substituents in the
nucleophile and leaving group, respectively.[30–32]
logðkXZ=kHHÞ ¼ rXsX þ rZsZ þ rXZsXsZ
rXZ ¼ @rX=@sZ ¼ @rZ=@sX
(2)
(3)
The sign and magnitude of the CICs have made it possible
to correctly interpret the reaction mechanism and degree of
tightness of the TS, respectively. In general, the rXZ has a negative
value (or sometimes a small positive value) in a concerted SN2
and a stepwise mechanism with a rate-limiting bond formation.
However, it has a positive value for a stepwise mechanism with a
rate-limiting leaving group departure from the intermediate.
The magnitude of rXZ is inversely proportional to the distance
between the nucleophile and leaving group in the TS.[30–32]
The sign of the CICs, rXZ, is negative for both electron-donating
and -withdrawing substituent Z (Table 1 and Fig. 4), indicating
that the reaction proceeds through a concerted mechanism.
However, the magnitude of rXZ (¼ ꢁ1.54) for electron-donating
substituents Z is much greater than that (rXZ ¼ ꢁ0.27) for
electron-withdrawing. The unusual large negative rXZ (¼ ꢁ1.54)
for the electron-donating substituents Z implies that the
nucleophile and leaving group are in close enough proximity
to interact strongly. This is in agreement with the proposed TS I
involving a frontside nucleophilic attack. The large magnitudes
of the rXZ values (rXZ ꢃ jꢁ0.4j)[33] were obtained due to the
frontside nucleophilic attack as follows: (i) the reactions of aryl
bis(4-methoxyphenyl) phosphates with the weakly basic pyr-
idines in MeCN (rXZ ¼ ꢁ1.98);[3] (ii) the anilinolysis of anilino
thioethers in MeOH (rXZ ¼ ꢁ1.70);[34] (iii) the anilinolysis of both
2- and 3-thiopheneethyl arenesulfonates in MeCN (rXZ ¼ ꢁ0.50);[35]
X=
0.3
4-MeO
4-Bn
H
0.0
3-Ac
4-Ac
4-CN
-0.3
3-CN
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
σ
Z
Figure 3. The Hammett plots (log k2 vs. sZ) of the reactions of
Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines in DMSO at
85.0 8C
X
O
N
P
Ph
N
Ph
Z
(iv) the anilinolysis of cumyl arenesulfonates in MeCN (rXZ
¼
H
ꢁ0.75);[36] (v) the anilinolysis of 1-phenylethyl arenesulfonates in
MeOH (rXZ ¼ ꢁ0.56);[37] (vi) the anilinolysis of 2-phenylethyl
arenesulfonates in MeOH (rXZ ¼ ꢁ0.45);[38] (vii) the benzylami-
TS I
Scheme 2. TS I for electron-donating substituents Z
nolysis of Z-aryl cyclopropanecarboxylates in MeCN (rXZ
¼
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 474–479

KINETICS AND MECHANISM OF THE PYRIDINOLYSIS
Figure 4. Determination of rXZ (¼ @rZ/@sX ¼ @rX/@sZ) of the reactions of Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines in DMSO at 85.0 8C.
The obtained values by multiple regression are: (a) rXZ ¼ ꢁ1.54 ꢄ 0.24 for electron-donating substituents Z ¼ 4-MeO, 4-Me, H; (b) rXZ ¼ ꢁ0.27 ꢄ 0.21 for
electron-withdrawing substituents Z ¼ H, 4-Cl, 3-Cl
þ1.06);[39] (viii) the benzylaminolysis of Z-aryl 2-furoates in
MeCN (rXZ ¼ þ1.19);[40] (ix) the benzylaminolysis of Z-thiophenyl
acetates in MeCN (rXZ ¼ þ0.90).[41]
through-space interaction with the adjacent electron-rich phenyl
ring. Thus, the TS I can be effectively stabilized by the two strong
p-acceptor substituents (X ¼ 4-Ac and 4-CN) more than expected
from their sX [and pKa(X)] values. As a result, the two strong
p-acceptor substituents yield exalted reactivity and exhibit
In contrast, the small negative rXZ (¼ ꢁ0.27) for electro-
n-withdrawing substituent Z indicates that the nucleophile and
leaving group are far apart due to backside nucleophilic attack.[42]
Thus, the TS II can be safely ruled out for electron-withdrawing
substituent Z. It is the proposal of the authors that the reaction
proceeds through TS III involving backside nucleophilic attack in
which both the nucleophile and leaving group occupy the apical
positions. Considerably smaller magnitudes of rZ (¼ 1.00 ꢁ 1.37)
for electron-withdrawing substituent Z than those (rZ ¼ ꢁ2.60 to
ꢁ4.10) for electron-donating substituent Z could be further
supporting evidence of TS III for electron-withdrawing substituent
Z. If the TS II is involved in the reaction path, the magnitude of the
positive rZ value would be great since the negative charge on the
N atom of the leaving group becomes greater.
¨
positive deviations from the Hammett (and Bronsted) plot.
Secondly, in the case of the TS III for electron-withdrawing
substituent Z, the exalted reactivity (or enhanced nucleophilicity)
of the strong p-acceptor groups would be owing to the
weak p-donor effects.[27,53–56] The Hammett sp values of the
p-acceptor substituents represent the inductive and p-electron-
withdrawing effects. However, the experimental pKa value only
represents the inductive effect of X, since protonation/deproto-
nation takes place at the s lone pair on N which is orthogonal to
the ring p-system.[27] As a result, the protonation/deprotonation
does not disturb the ring p-system, but the positive charge center
in the conjugate acid, naturally, attracts p-electrons inductively
without through-conjugation between the s-lone pair and the
p-acceptor para-substituent. Thus, the pKa values of p-acceptor
substituents correctly reflect the substituent effects when the
N atom of pyridine becomes positively charged in the TS, since
the determination of pKa involves a positive charge on N
(azonium type).
In general, the nonlinear free energy correlation of a concave
upward plot is diagnostic of a change in the reaction mechanism
where the reaction path is changed depending on the sub-
stituents, while nonlinear free energy correlation of the concave
downward plot is diagnostic of a rate-limiting step change from
bond breaking with less basic nucleophiles to bond formation
with more basic nucleophiles.[43–52] In the present work, the
nonlinear free energy correlation of a biphasic concave upward
plot for substituent Z variations in the leaving groups is due to a
change in the attacking direction of the nucleophile from a
backside (TS III) for electron-withdrawing Z to a frontside attack
(TS I) for electron-donating substituent Z in the leaving group.
The two strong p-acceptor para-substituents, X ¼ 4-Ac and
4-CN in X-pyridines, exhibit positive deviations from both the
The two p-acceptor substituents exhibited positive deviations
¨
from the Hammett plots, while no deviations from the Bronsted
plots, for the pyridinolyses of methyl chloroformates in MeCN[27]
and water,[57,58] benzenesulfonyl chlorides in MeOH,[59] benzyl
bromides in DMSO,[60] and phenacyl bromides in MeCN.[61] This
indicated that the N atom of pyridine becomes positively charged
in the TS. In the pyridinolysis of Y-aryl phenyl chlorophosphates,
the two p-acceptor substituents did not exhibit deviations from
[8]
¨
¨
Hammett (Fig. 1) and Bronsted plots (Fig. 2) for substituent X
either the Hammett or Bronsted plots. No positive deviations
variations in the nucleophiles (vide supra). This behavior indicates
that the two p-acceptor substituents yield exalted reactivity.
Since both positive deviations are exhibited regardless of the
nature of the substituent Z in the leaving group, electron-
donating or -withdrawing, the reason for such exalted reactivity
should be rationalized in two ways for TS I and III, respectively.
Firstly, in the case of the TS I for electron-donating substituent
Z, the pyridine ring becomes p-electron-rich due to the
for the p-acceptor in both plots were rationalized by the early
TS with little positive charge development on the N atom of
pyridine. The early TS, in which the extent of both the bond
formation and leaving group departure is small, was supported
by the small CIC, rXY ¼ ꢁ0.15.[62] In the present work, the two
strong p-acceptor substituents exhibit positive deviations from
¨
both the Hammett (Fig. 1) and Bronsted plots (Fig. 2) for
substituent X variations in the nucleophiles. This indicates that
J. Phys. Org. Chem. 2011, 24 474–479
Copyright ß 2010 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/poc

A. K. GUHA, C. K. KIM AND H. W. LEE
the degree of positive charge development on the N atom of
pyridine is substantial in the TS III.
then dried over anhydrous magnesium sulfate. The product was
isolated by evaporating the solvent under reduced pressure
after filtration. The GR grade dimethyl sulfoxide was dried over
molecular sieves and used after three distillations under reduced
pressure prior to use. The GR grade X-pyridines were used
without further purification. The analytical and spectroscopic
data of substrates are summarized in supporting information.
The second-order rate constants of the pyridinolysis (with
unsubstituted pyridine: C5H5N) of diphenyl phosphinic chloride
in MeCN at 35.0 8C[5] and N-phenyl-P,P-diphenyl phosphinic
amide in DMSO at 85.0 8C are k2 ¼ 5.46 ꢂ 10ꢁ4 and 2.37 ꢂ
10ꢁ4 Mꢁ1 sꢁ1, respectively. The difference between the two
substrates, Ph2P(¼O)Cl and Ph2P(¼O)NHPh, is the leaving group.
Taking into account the differences of solvent polarity and
reaction temperature between the two reactions; MeCN (dielectric
constant: er ¼ 35.94) versus DMSO (er ¼ 46.45) and 35.0 versus
85.0 8C, respectively, the leaving group mobility of NHPh is very
poor compared to Cl. This poor leaving group mobility of NHPh
leads to the late TS III, in which the extent of bond formation and
leaving group departure is great, and as a result, the positive
charge development on the N atom in the pyridine nucleophile is
substantial.
When the substituent Z in the leaving group changes from
electron-withdrawing to -donating, the leaving group mobility
becomes poor. This results in the change of the nucleophilic
attacking direction from backside to frontside. For electron-
donating substituent Z, thus, the reaction proceeds through the
TS I involving a frontside nucleophilic attack as discussed earlier,
and the Hammett plots for substituent Z variations in the leaving
groups exhibit biphasic concave upwards with a break point at
Z ¼ H. It is surprising substituent effects of the leaving group even
to yield the anomalous large negative rZ (¼ ꢁ2.60 to ꢁ4.10).
Kinetic procedure
Rates were measured conductometrically[3] in dimethyl sulfoxide
at 85.0 8C. The conductivity bridge used in this work was a
self-made computer automated A/D converter conductivity bridge.
Pseudo-first-order rate constants (kobsd) were measured by curve
fitting analysis in an origin program with a large excess of
pyridine, [Z-N-aryl-P,P-diphenyl phosphinic amides] ꢅ 3 ꢂ 10ꢁ3 M
and [X-pyridine] ¼ 0.1 ꢁ 0.3 M. The k2 values in Table 1 are the
average of more than three runs and were reproducible to within
ꢄ3%.
Product analysis
The N-(4-Chlorophenyl)-P,P-diphenyl phosphinic amide was
reacted with excess 4-methoxy pyridine for more than 15 half
lives at 85.0 8C in DMSO. The solvent was evaporated under
reduced pressure. The solid product was washed several times
with ether to remove excess pyridine. The analytical and
spectroscopic data of product are summarized in supporting
information.
CONCLUSIONS
Acknowledgements
A concerted SN2 mechanism for the pyridinolysis of N-aryl-
P,P-diphenyl phosphinic amides in DMSO at 85.0 8C is proposed
on the basis of the negative cross-interaction constants, rXZ. The
Hammett plots for substituent Z variations in the leaving groups
are biphasic concave upwards with a break point at Z ¼ H.
A frontside nucleophilic attack is proposed based on the
considerably large magnitude of rXZ (¼ ꢁ1.54) for the electro-
n-donating substituent Z of the leaving group. In contrast, a
backside nucleophilic attack is proposed based on the small
magnitude of rXZ (¼ ꢁ0.27) for the electron-withdrawing
substituent Z. The two strong p-acceptor substituents, X ¼ 4-Ac
and 4-CN in the X-pyridine, exhibit positive deviations from both
This work was supported by the Brain Korea 21 Program from
National Research Foundation of Korea.
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¨
the Hammett and Bronsted plots. Both positive deviations are
rationalized by TS stabilization for electron-donating substituent
Z and weak p-donor effect of the two p-acceptor substituents for
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Hammett plots for substituent Z variations in the leaving groups
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EXPERIMENTAL
Materials
Z-N-Aryl-P,P-diphenyl phosphinic amides were prepared by
reacting diphenyl phosphinic chloride with Z-anilines for 5 h in
methylene chloride at room temperature. The Z-anilinium
chloride salt was filtered out. The remaining product was treated
with ether and water for workup. The product mixture was
treated with ether and dilute hydrochloric acid to remove excess
anilines as Z-anilinium chloride salt. The remaining product was
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