Kinetics and mechanism of the anilinolysis of aryl phenyl isothiocyanophosphates in acetonitrile

Article abstract of DOI:10.3762/bjoc.9.68

Kinetic studies on the reactions of Y-aryl phenyl isothiocyanophosphates with substituted X-anilines and deuterated X-anilines were carried out in acetonitrile at 55.0 °C. The free-energy relationships with X in the nucleophiles were biphasic concave upwards with a break region between X = H and 4-Cl, giving unusual positive ρ_X and negative β_X values with less basic anilines (X = 4-Cl and 3-Cl). A stepwise mechanism with rate-limiting bond breaking for more basic anilines and with rate-limiting bond formation for less basic anilines is proposed based on the positive and negative ρ_XY values, respectively. The deuterium kinetic isotope effects involving deuterated anilines (XC₆H₄ND₂) showed primary normal and secondary inverse DKIEs for more basic and less basic anilines, rationalized by frontside attack involving hydrogen-bonded four-center-type TSf and backside attack TSb, respectively. The positive ρ_X values with less basic anilines are substantiated by the tight TS, in which the extent of the bond formation is great and the degree of the bond breaking is considerably small.

Full text of DOI:10.3762/bjoc.9.68

Kinetics and mechanism of the anilinolysis of aryl
phenyl isothiocyanophosphates in acetonitrile
Hasi Rani Barai and Hai Whang Lee*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 615–620.
Department of Chemistry, Inha University, Incheon 402-751, Korea
doi:10.3762/bjoc.9.68
Email:
Received: 10 December 2012
Accepted: 06 March 2013
Published: 26 March 2013
Hai Whang Lee* - hwlee@inha.ac.kr
* Corresponding author
This article is part of the Thematic Series "New reactive intermediates in
organic chemistry".
Keywords:
anilinolysis; cross-interaction constant; deuterium kinetic isotope
effects; phosphoryl transfer reaction; reactive intermediates; Y-aryl
phenyl isothiocyanophosphates
Guest Editor: G. Bucher
© 2013 Barai and Lee; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Kinetic studies on the reactions of Y-aryl phenyl isothiocyanophosphates with substituted X-anilines and deuterated X-anilines
were carried out in acetonitrile at 55.0 °C. The free-energy relationships with X in the nucleophiles were biphasic concave upwards
with a break region between X = H and 4-Cl, giving unusual positive ρX and negative βX values with less basic anilines (X = 4-Cl
and 3-Cl). A stepwise mechanism with rate-limiting bond breaking for more basic anilines and with rate-limiting bond formation
for less basic anilines is proposed based on the positive and negative ρXY values, respectively. The deuterium kinetic isotope effects
involving deuterated anilines (XC6H4ND2) showed primary normal and secondary inverse DKIEs for more basic and less basic
anilines, rationalized by frontside attack involving hydrogen-bonded four-center-type TSf and backside attack TSb, respectively.
The positive ρX values with less basic anilines are substantiated by the tight TS, in which the extent of the bond formation is great
and the degree of the bond breaking is considerably small.
Introduction
The nucleophilic substitution reactions of tetracoordinate phos- sition state (TS). To extend the kinetic studies on the phos-
phorus have been studied extensively, experimentally and phoryl transfer reactions, the reactions of Y-aryl phenyl isothio-
theoretically, in our lab. Two main types of displacement cyanophosphates (1a–e) with substituted anilines (XC6H4NH2)
processes are well known in neutral phosphoryl transfer reac- and deuterated anilines (XC6H4ND2) have been investigated
tions: a stepwise mechanism involving a trigonal bipyramidal kinetically in acetonitrile (MeCN) at 55.0 ± 0.1 °C (Scheme 1).
pentacoordinate (TBP-5C) intermediate and a concerted dis- The kinetic results of the present work are discussed based on
placement at phosphorus through a single pentacoordinate tran- the selectivity parameters and deuterium kinetic isotope effects
615

Beilstein J. Org. Chem. 2013, 9, 615–620.
Scheme 1: Reactions of Y-aryl phenyl isothiocyanophosphates (1a–e) with XC6H4NH2(D)2 in MeCN at 55.0 °C.
(DKIEs). The aim of this work is to gain further information on were obtained from the slopes of pseudo-first-order rate
the substituent effects, DKIEs and mechanism of the phos- constants (kobsd) versus aniline concentration (Equation 1).
phoryl transfer reactions.
Base-catalysis or noticeable side reactions could be safely ruled
out from the zero intercept, k0 ≈ 0, in MeCN.
Results and Discussion
Table 1 summarizes the second-order rate constants, kH and kD,
with X-anilines and deuterated X-anilines, respectively. Table 2
and Table 3 summarize the Hammett (ρX(H and D)) and Brönsted
(1)
(βX(H and D)) coefficients with X, and Hammett coefficients Figure 1 presents the Brönsted plots with X in the nucleophiles.
(ρY(H)) with Y, respectively. The second-order rate constants The substituent effects of X on the reaction rates are not
Table 1: Second-order rate constants (k(H and D) × 103/M–1 s–1) of the reactions of Y-aryl phenyl isothiocyanophosphates with XC6H4NH2(D2) in
MeCN at 55.0 °C.
X \ Y
4-MeO (1a)
4-Me (1b)
H (1c)
3-MeO (1d)
4-Cl (1e)
4-MeO
kH
kD
kH
kD
kH
kD
kH
kD
kH
kD
9.58 ± 0.02a
7.15 ± 0.04
10.2 ± 0.1
12.7 ± 0.1
8.92 ± 0.01
4.38 ± 0.01
3.44 ± 0.02
0.693 ± 0.004
0.647 ± 0.001
0.434 ± 0.001
0.492 ± 0.001
1.48 ± 0.01
2.12 ± 0.01
17.2 ± 0.1
27.5 ± 0.1
18.7 ± 0.1
–
–
4-Me
H
2.71 ± 0.01
3.02 ± 0.01
6.09 ± 0.03
9.51 ± 0.01
7.27 ± 0.05
1.89 ± 0.01
1.67 ± 0.01
1.67 ± 0.01
1.81 ± 0.02
5.62 ± 0.03
8.46 ± 0.04
2.30 ± 0.01
–
–
0.400 ± 0.001
0.384± 0.001
0.206 ± 0.001
0.244 ± 0.001
0.718 ± 0.002
0.994± 0.002
0.517 ± 0.005
1.04 ± 0.01
–
–
4-Cl
3-Cl
0.257 ± 0.001
0.787 ± 0.005
–
–
0.886 ± 0.008
–
2.68 ± 0.01
–
aStandard deviation.
Table 2: Hammett (ρX(H and D))a and Brönsted (βX(H and D))b coefficients with X for the reactions of Y-aryl phenyl isothiocyanophosphates with
XC6H4NH2(D2) in MeCN at 55.0 °C.
X \ Y
4-MeO (1a)
4-Me (1b)
H (1c)
3-MeO (1d)
4-Cl (1e)
4-MeO, 4-Me, H ρX(H)c
−5.09 ± 0.03
−4.69 ± 0.02
1.83 ± 0.08
1.69 ± 0.06
−4.76 ± 0.04
−4.68 ± 0.01
−4.23 ± 0.01
1.69 ± 0.04
1.53 ± 0.04
−4.51 ± 0.01
−4.29 ± 0.02
−3.87 ± 0.02
1.55 ± 0.07
1.40 ± 0.06
ρX(D)c
βX(H)c
βX(D)c
–
–
1.72 ± 0.09
–
1.63 ± 0.04
–
4-Cl, 3-Cl
ρX(H)
ρX(D)
βX(H)
βX(D)
3.87
4.35
3.84
–
3.81
4.53
3.80
–
3.76
4.78
−1.18
−1.33
−1.17
–
−1.16
−1.38
−1.16
–
−1.15
−1.46
aThe σ values were taken from [1]. bThe pKa values of X-anilines in water were taken from [2]. cCorrelation coefficients (r) of ρX and Brönsted βX
values for X = (4-MeO, 4-Me, H) are better than 0.996.
616

Beilstein J. Org. Chem. 2013, 9, 615–620.
Table 3: Hammett coefficients (ρY(H))a with Y for the reactions of Y-aryl phenyl isothiocyanophosphates with XC6H4NH2 in MeCN at 55.0 °C.
X
4-MeO
4-Me
H
4-Cl
3-Cl
ρY(H)b
0.88 ± 0.07
1.08 ± 0.05
1.27 ± 0.07
1.77 ± 0.09
1.75 ± 0.09
aThe σ values were taken from [1]. bCorrelation coefficients (r) of ρY(H) values are better than 0.962.
compatible with a typical nucleophilic substitution reaction. The values indicate that the nucleophilic N atom becomes more
Hammett and Brönsted plots with X are biphasic concave negative in the TS compared to in the ground state (GS). The
upwards with a break region between X = H and 4-Cl (break substituent effects of Y on the reaction rates are consistent with
point of σX ≈ 0.13), giving unusual positive ρX and negative βX a typical nucleophilic substitution reaction, and the rate
values with X = 4-Cl and 3-Cl. Positive ρX (and negative βX) increases with a more electron-withdrawing substituent.
The cross-interaction constant (CIC) is defined based on the
substituent effects of the nucleophiles, substrates and/or leaving
groups on the reaction rates [3-5]. In the present work, the CIC
(ρXY) between substituents X and Y in the nucleophiles and
substrates, respectively, is described in Equation 2 and Equa-
tion 3.
(2)
(3)
Figure 1: Brönsted plots with X [log kH versus pKa(X)] of the reactions
of Y-aryl phenyl isothiocyanophosphates with XC6H4NH2 in MeCN at
55.0 °C.
The two ρXY values are obtained because the Hammett plots
with X are biphasic. Figure 2 shows the plots of ρX versus σY
Figure 2: Plots of ρX versus σY and ρY versus σX of the reactions of Y-aryl phenyl isothiocyanophosphates with XC6H4NH2 in MeCN at 55.0 °C. The
obtained ρXY values by multiple regression are as follows: (a) ρXY = 1.40 ± 0.06 (r = 0.992) with more basic anilines and (b) ρXY = –0.18 ± 0.09
(r = 0.975) with less basic anilines.
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Beilstein J. Org. Chem. 2013, 9, 615–620.
and ρY versus σX to determine the ρXY values, according to
Equation 3. The signs of ρXY are positive with more basic
anilines (X = 4-MeO, 4-Me, H) but negative with less basic
anilines (X = 4-Cl, 3-Cl). The number of kinetic data points,
10 kH values with less basic anilines, to obtain the ρXY value is
not enough to overcome the experimental error. At least,
however, the sign of ρXY is acceptable, and the greater magni-
tude of ρXY value with more basic anilines than that with less
basic anilines is also acceptable. Accordingly, the authors
propose the following reaction mechanism: (i) a stepwise
process with rate-limiting leaving-group departure from the
intermediate for more basic anilines based on the positive sign
of ρXY (= 1.40), and (ii) a stepwise process with rate-limiting
bond formation for less basic anilines based on the negative
sign of ρXY (= −0.18) [3-5]. The greater magnitude of ρXY
value with more basic anilines compared to that with less basic
anilines suggests that the interaction between X and Y with
more basic anilines is larger than that with less basic anilines in
the TS (see below).
Table 4: The DKIEs (kH/kD) of the reactions of Y-aryl phenyl isothio-
cyanophosphates with XC6H4NH(D)2 in MeCN at 55.0 °C.
X \ Y
4-MeO (1a)
H (1c)
4-Cl (1e)
4-MeO
4-Me
H
1.34 ± 0.01a
1.18 ± 0.01
1.04 ± 0.01
1.42 ± 0.01
1.27 ± 0.01
1.07 ± 0.01
1.47 ± 0.01
1.31 ± 0.01
1.13 ± 0.01
4-Cl
3-Cl
0.844 ± 0.005 0.882 ± 0.003 0.922 ± 0.012
0.722 ± 0.002 0.698 ± 0.001 0.664 ± 0.004
aStandard error {= 1/kD[(ΔkH)2 + (kH/kD)2 × (ΔkD)2]1/2} from [16].
Scheme 2: Backside attack involving in-line-type TSb and frontside
attack involving a hydrogen-bonded, four-center-type TSf.
The DKIEs are primary normal with more basic anilines
(X = 4-MeO, 4-Me, H) while secondary inverse with less basic phosphorus with the Cl– leaving group have been extensively
anilines (X = 4-Cl, 3-Cl) as shown in Table 4. Primary normal studied in this lab, and the obtained data of primary normal
DKIEs (kH/kD > 1.0) indicate that partial deprotonation of the DKIEs involving deuterated anilines are rationalized by TSf-
aniline occurs in a rate-limiting step by hydrogen bonding type in which hydrogen bonding of an amine hydrogen atom
[6-12]. In contrast, secondary inverse DKIEs (kH/kD < 1.0) indi- occurs on the departing chloride [17-23]. The authors also
cate that the N–H(D) vibrational frequencies invariably increase suggested TSf-type, in which hydrogen bonding of an amine
upon going to the TS because of an increase in the steric hydrogen atom occurs to the departing phenoxy oxygen atom
congestion in the bond-making process in a stepwise process for the anilinolyses of aryl dimethyl, methyl and diphenyl phos-
with a rate-limiting bond formation (or a normal SN2 reaction) phinates [24]. Thus, at this point, the authors are in favor of TSf
[13-15]. The magnitudes of the kH/kD values invariably for the present work.
decrease as the aniline becomes less basic. The magnitudes of
the kH/kD values invariably increase for X = (4-MeO, 4-Me, H,
4-Cl) while they invariably decrease for X = 3-Cl as the
substituent Y changes from electron-donating to electron-with-
drawing. The maximum value of kH/kD = 1.47 with X = 4-MeO
and Y = 4-Cl indicates extensive hydrogen bonding whereas the
minimum value of kH/kD = 0.66 with X = 3-Cl and Y = 4-Cl
indicates severe steric congestion in the TS, suggesting a great
Scheme 3: Backside attack involving a hydrogen-bonded, four-center-
type TSb-H.
extent of bond formation. The secondary inverse and primary
normal DKIEs are substantiated by backside nucleophilic
attack involving in-line-type TSb and frontside attack
involving hydrogen-bonded, four-center-type TSf, respectively The focus will now shift to the unusual positive ρX and nega-
(Scheme 2).
tive βX values with X = 4-Cl and 3-Cl. These values can be
observed because of (i) desolvation of the GS [25,26] or (ii) TS
The hydrogen-bonded four-center type of TSb-H could be imbalance phenomenon [27-29]. However, in the present work,
another plausible TS structure, in which hydrogen bonding of the positive ρX and negative βX values for less basic anilines are
an amine hydrogen atom occurs on the P=O oxygen atom not ascribed to (i) a desolvation step prior to the rate-limiting
(Scheme 3). In the present work, three possible TSs could nucleophilic attack, because the aniline nucleophile is neutral
substantiate the primary normal DKIEs: (i) TSf, (ii) TSb-H or and the MeCN solvent is dipolar aprotic; and to (ii) a TS imbal-
(iii) both TSb-H and TSf. The anilinolyses of tetracoordinate ance phenomenon, because the leaving group of isothiocyanate
618

Beilstein J. Org. Chem. 2013, 9, 615–620.
is too poor to form an ion-pair type. The unusual positive ρX
(and negative βX) values with X = (4-Cl and 3-Cl) indicate that
the nucleophilic N atom becomes more negative in the TS
compared to in the GS. The authors, thus, propose that the
structure of the TS is similar to what would be if the isothio-
cyante were the nucleophile and aniline were the leaving group,
such that the nucleophilic N atom becomes more negative in the
TS compared to in the GS. In other words, the TS is very tight
and in which the degree of bond formation is very great while
the degree of bond breaking is considerably small, resulting in
the positive ρX. The very small value of kH/kD = 0.66 with
X = 3-Cl and Y = 4-Cl could be supporting evidence. The rela-
tively small magnitude of ρXY = −0.18 with less basic anilines
could be another piece of supporting evidence, because the
normal SN2 mechanism (or stepwise mechanism with rate-
limiting bond formation) gives a magnitude of ρXY ≈ −0.7
[3-5]. This would be attributed to the strong interaction between
the nucleophile (X) and leaving group over the nucleophile (X)
and substrate (Y) (see above).
Table 5: Activation parameters for the reactions of Y-aryl phenyl iso-
thiocyanophosphate with aniline (C6H5NH2) in MeCN.
∆H≠/kcal −∆S≠/cal
Y
t/°C kH × 104/M−1 s−1
mol−1
mol−1 K−1
4-MeO (1a) 45.0
2.91 ± 0.01
4.00 ± 0.01
5.29 ± 0.07
3.72 ± 0.01
5.17 ± 0.05
7.08 ± 0.01
4.96 ± 0.01
6.93 ± 0.01
9.51 ± 0.01
7.24 ± 0.01
10.4 ± 0.1
13.7 ± 0.1
13.5 ± 0.1
18.9 ± 0.1
25.5 ± 0.2
5.7 ± 0.1
56 ± 1
55.0
65.0
4-Me (1b)
H (1c)
45.0
55.0
65.0
45.0
55.0
65.0
6.2 ± 0.1
6.3 ± 0.1
6.2 ± 0.4
6.1 ± 0.1
55 ± 1
54 ± 1
54 ± 1
53 ± 1
3-MeO (1d) 45.0
55.0
65.0
4-Cl (1e)
45.0
55.0
65.0
Finally, the authors propose the following reaction mechanism
of the present work: (i) for more basic anilines (X = 4-MeO, concentrations of substrates and nucleophiles were as follows;
4-Me, H), a stepwise process with rate-limiting leaving-group [substrate] = 5 × 10−3 M and [X-aniline] = 0.10–0.30 M. The
departure from the intermediate, involving a predominant second-order rate constants (kH(D)) were determined for at least
frontside attack with a hydrogen-bonded four-center-type TSf five concentrations of anilines. The kobsd values were the
based on the positive ρXY and primary normal DKIEs; and (ii) average of at least three runs.
for less basic anilines (X = 4-Cl , 3-Cl), a stepwise process with
rate-limiting bond formation, involving a predominant backside Product analysis. Diphenyl isothiocyanophosphate was reacted
attack TSb, and very tight TS, in which the extent of the bond with excess aniline for more than 15 half-lives at 55.0 °C in
formation is great and the degree of bond breaking is very small MeCN. Acetonitrile was evaporated under reduced pressure.
based on the negative ρXY, secondary inverse DKIEs and The product mixture was treated with ether by a work-up
positive ρX.
process with dilute HCl and dried over anhydrous MgSO4. The
product was isolated through column chromatography (30%
Activation parameters, enthalpies and entropies of activation, ethyl acetate/n-hexane) and then dried under reduced pressure.
are determined as shown in Table 5. The enthalpies of acti- The analytical and spectroscopic data of the product gave the
vation are relatively low and entropies of activation are of rela- following results (see also Supporting Information File 1):
tively large negative value. The relatively low value of acti-
vation enthalpy and large negative value of activation entropy [(C6H5O)2P(=O)NHC6H5]. White solid crystal; mp
are typical for the aminolyses of P=O systems, regardless of the 132–133 °C; 1H NMR (400 MHz, MeCN-d3) δ 6.66 (br d,
mechanism, whether stepwise with rate-limiting bond forma- J = 8.8 Hz, 1H, aliphatic), 7.01–7.43 (m, 15H, aromatic);
tion (or a concerted) or stepwise with rate-limiting bond 13C NMR (100 MHz, MeCN-d3) δ 118.59–131.20 (m, 18C;
breaking.
aromatic); 31P NMR (162 MHz, MeCN-d3) δ 3.62 (d,
J = 8.6 Hz, 1P, P=O); GC–MS (EI, m/z): 325 (M+).
Experimental
Materials. HPLC grade acetonitrile (water content is less than
0.005%) was used without further purification. Deuterated
anilines were synthesized as previously described [17-24].
Substrates were prepared as described earlier [30].
Supporting Information
Supporting Information File 1
Spectra of product.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-68-S1.pdf]
Kinetics measurement. Rates were measured conductometri-
cally at 55.0 °C as described previously [17-24]. The initial
619

Beilstein J. Org. Chem. 2013, 9, 615–620.
27.Jencks, W. P. Chem. Rev. 1985, 85, 511–527.
Acknowledgements
This work was supported by the Inha University Research Fund.
doi:10.1021/cr00070a001
28.Bernasconi, C. F. Acc. Chem. Res. 1987, 20, 301–308.
doi:10.1021/ar00140a006
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