Kinetics and Mechanism of the Aminolysis of Aryl Phenyl

Jun 20, 2007 - ... Chloride in Acetonitrile. Keshab Kumar Adhikary , Hai-Whang Lee. Bulletin of the Korean Chemical Society 2011 32 (6), 1945-1950 ...
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Kinetics and Mechanism of the Aminolysis of Aryl Phenyl Chlorothiophosphates with Anilines Md. Ehtesham Ul Hoque, Shuchismita Dey,† Arun Kanti Guha,‡ Chan Kyung Kim, Bon-Su Lee,* and Hai Whang Lee* Department of Chemistry, Inha UniVersity, Incheon 402-751, Korea [email protected]; [email protected] ReceiVed January 28, 2007

Kinetic studies of the reactions of aryl phenyl chlorothiophosphates (1) and aryl 4-chlorophenyl chlorothiophosphates (2) with substituted anilines in acetonitrile at 55.0 °C are reported. The negative values of the cross-interaction constant FXY (FXY ) -0.22 and -0.50 for 1 and 2, respectively) between substituents in the nucleophile (X) and substrate (Y) indicate that the reactions proceed by concerted SN2 mechanism. The primary kinetic isotope effects (kH/kD ) 1.11-1.13 and 1.10-1.46 for 1 and 2, respectively) involving deuterated aniline nucleophiles are obtained. Front- and back-side nucleophilic attack on the substrates is proposed mainly on the basis of the primary kinetic isotope effects. A hydrogenbonded, four-center-type transition state is suggested for a front-side attack, while the trigonal bipyramidal pentacoordinate transition state is suggested for a back-side attack. The MO theoretical calculations of the model reactions of dimethyl chlorothiophosphate (1′) and dimethyl chlorophosphate (3′) with ammonia are carried out. Considering the specific solvation effect, the front-side nucleophilic attack can occur competitively with the back-side attack in the reaction of 1′.

Introduction Organothiophosphate compounds are useful as agricultural chemicals such as insectides, herbicides, acaricides, fungicides, and plant growth regulators.1 Hence, the chemistry of organothiophosphate compounds is important to a broad range of interests. A considerable amount of work has been completed † Present address: Department of Textile Engineering, Southeast University, Banani, Dhaka-1213, Bangladesh. ‡ Present address: Department of Textile Engineering, City University, Banani, Dhaka-1213, Bangladesh.

(1) (a) Nelson, R. C. J.-Assoc. Off. Anal. Chem. 1967, 50, 922. (b) Stewart, J. P. Proc. Pap. Annu. Conf. Calif. Mosq. Control Assoc. 1975, 43, 37. (c) Greenhalgh, R.; Dhawson, K. L.; Weinberg, P. J. Agric. Food Chem. 1980, 28, 102. (d) Engel, R., Ed. Handbook of Organophosphorus Chemistry; Marcel Dekker: New York, 1992; p 465. (e) Kamiya, M.; Nakamura, K.; Sasaki, C. Chemosphere 1995, 30, 653. (f) Vale, J. A. Toxicol. Lett. 1998, 102-103, 649. (g) Quin, L. D.; Quin, G. S. A Guide to Organophosphorus Chemistry; Wiley: New York, 2000; Chapter 11. (h) Vayron, P.; Rebard, P.-Y.; Taran, F.; Cre´minon, C.; Frobert, Y.; Grassi, J.; Mioskowski, C. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 7058. (i) Um, I. H.; Jeon, S. E.; Baek, M. H.; Park, H. R. Chem. Commun. 2003, 24, 3016.

on phosphoryl transfer reactions;1i,2 however, much less is known about thiophosphoryl transfer reactions. Because predicting the effects of replacing the oxygen atom in the phosphoryl group with sulfur is important, we investigated the kinetics and mechanism of thiophosphoryl transfer reactions from aryl phenyl (j) Lambert, W. E.; Lasarev, M.; Muniz, J.; Scherer, J.; Rothlein, J.; Santana, J.; McCauley, L. EnViron. Health Perspect. 2005, 113, 504. (k) Seger, M. R.; Maciel, G. E. EnViron. Sci. Technol. 2006, 40, 797. (l) Kabra, V.; Ojha, S.; Kaushik, P.; Meel, A. Phosphorus, Sulfur Silicon Relat. Elem. 2006, 181, 2337. (2) (a) Westheimer, F. H. Science 1987, 235, 1173. (b) Catrina, I. E.; Hengge, A. C. J. Am. Chem. Soc. 1999, 121, 2156. (c) Mol, C. D.; Izumi, T.; Mitra, S.; Tainer, J. A. Nature 2000, 403, 451. (d) Holtz, K. M.; Catrina, I. E.; Hengge, A. C.; Kantrowitz, E. R. Biochemistry 2000, 39, 9451. (e) Lahiri, S. D.; Zhang, G.; Dunaway-Mariano, D.; Allen, K. N. Science 2003, 299, 2067. (f) Catrina, I. E.; Hengge, A. C. J. Am. Chem. Soc. 2003, 125, 7546. (g) Hengge, A. C.; Onyido, I. Curr. Org. Chem. 2005, 9, 61. (h) Onyido, I.; Swierczek, K.; Purcell, J.; Hengge, A. C. J. Am. Chem. Soc. 2005, 127, 7703. (i) Um, I. H.; Shin, Y. H.; Han, J. Y.; Mishima, M. J. Org. Chem. 2006, 71, 7715. (j) Kumara Swamy, K. C.; Satish Kumar, N. Acc. Chem. Res. 2006, 39, 324. (k) van Bochove, M. A.; Swart, M.; Bickelhaupt, F. M. J. Am. Chem. Soc. 2006, 128, 10738.

10.1021/jo0700934 CCC: $37.00 © 2007 American Chemical Society

Published on Web 06/20/2007

J. Org. Chem. 2007, 72, 5493-5499

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Hoque et al. TABLE 1. Second-Order Rate Constants (k2 × 104/M-1 s-1) and Selectivity Parametersa of the Aminolysis of Y-Aryl Phenyl Chlorothiophosphates (1) with X-Anilines in Acetonitrile at 55.0 °C X\Y

4-OMe

4-Me

H

3-Cl

4-CN

FYd

4-OMe 4-Me H 4-Cl 3-Cl -FXb βXc

9.20 2.10 0.802 0.0790 0.0289 3.81 1.34

11.0 2.20 0.873 0.0861 0.0309 3.85 1.35

12.6 2.50 1.01 0.0951 0.0343 3.88 1.36

26.2 6.81 1.80 0.198 0.0670 3.98 1.40

43.1 12.6 3.70 0.314 0.117 4.01 1.41

0.73 0.89 0.70 0.67 0.67 FXYe ) -0.22

a σ values were taken from Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. The pK values were taken from Streitwieser, A., Jr.; Heathcock, a C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillan: New York, 1996; p 693. b Correlation coefficients, r, were better than 0.993. c r g 0.990. d r g 0.984. e r ) 0.990.

chlorothiophosphates (1) and 4-chlorophenyl aryl chlorothiophosphates (2) as the PdS counterparts of PdO substrates. Two main types of displacement processes are well known in neutral phosphoryl and thiophosphoryl group transfer reactions: the stepwise mechanism involving a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate2k,3 and the concerted displacement at phosphorus through a single pentacoordinate transition state (TS).2h,i,4,5 Another additional type is given by the reaction of chlorophosphates with strong organic bases.6 The base itself acts as a nucleophilic catalyst, and the leaving group is substituted by base leading to a very reactive zwitterionic intermediate. The nucleophile then reacts with this intermediate, giving the product. In previous work, we reported several phosphoryl transfer reactions.7 Anilinolysis of aryl phenyl chlorophosphates (3)7a and 4-chlorophenyl aryl chlorophosphates (4)7c in acetonitrile at 55.0 °C was proceeded by a concerted mechanism with the nucleophile (aniline) and leaving group (Cl) occupying apical sites in the TS. For both reactions, the cross-interaction constants,8 FXY in eqs 1a and 1b where X and Y are substituents (3) (a) Cook, R. D.; Rahhal-Arabi, L. Tetrahedron Lett. 1985, 26, 3147. (b) Cook, R. D.; Daouk, W. A.; Hajj, A. N.; Kabbani, A.; Kurku, A.; Samaha, M.; Shayban, F.; Tanielian, O. V. Can. J. Chem. 1986, 64, 213. (c) Friedman, J. M.; Freeman, S.; Knowles, J. R. J. Am. Chem. Soc. 1988, 110, 1268. (d) Hoff, R. H.; Hengge, A. C. J. Org. Chem. 1998, 63, 6680. (e) Admiraal, S. J.; Herschlag, D. J. Am. Chem. Soc. 2000, 122, 2145. (f) Harger, M. J. P. J. Chem. Soc., Perkin Trans. 2 2002, 489. (g) Harger, M. J. P. Chem. Commun. 2005, 22, 2863. (h) Hengge, A. C. AdV. Phys. Org. Chem. 2005, 40, 49. (i) Sorensen-Stowell, K.; Hengge, A. C. J. Org. Chem. 2006, 71, 7180. (4) PdO system: (a) Reimschu¨ssel, W.; Mikolajczyk, M.; SlebockaTilk, H.; Gajl, M. Int. J. Chem. Kinet. 1980, 12, 979. (b) Istomin, B. I.; Eliseeva, G. D. J. Gen. Chem. USSR (Engl. Transl.) 1981, 51, 2063. (c) Bourne, N.; Chrystiuk, E.; Davis, A. M.; Williams, A. J. Am. Chem. Soc. 1988, 110, 1890. (d) Humphry, T.; Forconi, M.; Williams, N. H.; Hengge, A. C. J. Am. Chem. Soc. 2004, 126, 11864. (5) PdS system: (a) Eliseeva, G. D.; Istomin, B. I.; Kalabina, A. V. J. Gen. Chem. USSR (Engl. Transl.) 1979, 49, 1912. (b) Istomin, B. I.; Voronkov, M. G.; Zhdankovich, E. L.; Bazhenov, B. N. Dokl. Phys. Chem. (Engl. Transl.) 1981, 258, 659. (c) Omakor, J. E.; Onyido, I.; vanLoon, G. W.; Buncel, E. J. Chem. Soc., Perkin Trans. 2 2001, 324. (6) (a) Pavan Kumar, K. V. P.; Praveen Kumar, K.; Vijjulatha, M.; Kumara Swamy, K. C. J. Chem. Sci. 2004, 116, 311. (b) Vijjulatha, M.; Praveen Kumar, K.; Kumara Swamy, K. C.; Vittal, J. J. Tetrahedron Lett. 1998, 39, 1819. (c) Silverberg, L. J.; Dillon, J. L.; Purushotham, V. Tetrahedron Lett. 1996, 37, 771. (d) Corriu, R. J. P.; Lanneau, G. F.; Leclercq, D. Tetrahedron 1989, 45, 1959. (e) Corriu, R. J. P.; Lanneau, G. F.; Leclercq, D. Tetrahedron 1986, 42, 5591. (7) (a) Guha, A. K.; Lee, H. W.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1999, 765. (b) Guha, A. K.; Lee, H. W.; Lee, I. J. Org. Chem. 2000, 65, 12. (c) Lee, H. W.; Guha, A. K.; Lee, I. Int. J. Chem. Kinet. 2002, 34, 632. (d) Lee, H. W.; Guha, A. K.; Kim, C. K.; Lee, I. J. Org. Chem. 2002, 67, 2215. (e) Adhikary, K. K.; Lee, H. W.; Lee, I. Bull. Korean Chem. Soc. 2003, 24, 1135. (8) (a) Lee, I. Chem. Soc. ReV. 1990, 19, 317. (b) Lee, I. AdV. Phys. Org. Chem. 1992, 27, 57. (c) Lee, I.; Lee, H. W. Collect. Czech. Chem. Commun. 1999, 64, 1529.

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in the nucleophiles and substrates, respectively, have negative values (FXY values are -1.31 and -0.31 for 3 and 4, respectively), in agreement with our proposed mechanistic criteria for the sign of FXY.

log(kXY/kHH) ) FXσX + FYσY + FXYσXσY

(1a)

FXY ) ∂FX/∂σY ) ∂FY/∂σX

(1b)

To further our understanding of the mechanism of thiophosphoryl transfer, as well as to compare the reactivity when the oxygen atom in the phosphoryl group is replaced with sulfur, here we examine the aminolyses of aryl phenyl chlorothiophosphates (1) and 4-chlorophenyl aryl chlorothiophosphates (2) with anilines in acetonitrile at 55.0 °C as PdS counterparts of PdO, 3 and 4.

Results and Discussion The pseudo-first-order rate constants observed (kobsd) for all reactions obeyed eq 3 with negligible k0 (≈0) in acetonitrile. The clean second-order rate constants, k2, obtained as the slope of the plot of kobsd against aniline concentration are summarized in Tables 1 and 2.

kobsd ) k0 + k2[An]

(3)

PdS substrates (1, 2) are less reactive than their PdO counterparts (3,7a 47c) by ca. 1 order of magnitude. This order of reactivity has been ascribed to the so-called “thio effect”.2d,h,5c,9 The rates are faster with a stronger nucleophile (δσX < 0) and a stronger electron acceptor substituent in the substrate (δσY > 0) which are compatible with typical nucleophilic substitution reactions where the reaction center, P, of the substrate becomes more negatively charged in the TS. The rates of 2 are faster (1.3-3.6 times) than those for the corresponding reactions of 1 (9) (a) Oivanen, M.; Ora, M.; Lonnberg, H. Collect. Czech. Chem. Commun. 1996, 61, S-1. (b) Hondal, R. J.; Bruzik, K. S.; Zhao, Z.; Tsai, M. D. J. Am. Chem. Soc. 1997, 119, 5477. (c) Gregersen, B. A.; Lopez, X.; York, D. M. J. Am. Chem. Soc. 2003, 125, 7178. (d) Liu, Y.; Gregersen, B. A.; Hengge, A. C.; York, D. M. Biochemistry 2006, 45, 10043.

Aminolysis of Aryl Phenyl Chlorothiophosphates TABLE 2. Second-Order Rate Constants (k2 × 104/M-1 s-1) and Selectivity Parametersa of the Aminolysis of Y-Aryl 4-Chlorophenyl Chlorothiophosphates (2) with X-Anilines in Acetonitrile at 55.0 °C

a

X\Y

4-OMe

4-Me

H

4-Cl

4-CN

FYd

4-OMe 4-Me H 4-Cl 3-Cl -FXb βXc

13.2 4.00 1.39 0.222 0.0628 3.49 1.23

14.6 4.40 1.40 0.234 0.0657 3.53 1.24

16.2 6.11 1.48 0.274 0.0659 3.63 1.28

41.8 13.4 3.00 0.706 0.0954 3.86 1.36

87.6 23.4 6.80 1.11 0.192 3.92 1.38

0.94 0.88 0.79 0.83 0.54 e FXY ) -0.50

Same as in Table 1. b r g 0.992. c r g 0.994. d r g 0.965. e r ) 0.990.

TABLE 3. Second-Order Rate Constants (k2 × 104/M-1 s-1) and Kinetic Isotope Effects, kH/kD, for the Reactions of 1 and 2 with Deuterated X-Anilines in Acetonitrile at 55.0 °C substrate

X

Y

kH

kD

kH/kD

1

4-OMe 4-OMe H H H 4-Cl 4-Cl 4-MeO 4-MeO H H H H 4-Cl 4-Cl 4-Cl

4-OMe H 4-OMe H 4-CN H 4-CN H 4-CN 4-MeO H 4-Cl 4-CN 4-OMe H 4-CN

9.20 12.6 0.0802 1.10 3.70 0.0951 0.314 16.2 87.6 1.39 1.48 3.00 6.80 0.222 0.274 1.11

7.80 9.90 0.0603 0.839 2.80 0.0853 0.276 15.2 82.1 1.27 1.39 2.79 6.00 0.170 0.206 0.759

1.18 1.27 1.33 1.20 1.32 1.11 1.14 1.10 1.10 1.10 1.10 1.10 1.13 1.31 1.33 1.46

2

because of the electron-withdrawing effect of the electronwithdrawing ligand (4-ClC6H4O) in 2 which implies the importance of bond making in the rate-determining step. In our previous work7a,c on the anilinolyses of 3 and 4 in acetonitrile, we proposed a concerted mechanism with a late, product-like TS in which both bond making and leaving group departure are extensive. In contrast to the back-side nucleophilic attack on the substrates in the anilinolyses of 3 and 4, in the present work, a front-side attack concerted mechanism through a hydrogen-bonded four-center-type TS is proposed on the basis of the following grounds: (1) The kinetic isotope effects (KIEs), kH/kD, involving deuterated aniline nucleophiles (XC6H4ND2) are determined as shown in Table 3. The kH/kD values are all greater than unity, kH/kD > 1, indicating that the rate-determining step is not a simple bond-making process. In such cases, an inverse secondary KIE, kH/kD < 1, would be expected because of an increase in the N-H vibrational frequency as a result of steric congestion of the N-H moiety in the bond-making step,10 as observed in the anilinolyses of 3 (kH/kD ) 0.61-0.87) and 4 (kH/kD ) 0.640.87). The kH/kD values (kH/kD > 1) in Table 3 suggest that partial deprotonation of the aniline occurs in the rate-determining step by hydrogen bonding.10 One plausible TS structure is shown in 5. Comparing the nonbonding orbital (NBO) charges of the phenoxy oxygens in 1 (-0.817, -0.824) and 3 (-0.806, -0.814) as shown in Figure 1, we find that the magnitudes of the phenoxy oxygen charges are similar and there is no possibility of a hydrogen bond between the phenoxy oxygen (10) Lee, I. Chem. Soc. ReV. 1995, 24, 223. (11) Hehre, W. J.; Random, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986; Chapter 4.

and N-H(D) moiety. This result is supported by the primary KIE (kH/kD ) 1.79) involving deuterated aniline nucleophile with diphenyl chlorophosphinate ((C6H5)2P(dO)Cl; 6) in acetonitrile at 55.0 °C: kH ) 1.73 × 10-3 M-1 s-1(approximately two times faster than 3) and kD ) 9.69 × 10-4 M-1 s-1. The kH/kD value for 6, kH/kD ) 1.79 (greater than unity), despite the absence of the phenoxy oxygen, strongly suggests that there is no hydrogen bonding between the phenoxy oxygen and hydrogen in N-H(D). Comparing again the charge of S (-0.450) in PdS (1) and that of O (-1.021) in PdO (3), we find that the hydrogen bonding between S and N-H(D) moiety is also not possible.

(2) The kH/kD value, which is greater than unity in the present work, could be attributed to the base-catalyzed reaction in which the second aniline acts as a base as shown in 7. If so, the reaction rate constant should be third-order. However, no third-order or higher-order terms were detected, and no complications were found in the determination of kobsd or in the linear plots of eq 2 with a negligible intercept (k0 = 0.0) in the present studies. This suggests that there is no base catalysis and the TS structure 7 can be neglected.

(3) The cross-interaction constants, FXY ) δFX/δσY ) δFY/ δσX, of 1 and 2 are -0.22 and -0.50, respectively. Figure 2 shows the good linearities (correlation coefficient r ) 0.990 for both reactions), which indicate that third-order (σX2σY, σXσY2) or self second-order (σX2, σY2) term interactions between the substituents of the substrate and/or nucleophiles are not involved; rather, only the second-order term (σXσY) interactions, represented in eq 1, between the substituents of the substrate and nucleophile are involved. The sequence of the FXY value magnitudes for 1 and 2 is reversed compared to that of 3 and 4: |FXY(1)| ) 0.22 < |FXY(2)| ) 0.50, |FXY(3)| ) J. Org. Chem, Vol. 72, No. 15, 2007 5495

Hoque et al.

FIGURE 1. B3LYP/6-311+G(d,p)11 geometries and NBO charges of 1 and 3 with Y ) H.

FIGURE 2. Plots of FX vs σY and FY vs σX for the reactions of substrates 1 and 2 with X-anilines in acetonitrile at 55.0 °C. TABLE 4. Selectivity Parameters and Kinetic Isotope Effects of the Anilinolyses of Substrates 1, 2, 3, and 4 in Acetonitrile at 55.0 °C substrate

-FX

βX

FY

-FXY

kH/kD

reference

1 2 3 4

3.8-4.0 3.5-3.9 3.4-4.6 4.0-4.2

1.3-1.4 1.2-1.4 1.2-1.7 1.4-1.5

0.7-0.9 0.5-0.9 0.2-0.9 0.1-0.3

0.22 0.50 1.31 0.31

1.1-1.3 1.1-1.5 0.6-0.9 0.6-0.9

this work this work 7a 7c

1.31 > |FXY(4)| ) 0.31 (Table 4). The smaller magnitude of |FXY(4)| compared to that of |FXY(3)| indicated partial electron loss, or shunt, toward the electron acceptor equatorial ligand (4-ClC6H4O) in the TBP-5C TS.7a,c The smaller magnitude of FY(4) ) 0.1-0.3 compared to FY(3) ) 0.2-0.9 was also explained in the same way (i.e., electron shunt; Table 4).7c If there is no partial electron shunt due to the electron acceptor ligand (4-ClC6H4O) and only the electron-withdrawing effect of the ligand is considered, the larger magnitude of |FXY(2)| ) 0.50 compared to that of |FXY(1)| ) 0.22 is taken for granted, as the cross-interaction between the substituents of 2 is stronger than that of 1. The different sequence of the FXY value magnitudes between the PdS (1, 2) and PdO (3, 4) systems strongly suggests that the TS structure of the PdS system differs from that of the PdO system. (4) The FX and βX values of the PdS (1, 2) and PdO (3, 4) systems have similar magnitudes, and the FY values of 1, 2, and 3 also have comparable magnitudes (Table 4). The sign of FXY values obtained in the present work is negative, as it is in PdO systems. The negative FXY value is characteristic of concerted nucleophilic substitution reactions, while the positive 5496 J. Org. Chem., Vol. 72, No. 15, 2007

FXY value is obtained in stepwise mechanisms with rate-limiting breakdown of the intermediate.8,12 The distinction between Pd S and PdO systems is in the kinetic isotope effects, that is, inverse secondary KIEs for PdO systems, kH/kD < 1, and primary normal KIEs for PdS systems, kH/kD > 1 (Table 4). The most plausible TS structure in the present work is the hydrogen-bonded, four-center-type with a front-side nucleophile attack; hydrogen bonding between the hydrogen atom of the N-H(D) moiety and the negative charge developed the Cl leaving group, as shown in 8 over the four-membered ring strain. We have previously studied7d the reactions of Z-aryl bis(4methoxyphenyl) phosphates ((4-MeOC6H4O)2P(dO)OC6H4Z) with pyridines (XC5H4N) in acetonitrile. In the case of more basic phenolate groups (Z ) 4-Cl-3-CN) and less basic pyridines (3-Cl-4-CN), we proposed the concerted mechanism with a front-side nucleophile attack on the basis of the FX, βX, βZ, and especially large negative FXZ () -1.98) values. (12) (a) Lee, I.; Shim, C. S.; Chung, S. Y.; Lee, H. W. J. Chem. Soc., Perkin Trans. 2 1988, 975. (b) Lee, I.; Kim, I. C. Bull. Korean Chem. Soc. 1988, 9, 133. (c) Lee, I.; Shim, C. S.; Chung, S. Y.; Kim, H. Y.; Lee, H. W. J. Chem. Soc., Perkin Trans. 2 1988, 1919. (d) Lee, I.; Shim, C. S.; Lee, H. W. J. Phys. Org. Chem. 1989, 2, 484. (e) Lee, I.; Hong, S. W.; Park, J. H. Bull. Korean Chem. Soc. 1989, 10, 459. (f) Lee, I.; Shim, C. S.; Lee, H. W. J. Chem. Res. 1992, (S) 90-91, (M) 0769-0793. (g) Lee, B. C.; Yoon, J. H.; Lee, C. G.; Lee, I. J. Phys. Org. Chem. 1994, 7, 273. (h) Oh, H. K.; Shin, C. H.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1995, 1169. (i) Koh, H. J.; Lee, H. C.; Lee, H. W.; Lee, I. Bull. Korean Chem. Soc. 1995, 16, 839. (j) Koh, H. J.; Lee, O. S.; Lee, H. W.; Lee, I. J. Phys. Org. Chem. 1997, 10, 725. (k) Koh, H. J.; Han, K. L.; Lee, H. W.; Lee, I. J. Org. Chem. 2000, 65, 4706. (l) Lee, K. S.; Adhikary, K. K.; Lee, H. W.; Lee, B. S.; Lee, I. Org. Biomol. Chem. 2003, 1, 1989. (m) Lee, I.; Lee, H. W.; Yu, Y. K. Bull. Korean Chem. Soc. 2003, 24, 993.

Aminolysis of Aryl Phenyl Chlorothiophosphates

In the anilinolysis of 1-phenyl ethyl benzenesulfonates (1PEB),13 we proposed a hydrogen-bonded four-center TS with a front-side nucleophile attack on the basis of the primary KIEs with deuterated aniline nucleophile, kH/kD ) 1.70-2.35, in acetonitrile at 30 °C. We also carried out optical rotation measurements and found that ca. 55% of the original reactant (1-PEB) stereochemistry was retained in the product because of the front-side attack and that ca. 45% of the remaining reactant was inverted because of the back-side attack.14 In the present work, we suggest that the nucleophile attacks the substrate from both directions, front- and back-side, the same as in the anilinolysis of 1-PEB.13 The observed primary KIE seems relatively small because of (a) inverse secondary KIE due to the partial participation of the back-side attack and (b) heavy atom (N, Cl) participation in the reaction coordinate motion. We also propose a four-center TS with a front- and back-side attack for the anilinolysis of diphenyl chlorophosphinate (6) on the basis of the primary normal KIE, kH/kD ) 1.79.15 (5) To examine the activation energy barriers for front-side and back-side nucleophilic attack of the substrate, MO theoretical calculations are carried out at the MP2/6-31+G(d) level16 of theory using the self-consistent reaction field (SCRF) methodology of the CPCM17 polarizable conductor calculation model in acetonitrile and in aqueous solution. All the trends in acetonitrile medium are very similar to those in aqueous solution (i.e., the TS structures and the relative energetics calculated by the CPCM method are very similar in both acetonitrile and aqueous solution). For example, bond lengths for bond making of the nucleophile and breaking of the leaving group at TSs are nearly the same within 0.01 Å, and the relative energetics also (13) (a) Lee, I.; Kim, H. Y.; Kang, H. K.; Lee, H. W. J. Org. Chem. 1988, 53, 2678. (b) Lee, I.; Koh, H. J.; Lee, B. S.; Lee, H. W. J. Chem. Soc., Chem. Commun. 1990, 335. (14) Lee, I.; Shim, C. S.; Lee, H. W.; Lee, B. S. Bull. Korean Chem. Soc. 1991, 12, 255. (15) One of the reviewers indicated the possible TS of 6 with hydrogen bonding involving the strong acceptor PdO. The proposed TS cannot be completely excluded, and further work on phosphinates will clarify the TS structure.

(16) (a) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (b) Head-Gordon, M.; Pople, J. A. J. Chem. Phys. 1988, 89, 5777. (c) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 275. (d) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 281. (e) Head-Gordon, M.; Head-Gordon, T. Chem. Phys. Lett. 1994, 220, 122. (17) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669.

show little difference within 1 kcal mol-1, respectively, in acetonitrile and in aqueous solution. However, since the experimental works are carried out in acetonitrile, we have discussed the theoretical results obtained in aceonitrile, and the results in aqueous solution are summarized in Supporting Information. In this calculation, substitution reactions of methyl chlorothiophosphate (1′) and methyl cholorophosphate (3′), in which two phenyl groups in 1 and 3 are replaced by methyl groups, with ammonia have been selected as model systems. The optimized TS structures and calculated activation energy barriers (∆Eq) are summarized in Figure 3 and Table 5, respectively. Examination of Table 5 shows that the ∆Eq for the back-side attack is more favorable by 0.98 kcal mol-1 for 3′ than for 1′. This fits well with the experimental results discussed earlier (i.e., PdS substrates are less reactive than their PdO analogues). On the other hand, the ∆Eq for the front-side attack is more unfavorable by 1.59 kcal mol-1 for 3′ than that for 1′. This strongly implies that the front-side attack mechanism could occur competitively with the back-side attack mechanism in the reaction of 1′ compared to that of 3′. Nevertheless, as seen in Table 5, the ∆Eq for the back-side attack of 1′ is lower than that for the front-side attack by 3.57 kcal mol-1; hence, the contribution of the front-side attack mechanism could be small. This seems inconsistent with the current proposed mechanism. However, the SCRF method employed in this theoretical approach considers only a bulk solvation effect without a specific solvation effect. Therefore, if specific solvation effects are included, the difference in ∆Eq between the back- and front-side attacks could be decreased or reversed. The TS structure for the front-side attack is a much more product-like, that is, charge-separated, late TS relative compared to that for the back-side attack (Figure 3), that is, the bond making and breaking of P-N and P-Cl are advanced by 0.462 [2.360(a) - 1.898(b)] and 0.274 [2.440(b) - 2.166(a)] Å, respectively, for the front-side attack TS structure compared to the back-side attack. As a result, in the front-side attack TS, the charge densities calculated by the natural population analysis (NPA)18 are more negative for Cl fragments by -0.195e and more positive for NH3 fragments by +0.239e, corresponding to a leaving group and a nucleophile, respectively, than in the back-side attack TS. This indicates that the degree of stabilization by specific solvation effects could be larger in the TS structure for the front-side attack. Indeed, when one acetonitrile molecule is included to examine the specific solvation effect, the difference between the activation energy barriers for the back-side and the front-side attack (δ∆Eq) is decreased (i.e., the δ∆Eq is decreased as 1.63 kcal mol-1 when the specific solvation effect of one acetonitrile molecule is considered).19 Therefore, a front-side attack concerted mechanism is a plausible prediction in the reactions of 1 and 2, if the specific solvation effects are fully considered. On the other hand, reaction intermediates cannot locate as stable species on the hypothetical potential energy surface. Therefore, the reactions are expected to occur via the concerted mechanism; this is consistent with the result obtained from the sign of FXY values (vide supra). (18) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (b) Carpenter, J. E.; Weinhold, F. J. Mol. Struct.: THEOCHEM 1988, 169, 41. (c) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19, 593. (d) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19, 610. (e) Glendening, E. D.; Badenhoop, J. K.; Weinhold, F. J. Comput. Chem. 1998, 19, 628. (19) The specific solvation effects are calculated by a simple supermolecular approach adding one acetonitrile molecule at the fixed TS geometries.

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Hoque et al.

FIGURE 3. TS structures optimized at the CPCM-MP2/6-31+G(d) level of theory for the reactions of (a) 1′ (back-side attack), (b) 1′ (front-side attack), (c) 3′ (back-side attack), and (d) 3′ (front-side attack) with ammonia in acetonitrile.

TABLE 5. Calculated Activation Energy Barriers (∆Eq in kcal

mol-1) at the CPCM-MP2/6-31+G(d) Level for the Reactions of 1′ and 3′ with Ammonia in Acetonitrile ∆Eq (back side)

∆Eq (front side)

δ∆Eq a

6.11 5.13

9.68 11.27

3.57 6.14

1′ 3′ a

δ∆Eq

)

∆Eq

(front side) -

∆Eq

(back side).

Summary The aminolyses of aryl phenyl chlorothiophosphates (1) and aryl 4-chlorophenyl chlorothiophosphates (2) with X-anilines in acetonitrile at 55.0 °C are investigated. The cross-interaction constants, FXY, are negative for both 1 and 2. The obtained kinetic isotope effects (kH/kD) involving deuterated aniline (XC6H4ND2) nucleophiles are greater than unity, kH/kD > 1, suggesting that the rate-determining step involves partial deprotonation of the aniline by hydrogen bonding. This is consistent with a front-side attack concerted mechanism through a hydrogen-bonded four-center-type transition state. On the basis of the cross-interaction constant and primary kinetic isotope effects, we propose a concerted SN2 mechanism with front- and back-side nucleophilic attack on substrate. A hydrogen-bonded, four-center TS is suggested for a front-side attack, while the TBP-5C TS is suggested for a back-side attack. The MO theoretical calculations of the model reactions of 1′ and 3′ with ammonia nucleophile are carried out. The charge densities calculated by the NPA18 are more negative and more positive for the leaving group (Cl) and the nucleophile, respectively, in the front-side attack TS than in the back-side attack TS, indicating that the degree of stabilization by specific solvation effects could be larger in the front-side attack TS. This is supported by the results of NBO analysis at the B3LYP/ 5498 J. Org. Chem., Vol. 72, No. 15, 2007

6-311+G(d,p) level and the calculated activation energy barriers (∆Eq) at the CPCM-MP2/6-31+G(d) level of theory. Experimental Section Materials. Diphenylphosphinic chloride, 98% (substrate), and HPLC-grade acetonitrile were used for kinetic studies without further purification. Anilines were redistilled or recrystallized before use as previously described.20 Deuterated anilines were prepared by heating anilines with D2O at 85 °C for 72 h, and after numerous attempts, anilines were deuterated more than 98%, as confirmed by 1H NMR. The substrates were prepared by the two-step synthesis.7a The starting materials, thiophosphoryl chloride, phenol, 4-chlorophenol, and triethylamine were G.R. grade and were used without further purification. Kinetic Procedure. Rates were measured conductometrically in acetonitrile at 55.0 °C. A self-made computer connected automatic A/D converter conductivity bridge was used in this work. Pseudofirst-order rate constants, kobsd, were determined as previously described7 with a large excess of anilines: [substrate] ) 3 × 10-3 M; [aniline] ) 0.1-0.5 M for 1, and [aniline] ) 0.1-0.3 M for 2. The second-order rate constants, k2, were also obtained as previously described.7 Product analysis is described in the Supporting Information.

Acknowledgment. This work was supported by a grant from KOSEF of Korea (R01-2004-000-10279-0). Supporting Information Available: Synthetic procedures, product analysis, and analytical and spectroscopic data for all compounds. The theoretical calculations containing the Cartesian coordinates and absolute energies of optimized structures of the compounds: (A) Reactant: aryl phenyl chlorothiophosphates (1) (20) Lee, I.; Lee, H. W.; Sohn, S. C.; Kim, C. H. Tetrahedron 1985, 41, 2635.

Aminolysis of Aryl Phenyl Chlorothiophosphates and aryl phenyl chlorophosphates (3); (B) modeling calculation (TS structures in acetonitrile): (i) PdS back-side attacking TS for dimethyl chlorothiophosphate (1′), (ii) PdS front-side attacking TS for dimethyl chlorothiophosphate (1′), (iii) PdO back-side attacking TS for dimethyl chlorophosphate (3′), (iv) PdO front-side attacking TS for dimethyl chlorophosphate (3′) with NH3, (v) PdS backside attacking TS for dimethyl chlorothiophosphate (1′) with one acetonitrile molecule, (vi) PdS front-side attacking TS for dimethyl chlorothiophosphate (1′) with one acetonitrile molecule; and (C) modeling calculation (TS structures in aqueous solution): (i) PdS back-side attacking TS for dimethyl chlorothiophosphate (1′), (ii)

PdS front-side attacking TS for dimethyl chlorothiophosphate (1′), (iii) PdO back-side attacking TS for dimethyl chlorophosphate (3′), (iv) PdO front-side attacking TS for dimethyl chlorophosphate (3′) with NH3, (v) PdS back-side attacking TS for dimethyl chlorothiophosphate (1′) with one water molecule, (vi) PdS front-side attacking TS for dimethyl chlorothiophosphate (1′) with one water molecule. This material is available free of charge via the Internet at http://pubs.acs.org. JO0700934

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