Indian Journal of Chemistry VoL 41A. April 2002, pp. 718-722

Studies on reactions of organic bases and nucleophiles with bis-(p-nitrophenyl ) methylphosphonothionate

Sherali Saxena & Purnanancl *

De/"ence Research and Deve lopment Establi shment , Gwalior 474 002, India

Received II April 200 I: revised 16 Jlllv 200 I

Bi s-(p-nitrophenyl) meth ylphosphonoth ionate (b-p-NPM PT) ( I ) reucts with umines in aqueous medium via general base catalysis o /" water attack. The rate is first order each in rsubstrate] and la mine]. Sol vent deuterium isotope effec t, k2(H20) I /;2(D20). is greater than 2 for amine reacti ons. Different classes of amines foll ow Bronsted relation wi th a slope of DAD. Fluoride and phosphate ions react w ith (I ) by nucleophili c mechani sm. At higher fluori de ion concentration, 2 mol of p-nitrophenol (p-N P) is released by a first order process. At lower Iluoride ion concentration less than 2 III 0 I of p -N P is

produced. /; 2( H20 )lk2(D10) for the reaction of fluoride and phosphate ions with b-p-N PMPT= I . It is believed that steric crowding in the transition st:.1le causes reaction of (I) wi th amine to proceed via general base mechani sm and tha t of Iluoride ion, oxyanions and phosphate anions through a nucleophili c mechani sm.

Bis-(p-nitrophenyl ) methylphosphonothionate is an important intermediate for the preparation of mixed esters by reac tion with alcohols in the presence of base via transesteri fication I. The genera] base and nucleophilic catalysis in displacement reactions at the tetrahedral organophosphorus center along with other determining factors have been described in the literature2

-5

. Reactions of neutral esters have also been shown to proceed by both path ways. Herein we report the reaction of nitrogen bases with bis-(p-nitrophenyl) methylphosphonothionate, which occurs by general base cata lys is of water attack, and that the reaction of flu oride ion and phosphate ion takes place through a nucleophilic mechani sm.

Materials and Methods Bi s- (p-nitrophenyl ) methylphosphonothionate (b­

p-NPMPT) was sy nthesized by the method reported in the literature6

; (b-p-NPMPT), mp=136°C; IH NMR (C DCI}) 8 2.35 (d, 3H); 7.35 (d, 2H); 8.3 (d, 2H). Liquid amines were distilled over potass ium hyd rox ide at atmospheric pressure. Deuterium ox ide (A ldrich) was of 98% purity. Acetonitr il e was disti lied and dried before use. Phosphate buffer was prepared according to the literature procedures7

. g For amines, parti all y neutrali zed so lutions of the nitrogen bases were used as buffers. Standardized hydrogen chl oride solutions were used to neutrali ze amines. Fraction of free base present in so lution is dependent on their pH values. pKa values are at observed pH va lues for amines in so lution as 50% acidic and 50%

basic forms, estab lished by quantitative addition of standard HC!. pKa va lues are in agreement with the literature va lues. Effect of HPO/ - was studied in unbuffered system containing varied amount of KH 2P04 . It was observed that p H va lues were dependent on KH 2P04 concentrations. Potass ium ch loride was employed to maintain the ionic strength at 0.5 M. pH measurements were made on a Control dynamics pH meter. Des ired pH values were eas il y maintained since low substi'ate concentrations were used.

NMR spectra were recorded on a Brucker Avance DPX-400 in CDCI]. Ultrav iolet spectra were obtained on a Specord 200 (Carl Zesis Technology) . Spectra of

. I 1<) 10 d . h I II . . Id d p-nltrop leno' an p-nltrop eno ate anion Yle e absorbance max ima at 3 17 nm (£=9500) and 400 nm (£= 18,200) respectively. p-N itrophenyl methylphos­phonothionate had a Am", in water at 279 nm. b-p­NPMPT in acetonitrile yie lded Ama" at 275 nm.

Reacti ons of b-p-NPMPT were foll owed by measuring the rate of format ion of p-nitrophenol or its anion, depending on pH . The reaction medium cons isted of th e reagent at the des ired co ncentrati on and pH given in Tab les I and 2. Ex ac tly 15 ml of this solution was taken in a test tube, fitted in a constant water bath at 25°C and thermostated for ca. 15 min. The reaction was initiated by the add ition of 0.45 ml of a slock solution of substrate di ssolved In ace tonitrile, so that fin al ester concentrati on was 3x 10-5 M. Aliquots withdrawn at regular time interval s were poured into the cuvette fo r

SAXENA er al.: STU DI ES ON REACTI ONS OF BIS-(p-N ITROPHENY L)METHYLPH OS PHONOTHI ONAT E 719

measurements. Reactions were carried out under pseudo-first order conditions with catalyst in excess and in all the cases fo ll owed first order kinetics. All reacti ons were all owed to proceed to 2-3 half li ves and rate constants were fo und in all the cases as slopes of log a/a-x versus time plots (where a and a-x are concentrati ons at zero and at any time respecti ve ly). Second order rate constants were determined by plotting observed pseudo-first order rate constants aga inst catalys t concentrati ons. The slope was taken as the second order rate co nstant and the intercept as the fi rs t order rate constant in the absence of added catalys t.

Results and Discussion The results of react ions of amines wi th b-p-N PMPT

are given in Tab le I. The free base fo rms are the only kinetica ll y acti ve amine spec ies detected. The rate law at pH va lues exa mined is given by Eq. ( I)

k Obs = [b-p-NPMPTl X (k2 [aminel + k'2 [OW]

+ k" 2 [H 20] ) ... (I)

where k2, k'2, k" 2 are the second order rate constants for the reacti on of amine, hydroxide ion and water respecti vely. Using partiall y neutralized am ine so lutions as buffe rs, k2 va lues are obtained fro m the slope of observed pseudo-first order rate constan t plotted as a functi on of amine ex isting in the free base fo rm . The intercept of plot of free [aminesl (e.g. butylamine, ethanolamine and piperidine) yields the k'2[OW] and the plots of free pyridine and phosphate bu ffe r, gives k" 2 rH 20l . Values of k2 being independent of p H exc lude spec ific ac id and base catalyzed path ways to be kineti ca ll y important. For all amines listed in Table I , p-nitrophenyl methyl­phosphonothi onate (p-NPMPT) and p-nit rophenol (or its ani on) depending on p H are fo rmed in mola r rati os. Above p H 9.0, spectra due to absorpti on of p-nitrophenolate ani on at 400 nm and that of p-N PMPT at 279 nm are eas ily separable. The am ines given in Table I follow Bronsted relation with a slope of 0.40 ( r = 0.997). These am ines have widely diffe ring environments about the nitrogen center and therefore one would ex pect considerabl y more scattered points on Bronsted line, if a nucleoph il ic

Tablc I- Gcnera l base CJtJ lys is fo r the hydrol ys is o f C Hr P(S)(OC6 H4 0 2h' (b-p- NPMPT) at 25°± I °C

Am ine pKah [Amine] (M") l i H k2(lf 1sec- l)d k2( H20)/k2( 0 20)

Pyrid inc 5.35

2-Meth yli midazole 7. 19

Ethanolam ine 9.50

II -B utyla mine 10.62

Piperid ine 11.1 2

0 .79-0.99

0. 164-0.S9" 0. IM-0.69r

0.76-0.93

0.63-0.S9

0.69-0.94

5.44-6. 16

7. 12-9.66c

7. 10-S .S4r

10.44- 11. 19

I 1.45- 12.26

9.74- 11.5 1

1.0x 10-5

S.Ox I 0-4"

2.0x I0-4f

2.63x I 0-2

"Concentration of b-p-N PMPT = 3x 10-5 M; ionic strength = 0.5 M with added KC I;

4.0

bof amine conj uga te ac id; C concentration of free amine; d k2 va lues were determined form the slope of the plots of pseudo-first orde r rate constant ve rsus concentration of amine ex isting in the free base fo rm ; cin H20; r in 0 20

Table 2-Nucleophili c catal ys is of the hydro lys is of C Hr P(S)(OC6 H4 N0 2)/ (b-p -NPMPT) at 2SOC ± 1°C

Nuc leophile p Kah [N] if p H k2• (If\ec- I/ d H20)/ C 0 20

F 3.45 0.00 1-0.03 7 .20 1.6x 10-2 1.06

0.00 1-0.03 7. 10 1.5x 10-2

HPO} - 7.2 1 0.005-0.2 7.2-9.32 6x 10-6 1.0

0.005-0. 15c 9.24" 6x IO-(lC

O H 15.74 10.92- 11 .45 3. 19

"[b-p-NPMPTI = 3x 10-5M: ion ic strength equals 0.5M with added KCI ; hpKa of the conjugate Jcid according to literature value: Ctotal nuc leophile concel1lration. in 0.2M phosphate buffer; dsecond order rate constJI1l for eJch step of the two step reactions; cin 0 20.

720 INDIAN J CHEM, SEC A, APRIL 2002

h . 4 1?· 15 . Th . d f mec anlsm . - were operative. e magnltu e 0

8rbnsted slope, for all amines are consistent with a general base catalysis. The deuterium isotope effect, k2 (H20)/k2 (020) for the reactions of 2-methyl­imidazole are in the range of 4.0. The value of isotope effect is as expected for general base catalysis4

.5.'7.

However, the nucleophilic catalysis would yield an . fl' . )1. 15Th I b I ' Isotope elect near unity ' ' . e genera ase cata YSls by amines of water attack is represented by Eq. (2).

S S

H3C- ~ 16c6H4N02-H3C- ~ - 0-+ -OC6H4N02

I

LO-H-Amine I + + ~ OC6H4N02 + amine H + H

... (2) OC6H4N02

The stoichiometry is shown by Eq. 2, and the rate law represented by Eq. 1, 8rbnsted correlation and solvent isotope effects are completely consistent with the reactions of b-p-NPMPT and amines proceeding exclusively by a general base mechanism. Earlier studied reactions of F- and oxyanions with monoanions and neutral phosphorus compounds have been shown to proceed by nucleophilic pathways 16. The kinetic data for reactions of title compound with the reagents are given in Table 2. Each reagent will be discussed separately.

Fluoride ion The second order rate constants for reactions of b­

p -NPMPT with different fluoride ion in phosphate buffer at pH 7.2 are given in Table 2. At high fluoride ion concentration reaction of b-p-NPMPT releases 2 mol of p-NP in first order process. This is explained by nucleophilic attack of fluoride ion at phosphorus atom of b-p-NPMPT followed by an attack at phosphorus atom of (II) (Eqns. a and b).

s s II kl II

H3C- P-OC6H4N02 + F-====", H3C-P- F + -OC6H4N02 I LI I OC6H4N02 OC6H4N02 ... (a)

(I) (II)

.. . (b)

However, at lower fluoride ion concentrations, less than 2 mol of p-nitrophenol was released (also in a first order process) corrected for reaction of buffer.

The possible reaction of bases, 8 (Buffer, -OH and H20) with (II) are shown by Eqs. (c) and (d).

s s II k II

H3C-P-OC6H4N02+ B~ H3C-P- B+ -OC6H4N02 I . F ~

(IV) . .. (c)

(V) . . . (d)

It may be pointed out that the plots of (A~-AI)/(2AI-A~) versus [F] for phosphate buffer gives straight line (r=0.9731) passing through the origin . Therefore, reaction step (c) appears smaller compared to those represented by Eqs. (b) and (d) (which are believed to proceed by comparable rates) and can be neglected. Equations b-d shows possible reactions of II. None of the reactions were followed kinetically . The rate for the slow step of the reaction (a) can be expressed as follows [Eq. (3)].

-d (I)/d, = d(II)/d, = d(NP)/d, .. . (3)

However, a rate expression can be written for the disappearance of II, which is given in Eq (4).

-d(II)/d, = k_1 [p-NP]+kb [II] [F-]+kc[II][B ]+ku [II] [8]

... (4) As the reagents are in sufficiently larger amounts than the substrates, the reactions are each first order in (II) and Eq. (4) can be transformed to Eq (5).

... (5)

where k'obs is the first order rate constant. Since p-NP is relatively poor nucleophile, k.1 term would be smaller and can be neglected. Now Eq. (5) can be written as Eq. (6).

k'obs = kb [F] + kc [8] + kd [B] ... (6)

kb and kc steps result in the release of 2 mol of p-NP while 1 mol of p-NP released as a result of steps kd and kb. The reactivity of II with F is much greater than 8 in producing p-NP (reactions b and c) yet, F and 8 react at comparable rates producing p-NP and F respectively (reactions b and d) . Equation (7) can be derived from Eq. (6).

SAXENA el al.: STUDIES ON REACTIONS OF BI S-(p-NITROPHEN YL)METHYLPHOSPHONOTHIONATE 721

where AI is the infinity value for the release of lmol of p-NP at a given substrate concentration [in the absence of F ion, A I = 0.230] , and A~ is the infinity value for the reaction in the presence of fluoride ion at the same concentration of substrate. The reaction of b­p-NPMPT in 0.2 M phosphate buffer (PH 7.2), with different concentrations of fluoride ions, 0.00 I M , 0.002M , 0.0 I M and 0.03M, were carried out, and A~ values were 0.309, 0.380, OA08 and OA39 respectively. Substituting these values in the formula [(A~-A I )/(2A I-A~) ], corresponding values 0.52, 1. 83, 3.50 and 9.95 respectively of p-NP were calculated. Equation (7) leads to the prediction that at constant [B] release of p-NP [(A~-A 1)/(2A I-A~)] is proportional to the fluoride ion concentration. The slow step (Eq. a) is independent of pH and buffer as expected for a rate determining nucleophilic attack of fluoride ion on b-p-NPMPT. The solvent isotope effect, which is near unity , is consistent with the reaction of fluoride ion and b-p-NPMPT proceeding via.

I h' I' I 13- 16 nuc eop I IC pat lway .

Phosphate dian ion The reactions of b-p-NPMPT in phosphate buffer

produce I mol of p-NP and 1 mol of p-NPMPT by a first order process. The reacti on of HPO/ - was studied and data are given in Table 2. The deuterium solvent isotope effect (k2(H 20)lk~(020), found unity for 0.05-0.2 M HPO/ -, suggests that it probably acts as nucleophile towards b-p-N PMPT forming poss ibl y pyrophosphonothionate, which subsequentl y decomposes to p-NPMPT and HPO/ -.

Hydroxide ion (and water) The reaction of hydrox ide ion and water with b-p­

NPMPT proceeds ' R according to Eq. (3). The rates of hydro lys is due to water and hydroxide ion were obtained from the intercepts of plots of k obs for reacti on of pyridinc/2-methylimidazole/phosphate buffer versus concentration of pyridine and 2-methyl imidazole in free base forms and k obs for reacti on of butylamine/piperi-dine/ethanolamine versus concentrati on of buty I-ami ne/pi peridi ne/etha­nolamine in free base form respecti vely. Above pH 9, the slope in Fi g. I is unity showing a first order dependence on [OH-]. The value of k2 for hydroxide ion has been calculated as 3. 19 lflsec- ' (Table 2). However, below pH 9, the water reaction becomes kinetically important. The second order rate constant

OT-----------------------------~

-1

-2

.: -3

PIp' B - Pip

-Bu E._ E. o

::.:: en o -4

I. I E. - .,. -5 P Py ~

1 -.Py _I. -6+-~.__T--._--.__.--_.--,__.--_.~

5 6 7 8 9 10 11 12 13 14 15

pH

Fi g. I- Pl ot of l ogko~s vs pH. Each point represe nt the observed rale in the presence of the labe lled amine, extrapolated to a illi ne concen tration

for water attack is found 1.34x lO- 1i If'sec- ' by dividing first order rate constant = 7AxI0- losec-1 by 55 M2. If the Brbnsted line is ex tended to 15 .74 (p Ka

9 A"I - I of H20), the second order rate constant, 3.1 / VI sec of hydroxide ion fall s only sli ghtly above the Brbnsted line. Thus, the general base mechani sm for hydroxide ion cannot be excluded. However, the attack of water, a poor nucleophile, involves general base catalysis by two (or more) water molecules. The slope for 2-methylimidazole reaction with b-p­NPMPT in H20 and 0 20 yields an isotope effect [k2 (H20 )lk2(0 20)] approximately 4, which is consistent with the general base catalysis of water attack . Neutral phosphate es ters, e.g. (2,4-d initrophenoxy)-2-oxo- I,3,2-dioxo-phosphorinane4 and methyl ethylene phosphate esterl7 which show the solvent isotope effect greater than 2, are also reported to give general base catalysis of water attack.

s s II II

H)C- P-OCr,H4N02+0H- - H1C-P-O- + -OC6H4N02 +H+ . I (H20) I (2H +)

OCr,H4N02 OC6H4N02

... (8)

Our data are consistent with the concerted reac ti on at phosphorus center by water attack catalyzed by amine and water. The general base catalysis by amines can be attributed to steric crowding in the transition state if amine attacked directly at phosphorus atom. A similar explanation has been advanced to ex plain general base mechani sm for steri cally hindered amine with sulphinyl sulphone l9, aryl diphenylphosphinate5 and meth yl ethylene

17 . d f l' phosphate esters . The magnltu e 0 so vent Isotope effect derived fo r reactions of strong nucleophiles

722 INDIAN J CHEM, SEC A, APRIL 2002

with b-p-NPMPT are consistent with nucleophilic pathway.

Acknowledgement We are th ankful to Dr. R V Swamy, Director, DRDE, Gwalior fo r their va luable advice throughout thi s work . We also thank Dr. L Singh for providing the spectrophotometer facility and Mr. T K Das for secretari al assistance.

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7 Datta S P & Grzybowski A L, Biochell1ists Halldbook. (C.Long.Ed. , Van Nostrand, New York), 1961 , 19.

8 Sorensen S P L, Biochem Z, 22 ( 1909) 352.

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II Behrman E J, Biall as M J, Brass H J. Edwards J 0 & Isaks M, J org chem. 35 ( 1970) 3063.

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15 DiSabato G & Jencks W P, J Am chem Soc, 83 ( 1961) 4393.

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19 Kice J L & Campbell J D, J org chem, 36 ( 1971) 229 1.