Mechanism of Reactions of Monosubstituted Phosphates in Water

Chapter 8 Mechanism of Reactions of Monosubstituted Phosphates in Water Appearance and Reality

Downloaded by CORNELL UNIV on September 19, 2016 | http://pubs.acs.org Publication Date: April 7, 1992 | doi: 10.1021/bk-1992-0486.ch008

William P. Jencks Graduate Department of Biochemistry, Brandeis University, Waltham, MA 02254-9110 18

Studies of structure-reactivity parameters, O isotope effects, solvent effects and other parameters have shown that the hydrolysis of monoanions and dianions of acyl phosphates and monoanions of phosphate esters proceeds through strongly dissociative transition states that closely resemble the monomeric metaphosphate monoanion. For example, acetyl phosphate monoanion undergoes hydrolysis in water with ΔS = +4 e.u.,kH2O/kD2O=1.0,smallsalt and solvent effects, and ΔV = -0.6ccmol . This might suggest that the reactions proceed through a metaphosphate intermediate. However, experiments are reviewed which show that monosubstituted phosphate derivatives react with water and other nucleophilic reagents through a fully coupled, concerted mechanism of displacement in aqueous solution. In particular, there is an interaction between the nucleophile and the leaving group in the transition state, which is not expected for a stepwise reaction through a metaphosphate intermediate. ≠

≠

-1

In 1955 Westheimer, Bunton, Vernon and their coworkers suggested that the hydrolysis of the monoanions of phosphate monoesters proceeds through the formation of an intermediate monomeric metaphosphate monoanion, which reacts rapidly with water to give inorganic phosphate (equation l)(i,2).

0097-6156/92/0486-0102$06.00/0 © 1992 American Chemical Society Walsh et al.; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

8. JENCKS

103

Monosubstituted Phosphates in Water

οΙ

OH

! P-OR

0

Η+ -— * · Ό - Ρ - O R

Ο

W

-— *

Ρ

:|

II ο

II ο

+

HOR

(1)

ο fast

Η 0 2

Η Ρ0 -


2

4

Such a mechanism might account for the rapid hydrolysis of phosphate monoester monoanions, which occurs some 10,000 times faster than that of the corresponding diester monoanions. It was suggested that the rapid reaction proceeds by transfer of a proton from the O H group to the leaving OR group of the monoester, followed by elimination to form metaphosphate, which then reacts rapidly with water as shown in equation 1. Such a mechanism is not possible for the slower hydrolysis of phosphate diesters. The dependence on pH of the rate of hydrolysis of the monoanion showed that most phosphate monoester monoanions undergo hydrolysis much faster than the dianion or the uncharged acid (3-5). Phosphate dianions with very good leaving groups, such as dinitrophenyl phosphate, acyl phosphates and phosphorylated pyridines, also undergo rapid hydrolysis; this might also proceed through monomeric metaphosphate (equation 2) (6-8).

cr I

ο \ »

Ό — P - X

II ο

* χ-

ο ·/ Ρ

H O > H P0 2

2

(2)

4

|: ο

The rates of hydrolysis of these compounds show a very large dependence on the pKj, of the leaving grouo, with values of B, in the range of -1.0 to -1.2 (6-8), and the hydrolysis of aryPO-^-dinitrophenyl phosphate dianion with O in the leaving oxygen atom shows a large oxygen isotope effect of 2.04% (9). g

l s

Walsh et al.; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

PHOSPHORUS CHEMISTRY

104

These results show that the reactions proceed with a large amount of bondbreaking in a transition state that resembles the monomeric metaphosphate monoanion. The monoanions of substituted benzoyl phosphates undergo hydrolysis rapidly, with only a small dependence of the rate on the p l ^ of the leaving group; the dianions also undergo rapid hydrolysis, with a value of B, = -1.2 (7). The rapid hydrolysis of the monoanions could be explained by intramolecular transfer of the proton to the leaving group (1), while the large dependence of the rate on the p l ^ of the leaving group in the dianion series indicates a metaphosphate-like transition state (2). The entropies of activation are near Downloaded by CORNELL UNIV on September 19, 2016 | http://pubs.acs.org Publication Date: April 7, 1992 | doi: 10.1021/bk-1992-0486.ch008

g

Ο

Ο

W

Ο

H

ρ....Ό-C—Ar

:l o

1

2

zero for the monoanion and dianion of acetyl phosphate, but AS* is -28.9 e.u. for the monoanion of acetyl phenyl phosphate. There is no significant solvent deuterium isotope effect for the hydrolysis of the monoanion and dianion of acetyl phosphate, while the hydrolysis of the monoanion of acetyl phenyl phosphate shows an isotope effect of k /k = 2.5. The addition of acetonitrile has little effect on the rates of hydrolysis of acetyl phosphate monoanion and dianion, while it causes a large decrease in the rate for acetyl phenyl phosphate (7). The volumes of activation are -0.6 to -1.0 cm /mol for acetyl phosphate monoanion and dianion, while A V * is -19 cm /mol for the monoanion of acetyl phenyl phosphate (10). It was concluded that "while no single one of these considerations should be taken alone as conclusive proof of the monomolecular or bimolecular nature of a reaction, taken together they constitute strong evidence that the neutral hydrolyses of acyl phosphates occur through a metaphosphate monoanion intermediate" (7). This is a conclusion that we remember well, because it should never have been reached. These data, and data from many other laboratories (11), are certainly consistent with a metaphosphate mechanism, but they are also consistent with a concerted substitution reaction; they do not distinguish which mechanism is followed. Structure-reactivity coefficients, isotope effects, volumes and entropies of activation and similar parameters are all measures of H O H

D O D

3

3



8. JENCKS

105


the structure of the rate-limiting transition state. They provide information regarding the structure of the transition state, but they provide no information as to whether the reaction occurs in one or two steps. They tell us about the appearance of the rate-limiting transition state; they do not determine the reality of the mechanism. They provide no information as to whether or not there is a metaphosphate intermediate with a significant lifetime that reacts with water in a second step, with a significant barrier after the rate-limiting transition state. All of the other criteria described above that were applied to different reactions of phosphate compounds also tell us that monosubstituted phosphate derivatives tend to react through dissociative transition states. They establish that the rate-determining transition state of the reaction is strongly dissociative in nature, but they do not tell us whether this transition state goes on to form a metaphosphate intermediate with a significant lifetime (equation 2, top) or gives the product directly with no intermediate P0 " ion (equation 2, center). 3

Ο Η,Ο

Ο Ρ

X'

(2)

Ο Η 0 + Ό-Ρ-Χ 2

Ο

Η 0 .Ρ.Χ2

cr ΗΟ—Ρ - Ο " + Χ~ + Η

+

ο

Phosphoryl Transfer between Amines The possibility that phosphoryl transfer between amines occurs through a metaphosphate intermediate was examined by determining the rate constants for reactions of phosphorylated pyridines with higher and lower pK compared with the leaving group (12,13). If the reaction proceeds by dissociation to pyridine and P0 ", followed by reaction with the acceptor pyridine (equation 3), there should be a change in the rate-limiting step for 3



106

(3) ^ l: < Ν Ρ Ν ' / Λ

Ν + Ό Ρ-Ν 3


Ν-ΡΟ,

+ Ν

ν

k-1

ο

ο

pyridine nucleophiles with ρΚ values that are higher and lower than the pK of the leaving group. With pyridines of higher pK than the leaving group the metaphosphate ion will almost always react with the attacking pyridine (k > k. equation 3), so that bond-breaking will be largely rate limiting and the rate will be almost independent of the pK of the attacking pyridine. However, if the attacking pyridine is less basic than the leaving group, the metaphosphate intermediate will generally be attacked by the leaving group to regenerate starting material (k > k ). Under these conditions, reaction with the attacking pyridine will be rate-limiting and the observed rate will depend on the basicity of the attacking pyridine. Examination of this reaction series by Bourne and Williams (12) and by Skoog and Jencks (13) showed no evidence for curvature of the Br0nstedtype plot of log k against the pK of the attacking pyridine with pyridines of higher and lower pK than the leaving group. Values of B describing the dependence of the rate on the pK of the attacking pyridine are small, close to 0.2, and the dependence on the pK of the leaving group is very large, with β, ~ -0.9. These values show that the transition state is strongly dissociative and resembles metaphosphate, as in the hydrolysis reaction. Strong evidence in support of a concerted, coupled mechanism for this reaction, even though the transition state is strongly dissociative, is provided by the interaction coefficient p^ (equation 4). A n increase in the dependence 2

l9

x

2

nuc

^nuc P x y

=

= φΚι

8

= 0.014

(4)

Φ*™

of the rate on the basicity of the nucleophile, B , is observed with increasing ρΚ,, of the leaving group. This corresponds to a coefficient of p^ = 0.014. The same coefficient describes the change in the dependence of the rate on the leaving group, with changing pK^ of the nucleophile. This represents a "Hammond effect", or an expression of the "Bema Hapothle" (14,15), in which the sensitivity to the basicity of the nucleophile increases with poorer leaving groups. The observed increase in B from 0.17 to 0.22 as the pK of the nuc

nuc


8. JENCKS


107

leaving group increases from pK 5.1 to 9.0 corresponds to a ρ coefficient of 0.014. Similar behavior was observed previously by Jameson and Lawler for the reactions of a series of amines with phosphorylated pyridines (8) and by Kirby and Varvoglis (16) for the reactions of substituted pyridines with /7-nitrophenyl phosphate and 2,4-dinitrophenyl phosphate. The values of B = 0.13 for the />-nitrophenyl and zero for the dinitrophenyl leaving group correspond to a ρ coefficient of 0.043. These interaction coefficients show that both the nucleophile and the leaving group are involved in the ratedetermining transition state, so that the reactions are concerted. Although metaphosphate is not an intermediate in the reaction, the fact that the transition state closely resembles metaphosphate indicates that the interaction of phosphorus with the attacking and leaving groups is weak in the transition state. It may be appropriate to compare metaphosphate to the proton - neither species exists as a free intermediate in water, but both are identifiable chemical species that are transferred from one nucleophilic reagent to another.


nuc

The Role of Nucleophile Solvation. The value of B = 0 for the reaction of substituted pyridines with 2,4-dinitrophenyl phosphate (16) is puzzling. If the value of B is a measure of the amount of the bond formation to the nucleophile in the transition state, this value might be taken to mean that there is no bond formation to the nucleophile in the transition state. This is obviously not the case, because there is a large increase in the rate of disappearance of the phosphate ester with increasing concentration of the nucleophile; the reactions follow simple second-order kinetics. The answer to this question only became apparent when it was observed that the reactions of substituted quinuclidines with 2,4-dinitrophenyl phosphate, /?-nitrophenyl phosphate and phosphorylated pyridine show a decrease in rate with increasing basicity of the attacking quinuclidine (17). These second-order reactions clearly cannot have a negative amount of bond formation with the nucleophile in the transition state, so that this result forced us to think harder about what could cause zero or negative slopes in Br0nstedtype plots against the pK of the nucleophile. The answer is that the amine nucleophile must lose solvating water that is hydrogen bonded to the lone-pair electrons of the nitrogen atom before it can react with the substrate. The strength of the hydrogen bond to the solvating water is expected to be stronger as the basicity of the amine increases, so that the concentration of the free amine will be smaller for more basic amines. The value of B for desolvation was estimated to be -0.2 for substituted quinuclidines (17). In a subsequent study the rate constants for dissociation of hydrogen-bonded water from the nitrogen atom of substituted quinuclidines were determined by examining inhibition of the exchange of protons on protonated quinuclidines by acid, according to the Swain-Grunwald mechanism. This work showed that the rate of dissociation of water decreases with increasing basicity of the amine. The decrease corresponds to a value of nuc

nuc


108



β = 0.25 for the formation of a hydrogen bond between the substituted quinuclidine and water (18). Therefore, the observed values of β for reactions of substituted quinuclidines should be corrected by 0.25 for reactions in which the β value is small; the correction becomes progressively smaller as the observed β value becomes larger (17). Hydrolysis of Phosphates. The conclusion that phosphoryl transfer between amines is concerted does not exclude the formation of a metaphosphate intermediate in reactions with weaker nucleophiles, including water. Pyridine is a much stronger nucleophile than water and it is possible that phosphoryl transfer between pyridines is concerted because the metaphosphate ion does not have a significant lifetime in the presence of pyridine; the metaphosphate ion might exist for a short time in water. However, it is much more difficult to determine whether or not the mechanism is concerted for a reaction with water than for a reaction with pyridine. Buchwald, Friedman and Knowles succeeded in preparing 2,4-dinitrophenyl phosphate in which the three free oxygen atoms on phosphate were labeled stereospecifically with different isotopes of oxygen. Solvolysis of this compound in methanol and analysis of the methyl phosphate product showed that the reaction had proceeded with inversion of configuration at phosphorus (19). This remarkable experiment supports a concerted bimolecular displacement mechanism, with no metaphosphate intermediate, for the solvolysis of 2,4-dinitrophenyl phosphate in methanol. However, it does not rigorously exclude a stepwise mechanism in which a metaphosphate intermediate with a very short lifetime is formed and reacts with methanol faster than it rotates, and it does not provide direct evidence for a bimolecular, concerted reaction with solvent water. Racemization was observed for the same reaction in t-butanol. This may well represent the transient formation of a metaphosphate intermediate in this solvent. However, it is conceivable that racemization arises from slow proton removal from the phosphorylated tertiary butanol, so that the phosphoryl group is transferred to the hydroxyl group of another molecule of tertiary butanol, with resulting racemization (20). If a series of compounds reacts by one reaction mechanism and another compound, B, reacts by a different mechanism, the rate constant for reaction of compound Β is not expected to fall on the structure-reactivity correlation that fits the first series of compounds. If the reaction of compound Β is fast enough to be observed, it will usually be faster than expected from the correlation of rate constants for the other mechanism. Phosphorylated γ-picoline was found to react with a series of anionic oxygen nucleophiles in a bimolecular concerted reaction, with a dependence on the pK of the nucleophile that is described by dlogk/φΚ = B = 0.25. Figure 1 shows that the bimolecular rate constant for the reaction of this compound with water falls on the correlation line that is defined by the rate constants for reaction of the anionic nucleophiles; there is no positive deviation nuc



8. JENCKS

109


that might suggest a different, monomolecular mechanism for the reaction with water (21). This result is particularly striking because it might be expected that an anionic substrate would react relatively slowly with anionic nucleophiles, compared with water, because of electrostatic repulsion. Evidently, electrostatic repulsion is not large at the anionic strength of 1.5 (KC1) in which these experiments were carried out. This result is consistent with reaction of phosphorylated γ-picoline with water through the same bimolecular, concerted mechanism that is followed for the anionic nucleophiles. Figure 2 shows that there is also no positive deviation of the rate constant for reaction with water in a correlation of log k for the reactions of oxygen nucleophiles with the monoanions of phosphorylated γ-picoline and with methyl dinitrophenyl phosphate (21). Methyl dinitrophenyl phosphate reacts with RO" by a concerted bimolecular substitution. The fit of the rate constant for water to this correlation is consistent with a concerted bimolecular reaction mechanism of water with phosphorylated γ-picoline. The reactions of anionic nucleophiles with a series of phosphorylated pyridines show an increase in selectivity with decreasing reactivity of the phosphorylated pyridine, as the p l ^ of the leaving pyridine is increased. This is shown in Figure 3, in which the ratios of the rate constants for reaction of oxygen nucleophiles and with water are plotted against the pKg of the leaving pyridine (21). The sensitivity of the rate to the base strength of the nucleophile increases as the substrate becomes less reactive, with increasing p l ^ of the leaving group, and there is a corresponding increase in the sensitivity of the rate to the leaving group, -β,^ as the nucleophile becomes less reactive. This behavior corresponds to a positive coefficient of ρ = 0.013 (equation 5). This ^nuc

=

= 0.013

(5)

interaction between the nucleophile and the leaving group in the transition state shows that both the nucleophile and the leaving group are involved in the rate-limiting step of the reaction. This interaction is not expected for a dissociative reaction mechanism with a metaphosphate intermediate, because the nucleophile and the leaving group are not both reacting in the rate-limiting transition state of the stepwise mechanism. Therefore, this interaction provides evidence for a concerted reaction mechanism. Figure 4 shows the dependence of -Bjg on the p i ^ of the nucleophile for reactions of phosphorylated pyridine monoanions with oxygen nucleophiles. The value of - i L decreases with increasing p l ^ of the nucleophile with a slope of = O.OIJ, as described by eq 5. The fact that the value of B, for the reaction with water fits the correlation supports a concerted mechanism for the reaction with water. In fact, the interaction coefficient of ρ = 0.013 for oxygen nucleophiles, including water, does not differ significantly from the value of p^ = 0.014 for the reaction of pyridines with a phosphorylated pyridine (equation 4). P x y

g



110


oh

-8

1

—'

'

1

1

0

ι

I

4

.

8 P nuc K

+ ,

1

•

1

12

1

16

°g P l / (

Figure 1. Br0nsted-type plot of log k against the ρΚ,, of oxygen nucleophiles for reactions with phosphorylated γ-picoline monoanion. The solid line has a slope of 6 = 0.13 and the dashed line has a slope of 0.25 (29). nuc

HO HCOO'n

-

Me NO*o 3

-

°CH C00" 3

H0 2

1

-9

1

1

l

1

1

1

-7 -6 -5 -4 -3 -2 -8 log k/q/M" s" Methyl Dinitrophenyl Phosphate, Monoanion 1

1

Figure 2. Correlation of the rate constants for the reaction of nucleophilic reagents with phosphorylated γ-picoline monoanion and methyl 2,4dinitrophenyl phosphate monoanion. The line has a slope of 0.51 (19).



8. JENCKS

111


I

ι

ι

ι

ι

5

6

7

8

9

J

PK G L

Figure 3. Br0nsted-type correlations with the pK^ of the leaving group of the ratio of the second-order rate constants for reactions of phosphorylated pyridines with anionic nucleophiles and water (Sue = succinate, TFE = trifluoroethanol) (29).




112

I

1

'

-2

0

2

'

'

4

I

6

'

8

'

10

I

12

14

I I

16

pKnuc + 9 P J , 0

/(

Figure 4. Plot of -fi^ against the pK^ of the nucleophile for reactions of phosphorylated pyridine monoanions with oxygen nucleophiles (Sue = succinate, Cac = cacodylate, TFE = trifluoroethanol) (79).


8. JENCKS

113


If the reaction with water were stepwise, the value of would be expected to be -1.25, which corresponds to the value of B, that is expected for complete breaking of the P-N bond to form the unsolvated pyridine (equation 6) (22). The reaction of the putative metaphosphate intermediate with pyridine g

(6) Ο


Ο

,

Ο

Q VO H ,

Η,ΡΟ;

Ο

+ H 0 2

is expected to be faster than with water; i.e. k > k , so that the rate-limiting step of the reaction would be the reaction of Ρ 0 ' with water. The equilibrium constants for the hydrolysis of phosphorylated pyridines follow a value of = -1.25 and the same value is expected for the observed rate constant for the reaction of equation 6, in which k is rate-limiting after the reversible cleavage of the phosphorylated pyridine (21). The observed value of B, = -1.02 is inconsistent with B, = -1.25. It is consistent with the observed values for the bimolecular reactions of pyridines and it supports a concerted bimolecular mechanism for the reaction with water. It was suggested in 1961 (7) that the formation of pyrophosphate from the anions of acetyl phosphate and inorganic phosphate in concentrated aqueous sodium perchlorate solutions occurs through the generation of a metaphosphate intermediate that reacts with inorganic phosphate to give pyrophosphate. No formation of pyrophosphate is observed in the absence of added salt. However, reexamination of this result has shown that pyrophosphate is formed in a bimolecular reaction that is first-order with respect to acetyl phosphate and inorganic phosphate and fourth-order with respect to N a (22). This result suggests that the reaction occurs through bimolecular displacement, with binding of the reacting anions by sodium ions. No pyrophosphate formation is observed at low salt concentrations because of electrostatic repulsion between the reacting anions. We can conclude that phosphate esters and other phosphate compounds react with water through bimolecular substitution in a concerted S 2, or A D , mechanism with no metaphosphate intermediate. The appearance of the transition state is that it resembles metaphosphate monoanion, but the reality of the mechanism is that the reaction is a one-step bimolecular substitution. The metaphosphate ion can be formed in the gas phase (23) and there is evidence that metaphosphate can exist briefly in nonnucleophilic solvents (11,24). The reason that it is not an intermediate in water is presumably that there is no significant barrier for its reaction with water. x

2

3

2

g

g

+

N

N

N



114

When the bond to the leaving group breaks, a water molecule attacks the incipient metaphosphate ion before bond breaking is complete, so that no intermediate is formed. The concerted mechanism is enforced, because a stepwise mechanism is not possible. Literature Cited


1. 2.

Butcher, W.; Westheimer, F. H. J. Am. Chem. Soc. 1955, 77, 2420. Barnard, P. W.C;Bunton, C. Α.;Llewellyn,D. R.; Oldham, K. G.; Silver, B. L.; Vernon, C. A. Chem. Ind. (London) 1955, 760. 3. Desjobert, A. C.R.Hebd.SeancesAcad.Sci.1947, 224, 575. 4. Desjobert, A.Bull.Soc. Chim. Fr. 1947, 14, 809. 5. Bailly, M. C.Bull.soc.Chim.Fr. 1942, 9, 314, 340, 421. 6. Kirby, A. J.; Varvoglis, A. G.J.Am.Chem.Soc. 1967, 89, 415. 7. DiSabato, G.; Jencks, W. P.J.Am. Chem. Soc. 1961, 83, 4400. 8. Jameson, G. W.; Lawler, J. M.J.Chem. Soc. Β 1970, 53. 9. Gorenstein, D. G.; Lee, Y.-G.; Kar, D.J.Am. Chem. Soc. 1977, 99, 2264. 10. DiSabato, G.; Jencks, W. P.; Whalley, E. Can. J. Chem. 1962, 40, 1220. 11. Westheimer, F. H. Chem. Rev. 1981, 81, 313. 12. Bourne, N.; Williams, A.J.Am. Chem. Soc. 1984, 106, 7591. 13. Skoog, M. T.; Jencks, W. P.J.Am. Chem. Soc. 1984, 106, 7597. 14. Hammond, G. S.J.Am. Chem. Soc. 1955, 77, 334. 15. Jencks, W. P. Chem. Rev. 1985, 85, 511. 16. Kirby, A. J.; Varvoglis, A. G.J.Chem. Soc. Β 1968, 135. 17. Jencks, W. P.; Haber, M. T.; Herschlag, D.; Nazaretian, K. L.J.Am. Chem. Soc. 1986, 108, 479. 18. Berg, U.; Jencks, W. P.J.Am. Chem. Soc. 1991, 113, 6997. 19. Buchwald, S. L.; Friedman, J. M.; Knowles, J. R.J.Am. Chem. Soc. 1984, 106, 4911-4916. 20. Friedman, J. M.; Freeman, S.; Knowles, J. R.J.Am. Chem. Soc. 1988, 110, 1268. 21. Herschlag, D.; Jencks, W. P.J.Am. Chem. Soc. 1989, 111, 7679. 22. Herschlag, D.; Jencks, W. P.J.Am. Chem. Soc. 1986, 108, 7938. 23. Harvan, D. J.; Hass, J. R.; Busch, K. L; Bursey, M. M.; Ramirez, F.; Meyerson, S.J.Am. Chem. Soc. 1979, 101, 7409. 24. Satterthwaite, A.C.;Westheimer, F. H.J.Am. Chem. Soc. 1981, 103, 1177. RECEIVED December 17, 1991


Mechanism of Reactions of Monosubstituted Phosphates in Water

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