Nucleophilicity Studies of Reactions in Which the ... - ACS Publications

19 Nucleophilicity Studies of Reactions in Which the Displaced Group Is a Neutral Molecule Downloaded by CORNELL UNIV on August 26, 2016 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch019

Dennis N. Kevill, Steven W. Anderson, and Edward K. Fujimoto Department of Chemistry, Northern Illinois University, DeKalb, IL 60115

A comparison of the rates of solvolysis of the tert -butyldimethylsulfonium ion and the 1-adamantyldimethylsulfonium ion presents strong evidence that the solvent dependence of the tert -butyldimethylsulfonium ion solvolysis rates is governed primarily by solvent nucleophilicity effects. Leaving-group contributions based upon 1-adamantyldimethylsulfonium ion solvolyses are better incorporated into the establishment of the solvent nucleophilicity scale based upon triethyloxonium ion solvolysis. Alternative solvent nucleophilicity scales based upon the solvolysis of S-methylbenzothiophenium ions are discussed. Analyses of the extent of nucleophilic participation by the solvent in the solvolyses of methyldiphenylsulfonium and benzhydryldimethylsulfonium ion will be presented. The relative nucleophilicities of various anionic and neutral nucleophiles toward the triethyloxonium ion in ethanol have been determined.

JL HEORETICAL TREATMENTS OF

REACTIONS,

S 2 in the presence of solvent molecules, using statistical mechanics simulations involving a large number of water molecules ( i , 2) or quantum mechanics calculations involving a few water molecules (3, 4\ are in their infancy. As an alternative, the roles of solvent nucleophilicity (N) and solvent ionizing power (Y) during solvolysis reactions can be treated in terms of a linear free energy relationship (LFER). In 1951, Winstein et al. (5) put forward equation 1 for the correlation of solvolysis rates of neutral substrates. In equation 1, k and k represent the specific rates of solvolysis of a substrate in a given solvent and in the standard solvent (80% ethanol), and / and m are measures of the sensitivity of the N

0

0065-2393/87/0215-0269$06.00/0 © 1987 American Chemical Society

Harris and McManus; Nucleophilicity Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

270

NUCLEOPHL IC IT IY

solvolysis of a given substrate toward changes in Ν and Y values. This equation can be considered as an extension of an earlier equation (without the IN term) put forward (6) to correlate the rates of unimolecular solvolyses. log (k/k ) = IN + mY

(1)

Downloaded by CORNELL UNIV on August 26, 2016 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch019

0

Recently, Swain et al. (7) have put forward equation 2, where Alog k is the incremented log k relative to the lowest value in the series, A and Β are measures of solvent electrophilicity and nucleophilicity, and a, b, and c are determined by details of the solvolysis under consideration. Although this equation appears to have an advantage over equation 1 in that A and Β values can be obtained without the study of solvolysis reactions, use of such values frequently leads to unreasonable predictions (8). Alog k = aA + bB + c

(2)

For the analysis of S 1 solvolyses, Abraham et al. (9) have proposed an equation (equation 3) based on sensitivities toward solvatochromatic proper ties. In equation 3, π * is a measure of solvent dipolarity-polarization, α is a measure of solvent hydrogen bond donor acidity, and β is a measure of solvent hydrogen bond acceptor basicity. More recently, a term governing cavity effects has been added, and this term is considered to represent an important contribution (10, 11). The cavity term can be directly related to the square of the Hildebrand solubility parameter (10-12). A similar analysis by Koppel and Palm (13, 14) involves terms governed by solvent polarity, solvent polarizability, electrophilic solvation ability, and nucleophilic solva tion ability. Recently, a cavity term has also been added to this analysis (12). N

log k = log k + sir* + aa + &β

(3)

0

For situations where solvent nucleophilicity may be a factor, Kevill (8) favors the use of the extended Grunwald-Winstein equation (equation 1). Scales of i V and Y values based upon the use of methyl tosylate and 2adamantyl tosylate as model S 2- and S l-reacting substrates have been developed (15, 16). Also Y scales have been developed for other anionic leaving groups using 1-adamantyl or 2-adamantyl derivatives (17-19), where S 2 reaction is impossible or severely hindered. 0 T s

O T s

N

N

N

Solvolyses of tert-Butyl Derivatives When a scale of Y values (17) was used together with N values within equation 2, an I value of 0.30 was determined for terf-butyl chloride sol volysis. A n S 2 (intermediate) mechanism, involving both an intermediate and covalent interaction by a nucleophilic solvent molecule, was proposed c l

O T s

N


271


19. KEVL IL ET AL.Neutral Molecule as a Displaced Group

(17). Abraham et al. (9) and Dvorko et al. (12) suggested that the data can be alternatively rationalized by assuming that the moderate differences in the response of specific rates to solvent variation between 1-adamantyl chloride and tert-butyl chloride arise not from a higher response to changes in solvent nucleophilicity for tert-butyl chloride but from a higher response to changes in solvent electrophilicity for 1-adamantyl chloride. The basis for this alter native explanation is the covarience between the Ν and Y scales usually observed for a binary solvent system. However, the proportionality factor involved in the covarience varies widely with the system, even changing from negative to positive, and by a judicious choice of standard solvents, i n c l u d i n g solvents from different systems, problems associated with covarience can be avoided (15, 20, 21). In an analysis of the solvolysis of tert-butyl chloride in terms of equation 3, Abraham et al. (9) found that the specific rates could be correlated against π * and a, without the need to include a term relating to basicity (β) values. However, a new analysis with inclusion of the cavity term has indicated (11) a minor contribution from the 6 β term. Conversely, an analysis (12) in terms of the Koppel-Palm equation, with inclusion of a cavity term, did not find any dependence on nucleophilicity (basicity). The situation regarding the analysis of tert-butyl chloride solvolyses in terms of a L F E R is clearly confused, a general acceptance of the beliefs that, at most, only a minor contribution arises from the term governed by solvent nucleophilicity and that the dominant contribution is from a combination of solvent dipolarity-polarizability, and that electrophilic solvation exists. We reasoned that the best approach toward estimating the extent of nucleophilic assistance during solvolysis of tert-butyl derivatives was not to try to refine the existing L F E R treatments (7, 9, 11-13, 17) by use of more sophisticated parameters or by addition of further parameters. Instead, the type of substrate studied should be changed in a manner that would remove, or greatly reduce in magnitude, the factors that dominate when the leaving group is anionic. A n excellent substrate for such a study is the terf-butyldimethylsulfonium ion (f-BuSMe ), conveniently used in conjunction with the essentially nonnucleophilic (22) trifluoromethanesulfonate (triflate) anion. Solvolysis of this cation will proceed with charge dispersal at the transition state and with ejection of a neutral molecule. This ion had been studied in a variety of solvents (23), and with the notable exception of the acetolysis, an approximately linear relationship, showing small rate reduc tions with increasing Y values, was found (m ~ —0.09). The data were interpreted in terms of the Hughes-Ingold concept, with increased solvent polarity preferentially stabilizing the more intensely charged initial state, which led to a reduction in the rate. A l l of the binary systems studied were of the type where an increase in the Y value is accompanied by a decrease in the Ν value. We extended the study to 2,2,2-trifluoroethanol (TFE)-containing solvents (24, 25), and in particular, we studied T F E - H Q mixtures, +

2

2


272

NUCLEOPHL IC IT IY

which are unusual in that both Ν and Y values increase together (26). Kevill et al. (25) found for T F E - H 0 mixtures that the rate increased with increas ing Y values (Figure 1). A rationalization of the variation in rates with solvent composition, consistent with all of the data, is that the specific rates increase with increasing solvent nucleophilicity. Consistent with this idea, acetic and formic acids, having very similar Ν values (15, 27) but very different Y values (15, 28), gave specific rates of solvolysis of £-BuSMe that were very similar (8.10 X l O ^ s " for acetolysis and 6.78 Χ 10" s" for formolysis, at 50.0 °C). When specific solvolysis rates were plotted logarithmically against N val ues (29), some lateral dispersion was found but the slopes of the plots (25) for a given binary system were around 0.35, consistent with the value estimated for tert-butyl chloride solvolyses (17). 2

+

2

1

6

1


KL

y Figure 1. Grunwald-Winstein plot for the solvolysis of the t-BuSMe ion. The line represents the best fit to the data of reference 23 (excluding acetolysis). The points represent our data for the solvolysis of t-BuSMe OTf~ at 50.0 °C. +

2

+

2

Solvolyses of the 1-Adamantyldimethylsulfonium Ion +

If the relative rates of £-BuSMe solvolyses are indeed governed by solvent nucleophilicity, then a study of 1-adamantyldimethylsulfonium ion (1A d S M e ) solvolyses (equation 4) should show quite different characteristics, because specific nucleophile solvation from the rear is prevented by the cage structure. 2

+

2


273

19. KEVILL ET AL.Neutral Molecule as a Displaced Group OR

•SMe

2

+SMe *ROh£*S0 CFâ 2

(4)

3

SO3CF3

(1-AdSMeî) The specific rates of solvolysis have been found to vary very little with the solvent. For 41 solvent systems, the specific rates at 70.4 °C varied only by a factor of less than 7, from 1.09 X 1 0 s in tert-butyl alcohol to 7.09 X 10" s in 97% hexafluoroisopropyl alcohol (HFIP). This lack of variation is even more remarkable considering the large temperature dependence, as reflected in enthalpies of activation of approximately 35 kcal/mol. Reasonable rates are observed at moderate temperatures only because of entropies of activation of approximately +18 eu. For each binary mixture within which both £-BuSMe and l - A d S M e were s t u d i e d ( T F E - E t O H , T F E - H 0 , E t O H - H 0 , M e O H - H 0 , a c e t o n e - H 0 , and d i o x a n e - H 0 ) , the specific rates of solvolysis of 1A d S M e always showed a variation with change in solvent composition in the opposite direction to that for £-BuSMe . Further, the rate variations were more modest for l - A d S M e . These observations can be rationalized in terms of an effect related to the Hughes-Ingold concept but with solvent nucleophilicity substituted for the more generalized concept of solvent polar ity. For both substrates, the effect will favor a reduction in rate with in creased solvent nucleophilicity because of a larger stabilizing effect at the more intensely charged initial state. However, for f - B u S M e solvolyses, this effect is outweighed by an additional effect involving specific nucleophilic solvation of the developing carbenium ion, an effect prevented for 1A d S M e solvolyses by the cage structure. For positively charged tert-butyl substrates, nucleophilic assistance from the solvent is the dominant influ ence in determining solvent composition induced changes in specific rates, whereas nucleophilic assistance was only a minor factor for neutral tert-butyl substrates. Another feature supporting the importance of nucleophilicity is the observation that the ratio of specific solvolysis rates (^-Bux^i-Adx) 97% H F I P gives a ratio of 2.56 (at 25 °C) for X = Br (30) and 2.60 (at 70.4 °C) and 2.71 (at 49.7 °C) for X = SMe +. Electrophilic solvation would not be expected to be a factor for a positively charged leaving group, and the similarity of the specific rate ratio for X = S M e or X = Br suggests that it is also not an important factor when X is a halogen; that is, f-BuCl and 1A d C l have similar responses to solvent electrophilicity. The value of approx imately 2.6 can then be considered as a base point, perhaps reflecting a slightly better nonspecific nucleophilic solvation of the t-BuX relative to that from 1-AdX. The ratio then increases for either X = S M e or C l as the - 6


6

_1

- 1

+

+

2

2

2

2

2

2

2

+

2

+

2

+

2

+

2

+

2

m

2

+

2

+

2


274

NUCLEOPHL IC IT IY

nucleophilicity of the solvent increases, because of the incursion of specific nucleophilic assistance for t-BuX substrates, but not for 1-AdX substrates. I£ as an alternative, the low ratio in 97% H F I P is ascribed as being due to a higher response to solvent electrophilicity for 1-AdCl relative to f-BuCl (9 12), then the similar ratio observed for the comparison with the terf-alkyldimethylsulfonium salts is difficult to explain. ;

+

Solvent Nucleophilicity Scales Based upon R-X

Substrates

Kevill and L i n (29) pointed out several years ago that solvent nucleophilicity scales are best developed by using a positively charged substrate, because the modified form required for the extended Grunwald-Winstein equation (equation 5) has solvent ionizing power (Y) replaced by a measure (Y ) of the influence of a given solvent upon the leaving-group ability of an initially positively charged leaving group. Because the range of Y values is much narrower than that of Y values, any errors in solvent nucleophilicity scales are reduced due to an incorrect estimate being made of the m value. Kevill and L i n developed (29) an N scale of solvent nucleophilicities using equation 6.


+

+

K

L

log (k/k ) = IN + m Y

+

(5)

0

N

K L

= log ( f c / g

E t 3 0 +

- 0.55Y+,

(6)

BuSMe2+

+

Two reasons led to the choice of f - B u S M e : the analogy with the use of tertbutyl chloride to establish the traditional Y scale (6) and the observation that several of the required values were available in the literature (23). Clearly, this choice was not good, and, indeed, the m Y corrections (fortunately small) were in the wrong direction. The required Y scale is better based on l - A d S M e solvolyses, and we have developed a revised scale (N '), defined as in equation 7. The 0.55Y term is, however, now so small that to a very good degree of approximation log (fc/& ) values can be directly used as a solvent nucleophilicity scale (designated N p ). Indeed, N ' and N Q+ values, for 34 solvents (Table I), correlate very well (r = 0.9958) with each other: 2

+

+

+

2

KL

+

0

Et3O+

Et

N

K L

N ' KL

' = log (k/k ) 0

= 1.06Ν

Εψ+

EhO+

+

KL

- 0.55 Y V

A

d

S

M

^

- 0.042

Et3

?

() (8)

Either the N ' or the N ^ scale can do an excellent job of correlating the rates of both positively charged and neutral substrates. The logarithms of the specific rates of solvolysis of the N-(methoxymethyl)-N,N-dimethyl-ranitroanilinium ion correlate linearly with N values in aqueous ethanol and aqueous T F E with a value of 0.75 for tne slope (31). A n analysis has K L

Et

Q+

+

Et

0


19.

KEVILL ET AL.

Neutral Molecule as a Displaced Group

275

Table 1. Solvent Nucleophilicity Values Based upon the Solvolysis of the Triethyloxonium Ion at 0.0 °C. ab

Solvent

0

N

C

Et 0+ 3

K L

'

Solvent *

d

100%

EtOH

0.55

0.56

80% dioxane

-0.10

-0.04

80%

EtOH

0.00

0.00

70% dioxane

-0.24

-0.22

60%

EtOH

-0.35

-0.38

60% dioxane

-0.34

-0.34

40%

EtOH

-0.64

-0.70

40% dioxane

-0.60

-0.60

20%

EtOH

-0.95

-1.03

20% dioxane

-0.80

-0.84

0.64

0.62

100% T F E

-2.17

-2.42

100%


N

a

MeOH

80%

MeOH

0.19

0.14

97% T F E

-2.28

-2.50

60%

MeOH

-0.25

-0.30

90% T F E

-1.73

-1.93

40%

MeOH

-0.49

-0.56

70% T F E

-1.28

-1.47

20%

MeOH

-0.88

-0.96

50% T F E

-1.05

-1.21

95% acetone

-0.23

-0.31

80% T - 2 0 % Ε

-0.92

-1.10

90% acetone

-0.12

-0.20

60% T - 4 0 % Ε

-0.42

-0.52

80% acetone

-0.22

-0.29

40% T - 6 0 % Ε

-0.05

-0.12

70% acetone

-0.36

-0.42

20% T - 8 0 % Ε

0.41

0.38

60% acetone

-0.46

-0.52

100%

H 0

-1.01

-1.15

40% acetone

-0.71

-0.78

100%

AcOH

-1.42

-1.46

20% acetone

-0.86

-0.96

100%

HC0 H

-1.71

-1.73

2

2

O n a v o l u m e - v o l u m e basis (at 25.0 °C), except for T F E - H 0 mixtures, which are on a weight percentage basis. Other component water, except for T F E - E t O H ( T - E ) mixtures. Decimal logarithm of the specific rate of solvolysis of E t 0 at 0.0 ° C relative to the specific rate of solvolysis in 80% aqueous ethanol (values from references 24 and 29). Defined according to equation 7, where Y is the decimal logarithm of the specific rate of solvolysis of l - A d S M e at 70.4 ° C relative to the specific rate of solvolysis in 80% aqueous ethanol. 2

h

c

+

3

d

+

+

2

+

a n (

recently been published comparing the use of ^ E t o ^ N , in conjunc tion with Y , for the correlation of the specific rates of solvolysis of allyl arenesulfonates (21). A minor source of irritation is that confusion can arise because I values based on the use of N ' or N ^ + values must be scaled before they can be compared with I values based on the use of A / values. The need for scaling can be avoided by using a positively charged methyl derivative as the standard substrate. This usage will also have the advantages of maximizing the range of Ν values and minimizing steric interactions. We have recently completed a study, at 25.4 °C, of the solvolyses of Smethyldibenzothiophenium trifluoromethanesulfonate (MeDBTh OTf~) in 37 solvents (equation 9): the 33 solvents studied for E t 0 solvolysis plus four HFIP-containing solvents. A plot (Figure 2) shows that, for the 33 solvents studied with both substrates, a good linear plot is obtained when log (k/k ) + 3

O X s

0 T s

KL

E

0

0 T s

+

+

3

0


Et

0

276

NUCLEOPHL IC IT IY Εφ*

UJ Ο


U) ο

Nucleophilicity Studies of Reactions in Which the ... - ACS Publications

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