THE GREAT TENDENCY OF CYCLOOCTANE OXIDE to react by

21 Equivalent Scales for Correlation Using Two Solvent Parameters Paul E. Peterson

Downloaded by RUTGERS UNIV on March 11, 2016 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch021

Department of Chemistry, University of South Carolina, Columbia, SC 29208

Conversion of the Swain A and Β solvent parameter scales to nu cleophilicity, N, and ionizing power, Y, is described. Results using Y values based on tert-butyl chloride rates are given, and the results of the comparable transformation using Y values based on adamantyl tosylate rates are presented. For hydroxylic "reaction" solvents and a few others, the converted values resemble published values or are reasonable. For other "nonreaction" solvents (the majority), we as sign no meaning to the converted values, although we list them for others to examine. The nucleophilicity ratio of two solvents, chosen to be acetic and formic acid, may be arbitrarily chosen to give equiv alent scales. The ratio may be adjusted to agree with chemical experi ence independent of the effect of solvent variation. The nucleo philicity of solvent component molecules in a constant solvent is an example of such independent chemical experience.

THE GREAT TENDENCY OF CYCLOOCTANE OXIDE

to react by transannular hydrogen shift in the solvent trifluoroacetic acid was ascribed to the suc cessful competition of internal hydrogen nucleophiles with relatively nonnucleophilic solvent (J). Subsequently, Peterson et al. (2) extended studies of neighboring-group participation in trifluroroacetic acid to other major types of solvolytic reactions, including tosylate solvolyses. 1,4-Halogen participa tion was clearly revealed through halogen shifts and kinetic evidence (3a), whereas only limited evidence for such participation had been found by Winstein and co-workers [discussed in Peterson (36)] working in more nu cleophilic solvents. With tosylate solvolysis rates available for the first time in trifluoroacetic acid, Peterson and Waller (4) noted that plots of log k versus the ionizing power, Y, for the solvolysis of methyl, ethyl and secondary tosylates showed deviations from linearity that could be attributed to solvent nucleophilicity. 0065-2393/87/0215-0299$06.00/0 © 1987 American Chemical Society

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

300

NUCLEOPHL IC IT IY


From the deviations, a scale of nucleophilicity was derived. Halogenated acetic acids were included, on the basis of reactivities with halonium ions. Other scales appeared from the Schleyer group (5, 6) at about the same time. The various nucleophilicity scales were used to correlate solvolysis rates by now familiar four-parameter equations A G = Ν + mY or A G = sN + mY. (G = free energy; Ν = solvent nucleophilicity; Y = solvent ionizing power; s = sensitivity; m = sensitivity.) Previously, parameters for such equations had not been determined. The availability of the above-mentioned scales and equations led Peter son et al. (7) to reexamine an existing correlation of reaction rates by an equation involving two solvent parameters—the S w a i n - M o s e l e y - B o w n equation (8): A G = Cjdj + c d 2

2

(1)

Here, the d's are solvent parameters and the cs are sensitivities to them. We were quite interested to find that a suitable transformation revealed an equivalent pair of solvent parameters that were i n t e r p r é t a b l e as nu cleophilicity and ionizing power. The parameters had been listed in a num ber of physical organic textbooks, but their significance had not been clear. Transformation of the A and Β Parameters Recently, Swain et al. (9) introduced another set of solvent and reaction parameters for the correlation of free energies. Further commentary regard ing them has appeared (JO, 11). The free energies are given by equation 1. G = αΑ + bB + C

(2)

In equation 2, A and Β are parameters characteristic of the solvent. The parameters α and b are characteristic of the reaction. I have now converted these parameters to obtain Ν and Y values. The conversion is described in the Appendix. Kevill (12) reported a related transformation of A and Β to obtain Ν and Y that utilizes the assumption that methyl tosylate solvolysis rates obey the equation N = log (k/k ) — 0 . 3 Y to determine the proportions of Ν and Y in A and Β. Kevill tabulated values of Ν and Y for hydroxylic solvents. In common with Kevills treatment, these solvents and a few others that we designate as reaction solvents are the only ones for which we have inter preted the converted values in terms of independent nucleophilicity and ionizing power parameters. In our conversion, the assumed nucleophilicity ratio for two chosen solvents is used to determine the proportions of Ν and Y in A and Β. We have given the formulas to determine sensitivity parameters, which were not 0 T s

Q

OTs


21.

PETERSON

301 Correlation Using Two Solvent Parameters


considered by Kevill (12). Although we give converted Ν values for all solvents in the Swain data set, we note that values for solvents that do not serve as nucleophiles in the Swain data set are presently regarded as arbi trary numbers arising from the assumptions made. We shall see that these Ν parameters for nonreaction solvents tend, in fact, to be linearly related to the Y parameters. We have not determined whether the modest improvement in correlations presumably made possible by Swains determination of two solvent parameters for the nonreaction solvents may be traced to any solvent property that is describable in familiar terms. Here, we give an abbreviated description of our conversion, which is outlined fully in the Appendix. We assume that the parameters are separable into components as follows: A

s i h e p

= n Nhe A

Shep

S1

= « N B

S1 P

s l h e p

+ !^hep sl

+ yY B

S1

hep

0) (4) s l

The superscripts and subscripts are solvent designations. The term A refers to the difference in A values between heptane, a zero point on the Swain scale, and solvent SI, for example. In our first investigated conver sion, we base the Y values on the rates of solvolysis of terf-butyl chloride, as calculated from the A , B, a, and b parameters. As shown in the Appendix, making the additional assumption that the nucleophilicities of two solvents, Srefl and Sre£2, have the ratio R that allows the calculation of the constants needed to apply equations 3 and 4. The results are h e p

Ώ A Srefâ _ A Srefl = fiâîiSE * P D V Srefe _ V Srefl hep hep A

Λ

ni

H

2

il

R A Srefâ _ A

η

RA

A

S2

Srefl

SI

— = — il RR, S2 _ y SI hep hep B R

1 1 0

FL

(5) '

V

= constant

(7)

1

N

8

*RX = RX A+ ^RX^B ^RX

=

+

KXVB

() ^

Values of n N or n N for all solvents were obtained from equations 3 or 4, because the y value was available from equation 5, when the nucleophilicity ratio, R, for the reference solvents acetic and formic acid was chosen to be 1. The nN values for solvolysis solvents were compared w i t h the Schadt-Bentley-Schleyer proposed set of nucleophilicity values. The linA

B


302

NUCLEOPHL IC IT IY Slope «6.41

\

0)

DC


.J

I

I 0.2

0

I

I 0.4

I

I—ÙL 0.6

Nucleophilicity, nN, from A and Β Figure 1. Plot of nucleophilicities of Bentley et al. (6) versus the unsealed nucleophilicities n , from transformation of the A and Β parameters. Solvents were ethanol, methanol, 50% ethanol in water, water, acetic acid, formic acid (superimposed), and trifluoroacetic acid. y

AN

earity of the plot (Figure 1) shows that the A and Β values are indeed interprétable in terms of familiar parameters. The slope of the plot was used to define the magnitude of n . The numerical values used in our conversion are as follows (parameter, value): y , 0.06274; y , 0.09534; n , -0.15595; and n , 0.20386 (all data are devised from data having two to three significant numbers; the additional numbers are supplied to facilitate the reproduction of our values). The equations involving them are summarized in the Box. In Tables I and II, the "tert-butyl chloride based" A and Β values and s and m values, respectively, are given. A

A

B

A

B

Summary of Equations s = 0.15595α + 0.203866 m = 0.06274α + 0.095346 A S2 S 1

= -0.15595N S2 + 0.06274Y S2 S1

S1

B S 2 = 0.20386N S2 + 0.09534Y S2 S1

N

S2

si

S1

A

= ( si

S2

S1

- 0.06274Y S2)/-0.15595 S1

Y S 2 = 7.37A S2 + 5.65B S2 S1

S1

S1


21.

303

PETERSON

Correlation Using Two Solvent Parameters

Table I. Solvent Nucleophilicity (JV) and Ionizing Power (F) Obtained by Transformation of A and Β Parameters: ferf-Butyl Chloride Based Solvent j from Ref9

CC1 CHC1

5 6 7

HCONH MeN0 MeOH

8

cs

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

A and Β

Structure

1 2 3 4

9 10


Ν from

4

3

CH2CI2

HCOOH 2

2

2

C1 CCC1 2

2

2

C1CH CH C1 MeCOOH EtOH MeSOMe HOCH CH OH MeCOMe HCONMe CH CH CH OH Me CHOH MeCOEt 2

2

2

2

2

3

2

2

2

1.86 -1.73

-2.25 2.38

1.15 1.92

1.25 -1.13

35 36 37

-0.27

-0.85

38

1.70 1.40

-6.31 -7.05 -4.97 3.48

39 40 41

1.85 2.01 -1.73 -0.08 2.46 0.40 2.15 2.25 0.01 0.14

(CH ) 0 MeCOOEt 0(CH CH ) 0 MeCONMe BuOH EtOEt Me COH MeOC H OMe BuNH

2.45 0.05 1.54 0.76 1.59 3.32

4

2

2

2

2

3

2

2

4

32 33 34

-6.61 -1.98

2.07 2.11 1.79 2.04

2

Solvent j from Kef 9

1.64 1.39

1.85 -4.75

C1CHCC1 CF3COOH MeCN

Y from A and Β

-1.62 -2.36 -1.61 -1.79 -0.60 1.29 -2.78 -1.74 -2.07 -2.36 -3.32 -4.16 -4.32 -4.01 -1.73 -2.27 -6.39 -3.06 -4.83 -1.49

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

Structure (CH) N PhBr 5

PhCl PhN0 PhH PhNH (CH ) C0 2

2

2

5

(CH ) Me(CH ) Me 2

6

2

3

5

3

2

2

5

PhCOMe o-C H Me m-C H Me p-C H Me Me CCH CHMe 6

4

6

6

3

2

4

2

4

2.53 1.92 1.96 2.00 2.00 2.64

-2.01 -3.85 -4.05 -4.76 -4.76 0.17

2.11 1.25

-2.90

3.61 2.12 1.96 2.12 2.23 2.40 1.18 2.43 2.17 1.18

3

PhMe PhOMe PhNHMe 2.6-C H NMe Me(CH ) Me

Y from A and Β

1.12 1.33

4

Et N (Me N) P0 PhCN 2

Ν from A and Β

2

2

Bu 0 H 0 96% M e O H 80% E t O H 2

2

60% E t O H 50% E t O H 80% MeCOMe 70% MeCOMe

2

2.11 1.08 1.61 0.00 -0.06 0.07 0.17 0.17 0.63 0.58

-8.71 -9.18 -7.53 -3.16 -2.08 -5.19 -3.47 -0.21 -3.30 -9.19 -2.42 -5.76 -9.19 -5.93 -9.29 -7.17 3.82 -0.15 0.00 1.05 1.36 -0.68 -0.16

Examination of the Α , Β , N , and Y Parameters Examination of the properties of only the hydroxylic solvents was facilitated by a plotting program I wrote for the I B M personal computer. Data from two separate files may be read to an array for plotting. A third file is read to discriminate points for plotting as large or small circles. As Taft et al. (10) noted, a large proportion of the solvents considered by Swain have A roughly linearly related to Β. As expected, the Ν and Y values in our conversion exhibit a similar phenomenon (Figure 2), as do related plots (Figures 3-6). Amines and amides, some of which served as nucleo philes in the Swain data set, also deviate from the linearity of the plots for nonreaction solvents.


304

NUCLEOPHL IC IT IY

Table II. Sensitivities of Nucleophilicity(s) and Sensitivity to Ionizing Power (m) Obtained by Transformation of a and b:terf-ButylChloride Based [from Ref9 1 2


3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

s from m from a and b a and b

Reactant MeBr

1.36 1.05

MeOTs BuBr

0.97 0.98 0.22 0.10 0.03 -0.13 -0.10 -0.20 -0.32 0.00 0.37 -0.46 -1.95 -1.39

PhCH Cl 2

Me CHOTs 2

cyclo-C H OTs q/c/o-C H OTs endo-C H OTs eio-C H„OTs Ph CHCl 2-AdOTs Me CCl, Y Me CBr 5

9

6

7

u

u

7

2

3

3

PhCMe 0 COPh Ph CF 2

2

3

Ph COAc Mel + (EtCH ) N Mel + PhNMe Mel + 3-ClPhNMe Mel + 4-ClPhNMe Mel + 3-MePhNMe Mel + 4-MeOPhNMe EtI + Et N E t 0 C C H B r + Et N E t 0 C C H I + Et N 4-0 NPhF + Et N + 3

2

3

2

2

2

2

2

3

2

2

2

2

3

3

2

3

27 28 29 30 31 32 33 34

2

2

2

Br + 1-pentene Br + Me Sn 2-PhSPhC0 CMe Berson Ω 2

2

4

3

3

35

sulfoxide rearrangement PhC0 H

36 37

2-0 NPhOH picramic acid

38

o-vanillin

2

2

Reactant

0.06

39

5-methylfurfural

0.23 0.29

40

l-nitroso-2-naphthol

41 42

2-nitroso-l-naphthol Et N + IKosower Ζ MPI 3-MeOC H N + 0" (1) 3-MeOC H N + 0" (2)

0.38 0.57 0.67 0.76 0.71 0.86 1.67 0.92 1.00 0.94

43 44 45 46 47 48 49 50 51 52

4

5

4

5

4

s from a and b

m from a and b

0.26

0.18

0.47 0.26 0.08 -5.36 5.30 -1.24 -2.23 -2.13 0.27 0.36 0.30

0.23 0.25 1.59 3.65 -3.53 0.53 0.56 0.49 0.21 0.25 0.36 0.11 0.12

3-MeOC H N + 0" (3) PhN0 4-MeOPhN0 4-Et NPhN0 Ph CO pyrimidine pyridazine pyrroline oxide iron imine oximate sulfoxide Dimroth E ^ O Dimroth E 26 BrookerxR Davis A Davis Β Davis E HCONMe POCl Me CHCH Cl, trans Me HPO, band 1 (Me C) NO, Ν piperidyloxy, Ν pyrrolinyloxy, Ν 4-AcC H NMe, 2-H 4-AcC H NMe, 3-H

-1.20 -0.63 -2.29 1.02 2.49 -4.14 -0.19 -0.17 -0.18 -0.17 -0.20

4-AcC H NMe, 5-H 4-AcC H NMe, 6-H

-0.20 -0.20

0.13 0.12 0.12 0.14

5

4

2

2

2

2

2

-0.30 -0.35 -0.53 -0.47

0.85 0.78 0.77 0.78 0.78

0.43 1.11 0.75 0.47 0.50 0.57 0.57 0.49

0.78 0.75 -0.08 -0.02

0.47 0.48 0.46 0.42

-1.52 -1.11 1.26

0.60 -0.11 0.34

0.52 0.17 0.17 -0.30

0.65 1.06 1.04 0.30

-0.05 -0.13

0.04 0.11

65 66 67 68 69 70 71 72 73 74

0.61

0.18

75

4-AcC H NMe, Ac-H 4

-0.61

0.26

0.50

0.12 0.23 0.22

76 77

2-fluoropicoline, F Et PO, Ρ

-1.09 -9.62

0.33 3.60

4

N PhS0 Cl + PhNH C1S0 NC0 + hexene TCNE + 4-methoxystyrene

[from Ref9

1.41 0.32

53 54 55 56 57 58 59 60 61 62 63 64

r

C t

2

3

2

2

2

3

2

5

4

5

4

5

4

5

4

5

3

-0.45 -3.15 0.57 -3.83 -3.70 0.19 -1.87

0.23 0.24 0.23 1.61 0.17 2.33 1.89 0.99 0.50 0.22 0.10 2.93 2.06 1.42 3.63 0.10 0.09 0.10

If the nonreaction solvents (the majority of the Swain solvents) exhibit linearity between two solvent parameters, the solvent properties can be represented by only one parameter. For the nonreaction solvents, the con verted Ν values are independent of the converted Y values only to the extent that small deviations from linearity exist in the plot of Ν versus Y. The scaling


21.

305

PETERSON


and zero points must arise from Swain s assumptions of the values of certain parameters, but we have not investigated this aspect. We list all of the converted values for the convenience of others who may discern meaning in the small deviations from linearity for the nonreaction solvents or for those who simply want to use the converted parameters for correlations.

DÛ "Ό C


< Ε

'ô

JΩE Ο _CD ϋ 13

-0.8

-0.4

0

-0.4

Ionizing Power, Y, from A and Β χ 10~

1

Figure 2. Plot of Ν from A and Β versus Y from A and Β (t-BuCl-based). Hydroxylic solvents are shown as dots.

Swain A Figure 3. Plot of Ν from A and Β (t-BuCl-based) versus A. Hydroxylic solvents are shown as the larger circles.


306

NUCLEOPHL IC IT IY

We now ask whether our converted Ν and Y parameters for the hydrox ylic reaction solvents resemble the A or β values. This resemblance is not easy to judge from the data of Table I because the range of the various parameters affects the size of the multipliers, y , y n , and n . Plots (Figures 3 and 4) where only the hydroxylic solvents (large circles) are considered significant show that our derived nucleophilicity, N, is roughly


A

0

B?

A

0.4

Swain Β Figure 4. Flot of Ν from A and Β (t-BuCl-based) versus B.

0

m

"D C

0

03

' f 'f I I I 1 0.8 0 0.4 S wain A

1

. . 1.2 1.6

Figure 5. Plot of Y from A and B (t-BuCl-based) versus A.


B

21.

PETERSON

307 Correlation Using Two Solvent Parameters


linearly related to the Swain A value (with a negative coefficient), whereas the Β value shows less correlation with Ν. Although it may seem surprising that the "electrophilic" A parameter correlates with — N , we surmise that the Ν parameter represents both bondforming "true nucleophilicity" and electrophilicity (the ability to promote the ionization of oxygen- or fluorine-containing leaving groups in solvents of low N). This hypothesis has been previously mentioned in connection with our conversion of Swains d parameters. (7). Apparently, neither A nor Β is closely correlated with Y (Figures 5 and 6).

Swain Β Figure 6. Plot of Y from A and Β (t-BuCl-based) versus B.

Examination of R We next investigate the effect of the ratio R in equation 5. In the original nucleophilicity scales, the nucleophilicities of acetic and formic acids were set equal, based on interpretations of solvolysis data in the literature and on the nucleophilicities of these in S 0 solvent, a property perhaps somewhat distantly related to the nucleophilicity in a pure solvent. We have now calculated y as a function of R; y changes sign at an R value near 1.2 (Figure 7). Equation 3 shows that Ν values become scaled A values when y becomes 0 at an R value near 1.2. We note that changing R is merely a roundabout way to vary the proportion of A and Β in the Ν and Y parameters. For purposes of calculating the free energy change for a change of solvent in a reaction, whether we use solvent parameters based on R = 1.0 or 1.2 or any value is irrelevant. A l l parameters are mathematically equiv2

A

A

A


308

NUCLEOPHL IC IT IY 0


1 -

ο

0

0

1

2

Assumed ratio, Ν (Acetic/Formic) Figure 7. Plot of y from equation 5 versus the chosen Ν ratio, R, for acetic and formic acid. A

aient! For computational purposes, whether acetic and formic acid are as signed the same nucleophilicity is irrelevant. Accordingly, concerns that the assignment of R = 1 has led to erroneous scales can be forgotten. Another way to view the situation is that Ν scales work when various proportions of the Y solvent property are added or subtracted, because any amount of the Y property may be resubtracted or added to Ν in choosing the proportion and sign of the two solvent parameters to represent a free energy change. What property independent of solvent effect correlation might be used to select a chemically reasonable Ν scale? One such possibility is to set the relative nucleophilicities of the reference solvents, acetic and formic acid in the example discussed here, to be the same as those values found for the molecules acting as nucleophiles in a constant solvent. That task, in fact, was done for acetic acid and formic acid reacting with halonium ions in S 0 . This assignment led to a nucleophilicity scale having additional properties in agreement with the properties found in the constant solvent. These proper ties are the lower nucleophilicity of trifluoroacetic acid and the higher nucleophilicity of alcohols. We see that the Ν scale can be chosen to be chemically reasonable and preserve an accuracy of calculation identical with that of the AB scale (subject only to the two-significant-figure accuracy available in making the conversion). 2

Examination of the Sensitivities We note that the sensitivities to nucleophilicity for methyl, primary, second ary, and tertiary halides resemble those in constant solvent in our converted



21.


PETERSON

309

V scale. This observation provides another reason to prefer our converted values in comparison to the original values, in which sensitivities were not readily interprétable in terms of independent chemical experience. The ethyl tosylate to methyl bromide s ratio in the Swain-Scott study is 0.66, compared to the butyl bromide to methyl bromide s ratio of 0.71 obtained from equation 8. The latter equation also gives a reasonable isopropyl tosy late to methyl tosylate s ratio (0.21). As Swain noted, the b parameters, discussed as comparable to s, have no familiar order. Both a and b have large magnitudes for tert-butyl chloride reactions, for example, whereas the sensi tivity of 0, set in the present conversion, leads to the familiar s order methyl > primary > secondary > tertiary. The s sensitivities are determined by the choice of a reaction whose sensitivity to Ν is set equal to 0 (see equation 8). If we explore parameter conversions that provide a closer match of sensitivities to those in constant solvent than those based on tert-butyl chloride rates, we can base Y on another standard reaction, or even an imaginary reaction correlated by any desired proportion of A and Β. Other Standard Reactions Because the solvolysis of adamantyl chloride has been used as a standard reaction, we list here the results of applying the conversion equations to this standard, using the equation given by Swain to represent the adamantyl rates. R of equation 5 was again set equal to 1 for acetic and formic acids. The y value (equation 5) was found to be 0.04684, compared with 0.0627 when tert-butyl chloride solvolysis was the standard reaction for obtaining Y. The n N values from solving equation 3 for nN, like those obtained by using Y values based on tert-butyl chloride, needed to be multiplied by a scale f a c t o r to o b t a i n Ν v a l u e s c o m p a r a b l e to t h o s e i n t h e Schadt-Bentley-Schleyer scale. For simplicity, we used the previously used scale factor n = 6.51 to derive the new adamantyl-based Ν scale. The spread of Ν values is slightly greater for the adamantyl-derived values obtained in this way. A plot (not given here) showed that the "Y _butyi based and "Y damantyi based Ν scales are very similar. The Y scales themselves as obtained from the A and Β parameters showed slightly greater differences. The adamantyl-based Ν and Y scales are given in Table III. Sensitivity parameters are given in Table IV. A

A

A

feri

a

Conclusion Perhaps a main benefit of converting A and Β parameters and sensitivities to them to other scales has been the clarification of how two-parameter equa tions work as applied to solvent effects. Swains statistical method, which avoids the assignment of solvent parameters based on any one reaction, is an


310

NUCLEOPHILICITY

Table III. Solvent Nucleophilicity (JV) and Ionizing Power (Y) Obtained by Transformation of A and Β Parameters: Adamantyl Tosylate Based


Solvent j from Ref9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 a

Structure

CC1 CHCLj CH C1 HCOOH HCONH MeN0 MeOH CS C1 CCC1 C1CHCC1 CF3COOH MeCN C1CH CH C1 MeCOOH EtOH MeSOMe HOCH CH OH MeCOMe HCONMe CH CH CH OH Me CHOH MeCOEt (CH^O MeCOOEt 0(CH CH2) 0 MeCONMe BuOH EtOEt Me COH MeOC H OMe BuNH 4

2

2

2

2

2

2

2

2

2

2

2

2

2

3

2

2

2

2

2

2

3

2

4

2

Ν from A and Β

Y from A and Β

1.41 1.19 1.60 -1.49 0.99 1.65 -0.23 1.46 1.21 1.85 -4.09 1.60 1.73 -1.49 -0.06 2.11 0.34 1.85 1.94 0.00 1.12 1.78 1.82 1.54 1.76 2.11 0.04 1.32 0.66 1.37 2.85

-6.56 -2.23 -2.63 2.75 0.81 -1.62 -0.67 -6.30 -6.88 -5.12 4.72 -2.05 -2.78 -0.90 -1.60 -1.31 1.08 -3.21 -2.29 -1.88 -2.19 -3.68 -4.46 -4.51 -4.31 -2.34 -2.08 -6.39 -3.02 -4.91 -2.40

Solvent j from Ref9 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

Ν from A and Β

Structure

(CH) N PhBr PhCl PhN0 PhH PhNH (CH ) CO (CH ) Me(CH ) Me Et N (Me N) PO PhCN PhMe PhOMe PhNHMe 2,6-C H NMe Me(CH ) Me PhCOMe o-C H Me m-C H Me p-C H Me Me CCH CHMe Bu 0 H 0 96% MeOH 80% EtOH 60% EtOH 50% EtOH 80% MeCOMe 70% MeCOMe 5

2

2

2

5

2

6

2

4

3

2

3

5

3

2

6

4

6

6

3

2

2

2

5

2

4

2

4

2

2

2

2.18 1.92 1.69 1.72 1.72 2.27 1.82 1.07 0.97 1.15 3.10 1.82 1.68 1.84 1.92 2.06 1.01 2.09 1.87 1.01 1.81 0.93 1.38 0.00 -0.05 0.06 0.14 0.14 0.54 0.50

Y from λ and Β -2.62 4.12 -4.32 -4.98 -4.98 -0.66 -3.30 -8.36 -9.75 -7.31 -4.02 -2.55 -5.36 -3.84 -0.88 -3.76 -9.78 -2.97 -5.95 -9.78 a

-9.29 -7.17 3.82 -0.15 0.00 1.05 1.36 -0.68 -0.16

Not determined.

interesting one. The A and Β parameters may be transformed to mathe matically equivalent ones by choosing any reaction to represent one of the new solvent parameters, provided the reaction is not one of those that actually requires only one parameter for a reasonable correlation. The other new solvent parameter is, by definition, not present in the correlation equation for the chosen reaction. This observation leads us to the relative amounts of the second parameter to be used in the transformation (equation 7).


21.

311


PETERSON

An infinity of scales for the second new solvent parameter are still available, because the reaction data offer no way to discern whether some of the first solvent property is admixed with the second. The chemist may elect to use one of the scales for the second parameter, which reflects his chemical experience under different circumstances—for example, experience with reactions in constant solvent.

Table IV. Sensitivities of Nucleophilicity (s) and Sensitivity to Ionizing Power (m): Adamantyl Tosylate Based


Solvent i 1 2 3 4 5

s from a and b

Reactant

MeBr MeOTs BuBr PhCH Cl 2

Me CHOTs 2

6 7 8 9 10 11 12 13 14 15 16 17 18

ct/c/o-C H OTs q/c/o-C H OTs endo-C \{ OTs exo-C H OTs Ph CHCl 2-AdOTs Me CCl, Y Me CBr PhCMe 0 COPh Ph CF Ph COAc Mel + (EtCH ) N

19 20 21 22 23 24 25 26 27 28

Mel + 3-ClPhNMe Mel + 4-ClPhNMe Mel + 3-MePhNMe

29 30 31 32 33 34 35 36 37 38 39

5

9

6

u

7

7

n

n

2

3

3

2

3

2

Mel + PhNMe

3

2

2

1.82 1.00 1.00 0.02 0.47 1.21 0.81 0.52 0.54 0.62 0.62 0.53

49 50 51 52 53 54

0.51 0.52 0.50 0.46

61 62

0.66 -0.11 0.37

TCNE + 4-methoxystyrene 0.86 Br + 1-pentene 0.62 Br + Me Sn 0.17 2-PhSPhC0 CMe -0.23 Berson Ω -0.04 sulfoxide rearrangement -0.10 PhC0 H 0.78 2-0 NPhOH 0.63 picramic acid 1.73 o-vanillin 0.32 5-methylfurfural 0.37

0.71 1.15 1.14 0.33 0.04 0.12 0.20 0.13 0.25 0.24 0.19

2

2

2

2

3

3

2

4

3

2

2

2

2

2

4

3

2

2

3

Et N + IKosower Ζ MPI 3-MeOC H N + 0~ (1) 3-MeOC H N + 0" (2) 3-MeOC H N + 0" (3)

45 46 47 48

3

2

42 43 44

0.73 0.83 0.78 0.94

1.10 Mel + 4-MeOPhNMe 1.09 EtI + Et N 1.06 E t 0 C C H B r + Et N 0.09 E t 0 C C H I + Et N 0.15 4-0 NPhF + Et N + N _ -1.52 PhS0 Cl + PhNH -1.33 C1S0 NC0 + hexene 1.26 2

l-nitroso-2-naphthol 2-nitroso-l-naphthol

0.39 0.34 0.14 0.23 0.45 -0.00 0.40 0.81

0.26 0.32 0.42 0.63

Reactant

40 41

0.07

1.12 1.13

2

Solvent i

1.61 1.32 1.24 1.30 0.49

-0.36 -1.82 -1.31 1.18 1.10

2

3

m from a and b

55 56 57 58 59 60

63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

4

5

4

5

4

5

4

PhN0 4-MeOPhN0 4-Et NPhN0 Ph CO pyrimidine pyridazine pyrroline oxide iron imine oximate sulfoxide Dimroth Ej30 Dimroth £ 2 6 BrookerxR 2

2

2

2

2

r

Davis A Davis B Davis E HCONMe P0C1 Me CHCH Cl, trans C T

2

3

2

2

s from a and b 0.64 0.41

0.25 0.28

0.73 -4.76 -4.73 -1.23

1.73 3.98 3.85 0.58

-2.37 -2.28 0.40 0.52 0.49 -0.31 -0.36 -0.53 -0.47 -0.43 -3.01 0.60 -3.51 -3.54 0.62 -1.97

0.62 0.53 0.21 0.27 0.40 0.12 0.13 0.26 0.26 0.25 1.75 0.18 2.54 2.07 1.08 0.55 0.24

-1.31 -0.69 -1.48 2.01 3.47

Me HPO, band 1 (Me C) NO, Ν

-3.35 -0.18

piperidyloxy, Ν pyrrolinyloxy, Ν 4-AcC H NMe, 2-H 4-AcC H NMe, 3-H 4-AcC H NMe, 5-H 4-AcC H NMe, 6-H 4-AcC H NMe, Ac-H 2-fluoropicoline, F

2

3

2

5

4

5

4

-0.16 -0.16 -0.14 -0.19 -0.18 -0.18 -0.61

Et PO, Ρ

-1.14 -9.73

5

4

5

4

5

3

4

m from a and b


0.11 3.20 2.25 1.55 3.96 0.11 0.10 0.11 0.15 0.13 0.13 0.15 0.28 0.36 3.92

312

NUCLEOPHILICITY

Appendix: A Full Description of the Conversion of A and Β to Ν and Y As already noted, the parameters are assumed to be separable into compo nents as follows: A

s l h e p

Bhe

= n JV Si + y Y e A

S1 P

hep

= n N B

A

s l

h

s

(3)

sl

+ yY

h e p

S1 P

(4)

hep

For a second solvent S2, equation 3 becomes S2

A Downloaded by RUTGERS UNIV on March 11, 2016 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch021

82

= nN ™

hep

A

+ ifcYhep

he

(10)

Calculating the y s and the Ratio of the ns. In our first calculation, we base the Y scale on terf-butyl chloride solvolysis rates as calculated from the expression given by Swain et al. (9): log k (tert-butyl chloride) = 7.37A + 5.65B — 6.10. We simply subtract the result of the calculation of log k (tertbutyl chloride) for the two solvents of interest to obtain the Y values to be used in equation 3. These Y values preserve the advantage of the Swain approach in that any unusually large error in one rate constant of a standard reaction is minimized, because the A and Β values are optimized for a number of reactions. We again note that the Swain parameters are compati ble with an infinite group of Ν scales. Any one of these scales may be obtained by assuming a value of the ratio, R, of nucleophilicity of two reference solvents designated as follows: SI = Srefl and S2 = Sref2 s l

S2

We replace N in equation 3 with RN . We then solve equation 3 for he - We solve equation 10 for the same quantity, where S2 is the reference solvent SrefS. Two equations that are equal to nN of solvent ref2 result. Equating these equations and solving for y gives h e p

nN

HEP

S2

P

A

v

= A

^hep RY

Srel2

h e p

~ Kef* ^ - Y s-n

(5)

hep

A similar expression having Β in place of A and the same denominator is obtained for y . Dividing the equations gives B

u !/A

VB

RA =

^

S r e i 2

RB ^ HEP

Srefl

—A

V

^ h e p _

-

Y

h e p

=

/

f

R

j

(

6

)

s-n

We may solve equations 3 and 4 for n and n in solvent SI where SI is any solvent. Dividing gives A

B


21.

PETERSON

313

Correlation Using Two Solvent Parameters ΏΑ

n

2*

S2

_

«*!SL

=

n

A

RB S2 -

B

SI

V

y

hep

_ hep

=

c

o

n

s

t

a

n

t

( 7 )

si


This equation says that the ratio of the proportions of nucleophilicity that are present in the A and Β parameters may be obtained from the parameters for any solvent by subtracting the contribution of ionizing power from each (A and B) and dividing. Although the y values depend on the assigned value of the ratio, K , as do the yly ratios (equation 6), the quotient in equation 7 is, remarkably, independent of the y values, within the error limits posed by significant figures. Sample calculations have confirmed this result. A proof that this statement is correct comes from an alternative way to get the ratio. Calculating Sensitivities to Nucleophilicity and Ionizing Power. If the right-hand terms in equations 3 and 4 are put into equation 11, we can collect the terms AG = aA Si +

feB^i

hep

(11)

that contain Ν and Y. The multipliers of Ν and Y are the sensitivities, usually called I and m (5) or s and m (7). As in our earlier conversion (7), sensitivities are = b^rig ™RX

=

(8)

+

(9) In the conversion of A and Β to Ν and Y, the Y values represent the best fit of equation 1 to the rates of solvolysis of terf-butyl chloride. Clearly, none of the Ν solvent property should be added to Y to give a better fit, because the fit is already optimized. Therefore, s = 0 for the reactions of this compound. Equation 8 with 5 = 0 leads to

n

A

— n

B

~^fBuCI

=— a

-5

64

=—ψ£ fBuCl

=-0.76526

12 V

7.37

'

Within the limits imposed by the availability of two significant figures in as and b s, this number agrees with the values obtained in equation 8 from A , B, and Y values in various solvents. Finding the Nucleophilicities. Values of n N or n N for any solvent can be obtained from equations 3 or 4, because we now have the y values from equations 5 and 6. The nN values may be compared with the proposed A

B


NUCLEOPHILICITY

314

sets of nucleophilicity values in the literature. Such a comparison will be appropriate only if we have chosen the ratio R for the nucleophilicities of acetic and formic acid to be 1, because this same assumption was made in setting up the scales in the literature. As has been noted, we have plotted the n N values versus the Ν values of Schadt, Bentley and Schleyer (Figure 1). The slope of the plot is the n value, which may be considered to be a scaling factor. Dividing nN by this η scale factor gives Ν values extracted from the A and Β parameters. This Ν scale initially has no chosen zero point, although a few values exist that are not far from zero. The Ν value for water may be subtracted from the Ν value in each solvent to get values that are comparable to the published scales. A l l of these Ν values based on A and Β are given in Table II. Y values obtained as described also appear in Table II. The s and m values (equations 8 and 9) are listed in Table III. In Table I, other numerical values and relationships are given. A


A

Literature Cited 1. Cope, A. C.; Grisar, J. M . ; Peterson, P. E. J. Am. Chem. Soc. 1959, 81, 1640-1642. 2. Peterson, P. E . ; Kelly, R. E . ; Belloli, R.; Sipp, K. A.J.Am. Chem. Soc. 1965, 87, 5169-5171. 3a. Peterson, P. E . ; Bopp, R. J.; Chevli, D. M . ; Curran, E. L.; Dillard, D. E . ; Kamat, R. J.J.Am. Chem. Soc. 1967, 89, 5902-5910. 3b. Peterson, P. Ε . Acc. Chem. Res. 1971, 4, 407-413. 4. Peterson, P. E . ; Waller, F. J.J.Am. Chem. Soc. 1972, 91, 991-992. 5. Schleyer, P. v. R.; Fry, J. L . ; Lam, L. K.; Lancelot, C. J. J. Am. Chem. Soc. 1970, 92, 2542-2544. 6. Bentley, T. W.; Schadt, F. L . ; Schleyer, P. v. R.J.Am. Chem. Soc. 1972, 94, 992-995. 7. Peterson, P. E . ; Vidrine, D. W.; Waller, F. J.; Henrichs, P. M.; Magaha, S.; Stevens, B. J. Am. Chem. Soc. 1977, 99, 7968-7976. 8. Swain, C. G.; Mosely, R. B.; Bown, D. E. J. Am. Chem. Soc. 1955, 77, 3732-3734. 9. Swain, C. G.; Swain, M . S.; Powell, A. L.; Alunni, S.J.Am. Chem. Soc. 1983, 105, 503-513. 10. Taft, R. W.; Abbout, J. M . ; Kamlet, M . J.J.Org. Chem. 1984, 49, 2001-2005. 11. Swain, C. G. J. Org. Chem. 1984, 49, 2005-2010. 12. Kevill, D. N . J. Chem. Res. 1984, 86-87. RECEIVED for review October 21, 1985. A C C E P T E D June 30, 1986.


THE GREAT TENDENCY OF CYCLOOCTANE OXIDE to react by

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