Solvolysis of Electron-Deficient 1-Arylethyl Tosylates - American

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Solvolysis of Electron-Deficient 1-Arylethyl Tosylates Kinetic and Stereochemical Tests for Nucleophilic Solvent Participation Annette D . Allen, V. M . Kanagsabapathy, and Thomas T. Tidwell* Department of Chemistry, University of Toronto, Scarborough Campus, Scarborough, Ontario M1C 1A4, Canada

The rates and stereochemistry of solvolysis of 1-arylethyl tosylates with electron-withdrawing substituents on the α-carbon or the aryl ring have been determined and suggest initial formation of ion pairs, which undergo competitive return to reactant or formation of prod ucts. The stereochemistry of substitution varies from major inversion to racemization to small net retention as a function of solvent and substrate. These electron-deficient carbocations show large magni tudes of ρ values but modest values of the solvent parameter m. These results indicate strongly nucleophilic solvents assist the ioniza tion of the less reactive substrates, whereas in the weakly nucleophilic media, solvent attack on reversibly formed ion pairs is rate-limiting. The first intermediate interacts with a solvent shell but without specific attachment to a single solvent molecule. +

H o w

NUCLEOPHILIC SUBSTITUTION OCCURS has been one of the preemi

nent problems in mechanistic organic chemistry for more than 50 years (1-3), and the importance of this process more than justifies this sustained interest. A n area that has received particularly close scrutiny is the "bor derline" or "combat zone" (2) region where potential competition between mechanisms of the S 1 and S 2 type exists. These processes involve forma tion of a nonspecifically solvated carbocation intermediate in the former case and a direct displacement of the leaving group by a solvent molecule in the latter and are designated k and k processes for solvolysis, respectively. N

N

Q

s

* To whom correspondence should be addressed.

0065-2393/87/0215-0315$06.00/0 © 1987 American Chemical Society

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

316

NUCLEOPHILICITY

The suggestion that a gradual transition occurs between these mecha nisms so that in the intermediate region the two processes merge, and a fivecoordinate intermediate exists on the reaction coordinate has received inter est recently. The possibility of partial covalent character in the solvolysis transition state has been considered for a long time (4) and was given particular prominence by the report of Doering and Zeiss in 1953 (5) that methanolysis of 3,5-dimethyl-3-hexyl phthalate proceeded with net 54% inversion of configuration and 46% racemization. The structure of this inter mediate was depicted as I (equation 1) (5), and the reaction coordinate for the merging of the mechanisms is depicted in Figure 1 (6).

I As the leaving group departs, the positive charge developing in the remaining organic moiety is not restricted to one atom but is delocalized throughout the structure where it interacts with many solvent molecules, and the electrophilic role of the solvent in assisting departure of the leaving group by hydrogen bonding has also been emphasized (1-7). The distinctive aspect of the "S 2-intermediate" (6) or Doering-Zeiss (5) proposals would appear to be the suggestion of a particularly strong backside interaction by one specific solvent molecule that exists for a sufficiently long time that a discrete chemical intermediate exists. As a test of the extent of nucleophilic solvent participation in transition states in the "borderline" or "combat zone" region, we studied 1-arylethyl N

A

Figure 1. Schematic repre mtation of thv upper jrtion of potential energy surfaces for merging oj substitution mechanisms (6). Reproduced from reference 6. Copyright 1981 American Chemical Society.


22. ALLEN ET AL.

Solvolysis of Electron-Deficient 1-Arylethyl Tosylates 317

systems that are deactivated by electron-withdrawing fluorine atoms. These systems are chiral and thus permit determination of the stereochemistry of the substitution and furthermore have been the subject of intensive previous study (1-3, 8) and are known to have a reactivity in the region where S 1 and S 2 processes are becoming competitive and a merging of mechanisms might occur. The presence of electron-withdrawing fluorine atoms either on the carbon adjacent to the substitution center or as part of ring substituents would be expected to destabilize (9) the developing carbocations by increas ing their electron deficiency (10) and should thereby favor nucleophilic participation by the solvent. N

N

Results The preparation of the l-aryl-2,2,2-trifluoroethyl tosylates I—III and the mesylate IV was described (11-16) as well as the solvolytic rate constants of these derivatives i n a range of solvents (11-16). Solvolytic and polarimetric rate constants were also obtained for optically active l-phenyl-2,2,2-trifluoroethyl triflate Va and solvolytic rate constants for the ot-deuterated analogue Vb (11). Solvolytic and polarimetric rate constants were also meas ured for optically active Vla-c (17).

la, lb, Ic, Id,

X X X X

CH 0 CH H Br 3

3

Ha,X Hb,X IIc,X Hd,X He,X Hf, X

CH ;

R

H; H; CH ; H;

R R R R

3

CH3O; R 3

IV


CF CF CF CN CN CH 3

3

3

1 3

3

318

NUCLEOPHILICITY

For comparative purposes, the solvolytic rate constants for I-VI at 25 °C in hexafluoroisopropyl alcohol (HFIP) are given in Table I, together with the m values, which measure the dependence of the rates on the solvent ionizing power parameter Y by the equation log (k/k ) = m Y (3, 6). O T s

0

O T s

Table I. Rate Dependence on Solvent Ionizing Power of Benzylic Sulfonates ArCR(CF )OTs Relative to C H CH OMs at 25 °C in HFIP 3

6

R

Ar

No.

5

2

k(25 °C)

m

Ref

Kel

ArCR(CF )OTs 0.583 5.66 x ΙΟ" 4.03 x ΙΟ" 2.95 x 10" 3

p-anis p-tol CH CH ^βΗ 6

5

6

5

Η Η Η CH CF CN CN CF Η Η

5

CH 6

3

3

5

p-tol p-anis 1-naphthyl 9-anthryl C H CH OMs 6

la lb Ic Ilf lie lid He lib III IV

5

3

0.76 0.94 0.69 1.01

63 6.1 x 4.4 x 0.32 1.2 x 6.1 X 2.1 x 25 0.16 13 1.0

5

8

3

4fl

0.66 0.76 0.94 0.64

1.94 x 10" 0.235a 1.50 x ΙΟ" 0.124 9.23 x ΙΟ" 3

3

2

11 11 11 14 12 12 15 12 16 16 32

3

ΙΟ" 10"

6

8

ΙΟ"

io-

7 2

ΙΟ"

ArCH(OTs)CH

3

m-BrC H p-CF C H 3,5-(CF ) C H 6

3

Via VIb Vic

4

6

3

4

2

6

3

29 2.4 1.5 X IO-

0.82* 0.267 0.78 2.19 Χ ΙΟ" 1.36 x 10-

2

fc

5

c

17 17 17

3

3

= k(25 ° C ) / ( 9 . 2 3 x 10"). Interpolated from the mY correlation. Derived from the plot excluding C H C H O H solvents. Derived from rates for T F A and H F I P only: log k = 1 . 3 6 Υ

NOTE: k

re]

a b

3

c

2

Ο Τ 5

-

9.74.

The products of the reactions involve substitution at the central carbon with the exception of l-(9-anthryl)-2,2,2-trifluoroethyl mesylate (VI), which undergoes extensive ring substitution at the 10-position (16), and lib (11,12, 14-17). The stereochemistry of substitution of Va and Vla-c was also deter mined (11, 17). ArCHCHo I OTs 0

Via, A r = m - B r C H Mb Ar = p - C F C H 6

Va, R = H Vb, R = D

3

Vic

4

6

4 H

Ar = 3 , 5 - ( C F ) C 3 3

2

6

The reaction of lib in T F E gave the expected substitution product Vila, but solvolysis in C H C H O H gave a mixture of Vllb and VIIc in a ratio of 50:50 as determined by vapor pressure chromatographic separation and isolation (Chart I) (15). Solvolysis in C D O H gave Vlld and Vile in a ratio of 3

2

3


22.

Solvolysis of Electron-Deficient 1-Arylethyl Tosylates

ALLEN ET AL.

319

CF„CH OH 9

C

H

3 ° \ U / -

C

(

C

V 2

O

T

s

lib C H

3

0 -

- C ( C F ) O C D 3

V U d , S = CHg

2

VII e ,

3

S = CD g

Chart I +

69:31 as analyzed by the relative intensity of their respective M peaks at 291 and 294. Trifluoromethyl Substituent Effects The destabilizing effect of the C F group adjacent to a developing carboca tion center is illustrated by the fact that CF -substituted alkenes are ΙΟ^-ΙΟ times less reactive i n protonation according to equation 2 (R = A r or C H C H O ) than the corresponding derivatives where hydrogen replaces C F (18, 19). These rates are predicted by equation 3, where the electrophile substituent parameters σ (21) for the group R and C F are summed (20). The strong electron-withdrawing power of the C F group is manifested by its σ value of 0.61 (21), and use of this parameter permits quantitative predic tion of the kinetic effect of this group by using equation 3. 3

7

3

3

3

+

p

3

3

+

p

R

/ CH =C ^ 2

R

/

„, H

+

CH C+ 3

CF, l°gV

= -10.5Σσ

( 2 )

CF, + p

- 8.92


(3)

320

NUCLEOPHILICITY

In solvolytic reactions, the presence of C F groups in positions rather remote from the reaction center leads to significant rate depressions com pared to hydrogen (22-26), as in examples V I I I - X (22-24), where the rate accelerations for replacement of the C F groups by hydrogen are shown in parentheses. 3

3

5

11

VIII (2 x 10 )

12

(22) I X (10 )

Χ (10 ) Mos

= p-CH OC H S0 3

6

4

Trifluoromethyl substituents in the α position are also strongly de stabilizing in the solvolysis of 2-(trifluoromethyl)propyl triflate (XI), which is less reactive than 2-propyl triflate by factors ranging from 1.5 x 10 in C H C H O H to 4 Χ 10 in T F A (27). Comparable factors were found for the tosylate corresponding to X I (28). These compounds may be interpreted as reacting through an ion-pair mechanism (equation 4) on the basis of the large rate deceleration due to the C F group, the exclusive formation of 2-(trifluoromethyl)propene as the only product in all solvents, the large rate enhancements caused by basic salts, and the noncumulative isotope effect k(d )/k(d ) = 1.78 and k(d )/k(d ) = 3.80 for deuteration of one or both methyl groups (27). 4

6

3

2

3

0

3

0

6

C F C ( C H ) O T f «=• C F < Î ( C H ) OTf- — C F C ( C H ) = C H 3

3

2

3

3

2

3

3

2

(4)

XI The observed absence of a correlation of the rates of X I with the Y solvent ionizing power parameter (3, 6) or with the new Y parameters for triflates (29, 30) is also indicative of a significant kinetic effect of the solvent acting as a base in assisting the elimination according to equation 4. This process is an example of the E 2 mechanism and has also been invoked to explain the results of solvolysis of some α-carbonyl- and a-phosphorylsubstituted mesylates (31-34). Such solvent-assisted elimination could result by the "S 2-intermediate" mechanism (6), but the high degree of carbocation character indicated by the large decelerating effect of C F in X I and the noncumulative isotope effect, which was quantitatively accounted for on the basis of the ion-pair scheme in equation 4 (27), argues against the former interpretation. 0 T s

O T f

+

C

N

3


2

Solvolysis of Electron-Deficient 1-Arylethyl Tosylates 321

22. ALLEN ET AL.

Solvolyses of systems A r A r ' C ( O T s ) C F and the related bromides have also been studied and also show rate decelerations of approximately 10 for C F relative to hydrogen (35, 36). The highly crowded nature of these substrates appears to preclude any kinetically important nucleophilic solvent participation. The constancy of the fc(H)/fc(CF ) rate ratios in comparison to less crowded systems suggests either that no steric effects involve the C F groups on the k reactivity of these substrates or that any such effects tend to cancel. 3

5

3

3

3

c

Tertiary Benzylic

Derivatives

The solvolyses of the tertiary α-trifluoromethyl-substituted benzylic tosy lates Ila-f would be expected to be protected from any involvement of direct nucleophilic solvent participation during departure of the leaving group for steric reasons, and the experimental evidence is consistent with this inter pretation. Thus, for the ring-substituted analogues of lie and Ilf p values (21) of - 10.7 (15) and - 6.85 (37), respectively, indicate a very high degree of cationic character with a high demand for electron donation by the aryl substituent, particularly in the former doubly destabilized system. For Ilf, the isotope effect k(CO )/k(CH ) of 1.3-1.6 (depending on solvent), the modest kinetic effects of added salts, and the calculated rate acceleration of 2 x 10 for replacement of the C F by H are all indicative of a carbocation route, as indicated in equation 5 (14). The question of the reversibility of the initial ionization cannot be definitely answered in the absence of any data on optically active or specifically O-labeled derivatives. +

3

3

5

3

ls

CF

^ C F

3

3

ArCOTs ^ = = ± A r C + OTs"

I

>· products

(5)

\

R

R

The dependence of the rates of solvolysis of the tertiary systems lib and Ilf on the solvent ionizing power by the single-parameter equation log (A/fc ) = mY (6) is listed in Table I along with a number of other values for destabilized systems. Interestingly, the m value of 0.76 for lib is consider ably less than that of 1.01 for Ilf, and a plausible explanation of this result is that the former system, which is destabilized by two C F groups, has greater charge derealization into the ring and a consequent lower demand for charge dispersal by the solvent. The product from lib in C H C H O H or C D O H involves attack of the solvent on the ring, as evidenced by the formation of the ipso substitution products VIIc and Vile (15). Evidently, the carbocation intermediate XII reacts with solvent to form the intermediate XIII, as shown in equations 6-8. Two processes may be envisaged for the conversion of XIII to the 0

O T s

3

3

2

3


322

NUCLEOPHILICITY

products, one involving reionization of XIII to XII and the other involving direct formation of VII from XIII by a 1,5 shift (equation 9).

C(CF3)2

CH30^^3V" \

SO

CFo

CH O

CF,

J==

(6)

+

/

H (k ) 3

3

XIII (a, S = Et; b, S = C D

XII

3

Vllb (S - Et) XII

SOH

(7)

C(CF ) OS

CH

3

2

Vlld (S = C D

3

VIIc (S = Et) C(CF ) OS

XIII

3

2

VIIe(S = C D )

(8)

3

CH,0

-A -#

Vllb, Vlld

(9)

However, the mechanism of equations 6-8 is evidently favored, as the results are consistent with this process but not that of equation 9. Thus, no product was observed from reaction of lib in either C H C H O H or C D O H that resulted from a shift of the C H 0 group to the benzylic carbon, and particularly for XHIb such a shift should be equally probable as the shift of CD 0. Mass spectrometric analysis of the product in C D O H shows that the ratio of M ions corresponding to Vlld and Vile is 69/31. A simple steadystate analysis of equations 6-8 predicts the ratio of products Vlllb/VIIc or Vlld/VIIe w i l l equal 2Jfc /Jfc , so that for C D O H , k = 0.9k F o r C H C H O H , Vllb equals VIIc, so k = 2k The greater preference of C H C H O H for attack at the ipso carbon relative to C H O H may reflect a steric preference with the larger nucleophile attacking at the para position. 3

2

3

3

3

3

+

1

3

2

3

2

2

3

2

2

v

v

3

Secondary Derivatives The solvolyses of the secondary l-aryl-2,2,2-trifluoroethyl derivatives I and III-V would be more likely candidates for kinetically significant solvent


22.


ALLEN ET AL.

323

participation than the tertiary derivatives because of the greater steric ac cessibility to the carbinyl center by solvent and because of the increased electron demand of the secondary derivatives compared to Ilf. However, by a number of criteria no strong evidence exists for a major role for rate-limiting solvent participation concurrent with leaving-group departure. Thus, the p values for I range from —6.7 to —11.9 depending upon the solvent, with the smallest magnitude of p occurring for T F A (II). The large magnitudes of these values provide strong evidence for carbocationic intermediates. The isotope effects k(H)/k(D) for V range from 1.21 to 1.34 depending upon the solvent, and this finding is also consistent with ratelimiting formation of carbocations (II). The evidence on the stereochemistry of the solvolysis was obtained from optically active C H C H ( O T f ) C F (Va), which gave ratios of k , the polar imetric rate constant, and fc , the rate of product formation, of 7.3, 27, 1.2, 1.1, and 1.0 in T F A , HFIP, T F E , H O A c , and C H C H O H , respectively (II). The acetolysis product from (-)-Va was 41% inverted and 59% racemized (II). These kinetic and stereochemical results for the tosylates I and triflates V may be consistently interpreted in terms of initial ionization to an ion pair that undergoes some return to optically inactive starting material and is converted to product by attack of solvent (Scheme I). The observation of substrate racemization during solvolysis appears to be incontrovertible evi dence for an ion pair that has a rather long lifetime in the poorly nucleophilic T F A and H F I P and hence has considerable opportunity for racemization and ion-pair return. In the more nucleophilic solvents, the ion pair is captured by the solvent more rapidly so that racemization and ion-pair return are no longer competitive. The fact that acetolysis of ( — )-Va gave partial net inver sion indicates the triflate leaving group effectively shielded one side of the cation so that the solvent preferentially attacked the other side. +

+

6

5

3

a

uv

3

ph

4 CF

V

2

V

COli

Ç OTf"

3

CF,

TfO"

+

C* * ^

,

PhCH(0S)CF

ψ TfOC^

CF,

3

Scheme I


CF

3

NUCLEOPHILICITY

324

The m values for I a - c , III, and I V from the relationship lot (k/k ) = mY (6, 38) are all less than 1.0; thus, a lower response to solvent ionizing power results for these benzylic substrates than for 2-adamantyl tosylate. A plausible explanation of this result is that the developing charge in the former systems is largely delocalized onto the aryl ring and therefore less demand exists for solvent stabilization. Also, a number of studies of formation of carbocationic systems with adjacent carbonyl groups show similarly low m values (31-34). These latter systems may have charge derealization onto the adjacent carbonyl group and in some cases adjacent aryl groups as well and, hence, a lower demand for solvent stabilization. These correlations of reaction rate with solvent ionizing power Y are somewhat scattered, so some uncertainty exists in the resultant m values, but nevertheless a sufficient number of examples are available to suggest that a charge derealization effect on the response to solvent ionizing power occurs. Such an effect might well also occur in systems reacting by & (neighboring group participation) routes in which the developing charge is partially dis persed onto a participating neighboring group. Studies of solvent effects on reactivity in such systems are just beginning to emerge and will provide an independent test of this theory of solvent effects. In one recent study, the solvolysis of mustard chlorohydrin ( H O C H C H S C H C H C l ) , a substrate known to react by a & path, showed abnormally low rates in mixed solvents containing C F C H O H , as compared to rates in aqueous C H C H O H and acetone (39). The cause of this effect has not been established, but the solvation requirements of the delocalized & transition state are clearly relevant to this phenomenon. The naphthyl and anthryl derivatives III and IV are considerably more reactive than the corresponding phenyl or tolyl compounds l b and Ic (Table I), a result expected for the greater electron-donating ability of the naphthyl and anthryl rings in carbocation stabilization (16, 40, 41). A value of 1.3 for kjk (the ratio of the polarimetric rate constant and the rate of product formation determined spectrophotometrically) was found for IV in T F E , but the high reactivity of IV precluded polarimetric rate measurements in other fluorinated solvents (16). Because of the stabilizing character and large size of the aromatic rings in III and IV, solvent participation at the central carbon is quite unlikely in these compounds, and the reactivity data obtained are consistent with an ion-pair mechanism similar to Chart I or equation 5. The formation of the ring-substituted products X I V during solvolysis of IV provides a unique probe of the reaction mechanisms in this case, as both the reactant and product are chiral. A possible mechanism for formation of the product could involve solvent attack at the relatively unhindered 10position of I V or on a chiral ion pair with a stereoelectronic preference for addition either syn or anti to the leaving group, analogous to proposals for the S 2 ' mechanism (42). However the products X I V from optically active I V were completely racemic, ruling out the operation of such a process. 0

0 T s

0 T s

Δ

2

2

2

2

Δ

3

2

3

2

Δ

uy

N


22.


ALLEN ET AL.

1-Arylethyl

325

Derivatives

The electron-withdrawing effect of the a - C F group in I - V would be ex pected to reduce the possibility of carbocation formation, but the net effect of an a - C F group is also known to inhibit nucleophilic attack, possibly due to the unfavorable interaction of the nucleophile with the lone pairs on fluorine (43). Therefore, the 1-arylethyl tosylates Vla-c bearing electron-withdraw ing halogen groups on the ring were examined as substrates where the effects of nucleophilic solvent participation would be likely to be manifested (17). The rates of Via and VIb in different solvents gave somewhat different correlations by the equation log (k/k ) = mY for aqueous ethanols and for other solvents, with m values of 0.82 and 0.78 for the latter, whereas Vic gave a very scattered correlation by this equation (17). Therefore, the rates were correlated by the equation log (k/k = m Y + IN (38), where Ν is a measure of solvent nucleophilicity and I expresses the dependence on Ν. For Via, VIb, and Vic, values obtained in this way for m and / were 0.90, 0.91, and 0.93 and 0.26, 0.37, and 0.72, respectively, with correlation coefficients of 0.994, 0.994, and 0.977, repsectively. Whatever the merits of the dual correlation with Y and N, all three substrates showed enhanced rates in the ethanol solvents compared with the trends for the other solvents; this result suggests that nucleophilic interactions are important. The p values derived from the rates of reaction of Vla-c in different solvents are - 4 . 9 (TFA), - 5 . 1 (100% HFIP), - 5 . 9 (97% HFIP), - 6 . 3 (100% T F E ) , - 5 . 1 (97% T F E ) , - 3 . 9 (HOAc), - 3 . 0 (80% EtOH), - 3 . 3 (90% C H C H O H ) , and - 3 . 4 (100% C H C H O H ) . These p values may be compared to those of —5.0 to —6.3 for 1-arylethyl derivatives attributed to reactions involving carbocations by Richard and Jencks (8) and that of —2.9 for the S 2 reaction of 1-arylethyl derivatives with azide (8). The net stereochemistry of the solvolysis of Vla-c in different solvents and the ratios of polarimetric rate constants k and the rate constants for product formation k are summarized in Table II (17). The results in the less nucleophilic solvents, particularly the large magnitudes of p and the high kjk ratios, are consistent with reversible 3

3

0

0Ts

0

0 T s

O T s

+

+

3

2

3

2

N

a

uy

+

uy


326

NUCLEOPHILICITY

Table II. kjk

uv

and Net Stereochemistry (in Parentheses) for Solvolysis of ArCH(OTs)CH (VI), 25 °C (17). 3

Solvent TFA HFIP TFE HOAc EtOH a b c d

3-BrC H 6

4

(Via) 3-CF C H 3

6

4

3 2

6

(Vic)

3

1.4* 2.1'

9.2* 1.4' 1.1* 1.0/

1.9 (rac) l.l(40%inv) l.l(72%inv)

3,5-(CF ) C H

(VIb)

e

g 77% inversion. f74% inversion. s 87% inversion. e

6% retention. Racemic. Product optically active, stereochemistry unknown. 46% inversion.

formation of ion pairs that undergo rate-limiting attack by the solvent as shown in Scheme I. The occurrence of 6% net retention in the trifluoroacetolysis of Vic could occur through an interaction of the leaving group with the solvent directing attack to the front side and has been observed in other systems (17, 34, 44, and references cited therein). The lower magnitude of the p values for Vla-c in the more nu cleophilic solvents, particularly those containing C H C H O H , is evidence for nucleophilic solvent participation in these solvents. A quantitative esti mate of the acceleration due to this cause in 100% C H C H O H may be obtained by comparing the observed rates to those calculated from m Y correlations for the remaining solvents in which such solvent participation is less important or absent. Thus, m values of 0.82, 0.78, and 1.35 are obtained for Vla-c, respectively, by using data for all the n o n - C H C H O H solvents in the former two cases and for T F A and H F I P in the latter. Rate constants calculated in this way in C H C H O H at 25 °C in the absence of nucleophilic solvent participation are 1.6 X 1 0 , 2.9 Χ 10" , and 8 X 10" s for Via, VIb, and Vic, respectively. The last value, based on extrapolation of a twopoint correlation, is only approximate. Comparison of the calculated nucleophilic accelerations of 9, 17, and 10 to the degree of inversion (72, 74, and 87%, respectively) for Vla-c shows that even though the kinetic effects of solvent assistance are significant, this fact does not lead to the complete inversion characteristic of the S 2 reaction as found in acetolysis of 2-octyl tosylate (45). The best description of the first intermediate in the solvolysis of Vla-c would thus appear to be the solvated ion pair shown in Scheme II. In this species, the solvent is involved in nucleophilic solvation of the central carbon as well as the remainder of the carbocation and also participates in electrophilic solvation of the anion. Numerous solvent molecules are involved, and no strong interaction of a single nucleophilic solvent molecule at the central carbon leading to ex clusive inversion occurs. Thus, these studies of 1-arylethyl systems provide excellent insight into the behavior of reactive systems where mechanistic changeover occurs. +

3

2

3

2

O T s

3

3

2

2

-5

6

13

_1

6

N


22.

ALLEN ET AL.


327

Scheme II. Solvated ion pairs in solvoly Other quite different approaches to the same problems exist, and the con tinued pursuit of such studies will provide increasingly more accurate under standing of transition-state structure. Acknowledgment This work was made possible by the contributions of our co-workers listed in the references, the continued research support of the Natural Sciences and Engineering Research Council of Canada, and a NATO grant that permitted collaboration with M . Charpentier and her associates at the C N R S , Thiais, France. Literature

Cited

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Solvolysis of Electron-Deficient 1-Arylethyl Tosylates - American

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