Electronic and steric effects on the solvation at the transition state in the solvolysis of some tertiary benzylic chlorides

Tetrahedron Letters. Vol. 33, No. 43, PP. 6499-6502.1992 Printedin GroatBritain

00404039,92 .SS.oO + .OO PergamonPressLtd

ELWG!J!ROWICAWD STERIC BYPECTS OW THE SOLYATIOW AT THE TRAWSITIOW STATE IW TWE SOLVOLYSIS OP SOWE TERTIARY BEWOYLIC CELORIDLee

Kwang-Ting Liu,* Pang-Shao Chen, Pao-Feng Chiu, and Meng-Lin Tsao Department of Chemistry, National Taiwan University, Taipei, Taiwan 107, Republic of China

Abstraat: The deviation from linear relationship in the regression analyses using Grunwald-Winstein equation for tertiary benzylic chlorides l-4 could be improved by increasing steric hindrance near the reaction site and/or by increasing electron-release of the substituent on aryl ring, which clearly demonstrates the importance of solvent intervention due to the solvation of cationic transition states in solvolysis. In the study of searching appropriate Y scales in Grunwald-Winstein type correlation of solvolytic reactivities, log(k/k,) = mY (l),l for benzylio substrates we recently demonstrated the necessity of using new YBnX values based on 2-aryl-2-adamantyl derivativess2 We have also shown the failure of obtaining linear logk - YBnCl relationship in the cases of 2-chloro-2-(3'chlorophenyl)propane

(lb) and 2-chloro-3,3-dimethyl-phenylbutane

(ra),

and

have attributed the anomaly to nucleophilic solvent intervention for lb and to the lack'of resonance stabilization for Ia, is

respectively.2a Probably it

the consequence of different extent of salvation at cationic transition

states. Wow we wish to report the evidence from electronic and steric effects of substituents on reactivities, which gives a strong proof of the importance of solvent intervention due to solvation at the solvolytic transition state of tertiary benzylic chlorides. 2-Chloro-2-phenylpropane phenyl)propane

(t-cumyl chloride, la), 2-chloro-2-(2'-methyl-

(2), 2-chloro-3-methyl-2-phenylbutane

chloro-3,3_dimethylbutane

(3), and 2-aryl-2-

(ra-4a) were prepared from the corresponding

alcohols by conventional method, and ,were solvolyzed in various solvents. The first order rate constants were measured at least in duplicate by using conductimetric

(+2%) method. The pertinent data are summarized in Table 1.

In the case of la,

some of the reported data2a were revised, for the less

accurate values of infinity conductance had been used.

la Z=H lb Z=3'-Cl

2

.3

4a Z=H

4b Z.=3'-Me 4c Z=3',4'-(Me)2 6499

6500

Table 1. Pertinent solvolytic rate constants for chlorides 1-4. Solvent

k, s-l (25OC) la

1OOE

3.80~10-~

2

3

1.30x10-3

3.71x10-5

la

4b

4c

3.91x10-3

1.21x10-2

3.44x10-4

2.54x10-6 a

2.76~10-~

5.38~10-~

80E

1.73x10-2

4.07x10-2

1.78~10-~

1.01x10-5 b

1.24~10-~

2.21x10-4

70E

6.87~10-~

0.124

5.43x10-3

2.06~10-~ b

4.24~10-~

7.42~10-~

7.52~10-~ b

1.35x10-3

2.18~10-~

70A

1.24~10-~

4.72~10-~

1.67~10-~

1.79x10-3

6.38~10-~

2.15~10-~

1.19x10-2

3.53x10-2

1.39x10-3

8.12~10-~ b

8.95x1O-5

1.47x10-4

60A

2.77~10-~ b

4.77x10-4

50A

1.47x10-4 b

2.24~10-~

7.90x10-4 3.42x10 -'3

3.53x10-6 a

6.38~10-~

1.37x10-4

1OOM

5.22~10-~

1.76~10-~

7.33x10-4

60M

2.83~10-~ b

1OOT

4.31x10-3

97T

3.98xlO-3

70T

0.166

4.84~10-~

SOT20E

0.451a

2.0a

0.133

3.82~10-~

1.47x10-2

4.68~10-~

60T40E

5.49x10-2

0.199

1.35x10-2

5 .19x1o-5

1.51x10-3

4.34x10-3

40T60E

8.60x10-3

3.08~10-~

1.68x10-3

1.65~10-~

4.34x10-4

aExtrapolated from data at other temperatures. bLiterature data.2a The results of correlation analyses are listed in Table 2, and the logk - yBnCl plots are displayed in Figure

1 and 2. Some data, such as k(60M),3

for la were not included, since no appropriate YBnCl valuelb was available. Excellent linear relationship ( R > 0.99) with all Ys could be found in the cases of 2, 3, 4b and Ic, while in other cases less satisfactory results were observed.

In addition, the deviation might be associated with a large

(e.g., la) or a small (e.g., ra) difference between the m value defined by the more nucleophilic solvents (m(AEM)) and that by the less nucleophilic ones (m(TE)). Despite the linear correlation with all solvents (R = 0.997 for m(Al1)) observed for Ib, there is still a splitting between m(AEM) and m(TE), Am = 0.11. 2-Aryl-2-propyl

(tert-cumyl) chlorides and p-nitrobenzoates have long

been considered to solvolyze via limiting Shl mechanism, and from which the &

constants are defined.4 However, the importance of different extent of solvation at delocalized benzylic transition states for solvolysis has been recognized.2'5 Since the Sp2 type nucleophilic solvent assistance was found to be absent in the solvolysis of la,6l7 the significant'deviation of data points corresponding to logk(TFE-EtOH) for la should be attributed

6501

Table 2. Correlation analyses of log ks for chlorides l-4 against YBnCl. m-value

Substrate

m(AEM)b

m(A1l)a la

0.789

2

0.767

3

0.852

Ca

0.748

4b

0.887

4a

0.954

0.959

(I?= 0.982, n= 12) (R = 0.991, n= 11) (R =, 0.994, 11) (::: 0.987, 11) (::: 0.997, 12) (::f 0.997, n= 11)

0.862 0.916 0.893 0.989 0.936

m(TE)=

(R = 0.991, n= 8) (; I 0;;92,

(R = n= (R = n= (R =

0.766

4) 0.999, 4) 0.999, 4) (::: 0.999, 3) (::2 0.999, 4) (::1 0.999, n= 3)

0.791

(R = 0.991,, n= 8) (R = 0.979, n =' 8,) (R = 0.997, n= 8) (R'= 0.999, n= 8)

0.999,

0.886 0.808 0.480 0.905

aAll solvents. bAqueous acetone, ethanol and methanol. =TFE-EtOH. to other kind of solvent intervention.

Obviously the introduction of an

ortho methyl group on the aryl ring (2) or a bulky isopropyl group at the reaction center (3) resulted in a better linear logk - YBnCl plot (Figure 1 and Table 2). Steric hindrance of solvation in the cationic transition state would be a proper explanation.' The increased bulkiness by a larger tert-butyl group is likely to make the planar benzylic cation less stable and the charge delocalization in the transition state less effective,' and thus causes dispersion in the linear plots with eq. 1.2as10

From Figure 2

and Tablo 2 it is remarkable that the linear relationship reappears if the aryl ring becomes electron releasing. The observed improvement of R, albeit small, of both m(Al1) and m(AEM) for 4b over those noted for

la

clearly

indicates that the increasing stability due to the presence of meta and/or para methyl substituents suppresses the steric destabilization due to the Figure 1 Correlation of logks for

la-3

against YgnCl.

1

o__

(la)

v,v

(2).

A.,

A

70E

--

;;i

8OA V

d

Y

.F

-2 --

-3

1OOE v 90A V

--

-5

A

b 0 i-Pro”

90E V

0

v

VVv v

A

o

O0 0

A

A AA A 4T6E IOOM

0

-- -1.5

8TZE

0

” 0 0

l -4 --

-- -0.5

(3) 70AaoE

-1

0.5

v

0.0’

A

-- -2.5

6T4E

A

-- -3.5

-- -4.5 A

A -5.5

-3

-2

-1

0

1

Ysncr

2

3

4

6502

Figurrn 2. Correlations of logks for Ia-la against YBnCl. coplanar arrangement. The small splitting between m(AEM) and m(TE) (vide supra)

might suggest an incomplete dominance of electronic effect for 4b.

A fine tuning of the structural change clearly affects logk -,YB,,K correlations. For the solvolysis of benzylic chlorides l-4, equation 1 is likely to be better than the single-parameter

(&)

analysis"

or the dual-

parameter Yukawa-Tsuno treatmentsa of rate data for the understanding of mechanisms.

The present results also provide strong evidence of solvent

intervention at the cationic transition state due to solvation. More work on other systems is in progress. Aeknowledgmnt:

We than& the National Science Council for financial support of this work.

RRIRRRWCRS AWR WOTRS 1. Grunwald, E.; Winstein, S. J. Am. Chem. Sot., 1948, 70, 846. 2. (a) Liu, K.-T.; Sheu, H.-C.; Chen, H.-I; Chiu, P.-F.; Hu, C.-R. Tetrahedron Lett. 1990, 25, 3611. (b) Liu, K.-T.; Sheu, H.-C. J. Org. Chem. 1991, 56,'3021. (c) Liu, K.-T.; Sheu, H.-C. J. Chinese Chem. Sot. 1991, 38, 29. (d) Liu, K.-T.; Chen, H.-I J. Phys. Org. Chem. 1991, 4, 463. (e) Liu, K.-T.; Yang, J.-S.; Chang, S.-M.; Lin, Y.-S.; Sheu, H.-C.; Tsao, M.-L. J. Org. Chem. 1992, 57, 3041. 3. The previously reported Y(BnC1) value for 60M was later found to be not verv reliable because of the low solubilitv of the reference substrate. 4. (a)-Brown, H. C.; Okamoto, Y. J. Am. Chem.-Sot. 1988, 80, 4979. (b) Brown, H. C.; Rao, C. G.; Ravindranathan, M. ibza 1977, 99, 7663. 5. Bentley, T. W.; Koo, I. S.; Norman, S. J. J. Org. Chem. 1992, 57, 2387. 6. Richard, J. P.; Amyes, T. L.; vontor, T. J. Am. them. Sot. 1991, 113, 5871. 7. Our unpublished data on the azide effect cannot be interpreted solely either. Cf. ref. 6. M.; Tsuno , Y. Tetrahedron Let. 1992, 33, 349. 8. $)S$$r~c?f'Fujio, (b) Liu, K.-T.; Chen, P.-S.; Chiu, P.-F.; Tsao, M.-L. J. Org. Chem., in press. 9. (a) Duismann, W.; Ruchardt, C Chem. Ber; 1973, 106, 1083. (b) Tanida, H.; Mafsumura, H. J. @. Chem. Sot. 1973, 95, 1586. 10. Regression analysis using Ycl values (from Bentley, T. W.; Llewellyn, G. Progr. Phys. Org. Chem. 1990, 17, 121; Kevill, D. N.; Kyong, J. B, J. Org. Chem. 1992, 57, 258.) yielded m =.0.737 and R = 0.958 (n = 12). 11. Liu, K.-T.; KUO, M.-Y. Tetrahedron 1988, 44, 3523. (Receivedin Japan24June1992)