13C NMR Chemical Shift Correlations in Application of ``Tool of Increasing Electron Demand''to Stable Long-Lived Carbocations: Comprehensive EvaluationAuthor(s): George A. Olah, Arthur L. Berrier and G. K. Surya PrakashSource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 78, No. 4, [Part 1: Physical Sciences] (Apr., 1981), pp. 1998-2002Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/9815 .

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Proc. Natl. Acad. Sci. USA Vol. 78, No. 4, pp. 1998-2002, April 1981 Chemistry

"C NMR chemical shift correlation increasing electron demand" to sta Comprehensive evaluation*

(stable carbocations/chemical shift-substituent constant relationships

GEORGE A. OLAH, ARTHUR L. BERRIER, AND G. K. SUR

Hydrocarbon Research Institute and Department of Chemistry, University of Southel

Contributed by George A. Olah, October 20, 1980

ABSTRACT The reliability of 13C NMR chemical shift cor- relations in the application of the "tool of increasing electron de- mand" to stable long-lived carbocationic systems is demonstrated by a comprehensive analysis of 22 stable aryl-substituted carbo- cationic systems. The observation of slopes of less than unity in such chemical shift correlations for several cationic systems has been attributed to additional charge delocalizing mechanisms present in the system (such as homoallylic, cyclopropyl, and Xr conjugations). The onset of nonclassical ar-delocalization in 2-aryl- 2-norbornyl cations with electron withdrawing-substituents pre- viously observed was further verified by using orc+ substituent constants. Difficulties in relating the CaNMR shifts in different carbocationic systems are also discussed.

In recent years considerable effort was expended in explaining and predicting the effects of substituents on various physical properties, equilibria, structures, and reactivities of organic molecules (2-8). These effects can be broadly divided into elec- tronic and steric components. For probing electronic effects the so-called tool of increasing electron demand is frequently uti- lized. Although it is only one of the many methods to probe into the nature of electron-deficient intermediates, recently there has been an upsurge in its applications in both stable ion (9-12) and solvolytic studies (13). The ability of this method to detect the onset of o- or XI participation or conjugation in many car- bocationic systems makes it a useful tool in determining the nature of carbocationic intermediates. In stable ion studies the method is generally applied by using l3C NMR chemical shifts as the structural probe (9-12) although 'H and 19F chemical shifts have also been used (14-16).

It is well understood that 13C NMR chemical shifts cannot be directly equated to charge densities. However, in closely related homologous carbocations of similar nature, there is con- siderable cancellation of factors other than charge distribution that enter into the makeup of the chemical shifts. Based on this assumption Farnum et al. (10, 17) and Olah et al. (9, 11, 12) have applied the tool of increasing electron demand to a wide variety of aryl-substituted carbocations, using 13C NMR spectroscopy as the structural probe. The method was successful in dem- onstrating the clearly different nature of certain carbocationic systems such as the 7-norbornenyl (9), 5-norbornene-2-yl (2) (10), 2-norbornyl (3) (10, 11) and 8-tricyclo[5.2.1.02'6]decyl (4) (10) cations compared to ordinary trivalent (classical) cations. Such 13C NMR chemical shift comparisons have been criticized by Brown and Rao (18) and by Kelly and Spear (19). In this paper we demonstrate that such comparisons are of general validity.

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertise- ment" in accordance with 18 U. S. C. ?1734 solely to indicate this fact.

1998

s in application of "tool of ble long-lived carbocations:

/onset of nonclassical cr delocalization)

YA PRAKASH

n California, Los Angeles, California 90007

RESULTS AND DISCUSSION

The 13C NMR data of various aryl-substituted carbocations are listed in Tables I and 2 (9, 10, 12, 20, 21). New data on the 2- adamantyl system are also presented. Table 3 lists the slopes of 13C NMR chemical shift plots. In the case of carbocations displaying nonlinear plots, the slopes were determined only with the substituents over which the carbocations displayed lin- ear behavior. Figs. 1, 2, and 3 show plots of the cationic center chemical shifts with either cr (20) or or+ (4) substituent constants.

Correlation of l3C NMR Chemical Shifts (Nature of the Slopes). Recently, attempts to relate ov+ constants and ground- state physical properties (such as 1H, 19F, and 13C NMR chem- ical shifts) obtained under stable ion conditions have been pub- lished. However, these or constants were derived from sol-

R

B R R

1 2 3 4

R R

R R R

5 6 7 8

/CH3 {^-R

+ RR R-C Li +

CH3

9 10 11 12 13

C/ CH, B B

H3C-C > C \>C\ >C C/ H,C-CR R

> CH2CH3 CH3

14 15 16 17

+/_ ?H + H ?+ / /H

18 19 20 21 22

R -= 4Q- X X=4-OCH3, 4-CH3, 3,4(CH3)2, 4-F, 4-Cl, 4-Br, 4-H. 3-CHa, 3-F, 3-Cl, 4-CFa. and 3,5(CF3)2

*This paper is part 233 in the series "Stable Carbocations." Part 232 is ref. 1.

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Chemistry: Olah et al.

Table 1. Cationic chemical shifts of carbocations studied

Shifts

Cation Ref. 4-OCH3 3,4(CH3)2 4-CH3 4-F 4-Cl

12 9 234.6 - 259.5 - - 11 20 219.0 - 243.6 248.4 251.4 1 9 229.9 - 252.5 - -

22 19 196.3 - 219.1 223.4 225.7 10 10 227 249 251 256 259 9 10 235 257 259 264 267 8 237.5 - 260.7 - - 7 10 235.5 256.4 - 262.3 - 6 12 213.6 - 234.3 - - 3 10 232 251.5 - 255.8 - 5 9 231.4 - 252.4 - - 4 10 231.9 250.4 - 255.0 -

13 9 235.0 - 251.1 - - 16 21 231.0 - 246.1 247.8 249.4 15 21 227.1 - 241.9 243.2 244.8 19 21 208.6 - 221.6 222.2 223.1 18 22 183.5 - 196.1 197.1 197.8 21 21 219.8 - 231.5 232.5 233.4 20 21 248.7 - 257.0 258.0 259.8 2 10 226.8 242.6 - - -

*At - 114?C. t At - 123.4?C. t Measured in FSO3H/SbF5/SO2C1F at -70?C for this study.

volytic studies (4) and do not properly reflect the greater electron demand of carbocations in non-nucleophilic superacid media. Indeed, it has been shown (19) that a plot of the 13C NMR chemical shifts of the cationic carbon in a series of substituted t-cumyl cations 11 against the or+ constants reveals a poor cor- relation. "Super sigma" constants, Cr++, have been derived by Kelly and Spear (19) to take into account the enhanced charge delocalization in ions containing electron-donating substitu- ents. However, the method used in deriving these constants assumes a linear correlation between the cationic chemical shift and o+ values for electron-withdrawing substituents.

Farnum et al. (10) observed that the 13C NMR chemical shifts of the cationic carbons of ordinary tertiary-aryl-substituted clas- sical ions are linearly related to one another with nearly unit slope. This finding has been exploited in recent studies estab- lishing the extraordinary behavior of several carbocationic sys- tems (9, 10). However, we have observed that the slopes in sev- eral instances fall substantially below the unity level observed previously. For example, the slope of the plot of the cationic carbons of 1-aryl-l-cyclopentyl 9 (10) and 2-aryl-2- bicyclo[2.1. 1]hexyl 5 cations (9) was 0.85, lower than the usually observed values of 1 ? 0.1. Even more striking was the slope of the 3-aryl-3-nortricyclyl cations 13 (9). A plot of the cationic (C-3) chemical shifts of these cations versus those of the model 1-aryl-l-cyclopentyl cations (10) reveals a deviation for electron- withdrawing groups due to increased cyclopropyl conjugation. However, a linear plot is observed for the electron-donating substituents with a slope of only 0.63, considerably less than the expected unity. One previous observation of a decreased slope was made by Farnum et al. (10) in the case of the 1, 1-diarylethyl cation 14, in which a slope of 0.71 was found. In order to un- derstand this phenomenon further, we have measured the slopes of the cationic chemical shift plots of various cationic sys- tems with the model-l-aryl-l-cyclopentyl system. These values are listed in Table 3.

The slopes of various alkyl-substituted cations such as the 1- aryl-l-cyclobutyl 12 (9), 2-aryl-2-adamantyl 8, and 7-aryl-7-nor- bornyl 1 (9) all fall close to unity. They range from 1.08 in the case of cyclobutyl to 0.97 for 2-adamantyl. This indicates that

Proc. Natl. Acad. Sci. USA 78 (1981) 1999

with aryl substituents 4-Br 3-CH3 4-H 3-F 3-Cl 4-CF3 3,5(CF3)2

- 269.7 271.7 - - 285.3* 290.1t 252.3 253.3 255.7 262.1 262.3 270.0 274.4

- - 263.9 - - 277.8 283.1 226.1 - 229.8 - - 244.1 248.8 260 - 263 267 - 276 279 267 268.3t 270 277 277 283 286

- - 271.3 - - 283.4 287.1 265.4 - 268.6 - - 279.5 282.2

- - 245.1 - - 254.4 258.2 258.5 - 260.8 - - 265.3 263.3

- - 261.3 - - 272.5 275.2 258.2 - 260.4 - - 268.0 268.4

- 256.5 257.1 - - 260.5 259.0 250.6 - 251.6 - - 254.8 - 245.3 - 246.2 - - 250.0 - 223.6 - 226.3 - - 225.9 - 198.1 - 199.8 - - 202.2 - 233.5 - 235.0 - - 235.3 - 260.0 - 261.0 - - 264.6 -

- - 247.4 - - 234.4 218.1

these cations behave similarly to the model system 9 (10). Slopes considerably less than unity can be attributed to the

presence of an additional charge delocalizing mechanism such as the interaction of either a cyclopropyl group or a phenyl ring with the electron-deficient carbocationic center. The presence of either a cyclopropyl or phenyl substituent causes a substantial decrease in the slope (0.64 or less). The alkyl-substituted cy- clopropyl phenyl cations (methyl 15 and ethyl 16) show similiar slopes (0.55 and 0.59, respectively), indicating that these ions involve stabilization by the cyclopropyl group approximately to the same extent. Replacing the alkyl group by hydrogen causes a marked decrease in slope (0.48), indicating increased cyclo- propyl conjugation. This observation is expected because the alkyl group (methyl or ethyl) should help stabilize the carbo- cationic center, resulting in less demand on the cyclopropyl ring. However, the presence of a hydrogen should destabilize the system, causing an increase in the involvement of the cy- clopropyl group with a resultant decrease in the slope. The same trend is also observed in the case of the phenyl-substituted cat- ions in which replacement of a methyl group (cation 17) by hy- drogen (cation 18) causes a decrease in slope from 0.58 to 0.46. Furthermore, the substitution of additional conjugative stabi- lizing groups results in a further decrease in slope. The aryl- phenylcyclopropyl and aryldicyclopropyl cations, 21 and 22, have the smallest slopes observed for classical systems (0.43 and 0.34, respectively).

It is interesting to note that a series of five bicyclic (tricyclic) systems, 3, 4, 5, 6, and 7, all have similiar slopes which are somewhat lower than other simple alkyl-substituted systems (0.92 to 0.81). Although structurely and electronically different, these systems appear to have similiar carbocationic environ- ments as indicated by their slopes. This similarity of slope may be due to an anisotropy effect of the bicyclic skeleton.

A analogous bicyclic system is the homoallylic 2-aryl-5-nor- bornene-2-yl cation 2 previously studied by Farnum et al. (10, 17). A substantial deviation in the chemical shift correlation plot was accommodated as due to either equilibrium 2 = 2b or Xr bridging (10). The slope of the line with electron-releasing sub- stituents only, for which a reasonable correlation is obtained

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2000 Chemistry: Olah et al.

Table 2. Additional "3C NMR chemical shifts of selected carbocations

Cation Ref. 4-OCH3 3,4(CH3)2 4-Me 4-F

3 11 C-i 52.0 - 56.3 - C-2 238.3 - 252.3 - C-3 45.6 - 48.6 -

5 9 C-i 59.5 - 63.3 - C-2 231.4 - 252.4 - C-3 42.8 - 45.6 -

7 10 C-5 49.8 53.1 - 55. C-6 235.5 256.4 - 262. C-7 44.5 47.5 - 49.

6 12 C-i 49.3 - 52.2 - C-2 213.6 - 234.3 - C-3 42.5 - 45.3 -

4 10 C-i 56.3 60.2 - 62. C-2 231.9 250.4 - 255. C-3 45.4 48.6 - 49.

13 9 C-1,6 43.4 - 54.7 - C-3 235.0 - 251.1 -

8 C-1,3 44.7 - 49.1 - C-2 237.5 - 260.7 - C-4,8,9,10 45.2 - 48.0 -

C-5,7 28.8 - 29.4 - C-6 36.2 - 36.4 - Ci 130.0 - 135.1 - Co 142.7 - 138.7 - Cm 119.9 - 134.3 -

Cp 182.7 - 173.0 -

C' 59.4 - 24.6 -

* Chemical shifts obtained in FSO3H/SbF5 (4:1)/SO2C1F at -60?C for this sti data (11).

t JCF coupling constant in Hz.

2 Ar 2b Ai

(r = 0.977), is 0.61 which is definitely lower than the slope of the other bicyclic systems. The presence of homoallylic con- jugation, even with electron-releasing aryl substituents, is clearly evident from the magnitude of the slope.

Substituent Constant-13C NMR Chemical Shift Relation- ships. Utilizing the approach previously used in determining or+ constants, Brown et al. (20) developed cr+ substituent con- stants which is claimed to take into account the increased elec- tron demand of carbocations in superacidic media. These or+ constants were derived from the cationic carbon chemical shifts of a series of meta- and para-substituted t-cumyl cations using a modified Hammett-Brown equation. Because these rc+" con- stants show a linear relationship with the 13C NMR chemical shifts of the cationic carbons of the I-aryl-l-cyclopentyl cations

(20), this approach is essentially an extension of the work of Farnum et al. (10) and Olah et al. (9).

The correlation coefficients of plots oC+ and the cationic car- bon chemical shifts for a number of reported trivalent aryl sub- stituted cations all show excellent linear relationships, as do plots with the model 1-aryl-1-cyclopentyl system 9.

Although the new rC+ substituent constants do not convey any new or unexpected information, we thought that the ap- plication of these rc+ constants to the controversial 2-norbornyl cation would be appropriate because Brown and Rao (18) have stated that, under solvolytic conditions, the tool of increasing

Proc. Natl. Acad. Sci. USA 78 (1981)

Chemical shift

4-Br 3-Me 4-H 4-CF3 4-N+(CH3)2H 3,5(CF3)2

- 58.3 59.3* 63.6 65.5 66.2 - 258.8 260.5* 264.5 264.6 262.8 - 49.8 50.5* 51.9 53.2 52.4 - - 65.0 70.0 - 72.5 - - 261.3 272.5 - 275.2 - - 46.3 49.4 - 50.5

2 55.9 - 55.7 59.1 - 60.8 3 265.4 - 268.6 279.5 - 282.2 1 49.6 - 49.6 52.5 - 53.6

- - 54.3 56.9 - 58.0 - - 245.1 254.4 - 258.2 - - 46.9 49.1 - 50.0

4 63.6 - 63.2 67.6 - 69.5 ) 258.2 - 260.4 268.0 - 268.4 ) 50.9 - 50.6 53.2 - 54.1

- 59.9 61.4 73.5 - 80.5 - 256.5 257.1 260.5 - 259.0 - - 51.4 56.7 - 58.6 - - 271.3 283.4 - 287.1 - - 49.3 52.8 - 54.3 - - 29.5 30.7 - 31.0 - - 36.3 37.0 - 37.1 - - 137.1 139.5 - 142.5 - - 138.1 137.7 - 135.8 - - 132.9 129.4 - 136.0(36)t - - 154.2 149.2 - 137.9

(33.9)t - - - 122.2 - 122.9(272.9)t

(233)t

idy. The C-2 chemical shift is slightly different from previously reported

electron demand fails to reveal any onset of or participation. A plot of the or+ constants versus the previously reported (11) carbocationic chemical shifts of the 2-aryl-2-norbornyl cations is shown in Fig. 1. In contrast to the linear plots observed with classical ions, the plot shows a distinct break with the electron- withdrawing 4-trifluoromethyl and 3,5-bistrifluoromethyl sub- stituents. This is consistent with the previous analysis by Farnum et al. (10) of the 2-aryl-2-norbornyl cation in which they observed an identical deviation in the plot of the model 6-aryl- 6-bicyclo[3.2. 1]octyl system and the 2-aryl-2-norbornyl cations. Thus, the application of the new crc constants of Brown et al. (20) further verifies the onset of nonclassical (r delocalization in the 2-aryl-2-norbornyl cations.

A similar onset of ao delocalization (10) in the 8-aryl-8-tricy- clo[5.2.1.02'6]decyl cations 4 is shown in a plot of the cationic chemical shifts (10) and the crc constants (Fig. 2).

The onset of cr delocalization in the 2-aryl-2-norbornyl cation occurs with the electron-withdrawing 4-trifluoromethyl and 3,5-bistrifluoromethyl substituents. However, these conclu- sions have been disputed by Kelly and Spear (19) who ques- tioned the validity of the o'+ and C(1) chemical shift plot in which a deviation was found and interpreted in terms of o- delocaliza- tion (11). We agree that such v+ correlation plots are inconclu- sive because similiar deviations have been found in numerous purely classical systems in which there is no onset of either cr or XT participation.

The criticism (19) of the C(1) versus C(3) plot which likewise shows a deviation with electron-withdrawing groups, however, is unjustified. The plot (10, 11) shows a distinct break in the curve between electron-releasing and electron-withdrawing

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Chemistry: Olah et al.

Table 3. Slope of cationic chemical shift vs. SC-1 of 1-aryl-1- cyclopentyl cations

No. of Cation Slope, r points Ref.

12 1.08 0.999 5 9 11 1.07 0.997 10 20 1 1.03 0.997 5 9

22 1.02- 0.993 8 19 10 1.01 0.998 10 10 8 0.97 0.999 5 7 0.92 0.999 7 10 3 0.88* 0.999 5 10 6 0.87 0.998 5 12. 5 0.86 0.999 5 9 4 0.81* 0.999 5 10

14 0.71 0.998 4 10 13 0.64* 0.998 4 9 2 0.61* 0.977 3 10,

16 0.59* 0.995 6 21 17 0.58* 0.996 6 10 15 0.55* 0.992- 6 21 19 0.48 0.988 6 21 18 0.46* 0.987 6 22' 21 0.43* 0.989 6 21. 20 0.34 0.994 7 21

* Slope of linear portion :of plot.

substituents. This indicates that, as the aryl ring becomes less effective in delocalizing the positive charge at the C(2) position, the neighboring C(1)-C(6) o bond begins to interact with the empty p orbital at the cationic center-i.e., the onset of ar delocalization.

A recent analysis (12) ofthe 13C NMR chemical shift:data of the 2,5-diaryl-2,5-norbornyl dications 6 shows that this ap- proach is valid. The plot of C(1) versus C(3) in this system shows an excellent linear relationship (r = 0.998). This excellant linear fit demonstrates the similiar neighboring group deshielding effects experienced by both the C(1) and C(3) carbons. No C(1)-C(6) or C(3)-C(4) o bond delocalization is occurring in these dications because the presence of two carbocationic cen- ters renders the entire bicyclic skeleton highly electron defi- cient and unable to delocalize any additional charge via o. de- localization. These 2,5-diaryl-2,5-norbomyl dications are regular classical carbocations and do not show any ordelocalization (12).

The analysis of several other norbornyl-type systems shows

265 ",4-CF3 e

260 4-H 3,5-(CF3)2 3-CH3

1 255 / c'z ' 4-CH3

i 250 /

245 / 4-OCH3/

240 /

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

FIG. 1. Plot of aC(2) chemical shifts of the 2-aryl-2-norbornyl cat- ions 3 vs. or+ constants.

Proc. Natl. Acad. Sci. USA 78 (1981) 2001

270 - /3,5-(CF3)2

' 4-CF3 265

260- 4-H 4-Br/

255 4-Fe v 4 /

?'250

245

240 /

235

/ 4-OCH3 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

ac+

FIG. 2. Plot of 8C(8) chemical shifts of the 8-aryl-8- tricyclo[5.2.1.02'6]-decyl cations 4 vs. SC+ constants.

that this' phenomenon is observed in all cases. The plots of the carbocationic carbon chemical shifts, of the 1-aryl-l-cyclopentyl cations (10) versus the cationic chemical shifts of the 2-aryl-2- bicyclo[2.1.1]hexyl 5 (9) and the 6-aryl-6-bicyclo[3.2.1]octyl 7 (10) cations show linear relationships indicating the classical behavior of these aryl substituted cations. The C(1) versus C(3)- [C(5) versus C(7) for cation 7] plots for these cations are also linear. The 2-aryl-2-norbornyl 3 (10, 11) and 8-aryl-8- tricyclo[5.2.1 .02'6]decyl 4 (10) cations both show a nonlinear plot of the cationic chemical shifts and.the C(1) chemical shift of the 1-aryl-1-cyclopentyl cations. Plots of C(1) and C(3) [C(7) and C(9)] again show deviations. The data available are therefore consistent with the explanation of the. onset of oC delocalization causing deviations in either cationic or C(1) and C(3) chemical shift plots..

Kelly and Spear, (19) further argued that breaks in plots of the C(1) chemical shifts of the 2-aryl-2-norbornyl cations, does not. imply the onset of oa. delocalization because they found that, when the C(1) chemical shift of the 2-aryl-2-norbornyl cation is plotted against the chemicalshift of the methyl carbons of the t-cumyl cations 11 in which no (r or 7r bridging is possible, an excellent straight line is, obtained.. They then concluded "that breaks in plots of C(1) chemical shifts of 2-norbornyl derivatives cannot be used as evidence for the presence of or-bridging in. these cations'." However, this argument is misleading. We plot- ted C, shifts of several systems and observed excellent corre- lations. The Ca shifts of the I-aryl-l-cyclopentyl cations. (10)' relate with the C(1), (3) shifts of the 2-aryl-2-adamantyl cations (r = 0.992), C(1) of the' 2-aryl-2-norbornyl cations, (11) (r =

3,5-(CF3)2 80 /

' 70 4CF3 70

60 [3-CH3 4-

50 X . 4-OCH3 1

40 7 ... v, - 1.0 -0.5 0.0 0.5 1.0

FIG. 3. Plot of aC(1),C(6) chemical shifts of the 3-aryl3-nortricy- clyl cations 13 vs. a+ constants.

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2002 Chemistry: Olah et al.

0.994), and the methyl carbons of the t-cumyl cations (20) (r = 0.996). However, we think that these plots can be deceptive and are, in fact, not a good representation of the nature of the carbocation. It is the carbocationic carbon that the tool of in- creasing electron demand affects the most. The difficulty in re- lating C, shifts of different carbocationic systems is that the fac- tors influencing the chemical shifts of these carbons could be quite different and dependent upon the system studied. We think that the linearity observed in the case of the C(l1 chemical shifts of the 2-aryl-2-norbornyl cation versus the methyl carbon chemical shifts of the t-cumyl cations is an example of such a misleading plot and that the correlation observed is highly co- incidental. A careful look at the 2,raryl-2-norbornyl data (Table 2) reveals. the fallacy of this comparison. The carbocationic chemical shifts [C(2)] get progressively deshielded as stronger electron-withdrawing groups are introduced. This happens un- til the p-dimethylammonium substituent (613C = 264.6) after which a shielding is observed for the more strongly electron- withdrawing 3,5-bistrifluoromethylphenyl group (813C = 262.8).

Similiar observations can be seen for the C(3) carbon chem- ical shifts. Introduction of a 3,5-bistrifluoromethylphenyl group causes a.slight shielding with respect to the dimethylammonium substituent (8'3C 53.2-52.4). ,This neighboring group effect is expected for carbons a to a carbocationic center (e.g., t-cumyl or 1-aryl-l-cyclopentyl cations). However, the C(1) chemical shifts do not follow the trend of the cationic carbon but get pro- gressively deshielded even with' the 3,5-bistrifluoromethyl sub- stituent. The two a-carbons [C(1) and C(3)]'in the 2-aryl-2-nor- bornyl cations thus display different neighboring group effects. The C(3) trend of being closely. related to the carbocationic chemical shift is analogous to the trend observed for the methyl carbons of the cumyl series and C, of the cyclopentyl series. It is the C(1) shifts that, are abnormal and do not follow the trend of the cationic center's chemical shifts. 'Because the plot of the cumyl cationic chemical shifts and the 2-aryl-2-norbornyl cat- ionic chemical shifts is not linear, a plot of C(1) of the norbornyl system versus the methyl carbon of the t-cumyl cation is mean- ingless. As discussed before, a plot of two carbons within a sys- tem, such as C(1) and C(3) in the norbornyl cations, is reasonable and does reveal structural changes. But plots of a-carbons be- tween systems should not be made unless the systems them- selves behave similarly (i.e., the cationic centers are linearly related).

Attempts to correlate C, and other neighboring group chem- ical shifts with various substituent constants. have appeared re- cently. For example, the methyl carbon chemical shifts of the I-aryl-1-ethyl cations 22 and 1-methyl-l-arylethyl cations 11 can be correlated to a limited degree by r+ constants (r = 0.989 and 0.990, respectively) (19). Likewise, the C(1) chemical shifts of the 2-aryl-2-norbornyl cation can be correlated quite well with or++ constants (19) (r = 0.997) and the para carbon of the unsubstituted ring of a series of 4-substituted benzhydryl cat- ions 18 shows a good correlation with oa+ (22). Although these characteristics are dependent upon the electron demand, it is only a secondary effect and, as such, is not a true measure of the electron demand of the cationic center. These effects should not be dependent upon ce+ and indeed, they are not. One can rationalize that these diminished characteristics of the electron demand should show reasonable correlations with substituent constants which are not a true measure of the electron demand such as the cr+ or r++ constants.

A further example of such correlations is in the 3-aryl-3-nor- tricyclyl cation 13. The onset of increased cyclopropyl conju- gation can be observed with highly electron-demanding groups which consequently shields the carbocationic center. The Cp [C(1) and C(6)] carbons, however, are significantly deshielded

Proc. Natl. Acad. Sci. USA 78. (1981)

due to the increased dispersal of charge into the rigid cyclo- propyl skeleton. The CG chemical shifts should be dependent on the electron demand, although not as strongly as-the cationic carbon. A plot of C. and or+ (Fig. 3) shows a good linear cor- relation (r = 0.994). It seems apparent that the diminished electron density substituent constants such as + or r++ can measure, to some extent, the effect of increasing electron de- mand at remote sites in several carbocationic systems. How- ever, this ability to measure a diminished electron demand in a few instances does not imply that similiar. correlations can be made in different systems. Other factors play important.roles in determining the effect of electron demand at a remote site [e.g., C(1) and C(3) in the 2-aryl-2-norbornyl cations] and ex- treme care must be exercised in attempting such correlations.

CONCLUSIONS In conclusion the application of the tool of increasing electron demand to stable carbocations utilizing the recently developed oC+ substituent constant-13C NMR chemical shift relationship fully substantiates our previous finding from chemical shift cor- relations on the onset of nonclassical or delocalization in 2-aryl- 2-norbornyl cations 3 with. electron-withdrawing substituents. Also, the observation of slopes less than unity in chemical shift correlations can reveal the nature of additional.charge delocal- izing mechanisms operating in the system.

EXPERIMENTAL The 2-aryl-2-adamantyl cations were generated from their al- cohol precursors and studied as described (9).

Support of our work by the National Science Foundation and by the National Institutes of Health is gratefully acknowledged.

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