J . Org. Chem. 1987, 52, 4164-4171
4164
Substituent Effect of the Pentafluorophenyl Group: Is There a Correlation between X-ray Crystal Structures and Reactivity in Carbocation Formation? Annette D. Allen, Jean Marc Kwong-Chip, Jayant Mistry, Jeffery F. Sawyer, and Thomas T. Tidwell* Department of Chemistry, University of Toronto, Scarborough Campus, Scarborough, Ontario, Canada M l C lA4 Received March 17, 1987
The reactivity of C6F5CH(OTs)CH3 (7) in various solvents is found to be less than that of PhCH(OTs)CH, by factors of 6 X lo3to 5 X lo5,but the CsF5substituent is still much more activating than hydrogen. The reactivity of 7 gives a poor correlation with the YoTsmeasure of solvent ionizing power, with an m value of 0.44, and while the reaction in TFA or HFIP evidently involves initial formation of an ion pair, there is strong nucleophilic assistance to ionization by less ionizing solvents. Crystal structures of C6H5C(CH3)20PNB (lo), C6F5C(CH3),0PNB(111, and 3,5-(CF3)2C6H3CH(OTs)CH3 (12) have been determined and used to evaluate the proposal of Kirby et al. ( J . Am. Chem. SOC. 1986, 108, 706777073] that more reactive phenylethyl systems have geometries that more closely resemble solvolysis transition states. The geometries of 10 and 11 are similar, and the differences are predicted by the Kirby proposal, but the very short C-OTs bond length in 12 is contrary to expectations,although disorder in the CF, groups complicates the interpretation. It is concluded that the above simple structure/reactivity correlation should only be applied with due caution.
Introduction The substituent properties of trifluoromethyl (CF,) groups in the formation of electron-deficient destabilized carbocations have been under recent investigation in our laboratory’ as well as others.* We have now extended these studies to include the effect of the pentafluorophenyl (C6F,) group on reactivity, and to utilize CF3 and C6F5 groups in critical tests of the general applicability of the proposed3i4correlation of structure, as measured by X-ray crystallography, with reactivity. Fluorinated groups are well-suited as probes of this correlation, as they can cause drastic changes in reactivity but have rather modest steric requirements. The study of the pentafluorophenyl substituent has already a t t r a ~ t e dconsiderable ~,~ attention, but many of the previous results appear contradictory or unexplained. Thus while C6F5C02His a stronger acid than C6H5Co2H, as expected for the electron-withdrawing effect of the fluorines, m-C6F5C6H4CO2H is a weaker acid than benzoic.6 This anomaly was attributed6 to “some unusual solvent interaction associated with the highly fluorinated substituent” and to p-x interactions of the fluorine lone pairs with the aryl x system. In studies of long-lived carbocations by I3C NMR the chemical shift of C2 of the 2-(pentafluorophenyl)-2-nor(1) (a) Allen, A. D.; Kanagasabapathy, V. M.; Tidwell, T. T. J . Am. Chem. SOC.1986,108,3470-3474. (b) Allen, A. D.; Girdhar, R.; Jansen, M. P.; Mayo, J. D.; Tidwell, T. T. J. Org. Chem. 1986,51,1324-1329. (c) Tidwell, T. T. Angew. Chem., Int. Ed. Engl. 1984, 23, 20-32. ( 2 ) (a) Gassen, K.-R.; Kirmse, W. Chem. Ber. 1986, 119, 2233-2248. (b) Kirmse, W.; Mrotzeck, U.; Seigfried, R. Angew. Chem., Int. Ed. Engl. 1985,24, 55-56. ( c ) Gassman, P. G.; Hall, J. B. J. Am. Chem. SOC.1984, 106, 4267-4269. (d) Richard, J. P. J . Am. Chem. SOC.1986, 108, 6819-6820. (e) Liu, K.-T.; Kuo, M.-Y. Tetrahedron Lett. 1985, 26, 355-358. (0Guo, Z.; Fry, A. Ibid. 1986,27, 5059-5062. (9) Gassman, P. G.; Harrington, C. K. J . Org. Chem. 1984,49, 2258-2273. (h) For studies of polyfluoroarenium ions, see: Koptyug, V. A. Top. Curr. Chem. 1984, 122, 1-250. (i) Barkhash, V. A. Ibid. 1984, 116/117, 1-265. (3) Burgi, H. B.; Dunitz, J. D. Ace. Chem. Res. 1983, 16, 153-161. (4) (a) Edwards, M. R.; Jones, P. G.; Kirby, A. J. J. Am. Chem. Soc. 1986,108,7067-7073. (b) Jones, P. G.; Edwards, M. R.; Kirby, A. J. Acta Crystallogr., Sect. C 1986, C42, 1355-1358; (c) 1359-1360; (d) 1365-1367; (e) 1368-1369; (0 1370-1372. (9) Jones, P. G.; Sheldrick, G. M.; Edwards, M. R.; Kirby, A. J. Ibid. 1986, C42, 1361-1364. (h) Jones, P. G.; M e y er-Base, K.; Sheldrick, G. M. Ibid. 1987, C43, 366-367. ( 5 ) Filler, R. Fluorine Chem. Rev. 1977, 8 , 1-37. (6) Sheppard, W. A. J . Am. Chem. SOC.1970, 92, 5419-5422.
0022-3263/87/1952-4164$01.50/0
bornyl carbocation (1)was upfield from C2 of the 2-m-tolyl analogue, while C1 of 1 was downfield of C1 for the 3,s(CF3)2C6H3 ion.7a Superficially, the former result indicates that C6F, is acting as a good donor (better than m-tolyl) but the latter suggests that C6Fj is strongly electron withdrawing (more so than 3,5-(CF3),CGH3).While it has been empha~ized’~ that 13C chemical shifts cannot be directly equated with charge densities of cations or with donor properties of substituents, nevertheless consistent trends in restricted series of such data were reported,’a and the contradictory trends of the chemical shift values for the C,F,-substituted carbocation noted have not been explained. Results for the 2,5-diarylnorbornyl carbodications were completely analogous.7b
J+ c6F5
1
Protiodetritiation of tritiated pentafluorobiphenyl was slower than the same reaction of benzene, indicating the C6F, group to be electron withdrawing and deactivating relative to hydrogen with um+ of 0.285 and up+ of 0.225.’ The smaller value of the latter was considered noteable and was attributed8 to modest r-donation by C6F5, reducing up+. However despite the high electron demand in the hydrogen exchange reaction, manifested in the p value of -8.0,’ the net effect of C6F5is strongly electron withdrawing in this system. Thus the previous studies of the substituent effect of the pentafluorophenyl group lead to widely different conclusions, with an indicated electron donor ability ranging from much poorer than hydrogen to better than m-tolyl. Therefore solvolytic reactivity has now been utilized as a measure of substituent properties of C6F5,as this process provides a well-established and reliable criterion of such effects. (7) (a) Olah, G. A.; Prakash, G. K. S.; Liang, G. J . Am. Chem. SOC. 1977,99, 5683-5687. (b) Olah, G. A.; Prakash, G. K. S.; Rawdah, T. N. Ibid. 1980, 102, 6127-6130. ( 8 ) Taylor, R. J . Chem. SOC.,Perkin Trans. 2 1973, 253-258.
0 1987 American Chemical Society
J. Org. Chem., Vol. 52, No. 19, 1987 4165
Substituent Effect of the Pentafluorophenyl Group Table I. Solvolytic Rate Constants of C6F&H(OTs)CHa(7) k,l's k, s-' solvent (Y) (25 "C) solvent (Y) (25 "C) CF3C02H(4.57) 3.90 X lo4 HOAc (-0.61) 1.56 x 10-74 97% HFIP (3.61) 5.42 X 80% EtOH (0.0) 1.44 X lo+" 97% TFE (1.83) 9.04 X 10"' 100% EtOH (-1.75) 1.40 X 10"' HC02H (3.04) 1.74 X OExtrapolated from the measured rate constants, s-l (T, "C): HOAc, 3.10 X (90.6),8.73 X (75.8), 2.36 X 10" (65.8),AH* = 24.5 kcal/mol, AS* = -7.5 eu; 80% EtOH, 1.95 X (75.7),8.19 X (65.8),1.99 X (50.0, AH* = 19.3 kcal/mol, AS* = -16.0 eu; 100% (57.8),7.95 X lo4 (40.0),AH* = EtOH, 1.66 X (70.4),5.34 X 20.8 kcal/mol, AS* = -15.4 eu; 97% TFE, 9.34X lo4 (72.6),2.92 X lo4 (59.2),8.08 X (45.9),AH* = 19.5 kcal/mol, AS* = -16.4 eu.
W
Crystal structures of suitable substrates bearing C6F5 and 3,5-(CF3)2C6H3groups have also been determined, as we have previously demonstrated that this technique readily detects the effects of steric strain on reactivity in 2,9 but did not reveal a simple relationship between molecular geometry and the reactivity differences due to electronic effects in 3-6.1° However, it has recently been suggested by Kirby et aL4 that there is a close relationship between structure and reactivity in 1-phenylethyl derivatives, and the current studies provide further information relevant to this proposal.
views of C6H5C(CH3)20PNB (10) and C6F5C(CH,),OPNB (11). The view of 10 is drawn with inverted co-
Figure 1.
ORTEP
ordinates.
R
I
-
t B u 3COP NB
A'1L'
PhkOTs
I
2
i /
c F3
Figure 2. ORTEP views of 12 showing the s y n conformation and the overlap of the two ring systems. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are drawn with uniform isotropic temperature factors. Both orientations of the disordered CF, groups are indicated.
3,R=CF, 4,R;CN
5,R.H 6 , R = CH3
Results 1-(Pentafluoropheny1)ethyl tosylate (7) was prepared by the reaction of 1-(pentafluoropheny1)ethanol(8) with NaH in ether followed by reaction with p-toluenesulfonyl chloride (eq 1).
(F)
1. NaH, ether 2, TsCl
C~F~CH(OTS)CH (1)~ 8 7 The kinetics of reaction of 7 in CF3C02H (TFA), 97% (CF,),CHOH (HFIP), 97% CF3CHZOH (TFE), HCOZH, CH3C02H,C2H50H,and 80% C2H50H/H20were monitored by UV spectroscopy or conductivity and the rate constants obtained are given in Table I. The progress of the reaction of 7 in TFA, HC02H, CD3CD20D,and CD3C02Dwas observed by dissolving 7 in the appropriate solvent in an NMR tube and scanning the 'H NMR spectrum at intervals. In each case complete conversion to the corresponding substitution product (ga-d, respectively), was observed (eq 2), and no signals due to other products could be detected. C6F5CH0HCH3
SOH(D)
C~F,CH(OTS)CH~-CGF~CH(OS)CH~ 7 9a, S = CF3C0 9b, S = HCO
1.378 1.381
~ ( 3 ) .C
(2)
9 ~ S. = CQDz 9d. S = CGQC"0 9e s = c2H 9f, d = CH3C6
(9)(a) Cheng, P.-T.; Nyburg, S. C.; Thankachan, C.; Tidwell, T. T. Angew Chem., Int. Ed. Engl. 1977,16,654-655. (b) Cheng, P.-T.; Nyburg, S. C. Acta Crystallogr., Sect. B 1978,B34, 3001-3004. (10)Kanagasabapathy, V. M.;Sawyer, J. F.; Tidwell, T. T. J. Org. Chem. 1985,50, 503-509.
(L)-C(I)
110 I10
+ 7
Figure 3. Bond distances (A) and bond angles (degrees) for (10) and C6F5C(CH3)20PNE3 (11). Top values C6H6C(CH~)@Pb?B distances are for compound LO, the bottom values
in any paired
for 11.
Authentic samples of 9a and 9f were prepared by reaction of 8 with (CF3C0)20and CH3COC1,respectively, and 9a, 9b, and 9e were isolated from the solvolysis reactions and characterized. The crystals of 7 were not satisfactory for a structure determination, but structures were obtained for 2phenyl-2-propyl p-nitrobenzoate ( l o ) , 2-(pentafluoro-
4166
Allen e t al.
J. Org. Chem., Vol. 52, No. 19, 1987 Table 11. Crystal Data, Details of Intensity Measurements, a n d S t r u c t u r e Refinements" comDound 10 11
fw/F(000) Z / D , (g cm-? space group ~ ( M Ko n ) (cm-') reflns used in cell determ (no./0 range in deg) scan width (deg) max scan time (s) std reflns (no. interval in sIb max 20 (deg)/octants total no. reflns collected unique data no. data I 2 3 u ( n Ri (R&' max A / u weights, value of pB esd O.U.W. (e) max peak final AF Fourier (e k3) extinction corr.
monoclinic 6.417 (2) 10.962 (2) 20.515 (4) 91.69 (2) 1442 285.3/300 4 f 1.31 P21fC 0.9 25 (9.4 < 0 < 16.5) 0.65 + 0.35 tan0
monoclinic 15.980 (7) 6.122 (3) 16.199 (4) 93.44 (2) 1582 375.31760 411.58 P2,la 1.4 25 (10.8 < 0 < 15.8) 0.65 + 0.35 tan0 65 317500 50/h, k , f 1 3267 2363d 1309 0.0353 (0.0391)
