Experimental and theoretical investigation of the

Experimental and theoretical investigation of the unusual substituent effect of the vinyl group W . F. REYNOLDS A N D T . A . MODRO Department of Chcmistry, Uni~ersiryof Toronto, Toronto, Ont., Catzada M5S 1Al

P. G. MEZEY Department of Chemistry, Unir-ersity of Saskatche~tan,Saskatootz, Sash., Crrnada S7,V OW0

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 41.76.170.17 on 06/03/13 For personal use only.

AND

E. SKURUPVWA AI\ID A. MARON Irzstitute of Chemistry, Unitersity of Gdur~sk,Gdansh, Poland Received June 28, 1979

W. F. REYNOLDS, T. A. MODRO,P. G . MEZEY.E. SKORLPOWA. and A. MARON.Can. J . Chem. 58,412 (1980) o- Substituent constants for the ortko- and para-vinyl group have been determined by the application of the linear free-energy relationship to the nitration of the P-substituted styrene derivatives. Energy changes (relative to benzene system) for the proton and hydride ion transfer to individual positions in the styrene mo1ecule have been calculated. Both approaches indicate that the vinyl group is capable of stabilizing both positively and negatively charged transition states. The interactions of the vinyl group with other substituents in the phenyl ring are also determined. Again, stabilizing effects with respect to both rr-donor and rr-acceptor substituents have been demonstrated for the vinyl group. W. F. REYNOLDS. T. A . MODKO.P. G. MEZEY.E. SKORUPOWA e t A. M A R O NCan. . J. Chem. 58.412 (1980). On a determine les constantes de substituant IS+du groupe vinyle en position ortho e t para en appliquant la relation lineaire d'energie libre a la nitration des derives du styrene substitues en position P . On a calcule les variations d'energie (par rapport au systeme benzenique) resultant du transfert du proton et de I'ion hydrure a des positions individuelles dans la molecule de styrene. Les deux approches indiquent que le groupe vinyle peut stabiliser d e f a ~ o na la fois positive e t negative les etats de transition charges. On a egalement determine les interactions du groupe vinyle avec les autres substituants du phenyle. On a encore demontre l'effet stabilisateur d u groupe vinyle par rIpport aux substituants donneur rr e t accepteur n . [Traduit par le journal]

Introduction Most substituent groups attached to organic molecules can be classified as either electron donor or electron acceptor groups. Electron donor groups accelerate rates of reactions in which positive charge is developed in the transition state and slow down reactions involving development of negative charge, while electron withdrawing groups have the opposite effect. In the context of aromatic reactivity, substituent effects on a group X in a system RX can be predicted by the Hammett equation (1)

[I1 log (kxlktf) = pox The substituent constant, ox,is respectively negative and positive for donor and acceptor groups while p is respectively negative and positive for reactions involving development of positive and negative charge in the transition state. A large number of o constant scales have been proposed (2), including o+ (3) and o- (4) scales which are respectively intended for systems in which there is a large degree of positive or negative charge development in the transition state. Nevertheless, for most substituents o+ and o- have the same sign,

characteristic of the donor or acceptor nature of the group. However, there are a number of exceptions. For example, the halogens and most oxygen-bonded substituents have opposing fieldlinductive and resonance effects (e.g., o,= f 0 . 2 7 , oRO= -0.45 for OCH, and o, = f 0 . 5 0 , oRO= -0.34 for F according to Taft's dual substituent parameter scale (5)). Thus the net effect of the substituent depends upon the relative magnitudes of the two effects and may change sign for different positions of substituents (e.g.,ometa= +0.12,oD.,, = -0.27forOCH3 (6))or for different types of reaction sites (e.g., 0,- = f 0 . 0 6 , o,+ = -0.07 for F(6)). Another more unusual class of exceptional substituents are those which have intrinsically small substituent effects in neutral systems but which are capable of stabilizing both negatively and positively charged species. For example, alkyl groups are both acid-strengthening and base-strengthening in gas phase proton transfer reactions, apparently due to the polarizability of these groups (7). However, for aromatic reactions in solution, this effect must be relatively unimportant since alkyl groups act as weak electron donors in this case (e.g., o,+ =

0008-4042/80/0404 12-06$0 1 .00/0 0 1 9 8 0 National Research Council of Canada/Conseil national de recherches du Canada

413

REYNOLDS ET AL.

TABLE1. Nitration of P-substituted styrenes in TFA/H,SO,, 25°C Orientation (70)


k2 (ssl M - ' )

Pnrcz

Ortho

k , (s-I M s l ) Para

Ortho

0.726 f 0.080 0.118 f 0.013 0.0720 f 0.0072 0.0252 f 0.0035 0.0117 f 0.0010 0.107 f 0.015

0.963 f 0.106 0.138 f 0.015 0.0750 f 0.0075 0.0175 0.0024 0.00848 rt 0.00075 0.306 f 0.043

+

"Reference 6. OReference 14. T h i s \slue gives the relatlve rate of nitration of cinnamic a c ~ d(relative to benzene), h,,, = 0.31 Early value (16) estimated in acetlc anhydride g i v e i for cinnamic acid k,,, 0 11

-0.31, a,- = -0.17 for CH, (6)). Two other substituents which should fall into this category even in solution are the phenyl and vinyl groups. In fact oPi (= -0.20 (6)) and 0,- ( = a , a, = +0.14 (6)) constants for the phenyl group confirm that it will have a rate-accelerating effect on reactions involving the development of either positive or negative charge in the transition state. This presumably reflects the ability of the phenyl group to act as either a n-donor or a n-acceptor. Similarly, the vinyl group has a very small substituent effect in neutral systems (e.g., a , = +(0.01-0.05), oRO= -0.03 (8)) but can act as a n-donor or n-acceptor, on demand:

+

=

densities and stabilization energies, such as has been recently noted in other systems (13). Results of these investigations are reported below. Results and Discussion Experimei7tal Data Data for nitration of C,H,-CH==CH-X derivatives are summarized in Table I . Rates of substitution at positions para and ortho correlate well with the values of o+for the groups X:

+ 0.253 (r = 0.998) log k,, = -3.28~,+ + 0.477 (r = 0.998) log k,

=

-2.800x+

(see Fig. 1 . ) I The plot yields k, = 1.79 s-' M-I for nitration of the para position of styrene. Similarly k,, = 3.00 s-lM-l for nitration of the ortho position (1.50 s-lM-' for nitration at each carbon). By comparison, the determined value of k, for benzene = 0.832 sslM-' (0.139 sP1M-l at each carbon atom); o+ for the nitration of benzene derivatives = -6.0 (14). Therefore,

Thus one might expect opposite signs fore.+ and for the vinyl group. The evidence on this point is contradictory. up- = $0.14 for the vinyl group (9). However, values of a,+ = -0.16 and +0.10 have been r e ~ o r t e dfor this substituent. based respectively on rates of solvolysis of cumyl chlorides [3] log (k(styrene)lk( l H , benzene)) (10) and 13C chemical shifts of mono-substituted = -~.OD(CH=CH,)+ benzenes (1 I). Consequently, we decided to redetermine ap+for the vinyl group, based upon rates Using this equation the values of a," = -0.18' and of nitration (12). One cannot investigate the nitra- o,,+= -0.17 are obtained for the vinyl group. The tion of styrene directly since the favoured site for data also allow one to set a limit on the value ofom+. electrophilic attack is the vinyl group. However, ITable 1 also includes nitration data obtained for the Pone can determine rates of nitration for trans-0substituted styrenes, C,H,CH=CHX where X is styrylphosphonic acid. From these data, together with Fig. 1, (previously unreported) o+ value for the phosphonic group an electron withdrawing group. One can then plot the could be evaluated. It follows that oPo,H2t= 0.44, similar to that rate of nitration vs. ox+and extrapolate t o o + = 0.0, for the carboxylic group (oCO,H+= 0.42 (14)). Close similarity of substituted effects of groups CO,H and PO,H, has been demoni.e., X = H , using a Hammett plot. It was also decided to carry out a .theoretical strated before (15). The excellent correlations in Fig. 1 justify investigation of the stabilizing effect of a vinyl the use of op+values for side-chain substituent groups. ZThe different value estimated by Levy and co-workers (1 I) group in aromatic systems, both for comparison presumably is in error because para I3Cchemical shifts are not. with the experimental results and to check whether in fact, related to op+ but rather must be correlated with o, and there was any relationship between induced charge OR" (5).

0,-

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C A N . J . CHEM.


energy change for the isodesmic proton transfer reaction (18):

and the similar reactions involving ortho- and rnrta-protonated styrenes. Similarly, the effect of a vinyl group on nucleophilic aromatic substitution can be estimated from the corresponding hydride transfer reactions:

0-+

FIG. 1. Log k 2 for nitration of P h - C H S H - X vs. ox+. TFAIH,SO,, 25°C. (0) para nitration; (@) ortho nitration.

The maximum concentration of meta-nitro product is 2% (see Experimental). Thus, the rate constant for ortho-nitration is at least 30 times as great as for nleta-nitration. This corresponds to om+> +0.06. The o,+ and om+values are close to those estimated from the solvolysis of p- and m-vinylcumyl chloride (op+ = -0.16, om+= +0. 1 1 (1)). Thus, o,+ is in fact negative for the vinyl group. Comparison with 0,- ($0.14 (9)) confirms that, as suggested in the Introduction, the vinyl group can act as both a donor (to positively charged groups) and an acceptor (from negatively charged groups). In fact, the vinyl group appears to be almost equally effective at stabilizing positively and negatively charged transition states in solution, judging from the similar magnitudes of o,+ and o,. As shown in Table I , the extent ofpara substitution increases with increasing polarity of X in C,H,-CH=CHX derivatives. This is consistent with the expected through-space field effect of X which should disfavour approach of an electrophile to the ortho position (17). Theoretical (ab initio) Calculations The effect of a vinyl group on electrophilic aromatic substitution can be estimated from the

Results for these calculations are summarized in Table 2. The calculations clearly demonstrate the ability of the vinyl group to stabilize positively and negatively charged transition states to a similar degree. They also show that this results from n charge transfer between the vinyl and phenyl groups. In fact, there is a close parallel between AE and lq, I, the magnitude of n charge transfer. The calculations also indicate that the vinyl group stabilized the para-protonated intermediate slightly more than the ortho-protonated intermediate. consistent with the experimental o,+ (-0.18) and oof(-0.17) values. However, the calculations differ from the experimental results in predicting a slight stabilization of electrophilic substitution at the meta position. This may reflect a difference between gas phase and solution reactivity. In view of the effect of a vinyl group on charged systems, it is also interesting to consider the effect of the vinyl group in neutral aromatic species. The calculated n charge distribution in styrene is shown in Fig. 2. It is seen that the group is essentially neutral (with only 0.0001 n electron transferred to the phenyl group. compared to almost k0.2 n electron transfer between the vinyl group and charged T A B L E2. Energy changes (in kcal/mol) for isodesmic proton transfer reactiona and hydride reactionb and extent of .ir charge transfer. Aq,, to or from vinyl group Species

AE (kcallmol)

kn

"Equation 141 'Equation [ S ] .

dTotal change in rr charge d e n s ~ t yfor the vinyl group, relative to styrene. Positive sign corresponds to decreased electron density. i.e.. n electron transfer to ring.

REYNOLDS ET AL

TABLE 3. Energy changes (in kcal/mol) for isodesmic reactiona measuring the energy of interaction between a variable substituent X and a fixed substituent Y in apara-disubstituted benzene XC,H,Y


X

ox+@~

Y=C H S H ,

-H

e

y

+

H+H

phenyl groups, see Table 2). However, there is significant n charge redistribution within the phenyl and vinyl groups. Within the orbital approximation. charge redistribution schemes may be modelled in terms of orbital interactions. It has previously been suggested in the case of halobenzenes and alkylbenzenes that repulsion between a filled orbital on the substituent and the phenyl 7c system will lead to an increase in n electron density at the ortho and para carbons (19). However, in these cases, it is difficult to separate this orbital repulsion effect from the electron donating resonance effects of these groups. With no net 7c electron transfer, the vinyl group provides very clear evidence for the existence of an orbital repulsion effect. This may account for the o values of the vinyl group in neutral systems (om= +0.08, o, = -0.08 (6)).

FIG.2. Calculated n charge distribution in styrene.

It is also informative to consider the interaction of the vinyl substituent with other groups. This can be determined from the energy change for the isodesmic reaction:

Y = CH3

+

X+Y

Y = CO,H

Y = CO,

3E

Results are summarized in Table 3. The interaction of the vinyl group with both electron donor groups (e.g., NH,) and electron acceptor groups (e.g., NO,) is stabilizing, although weak in the case of electron donor groups. The latter weak interaction may reflect a par-tial cancellation of the stabilizing 7c acceptor ability of the vinyl group.by its orbital repulsion effect. The ability of the vinyl group to have stabilizing interactions with both donor and acceptor groups is most unusual. This can be seen by comparing with three other substituent groups with differing properties: a weak donor group. CH, (oPo= -0.15); a moderately strong acceptor group, C0,H (oP0= f0.44); and a charged group CO,-. These data are shown in Table 3. The interactions of the methyl group with acceptor groups and donor groups are respectively stabilizing and destabilizing (as expected from the same signs for of and o - , see above). The CO,H group shows the exact opposite pattern of stabilizing and destabilizing interactions, as expected from its opposite substituent effect. Finally, interactions of CO,- with acceptor and donor groups are respectively stabilizing and destabilizing. Interactions with highly polar groups (e.g., NO,) are particularly strong. This presumably reflects the strong (through-space) field interaction between a polar and a charged group (20). This is also reflected in the apparently anomalous interactions with OH and F. These two groups have opposing field and resonance interactions (see Introduction). The former must dominate in the interaction with a

416

C A N . J . CHEM. VOL. 5 8 . 1980


charged group while the latter dominates in the interaction with neutral groups (where field effects should be less important (20)). Finally, it is noteworthy that, of the various X groups, only vinyl shows stabilizing interactions with all three CH,. C 0 2 H ,and C O , . Summary and Conclusions Experimental and theoretical investigations of the substituent effect of the vinyl group indicate that this group is capable of stabilizing both positively and negatively charged transition states by acting as either a n donor or a n acceptor, depending upon the demand of the system. A close parallel is noted between the stabilization energy and the extent of n charge transfer. The vinyl group appears to have an intrinsically small substituent effect in neutral systems; mainly a weak orbital repulsion effect. However, the interaction ofthe vinyl group with both n donor and n acceptor substituents in p-disubstituted benzenes is a stabilizing interaction. Experimental Solvents were purified by conventional methods. AnalaR nitric acid (d. 1.42) was distilled from an equal volume of concentrated H,SO, immediately before use. Trifluoroacetic acid was distilled from trifluoroacetic anhydride immediately before use. Ultraviolet spectra were determined with a Perkin-Elmer 402 spectrophotometer. 'H nuclear magnetic resonance spectra were obtained with a Tesla BS-487 (80 MHz.) spectrometer. Gas chromatography was performed using a Pye Unicam. series 104 instrument. Substrates Benzene (1). P-bromostyrene (2), cinnamic acid (3), ethyl cinnamate (4), cinnamic nitrile (S), and P-nitrostyrene (6) were obtained commercially and were purified by distillation or crystallization until chromatographically pure (glc). P-Styrylphosphonic acid (7) was prepared according to Bergmann and Bondi (21); mp (from H,O) 149-150°C (lit. (21). mp 146°C). Product Detrrminarion M ) was dissolved in TFA containing conSubstrate (ca. centrated H,S04 in the amount equimolar to that of the aromatic compound. The solution of nitric acid (slight excess) in TFA was added slowly at 0°C with stirring. The mixture was then left at room temperature until tlc showed that the reaction was complete. The product was then isolated by evaporation of TFA and extraction; in all cases the yield was practically quantitative and the elemental analysis (nitrogen) of the product agreed with the composition of a mononitro derivative. The nitration product was examined by tlc to determine the number of isomers formed and then oxidized with alkaline permanganate (22) to a mixture of nitrobenzoic acids (in all cases only ortho and para isomers were observed). The proportion of isomers was determined by quantitative separation of individual compounds and by comparison of the uv spectrum of the oxidation product with the spectra of model mixtures of known composition. The oxidation product was also examined by nmr spectroscopy and the proportion of ortho- and para-nitrobenzoic acids was determined from the integrated signals of the aromatic protons of the ortho (6 7.50-7.80) and para (6 7.95-8.25) iso-

mers. All determinations gave good internal agreement. Control experiments showed that the amount of mefa-nitro product should not exceed 2%. Rates of Nitration A solution of HNO, in TFA was added to a stirred solution of an equimolar amount of substrate in TFA containing one equivalent of concentrated H,S04 at 25.0°C. and aliquot portions were withdrawn at suitable times. For substrates 1 . 2 , 3 , 4 . 5 , and 7 aliquots were quenched in water and the concentration of nitro compounds in the reaction mixture was determined spectrophotometrically (uv) in the usual way. The following wavelengths and extinction coefficients of substrate (EJ and product (E,) were used: 1, h = 258 nm; &, = 150, E,= 8 500; 2. h = 265nm, E , = 17000, E , = 4500; 3, h = 273nm, E , = 21 500, E , = 10 800; 4, h = 275 nm, E, = 22 000, E, = 11 200; 5, A. = 271 nm, E, = 2 2 6 0 0 . ~ , = 13 100;7,h= 258nm.&,= 1 8 7 0 0 . ~ , =11500. For P-nitrostyrene (5) aliquots were extracted with n-hexane. extracts were evaporated under reduced pressure. and the residue was dissolved in n-hexane containing a known amount of 4bromodiphenyl as a standard. The mixture was then separated by gas chromatography (column: 3% OV-225 on a Chromosorb W (AW-DMCS) 60180 mesh, 1 m; temperature 115°C; flow rate (argon) 25 cms/min) and concentrations of individual nitro products were determined by the planimetric estimation of the peak areas. Kinetic runs were followed to ca. 80% of conversion. Good second-order kinetics were obtained in all cases. Values of k, for nitration of benzene and substrates listed in Table 1 represent the average of 4-6 independent runs. Individual rate constants are reproducible to within k5-15%. Details ofCa1c~tlation.s A STO-3G basis set as contracted to a minimal basis set (23) was used for the ub itzitio molecular orbital calculations. Calculations were performed on an IBM 3701168 computer using a version of the Gaussian 70 program (24). Standard geometries were assumed for phenyl and vinyl groups (25) while the geometry of the protonated phenyl group was that given by McKelvey et a / . (26). Other substituent geometries were identical to those previously reported (20).

Acknowledgements The financial assistance of the National Research Council of Canada and of the Polish Academy of Sciences is gratefully acknowledged. 1. L. P. HAMMETT. J . Am. Chem. Soc. 59,96 (1937). 2. C. G. S W A I Nand E. C. LUPTON.J. Am. Chem. Soc. 90. 4328 (1968). 3. H . C. BROWNand Y. O K A ~ ~ O T J . OAm. . Chem. Soc. 80, 4979 (1958). 4. R. W. TAFT.J . A m . Chem. Soc.79, 1045(1957). 5. E. EHRENSON,R. T. C. B R O W N L E E . ~W. ~ ~TAFT. R . Prog. Phys. Org. Chem. 10. l(1973) and references therein. 6. C. D. JOHN SO^. The Hammett equation. Cambridge University Press, London. 1973. and L. K. BLAIR.J . Am. Chem. Soc. 90, 7. J . I. BRAUMAN 5636 (1968); YO, 6501 (1968). 8. P. R. WELLS.S. EHRENSON, and R. W. TAFT.Prog. Phys. Org. Chem. 6, 147 (1968). 9. T. MILLER.Aromatic nucleophilic substitution. Elsevier, Amsterdam. 1968. Chapts. 4,5. and 6. 10. E . N . PETER.J. Polym. Sci. Polym. Lett. Ed. 13,479(1975). 11. C . L. NELSON.G. C. LEVY,and J . D. CARGIOLI. J. Am. Chem. Soc. 94,3089 (1972).


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19. D. T. CLARK,J. N.MURRELL,and J. M . T E D D E RJ.. Chem. I?. J. G . HOGGETT.R. R . MOODIE,J. R. P E ~ S O Nand , R. SOC.1250 (1963). S C H O F I ~ L Nitration D. and aromatic reactivity. Cambridge S . J. Chem. 55. 1567 20. P. G. MEZEYand W. F. R E ~ N O L DCan. University Press, London. 1971. Chapts. 3 and 7. (1977). S . G . MEZEY,W . J. H E H R ~R. , D. 13. W. F. R E Y ~ O L D P. and A . BONDI.Ber. 63, 1158 (1930). T o ~ s o b r ,and R. W. TAFT. J. Am. Chem. Soc. 99. 5821 21. E. BERGRIANN . Chem. 22,762(1957). 22. W. E. TRUCEand J. A . S I R ~ ~J.I SOrg. (1977). , F. STEWART,and J . '4.POPLE.J. Chem. 14. L. M. STOCKand H . C. BROWN.Adv. Phys. Org. Chem. 1, 23. W. J. H E H R E R. Phys. 51,2657 (1969). 35 (1963). 15. T. A. MODRO.W. F. REYUOLDS, and E. SKORUPOWA. J . 24. Gaussian 70. Quantum chemistry program exchange. Indiana University, Bloomington. IN. Chem. Soc. Perkin Trans. 11, 1479 (1977). 16. F. G. BORDWELL and K. ROHDE.J. Am. Chem. Soc. 70. 25. J. A . POPLEand M. G O R D O ~J.. Am. Chem. Soc. 89,4253 1191 (1948). (1967). Y .ALEXANDRATOS. A. S T R E I T W I E S ~ R , 26. J. M. M C K ~ L V E S. 17. D. hf. B r s ~ o p a n dD. P. CRAIG.MoI. Phys. 6, 139 (1963). J-L. M. ABBOUD,and W. J. HEHRE.J. Am. Chem. Soc. 98. L. RADOM.and J . A . POPLE. 18. W. J. H E H R ~R.. DITCHFIELD, 244 (1976). J . Am. Chem. Soc. 92.4796(1970).