WILLIAM F. REYNOLDS AND ROBERT A. MCCLELLAND. Received June 8, 1976

Transcription

1 Chemical shifts for Qsubstituted phenylvinyl ethers, sulfides, and selenides: evidence concerning the relative abilities of 0, S, and Se to transmit electronic effects WILLIAM F. REYNOLDS AND ROBERT A. MCCLELLAND Depurtmrnt of Chemrstry, Ut~ii'ersity of Toronto, Toronto, Orlt., Cunuda M5S 1Al Received June 8, 1976 WILLIAM F. REYNOLDS and ROBERT A. MCCLELLAKD. Can. J. Chen~. 55, 536 (1977) 13C chemical shifts for B carbons of 4-substituted phenylvinyl ethers, sulfides, and selenides plus previous data for styrenes indicate that the relative ability of link groups to transmit electronic effects between conjugative groups is S > Se %- > 0 (where- refers to no link group, i.e. styrene). However, marked deviations from additivity are noted for C(l) chemical shifts which may indicate that 0 deactivates the ring to electronic substituent effects while S and Se activate the ring. If this explanation is valid then the actual ability of the link atom to transmit electronic effects is - > 0 > S > Se. WILLIAM F. REYNOLDS et ROBERT A. MCCLELLAND. Can. J. Chem. 55, 536 (1977). Les deplacements chimiques du carbone-13 des carbones-b d'ethers sulfures et stltnures de vinyle et de phknyles substitues en position 4, utilises de concert avec des donnees anterieures pour les styrines, indiquent que l'habilite relative des groupes liants a transmettre les effets electroniques entre les groupes conjugues est S > Se a - > 0 (oh- se refere au cas ou il n'y a pas de groupe liant, i.e. le styrene). Toutefois on note des deviations importantes de I'addivite pour les deplacements chimiques du C(1); ces deviations peuvent indiquer que I'oxygene dcsactive le cycle par rapport aux effets electroniques des substituants alors que le S et le Se activent le cycle. Si cette explication est valide, l'habilite reelle des groupes liants a transmettre les effets electroniques serait - > 0 > S > Se. [Traduit par le journal] There has been considerable interest in the relative abilities of 0, S and, to a lesser extent, Se to transmit electronic effects, particularly in the case where these atoms act as a link or bridge between two conjugative groups. The initial investigation was by Litvinenko who investigated the reactivity of the amino group to acylation in compounds of the type X-C,H,- Y-C,H,NH, where X is the variable substituent and Y = 0, S, etc. (1). He concluded that the ability to transmit electronic substituent effects decreased in the order S > Se > 0 > -.I Subsequent investigations involving 'H (2, 3), 19F (4, 5), and 13C (6) nmr spectroscopy supported the order S > in cases involving conjugative transmission of substituent effects. Perhaps the most striking feature of these investigations is the general conclusion that interposition of 0 or S between two con- phenomenon which has been referred to as a positive bridge effect (1) or superconductivity (2). The most recent investigation involved 'H and 13C chemical shifts measurements for phenylvinyl ethers and sulfides (6). However, measurements were carried out for a limited number of derivatives (including only three para derivatives in each case) and I3C chemical shifts were measured for neat liquids. Since we had a large number of phenylvinyl ethers, sulfides, and selenides as part of another investigation (7), a further more careful 13C nmr spectroscopic investigation of this system seemed desirable. It was hoped that this would provide further evidence concerning the relative position of Se on the transmission scale and possibly also help to elucidate the origin of the positive bridge effect. In fact, the investigation has shown jugative groups apparently enhances the trans- that the previous order S > Se > 0 > - mission of electronic effects relative to the case possibly does not reflect the ability of these where the two groups are directly linked, a linking groups to transmit substituent effects. 'Throughout this manuscript the symbol - refers to the case where the two conjugative groups are directly linked, i.e. biphenyl in this case. Results and Discussion 13C chemical shifts for 4-substituted phenylvinyl ethers, sulfides, and selenides are listed

2 REYNOLDS AND McCLELLAND 537 TABLE 1. I3C chemica! shiftsa for 4-substituted derivat~ves X-C6H4YCH=CH2 Y X 8~~41 &c(3) Sc(2) ~C(I) fic(z) ficcp) A6c(11b A8c(plb 0 H NH, O OCH GH O F C Br htoz i-3.95 S H S NHz S OCH S ch S F S C ' ' S Br "31.72d S CF S NO, i6.30 Se H Se NH, Se OCH, Se CH Se F Se C Se Br i0.97 Se CF Se NO f5.44 Oln ppm to low field for tetramethylsilane. bchemical shift- relative to unsubstltuted derivatibe. 'Overlapping peaks. dassignments uncertain. in Table I. Based on both the maximum range of substituent effects upon p carbon 13C chemical shifts (from NH, to NO, derivatives) and on correlations of p carbon chemical shifts against one another, the apparent order of transmission of substituent effects is S > Se = - > 0 (see Table 2). This parallels the order of Litvinenko (I) and subsequent workers (2-6) with respect to 0, S, and Se. However, the directly bonded system (styrene) appears to be more efficient at transmitting substituent effects than previously deduced from systems of the type X-C,H,-Y-C,H,Z (1, 3-6). It could be argued that the present system supplies a more reliable order since styrene (8) (and presumably the other derivatives) are planar while the systems X-C,H,-Y-C,H,Z undergo uncertain conformational changes as Y is varied (9). However, inspection of the phenyl carbon chemical shifts indicates an additional complexity. The effect of substituents on C(l) chemical shifts varies markedly from system to system (Table 1). Taft and co-workers have -- ~ recently reported similar marked deviations from additivity of 13C chemical shifts in yaradisubstituted benzenes (10). They suggested that o electron-withdrawing groups deactivate the ring towards both n-inductive and resonance effects of substituents (10). The obvious corollary of this is that o-donor groups should activate the phenyl group; a trend which is apparent in 13C chemical shift data for 4- substituted trimethylsilylbenzenes (1 1). On this basis, it would appear from our results that -SCH=CH, and -SeCH=CH, groups both activate the phenyl groups (see &A,,,,). While S and Se are normally considered to have electronegativities similar to carbon (12), it is possible that the polarizabilities of these atoms contribute to their apparent activating effect.' By contrast -OCH=CH, appears to be a de- 2Lynch has collected data for a large number of p- disubstituted benzenes and has concluded that the deviations from additivity for the I3C chemical shifts of the carbon para to the variable substituent depend upon the polarizability of the fixed substituent (13). The other I3C chemical shifts show little deviation from additivity.

3 CAN. J. CHEM. VOL TABLE 2. Various parameters which measure the relative abilities of 0, S, and Se to transmit electronic effects, relative to the case where the two conjugative groups are directly linked Values Parameter - d 0 S Se Chemical shift difference between KO, and NH2 derivative. bsslpe of a correlation of C(P) chemical shifts us. C(B) for 4-substituted styrenes. "0, values from Table 3. ddata from ref. 18. activating group (as observed for the o-withdrawing OCH, group (10)) while -CH=CH, is essentially neutral (the total range of para carbon chemical shifts from NH, to NO, derivatives of benzene is 16.0 ppm (10) compared to 15.0 ppm for C(l) in styrene, see Table 2). Taking our results in conjunction with those of Taft and co-workers (lo), total range of substituent-induced para carbon chemical shifts (from NH, to NO,) varies from 9.9 ppm in 4- substituted fluorobenzenes to 25.1 ppm in 4- substituted phenylvinyl sulfides and selenides. This raises the possibility that these deviations from non-additivity reflect variations in the sensitivity of 13C chemical shifts to electron density changes rather than variations in electron density changes. Put in empirical terms, this would indicate that the scaling factor relating 13C chemical shifts to.rr. electron density changes is altered from the normal value of ppmln electron (14, 15) by the effect of the directly bonded atom. In support of this view, ab initio (ST0/3G minimal basis set) calculations for 4-substituted fluorobenzenes and substituted benzenes indicate much smaller deviations from additivity than noted from the I3C chemical shifts (16). However, these calculations assume a regular hexagonal structure for the benzene rings. Available structural data indicates that substituents can induce significant distortions of the benzene ring in poly-substituted benzenes (17) which might result in non-additive electronic effects not predicted by calculations based on idealized geometries. Obviously this is an area worthy of further investigation, although geometry-optimization calculations on poly-substituted benzenes would be prohibitively expensive. If it is assumed that these non-additive effects on 13C chemical shifts accurately reflect ground state electron density changes (previous investigations of 13C chemical shifts in benzene derivatives support this view (18-21)), the phenyl carbon chemical shift data allow an alternative interpretation of transmission of electronic effects by 0, S, and Se. The effect of the link group can be divided into two parts: (1) its ability to activate (or deactivate) the directly bonded carbon of phenyl group towards electronic substituent effects and (2) the ability to transmit these effects to the second conjugative group. On this basis, a better measure of the actual ability of the substituent group to transmit electronic effects should be the ratio of C(P) to C(1) chemical shifts. These ratios are given in Table 2. They indicate a much different order of transmission ability, i.e. - > 0 > S > Se. If this approach is correct, it indicates that the apparent 'superconducting' or 'positive bridge' effect of S and Se is not really a transmission effect but rather reflects the ability of these linking atoms to activate the phenyl group towards substituent effects. In many ways, this seems more satisfactory than an explanation involving positive bridge effects. It is also consistent with previous conclusions that 0 is more capable of conjugative interactions than S (22). Assuming that the phenyl carbon shifts parallel electronic effects, a more detailed picture of these electronic effects can be obtained by correlating 13C chemical shifts for C(1), C(a), and C(P) with Taft's o,.and oro substituent constants (23) (Table 35. These correlations indicate that C(P) and C(1) show different relative sensitivities to fieldlinductive and resonance effects. Considering the resonance

4 REYNOLDS AND McCLELLAND TABLE 3. Correlations of C(1), C(a), and C(p) chemical shifts with (31 and oro -" c(1) C(a) C(P) c(1) c(a) c(p) S c(1) S c(a) S c(b) Se C(1) Se c(a) Se c(p) aexperimental data from ref. 18. Correlations were based on the same substituents as for phenylvinyl sulfide. Inclusion of other substituents gives sllght different values (19). OWeighting coefficients for equation 6 = p,ol + p,ano t So. 'h = PRIPI. dcorrelat~on coefficient. effects first, the p, values indicate an enhancement of resonance effects at C(l) on going from phenylvinyl ether to the sulfide and selenide analogues. The correlations for the vinyl group indicate the alternation in sign and magnitude of p, which seems typical of resonance effects. Since C(P) and C(l) have different sensitivities to fieldlinductive and resonance effects, p,c(o)/ prc(l) should provide a better measure of the ability of the bridge group to transmit conjugative effects. These ratios are given in Table 2. While quantitatively different from AAC,,,/., AAq,,, they predict the same order. There is an even greater variation of p, values for C(l) since these vary by a factor of three. Taft and co-workers also noted greater variations in p, than p, (10). We have previously suggested that polar substituents can polarize the entire.n electron system towards the substituent (20) Taft et al. suggested that o-withdrawing groups para to X would inhibit this polarization by making it more difficult to remove n electron density from the para carbon (while o donor groups should enhance the polarization) (10). Our results are consistent with this explanation. In this regard, it may or may not3 be significant that carbons ovtho to the variable substituent show much smaller deviations from additivity (19, 20b and Table 1) than in the case of the para carbon. This is consistent with.n polarization effects, 1, as well as resonance effects, 2, involving quininoid structures : It is interesting to note that C(P) chemical shifts show a large sensitivity to fieldlinductive effects (in fact p, is larger for C(P) than for C(l) in phenylvinyl ether). This indicates the present of polar n inductive effects, i.e. effects which can alter the.n electron distribution without charge transfer to or from the substituent. We have previously shown that n polarization in styrene involves two components (1, 24); polarization of the entire conjugated system from vinyl to phenyl groups and through space polarization of the vinyl n bond towards the substituent. The latter effect can account for the relatively large sensitivity of C(P) to fieldlinductive effects. Although the vinyl group is further removed from the substituent in ethers, sulfides, and selenides than in styrene, it is aligned more directly with the C-X bond axis: 31t may not be significant because it could equally well be argued that this demonstrated that the deviations from additivity for the para carbon reflected different sensitivities of these carbons to electron density changes.

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