Pyrolysis Of 1-arylethyl Acetates Research Articles

Electrophilic aromatic substitution. Part 33. Partial rate factors for protiodetritiation of benzo[b]thiophen; the resonance-dependent reactivity of the ring positions

Rate coefficients for detriation of benzo[b]thiophen, specifically labelled with tritium at each position, have been measured in anhydrous trifluoroacetic acid, and, for [2-3H]benzo[b]thiophen, in a range of trifluoroacetic acid–acetic acid mixtures, all at 70°. The dependence of rate upon acidity is consistent with the effect of hydrogen bonding between sulphur and trifluoroacetic acid, as found previously for other sulphur-containing heterocycles. Partial rate factors, corrected for the effect of hydrogen bonding are (positions in parentheses) : 8.84 × 105(2); 1.18 × 106(3); 360 (4); 13 200 (5); 16 200 (6); and 26.2 (7); the corresponding σ+ values are –0.68, –0.695, –0.29, –0.47, –0.48, and –0.16. The positional reactivity order is the same as found in pyrolysis of 1-arylethyl acetates, and solvolysis of 1-arylethyl chlorides, but the σ+ values are more negative due to the greater demand for resonance stabilization of the transition states in hydrogen exchange. Comparison of the 2-: 3-isomer ratios in various substitutions leads to the unexpected conclusion that substitution at the 2-position is subject to steric effects, probably due to the proximity of the sulphur d-orbitals.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part 14. Non-additivity of methyl substituent effects: the reactivity selectivity principle

Rates of gas-phase elimination of acetic acid from polymethyl-substituted 1-arylethyl acetates, measured between 613.8 and 667.2 K, have shown that the effects of the methyl groups are not strictly additive in this reaction. The reactivity is greater than that calculated on the basis of the additivity principle, log krel.(obs.)– log krel.(calc.)/[log krel,(calc.)]2 being approximately constant. This contrasts with the related electrophilic aromatic substitutions, in which the reactivities for polymethyl compounds are less than calculated from the additivity principle. The results confirm that the transition state for ester pyrolysis becomes more polar, the greater the electron supply to the α-carbon atom. Greater electron supply causes a shift in the transition state away from the ground state, whereas in electrophilic substitutions the transition state is shifted towards the ground state. These observations demonstrate the differences arising from the participation in the transition state of bond-breaking processes in the elimination as compared with bond-making processes in the substitution. As in solution reactions, the ratio of the reactivities of the 4- and 3-positions in o-xylene is less than that of the para- and ortho-positions in toluene, so this anomaly cannot arise from solvation effects.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part 15. Non-additivity of chloro-substituent effects: mechanism of the elimination

Rates of gas-phase elimination of acetic acid from polychloro-substituted 1-arylethyl acetates, measured between 642.0 and 692.6 K, show that the effects of the chloro-substituents are not additive in this reaction. The reactivities of the esters are greater than calculated on the basis of additivity of the substituent effects, the deviation being greater the less reactive the ester. This result, which parallels that observed with the polymethyl -substituted esters, is attributed to variation in the transition state polarity such that the latter decreases with decreasing ester reactivity. Compounds with buttressed ortho-chloro-substituents show particularly enhanced reactivity probably due to enhanced direct p-orbital overlap between the ortho- chlorine atom and the incipient side-chain α-carbocation. As in nitration pentachloro-substitution produces less deactivation than the 2,3,5,6-tetrachloro-substitution, and the increased buttressing in the former may be reponsible: in nitration the effect would appear as increased co-ordination to the electrophile facilitating substitution. Reduced C–C σ-bond overlap (and hence a reduced –I effect) produced by steric crowding cannot at present be ruled out as an alternative explanation.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part XII. Total reactivity of isoquinoline

All seven 1-(isoquinolyl)ethyl acetates have been prepared and their rates of elimination of acetic acid determined together with that of 1-phenylethyl acetate at temperatures between 351.8 and 426.3°. The results give the first measure of the quantitative electrophilic reactivity of the neutral isoquinoline molecule and the positional reactivity order is: 4 > (phenyl) > 5 = 7 > 8 > 6 > 3 > 1. Each position is less reactive than the corresponding position in naphthalene and more reactive than the corresponding position in pyridine. The electrophilic substituent constants are: –0.025(4); +0.07(5,7); +0.255(8); +0.31(6); +0.41(3); +0.51(1). The reactivity pattern shows that the formation of mainly 4-derivatives in electrophilic substitutions under neutral conditions is not anomalous as hitherto supposed, and the special mechanism invoked to account for this substitution pattern is no longer necessary. Isoquinoline is more reactive overall than quinoline; the benzenoid ring is much less reactive than in quinoline, but the pyridinoid ring is correspondingly much more reactive than in quinoline. The deactivating effect of the nitrogen in quinoline and isoquinoline is closely paralleled by the effects of substituents in hydrogen exchange of naphthalenes. The weak deactivation of the 5-position in quinoline, and the greater deactivation at the 1-relative to the 3-position in isoquinoline are shown to be due to bond fixation effects. As with pyridine and quinoline, Huckel π-electron densities are found to be the best theoretical indices of reactivity, and with an auxiliary inductive parameter of [graphic omitted] 0.018 they predict not only the observed reactivity order, but also the activation at the 4-position. The reactivity order is also predicted almost quantitatively by the difference between the localization energy at position j of i-methylenenaphthalene and that at position j of naphthalene.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part X. Pyridine N-oxide

All three 1-acetoxyethylpyridine N-oxides have been prepared and the rates of elimination of acetic acid from these esters and from 1-phenylethyl acetate have been measured over a 50° range for each compound at temperatures between 286 and 402°. The rates lead to σ+ values of +0·81 and +0·016 for the 3-and 4-positions respectively, in good agreement with both theoretical predictions and the qualitative observations of electrophilic substitution. The reactivity of the 4-position lends strong support to the belief that nitration of pyridine N-oxide does not take place on the free base. The 2-compound is very much more reactive than the other isomers (contrary to theoretical prediction) and gives 2-acetylpyridine instead of the expected 2-vinylpyridine N-oxide as the elimination product. 2-Acetylpyridine is also obtained on elimination of water from 2-(1-hydroxyethyl)pyridine N-oxide which proceeds at almost the same rate as elimination of acetic acid from the acetate. The high reactivity is attributed either to acceleration of the loss of the functional group in the side chain through a neighbouring group effect (followed by rearrangement) or to an initial insertion of the oxygen into the side chain α-C–H bond followed by rapid elimination from the 1,1-diol (and its acetyl derivative). The literature mechanism for the 3-nitration of quinoline N-oxide is shown to be highly unlikely, and an alternative is suggested.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part IX. The effect of the trimethylsilyl substituent

The three isomeric 1-(trimethylsilylphenyl)ethyl acetates have been prepared and their rates of pyrolysis together with that of 1-phenylethyl acetate measured between 340·6 and 392·9°. The trimethylsilyl substituent activates in all positions according to the sequence o > m > p as theoretically predicted and found in protodesilylation, but contrary to the activation order observed in nitration, and the deactivation order found in solvolysis of 1-aryl-1-methylethyl chlorides. The variability of the substituent effect in these and some other reactions is attributed mainly to varying demands for resonance stabilisation of the transition states together with some solvent–substituent interactions. The variation in the meta-substituent effect coupled with the high activation by the ortho-substituent indicates that the trimethylsilyl group supplies electrons inductomerically. In the light of the present results the reactivity pattern in nitration of phenyltrimethylsilane is seen to be affected by steric hindrance. Values of σ+ are –0·16 (m-SiMe3) and –0·09 (p-SiMe3).

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VIII. The 1-position of biphenylene

1-Biphenylen-1-ylethyl acetate has been prepared and its rate of pyrolysis (togther with that of 1-phenylethyl acetate) measured between 399·3 and 349·2 °C. The former ester eliminates 1·205 times faster than the latter at 625 K which leads to a σ+-value of –0·13 for the 1-position of biphenylene in this reaction. The weak activation of the 1-position is less than in hydrogen exchange but contrasts with protodesilylation where weak deactivation occurs. The reactivity in the pyrolysis is predicted by our theory attributing the differential reactivity of the α- and β-positions in biphenylene (and strained aromatic molecules in general) to bond strain in the transition state, and is counter to the prediction of the alternative theory based upon increased electronegativity of bridgehead carbons in strained rings.

Determination of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VII. The total reactivity of quinoline

All seven 1-(quinolyl)ethyl acetates have been prepared and their rates of elimination of acetic acid together with those of 1-phenylethyl acetate and the three 1-(pyridyl)ethyl acetates measured at temperatures between 348·2 and 413·5°. The results yield the first measure of the quantitative electrophilic reactivity of the neutral quinoline molecule and coupled with previous data show the relative reactivities as follows: α-naph > β-naph > 5-Q > phenyl > 8-Q = 6-Q > 3-Q > 7-Q > 3-P 2-Q > 4-Q > 2-P > 4-P. Each position in quinoline is consequently shown to be less reactive than the corresponding positions in naphthalene, more reactive than the corresponding positions in pyridine, and only the 5-position is activated relative to a position in benzene. Electrophilic substituent constants σ+ are derived for the positions (in parentheses) as follows: –0·108(5); +0·063(8,6); +0·079(3); +0·152(7); +0·730(2); +0·747(4). Taken in isolation the positional reactivities are predicted satisfactorily by molecular orbital calculations of π-electron densities, but these are less satisfactory in predicting the reactivities relative to other molecules; calculated localisation energies are poor indicators of the positional reactivities, which appear to be particularly dependent upon inductive and/or field effects.

Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VI. Further evidence for the direct field effect, and the invalidity of ? 0 + values

The relative rates of pyrolysis of 1-arylethyl acetates XC6H4CHMe·OAc (measured over a 50° range for each compound) are as follows at 600 °K: (X =) H, 1·00; 2-PhS, 0·89; 3-PhS, 0·805; 4-PhS, 1·32; 2-PhO, 1·20; 4-PhO, 2·14; 2-MeS, 1·02; 4-MeS, 1·48; 2-PhCH2, 1·51; 4-PhCH2, 1·51. The abnormally low activation by ortho- relative to para-oxygen- and -sulphur-containing substituents is shown to be general for reactions involving formation of carbonium ions at the side-chain α-positions. The results parallel those found for ortho-halogeno-substituents and show that the rules required to describe the effect of ortho-substituents in these reactions are different from those required for electrophilic substitutions; the difference is ascribed to the particular effectiveness of the direct field effect operating between the ortho- and the side-chain α-positions, and any attempt to determine σ+ values from these reactions for application to electrophilic substitutions is invalid. The abnormally high activation by the ortho-benzyl substituent is attributed to interaction between the substituent π-electrons and the incipient carbonium ion.Values of σ0+ have been evaluated as –0·27(4-PhCH2), –0·53(4-PhO), and +0·145(3-PhS), but for the sulphur-containing para-substituents, correlation with electrophilic substitution data is poor, indicating substantial dπ–pπ conjugation in the gas-phase reaction, and this may arise from the absence of solvent.Lithiation of diphenyl sulphide and coupling of the intermediate with acetaldehyde produces ca. 40% of meta-product in contrast to other coupling reactions. Caution is therefore recommended in using lithiation reactions for the preparation of specific isomers.

Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part IV. The deactivating effect of the pentafluorophenyl group

The rates of pyrolysis of 1-phenylethyl acetate and 1-(pentafluorophenyl)ethyl acetate have been measured between 330·6 and 409·6 °C. At 625 K the latter undergoes elimination 4·60 times slower than the former in contrast to the value of 7·75 predicted from additivity of the effects of the individual fluorine atoms. This confirms the exalted reactivity of pentafluorobenzene obtained in electrophilic aromatic demetallations (carried out in solution) showing this effect to be independent of solvent influences and also not to arise solely from steric acceleration.

Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part V. Abnormal deactivation by some ortho-substituents: the direct field effect

The rates of pyrolysis of 1-arylethyl acetates containing the 2-CF3, 3-CF3, 4-CF3, 2-NO2, 2-F, 2-Cl, 2-Br, and 2-l substituents have been measured at temperatures between 330·6 and 409·6 °C. A value of +0·565 is obtained for σ+3–CF3 which gives a much better correlation of the data for all electrophilic aromatic substitutions and related reactions, than does the value of 0·52 derived from the solvolysis of t-cumenyl chlorides; use of the latter value should be discontinued. The o-trifluoromethyl substituent is no more deactivating than the meta-substituent and this is attributed to direct stabilisation through space of the electron-deficient reaction centre by the fluorine lone pair. The effect of the o-nitro- and o-halogen-substituents are consistent with the linear correlation of kinetic data for 1-arylethyl acetate and 1-arylethyl methyl carbonate pyrolysis (for which further evidence is given) but the recent use of such correlations to derive σ+(ortho) values is shown to be invalid. A survey of reactions with electron-deficient transition states for which the ortho-halogen substituent effects have been measured shows that where reaction occurs at the α-side chain atom, and where the solvent is either absent or of low polarity, these substituents deactivate more than from the meta-positions; this is rationalised in terms of the direct field effect. The greater deactivation by the o-nitro-substituent relative to the para, parallels that in protodesilylation but not nitration and chlorination which is however not inconsistent with recent theories of substituent effects.

Electrophilic aromatic substitution. Part V. Partial rate factors for protodesilylation of biphenylene: a novel polycyclic reactivity

Both trimethylsilyl derivatives of biphenylene have been prepared and their rates of protodesilylation by aqueous perchloric acid and methanol (2:5, v/v) at 50 °C measured spectrophotometrically giving the partial rate factors 27·8 (2-position) and 0·52 (1-position). The 2-position is close in reactivity to that of the 2-position of fluorene, as found previously in protodetritiation and pyrolysis of 1-arylethyl acetates. The deactivation at the 1-position contrasts with its considerable activation in detritiation, and this therefore constitutes the first example of a polycyclic aromatic position being deactivated or activated according to the demands for resonance stabilisation of the transition states.The ratio of 2 : 1 substitution (53) is considerably greater than for the related benzocyclobutene (9·6) in accord with the prediction of theories which attribute this reactivity difference to strain in the four-membered ring. The similar reactivities of the 2-positions of fluorene and biphenylene is predicted by our theory but not by that of Streitwieser et al.

Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part II. The 2- and 3-positions of furan and thiophen

Rates of pyrolysis of 1-arylethyl acetates (where aryl = 3-furyl, 2-furyl, 2-thienyl, 3-thienyl, and phenyl) have been measured over a range of more than 50° for each compound between 311·3 and 379·1°. The relative rates of pyrolysis of the heterocyclic esters are found at 600°K to be: 2-furyl, 3·84; 2-thienyl, 3·31; 3-furyl, 1·88; 3-thienyl, 1·785, which coupled with the previously determined ρ-factor for the reaction leads to the corresponding σ+-values of –0·890, –0·795, –0·415, and –0·38. This is the first quantitative determination of the electrophilic reactivity of all four positions in one reaction and in the absence also of solvent, and shows the 3-position of furan to be slightly more reactive than that in thiophen, but the difference in reactivities between the 2- and 3-positions is greater in furan as indicated from qualitative work with reactions in solution. Comparison of the results with those obtained in electrophilic substitutions shows excellent agreement except in those reactions for which solvent effects have previously been postulated as important. Correlation of reactivity at the 3-position of thiophen is less good than that at the 2-position and indicates that the former is more susceptible to resonance and/or solvent effects.Molecular orbital calculations show that for furan it is possible to correlate the σ+-values with those for pyridine (by means of either π-electron densities or localisation energies) by using reasonable parameters for the coulomb and resonance integrals, but for thiophen this approach fails completely indicating that the d-orbitals of sulphur play a significant role in determining the reactivity.

953. The electrophilic reactivity of pyridine, deduced from the pyrolysis of 1-arylethyl acetates

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