The rates of solvolysis of twenty-two m- and p-substituted α-methylbenzyl chlorides were determined in 80% aqueous acetone. The relative rates at 45 °C are not correlated linearly with σ0 or σ. The use of Brown’s σ+ improves the fit but the resulting correlation is still concave. An excellent correlation, logk⁄k0=−4.950(σ0+1.147Δ\barσR+), was obtained by applying the LArSR equation. The r value 1.15 suggests that the π-electronic contribution relative to the inductive contribution of −R  para substituents at the transition states differs with reaction, and is more important in the present α-methylbenzyl than Brown’s t-cumyl solvolysis (r=1.00 by definition). A set of Δ\barσ+ and σ+ corresponding to the substituent constants for unit r were calculated. The calculated σ+ values are in precise agreement with Brown’s primary σ+ values. The generality of the LArSR equation and some new substituent constants are illustrated.

The intramolecular interaction between a sulphur atom and a methoxycarbonyl group was found to affect their chemical reactivity. The rates of C-S bond cleavage in methyl 9,9-bis( ethy1thio)fluorene-xcarboxylate, where x is 1, 2, or 3, with sodium ethanethiolate show that the rate is minimal, though comparable with the others, when x = 1, because of steric effects. In contrast, methyl 9-(ethy1thio)fluorene-x-carboxylates, where x is 1 , 2, or 3, exhibit the maximum rate when x = 1. The rates of ester exchange in the presence of toluene-p-sulphonic acid indicate that the reaction is accelerated by the presence of two ethylthio groups at the 9-position of methyl fluorene-1-carboxylate but no significant effect is detected if there is only one ethylthio group. The hydrolysis of the thioketal group in 9,9-bis(ethylthio)fluorene is also accelerated by the presence of a 1 -methoxycarbonyl group. The implications of these results are discussed with the use of a model compound.Carbonyl compounds carrying an alkyl(or ary1)thio group at the a-position can be reduced by soft bases, including thiolate anions2 During the course of a study of the enhanced reactivity of these compounds towards soft bases, we encountered a C-S bond cleavage in the reaction of 9,9-bis(ethylthio)fluorene (1) with ethanethiolate in methanol to give 9-(ethy1thio)fluorene (5) and diethyl disulphide. We have examined various sulphides which should give stable anions on cleavage of the C-S bond. As a result, it has become apparent that: (i) if there is no assistance of a carbonyl group, cleavage of the C-S bond requires a leaving group whose conjugate acid has a pK, of less than 20; and (ii) the C-S bonds in a-alkyl(or ary1)thio carbonyl compounds react easily, irrespective of their rather high ~K , s . ~The cause of this facile C-S bond cleavage in these compounds is worth investigation.If a sulphur atom and a carbonyl group are placed in proximity to each other, there should be a charge-transfer-type interaction, as has been found in thiacyclo-octan-5-0ne.~ Similar interactions have been studied by u.v.,' i.r.,6 n.m.~-.,~*' and photoelectron spectroscopy.' The importance of 1,3through-space interactions was pointed out by lnamoto and Masuda to explain "C n.m.r. chemical shifts.' We have looked for model compounds which might exhibit enhanced rates of C-S bond scission as a result of a through-space interaction between a sulphur atom and a carbonyl group. This paper reports one such example, methyl 9-(ethylthio) fluorene-1carboxylate. (It was, however, not possible to completely erase the inductive and mesomeric effects of the carbonyl group.) Enhanced rates of ester exchange and hydrolysis of the thioketal group in methyl 9,9-bis(ethylthio) fluorene-1carboxylate were also observed. These reactions provide another intriguing example of the chemical consequences of intramolecular interactions.

The PIES and UPS spectra of xanthone (1a), thioxanthone (1b), and selenoxanthone (1c) were measured, together with those of the corresponding xanthene (2a), thioxanthene (2b), and selenoxanthene (2c). Ab initio MO calculations were performed with the 6-311+G(d,p) basis sets at the DFT (B3LYP) level on 1a − c and 2a − c. The results of the measurements and of the MO calculations explained well the striking difference in the reactivity of the compounds, especially for 1c versus 2c. The ionization potentials (IP) of π6(n) (HOMO), n(p z ) (π2(n) for 1a and 2a), and n(p y ) were in the order IP(1a) > IP(1b) > IP(1c) and IP(2a) > IP(2b) > IP(2c). The IP of the xanthones were larger than those of the corresponding xanthenes. The IP of n(p z ) of 1c was demonstrated to be larger than those of 2b and PhSPh (3b), which showed that 1c was less reactive than 2b and 3b toward electrophiles. These results accounted for the reactivity of 1c to give a molecular complex (MC) with bromine similar to the case of the MC structure of the sulfide dibromides. The relative intensity of PIES of HOMO in 1c was smaller than that in 2c. The electron density outside the molecular surface of the orbital of 1c must be smaller than that of 2c, due to the strong electron-withdrawing carbonyl group in 1c, which was supported by the calculated natural populations at the chalcogen atoms of the compounds.

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