Arenesulphonates

The yields of arenesulphonic acids (-80%) via the reaction of activated haloarenes with potassium sulphite under phase-transfer catalytic conditions [62, 63] are no better than conventional non-catalytic methods, although reaction conditions are less severe. There is evidence that indicates the initial attack by the sulphite anion is at C-5. Surprisingly, tri-n-butylamine is a better catalyst, producing higher yields (>90%). 

If the family of nucleophiles is greatly strengthened in comparison to the family of arenesulphonates used by Lewis, then the p-value is predicted to decrease since the stronger nucleophile is expected to stabilize [23] in comparison to [22]. Thus (98) is predicted to result in a p-value less than + 0.6 (obtained for the weak nucleophile ArSOj). 

Most solvolyses of /3-arylalkyl arenesulphonates involve a mechanistic complication arising from a concurrent solvent-assisted (ks) process, as shown in Scheme 6 (Schadt el al, 1978 Fujio et a/., 1987b,c, 1992a Goto et al., 1989). The measured rate constant A , of the solvolysis should be the sum of the rate constants for the aryl-assisted and the -unassisted As processes. In practice, substituent effects on these competitive processes can be directly analysed by using a non-linear least-squares program based on the assumption of independent Y-T correlations for both A and As processes, i.e. equation (9). The treatment has been simplified by taking rs = 0 for the As process of this reaction. 

A different type of a-sulphonyl radical is obtained when arenesulphonic acids or their sulphonyl fluorides are reacted with Pb02 in HS03F at ca 200 K. Under these conditions, electron transfer takes place to generate the radical cations of the parent compound (equation 17)40. 

Table 9 contains the data for radical cations of arenesulphonic acids and arenesulphonyl fluorides40,41. 

Alkene- and arenesulphonic acids have been studied in strong or super acid media by NMR spectroscopy. Olah and coworkers64 in 1970 examined sulphonic acids in fluorosulphuric acid-antimony pentafluoride-sulphuryl chloride fluoride solution at low temperature ( — 60 °C). In S02 solution methanesulphonic acid has a chemical shift of <5 3.1 ppm for the methyl group. In the superacid medium there are two singlets at S 4.15 and 4.07, with ratio 60 40, for the methyl protons. It was suggested that this indicated the presence of two isomers, possibly 27 and 28, due to restricted rotation about the SO bond. 

TABLE 1. Isomer distribution for the rearrangement of some arenesulphonic acids... 

SCHEME 1. Mechanism for the isomerization of substituted arenesulphonic acids... 

Arenesulphonate salts undergo rearrangement on heating, but there has been no subsequent work reported since this area was last reviewed1. 

In contrast to these observations, Snell studied the photo-Fries rearrangement of sulphonate esters of 6-substituted 4-hydroxypyrimidines (equation 9) and found efficient conversion for a variety of alkane- and arenesulphonates [e.g. R1 = Me, Et, Bu, (CH2)3C1, Ph, 4-Tol, 4-ClC6H4, 4-BrC6H4, 4-An, 2,5-Me2C6H3, 2,4,6-Me3C6H2]47. Yields vary between 20-60%, and the reaction does not proceed for R3 = NH2 or NHAc. The reaction has synthetic utility, since the product sulphones are inaccesible via the Friedel-Crafts procedure. 

However, in non-aqueous media the cathodic cleavage of arenesulphonates is far from... 

The electrochemical behaviour of other substituted arenesulphonic acids can be related to the cathodic or the anodic properties of substituents prior to the cleavage of C—S bonds as mentioned above. For example, an aromatic nitro group will be reduced55 before the S03H group ... 

Substituent effects on the reactions of arenesulphonates with trimethyloxonium ion (equation 13) in acetonitrile70 (and with methyl triflate71) show p = —1.1, e.g. p-methoxy-benzenesulphonate anion reacts about 15-fold faster than p-nitrobenzenesulphonate... 

Patents are still appearing on further substitution reactions of naphthalene disulphonic acids by both kinetically and thermodynamically controlled processes, which have been of great commercial importance for many years. Isomer distributions for the sulphonation of other arenesulphonic acids and some alkyl derivatives have been compiled142. 

The arenesulphonic acid salt of emetine has been prepared for qualitative analysis. 

Nonclassical ions with a norbornane skeleton, as has l en already noted, have been studied extensively, though this system permits a far greater number of ambiguous interpretations than the bicyclo[2,2,2]octane system Studies on the 2-norbomyl ion were started by the solvolysis of 2-norbomyl arenesulphonates as these works have served as models for all subsequent... 

Table 4. Calculation of the Acetolysis Rates of Arenesulphonates at 25 °C and Estimation of the Magnitude of Anchimeric Assistance...

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