Sulphonates solvolysis

The C-nor-D-homo rearrangement has been effected by methods other than i2j sulphonate solvolysis, including the deamination of a i2jS-amine (7) [iii] and the decomposition of i2-toluene- sulphonylhydrazones (8) under basic conditions [io8,iog,ii2]. It is both convenient and instructive to consider these reactions together, especially since the com-... 

Leffek, Llewellyn and Robertson (1960a, b) made careful conductometric determinations of deuterium kinetic isotope effects on the solvolysis rates (in water) of some ethyl, isopropyl and n-propyl sulphonates and halides. In the case of the n-propyl compoimds,... 

A stereoselective total synthesis of the antifungal mould metabolite ( )-LL-Z1271a (165) from the readily available Wieland-Miescher diketone, via the keto-lactone (164), has been described. A synthesis of grindelic acid (167) from the unsaturated 7-toluene-p-sulphonate (166) utilized an intramolecular solvolysis of the toluene-p-sulphonate to construct the 9—13 ether bridge. 

Shiner and coworkers also studied optically active acyclic secondary sulphonate esters and found rate accelerations of about 100 in solvolysis in TFA at 25 °C compared with the alkyl reference (Table 8)93-95. From the product distribution and the stereochemical outcome of the solvolysis Shiner and coworkers proposed the reaction course outlined in equation 37 for the brosylate 23994. The transition state for the ionization adopts the W conformation and, to a smaller extent, the ewrfo-Sickle conformation and results in the formation of /-silicon carbocation which is stabilized by a percaudal interaction. The incoming nucleophile attacks the bridged ion from the front side94. 

Probably both reactant stabilization and the already evaluated relative instability of the cationic transition state contribute to the slowness of the solvolysis of vinyl components, but other factors are certainly involved. The most obvious experimental problem is whether the compounds compared react by a unimolecular mechanism or nucleophilic attack by the solvent is involved to a certain extent. In the case of vinylic systems, for instance, nucleophilic solvation from the rear is in general much more hindered than in the case of saturated compounds and the transition state is likely to be stabilized only by electrophilic solvation of the leaving group (Rappoport and Atidia, 1970). The low m values observed in the case of vinyl halides or sulphon-ates may be taken as a strong indication of poor solvation of the transition state in solvolytic reactions of vinyl derivatives. These and other complications, such as differences in hyperconjugation, differences in electronegativity of the -—C= and —- bonds (Jones and Maness,... 

Let us now consider Scheppele s discussion of the solvolysis of 2-d-exo- and -endo-bicyclo[2.2.1] hept-2-yl p-nitrobenzoates and p-toluenesulfonates. A maximum a isotope effect of 1 22 is suggested for kH/kD for the limiting solvolysis of simple secondary sulphonates (solvolyses where the rate is not affected by the nucleophilicity of the medium). The endo-esters yield a similar value which might be taken to imply the absence of steric hindrance in their reaction. However, whereas the maximum isotope effect for the simple sulphonates is believed to arise partly from an activated complex in the conversion of the internal ion pair into a solvent-separated ion pair, B - C, the effect in the endo-norbomyl compounds is suggested to arise during the transition state involved in the formation of the intimate ion pair, A - B. The latter is deduced from the fact that polarimetric and titrimetric rate constants are the same in acetic acid, aqueous acetone and ethanol (Winstein and Trifan, 1949 and 1952). If this is the case, the endo-effect of 1-20 should be... 

Solvolysis of 1-arylethenyl sulphonates forming vinyl cations 303 Highly electron-deficient carbocation systems 304 Carbocation formation equilibria 315 Triarylmethyl cations 315 Benzhydryl cations 319 1,1-Diarylethyl carbocations 322... 

SOLVOLYSIS OF 1-ARYLETHENYL SULPHONATES FORMING VINYL CATIONS... 

The solvolysis rate constants of l-aryl-2-methylpropenyl sulphonates, extrapolated to the tosylate solvolysis in 80% aqueous ethanol at 25°C, give an excellent Y-T correlation with r = 1.06, as shown in Fig. 11. The solvolysis of ( )-l-aryl-3,3-dimethylbutenyl bromides also gives an excellent correlation with r = 1.02. 

Fig. 11 The Y-T plot of the solvolysis of phenyl-2-methylpropenyl sulphonates in 80% aqueous EtOH at 25°C r = 1.06 (Matsumoto, 1997). For interpretation of...

The products from solvolysis of 4a-methyl cholesteryl sulphonates are analogous to those in the 4-unsubstituted compounds, the major component being the b S-hydroxy-3,5-cyclo compound. The minor products, as reported by different workers [2g,4y], display curious differences in com-... 

A few related observations deserve comment. The presence of an ii-keto function completely inhibited solvolysis of the I2j0-sulphonates by its opposed inductive effect, but did not prevent the tosylhydrazone decomposition and rearrangement [1087. This may be a reflection of the exceptionally low activation energy for the decomposition of an aliphatic diazo-nium ion into a carbonium ion and molecular nitrogen (p. 316). Rather surprisingly, the Cps) cyanohydrin (15) undergoes similar rearrangement on dehydration with thionyl chloride/ pyridine [108,11 ]. 

One case of apparent defiance of the trans requirement for oxetane formation has recently been reported in the solvolysis of the 3"tosylate (i6) of 3j5,5jS dihydroxycholestan"6"One [ 138]. Under strictly anhydrous conditions the major product was the 3 ff,5j5 oxetane (17), although aqueous solvents gave mainly the 3a,5jS-diol. The mechanism of internal cis substitution at C(3) is not known. Various possibilities include the formation of an internally "solvated ion-pair (18), as a consequence of the extra acidity imposed upon the 5i -hy-droxylic proton by the 6 oxo group this could be followed by oxetane formation with extrusion of a molecule of toluene- -sulphonic acid. 

Solvolysis of (S)-2,6-dimethyloct-5-yl toluene-p-sulphonate gives a tetrahydro-linalool, (i )-2,6-dimethyloctan-6-ol, with about 60% retention of asymmetry. Kirmse and Arold have described several other similar reactions and suggest that a hydrophobic anchimeric interference of alkyl residues persists during the rearrangement, giving chirality to the carbonium ion. ... 

These conclusions contradict some widely held views. Sulphonates undergo solvolysis more rapidly than the corresponding chlorides (see Tables 4r-6) amd it has often been assumed that the better leaving group (sulphonate) will induce pure solvolysis at an earlier stage in a series of increasing electron accession to the reaction centre. Thus mechanism S l has been thought to operate in the hydrolysis of benzyl... 

The solvolysis of isopropyl p-toluenesulphonate in 50% aqueous acetone shows a value of ACp JAS which is very close to that observed for established reactions and the same mechanism can therefore be assumed. However, a different conclusion is required when the solvent is 85% acetone, a poorer ionising medium, since AC I AS is now a little smaller than would have been expected (see Table 10). These observations and their interpretation are fully consistent with solvolysis near the mechanistic border-line. Completely Sj l solvolysis should therefore also occur in water but the validity of this conclusion cannot be definitely established since AC appears to be independent of mechanism in the solvolysis of sulphonates (see Section IVC4). However, for the hydrolysis of isopropyl methanesulphonate and benzenesulphonate in water is about 10 caldeg. greater than for the Sjj2 reactions of the primary compounds, a difference often found between S l and Sif2 solvolysis, but isopropyl -toluenesulphonate is clearly anomalous (see Table 6). 

The conclusion that secondary sulphonates undergo 8 1 solvolysis, except in poor ionising solvents, appears to be contradicted by the... 

The absence of a distinct maximum for AC for the solvolysis of substituted benzyl chlorides in 70% acetone (see Table 8) could be a consequence of the fact that the difference between AC for S l and Sif2 reactions is generally greater for reaction in this solvent than in 50% acetone. However, the similar observation for the solvolysis of the benzyl -toluene-sulphonates (Table 9) cannot be explained in this manner since all the available evidence suggests that a change of mechanism only has a very small effect on AC for these reactions. 

The outcome of solvolysis of a sulphonic ester of a 19-hydroxy-steroid apparently depends upon functional groups in rings a and b. The 19-tosylates (501 3/3-OAc, 5a or SP-3-oxo) react by Wagner-Meerwein migration of C-1, giving the... 

The well-known contraction of ring a by solvolysis of 3-sulphonates in... 

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