Thiol comparison with alcohol

Thiols react with aldehydes and ketones to form thioacetals (mercaptals). The reaction equilibrium is, in comparison with the reaction of alcohols, shifted more in favour of products that are mainly involved as flavour-active components of meat, vegetables and mushrooms (Figure 8.27). The reaction intermediates (semithioacetals) give rise to a number of other compounds. 

In the absence of added nucleophiles, nitrosation occurs virtually irreversibly by an acid-catalysed pathway, presumably by attack by HjNO or NO". The third order rate constant from the rate equation equivalent to (46) has a value of 840 dm moF s- at 31°C (c/. 456 and 6960 dm mol- s for cysteine and thiourea respectively at 25°C) which suggests that for this neutral substrate the reaction rate is somewhat less than that expected for an encounter-controlled process. There is a major difference between the nitrosation of alcohols and that of thiols in that, whilst the former reactions are reversible (with equilibrium constants around 1), the reactions of thiols are virtually irreversible. It is possible to effect denitrosation of thionitrites but only at high acidity and in the presence of a nitrous acid trap to ensure reversibility (Al-Kaabi et al., 1982). Direct comparisons are not possible, but it is likely that nitrosation at sulphur is much more favoured than reaction at oxygen (by comparison of the reactions of N-acetylpenicillamine and t-butyl alcohol). This is in line with the greater nucleophilicity expected of the sulphur atom in the thiol. For the reverse reaction of denitrosation [(52) and (53)], the acid catalysis observed suggests the intermediacy of the protonated forms... 

A hydrogen-bond acidity scale has been constructed by Frange and coworkers and by Sraidi based on log K values for complexation with pyridine N-oxide in cyclohexane (equations 22 and 23). Values for phenols and, for comparison, thiols, chloroform, pyrrole and alcohols are collected in Table 5. There is a fair measure of agreement between log K and log Kf n = 16, r = 0.992) and log (n=9,r = 0.972). 

The same research team used a,fl>dienes derived from oleic acid and erucic acid to be copolymerised further with a derivative of ferulic acid obtained from rapeseed cake (Scheme 6.12). Methyl erucate and methyl oleate were epoxidised and subsequently reacted with allyl alcohol in acidic conditions to yield the corresponding ct,(0-dienes [21, 34]. These monomers were polymerised by ADMET or thiol-ene reactions for the sake of comparison. For thiol-ene reactions, 1,4-butanedithiol was used to give polyhydroxyesters with Mn values of 4-14 kDa and Tg values ranging from -63 to -33 °C. 

For other metals with analogous ligand sets, the situation remains uncertain, with most of the mechanistic inferences coming from theory. Results for Rh have been discussed already. Meijer and co-workers have done a theoretical comparison of (T -arene)Ru and (l,5-COD)Ir systems with amino alcohol and amino thiol chelates. They concluded that the Ru system uses the bifunctional mechanism, but the Ir system proceeds by direct transfer with chelate ring opening to accommodate the alcohol and ketone ligands and no formation of a hydride. 

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