Thioesters and thiocarbonates

The kinetics and mechanisms of reactions of thiol, thiono, and dithio analogues of carboxylic esters with nucleophiles have been reviewed (115 references). Studies of the aminolysis (PhCH2NH2) of thiophenyl-2-furoates (117 X = O) and thiophenyl- 

2-thiophenates (117 X = S) in MeCN at 50 °C were consistent with rate-limiting expulsion of thiophenate from a tetrahedral intermediate.  

The pyridinolysis rates of carbonyi compounds with good leaving groups usually exhibit a biphasic dependence on pyridine basicity. This has now been shown to be the case for the pyridinolysis of thiophenyl 4-nitrobenzoates in MeCN. The reactions between X-pyridines (118 X = 4-MeO, 4-Me, 3-Me, 3-CONH2, 3-MeCO, 

4-Br) were investigated in MeCN at 55 °C. The Brpnsted plots obtained for the pyridinolysis of thiophenyl benzoates are curved, with the centre of curvature at pK, 4.2(pX°). The Brpnsted plots for these nucleophilic reactions show a change in slope from a large (y3x 0.64-0.72) to a small (y3x 0.19-0.23) value, which can 

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Sulfur as Heteroatom. Thiols and sulfides are protonated on sulfur in superacid media and give mono- and dialkylsulfonium ions, respectively.136 Thio-carboxylic acids, 5-alkyl esters, thioesters, dithioesters, and thiocarbonates in similar media also form stable protonated ions541,647 such as cations 348-353. 

Aminolysis of a series of aryl 2,4-dinitrophenyl carbonates by a series of quinuclidines gave linear Br0nsted-type plots, the magnitudes of their slopes confirming their mechanisms as concerted.40 A comparison41 of the aminolysis, by primary amines, of 4-nitrophenyl phenyl carbonate (31 X = O) with its thiono analogue (31 X = S) is discussed in the section Thioacids, Thioesters, Thiolactones, and Thiocarbonates below. 

The newest LFRP system involves reversible addition fragmentation chain transfer (RAFT) (259). PS, PMA, and PMMA having a polydispersity of < 1.3 and block copol5oners containing these monomers have been produced utilizing this novel chemistry. The initiators are either thioesters or thiocarbonates (260). The RAFT process is shown in Figure 48. 

A new mild reaction for the deoxygenation of secondary alcohols has been applied successfully to sugars the reaction, which is radical in character, involves the reduction of a thiobenzoate with tributylstannane (see Scheme 25). Other references to the use of thioesters and the photochemical transformation of dithiobis(thioformates) into thiols are reported in Chapter II, the reaction of cyclic thiocarbonates with methyl iodide to give tu-iodo-iS-methyl thiocarbonates... 

The utility of thiol esters and carbonates as protecting groups is limited by their vulnerability to hydrolysis, The poor overlap between the non-bonded electrons on the sulfur atom (3p) and the n-system of the carbonyl (2p) precludes or diminishes resonance stabilisation of the type enjoyed by normal esters thereby raising their ground state energy, The carbonyl group of the thioester is more electrophilic than a normal ester and hence more reactive. Thiocarbonate derivatives are marginally more stable. The 5-benzoyl derivative of cysteine is 95% hydrolysed in 30 minutes with 2 M ammonia whereas the 5-benzyloxycarbonyl derivative is only 20% hydrolysed in 30 minutes under the same conditions.54... 

Thiones ketones. Xanthates, thioesters, thiocarbonates, thioamides, and thiones are converted into the parent carbonyl compounds when treated with this anhydride at 20° in THF. ScOa is not a satisfactory alternative for this reaction. Water and oxygen are not involved. 

Tang, M.C., He, H.Y., Zhang, F., and Tang, G.L (2013) Baeyer-Villiger oxidation of acyl carrier protein-tethered thioester to acyl carrier protein-linked thiocarbonate catalyzed by a monoo ge-nase domain in FR901464 biosynthesis. ACS Catal, 3, 444-447. 

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