Thiol carbonylation

These oxidants are generally too feeble to attack monofunctional compounds except thiols, carbonyl- and nitro-compounds in their enolic forms, phenols and aromatic amines. However, ferric rWj-o-phenanthroline readily oxidises cyclohexanone. 

Internal thiol-carbonyl interactions were extensively investigated in the field of thiosugars, and were applied to the syntheses of the thiofuranose , and thiopyranose and thioseptanose systems. 

Platinum complexes also exhibit catalytic activity toward the carbonylation of allene with CO and aliphatic thiol can react with allene regioselectively [12]. The PtCl2(PPh3)2-catalyzed carbonylative thiolation of cyclohexylallene with I " Hex-SH) as an aliphatic thiol carbon monoxide by the use of aliphatic thiol ( Hex-CH) proceeds regioselectively at the terminal double bond of the allene. In the case of Pt(PPh3)4-catalyzed reaction, the thiolative carbonylation takes place at both terminal and central double bonds (Scheme 11.9). 

The carbonyl band of thiol carbonyl esters -S-CO- " occurs at lower frequencies than that of normal esters. 

We 11 Start by discussing m more detail a class of compounds already familiar to us alcohols Alcohols were introduced m Chapter 4 and have appeared regularly since then With this chapter we extend our knowledge of alcohols particularly with respect to their relationship to carbonyl containing compounds In the course of studying alco hols we shall also look at some relatives Diols are alcohols m which two hydroxyl groups (—OH) are present thiols are compounds that contain an —SH group Phenols, compounds of the type ArOH share many properties m common with alcohols but are sufficiently different from them to warrant separate discussion m Chapter 24... 

A major difference between alcohols and thiols concerns their oxidation We have seen earlier m this chapter that oxidation of alcohols gives compounds having carbonyl groups Analogous oxidation of thiols to compounds with C=S functions does not occur Only sulfur is oxidized not carbon and compounds containing sulfur m various oxida tion states are possible These include a series of acids classified as sulfemc sulfimc and sulfonic according to the number of oxygens attached to sulfur... 

Analogously, aldehydes react with ammonia [7664-41-7] or primary amines to form Schiff bases. Subsequent reduction produces a new amine. The addition of hydrogen cyanide [74-90-8] sodium bisulfite [7631-90-5] amines, alcohols, or thiols to the carbonyl group usually requires the presence of a catalyst to assist in reaching the desired equilibrium product. 

Conversion of carbon in the coal to gas is very high. With low rank coal, such as lignite and subbituminous coal, conversion may border on 100%, and for highly volatile A coals, it is on the order of 90—95%. Unconverted carbon appears mainly in the overhead material. Sulfur removal is faciUtated in the process because typically 90% of it appears in the gas as hydrogen sulfide, H2S, and 10% as carbonyl sulfide, COS carbon disulfide, CS2, and/or methyl thiol, CH SH, are not usually formed. 

A-/-Butyldimethylsilyl-7V-methyltrifluoroacetamide, CH3CN, 5 min, 97-100% yield.This reagent also silylates thiols, amines, amides, carboxylic acids, and enolizable carbonyl groups. 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

The thiation procedure described here is an example of a general synthetic method for the conversion of carbonyl to thiocarbonyl groups. Similar transformations have been carried out with ketones, carboxamides,esters,thioesters, 1 actones, " thiol actones, - imides, enaminones, and protected peptides. ... 

General acid catalysis is a catalysis by a Br(4nsted acid (other than the lyonium ion) acting by donating a proton. The addition of thiols to the carbonyl group is general acid catalyzed. ... 

The product is a hemithioacetal. In the rate-determining step, the general acid HA donates a proton to the carbonyl oxygen, thus assisting the nucleophilic attack of the thiol on the carbonyl carbon. 

The reaction of hydroxyethyl-TPP with the oxidized form of lipoic acid yields the energy-rich thiol ester of reduced lipoic acid and results in oxidation of the hydroxyl-carbon of the two-carbon substrate unit (c). This is followed by nucleophilic attack by coenzyme A on the carbonyl-carbon (a characteristic feature of CoA chemistry). The result is transfer of the acetyl group from lipoic acid to CoA. The subsequent oxidation of lipoic acid is catalyzed by the FAD-dependent dihydrolipoyl dehydrogenase and NAD is reduced. 

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA. 

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