Thiols from alcohols, 28, Table

Double nucleophilic substitution can similarly be achieved by alkoxides of sufides generated in situ from alcohols of thiols in the presence of K2C03 [99] Eq. (29) and Table 9. 

Wallace and coworkers had reached similar conclusions by studying the oxidation rates of several thiols. They also observed that the solvent has a quite large effect, which, in a general way, may be explained on the same basis. As shown in Table 8, the rate increases quite steadily on passing from alcoholic to non-protic and to dipolar aprotic solvents. 

Starters. Nearly any compound having an active hydrogen can be used as starter (initiator) for the polymerization of PO. The common types are alcohols, amines, and thiols. Thus in Figure 2 ROH could be RNH2 or RSH. The fiinctionahty is derived from the starter, thus glycerol results in a triol. Some common starters are shown in Table 4. The term starter is preferred over the commonly used term initiator because the latter has a slightly different connotation in polymer chemistry. Table 5 Hsts some homopolymer and copolymer products from various starters. 

A thiol contains an —SH group covalently bonded to carbon. Sulfur is just below oxygen in the periodic table, so a thiol is somewhat similar to an alcohol. Still, the chemical and physical properties of thiols differ significantly from those of alcohols. For example, whereas alcohols have inoffensive odors, thiols smell bad. The stench of skunk scent is due to thiols, including 3-methylbutanethiol. Thiols are important in proteins because of their abilities to form S—S linkages, which we describe in Section 13-1. 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

Isothioureas can be prepared on insoluble supports by S-alkylation or S-arylation of thioureas (Entry 7, Table 14.6). Further methods for the preparation of isothioureas on insoluble supports include the N-alkylation of polystyrene-bound, A/,/V -di(alkoxy-carbonyl)isothioureas with aliphatic alcohols by Mitsunobu reaction (Entry 7, Table 14.6) and the addition of thiols to resin-bound carbodiimides [7]. Resin-bound dithio-carbamates, which can easily be prepared from Merrifield resin, carbon disulfide, and amines [76], react with phosgene to yield chlorothioformamidines, which can be converted into isothioureas by treatment with amines (Entry 8, Table 14.6). The conversion of support-bound a-amino acids into thioureas can be accompanied by the release of thiohydantoins into solution (see Section 15.9). The rate of this cyclization depends, however, on the type of linker used and on the nucleophilicity of the intermediate thiourea. 

Organic chemicals that are susceptible to oxidation and are of concern from the perspective of contamination and environmental degradation include aliphatic and aromatic hydrocarbons, alcohols, aldehydes, and ketones phenols, polyphenols, and hydroquinones sulfides (thiols) and sulfoxides nitriles, amines, and diamines nitrogen and sulfur heterocyclic compounds mono- and di-halogenated aliphatics linear alkybenzene-sulfonate and nonylphenol polyethoxylate surfactants and thiophosphate esters. Table... 

The esters derived from Merrifield resin are only cleaved by strong acids such as HF, TFMSA/TFA, TMSOTf/TFA, HBr/TFA and TMSBr/TFA (Table 14.4). Weaker acids like TFA may also be used but the reaction is sluggish and requires catalysis with reagents like soft nucleophiles such as thioanisole. Similar methods should also work for alcohols and phenols, whereas thiols and amines can not generally be cleaved from this resin by acidolysis. 

Esters, ethers and hydroxamates derived from these linkers can be cleaved with mild acids such as TFA or HC1 in dioxane (Table 14.4). If TFA is used for release of alcohols, trifluoroacetates can be formed [2], but these can be easily hydrolyzed later with aqueous base. Simple amines cannot be cleaved from this type of linker by acidolysis. Thiols may be cleaved with HF. 

Photoinitiator systems from hydrogen abstraction or electron transfer usually contain two components a photoinitiator (typically an aromatic ketone) and a co-initiator with a weak covalent bond. Some examples for this system include benzophenones (1), thioxanthones (2), benzyls (3), camphor-quinones (CQs) (4), and ketocoumarins (5), which can be used in the presence of H donors (alcohols, THE, and thiols) or electron donors (such as amines) (Table 2). One of the drawbacks of photoinitiation requiring electron transfer is back electron transfer that limits their practical utility. For this reason, the acidity of the C-H bond of the co-initiator is of great importance. A few initiators are also able to undergo cationic and radical photoinitiation such as iodium and sodium salts and arene complexes. ... 

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