Phase thiocyanates from halides

In the unconventional synthesis of thioethers (Scheme 4.11), cyanide ion is displaced from thiocyanates by carbanions [52, 53], which have been generated under phase-transfer catalytic conditions (cf. 4.1.12). Thiocyanates are readily obtained by a standard catalysed nucleophilic substitution reaction [4, 54-58] (see Table 4.19). Aryl thiocyanates are obtained from activated aryl halides [4, 57] (see Chapter 2). 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

Mechanistically, typical PTC reactions take place via two main mechanisms in liquid-liquid systems. The extraction mechanism (Starks, 1971) is the most commonly accepted one for simple nucleophilic substitution under neutral conditions for reactions with a range of anions (e.g halides, cyanide, thiocyanate, sulfite, nitrite, acetate, carbonate, etc.). According to this mechanism (Figure 16.2a), the PT catalyst, denoted by Q+X , is a vehicle to transfer the reactive anion Y of the metal salt from the aqueous phase into the organic phase, where it reacts with the organic substrate, RX, to give the desired product RY and regenerating Q+X , which can continue the PTC cycle. Typically, the active form of... 

The reactions of halide and azide ions (see above), cyanide ion (see Chap. 7) and thiocyanate ions (see Sect. 13.7) have all been discussed. In the general context of halides and pseudohalides, it should be noted that nitrite ion has been successfully phase-transferred [31, 32]. The yields for the formation of nitroalkanes in either crown [31 ] or quaternary ion catalyzed [32] processes are fair to good for primary alkanes and poor for secondary systems. Bromocyclohexane, for example, afforded only traces of nitrocyclohexane when treated with nitrite ion, the by-products resulting presumably from oxygen attack (nitrite ester formation) or elimination. The conversion of n-octy bromide into the corresponding nitro-compound is formulated in equation 9.14 and several examples of the transformation are recorded in Table 9.6. 

Amines as Catalysts. Some reports have appeared on the use of amines as catalysts in PTC nucleophilic substitution methods. For example, the preparation of alkyl thiocyanates or nitriles from alkyl bromides in two-phase systems may be assisted by a variety of primary, secondary, or tertiary amines as alternatives to quaternary ions. Efficient catalysis seems to require a sterically unhindered amino group with relatively high basicity (J.e. t-alkyl and aromatic amines are not fully efficient), and a total number of carbon atoms in the amine of greater than six to achieve good phase distribution of the catalysts. A similar study on the alkylation of benzyl methyl ketone reached the same conclusions, and from various observations e.g. that the reaction displayed an induction period at low catalyst concentration) it was postulated that initial alkylation of the amine by the alkylating agent (usually a halide) was essential to provide quaternary ions as the actual catalyst,... 

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