Thiocyanates aromatic nucleophilic substitution

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,... 

It has also been argued10,40 that the second mechanism (rapid, reversible interconversion of II and IV) cannot be general. The basis for this contention is the fact that electrophilic catalysis is rare in nucleophilic aromatic substitution of non-heterocyclic substrates, an exception being the 2000-fold acceleration by thorium ion of the rate of reaction of 2,4-dinitrofluorobenzene with thiocyanate... 

The exposure of aromatic ethers to BTIB in (CF3)2CHOH or CF3CH2OH (TFE) leads to arene cation-radical intermediates and permits the introduction of nucleophiles (N, AcO-, NCS-, ArS-) into the alkoxyarene nucleus (94JA3684, 95JOC7144). For example, the treatment of / -substituted anisoles with BTIB in the presence of trimethylsilyl azide or -isothiocyanate affords the 2-anisyl azides 98 or 2-anisyl thiocyanates 99, respectively (Scheme 30). 

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