Reductive dealkylation of quaternary ammonium salts

Sodium hydrogen telluride efficiently dealkylates quaternary ammonium salts to the corresponding tertiary amines in high yields.  

18-crown-6-ether, reflux Ar = Ph R = Me, PhCHg, p-MeC6H4, p-MeOCsH4, P-CIC6H4, p-BrCgH4CH2 

Method B. To a solution of NaHTe (5 mmol), prepared by heating a mixture of powdered Te (0.77 g, 5 mmol) and NaBH4 (0.56 g, 12 mmol) in EtOH (15 mL), is added a solution of the same ketosulphone (2 g, 5 mmol) and catalytic amounts of 18-crown-6 (0.02 g) in THE (20 mL) at room temperature under N2. The final work-up is similar to that of method A to give the ketone (1.05 g (90.1%) m.p. 66-67°C). 

The above-described desulphonylation, combined with the previously described selective reduction of a, -unsaturated carbonyl compounds, can be applied to a-alkylidene 

Otherwise, the same products can be obtained by a Knovenagel-type reaction between )3-cyanosulphones and aromatic aldehydes in the presence of sodium telluride (behaving as a base). This result is consistent with the above-formulated desulphonylation of an intermediate a-alkylidene )3-cyanosulphone at the expense of the sodium hydrogen telluride formed during the reaction. 

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