Benzoin condensation Subject

The heterocyclic equivalent of the benzoin condensation offers a route to the important herbicide paraquat (Scheme 128) (73JOC3993) (see also Section 2.05.7). N- Alkylnicotinic acid and its derivatives readily undergo addition of cyanide and many other nucleophiles (see Section 2.05.4.8). In the example shown in Scheme 129, 1,2-addition is subject to kinetic, and 1,4-addition to thermodynamic, control. 

AG(E)508> of -l,2-bis(tri-/i-butylstannyl)ethylene with a 2-iodo-3-substituted thiophene affords monomer (147) <93SM1188>. When 2,5-diiodothiophene derivatives are used, the polymer (148) is obtained directly <93MI 212-12, 93SM1255>. Thiophene-2,5-dicarbaldehyde can also be subjected to benzoin condensation conditions to afford (149) after methylation with dimethylsulfate <93MI 212-13>. 

Phase transfer catalysis found one of its earliest applications in cyanide displacement reactions. Starks studied this reaction in some detail and it was due in large part to his work that the concepts of phase transfer were so clearly understood at an early stage [1, 2]. There was, of course, other early interest in cyanide ion and these efforts should not be overlooked. Brandstrom had solubilized cyanide ion by his ion pair extraction technique [3] Solodar had facilitated the benzoin condensation by utilizing tetrabutylammonium cyanide [4] and Durst [5] and Liotta [6] both used crown ethers to phase transfer cyanide ion. A good deal of other work has now been carried out on phase transferred cyanide ion and is the subject of this chapter. 

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