Stetter reagent

If this is really true and not just a theoretical analogy, it ought to be possible to learn from Nature and design useful d1 reagents based on thiamine. This was done by Stetter using simplified thiamines. The pyrimidine is replaced by a benzene ring and the pyrophosphate is removed. This leaves a simple thiazolium salt called a Stetter reagent. 

Novel compounds for use in liquid crystal displays have been prepared via the Stetter procedure followed by Paal thiophene formation with Lawesson s reagent. Thus... 

Any of the d1 reagents suggested in chapter 25 would do but they preferred a catalytic method using the thiazolium salts 25 devised by Stetter.4 These are also aromatic heterocyclic compounds... 

It is only a small step from the asymmetric benzoin condensation to the asymmetric Stetter reaction, the aliphatic variant of the benzoin condensation. The literatnre refers to the Stetter reaction when at least one of the two reactants is an aliphatic aldehyde. Normally, the reaction is performed as a cross-coupling reaction with two different reactants, one of which is not an aldehyde, bnt an a, 3-unsaturated ketone. Strictly speaking, most thiazole catalysed reactions referred to as Stetter reactions are in fact Michael-Stetter reactions [21,22] (see Fignre 6.4). The reaction received the name because Stetter used a Michael reagent, an acceptor with an activated double bond, as the second component of a cross-coupled Stetter reaction [11]. 

The full structures of thiamine and of Stetter s catalyst appear in the chapter (pp. 1392-7). We use an abbreviated structure for the mechanism of conjugate addition. The essential features of this catalyst are that it is biomimetic and a d reagent that does conjugate additions. The first stages are the formation of the ylid, direct attack on the most reactive carbonyl group, the ketone of the a-keto-acid, and loss of CO2 to give the key intermediate. 

The useful Stetter reaction38 is biomimetic - the idea and the reagent came from the coenzyme thiamine pyrophosphate 187. Removing unnecessary substituents left the core of the thiazolium species 188 where R is usually benzyl (catalyst a) but can be various other primary alkyl groups. 

We have synthesized such a precursor polymer by using the Stetter reaction (scheme 2). Poly-1,4-phenylene-1, 4 -butanedione 9 was formed in the reaction of terephthalic-dicarboxaldehyde 10 and the bis-Mannich base of 1,4-diaeetylbenzene 11 in a yield of 81%. The conversion of 9 into the alternating copolymers of p -phenylene-2,5-pyrrole 12 and p-phenylene-2,5-thiophene 13 is performed with liquid NH3 and Lawesson s reagent, respectively (scheme 3). Upon doping with either Ig or AsFg both polymers 12 and 13 become electrically conducting with specific conductivities up to 0.1 S/cm. 

Polymer (63) was produced in an 81% yield. This new condensation polymerization represents the first example of polymerization using the Stetter reaction. Since a variety of aromatic aldehydes and Mannich bases undergo the Stetter reaction, this procedure is useful for the preparation of new polymers. Ring closure of the polymer (63) by Lawesson s reagent gave the polymer (64), which showed a conductivity of 10 Scm after doping with AsFs. 

Stetter, H., Ramsch, K.-D., and Elfert, K., Compounds with urotropine structure. LIV. 2,2-B/s(chlo-romethyl)acetophenone as a new a,a -fusion reagent, Liebigs Ann. Chem., 1322, 1974. 

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