Chlorine Bromine Precursor Route

1 Preparation of Poly 2-[2 -ethylhexyloxy]-5-[4 -methylsulfonyl phenyl]-1,4-phenylenvinylene (70) (Chart 2.148) 

From Boardman, F.H., Grice, A.W., Riither, M.G., Sheldon, T.J., Bradley, D.D.C., and Burn, P.L., Macromolecules, 32 111-117, 1999. Copyright 1999, American Chemical Society, Washington, D.C. With permission. 

Thin films of 619 were heated at 160°C under a dynamic vacuum for 14h to yield 70. Amax (film on KBr dis /cm-1 955 (C=CH), 1154 (S02), 1315 (S02) Amax (thin film)/nm 285sh, 437. 

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The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

A convenient synthesis of pyridazine-3,4-dicarboxylic acid based on [4 + 2] cycloaddition of an electron deficient diazadiene has been developed (Scheme 102) and carried out on a molar scale. The azadiene is generated in situ from the precursor (124) (prepared from diethyl oxaloacetate and methyl carbazate followed by chlorination with sulfuryl chloride) and trapped with ethyl vinyl ether. The regiospecific cycloaddition gives the tetrahydropyridazine (125) as a mixture of cis and trans isomers which, with bromine in acetic acid, are transformed into diethyl pyridazine-3,4-dicar-boxylate. Saponification gives the diacid in an overall yield of 52% <90JHC579>. In a similar approach from the readily available ethyl 2-chloro-3-oxobutanoate, an efficient route to ethyl 3-methylpyridazine-4-carboxylate has been developed which gives the product in 56% overall yield. This route is claimed to be a practical alternative to the radical ethoxycarbonylation of pyridazines <91JHC1043>. 

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