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1. $ \ce{ArN2+Cl-}$ gives addition product with $\alpha,\beta$ unsaturated acids, with decarboxylation being integral part of reaction. For example, $$\ce{ArN2+Cl- + Ph-CH=CH-COOH -> Ph-CH=CH-Ar +CO2 + HCl}$$

2. With $\ce{SnCl2/HCl, ArN2+Cl-}$ given phenyl hydrazine $\ce{ArNH-NH2.HCl}$

I can not find mechanism of any of these reactions, first reaction seems similar to Meerwein reaction (without $\ce{Cu^2+}$), but certainly can't follow similar mechanism. Second reaction is completely different from either. What are the reaction mechanisms of each?

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As to your first reaction that does not utilize copper, a reference to the source would be useful. Nonetheless, the Meerwein Arylation Reaction has been reviewed in Organic Reactions. The reaction is conducted in the presence of catalytic cuprous ion. Meerwein proposed an aryl cation but a radical mechanism appears plausible. Reduction of the aryldiazonium salt 1 produces aryl radical 2 and cupric ion. Addition of the aryl radical to cinnamic acid 3 to the double bond to give the more stable of two possible radicals, ---namely--- the benzylic radical 4. This radical is oxidized by cupric ion to produce benzylic cation 5 and the catalyst cuprous ion. Loss of CO2 forms the stilbene 6.

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