Additions onto Olefins Meerwein Arylation

The cupric halide-catalyzed reaction of an aryl diazonium salt with an olefin-known as the Meerwein arylation-is a powerful and well-developed method for the arylation of olefins (136, 137). Good results have been obtained for olefins bearing electron-withdrawing groups such as a carbonyl, a cyano, an aryl or an alkenyl group. Depending on the nature of the olefin and the diazonium salt, the reaction can lead either to the oxidation product 88 or to the Cl-atom transfer product 89 (Equation 13.10) [138]. 

it is widely accepted that the Cu(I)-salt which serves as the catalytically active species is formed by the reduction of a Cu(II)-salt, either by the solvent or 

Nonconjugated olefins, acetylenes, styrenes, a,P-unsaturated aldehydes, a,P-unsaturated ketones, a,P-unsaturated acids and quinones are all arylated under Meerwein conditions. Conjugated dienes and ene-ynes can be also used as substrates. 

The synthesis of substituted quinolones [141], benzothiophenes [142] and ben-zylthiazoUdine-2,4-diones [143] has also been achieved using the Meerwein reaction. Ti(III)chloride has been shown to be suitable for the generation of aryl radicals from diazonium salts [144]. Interestingly, a metal-free version of the Meerwein arylation has recently been applied to the synthesis of a-aryhnethyl 

The carbodiazenylation of olefins is related to the Meerwein arylation. In this case, the aryl radical addition intermediates are not halogenated (as in the original Meerwein protocol) but are trapped with a diazonium salt so as to provide the corresponding radical cation. Reduction finally leads to aryl diazenes. Fe(ll)-salts, Ti(III)chloride and Cu(l)-salts or mixtures of these reducing reagents have been used to conduct carbodiazenylations, some reported examples of which are presented in Equation 13.12 [146]. 

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