Electrophilic aromatic substitution alkynylation

Pd-catalyzed cycloisomerization of (Z)-2-en-4-yne-l-thiols 111 gives substituted thiophenes 112. The mechanism involves electrophilic activation of the alkyne moiety by Pd(ll) followed by intramolecular cyclization, protonolysis, and aromatization (Scheme 25) <20000L351>. S-Endo cyclization of alkynyl thiols 113 using a Mo, W, or Cr catalyst affords dihydrothiophene 114 <2000S970>. 

Application of this mechanochemical halogenation methodology to aromatics 106-108 possessing unsaturated functionalities like alkenyl or alkynyl groups led to the halogen addition to the C—C double or triple bond, without formation of electrophilic substitution products (Scheme 4.29). 

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