The dual role of the bicyclic amidine base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was demonstrated in a synthesis of terminal aryl- and styryl-acetylenes. Mechanistically, a tandem process involving elimination/Umpolung/protonation occurs in a single step to generate terminal aryl- and styryl-acetylenes from geminal dibromoalkenes. The key to the success of this transformation lies in the organobase-mediated generation of the acetylide from the 1-bromoalkynes at room temperature. The unique characteristics of DBU as an inherently safer reagent make it an attractive alternative to previous systems wherein required pyrophoric reagents and nonambient temperatures remain unsolved issues. The procedure does not work for the synthesis of alkyl-acetylenes.
Herein, the unique method reports the efficacy of abundantly available urea for the generation of alkynyl radicals from 1‐bromoalkynes under metal‐free conditions. Urea as a sole reagent enabled the transformation of 1,1‐dirbomoalkenes or 1‐bromoalkynes to 1,3‐diynes via Homolytic Alkynyl Substitution (HAS). Urea further showed efficiency in synthesizing disubstituted alkyne via Sonogashira type cross‐coupling of 1,1‐dibromalkene/1‐bromoalkyne with aryl iodide. The Azobisisobutyronitrile (AIBN) trap detected the existence of short‐lived alkynyl radical during the reaction. This method might be the first report with Homolytic Alkynyl Substitution under metal‐free conditions, whereas Homolytic Aromatic Substitution is well reported.
The reaction of 1,1‐dibromoalkenes and aryl iodides in the presence of DBU and a PdII/CuI catalytic system afforded diaryl alkynes and enynes at ambient temperature. Control experiments demonstrated the essential ligand role of DBU in this transformation. The unusual stability of the key catalyst, Pd0(DBU)2, allowed it to be characterized by HRMS analysis.
A three‐component reaction of 1,1‐dibromoalkenes, sodium sulfide, and N‐substituted formamide for the synthesis of disubstituted thioamides has been developed. Various dibromoalkenes were found to be compatible under these conditions and gave corresponding thioamides in good to excellent yields.
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