Self-coupling, terminal acetylenes

The self-coupling of terminal acetylenes is more severe under dilute conditions and is catalyzed by adventitious molecular oxygen. It was found that the strict exclusion of oxygen, low reaction temperatures, and extended reaction times were optimal. Extensive experimentation established that a 50% molar ratio of catalyst to sequence and 0.002 M sequence concentration in dry triethylamine solution gave the highest yield of the 18-unit tetrafunctional sequence 24. Interestingly, the best mode of addition was to add both sequences 22 and 23 together at the start, rather than attempt a controlled slow addition of one sequence to the other. 

Self-condensation of but-3-yn-2-one occurs in the presence of 0.1m DABCO, to give ( )-3-(but-l-en-3-yn-2-oxy)buten-2-one. ° This is not possible for 4-substituted-but-3-yn-2-ones, which can, however, form products of cross-coupled condensation on reaction with a terminal acetylenic ketone. 

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