Reppe cyclization

What is Reppe cyclization?

The Reppe cyclization is a reaction that was first reported by Reppe in 1948. It involves a nickel complex-catalyzed [2+2+2+2] cyclotetramerization of acetylene to cyclooctatetraene (COT). It is sometimes referred to as the Reppe reaction, Reppe synthesis, or Reppe cyclooctatetraene synthesis.

Reppe cyclization - general reaction scheme - Reppe reaction - Reppe synthesis - Reppe cyclooctatetraene synthesis
Reppe cyclization

The nickel catalysts used for this reaction include Ni(acac)2, Ni2(COT)2, 1,2-diaza diene nickel complex Ni(dad)2, and even vapor-deposited nickel (see list of acronyms). Reppe cyclization is highly efficient and has a high turnover number of up to 106 in neat propargylic alcohol, regardless of the catalyst. Disubstituted alkynes may not work for this reaction, but in some cases, they form octa-substituted cyclooctatetraenes.

Reppe cyclization does not hold regioselectivity and can produce various types of substituted cyclooctatetraenes. However, it often accompanies side reactions that yield benzene derivatives. Overall, the Reppe cyclization is an important reaction in organometallic complex-catalyzed reactions.