This two step methodology allows the preparation of terminal alkynes by one-carbon homologation of an aldehyde. The first step is comparable to a Wittig Reaction, and leads to a dibromoalkene. Treatment with a lithium base (BuLi, LDA) generates a bromoalkyne intermediate via dehydrohalogenation, which undergoes metal-halogen exchange under the reaction conditions and yields the terminal alkyne upon work-up.
A modification of the Corey-Fuchs Reaction involves the reaction of the intermediate alkynyllithium with an electrophile prior to aqueous work-up, giving a chain extension product:
Mechanism of the Corey-Fuchs Reaction
In the formation of the ylide from CBr4, two equivalents of triphenylphosphine are used. One equivalent forms the ylide while the other acts as reducing agent and bromine scavenger.
The addition of the ylide to the aldehyde:
In the 2nd part of the reaction, which is conducted in apolar solvents such as n-hexane, the more sterically hindered bromine undergoes halogen-metal exchange followed by α-elimination. The resulting carbene isomerizes in a 1,2-hydride shift (Fritsch-Buttenberg-Wiechell Rearrangement, FBW) to yield the desired product (B. Sahu, R. Muruganantham, I. N. N. Namboothiri, Eur. J. Org. Chem., 2007, 2477-2489. DOI).