homoallyl

Homoallyl is a term used in organic chemistry to describe a structural motif that is an extended analogue of the allyl group. An allyl group is the three-carbon fragment CH2-CH=CH2. A homoallyl fragment extends this chain by one methylene unit, typically represented as CH2-CH2-CH=CH2. In practice, the descriptor may refer to a substituent attached to a molecule through the first methylene of that four-carbon chain or to a reactive intermediate that bears a homoallyl arrangement.

In synthetic contexts, homoallyl motifs appear in the preparation of homoallylic derivatives, including homoallylic alcohols and homoallyl ethers. Reagents and intermediates bearing homoallyl groups are used in various carbon–carbon bond-forming strategies, including cross-coupling and allylation-type transformations, to install a four-carbon fragment featuring a terminal alkene. The term is also employed when discussing reaction mechanisms that involve homoallyl cations, radicals, or anions as intermediates, where the reactive center is separated from the double bond by two methylenes rather than one.

Related concepts include the allyl group and the broader class of homoallylic or homoallyl-containing compounds. The nomenclature is part of the broader practice of naming extended or shifted allyl systems in organometallic and synthetic chemistry.