In the presence of boron trihalides, aryl aldehydes couple readily
with allylmetals to afford haloallylated products. The reactions
tolerate a variety of functional groups. Simple aqueous workup of
haloallylation reactions, followed by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene,
provides a straightforward route to synthetically useful (E)-1,3-dienes. The mild reaction conditions, readily
available starting materials, and high stereoselectivity make this
two-step (E)-1,3-diene synthesis quite
attractive.
boron trihalide - allylmetal - haloallylation - diene - stereoselectivity