1,4-Brook Rearrangement of β-Silyl Allylic Alcohols: Trisubstituted Alkenes by Copper-Mediated Stereoretentive ipso-Substitution Reactions.

When [Cp*Ru(MeCN)3]PF6 is used as the catalyst for the trans-hydrosilylation of propargyl alcohol derivatives, the silyl group is transferred with appreciable levels of regioselectivity to the alkyne C-atom distal to the protic substituent. A copper-mediated 1,4-Brook rearrangement converts the resulting alkenylsilanes into alkenylcopper species, which can be intercepted with a multitude of carbon- or heteroatom-based electrophiles; this formal ipso-substitution of the R3Si-group allows a variety of trisubstituted olefins to be formed in stereodefined format, including fluoroalkenes and olefins bearing trifluoromethyl substituents. Best results are obtained with copper amides such as [Cu(NEt2)]4 as the promoter in THF/DMPU; they are sufficiently basic to deprotonate the ─OH group in the first place, thereby triggering the critically important C-to-O silyl migration, yet mild enough to ensure compatibility with a variety of functional groups, including even C─H acidic sites adjacent to an ester or ketone. In cases where the trapping of the organocopper intermediates primarily formed is unduly slow or inefficient, the addition of either 2-thienyllithium or tetra-n-butylammonium cyanide is recommended; these additives lead to more highly nucleophilic cuprate species that show the right reactivity profile for efficient coupling of otherwise noncompliant electrophilic partners.