Iron‐Catalyzed <i>trans</i>‐Hydrostannation of Terminal Alkynes

The readily accessible iron complex [Cp*FeCl(tmeda)] is an effective catalyst for the highly regio- and stereoselective trans-hydrostannylation of terminal alkyl alkynes, affording a type of alkenylstannanes that is difficult to make otherwise. The R₃Sn- moiety is faithfully delivered to the terminal C-atom, unless a propargylic or homo-propargylic ─OH or ─NH₂ group is present in the substrate, which (partly or fully) inverts the regiochemical course; this steering effect, however, can be switched off upon protection of the protic site. The trans-addition likely starts by insertion of the [Cp*FeCl] fragment into the R₃Sn─H bond, followed by a migratory insertion of the ligated alkyne into the Fe─Sn unit of the Fe(IV) species thus formed. This "modified Chalk-Harrod" mechanism is manifested in a stannylative cyclization of 1,6-enyne derivatives, which has hardly any precedent either; a pathway via iron vinylidene intermediates can be excluded on the basis of deuterium labeling experiments.