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Iron-Catalyzed trans-Hydrostannation of Terminal Alkynes
Soumyadeep Chakrabortty1, Alois Fürstner1
1Max-Planck-Institut für Kohlenforschung, Mülheim/Ruhr, 45470, Germany.
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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.
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02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:40Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration
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02:35Electrophilic Addition to Alkynes: Hydrohalogenation
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