Difluoro-λ³-Bromane-Induced Oxidative Carbon−Carbon Bond-Forming Reactions:  Ethanol as an Electrophilic Partner and Alkynes as Nucleophiles

Abstract

Reported here for the first time are the oxidative couplings of alkynes and primary alcohols yielding conjugated enones. Although the BF3-catalyzed reaction of terminal alkynes with p-trifluoromethylphenyl(difluoro)-lambda3-bromane results in the fluoro-lambda3-bromanation of triple bonds to afford (E)-beta-fluorovinyl-lambda3-bromanes, reaction of an alkyne with the difluoro-lambda3-bromane in the presence of an alcohol and BF3-Et2O affords directly conjugated enones in good yields. The reaction proceeds in a highly stereo- and regioselective manner under metal-free conditions. Interestingly, no formation of enones was detected, when difluoro-lambda3-iodane p-CF3C6H4IF2 was used instead of the lambda3-bromane. A mechanism involving a lambda3-bromane-induced oxidation of an alcohol to an aldehyde, [2 + 2] cyclization with alkynes yielding 2H-oxetes, and finally the electrocyclic ring opening is discussed.

Introduction of Fluorine and Fluorine‐Containing Functional Groups

“…Another approach for the synthesis of fluoromethylated ethers involves the oxidative rearrangement of benzyl alcohols induced by XeF 2 177 or aryl difluoro‐λ 3 ‐bromane (Scheme )178 through participation of the phenyl group in the aryl carbon–oxygen bond‐forming step in the synthesis of aryl α‐fluoromethyl ethers. The electrophilic fluorination of O , S ‐acetals179 and α‐carboxymethyl ethers99, 180 can also afford α‐fluoromethyl ethers.…”

Synthesis of (E)‐α,β‐Unsaturated Carbonyls via Silver‐Catalyzed Tandem Epoxide Rearrangement/Intermolecular Carbonyl‐ Heteroalkyne Metathesis

“…In this respect, Ochiai and co‐workers reported a one‐pot synthesis of conjugated enones via the difluoro‐λ 3 ‐bromane‐induced oxidative coupling of primary alcohols with alkynes, but under harsh reaction conditions and with limited substrate scope [Eq. (2)] . Recently, a well‐designed protocol for the construction of medium and large ring lactones has been achieved by the group of Li and Sun using the ACM reaction of ynol ethers with cyclic oxocarbeniums, generated in situ from the BF 3 ⋅OEt 2 ‐mediated elimination of acetals [Eq.…”

Einführung von Fluor und fluorhaltigen funktionellen Gruppen

“…Eine weitere Synthesemethode für fluormethylierte Ether ist die durch XeF 2 [177] oder Aryldifluor-l 3 -broman induzierte oxidative Umlagerung von Benzylalkoholen (Schema 41) [178] , die unter Phenylgruppenbeteiligung an der Aryl-C-O-Bindungsbildung in der Synthese von Aryl-a-fluormethylethern abläuft. Auch durch elektrophile Fluorierung von O,S-Acetalen [179] oder a-Carboxymethylethern [99,180] lassen sich a-Fluormethylether erhalten.…”