Silyl dienol ethers prepared from α,β-unsaturated ketones underwent nickel-catalyzed difluorocyclopropanation of the electron-rich alkene moiety with trimethylsilyl 2,2-difluoro-2-(fluorosulfonyl)acetate. The subsequent vinylcyclopropane-cyclopentene rearrangement afforded silyl 5,5-difluorocyclopent-1-en-1-yl ethers in good yields. The obtained five-membered silyl enol ethers were demonstrated to be versatile intermediates for the synthesis of di- and monofluorinated cyclopentanones and cyclopentenones. A nickel difluorocarbene complex is proposed as a key intermediate in the difluorocyclopropanation.

Silyl dienol ethers readily underwent copper(I)-catalyzed [4 + 1] cycloaddition with sodium bromodifluoroacetate to afford 4,4-difluorocyclopent-1-en-1-yl silyl ethers. On the basis of high-resolution mass spectroscopy analysis, annulation presumably proceeded via a copper(I) difluorocarbene complex, which represents an unprecedented example of [4 + 1] cycloadditions promoted by a transition metal difluorocarbene complex.

Controlled generation of difluorocarbene was effected by an NHC catalyst under mild conditions starting from trimethylsilyl 2,2-difluoro-2-fluorosulfonylacetate (TFDA). Cyclohexenones and tetralones were treated with TFDA in the presence of catalytic amounts of N,N'-dimesitylimidazolium chloride and sodium carbonate. The ketones were difluoromethylated with the generated difluorocarbene to afford enol difluoromethyl ethers without difluorocyclopropanation. The ethers thus obtained were dehydrogenated with DDQ to furnish aryl difluoromethyl ethers in high yield. Under similar conditions, secondary amides underwent difluoromethylation selectively on the oxygen atom to give difluoromethyl imidates, which allows the formation of 2-difluoromethoxypyridines.

The syntheses of fluorinated cyclopentanone derivatives, which have pharmaceutical and agrochemical importance are reported. The catalytic reaction of copper(I) and nickel(II) difluorocarbenes with silyl dienol ethers afforded 4,4-difluoro- and 5,5-difluorocyclopent-1-en-1-yl silyl ethers, respectively. The fluorine-directed and -activated Nazarov cyclization of 1-fluorovinyl vinyl ketones, which were prepared from silyl dienol ethers and free difluorocarbene, proceeded efficiently to afford 2-fluorocyclopent-2-en-1-ones. Moreover, fluorine-­directed Nazarov cyclizations of 2,2-difluorovinyl vinyl ketones and 1-(trifluoromethyl)vinyl vinyl ketones afforded 3-fluorocyclopent-2-en-1-ones and 5-(trifluoromethyl)cyclopent-2-en-1-ones, respectively. In addition, derivatization of ring-difluorinated products also provided 3-fluorocyclopent-2-en-1-ones.

Regioselective Synthesis of ,-Difluorocyclopentanone Derivatives: Domino Nickel-Catalyzed Difluorocyclopropanation/Ring-Expansion Sequence of Silyl Dienol Ethers. -The utility of the method is demonstrated by further transformations of the products into ,-difluorocyclopentanone derivatives as well as mono-and difluoropentenone derivatives. -(AONO, T.; SASAGAWA, H.; FUCHIBE, K.; ICHIKAWA*, J.; Org. Lett. 17 (2015) 23, 5736-5739, http://dx.doi.org/10.