gem-Difluoro Compounds: A Convenient Preparation from Ketones and Aldehydes by Halogen Fluoride Treatment of 1,3-Dithiolanes

gem-Difluoro compounds can be prepared from ketones and aldehydes by formation of the corresponding 1.3- dithiolanes, followed by reaction with l,3-dibromo-5,5-dimethylhydantoin and pyridinium poly(hydrogen fluoride) (HF-pyridine) in methylene chloride. The reaction requires 2 equiv of Br⁺and appears to proceed through a sequence of two bromosulfonium ions that open and cleave, respectively, to sulfur- and fluorine-stabilized carbocations that are trapped by fluoride ion. Formation of gem-difluoro compounds from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas reaction of dithiolanes derived from aldehydes proceeds rapidly only at higher temperature (0 °C). While the reaction operates successfully on a variety of ketones and aldehydes, brominated byproducts are observed with electron-rich aromatic systems, and side reactions take place in systems prone to carbonium ion rearrangements. The successful geminal difluorination of the related 1.3- dithiane system, although slower than the 1,3-dithiolanes, makes the former system a synthetic equivalent to the difluoromethyl 1,1-dianion. The two-step geminal difluorination of ketones and aldehydes by treatment of the 1,3-dithiolanes with halogen fluoride is a convenient alternative to the use of sulfur tetrafluoride: It is efficient, proceeds at low temperatures, and can be carried out in conventional glassware.