gem-Difluoro compounds can be prepared from ketones and aldehydes by formation of the corresponding 1, 3-dithiolanes, followed by reaction with 1, 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- ...
