Difluoromethylene derivatives reaction

Much better known are the fluonnatedphosphoranes, which have been widely used m the Wittig reaction for the preparation of fluoroolefms Difluoromethylena tion reactions have been effected by using a variety of conditions Treatment of dibromodifluoromethane with two equivalents of tns(dimethylammo)phosphine m carefully dried tnglyme yields a solution of bromodifluoromethylphosphonium broomide, which very effectively converts ketones to difluoromethylene derivatives A more sensitive reagent is prepared by the addihon of two equivalents of the phosphine to the reaction mixture of fluorohalomethane and a carbonyl compound [39, 40] (equation 40) (Table 14)... 

Aminosulphur trifluorides, which are easier to handle than SF4, can also be used for the conversion of most aldehydes and ketones to difluoromethylene derivatives numerous examples have been documented [12] (Table 3.6). A similar reaction mechanism to that for SF4 may be assumed. 

Novel chemistry that was initiated by Nakai and co-workers [36, 37] involves elimination of hydrogen fluoride from the tosylate of trifluoroethanol, followed by reaction of the intermediate with appropriate electrophiles (Figure 6.27). A wide range of approaches to the synthesis of difluoromethylene derivatives has ensued. 

Ishihara and coworkers have reported that the reaction of 2-[(trimethylsilyl)methyl]-3-chloro-3,3-difluoropropene couples regioselectively with a variety of carbonyl compounds in the presence of zinc-copper chloride or silver acetate to give 2,2-difluoro-3-(trimethylsilyl)methyl-3-buten-l-ol derivatives (equation 87)81. Note again that the difluo-roallyl zinc species generated in situ reacts exclusively on the difluoromethylene terminus. 

Similarly, partially fluorinated and perfluonnated methylenecyclopropanes [87, 82], cyclopropenes [85, 84, 85], cyclobutenes [75, 86], and bicychc alkenes [87, 88, 89, 90] apparently derive dienophihc reactivity from relief of their ground-state strain during reaction Thus 2,2-difluoromethylenecyclopropane and perfluoromethylenecyclopropane undergo exclusive [2+4] cycloadditions [87, 82] (equations 72 and 73), whereas (difluoromethylene)cyclopropane undergoes only [2+2] cycloadditions [87]... 

Hine, J. Porter, J. J. Methylene derivatives as intermediates in polar reactions. VIII. Difluoromethylene in the reaction of chlorodi-fluoromethane with NaOMe. J. Am. Chem. 

As in Explanation 89, these two compounds do not react in the sense of the Diels-Alder reaction. Instead, a cyclobutane derivative is formed by cycloaddition. The reaction is a free-radical process starting with single electrons on carbons 1 and 2 of perfluorovinylsulfur pentafluoride. Carbon 1 of the 1,3-butadiene joins the difluoromethylene group of the perfluorovinylsulfur pentafluoride because in this way, the newly formed biradical can better accommodate the single electron on the carbon next to the SF5 group. The biradical closes the ring in such a way that two stereoisomers, P and Q, cis- and trans-2,3,3-trifluoro-l-vinyl-2-cyclobutyl-sulfur pentafluoride, are formed in equal amounts [122]. 

Functional groups containing the C = N bond, such as hydrazones, oximes and diazo compounds, can also be used in the synthesis of fluorinated derivatives and these reactions are the subject of this section. The reaction products in these procedures are usually the corresponding g cw-difluoromethylene compounds. Since oximes and hydrazones arc obtained from aldehydes and ketones, these reactions can be considered as an alternative to the direct transformation of a carbonyl group into the difluoromethylene group. As indicated in Section... 

The mixture SmI2/HMPA has been applied to the conversion of vicinal bis (sulfonyl) derivatives into the corresponding alkenes.100 An application of this reaction to the synthesis of a difluoromethylene nucleoside, where conventional difluoromethylation strategies (e.g., Wadsworth-Emmons and Wittig reactions) failed, is shown in Eq. 132.234... 

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