Difluoro sulfane

Ketones are less reactive than the aldehydes (see Section 8.4.2.). Ketones, e.g. 1 and 3, can be converted to the corresponding gem-difluoro compounds, e.g. 2 and 4, respectively, with (dia-lkylamino)trifluoro-A4-sulfanes at temperatures ranging from 20 to 80 C (see Table 4). 

DialkyIamino)trifluoro-A4-sulfanes are unable to convert esters into the corresponding a,a-difluoro ethers due to the low electrophilicity of the carbonyl group. However, this transformation can be carried out if the ester is first converted to a thioester with Lawesson s reagent,56 followed by fluorination with DAST.57 The fluorination step occurs at room temperature and in high yield (see Table 6). 

The addition of equivalent quantities of a (dialkylamino)trifluoro-/l4-sulfane (see Section 8.4.) to a (dialkylamino)trimethylsilane results in the formation of a bis(dialkylamino)difluoro-/l4-sulfane.71 The dialkylamino moieties of the reactants can be different, resulting in the synthesis of unsymmetrical products. This class of compound is not stable to distillation and is generally utilized without prior purification. For small-scale fluorinations, the bis(dialkylamino)difluoro-/l4-sulfanes are generated in situ, for example, by the addition of one equivalent of commercial-grade DAST to a solution of an appropriate (dialkylamino)trimethylsilane at low temperature.72 73 Examples for the preparation of a bis(dialkylamino)difluoro-/l4-sulfane l71 and dif-luoro-/l4-sulfane 25,11 with different dialkylamino groups are given below. 

Bis(dialkylamino)difluoro-/4-sulfanes, e.g. 3, have also been synthesized by the fluorination of A,/V,/V ,7V -tetraalkylsulfinyldiamines with (dialkylamino)trifluoro-A4-sulfanes.11... 

Reactions with bis(dialkylainino)difluoro-/l.4 -sulfanes can be performed in normal glassware under anhydrous conditions. Caution in the presence of moisture hydrolysis takes place liberating hydrogen fluoride. Bis(dialkylamino)difluoro-24 -sulfanes are less reactive fluorinating reagents than (dialkylamino)trifluoro-24 -sulfanes. This fact coupled with their lengthier syntheses and their noncommercial availability has limited their application. 

In certain cases, bis(dialkylamino)difluoro-/l4-sulfanes have advantages over (dialkyl-amino)trifluoro-24 -sulfanes, for example, in the lluorination of alcohols which are susceptible to rearrangement and/or dehydration.71 The fluorination of but-2-en-l-ol with DAST in isooctane gives the rearrangement product 4 as the major product, whereas (diethylamino)di-methylaminosulfur difluoride gives the unrearranged product 5 as the major product. 

Fluorination of cyclohexanol with bis(dialkyl amino )difluoro-z4-sulfanes results in significantly less dehydration than when the identical reaction is performed with DAST. 

The addition of one of the fluorinating reagents of the bis(dialkylamino)difluoro-24-sulfanes type (see Section 8.5.) to one equivalent of a (dialkylamino)trimethylsilane results in the formation of a tris(dialkylamino)sulfonium difluorotrimethylsilicate. Alternatively, these compounds can be directly synthesized by the addition of sulfur tetrafluoride to 3 equivalents of a (dialkylamino)trimethylsilane, e.g. 1 and 2.71,74... 

SF,C2H, X, -Sulfane, (2,2-difluoro-ethenyl)pentafluoro-, 29 35 SFgCj, X, -Sulfane, pentanuoro(trifluoro-ethenyl)-, 29 35... 

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