Fluoroolefins Fluoroalkenes

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

Synthesis of funtionalized (Z)-fluoroalkene-type dipeptide isosteres (36) via Sml2-mediated reduction of y,y-difluoro-ot, -enoates 2.3.19. Reductive formation of fluoroolefins and subsequent conversion to diketopiperazine mimics (71). Nonpeptidic amide bond replacement... 

Methyl fluoro(diethoxyphosphono)dithioacetate (34) has been prepared from difluorinated precursors [56], Fluorophosphonothioacetamides (35) derived from this dithioester, have been successfully transformed into highly functionalized fluoroalkenes (36). Judicious selection of the aldehyde coupling partner can lead expeditiously to the preparation of fluoroolefin dipeptide isosteres following elaboration of the carboethoxy group and desulfurization (Scheme 11). 

Alternatively, the amino group was introduced subsequent to the reductive formation of the fluoroalkene [72]. The regio- and stereoselective conversion of the C5-hydroxyl group of the fluoroolefin to an amino group could be achieved through one-pot mesylation and azidation reaction (Scheme 25). 

Scheme 1. Mechanism of telomerisation of fluoroalkene F2C = CRR with RpOF (in order to generalise the formulae, the expression C2F2RR has been prefered to CF2CRR or CRR CF2 since the nature of R and R (which can be either electron-withdrawing or electron-donating groups) influences the sense of addition of RfO to a preferential site of the fluoroolefins).

Beside most important and commercially available fluoroalkenes, a large variety of different fluoroolefins have been successfully telomerised. Although most of them led to monoadducts mainly, devoted to the perspective of basic... 

Under the same conditions, two fluoroolefins are not supposed to react similarly with a given telogen. For instance, HFP requires much energy to react with a telogen compared to TFE or VDF and it homopolymerises with difficulties in contrast to other fluoroalkenes. Below are presented several examples of compared reactivities of fluoroolefins in telomerisation involving the same telogen. 

By reaction of a 1,1-difluoroethene with tributylphosphane in the presence of boron trifluoride-diethyl ether complex, the vinylphosphonium salt is obtained. On subsequent hydrolysis the selectivity of formation of the Z- and F-isomer is 100%, but it diminishes after the introduction into the alkene. together with the perfluoroalkyl group, of substituents such as phenyl and 4-methoxyphcnyl. In accordance with this methodology, a method of substitution of fluorine by hydrogen at the fluoroolefinic double bond has been developed.194,206 The first stage of this reaction includes the formation of a fluoro-/5-phosphane from the fluoroalkene and tributylphosphane. The decomposition of this /l5-phosphane in the aqueous solvent leads to a fluoroalkene, e.g. formation of l.206... 

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