Insertion into carbon-fluorine bond

A novel approach to perfluoroarylzinc reagents was recently reported by Miller, Platonov and coworkers. This route involves direct insertion of zinc into carbon-fluorine bonds in the presence of metal salts, such as SnCl2, CuCl2 and ZnBr2. These reactions are accelerated by ultrasound (equation 75)71. The reaction rate decreased in THF or if ZnBr2 or CuCl2 were used. 

The insertion of calcium atoms into vinyl and aryl carbon-fluorine bonds has been reported Only the resulting organometallic from the aryl derivatives appears to live long enough to be trapped by water [J]... 

This chapter does not attempt to cover electrochemical processes which involve the insertion of a group already containing carbon-fluorine bonds into an existing molecule (the building block approach), e.g., perfluoroalkyl group insertion via the Kolbe reaction, or fluoroacyloxylations or fluoroalkoxylations via electro-oxidation, or fluoroalkylation via electro-reduction. These have been reviewed elsewhere [1,2]. 

Oxidative addition of fluorinated aromatics to give metal(aryl) fluoro complexes has been observed frequently for complexes of group 10 metals, but rarely otherwise. The reactions include the insertion of an Ni(PEt3)2 unit into a carbon-fluorine bond in hexafluorobenzene (Scheme 5). The very slow conversion (4 weeks) was initially reported by Fahey and Mahan full spectroscopic and crystallographic characterization of the product was performed by Perutz et al " Mechanistic studies and density functional theory (DFT) calculations give strong evidence for precoordination of the aromatic substrate followed by a concerted oxidative addition. " " Oxidative addition has also been... 

Figure 5.1 Proposed transition state for the insertion of Ni(0) into the carbon-fluorine bond in the presence of HASPO ligands [44].

Vinyl fluorinated zinc reagents can be prepared by two different methods (1) capture of the corresponding vinyllithium reagent at low temperatures with a zinc salt and (2) direct insertion of zinc into a carbon-halogen bond. 

The fluorinated vinylzinc reagents formed via direct insertion of zinc into the carbon-halogen bond of a vinyl halide have served as useful synthons for the stereospecific synthesis of a wide variety of fluorinated compounds. Some illustrative examples of some of the synthons developed are summarized in the following sections. 

Fluorinated alkenes are able to insert into weak C-H bonds of various compounds branched alkanes, haloforms. alcohols, ethers, aldehydes and their corresponding ketals. These reactions usually involve the use of UV irradiation or radical-initiation catalysts, such as peroxides or azobisisonitriles. Variable amounts of telomcric products are also formed. Under the influence of ) -irradiation ( °Co source), one-to-one adducts are obtained predominantly. Attack of the intermediate radical occurs preferentially on the less-hindered carbon of the fluorinated alkene. 

The effect of fluorinated systems on the reaction process shown in Figure 10.67 is of interest [139] it is to be noted that insertion of the palladium catalyst into the carbon-halogen bond may be considered as a nucleophihc attack by the palladium centre, albeit a soft nucleophile, which prefers to attack C—Br over C—F [138]. This process is, of course, aided by the presence of electron-withdrawing groups (EWG) in the organic system. It is likely, however, that co-ordination of the other reactant, e.g. alkyne, to the palladium is the rate-determining step [137], but this will be aided by EWGs attached to the metal. 

As mentioned previously (see p. 138), carbon disulphide inserts into the platinum-fluorine bond of [PtF(PPhs)j]+ to give the hydrolytically unstable complex cation (137). ... 

Fluorinated organoboranes reacted with internal alkynes through 1,2-alkyl/aryl migration to afford tetrasubstituted alkenes bearing a boron atom (Scheme 1.68) [89]. In a formal sense, one substituent on the alkyne migrated to another alkyne carbon to result in a vinylidene, which was inserted into the C-B bond. 

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