Pentafluorophenyl derivatives boron

Cavell and Dobbie (214-216) have suggested that halogen transfer rearrangements in trifluoromethylphosphines arise from interactions of nonbonding fluorine p orbitals with vacant d orbitals on phosphorus. Such an explanation is consistent with observations for the Groups IV and V pentafluorophenyl derivatives, exclusive of carbon and nitrogen, and similarly fits the behavior of boron with its vacant p orbital. 

Pentafluorophenyllithium in ether reacts slowly with mercury at 15° C to afford bis(pentafluorophenyl)mercury, a compound recently prepared by another route (Section VII, B). The lithium reagent has very recently been used to prepare pentafluorophenyl derivatives of boron (79a), tin 19b), lead 19b), and several transition metals 19c). 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

The benzylation of both activated and deactivated arenes has been achieved using the boron trifluoride activation of A -methyl hydroxamate leaving groups to liberate the reactive benzyl cations. Elimination of p-toluenesulfinic acid from a-amidosulfones under the influence of tris(pentafluorophenyl)borane gives acyl iminium ions, which may attack activated arenes to yield p-tolylsulfone derivatives (12), as indicated in Scheme 1 (where Cbz represents the fV-benzyloxycarbonyl leaving group). Elimination... 

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

After the discovery of MAO by Kaminsky, many chemicals had been examined as cocatalysts. For an SPS production catiyst, Campbell found that boron compounds based on tris(pentafluorophenyl)boron and its derivatives are suitable as cocatalysts in the syndiotactic polymerization of styrene [1]. 

Some more exotic electrophilic reagents have been applied successfully to thiophens. Thus, pentafluorobenzenesulphenyl chloride reacts with thiophen in ether in the presence of catalytic amounts of SnCl to give 2-pentafluorophenyl-thio-thiophen. Thiophen and 2-methylthiophen give trifiuoromethylthio-derivatives with trifluoromethanesulphenyl chloride in chloroform in the presence of pyridine. Electron-deficient aromatics such as 2,5-bis(trifluoromethylthio)-thiophen react with trifluoromethanesulphenyl chloride, in the presence of trifluoromethanesulphonic acid as catalyst, to give mainly the 3-chloro-derivative. Fluorinated immonium salts (103), obtained by the action of boron trifluoride on a-fluorinated amines, can be used for fluorinated acylation of... 

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