Pentafluorophenyl transfer

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Rieger et al. described a heteroatom-containing C, symmetric metallocene 13 whose stereoselectivity depended on the activator [28, 29], The resulting PP contained fewer stereoerrors when activated with a combination of TiBA and trityl tetrakis(pentafluorophenyl)borate than with MAO. In addition, the molecular weight was lower with MAO. To explain this, it was proposed that some of the stereoerrors arise by reversible chain transfer to aluminum. 

The pentafluorophenyl derivatives are particularly revealing, since the polarization transferred to the individual 19F positions can be compared and correlated with different concepts of their interactions with the primarily spin polarized protons [48]. The enhancement factors achieved in the hydrogenation products of monosubstituted compounds at low field under ALTADENA conditions are given in Figure 12.32. 

Since silyl cations are highly reactive and moisture sensitive, the salts (S)-2a and (S)-2b were prepared in situ from the air and moisture stable precursor (S)-5 via a hydride transfer [34, 35] with trityl tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [Tr][TFPB] or trityl tetrakis[pentafluorophenyl]borate [Tr][TPFPB], The authors showed by Si-NMR studies that the desired salts were formed. The silyl salt (5)-2a was then tested in the Diels-Alder reaction as shown in Scheme 5. A good reactivity was found, and the product was obtained in 95% yield with higher than 95% endo selectivity at -40 °C in 1 h. However, only 10% ee was achieved. 

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. 

Pentafluorination diminishes n electron density of phenyl, making it more suitable for participation in stacking or charge transfer with the phenyl groups of other aromatic amino acids of the active site. As an example, the inhibitory power of the carbonic anhydric hydrolase by the pentafluorophenyl analogue of methazolamide is tenfold increased (Fig. 5) [31]. 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

Irradiation of isoquinolinium hydroxytris(pentafluorophenyl)borate 232 resulted in C,T S transfer to the isoquino-linium cation to yield 2-m cthy 1-1-(2,3,4,5,6-pentafluoropheny I)-1,2-dihydroisoqu incline 233 <2005JOC10653>. The mechanism is proposed to be due to a photoinduced electron transfer from the singlet state of the iV-methylisoqui-nolinium cation, confirmed using fluorescence quenching (Scheme 41). 

These results clearly show that the potential energy surface can contain a series of minima. The fact that selectivity in re-attack by the F ions can be observed indicates that the differences between the energy barriers for the secondary reactions control the distribution of the final products. The multistep character of these processes is further illustrated by the reactions observed when enolate anions are used as reactant ions. The ambident enolate anions may react with methyl pentafluorophenyl ether at the carbon or the oxygen site. If they react with the carbon site at the fluorine-bearing carbon atoms, then the molecule in the F ion/molecule complex formed contains relatively acidic hydrogen atoms so that proton transfer to the displaced F ion may occur. An example is given in (47) where the enolate anion, generated by HF loss, is not observed. An intramolecular nucleophilic aromatic substitution occurs instead and leads to a second F ion/ molecule complex. The F" ion in this complex then re-attacks the substituted benzofuran molecule formed, either by proton transfer or SN2 substitution. 

The reaction of pentafluoroiodobenzene with aromatic compounds such as anilines, pyrroles, indoles, imidazoles, aromatic ethers and phenols leads to aryl—aryl coupling477. The reactions proceed via pentafluorophenyl radicals which are generated by photoin-duced electron transfer (PET) and loss of iodide ion. Coupling between the pentafluorophenyl radical and the radical cation of the donor gives biaryl cations (138,139) which lose a proton. The reaction is illustrated for N, A-dimethylaniline (equation 125). 

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