Lewis base catalysts halogenations

The addition of a third component to the metal-alkyl is a widespread practice with MgCI2 catalysts in order to improve their performance and to control the polymer molecular structure. In ethylene polymerization the addition of modifiers (alkyl-halides, Lewis acids such as A1C13, halogens such as I2, and others) is rather limited and is principally used to modify the MWD (see Table 11 in Ref.53)). On the other hand, the addition of modifiers is almost indispensable to obtain satisfactorily stereoregular propylene polymers. The additives used for this purpose are generally electron donor compounds (Lewis bases) and a wide variety has been described in patent and scientific literature. 

Desymmetrization with halogen nucleophiles was effectively demonstrated with two mechanistically-divergent chiral catalysts. Denmark disclosed a Lewis-base activated delivery of chloride that was catalyzed by the enantiopuxe phos-phoramide 16. Binding of the phosphoramide was believed to induce dissociation of SiCl4 into the chiral phosphorus/silicon cation and chloride anion, which subsequently ring-opened the activated epoxide. The best enantioselectivity was observed with cfs-stilbene oxide, which was formed in 94% yield and 87% ee (Scheme 14) [28]. 

Catalysts (25) are the Lewis acid-Lewis base bifunctional catalysts in which Lewis acid-Al(III) moiety activates acyl iminium ion and the Lewis base (oxygen of phosphine oxide) does TMSCN, simultaneously (Scheme 5.7). Halogen atoms at the 6-position enhanced both yields and enantioselectivity in Reissert-type cyanation of the imino part of 26. However, the order for the activation is not parallel to the electronegativity of the halogen atoms and, moreover, the strong electron-withdrawing trifluoromethyl group provided unexpectedly the worst result for the activation [13]. It is not simple to explain this phenomenon only in terms of the increased Lewis acidity of the metal center. Trifluoromethylated BINOL-zirconium catalysts (28) for asymmetric hetero Diels-Alder reaction (Scheme 5.8) [14], trifluoromethylated arylphosphine-palladium catalyst (32) for asymmetric hydrosilylation (Scheme 5.9) [15], and fluorinated BINOL-zinc catalyst (35) for asymmetric phenylation (Scheme 5.10) [16] are known. 

There are indications in the literature suggesting the formation of electron deficient metal particles in e.g. AbOs-based and halogenated solid catalysts. However, the mechanism of this process and the nature of anchoring sites are not quite clear. Broensted acid sites, as well as strong Lewis acid sites may be considered as surface centers stabilizing small metal particles (Pt, Pd, Ir, Ni) and causing their positive charging. ... 

In the presence of a strong Lewis acid, bromine and chlorine halogenate aromatic rings. Bromina-tion requires both Br and a Lewis acid catalyst, FeBr. The catalyst generates a Lewis acid-Lewis base complex with a weakened Br—Br bond. The bromine atom bonded to iron carries a formal positive charge. It is the electrophile. 

Nickel-based Ziegler catalysts can be prepared using halogen-free or-ganoaluminum compounds of low Lewis acidity, e.g., dialkylaluminum alkoxides. However, the catalytic properties of these systems differ remarkably from those described above. The nickel components in such a case may be nickel acetylacetonate, or the nickel enolates of various other /3-dicarbonyl compounds (44, 45), in particular such halogenated /3-dicarbonyl compounds as hexafluoroacetylacetone (44, 46). 

The most studied ionic liquids (ILs) in the literature are chloroaluminate-based ionic liquids. Chemical properties such as Lewis acidity depend on the nature of the anion, namely AI2CI7. Considering the acidity parameter as the key point to design a liquid-phase fluorination catalyst, a new range of acidic ILs has been set up [9]. These ILs have been synthesized starting from halogenated metallic acids MX, (with M = Ti, Nb, Ta, Sn, Sb and X = Cl and/or F), such as Ta(V)Cl Fj, Nb(V)Cl Fj, Ti(IV)Cl Fj, Sn(IV)Cl Fj, Sb(V)Cl Fj,. Among these compounds, anions derived from SbCl F, (x -i-y = 5,0[Pg.536]

When halogen atoms are present in the epoxide such as in epichlorohydrin, 3,3,3-trichloropropylene oxide (TCPO) or 4,4,4-trichloro-l,2-butylene oxide (TCBO), or in the initiator, acid catalysts, e.g. boron trifluor-ide etherate, may be used (13-18). Vogt cind Davis (16) found that, if the concentration of catalyst/ini-tiator (polyol) complex is decreased with respect to TCPO in order to obtain higher molecular weight products, side reactions such as cyclization reactions become increasingly important. Boron trifluoride also promotes dimerization of alkylene oxides to dioxane or alkyl derivatives of dioxane as described by Fife and Roberts ( ). The use of acid catalysts, e.g. Lewis acids, promotes formation of a greater amount of terminal primary alcohol groups when compared to base catalysis of epoxides. 

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