Lewis acid group 15 halides

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

Nucleophilic carbenes have been reported to react with Lewis acidic group 14 complexes with the isolation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. The range of carbene complexes of silicon, germanium, tin, and lead, as well as some related cyclopropenylidene complexes are described below. 

Lewis acid with Halides. Ag20 can activate halide as a leaving group by coordination. Methylation of carbohydrates and 5-benzyUdenebarbituric acid have been carried out with Ag20 and iodomethane in DMF. Triphenyltin trifluoroacetate was obtained from the reaction of triphenyltin iodide and trifluoroacetic acid in the presence of silver oxide. Ag20 converts trawi -halohydrins to epoxides(eq 12) or rearranged products (eq 13). It is reported that the reaction is nonstereoselective, and that tra/w-epoxides are the major products when c/s-alkenes are the reactants. ... 

This study suggests a radically new explanation for the nature of Lewis acid activation in the Simmons-Smith cyclopropanation. The five-centered migration of the halide ion from the chloromethylzinc group to zinc chloride as shown in TS2 and TS4 has never been considered in the discussion of a mechanism for this reaction. It remains to be seen if some experimental support can be found for this unconventional hypothesis. The small energy differences between all these competing transition states demand caution in declaring any concrete conclusions. 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

To optimize the alkylation conditions, ferrocene was reacted with allyldimethyl-chlorosilane (2) in the presence of various Lewis acids such as aluminum halides and Group lO metal chlorides. Saturated hydrocarbons and polychloromethanes such as hexane and methylene chloride or chloroform were used as solvents because of the stability of the compounds in the Lewis acid catalyzed Friedel-Crafts reactions. The results obtained from various reaction conditions are summarized in Table IV. 

As shown in Table IV, the highest catalytic activity of metal halides used as Lewis acid for the alkylation reaction of ferrocene with 2 was observed in methylene chloride solvent. Among Lewis acids such as aluminum chloride, aluminum bromide, and Group 4 transition metal chlorides (TiCl4, ZrCU, HfCU), catalytic efficiency for the alkylation decrea.ses in the following order hafnium chloride > zirconium chloride > aluminum chloride > aluminum bromide. Titanium chloride... 

Group 14 metal halides also undergo metathesis reactions. For instance, organotin compounds are prepared on an industrial scale using organoaluminum reagents. These reactions take place because tin is a softer Lewis acid than aluminum, and carbon (in an alkyl group, represented by R) is a softer Lewis base than chlorine ... 

Silyl enol ethers and silyl ketene acetals also offer both enhanced reactivity and a favorable termination step. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCl4. This reaction provides a method for introducing tertiary alkyl groups a to a carbonyl, a transformation that cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

The stereochemistry of the reaction depends on the Lewis acid. Protic acids favor retention of configuration, as does TMSOTf. Most metal halides give mixtures of inversion and retention, but A1(CH3)3 gives dominant inversion.142 Inversion is suggestive of direct carbonyl group participation. 

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