Lewis basicity boron trifluoride

Figure 5.19. Illustration of the formation of a donor-acceptor complex by the sequential adsorption of trimethylamine and boron trifluoride. The Lewis-basic trimethylamine forms a dative bond with the electrophilic Si dimer atom, and the Lewis-acidic boron trifluoride bonds to the nucleophilic Si dimer atom [278].

The addition of phosphine to olefins is accelerated by acidic and basic catalysts. Under the influence of non-oxidising acids or Lewis acid such as, for example, methanesulphonic acid, benzenesulphonic acid, trifluoro-acetic acid or boron trifluoride phosphine is quickly added to olefins at pressures of 20-40 at, and temperatures of 30-60 °C. It is assumed that the reaction proceeds via a carbonium ion which is first formed thus ... 

Ethers are weakly basic and are converted to unstable oxonium salts by strong acids such as sulfuric acid, perchloric acid, and hydrobromic acid relatively stable complexes are formed between ethers and Lewis acids such as boron trifluoride, aluminum chloride, and Grignard reagents (qv) (9) ... 

Another remarkable Lewis basicity scale for 75 non-HBD solvents has been established by Gal and Maria [211, 212]. This involved very precise calorimetric measurements of the standard molar enthalpies of 1 1 adduct formation of EPD solvents with gaseous boron trifluoride, A//p gp, in dilute dichloromethane solution at 25 °C, according to Eq. (2-10a). 

A selection of A//p gp values has already been given in Table 2-4 in Section 2.2.6. This new Lewis basicity scale is more comprehensive and seems to be more reliable than the donor number scale. Analogously, a Lewis basicity scale for 88 carbonyl compounds (esters, carbonates, aldehydes, ketones, amides, ureas, carbamates) has been derived from their standard molar enthalpies of complexation with gaseous boron trifluoride in dichloromethane solution [143]. The corresponding Aff Q gp values range from 33 kJ mol for di-t-butyl ketone to 135 kJ mol for 3-diethylamino-5,5-dimethyl-cyclohexen-2-one. 

We can conclude this section on Lewis acid catalysis with a striking example of its use in solving a problem in steroid synthesis. We saw, on p. 309, how 2,6-xyloquinone 6.230 added to the diene 6.229 with inappropriate regioselectivity for steroid synthesis. When boron trifluoride was added to the reaction mixture, it formed a salt 6.284 at the more basic carbonyl group, the one conjugated to the two methyl groups, which is also the less hindered. The result was that the polarisation of the LUMO of the C=C double... 

A facile explanation of the failure of acidity to enhance activity of any of the catalysts while base enhances the activity of a poor catalyst without affecting the active ones is that the Claus active catalysts owe their activity to basic sites. In general, alumina shows strong surface acidity when activated at 450 °C in vacuum, and intensive investigation has shown this to happen in Lewis acid sites (8). There is also evidence that basic sites occur on alumina. Schwab and Krai (9) obtained evidence for basic sites by boron trifluoride adsorption on alumina, and Yamadaya et al. (10) reported up to 0.4 mmole/g of basic sites on alumina from benzoic acid titrations. Pines and Manassen (11) obtained indirect evidence of such sites from studies of the dehydration of primary alcohols. 

Acidity and basicity are fundamental properties of organic compounds, and acid-base reactions are essential steps in many organic transformations. Although there are several definitions of acidity and basicity, the Bransted theory and the Lewis theory are used most often in organic chemistryIn Lewis theory, an acid is an electron pair acceptor and a base is an electron pair donor, as in the reaction of a trialkylamine as Lewis base with boron trifluoride as Lewis acid (equation 7.1). 

Aminoquinolines have been synthesized in a manner analogous to the Niementowski reaction, wherein the carboxylic acid in 1 was replaced with a nitrile (49). This transformation has been carried out under either acidic or basic conditions. Under the former conditions, a variety of Lewis acids have been employed to effect this transformation, including zinc chloride, aluminum trichloride, boron trifluoride diethyl etherate, and titanium tetrachloride. In a representative example, 49 was combined with 50 and heated at reflux in the presence of boron trifluoride diethyl etherate to furnish 51, which contains an embedded 4-aminoquinoline ring system. ... 

Maria, P.C. and Gal, J.F. (1985) A Lewis basicity scale for nonprotogenic solvents enthalpies of complex formation with boron trifluoride in dichloromethane. J. Phys. Chem., 89,1296—1304. 

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