Methyl groups 1,1,1-trifluorides

Triethylsilane in the presence of boron trifluoride [772] or trifluoroacetic acid [777] also reduced the aldehyde group to a methyl group. 

The methyl group in aromatics could be converted similarly to the trifluoromethyl group using the chlorine, hydrogen fluoride and aluminum trifluoride system as a catalyst.21 22... 

In the case of compound 216, treatment with boron trifluoride etherate gave a mixture of 217 and 218 (75). The transformation 216 217 must have occurred via the intermediate 219 as described above. The formation of 218 is the result of the migration of the methyl group followed by the loss of a proton (219 - 220 218) These two steps are equivalent to 200 201 and 195 198. 

The ethyl ester 258 (Eq. 103) has been recovered unchanged after treatment with the boron trifluoride acetic acid complexin>, whereas cyclopropane 255 with an additional 2-methyl group opens under these conditions to provide y-butyrolactone 257112). Apparently the intermediate tertiary carbenium ion 256 is sufficiently stabilized by the trimethylsilylmethyl and the methyl group to be generated from 255. 

Carbon-centered nucleophiles can also be used to advantage in the reaction with epoxides. For example, the lithium enolate of cyclohexanone 96 engages in nucleophilic attack of cyclohexene oxide 90 in the presence of boron trifluoride etherate to give the ketol 97 in 76% yield with predominant syn stereochemistry about the newly formed carbon-carbon bond <03JOC3049>. In addition, a novel trimethylaluminum / trialkylsilyl triflate system has been reported for the one-pot alkylation and silylation of epoxides, as exemplified by the conversion of alkenyl epoxide 98 to the homologous silyl ether 99. The methyl group is delivered via backside attack on the less substituted terminus of the epoxide <03OL3265>. 

Boron trifluoride has electron-withdrawing fluorine atoms trimethyl boron has electron-donating methyl groups. As a result, boron trifluoride should be a stronger acid, with a more positive boron atom, toward ammonia. There are no significant steric interactions with ammonia. 

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... 

Reactions of Enol Ethers and Esters.—When enol acetylation of a steroid 4-en-3-one with acetic anhydride is catalysed by perchloric acid (or boron trifluoride ), C-acetylation of the enol acetate (373) occurs on prolonged reaction. This step, probably involving acetyl perchlorate ior airs, is akin to Friedel-Crafts acylation of aromatic compounds. Both 6-acetylation and 2-acetyIation are known, although each is prevented by the presence of a methyl group at the site. The products can be of various enolic types (e.g. 374 and 375). 

Similar attempted functionalisation of the 5)5-methyl group of Westphalen s diol, using either the Barton or hypoiodite reactions, was unsuccessful although lead tetra-acetate gave the epoxide (607), which could be opened with boron trifluoride etherate after reduction of the 9,10-unsaturation. 

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