Boronate activation, with Lewis adds

Boron trifluoride is also widely used as a Lewis add in industrial processes. It is less active than aluminium trichloride but has the advantages of being more tolerant of air exposure (it is deactivated but not destroyed by water). It is normally used as a complex, such as the etherate, which gets around the difficulties of working with a gas. Several versions of supported BF3 have been reported. The most recent and possibly most interesting for liquid-phase organic reactions involves the complexation of the Lewis acid with the surface of a silica support in the presence of a protic molecule, typically an alcohol (Figure 4.12).119 120... 

Another strategy used to overcome the low activating effect of the boronate substituent was to add Lewis acids to activate the dienophile [74], With methyl acrylate and a stoichiometric amoxmt of EtAlCl2, the reaction occurred at a lower temperature (20 °C, CH2CI2) and shorter reaction time (6 h) to afford the cycloadduct in a better stereocontrol (endo/exo >95 5). 

Allyl silanes react with a wide variety of electrophiles, rather like the ones that react with silyl enol ethers, provided they are activated, usually by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trim ethyls ilyl tri-flate. Electrophiles include the humble proton generated from acetic add. The regiocontrol is complete. No reaction is observed at the other end of the allylic system. All our examples are on the allyl silane we prepared earlier in the chapter. 

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