The second trial was carried out using ethanolic solutions of aromatic aldehydes, malononitrile, and primary amines in the presence of Lewis acids
as catalysts such as Al[Cl.sub.3], Zn[Cl.sub.2], and Fe[Cl.sub.3].
This might be due to a decrease in Lewis acidity of the modified HY catalyst and a decrease in contact probability of Lewis acid
sites with sugar caused by the outer silicalite-1 layer.
It is noteworthy that the addition of strong Lewis acids
is not necessary to promote the glycosylation; the use of relatively weak Lewis acids
like Sn[Cl.sub.2] is sufficient to activate the considerably strong C-F bond of glycosyl fluorides.
Relatively high polymerization activities are achieved if strong Lewis acids
or combinations thereof are selected which achieve both, (i) alkylation of the metal complex and (ii) formation of separated non-coordinating ions upon contact with the alkylated lanthanide complex.
Among the Lewis acids
, [BF.sub.3] is the most frequently used as an initiator for cationic ring-opening polymerization.
Complexes formed by Lewis acids
and bases are widely known and of great importance in modern chemistry.
de Nice-Sophia Antipolis-CNRS, France) should allow for the judicious choices of a few reference Lewis acids
in order to construct Lewis basicity scales that cane be used as a general guide to basicity.
The procedure is based on molecules called Lewis bases, which carry a lone pair of electrons and can bond with so-called Lewis acids
to make a molecule called an adduct.
These Lewis acids
can bind the cis-1,2- or -1,3diols of glucose covalently to form 5- or 6-part rings.
Metallic catalysts have been reported as enhancers to the production of furfural, they may act as Lewis acids
promoting the reaction or may stabilize the intermediates in the dehydrocyclization of pentoses.
For example, dehydrogenation cycling taking place in the presence of Lewis acids
. However, the increase of the carbonisation residue is not very significant (Mochida, 1975).