1.2: Lewis Acid-Assisted Lewis Acid (LLA)
In Lewis acid assisted chiral Lewis acids (LLAs), achiral Lewis acid is added to activate chiral Lewis acid via complex formation. The reactivity of LLA is much greater compared to that of achiral Lewis acid, and thus, the latter's presence does not affect the selectivity of the reaction.
Diels-Alder Reaction
The LLA 2a , derived from chiral valine-based oxazaborolidine and SnCl 4 as an activator, has been utilized as an efficient catalyst the for Diels-Alder reaction of wide range of substrates (Scheme \(\PageIndex{1}\)). In this system, the LLA 2a is more reactive compared to SnCl 4 and the ee is not affected because of the addition of excess SnCl 4 .
Additional examples:
The LLA 2b , derived from the complexation of AlBr 3 with chiral oxazaborolidine, has been shown as useful catalyst for Diels-Alder reaction (Scheme \(\PageIndex{2}\)). The observed results suggest that LLA 2b is considerably is more efficient catalyst than the corresponding BLA 1a or 1b since 10-20 mol% of BLA is usually needed for the optimum results.
Additional examples:
[2+2]-Cycloaddition
The utility of LLA 2b has been further extended to [2+2]-cycloaddition reactions of trifluoroethyl acrylate with enol ethers (Scheme 1.1.1). The protonated BLA 1a was found to inferior to LLA 2b in catalyzing the [2+2]-cycloaddition due to side reactions involving the enol ether component. The stereochemical outcome could be predicted using the transition states proposed earlier in Scheme 1.1.3, Lecture 1.
Examples:
Allylation
Maruoka group has developed chiral bis -Ti oxide complex 2c as LLA (Lewis Acid-Assisted chiral Lewis Acid) for the enantioselective allylation of aldehydes with allylbutyltin (Scheme \(\PageIndex{5}\)).
Examples:
For the high reactivity of the catalyst 2c , two different transition states are proposed (Scheme \(\PageIndex{6}\)). In the first, intramolecular coordination of one isopropoxy oxygen to the other titanium has been proposed which could lead to enhancement in Lewis acidity of the original Ti center for the carbonyl activation. In the second system, the simultaneous coordination of the two Ti centers to the carbonyl group has been proposed which may also lead to the high reactivity.
The catalyst 2c has also been found to effective for 1,3-dipolar cycloaddition reaction between diazoacetates and α -substituted acroleins to give 2-pyrazolines with a quaternary carbon centre (Scheme \(\PageIndex{7}\)).
Examples: