Carbonyl-Lewis Acid Complexes

T. Ooi, K. Maruoka, Carbonyl-Lewis Acid Complexes, in Modem Carbonyl Chemistry (J. Otera, Ed.), Wiley-VCH, Weinheim, 2000, 1-32. 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

When considering factors that may influence the reactivity and conformation of carbonyl-Lewis acid complexes, primary attention should be given to the modes of coordination of Lewis acids to carbonyl groups, analyzing the exact location of the Lewis acid with respect to its carbonyl ligand. There are several different possible modes of coordination. First, purely electrostatic interaction can be considered, in which the metal is situated at the negative end of the C=0 dipole [C-0-M= 180°... 

Additionally, Lewis acid complexes of carbonyl compounds bearing heteroatom-containing functionality (X) in appropriate proximity are an interesting subject to be addressed. Such chelate-type carbonyl-Lewis acid complex formation is generally a favorable process, and can bring an enhancement of reactivity and selectivity by the effective activation of the carbonyl moiety compared to the nonchelation case, implying considerable utility in organic synthesis [7]. 

The first chapter on these carbonyl-Lewis acid complexes uses information on (1) theoretical study, (2) NMR study, and (3) X-ray crystallographic study. For the rest, the subjects of chelate complexes and bidentate Lewis acid complexes, mainly featuring recent advances, are discussed. 

Theoretical Study of Carbonyl-Lewis Acid Complexes... 

NMR studies have been able to provide a lot of information on the structure of carbonyl-Lewis acid complexes in solution including stoichiometry and conformation, which are of direct importance with respect to the stereochemical outcome of the reactions involving these complexes. 

The aldol adduct (120) is obtained in 59% yield with 2 equiv. of the carbonyl-Lewis acid complex and in 67% with 10 equiv. 

The tridimensional structure of carbonyl-Lewis acid complexes has been determined by X-ray studies. The crystal structure of the benzaldehyde BFs complex, shows that the BF3 group coordinates anti to the Ph group as in I iP Likewise, the crystal structure of (4-(-/-butyl)benzaldehyde)2 SnCl4 shows an anti coordination of SnCU. Importantly, a formyl hydrogen bond has been postulated by Corey as an organizing element to rationalize the outcome of a number of reactions involving aldehyde-metal complexes... 

---