Carbonyl-Lewis Acid Chelation Complexes

The most feasible pathway to coupled products is intramolecular ketyl addition to the Sm -coordi-nated ketone (see Scheme 6). Several examples of ketyl addition to Lewis acid activated carbonyls have been reported in the literature. Clerici and Porta have demonstrated in detailed experiments that intermolecular addition of ketyls to carbonyls can be a rapid process. Generally, ketyl addition to carbonyls is a reversible reaction. However, reversibility can be greatly affected by Lewis acid chelation of the complex, and further reduction of the radical intermediate (8) by the second equivalent of Smh would serve to make the process irreversible. ... 

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. 

Chelation Control. The stereoselectivity of reduction of carbonyl groups can be controlled by chelation when there is a nearby donor substituent. In the presence of such a group, specific complexation among the substituent, the carbonyl oxygen, and the Lewis acid can establish a preferred conformation for the reactant. Usually hydride is then delivered from the less sterically hindered face of the chelate so the hydroxy group is anti to the chelating substituent. 

The adduct Ru3(CO)12,AlBr3 is formed via treatment of Ru3(CO)12 with AlBr3 in toluene.5 This diamagnetic red solid reverts to the parent on exposure to air or acetone. It displays a low-frequency carbonyl stretching band at 1535 cm-1 characteristic of Lewis-acid-co-ordinated bridging carbonyls. Reaction of Ru3(CO)12 with l,2-bis(dimethylsilyl)ethane has been shown6 to yield the stable white chelate complex (1 M = Ru) 13C n.m.r. studies indicate that this species is stereochemi-cally rigid. 

The a-chiral ketone from Figure 10.18—the a-substituent is a benzyloxy group—is reduced to the Cram chelate product by Zn(BH4)2, a Lewis acidic reducing agent. The Zn2 ion first bonds the benzyl and the carbonyl oxygen to a chelate. Only this species is subsequently reduced by the BH 4 ion because a Zn2 -complexed C=0 group is a better elec-... 

Recently, catalytic amounts of Lewis acids have been used in the reaction of allyllic tri- -butyltins with carbonyl compounds. Maruoka et al. report the remarkably chemoselective allylstannation of o-anisaldehyde over/>-anisaldehyde, catalyzed by BlCgFsL.1 z Piers et al. report that the chemoselectivity observed does not rely on classical chelation control. They conclude that stannylium ion pair [Bu3Sn(o-anisaldehyde)2]+[o-ArCH(allyl)OB(G6Fs)3] is the active species which is preferentially formed over the complex of/>-anisaldehyde with BlCgFsL (Equation (44)).143 Lambert et al. report a similar formation of stannyl cation from allyltri- -butyltin and trityl (CgFsLB-.144... 

Lewis acids that possess two empty sites of coordination are ct )able of imposing conformational constraints on a carbonyl ligand either by forming 2 1 complexes or via chelation when a second basic site is present in the ligand. Despite the popularity of the chelation-controT model since its original proposal in 1952 by Cram and coworkers, ° rigorous and direct experimental evidence for chelation in a- and (3-alkoxycarbonyls was only obtained recently from variable temperature H and... 

Although crystal structures of other bimolecular complexes of carbonyls with boronic Lewis acids have not been reported, a number of intramolecular chelates have been detected in the solid state. In all these cases boron is found to lie in the direction of the carbonyl lone pair with no more than 11 distortion away from the best plane of the carbonyl group. The average B—O bond length is 1.581 0.019 A and the B—O—C angle lies between 112 and 119. ... 

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