Carbonyl organoaluminum complexes

Enantioselective addition of various nucleophiles to carbonyl compounds has received significant attention in modem asymmetric synthesis. This objective has been accomplished with the aid of chiral organoaluminum complexes. Kee et al. examined organoaluminum-chiral salen complex 55 catalyzed addition of dialkylphosphites 56 to aldehydes, known as Pudovik reaction, to give a-hydroxyphosphonates 57 (Scheme 39) [70, 71]. 

Other than listed above, the asymmetric conjugate addition to a,(3-unsaturated carbonyl compounds has been also conducted by using organoaluminum species. In 1999, Jacobsen et al. described the highly enantioselective conjugate addition of hydrazoic acid (HN3) to a,(3-unsaturated imides catalyzed chiral salen-organoaluminum complex 59 (Scheme 42) [75]. 

In 2002, Mamoka et al. reported the enantioselective Claisen rearrangement of allyl vinyl ethers 61 induced by chiral bis-organoaluminum complex 62 (Scheme 44) [77]. Reactions using mono-organoaluminum complex 63 resulted in the decrease of both chemical yields and enantiomeric excess, indicating the importance of the double coordination between carbonyl group and two aluminum centers of 62. 

As an example of asymmetric intermolecular sigmatropic rearrangement, chiral Lewis acid promoted carbonyl-ene reactions have been utilized as the powerful tool for stereocontrolled carbon-carbon bond formation processes. In 2004, Mamoka et al. demonstrated enantioselective hetero-carbonyl-ene reaction of aldehydes and 2-methoxypropene catalyzed by chiral organoaluminum complex 64, giving enantiomerically enriched (3-hydroxymethylketones (Scheme 45) [78]. 

To realize high activation of carbonyl compounds, Ooi and Maruoka have reported bis(organoaluminum) complex (17) as the bidentate Lewis acid [51]. This Lewis acid (17) prepared by treatment of the corresponding biphenylenediol (18) with 2 equivalent of MesAl in CH2CI2 reacts with carbonyl compounds to form di-a-bonding (double coordination mode such as H) (Scheme 6.42) [52]. In general, the formation of such double coordination mode from excess amount of monodentate Lewis acids thermodynamically disfavors, and single coordination mode is favorably formed. 

Although the ultimate fate of the nitrosyl ligands has not been established, a reasonable pathway for their removal can be postulated based on our IR data and recent literature results. Shriver et al. (42) have found that organoaluminum species will complex with the oxygen of metal-coordinated carbonyl groups. For example, the six-coordinate... 

Organoaluminum compounds are highly oxygenophilic, and hence are capable of forming long-lived monomeric 1 1 complexes with carbonyl substrates. For example, the reaction of benzophenone with McsAl in 1 1 molar ratio gives a yellow, long-lived monomeric 1 1 species which decomposes unimolecularly to dimethylaluminum 1,1-diphenylethoxide after some minutes at 80 °C or many hours at 25 °C [124]. 

Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of ir-Complexes, 1, 239 Insertion Reactions of Compounds of Metals and Metalloids, 5, 225 Isoelectronic Species in the Organophosphorus, Organosilicon, and Organoaluminum Series, 9, 259... 

The chiral oxygenophilic organoaluminum catalyst 12 bearing such a sterically hindered chiral auxiliary unit may form a stable 1 1 complex with benzaldehyde, allowing enantioselective activation of the carbonyl moiety as illustrated in Fig. 1. Then the diene 10a would approach benzaldehyde with an endo alignment of the aldehyde phenyl residue and 10a in order to minimize the steric... 

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