Benzaldehyde acetal chromium tricarbonyl complex

Chromium tricarbonyl complexed to benzaldehyde acetals can be reduced using lithium 4,4 -di(t-butyl)biphenyl to give benzyllithium complexes that can be further functionahzed using electrophiles (Scheme 94). 

Acetals and ethers can also be used to produce benzylic car-bocations by reacting with a Lewis acid. Treatment of acetals derived from chiral 2-substituted benzaldehyde chromium tricarbonyl complexes with trimethylsilyl triflate followed by reaction with a silyl enol ether give diastereomerically pure alkylation products (Scheme 98). Benzylic alcohols can be... 

Chiral 2-substituted benzaldehyde chromium tricarbonyl complexes have been reacted with chloroacetophenone in the presence of KO-fert-Bu [544], After decomplexation, the E-epoxyketone is obtained with a high selectivity (Figure 6.88). This Darzens reaction with ClCF COO-tert-Bu is poorly stereoselective. Condensation of the same aldehydes with methyl aaylate or acrylonitrile in the presence of DABCO, followed by decomplexation, also leads highly selectively to P-hydroxyesters or -nitriles 6.105 (Y = COOMe or CN) [547] (Figure 6.88). An anti aldol product is also obtained with a high selectivity from a chromium complex and the titanium enolate of PhCF OCF COS-tert-Bu at -78°C [1281, 1282], Chiral aminals of a-ketoaldehydes react with lithium or sodium enolates of ethyl acetate. After treatment with acid, compounds 6.106 are obtained with a high enantiomeric excess (Figure 6.88). 

Although many oxidizing reagents remove the chromium tricarbonyl group, benzylic alcohols can be oxidized to benzaldehydes using dimethyl sulfoxide with acetic anhydride, trifluoroacetic anhydride, or sulfurtrioxide with minimal decomplexation. Asymmetric oxidation of aUcylthio-substituted complexes can be achieved using titanium tetraisopropoxide and an optically active tartrate ester (Scheme 108). Dimethyloxirane can also be used to oxidize sulfides to sulfoxides. 

It has been observed that in basic media aromatic hydrocarbons and benzylmethyl ethers bound to -Cr(CO)3 are easily nitrosated at the benzylic position. A review type article discusses the stereoselective manipulation of acetals derived from o-substituted benzaldehyde chromium tricarbonyls. The diastereoselective synthesis of a range of substituted cyclohexadienes has been reported from enandomerically pure (2-phenyl-4,S-dibydroxazole)chromium tiicaibonyl complexes. Aromatic nucleophilic substitution on halaogenoarene tricarbonylchromium complexes gives rise to a series of complexed aniline derivatives. The diastereoselective 1,4-addition of organocuprates to o-substituted-phenyI-(E)-enone chromium carbonyl conq)lexes provides a new method for remote stereocontrol at the 1,3, and S-positimis of the side chains. ... 

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochi-ral and therefore the corresponding chromium tricarbonyl compounds are chiral. (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25). Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26). The optically active chromium complexes can be prepared by di-astereoselective ortho lithiation of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation of the chiral ort/io-substituted benzaldehyde am-inals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes. The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions. ... 

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