Tricarbonyl chromium reaction with benzaldehyde

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... 

As with the corresponding allylboronate, the enantioselectivity of reactions with (3-alkoxy and conjugated aldehydes are lower (55-74% ee). In the case of benzaldehyde (91%, 66% ee), selectivity can be improved by the use of the derived chromium tricarbonyl complex. The homoallylic alcohol is obtained after... 

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). 

Reactions of reagents 1 and 2 with metal-complexed aromatic, propargylic and dienylic aldehydes provides homoallylic alcohol products with improved selectivity compared to their uncomplexed counterparts. The reaction of benzaldehyde chromium tricarbonyl complex 14 with (R,R)-1 followed by oxidative decomplexation provided ( -15 in 90% yield and 83% ee. The (JE)-crotylboration of 14 with (R,R)-2 provided 16 in 90% yield and 92% ee. Reaction of aldehyde 14 with (. -crotylboronate 3, however, provided adduct 17 in only 41% ee. 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

Ort/io-substituted benzaldehyde complexes have been prepared in high enantiomeric purity (97% ee), and in a one-pot sequence, from an optically pure hydrazone derivative, readily available from -q -benzaldehyde chromium tricarbonyl and SAMP [(S)-l-amino-2-(methoxymethyl)pyrrolidine]. The novelty derives from the combined use of a diastereoselective orthoaddition reaction of an organolithium nucleophile and a hydride abstraction with a triphenylmethyl cation. The subsequent acid hydrolysis serves to remove the hydrazone group, thus liberating the aldehyde functionality (Scheme 6.13). 

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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