Chirale Alkoxytitan(IV)‐Komplexe für enantioselektive nucleophile Additionen an Aldehyde und als Lewis‐Säuren in Diels‐Alder‐Reaktionen

Abstract

Chiral Alkoxytitanium(iv) Complexes for Enantioselective Nucleophilic Additions to Aldehydes and as Lewis Acids in Diels‐Alder ReactionsA Number of chiral 1,2‐1,3 and 1,4‐diols were prepared and used as alkoxy ligands on Ti for enantioselective nucleophilic transfer of methyl, butyl, cyclopropyl, allyl, alkinyl, and phenyl groups to aromatic aldehydes, as well as for the enantioselective[4+2]cycloaddition of acrylate to cyclopentadiene. The 1,2‐diols were pinane diol 7 and 1,2:5.6‐diacetonide‐protected mannitol 9 (Scheme 3) and tartrates. The 1,3‐diols were obtained from the yeast‐reduction products of 2‐oxocyclopentane‐ and 2‐oxocyclohexanecarboxylates and excess MeLi, BuLi, or PhLi (or the corresponding Grignard reagents; see4–6.) As 1,4‐diols, we used the products 2 and 3 from tartrate acetals and methyl or Pheny1 Grignard reagents, the bis(benzaldehyde) acetal 8 of d‐mannitol and o,o'‐binaphthol (22). These diols were attached to the Ti‐atom by azeotropic removal of i‐PrOH from a mixture with [TiCi(i‐PrO)3]. Addition of various organometallic reagents R‐metal (metal = Li, BR3, MgX, MnC1, CuLiR) was followed by combination with aldehydes at – 75., a warm up period, quenching with aqueous KF solution, and workup (for results see Tables 1–6 and Formulae 17–20). The enantiomeric excess of the secondary alcohols obtained varies greatly, certain combinations of chiral ligands, nucleophilic groups, and aldehyde substrates give rise to values as high as 90% ee; see e.g. Table 4. The Ti‐complexes of the general formula [Ti(R*O)2Ci2] or [Ti(R*O)2(i‐PrO)CI] induced the Diels‐Alder addition of methyl acrylate to cyclopentadience to take place at –30.. The best enantioselectivity (50% ee) was observed with the binaphthol derivative (Table 7). The structures of the complexes involved in these reactions are unknown. The substitution on C(2) of the dioxolanes 2 and 3 (derived from tartaric acid) has a pronounced effect on the selectivities of both reactions studied here (Tables 2, 3, and 7). This remote effect (1,6‐distance between the stereogenic acetal‐C‐atom and the Ti‐centers) must be caused by conformational changes in the vicinity of the reactive site, i.e. the TiC bond in the nucleophilic addition reactions and the Ti‐acrylate‐oxygen complexation in the Diels‐Alder reaction.