Catalysis by BINOL

Hetero Diels-Alder Reaction. The hetero-Diels-Alder reaction involving glyoxylate as the dienophile provides an efficient access to the asymmetric synthesis of monosaccharides. The hetero Diels-Alder reaction with methoxydienes proceeds smoothly with catalysis by BINOL-TiCl2 to give the cis product in high enantiomeric excess (eq 14). The dibromide affords a higher cis selectivity, however, with a lower enantioselectivity, particularly in the trans adduct. The product thus obtained can be readily converted to the lactone portion of HMG-CoA inhibitors such as mevinolin or compactin. ... 

Reetz reported the catalysis by BINOL-TiCl2 of aldol reactions with aliphatic aldehydes [88]. BINOL-TiCli was prepared by treatment of the lithium salt of BINOL with TiCl4 in ether. After removal of the ether the residue was treated with dry benzene and the solid was separated under nitrogen. Removal of the solvent provided the red-brown complex, which was used as the catalyst for the aldol reaction to give 8 % ee. Later, Mukaiyama reported that use of BINOL-Ti oxide prepared from (i-PrO)2-Ti=0 and BINOL resulted in moderate to high enantioselectivity (Sch. 30) [89]. 

We have previously reported that the hetero Diels-Alder reactions of glyoxylates with 1-methoxy-l,3-butadienes proceed smoothly under catalysis by BINOL-Ti complex to give the cis product with high ee (Sch. 48) [128]. The hetero Diels-Alder products thus obtained can be transformed into monosaccharides [129], The hetero Diels-Alder product can, furthermore, readily be converted into the lactone portion of HMG-Co A inhibitors such as mevinolin or compactin [106] in few steps. 

Fig. 10.2. Structures of complexed aldehyde reagent (a) and transition structure (b) for enantios-elective catalysis of the carbonyl-ene reaction by BINOL-Ti(IV). Reproduced from Tetrahedron Lett., 38, 6513 (1997), by permission of Elsevier.

Asymmetric catalysis of BINOL-Ti complexes in the reaction of aliphatic and aromatic aldehydes with an allylstannane has also been reported independently by Umani-Ronchi [54] and Keck [55]. The former group has suggested that a new complex generated by the reaction of the BINOL-Ti complex with allylstannane is the catalytic species that provides remarkably high enantioselectivity (Scheme 8C.23). It is interesting that no reaction occurs if dry MS 4A... 

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. 

In the aldol reaction of 2-trimethylsilyloxyfuran with aldehydes catalyzed by (BINOL)2Ti complex, a significant impact of the product on the enantioselectivity of the catalysis was observed [124]. As shown in Scheme 14.44, the addition of 5 mol% of the product (82% ee) in the catalyst can enhance the enantiomeric excess of the product from 70% to 96%. Therefore, an asymmetric autoinduction might be involved in the catalytic system. On the basis of this observation. 

For the catalytic asymmetric FC reaction, the application of chiral titanium complexes of BINOL derivatives was first realized by Mikami and coworkers in the reaction of electron-rich aryl and vinyl ethers with fluoral [245]. For the FC reaction of anisole, it was found that the catalytic activity and enantioselectivity of BINOL-Ti catalysts were critically influenced by the substituents of BINOL derivatives. The electron-withdrawing bromo atoms at 6,6 -positions of BINOL turned out to be beneficial to the catalysis, the trifluoroethanol derivatives were obtained in high yield (89%) and up to 90% ee with a p o isomer ratio of 4 1. The FC reaction of vinyl ether with fluoral catalyzed by BINOL-TiCb (10 mol%) gave a mixture of allylic alcohols in which the major isomer was usually the Z-alkene. The increase of bulkiness of silyl group is favorable for the formation of FC products with very high enantiomeric excess (Scheme 14.105). 

The structure of the active catalyst and the mechanism of catalysis have not been completely defined. Several solid state complexes of BINOL and Ti(0-/-Pr)4 have been characterized by X-ray crystallography.158 Figure 2.4 shows the structures of complexes having the composition (BIN0Late)Ti2(0-/-Pr)6 and (BINOLate)Ti3(O-/-Pr)10. 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

In the neutral BIPHEP-Pt complex, the axial chirality of BIPHEP moiety is controlled by chiral diol BINOL as shown in Scheme 8.29. However, the diastereo-meric purity is not high enough (95 5). Therefore, recrystallization is essential to obtain the single BIPHEP-Pt diastereomer and subsequent enantiomer. It has thus been required that complete chirality control of both neutral and cationic BIPHEP-Pt complexes without recrystallization and its application to asymmetric Lewis acid catalysis (Scheme 8.32)." Interestingly, both enantiopure (5)- and (7 )-BIPHEP-Pt complexes can be obtained quantitatively through the... 

Other successful H-bond catalysis apphcations have been introduced by Schaus and Sasai involving asymmetric Morita-Bayhs-Hilhnan (Scheme 11.13c) and aza-Morita-Baylis-Hillman reactions (Scheme 11.13d), respectively. Intriguingly, derivatized BINOL systems 33 and 34 provided optimal selectivities. 

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