Catalysts borane

The borane catalyst 4 is also effective in the Diels-Alder reaction of furan (Scheme 1.11). In the presence of a catalytic amount of this reagent a-bromoacro-lein or a-chloroacrolein reacts with furan to give the cycloadduct in very good chemical yield with high optical purity [6d]. 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Entry Alkene R= Borane Catalyst (solvent) Yield/% 11 12 Rrf. 

Figure 11.3 Mechanism of the reduction of ketone by borane catalysts. ...

To obtain a good enantiomeric excess, the ligand synthesis and the reduction reaction need to be carried out under strictly anhydrous conditions. The addition of the substrate needs to be as slow as possible. Table 11.3 gives some examples of the different substrates that can be reduced by the hydro-xysulfoximine-borane catalyst described. Other examples are given in the comparative Table 11.4. Concerning the synthesis of the catalyst, the yield can dramatically decrease if the reaction conditions are not strictly anhydrous. 

Table 11.3 Reduction of ketones by hydroxysulfoximine-borane catalyst (results according to the literature).

Some chiral oxazaphospholididine-borane catalysts can be used for enantioselective reduction of prochiral ketones by borane-THF or bor-ane-dimethyl sulfide complex (Scheme 19) (44). 

Kumagai, T., Itsuno, S. Asymmetric Aiiyiation Polymerization Novel Polyaddition of Bis(allylsilane) and Dialdehyde Using Chiral (Acyloxy)borane Catalyst. Macromolecules 2000, 33,4995-4996. 

Masamune et al. examined the catalytic activity of several boron Lewis acids derived from BH3 THF and the p-toluenesulfonamides of simple a-amino acids towards the aldol reaction of benzaldehyde with TMS enolate 48 [121]. As a result, the borane catalysts derived from a,a-disubstituted glycine p-tolueriesulforiarriides were found to have high activity. The disubstitution would accelerate the second step (Step II) of the catalytic cycle (Scheme 10.43). On the basis of this observation, they developed chiral borane catalysts 47 c and 47 d, which enable highly enantioselective aldol reactions of KSA and thioketene silyl acetals (84—99% ee with 48). 

Chiral borane catalyst 47g, prepared from N-losyl-(a.S, /j R)-/i-melhyltryptophari and (p-chlorophenyl)dibromoborane, is fairly effective in asymmetric aldol-type reaction of 1,3-dioxolanes bearing an aryl or vinyl group at fhe 2-position (Scheme 10.45) [125]. The ring-cleavage products can be converted into free aldols without epimerization by iodination and subsequent reduction. The chiral borane-promoted reaction wifh 48 is very valuable for asymmetric desymmetrization of symmetric 1,3-dioxolanes and 1,3-dioxanes leading to mono protected 1,2- and 1,3-diols, respectively [126]. 

With the exception of 3.9, these borane catalysts give lower selectivities with enoxysilanes of propionic esters or ethylketones (< 80%) [796, 1302], Using 3.9 as a catalyst, high diastereo- and enantioselectivities are observed in the reactions of the E-ketene silylacetal of phenyl propionate with a,P-unsaturated aldehydes [788] and in the reaction of the enoxysilane of diethylketone with PhCHO [787] (Figure 6.95). All these results are interpreted by acyclic transition state models in which steric repulsions are minimized (Figure 6.95). 

Recognizing that FLP activation of H2 requires a bulky base and acid combination, we probed the question of whether a bulky imine could act both as the base component of an FLP and as the substrate for catalytic reduction. Indeed, a number of aldimines and ketimines could be hydrogenated with a catalytic amount of B(QF5)3 under H2 in a facile manner (Table 11.2, Scheme 11.13) [47]. As with the phosphino-borane catalyst, dx-triphenylaziridine is also reduced under these conditions to N-(l,2-diphenylethyl)aniline (Table 11.2) [47]. 

In a very recent paper, this group has extended this strategy, developing chiral borane catalysts for the enantioselective imine reduction with enantioselectivities as high as 84% [20]. 

Hydrosilylation ofmethyldiundecenylailane (68) yielded hyperbranched polymer (69) having terminal olefins [40]. Subsequent hydroboration with bis(pentafluoroph-enyl) borohydride (70) gave the polymeric borane catalyst (71) [41] (Scheme 19.17). The polymeric borane was tested as cocatalyst in the metallocene-catalyzed polymerization of propylene. Activation through the polymeric borane (71) led to higher activities compared to that of B(C6Fs)3. 

In another frustrated Lewis pair route, a highly enantioselective metal-free hydrogenation of imines uses a BlNAP-derived diene as a ligand hydroboration of the alkenes in situ with HB(CgF5)2 generates a chiral bis-borane catalyst. ... 

Reetz and coworkers introduced the cyclic chlorodialkylboron Lewis acid (75) (Equation 48) [46], and Kiyooka and coworkers made use of acyloxyborane (76) (Equation 49) [47] in enantioselective Mukaiyama-aldol reactions that employ stoichiometric amounts of the respective boron Lewis acids. Both species give high enantioselectivity in the formation of the desired aldol adducts. After Kiyooka s report of (76), various boron catalysts derived from chiral amino acids appeared in the literature. As such, Masamune and coworkers introduced (77) and (78) [48], Kiyooka and co workers introduced (79) [49], and Corey and co workers introduced (80) [50] as chiral acyloxy borane catalysts for enantioselective aldol reactions (Figure 5.7). 

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