BINOL monomethyl ether

Shibasaki et al. also developed a barium complex (BaB-M, 14, Scheme 5) for the aldol reaction of acetophenone (la), making use of the strongly basic characteristic of barium alkoxide. The catalyst was prepared from Ba(0-z-Pr)2 and BINOL monomethyl ether, and the products were obtained in excellent yield with up to 70% ee (Scheme 6) [8], Shibasaki et al. attempted to incorporate a strong Bronsted base into the catalyst and developed a lanthanide heterobime-tallic catalyst (15) possessing lithium alkoxide moieties, which promoted the aldol reaction with up to 74% ee (Scheme 6) [9]. Noyori and Shibasaki et al. reported a calcium alkoxide catalyst (16) that was prepared from Ca[N(SiMe3)2]2,... 

Yamamoto and coworkers protonated silyl enol ethers with a stoichiometric amount of a complex derived from BINOL and SnCLj giving optically active a-alkyl ketones. A catalytic reaction was developed employing another tin complex derived from BINOL monomethyl ether (LBA), in which 2,6-dimethylphenol was used as the proton source (equation 65). ... 

Shibasaki et al. first reported a chiral barium catalyst prepared from BINOL monomethyl ether and barium alkoxide, which was a good catalyst for asymmetric direct-type aldol reactions (Table 1) [21]. In the presence of a barium catalyst, several aliphatic aldehydes were tested, and the desired cross aldol products were obtained in good yields with moderate to good enantioselectivities. For the substrate BnOC(CH3)2CHO, the best enantioselectivity was observed (Table 1, entry 6). Although there is no definitive experimented proof, the barium catalyst was assumed to be monometallic and could be stored for several months under argon atmosphere. 

Stoichiometric use of BINOL, activated by tin(IV) chloride, for the highly enantioselective protonation of silyl enol ethers and silyl ketene acetals was reported by Yamamoto and coworkers. A remarkable progress came from the use of BINOL monomethyl ether 481 in catalytic amounts, while 2,6-dimethylphenol serves as the stoichiometric proton source. Again, activation by tin(IV) chloride was required to convert silicon enolates 482 and 484 into ketone 483 and carboxylic acid 485, respectively, with high enantiomeric excess. This is one of the very few enantioselective protonations that is not restricted to cyclic enolates, as illustrated by the application of the protocol to sUyl ketene acetal 484. Racemization of the products does not occur to a significant extent (Scheme 5.121) [239]. 

Scheme 5.121 Enantioselective protonation of silicon enolates 482 and 484 using BINOL monomethyl ether 481 in catalytic amounts proposed catalytic cycle.

Yamamoto and Ishihara et al. have revealed that acidic protons in the optically active binaphthol (BINOL) or BINOL monomethyl ether are activated by coordination of SnCU to form a Lewis acid-assisted chiral Bronsted add (LBA) (43) and can be used a s a chiral proton source f or the synthesis of chiral ketones or carboxylic acids from silyl enol ethers or ketene bis (trialky Isilyl) acetals [44]. The chiral LBA system is also effective in the catalytic version with 5-10 mol% of the optically active BINOL monomethyl ether and SnCU in the presence of 2,6-dimethylphenol as a stoichiometric proton source (Scheme 10.22). Further details and backgrounds of the reactions using LBA are reviewed in the first edition of this series [12]. 

The fir st examples of the highly enantioselective protonation of silyl enol ethers, such as (32), have been reported (68-94% ee), using a complex of SnCLt and the monomethyl ether of BINOL (i )-(33). hr this catalytic cycle, the active catalyst is reprotonated by a bulky phenol (Scheme 10).43... 

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See also in sourсe #XX -- [ ]

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