Boronic chiral Lewis acid catalysts

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

The second well-known and much-used carbon-carbon bond forming reaction is a [4 + 2]-cycloaddition, the Diels Alder reaction. Very many chiral Lewis acid catalysts have been used to promote this reaction and a pot-pourri of organo-aluminium, -boron and -copper catalysts are described, in brief, below. 

The first asymmetric Simmons-Smith reaction with a chiral Lewis acid catalyst was introduced in 1994 by Charette and Juteau and featured a chiral boron Lewis acid prepared from tartaric acid [32]. Although this process resulted in excellent enantioselec-tivity, it would not turnover, i.e. the yield was less than 10 %. In the same year Imai, Takahashi and Kobayashi introduced a chiral aluminum Lewis acid that would catalyze the cyclopropanation of allylic aleohols with significant turnover numbers but their system did not lead to asymmetric induction as high as that resulting from the dioxaborolane catalyst [33]. The catalyst is prepared from the bis-sulfonamide 132... 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

CHIRAL LEWIS ACID CATALYSTS ORGANOALUMINUM AND BORON REAGENT... 

Allenyl boronic ester. In principle, using a chiral Lewis acid catalyst, it should be possible to activate carbonyl compounds asymmetrically. In our early work, we focused... 

Asymmetric Aldol Reaction of Difluoroketene Silyl Acetal 1 Catalyzed by Chiral Lewis Acids. We turned our attention to evaluating several chiral Lewis acid catalysts, which are known to be capable of serving as asymmetric catalysts in the aldol reaction of nonfluorinated ketene silyl acetals, for Ifaeir usefulness in the reaction of the difluoroketene acetal 1 with aldehydes. A couple of boron complexes, Masamune s catalyst 6 (55,56) and Kiyooka s catalyst 7 (57, 58), were found to be effective for our study. For Masamune s catalyst 6, the reaction was carried out by adding an aldehyde in nitroethane to a solution of the acetal 1 and the catalyst 6 in the same solvent over 3 h at -78 C with stining at fliat temperature for an additional hour prior to quenching. With Kiyooka s catalyst 7, an aldehyde in nitroethane was added to a solution of the acetal 1 and the catalyst 7 in nitroethane at -45 C for 5 min, followed by stirring at -45 C for 2 h (28,29), Nitroethane is the best medium for the enantioselectivity. 

The most successful approach in this reaction category has been the use of chiral boron Lewis acid catalysts, in the addition of ethyl diazoacetate to imines reported by Wulff (Scheme 1.33) [59-60]. 

Allyl boronates react very slowly with carbonyl compounds as compared to the corresponding allyldialkylboranes, due to the presence of two oxygen atoms on boron which diminish the Lewis acidity of boron. However, the activity of the allyl boronates can be enhanced by the addition of Lewis acid catalysts. There have been two complementary approaches described for the stereoselective allylation with allyl boronates, one involving the use of chiral Lewis acid, and the other involving chiral allyl boronates in conjunction with achiral Lewis acid catalyst. Several chiral fVsymmetric-based 1,2-diols 197 have been employed in combination with SnCLj as a Lewis acid and excellent level of enantioselectivity has been observed for the allylation to furnish homoallylic alcohols 198 with high ee (Equation 8) <2006AGE2426>. 

Alternatively, the allylboration of aldehydes 200 with chiral allyl boronates 199a-d (conveniently prepared from camphorquinone in four steps) also provided the optically active homoallylic alcohols 201a-d with high ee in the presence of achiral Lewis acid catalysts. These boronates are relatively unreactive with aldehydes at low temperatures in the absence of Lewis acid catalyst. However, they furnish low to moderate ee for the allylation at higher temperatures. Hall and co-workers were able to increase the reactivity of the allyl boronates at low temperatures by the addition of strong Lewis acids such as Sc(OTf)3 and obtained the homoallylic alcohols with high ee at low temperatures (Equation 9) <2003JA10160>. 

These chiral C2 symmetric 1,2-diamines have been used to prepare chiral reagents containing aluminum or boron, which have proved to be highly effective Lewis acid catalysts for several synthetic reactions. 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

We have found that chiral boronate complexes with BLA (Brpnsted acid-assisted chiral Lewis acids) to give new catalysts for enantioselective synthesis which achieve selectivity by a double effect of intramolecular hydrogen-bonding interaction and attractive ji-ti donor-acceptor interaction in the transition state by a hydroxy aromatic group [27a],... 

Chiral alkyldihaloboranes are among the most powerful chiral Lewis acids. In general, however, because alkyldihaloboranes readily decompose to alkanes or alkenes as a result of protonolysis or /3-hydride elimination, it is difficult to recover them quantitatively as alkylboronic acids. Aryldichloroborane is relatively more stable and can be reused as the corresponding boronic acid. We have developed chiral aryldichlorobor-anes 23 bearing binaphthyl skeletons with axial chirality as asymmetric catalysts for the Diels-Alder reaction of dienes and a,/3-unsaturated esters (see, e.g., Eq. 37) [36]. 

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. 

During recent years, the homogeneous Lewis acid-catalyzed asymmetric Diels-Alder reactions and hetero-Diels-Alder (HDA) reactions have each undergone extensive study. Various chiral Lewis acids including aluminum, titanium or boron, and chiral ligands such as chiral amino alcohols, diols, salen, bisoxazoline or N-sulfonylamino acids have been used as the catalysts [84]. Much efforts have also been made in the investigation of heterogeneous diastereoselective Diels-Alder reactions. 

Though either enantiomer of a yyn-aldol can be made by using the right auxiliary in an Evans aldol reaction the anti aldols cannot be made this way. The addition of a Lewis acid catalyst transforms the situation.13 Using the valine-derived chiral auxiliary 89, the same Z-boron enolate 111 is used but the aldehyde is added in the presence of a threefold excess of the Lewis acid Et2AlCl. The product is predominantly one enantiomer of an anh-aldol 112. 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

Both enantiomers of binaphthol have found many uses as chiral reagents and catalysts. Thus, they modify reducing agents (e.g., lithium aluminum hydride) for the reduction of ketones to chiral secondary alcohols (Section D.2.3.3.2.) or react with aluminum, titanium or boron compounds to give chiral Lewis acids for asymmetric Diels-Alder reactions (Section D. 1.6.1.1.1.3.) and ene reactions (Section D.I.6.2.). They have also been used as chiral leaving groups in the rearrangement of allyl ethers (Section D.l.1.2.2.) and for the formation of chiral esters with a-oxo acids (Section D. 1.3.1.4.1, and many other purposes. 

Boronic acids and their derivatives are very popular as components of chiral Lewis acids and promoters for various reaction processes [481]. Indeed, the chiral acyloxyb-oranes and the oxazaborolidines (Section 1.2.3.5) described in Chapter 11 made a mark in organic synthesis. Recently, Ryu and Corey extended the apphcation of chiral oxaborolidinium catalysts to the cyanosilylation of aldehydes [482]. Chiral diaz-aborohdine salts were evaluated in the enantioselective protonation of enol ethers [145]. Likewise, a tartramide-derived dioxaborolane is key as a chiral promoter in the asymmetric cyclopropanation of allyhc alcohols [483]. More examples and details on the applications of boronic add derivatives as reaction promoters and catalysts are provided in Chapter 10. 

---