Lewis binol complexes

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... 

Since dienolates 1 and 2 represent diacetate synthons, the dienolate derived from 6-ethyl-2,2-dimethyldioxinone can be seen as a propionate-acetate syn-thon. The synthesis of the corresponding dienolate provides a mixture of the E and Z enolates in a 3 5 ratio. The reaction with Ti-BINOL complex 5 generates a 5 1 mixture with the syn isomer as the major diastereomer. After separation of the diastereomers, the enantiomeric excess of the syn isomer was determined to be 100%. The anti isomer was formed in 26% ee. The same transformation performed with boron Lewis acid 7 gave the anti isomer as the major compound, but only with 63% ee. The minor syn isomer was produced with 80% ee. The observed selectivity could be rationalized by an open transition state in which minimization of steric hindrance favors transition state C (Fig. 1). In all three... 

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

A spectacular activation of the chiral zirconium-BINOL Lewis acid complex was achieved by the addition of the (achiral ) r-butyl-calix[4]arene. Less than 2% of the catalyst were sufficient in the enantioselective allylation of various aldehydes by allyltributyltin to reach enantiomeric excesses of more than 90%, see Casolari, S. Cozzi, P. G. Orioli, P. Tagliavini, E. Umani-Ronchi, A. Chem. Commun. 1997, 2123-2124. 

Kobayashi and co-workers. used zirconium-based bromo-BINOL complex for the catalytic enantioselective Mannich-type reaction. The o-hydroxyphenyl imine 3.36 chelates the Zr(IV)(BrBINOL)2 to form the activated chiral Lewis acid complex A. The ketone acetal 3.37 reacts with the Lewis acid complex A to give the complex B. The silyl group is then transferred to the 3-amino ester to form the product 3.38 and the catalyst Zr(BrBINOL)2 is regenerated, which is ready for binding with another imine molecule (Scheme 3.16). 

Shibasaki and Groger developed lanthanide/alkali binapthoxide-based Lewis acid-Brpnsted base bifunctional catalysts [44]. One such example, the (R,R)-Ln-M-linked BINOL complex. 

Aluminmn catalysts derived from the three BINOL derivatives outlined in Table 14 have been used in the asymmetric cycloaddition of the A-crotyloxazolidinone 175 and cyclopentadiene. These reactions are slower and require the use of stoichiometric amounts of catalyst. Although the dienophiles 175 are bidentate and should lead to a more conformationally restrained dienophile-Lewis acid complex, asymmetric induction is quite low. 

The catalytic asymmetric epoxidation of a,/5-unsaturated ketones with hydroperoxides such as tert-butyl hydroperoxide (TBHP) and cumene hydroperoxide (CMHP) can be carried out at ambient temperature by using alkali-metal free Ln-BINOL complexes (eq. (22)) [184]. The oligomeric structure of the catalyst is assumed to play a key role that is, the Ln alkoxide moiety acts as a Brpnsted base, activating a hydroperoxide molecule, while another Ln metal ion acts as a Lewis acid, both activating and controlling the orientation of the enone. 

The BINOL/BINAP Lewis acid complexes and the CAB catalyst are complementary in the following respects in general, the BINOL/BINAP-Lewis acid complexes provide excellent enantiocontrol in the reactions of aldehydes with allyltri-n-butylstannane, but poor diastereocontrol (syn anti) in the reactions of aldehydes with crotyltri- -butylstannane. In contrast, when the CAB catalyst is used to promote the reaction of aldehydes and crotylsilane or crotylstannane reagents, excellent levels of diastereo- and enantioselectivity are achieved, while in the corresponding reactions with allyltri-n-butylstannane poor levels of enantioselectivity are realized. 

Radical reactions are also valuable strategies for the formation of quaternary carbon centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to a chiral binol ligand (103) in the allylation of -iodolactones 101 (Eq. (13.31), Table 13-6) [43]. It was established that diethyl ether as an additive in these reactions dramatically increases product enantioselectivities (compare entries 1 and 2, Table 13-6). Catalytic reactions were also demonstrated (entry 3) with no appreciable loss of selectivity. A proposed model for how diethyl ether functions to enhance selectivity in the enantioselective formation of these quaternary chiral centers is shown in 104. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

Radical reactions are also valuable strategies for the formation of quaternary carbon-based centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to chiral BINOL ligand 26 in the allylation of a-iodolactones 24 (Eq. 9, Table 1) [13]. 

Various asymmetric ene reactions have been reported and particular success has been achieved with the carbonyl ene reaction of glyoxylate esters and chiral Lewis acid catalysts. For example, 2,2 -binaphthol (BINOL) complexes of titanium(IV)... 

Titanium complexes are often encountered in Lewis acid-catalysed reactions. This is certainly true for catalysed aldol reactions. Mikami and Matsukawa demonstrated that titanium/BINOL complexes e.g. complex (7.20) afforded high yield and enantioselectivity in the aldol reactions of thioester ketene silylacetals with a variety of aldehydes. In contrast to some of the aldol reactions described above, the stereochemistry of the adducts is dependant on the geometry of the enol ether. Thus, reaction of the (B)-enol ether (7.21) with aldehyde (7.22) yields the sy -aldol adduct (7.23) predominantly while the (Z)-e.no ether (7.24) results in isolation of the anti-adduct (7.25) as the major product. The authors invoke a closed silatropic ene transition state (structure (7.26) for syn-transition state), substantiated by suitable crossover experiments, to explain the diastereoselectivities... 

Markd and co-workers also explored the use of binol complexes as Lewis acid catalysts for [4-t-2] cycloadditions of 3-methoxycarbonyl-2-pyrone. Marko reported modest to excellent levels of stereocontrol in the cycloadditions of a variety of vinyl ethers with this pyrone (Table 1 i)."b-d,6i gjjjj better results are reported when vinyl thioethers function as the dienophile. It is important to note that these impressive stereochemical results are obtained with catalytic amounts (0.1-0.2 equivalents) of Lewis acid. 

The postulated catalytic cycle of the asymmetric epoxidation reaction is shown in Figure 13.10. A lanthanide metal alkoxide moiety changes to a rare earth metal-peroxide through proton exchange (I). In this step, lanthanide metal alkoxide moiety functions as a Bronsted base. The rare earth metal-BINOL complex also functions as a Lewis acid to activate electron-deficient olefins through monoden-tate coordination (II). Enantioselective 1,4-addition of rare earth metal-peroxide gives intermediate enolate (III), followed by epoxide formation to regenerate the catalyst (IV). 

The use of lanthanide complexes in asymmetric catalysis was pioneered by Danishefsky s group with the hetero-Diels-Alder reaction,and their utility as chiral Lewis acid catalysts was shown by Kobayashi. The Brpnsted base character of lanthanide-alkoxides has been used by Shibasaki for aldol reactions, cyanosilylation of aldehydes and nitroaldol reactions.The combination of Lewis acid and Brpnsted base properties of lanthanide complexes has been exploited in particular by Shibasaki for bifunctional asymmetric catalysis. These bimetallic lanthanide-main-group BINOL complexes are synthesized according to the following routes ... 

Noteworthy is the fact that the use of Ti(IV)-(S)-BINOL complexes resulted in low yields with moderate enantioselectivities, implicating the importance of enhanced Lewis acidity in chiral bis-titanium catalyst (12). 

In 1995, Shibasaki s group disclosed the first example of multifunctional heterobimetallic complex-catalyzed Michael reaction of malonate to enone. The chiral catalyst, lanthanum-sodium-BINOL complex (/ )-LSB, was prepared from La(Of-Pr)3, (/ )-BINOL, and NaOt-Bu. Two different metals indeed play their unique roles to enhance the reactivity of both substrate partners by locating them in designated positions. The Lewis acidic metal (lanthanides or group 13 elements) has been found capable to activate the acceptor, whereas the second metal center (alkali metals bound to a Brpnsted base) assists the coordination of enolate. The proposed catalytic cycle is shown in Scheme 9.5. 

In 1986, Reetz et al. provided the first indication that asymmetry in catalyzed Mukaiyama aldol reactions could be induced by substoichiometric quantities of chiral Lewis acid complexes. The Ti(IV)-BINOL complex 71 and the Al(III)-based Lewis acids 72 and 73 were evaluated as... 

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