Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

BINOL 3,3 -silylated

Several catalysts based on Ti(IV) and BINOL have shown excellent enantiose-lectivity in Mukaiyama aldol reactions.156 A catalyst prepared from a 1 1 mixture of BINOL and Ti(0-i-Pr)4 gives good results with silyl thioketene acetals in ether, but is very solvent sensitive.157... [Pg.128]

Enantioselective protonation of silyl enol ethers using a SnCl4-BINOL system has been developed (Scheme 83). 45 This Lewis-acid-assisted chiral Bronsted acid (LBA) is a highly effective chiral proton donor. In further studies, combined use of a catalytic amount of SnCl4, a BINOL derivative, and a stoichiometric amount of an achiral proton source is found to be effective for the reaction.346 347... [Pg.435]

A similar enantiomer-selective activation has been observed for aldol " and hetero-Diels-Alder reactions.Asymmetric activation of (R)-9 by (/f)-BINOL is also effective in giving higher enantioselectivity (97% ee) than those by the parent (R)-9 (91% ee) in the aldol reaction of silyl enol ethers (Scheme 8.12a). Asymmetric activation of R)-9 by (/f)-BINOL is the key to provide higher enantioselectivity (84% ee) than those obtained by (R)-9 (5% ee) in the hetero-Diels-Alder reaction with Danishefsky s diene (Scheme 8.12b). Activation with (/ )-6-Br-BINOL gives lower yield (25%) and enantioselectivity (43% ee) than the one using (/f)-BINOL (50%, 84% ee). One can see that not only steric but also electronic factors are important in a chiral activator. [Pg.231]

BINOL derivative SnCl4 complexes are useful not only as artificial cyclases but also as enantioselective protonation reagents for silyl enol ethers. " However, their exact structures have not been determined. SnCl4-free BINOL derivatives are... [Pg.373]

The Akiyama group tested various BINOL phosphates 3 as catalysts for the indirect Mannich reaction of aldimines 8 derived from 2-aminophenol with silyl ketene acetals 9 (Scheme 4). All of these Brpnsted acids furnished P-amino ester 10a in (nearly) quantitative yields. Both the reaction rates (4-46 h) and the enantioselectivities (27-87% ee) were strongly dependent on the nature of the substituents at the 3,3 -positions. [Pg.400]

Three years after the discovery of the asymmetric BINOL phosphate-catalyzed Mannich reactions of silyl ketene acetals or acetyl acetone, the Gong group extended these transformations to the use of simple ketones as nucleophiles (Scheme 25) [44], Aldehydes 40 reacted with aniline (66) and ketones 67 or 68 in the presence of chiral phosphoric acids (R)-3c, (/ )-14b, or (/ )-14c (0.5-5 mol%, R = Ph, 4-Cl-CgH ) to give P-amino carbonyl compounds 69 or 70 in good yields (42 to >99%), flnfi-diastereoselectivities (3 1-49 1), and enantioselectivities (72-98% ee). [Pg.416]

BINOL-derived titanium complex was found to serve as an efficient catalyst for the Mukaiyama-type aldol reaction of ketone silyl enol ethers with good control of both absolute and relative stereochemistry (Scheme 8C.24) [57]. It is surprising, however, that the aldol products were obtained in the silyl enol ether (ene product) form, with high syn-diastereoselec-tivity from either geometrical isomer of the starting silyl enol ethers. [Pg.562]

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... [Pg.400]

Regioselective substitution reactions of a series of 2- and 3-hydroxybiaryls including BINOL have been performed via a new directed orf/io-metallation procedure.75 O-Aryl AMsopropylcarbamates, conveniently prepared from phenols and isopropyl isocyanate, have been temporarily and in situ N-protected by means of silyl inflates to form stable intermediates for low-temperature lithiation reactions using n-BuLi-TMEDA in diethyl ether. The IV,IV-dialkyl aryl O-sulfamate has been reported as a new directed metallation group.76... [Pg.261]

Yamamoto et al. reported the asymmetric (9-nitrosoaldol reaction using silyl enol ethers in the presence of the silver catalyst.32 In order to achieve this reaction, they developed a novel combination of silver and a chiral phosphite derived from BINOL. The disilanyl enol ether was used to ensure high yield and enantioselectivity. The reaction was conducted with disilanyl enol ether and nitrosobenzene in the presence of AgBF4 and the chiral phosphite ligand in THF to produce the O adduct with high regio- and enantioselectivity (Table 9.14). In addition, a chiral silyl enol ether could be used as a substrate. The reaction was conducted with chiral silyl enol... [Pg.280]

If the metal-binaphthyl complex is not fitted directly into the cyclic transition state, it becomes difficult to explain the asymmetric inductions observed. The following rule seems to be generally valid for both BINOL and BINAP complexes The complexation of carbonyl or imine moieties by (R)-binaphthyl-metal complexes results in a shielding of the si face, the reaction proceeds from the re face. Correspondingly, the opposite principle applies when (STbinaphthyl complexes are used. All aldol reactions and carbonyl-ene reactions which are catalyzed by binaphthyl complexes abide by this rule [18], and the scheme can also be applied to the addition of ketene-silyl-acetals to imines with boron-BINOL catalysts [19]. [Pg.161]

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). [Pg.129]

Mukaiyama Aldol Condensation. The BINOL-derived titanium complex BINOL-T1CI2 is an efficient catalyst for the Mukaiyama-type aldol reaction. Not only ketone silyl enol ether (eq 25), but also ketene silyl acetals (eq 26) can be used to give the aldol-type products with control of absolute and relative stereochemistry. [Pg.89]

Mukaiyama Aldol Condensation. As expected, the chiral titanium complex is also effective for a variety of carbon-carbon bond forming processes such as the aldol and the Diels-Alder reactions. The aldol process constitutes one of the most fundamental bond constructions in organic synthesis. Therefore the development of chiral catalysts that promote asymmetic aldol reactions in a highly stereocontrolled and truly catalytic fashion has attracted much attention, for which the silyl enol ethers of ketones or esters have been used as a storable enolate component (Mukaiyama aldol condensation). The BINOL-derived titanium complex BINOL-TiCl2 can be used as an efficient catalyst for the Mukaiyama-ty pe aldol reaction of not only ketone si ly 1 enol ethers but also ester silyl enol ethers with control of absolute and relative stereochemistry (eq 11). ... [Pg.93]

BINOL-Me, and stoichiometric amounts of 2,6-dimethylphenol as an achiral proton source, protonation of the ketene bisftrime-thylsilyl)acetal derived from 2-phenylpropanoic acid proceeds at —80°C to give the (5)-carboxylic acid with 94% ee. (/ )-BINOL-Me is far superior to (/ )-BINOL as a chiral proton source during the catalytic protonation, and 2,6-dimethylphenol is the most effective achiral proton source. In addition, it is very important that the molar quantity of SnCU should be less than that of (/ )-BINOL-Me to achieve a high enantioselectivity. For the reaction of 2-phenylcyclohexanone, however, the use of tin tetrachloride in molar quantities lower than BINOL-Me remarkably lowers the reactivity of the chiral LBA (eq 3). Excess SnCLt per chiral proton source, in contrast, promotes this protonation. In the protonation of silyl enol ethers less reactive than ketene bis(trialkylsilyl) acetals, chelation between excess tin tetrachloride and 2,6-dimethylphenol prevents the deactivation of the chiral LBA. [Pg.366]

To demonstrate the synthetic usability of the isomerization, a one-pot procedure from the racemic silyl enol ether to the (5)-2-phenylcyclohexanone was developed by combining the isomerization with subsequent enantioselective protonation catalyzed by (i )-BINOL-Me in the presence of 2,6-dimethylphenol, tin tetrachloride, and MeaSiCl (Eq. 96). We also succeeded in the enantiomer-selective isomerization of racemic silyl enol ethers. For example, during isomerization of the same racemic silyl enol ether with 5 mol % (i )-BINOL-Me-SnCl4 at -78 °C for 2 min, the (f )-silyl enol ether was recovered in 42 % yield with 97 % ee. This absolute stereopreference is consistent with that in the above enantioselective protonation (Eq. 97). [Pg.435]

The aldol reaction of a silyl enol ether proceeds in a double and two-directional fashion, upon addition of an excess amount of an aldehyde, to give the silyl enol ether in 77 % isolated yield and more than 99 % ee and 99 % de (Sch. 33) [92]. This asymmetric catalytic aldol reaction is characterized by kinetic amplification of product chirality on going from the one-directional aldol intermediate to the two-directional product. Further transformation of the pseudo C2 symmetric product still protected as the silyl enol ether leads to a potent analog of an HIV protease inhibitor. Kinetic resolution of racemic silyl enol ethers by the BINOL-Ti catalyst (1) has been reported by French chemists [93]. [Pg.819]

Carreira employed a chiral BINOL-derived Schiff base-titanium complex as a catalyst for aldol reactions with acetate-derived ketene silyl acetals (Sch. 38) [100]. The catalyst was prepared in toluene in the presence of salicylic acid, which was reported to be crucial to achieving high enantioselectivity. A similar Schiff base-titanium complex is also applicable to the carbonyl-ene type reaction with 2-methoxypropene (Sch. 39) [101]. Although conducting the reaction in toluene or ether solution provided no addition product, excellent chemical yield and enantioselectivity were attained by the use of 2-methoxypropene as a solvent. [Pg.822]

Keck also investigated asymmetric catalysis with a BINOL-derived titanium complex [102,103] for the Mukaiyama aldol reaction. The reaction of a-benzyloxyalde-hyde with Danishefsky s dienes as functionalized silyl enol ethers gave aldol products instead of hetero Diels-Alder cycloadducts (Sch. 40) [103], The aldol product can be transformed into hetero Diels-Alder type adducts by acid-catalyzed cyclization. The catalyst was prepared from BINOL and Ti(OPr )4, in 1 1 or 2 1 stoichiometry, and oven-dried MS 4A, in ether under reflux. They reported the catalyst to be of BINOL-Ti(OPr% structure. [Pg.823]

Trimethylsilyloxy)furan can also be used as a functionalized silyl enol ether for the asymmetric catalytic aldol-type reaction. Figadere has reported that the reaction of aliphatic aldehydes with the siloxyfuran catalyzed by BINOL-derived titanium complex provides the diastereomeric mixtures with high enantioselectivity (Sch. 42) [107], The addition reaction proceeds at the y position of the siloxyfuran to give butenolides of biological and synthetic importance. [Pg.824]

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]. [Pg.825]

As part of a series of studies on the use of BINOL-Ti(IV) complex 53 as a catalyst in a number of C-C bond-forming reactions, Mikami has reported the aldol addition reactions of thioacetate-derived silyl ketene acetals 55, 56 to a collection of highly functionalized aldehydes (Eq. (8.13)) [28]. As little as 5 mol% of the catalyst mediates the addition reaction and furnishes adducts 57 in excellent yields and up to 96% ee. One of the noteworthy features of the Mikami process is the fact that aldehyde substrates containing polar substituents can be successfully employed, a feature exhibited by few other Lewis-acid-catalyzed aldehyde addition reactions. [Pg.238]


See other pages where BINOL 3,3 -silylated is mentioned: [Pg.123]    [Pg.276]    [Pg.31]    [Pg.411]    [Pg.49]    [Pg.403]    [Pg.448]    [Pg.122]    [Pg.782]    [Pg.783]    [Pg.528]    [Pg.531]    [Pg.250]    [Pg.213]    [Pg.365]    [Pg.366]    [Pg.366]    [Pg.367]    [Pg.367]    [Pg.431]    [Pg.432]    [Pg.433]    [Pg.434]    [Pg.534]    [Pg.109]   
See also in sourсe #XX -- [ Pg.336 ]




SEARCH



BINOL

© 2024 chempedia.info