Aluminum, chiral catalysts

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

Some of the developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds have origin in Diels-Alder chemistry, where many of the catalysts have been applied. This is valid for catalysts which enable monodentate coordination of the carbonyl functionality, such as the chiral aluminum and boron complexes. New chiral catalysts for cycloaddition reactions of carbonyl compounds have, however, also been developed. 

Yamamoto et al. were probably the first to report that chiral aluminum(III) catalysts are effective in the cycloaddition reactions of aldehydes [11]. The use of chiral BINOL-AlMe complexes (R)-S was found to be highly effective in the cycloaddition reaction of a variety of aldehydes with activated Danishefsky-type dienes. The reaction of benzaldehyde la with Danishefsky s diene 2a and traws-l-methoxy-2-methyl-3-(trimethylsilyloxy)-l,3-pentadiene 2b affords cis dihydropyrones, cis-3, as the major product in high yield with up to 97% ee (Scheme 4.6). The choice of the bulky triarylsilyl moiety in catalyst (J )-8b is crucial for high yield and the en-antioselectivity of the reaction in contrast with this the catalysts derived from AlMe3 and (J )-3,3 -disubstituted binaphthol (substituent = H, Me, Ph) were effective in stoichiometric amounts only and were less satisfactory with regard to reactivity and enantioselectivity. 

A remarkable change in reaction course is notable when changing the metal from aluminum to titanium for cydoaddition reactions using BINOL as the chiral ligand. When the chiral aluminum(III) catalyst is applied the cydoaddition product is the major product, whereas for the chiral titanium(IV) catalyst, the ene product is the major product. The reason for this significant change in reaction course is not fully understood. Maybe the glyoxylate coordinates to the former Le-... 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

A chiral aluminum-salen catalyst gives good enantioselectivity in the addition of cyanide (from TMS-CN) to unsaturated acyl imides.338... 

This procedure describes the preparation and application of an effective chiral catalyst for the enantioselective Diels-Alder reaction.11 The catalyst is derived from optically active 1,2-diphenylethylenediamine, the preparation of which (either antipode) was described in the preceding procedure. The aluminum-based Lewis acid also catalyzes the cycloaddition of crotonoyl oxazolidinones with cyclopentadiene,11 and acryloyl derivatives with benzyloxymethylene-cyclopentadiene. The latter reaction leads to optically pure intermediates for synthesis of prostaglandins.11... 

Perhaps the most attractive method of introducing enantioselectivity into the Diels-Alder reaction is to use a chiral catalyst in the form of a Lewis acidic metal complex. In recent years, this area has shown the greatest progress, with the introduction of many excellent catalytic processes. Quite a number of ligand-metal combinations have been evaluated for their potential as chiral catalysts in Diels-Alder reactions. The most commonly used metals are boron, titanium, and aluminum. Copper, magnesium, and lanthanides have also been used in asymmetric catalytic Diels-Alder reactions. 

Corey and colleagues215 prepared chiral aluminum complexes from chiral bis(sulfona-mides) and trimethylaluminum. These were successfully applied in the cycloadditions of 3-acryloyl-l,3-oxazolidin-2-one (17a) with substituted cyclopentadienes. Thus, the reaction of 3-acryloyl-l,3-oxazolidin-2-one with 5-(benzyloxymethyl)cyclopentadiene (331) afforded 332 with 94% ee (equation 93). A transition state was proposed based on the X-ray structure of the chiral catalyst and on NMR data of the 1 1 complex between 333... 

The real promise of this catalytic reaction is the eventual development of an efficient enantioselective allylboration catalyzed by chiral Lewis acids. A stereoselective reaction using a substoichiometric amount of a chiral director has been reported, but only modest levels of stereo-induction were achieved with an aluminum-BINOL catalyst system (Eq. 19)P Recently, a chiral Brpnsted acid catalyzed system has been devised based on a diol-tin(IV) complex (Eq. 80). In this approach, aliphatic aldehydes provide enantioselectivities (up to 80% ee) higher than those of aromatic aldehydes when using the optimal complex 114. Although the levels of absolute stereoselectivity of this method remain too low for practical uses, promising applications are possible in double diastereoselection (see section on Double Diastereoselection ). 

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. 

The reaction of methacrolein with cyclopentadiene catalyzed by a chiral menthoxyaluminum complex gives adducts with ee s of up to 72%, but with other dienophiles little, if any, induction was noted.9495 A chiral cyclic amido aluminum complex 2 catalyzes the cycloaddition of cyclopentadiene with the fran.v-crotyl derivative 3 in good yield and enantioselectivity (Scheme 26.2).47 This chiral catalyst can also be easily recovered. 

The vast majority of successful chiral catalysts to date are based on tartaric acid, BINOL, or oxazolidinone derivatives (Table 26.1). Because derivatives of both of these compounds are commercially available, scale up should not present a problem. If the observed asymmetric induction is found to be low with catalysts based on tartaric acid or oxazolidinones, the sterically hindered titanium BINAP-type complexes should allow for increased selectivity. In addition, nontoxic metal counterions, such as iron and aluminum, do not appear to compromise the asymmetric induction. 

Claisen Rearrangements. A modified BINOL-derived aluminum reagent is an effective chiral catalyst for asymmetric Claisen rearrangement of allylic vinyl ethers (eq 29). The use of vinyl ethers with sterically demanding C-3 substituents is necessary for the high level of asymmetric induction. 

Atropisomers of conformationally restricted bis-phenols have been popular ligands in several applications (Sch. 5, 8, 9, 11-13 Tables 4 and 5). Aluminum compounds prepared from bis-l,T-binaphth-2,2 -ol (BINOL) 40 and the derivatives 97 were examined as chiral catalysts in the reaction of methyl acrylate and cyclopentadiene by Maruoka, Concepcion and Yamamoto [50] and by Ketter, Glahsl and Hermann [47]. Four catalysts prepared from four derivatives of the 3,3 -bis-triarylsilyl derivatives of 97 and trimethylaluminum were examined in both toluene and dichloromethane the results are summarized in Sch. 26 [50]. Slightly higher asymmetric induction was observed in toluene and for the f-butyldiphenysilyl derivative 97b. The catalyst prepared... 

The only known metal catalyst for the asymmetric catalytic Strecker reaction is the aluminum salen catalyst 465 (Sch. 65) recently reported by Sigman and Jacobsen [97]. They prepared 11 different chiral salen complexes from different transition and main group metals and screened these complexes for the addition of trimethylsilyl cyanide to imine 460 at room temperature. The aluminum catalyst 465 was optimum in terms both of asymmetric induction and rate. This constitutes the first aluminum salen complex successfully developed for an asymmetric catalytic reaction. 

Optically active l,l -binaphthols are among the most important chiral ligands of a variety of metal species. Binaphthol-aluminum complexes have been used as chiral Lewis acid catalysts. The l,T-binaphthyl-based chiral ligands owe their success in a variety of asymmetric reactions to the chiral cavity they create around the metal center [107,108]. In contrast with the wide use of these binaphthyls, the polymer-supported variety has been less popular. The optically active and sterically regular poly(l,l -bi-naphthyls) 96 have been prepared by nickel-catalyzed dehalogenating polycondensation of dibromide monomer 95 (Sch. 7) [109] and used to prepare the polybinaphthyl aluminum(III) catalyst 97 this had much greater catalytic activity than the corresponding monomeric catalyst when used in the Mukaiyama aldol reaction (Eq. 29). Unfortunately no enantioselectivity was observed in the aldol reaction. 

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

The cycloaddition of functionalized cyclopentadiene and dienophile 83 was better performed by use of an entirely different, non-phenoxide-type aluminum complex (Scheme 6.53). Thus a chiral catalyst endowed with the more electron-with-drawing bis (sulfonamide) ligand was explored by Corey and coworkers [73]. The reaction of the trons-crotyl derivative 83 and cyclopentadiene with 20 mol% 84 as catalyst at -78 °C for 16 h provided adduct 85 in 88% yield and 94% ee. The ad-... 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

The mechanistic complexities of stereoselectivity is further evidenced by a recent report by Maudoux et a/. who describe a chiral aluminum salen catalyst that generates highly isotactic PLA from rac-lactide (Pm-0.90). In this example, the kinetics indicated a dominant chain-end control mechanism, which contrasts to other chiral aluminum salen catalysts where enantiomorphic site control is thought to predomi-nate. ° All the previously mentioned chiral aluminum salen alkoxide systems require multiple days at elevated temperatures to polymerize -200 equiv. of lactide. The low activity of chiral aluminum salen systems towards lactide polymerization is a major drawback of these systems. 

The first catalytic asymmetric radical-mediated allylation was reported in late 1997 by Hoshino and coworkers, who studied the allylation of an a-iodolac-tone substrate, Eq. (19) using trimethylaluminum as Lewis acid and a silylated binaphthol as the chiral catalyst, with triethylborane as radical initiator [62]. Use of one equiv. of diethyl ether was crucial for high enantioselectivity, providing an ee up to 91% in the presence of one equiv. of catalyst, with only a 27% ee in the absence of ether, and poorer ee s when other ethers were employed. In the catalytic version, the ee s dropped off vs. the stoichiometric reaction, with an ee of 81% with 0.5 equiv., and 80% with 0.2 equiv., and 72% with 0.1% catalyst. As in the above example, the presumed chiral intermediate involves complexation of the lactone radical with the Lewis acid-binaphthol complex, with the diethyl ether perhaps as a ligand on the aluminum. 

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