Binol catalyst complex

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. 

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

The best procedure reported to date for the asymmetric allylation of aldehydes using tributyl(2-propenyl)stannane involves the catalyzed addition with the BINOL-TiCl2 complex as catalyst. Good yields and ee s were obtained for both aromatic and aliphatic aldehydes using 20 mol% of the catalyst127. 

As shown in Figure 4.5, a remarkably high positive nonlinear effect was observed in the La-BINOL-Ph3PO complex-catalysed epoxidation of chalcone (either with CMHP or with TBHP as an oxidant)1121, which strongly suggests that the active catalyst leading to high enantioselection does not have a monomeric structure but may exist as a thermodynamically stable dinuclear complex. 

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 ). 

The data in Scheme 3 illustrate that a wide range of 1,7-dienes can be resolved with high selectivity and efficiency. These findings provide further evidence regarding the importance of the availability of a diverse set of chiral catalysts. Although BINOL-based complexes (e.g., 11a) typically promote ARCM... 

Throughout this article, the identity of the recovered enantiomer shown is that which is obtained by the catalyst antipode illustrated. Moreover, transformations with BINOL-based complexes (e.g., 11) were carried out with the opposite antipode of the catalyst versus that illustrated. Because (S)-biphen and (R)-BINOL complexes were employed in our studies, this adjustment has been made to facilitate comparison between biphen- and BI-NOL-based catalysts... 

Ziegler et al. have reported the asymmetric desymmetrization approach to the synthesis of tricothecene, anguidine, via an ene cyclization (Scheme 8C.11) [31], The (2,4) ene cyclization (vide infra) of the prochiral aldehyde on silica gel gives a 1 1 diastereomeric mixture. Cyclization with purified Eu(fod)3 as Lewis acid catalyst gives an 8 1 mixture. The major isomer is a potential intermediate for the synthesis of anguidine. However, use of (+)-Eu(hfc)3, (+)-Eu(dppm)3, or (S)-BINOL-TiCl2 complex as chiral Lewis acid affords the major product with only 20-38% ee. 

In an effort to develop new chiral BINOL-Ti complexes, chemical modifications of the chiral complex (f )-BINOL-Ti(OPr )2 (R-2) that can easily be prepared by simply mixing ( PrO)4Ti and (/ )-BINOL in the absence ofMS4A have been studied [37c-e]. A dimeric form has been reported for the single-crystal X-ray structure of complex R-2 [38], (I )-BINOL-Ti-p3-oxo complex, prepared via hydrolysis of complex R-2 has been shown to serve as an efficient and moisture-tolerable asymmetric catalyst [37d,e]. It is noteworthy that the (/ )-BINOL-Ti-)i3-oxo catalyst [37e] shows a remarkable level of (+)-NLE (asymmetric amplification), thereby attaining the maximum enantioselectivity for this system by using (/ )-BINOL with only 55-60% ee as the chiral source (consult Scheme 8C. 14). 

BINOL-Ti complexes (1) has been shown to serve as efficient asymmetric catalysts for the carbonyl addition reaction of allylic stannanes and silanes 152,53]. The addition reactions to glyoxylates of ( )-2-butenylsilane and -stannane proceed smoothly to afford the corresponding syn-product with high enantiomeric excess (Scheme 8C.21) [52]. 

A Et2Zn-(5, S)-linked-BINOL (21) complex has been found suitable for chemos-elective enolate formation from a hydroxy ketone in the presence of isomerizable aliphatic iV-diphenylphosphinoylimines.103 The reaction proceeded smoothly and /9- alkyl-yS-amino-a-hydroxy ketones were obtained in good yield and high enantioselectivity (up to 99% ee). A titanium complex derived from 3-(3,5-diphenylphenyl)-BINOL (22) has exhibited an enhanced catalytic activity in the asymmetric alkylation of aldehydes, allowing the reduction of the catalyst amount to less than 1 mol% without deterioration in enantioselectivity.104... 

The Ln-BINOL derivative complexes are efficient asymmetric catalysts for Michael reactions and the epoxidations of enones. However, as was mentioned above, almost racemic products are obtained in the case of the asymmetric nitroaldol reaction of 2 with 12. For this transformation, a new class of catalysts, heterobimetallic species, have been developed. 

Mikami et al. studied the Diels-Alder reaction between a-methylstyrene and n-butyl glyoxylate catalyzed by a titanium binolate catalyst.76-78 Addition of 0.5 equivalents of (Zf)-BINOL to 1 equivalent of the racemic catalyst accelerated the reaction and gave the product with 89.8% ee (Scheme 20). Enantiopure catalyst derived solely from (/ )-BINOL gave the product with 94.5% ee. Here the amplification originates from the creation of a new chiral complex 9 of higher efficiency (rate and enantioselectivity) with respect to each enantiomer of the original racemic catalyst. 

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]. 

The BINOL-Ti complex-catalyzed addition of allylsilane to aliphatic and aromatic aldehydes has been reported by Carreira [43], The catalyst is prepared from BINOL and polymeric Tip4 (Sch. 9). The presence of a small amount of CH3CN is crucial to achieving not only high catalytic activity but also high enantioselectivity. 

The Diels-Alder reaction of methacrolein with 1,3-dienol derivatives can also be catalyzed by the BINOL-derived titanium complex, although the catalyst must be freed from molecular sieves (MS) to give the endo adduct with high enantioselectivity (Sch. 50) [131], because MS act as achiral catalysts in the Diels-Alder reaction. The asymmetric Diels-Alder reaction catalyzed by the MS-free (MS-(-)) BINOL-Ti complex (L) can be applied naphthoquinone derivatives as dienophiles to provide entry to the asymmetric synthesis of tetra- and anthracyclinone [132] aglycones (Sch. 51). The sense of asymmetric induction is exactly the same as that observed in the presence of MS in the asymmetric catalytic reactions described above. 

The reaction of methallyltri-n-butylstannane 117 with achiral aldehydes is also effectively promoted by the binol-Ti complex [89 c]. In all but one case (cyclo-hexanecarboxaldehyde), the yields and enantioselectivities observed with the methallylstannane are identical or higher than those obtained in the reactions with allyltributylstannane with only 10 mol% of the binol-Ti complex (Scheme 10-50). Insight into the nature of the titanium catalyst is provided by the observation of asymmetric amplification [89 b] and chiral poisoning [89 g]. An intruiging hypothesis on the origin of enantioselection in allylation and related reactions [89 h]. 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

Of the BINOL/BINAP-metal catalyst complexes, only the allylation procedure described by Keck using the BINOL-Ti(IV) catalyst 451 has been applied in the synthesis of natural products, presumably because it has the most substrate generality and the field is so new. In a preliminary report, Evans disclosed the synthesis of the 4-hydroxy buteneolide terminus 470 of mucocin, where he uses Keck s original catalytic asymmetric allylation procedure to effect conversion of aldehyde 469 to the homoallylic alcohol 470 in good yield and high diastereoselectivity (Scheme 11-36) [312]. 

ASYMMETRIC NITROALDOL REACTION USING LANTHANUM-LITHIUM-BINOL DERIVATIVE COMPLEX AS A CATALYST... 

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