BINOL-derived catalysts

In related work, Sasai developed several bifunctional BINOL-derived catalysts for the aza-Morita-Baylis-Hillman (aza-MBH) reaction [111]. In early studies, careful optimization of the catalyst structure regarding the location of the Lewis base unit revealed 41 as an optimal catalyst for the aza-MBH reaction between acyclic a,P-unsaturated ketones and N-tosyl imines. Systematic protection or modification of each basic and acidic moiety of 41 revealed that all four heterofunctionalities were necessary to maintain both chemical and optical yields. As seen in Scheme 5.58, MO calculations suggest that one hydroxyl groups forms a... 

The authors also investigated the mode of activation of these BINOL-derived catalysts. They proposed an oligomeric structure, in which one Ln-BINOL moiety acts as a Brpnsted base, that deprotonates the hydroperoxide and the other moiety acts as Lewis acid, which activates the enone and controls its orientation towards the oxidant . This model explains the observed chiral amplification effect, that is the ee of the epoxide product exceeds the ee of the catalyst. The stereoselective synthesis of cw-epoxyketones from acyclic cw-enones is difficult due to the tendency of the cw-enones to isomerize to the more stable fraw5-derivatives during the oxidation. In 1998, Shibasaki and coworkers reported that the ytterbium-(f )-3-hydroxymethyl-BINOL system also showed catalytic activity for the oxidation of aliphatic (Z)-enones 129 to cw-epoxides 130 with good yields... 

Masakatsu Shibasaki of the University of Tokyo reports (J. Am. Chem. Soc. 2004,126, 7559) that use of a BINOL-derived catalyst with cumyl hydroperoxide enables the enantioselective epoxidation of unsaturated N-acyl pyrroles such as 7. The pyrroles 7, prepared from the precursor aldehydes such as 5 with the reagent 6, can be used directly, without further purification. 

Figure 5. Preparation of chiral lanthanoid-BINOL derivative catalysts.

An improved ligand design by Manioka, Saito and Yamamoto has led to a catalyst that is effective for allyl vinyl ethers that are unsubstituted on the vinyl group [27]. This was a serious limitation of the BINOL-derived catalyst 98 which induced only C-O bond cleavage with these substrates (Table 1, entry 1). The ligand (f )-115 was prepared in six steps from (/ )-BINOL and was reacted with trimethylaluminum to give C3 symmetrical Lewis acid Five substituted catalysts were prepared and as... 

The aza-Morita-Baylis-HiUman reaction is known to be a useful and atom-economical C-C bond-forming reaction of electron-deficient alkenes with imines usually catalyzed by Lewis bases [202]. It formally involves a sequence of reactions including a Michael addition, a Mannich reaction, a proton transfer, and a retro-Michael reaction ( -elimination). Although there are many reports in the field of the enantioselective aza-Morita-Baylis-Hilhnan reaction, only rare examples of asymmetric domino reactions initiated by this reaction have been reported. In 2010, Sasai et al. [203] developed the first organocatalyzed asymmetric domino aza-Morita-Baylis-Hillman/aza-Michael reaction of a,p-unsaturated carbonyl compounds with N-tosylimines, allowing an easy access to chiral cis-1,3-disubstituted isoindolines as single diastereomers. The process was induced by a Hg-BINOL-derived catalyst and provided these products in high yields and enantioselectivities, as shown in Scheme 10.18. 

The Zr(OiPr)4 precursor with YANOL or VAPOL ligands forms highly active catalyst for asymmetric imine aldol reaction. Both ligands exhibit much higher optical induction than the corresponding BINOL-derived catalyst (140). 

Jprgensen et al. and Macmillan et al. independently reported on the successful enantioselective a-chlorination and a-bromination of carbonyl groups using amino catalysts. a-lodination remained unsolved until the recent discovery of the binol-derived catalysts by Maruoka et A general catalytic cycle, involving an enamine... 

Table 1.13 BINOL-derived phase-transfer catalysts for asymmetric Darzens reactions.

The enantioselection depends greatly on the nature of the R2 group at the boron atom, and the ee values were as high as 97 %. High enantioselectivity was observed in the synthesis of 4-dihydropyranones, based on the Diels-Alder reactions of aldehydes 74 and Danishefsky s diene, catalyzed by a BINOL-Ti(0-i-Pr)4-derived catalyst [75] (Equation 3.23). 

The adduct derived from (a-benzyloxyacetaldehyde (97 % ee) is an important intermediate en route to compactin and mevinolin [76]. In contrast, modest enantioselectivity was attained when the cycloadditions were catalyzed by a chiral BINOL-ytterbium-derived catalyst [77]. Pyridines were used as additives, and the best enantioselection (93% ee) was attained only in the case of p-methoxybenzaldehyde using 2,6-lutidine. 

The asymmetric hydrogenation of quinoline continues to be of interest. Li et al. reported the asymmetric hydrogenation of a variety of 2-substituted-quinolines to the corresponding tetrahydroquinolines using an Ir-catalyst with a BINOL-derived diphosphonite ligand... 

Mikami and co-workers16-19 have done extensive work for developing catalysts for the asymmetric carbonyl-ene reaction. Excellent enantioselectivites are accessible with the binol-titanium catalyst 17 (Equation (10)) for the condensation of 2-methyl butadiene (R1 = vinyl) and glyoxalates (binol = l,T-binaphthalene-2,2 -diol).16 The products were further manipulated toward the total synthesis of (i )-(-)-ipsdienol. The oxo-titanium species 18 also provides excellent enantioselectivity in the coupling of a-methyl styrene with methyl glyoxalate.17 Reasonable yields and good enantioselectivites are also obtained when the catalyst 19 is formed in situ from titanium isopropoxide and the binol and biphenol derivatives.18... 

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

In 2008, the Ackennann group reported on the use of phosphoric acid 3r (10 mol%, R = SiPhj) as a Brpnsted acid catalyst in the unprecedented intramolecular hydroaminations of unfunctionaUzed alkenes alike 144 (Scheme 58) [82], BINOL-derived phosphoric acids with bulky substituents at the 3,3 -positions showed improved catalytic activity compared to less sterically hindered representatives. Remarkably, this is the first example of the activation of simple alkenes by a Brpnsted acid. However, the reaction is limited to geminally disubstituted precursors 144. Their cyclization might be favored due to a Thorpe-Ingold effect. An asymmetric version was attempted by means of chiral BINOL phosphate (R)-3( (20 mol%, R = 3,5-(CF3)2-CgH3), albeit with low enantioselectivity (17% ee). 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

In 2006, Yamamoto and Nakashima picked np on this and designed a chiral A -triflyl phosphoramide as a stronger Brpnsted acid catalyst than the phosphoric acids based on this concept. In their seminal report, they disclosed the preparation of new chiral BINOL-derived A -triflyl phosphoramides and their application to the asymmetric Diels-Alder (DA) reaction of a,p-unsaturated ketones with sily-loxydienes [83], As depicted in Scheme 59, chiral A-triflyl phosphoramides of the general type 4 are readily synthesized from the corresponding optically active 3,3 -substituted BINOL derivatives 142 through a phosphorylation/amidation route. 

Dixon reported that saturated BINOL 45 sufficiently activates various N-Boc aryl imines toward Mannich reaction with acetophenone-derived enamines to yield P-amrno aryl ketones in good yields and enantioselectivities (Scheme 5.62) [116]. The same group applied a BINOL-derived tetraol catalyst to the addition of meth-yleneaminopyrroHdine to N-Boc aryl imines. Interestingly, appendage of two extra diarymethanol groups to the BINOL scaffold resulted in a marked increase in enantiomeric excess [117]. 

M. Shi and Y.-L. Shi reported the synthesis and application of new bifunctional axially chiral (thio) urea-phosphine organocatalysts in the asymmetric aza-Morita-Baylis-Hillman (MBH) reaction [176, 177] of N-sulfonated imines with methyl vinyl ketone (MVK), phenyl vinyl ketone (PVK), ethyl vinyl ketone (EVK) or acrolein [316]. The design of the catalyst structure is based on axially chiral BINOL-derived phosphines [317, 318] that have already been successfully utilized as bifunctional catalysts in asymmetric aza-MBH reactions. The formal replacement of the hydrogen-bonding phenol group with a (thio)urea functionality led to catalysts 166-168 (Figure 6.51). 

There has been a continuing effort to make the Baylis-Hillman reaction a catalytic asymmetric process. Scott Schnauss of Boston University recently reported (J. Am. Chem. Soc. 125 12094, 2003) an elegant solution to this problem, based on the use of Binol-derived Bronsted acids as catalysts. The product hydroxy enones such as 6 are interesting in themselves, and also as substrates for further transformation, for instance by Claisen rearrangement. 

Shibasaki and co-workers applied (BINOL)Al(III)-derived catalyst 5a, previously developed for the cyanation of aldehydes [28], to the asymmetric Strecker reaction. This catalyst proved to be highly enantioselective for both aromatic and a,p-unsaturated acyclic aldimines (>86% ee for most substrates) (Scheme 8) [63-65]. Aliphatic aldimines underwent cyanide addition with lower levels of enantioselectivity (70-80% ee). A significant distinction of 5 relative to other catalysts is, undoubtedly, its successful application to the hydrocyanation of quinolines and isoquinolines, followed by in situ protection of the sensitive cx-amino nitrile formed (this variant of the Strecker reaction is also known as the Reissert reaction [66]). Thus, Shibasaki has shown that high enantioselectivities (>80% ee for most substrates) and good yields are generally obtainable in the Reissert reaction catalyzed by 5b [67,68]. When applied to 1-substituted... 

Yamamoto et al. have reported the asymmetric catalysis of a chiral Lewis acid in a carbonyl-ene reaction, which uses chloral as the enophile and an aluminum catalyst with enantiopure 3,3 -bissilylated binaphthol (BINOL) to give the corresponding homoallylic alcohol with 78% ee in 79% yield (Scheme 8C.2) [6]. It should be noted that 3,3 -diphenyl-BINOL-derived aluminum catalyst provides the racemic product in low yield. 

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