Lewis binap complexes

Jorgensen has recently reported similar enantioselective reactions between N-tosylimines 107 and trimethylsilyldiazomethane (TMSD) catalyzed by chiral Lewis acid complexes (Scheme 1.32) [57, 53]. The cis-aziridine could be obtained in 72% ee with use of a BINAP-copper(i) catalyst, but when a bisoxazoline-copper(i) complex was used the corresponding trans isomer was fonned in 69% ee but with very poor diastereoselectivity. 

The imines 12 (X = 4-CH3-QH4-SO2 (Ts), Ar, C02R, COR, etc.) preformed or generated in situ from N,0- or N,N-acetals or hemiacetals are another important class of Mannich reagents frequently used for diastereo- and/or enantioselective aminoalkylation reactions catalyzed by chiral Lewis acids (usually copper or palladium BINAP complexes such as 13). Among other things excellent results were obtained in the aminoalkylation of silyl enol ethers or ketene acetals [24], A typical example is the synthesis of Mannich bases 14 depicted in Scheme 5 [24b], Because of their comparatively high electrophilicity imines 12 could even be used successfully for the asymmetric aminoalkylation of unactivated alkenes 15 (ene reactions, see Scheme 5) [24h, 25], and the diastereo- and/or enantioselective aminoalkyla-... 

Addition of [1-ketoesters to unsaturated A -acylthiazolidinethiones 836 is catalyzed by the Ni(ll) Tol-BINAP Lewis acid complex 837. The initial addition products 838 cyclize upon treatment with base to afford enantiopure 3,4-dihydropyran-2-ones 839 in excellent yield (Scheme 236, Table 35) <2005JA10816>. 

Fluoride ion (from CsF, CdF2, or AgF) can also catalyze the allylation using AllylSiF3 and AllylSi(OMe)3, respectively [10, 11] the asymmetric version (with <56% ee) requires a combination of Bu4N+[SiPh3F2] with the Lewis acidic complex of CuCl and BINAP [21]. 

Lewis acid catalysts activate the aldehyde by coordination to the carbonyl oxygen. Shibasaki et al. [13] were able to demon,strate that the activation of the enol ether is possible too. The reaction of the aldehyde 37 with the silyl enol ether 38 in the presence of the catalyst 39 proceeds with good, but still not excellent enantioselectivity to yield the aldol adduct 40. Only 5 mol % of the chiral palladium(II) complex 39 was used (Scheme 6a). Activation of the Pd(lI)-BINAP complex 39 by AgOTf is necessary. Therefore, addition of a small amount of water is important. 

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. 

Lectka et al. have reported that Tol-BI NAP-coordinated CUCIO4 catalyzes fhe addition of SEE to a-N-tosylimino esters wifh high enantioselectivity (Scheme 10.81) [227]. The use of a-substituted SEE gives anti adducts wifh high diastereoselectivity. The Tol-BINAP complex is superior to fhe corresponding BINAP complex in diastereo- and enantioselectivity. It has been proposed that this asymmetric Mannich-type reaction proceeds by a Lewis acid-catalyzed mechanism, not a transmetalation mechanism as reported by Sodeoka et al. 

Yamamoto and coworkers have developed an asymmetric a-aminoxylation of cyclic tributyltin enolates using nitrosobenzene (5.85) as oxygen source in the presence of Lewis acidic 1 1 silver-BINAP complexes. For example, the enolate (5.86) is converted into the a-aminoxyketone with high ee using this procedure. Cleavage of the N-0 bond is then effected with no racemization, using catalytic copper sulfate in methanol. 

The asymmetric fluorination of 3-ketoesters has been achieved in 62-90% ee using F-TEDA (Selectfluor) as fluorine source in the presence of 0.5 mol% of the chiral nonracemic titanium-based Lewis acid (5.108). ° A greater range of p-ketoesters are fluorinated with higher ee using catalytic quantities of the palladium-BINAP complex (5.109) and N-fluorobenzenesulfonamide (NFSI). ° In both cases the reaction proceeds through the intermediacy of a chiral enolate. 

Several other chiral Lewis acids have also been reported to effect asymmetric aldol reactions. Kruger and Carreira59 reported a catalytic aldol addition of silyl dienolate to a range of aldehydes in the presence of a bisphosphanyl-Cu(II) fluoride complex generated in situ from (iS )-Tol-BINAP, Cu(OTf)2, and (Bu4N)Ph3SiF2. Aromatic, heteroaromatic, and a,/ -unsaturated aldehydes provided the aldol adducts with up to 95% ee and 98% yield (Scheme 3-33). 

Besides the silyl enolate-mediated aldol reactions, organotin(IY) enolates are also versatile nucleophiles toward various aldehydes in the absence or presence of Lewis acid.60 However, this reaction requires a stoichiometric amount of the toxic trialkyl tin compound, which may limit its application. Yanagisawa et al.61 found that in the presence of one equivalent of methanol, the aldol reaction of an aldehyde with a cyclohexenol trichloroacetate proceeds readily at 20°C, providing the aldol product with more than 70% yield. They thus carried out the asymmetric version of this reaction using a BINAP silver(I) complex as chiral catalyst (Scheme 3-34). As shown in Table 3-8, the Sn(IY)-mediated aldol reaction results in a good diastereoselectivity (,anti/syn ratio) and also high enantioselectivity for the major component. 

Lectka and co-workers found that cationic Cu phosphine complexes are efficient Lewis acids in the activation of a-imino esters (248). The Tol-BINAP was found to be the most effective ligand providing the adduct of acetophenone enol-... 

Takaya and co-workers (256) disclosed that chiral copper alkoxide complexes catalyze the transesterification and kinetic resolution of chiral acetate esters. Selec-tivities are very poor (E values of 1.1-1.5) but it was noted that the Lewis acid BINAP CuOTf was not an effective catalyst. The observation thatp-chlorophcnyl-BINAP-CuOf-Bu complex gave faster rates than BINAP-CuOt-Bu suggests that both the Lewis acidic and Lewis basic properties of the copper alkoxide are required for optimal reactivity. 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

The allylation of a-iV-tosylimino esters, highly activated imines, proceeds by catalytic use of a Lewis acid. Tol-BINAP-Cu(l) complexes are effective catalysts of asymmetric allylation of these imines with allylsilanes (Equation (28)).124 125 The presence of an aryl group at the position j3 to silicon improves the enantioselectivity. 

The use of Lewis acid drastically changes the regioselectivity. The highly enantioselective and O-selective nitroso aldol reactions of tin enolates with nitrosobenzene have been developed with the use of (i )-BINAP-silver complexes as catalysts. AgOTf and AgCICL complexes are optimal in the O-selective nitroso aldol reaction in both asymmetric induction (up to 97% ee) and regioselection (0/N= > 99/1), affording amino-oxy ketone. The product can be transformed to a-hydroxy ketone without any loss of enantioselectivity (Equation (71)).224... 

The regio- and enantioselective pentadienylation of aldehydes is also catalyzed by the BINAP/silver complex.312 The reaction of dienylstannanes 351 with various aldehydes takes place in the 7-position exclusively to afford the products 352 with good to high enantioselectivities (Scheme 100). The reaction was proposed to occur either via a Lewis acid mechanism (transition state 353) or by transmetallation (transition state 354). 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Several other chiral Lewis acids have been developed for the addition of allyl and methallyl tributylin to aldehydes [28]. These additions usually proceed slowly with reaction times of days. Less reactive stannanes, for example crotyl tributyltin, require even longer times and diastereoselectivity is poor. The allyl and methallyl additions, however, afford products in high yield and ee. The most successful ligands are BINOL and BINAP as Ti or Zr complexes in the former case and an AgOTf complex in the latter. 

The BINAP silver(I) complex can be further applied as a chiral catalyst in the asymmetric aldol reaction. Although numerous successful methods have been developed for catalytic asymmetric aldol reaction, most are the chiral Lewis acid-catalyzed Mukaiyama aldol reactions using silyl enol ethers or ketene silyl acetals [32] and there has been no report which includes enol stannanes. Yanagisawa, Yamamoto, and their colleagues found the first example of catalytic enantioselective aldol addition of tributyltin enolates 74 to aldehydes employing BINAP silver(I) complex as a catalyst (Sch. 19) [33]. 

This BINAP silver(I) complex was subsequently used by Lectka and coworkers as a catalyst for Mannich-type reactions [35]. Slow addition of silyl enol ether 49 to a solution of tosylated a-imino ester 80 under the influence of 10 mol % (i )-BINAP AgSbFg at -80 °C affords the corresponding amino acid derivative 81 in 95 % yield with 90 % ee (Sch. 20). They reported, however, that (R)-Tol-BINAP-CuC104-(CH3CN)2 was a more effective chiral Lewis acid for the reaction and gave the highest yield and ee at 0 °C. 

---