BINOL system

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... 

Shibasaki s lanthanide-alkaline metal-BINOL system, discussed in Chapters 2 and 3, can also effect the asymmetric conjugate addition reaction. As shown in Scheme 8-35, enantioselective conjugate addition of thiols to a,/ -unsaturated carbonyl compounds proceeds smoothly, leading to the corresponding products with high yield and high ee.76... 

This trend observed for 3-formylfuran (11) (Scheme 5) could also be confirmed for other aromatic and unsaturated aldehydes (Table 5, Scheme 6). The yields that were achieved using 50 mol% together with dienolate 2 were matched with dienolate 1 using only 17 mol% of the Ti-BINOL system. Interestingly, the reaction of cinnamaldehyde with 2 gave no conversion. When aliphatic aldehydes were used, only notoriously poor yields were observed, indicating the constant problem in this type of transformation that is also encountered by other groups with different dienolates in one way or the other vide infra). 

Other successful H-bond catalysis apphcations have been introduced by Schaus and Sasai involving asymmetric Morita-Bayhs-Hilhnan (Scheme 11.13c) and aza-Morita-Baylis-Hillman reactions (Scheme 11.13d), respectively. Intriguingly, derivatized BINOL systems 33 and 34 provided optimal selectivities. 

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... 

A general review of the advances in homogeneous and heterogeneous catalytic asymmetric epoxidation covering Sharpless, porphyrin, A, A -bis(saclicylaldehydo)ethylenediamine (salen), and l,T-bi-2-naphthol (BINOL) systems, carbonyl-derived dioxiranes and iminium species, as well as their supported counterparts, has recently been published <2005CRV1603>. 

Bulky ligands accomplish the desired, well-defined monolanthanide precatalyst species and their fine tuning directly affects the reactivity, as impressively demonstrated by tied-back cyclopentadienyl complexes and even water-stable BINOL systems. Simultaneously, strongly chelating ligands provide a sterically rigid ligand frame ( spectator area ) which is a prerequisite for induction of asymmetry at the lanthanide center. 

Hence, synthetic organolanthanide chemistry puts the main emphasis on the adaption of prevailing precatalyst types to the requirements of highly enantiose-lective catalysis. This is impressively demonstrated by tied-back cyclopentadienyl complexes [207], even water-stable BINOL systems [43], and fluorinated (3-diketonate complexes (Fig. 11) [208]. 

Significant counterion effects are observed in the enantioselectivity of addition of diethylzinc to benzaldehyde using a titanium/a-acetyl-(5 )-BINOL system. rran5-l,2-Diaminocyclohexane - a common motif used in asymmetric catalysis - exhibits a dihedral angle of ca 60° between the amino groups, whereas rranx-ll,12-diamino-9,10-dihydro-9,10-ethanoanthracene (124) is constrained to >110°. Further functionality and chirality has been incorporated by the formation of bis-sulfonamides of (124), using (S j-camphor sulfonyl chloride. The... 

But, Yukawa et al. used lanthanum isopropoxide/BINOL system, 128, for the catalysis of asymmetric aza-Morita-Baylis-Hillman reaction (reaction 7.28), in which the electron-deficient alkenes, 129, reacted with... 

Kantam, M. L., Arundhathi, R., Likhar, R R., and Damodara, D. 2009a. Reusable copper-aluminum hydrotalcite/rac-BINOL system for room temperature selective aerobic oxidation of alcohols. Arfv. Synth. Catal. 351(16) 2633-2637. 

An extension of the Ti/BINOL system was developed by Maruoka and co-workers wherein a p.-oxo titanium dimer 71 was generated in situ. This method led to meaningful improvements in the enantioselectivity and reaction time (Table 9). ... 

A one-pot titanium-catalyzed tandem sulfoxidation-kinetic resolution process was developed by Chan using TBHP as the oxidant This process combines asymmetric sulfoxidation (at 0°C) and kinetic resolution (at room temperature). Excellent enantiomeric excesses (up to >99.9%) and moderate to high chemical yields of sulfoxides were obtained [270] (Scheme 14.113). The effect of fluorine substitution at the backbone of BINOL on the catalytic activity in titanium-catalyzed sulfide oxidation with TBHP or cumyl hydrc en peroxide (CHP) was studied by Yudin [271]. Introduction of fluorines into the BINOL scaffold was found to increase the electrophilic character of the Lewis acidic titanium center of the catalyst The most intriguing difference between the FsBINOL and BINOL systems is the reversal in the sense of chiral induction upon fluorine substitution. A steroid-derived BINOL ligand has also been used for the same reaction [272]. 

The TiCl2(0/-Pr)2/BrN0L system is only weakly acidic, and therefore limited to reactive aldehydes such as glyoxaldehyde. To circumvent the problem, Carreira introduced the TiF4/BINOL system, which provides a more Lewis acidic, and more reactive catalyst (Table 12). In addition, the greater strength of the Ti-F bond compared to the Si-F bond assists the catalyst turnover. ... 

In 1993, Mikami and Matsukawa reported on an interesting reactivity while attempting enantioselective catalysis of the Mukaiyama aldol reaction using the 1, r-binaphthyl-2,2 -diol (BINOL) system (Scheme 10.21). ° The method used condensation of 3-pentanone-derived enoxysilane 91 with glyoxalate 92 in the presence of enantiomerically pure catalyst 93. Silyl enol ether 94 was obtained as an c/jc-type product with excellent diastereo- and enantioselectivity. 

In parallel with the search for catalytic systems, has emerged an impressive amount of results in the field of enantioselective allylation. The pioneering work of Marshall using a chiral (acyloxy)borane (CAB) system [216] was readily followed by titanium/BINOL catalysts [217], leading to homoallylic alcohols with enantiomeric excess up to 98%. An extension of this work in fluorous phase was also developed with 6,6 -perfluoroalkylated BINOLs [218]. Replacing the titanium by zirconium (IV) salts, led to more reactive catalyst for the allylation of aromatic and aliphatic aldehydes [219]. One of the more active catalyst is the zirconium-BINOL system associated with 4-tert-butylcalix [4]arene, which remains active with only 2% of the chiral inductor [220]. The use of activators, such as iPrSSiMe3, iPrSBEt2,... 

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