PyBox catalyst

Scheme 8.9 Asymmetric Mukaiyama aldol reactions with Fe(ll)-PYBOX catalysts.

An immobilized Ru-PyBOx catalyst 56 was synthesized by thermally induced radical copolymerization and used to catalyze the flow cyclopropanation reaction between styrene and ethyldiazoacetate (Scheme 4.79), building on initial work performed in batch [179]. For the flow example, a range of experiments using the starting materials in CH2C12 solution, neat or in scC02 were assessed and the results are summarized in Table 4.2 [180],... 

In 1994, asymmetric cydopropanation (ACP) with ruthenium catalysts was first reported by Nishiyama and coworkers [ 19,20] by adoption of their chiral bis(oxazolinyl)pyridine (Pybox) ligands. The reaction profiles of Ru Pybox catalysts reveal extremely high trans selectivity with high enantioselectivity (or di-astereoselectivity) of cyclopropane products at the relatively low reaction temperatures (around 20-50 °C) so far reported for ruthenium catalysts. After 1997,... 

Other terminal olefins were transformed to the corresponding cyclopropane esters with Z-menthyl and d-menthyl diazoacetates with high stereoselectivity up to 98% ee (Scheme 3). Intramolecular reaction of the phenyl-allyl ester 9 was carried out to give the bicyclic compound 10 with 86% ee and 93% yield. The enantioselectivity for intramolecular cyclopropanation of the 3-methylbutenyl ester 11 was compared with chiral Cu(I), Rh(II), and Ru Pybox catalysts Rh>Ru>Cu [26]. 

In an exciting new challenge the Bristol-Myers-Squibb group carried out an ACP on a 100-kg scale with a chiral Ru Pybox catalyst, especially in two-phase media of water and ferf-butyl methyl ether (Scheme 5) [35]. The operations produced good yields and enantioselectivity,but separation was difficult. Similarly, Wurz and Charette [36] demonstrated ACP in aqueous media by using Ru, Rh, and Co catalysts including an O-H insertion reaction of carbenes. 

In 1997 after the introduction of Ru Pybox catalysts, chiral ruthenium porphyrin derivatives were found by three groups to be effective catalysts for ACP... 

Table 7.1 Catalytic asymmetric cyclopropanation of styrene and diazoacetates with Ru-Pybox catalysts.

Scheme 12.19 Ru (I I) pybox catalyst and its application to enantioselective C H amination.

Rli2(S nap)2 is an effective catalyst for the enantioselective C H amination of the cis olefin but gives lower yield and poor enantioselectivity in the case of the corresponding trans olefin. The opposite behavior is observed with the RuBr2 pybox catalyst 43. 

Origins of enantioselectivity in catalysed Diels-Alder reactions Box and pybox catalysts in asymmetric Diels-Alder reactions Cyclopropanation... 

Box and pybox catalysts in asymmetric Diels-Alder reactions... 

Kobayashi et al. developed catalytic asymmetric 1,4-additions using chiral calcium species prepared from calcium isopropoxide and chiral bisoxazoline ligands 39, 166 and 168. They found that calcium pyBOX catalysts could effectively mediate catalytic asymmetric additions of 1,3-dicarbonyl compounds 4 to nitroalkenes 86, N-Boc-imines 138 or unsaturated amides 49 giving products 165,167 and 170, respectively. Neutral coordinative ligands worked well in these reactions, giving a noticeably faster rate of reaction... 

Towers, M. D. K. N., Woodgate, P. D. and Brimble, M. A. 2003. Addition of 2-[(trimethylsi-lyloxy)]furan to 2-acetyl-1,4-benzoquinone using chiral non-racemic copper(II)-pybox catalysts. Arfa voc (i) 43-55. 

Shao, Chan, and coworkers have developed the first catalytic asymmetric synthesis of chiral p.y-alkynyl a-amino acid derivatives 404 in 61-80% yields and moderate enantioselectivities (66-74%), using ethyl glyoxylate 400, p-anisidine 401, and aliphatic or aromatic alkynes 402 (Scheme 6.60) [126]. This process is catalyzed by a catalyst system between Cu(I) triflate benzene complex and 10mol% of pybox catalyst 403. 

A chiral Sc-pybox catalyst from Sc(OTf)3 catalyzes a highly enantioselective Michael-type indole Friedel-Crafts reactions with a variety of )3-substitnfed a, -unsaturated acyl phosphonates and -substituted a,j8-unsaturated 2-acyl imidazoles (Scheme 2). The acyl phosphonate products were efficiently ttansformed into the corresponding esters and amides, whereas the acyl imidazole prodncts were converted to more diverse functionalities snch as esters, amides, carboxyhc acids, ketones, and aldehydes. A nuld and efficient cleavage protocol for the diversification of the 2-acyl imidazole prodncts ntihzing methylating conditions was also developed. 

A chiral Sc-pybox catalyst catalyzes Lewis acid-catalyzed enantioselective alkenylation of air- and moisture-stable trimethylvinylsilanes under nuld conditions. Most products are highly crystalline sohds, which are ideally suited for industrial process chemistry (Scheme 4). 

Recently enantioselective oxidative functionalization of C—H bonds was reported. Intramolecular C—H amination proceeds in the presence of ruthenium pybox catalyst with bis(acyloxy)iodobenzene (Eq. (7.88)) [145]. 

In 2004, Hossain and Redlich reported the synthesis of a series of iron-pybox complexes and their employment in the catalytic asymmetric aziridine forming reaction with imine (109a) and ethyl diazoacetate (10) (Scheme 16.31) [35]. When combined with AgSbFe, the isopropyl- and tert-butyl-pybox complexes (112a) and (112b) produce 47% of the cis-aziridine (110) in moderate enantiomeric excesses. The best overall results came when the tert-butyl pybox catalyst was used, although results obtained with the isopropyl pybox catalyst are very similar. 

Scheme 119 (PBB = p-BrCgH4CH2). Synthesis of (-t-)-tashiromine (929) by Suga et al. Reagents and conditions (a) Pybox catalyst 967—Sm(OTf)3 (10 mol%), 965, Rh2(OAc)4 (2 mol%), 4 A molecular sieves, CH2CI2, 10 °C, then 966, 6h (syringe pump) (b) p-BrCgH4CH20H—n-BuLi, THF, then 968, rt, 3 h, then recrystallization (c) EtsSiH, BF3-OEt2, CH2CI2, -20 °C, then rt, 2 days (d) PhOC(=S)CI, py, CH2CI2, 0 °C, 1 h, rt, 5 h (e) BusSnH, AIBN, CgHs, reflux, 5 h (f) LIAIH4, THF, reflux, 2 h.

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