Benzene, 1 - complexes chirality

One of the earliest reports of enantioselective transfer hydrogenation was by Alper et al., who used chiral Schiff bases and a dichlororuthenium(II)benzene complex employing the IPA system [5]. In another report, Lemaire et al. utilized... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Benzene, l-(methylphenylarsino)-2-(methylphenyl-phosphino)-complexes chirality, 199... 

Benzene-4-sulfonic acid, l-hydroxy-2-(2-hydroxy-6-suIfo-l-naphthylazo)-6-(2-hydroxy-l-naphthylazo)-metallochromic indicator, 556 Benzimidazole, 2-(o-hydroxyphenyl)-raercury determination, 532 Betrzoic acid complexes linkage isomers, 186 a-Benzoin oxime in gravimetry, 530 Benzothiazole, hydroxy-in gravimetry, 531 Benzothiazole, mercapto-in gravimetry, 534 Benzothiazole, 2-mercapto-cadmium complexes liquid-liquid extraction, 547 Benzoxazole, 2-(o-hydroxyphenyl)-cadmium determination, 531 Benzylamine, a-methyl-Al,7V-dimethyl-complexes chirality, 199 Berry pseudorotation, 39 Beryllium analysis, 523... 

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. 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

In a rather different approach optically active chromium complexes of 2,3-dihydro-1 H-in-denone are used as chiral enolate precursors. These chiral complexes react with 3-buten-2-one in benzene using l,5-diazabicyclo[4.3.0]non-5-ene as the base. The diastereomeric ratio of the product is the same irrespectively of whether the exo- or the Noisomer of the chromium... 

Significant advance in the field of asymmetric catalysis was also achieved with the preparation of l,2-bis(phospholano)benzene (DuPHOS 4) and its confor-mationally flexible derivative (l,2-bis(phospholano)ethane, known as BPE) by Burk et al. [59]. Two main distinctive features embodied by these Hgands, as compared to other known chiral diphosphine ligands, are the electron-rich character of the phosphorus atoms on the one hand and the pseudo-chirality at phosphorus atoms, on the other. These properties are responsible for both the high activity of the corresponding metal complex and an enantioselection indepen-... 

Photoirradiation of both neat and benzene solutions of 2-cyclohexenone (66b) gives a complex mixture of photodimers [40]. However, photoirradiation of a 1 1 complex of 66b with the chiral host (S,S)-(-)-l,4-bis[3-(o-chlorophenyl)-3-hydroxy-3-phenylprop-l-ynyl]benzene (167) in the solid state (Scheme 24) gave (-)-anf/-head-to-head dimer 168 of 46% ee in 75% yield [40]. This reaction was found to proceed in a single crystal-to-single crystal manner. The mechanism of the reaction was studied by X-ray crystal structural analysis [41]. 

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

In the case of 67g which has a chiral alkyl group, optically pure 67g was included at the complexation process with 2a, and (—)-67g of 100% ee [[a]D —66.8° (c 0.22, CHClj)] was obtained. Irradiation of the 1 1 inclusion complex of 2a and (—)-67g of 100% ee gave 69g of 100% ee which has three optically pure chiral centers 40). This is not the result of a chiral induction by the optically active alkyl group, since irradiation of 67g of 100% ee in benzene gave 69g of only 12% de (diastereomeric excess). 

A second example of homochiral columns formed by discotics are the complexes of tetrazoles (59 and 60) with l,3,5-tris(4,5-dihydroimidazol-2-yl)benzene (61).74 Four molecules self-assemble to give a supramolecular disc and these discs subsequently form columns in nonpolar solvents. Chiral discs were obtained from the self-assembly of the chiral tetrazole (60) with 61. The chirality of the side chains was found to induce a bias in the helic-ity of the supramolecular assembly. Sergeants-and-soldiers measurements75 were performed for which chiral (60) and achiral (59) molecules were mixed. The experiments showed no amplification of chirality, thus revealing that in these systems chirality transfer from the side chains into the column is... 

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