Backbone binaphthyl

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

Besides the function of the l,l -binaphthyl backbone, which is very important for high enantioselectivity, configuration of the central chirality on the... 

The binaphthyl backbone of BINAP has inspired many variations of atropisomeric biaryl bisphosphines. One approach by Roche was to substitute the binaphthyl backbone with a 6,6 -dimethox-ybiphenyl backbone. MeO-Biphep (96a) was synthesized in approximately 26% yield in 6 steps from 3-bromoanisole (97a) (Scheme 12.30). MeO-Biphep can also be synthesized in 5 steps from 2-iodo-3-nitroanisole in approximately 18% yield. Several phosphine analogues can be prepared by the addition of R2PC1 to the lithio intermediate.117... 

A third example comes from Clyne et al. [358] and concerns the axial chiral binaphthyl backbone [359,360], itself known from phosphorus chemistry [361]. The synthesis starts from the trifluoromethylsulfonato substituted binaphthyl with a Kumada coupling reaction [291,292] with methytmagnesiumbromide. Oxidation with NBS yields the methyl brominated derivative that can be attached to the imidazole ring. Subsequent methylation results in the bis-imidazolium salt that is deprotonated to the bis-carbene and coordinated to the transition metal halide (Pd, Ni), a rather straightforward reaction sequence (see Figure 3.113). The overall yield for the four-step reaction to the bis-imidazolium salt is surprisingly good (65%). 

We are already well accustomed with the historical relationship seen in the development of NHC ligands with special features. They often follow the example of successful phosphane ligands. Chiral NHC are no exception, although in key areas the very difference in shape between NHC and phosphanes prevents the development of NHC ligands as structural phosphane mimics. The axial (atropisomers with binaphthyl backbone [6,7]) and planar (ferrocene derivatives [8,9], [2,2]-paracyclophanes [10,11]) chiral examples, however, are styled on their phosphane predecessors [12-15]. We will first look at the unique options for NHC ligands before we turn to the more familiar phosphane mimics. 

It is not strictly necessary to introduce the 1,1 -binaphthyl backbone. For axial chirality, the biphenyl scaffold is sufficient, provided that rotation around the phenyl-phenyl axis is sufficiently hindered. Hoveyda combined this reduced axial chiral motif with additional central chirality in the imidazole backbone (O and C ) [6,7], Synthetically, the task is accomplished by Buchwald-Hartwig amination of enantiomerically pure (H ,21 )-diphenylethylenediamine with 1-methoxy-I -iodo-biphenyl and subsequent reaction with mesityl bromide to introduce the bulky wingtip group on the second amino group of the chiral starting material. Ring closure reaction with triethyl orthoformate and hydrolysis of... 

Simple L-alanine, L-valine, L-norvaline, L-isolecucine, L-serine and other linear amino acids [ 121 ] or chiral amino acids with a binaphthyl backbone [ 122] and peptides have also been used as asymmetric catalysts [123,124,125,126]. Solid-supported proline-terminated peptides have been used for heterogeneous catalysis of the asymmetric aldol reaction [ 127]. Apart from proline and derivatives, other cyclic compounds such as 5,5-dimethyl thiazolidinium-4-car-boxylate (DMTC) [128], 2-fert-butyl-4-benzyl imidazolidinones [129], (l/ ,25)-2-aminocy-clopentanecarboxylic acid [130], (5 -5-(pyrrolidin-2-yl)tetrazole, (5)-l,3-thiazolidine-4-car-boxylic acid, (5)-5,5-dimethyl-l,3-thiazolidine-4-carboxylic acid, and (5)-hydroxyproline are effective catalysts in asymmetric aldol reactions [126,131,132,133,134,135]. 

The chiral phosphines (219) and (220), based on camphor lactam or binaphthyl backbones, were obtained by treatment of the precursor amine with BuLi or NaH, followed by quenching with Ph2PCl.465,466... 

Recently, Fan et al. reported a series of chiral BINOL-cored dendrimers via substitution at the 3,3 -positions of the binaphthyl backbone by different generations of Frechet-type dendrons (Figure 4.18) [73]. The proximity of the dendritic wedges to the catalytic center is expected to induce catalytic properties different from BINOL. In the absence of Ti(Oi Pr)4, the chiral dendrimer ligands showed much... 

Several miscellaneous polymer-supported chiral phosphine ligands and oxazoline ligands are depicted in Figure 6.8 and Figure 6.9, respectively. Binaphthyl monophosphine MOP [36] was anchored onto PS-PEG resin at the 2 -, 6- and O -positions of its binaphthyl backbone (compounds 84, 85 and 86) [37]. The polymeric phos-... 

The asymmetric synthesis of a-alkyl-a-amino acids using a chiral catalyst is a useful method for the preparation of both natural and unnatural amino acids. O Donnell et al. developed the cinchona alkaloid-catalyzed alkylation of glycine derivatives [49]. However, almost all of the chiral phase-transfer catalysts were restricted to cinchona alkaloid derivatives. In 1999, Maruoka and co-workers designed a chiral ammonium salt bearing a binaphthyl backbone as a chiral phase-transfer catalyst (10a) (Figure 10.11), and demonstrated its catalytic activity... 

Imai, Y, Zhang, W., Kida, T. et al. (1998) Diphenylphosphinooxazoline hgands with a chiral binaphthyl backbone for Pd-catalyzed aUylic alkylation. Tetrahedron Lett., 39,4343-6. 

In the area of organocatalysis, proline has been utilised in various asymmetric reactions including direct asymmetric aldol reactions. Some such proline-catalysed aldol reactions, however, have serious limitations with respect to reactivity and selectivity. Although these problems were overcome through the development of new catalysts derived from proline, there is still an urgent need for structurally and electronically novel catalysts due to the difficulty in appropriate modification of proline. In this context, we have designed and prepared artificial amino acid catalyst (S)-l having a binaphthyl backbone as a frequently utilised chiral unit in asymmetric catalysts. ... 

Many metal-catalysed allylic substitution reactions have been reported. Adding a terminal olefin to the binaphthyl backbone of a phosphorus amidite ligand in a [PdCl(C3H5)]2-ligand-K2C03 catalysed reaction between indoles and l,3-diphenyl-2-propenyl acetate has enabled the allylic substitution to occur, with yields ranging from 72 to 96%, with 91-98% ee. 

This ligand has been unique in the following aspects ii) It is fully aromatic and conformationally flexible. The binaphthyl backbone can change its geometry from synclinal to anticlinal by rotation around Cd)-C(l ) axis. Hi) The seven-membered chelate rings thus produced are conformationally unambiguous because they contain only sp carbon atoms. 

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