QUINAP complexes

Similar to the CuOTf/PyBox system, the CuBr/QUINAP system also gave high enantioselectivities of the three component reactions to construct propargyl amines from aldehydes, amines, and alkynes (Scheme 5.6). In this system various aldehydes including aromatic aldehydes and aliphatic aldehydes could be used and a wide range of chiral propargyl amines were prepared in good yields and enantioselectivities. Mechanistic studies showed that the dimeric Cu/QUINAP complex is the catalytically active species that differs from the previous reaction. 

Fig. 2.13 Examples of hydroboration/amination using Rh-QUINAP complexes and electrophilic aminating agents. (I) Catecholborane, 1 mol% (S)-QUINAP-Rh catalyst, THE, RT, 1 h 2 equiv. MeMgCI in THE, 30 min solid H2NOSO3H,...

This reaction is believed to proceed via nucleophilic combination of in situ generated Cu-acetylide and iminium ion. Mechanistic studies indicate a strong positive non-linear effect based on which a catalytic cycle is proposed that involves a dimeric Cu/quinap complex as the active catalytic species. 

The hydroboration of stilbenes and related disubstituted alkenes catalysed by QUINAP complexes may proceed with high enantio- and regio-selectivity [(48) (49)] rhodium and iridium catalysts give the same regioisomer but opposite enantiomers.58... 

Figure 2.10 Hydroboration of stilbenes catalyzed by QUINAP complexes [64]. (Reproduced with permission.)...

Among them are planar chiral chelating iridium complexes bearing phosphinyl-imidazolylidene ligands 238 " dihydrooxazole-imidazolylidene ligands 239 " axial chiral Ir(I)-QUINAP-complexes 240 " planar chiral Ir(I)-FcPHOX-complexes 241 and analogs, " and so on. 

Li X, Kong L, Gao Y, Wang X. Enantioselective hydrogenation of olefins with axial chiral iridium QUINAP complex. Tetrahedron Lett. 2007 22(28) 3915-3917. 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

Cationic rhodium complexes of these ligands were prepared and applied in the enantioselective hydroboration-oxidation of a range of vinylarenes,106,107 carefully chosen to highlight the effect on reactivity and enantioselectivity of different aryl substituents and / -substitution. Like QUINAP 60 and PHENAP 65, the ( -ligand gave rise to the (A)-secondary alcohol. 

The two-substituted-Quinazolinap-derived rhodium complexes proved extremely efficient catalysts for the hydro-boration-oxidation of vinylarenes (Table 6). For styrene derivatives, in most cases quantitative conversions were obtained after just 2 h at the relevant temperature (entries 1-6). Higher enantioselectivities were afforded with a 4-methoxy substituent (up to 95% ee, entry 3) compared to the 4-chloro or unsubstituted styrene analogs (entries 5 and 1), a trend also observed in hydroboration with rhodium complexes of QUINAP 60. This highlights that both the electronic nature of the substrate combined with the inherent steric properties of the catalyst are important for high asymmetric induction. It is noteworthy that in most cases, optimum enantioselectivities were afforded by the... 

In an attempt to rationalize the factors that control selectivity in the Rh- and Ir-catalyzed hydroboration reactions, Fernandez and Bo [35] carried out experimental and theoretical studies on the H—B addition of catecholborane to vinylarenes with [M(C0D)(R-QUINAP)]BF4, (QUINAP = l-(2-diphenylphosphino-l-naphthyl) isoquinoHne). A considerable difference was found in the stability of the isomers when the substrate was coordinated to the iridium(I) or rhodium(I) complexes. In particular, the difference between pro-R B1 and pro-S B2 isomers was not so great when the metal center was iridium and not rhodium (Figure 7.1), which explains the low ee-values observed experimentally when asymmetric iridium-catalyzed hydroboration was performed. Structurally, the energy analysis of the n2 and Tti interactions [36] seems to be responsible for the extra stabilization of the B2 isomer in the iridium intermediates (Figure 7.1). The coordination and insertion of alkenes, then, could be considered key steps in the enantiodifferentiation pathway. 

Figu re 7.1 Relative stability of the most stable isomers in the hydroboration of vinylarenes with cationic metal complexes modified with QUINAP. 

Asymmetric hydroboration of styrenes employing diphosphine complexes provides a successful solution to the generation of chirality at a benTyhc site, a potentially important route to many bioactive molecules. Since substitution on the double bond leads to severe loss of enantioselectivity, the apphcations are necessarily hmited. It was adventitiously discovered that the P,N-ligand QUINAP [47], which is effective in asymmetric... 

Fig. 2.12 (a) Hydroboration/oxidation with (S)-QUINAP-Rh complexes at ambient temperature. The numbers refer to the ee of the S-enantiomer of product, (b) Kinetic resolution with (R)-QUINAP-Rh conditions 0.5 equiv. catecholborane, 1 mol% catalyst, toluene, 2 h H202/NaOH. 

Li and co-workers examined a variety of ligands such as chiral bisoxazolines, Quinap, and Binap as ligands in conjugation with various copper salts. Li found that CuOTf with PyBox provided the best results for 29 and found that aromatic substituted alkynes gave the best yields and enantioselectivity relative to the aliphatic substituted alkynes. Li also found that the presence of an ort/io-methoxy substituent improved enantioselectivity and attributed the improvement in selectivity to potential coordination of the oxygen atom to the copper/Ugand complex. 

An enantioselective synthesis of chiral QUINAP 234 was reported by Knochel et al. (07SL2655). The organolithium species obtained from l-(2-bromo-l-naphthyl)isoquinoline by treatment with f-BuLi reacted with (—)-menthyl (S)-p-toluene-sulfinate at-78 °C. The resulting diastereomers were separated via column chromatography. One pot sulfoxide lithium exchange at low temperature, Ph2PCl reaction, sulfur protection with Ss and a Raney-Ni desulfurization step afforded optically pure QUINAP (99% ee) in 60% yield. The s)mthetic route avoided the use of Pd complexes for the resolution. The ees were determined after resulfurization on Chiralcel OD-H. 

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