QUINAP complexes hydroboration

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,...

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.)...

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. 

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