Hinder catalyst

Successful solutions to the a-halogenation of ketones have also emerged, but here the problem is that sterically very hindered catalysts that work very nicely with aldehydes are not active with ketones. Some current solutions to this problon are summarized in Scheme 31 [314,317,318]. 

Use of a More Hindered Catalyst. Using the more steri-cally hindered catalyst RhCl(Cyttp), where Cyttp represents the... 

In the cases of catalysts containing the naphthalene linker (30-35), the 1,4-, 1,5-, and 2,7-substituted catalysts 30, 31 and 35 gave good enantioselectivities [13], Whereas, the 2,6-substituted catalyst 34 yielded a moderate result, the sterically hindered catalysts 32 and 33 showed poor enantioselectivities. Especially, the 2,7-naphthalene-linked catalyst 35 was found to possess excellent catalytic activity for this alkylation from the viewpoints of enantioselectivity and chemical yield. 

The formation of 64 using catalyst (S,S)-62 exhibits a positive nonlinear effect, fitting well with Kagan s two ligand model [78] whereas the more hindered catalyst (S,S)-63 led to a perfect linear asymmetric induction suggesting that the product arose from a transition structure involving only one chiral phosphoramide. The kinetic study of this aldol reaction is in accordance with these re-... 

Some fairly hindered catalysts, such as the rosindones [97] and metal porphyrins [98] are as efficient as AQ at low concentrations in delignifying wood and/or fragmenting models. This observation is contrary to what one would expect with an adduct mechanism metal ion complexes have been proposed to fragment lignin models by SET pathways. Moreover, carbohydrates have been shown to facilitate p-ether cleavage in lignin models [99] and can also transfer electrons to QMs [87]. 

They further feel that isotactic-stereoblock nature of the polymers obtained with this catalyst proves unambiguously that the active cation oscillates between the two enantiomorphous racemic-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less hindered catalyst gives instead a largely stereoirregular... 

Very hindered catalysts 96a, b (20 mol%) permitted to reach very good level of enantioselectivity (74-91% ee) in the reaction of acetone with aromatic aldehydes using a DMF/acetone mixture as reaction media at 25°C, with the camphorsulfonyl derivative 96b giving better results even using only 10 mol% of catalyst [173]. 

The reaction was performed with CAL-B in the presence of isoproprenyl acetate as the acyl donor in toluene. This new catalyst showed a higher stability combined with an improved performance as an alcohol racemisation catalyst in comparison with its well-known analogue, Backvall s catalyst. The enhanced stability of this catalyst compared with Backvall s catalyst could be related to the better shielding of the metal centre towards hydrolysis, thus hindering catalyst decomposition. As shown in Scheme 4.34, the corresponding acetates were obtained in high to excellent yields and excellent enantioselectivity of >97% ee. 

Catalysts such as 56 also lead to alternating isotactic ethylene—propylene copol5uners through a similar mechanism (198), since conditions can he arranged such that ethylene incorporates predominantly on the more hindered catalyst side. Other Monomers. 

For reference, the polymerization of 1-hexene with several catalysts under similar conditions is also included in Table 1. The productivities for both monomers with the most sterically hindered catalyst precursor, Cp 2ZrMe2, are almost identical. In contrast, the productivities for the amine monomer using the less sterically hindered precursor, rac-(EBI)ZrMe2, are two orders of magnitude lower than those observed with 1-hexene. This is likely a result of coordination of the Lewis basic amine nitrogen to the Lewis acidic metal center. Experiments to address the observed difference in monomer reactivities are currently in progress. 

In 2009, Fu et al. developed highly 4-phenoxy-substituted prolinamide phenols, which could promote the asymmetric aldolisation of cyclohexanone with a range of aldehydes with a high degree of diastereo- and enantioselectivity in a large amount of water (Scheme 2.12). The best enantioselectivities of up to 97% ee were obtained with the most steric hindered catalyst that bore a tert-butyl group on the phenol moiety. The scope of the reaction could be extended to aliphatic ketones with enantioselectivities of up to 94% ee, albeit with low to moderate diastereoselectivities of up to 50% de. 

Diastereomeric phosphoramides have been employed to catalyse the asymmetric aldol addition of trichlorosilyl enolates to benzaldehyde. Good anti/syn product ratios were achieved, but these were reversed on employing a more hindered catalyst, and the ratios were also affected by the catalyst concentration. A mechanistic switchover is proposed one transition-state geometry involves a 1 1 complex (cat-alyst enolate) favoured by a hindered catalyst in low concentration, while the other route involves a 2 1 stoichiometry. 

More recently, a Au(I)-derived phosphine enabled the formation of bicy-clo[5.2.0]nonadienes 122 from simple 1,8-enynes with great diversity of possible substituents (Scheme 5.44) [126]. Yields and diastereoselectivities were high if the shown sterically hindered catalyst was used instead of the known (Ph3P)AuNTf2. 

The abiUty of enals to react as homoenolates [84] in the presence of NHC catalysts was first noted in 2004 independently by the groups of Glorius and Bode [85]. In both cases, the use of stericaUy-hindered catalyst 99 and a strong base (DBU or Bu OK) allowed a -d umpolung through the extended Breslow intermediate (101), thereby attacking an aldehyde electrophile and ultimately forming a y-lactone diastereoselectively (105, = H) (Scheme 18.18). Whereas two aldehydes can... 

The synthesis of the bicyclo heptanone synthon 21 was achieved in five steps from isovaleraldehyde. The silyloxy aldehyde synthon 22 was synthesized from the readily available isobutenol. The cycloaddition was first attempted using standard Lewis acid conditions, with either tin chloride or tin triflate however, this led to competitive elimination and the cyclic adduct was not obtained. It was later discovered that the use of the hindered catalyst MADNTfa [(ArOjaAlNTfa] resulted in the cycloaddition, afrbrding... 

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