Substituted catalysts

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

Comparing the reactivity of the catalysts (Fig.l and Table 1), the -OH substituted catalyst showed highest activity with EAS and the least to -OMe substituted catalyst. But with MAO the trend was reversed. Even though it is difficult to establish a direct influence for these push-pull substituents on the metal center, their roles have been try to investigate by cyclic... 

The catalyst jnecursors were tested in solution polymerization runs at 1.3 bar of elhylene pressure in toluene at tempraature (Tp) between 10 and 50 C and the results are summarized in Table 1. The active catalysts are generatKl in situ in toluene by the addition of MAO to the catalyst precursor in the prraence of ethylme. Methyl substituted catalyst (3a/MAO) showed the highrat activity while isopropyl homologue (3c/MAO) the lowest activity. The 3a/MAO catalyst showed higher activity than 3c/MAO by 2-fold at low Tp (say 10 C). However, as Tp... 

Unfortunately, the ethylene oligomers produced by the Ph-substituted catalyst are a complex mixture of 1-alkenes, 2-alkyl-1-alkenes, and 2-alkenes (Scheme 25). In contrast, the OEt-substituted catalyst selectively affords 1-alkenes (average number of inserted ethylene units 7), due to its relative reluctance to react with olefins other than ethylene 44 An X-ray crystal structure of the OEt-substituted... 

Ionkin has reported a similar series of Ni(II) catalysts 1.50a and b bearing ortho-difuryl substituents that are noteworthy for their high thermal stability [127], The bulkier benzofuranyl-substituted catalyst 1.50b possesses the most attractive catalytic properties (Table 5, entry 9) the ability to form high molecular weight polymers (albeit in high polydispersity) and reasonable activity even at 150 °C. Even under these harsh conditions, the polymer branching density is still relatively low. 

The hydrogen consumption and enantioselectivities for the asymmetric hydrogenation of dimethyl itaconate with various substituted catalysts of the basic type [Rh(PROPRAPHOS)COD]BF4 are illustrated in Figure 10.13 [61]. The systems are especially suitable for kinetic measurements because of the rapid hydrogenation of COD in the precatalyst. There are, in practice, no disturbances due to the occurrence of induction periods. 

E. HCA 53,50(1970) (see JMC 14,375(1971) or the following for other methods of reducing (IV)). 40 g (IV) in 90 ml methanol and 10 g aqueous 50% Raney-Ni (or substitute catalyst described in chemical hints section). Add dropwise over ten minutes with vigorous stirring a solution of 7.6 g NaBH4 in 25 ml 8N NaOH and cool to keep temperature at 50°. H2 evolution stops in about five minutes. Filter, wash with methanol and evaporate in vacuum to get mescaline (V0. 

Mechanism of Titanocene-Mediated Epoxide Opening by Homolytic Substitution Catalyst Structure,... 

The ion pairing between the enolate of 5 and the catalyst should make the asymmetric induction sensitive to the electronic effects of substituents on the N-benzyl group. A Hammett plot of log ee/eeQ vs the substituent constant o of the para N-benzyl substituted catalysts (R = CH3O, CH3 H, F, Cl, CF3) gave a reaction constant of p - 0.21 + 0.02 with ee s in the range of 60% to 92% demonstrating that substituents with increasing... 

MPa O2). The role of the encapsulated [Co(salophen)] complexes is to catalyze the aerobic oxidation of hydroquinone to p-benzoquinone, which in turn oxidizes Pd(0). For the oxidation of 1,3-cyclohexadiene to l,4-diacetoxy-2-cyclohexene, the most active catalyst system involved the encapsulated complex [Co(tetra-tert-butyl-salophen)], which afforded product yields of 85-95% after 3 h at room temperature with greater than 90% trans-selectivity. This complex displayed significantly higher activity than the encapsulated [Co(salophen)] complex (72% yield in 3h) and the analogous homogeneous complex (86% yield in 5h). The increased activity of the t-butyl substituted catalyst was attributed to distortion of the bulky complex by the... 

For the calculations we used a simplified model system in which all substituents were replaced by methyl groups (Scheme 4). Experimentally, the methyl substituted catalyst and methanol as nucleophile are active, but the enantiomeric excesses obtained fall below those obtained with the tert-XmcinQ amide-derived catalyst in combination with allyl alcohol (Scheme 3). 

Recovery of the metallodendritic catalysts with Cy substituents was achieved by precipitation with pentane, and the dendrimers were reused five times without significant loss of activity. However, the tBu-substituted catalysts could not be recovered by this method as a consequence of their high solubility in pentane. 

Widenhoefer and co-workers have developed an effective Pd-catalyzed protocol for the asymmetric cyclization/ hydrosilylation of functionalized 1,6-dienes that employed chiral, non-racemic pyridine-oxazoline ligands." " " Optimization studies probed the effect of both the G(4) substituent of the pyridine-oxazoline ligand (Table 7, entries 1-6) and the nature of the silane (Table 7, entries 6-15) on the yield and enantioselectivity of the cyclization/ hydrosilylation of dimethyl diallylmalonate. These studies revealed that employment of isopropyl-substituted catalyst (N-N)Pd(Me)Gl [N-N = (i )-( )-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] [(i )-43f and a stoichiometric amount of benzhydryldimethylsilane provided the best combination of asymmetric induction and chemical yield, giving the corresponding silylated cyclopentane in 98% yield as a single diastereomer with 93% ee (Table 7, entry 15). 

Pyridine adsorption experiments have showed that the nickel containing smectites have Lewis acid sites and do not have Bronsted acid sites [9]. The Ni2+-substituted smectite catalysts have large surface areas even after 873 K treatment because many small fragments with the same smectite structure are intercalated in the interlayer region. The activities of the Ni2+ substituted catalysts are derived from Ni2+ Lewis acid sites located on the edge framework. 

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. 

With bis(3,4,5-trifluorophenyl)-substituted catalyst (S)-16A, the substituent effect of R was examined by changing the number of straight alkyl chains (Figure 5.1). In the asymmetric benzylation of N-(diphenylmethylene)glycine tert-butyl ester 2, use of (CH3(CH2) ,1)2NH (n > 4) gave a uniformly high asymmetric induction. 

As an extension of this research, Maruoka and coworkers succeeded in the catalytic asymmetric conjugate addition of nitroalkanes to cyclic a,[S-unsaturated ketones under phase-transfer conditions (Scheme 5.40) [39]. Here, the use of 3,5-bis(3,4,5-trifluorophenyl)phenyl-substituted catalyst (S,S)-lj is crucial for obtaining the high enantioselectivity. 

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