Reactions Using Catalysts

Compound 36 when treated by sulfur under nitrogen, leads to the thiazoline-2-thione (37) (Scheme 16) (43). Oxidation by O- or air of 36 (43) or 38 (45, 46) leads to the corresponding thiazoiine (39 or 40). Consequently, condensation reactions using catalysts like 36 must be run in strictly oxygen-free atmosphere (47-50). The isolation of traces of 3-benzyl-4-methyl-A-4-thiazoline-2-one (42) as a product of the oxidation of... 

Scheme 20 Vinylogous aldol reactions using catalyst 48...

Grubbs and co-workers have further investigated the influence of allylic substitution on E/Z diastereocontrol in olefin CM reactions using catalyst 5. In some cases, it was found that secondary and tertiary allylic alcohols could afford complete -selectivity, particularly when a cross-partner bearing allylic heteroatom substitution was used. Also, in contrast to the less reactive catalyst 2, catalyst 5 was found to promote the CM reaction of olefins bearing quaternary allylic substitution (Scheme 7). The cross-partners in these examples represent type III olefins with respect to 5 therefore, they can be applied either stoichiometrically or in excess without a reduction in yield. E/Z ratios of >20 1 were typically observed. 

One of the most unique applications of CM to reagent synthesis is described in a report by Zhang and Herndon, where catalysts 2 and 5 were used to effect the homologation of homoallylic Fischer carbene complexes (Equation (4)). The E/Z ratios observed in these reactions using catalyst 5 were typically 9 1. Highly reactive, electron-neutral olefins were... 

The acetate aldol addition reactions using catalyst 72 have found application in a number of syntheses. S imon and co-workers util ized aldol adduct 74 as a key building block in the total synthesi s... 

Aldol reactions using catalyst 12 were also performed with water without any organic solvent (Table 2.9) [15]. In these examples, it is assumed that the reaction occurred in the organic phase composed of reactants and the catalyst separated from the water phase. A large excess of ketone was used in these reactions. Catalysts possessing a tert-butyldimethylsilyl (TBS) or a triisopropylsilyl (TIPS) group instead of the tert-butyldiphenylsilyl (TBDPS) group on 12 also effectively catalyzed the reaction with water, but hydroxyproline 2 did not catalyze the reaction under the same conditions this indicated that the hydrophobic substituents of... 

Seiders et al. [92] have prepared enantiopure ruthenium-based catalysts 41a and 41b for use in asymmetric RCM reactions. Using catalyst 41b, the desym-metrisation of achiral triene 46 was not as selective as was reported by Hoveyda and Schrock [85] using 40a. Nonetheless, under optimised conditions (involving the formation of an iodo derivative of 41b in situ) a product (47) enantiomeric excess of 90% was possible (Scheme 15), thus showing that there is some potential for analogues of the more robust 4 to serve as efficient chiral catalysts. 

An extended addition increases the effective mole ratio of reaction components, which can have a beneficial impact on reaction selectivity. This is particularly true for reactions using catalysts. In the dihydroxylations in Figure 8.7, olefin 13 was added over 6 hours in order to ensure high enantiomeric purity of the product [6], Excess olefin, which accumulates if the addition rate is faster than the reaction rate, is thought to coordinate with the catalyst in a fashion that leads to non-enantioselective dihydroxylation [6]. In the osmium-mediated dihydroxylation of... 

Precursors and catalysts were characterized in ambient conditions by X-ray diffraction (XRD) on a Rigaku Powder Diffractometer using CuK radiation with a Ni filter. LiF was used as an internal standard for the activated catalysts. Laser Raman spectra (LRS) were collected using Ar ion laser excitation (514.5 nm) at a power of 25 mW at the sample. Spectra for the precursors were collected in ambient conditions, and reaction-used catalysts were characterized in-situ at 400°C in a 70 ml/min flow of C4H,(/02/He (0.99/10.2/88.81). Phosphorus to vanadium ratios (molar) were determined by inductively coupled plasma (ICP). Diffuse reflectance spectra (DRS) were collected in ambient conditions using polytetrafluoroethylene as a reference. 

Scheme 11.20 Asymmetric aldol reaction using catalyst 24 in the presence of water .

Scheme 14.9 A tandem, catalytic RCM, and atom-transfer radical reaction using catalyst 18.

SCHEME 9.17 Enantioselective Biginelli reaction using catalyst 47. 

Organozinc compounds, as described above, become highly reactive by using catalysts such as palladium, nickel and copper compounds. Therefore, even if the reaction could not proceed since the reactivity of the organozinc compound is too low, it is able to be carried out. Recently, these reactions using catalysts are noted because the selectivity of the reaction is high [15,35—47]. 

Cyclic amino acid and fluorinated amino acid derivatives and their phos-phonate analogues have been readily obtained via RCM reaction using catalyst 1 [25] and allenylidene 6 [26], Medium size cyclic amino esters have been obtained using catalyst 6 (Scheme 8) [27]. 

Example 1.3-2 Reactions in porous catalysts Many industrial reactions use catalysts containing small amounts of noble metals dispersed in a porous inert material like silica. The reactions on such a catalyst are sometimes slower in large pellets than in small ones. This is because the reagents take longer to diffuse into the pellet than they do to react. How should you model this effect ... 

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