Catalytic examples

In 1975, Heck described the first non-catalytic example of this reaction, nsing organoboronic acids as precursors and stoichiometric amounts of palladium acetate... 

Maruoka and co-workers recently reported an example of a Zr-catalyzed cyanide addition to an aldehyde [64]. As is also illustrated in Scheme 6.20, the reaction does not proceed at all if 4 A molecular sieves are omitted from the reaction mixture. It has been proposed that the catalytic addition proceeds through a Meerwein—Ponndorf—Verley-type process (cf. the transition structure drawn) and that the crucial role of molecular sieves is related to facilitating the exchange of the product cyanohydrin oxygen with that of a reagent acetone cyanohydrin. The example shown is the only catalytic example reported to date the other reported transformations require stoichiometric amounts of the chiral ligand and Zr alkoxide. 

The general concept of cascade conversions is demonstrated by a representative selection of illustrative bio-bio, chemo-chemo, and bio-chemo catalytic examples on a laboratory scale as well as on a pilot- or industrial-scale. 

Up to the late 1990s, combined multi-step chemo-chemo conversions were restricted to a few catalytic examples. Apparently, there has been little effort or interest in developing a toolkit of chemocatalytic reactions that are mutually compatible with respect to reaction conditions. Consequently, chemocatalysts have not yet reached the same level of mutual compatibility as biocatalysts. Some recent examples prove, however, the potential power of chemo-chemo catalytic cascades. 

A catalytic example of C-S bond breakage in benzothiophene has been reported by Bianchini [47], A catalytic desulfurisation was not yet achieved at the time as this is thermodynamically not feasible at such mild temperatures because of the relative stability of metal sulfides formed. Bianchini used a water-soluble catalyst in a two-phase system of heptane-methanol/water mixtures in which the product 2-ethylthiophenol is extracted into the basic aqueous layer containing NaOH. Figure 2.43 gives the reaction scheme and the catalyst. The 16-electron species Na(sulfos)RhH is suggested to be the catalyst. Note that a hydrodesulfurisation has not yet been achieved in this reaction because a thiol is the product. Under more forcing conditions the formation of H2S has been observed for various systems. 

In addition, a collection of carefully selected representative catalytic examples, organized by reaction type, is given in the Appendix. These tables will offer a great deal of information and be invaluable to anyone who wants to get the information of the current state of the art on this topic within a short time. 

When the alkali-metal cation of a sulfonated phosphine or phosphite is replaced by ammonium ion it is possible to devise applications in nonaqueous solvents. Al-kylammonium salts of TPPTS were reported in the first patents of Kuntz [5] and catalytic examples were given in which water alone was the solvent. More recently Fell et al. prepared a series of sulfonated phosphites including the example shown below, 1 [6], and examined these in a variety of solvents for the hydroformylation of tetradecene. Catalytic results are reported with acetone and acetophenone as the... 

The complex experimental evidence points to the fact that difference in orientation, more than the actual arrangement of the outer planes, may govern the speed of a catalytic reaction. Whether a certain orientation promotes the speed or slows it down may well depend on the mechanism of the slowest of the various consecutive reactions of the whole sequence of the catalytic example which is studied. 

In this chapter, the aerogel synthesis method is detailed and recent catalytic examples, from 1999 up, will be presented and discussed. Because of their potentialities, ambigels, which do not require the use of an autoclave, are also briefly described. 

Further catalytic examples are discussed in the section Chiral Catalysts Containing Transition Metals.)... 

A catalytic example is provided by the autocatalytic reaction scheme in which x multiplies ... 

The most widely used electron-donating a-substituents are alkoxy groups. Indeed, these substrates are so reactive that they enabled the first truly catalytic examples of Lewis acid-catalyzed Nazarov reactions. Whereas stoichiometric quantities of Lewis acid are often required due to slow protonation of the Lewis acid enolate, a-alkoxy appended systems show efficient catalytic turnover. For example, Trauner and co-workers reported the efficient Nazarov cyclization of 37 in the presence of 10 mol % of aluminium trichloride. " Not only do a-alkoxy groups lower the activation barrier to cyclization, but they also localize the resultant positive charge at one a-carbon, ensuring a highly regioselective elimination. 

Polymerization Catalytic Example Tacticity R.l. Comonomer References... 

Although a multicomponent [4+2] cycloaddition/allylbo-ration reaction is not a new protocol [44] and this sequence has been explored to trap the apparently unstable intermediate with an aldehyde to afford the corresponding alcohols directly after hydrolysis, the pioneering chiral catalytic example was independently reported by Hall s [45] and Carreaux s [46] groups in 2003, and afterward. Hall s research group has extended the methodology (Scheme 4.25) [47],... 

Rh and Ir complexes bearing at least two N-heterocyclic carbene (NHC) moieties within a tridentate ligand framework, mostly in form of a pincer ligand, are the subject of this chapter. Special properties of each ligand type, its complexes, and catalytic examples will be discussed. 

Although pincer complexes are in general ubiquitous in catalysis [7,10], the reactions evaluated for Rh andir pincer complexes are still quite limited. Therefore, the few catalytic examples ofthose pincer complexes bearing two NHC moieties show the potential of these complexes rather than their limits, as the catalytic applications are highly diverse. 

Aiming to develop an efficient route to 1,2-dihydropyridines, Ogoshi et al. performed extensive mechanistic studies with a stoichiometric amount of nickel complex [16]. These studies revealed a plausible mechanistic pathway for this particular cycloaddition reaction. Further experimentation revealed that the use of electron-donating, sterically hindered phosphines actually renders the reaction catalytic. Three catalytic examples were reported. These include the cycloaddition of alkyl-aryl alkynes (72 and 73) and trimethylsilyl acetylene (74) (Scheme 2.19). The moderate yields of cycloadducts and small substrate scope still need to be addressed. 

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