Catalytic Substitution Reactions

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Piperazinothiazoies (2) were obtained by such a replacement reaction, Cu powder being used as catalyst (25. 26). 2-Piperidinothiazoles are obtained in a similar way (Scheme 2) (27). This catalytic reaction has been postulated in the case of benzene derivatives as a nucleophilic substitution on the copper-complexed halide in which the halogen possesses a positive character by coordination (29). For heterocyclic compounds the coordination probably occurs on the ring nitrogen. 

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

This class of substrate is the only real problematic substrate for the AE reaction. The enantioseleetivity of the AE reaction with this class of substrate is often variable. In addition, rates of the catalytic reactions are often sluggish, thus requiring stoichiometric loadings of Ti/tartrate. Some representative product epoxides from AE reaction of 3Z-substituted allyl alcohols are shown below. 

See also Enzyme cofactors downhill trajectories for, 196,197 mechanism of catalytic reaction, 190-192 metal substitution, 200-204 potential surfaces for, 192-195,197 rate-limiting step of, 190 reference solution reaction for, 192-195,... 

These nano-objects display an organometalhc surface chemistry comparable to usual organometalhc moieties and which can be studied by classical spectroscopic methods substitution reactions leading to structural changes in the particles, the fluxional or non-fluxional behavior of surface hgands, the formation and observation of surface hydride species, the monitoring of catalytic reactions etc. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Van Eldik attempted to achieve catalytic reactions with substituted nitrogen macrocycles and carbon dioxide but was unsuccessful in the synthesis of alkyl and aryl carbonates.458 A bridging... 

Ideally, those molecules that are involved in the catalytic reaction should be the best characterizers of catalytic sites. Indeed, the path of the development of organic reaction mechanisms is paved with clever examples of stereochemistry and isotopic substitution that reveal the nature of activated complexes and intermediates and allow the unambiguous interpretation of the stereorelations... 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

Related phosphine-substituted derivatives were also obtained, and the chirality of the clusters demonstrated by the NMR spectra of the compounds (246). Molecules of this type have obvious utility in establishing the potential intermediacy of the polynuclear adduct in a catalytic reaction by the formation of optically active products. 

The absolute stereochemistry for 150 (entries 2 and 3) was determined by hydrolysis and conversion to known compounds. Assuming a tetrahedral or cis octahedral geometry for the magnesium [110], the product stereochemistry is consistent with si face radical addition to an s-cis conformer of the substrate. This is the same sense of selectivity as that obtained with oxazo-lidinone crotonates or cinnamates suggesting that the rotamer geometry of the differentially substituted enoates is the same. The need for stoichiometric amount of the chiral Lewis acid to obtain high selectivity with 148 in contrast to successful catalytic reactions with crotonates is most likely a reflection of the additional donor atom present in the substrate. 

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