Cascade reactions Subject

TTF (tetrathiafulvalene) and related compounds have been the subject of intense interest in the materials chemistry community because of their semi-conduction and superconduction properties. Recently, TTF has emerged as a unique radical initiator because its radical cation can be easily formed. The ease of formation is presumably derived from the favorable structure of the radical cation that incorporates an aromatic disulfonium salt and a very delocalized radical [49a]. Murphy et al. demonstrated a novel one-pot reaction cascade... 

Arylazo-4-(3-ethoxycarbonylureido)furoxans 62, which were synthesized by the reactions of 4-amino-3-arylazo-furoxans with ethoxycarbonyl isocyanate, were subjected to cascade rearrangements under the action of potassium r/-butoxidc in dimethylformamide or by heating in dimethyl sulfoxide to form 4-amino-2-aryl-5-nitro-2//-l,2,3-triazoles 63 (Scheme 13) <2001MC230, 2003RCB1829>. 

These examples demonstrate that a selective Heck-Diels-Alder sequence with two different alkenes is only possible either in a stepwise manner, if an alkene reacts much faster in the Heck reaction than in the subsequent cycloaddition so that the 1,3-diene can be isolated, or as a real cascade reaction if one alkene is more reactive and thus selectively reacts as a coupling partner, whereas the other one is a better dienophile. Both concepts have been used by Kollar et al. for the annelation of cyclohexene rings onto the steroidal skeleton 26 (Scheme 4) [28-30]. At 60 °C the cycloaddition was sufficiently suppressed so that the Heck coupling product 29 could be isolated and subsequently subjected to Diels-Alder reactions with different dienophiles. For a domino reaction with both methyl acrylate and dimethyl fumarate (28) present in the reaction mixture, the conditions had to be precisely adjusted so that the mixed products 31 and 32 were formed predominantly along with only small amounts of the products of a twofold reaction of either 27 (R = CC Me) or 28 with 26. These conditions also proved suitable for a cascade reaction of 26 involving allyl alcohol 27 (R = CH2OH) or allyl acetate 27 (R = CH2OAc) and dimethyl fumarate (28). 

It is very well known that jr-allyl palladium complex 1, which is a key intermediate for the Tsuji-Trost type allylation, has an electrophilic character and reacts with nucleophiles to afford the corresponding allylation products. We discovered that bis 7r-allyl palladium complex 2 is nucleophilic and reacts with electophiles such as aldehydes [27] and imines [28-32] (Scheme 2, Structure 2). We have also shown that bis 7r-allyl palladium complex 2 can act as an amphiphilic catalytic allylating agent it reacts with both nucleophilic and electrophilic carbons at once to produce double allylation products [33]. These complexes incorporate two allyl moieties that can bind with different hapticity to palladium (Scheme 3). The different complexes may interconvert by ligand coordination. The complexes 2a, 2b and 2c are called as r]3,r]3-bisallypalladium complex (also called bis-jr-allylpalladium complex), r)l,r)3-bis(allyl)palladium complex, -bis(allyl)palladium complex, respectively. Bis zr-allyl palladium complex 2 can easily be generated by reaction of mono-allylpalladium complex 1 and allylmetal species 3 (Scheme 4) [33-36]. Because of the unique catalytic activities of the bis zr-allyl palladium complex 2, a number of interesting cascade reactions appeared in the literature. The subject of the present chapter is to review some recent synthetic and mechanistic aspects of the interesting palladium catalyzed cascade reactions which in-... 

It is worthy of note that this reaction is still the subject of solid and productive interest, as shown by the following recent examples. Chiu has exploited a rhodium carbene-promoted intramolecular formation of a carbonyl ylid - cycloaddition cascade as the key reaction in the synthesis of the nucleus of the cytotoxic diterpenoids pseudolaric acids A and B [54]. Although the diastereoselectivity was preferential for the undesired isomer 64, use of Hashimoto s chiral rhodium catalyst Rh2(SBPTV)4 reversed the selectivity in favor of 65 (64 65, 1 1.4) [55] (Scheme 29). 

We can easily predict that cascade and sequential catalytic reactions will be the subject of important investigations not only by promoting the cooperation of several metal catalysts but also by organizing the tolerance and the cooperative work of metal and organo catalysts, and of metal and enzyme catalysts. This will be possible through a deep understanding of the mechanisms of each catalytic system, so as to organize their mutual tolerance. 

A radical cascade reaction has been accompHshed by StaHnski and coworkers which converts dipeptide derivatives to nitrogen-containing heterocycles (Scheme 6) [9]. In this work, N-bromobenzyl-hf-propargyl-substituted dipeptides such as 10 were subjected to Stork s catalytic procedure with tributyltin hydride [10]. An aryl radical is formed followed by a 1,5-hydrogen shift, generating the a-centered carbon radical 11. 5-Fxo-dig radical cyclization... 

Much effort has been devoted to the extension of this chemistry by the use of new starting materials and by combining the carbonylation step with new modes of trapping reactions of the intermediate acylmetal complex. The combination of metal-catalyzed C-C coupling reactions with carbonylation chemistry to develop regio- and stereoselective atom-economic cascade reactions has been a major subject of interest. This resulted in a number of new inter- and intramolecular methods for the synthesis of complex organic intermediates, mainly heterocycles. 

If particulate matter has to be dissolved in a liquid or if a chemical reaction catalyzed by a solid is involved, the particles must be suspended from the vessel bottom, so that the total surface can participate in the process. In continuous processes a stochastically homogeneous distribution of the solid in the bulk of the liquid is required, so that the solid particles can be transported with the liquid from stage to stage (for example in a cascade crystallization process). In this intensive suspension process, the solid is, as a rule, subjected to high mechanical stress, which can result in its attrition. 

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