Metal-allyl complexes Synthetic applications

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Among 7i-allyl complexes of several transition metals, the chemistry of n-allylpalladium has been studied most extensively. From the standpoint of organic synthesis, reactions involving 7i-allylpalladium complexes are by far the most important therefore, their synthetic applications are mainly treated in this chapter. 

Electrophilic substitution of r/ -allyl complexes, especially those of Si and Sn, has found extensive synthetic applications, but the overall transformation is stoichiometric with regard to the amount of the metal atom. A catalytically useful reaction of r/ -allyl intermediate was involved in telomerization of 1,3-dienes in the presence of Pd catalyst shown in Scheme 8.41. r/ -Allylmetal complexes were reactive with not only H+ but also electrophilic alkenes (e.g. Scheme 8.72) [62,133]. Recent development in Pd-catalyzed amphiphilic allylation of alkenes and imines (e.g. Scheme 8.73) relied on the high susceptibility of Pd-bound r -allyl ligand to the attack of unsaturated carbon electrophile [134]. 

Allylic substitution is well known and other metal-based complexes have been reported to catalyze reactions with hydroxyl leaving groups/ Indeed, with homogeneous Au-catalysis, a variety of reactions that proceed via cationic pathways have been reported. " While discussion is beyond the scope of this chapter, it is worth noting that the mechanism for this reaction has been extensively studied and does not appear to proceed via an allylic cation/ Further developments in this area have been reported by a variety of groups and now involve the use of nitrogen nucleophiles, carbon nucleophiles, and inter-molecular reactions as well as enantioselective variants/ While many of those reports came from our laboratory, our interests took us in a different direction and we became keen on using modified substrates to develop spiro-ketalization methods for applications in synthetic schemes. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

Supramolecular chemistry has been a very popular research topic for three decades now. Most applications are foreseen in sensors and opto-electronical devices. Supramolecular catalysis often refers to the combination of a catalyst with a synthetic receptor molecule that preorganizes the substrate-catalyst complex and has also been proposed as an important possible application. The concept, which has proven to be powerful in enzymes, has mainly been demonstrated by chemists that investigated hydrolysis reactions. Zinc and copper in combination with cyclodextrins as the receptor dramatically enhance the rate ofhydrolysis. So far, the ample research devoted to transition metal catalysis has not been extended to supramolecular transition metal catalysis. A rare example of such a supramolecular transition metal catalyst was the results of the joined efforts of the groups of Nolte and Van Leeuwen [SO], They reported a basket-shaped molecule functionalized with a catalytically active rhodium complex that catalyzed hydrogenation reactions according to the principles of enzymes. The system showed substrate selectivity, Michaelis Menten kinetics and rate enhancement by cooperative binding of substrate molecules. The hydroformylation of allyl catachol substrates resulted in a complex mixture of products. 

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