Platinum complexes cycloisomerization

From this overview it also appears that in most cases, catalytic tests have been performed with catalysts formed in situ from a metal precursor and the desired chiral phosphine, according to usual procedures, while specific catalyst design has been done only sporadically. Relevant examples are the DuPHOS/diene rhodium complexes mentioned in Fig. 10.31 (Sect. 10.3.2) and the platinum/NHC/phos-phine derivatives C4 which allowed highly enantioselective platinum promoted cycloisomerizations to be carried out (Fig. 10.44). 

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Rhodium is a rare white-silvery metal classified as a member of the platinum group metals. As a result, rhodium is commonly used as a catalyst in chemical reactions. It is also used in several chemical feedstock processes, including hydroformylation. Furthermore, rhodium has been used to catalyze more complex processes, such as higher order cycloaddition reactions. In addition, rhodium has been used in simpler reaction types such as hydrogenation and cycloisomerization. The fact that rhodium is effective for a wide range of chemical processes makes it an attractive metal for catalysis. 

The cycloisomerization of l, -enynes promoted by gold complex catalysts, are known to go through only the third pathway. This high selectivity is due to two main reasons (1) the fragment [AuL]" has only one vacant site, thus it can not coordinate simultaneously the alkyne and the alkene moieties, (2) oxidative addition processes are not facile for gold complexes [24, 157, 158]. In general, gold(I) complexes surpass the reactivity shown by platinum(ll) and other electrophilic metals for the reaction of enynes. ... 

SCHEME 747 (a) Cycloisomerization of 1,6-dienes using cationic platinum pincer complexes and (b) mechanism of platinum pincer complexes. 

The six-membered rings are assumed to be formed via 7t-complexation of the triple bond by the electrophilic metal complex, followed by a nucleophilic attack of the olefin moiety on the activated alkyne (Fig. 10.14) and a subsequent H-migration step. The electrophilic activations of enynes by 7r-complexation are common cycloisomerization pathways especially for platinum and gold catalysts. Other examples are presented in Sects. 10.4 and 10.5 of this book. 

Based on this working hypothesis, Marinetti et al. have designed the first series of efficient chiral platinum pre-catalysts for the cycloisomerization of 1,6-enynes, i.e. complexes C3 and C4 in Fig. 10.42. These are well defined square planar complexes combining N-heterocyclic carbenes (NHC), chiral phosphorus ligands and an iodide as the arguably labile ligand [62-64]. 

Soriano E, Marco-Contelles JM (2009) Mechanistic insights on the cycloisomerization of polyunsaturated precursors catalyzed by platinum and gold complexes. Ace Chem Res 42 1026-1036... 

Brissy D, Skander M, Retailleau P, Frison G, Marinetti A (2009) Platinum(II) complexes featuring chiral diphosphines and N-hetetocyclic carbene ligands synthesis and evaluation as cycloisomerization catalysts. Organometallics 28 140-151... 

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