Pauson-Khand reaction, asymmetric

Asymmetric Pauson-Khand reaction in syntheses of heterocycles fused with five-member carbocyclic fragment 980PP121. 

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

Ingate, S. T. Marco-Contelles, J. The Asymmetric Pauson-Khand Reaction, Org. Prep. Proc. Inti. 1998, 30, 121-143. 

Recently, the enantioselective Pauson-Khand reaction has been developed using chiral ligands.111 Hicks and Buchwald reported that, in the presence of (S,S)(EBTHI)Ti(CO)2, the asymmetric Pauson-Khand reaction of the 1,6-enyne 94 gave the bicyclic heterocycle 95 in a high yield with good enantiomeric excess (Scheme 32).lllab... 

Enantioselective versions of the Pauson-Khand reaction have been reported.3 8 Brucine-A-oxide promotes an asymmetric Pauson-Khand reaction, for example.3 9 enantiospecific variation has been reported by... 

The Pauson-Khand reaction is also mediated by complexes of other transition metals including those of titanium, rhodium and iridium. A number of complexes of these metals have been used to catalyse the intramolecular asymmetric Pauson-Khand reaction of 1,6-enynes to give bicyclo [3.3.0] octenes. The... 

Rhodium and iridium catalysts incorporating bisphosphine ligands such as BINAP and derivatives have also been shown to be effective in the asymmetric Pauson—Khand reaction of 1,6-enynes, and some high ees have been obtained with substrates incorporating heteroatoms. As an example, the allylpropargylamine (8.243) is converted into the bicychc product (8.244) with high ee in the presence of an iridium/ToIBINAP catalyst. It has been shown that, in some cases, aldehydes... 

Although tremendous advances in the catalytic Pauson-Khand reaction have been made, the development of an asymmetric version did not share the same degree of success. Several asymmetric Pauson-Khand reactions were reported using chiral auxiliaries. However, those systems required stoichiometric amounts of cobalt as well as the chiral source. Attempts at using a catalytic amount of cobalt did not give satisfactory results. By contrast, the use of titanium chiral catalyst S,Sy (EBTHI)Ti(CO)2 (EBTHI = ethylene-l,2-bis(tiM,5,6,7-tetrahydro-l-indenyl)... 

Catalyst for the first catalytic asymmetric Pauson-Khand reaction. 

---