Diene cycloisomerization enantioselectivities

Feducia JA, Campbell AN, Doherty MQ, Gagne MR (2006) Modular catalysts Pot diene cycloisomerization rapid and enantioselective variants Pot bicycloptopane synthesis. J Am Chem Soc 128 13290-13297... 

Asymmetric cycloisomerization of nitrogen-bridged 1,6-enynes occurs in the presence of a cationic rhodium complex coordinated with a chiral diene/phosphine tridentate ligand to give chiral 3-azabicyclo[4.1.0]heptenes with high enantioselectivity (Scheme 149). " ... 

The formation of chiral cyclic 1,4-dienes from cycloisomerization of 1,6-enynes and 1,7-enynes has triggered the search for enantioselective catalytic... 

Inducing Asymmetry in -Elimination Pathway Apart from its racemic variants, cycloisomerization of enynes has been successfully employed in asymmetric synthesis. In the first example of an asymmetric cycloisomerization reaction, Trost et al. used a combination of palladium(O) and a chiral acid to catalyze the cycloisomerization of 1,6-enynes resulting in a rather low enantioselectivity of 33% ee for the formation of 1,4-diene (35) (Scheme 7.17(a)) [35]. Modem development of chiral phosphoric acids has allowed the reexamination of this approach. Thus, when palladium(O) and chiral (5)-TRIP were used with enyne 36 a 71% yield of diene 37 was isolated in 88% ee [36]. Under the same eon-cept, Mikami and coworkers found that cationic palladium... 

SCHEME 7.46 Moderate enantioselectivities in palladium-catalyzed cycloisomerization of dienes. 

Fig. 10.36 Enantioselective cycloisomerization of dien-ynes into tricyclic cyclohexenes [49]...

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). 

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