Asymmetric cycloisomerization diene

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). " ... 

Using the catalyst system based on [Ni(allyl)(cod)l [BARF] and Wilke s aza-phospholene ligand, several 1,6-dienes including diene 48 were converted to exo-methylenecyclopentanes 49 through the asymmetric cycloisomerization with enantiomeric excess of up to 91% (Scheme 28) (66). 

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 7.37 Variations of asymmetric cycloisomerization of 1,6 dienes under nickel catalysis. 

TABLE 7.1 Dependence of Counteranions of the Catalyst on the Asymmetric Cycloisomerization of 1,6-Dienes... 

Zhang54 published the first and only account of a non-asymmetric rhodium-catalyzed Alder-ene cycloisomerization of 1,6-enynes.55 The conditions developed by Zhang and co-workers are advantageous in that, similar to the ruthenium conditions developed by Trost, selectivity for 1,4-diene products is exhibited. The rhodium conditions are dissimilar from many other transition metal conditions in that only (Z)-olefins give cycloisomerization products. 

Lewis acids as cocatalysts are effective for Pd-catalyzed cycloisomerization. The reaction of diallylmalonate 36 in the presence of AgOTf afforded 63 with high selectivity [31]. Asymmetric cyclization of 1,6-diene 36 was carried out using PdCl2(MeCN)2-AgBF4 in the presence of a chiral ligand [32]. Cycloisomerization... 

RajanBabu has developed an asymmetric protocol for the heterodimerization of vinyl-arenes and ethenej The use of Hayashi s novel, weakly chelating phosphine 91 is critical to the success of this asymmetric reaction (Scheme 68). 1,6-Dienes (e.g., 92) also undergo direct cycloisomerization in the presence of bis[allyl(bromo)nickel] to afford meth-ylenecyclopentane products (e.g., 93 Scheme 69). The scope of the intramolecular process allows preparation of a variety of carbocyclic and heterocyclic ring systems. A reaction mechanism involving in situ generation of a nickel hydride catalyst, alkene hydro-metalation, cyclization, and p-hydride elimination has been proposed. ... 

In summary, cycloisomerizations of enynes and dienes represent a powerfnl tool in the hand of organic chemists. Although in the past 20 years, the field has witnessed remarkable advances especially in their asymmetric variants in the years to come, cycloisomerizations will continue to enrich onr knowledge with atom-economical... 

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