Cationic rhodium catalysts with complexes containing

The reaction of 3,4-dihydroisoquinoline A-oxide (74) and methacrylonitrile in the presence of cationic half-sandwich rhodium and iridium complexes containing a chiral diphosphine ligand was analyzed. The cycloadditions occurred with excellent regio- and diastereoselectivity and low-to-moderate enantioselectivity. Analysis of the catalytic system showed the formation of two epimeric complexes 75 containing the dipolarophile methacrylonitrile. The reaction of one of the isolated diastereopure complexes 75 with 74 afforded cycloadduct 76 with high enantioselectivity. A recycling procedure was developed in order to increase the adduct/catalyst ratio <07CEJ9746>. 

This procedure resulted in an efficient and selective substrate conversion and it was established by spectrophotometry that there was no catalyst leaching to the apolar phase. However, a marked decrease in the catalytic activity was observed after the third cycle. This was probably caused by a continuous loss of free triphenylphosphine ligand present in equilibrium with the rhodium complex, ultimately generating inactive species. To overcome this problem the cationic rhodium complex (Rh(cod)(dppe)]PFg was tested. In the ternary mixture containing CH2CI2 this complex showed poor catalytic activity. Using methanol instead clearly increased the activity. This effect of increased activity in methanol is well known for rhodium complexes. 

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