Norbornadiene catalysts, palladium complexes

Chiral methyl chiral lactic acid (5). This labeled molecule, useful for study of stereospecificity of enzymic reactions, has been prepared in a way that allows for synthesis of all 12 possible isomers. One key step is the stereospecific debromination of 1, accomplished by conversion to the vinyl-palladium cr-complex 2 followed by cleavage with CF3COOT to give the tritium-labeled 3. The next step is the catalytic deuteration of 3, accomplished with a rhodium(I) catalyst complexed with the ligands norbornadiene and (R)-l,2-bis(diphenylphosphino)propane. This reaction gives 4 with an optical purity of 81%. The product is hydolyzed to 5, which is obtained optically pure by cr3rstallization. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

In a more detailed study, the structure of the catalyst precursor was determined and found to be Pd(Diop),32. Other L2Pd and L2Ni complexes [L = Diop, BPPM, BINAP, etc.] were prepared [e.g., by in situ reduction of Pd(Il)Cl,L with sodium borohydride or as isolated palladium(O) complexes] and used as catalysts for the asymmetric addition of hydrogen cyanide to norbornene. norbornadiene, benzonorbornadiene, and cyclopentadiene dimer. In the presence of excess ( + )-Diop and L,Pd, norbornene gives 91 -95% of exo-2-cyanonorbornane with 24% cc of the ( + )-(15.25,4/ )-isomer. Similarly, use of the ( —)-Diop complex leads to the (-)-(l/ ,2f ,4S)-isomer with 24% ee (95% yield). Lower reaction temperatures, instead of the 120 "C used above, give better ee values (80 =C 32% ee with 94% yield 35 °C 35 % ee with 6% yield)32. 

Similar studies have been reported for the complexes of norbornadiene with cuprous chloride (5) and palladium chloride 4). A complex of 1,3- and 1,4-cyclooctadiene with palladium chloride has been reported to be a useful catalyst for the polymerization of olefins (5). 

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