Controlling the regiochemistry of nucleophilic attack on unsymmetrical allyl complexes

Article abstract of DOI:10.1016/0022-328X(90)85129-M

The NO and CO ligands in a 3-allyl)>⁺ complex exert differential electronic effects on the allyl moiety and control the regiochemistry of nucleophilic attack.These directing influences are sufficiently strong that they overcome the normal directing influences of substituents of the allyl moiety.An essential task, therefore, is arranging for the appropriate terminus of the allyl to have the relationship to the nitrosyl group that will result in attack at the desired location.Replacement of a single CO ligand in a neutral monosubstituted allyl complex of CpMo(CO)2 with NO⁺ yields a product for which addition occurs at the unsubstituted end of the allyl.Hence, an E-olefin with no newly created chiral centres is formed upon nucleophilic attack.A new synthetic strategy has been developed which allows us to build a chiral center in the allylic position of a terminal olefin.Sequential addition of two nucleophiles to a CpMo(NO)(CO)(allyl)⁺ cation creates these chiral centers, which were not accessible by addition of NO⁺ to the dicarbonyl.Addition of nucleophiles to homochiral (Neomenthylcyclopentadienyl)Mo(NO)(CO)(phenylallyl)⁺ complexes yields chiral olefins in high enantiomeric purity.The cationic phenylallyl complexes prepared by different routes were identified by single crystal X-ray diffraction determinations. endo-cis-syn-3-CH(Ph)CHCH2)>BF4 crystallizes in the monoclinic space group P2₁/n with a 8.226(2), b 12.273(2), c 16.051(2) Angstroem, β 100.02(2)deg, V 1595.7(9) Angstroem³, Z = 4. exo-cis-syn-3-CH(Ph)CHCH2)>BF4 crystallizes in the monoclinic space group P2₁/c with a 10.706(3), b 14.700(4), c 10.491(3) Angstroem, β 96.48(2)deg, V 1640.5 Angstroem³, Z = 4.