Biphenyl tricarbonylchromium complexes,part 10

One CO group of the dimethyldihydrophenanthrene mono-Cr(CO)₃ complex (1) was photochemically substituted for CS (2) or P(C₆H₅)₃ (3). Separation of all four possible stereoisomers (R) _m (R) _b /(S) _m (S) _b and (R) _m (S) _b /(S) _m (R) _b ] of the complexes1–3 was accomplished by two successive chromatographic steps: Separation of the enantiomers on triacetylcellulose was followed by MPLC at low temperatures to yield both epimers (exo andendo). Their configurational assignment is based on optical comparison of the CD-spectra and on¹H-NMR-spectroscopy. The kinetics of the biphenyl flip were followed by CD and NMR. The results revealed that the rotational barriers around the biphenyl bond are hardly altered by the substitution of CO for CS or even P(C₆H₅)₃. Whereasexo andendo-isomers of1 and2 are obtained in appr. equal amounts, in the more crowded complex3 theexo-isomer predominates over theendo-form by 80:20%.Dedicated to Prof.K. Komarek with cordial wishes on the occasion of the 60th anniversary of his birthday.