Stereochemical studies on the interconversion of alkylidenecarbene, carbene, and alkyl ligands in chiral cyclopentadienyl ruthenium complexes

Article abstract of DOI:10.1021/om00141a008

Pseudotetrahedral (S,S)-(η-C₅H₅)Ru{Ph₂PCH(CH ₃)CH(CH₃)PPh₂}Cl (1), (S_Ru,R_C)-1′a, and (R_Ru,R_C)-(η-C₅H₅)Ru{Ph ₂PCH(CH₃)CH₂PPh₂}Cl (1′b) have been used to investigate the stereochemistry of alkylidenecarbene and carbene formation. Depending on conditions 1′a and 1′b can react stereospecifically with terminal acetylenes, in the presence of halogen scavengers, to give the corresponding alkylidene complexes [{Ru}=C=CHR]PF₆ (R = C₆H₅,2′a and 2′b; R = CH₃, 6′a and 6′b). Retention of configuration at ruthenium was confirmed by an X-ray crystal structure of 6′a. 2′a and 2′b are converted with base to the corresponding phenylacetylide complexes {Ru}C≡CC₆H₅ (5′a and 5′b), which, in turn, regenerated 2′a and 2′b on reaction with HPF₆. Neither reaction changes the stereochemistry at the metal. 2′a and 2′b react further with CH₃OH stereospecifically and with retention of the configuration at ruthenium to give the methoxycarbene complexes [{Ru}=C(OCH₃)CH₂C₆H₅]PF₆ (3′a and 3′b). The crystal structure of 3′b has also been determined. 3′a and 3′b react with LiAlH₄ in THF at low temperature to give the 2-phenylethyl complexes {Ru}C-H₂CH₂C₆H₅ (4′a and 4′b) with retention of configuration. Labeling studies have demonstrated that the two hydrogen atoms of the new formed methylene group arise from LiAlH₄. The summary of crystal data is as follows. For 6′a: a = 11.469 (4) A?, b = 14.976 (4) A?, and c = 20.696 (7) A? with Z = 4 in space group P2₁2₁2₁ (no. 19). For 3′b: a = 12.555 (4) A?, b = 12.651 (4) A?, and c = 24.198 (8) A? with Z = 4 in space group P2₁2₁2₁ (no. 19).