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

Pseudotetrahedral (S,S)-(η-C5H5)RuPh2PCH(CH 3)CH(CH3)PPh2Cl (1), (SRu,RC)-1′a, and (RRu,RC)-(η-C5H5)RuPh 2PCH(CH3)CH2PPh2Cl (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 RuC≡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 RuC-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) Å, b = 14.976 (4) Å, and c = 20.696 (7) Å with Z = 4 in space group P212121 (no. 19). For 3′b: a = 12.555 (4) Å, b = 12.651 (4) Å, and c = 24.198 (8) Å with Z = 4 in space group P212121 (no. 19).