Reactions of benzylrhenium complexes (η5-C5H5)Re(NO)(L)(CH2Ar) with Ph3C+PF6-. Analysis of the Re-Cα rotamers involved in α-hydride abstraction

Article abstract of DOI:10.1021/om00084a010

Sequential reaction of [(η⁵-C₅H₅)Re(NO)(PMe₃)(CO) ⁺BF₄^- (4) with CH₃ONa, C₆H₅MgBr, and then BH₃·THF gives (η⁵-C₅H₅)Re(NO)(PMe₃)(CH ₂C₆H₅) (7, 15%). Reaction of [η⁵-C₅H₅)Re(NO)(PPh₃)(= CH₂)]⁺PF₆^- with o-CH₃C₆H₄MgBr and mesitylmagnesium bromide gives (η⁵-C₅H₅)Re(NO)(PPh₃)(LH ₂-(2-C₆H₄CH₃)) (8,52%) and (η⁵-C₅H₅)Re(NO)(PPh₃)(CH ₂(2,4,6-C₆H₂(CH₃)₃)) (9, 78%), respectively. Reactions of Ph₃C⁺PF₆^- with 7, 8, and 9 are examined and compared to that of Ph₃C⁺PF₆^- with (η⁵-C₅H₅)Re-(NO)(PPh₃)(CH ₂C₆H₅) (1). With 1, the pro-R H_α is abstracted to give sc-[(η⁵-C₅H₅)Re(NO)(PPh₃)(= CHC₆H₅)]⁺PF₆^- (2k). In contrast, 9 undergoes exclusively pro-S H_α abstraction to give ac-[(η⁵-C₅H₅)-Re(NO)(PPh ₃)(=CH(2,4,6-C₆H₂(CH₃) ₃))]⁺PF₆^- (12t). With 8, both the pro-R and pro-S H_α are abstracted to give approximately equal amounts of sc- and ac-[(η⁵-C₅H₅)Re(NO)(PPh ₃)(=CH(2-C₆H₄CH₃))] ⁺PF₆^- (11k and 11t). With 7, the pro-S H_α is abstracted to give aC-[(η⁵-C₅H₅)Re(NO)(PMe ₃)(=CHC₆H₅)]⁺PF₆ ^- (13t). These data are discussed within the context of the Curtin-Hammett principle. Photolysis of 12t and 13t at -78°C gives ca. 50:50 mixtures of t/k (ac/sc) Re=C isomers, but in the dark at 25°C ?99:?1 equilibrium mixtures are reestablished. For 12k → 12t, ΔH? = 18.8 ± 0.3 kcal/mol and ΔS? = 0.5 ± 1.1 eu. Reaction of 13t with Li(C₂H₅)₃BD gives a (77 ± 1):(23 ± 1) mixture of (η⁵-C₅H₅)Re(NO)(PMe₃)(CHDC ₆H₅) diastereomers.