Frustrated β-chloride elimination. Selective arene alkylation by α-chloronorbornene catalyzed by electrophilic metallocenium ion pairs

Single-site polymerization catalysts generated in situ via activation of Cp*MMe₃ (Cp* = C₅Me₅; M = Ti, Zr), (CGC)MMe₂ (CGC = C₅Me₄SiMe₂NBu ^t; M = Ti, Zr), and Cp₂ZrMe₂ with Ph ₃C⁺B(C₆F₅)₄^- catalyze alkylation of aromatic molecules (benzene, toluene) with α-chloronorbornene at room temperature, to regioselectively afford the 1:1 addition products exo-1-chloro-2-arylnorbornane (aryl = C₆H ₅ (1a), C₆H₄-CH₃ (1b)) in good yields. Analogous deuterium-labeled products exo-1-chloro-2-aryl-d _n-norbornane-7-d₁ (aryl-d_n = C ₆D₅ (1a-d₆), C₆D₄CD ₃ (1b-d₈)) are obtained via catalytic arylation of α-chloronorbornene in either benzene-d₆ or toluene-d ₈. Isolated ion-pair complexes such as (CGC)ZrMe(toluene) ⁺B(C₆F₅)₄^- and Cp*₂ThMe⁺B(C₆F₅) ₄^- also catalyze the reaction of α-chloronorbornene in toluene-d₈ to give 1b-d₈ in good yields, respectively. Small quantities of the corresponding bis(1-chloronorbornyl)aromatics 2 are also obtained from preparative-scale reactions. These reactions exhibit turnover frequencies exceeding 120 h^-1 (for the Cp*TiMe ₃/Ph₃C⁺B(C₆F₅) ₄^--catalyzed system), and chlorine-free products are not observed. Compounds 1 and 2 were characterized by ¹H, ²H, ¹³C, and 2D NMR, GC-MS, and elemental analysis. The aryl group exo-stereochemistry in 1a and 1b is established using ¹H- ¹H COSY, ¹H-¹³C HMBC, and ¹H- ¹H NOESY NMR, and is further corroborated by X-ray analysis of the product 1,4-bis(exo-1-chloro-2-norbornyl)benzene (2a). Control experiments and reactivity studies on each component step suggest a mechanism involving participitation of the metal electrophiles in the catalytic cycle.