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 Ph3C+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 = C6H5 (1a), C6H4CH3 (1b)) in good yields. Analogous deuterium-labeled products exo-1-chloro-2-aryl-d n -norbornane-7-d 1 (aryl-d n = C6D5 (1a- d 6 ), C6D4CD3 (1b- d 8 )) are obtained via catalytic arylation of α-chloronorbornene in either benzene-d 6 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-d 8 to give 1b- d 8 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*TiMe3/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.