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 = C5Me4SiMe2NBut; 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-dn-norbornane-7-d1
(aryl-dn = C6D5 (1a-d6), C6D4CD3 (1b-d8)) are obtained via catalytic arylation of α-chloronorbornene in
either benzene-d6 or toluene-d8. 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*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.