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; M = Ti, Zr), and Cp2ZrMe2 with Ph3C+B(C6F5)(4)(-) catalyze alkylation of aromatic molecules (benzene, toluene) with alpha-chloronorbornene at room temperature, to regioselectively afford the 1:1 addition products exa-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 alpha-chloronorbornene in either benzene-d(6) or toluene-os. Isolated ion-pair complexes such as (CGC)ZrMe(toluene)B+(C6F5)(4)(-) and CP*2ThMe+B(C6F5)(4)(-) also catalyze the reaction of a-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 H-1, H-2, C-13, and 2D NMR, GC-MS, and elemental analysis. The aryl group exo-stereochemistry in 1a and 1b is established using H-1-H-1 COSY, H-1-C-13 HMBC, and H-1-H-1 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.