Highly Anti-Markovnikov Selective Oxidative Arene Alkenylation Using Ir(I) Catalyst Precursors and Cu(II)...

The Ir(I) complex [Ir(μ-Cl)(coe)₂]₂ (coe = cis-cyclooctene) is a catalyst precursor for benzene alkenylation using Cu(II) carboxylate salts. Using [Ir(μ-Cl)(coe)₂]₂, propenylbenzenes are formed from the reaction of benzene, propylene, and CuX₂ (X = acetate, pivalate, or 2-ethylhexanoate). The Ir-catalyzed reactions selectively produce anti-Markovnikov products, trans-β-methylstyrene, cis-β-methylstyrene, and allylbenzene, along with minor amounts of the Markovnikov product, α-methylstyrene. The selectivity for the anti-Markovnikov products changed as the reaction progressed. For example, in a reaction that uses 240 equiv of Cu(OHex)₂ (related to Ir), the selectivity for the anti-Markovnikov products increases from 18:1 at 3 h to 42:1 at 42 h with 30 psig of propylene at 150 °C. Studies of product stability have revealed that the increase in the selectivity for anti-Markovnikov products is not the result of an isomerization process or the selective decomposition of specific products. Rather, the change in selectivity correlates with the ratio of Cu(II) to Cu(I) in the solution, which decreases as the reaction progresses. We propose that the identity of the active catalyst changes as Cu(I) is accumulated, resulting in the formation of an active catalyst that is more selective for anti-Markovnikov products. Using a 4:1 Cu(I)/Cu(II) ratio at the start of the reaction, a 65(3):1 anti-Markovnikov/Markovnikov ratio is observed.