Abstract
Alkyl and alkenyl arenes are used for a wide variety of products including plastics, detergents, and pharmaceuticals. The chemical industry produces alkyl arenes through acid catalyzed methods including Friedel Crafts alkylation or related zeolite processes. Dehydrogenation of the alkyl arenes results in alkenyl arenes. The synthesis of alkyl and alkenyl arenes has been reported using transition metal catalysts that operate through C–H activation. Benefits of using transition metal catalysts include a difference in Markovnikov/anti-Markovnikov selectivity when using 𝛼-olefins, inhibition of poly alkylation, and the direct synthesis of alkenyl arenes upon the addition of an oxidant.
Arene alkenylation using [Ir(μ-Cl)(coe)2]2 (coe = cis-cylcooctene) as a catalyst precursor in combination with Cu(II) carboxylates and propylene produces alkenyl arenes with a high anti-Markovnikov:Markovnikov (Linear:Branched, or L:B) ratio. The selectivity of the reaction increases as the reaction progresses, with a 18:1 L:B ratio at 3 h, and a 42:1 L:B at 42 h. The increase in selectivity for linear products is not from an isomerization or consumption of products, but rather due to an change in the catalyst structure as the reaction progresses. As the catalyst precursor turnover the Cu(II) is used to oxidize the Ir–H intermediate which results in Cu(I), as the amount of Cu(I) in solution increases the selectivity for linear products increase. The catalyst precursor produces low TOs (TOs = turnover) of styrene and propenyl benzenes with respect to the limiting Cu(II) oxidant. A competitive side reaction is identified in which the Ir catalyst precursor decarboxylates the Cu(II) oxidant.
Other oxidants for arene alkenylation were explored including benzoquinones. The TOs of alkenyl and alkyl arenes were dependent on which benzoquinone was used. The selectivity for alkyl and alkenyl arenes depends on the substitution of the benzoquinone, in which ortho benzoquinones led to alkyl and alkenyl arenes and para benzoquinones resulted in only alkenyl arenes. The difference in the product selectivity (saturated and unsaturated products) is likely due to the ortho benzoquinones binding in a bidentate fashion to the Ir center forming a monomeric Ir catalyst. Stirring [Ir(μ-Cl)(coe)2]2 with 2 equiv of 9,10-phenanthrenequinone resulted in a new Ir(III) semiquinone species, Ir(phenSQ)2Cl(coe).
The stability of capping arene complexes (5-FP)RhCl(C2H4), (5-NPFP)RhCl(C2H4), and (6-FP)RhCl(C2H4) (5-FP = 1,2 bis(N-7-azaindolyl)benzene, 5-NPFP = 1,2 bis(N-7-azaindolyl)naphthalene, 6-FP = 8,8’-(1,2-phenylene)diquinoline) were studied upon the addition of ethylene, 1-pentene, and styrene in order to mimic conditions for arene alkenylation. Equilibrium constants were calculated upon the addition of ethylene. The stability of the capping arene complexes is dependent on the identity of the olefin added, and differs for all complexes examined, highlighting the difference in the capping arene ligand design. Additionally, new complexes were formed upon the addition of 1-pentene and styrene. The new complexes formed upon the addition of 1-pentene and styrene had different stability under the conditions probed, in which (5-FP)RhCl(C2H4) and (6-FP)RhCl(C2H4) went to full dissociation of ligand and (5-NPFP)RhCl(C2H4) reached equilibrium.
New capping arene ligands including N–H capping arene and phosphine capping arene ligands were synthesized and coordinated to Rh and Ir. The synthesis and characterization of C2-FP is reported (C2-FP = 1,2-di(9H-carbazol-1-yl)benzene) as well as the coordination to Ir and Rh to form [(C2-FP)Ir(C2H4)2][K] and [(C2-FP)Rh(C2H4)2][K] respectively. Both [(C2-FP)Ir(C2H4)2][K] and [(C2-FP)Rh(C2H4)2][K] are tested for oxidative arene alkenylation using propylene, and the TOs and L:B ratios of both complexes are similar to Rh(I) and Ir(I) catalyst precursors. New phosphine ligands are synthesized and characterized as I2-IPPFP (I2-IPPFP = 1,2-bis(9-(diisopropylphosphaneyl)-1H-indol-1-yl)benzene) and I2-PhPFP (I2-PhPFP = = 1,2-bis(9-(diphenylphosphaneyl)- H-indol-1-yl)benzene). Additionally, a new Rh complex using previously synthesized C2-IPPFP (C2-IPPFP = 1,2-bis(9-(diisopropylphosphaneyl)-9H-carbazol-1-yl)benzene) and was characterized as (C2-IPPFP)Rh2Cl2.
