Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(iii) complexes

Abstract

The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(m-acac-O,O,C 3 )-(acac-O,O) 2 ] 2 and [R-Ir(acac-O,O) 2 (L)] (R = acetylacetonato, CH 3 , CH 2 CH 3 , Ph, or CH 2 CH 2 Ph, and L = H 2 O or pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61 : 39 for benzene + propylene and 98 : 2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. 13 C-labelling studies with CH 3 13 CH 2 -Ir-Py showed that reversible b-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the 13 C-label from the a to b-positions was found to be slower than the C-H activation of benzene (and thus formation of ethane and Ph-d 5 -Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and b-hydride elimination (k CH : k b ) of~0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O) 2 -Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C-H bond of an incoming benzene to generate the product and regenerate the catalyst.

Beyond Friedel and Crafts: Directed Alkylation of C−H Bonds in Arenes

“…[165] As eries of mechanistic studies were also reported, including DFT calculations and organometallic studies,which highlighted aC ÀHactivation mechanism based on an iridium(III)-alkyl-mediated concerted metalationdeprotonation (CMD) pathway. [166] Besides iridium(III) catalysts,t he use of well-defined ruthenium(II) catalysts was described almost simultaneously. Indeed, Gunnoe first reported in 2003 the use of ah ydridotris(pyrazolyl)borate-derived ruthenium complex for the direct alkylation of benzene with ethylene (51 turnovers after 4h)o rp ropene (14 turnovers after 19 h) with a1 .6:1.0 ratio of n-propyl-versus isopropylbenzene in the latter case (Scheme 61).…”

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…In addition to BTX, there is also a high demand for n-alkyl arenes, which are precursors for surfactants with high detersive power [128]. Synthesis of n-alkyl arenes requires the challenging anti-Markovnikov arylation of olefins [129][130][131], and hence branched alkyl arenes synthesized via FriedelÀCrafts alkylation currently dominate the surfactant industry. Goldman and Brookhart have reported an efficient route to n-alkyl arenes directly from n-alkane as starting materials via dehydroaromatization reaction (14) [61].…”

Oxidative Alkenylation of Arenes Using Supported Rh Materials: Evidence that Active Catalysts are Formed by Rh Leaching

“… [14,16–20] These potential advantages include: a) direct arene alkenylation (rather than alkylation) via β‐hydride elimination after the olefin insertion step, b) selective production of 1‐aryl alkane/alkene by circumventing carbocationic intermediates that lead to Markovnikov selectivity, c) conversion of electron‐deficient arenes, d) new regioselectivity for alkenylation or alkylation of substituted arenes, and e) inhibition of polyalkylation since alkylated or alkenylated products can be less reactive than starting arenes [14] . Molecular catalysts involving Ni, Ir, Ru, Pt, Pd and Rh have been studied for catalytic C−H alkylation or alkenylation of arenes with α‐olefins [15,16,21–35] …”

“…[14] Molecular catalysts involving Ni, Ir, Ru, Pt, Pd and Rh have been studied for catalytic CÀ H alkylation or alkenylation of arenes with α-olefins. [15,16,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Recently, we reported Rh-catalyzed arene alkenylation to directly synthesize styrene and 1-aryl alkenes using molecular catalysts in homogeneous processes. [13,14,[36][37][38][39][40][41] Based on the mechanistic studies, our initial proposed catalytic cycle involves (Scheme 3): a) Rh-carboxylate assisted arene CÀ H activation, b) ethylene coordination and insertion into a Rh-aryl bond, c) βhydride elimination, and d) alkenyl arene dissociation and oxidation of a RhÀ H intermediate with CuX 2 (X = carboxylate) to regenerate the starting catalyst.…”