Markovnikov hydroarylation

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

In 2000, the catalytic properties of (acac-<9,<9)2Ir(III)(R)(L) were discovered as a part of a program on developing thermally stable, group VIII metal complexes containing O-donor ligands. For example, [Ir( J,-acac-<9,<9,C3)(acac-<9,<9)(acac-C3)]2 catalyzes the anti-Markovnikov hydroarylation of benzene with propylene to yield ra-propylbenzene via a well-defined CH activation reaction.6... 

In a seminal paper, Matsumoto and coworkers reported an anti-Markovnikov hydroarylation of olefins catalyzed by a binuclear iridium(III) complex (Scheme 19.91) [133, 134], In contrast to conventional Friedel-Crafts alkylation of aromatic compounds with olefins, which follow Markovnikov s mle, mainly linear alkylbenzenes were obtained in this case, suggestive of a C-H activation mechanism. Although highly efHcient, the reaction led to regioisomeric mixtures. 

Loss of pyridine from 349 and trans-cis isomerization should give an intermediate accounting for the oxidative addition of the hydrocarbon and the H/D isotopic exchange between CsDs and hydrocarbons catalyzed by 349 (for cyclohexane/CsDg, measurements yielded TON = 240). The related phenyl derivative [IrPh(re -0,0-acac)2(py)] 352 catalyzes the anti-Markovnikov hydroarylation of unactivated alkenes. ° The mechanism of the alkene hydroarylation catalyzed by 352 was theoretically analyzed by DFT methods. ... 

Homogeneous iridium(m) catalysts mediate arene C-H activations to form anti-Markovnikov products as in the hydroarylation of propene (Equation (631).64... 

Apart from our work, few publications have appeared so far concerning this type of hydroarylation of styrene. For example, Periana s group described in 2005 the hydroarylation of styrene with benzene using a bis-tropolonato-iridium(III) organ-ometallic complex [59]. However, in contrast to the results above, the linear product (anti-Markovnikov product) was preferentially generated. 

In the presence of BiCU, the hydroarylation of styrenes Ar,C(R)=CH2 with electron-rich arenes ArH afforded Markovnikov adducts Ar C(R)(Ar)Me selectively in good to high yields as a result of the C-H activation of ArH. Under arene-free conditions, the intermolecular hydroarylation of a-substituted styrenes and subsequent intramolecular hydroarylation produced the cyclic dimers of a-substituted styrenes in good yields.122,123... 

These catalytic reactions provide a unique pathway for addition of aromatic C-H bonds across C=C bonds. In contrast with Friedel-Crafts catalysts for olefin hydroarylation, the Ru-catalyzed hydrophenylation reactions of a-olefins selectively produce linear alkyl arenes rather than branched products. Although the selectivity is mild, the formation of anti-Markovnikov products is a unique feature of the Ru(II) and Ir(III) catalysts discussed herein. Typically, the preferred route for incorporation of long-chain linear alkyl groups into aromatic substrates is Friedel-Crafts acylation then Clemmensen reduction, and the catalysts described herein provide a more direct route to linear alkyl arenes. 

A series of arylations of olefins by C-H bond cleavage without direction by an ortho functional group has also been reported, and these reactions can be divided into two sets. In one case, the C-H bond of an arene adds across an olefin to form an alkylarene product. This reaction has been called hydroarylation. In a second case, oxidative coupling of an arene with an olefin has been reported. This reaction forms an aryl-substituted olefin as product, and has been called an oxidative arylation of olefins. The first reaction forms the same t)q)es of products that are formed from Friedel-Crafts reactions, but with selectivity controlled by the irietal catalyst. For example, the metal-catalyzed process can form products enriched in the isomer resulting from anti-Markovnikov addition, or it could form the products from Markovnikov addition with control of absolute stereochemistry. Examples of hydroarylation and oxidative arylation of olefins are shown in Equations 18.63 - and 18.64. ... 

Indeed, Cu can be used in combination with tungstophosphoric acid to realize solvent-free acid-catalyzed hydroarylation and hydroamination of alkynes to give the Markovnikov products with a variety of arylamines [211]. Furthermore, gold catalysts have been used extensively with acid additives for a variety of amine and protected amine substrates [120,212]. More recently, Bertrand [189] was able to illustrate the useful apphcation of his [(CAACjAulBlCgFj) complex for intermolecular alkyne hydroamination with both primary and secondary amines (Table 15.14). 

Scheme 19.91 The anti-Markovnikov olefin hydroarylation catalyzed by an iridium complex.

The same research group also investigated the hydroarylation of the vinyl side chain of poly(vinylmethylsiloxane) via ruthenium-catalyzed alkenylation of the ortho C—H bonds of aromatic ketones with vinylsilanes (Fig. 18B), a reaction originally developed by Murai. The functionalization reaction was regioselective, giving anti-Markovnikov addition of the 10-position C—H bond of 9-acetylphenanthrene across the pendant... 

---