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= Murai Reaction = The Murai reaction is a chemical reaction that uses C-H activation to create a new C-C bond between a terminal or strained internal alkene and an aromatic compound using a Ruthenium catalyst. The reaction, named after Shinji Murai, was first reported in 1993. While not the first example of C-H activation, the Murai reaction is notable for being the first to show high efficiency, and wide applicability.

Previous examples required large amounts of catalyst, excess reagent, long reactions times, or only worked for a very specific family of reagents.



The Murai reaction uses a directing group coordinated to the ruthenium catalyst to direct selective ortho C-H activation. Ru0 and RuII catalysts are used, though the former catalysts must be heated to generate its active Ru0 form. The reaction is tolerant of both electron withdrawing and electron donating substituents on the aromatic ring, and is highly tolerant of various functional groups. The reaction was first reported using a ketone as the directing group, but other functional groups have been reported, including esters, amines, nitriles, and imidates directing groups. Murai reactions have also been reported with  disubstituted alkynes. In 2013, Chatani et al. showed it was possible to direct ortho alkylation of aromatic rings with α,β-unsaturated ketones (which typically are unreactive in Murai reactions) by using a bidentate directing group.

A variety of commercially available Ru catalysts have been shown to catalyze the Murai reaction, including RuH2(CO)(PPh3)3, RuH2(PPh3)4, Ru(CO)2(PPh3)3, and Ru3(CO)12.

Ru(0) Catalysts
A detailed mechanism for the Murai reaction has not been fully elucidated. Experimental and computational studies give evidence for at least two different mechanisms, depending on the catalyst used. For catalysts such as [Ru(H)2(CO)(PR3)­3] which are active as Ru0, a combination of computational density functional studies and experimental evidence has resulted in the following proposed mechanism:

It is proposed that the Murai catalyst mechanism requires high temperatures to activate Ru(H)2(CO)(PPh3)3 to the Ru0 active species [Ru(CO)(PPh3)n] (n=2, 3). DFT studies suggest that the active form of the catalyst is three-coordinate Ru(0), although it has yet to be isolated. The catalytic cycle begins with coordination of the ketone oxygen to a three-coordinate Ru complex (2) followed by oxidation of the Ru center to give intermediate 3, a five-coordinated metallocycle that has an agostic interaction with the C-H via its empty coordination site. Aromatization gives 4, the product of the two-step oxidative addition. Coordination to the π orbitals of the ketone is theoretically possible and could give 4 via a more typical one-step oxidative addition. However, density functional studies show that the activation energy is much higher that the two-step process. Substitution of ethylene for one of the ligands cis to the hydride gives 5, which can undergo migratory insertion with the hydride to give 6. Reductive elimination, potentially proceeding through a stepwise mechanism, gives the product and ligand coordination regenerates the active catalyst 1. The C-C bond formation is the rate limiting step.

Ru(II) Catalysts
Work by Chaudret et al. and others on [Ru(o-C6H4PPh2)(H)(CO)(PPh3)­2] and other RuII catalysts demonstrated that RuII catalysts can catalyze the Murai reaction at room temperature. DFT studies by Helmstedt and Clot on model catalyst [Ru(H)2(H2)2(PR3)2] show that the hydride ligands likely play a large role in the reaction mechanism. They propose that the active complex is actually the dihydride complex [Ru(H)2(PMe3)2], which they suggest is generated by the loss of one coordinated H2 by substitution of ethylene, and another by the hydrogenation of ethylene to ethane.



Their proposed mechanism for the ortho-alkylation of acetophenone after activation is as follows:

After the active form of the ruthenium catalyst complex is generated from 1, acetophenone coordinates to the complex via its carbonyl oxygen and agostically via its ortho C-H bond (2). As in the Ru0 proposed mechanism, this agostic interaction leads to the oxidative addition of the ortho C-H. Reductive elimination releases H2, which remains coordinated, giving complex 3. Coordination of ethylene and decoordination of the ketone results in complex 4 which then undergoes migratory insertion of ethylene into the hydride to give 5. Following oxidative addition of H2 (6), the complex reductively eliminates the product to give the product agostically bound to the complex. Coordination of another acetophenone molecule regenerates complex 2.

Regioselectivity
Early examples of the reaction suffered from side products of alkylation at both ortho positions. This was initially treated by the addition of an ortho methyl blocking group. Unfortunately, this reduced the rate of reaction, and restricted the reaction’s applicability. In 1997 Murai et al. showed that a substituent at the meta position could influence regioselectivity. The reaction preferentially adds at the least sterically hindered ortho position, except when there is a meta group capable of coordinating with the Ru catalyst. As seen in Figure 5, the methoxy- substituted acetophenone showed preferential reaction at the more hindered position. Fluorinating the methyl group blocked this preference.