Hydroarylation of alkenes

FIGURE 25.25 Computed eneigy profile for the hydroarylation of ethene catalyzed by [Irfacac ljfPhXH O)] (49, acac = H3C3O2). Energies (kcal/mol) are reported relative to free ethene and 49 [23b]. 

FIGURE 25.26 Computed reaction for the (1—H eUmination from [Ir(acac )(C,H3Ph)], 51 [23b], 

The second side reaction, the addition of a second ethylene molecule to 51, was also favorable (Fig. 25.27). Complex 58 can promote an insertion of the second ethylene molecule (59) or the C—H activation to form the vinyl complex 60. Both reactions were computed to be competitive with ethylene insertion. 

FIGURE 25.27 Computed sides reactions of [Ir(acac )(C,H.Ph)], 52, with ethene [23b], 

Some years later, Gunnoe and Cundari [39] reported how the nature of the para-substituents could affect the C—H activation step. Their efforts were focused on comparing two diffoent complexes, [RuTp(CO)(Me)(C HjX)] and [RuTpCPMejXMeXCgH X)] (X=CN, H, NH, NO, Br, Cl, F, or OCHj). The analysis of the structures of the transitions states led to longer Ru—H bond distances for electron-donating substituents. The metal cento- coordinates to the C—H bond and promotes an intramolecular transfer (Fig. 25.29) similar to a 1,2-addition across Ru—X (X=OH orNHPh). 

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Hydroarylation of alkenes is applied to achieve step-growth co-polymerization of aromatic ketones and ct,c< -dienes such as 3,3,6,6-tetramethyl-3,6-disila-l,7-octadiene and 1,3-divinyltetramethyldisiloxane. Co-polymerization of acetophenone and 3,3,6,6-tetramethyl-3,6-disila-l,7-octadiene is catalyzed by Ru species which has been previously activated by treatment with styrene, and a significantly high molecular weight co-polymer, co-poly(3,3,6,6-tetra-methyl-3,6-disila-l,8-octanylene/2-acetyl-l,3-phenylene), is obtained (Scheme 21 ).166... 

The Brpnsted acidic anilinium salt [PhNH3+][ B(C6F5)4] has been developed as a catalyst for the hydroamination and hydroarylation of alkenes, such as styrenes, nor-bomene, cyclic alkenes, and cyclohexadiene, with anilines. The weakly coordinating... 

It is considered that the hydroarylation of alkenes proceeds with a mechanism similar to that depicted in Scheme 6. 

Periana and coworkers have recently made computational studies with respect to the mechanism of regioselective hydroarylation of alkenes reported by Periana, Matsumoto, and coworkcrs- " induced by the Ph-Ir(acac)2L catalyst in the formation of ethylbenzene. Iridium inserts into the benzene C-H bond, resulting in an oxidative hydrogen migration through transition... 

Quite recently, Diaz and co-workers have reported an intfamolecular asymmetric hydroarylation of alkene 77, which gives satisfactory ees and moderate yields and also showed that silver zeolites are very effective silver salts to obtain high enantioselectivity in this system (Scheme 19b). 

Scheme 10 Rhodium-catalyzed hydroarylation of alkenes with pyridine N-oxides...

In fact, intermolecular hydroarylation of alkenes other than norbornenes was also realized, with Kulagowski et al. exploiting the diastereospecificity of both aUcene insertion and 3-hydride elimination in the sterically driven regioselec-tive coupUng of 170 and 171 (170 172 Scheme 7.41) [110]. However, it was emphasized that an excess of chloride anions was required to prevent dehalogenation [95]. 

In this chapter, the recent progress in transition-metal-catalyzed C-H alkylation (until the end of 2011) will be presented. These reactions have been divided into three categories differing in the nature of the alkylating agent an electrophilic alkyl source (Scheme 19.2a, part 2), an alkyl metal (Scheme 19.2b, part 3), or an alkene (hydroarylation of alkenes. Scheme 19.2c, part 4). Reactions that do not involve the formation of a (hetero)arene-transition metal intermediate during the catalytic cycle have been excluded. Extensions to nonaromatic C(sp )-H bonds, that is, C(sp )-H of alkenes, and C(sp )-H bonds wiU also be reviewed. As for... 

To date, most research on NHCP transition-metal catalysis has been devoted to cross-couplings [13] or related reactions such as hydroarylation of alkenes [18], direct arylation of alkynes [17], or oxidative homocoupling of terminal alkynes [19]. All NHCP systems used in these studies feature one or two... 

Abstract The selective catalytic activation/functionalization of sp C-H bonds is expected to improve synthesis methods by better step number and atom economy. This chapter describes the recent achievements of ruthenium(II) catalysed transformations of sp C-H bonds for cross-coupled C-C bond formation. First arylation and heteroarylation with aromatic halides of a variety of (hetero)arenes, that are directed at ortho position by heterocycle or imine groups, are presented. The role of carboxylate partners is shown for Ru(II) catalysts that are able to operate profitably in water and to selectively produce diarylated or monoarylated products. The alkylation of (hetero)arenes with primary and secondary alkylhalides, and by hydroarylation of alkene C=C bonds is presented. The recent access to functional alkenes via oxidative dehydrogenative functionalization of C-H bonds with alkenes first, and then with alkynes, is shown to be catalysed by a Ru(ll) species associated with a silver salt in the presence of an oxidant such as Cu(OAc)2. Finally the catalytic oxidative annulations with alkynes to rapidly form a variety of heterocycles are described by initial activation of C-H followed by that of N-H or O-H bonds and by formation of a second C-C bond on reaction with C=0, C=N, and sp C-H bonds. Most catalytic cycles leading from C-H to C-C bond are discussed. 

Hydroarylation of alkenes offers a more atom-economical process of catalytic C-H bond alkylation. It involves the transition-metal catalysed formal addition of the arene C-H bond onto C=C bonds. Especially T. B. Gtiimoe and his group have shown that ruthenium(II) catalysts favour the addition of aromatic C-H bonds to an olefin C=C bond. They have used ruthenium(II) catalysts of type TpRu-R(L) (NCMe) (Tp=hydrotris(pyrazolyl)borate) and demonstrated the efficiency of the formation of ethylbenzene via catalytic addition of benzene to ethylene [(Eq. 37)] [113-118]. Analogously the same type of catalyst TpRu-R(L)(NCMe) promotes the alkylation by ethylene at C2 position of furan and thiophene [119]. 

Intramolecular hydroarylation of alkenes through direct aromatic C—H activation was applied to the asymmetric synthesis of the potent protein kinase C inhibitor tricyclic indole (Scheme 18.54) [44]. 

Jung and Chang developed also a rare kind of C-H bond functionalization of 2,2 -bipyridines and 2,2 -biquinolines based on Rh-catalyzed hydroarylation of alkenes [eqn (4.5)]. Since 2,2 -bipyridines tend to form very stable chelates with transition metals, their functionalization had remained a challenge. The authors reasoned that the use of a NHC as external ligand may circumvent this issue. Indeed, while PPha led to traces of the desired double functionalization product 79, IMes provided it in excellent yield. 

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