Arylation of C-H bond

This chapter mainly treats transition metal-catalyzed direct functionalization of carbon-hydrogen bonds in organic compounds. This methodology is emphasized by focusing on important functionalizations for synthetic use. The contents reviewed here are as follows (i) alkylation of C-H bonds, (ii) alkenylation of C-H bonds, (iii) arylation of C-H bonds, (iv) carbonylation of C-H bonds, (v) hydroxylation and the related reactions, and (vi) other reactions and applications. 

Arylation of C-H bonds is achieved by coupling reactions of C-H bonds with aromatic compounds such as halides, triflates, and organometallic reagents. Early works in this field involve the reaction of aryl halides with norbornene. As shown in Scheme 5, the coupling reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as a... 

The palladium-catalyzed arylation of 2-phenylphenols and naphthols shows an interesting feature of arylation of C-H bonds, leading to the formation of an (aryl)(aryloxy)palladium(n) intermediate.65,65a,65b The phenolates are suitable as precoordinating groups. The reaction of 2-hydroxybiphenyl with an excess of iodobenzene occurs regioselectively at the two ortho-positions of phenyl group under palladium catalysis (Equation (57)). In the case of 1-naphthol, the peri-position is phenylated (Equation (58)). 

A ruthenium catalyst derived from a secondary diaminophosphine oxide has been used successfully in the arylation of C-H bonds by aryl tosylates.41... 

Two closely related yet distinct pathways can be proposed for the arylation of C-H bonds based on coordination-directed C-H bond activation (1) cyclometalation with an MXn fragment followed by transmetalation with Ar-M and reductive elimination, or (2) cyclometalation with an ArMX fragment followed by reductive elimination (Scheme 1). Analogously, two catalytic cycles can be written for the current transformation. The first one would be Cycle 1 which proceeds via cyclo-palladation (with Pd(OAc)2) followed by transmetalation (Scheme 3). An alterna-... 

Very recently, Kakiuchi reported on the ruthenium-catalyzed arylation of C-H bonds using organoborane reagents. The reaction of aromatic ketones with arylboro-nates using a ruthenium catalyst resulted in the production of arylated aromatic ketones (Eq. 9.36) [50], This arylation reaction using arylboronates can be applied to a variety of aromatic ketones and arylboronates. The authors proposed that this reaction involves the oxidative addition of a C-H bond to a Ru(0) species. 

Palladium and copper-catalyzed arylation of C-H bonds by aryl halide reagents is reviewed. The emphasis of the review is on directing-group-containing arene and alkane arylation catalyzed by palladium and on sp2 C-H bond arylation catalyzed by copper. Literature up to early 2009 is covered. 

Scheme 10 Ni-catalyzed arylation of C-H bonds with aryl iodides...

Since Sanford reported the first example of the Pd-catalyzed arylation of C-H bonds with diaryliodonium salts as coupling partners [49], the utilization of diaryliodonium salts in the functionalization of C-H bonds has been of great interest. However, all examples involved the use of Pd, Pt, and Cu as the catalyst. Chatani reported that diaryliodonium salts can also be used as coupling partners for the arylation of C(sp )-H bonds in place of aryl iodides using Ni(II) as the catalyst (Scheme 22) [50]. Arylated products were obtained in good yields even in the... 

With Nickel In 2013, Liu et al. [109] reported the Ni-catalyzed intermolecular oxidative arylation of C-H bonds using arylboronic acids to afford 2-aryltetrahydrofurans. The active catalyst was prepared from Ni(acac)2 (10 mol%) and PPhj (10 mol%). Various aryl boronic acids were evaluated, but only two cyclic ether substrates. 

The cmcial role of carbonate that can coordinate to Ru(ll) sites [64] was shown to be responsible for the initial intramolecular C-H bond deprotonation leading to ruthenacycle intermediate C, as supported by DPT calculations [(Eq. 7)] [46]. This observation led later to discover better catalysts for arylation of C-H bonds... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

The use of Mn-salen catalysts for asymmetric epoxidation has been reviewed.30 Oxo(salen)manganese(V) complexes, generated by the action of PhIO on the corresponding Mn(III) complexes, have been used to oxidize aryl methyl sulfides to sulfoxides.31 The first example of C—H bond oxidation by a (/i-oxo)mangancsc complex has been reported.32 The rate constants for the abstraction of H from dihydroanthracene correlate roughly with O—H bond strengths. 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

Alkyl aryl ketones are known to be arylated at the a-position of the alkyl groups, via the corresponding enolates, by treatment with aryl halides in the presence of palladium catalysts [4, 9]. The ortho arylation of alkyl aryl ketones is also possible. For example, in the reaction of benzyl phenyl ketones with bromobenzenes, the arylation first occurs at the benzylic position the ortho positions are then arylated via C-H bond cleavage (Eq. 8) [15]. The ortho arylation is believed to occur after coordination of the enol oxygen to ArPd(II), which is followed by ortho palladation as in the reaction of 2-phenylphenols shown in Scheme 2. 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

The use of C-H bonds is obviously one of the simplest and most straightforward methods in organic synthesis. From the synthetic point of view, the alkylation, alkenylation, arylation, and silylation of C-H bonds are regarded as practical tools since these reactions exhibit high selectivity, high efficiency, and are widely applicable, all of which are essential for practical organic synthesis. The hydroacylation of olefins provides unsymmetrical ketones, which are highly versatile synthetic intermediates. Transition-metal-catalyzed aldol and Michael addition reactions of active methylene compounds are now widely used for enantioselective and di-astereoselective C-C bond formation reactions under neutral conditions. 

The stirring discovery, that transition metal alkyls and aryls were accessible via oxidative addition of C—H bonds to electrophilic metal centers, soon brought up the question whether dialkyl, diaryl or alkyl(aryl) transition metal... 

Reactions of C—H bonds attached to donor ligands. These may lead to the formation of rings of various sizes. The orthometallation of aryl substituents is particularly common those involving PPh3 may contribute to catalyst deactivation reactions. The metallation of ligands may be reversible, particularly where 3- and 4-membered rings are formed.117... 

Dibenzylideneacetone Pt° complexes catalyze the arylation of Si—H bonds via C—H bond activation of substituted arenes.139... 

A second, very important decomposition pathway involves the activation of C-H bonds on the N-alkyl [142-147] or N-aryl [148-151] sidechains. OccassionaUy, even C-C activation in the sidechain is observed [152]. Similar C-H activation is observed in transition metal phosphides, especially when the phosphorus ligand has a SMes substituent [153]. 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

The oxidative addition of C H bonds of ligands is very common and this reaction forms metal alkyl or metal aryl complexes. In osmium triarylphosphine complexes, orthomet-allation gives four-membered metaUocycles. When the ortho... 

The activation of C-H bonds by d° metal centers can be investigated very effectively with complexes of the type t-Bu3SiNH)3ZrR, where R is an alkyl or aryl group, as these compounds undergo reversible elimination of hydrocarbon. In these examples, concerted mechanisms involving R-H bond interaction at d° metal imido complexes, either isolable or transient, give metathesis products via R-H elimination. ... 

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