Alkyl C-H bond activation

This then was the first report of a compound in which alkyl C—H bond activation by a transition metal had occurred. In the solid state, this equilibrium is also in favor of the hydrido complex (V), and its crystal structure has recently been determined (15). It shows compound V to be a dimer (VI), the oxidative addition of the methyl group of a ligand on each ruthenium atom being to a second ruthenium atom. Presumably one reason why this occurs is because the product of intramolecular ring closure would contain a highly strained three-membered Ru—P—C ring (i.e., in monomer V). 

X-ray analysis shows distorted square planar coordination around Pd. This is the first example of alkyl C-H bond activation with a cationic diimine Pd(II) complex under such mild conditions. 

Since aryl halides are fairly cheap reagents, there has been less recent emphasis (see Section 3 below) on the development of aryl relative to that of alkyl C-H bond activation [13-17]. However, the manufacture of aryl halides is not an environmentally friendly process and thus the future of bulk aromatic synthesis may he in the direct activation of C-H bonds. For example, the formation of ben-zaldehyde from the insertion of CO into a C-H bond of benzene is a recent development in this area [17]. 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

The C-H bond activation followed by addition to a double bond leads to the formation of alkylated compounds (Equation (1)). This reaction involves aromatic, aliphatic, olefinic, and acetylenic C-H bonds. 

The carbonylation of the sp3 C-H bond adjacent to a nitrogen atom is also possible by means of chelation-assisted C-H bond activation.121 The carbonylation reaction of A-(2-pyridyl)pyrrolidine occurs at the a-position of the pyrrolidine ring by using [RhCl(cod)]2 as a catalyst and 2-propanol as a solvent. Cyclic amines exhibit a high reactivity (up to 84%) (Equation (93)), while acyclic amines show relatively low reactivity (18%). The use of Ru3(CO)i2 as a catalyst does not result in a carbonylation reaction, but instead the addition of the sp3 C-H bond across the olefin bond to give an alkylation product, as mentioned before (Section 10.05.4). 

Control of H-C(sp3) Bond Cleavage Stoichiometry Clean Reversible Alkyl Ligand Exchange with Alkane in [LPt(Alk)(H)2]+ (L=[2.1.1]-(2,6)-Pyridinophane) (226) this complex activates hydrocarbons RH to yield LPtRHjT. This is similar to the C-H bond activation shown in Scheme 17 but occurs without added acid. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

Binuclear [RuX2(arene)]2 (1) and mononuclear RuX2(L) (arene) (3) derivatives have been shown to be useful precursors for access to alkyl-or hydrido(arene)ruthenium complexes. The latter are key compounds for the formation of arene ruthenium(O) intermediates capable of C—H bond activation leading to new hydrido and cyclometallated ruthenium arene derivatives. Arene ruthenium carboxylates appear to be useful derivatives of alkyl-ruthenium as precursors of hydrido-ruthenium complexes their access is examined first. 

Sixteen-electron ruthenium(O) species of type (rj6-arene)(L)Ru(0) and containing two-electron ligands are probable intermediates for C—H bond activation and formation of metallacyclic complexes (Section II,A,3,c). A variety of 18-electron complexes of general formula (arene)(L1)(L2)Ru(0) have been prepared by H. Werner and co-workers either by deprotonation of hydride ruthenium(II) complexes or by reduction of cations RuX(L)2-(arene)+. Some of these Ru(0) complexes have already been discussed with the formation of alkyl or hydridoruthenium complexes (Sections... 

The industrially important direct methane conversion processes comprise oxidative coupling, reductive coupling including pyrolysis reactions, partial oxidation, halogenation and oxyhalogenation,26 and ammoxidation. Other direct conversions include alkylation, electrophilic substitution, and C-H bond activation over various complex and super acid catalysts. Several of these direct conversion technologies remain to be exploited to achieve their full commercial potentials. 

Scheme 9.9 Preparation of the cycloalkyl(hydrido) and alkyl(hydrido) iridium(III) complexes 24a,b and 25a,b by intermolecular C—H bond activation from the dihydrido iridium(III) and the dicarbonyl iridium(I) compounds 21 and 22 as the precursors (a L = PMe3 b L = CO)...

An enantioselective Friedel-Crafts alkylation of pyrroles with /V-acylimincs has been reported <070L4065>. The reactions were run in the presence of chiral phosphoric acids. A novel C-H bond activation procedure was developed for the preparation of heteroarylamides including pyrrole-3-carboxamides <07CL872>. The reactions involved imine-substituted pyrroles, isocyanate electrophiles, and a rhenium catalyst. 

Aromatic C-H bond activation opens an attractive pathway to achieve cyclizations with tethered alkenes for the synthesis of dihydrobenzofurans <2001JA%92> <20030L1301>. An auxiliary-directed asymmetric alkylation via C-H bond activation to yield a virtually enantiomerically pure 2,3-enantioselective synthesis of (-l-)-lithospermic acid has been reported by Bergman, Ellman, and co-workers (Scheme 97) <2005JA134%>. 

Detailed theoretical work has shown that alkane C-H activation by the 3Pi excited state Hg proceeds by initial formation of a weak [Hg (alkane)] exciplex, followed by C-H cleavage via a mechanism described as a mixture of oxidative addition and H atom abstraction. While alkanes react exclusively via C-H bond activation, sensitization of monosubstituted arenes ArCH2 -X (Ar = aryl X = H, alkyl, OR, NR2) occurs with substantial C-X bond cleavage (equation 25). 

A less common method to give -alkyl or aryl complexes is the C H bond activation reactions. Elimination of HNMe2 from Ta(0-2-Ind-4,6-Bu 2-C6H3)(NMe2)4 yields (14). Activation of a C-H bond in a Ta- NMc2 group led I----------------------------1... 

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Some lanthanide alkyl complexes can induce intramolecular C-H bond activation via metalation of the ligand with elimination of CH4 or SiMc4 under suitable conditions [63-65]. A representative example of the sp -hybridized C-H activation is presented in Figure 8.15 [65]. 

This is a new entry for alkylation of benzene, though the applicability of this reaction is narrow. These authors proposed that a catalytic cycle involves olefin/acetonitrile ligand exchange followed by olefin insertion into the Ru-Ar bond. The C-H bond activation in another arene allows elimination of alkylbenzenes. 

---