Electrophiles, transition-metal complexes attacked

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Electron-rich, unsaturated hydrocarbons, which are normally resistant to nucleophilic attack, become generally reactive towards nucleophiles upon complexation to an electrophilic transition metal such as palladium(II), platinum(II) or iron(II). Complexation also directs the regio- and stereo-chemistry of the nucleophilic attack, the result of which is a new organometallic complex, which can often be used to promote additional functionalization of the original substrate. Synthetically useful examples of such processes are presented in the following sections. 

As stated above, aliphatic amines are potent ligands for electrophilic transition metals and are efficient catalyst poisons in attempted alkene animation reactions. However, tosylation of the basic amino group greatly reduces its complexing ability, yet does not compromise its ability to nucleophilically attack complexed alkenes. Thus, a variety of alkenic tosamides efficiently cyclized under palladium(II) catalysis producing N-tosylenamines in excellent yield (equations 17 and 18).32 Again, this alkene amination proceeded through an unstable a-alkylpalladium(II) species, which could be intercepted by carbon monoxide, to result in an overall aminocarbonylation of alkenes. With ureas of 3-hydroxy-4-pentenyl-amines (Scheme 7), this palladium-catalyzed process was quite efficient but it was somewhat less so with... 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

The imido groups of transition metal complexes undergo a wide range of reactions including electrophilic attacks by protons, NR transfer, and many other reactions.154... 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

The cationic dinitrosyl complexes (146) are apparently much more electrophilic than (143). Studies of the reactivity of (146) have been focused on attack of coordinated organic ligands of other transition metal complexes, such as the cyclooctatetraene ligand in L3M(cot) complexes (M = Fe, Ru) and Cp Co(cot) complexes (Cp = Cp, Cp ), the cyclopenta-dienyl ligand of CpRh(cot) complexes, and the cyanide ligand... 

Reactions are achieved with benzene and its derivatives (including those deactivated for electrophilic attack), polycyclic aromatics, heteroaromatics and transition-metal complexes of aromatic ligands. 

Each of these classes of electrophiles may attack transition metal complexes. We will encounter examples of all three classes during the remainder of the discussion in this section of Chapter 8. 

A number of transition metal complexes containing weakly basic (5-member ring) HDN-related ligands are known. The authenticated bonding modes of pyrrole (Pyr) and pyrrolyl ions (Pyl) -or their alkylated analogues- in mononuclear metal complexes are summarized in Fig. 6.1. Pyrrole is a 5-member aromatic heterocycle in which the lone pair is delocalized over the n system of the ring, and it is therefore an electron rich molecule that reacts readily with electrophiles but is not susceptible to nucleophilic attack. 

Nonconjugated dienes bound to electrophilic transition-metal centers are activated toward nucleophilic attack analogous to olefin activation by transition metals discussed in 5.8.2.3.4. Although secondary reactions or rearrangements can occur, the pattern of the nucleophilic addition step (Nuc = nucleophile) leading to a, n complexes is shown... 

One important mechanism for homogeneous catalytic activation of aromatic C—H bonds is electrophilic attack by transition-metal complexes on the aromatic substrates. It is presumed t -aryl complexes are important intermediates in these reactions, but they are rarely isolated. Direct electrophilic metallation of aromatic substrates is closely related to reactions observed with nontransition metals ( 5.6., auration 5.7.2., mercuration and 5.3., thallation - ). References to metal-aryl complexes synthesized by electrophilic attack on arenes by transition metals are sununarized in Table 1. Reviews are available " . 

The carbon-metal a bond of these heterometallocyclic compounds is obtained by intramolecular activation of a C—H bond by a transition-metal complex. This activation results either from the oxidative addition on a basic metal in a low oxidation state such as Fe(0), Ru(0), Ir(I), or by electrophilic attack of a transition metal in a di- or trivalent states [Ru(II), Pb(II), Pt(II), Ir(III),...]. 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

Among the earliest reports of alkane activation by a transition metal complex were the articles by Shilov in which Pt(n) served as a catalyst for methane oxidation and Pt(iv) served as a stoichiometric oxidant." The mechanism of C-H activation was termed electrophilic, as the cationic metal was postulated to interact with the electrons of the C-H bond which then lost a proton, forming a metal-carbon bond without a change in oxidation state. Oxidation of the complex by two electrons was then followed by nucleophilic attack at carbon, giving a functionalized hydrocarbon (Scheme 1). 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

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