Metal carbene complexes electron-deficient

Electron-deficient (having less than 18 electrons) and coordinatively unsaturated (containing less than 6, usually only 4 or 5 ligands) metal carbene complexes are highly effective in metathesis.191-194... 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

C-NMR chemical shifts are a useful diagnostic tool for carbene complexes, with Ccarb being substantially deshielded. The Ccarb resonance is seen to shift to higher fields as the electron deficiency of the metal center is increased (i.e., with electron-withdrawing ligands and in cationic complexes). 

Fischer carbenes characteristically contain a number of electron-withdrawing carbonyl ligands while the typical Ru, Os, or Ir carbene complexes described above frequently contain several cr-donor ligands. The metal centers in these former compounds, then, are rather electron-deficient, with nucleophilic attack at Ca being a favorable reaction. 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

One of typical reactions of the Fisher-type metal carbene is interaction of the electron-deficient carbenic carbon with a pair of non-bonding electrons contributed by a Lewis base (B ) to generate a metal complex-associated ylide or a free ylide. The ylide intermediate thus generated is usually highly reactive and undergoes further reactions to give stable products (Figure 1). 

Downfield shifts due to electron deficiency are observed for carbene metal complexes [80 a] and for the sp carbons of metal carbonyls [80 b],... 

The cyclopropanation reactions of the cationic iron carbene complexes occur most efficiently with alkenes of normal electronic characteristics. Veiy electron deficient alkenes such as a,(3-unsaturated carbonyl compounds are veiy poor substrates. Veiy electron rich alkenes such as enol ethers react rapidly, but the expected cyclopropanes generally cannot be isolated if they are indeed formed, they apparently undergo further reactions, peihaps promoted by the metallic species present in the reaction mixtures. 

C60 chemically behaves as an electron-deficient polyalkene, with rather localized double bonds. Thus, the reactions that mainly take place involve cycloadditions, additions of nucleophiles, free radicals, and carbenes, as well as rp-complexation with various transition metal elements. 

These compounds are nucleophihc at the noncoordinated sulfur, and undergo alkylation there with iodomethane or benzyl bromide to give initially (128) (which may be isolated as the PFe salt) and ultimately (129) (equation 28). They also react with electron-deficient alkynes, possibly by a dipolar mechanism, to afford cyclized iron carbene complex (130). Finally, compounds (129) are subject to attack by some nucleophiles hydride attack occurs initially at the metal, but ultimately gives ) -dithioester complexes, such as (131). ... 

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