Electrophiles neutral complexes

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Because electrophilic carbene complexes can cyclopropanate alkenes under mild reaction conditions (Table 3.1) [438,618-620], these complexes can serve as stoichiometric reagents for the cyclopropanation of organic compounds. Thoroughly investigated carbene complexes for this purpose are neutral complexes of the type (C0)5M=CR2 (M Cr, Mo, W) and cationic iron(IV) carbene complexes. The mechanism of cyclopropanation by electrophilic carbene complexes has been discussed in Section 1.3. 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

In a mechanistically similar process, the neutral palladium(II) dipyridylamine complex (24), obtained by deprotonation of complex (23), underwent reaction with benzoyl chloride to give the substituted complex (25) together with some free ligand (Scheme 8).33 This particular reaction sequence could not be generalized because of the relative instability of other metal complexes related to compound (24). However, a more extensive series of electrophilic substitutions could be carried out on the neutral complex (26), which displayed ambident nucleophilic behaviour by reaction with benzyl chloride and benzoyl chloride at nitrogen and reaction with benzenediazonium fluoroborate at carbon (Scheme 9). 

The effect of the charge may also be seen in some nickel(n) complexes. The neutral complex 4.7, containing two anionic ligands obtained by the deprotonation of the salicylaldehyde derivative, is completely stabilised towards hydrolysis. In contrast, the monocationic complex 4.8, containing two neutral 1,10-phenanthroline ligands, is rapidly hydrolysed to the corresponding salicylaldehyde complex (Fig. 4-25). Presumably, the overall positive charge of the complex promotes the attack of the nucleophile upon the electrophilic carbon centre. 

Two electrophilic substitution reactions of neutral complex A have been reported. These are Friedel-Crafts acetylation and bromination. The proposed positions of substitution are indicated in Fig. 11 (20). 

Importantly, H2 gas can be turned into a strong acid on binding to electrophilic cationic complexes. Free H2 is an extremely weak acid with a TpKa near 35 in THF (23), and heterolytic splitting of r 2-H2 in relatively electron-rich neutral complexes is usually achieved only by strong bases. For example, we have shown Eq. (4) that copper alkoxides deprotonate W(CO)3(PR3)2(H2) and FeH2(H2)(PR3)2 to give heterobi-metallic species with bridging hydrides (24). 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

The overlap between the metal and ligand orbitals in the 5i>2g orbital (LUMO for Z = 0) provides an efficient pathway for ligand-to-metal electron donation and confers electrophilic character on the ligands in neutral complexes. This finding affords a reasonable explanation for the electrophilic mechanism proposed by Wang and Steifel for the nonclassical reaction of neutral Ni-bis(dilhiolene) complexes wilh olefins, which can be separated and purified by a novel melhod. The LUMO of Ihe dilhiolene complex... 

The tungsten-centered anions of 2a,b offer reaction with various electrophilic reagents as HOAc, Mel, ClSiMe2H, or ClSiMes, which lead to the neutral complexes 3 and 4a-c (Fig. 1). 

Aschi, M., Attina, M., Cacace, F. An Alternative Route To Electrophilic Substitution. 2. Aromatic Alkylation in the Ion Neutral Complexes Formed Upon Addition of Gaseous Arenium Ions to Olefins. J. Am. Chem. Soc. 1995,117,12832-12839. 

Scheme 12 summarizes a y-radiolysis performed on a mixture of benzene-CF4, and ethylene. Ionization of CF4 yields CFj, which has been called an ionic Lewis superacid.Electrophilic attack on benzene-(i6 gives the conjugate acid of perdeuterated trifluorotoluene, CF3C6DJ, as the first step of Scheme 12. Trifluorotoluene is more basic than ethylene hence, the ion-molecule reaction of CF3C6DJ with ethylene cannot give a Bronsted acid-base reaction. The only thermochemically accessible pathway to ethyl trifluorotoluenes is via ion-neutral complexes containing the ethyl cation...

Neutral Complexes. Interaction of acetylacetone and hydrous Rh2G3 gives the trisacetylacetonate, which has been resolved into enantiomeric forms. It undergoes a variety of electrophilic substitution reactions of the coordinated ligand, such as chlorination. The stereochemistry and racem-ization of the cis- and trans-isomers of the unsymmetrical trifluoroacetyl-acetonate have been studied by nmr spectroscopy the compound is extremely stable to isomerization. 

If the trans ligand is a strong a donor such as a hydride, there is a powerful trans labilizing effect that reduces donation from H2, which once again weakens M-H2 binding and contracts dm. Even relatively electron-rich neutral complexes with hydride trans to H2 such as OsHCl(H2)(CO)(PR3)2 and trans-IrCl2H(H2)(PR3)2 (10) often bind H2 more weakly than comparable electrophilic cationic systems which rely on enhanced a donation from H2 for stability. For the isomer with Cl trans to H2, IrCl2H(H2)(PR3)2 (11), JHD decreases dramatically to... 

Electrophilic attack sites In neutral complexes, producing a- nyl and alkylldyne complexes... 

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