Metals/metal ions electrophilic addition with

Unactivated cyclopropanes can be cleaved by strongly electrophilic reagents like the proton, halogens or certain metal ions, unfortunately often with low regioselectivity only. So far, only mercury(Il)-induced openings have been of synthetic value. They incorporate suitable nucleophiles and reduction of the products can form radicals capable of additions to electrophilic olefins (equation 14) . ... 

Electrophilic catalysis may play an important role in the case of the similar benzylic carbon, too. For an O-benzyl system, it was found in a 1997 experiment that palladium oxide is a much more effective catalyst than palladium metal when the catalyst has been prereduced with chemical reducing agents. This finding shows very clearly that the electrophilic character of the unreduced metal ions plays an important role in the hydrogenolysis of the benzyl C—O bonds. Additional support for this mechanism is the fact that a small amount of butylamine can inhibit the hydrogenolysis of the benzyl C—O bond. 

Subsequent to CO2 association in the hydrophobic pocket, the chemistry of turnover requires the intimate participation of zinc. The role of zinc is to promote a water molecule as a potent nucleophile, and this is a role which the zinc of carbonic anhydrase II shares with the metal ion of the zinc proteases (discussed in the next section). In fact, the zinc of carbonic anhydrase II promotes the ionization of its bound water so that the active enzyme is in the zinc-hydroxide form (Coleman, 1967 Lindskog and Coleman, 1973 Silverman and Lindskog, 1988). Studies of small-molecule complexes yield effective models of the carbonic anhydrase active site which are catalytically active in zinc-hydroxide forms (Woolley, 1975). In addition to its role in promoting a nucleophilic water molecule, the zinc of carbonic anhydrase II is a classical electrophilic catalyst that is, it stabilizes the developing negative charge of the transition state and product bicarbonate anion. This role does not require the inner-sphere interaction of zinc with the substrate C=0 in a precatalytic complex. 

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

The extensive data accumulated by Nakamura and Otsuka, although interpreted by them as being due to the intervention of metal carbene and metallocyclobutane intermediates, can also be rationalized by an alternative mechanism in which coordination of the chiral Co(II) catalyst with the alkene activates the alkene for electrophilic addition to the diazo compound (Scheme 5.4). Subsequent ring closure can be envisioned to occur via a diazonium ion intermediate, without involving at any stage a metal carbene intermediate. 

Let us start by considering the reaction of the copper(n) complex 6.49 with formaldehyde. Initially we might expect the diimine 6.50 to be formed, but this ignores the nature of the intermediates. As we saw earlier, the reaction of an amine with an aldehyde initially produces an aminol. Consider the addition of the second molecule of formaldehyde to 6.49. The product will be 6.51, which contains an imine and an aminol (Fig. 6-43). The imine is co-ordinated to a metal ion, and the polarisation effect is likely to increase the electrophilic character of the carbon. The hydroxy group of the aminol is nucleophilic and it is correctly oriented for an intramolecular attack upon the co-ordinated imine. The result is the formation of the copper(n) macrocyclic complex 6.52. 

The effects of the cr—JT interaction on the ground-state properties of allyltrimethylmetal compounds are paralleled by the effect on reactivity towards electrophilic reagents. Mayr demonstrated that allyltrialkylsilanes, allyltrialkyl-germanes, and trialkylstannanes react with diphenylcarbenium ions at rates 105,5.6 x 105, and 109, respectively, relative to propene.158 The reaction rates were also found to be sensitive to the inductive effects of the other substituents attached to the metal. A theoretical evaluation of the factors determining the regiochemistry and stereochemistry of electrophilic addition to allylsilanes and other allyl systems is reported by Hehre et al.159 They predict a preference for electrophilic attack anti with respect to the silane substituent, a prediction that is supported by many experimental studies.82,160... 

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

The cleavage of the C—H bond by direct participation of a transition metal ion proceeds via an oxidative addition mechanism or an electrophilic substitution mechanism. Metals in low oxidation states undergo oxidative addition while high oxidation state metals take part in electrophilic substitutions. Another function of the metal complex in these reactions consists of abstracting an electron or a hydrogen atom from the hydrocarbon, RH. The RH radical ions or R radicals which are formed then interact with other species, such as molecular oxygen which is present in the solution or in one of the ligands of the metal complex (21). 

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