External attack

The reverse reaction (ion formation) can occur in two ways internally, by attack of the penultimate polymer oxygen atom, or externally, by attack of a monomer oxygen atom (chain growth). The external process is about 10 times slower than the internal process in bulk THF (1). Since ion formation is a slow process compared to ion chain growth, chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerization process. 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

Based on detailed kinetic investigations, a tentative mechanism for this asymmetric oxidation was proposed (Scheme 2) according to which optically active sulphoxides may be formed by two pathways external attack on the sulphur atom by the chiral titanium hydroperoxide (path A) or coordination of sulphur to titanium prior to the oxidation step (path B). Although paths A and B could not be distinguished experimentally, the temperature effect was tentatively ascribed to a change of the mechanism, path A being predominant above — 20 °C and path B becoming competitive at lower temperatures (or vice versa). 

Pd(H20)4] at 40°C [73]. A kinetic study indicated that internal attack on the Pd-co-ordinated nitrile ligand by the aqua (not hydroxide) ligand and external attack on the nitrile ligand by solvent water occur at a similar rate. 

Mechanistically, the nucleophilic addition can occur either by internal ligand transfer or by external attack. Generally, softer more stable nucleophiles (e.g., malonate enolates) are believed to react by the external mechanism and give anti addition, whereas harder nucleophiles (e.g., hydroxide) are delivered by internal ligand transfer with syn stereochemistry.120... 

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the rr-diene complex and the second on the intermediate jr-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. 

The involvement of ion pairs in the addition process has also been related to the stereochemical behavior. The remarkable difference in configuration between the rearranged chlorides and acetates has been rationalized, as shown in equation 113, on the basis of a syn internal attack of Cl- on ion c and anti external attack of AcOH from the solvent pool. 

A bigger effect for H2O than OH is very unusual and is a behavior certainly not shown by the uncoordinated amide. The effect is ascribed to a benefit from cyclization and concerted loss of protonated amide, without formation of the tetrahedral intermediate. Although the coordinated OH is some 10 times less effective than coordinated HjO (Table 6.4), it is still about 10 times faster with 15 than via external attack by OH at pH 7 on the chelated amide 13. Early studies showed that complexes of the type CoN4(H20)OH can promote the hydrolysis of esters, amides and dipeptides and that this probably arises via formation of ester, amide or peptide chelates. These then hydrolyze in the manner above. 

The formation of olehnic dimerization products as the main product is rare in the decomposition of diazo compounds, whereas formation of ketazine is virtually omnipresent. The authors explained these data by assuming that the hindered diarylcarbenes do not have accessible singlet counterparts, because the singlet would require a smaller carbene angle and incur severe aryl-aryl repulsion. As a result of severe steric hindrance and consequent resistance to external attack by solvent, the... 

Contrary to the ionic mechanism suggested by Tsuji, an insertion mechanism explains the facts much better. An external attack of carbon monoxide at the most positive carbon atom of propylene in a palladium chloride complex, as Tsuji proposed, would be expected to produce 3-chloro-2-methylpropionyl chloride rather than the observed product, 3-chlorobutyryl chloride. Since oxidation of propylene by Pd (II) ion gives acetone rather than propionalydehyde, a CO insertion reaction and elimination should produce the observed compound, 3-chlorobutyryl chloride... 

The reaction of C02 with Ir(CH3)CO(02)[P(p-tolyl)3]2 also results in the formation of a peroxycarbonate complex (191) via external attack by carbon dioxide. In this case, however, only gaseous carbon dioxide is required, rather than the more strenuous conditions of liquid C02. This same complex reacts with gaseous carbon monoxide to form the carbonate complex. Labeling experiments demonstrate that the coordinated CO does not participate in the reaction External attack by the added CO is responsible for the reaction (191). Coordinated CO has been shown to react with bound dioxygen, as is seen in Scheme 16. In this case, the chelating triphos ligand obviously has a significant effect on the reactivity (189). 

Once activated, the substrates are transformed via a number of different possible steps including ligand migration, insertion, elimination or extrusion, and external attack on bound substrate. Of these, the last is most easily envisioned—a reagent not coordinated to the metal center of the catalyst attacks the bound substrate whose coordination has rendered it chemically reactive. 

Dissociation of DIBAH from (16) then creates a vacant site for CO coordination. Hydride migration from Zr or external attack by hydride from DIBAH leads to the formation of a formyl that is then reduced by the aluminum hydride species (81). The steps for chain propagation and termination are outlined below. Since the exact nature of the Zr and A1 species is unknown in these steps, they are represented by (Zr) and (Al), respectively. 

Most of the reactions listed in Table 6 involve prior activation of the substrate by coordination to palladium in the form of a v-, a 77-ally lie, a 77-benzylic, or an alkyl or aryl complex. Once coordinated to the metal, the substrate becomes an electron acceptor and can react with a variety of different nucleophiles. The addition of nucleophiles (Nu) to the coordinated substrate may occur in two different ways, as shown by Scheme 9 for 7r-alkene complexes 397"399 (a) external attack leading to trans addition of palladium and nucleophile across the 77-system (path A) or (b) internal addition of the coordinated nucleophile to the complexed alkene resulting in cis addition of palladium and nucleophile to the double bond. The cis and trans adducts (120) and (121) may then undergo /3-hydride elimination (/3-H), producing the vinylic oxidation product... 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

The platinum complex (diphoe)Pt(CF3)(OH) is an effective catalyst for the selective epoxidation of terminal alkenes by dilute H202 under mild conditions (20 °C). The reaction is thought to proceed via external attack of the HOO- anion on the coordinated alkene (equation 296).636... 

Further investigations of the reaction suggest that the bidentate complex (154) is the reactive species which undergoes external attack by water or other nucleophiles present in solution. The enzymic hydration of epoxides and the possible role of metal ions has been discussed 501 the results obtained suggest that a metal ion is not involved at the active site of epoxide hydrase. 

Within the past 20 years, ferrates, i.e. anions possessing iron as the center atom, have found increasing application as nucleophilic complexes in substitution chemistry. In these reactions, the ferrate replaces the leaving group X in a first nucleophilic substitution event. A transfer of one ligand from the metal atom (i.e. a reductive elimination, path A, Scheme 7.2) or substitution of the metal atom via external attack of the nucleophile (path B) concludes this mechanistic scenario. However, the exact mechanism in ferrate-catalyzed nucleophilic substitutions is still under debate. Apart from the ionic mechanism, radical processes are also discussed in the literature. 

How may we distinguish between pathways that involve external attack by hydroxide and those that involve co-ordinated hydroxide There is a considerable accumulation of data that suggest the two latter pathways are the most important (i.e., attack of external hydroxide upon monodentate amino acid ester is not greatly accelerated). The attack by external hydroxide may be studied independently and accurate rate constants may be determined for insertion in the composite rate equation with the two competing processes. In some cases it is possible to detect the five-co-ordinate and the other intermediates. Finally, some elegant labelling studies have provided very strong evidence for the exi-... 

Because of its molecular size and polarity, water is absorbed into polyurethane and an equilibrium set up. The presence of water in the urethane will aid in certain external attacks by dilute acids and alkalis. 

The effect of the charge is stronger than the effect of oxidation state. The chemist must be aware of the simplicity of these rules which, however, provide an indication of the reactivity of the coordinated ligands. In some external attacks it appears that other factors may be at the origin of the driving forces (88a). 

The peroxocarbonato iridium complexes [Ir(OC03)(CO)(PPh3)2(R)] (R = Me, Ph) have been synthesized from [Ir(CO)(Q2)(PPh3)2(R)] and liquid C02, probably via external attack by C02 on... 

The Pd(II) complex (41) promotes stoichiometric alcoholysis of urea according to Scheme 8, giving the carbamate esters of the ligand (101). For methylurea as the substrate, the major product is the one with R = H (75%), while the product with R = Me is the minor one (25%). This intramolecular alcoholysis is 240-380 times faster than the intermolecular alcoholysis involving external attack of free ethanol. The O-bound 1,3-dimethylurea does not undergo any detectable intramolecular or catalytic alcoholysis, since the N-bound isomer, which is the much more reactive one, is practically absent due to steric reasons. 

The explanation for the dual stereocontrol in the oxyamination reaction is as follows in the absence of chloride ligand, cis migration of coordinated acetate is observed from the intermediate 7t-allyl complex, whereas added chloride blocks the coordination of acetate to palladium and forces the external attack of acetate to take place. The relative stereochemistry, referring to the addition across the diene system, was established by H-NMR spectroscopy. 

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