Lewis acids complex ions

Titanium tetrachloride and tin tetrachloride can form complexes that are related in character to both those formed by metal ions and those formed by neutral Lewis acids. Complexation can occur with an increase in the coordination number at the Lewis acid or with displacement of a chloride from the metal coordination sphere. 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acylium ion and a Lewis acid-bound phenolate. A fr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

Remarkable positive shifts of the °red values of the singlet excited states of the metal ion-carbonyl complexes as compared to those of the triplet excited states of uncomplexed carbonyl compounds (Table 2) result in a significant increase in the redox reactivity of the Lewis acid complexes versus uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. For example, photoaddition of benzyltrimethylsilane with naphthaldehydes and acetonaphthones proceeds efficiently in the presence of Mg(C104)2 in MeCN, although... 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

Because metal ions bind to and modify the reactivity and structure of enzymes and substrates, a wide spectrum of techniques has been developed to examine the nature of metal ions which serve as templates, redox-active cofactors, Lewis acids/bases, ion-complexing agents, etc. 

In the previous examples, the sulfur atom acted as a nucleophile. Electron-deficient sulfur species such as sulfenyl ion and its equivalents (e.g. disulfide/Lewis acid complexes, sulfenic acids, sulfenyl halides, sulfonium ions, sulfines, etc.), can also serve as an electrophile. Oxidative ring closure of enethiols (a-thioketocarboxylic acid) (124), which proceeds via disulfides, produces thiophenes (125) in good yields (86EUP158380, 88JHC367). 

Oxenium Ions. Oxenium ions similar to nitronium ions are, in general, too reactive to be observed. The parent ion—that is, the hydroxyl cation HO+—is elusive, and it is improbable that it can be observed in its free form in the condensed state. However, the incipient hydroxyl cation is involved in acid-catalyzed electrophilic hydroxylation with protonated (or Lewis acid complexed) hydrogen peroxide (HO—OH2+) or ozone (HO—O—0+).125 Nitrous oxide is also apotential precursor for the hydroxyl cation (in its protonated form). The hydroxy diazonium ion HON2+ has not yet been observed. 

Fluoride ion (from CsF, CdF2, or AgF) can also catalyze the allylation using AllylSiF3 and AllylSi(OMe)3, respectively [10, 11] the asymmetric version (with <56% ee) requires a combination of Bu4N+[SiPh3F2] with the Lewis acidic complex of CuCl and BINAP [21]. 

In the presence of an electrophile, tautomerization of a substrate with a C=0 double bond to its enol only takes place when catalyzed by either a Bronsted- or a Lewis acid. The proton-catalyzed mechanism is shown for the ketone — enol conversion B — iso-B (Figure 12.4), the carboxylic acid —> enol conversion A — E (Figure 12.6), the carboxylic acid bromide — enol conversion E —> G (Figure 12.7) and the carboxylic acid ester — enol conversion diethyl-malonate —> E (Figure 12.9). Each of these enol formations is a two-step process consisting of the protonation to a carboxonium ion and the latter s deprotonation. The mechanism of a Lewis acid-catalyzed enolization is illustrated in Figure 12.5, exemplified by the ketone —> enol conversion A —> iso-A. Again, a protonation to a carboxonium ion and the latter s deprotonation are involved the Lewis acid-complexed ketone acts as a proton source (see below). 

In eutectic-based ionic liquids, the chloride ions act as strong ligands for the oxidized metal ions, forming a range of chlorometallate anions. The free chloride ions are present in very low concentrations as they are complexed with the Lewis acidic metal ions and so the dissolution of metal ions must lead to a complex series of equilibria such as... 

As a first approximation, the reactions of pyridines with electrophiles can be compared with those of trimethylamine and benzene. Thus, pyridine reacts easily at the nitrogen atom with reagents such as proton acids, Lewis acids, metal ions, and reactive halides to form salts, coordination compounds, complexes, and quaternary salts, respectively. Under much more vigorous conditions it reacts at ring carbons to form C-substitution products in nitration, sulfonation, and halogenation reactions. 

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

Tellurophenes readily form metal complexes with Lewis acids, metal ions, and metal carbonyls. A mixture of mono-and binuclear complexes 18 and 19 is obtained by coupling tellurophene (L ) with Na2PdCL <1972JOMC87>. With tetrachlorotellurophene (L ), the complex 20 with /ra r-configuration was prepared <1972JOMC87>. 

The large number of lanthanide alkoxide complexes featuring the inclusion of oxo or hydroxo groups unmistakably suggests that the coordination chemistry of lanthanide hydroxide is intimately associated with that of the alkoxides. Arguably the simplest and most ubiquitous O-containing ligand, the water molecule, occupies a special position in the development of lanthanide coordination chemistry. Upon coordination to the Lewis acidic lanthanide ion, an... 

Since the initial report in 1962 by Merten and Muller of the cycloaddition of an -acyl imine with a conjugated diene,a number of examples of this type of reaction have appeared. In general, IV-acyl im> ines are highly reactive, unstable species which are rarely isolated, but rather are generated in situ from stable precursors. Depending upon the method of formation of the particular dienophile and the reaction conditions, a neutral A -acyl imine or a protonated (or Lewis acid complexed) IV-acyl immonium ion may be involved in the Diels-Alder reaction. 

The reaction of aldehydes 3 with crotyl silanes (e.g. 5) yields 3-methylated homoallylic products such as 6 and 9. Since crotyl silanes are only weak nucleophiles, the carbonyl compound 3 must be activated. This can be done by addition of a Lewis acid (LA) to form the C2ixhony -Lewis acid complex 4. After addition of 5 and aqueous workup, the homoallylic alcohol 6 is obtained. An alkyl-oxo-carbenium ion 8 is available when treating an acetal 7 with acid or when the aldehyde 3 reacts with a silyl ether 10 in the presence of a Lewis or a Brousted acid (multicomponent crotylation). Crotylation of this alkyl-oxocarbenium ion 8 yields homoallylic ethers 9. 

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