Nucleophilicity scale for

Recent works of Mayr and co-workers4-14 have illustrated this trend. In fact, these authors have established, in contrast to the accepted opinion about the relative character of the experimental electrophilicity/nucleophilicity scales for many reactions in organic and organometallic chemistry, that it would be possible to define nucleophilicity and electrophilicity parameters that are independent of the reaction partners. Mayr et al. proposed that the rates of reactions of carbocations with uncharged nucleophiles obey the linear free energy relationship given by 4-14... 

Although a large number of Au systems have been studied, there has been no serious attempt to establish a set of nucleophilicity scales for gold. Reactions of a variety of Au complexes with amines and heterocyclic nitrogen bases indicate a dependence on proton basicity that is frequently greater than that observed for Pt (Table 15). A number of rate constants for the reactions involving [AuClJ" are collected in Table 16. Reactions of anions with [AuC " and similar anionic... 

In Section 4 the problems associated with establishing a nucleophilicity scale for metal complexes was discussed we must await, with some impatience, the establishment of such a scale. 

The second well-known electrophilicity or nucleophilicity scale was by Legon and Millen [13,14]. In this scale, the assigned intrinsic nucleophilicity is derived from the intermolecular stretching force constant k, recorded from the rotational and infrared (IR) spectra of the dimer B. .. HX formed by the nucleophile B and a series of HX species (for X halogens) and other neutral electrophiles. The nucleophilicity number in this case is obtained from the empirical relation... 

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

A point of key importance in study of solvolysis is the nucleophilicity of the solvent. Whereas the Y and other scales have been available for measuring ionizing power for some years, there has been no satisfactory scale for nucleophilicity. Swain, Mosely, and Bown attempted to set up an equation for correlation of solvolysis rates that included both nucleophilicity and ionizing power 112 their system did not prove particularly helpful for understanding mechanism.113 The Swain-Scott equation, discussed in Chapter 4 (p. 185), was not evaluated for solvents. 

Activation of the aromatic ring towards nucleophilic attack is required for all these reactions to proceed at reasonable rates and is commonly provided by strongly electron-withdrawing substituents. Reactivity also strongly depends on the leaving group and on the nucleophile. Scales of activating power of... 

Ketene alkyl sllyl acetals may also be used as nucleophiles for the formation of 8-hydroxy esters. The present reaction can be carried out equally well on large or small (mmole) scales. For small scale applications,... 

Dinitrobenzofuroxan (DNBF) is known as a superelectrophile due to its high reactivity both as an electrophile and in its pericyclic addition reactions. NMR studies show that reaction with 2-aminothiazole and its 4-methyl derivative yield anionic carbon-bonded adducts such as (11) by reaction at the 5-position, whereas the 4,5-dimethyl derivative reacts via the exocyclic amino group. Kinetic studies of the first two compounds, both in acetonitrile and in 70 30 (v/v) water-DMSO, have been used to assess their carbon nucleophilicities and place them on the Mayr nucleophilicity scale.55 In a related study, the nucleophilic reactivity, in acetonitrile, of a series of indoles with both DNBF and with benzhydryl cations have been compared and used to determine nucleophilicity parameters for the indoles.56... 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

NMR time scale for all counterions studied, i.e., Br, I-, C104, CF3S03, including also Cl- at low temperature. The rate constant was estimated to be at least of the order of 103 second-1 at 30°C. In light of this result it is reasonable to postulate a ligand exchange process for the nucleophile-induced racemization of optically active silanes (discussed in Section V,F). 

Various scales of nucleophilic reactivity for anions have been in use for several years (Swain and Scott, 1953 Edwards, 1956 Pearson et al., 1968 Ritchie, 1972 see also Bunnett, 1963 Ibne-Rasa, 1967 Hartshorn, 1973b), but quantitative scales of solvent nucleophilicity have only recently been developed. Peterson and Waller (1972) derived a scale of solvent nucleophilicity (APW) based on the... 

According to Scheme 51, at - 70° C alkynes are somewhat less reactive toward carbocations than alkenes with analogous substitution, but the reactivity scales for alkynes and alkenes overlap, and it is certainly incorrect to say that alkynes are generally weaker nucleophiles than alkenes. 

As already established for combinations of cations with n-nucleo-philes [33,160,219], the situation is less complicated for the reactions of carbocations with 7r-systems. Solvent polarity plays only a minor role (Section III.D.3) and, for many 7r-systems, the relative reactivity has been found to be electrophile-independent (Fig. 10, Section III.D.4.b). Also for these systems, the construction of a universal nucleophilicity scale is not unproblematic, however. Remember Fig. 11, which shows that An2CH + reacts 3.4 times faster with allyltrimethylsilane than with 2-methyl-2-bu-... 

A considerable improvement in the construction of reference scales for the quantitative characterization of nucleophiles and electrophiles in solution has been made by Mayr et al. [598] by means of Eq. (5-109a),... 

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