Aptitudes of

The migratory aptitudes of the metal-boimd group R in carbonyl insertion reactions vary widely. The insertion of CO into some groups is thermod)mamically unfavorable, and the migratory aptitudes result from thermod)rnamic effects, rather than kinetic effects. In other cases, the insertions of CO into two different M-R bonds in the same complex or the insertion of CO into the M-R bonds in two different metal complexes are all favored thermodynamically, and the relative rates are controlled by kinetic effects. Examples of measurements of the relative rates for insertion in these two situations are described in this section. 

---

Examination of the migratory aptitude of groups in a variety of unsym-metrical ketones indicates that the group best able to accept a positive charge will usually migrate most readily. ... 

From the data eolleeted in Table VII, it follows that the migratory aptitude of the Si-, Ge-, and Sn-eentered groups in the rearrangement 40a 40b inereases from SiRj to SnRj groups. 

The carbocations generated by non-Kolbe electrolysis can rearrange by alkyl, phenyl or oxygen migration. The migratory aptitudes of different alkyl groups have been studied in the rearrangement of a-hydroxy carboxylic acids (Eq. 34) [323]. 

With unsymmetrical cyclic ketones, however, mixtures due to similar migratory aptitudes of the different groups are obtained (Table 13, Nos. 2, 3). The rearrangement has also been used as key step in a d,/-muscone synthesis (Table 13, No. 4). 

A number of experiments have been carried out to determine the relative migratory aptitude of groups in pinacol/pinacolone type rearrangements, and in general the relative ease of migration is found to be ... 

The ambivalent aptitude of sulfur [19] to stabilize adjacent anionic as well as cationic centers is a remarkable fact that has shown to be a reliable feature for the assembly of four-membered ring scaffolds utilizing cyclopropyl phenyl sulfides [20]. Witulski and coworkers treated the sulfide 1-69 with TsOH in wet benzene (Scheme 1.19) [21]. However, in addition to the expected cyclobutanone derivative 1-70, the bicyclo[3.2.0]heptane 1-70 was also obtained as a single diastereoisomer, but in moderate yield. Much better yields of 1-71 were obtained using ketone 1-72... 

The decomposition of the peroxide IX, which gives acetophenone when the conditions are such as to favor the radical mechanism, gives methyl ethyl ketone if the reaction is run in acid. This is because of the superior migration aptitude of phenyl groups in real or incipient oxygen cations.112... 

The product ratio a /b (statistically corrected for the number of competing H migrants), gives the relative migration rate of Ha vs. Hb, or k /kub- The rate constant for the migration of Ha corresponds to the intrinsic migratory aptitude of Ha (M[H]) multiplied by the bystander assistance factor for Y, B[Y], The carbon atom that bears Hb has no bystander substituent, so that km, is simply A/[H], We thus obtain Eq. 24. 

Spectator substituents, bonded to the carbene s migration terminus (Ci), directly influence the lifetime and philicity of the carbene, but they do not primarily alter the migratory aptitudes of migrants on C2. Oxa spectator substituents stabilize singlet carbenes by electron donation to the vacant carbenic p orbital (LUMO) cf. resonance hybrid 69. 

This rearrangement can also be effected with catalytic amounts of either (CH3)3SiI or (CH3)3SiOTf in the case of a,(3-epoxy trimethylsilyl ethers. Yields are dependent on the migratory aptitude of the group that rearranges.2... 

Exploratory data analysis shows the aptitude of an ensemble of chemical sensors to be utilized for a given application, leaving to the supervised classification the task of building a model to be used to predict the class membership of unknown samples. 

It is considered that [1,5] alkyl shifts usually require temperatures above 330 °C and proceed with free energies of activation greater than 180 kJmol-1. The 1,5-migratory aptitude of the formyl group is comparable with that of a trimethylsilyl group. However, 5-acetyl- and 5-ethoxycarbonyl-PMCPDs under the same conditions show a temperature-invariant XH NMR spectra, i.e. the migratory aptitude decreases in order CHO COMe COOMe114. 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

As a further confirmation of the extended redox aptitude of polypyridine ligands, Figure 15 shows the cyclic voltammetric behaviour of the heteroleptic chromium(III) complex with 2,2/-6/,2"-6//,2"/-quaterpyr-idine (qpy), together with its molecular structure.28... 

In confirmation of the redox aptitude of the diamantoidal derivatives, the complex [Mn406(bpea)4]4+ (bpea = iV,iV-bis(2-pyridylmethyl)ethy-lamine), Figure 43a,58 displays in acetonitrile solution a reversible one-electron reduction Mn4IV — Mn3IVMnm (Ecl = +0.10 V), followed by... 

In order to illustrate the possibly extended redox aptitude of the ferrocene complexes, Figure 7 shows the cyclic voltammetric response of the ferrocenyl-terpyridine-Ru(II) complex [Fc -(C6H4)-terpy-Ru-terpy-(C6H4)-Fc ]2+ [Fc = (fj5-C5Me4H)Fe(i 5-C5Me4)].5... 

The high redox aptitude of the neutral bis-complexes [MII o-C6H4(NH)2 2 (M = Co, Ni, Pd, Pt) is long known.62 Also in this case, under the assumption of the invariance of the oxidation state of the metal, such derivatives would display the redox path illustrated in Scheme 5. 

Since the electrochemical behaviour of metallophthalocyanines has been reviewed,103 we will simply demonstrate the ligand based nature of their redox processes. As a typical example, Figure 57 shows the redox aptitude of zinc-phthalocyanine [Zn(Pc)] CH2C12 in solution,96 together with its molecular structure.104... 

Comparison with the redox aptitude of [Zn(TPP)] (Figure 52) points out a minor redox flexibility of phthalocyaninates with respect to porphyrinates, in that in the former either the second reduction or the second oxidation are complicated by degradation of the corresponding congeners. We also note that the HOMO/LUMO separation for [Zn(Pc)] is 1.55 eV, i.e. lower than that of [Zn(TPP)] (2.15 eV). 

Whereas a large number of metal-sulfur clusters are present in nature, carbonyl clusters are exclusively products of chemical synthesis. They have been widely used in industrial catalytic processes17 and some of these processes are triggered by the redox aptitude of these species.lc g As for the metal-sulfur clusters, we will briefly discuss their structures and their propensity to donate/accept electrons in order of increasing nuclearity. We will consider only homonuclear and homoleptic metal-carbonyl derivatives. However, it is noted that heteronuclear derivatives are gaining considerable interest due to the synergistic effect of metal-metal bonds possessing a polar character.lc,ld... 

In conclusion, the low redox aptitude of [M4(CO)12] derivatives is related to their electronic saturation (60 valence electrons). 

At this time we are not aware of electrochemical studies pertinent to the redox change PN — Pox + 2e. In contrast, the redox aptitude of FeMoco has been electrochemically proved. The cyclic voltammogram of oxidized FeMoco in nmf (HCONHCH3) solution is shown in Figure 34.43,44... 

---