Changing the Heteroatoms

Like the variations of the metal (Section 3.1) and of the additional ligands (Section 3.2), the variation of the heteroatom in the 1,3-dipole cannot be done arbitrarily. A suitable synthetic route to and the stability of the resulting starting compound are the limiting factors. So, like the complexes [Ru(CO)3(dab)] (13) and [Fe(RNC)3(dab)] (25), several of the 1,3-dipoles in this section had to be prepared and reacted in situ. Their identity, however, was beyond doubt - either through spectroscopic characterisation and comparison with stable representatives of the same type, or from a complete characterisation of their reaction products. 

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The regiochemistry of the coupling products between aryl radicals and ambident nucleophiles was at the center of the recent work of Pierini et al. [116] (see Table 4). Changing the heteroatom of the nucleophile into a softer one by going down in the periodic table (from naphtholate to naphthylthiolate for instance) leads to an increase in heteroatom substitution C substitution increases when the nucleophile heteroatom is more electronegative or if the aryl moiety of the nucleophile is a more delocalized one. 

NMR data for the series of 2,6-di-rerr-butyl chalcogenopyrylium ions 25-28 are summarized in Table IV. Noteworthy is the increase of V(H-3,H-4) on changing the heteroatom from selenium to tellurium. 

HPA have become interesting materials for catalysis because of their tunable redox and acid-base properties and their stability in the solid state. In fact it is not only possible to change the heteroatom and the framework metal atom, drastically affecting the chemistry of the compound, but it is also possible to exchange the counter cations, thus modifying the overall acid site strength of the catalyst and ultimately the activity and selectivity of the catalyst towards a particular reaction. 

Changing the heteroatom to oxygen (the element between nitrogen and fluorine), it was intriguing to observe that it exhibited reactivity patterns of both its neighbors [4, 8]. Conducting the reaction of methoxytris(trimethylsilyl)silane with potassium te/t-butoxide in benzene in the presence of 18-crown-6 led to the formation of the a-alkoxysilyl anion 2. The compound was meta-stable at room temperature and displayed slow self-condensation. But it could be derivatized with electrophiles like ethyl bromide and trimethylchlorosilane to give the respective methoxysilanes. 

A particularly interesting feature of the above reaction is the change of the stereoselectivity of the process upon changing the heteroatoms of the side arms (Table 4.1). 

As a main conclusion of this work, a summary of the influence of the vinyl groups on the electronic properties of the parent polymers is presented in Fig. 6. The same trends are observed, when changing the heteroatom, for the ionization potential and the energy bandgap of polyheterocycles and poly (heteroaromatic vinylenes). A continuous decrease of IP and increase of Eg are obtained on going from sulfur to oxygen-containing polymers and from... 

The resonance integral of the 7r-bond between the heteroatom and carbon is another possible parameter in the treatment of heteroatomic molecules. However, for nitrogen compounds more detailed calculations have suggested that this resonance integral is similar to that for a C—C bond and moreover the relative values of the reactivity Indices at different positions are not very sensitive to change in this parameter. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

The substituents and heteroatoms can be used to tune the spin preference of the acyclic diradicals by changing the energy levels of electron-donating and -accepting orbitals and hence the donor-acceptor interaction. 

The heteroatom content and viscosity are reduced while the gross hydrocarbon structure is little changed. However, for economic reasons high catalyst concentrations are unlikely to be used. 

On the other hand, the situation is more complex for the 3-substituted derivatives. In such compounds the substituent can be considered electronically to be located either meta or para to the selenium ring atom (cf. resonance formulas 8 and 9 for a -I -M substituted derivative). However, the substituent-caused shifts in the 3-substituted derivatives indicate that the heteroatom and the substituents are para and not meta related. 77Se chemical shifts are more sensitive than the 13C shifts to changes in electron... 

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