Heteroatom-cation interactions

Cyclic compounds containing two S, Se, or Te atoms may display transannular or intramolecular interactions between the two heteroatoms depending on their positions. If a cation radical is generated on one of the heteroatoms, the second heteroatom can interact to stabilize the cation radical, resulting in the formation of a three-electron-bonded cation radical which further oxidizes to give a dication. Such systems joined by two positively charged heteroatoms have attracted considerable interest. The studies in this field have been hampered by the difficulty of their preparation and further by their instability. 

Because the EHT-MO calculations on the model cation [Ru(ti5-C5H5) (C=C=CH2 (C0)(PH3)]+ indicate that the C and Cp atoms of the allenylidene unit are electrophilic and nucleophilic centers, respectively, and the H-O hydrogen atoms of water and alcohols are electrophilic, it has been proposed that the transition states for the above mentioned additions require heteroatom-C interactions, which labilize the O-H bonds, favouring the migration of the H-O hydrogen atoms to the Cp atom of the allenylidene. Thus, the lower nucleophilicity of the H-C(sp)carbon atom of phenylacetylene and H-C(sp ) carbon atoms of methane and acetone could explain why the additions of the latter substrates to the allenylidene ligand are kinetically disfavored processes [23]. 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

The cation radical at the heteroatom generated by one-electron oxidation is stabilized by a silyl group situated at the jS-position. The C-Si a orbital interacts with the SOMO to stabilize the cation radical when they are in the same plane. 

Another point of such coordination activity is the specific interaction of the solvents containing heteroatoms with cation-radicals having a suspended unpaired electron (cf Chapter 3). A pertinent example in this context is the interaction between the dialkylsulfide cation-radicals and the oxygen belonging to the water molecule. Such interaction enhances stability of the coordinated cation-radical ... 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

Noyori et al.1 have reviewed uses of this triflate (1) for silylation and as a catalyst in various nucleophilic reactions. The review includes several unpublished results obtained by the Nagoya group. The Si atom in the triflate can interact with various heteroatoms, particularly with oxygen to form onium cations, which can act as supercations in aprotic solvents. For example, the reaction of 1 with the keto epoxide 2 involves ring fusion followed by cyclization to 3. 

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