Onium complexes

Organometallic compounds are considered only with strong restrictions, namely when organic ligands, e.g., methyl groups, are introduced to avoid formation of coordination polymers, as is the case with mononuclear metallo-onium complexes. 

Dialkylborylalkylidenetriphenylphosphoranes react with polar or polarizable compounds via onium complexes to give new ylides, in which the dialkylboryl group has been substituted, and the corresponding organoboron compounds (equation 89). ... 

Wittig coined a large number of technical terms— valence tautomerism, ylide, onium complexes, halogen-metal exchange, dehydrobenzene, and umpolung (polarity reversal). Wittig died in 1987 at the age of 90. °... 

A second control element will be directed complex formation of the 4ii-system as a whole, intervention of a nucleophile at Cj or C, and the subsequent fate of the complex formed at this stage. To answer the first question, the onium complex 274 should be captured by different nucleophiles with the expectation that low nucleophilicity should lead to 1,4-adducts as the major product. 

Other complexing agents sometimes advocated are cryptates, especially the compound dubbed [2.2.2] (Kryptofix 222) [23978-09-8] (see Chelating agents). Crown ethers were originally advocated for reactions in the presence of soHd reagents (Uquid-soHd PTC). It is now known, however, that onium salts are equally suitable in many cases. 

The ate complexes are analogous to the onium salts formed when a Lewis base expands its valence, for example. 

Formulas of phosphonium/sulfonium ylide complexes should be written with onium charges at the P or S atoms and negative charges at the metal atom, but for the sake of convenience and clarity, these charges are omitted. 

Early work focused on compounds with open-chain carbenes generally synthesized in the coordination sphere of the gold atom, for example, by addition of amines or alcohols to isocyanide ligands in the corresponding gold complexes. Subsequent synthetic approaches have relied on the in situ deprotonation of onium salt precursors by a... 

Thiols like pyridine-2-thiol yield (carbene)gold thiolates, and onium salts give cationic (carbene)gold ylide complexes, isolated and characterized as the perchlorates.170 The reaction of dithiocatechol with (cyclohexylisocya-nide)gold chloride affords a carbene complex [(CyNC)AuC(NHCy)2]+Cl-, which co-crystallizes with a neutral tetranuclear complex [(CyNCAu)2Au2(S2C6H4)2].224... 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

The synthetic principles underlying the preparation of the 1 1 carbene complexes (above) are also applicable to products with a 2 1 ligand/metal ratio . The carbene components may be introduced as free carbenes or their dimers, or generated in situ from the onium salts. A second major pathway uses anionic hetero-... 

In order to compare donor strengths of different donor ligands, Su and Collette found it convenient simply to use the pKa values (23) of the onium salts of various donors. This is based on the assumption that the complexation of Rh with a donor is analogous to an acid-base interaction ... 

UOP in a joint venture with ChevronTexaco developed an additive technology named Alkad . The additive is based on HF salts of amines, which form liquid onium polyhydrogen fluoride complexes with HF, reducing the vapor pressure of the catalyst 65% to more than 80% aerosol reduction is claimed with this additive. As in the ReVap technology, additional separation columns have to be installed. Both additives are claimed to increase the product octane number, especially when propene, isobutylene, and pentenes are employed in the feedstock. 

The commonly used quaternary onium salts work by associating with the inorganic anion and transferring it into the organic phase. Complexing agents such... 

Bowen, R.D. The Role of Ion-Neutral Complexes in the Reactions of Onium Ions and Related Species. Org. Mass Spectrom. 1993,25,1577-1595. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

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