Onium stability

Ylides are 1,3-dipolar compounds, RjE—CRjRj, where E = N, P, As is a group-V element and RjRj is H, alkyl or aryl. The onium-stabilized carbanion is planar, or ca. sp -hybridization about the carbon atom. ... 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

In the absence of the carbonyl or similar stabilizing group, the onium salts are much less acidic. The pATp so of methyltriphenylphosphonium ion is estimated to be 22. Strong bases such as amide ion or the anion of DMSO are required to deprotonate alkylphos-phbnium salts ... 

Arenediazonium salts also react with stabilized phosphonium, arsonium, pyridinium, and sulfonium ylides (12.111) in acetonitrile, yielding via the azo-onium salt (12.112) the azo-onium ylide (12.113, yellow to red), and in some cases the for-mazane (12.114) (Froyen and Juvvik, 1992). 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

What is retained nowadays of the initial mechanism (Scheme 1) is the occurrence of a cationic intermediate. But bromine bridging is not general, and its magnitude depends mainly on the double bond substituents (Ruasse, 1990). For example, when these are strongly electron-donating, i.e. able to stabilize a positive charge better than bromine, / -bromocarbocations are the bromination intermediates. The flexibility of transition state and intermediate stabilization puts bromination between hydration via carbocations and sulfenylation via onium ions. 

Although diselenonium-, ditelluronium- and mixed sulfonium-selenonium dications can exhibit either oxidative or electrophilic properties in reactions with nucleophiles, substitution at the onium chalcogen atom is more typical.96 Owing to the increased stability of heavier dichalcogenium-dications, they react only with highly activated substrates such as aniline and tV,A-dimethylaniline, while no reaction is observed with phenol and diphenylamine.113 Reactions of ditelluronium dications with activated aromatics are also not known (Scheme 44).114... 

The increasing stability toward racemization was supported by theoretical calculations carried out on the dimethyl chalcogen-onium ylides.20 In fact, the... 

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. 

The onium form of the ethyl acetate cation-radical is more stable by 50 kJ moH than the corresponding carbonyl form (Rhodes 1988). The CHj fragment is stabilized by the three-electron bonding with the neighboring oxygen in the following manner -O.. CH2. Oxidation of the carbonyl... 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Onium-Salze sind in der Literatur nur vereinzelt beschrieben worden, so bei der Ringoffnung von 1,3-Benzoxazinen mit Hydroxylamin358 oder als stabile Zwischenprodukte bei der Isomerisierung von 3-(Aryl-nitrono-mcthyl)-l,2,4-oxadiazolcn, die mit Diazomethan N-methyliert werden konnen253 ... 

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

For the purposes of this review, we include probe molecules that can be either directly adsorbed or formed in situ. Examples of the latter case are carbenium ions and related electrophilic species. We will also consider several important heteroatom-substituted carbenium ions and heteroatom analogs of carbenium ions. Acylium ions are the intermediates in Friedel-Crafts acylation reactions (96). The most simple, stable acylium ion is the acetylium ion, 1, and others are formally derived by replacing the methyl group with other R groups. Oxonium ions, formed by alkylation of an ether, resemble carbenium ions but are in fact onium ions in terms of their structures. Their stabilization requires strongly acidic media, and like carbenium ions, oxonium ions have been proposed as intermediates in a... 

This reacts with a second molecule of alkene to form the 18 carbocation which stabilizes by a proton loss to form the p,y-unsatu rated ketone. The 19 cyclic carbox-onium ion was observed by CP-MAS NMR spectroscopy. 

The tetra alkyl amm onium salts of [B Hg]- , formed by ion-exchange reactions, have proven to be useful synthetic reagents because of their thermal and air stabilities. The structure of the [B Hg]- ion has been determined by an x-ray study (66) and shown to have the 2013 styx structure, C2j) symmetry. Mechanisms for the formation of this ion have been proposed (67). Tetraborane(lO) can be easily obtained from salts of [B3H8] (eq. 9). 

The majority of gold(I) ylide complexes prepared have been of a binu-clear constitution. The first compound of this type was obtained (56) when [Au(CH2PMe3)2]Cl was allowed to stand in the presence of the ylide for seven days [Eq. (19)]. The 3IP-NMR spectrum showed only one signal, indicating that the two phosphorus centers were in identical environments, and the H-NMR spectrum exhibited two doublets with 2/(P—CH3) and 2J(P—CH2) having the same sign. Thus the symmetrical structure depicted in Eq. (19) was invoked, and the presence of two onium centers adjacent to the Au—C bonds was believed to stabilize this unlikely species. 

Cavicchio c-Y al. (14) studied the reaction of stabilized ylides and related onium salts with tropylium ion and found that the reaction between a tropylium salt and an arsonium ylide led to tra .v-chalcone (48) along with triphenylarsine. 

Ionic Photoacid Generators. Ease of synthesis, high thermal stability, and good quantum yield have made sulfonium and iodonium salts the most widely used onium salts. Figure 19 depicts some representative examples. 

In the reaction of activated aryl halides with phenoxide and thiophenoxide in chloro- or o-dichloro-benzene at reflux, pyridinium salts (48 R, R = —-CH2CH2CHMeCH2CH2— R = Me, Bu", n-C6Hi3 R = CH2CHEtBu, CH2BU1) proved to be superior catalysts to simpler onium salts, like TBAB, due mainly to their greater thermal stability.162... 

---