New Cations

One of the early applications of a superacid medium was the preparation and identification of the nitronium ion NOJ from nitric acid dissolved in 100% H2S04. 

A very early application of a superacid solvent was the use of 100% H2S04 for the preparation of stable solutions of carbonium ions, such as the triphenylcarbonium ion (Hantzsch, 1908c) (C6H5)8C+, e.g., 

More recently a large variety of different carbonium ions have been prepared in various highly acidic media. For example t-butanol is completely converted to the trimethylcarbonium ion in HS08F— SbF5—S02 at — 60° (Olah et al., 1967). Stable solutions of the same carbonium ion can be obtained by protonation of 2-methylpropene in HF—SbF8 at low temperature (Brouwer et al., 1968). 

In the same medium methylcyclopropane is protonated to give the methylethylcarbonium ion 

Many aromatic hydrocarbons can be protonated in various superacid 

---

The classification fragmentation applies to reactions in which a carbon-carbon bond is broken. One structural feature that permits fragmentation to occur readily is the presence of a carbon that can accommodate carbocationic character (3 to a developing electron deficiency. This type of reaction, known as the Grob fragmentation, occurs particularly readily when the y-atom is a heteroatom, such as nitrogen or oxygen, that has an unshared electron pair that can stabilize the new cationic center.96... 

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes clear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for cluster synthesis in the autoclave. 

Nakazumi H, Natsukawa K, Nakai K, Isagawa K (1994) Synthesis and structure of new cationic squarylium dyes. Angew Chem Int Ed Engl 33 1001-1003... 

Until recently, the hydroformylation using palladium had been scarcely explored as the activity of palladium stayed behind that of more active platinum complexes. The initiating reagents are often very similar to those of platinum, i.e., divalent palladium salts, which under the reaction conditions presumably form monohydrido complexes of palladium(II). A common precursor is (39). The mechanism for palladium catalysts is, therefore, thought to be the same as that for platinum. New cationic complexes of palladium that are highly active as hydroformylation catalysts were discovered by Drent and co-workers at Shell and commercial applications may be expected, involving replacement of cobalt catalysts. 

A mechanism involving the generation of a cationic alk(en)ylzirconocene (I X = R ) through chloride abstraction by silver(I) has been postulated (Scheme 8.22). This cationic intermediate is capable of activating the carbonyl group towards addition (II). Irrespective of whether an alk(en)yl group is added intra- or intermolecularly, a new cationic species is generated (either by R -transfer or Cl-abstraction from III) and the reaction thus proceeds in a zirconium-catalyzed manner [50]. 

If the new cation formed by union with an alkene further combines with an alkene, this will result in the formation of a polymer. 

Lenssen K, Jantscheff P, von Kiedrowski G, Massing U. Combinatorial synthesis of new cationic lipids and high-throughput screening of their transfection properties. Chembiochemisty 2002 3(9) 852-858. 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

Essentially two mechanisms have been offered for polar cycloaddition. The first of these is a nonsynchronous or two-step mechanism which involves attack of the electrophilic cation on the nucleophile to produce a new cation, which undergoes cyclization by attack on suitably located TT-electrons or an unshared electron pair. This mechanism has been advocated at some time by seemingly all the major workers in the area with the exception of Fields et al. The alternative view, now held by Schmidt, is that the reaction is synchronous. In this section, an effort will be made to reconcile each of the two theories to some observations which have been made concerning polar cycloaddition. 

The nitrogen Is binding energy in [MoCl2(NO)2diars] has been reported. [M(CO)4(diphos)] (M = Mo or W) react with NO" to form the new cationic nitrosyl complexes [M(CO)3(NO)(diphos)], isoelectronic with [V(CO)3-N0(PR3)2]. NOCl reacts with [M2(CO)8Cl4,] (M = Mo or W) to give new mononitrosyl derivatives which have been tentatively formulated as [M(NO)-... 

Nitrosyl complexes. The new cationic species [Os(NO)(CO)2(PPh3)2] has been isolated as its BPhJ salt from the reaction of CO and NaBPh4 with [Os(NO)(CO)(PPh3)2Cl]. The complex undergoes an interesting reversible reaction with methoxide ion ... 

Similar bridge-splitting processes have been independently used to prepare the new cationic series trans-[PtX(CO)L2]BF4 (X = anions L = PEt3, PPhj, or PMe Ph), and cis-[PtClYLj (Y = SCN, NO3, or NOj). The geometries were suggested from their n.m.r. spectra. A detailed study was made of the effect of the trans influence of X on the reactivity of the trans-[PtX(CO)L2]BF4 complexes. 

A number of ionic liquids incorporating new cations and anions have shown decreased viscosity and stability (/). There is no doubt that more applications of ionic liquids in catalysis are yet to be developed. 

Nucleophiles such as water, alcohols, esters, acetals, and ethers can also act as transfer agents by reacting with the macrocations, at the same time forming a new cation that initiates the growth of a new chain the kinetic chain thus remains unbroken. 

Easy and well-defined olefin insertion has not yet been observed for these new cationic alkyls. However, with terminal alkynes - which are much more... 

The displacement of the chlorine atom in 395 by triphenylphosphine or other phosphorus derivatives leads to the corresponding phosphorylated oxazolones 397 or 398 that have been used to prepare new and interesting substituted vinylphos-phonium salts (Scheme 7.127). Of particular interest is the synthesis of N-acyl-a-(triphenylphosphonio)glycinates as new cationic glycine equivalents. ... 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

---