Effect of tetramethylammonium cation

A zeolite reactant mixtures, effect of tetramethylammonium cation on crystallinity, 156,157/... 

Weng et al. [113] studied the effect of tetramethylammonium cations (TMA"") on HO2 crystal morphology under hydrothermal conditions. The as-synthesized samples were characterized by XRD, TEM and SEM methods (see Table 2). The observed morphologies include besom-like particle, nanosheet and nanotubes. The mechanism to accelerate the formation of nanotube in the base of NaOH/TMAOH mixture is illustrated in Eigure 7. Bulk HO2 is first exfoliated to be layered protonic titanate by the mineralization effect of Na". In the presence of TMA"" cations, the separation of layered protonic titanate is accelerated by intercalating TMA"" cations in layered titanate. As a result of the presence of more layered titanate in the hydrothermal solution, nanotubes are formed ahead of schedule by curliness of layered titanate. Thus, the mechanism through which TMA cations affect crystal growth in the conditions of this study is different. 

It is interesting to note that although the first examples of template effects were observed in nitrogen macrocycles (see chapter 2) no template effect appears to operate in the synthesis of 72. Richman and Atkins note this in their original report . The authors replaced the sodium cation with tetramethylammonium cations and still obtained greater than 50% yield of tetra-N-tosyl-72. Shaw considered this problem and suggested that because of the bulky N-tosyl groups, .. . the loss of internal entropy on cyclization is small He offered this as an explanation for the apparent lack of a template effect in the cyclization. 

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

Solid lithium aluminium hydride can be solublized in non-polar organic solvents with benzyltriethylammonium chloride. Initially, the catalytic effect of the lithium cation in the reduction of carbonyl compounds was emphasized [l-3], but this has since been refuted. A more recent evaluation of the use of quaternary ammonium aluminium hydrides shows that the purity of the lithium aluminium hydride and the dryness of the solvent are critical, but it has also been noted that trace amounts of water in the solid liquid system are beneficial to the reaction [4]. The quaternary ammonium aluminium hydrides have greater hydrolytic stability than the lithium salt the tetramethylammonium aluminium hydride is hydrolysed slowly in dilute aqueous acid and more lipophilic ammonium salts are more stable [4, 5]. 

Effect of cations other than TPA has also been studied and the results are given in Table 2 and Fig.3. The molar ratio of additional cation/CTA" was kept at 1.4. It can be seen that cations like tetramethylammonium or tetraethylammonium ions or even sodium also gives highly ordered MCM-41 structure The BET surface area of these samples was found to be >1000 m2/g. Fig.3 shows that the MCM-41 structures obtained with these additional cations have improved hydrothermal stability. The long range ordering was unaffected by 4 days of hydrothermal treatment. Moreover, the surface area and pore volume of the water-treated samples was only marginally lower than that of the calcined samples. 

In an attempt to demonstrate the existence of pentavalent nitrogen, Schlenk and Holtz studied the reaction of triphenylmethyl sodium with tetramethylammonium chloride (52). The highly colored material was strongly conducting in polar solvents and could be identified as a salt, the stability of which is due to the resonance stabilization of the triphenyl-methide anion. In the absence of such stabilizing substituent effects (53), as with n-butyl or another alkyllithium reagent, a metalation of the tetramethylammonium cation occurs, which leads to type I products (18) ... 

Effect of the Structure of Organic Quaternary Ammonium Ions. The tetramethylammonium ion (N C J ), first introduced in zeolite synthesis by Barrer and Denny (30), and Kerr and Kokotailo (21) is effective in forming the cubic octameric silicate anion (Sig02Q°, cubic octamer) (2-16). In the tetramethylammonium silicate aqueous solutions at higher S3.O2 concentrations or cation-to-silica molar ratios (abbreviated to the N/Si ratios), the cubic octamer is singularly formed. 

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

Figure 5. Effect of the supporting electrolyte cation in the nitrobenzene phase on the relationship between interfacial potential and dye iodide (DiOC2(3)1) concentration. A, 0.01 mol /L tetramethylammonium tetrapheny lb orate B, 0.01 mol/L tetraethylammonium tetrap heny lb orate C, 0.01 mol/L tetrabuty-lammonium tetrapheny lb orate D, two identical curves, 0.01 mol / L tetraphenylarsonium tetraphenylborate and 0.01 mol / L crystal violet tetraphenylborate. (The crystal violet has no supporting electrolyte in the nitrobenzene phase.) The supporting electrolyte in the water phase is always 0.01 mol/L LiCl, except for curve E, which has no supporting electrolytes.

The specific effects of salts come from differences in their screening ability. The larger size of the hydrated positively charged cation in the neutral salt, the more efficiently it can screen and the more rapid the gel time. The effect is particularly pronounced with large organic cations such as guanidine, tetramethylammonium, and tetraethanol-ammonium ions. 

Whereas the radius of every inorganic ion, in the anhydrous state, is well established, those of low mass are strongly hydrated in solution so that their effective radius is much greater. Just how much greater cannot be said because an exact method for measurement is lacking. However, the figures for Li and Na" " in the last column of Table 12.2 are indicative. The cationic head is the widest part of the acetylcholine molecule. What happens if it is made wider First, let us note the acetylcholine-like effect of simple aliphatic quaternary amines. These have only a feeble action on muscarinic sites, although their action on nicotinic sites is considerable (see below). Tetramethylammonium salts, for example, have only about one-thousandth of the activity of acetyl-... 

In the alkyltrimethylammonium cation series (of which tetramethylammonium is the foundation member) maximal activity is reached at the n-pentyltrimethylammonium salts which are about eight times as active as acetylcholine (and about as active as nicotine) at nicotinic receptors (Willey, 1955). D-Lactoylcholine has strong nicotinic, but little muscarinic, potency (Sastry, Lasslo and Pfeiffer, 1960). Phenolic ethers of choline, such as (12.86), have a strong nicotinic (but little muscarinic) activity (Hey, 1952). However, this ratio is reversed by inserting methyl-groups into the two or/Ao-positions of the benzene ring, a reminder that nicotinic action is easily repressed by steric hindrance to which muscarinic action is indifferent (note the effect of the C-methyl-group in methacholine, p. 523). 

---