Counterions bulky

Polk et al. reported27 that PET fibers could be hydrolyzed with 5% aqueous sodium hydroxide at 80°C in the presence of trioctylmethylammonium bromide in 60 min to obtain terephthalic acid in 93% yield. The results of catalytic depolymerization of PET without agitation are listed in Table 10.1. The results of catalytic depolymerization of PET with agitation are listed in Table 10.2. As expected, agitation shortened the time required for 100% conversion. Results (Table 10.1) for the quaternary salts with a halide counterion were promising. Phenyltrimethylammonium chloride (PTMAC) was chosen to ascertain whether steric effects would hinder catalytic activity. Bulky alkyl groups of the quaternary ammonium compounds were expected to hinder close approach of the catalyst to the somewhat hidden carbonyl groups of the fiber structure. The results indicate that steric hindrance is not a problem for PET hydrolysis under this set of conditions since the depolymerization results were substantially lower for PTMAC than for die more sterically hindered quaternary salts. 

ESR spectra (Table 1). The JV-cyclohexylthiosemicarbazone, 13, complex formed the expected [Fe(13-H)2] with FeCl as the counterion [141]. However, [Fe(13) (13-H)H20]C104 was isolated from ethanol. Bulkiness of the cyclohexyl group, and the perchlorate ion s greater ability to hydrogen bond are probably both important to the stability of this cation. The iron(III) center is considered six-coordinate with a tridentate 13-H, bidentate 13, and a coordinated water molecule. 

Oxidation of the less bulky substituted cyclotrisilene 19 with [Et3Si(C6Hg)]+ TPFPB in benzene surprisingly resulted in the formation of the cyclotetrasileny-lium ion 20+ TPFPB , which was also free from any counterion or solvent interaction in both the solid state and solution (Scheme 2.18). In the cationic portion of the molecule 20+, the Sil, Si2, and Si3 atoms exhibited a planar geometry whereas... 

While related to its carbon analogs, the existence of the RsSi species as a free ion in condensed phases had been doubted for a long time. However, NMR characterization using bulky aryl substituents has provided evidence for the triply coordinated silicon cation. " However, definitive evidence was recently reported by the groups of Reed and Lambert with a silyl cation species bound to three mesityl groups and a carborane [HCBnMesBrg] counterion (Eig. 7.5). It was suggested that... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

How can these bulky, extremely weakly coordinating anions prevent catalyst deactivation A comparative kinetic study of catalysts with different anions provided a plausible answer [19]. With PFg as a counterion, the rate dependence on olefin concentration was first order, whereas the rate order observed for the corresponding BArp complex was close to zero. This striking difference may be explained by the stronger coordination of PFs or formation of a tight anion pair, which slows down the addition of the olefin to the catalyst to such an extent that it becomes rate-limiting. In contrast, the essentially noncoordinating BArp ion does not interfere with olefin coordination, and the catalyst remains saturated with olefin even at low substrate concentration. The slower reaction of the PFg salt with the olefin could... 

Both theory and experiment point to an almost perpendicular orientation of the two butadiene H2C=C(t-Bu) moieties (see Scheme 3.53). On passing from the neutral molecule to its anion-radical, this orthogonal orientation should flatten because the LUMO of 1,3-butadiene is bonding between C-2 and C-3. Therefore, C2-C3 bond should be considerably strengthened after the anion-radical formation. The anion-radical will acquire the cisoidal conformation. This conformation places two bulky tert-butyl substituents on one side of the molecule, so that the alkali metal counterion (M+) can approach the anion-radical from the other side. In this case, the cation will detain spin density in the localized part of the molecular skeleton. A direct transfer of the spin population from the SOMO of the anion-radical into the alkali cation has been proven (Gerson et al. 1998). 

Control of the side reactions is achieved through two factors (1) reversible complexation of the anionic propagating species XXVII by the silyl ketene acetal polymer chain ends XXVI maintains the concentration of the anionic propagating species at a low concentration and (2) the bulky counterion W+ (e.g., tetra-ra-huty I ammonium, tris(dimethylamino)sulfonium) decreases the reactivity of the anionic propagating centers toward the terminating side reactions. 

A small number of carbocations have been crystallized and their geometries determined by X-ray diffraction. Although the influence of counterions is of some concern, in many cases these can be made sufficiently bulky to preclude their encroaching too close. Here, the measured crystal geometry should closely reflect that of the (isolated) charged species. 

Optimum enantiomeric excesses are obtained when a bulky substituent R1 sterically shields the top face of the 4,5-dihydrooxazole. The introduction of the methoxymethyl side chain, serving as an additional ligand of the azaenolate counterion to generate a rigid chelate, was essential for achieving maximum asymmetric induction. This principle, initially demonstrated in the asymmetric alkylation of 4,5-dihydrooxazoles, has found further application in the asymmetric alkylation... 

A variety of preparative methods are available for the synthesis of metal pseudohalide complexes.202 209 The use of a large counterion like NEt4+, PPh4+ or (PPh3)2N+ is often important for the isolation of the water-free complexes. Furthermore, the presence of bulky counterions can have a stabilizing influence on the more or less explosive complexes containing azide anions. 

Attachment of bulky chiral barriers to achieve enantiomeric selection is exemplified by ligands (61),204 (63)69 and (64).19 Ligands of type (61) are especially efficient chiral compounds with binap-thyl hinges . The orientation of the binaphthyl rings for (61) is shown more explicitly in (62).234 With optically pure (61), complete enantiomeric separation of ammonium salt racemates such as (67) is obtained.235-237 The structure has been determined crystallographically for the less stable d form of the PF6- salt of (67) with the (S,S) host (61). The results indicate that the receptor, substrate, counterion and solvent have undergone several steric concessions in order to relieve the strain imposed on the system by the visit of an unwanted guest.238... 

It became clear after these efforts that the successful synthesis of free silicenium ion requires the use of a counterion as inert as possible. Furthermore, it is necessary to hinder the Si+ environment with bulky, sterically demanding groups and electronically stabilize the cation by appropriate substituents to suppress any interaction with the anion or the solvent.643,644... 

Several explanations were proposed for this disappointing result. According to Bushby et al. (1997b), for the cross-linked polymers, a more likely interpretation is the steric difficulty of incorporating counterions into a relatively rigid polymer network. The authors consider the PF<7 anion as the counterion to each cation radical part of the cross-linked polymer. If this is so, it is unlikely to be insurmountable. A bulky anion can be replaced by an anion of small size. 

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

Tetraalkylammonium and Other Bulky Counterions for Anionic Polymerization... 

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