Cationic centers

Proton loss from alkyl groups a or 7 to a cationic center in an azolium ring is often easy. The resulting neutral anhydro bases or methides (cf. 381) can sometimes be isolated they react readily with electrophilic reagents to give products which can often lose another proton to give new resonance-stabilized anhydro bases. Thus the trithione methides are anhydro bases derived from 3-alkyl-l,2-dithiolylium salts (382 383) (66AHC(7)39). These... 

When an exocycllc function has a higher priority, it may be necessary to name a cationic heterocyclic substituent group. The most important case is that in which the heterocyclic substituent is bonded through its cationic center. Such cases may be named in two ways, as in (182) and (183). The simplest is to use the suffix -io , as used for the H3N— substituent, ammonio terminal e is elided. More generally, however, an -yl suffix is appended after -I um , as shown in the second names given for the examples. This method applies equally well to situations with other sites of attachment, and also allows one to name divalent substituents, e.g. (184) and (185). 

The methods for naming cations and anions may be combined to name zwitterions the cationic center is cited before the anionic center, both in the replacement prefixes and in the suffixes, e.g. (204). One should be aware, however, that Chemical Abstracts names zwitterions by an extraordinary circumlocution, in which water is hypothetically added and then hypothetically taken away, to give names consisting of four separate words ending in inner salt, e.g. (205). 

Direct resonance interaction with a cationic center... 

In ion D, in which the phenyl group would be expected to be coplanar with the cationic center to maximize delocalization, the observed angle is 25-30°. This should permit effective benzylic stabilization. The planes of the cyclopropyl groups in both structures are at 85° to file plane of file trigonal carbon, in agreement with expectation for the bisected... 

P-Fluonne or fluonne further removed from the cation center always inductively destabilizes carbocabons [115, 116] No simple p-fluoroalkyl cations have been observed in either the gas phase or solution, and unhke the cases of the other halogens, there is no evidence for formation of alkyl fltioronium ions (5) in solution [117, 118], although (CH3)2F is long-hved m the gas phase [119] The only P-fluonnated cations observed in solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [112] and per-fluonnated allyl [120], cydopropenium [112], and tropylium [121] ions... 

The familiar pattern of 2-amination with sodamide ( — 33°, 90% yield) occurs also with 1,5-naphthyridine. Greater reactivity at the 2-position is attributed, as before, to a cyclic transition state with electrophilic attack at a ring-nitrogen concomitant with nucleophilic attack adjacent to the cationic center thus formed. 

The anionic moiety can substitute chlorines in PVC by an Sn mechanism [Eq. (31)]. The reaction can also take place by an Sn mechanism. This would involve the formation of a cationic center on the polymer backbone... 

Step 4 of Figure 27.14 Final Cyclization The fourth and last cyclization occurs in step 4 by addition of the cationic center at Cl3 to the 17,20 double bond, giving what is known as the protosteryl cation. The side-chain alkyl group at... 

Hydride shift (Section 6.11) The shift of a hydrogen atom and its electron pair to a nearby cationic center. 

In contrast, reaction of ligand 72 with 4,4 -biphenyldiboronic acid has been successful and diboronate 73 is obtained in yields of 33%. This complex acts as a receptor for the paraquat dication forming a 1 1 complex with an association constant of 320 in acetone. The intermolecular forces responsible for the complexation are ion-dipole stabilization between the dative N B dipoles and the two cationic centers in paraquat, attractive tz-tz interactions between... 

Another structural feature that increases carbocation stability is the presence, adjacent to the cationic center, of a heteroatom bearing an unshared pair," for example, oxygen," nitrogen," or halogen. Such ions are stabilized by resonance ... 

In the interaction of a pair of atomic orbitals, two electrons form a bond and four electrons form no bond (Sect. 1.1). The snbstitnted carbocations are stabilized by the electron delocalization (hyperconjngation and resonance) through the interaction of the doubly occupied orbitals on the snbstitnents with the vacant p-orbital on the cation center. The exchange repulsion (Sect. 1.5) is cansed by four electrons. Now... 

Electron-donating orbitals are those occupied by electrons, i.e., bonding orbitals of bonds, non-bonding orbitals of lone pairs, HOMOs of molecnles, gronps and others. Electron-accepting orbitals are vacant orbitals, i.e., antibonding orbitals of bonds, vacant atomic orbitals on cationic centers, LUMOs of molecnles, gronps, etc. 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Scheme 7.7, ORTEP adapted from reference 32), with [BCCgFj) or [CBnHgBrg] as the counterions. The X-ray structure clearly shows evidence of an sp hybridized center with a C -C -C angle of 178.8° and an abnormally short C -C double bond distance of 1.22 A (compared to 1.32 A, Table 7.1), and a nearly normal C -CA (sp -sp) distance of 1.45 A. The most striking feature of 8, however, is the very long C -Si distance of 1.97 A (compared to 1.87 A, Table 7.1), which is attributed to hyperconjugation, as shown in the scheme. Elongation of the bonds a- to the cation center is a characteristic of hyperconjugation, also observed in the structure of the adamantyl cation. ...

A common feature of the compounds that give extensive syn addition is the presence of a phenyl substituent on the double bond. The presence of a phenyl substituent diminishes the strength of bromine bridging by stabilizing the cationic center. A weakly bridged structure in equilibrium with an open benzylic cation can account for the loss in stereospecificity. 

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... 

As illustrated in the previous sections, the uninegative poly(pyrazo-lyl)borato ligands provide a versatile system for coordination to cationic centers. However, in their triprotonated form, such ligands offer potential for acting as receptors for anions (Scheme 28). Indeed, since the coordination chemistry of anions (199) is significantly less developed than that of cations, protonated poly(pyrazolyl)borato ligands offer potential for exploring the chemistry of such systems. 

In 1956 Brown, in a series of patents(68-75), disclosed that clays could be treated with di-, tri-, or tetra-substituted ammonia derivatives. Later, McLaughlin, et al.(76,77), introduced cationic polymers as permanent clay protective chemicals. A series of published results describing laboratory and field applications soon became available(78-81). Structural details of the cationic polymers appeared in patents(82-85). In general the polymers are polyamine derivatives, mostly quaternary in nature. Theng(86,87) has discussed how the multiple cationic centers in these polymers can interact and permanently protect clays. Callaway(88) et al. has noted that cationic polymers may interfere with the performance of crosslinked fracturing fluids. 

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