Cationic Substitutions

Ab initio calculations suggested that the oxygen-substituted compounds 40 and 41 should be much better candidates for nucleophilic substitution reactions that follow pathway b than the halo salts belonging to the 7a-7i family (Scheme 9). Tire properties of the CCls-substituted cation of 43 should reveal more similarities with those of 7a than with those of 40 and 41. 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

The first stable derivative of methylene cyclopropene was the quinocyclopropene 108 reported in 1964 by Kende98 it was prepared from the cyclopropenium cation 106 which underwent pyrolysis and bromination with NBS to the p-hydroxy-phenyl substituted cation 107, which gave quinocyclopropene 108 by deprotonation ... 

As the incorporation of one substitutional cation results in the production of one electron ... 

CH3)C(-0C(CH3)2C(CH3)20-)+,41 Until the work described in this Review, the CI3+ and C(OH)3+ cations were the only trihalo- and trioxy-substituted cations to have been characterized by X-ray crystallography.23,42... 

Lambrechts SA, Aalders MC, Langeveld-Klerks DH, Khayali Y, Lagerberg JW (2004) Effect of monovalent and divalent cations on the photoinactivation of bacteria with meso-substituted cationic porphyrins. Photochem Photobiol 79 297-302. 

Merchat M, Bertolini G, Giacomini P, Villanueva A, Jori G (1996) Meso-substituted cationic porphyrins as efficient photosensitizers of gram-positive and gram-negative bacteria. J Photochem Photobiol B 32 153-157. 

In addition to this method, Yoshida and coworkers [183] have electrochemically prepared, at low temperatures, stable N-substituted cations (Scheme 100) that can directly react with various nucleophiles after electrolysis. 

A new class of water-soluble materials [10,11], was developed as a result of such design parameters that will be referred to as double substituted cationic cellulose ethers (DCEs). These materials contain both a cationic substituent and a hydrophobic substituent, attached to a cellulose ether backbone. The use of a double-substituted hydrophobe modified cationic polysaccharide is fundamentally different from current commercial vaginal formulations, which rely exclusively on nonionic or anionic vehicles. 

One path for HIV infection may involve damaged (or healthy) vaginal epithelia [41,42] through cell surface adhesion [40]. Recently it has been reported that the major HIV envelope protein is held intact through hydrophobic residues [43]. For these and other reasons, we sought to combine hydrophobe substituted cationic polymers, particularly DCEs with active anionic polymers, with or without N9 present. 

Because the three-electron-bonded radicals are formed at the cost of the removal of the nitrogen p-electron, such cation-radicals should be considered as p-acids. Of course, the formation and behavior of these p-acids have to be dependent on steric factors. Works by Tomilin et al. (1996, 2000), Bietti et al. (1998), Dombrowski et al. (2005), and Yu et al. (2007) should be mentioned as describing stereoelectronic requirements to formations and configurational equilibria of A-alkyl-substituted cation-radicals. 

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

Chemical Shifts of Some Methyl Substituted and Unsubstituted Allylic Cations" Case Position Unsubstituted cation Methyl substituted cation A5 1. 

There is no general theoretical study for trialkyl-substituted cations R3E, which investigates the relationship of the classical planar trigonal structure to isomeric complexes RE /R2 and its relative energy compared to the dissociation products, the singly coordinated four-valence-electron species R E and the hydrocarbon R2. The only exceptions are 7-norbornadienyl cations 37 for which the germyl and silyl cation has been intensively studied theoretically by Radom and Nicolaides. ... 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

Mineral series are composed of species whose basic chemistry and three-dimensional structure are so similar that they differ in a well-documented, predictable manner, based on the substitution of comparable chemical elements. Such a series is usually defined by the end members, that is, the compounds that contain only one of the substituting cations. End members... 

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