Cations with conjugated

Cholesterol, on reaction with dibenzoyl peroxide followed by trichloroacetic acid, gave a blue colour attributed to cations with conjugated unsaturated structures. Cholesta-2,4,6-triene and 3-(cholest-5-en-3/8-yl)cholesta-2,4,6-triene were isolated from the product mixture. Some other sterols gave diverse colours.The coloured solutions formed from cholesterol or oestrone with antimony trichloride give e.s.r. spectra suggesting the presence of radical-cations. Cholesta-3,5-diene and 3,3 -bicholesta-2,4-diene were isolated from the reaction of cholesterol, and the hexaene (131) from oestrone. Dimeric and trimeric products have been... 

The SSPs are prepared by the reaction of a stabilized Cu(I) cation, with an indium(III) or gallium(III) chalcogenide anion, prepared in situ by reaction of the conjugate acid of the thiol or selenol with NaOMe in methanol (Scheme 6.1 below).1,3... 

With conjugated enone substrates, the alkoxymetallation leads to the formation of a metal enolate that can undergo a facile protonation to accomplish the hydroalkoxylation. Following this mechanism, various /3-alkoxyketones were obtained in good yields by the addition of primary and secondary alcohols to methyl vinyl ketone under cationic Pd(n) catalysis.443 Similarly, [Rh(COD)(OMe)]2 was found to catalyze the hydroalkoxylation of both methyl vinyl ketone and phenyl vinyl ketone (Equation (121)).444... 

For the design of mitochondriotropic liposomes, we have used a method, that has been a standard procedure in liposome technology for over 30 years the lipid-mediated anchoring of artificially hydrophobized water-soluble molecules into liposomal membranes (25-28). We have hydrophobized mitochondriotropic TPP cations by conjugating them to long alkyl residues specifically, we have synthesized stearyl TPP (STPP) salts (29). Following liposome preparation in the presence of STPP, the liposomal surface became covalently modified with TPP cations, thereby rendering these liposomes mitochondriotropic as verified in vitro by fluorescence microscopy (30). 

Subsequent ion exchange of the metal cation with the quaternary ammonium ion catalyst provides a lipophilic ion pair (step 2), which either reacts with the requisite alkyl electrophile at the interface (step 3) or is partitioned into the electrophile-containing organic phase, whereupon alkylation occurs and the catalyst is reconstituted. Enantioselective PTC has found apphcation in a vast number of chemical transformations, including alkylations, conjugate additions, aldol reactions, oxidations, reductions, and C-X bond formations." ... 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

The homoallylic conjugation of the derived cation with the enolate is easily achieved. There are two possibilities for the transformation of 9 ... 

Improved yields of adamantane up to 65% were achieved by using superacidic systems [HF-SbF5, CF3S020H-B(0S02CF3)3, various solid superacids]40"42 and chlorinated Pt on alumina.43 More recently near-quantitative isomerization with conjugate superacids promoted by 1-haloadamantanes as the source of 1-ada-mantyl cation and sonication was reported.44... 

A secondary reaction pathway also is possible in formaldehyde-facilitated conjugations. Formaldehyde may react with a primary amine to form a quaternary ammonium salt. This intermediate spontaneously reacts to create a highly active immonium cation with loss of one molecule of water (Ji, 1983 Blass et al, 1965). The immonium cation... 

Conducting polymers based on polymer chains with conjugated double bonds are electroactive materials that have found widespread use also in the field of chemical sensors [11-41], Oxidation of the conjugated polymer backbone is accompanied by anion insertion or cation expulsion, as follows ... 

Y. Saeki, N. Matsumoto, Y. Nakano, M. Mori, K. Awai, and Y. Kaneda. Development and characterization of cationic liposomes conjugated with HVJ (Sendai virus) reciprocal effect of cationic lipid for in vitro and in vivo gene transfer, Hum. Gene Ther. 8 2133-2141 (1997). 

Cations containing four sulfur atoms in line are obtained by reacting a 3-methylthio-l,2-dithiolylium cation with the conjugate base of a 3-alkyl-l,2-dithiolylium cation as shown in equation (18) (71AHC(13)161,p. 205,73BSF1659,80AHC(27)151,p. 191). 

Using a simple resonance approach, we might incorrectly expect both of the cyclopentadienyl ions to be unusually stable. Shown next are resonance structures that spread the negative charge of the anion and the positive charge of the cation over all five carbon atoms of the ring. With conjugated cyclic systems such as these, the resonance approach is a poor predictor of stability. Hiickel s rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems. 

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