Persistent vinyl cations

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

Stabilization of the vinyl cations 8-10 by aryl substituents is important but even substituents without strong resonance effects provide adequate stabilization to allow the synthesis of persistent vinyl cations at ambient temperature. This is demonstrated by the synthesis and isolation of the salts of alkyl-substituted vinyl cations 19 and 20, and their characteristic NMR parameters are summarized in Table 1. 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

The l3C NMR spectrum of 31 (R = H) is shown in Figure 8. This is likely to be the the smallest member of P-silyl substituted vinyl cations which can be generated and observed as persistent species in solution. 

Not only the ring size but also the number of stabilising silyl groups in the -position is essential for the stability of the vinyl cations. Thus, reaction of alkyne 16 with tityl cation gave both stereoisomers of aikenylsilane 18 as the only products in 80-85% isolated yield (Scheme 3). This result suggests, that the generated / -silyl-substituted vinyl cation intermediate 17 did not persist under the applied reaction conditions but underwent a second hydride transfer with the formation of compound 18. 

The silyl fragmentation in superacids initiated by a controlled temperature increase is a method to generate persistent carbocations, such as the vinyl cation 378, which are not accessible by direct protonation of unsaturated hydrocarbons because of excessive formation of oligomeric and polymeric products. 

Siehl and, recently, Muller et al. have generated and observed a variety of (3-silylated vinyl cations. The first successful observation by 13C NMR of a persistent (3-silyl-substituted vinyl cation was reported by Siehl et al.329 [Eq. (3.42)]. The... 

Photolysis of 2-bromo-4,4-dimethyl-2-cyclohexenone only affords reduction, even in a nucleophilic medium343,344. Apparently, this substrate is structurally not suitable to form a vinyl cation. Formation of vinyl radical-derived products is also the main process for all vinylic halides, if their irradiation is performed in an apolar medium. Such photochemical reductive dehalogenation and especially dechlorination reactions have been extensively studied in the past, not in the least because of their importance as abiotic transformation of persistent polychlorinated environmental pollutants. Examples are the cyclodiene insecticides aldrin and dieldrin, which contain a vicinal dichloroethene chromophore. In recent... 

Peptide-based gene delivery, 334 Peptide nucleic acid (PNA), 73,252 Pharmacokinetics, 409 Polyethyleneimine (PEI), 369 Peptides with thiol groups, 348 Persistence of gene expression, 10,463 Physiological Pharmacokinetics, 423 PNA binding affinity, 75 Poly anion interchange, 179 Poly cations, 164 Polymeric gel, 447 Polyamidoamine (PAMAM), 376 Polyaminomethacrylates, 379 Poly(L-lysine), 336, 369 Poly(vinyl pyrrolidone) (PVP), 382 Prophylactic vaccines, 474... 

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