Phenyl cations, preparation

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither the vinyl nor the phenyl cation has as yet been prepared as a stable species in solution. ... 

With the same methodology, the preparation of 3-phenyl-substituted THFs can be assessed by irradiating ArCl in the presence of a nonterminal alkene. Hence, the photogenerated phenyl cations react with the (E)-3-hexen-l-ol, and this results in the stereoselective formation of traws-2-ethyl-3-aryltetrahydrofuran derivative in good to very good yields (52-88%) [72],... 

The evolution of the mass spectra of the ion-molecule reaction products of m/z 77 ions with ammonia as a function of the nature of the precursor halobenzene is also of interest. The m/z 94 m/z 93 branching ratio measured as 22 78 for the chlorinated precursor becomes 30 70 for the brominated precursor and 49 51 for the iodinated precursor. Moreover, this ratio is still significantly modified (60 40) if ionized iodobenzene is prepared by charge exchange with methylene chloride in the Cl source. The degree of fragmentation of the m/z 94 ions thus appears extremely dependent on the distribution energies of the precursor m/z 77 ions, but the occurrence of isomeric species of the phenyl cation cannot also be ruled out completely. 

However, there are a few known quaternary bismuth compounds that can be prepared with this methodology. Tetramethylbismuthonium triflate, which is the only known tetraalkylbismuth compound, is synthesized by the reaction between trimethylbismuthine and methyl triflate in acetonitrile [94AG(E)976]. The tritium-labeled phenyl cation, generated from the... 

Fluorescence lifetimes have been determined for several 9-arylxanthyl cations generated Irom their corresponding alcohol in strongly acidic media, or as their tetrafluoroborate salt in acetonitrile (Table 2) [11,28,30]. Excellent agreement between the two methods was found for the parent 9-phenyl-, p-Me-, and m-Me-substituted cations, with poorer agreement in the case of the tn-OMe cation. The error associated with the very weak fluorescence of the m-OMe-substituted cation may be responsible for the difference in the lifetime estimates. The two values reported for the fluorescence lifetime of the p-F xanthyl cation show the widest variation, with a report of 18.8 ns for the cation prepared as its tetrafluoroborate salt in acetonitrile versus 47 ns in TFA-TFE [13,28,30]. 

In the literatures we can find out a number of data which indicate the formation of heterotactic polymers, although the authors have not commented about it. These Include poly(methyl methacrylate) and poly(ethyl methacrylate) obtained by octyl-potassium, biphenylpotassium, fluorenylpotassium (Table 5), benzylcal-dum or potassium alkoxide in polar solvent, and poly(phenyl methacrylate) prepared by n-, iso- and sec-butylmagnesium bromide in toluene at low temperature (Table S). These clearly indicate that not only the size of the substituent of monomer but also the nature of the counter cation or the pdymerization medium is important factor for the formation of heterotactic polymer. 

A nucleogenic phenyl cation has been generated by -decay of tritium-labeled benzene and reacted with methyl-substituted quinolones. A double tritium-labeled benzene (216) was prepared from 1,4-dibromobenzene. Decay of a tritium leads to the phenyl cation (217) and this cation reacts with 8-methylquinoline to give products (218) and (219). 

The tricarbonylcyclohexadienyliumiron salts (32) are synthetically equivalent to substituted phenyl cations/ and react with silyl enol ethers to give, after decomplexation, the a-(cyclohexa-l,3-dienyl) ketones, which are readily oxidized to a-aryl ketones. 2-Substituted cyclopent-2-enones can be prepared from the salts and l,2-bis(trimethyIsilyloxy)cyclopentene (Scheme 52).A... 

The direct photolysis of alkyl or aryl halides in solution to form carbon-centered radicals is rarely used in organic synthesis." Alkyl iodides usually afford mixtures of radical and ionic products, while alkyl bromides can produce radical-derived products but in low yield. A notable exception is the photocycliza-tion of haloarenes, which has been shown to produce carbon-centered radicals that can add to aromatic rings. A similar reaction has recently been observed on irradiation of iodoheterocycles, with substituted benzenes or electron-poor alkenes, to form arylated or alkylated heterocycles in good yield. Related reactions have also been reported on irradiation of 4-chloroanilines in the presence of (electron-rich) alkenes, although in this case, the alkylations appear to involve the formation of a phenyl cation. An alternative approach to form carbon-centered radicals is to irradiate the alkyl iodide or bromide in the presence of triethylamine this is proposed to form an amine-haHde exciplex, which cleanly breaks down to give a carbon-centered radical and a halide anion. Cossy and co-workers have shown this to be a fast, convenient, and chemoselective method of radical generation, which has recently been used to prepare the bicyclic core of ( )-bisabolangelone (Scheme 1). ... 

Less reactive electrophilic reagents like those involved in acylation or alkylation apparently do not react with phenyl-substituted pyrylium salts the p-acylation of a phenyl group in position 3 of the pyrylium salt obtained on diacylation of allylbenzene (Section II, I), 3, a), and the p-l-butylation of phenyl groups in y-positions of pyrylium salts prepared by dehydrogenation of 1,5-diones by means of butyl cations (Section II, B, 2, f) probably occur in stages preceding the pyrylium ring closure. 

Chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with /V-phenyl-TV -OS )-(l-phenylethyl)thiourea associated with a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved.387,388 Chiral polyether-phosphite ligands derived from (5)-binaphthol were prepared and combined with [Rh(cod)2]BF4. These systems showed high activity, chemo- and regio-selectivity for the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions. Ee values of up to 25% were obtained and recycling was possible without loss of enantioselectivity.389... 

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