Super acid media

Hydrocarbons undergo related reaction.s in the super-acid media, such as fluorosuiphuric acid and antimony pentachloride. It has been suggested that the initial one-electron processes during the electrochemical oxidation of alkanes in fluorosuiphuric acid involve a protonated carbon-hydrogen bond with formation of a carbon radical and release of two protons [15]. 

Vinca dimer indole alkaloids (e.g., vinblastine) act as spindle poison. They bind tubulin, inhibiting polymerization into microtubules, major elements of the cytoske-Vinblastine itself and its analogue vinorelbine (Navelbine ) are marketed for cancer therapies (Figure 4.48). Because of the obvious difficulty in synthesizing such highly complex structures, there were no reports on the preparation of fluorinated derivatives until the remarkable work of Jacquesy s group on the synthesis in super-acidic media. 5 ... 

Lemairc Doussot Guy Chem. Lett. 1988, 1581. See also Hayashi Mukaiyama Chem. Lett. 1987, 1811, 2ll01ah Mo Olah J. Am. Chem. Soc. 1973, 95, 4939. For reviews, see Olah Farooq Prakash, in Hill Activation and Functionalization of Alkanes, Wiley New York, 1989, pp. 27-78 Olah Prakash Sommer, Ref. 44, pp, 270-277 For a review of the thermodynamic behavior of alkanes in super-acid media, see Fabre Devynck Trdmillon Chem. Rev. 1982, 82, 591-614. See also Ref. 46. 

Despite the seeming stability of 80, all attempts to prepare the cation as a stable species in super-acid media have been unsuccessful191 192. The retention of configuration at C(4) in solvolysis of cyclopentene derivatives suggests that there is some involvement of the... 

Both 84 and 85 have been prepared as stable ions in super-acid media and their NMR properties studied198-201. The conclusions reached from the extensive amount of work done with the parent and a variety of substituted systems is that both ions can be considered to be bishomoaromatic. In the case of 85 it should be noted that the C(7) bridge was found to lean towards and interact with one of the double bonds. Cation 85 was found to undergo an inversion process in which there is an interchange of the participating double bond (Scheme 35). Winstein and coworkers were able to place a lower limit of 19.6 kcalmol 1 on... 

For a molecule with such a low basicity (only protonated in super-acidic media), CO is a surprisingly versatile ligand. The homoleptic (or binary) carbonyls (i.e. in which CO is the only ligand) already span oxidation states from -IV to +III, and higher oxidation states are known for heteroleptic complexes (mixed ligand sets) which have good 71-donor co-ligands, e.g. RuIV(CO)(SR)4. 

Kinetic measurements involving the four p-nitrobenzoates revealed their rates of ionization to differ by a factor of only 43 at 111 °C, with the more highly unsaturated molecules again proving to be the less reactive.392 These findings were interpreted in terms of the absence of neighboring double bond participation. Neither could evidence for trishomotropylium ion character be gained from studies in super-acidic media.391,392 ... 

The difference in reactivity compared to organic ketones is caused by the strong electron-withdrawing effect of the fluorine atoms in HFA, which leads to an electron-deficient carbonyl group. This is manifested in the inability to protonate the oxygen atom in super acidic media 203). 

The reader is referred to the second chapter Preparative Methods by J. Grannec and L. Lozano in Inorganic Solid Fluorides , edited by P. Hagenmuller [3], for information on chemical syntheses and crystal growth techniques anterior to 1985, and for general instructions in major preparative techniques in the field of inorganic fluorine chemistry. Some excellent reviews of different topics in the field have been published past 1985 [4-9]. The wide synthetic possibilities of super-acidic media are described in T.A. O Donnell s book [10]. Synthetic and characterization methods with emphasis on solid compounds have been summarized in a book edited by A.K. Cheetham and P. Day [11],... 

Winstein, Sonnenberg and de Vries first proposed the intermediacy of 91 in order to account for the solvolytic properties and products of derivatives of 92 . A major advance in understanding the properties and structure of 91 came from the laboratories of Masamune and colleagues, who reported the preparation of the cation as a stable species in super-acid media a preparation later repeated by Olah and colleaguesand Kelly and coworkers (Scheme 36). ... 

The complexity of the chemistry involved is apparent from the variety of plausible pathways which can be invoked to explain that part of the reaction we can measure. The work to dote clearly indicates that differences in reaction products will result and should be expected in comparing the behavior of carbocations generated from the alkane versus the complex mixture of cationic species generated from the alkyl chloride in super acid media. 

The elusiveness of "free" silyl cations in condensed phases stands in remarkable contrast to the behavior of the isoelectronic carbocations [11] which Olah s school has explored so extensively in super acid media [12]. Laube s splendid recent series of carbocation X-ray structures [13, 14] illustrate what to expect truly "free" cations will be separated from the nearest atoms of the counteranions by 3 A or so. Despite the larger size of silicon, the Si-ligand distances in the Lambert-Reed structures are only modestly longer than typical covalent bonds, and are in the range expected for dative bonds [IS]. 

Photoheterolysis of benzylic chlorides [204] yielded results signifying that simple benzyl cations, such as cumyl and 1-phenylethyl cations, can exist in the solution as free ions radicals arising from a competing photohomolysis are also observed frequently. Haloalkyl-carbocations are studied by heterolysis of the corresponding dihalides in super acid media [205]. NMR chemical shifts are interpreted as evidence for an interaction between the vacant orbital of cationic center of the haloalkyl carbocations with a lone electron pair of the halogen atom. 3-chloro-l-methylcyclopentyl cation 73, thermally eliminates hydrogen chloride and yields l-methyl-2-cyclopentyl cation 74, a similar behavior reported for y-chloroalkyl carbocations [206] (Scheme 5). 

Carbocation Intermediates.—Though the nature of the active centres in polymerizations propagated by carboanions has been well known for some time, and reasonably adequately characterized, the corresponding species in cationic propagations has proved difficult to identify with anything like the same certainty. However, the use of super acid media, e.g. FSOaH/SbFj, has now enabled Olah to obtain a n.m.r. spectrum of the elusive styryl carbocation, following experiments such as that in which a racemic mixture of products was obtained from the reaction of (—)-l-phenylethyl chloride with EtaAl, which provided powerful circumstantial evidence for the existence of the ion. More recently a particular definitive paper on and n.m.r. spectra of styryl species has appeared. 

NMR studies in super acid media and theoretical studies have suggested that the bicyclo-butonium ion and the classical cyclopropylmethyl cation should be close in energy. 

Oxygenation with Hydrogen Peroxide Hydrogen peroxide in super-acidic media is protonated to hydroperoxonium ion (H3O2+). Christe et al. " reported the characterization and isolation of several peroxonium salts. The... 

Vinblastine and vincristine have been used clinically for many years. The major importance of vinblastine is as part of a combination treatment for Hodgkin s disease, while vincristine is used in combination chemotherapy of acute lymphoblastic leukemias and lymphomas. Several analogues of the vinca alkaloids have entered clinical use or clinical trials, including vindesine (3), vinorelbine (4), " the naturally occurring anhydrovin-blastine (5), and vinflunine (6). ° Several synthetic approaches to the vinca alkaloids have been developed, especially by Kuehne, while studies of the chemistry of the alkaloids in super acidic media have yielded new alkaloids such as vinflunine. ... 

Alkene- and arenesulphonic acids have been studied in strong or super acid media by NMR spectroscopy. Olah and coworkers64 in 1970 examined sulphonic acids in fluorosulphuric acid-antimony pentafluoride-sulphuryl chloride fluoride solution at low temperature ( — 60 °C). In S02 solution methanesulphonic acid has a chemical shift of <5 3.1 ppm for the methyl group. In the superacid medium there are two singlets at S 4.15 and 4.07, with ratio 60 40, for the methyl protons. It was suggested that this indicated the presence of two isomers, possibly 27 and 28, due to restricted rotation about the SO bond. 

Persistent radical cations of Cgg have been observed in super acidic media, for example in magic acid (FSO3H, SbFj) [168]. Multiply charged fuUerene radical cations were also stabifized in a mixture of fuming sulfuric acid and SO2CIF at low temperatures [169]. The reaction of Cgo with sulfuric acid and nitric acid and the subsequent hydrolysis of the intermediates with aqueous base resulted in the formation of fuUerenols [170,171]. 

Correspondingly, diazomethane (42) which is isosteric with 43 may be protonated at the terminal C and N atoms, respectively. Molecular orbital theoretical studies of the protonation of ketene (102) and diazomethane (101) have revealed that the products of the thermodynamically controlled protonation reactions in the gas phase are the C-protonated species. And it appears that in solution C protonation is still favored thermodynamically. C-protonated derivatives of ketenes (103) and diazocompounds (104) have been obtained in super-acid media at low temperatures. O-protonated ketene 43a in its most stable conformation of C symmetry (102) is Isosteric with ketene imine (49) and the most stable allenyl anion 11a (1 ). 

Scheme 5.2 illustrates the use of super acid media in the study of carbonium ion generation, structure, and reactions by NMR spectroscopy. The behavior of primary, secondary, and tertiary aliphatic alcohols on being dissolved in FS03H-SbF5-S02 at... 

Diacids.— Dicarboxylic acids are obtained by the reaction of y-butyrolactones with carbon monoxide at atmospheric pressure and room temperature in HF-SbFs super acid media. A double-bond cleavage reaction of silyl enol ether using Mo02(acac)2 Bu OOH as oxidizing agent leads to a diacid in the case of (11) and to a ketoacid with (12) yields are high for the nine examples studied. ... 

The bridged form of the j8-phenylethyl ion can be observed in super acid media, and can be characterized by carbon and proton NMR spectra. The bridged ion subsequently rearranges to the more stable a-methylbenzyl cation with Ea for the rearrangement being 13 kcal/mol. With more substituted cations the rearrangement to benzyl cations occurs too rapidly for the bridged ion to be observed. 

The area of super acidic media, defined as media with Ho values more negative than that of 1(X)% H2SO4 (-11.9) has been reviewed fairly recently, with emphasis on fluorine-containing acids. ... 

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