Olah group

The arrival of the Olah group at the USC campus with its moving vans caused quite a stir. Whatever shortcomings our temporary quarters had were overcome by our enthusiasm, and, miraculously, in 3 weeks we were back doing research. I am not sure whether everybody at USC was pleased by this invasion and our determination to over-... 

The Olah group the day my Nobel Prize was announced with friends Bob Williams, Surya Prakash, Joe Casanova, and Reiko Choy, my secretary... 

Recently, analogues of nucleosides [60], natural products Huperzine-A [61] and Hydroartemisinin [62], and inhibitors of metallo-/ -lactamases have been synthesised [63]. With acylsilane electrophiles, the initial adducts undergo Brook rearrangement which is interrupted by -Si bond fission with loss of fluoride anion (Eq. 16), leading to the formation of extremely useful difluoro-enol silanes [64]. Of the various fluoride sources employed, the tetrabutylam-monium triphenyldifluorostannate described by Gingras appears to be particularly effective. The numerous other methods for trifluoromethylation formed the subject of an exhaustive review [65]. More recently, the Olah group described a chlorodifluoromethyl trimethylsilane which is expected to have a rich chemistry [66]. 

The concept of superelectrophilic activation was first proposed 30 years ago.20 Since these early publications from the Olah group, superelectrophilic activation has been recognized in many organic, inorganic, and biochemical reactions.22 Due to the unusual reactivities observed of superelectrophiles, they have been exploited in varied synthetic reactions and in mechanistic studies. Superelectrophiles have also been the subject of numerous theoretical investigations and some have been directly observed by physical methods (spectroscopic, gas-phase methods, etc.). The results of kinetic studies also support the role of superelectrophilic activation. Because of the importance of electrophilic chemistry in general and super-acidic catalysis in particular, there continues to be substantial interest in the chemistry of these reactive species. It is thus timely to review their chemistry. 

Carbo-acyl dicationic species have been proposed as intermediates in several reports, but these types of distonic superelectrophiles have not yet been sufficiently studied. Work by the Olah group has shown that pro-tonated carboxylic acids cleave to the acyl ions in superacidic media at temperatures above — 10°C.32 In principle, ionization of a second group (such as hydroxyl or olefinic) can generate a carbocationic site adjacent to... 

A procedure for alkylation of C=0 double bonds in the presence of (metal-free) organocatalysts and non-metallic nucleophiles has been reported by the Iseki group for trifluoromethylation of aldehydes and ketones [185]. On the basis of a previous study of the Olah group [186, 187] which showed the suitability of non-chiral phase-transfer catalysts for trifluoromethylation of carbonyl compounds, Iseki et al. investigated the use of N-benzylcinchonium fluoride, 182, as a chiral catalyst. The reaction has been investigated with several aldehydes and aromatic ketones. Trifluoromethyltrimethylsilane, 181, was used as nucleophile. The reaction was, typically, performed at —78 °C with a catalytic amount (10-20 mol%) of 182, followed by subsequent hydrolysis of the siloxy compound and formation of the desired alcohols of type 183 (Scheme 6.82). 

The scope of nitrating agents, however, by now is much wider (Table I) 13). The discussion in this chapter will primarily emphasize reagents and methods developed by the Olah group in its study of nitration chemistry. 

Olah s original preparative nitrations were carried out with mixtures of the aromatic compound and nitronium salt alone or in ether, and later with sulpholan as the solvent. High yields of nitro-compounds were obtained from a wide range of aromatic compounds, and the anhydrous conditions have obvious advantages when functional groups such as cyano, alkoxycarbonyl, or halogenocarbonyl are present. The presence of basic fimctions raises difficulties with pyridine no C-nitration occurs, i-nitropyridinium being formed. ... 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

The imaginative exploitation of these and related reactions by G. A. Olah and his group have had an enormous impact on our understanding of organic catalytic processes and on their industrial application, as recognized by the award to Olah of the 1994 Nobel Prize for Chemistry. 

The reaction processes shown in Scheme 8 not only accomplish the construction of an oxepane system but also furnish a valuable keto function. The realization that this function could, in an appropriate setting, be used to achieve the annulation of the second oxepane ring led to the development of a new strategy for the synthesis of cyclic ethers the reductive cyclization of hydroxy ketones (see Schemes 9 and 10).23 The development of this strategy was inspired by the elegant work of Olah 24 the scenario depicted in Scheme 9 captures its key features. It was anticipated that activation of the Lewis-basic keto function in 43 with a Lewis acid, perhaps trimethylsilyl triflate, would induce nucleophilic attack by the proximal hydroxyl group to give an intermediate of the type 44. 

There is one other substituent which is comparable with the diazonio group in the sense that it is cationic and that it has, in one of its mesomeric structures, a triple bond between the atom attached to the aromatic system and the second atom. It is the acylium group in 7.9. However, no substituent constants are known for this group, obviously because this cation is detectable in measurable concentrations only in superacidic media (see review by Olah et al., 1976). 

Aromatic diazonium salts can, of course, be isolated (see Chapter 13), but only a few aliphatic diazonium salts have been prepared (see also Ref. 383). For reviews see Laali, K. Olah, G.A. Rev. Chem. Intermed., 1985, 6, 237 Bott, K. in Patai Rappoport The Chemistry of Functional Groups, Supplement C, pt. 1 Wiley NY, 1983, p. 671 Bott, K. Angew. Chem. Int. Ed. Engl., 1979, 18, 259. The simplest aliphatic diazonium ion CH3N2 has been prepared at — 120°C in superacid solution, where it lived long enough for an nmr spectrum to be taken Berner, D. McGarrity, J.F. J. Am. Chem. Soc., 1979, 101, 3135. 

For a review of selectivity in this reaction, that is, which group preferentially attacks when the reagent contains two or more, see Olah, G.A. in Olah, Ref. 240, vol. 1, p. 881. This review also covers the case of alkylation vs. acylation. 

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

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