Stable ions superacids

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Computed GIAO-NMR chemical shifts were compared with the experimental data when available, and the optimized geometries were compared with the X-ray data if known. The lowest energy annulenium cation derived from 76 and 77 (both cis) was formed by protonation at C-1, and this was in concert with experimental observation of 76H" in superacid media. " The resulting annulenium ions were less delocalized than those via 74 and 75. DFT and stable ion study were also in concert regarding the protonation of 78. The availability of X-ray structures and stable ion NMR data for the dieations and dianions of 78 and 79 enabled comparison with the DFT-optimized structures and GIAO-NMR shifts. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

The unsubstituted phenonium ion, as well as other phenonium ions substituted with electron-donating groups, have been recently observed as stable ions in superacid medium.34 That the structure is actually 18 and not an unsym-metrically bridged ion (19) nor a nonclassical ion (20) (see Section 6.2) in which there are three-center bonds was shown by the nmr evidence. The ring carbon that is bonded to the aliphatic carbons was established by 13C shifts to be tetrahedral in nature and 13C and proton chemical shifts in the ring were similar to those of cations shown to have Structure 21. 

Cubylcarboxonium ions have been also studied by Prakash, Olah, and co-workers.579,580 The parent cation 281 prepared under superacid conditions was stable at low temperature but decomposed to cubylacylium cation 282 as a result of further protonation and dehydration [Eq. (3.71)]. In addition to cation 281, di- and tetra-carboxonium ions and the corresponding protonated methyl esters were also observed as long-lived species stable under superacidic conditions. Experimental evidence and theoretical data indicated that the strained cubyl system effectively stabilizes the carbocationic centers through C—C bond hyperconjugation (283). On the basis of 13C data, three conformers of protonated dimethyl cubane-l,4-dicarboxylate (284-286) could be identified. 

The 7-Norbornyl Cation. 7-Norbomyl derivatives were found to be extremely unreactive in solvolysis studies and product formation was shown to occur with predominant retention of configuration.917 920 These observations led to the suggestion by Winstein et al.917 that the cationic intermediate is a nonclassical ion. Attempts to isolate the 7-norbomyl cation under stable ion conditions in superacid... 

To improve the understanding of these alkane alkylation reactions, Olah and his group carried out experiments involving the alkylation of lower alkanes by stable car-benium ions under controlled superacidic stable ion conditions128 145 146 [Eq. (5.60)]. 

By studying stable ion chemistry of polycyclic aromatic systems, Laali et al.862 observed the ring closure of dicyanometacyclophanediene 254 with the involvement of diprotonated intermediate 255 [Eq. (5.318)]. When product 256 was treated again in superacids under different conditions, rearrangement took place to yield 1-cyanopyr-ene through mono- and diprotonated intermediates [Eq. (5.319)]. 

Other oxygen stabilized ethylene dications have been prepared and studied using stable ion conditions. For example, Olah and White were able to observe as early as 1967 diprotonated oxalic acid (74) in FSOsH-SbFs solutions.33 This species and related dications were also studied subsequently by theoretical methods. Diprotonated a-keto acids and esters have likewise been generated in superacidic media, such as 75-77 34... 

Another related class of gitonic superelectrophiles are the superelectrophilic halocarbonyl dications. The halocarbonyl cations (XCO+, X = F, Cl, Br, I) have been prepared under long-lived stable ion conditions and characterized by 13C NMR spectroscopy.68 Sommer and co-workers studied the bromine-assisted carbonylation of propane in superacids, and... 

The dicationic species have also been obtained from /3-ketoacids, fi-ketoesters, and /-i-ketoamides in superacid solutions (Table 1, entries 2-4). Diprotonated acetoacetic acid (75) can be observed by low-temperature NMR under stable ion conditions.34 Likewise, diprotonated methylacetoacetate (77) can be observed by NMR at temperatures lower than — 80°C in FS03H-SbF5-SC>2 solution.35 With ethyl acetoac-etate in HF-SbFs, the equilibrium constant for the dication-monocation equilibrium has been estimated to be at least 107, indicating virtually complete conversion to the superelectrophile.35 The /3-ketoamide (78) is found to give the condensation products 95 in good yield from CF3SO3H and the superelectrophile 79 is proposed as the key intermediate in the condensation reaction (eq 25 ).27... 

There has always been considerable interest in the halogen cations and it has often been postulated that the simple ions I+, Br+, and Cl+ are the reactive intermediates in aromatic halogenation reactions. No convincing evidence has ever been obtained for these cations however, the search for them has led to the discovery of the I cation and a number of other related halogen cations (Gillespie and Morton, 1970) which are stable in superacid media. The I cation can be conveniently prepared by the oxidation of I2 with S206F2 in solution in HSOsF. 

These results by Olah and coworkers that the norbornyl cation adopts a corner protonated nortricyclane structure [4] in superacid media have recently got strong support. Saunders isotopic perturbation technique (Section 4) has been applied to this ion (Saunders and Kates, 1980). They prepared the dideuterio derivative [210] of [4] under stable ion conditions and... 

The pathway of these alkylations was clearly demonstrated by Olah et al from their extensive work on the alkylation of the lower alkanes by stable carbocations under superacidic, stable ion conditions. They found that the order of reactivity of C—C and C—H bonds reflected their donor abilities and was in the order tertiary C—H > C—C > secondary C—H primary C—H, although various specific factors, such as steric hindrance, can influence the relative rates. 

The trishomocyclopropenium ion (CeH/, 199) was first proposed by Winstein and coworkers as an intermediate in the solvolysis of czs-bicyclo[3.1.0] hexyl tosylate and extensive efforts were directed toward its generation under stable ion conditions. The persistent cation 199 was first prepared by Masamune et al. by the ionization of czx-bicyclo[3.1.0]hexane in superacid media and it has since been generated from the corresponding alcohol [Eq. (5.29)]. The NMR spectra of structure 199 are consistent with an ion of Csv symmetry. The three equivalent C-H groups are found at high field in the C NMR spectrum (8 C 4.9,7c h = 195.4Hz) in accordance with their hypercoordinate environments. 

This is a remarkably stable ion, already observed in 1955. Its first NMR spectrum (in H2SO4 at room temperature ) was reported in 1962 and so were a little later, the spectra of a variety of cyclopropyl carbocations in superacid media. Its spectrum has been carefully analyzed. 

Indicate the structure of the final stable ion that would be formed from each of the following reactants in superacid media. 

Can the data on the structure of a stable 2-norbomyl ion be applied to the intermediates in the solvolysis of 2-substituted norbornanes Brown says no Indeed, in superacids the solvation of the cation, though feeble, is unspecific. Nucleophilic media exhibit specific interactions with the counter-ion and one or more solvent molecules, which may alter the structure, energies and properties of intermediates. Nevertheless, if we compare the same substrates in solvolytic media and in superacids, we obtain related information provided we really compare carbocations in different media, and not processes of S 2 type and stable ions. 

Alkylation of methane by olefins under superacid catalysis was demonstrated both in solution chemistry under stable ion conditions and in heterogeneous gas-phase alkylations over solid catalysts using a flow system. Not only propylene and butylenes but also ethylene could be used as alkylating agents. 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

C provides direct support for the dictum that tertiary carbonium ions are more stable than secondary, which are more stable than primary. Primary and secondary alcohols are protonated under these conditions, and the protonated alcohols are the species observed (entry 1), while tert-butyl alcohol yields ter/-butyl cation at rates too fast to measure. The rates of cleavage of protonated primary and secondary alcohols depend on their structure. Protonated sec-butanol cleaves with rearrangement to (CH3)3C and water slowly at -60°C, protonated isobutanol cleaves with rearrangement at —30°C, and protonated n-butanol at 0°C. It is typical of reactions in superacid media that the most stable ion of a particular class is observed because, under conditions in which the ions are long-lived, intramolecular hydride shifts and rearrangement processes occur that lead ultimately to the most thermodynamically... 

---