Early Unsuccessful Attempts

Early Unsuccessful Attempts. Until the early 1960s, simple alkyl cations were considered only as transient species.15 Their existence has been inferred from the kinetic and stereochemical studies of reactions. No reliable physical measurements, other than electron impact measurements in the gas phase (mass spectrometry), were known. The formation of gaseous organic cations under electron bombardment of alkenes, haloalkanes, and other precursors has been widely investigated in mass spectrometric studies.81 No direct observation of carbocations in solutions was achieved prior to the early 1960s. 

The observation of alkyl cations such as the ferf-butyl cation [trimethyl-carbenium ion, (CH3)3C+] 1 and the isopropyl cation [dimethylcarbenium ion, (CH3)2CH+] 2 was a long-standing challenge. The existence of alkyl cations in systems containing alkyl halides and Lewis acids has been inferred from a variety of observations, such as vapor pressure depressions of CH3C1 and C2H5CI in the 

Electronic spectra of alcohols and alkenes in strong proton acids (H2SO4) were obtained by Rosenbaum and Symons.86 They observed, for a number of simple aliphatic alcohols and alkenes, absorption maxima around 290 nm and ascribed this absorption to the corresponding alkyl cations. 

Finch and Symons,87 on reinvestigation of the absorption of aliphatic alcohols and alkenes in sulfuric acid solution, showed that the condensation products formed with acetic acid (used as solvent for the precursor alcohols and alkenes) were responsible for the spectra and not the simple alkyl cations. Moreover, protonated mesityl oxide was identified as the absorbing species in the system of isobutylene, acetic acid, and sulfuric acid. 

Deno88 has carried out an extensive study of the fate of alkyl cations in undiluted H2SO4 and oleum. He obtained equal amounts of a saturated hydrocarbon mixture (C4-C19) insoluble in H2SO4 and a mixture of cyclopentenyl cations (Cq-Cjo) in the H2SO4 layer. These cations exhibit strong UV absorption around 300 nm. 

It must therefore be concluded that earlier attempts to prove the existence of stable, well-defined alkyl cations were unsuccessful in experiments using sulfuric acid solutions and inconclusive in the interaction of alkyl halides with Lewis acid halides. Proton elimination reactions or dialkyl halonium ion formation may have affected conductivity studies. 

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Despite early, unsuccessful attempts to further expand the range of dendrimeric compounds that contain heterocyclic residues, 1,2,4-triazoles were eventually incorporated into large dendrimeric arrays <2002ARK17, 2002JOM(660)50, 2006T2677>. 

After early unsuccessful attempts to direct the photoreduction of ketones with chiral secondary alcohols [8-10]. Weiss et al. examined the sensitized cis-trans photoisomerization of 1,2-diphenylcyclopropane in chiral solvents but obtained the product without detectable optical rotation [11]. Seebach and coworkers were the first to achieve asymmetric induction for a photochemical reaction by a chiral solvent [12-15]. They examined the photopinacolization of aldehydes and ketones in the chiral solvent (S,S)-( + )-l,4-bis(dimethylamino)-2,3-dimethoxybutane (DDB, 4). Irradiation of acetophenone in the presence of 7.5 equiv. of DDB yielded a mixture of chiral d,/-pinacols 3/ent-3 and achiral meso-pinacol 2. At 25°C pinacol 3 was obtained with 8% ee, with the (R, / )-( + )-enantiomer prevailing. At lower temperatures the asymmetric induction was more effective, up to 23% ee at — 78°C in a 1 5 mixture of DDB and pentane (Scheme... 

It is the similarity between silicates and phosphates that suggested that phosphorus might be substituted for silicon in molecules of serpentine mineral types. If properties of silicates can be retained while adding known safety properties of phosphates, a safe asbestos -type product is possible. An early unsuccessful attempt was made by the author to prepare a safe asbestos by inserting phosphate groups within silicate chains of chiysotile. With some additional work this should still be possible and no more than two or three percent of the weight of the chrysotile product would be required as inserted phosphates, and probably inexpensively. 

The corresponding alcohol elimination from 2-alkoxyimidazolidines 80 to give imidazolin-2-ylidenes of type 81 was reported by Grubbs and co-workers after early unsuccessful attempts by Wanzlick and Kleiner. Imidaz-olin-2-ylidenes 81 were also accessible by a-elimination of fluorinated aryls from 2-(fluorophenyl)imidazolines 82 (Scheme l.lOb). " The a-elimination of acetonitrile from 83 to yield the benzimidazol-2-ylidene 84 has also been... 

In about 1830 an unsuccessful attempt was made to determine the nature of a peculiar ore from die Saucon Valley near Bethlehem, Pennsylvania. Mr. W. T. Roepper, who afterward became the first professor of mineralogy at Lehigh University, identified it as calamine, zinc hydrosilicate, and produced brass by smelting it with native copper (110). The history of early zinc works in die Lehigh Valley has been ably presented in die Journal of Chemical Education by R. D Billinger (110). 

Methylphenidate was synthesized in 1944 in Europe in an unsuccessful attempt to create a stimulant that would not produce addiction or tolerance (Figure 1.1). Introduced in the United States in 1955, it was first approved by the FDA for the treatment of drug-induced lethargy, mild depression, and narcolepsy. In the early 1960s, the drug was first marketed under the name Ritalin to improve memory in elderly patients and to treat several behavioral problems in children. 

The problem of the dependence of E on the number of Kekule structures is examined in some detail in the subsequent section. Nowadays it is fairly well established that this dependence is linear. In former times, when this simple fact was not conceived, an astonishing variety of mathematical expressions was proposed for the description of the K-dependence of E. A comparative study of these early (in most cases unsuccessful) attempts is reported in [89], The data collected in Tables 3 and 4 are analogous to those given in Tables 1 and 2. They are obtained using the same sample of 104 benzenoids from the book [92]. The following (n,m,K)-type expressions for E have been examined ... 

Historically it is of interest that an early, albeit unsuccessful, attempt to detect NMR in bulk matter was performed with powdered lithium chloride [25]. Only later did it become clear that under the experimental conditions used the Li resonance was probably saturated. The first Li NMR signal was recorded in 1946 [26], while Li was measured a few years later [27]. High-resolution spectra for Li were then reported in 1962 [28] and for Li in 1976 [29]. For completeness we mention that nuclear properties of the short-lived... 

The first unsuccessful attempts to convert alcohols directly into alkylfluorides were documented in 1782, when Scheele treated ethanol with hydrofluoric acid vapor obtained from the reaction of fluorspar (CaF2) with sulfuric acid [115]. Despite this and other early failures, a variety of successful and convenient strategies designed to solve this general synthetic problem have been developed, especially since the 1920s. 

The first, unsuccessful, attempts to prepare alkylsilver compounds were made as early as 1859 by Buckton5a and 1861 by Wanklyn and Carius5b. The reaction of silver chloride with diethylzinc yielded only metallic silver, and a mixture of ethylene, ethane and butane. Due to their low thermal stability alkylsilver compounds were entirely unknown until 1941, when Semerano and Riccoboni6 reported the formation of some alkylsilver complexes by the reaction of silver nitrate with tetraalkyllead in alcoholic solution at low temperatures (equation 1). The yellow to brown precipitates of the formula RAg (R = methyl, ethyl, propyl) were stable at —80 °C for several hours, but upon warming to room temperature rapid decomposition took place with formation of metallic silver and... 

The list of unsuccessful attempts to find new chemical laser reactions is very long and will not be discussed in detail here. The reader is referred to the discussion of prospects at the first chemical laser conference which appeared as a supplement to Applied Optics 32>. A new approach of a more general nature is the photorecombination laser first suggested by R. A. Young 85> as early as 1964 and treated in detail by Kochelap and Pekar 86>. In describing this principle, we follow in part the argument of A. N. Oraevskii 87>. A number of chemical processes give rise to the emission of a photon such that this emission is not a consequence but a necessary condition of the elementary act. 

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