Flash photolysis carbocations

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

The transient zwitterion (15), obtained from the /(-hydroxy acid precursor by laser flash photolysis, has been characterized 46 it reacts with nucleophiles more slowly than does the 9-fluorenyl cation itself. The parent acid was also characterized.46 Evidence has been presented that (16) undergoes substantial El elimination via a primary carbocation.47 An analysis of solvolysis results for (17) is indicative of extensive charge delocalization throughout the fluorenyl ring at the transition state apparently... 

The photoionization of benzylic alcohols leading to carbocations has recently been reported197. Taking advantage of the stabilization provided by the /J-silicon effect, flash photolysis of 380 in trifluoroethanol gave rise to the fluorenyl cation 381 which was characterized spectroscopically (equation 46). 

The extension of equilibrium measurements to normally reactive carbocations in solution followed two experimental developments. One was the stoichiometric generation of cations by flash photolysis or radiolysis under conditions that their subsequent reactions could be monitored by rapid recording spectroscopic techniques.3,4,18 20 The second was the identification of nucleophiles reacting with carbocations under diffusion control, which could be used as clocks for competing reactions in analogy with similar measurements of the lifetimes of radicals.21,22 The combination of rate constants for reactions of carbocations determined by these methods with rate constants for their formation in the reverse solvolytic (or other) reactions furnished the desired equilibrium constants. 

Using time-resolved laser flash photolysis techniques, transient carbocations in the photolysis of benzyl halides have been widely observed. A variety of phenylmethyl halides157-160, substituted diphenylmethyl halides154,155,161-163 and substituted triphenyl-methyl halides162,164 has been successfully used as precursor under various reaction conditions. The photogeneration of the transient cations is often accompanied by that of the corresponding transient radicals. 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

Two transients have been observed during flash photolysis of 9-fluorenol and both have been assigned the fluorenyl cation structure.A new study has concluded that while one of these transients is indeed the cation, the other corresponds to the fluorenyl radical cation.A transient assigned as the carbocation obtained by loss of hydroxide has also been observed by flash photolysis of arylxanthenol (417) in aqueous ethanol. 

Because the substituted benzene chromophores absorb in the 205- to 280-nm range and have low e values, the solvents used must be transparent down to at least 250 nm. This requirement is unnecessary for the more conjugated naphthalene and anthracene chromophores. The usual polar, nucleophilic solvents that have been used to observe ions and ion-derived products are various alcohols, water, or water mixed with a cosolvent such as dioxane for solubility reasons. Recently, and particularly for the observation of the intermediate carbocations by laser flash photolysis (LFP) methods, the strongly ionizing (high Tore values) but weakly nucleophilic Oow N values) alcohols, 2,2,2-trifluoroethanol (TFE) and l,l,l,3,3,3,-hexafluoro-2-propanol (HFIP), have been more commonly used. A limited list of polar solvents and their properties is given in Table 2 [23,24]. 

Triplet-excited nitrobenzyl acetals 18-19 were proposed to undergo direct carbon-carbon bond photoheterolysis to generate the nitrobenzyl carban-ion and the corresponding a-dialkoxy carbocation [Eq. (7)] [81,82]. Transient spectra obtained following laser flash photolysis of 19 showed the formation of the /7-nitrobenzyl anion and the oxocarbocation [83] ... 

Recently, such carbocation formations were studied kinetically by stopped-flow laser-flash photolysis by Scaiano s group (Belt et al., 1993). The quantum yield of diphenylcarbocation formation is low (0.007). 

In polar solvents, a-halomethyl aromatics give rise to photochemical reactions that can be explained by both radical and ionic mechanisms. Equation 12.77 shows the results for irradiation of 1-chloromethylnaphtha-lene (119) in methanol. The most direct pathway for formation of the methyl ether 120 is heterolytic dissociation of the C-Cl bond to give a chloride ion and a 1-naphthylmethyl carbocation, the latter then undergoing nucleophilic addition by the solvent. Indeed, naphthylphenylmethyl carbocations were detected spectroscopically following laser flash photolysis of (naphthylphenylmethyl)triphenylphosphonium chlorides. On the other hand, products 121, 122, and 123 appear to be formed via the 1-naphthylmethyl radical. Therefore, an alternative source of the carbocation leading to 120 could be electron transfer from the 1-naphthylmethyl radical instead of direct photochemical heterolysis of 119.215-216 jaj-g g p. 

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