Transient carbocation

The situation just discussed probably applies also to the attack of water on other kinds of stabilized carbocations. For example, some of the many transient carbocations studied in recent years by McClelland and Steenken, and their coworkers (e.g. Steenken et al., 1986 McClelland and Steenken, 1988), have relatively constant kOH/kw ratios. For alkyldialkoxy cations [41], oh w = 103 to 104 m-1 and so pKt = 10 to 11 for the trialkoxy analogues [42], kon/kw == 104 to 106m i and p/Cf = 8 to 10 (Table A6.3), suggesting a more acidic transition state for [42], due to the extra oxygen atom. Within each series there is a systematic variation of p/C, since logfc0H correlates with logfcw, with a slope of approximately 0.6 (McClelland and Steenken, 1988), rather than 1. Estimates of the Leffler indices for the two series of... 

Fluoroalkylamines react with nitrous acid to produce the corresponding unstable fluoroahphatic diazomum ions Placement of the tnfluoromethyl group at a carbon position a, (i, or y to a diazomum ion was used to probe the inductive effect on the chemistry of the transient carbocation resulting from dediazomation [7] If the fluoroalkyl group is bound to the same carbon as the amino group, conversion to the more stable diazo compound occurs For example, 4-diazo-l,l,l,2,2-pentafluoro-3-pentafluoroethyl-3-tnfluoromethylbutane is obtained from the reaction of the poly-fluoroalkylamine salt with sodium nitrite [8, 9] (equation 8)... 

A methyl substituent at a double bond of benzobicyclootadienes has a pronounced effect upon the course of addition of hypoiodous acid. While the unsubstituted derivative 80 has been known to give solely rearranged products, the stabilization of the transient carbocation by the methyl group prevents the rearrangement to some extent188. 

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. 

Many other metabolic degradation pathways are either initiated by reactions involving generation of transient carbocations or pass through a transition state with highly positive charge density on a carbon center. Reactions in this category are the acidic hydrolysis of acetals, aminals, or enol ethers, and the oxidation of alcohols to ketones via a hydride-transfer mechanism. 

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

Formed by either process, carbocations are most often short-lived transient species and react further without being isolated. The intrinsic barriers to formation and reaction of carbocations has been studied. ... 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

When a positive charge is carried on carbon the entity is known as a carbocation, and when a negative charge, a carbanion. Though such ions may be formed only transiently and be present only in minute concentration, they are nevertheless often of paramount importance in controlling the reactions in which they participate. 

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

Reference has already been made in the last chapter to the generation of carbocations, in ion pairs, as intermediates in some displacement reactions at a saturated carbon atom, e.g. the solvolysis of an alkyl halide via the SN1 mechanism. Carbocations are, however, fairly widespread in occurrence and, although their existence is often only transient, they are of considerable importance in a wide variety of chemical reactions. 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

Positively charged amines that are structural analogues and are isosteric with putative carbocation intermediates in enzymic reactions. These compounds have proved their value in efforts to characterize enzyme mechanisms that proceed by the transient formation of carbocation intermediates. 

In the absence of either HPI or Co(acac)2, no appreciable conversion into products was obtained. EPR evidence for the formation of an aminoxyl radical intermediate was acquired, and a KIE of 3.8 determined . Generation of PINO in situ as the reactive intermediate was postulated . A subtle alternative enables the functionalization of hydrocarbons through the formation of carbocations as transient intermediates whenever PINO is formed and reacts in the presence of NO (e.g. 1 atm) . 

---