Carbonium ions delocalization

How then is the positive charge in the (OC)9Co3C-substituted carbonium ions delocalized into the cluster substituent In our first preliminary report on this new class of carbonium ions (45), we suggested that their structure presents an especially favorable opportunity for lateral overlap of a filled er-bonding orbital of a metal-carbon bond and a vacant p orbital on an electron-deficient carbonium ion center /3 to the metal (VIII), that is believed responsible for the high... 

Whereas the differentiation of trivalent carbenium and pentacoor-dinated carbonium ions serves a useful purpose in defining them as limiting cases, it should be clear that in carbocationic systems there always exist varying degrees of delocalization. This can involve participation by neighboring -donor atoms, 7r-donor gronps, or [Pg.148]

Kinetic data on acetate displacement from C-3 using a number of sulfur and nitrogen nucleophiles in aqueous solution at near neutral pH demonstrate that the reaction proceeds by an 5 1 mechanism (B-72MI51004). The intermediate in this reaction is depicted as a dipolar allylic carbonium ion (9) with significant charge delocalization. Of particular significance in this regard is the observation that the free carboxylate at C-4 is required since... 

With regard to the composition of the electrical effect, examination of the p values reported in Table XVII shows that in six of the sets which gave significant correlation, the localized effect is predominant (in these sets, either Pr < 50 or / is not significant). Thus it would appear that in so far as substituent effects are concerned, there are two major classes of electrophilic addition to the carbon-carbon double bond predominance of the localized effect or predominance of the delocalized effect. This behavior may well be accounted for in terms of the reaction mechanism. The rate-determining step in the electrophilic addition reaction is believed to be the formation of an intermediate which may be either bridged or a free carbonium ion. 

It is well known that allylic substrates are more reactive under solvolytic conditions than their saturated counterparts because of the delocalization of the positive charge in the developing carbonium ion over the tt system and the overlap of the empty p orbital with the double bond in the intermediate ion. 

Hydrolysis of optically pure bromide 100 a in dioxane/water gives the optically pure alcohol. This is consistent with the transannular p-xylylene ring participating in carbonium ion formation only through tz-g charge delocalization (iz-c resonance) of the type 104 rather than by direct participation in replacement of bromide via a transannularly bridged ion such as 102a. In the latter case, racemization would be expected to take place ... 

The investigations were also extended to the triphenyl-C -carbonium ion (Olah et al., 1964a), where charge delocalization through conjugative interaction with the three benzene rings was expected to decrease the C -chemical shift of the carbonium ion, as compared with that of the trimethylcarbonium ion. 

Olah and Westerman (1973) have determined the carbon-13 spectra of a number of phenyl and cyclopropyl carbonium ions in order to assess the relative abilities of phenyl, cyclopropyl, and methyl to delocalize charge. In Table 8 their results are presented in such a way as to simplify the assessment of their conclusion that... 

Quantitative data are available for the solvolysis of l-(2-aryl)ethyl acetates (where 2-aryl = 2-furyl, 2-thienyl, 2-selenienyl and 2-tellurienyl) in 30% ethanol (77AHC(21)119). The rates at 60 °C relative to the thienyl derivative (1) are selenienyl (1.67), furyl (3.03) and tellurienyl (5.63). These data provide a measure of the effectiveness of the four ring systems in delocalizing the positive charge of the intermediate carbonium ions (80). The rates of the 5-methyl substituted compounds relative to the unsubstituted compounds at 25 °C are thienyl (70), selenienyl (23.3), furyl (160) and tellurienyl (11.8). Thus the furyl derivative is most sensitive to the substituent effect and the tellurienyl the least. The effect of benzo fusion on solvolysis rates has also been studied the solvolysis rate of 2-benzo[6]-thienyl-, -selenienyl- and -tellurienyl-ethyl acetates is less than that of the corresponding monocyclic esters by a factor of about 100. 

Other noteworthy achievements of solid-state 13C CP/MAS NMR in the context of carbonium ions are that (1) the sec-butylcarbonium ion can be identified at low temperatures in a sec-butyl chloride/antimony pentafluoride matrix in the temperature range 80-190 K (2) the norbornyl carbonium ion has been characterized (356,396) at temperatures down to 5 K, there being a strong (but not yet incontrovertible) indication that the controversial non-classical ion (398) exists and (3) the homotropylium ion is best represented (399) by the completely delocalized (homoaromatic) seven-membered state (a below) rather than the incompletely delocalized state (b). 

Roberts suggested that a set of charge-delocalized, rapidly equilibrating carbonium ions, which he called bicyclobutotiium ions, areThe first-formed ions from all three systems, In Scheme 3 are shown the bicvclobntnninm inns fm-mad fmm the deamination of 14C-labeled cyclopropylcarbinyl amine (Equation 6.32). According to Roberts, there would be two equivalent first-formed rarhonium... 

Positive charge in each of the bicylobutonium ions would be distributed between the three carbon atoms of the three-center bond. For example, the positive charge on carbonium ion 64 would be delocalized over C4, Cl5 and C3. If water attacked this ion at C4, unrearranged cyclopropylcarbinol would be obtained if water attacked at C1 cyclobutanol would be the product and if it attacked at C3, allylcarbinol would be formed (Scheme 4). Addition of water to... 

It is well known that trivalent carbenium ions play an important role in electrophilic reactions of 7t- and -donors systems. Similarly, pentacoordinate carbonium ions are the key to electrophilic reactions of o-donor systems (single bonds). The ability of single bonds to act as o-donors lies in their ability to form carbonium ions via delocalized two-electron, three-center (2e-3c) bond formation. Consequently, there seems to be in principle no difference between the electrophilic reactions of n- and O-bonds except that the former react more easily even with weak electrophiles, whereas the latter necessitate more severe conditions. 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required. ... 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity), primary cyclopropylmethyl cations show completely o-delocalized nonclassical carbonium ion character (see Section 3.5.2.5). Also, some of the secondary cyclopropylmethyl cations undergo rapid degenerate equilibrium (see later discussion). 

The addition of a proton to a metal carbonyl compound may occur in either of two modes the formation of metal-hydrogen bond, or protonation of a ligand attached to the central metal atom. If the ligand protonated is an organic radical, a carbonium ion is produced, which may be stabilized by suitable delocalization of charge over the complex, including the central metal atom. Consequently, such protonated species may be legitimately considered as examples of cationic metal carbonyl compounds. 

The data in Table XIV suggest that the positive charge in complexes Via, b, and c has been delocalized to a large extent into the cluster substituent. The observed slight increase in shielding of the carbon atoms of the carbon monoxide ligands when the alcohols are converted to the carbonium ions speak in favor of this view. If the cobalt atoms are more... 

In organic reactions there is abundant evidence for transient carbonium ions (R3C+), carbanions (R3C ), and carbenes ( CR2). Some stable carbonium ions like Ph3C+ and carbanions like C(CN)3 can be isolated as well as radicals like Ph3C In most of these cases the charge on the electron must be delocalized over the entire system for stability. Transition metal complexes with carbene or carbyne ligands, L M=CR2 and L M=CR, are discussed in Chapters 16 and 21. 

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