Protonated carbenes carbocations

This distinction between the two types of carbocations was championed by Olah, and is consistent with other nomenclature. Just as hydronium and ammonium ions represent pro-tonated forms of water and ammonia, carbonium ions can be considered to contain proton-ated forms of methane and other typical, tetravalent carbon compounds. Likewise, carbe-niumions can be thought of as protonated carbenes ( CH2 + givesCHs ), consistent with... 

Carbenes from Sulfonylhydrazones. The second method listed in Scheme 10.8, thermal or photochemical decomposition of salts of arenesulfonylhy-drazones, is actually a variation of the diazoalkane method, since diazo compounds are intermediates. The conditions of the decomposition are usually such that the diazo compound reacts immediately on formation.147 The nature of the solvent plays an important role in the outcome of sulfonylhydrazone decompositions. In protic solvents, the diazoalkane can be diverted to a carbocation by protonation.148 Aprotic solvents favor decomposition via the carbene pathway. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

The proton affinity of a carbene can be derived by means of Eq. 9 (M = RR C ) if AHf of the carbene and of the corresponding carbocation have been estimated independently (Table 7). Appearance potentials (AP) are convenient, although sometimes inaccurate, sources of AHf (RR CH+).129 Adiabatic ionization energies (IEa) of free radicals, in combination with dissociation enthalpies... 

The protonation of carbenes by acids has been studied by PAC. Using sterically hindered acids to prevent collapse of the ion pair, the heat of reaction can be obtained and used to determine the p as of the resultant carbocations (Fig. 4).56 The results indicate that deprotonation of the carbocations to generate carbenes should be possible with strong hindered bases. This method has only rarely been used in the literature.1-57... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

Carbocations have also been obtained by protonation of photochemically generated carbenes (see Eq. 17), by the fragmentation of photochemically generated cation radicals (see Eq. 18), and by the addition of one photochemically generated cation to an arene (or aUcene) to generate a second cation. As illustrated in Eq. 19, the last method has been employed to convert invisible carbocations into visible ones. Short-hved aryl cations and secondary alkyl cations are quenched by electron-rich aromatics such as mesitylene and 1,3,5-trimethoxybenzene in HEIP to give benzenium ions that can be observed by LEP in this solvent. 

A variation on Scheme 27 and Equation (19) has been utilized by Keeffe and the author to evaluate p fas of alkyl, aryl and alkoxy carbenes.26 For carboca-tions for which the pKA for loss of a proton from a (3-carbon atom is known, combination of this pKA with the experimental or calculated energy difference between alkene and carbene conjugate bases leads to the pAa for protonation of the carbene, provided it can be assumed that the energy difference between alkene and carbene is insensitive to solvent. Where a pKA for loss of a (3-hydrogen of the carbocation is not accessible, for example, for carbenes lacking a (3-hydrogen, p fR can be used instead. Thus the cycle of Scheme 28 relates a pKa for protonation of the carbene to an experimentally measured... 

A surprising conclusion from Table 2 is that methoxy substituents are more strongly stabilizing for a carbene than a carbocation, and yield smaller than normal pAas, comparable to those for the imidazolium and related ions (46-49). This is consistent with Kirmse s finding that relative rates of protonation of carbenes fall in the order Ph2C > Ph(MeO)C > R(MeO) C > (MeO)2C. As Kirmse and Steenken pointed out, this does not correspond to the order of stability of the carbocation products.213... 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES shown by the following facts (i) A proton was abstracted ordinary... 

At least two types of nitrenium ions the nitrogen analogs of carbocations, can exist as intermediates, although much less work has been done in this area than on carbocations. In one type (65), the nitrogen is bonded to two atoms (R or R can be and in the other (66) to only one atom. " When R = H in 65 the species is a protonated nitrene. Like carbenes and nitrenes, nitrenium ions can exist in singlet or triplet states." ... 

NaY FAU. Since carbenes, e.g., phenylcarbenes, can sometimes act as Bronsted-Lowry bases, acidic media should be avoided. In principle, protonation of carbenes will give carbenium ions, their conjugate acids. This will, of course, complicate product studies because carbenes and carbocations behave differently. Therefore, HX and HY zeolites, i.e., NH4X and NH4Y zeolites,45 were not used. 

Considerable amounts of 2-methoxy-l-methylcyclohexane (36b) were formed in MeOH (Table 3). Methyl ether 36b could stem from two alternative routes (1) insertion of carbene 27 into the O-H bond of MeOH,120 or (2) protonation of 27 and/or 32 to give carbocation 35, which then incorporates MeOH. Therefore, the actual carbenic ratios of 28 29 might have been obscured due to the possible involvement of cation 35. However, the absence of products stemming from tertiary carbocation iso-35, e.g., iso-36 and 37,121 indicates that carbocation 35 was probably not generated during photolysis of diazirine 26. 

These trials showed a reluctance of carbene 46 to undergo intramolecular fragmentation to 47 also in the condensed phase. The hydrolysis reaction of 46 with trace amounts of H20, giving 55f, was evident in each medium. Yet, the formation of 5-norbornen-2-yl-substituted products iso-55 in protic media is noteworthy. Unsaturated iso-55 cannot directly stem from carbene 46 (i.e., no 46-> iso-55) it derives from carbocation 58. So, carbene protonation is indicated (46 —> 58) 69-120 135 However, the typical product ratio that is characteristic of nonclassical carbocation 58 (i.e., 55 iso-55 — ca. 3.5) was not observed, even in the presence of diethyl fumarate (DEF).136,137 Thus, the paucity of iso-55 implies a deviation due to ion pairing of 58 with its counterion. 

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