Carbenes, carbenium ions

While the choice between carbene, carbenium ion, and methyl oxonium ion as methylation intermediates still remains open, it is clear that the lowering of i/n butane ratio should not be cited as evidence in favour of carbene. 

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

CARBENIUM CENTER CARBENIUM ION CARBONIUM ION CARBENE Carbenoid,... 

As a class, nitrenium ions are rather poorly characterized relative to similar reactive intermediates such as carbenes and carbenium ions. This simation alone is sufficient to motivate many fundamental studies into their structures and behavior. There are also several practical considerations that motivate their study. The following is intended as a brief overview of these latter areas. 

Our understanding of the chemistry of N-arylnitrenium ions is significantly more advanced than it was a decade ago. Nevertheless, this field of research is still considerably less developed than that of carbenium ions, carbenes, or nitrenes. For example, although singlet nitrenium ions behave as one might expect that their 4-imino-2,5-cyclohexadienyl resonance contributors would in their reactions with H2O, NJ, or Cl, their reactions with carbon, nitrogen, and sulfur nucleophiles, particularly d-G, are not so easily rationalized. Except for d-G, these reactions with soft nucleophiles have not been examined systematically and the regiochemistry exhibited by these nucleophiles is incompletely understood. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

If an organolithium is used as the base, the reaction follows another mechanism without occurrence of carbenium ions and carbenes. 

The advantage of the Shapiro over Bamford-Stevens Reaction is, that the resulting dianion does not tend to rearrange, which can occur with intermediate carbenes and carbenium ions. However, the Shapiro reaction does not lead to high stereoselectivity between the -/Z-isomers. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioseleetive intramolecular cyclopropanation, even though the sterie bulk of their chiral ligand attachments (COOMe versus /-Pr or C Ph) are similar. Significantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either Rh2(4S-IPOX)4, Rh2(4S-BNOX)4, or Rh2(4R-BNOX)4 (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme II), was of considerable importance in reactions performed with 5-7 but was only a minor constituent ( 1%) from reactions catalyzed by Rh2(5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the earboeation form of the intermediate metal carbene. 

Product analysis shows that the thermal and metal induced reactions follow similar courses. The former reaction may proceed by opening of one ring to produce a formal vinyl carbene, followed by ring expansion the latter may be rationalised in terms of electrophilic attack by silver ion at one of the cyclopropenes to produce a carbenium ion (355) — that is a silver-coordinated equivalent of the carbene — followed by ring expansion of the second cyclopropene. 

It was pointed out in the discussion of the lower part of Figure 11.1 that leaving groups cannot be dissociated from O or N atoms, respectively, if these dissociations resulted in the formation of oxenium or nitrenium ions, respectively. The same is true if a nitrene (R—N ) would have to be formed. These three sextet systems all are highly destabilized in comparison to carbenium ions and carbenes because of the high elec-... 

There is some divergence in the nomenclature various names of these species are used, including silicenium, siliconium, and silylenium ion. According to Barton et al. (19), silylenium ion is the proper name since it is derived in a logical way from the name silylene in analogy to the carbenium ion originating from carbene. 

Carbenium ions too have only six valence electrons, but, of course, unlike carbenes they are charged. 

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