Carbanion carbenium ion

However, it is not clear from the foregoing as to what type of 7r-allyl species is involved, carbanionic, carbenium ion, or radical, although Kokes infers that it possesses at least partial carbanionic character. 

This section deals with the generation and reactivity of carbanions, carbenium ions and carbon radicals adjacent to the thiophene ring. A few miscellaneous substituted alkyl groups will also be covered. 

The pivotal step in this sequence is an electrophilic substitution on indole. Although the use of l,3-dithian-2-yl carbanions is well documented, it has been shown only recently that 1,3-dithian-2-yl carbenium ions can be used in a Priedel-Crafts type reaction. This was accomplished initially using 2-methoxy-l,3-dithiane [1,3-Dithiane, 2-methoxy-] or 2-metlioxy-l,3-dithiolane [1,3-Dithiolane, 2-methoxy-] and titanium tetrachloride [Titanate(l —), tetrachloro-] as the Lewis acid catalyst.9 2-Substituted lysergic acid derivatives and 3-substituted indoles have been prepared under these conditions, but the method is limited in scope by the difficulties of preparing substituted 2-methoxy-1,3-dithianes. l,3-Dithian-2-yl carbenium ions have also been prepared by protonation of ketene dithioacetals with trifluoroacetic acid,10 but this reaction cannot be used to introduce 1,3-dithiane moieties into indole. 

It follows from the present argument that the ideas of the intimate and solvent-separated ion pairs, which are useful in the context of anionic polymerisations, are not relevant for cationic polymerisations. The reason is that in contrast to the carbenium ions, the carbanions are tetrahedral, the fourth valency being the lobe of the electron pair. Therefore, a solvent molecule can find a potential well between the anion and the cation, but there is no suitable site on the opposite side of the anion. 

Arnett and colleagues [219,220] measured the enthalpies of a considerable number of processes where a resonance-stabilized carbenium ion R was reacted with a resonance stabilized carbanion, oxanion, thioanion, or nitroanion, R(T, in mixtures of sulfolane (95%) and 3-methylsulfolane (5%), at 298.15 K ... 

However, acid-catalyzed isomerization attracts more attention, probably due to its connection with the recent intensive development of carbenium ion chemistry. It is common knowledge that effective methods for stabilization of reactive carbocations have been known since 1962 while base-catalyzed processes with the participation of carbanions were developed more than 100 years ago. 

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. 

Similar effects could be expected for carbanions. Normally kn/kp ratios in the region 1.15-1.20 are found per deuterium in the formation of carbenium ions. If this ratio is translated into the separation of C-13 shifts expected by deuteration of one side of an equilibrating ion, then 6/A a, 0.1 per deuterium, where 6 is the difference between the observed lines and A is the separation expected between the shifts of the two positions in each tautomeric structure (W). If, however, there is resonance between the two structures, this ratio appears to be considerable smaller (lU). 

In another early application to natural product synthesis, Fleming and coworkers utilized this approach in the efficient formation of the gelsemine model (47) from 45 according to Scheme 8. The cyclization step to form the spiro-oxindole (46) proceeded in 85% yield and provided a means of generating the spiro-fused quaternary carbon without the need for carbenium ion or carbanion chemistry. 

The effect of the 1,2,5-thiadiazole system on the reactions of carbon-bound substituents can be summarized as follows (i) stabilization of carbanions (ii) destabilization of carbenium ions (iii) enhanced Sn2 reactivity and repressed SnI reactivity <68AHC(9)107>. Aryl substituents are rendered more reactive to nucleophiles <72US(A)25> and deactivated in reactions with electrophiles, which are directed to the orthojpara positions by the thiadiazole ring <72IJS(A)25,78MI409-01>. 

Finally, we point to the possibility of P = 0 bond formation from 1-alkoxy-X -phosphorin derivatives 124 or 125 by cleavage of alkyl cations. Also the reverse process, /. e. alkylation of the P = O moiety to form P—O—R groups is possible. The synthesis of X -phosphorins having functional groups at the C-atoms of the phosphorin ring was first made possible by the preparation of new stable X -phosphorin carbenium ions 140. Here again, the fundamental difference between phosphorin and pyridine systems comes to light Whereas carbanionic structures 139 b are stabilized in the pyridine series, in the X -phosphorin series carbenium ions as 140 b are stabilized. 

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-... 

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

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. 

Polymer-supported carbanion equivalents are the obvious supplement to polymer-supported access to reactive electrophiles either by oxidizing polymers or by release of carbenium ions. The combination of an oxidizing resin with a support carrying carbanion equivalents will be especially rewarding, enabling reaction sequences with C-C coupling steps and thus opening access to a wealth of potentially relevant products. 

Bonding and Preferred Geometries in Carbon Radicals, Carbenium Ions and Carbanions... 

A carbon radical has seven valence electrons, one shy of the octet of a valence-saturated carbon atom. Typical carbon-centered radicals have three substituents (see below). In terms of electron count, they occupy an intermediate position between the carbenium ions, which have one electron less (a sextet and a positive charge), and the carbanions, which have one electron more (an octet and a negative charge). Since both C radicals and carbenium ions are electron deficient, they are more closely related to each other than to carbanions. Because of this, carbon radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

What are the geometries of carbon radicals, and how do they differ from those of carbenium ions or carbanions And what types of bonding are found at the carbon atoms of these three species First we will discuss geometry (Section 1.1.1). and then use molecular orbital (MO) theory to provide a description of the bonding (Section 1.1.2). 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

The preferred geometries of carbenium ions and carbanions are correctly predicted by the valence shell electron pair repulsion (VSEPR) theory. The theory is general and can be applied to organic and inorganic compounds, regardless of charge. 

Fig. 1.3. Energy levels and occupancies (red) of the MOs at the trivalent C atom of planar carbenium ions R3C (left) and pyramidal carbanions RjC (right). The indices of each of the four MOs refer to the AOs from the central C atom.

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