Carbanions special

Strangely enough, cyanide ion is also involved in one special reaction giving an a-hydroxy-ketone. Can you show how the adduct A of benzaldehyde and cyanide ion can give a stable carbanion ... 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

This chapter describes the chemical behavior of a-sulfinyl and a-sulfonyl carbanions. The stereoelectronic effects of these sulfur-containing groups have been the subject of much controversy for more than a decade which has now gradually settled down. Meanwhile, the special features of the chemical behavior of these groups have been utilized for syntheses of thousands of useful organic substances. This chapter deals with the... 

Among special chemical methods that facilitate the Diels-Alder reaction can be included the temporary metal connection strategy [101] that is illustrated in Table 4.27. Si, Mg and A1 are used as temporary connectors of diene and dienophile moieties. The cycloaddition occurs easily due to its intramolecular nature and because the dienophilic component of reagent is now formally a vinyl carbon ion (i.e. a vinyl carbanion in 154 with M = AlEt ). Thus the metal-tethered 154, prepared from lithium alkoxide of 153 with the suitable metal vinyl halide, gives, by heating, the cycloadducts 156 and 157, through the... 

Although the reaction mechanism of this type of reactions is not fully elucidated, it is easily anticipated that no intramolecular special stabilization effect for the carbanion generated from decarboxylation is expected, different from the case of malonic acid-type compounds. Moreover, cinnamic acid derivatives that have both the electron-donating and withdrawing substituents have been reported to undergo this reaction. This fact suggests that the enzyme itself stabilizes the transition state without the aid of mesomeric and inductive effects of the other part of the substrate molecule itself. If such unknown mechanism also works for other... 

A major advantage of the sequence presented here is that the aldehyde group is protected at the siloxycyclopropane stage, which allows convenient storage of this stable intermediate. Of equal importance is the valuable carbanion chemistry that can be carried out a to the ester function. Efficient substitution can be achieved by deprotonation with LDA and subsequent reaction with electrophiles.12-13-6 This process makes several a-substituted [1-formyl esters available. Other ring opening variants of siloxycyclopropanes - mostly as one-pot-procedures - are contained in Scheme I. They underscore the high versatility of these intermediates for the synthesis of valuable compounds.6 Chiral formyl esters (see Table, entries 2-5) are of special... 

The stable carbanions may belong in a special category since their stability is in most cases due to resonance, and the resonance has geometrical requirements that might or might not be the same as those of the bond hybridization of an ordinary carbanion. The central hydrogen of triptycene has none of the acidity of the central hydrogen of triphenylmethane.364... 

Extensive theoretical studies have been carried out to probe the nature of the allyl anion. These studies supplement and extend the experimental results. Allyl anion is of special interest because it is the simplest 7r-delocalized carbanion with 4 electrons and 3 Pjr-centers. Much recent theoretical discussion has concerned the role of resonance in the stabilization of such conjugated systems, a stabilization defined as the enthalpy difference between the localized double-bonded system and its conjugated state. The stabilization of allyl anion has generally been attributed to the delocalization of charge associated... 

Addition of C-nucleophiles to nitrile oxides is of special interest. There are examples of reactions with both carbanions and neutral carbon nucleophiles. To the former group belong reactions of nitrile oxides with organometallic... 

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Ketones can be prepared from special carbanions types by elimination reactions which follow oxygenation. 

In recent years Pagani and coworkers have made detailed studies of the problem. In the space available we can only outline their work and interested readers should consult the very detailed papers. The authors have developed special scales of substituent constants for dealing with contiguous functionalities 193. These new substituent constants are a (which seems to be related fairly closely to the ordinary a ), aIB (which bears some relationship to o/, but not that close), and (Tr-, a special delocalization parameter. It is claimed194 that these scales are appropriate for describing interactions between contiguous functionalities, as opposed to literature values which account for remote interactions . Various C—H acidities in gas phase and in solution were successfully correlated by means of multiple regressions on am and chemical shifts for the central carbon in the carbanions. 

Special interest has been devoted to the reaction of oxygen on carbanionic sites.- It has been established that triplet oxygen acts as an electron acceptor, and that the reaction may yield either terminal hydroperoxy or alcohol functions. Depending on the reaction conditions, the latter group which originates from a disproportionation process between a hydro-peroxidate and a carbanlon may be observed. By choosing proper conditions one can get polymers terminated quantitatively with hydroperoxide groups. The latter can be used in a later step as a radical initiator to prepare new block copolymers. 

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

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. 

Living polymers usually require special reagents to achieve termination. Any electrophilic reagent that reacts with the carbanion active center and also allows preparation of polymers with desired terminal functionalities can be used for this purpose.168,174,181 Hydrogen-terminated polymers can be produced by proton donors, whereas carbon dioxide results in a carboxylate end group. Terminal alcohol functionalities can be formed through reaction with ethylene oxide and carbonyl compounds. 

Cyclic vinyl ethers have been deprotonated using f-butyllithium in THF (77TL4187). The carbanions formed, which react readily with a variety of electrophiles, constitute a special type of protected acyl anion. Compound (658), available from the metallated pyran (657) and methallyl bromide, gave the /8-vinyl enone (659) on hydrolysis and cyclization (Scheme 152). 

In addition to the alkylations discussed above, some special reactions have been reported that enable the solid-phase synthesis of cycloalkanes. These include the intramolecular ene reaction and the cyclopropanation of alkenes (Figure 5.5 see also [44]). Cyclobutanes have been prepared by the reaction of polystyrene-bound carbanions with epichlorohydrin, and by [2 + 2] cycloadditions of ketenes to resin-bound alkenes. 

Some non-cyclic thioacetals have been used sometimes, particularly those bearing phenylthio groups or some special feature such as an extra electron-withdrawing group. The corresponding carbanion can then be used for a 1,4 addition on an a,(J-unsaturated ketone as reported [282],... 

At least two other special mechanisms exist that are not considered in this chapter. The first is electrophilic aromatic substitution via a carbanion. This pathway is sometimes followed if a strong base is present or if the substrate is a metal-substituted aromatic. For example, Mach and Bunnett have found that the presence of i-BuOK, i-BuOBr brominates 1,3,5-tribromobenzene by the mechanism shown below ... 

Several new leaving groups have been discovered recently which merit special discussion. Allyl sul-fones, surprisingly, function as substrates for palladium catalysis.86 As the sulfone group had previously been proven to be able to stabilize an adjacent carbanion, this result allowed allyl sulfones now to be considered as synthons for 1,1- and 1,3-dipoles (equation 10). That is, the allyl sulfone can be used alternately as a nucleophile and electrophile, greatly extending its synthetic utility. 

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