Carbanions structure

Since a carbanion is what remains when a positive species is removed from a carbon atom, the subject of carbanion structure and stability (Chapter 5) is inevitably related to the material in this chapter. So is the subject of very weak acids and very strong bases (Chapter 8), because the weakest acids are those in which the hydrogen is bonded to carbon. 

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. 

Influence of the Anionic Ends Structures In adding different monomer units at the end of monocarban-ionic polystyrenes, we obtain a set of carbanionic structures which habe been deactivated In the same way. The results (Table VI) show that the terminal unit, which allows the more delocalized anion or radical charge, and presents the more sterlc hindrance, gives the lower coupling ratio, and the best functionality. 

SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 300) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion structure is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, i.e., if there is pyramidal inversion as with amines (p. 98). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 181). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

This reaction is possible, but the actual product obtained is more like to arise from the reaction of the alternative carbanion structures reacting with methyl iodide (Following fig.). This is a more useful reaction as it involves the formation of a carbon-carbon bond and allows the construction of more complex carbon skeletons. 

A second peculiarity of 77 expresses itself in the reaction with n-butyllithium inJ which the hydrogen atom at the phosphorus atom is displaced nucleophilically as hydride ion to give butyl-bis-2,2 -biphenylylenephosphorane (25b). Deprotonation to the phosphoranyl anion 79 occurs only to a minor extent here. For 79, an equilibrium with the ring-opened carbanionic structure 86 was proven 94,95,97). 

Remember that the oxyanion and carbanion structures are just two different ways to represent the same thing. We shall usually prefer the oxyanion structure as it is more realistic. You can say the same thing in orbitals. 

This discussion of aliphatic carbanion structures has included mainly organolithium compounds simply because the structures of most aliphatic caibanions incorporate lithium as the counterion and also because this alkali metal cation is the most widely used by synthetic organic chemists. For comparison the entire series of Group la methyl carbanion structures, i.e. MeNa, MeK, MeRb and MeCs, have been determined. Methylsodium was prepared by reaction of methyllithium with sodium r-butoxide. Depending upon the reaction conditions, the products obtained by this procedure contain variable amounts of methyllithium and methylsodium (Na Li atom ratios from 36 1 to 3 1). Hie crystal structure of these methylsodium preparations resembles the cubic tetramer (38) obtained for methyllithium with the Na— Na distances of 3.12 and 3.19 A and Na—C distances of 2.58 and 2.64 A. 

A systematic investigation of alkynic carbanion structures has been reported by Weiss and cowoikers. These structures all incorporate either r-butylacetylide or phenylacetylide anion. They differ by the ligand that is incorporated. Perhaps this sequence of structures most tq)tly demonstrates the rich variety of aggregate structural types that an acetylide anion can choose. 

Other aliphatic carbanion structures associated with Group Ila cations are known. Some examples of these are dimethylberyllium and lithium tri-r-butyl beryllate. Since the beryllium alkyl carbanions have not yet been utilized as common synthetic reagents, these structures will not be discussed further. 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

C2a-carbanion or enamine as suggested 35 years ago by Breslow. After considerable research by many groups " , the true structure of the former is still unknown, as its existence is too fleeting for direct detection. Suffice it to say that more recent kinetic studies suggest that it is formed with a pA" near 17-19 and its behavior is consistent with a localized carbanion structure similar to that of cyanide ion. While there were some reports favoring a carbene °, rather than nucleophilic reactivity for the C2-carbanion vis-a-vis electrophiles, more recently there was evidence presented reconfirming that the reactivity of this C2-carbanion is best reflected by nucleophilic pathways, and the mechanistic probes developed provided evidence inconsistent with carbene insertion into C—H single bonds or addition across C=C double bonds ... 

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