Carbanion moiety

As shown in Fig. 3, the observed values of A/fhet(R-R ) in DMSO for the hydrocarbons [3-2] and [25-2] are 15kcalmol less than the values predicted from (28) and (29), respectively, while A/fi,et(R-R ) for the nitro-cyano compound [4-102] in acetonitrile (30) is close to the value predicted from (29). Therefore, this departure of the dissociative hydrocarbons from the predicted behaviour can be ascribed to steric congestion in the hydrocarbon, predominantly caused by the carbanion moiety [2 ]. 

Considering the long saga of hydrocarbon chemistry, it is surprising that two new classes of hydrocarbon - ionically dissociative hydrocarbons and hydrocarbon salts - have been discovered in the last decade. The syntheses of authentic samples as analytically pure solids have revealed the very existence of such novel hydrocarbons in an unquestionable way, but the investigation of their basic features is just in the inchoate stage. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. As mentioned above, until now such elaboration has been concentrated on the carbocation side, and examination of the carbanion moiety has only just started. 

Only a few reports on studies of the mass spectra of arsonium ylides have appeared in the literature. Gosney and Lloyd (32) studied the mass spectra of bis(carbethoxy)methylene triphenylarsorane and nitromethy-lene triphenylarsorane and found that preliminary fragmentation resulted in loss of the carbanionic moiety. We studied the mass spectra of 11 arsonium ylides and of triphenylarsine difluoride (30). 

Current results indicate that stabilized arsonium ylides such as phenacylide, carbomethoxymethylide, cyanomethylide, fluorenylide, and cyclopentadienylide afford only olefinic products upon reaction with carbonyl compounds. Nonstabilized ylides such as ethylide afford almost exclusively epoxides or rearranged products thereof. However, semi-stabilized arsonium ylides, such as the benzylides, afford approximately equimolar amounts of olefin and epoxide. Obviously, the nature of the carbanion moiety of the arsonium ylide greatly affects the course of the reaction. It is reasonable to suppose that a two-step mechanism is involved in the reaction of heteronium (P, S, and As) ylides with carbonyl compounds (56). 

Among some unstable phenyliodonium ylides the most interesting are those with a monocarbonyl carbanionic moiety which are formed from / -ace-toxyalkenyl(phenyl)iodonium precursors upon reaction with EtOLi (Scheme 52) [160]. These reagents are useful for the synthesis of a,/ -epoxy ketones and 2-acylaziridines. 

In this chapter, reagents that transfer a carbanion (in contrast to an enolate ion) to the C atom of a C=0 double bond are referred to as C nucleophiles. The most important nucleophiles of this kind are organolithium compounds and Grignard reagents. Organocopper compounds transfer their carbanion moieties to the carbonyl carbon far less easily and usually not at all. 

The reason for this complementarity is that the CH2 group of the sulfoxonium ylide is less nucleophilic than that of the sulfonium ylide. In the first case the CH2 group is adjacent to the substituent Me2S+=0 and in the second case it is adjacent to the substituent Me2S+. The extra oxygen in the first substituent reduces the nucleophilicity of the sulfoxonium ylide because it stabilizes the negative charge of its carbanionic moiety especially well. 

In further studies of ion-pairing, a variety of sec-a-silyl benzylic lithium compounds 39, 40, 41 and 42, were prepared, both externally and internally solvated, the latter by means of a potential ligand attached to the carbanionic moiety. Ion-paired carbanide salts tend to assemble into several arrangements which differ in aggregation, solvation and in the proximity of anion to cation. Many of these species interconvert rapidly relative to the NMR time scale even at quite low temperatures. An internally solvated ion-pair carbanide salt is more likely to assume a single molecular structure, to undergo the latter exchange processes more slowly and thus be more amenable to NMR spectroscopic studies of structure and dynamic behavior. 

However, the available examples, 1-4, have stabilized carbanion moieties and the lithiums also are stabilized by coordination with electron pair donor atoms. Two compounds in the literature involve an sp -hybridized carbon atom cr-bonded to a lithium atom [1 (2) and 3 (3)]. Compound 2 (4) also involves a terminally bonded lithium atom but the more or less planar benzylic carbon can perhaps be described as being sp hybridized. 

Reactivity. The basicity of organolithium reagents decreases with increasing stability of the carbanion moiety (e.g., t-BuLi > 5-BuLi > n-BuLi). 

In the semibenzylic pathway there is a nucleophilic attack by the base at the carbonyl carbon, followed by a concerted 1,2-migration of a carbanionic moiety with concomitant expulsion of halide. Implicit in this mechanism is the requirement for an antiparallel arrangement of the C—C bond that is broken and the carbon-halogen bond. This further requires an inversion of configuration at the carbon center initially bonded to the halogen atom, and an elegant demonstration of this feature has been provided by Charpen-tier and coworkers (Scheme 9). ... 

Another method to introduce a substituent into the 1,2,4-triazine system is vicarious nucleophilic substitution. Here, a carbanion with a leaving group at the carbanionic center reacts with a 1,2,4-triazine replacing protons in the 5-, 3- and 6-position with the carbanionic moiety. The reactivity of the various positions decreases in the order 5 > 3 > 6. This reactivity differs from that toward Grignard reagents. Thus, substitution of the 5-position occurs in the 1,2,4-triazine to afford 15 which undergoes elimination to 16 followed by protonation to yield 17 275,276 Carbanions of the following compounds were used in this reaction nitrones, chloromethyl phenyl sulfones, chloromethanesulfonamides and acetonitriles.274-378... 

Contribution from this type of structure is not likely to be great, however. For ylides having hetero-atoms from other than the first row of the periodic table the doubly-bonded structure (B) may well be important-since these atoms can expand their valence shell. However it would be expected that as one goes down the periodic table the increasing size of the hetero-atom and its more diffuse d-orbitals would lower the effectiveness of the overlap between the 2p-oibitals of the carbanionic moieties and the vacant nd-orbitals of the hetero-atoms, leading to decreased double-bonding between them. 

Oxidation of these a -adducts with DMD results in an introduction of />-hydroxyaryl substituents into a-positions of amino acids [15]. The formation of a -adducts is connected with creation of a stereogenic center. When the carbanions of protected amino acids contain a chiral center in the vicinity of the prochiral carbanion moiety, the formation of the o"-adducts proceeds with high stereoselectivity. Since the oxidation of the a -adducts does not affect the newly formed stereogenic center, the ONSH in such cases proceeds with high diastereoselectivity and, as a consequence, enantioselectivity (Scheme 11.8) [16]. 

From many new applications of two-phase systems to reactions of carbanions some processes, connected with the direct introduction of a heteroatom to the carbanionic moiety, will be discussed. 

The carbanionic moiety is bent (/.CCC = 157°), due to contacts of lithium to the two largely negatively charged terminal carbons. These same structural motifs apply to computed structures of allenyllithum dimers. 

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