Inversion carbanions

Pechhold, E. Adams, D. G. Fraenkel, G. On the rigidity to carbanion inversion of four-, five-, and six-membered cyclic organomagne-sium compounds./. Org. Chem. 1971, 36, 1368-1374. 

For labile metals, the sequence of alkyl dissociation and carbanion inversion could lead to racemization. Elimination, followed by olefin dissociation and coordination of the opposite face results in loss of stereochemistry. Racemizations of Be, Al, and B alkyls proceed by the jS-elimination mechanism... 

By the isolation of nonracemic chiral it-allylpalladium complexes82,83, ss, it was unambiguously demonstrated that the oxidative addition step occurs with inversion of configuration. Loss of chirality is mainly due to anti attack of free palladium(O) species on the rr-allyl complex (sec Section 1.5.6.1.2.1.). When the chiral allyl complexes are subjected to nucleophilic addition with soft carbanions, inversion of configuration is observed and thus retention of configuration for the overall process results. 

Scheme 9.78. Representation of a pathway that accounts for a change in the ratio of (Z)- to fitj-l-phenylpropene by change in the reaction conditions (lower temperature and additional strong base, sometimes called the Schlosser modification of the Wittig reaction). Early phos-phetane fragmentation to alkene is avoided and rotation (or carbanion inversion) to change the isomer ratio is allowed. An excellent discussion of various permutations can be found in Schlosser, M. Strunk, S. Tetrahedron, 1989,45,2649.

Because of the potential for carbanion inversion in the intermediate in ElcB reactions, a mixture of stereoisomers is usually obtained. If the reaction of 10.17 had been strictly an E2 process, only the Z-isomer would have been obtained. 

A great number of N-substituted 4-hydroxy-3-nitro-l,8-naphthyridin-2 (IH)-ones are obtained by reaction of N-substituted azaisatoic anhydrides with ethyl nitroacetate carbanion (Section II,A,4,a). A very specific method, more recently developed, is that of the inverse Diels-Alder method, involving the reactions of enamines with 5-nitropyrimidine (Section II,A,4,b). 

The carbanion is found to react with esters, ketones or aldehydes21 with retention, whereas with carbon dioxide or carbonyl chlorides19,19 a inversion is observed. 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

One commonly used procedure for studying the steric stability of the carbanion is comparison of the rate of base-catalyzed H/D exchange reaction (kel) on the chiral carbon with the rate of racemization (fcrac). By this comparison, inversion, racemization or... 

The mechanism of these reactions is usually Sn2 with inversion taking place at a chiral RX, though there is strong evidence that an SET mechanism is involved in certain cases, ° especially where the nucleophile is an a-nitro carbanion and/or the substrate contains a nitro or cyano group. Tertiary alkyl groups can be introduced by an SnI mechanism if the ZCH2Z compound (not the enolate ion) is treated with a tertiary carbocation generated in situ from an alcohol or alkyl halide and BF3 or AlCla, or with a tertiary alkyl perchlorate. ... 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) 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 stracture 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, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). 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. 

However, even planar carbanions need not give racemization. Cram found that retention and even inversion can occur in the alkoxide cleavage reaction (12-39) ... 

When C—H bond formation occurs, the result is inversion. Racemization results in polar aprotic solvents such as DMSO. In these solvents the carbanions are rela-... 

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