Dipole stabilization

Dimethylpropanoyl)-l, 2,3,4-tetrahydroisoquinolincs 16 form dipole-stabilized lithium carbanions on deprotonation, but their addition to aldehydes or methyl ketones proceeds nevertheless with low simple diastereoselectivity22 23. However, a high preference for the formation of the w-diastereomer is observed after transmetalation with magnesium bromide22"24. 

In contrast, reaction of ligand 72 with 4,4 -biphenyldiboronic acid has been successful and diboronate 73 is obtained in yields of 33%. This complex acts as a receptor for the paraquat dication forming a 1 1 complex with an association constant of 320 in acetone. The intermolecular forces responsible for the complexation are ion-dipole stabilization between the dative N B dipoles and the two cationic centers in paraquat, attractive tz-tz interactions between... 

Configurational stability has also been confirmed for various metalated carbamates by Hoppe and coworkers. Remarkably, carbamate-protected alcohols such as 20 are deprotonated enantioselectively, when treated with i-butyllithium in the presence of (—)-sparteine. The lithium carbenoids like 21 (R = alkyl) thus generated turn out to retain their configuration (equation 11). Similar results have been obtained for a-lithiated amines and carbamate protected amines " . As a rule, dipole stabilization of the organolithium compounds in general also enhances the configurational stability of a-oxygen-substituted lithium carbenoids. 

One of the most important features of dipole-stabilized a-amino-organolithium compounds is the fact that the carbon-lithium bond is in the nodal plane of the Jt orbitals... 

As indicated in Section ni.B, deprotonation of a carbamate affords a dipole-stabilized a-amino-organolithium that can be transmetalated with copper salts to form cuprates, thereby expanding the versatility of the organolithium. Suitable electrophiles include enones, alkenyl, alkynyl, allenyl and dienyl carboxylic acid derivatives, nitriles and sulfoxides. Dieter and coworkers have shown that the same process can be accomplished via transmetalation of a stannane (Scheme 36). The procedure is particularly... 

II. NON-MESOMERICALLY STABILIZED ORGANOLITHIUM COMPOUNDS A. Acidification by Dipole Stabilization... 

Deprotonation is the most convenient manner for the generation of alkyllithium derivatives. However, most of the potential precursors have insufficient acidity. Notable exceptions are those substrates which lead to dipole-stabilized carbanions , among them O-alkyl iV,A-dialkylcarbamates 15 ° and A-Boc-(cyclo)alkylamines 18 ° (equation 5). A... 

However, dipole stabilization is less significant as an aid to carbanion formation with unsaturated azaheterocycles, since the nitrogen lone-pair electrons are normally incorporated into the 77-electron system of the heterocycle, and are therefore less readily available for donation to the substituent group. In fact, dipole-stabilization can even be a hindrance in some systems where exocyclic ip -carbanion formation is competitive with internal ip -carbanion formation. In these cases it is the heterocycle itself that is responsible for the dipole-stabilization of the external carbanion. 

Attempts to extend this process to the related 2-pyridone system have been less successful, with only a small proportion of C-6 lithiation being observed with l-methyl-2-pyridone [85CC1021 88JCS(P1)1]. In fact, the major mode of reaction involves lithiation of the exocyclic methyl group, to give a carbanionic species that is dipole-stabilized by the heterocyclic amide group (Scheme 120) (83MI1). 

Interestingly, the protected pyrroline starting material is obtained from the analogous pyrrolidine via the intermediacy of the dipole-stabilized sp3-carbanion and diphenylselenide derivatives (85JOC1019) (Scheme 135). 

A study of gas-phase negative-ion chemistry of Lewis acid-base complexes of BH3 with Me2S, Me3N, Me3P, and Et3N has shown that the a-CH acidities in the complexes (to form dipole-stabilized carbanions) are up to 20 kcal mol-1 higher than in the uncomplexed molecules.150... 

Hay, D. R. Song, Z. Smith, S. G. Beak, P. Complex-induced proximity effects and dipole-stabilized carbanions kinetic evidence for the role of complexes in the a -lithiations of carboxamides./. Am. Chem. Soc. 1988, 110, 8145-8153. 

Beak, P. Reitz, D. B. Dipole-stabilized carbanions novel and useful intermediates. Chem. Rev. 1978, 78, 275-316. 

Meyers, A. L Rieker, W. F. Fuentes, L. M. Initial complex and the role of solvent in metalations leading to dipole-stabilized anions./. Am. Chem. Soc. 1983, 105, 2082-2083. 

L. B. Dipole stabilized carbanions in series of cyclic aldonitrones. Aldonitrone metalation and dimerization in LDA and BuLi solutions. Heterocyclic Comm. 1998, 4, 261-270. 

Carbanions which bear an a-heteroatom, which is bonded to a functional group capable of inducing a partial positive charge on that heteroatom (64), are both of theoretical interest and of synthetic importance (Seebach et al., 1983). Such have been termed dipole stabilized carbanions (Beak and Reitz,... 

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