Hydrogen with carbanions

Me3SiCl reacts with phosphinomethanides I (R=Me) with at least one hydrogen as carbanion substituent (X = Y=H X=H, Y=SiMe3, PMej) via Si-C bond formation to give heteroelement substituted phosphinomethanides [4]. With fully C-heteroatom-substituted I, the reaction depends on the nature of X and Y, as shown by Eqs. (l)-(3) ... 

The two bridgehead hydrogens Ha and Hb in the anisaldehyde dithioace-tal (130) have a large difference in their acidities, as indicated by the NMR chemical-shift difference (4.88 and 5.03 ppm). Preparation of the monocarbanion (1 BuLi in THF at -78°C) and quenching with DC1 removed only the higher field hydrogen. The carbanion can be reacted with electrophiles such as primary halides, acid halides, or aldehydes to produce (135). Carbanion generation and alkylation can be repeated on (135) to yield the disubstituted derivative (136) as shown in Scheme 44. 

There has been a short review of the oxidative nucleophilic substitution of hydrogen in nitroarenes in which recent results with carbon, nitrogen, and oxygen nucleophiles are summarized and the preferred oxidants are discussed.11 The oxidative substitution of nitroarenes with carbanions of isopropyl phenylacetate in liquid ammonia-KMn04 initially yields products (4) which may suffer hydroxylation at the o -position, and dimeric and trimeric products may be formed by couplings of nitrobenzylic radicals formed during the reaction.12... 

A substitution reaction on 153 with carbanions (active methylene compounds in the presence of sodium hydride) gave the 7-substituted derivatives 159. The oxo group of the latter products enolized to provide a hydrogen bond with the adjacent ring nitrogen (79YZ1031 80CPB337) (Scheme 31). 

The reaction of an isocyanide containing an acidic hydrogen with copper(I) oxide and an activated olefin or a ketone [Eq. (123)] provides a synthesis of either pyrrolines or oxazolines, respectively (251,252). Addition of allyl bromide gave the coupling product with the allyl carbanion derived from allyl isocyanide. Oxazolines are obtained in yields as high as 957o> not pyrrolines because of competing dimerization... 

A major structural difference between aromatic aldehydes and most aliphatic analogues is that the former lack an a-hydrogen atom. As a consequence, they are unable to enolize and so enolates/carbanions cannot be generated from them. Nevertheless, aromatic aldehydes can react with carbanions derived from, for example, aldehydes, ketones, esters and anhydrides, and so undergo a range of condensation reactions. 

Dithiolium ions, particularly those having a hydrogen atom in position 3 or 5, have oxidative properties that appear, for example, in dehydrogenation processes which follow condensation of 3-aryl-1,2-dithiolium ions with carbanionic reagents. No precise information is available on the nature of the reduction products arising from 1,2-dithiolium ions in these dehydrogenations. 

The Tollens condensation is a crossed aldolization employing formaldehyde and an aldehyde or a ketone in the presence of a weak base. Since formaldehyde has no -hydrogen atoms, it is an extremely reactive carbanion acceptor (p. 155). Consequently, reaction occurs readily with carbanions and leads to polymethylol derivatives 6... 

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