Active carbanionic

The only currently unrefuted evidence even suggestive of an optically active carbanion is the report of optically active 2-octyl-lithium.899 It should be recalled, however, that ordinary alkyl lithium compounds are volatile, soluble in non-polar solvents, and generally more covalent than ionic in their behavior. 

Benedetti, E Berti, R Fabrissin, S. Gianferrara, T. Intramolecular ring opening of epoxides by bis-activated carbanions. The influence of ring size on reactivity and selectivity. J. Org. Chem. 1994, 59, 1518-1524. 

The use of alkyllithium initiators which contain functional groups provides a versatile method for the preparation of end functionalized polymers and macromonomers. For a living anionic polymerization, each functionalized initiator molecule produces one macromolecule with the functional group from the initiator residue at one chain end and the active carbanionic propagating species at the other chain end. 

The formation of active carbanion species and their addition to electrophiles have also been observed in the cathodic reduction of carbon tetrachloride and trichloroacetic acid ester 34), though yields are not always satisfactory. 

Another methods for the electrochemical formation of the active carbanion is the cathodic reduction of iminium salts. When an iminium salt is reduced in the presence of a suitable alkylating agent, an alkyl group is introduced to the carbon atom of the imine. Some isoquinoline and indole type alkaloids are synthesized by using this substitution method as exemplified bellow by the synthesis of laudanosine 51 37>. 

A polymer with an active carbanionic end will initiate the polymerization of a different monomer if the initial macroion is more nucleophilic than the anion formed from the second monomer in the particular solvent/counterion environment. Thus alpha-methylstyryl anion (9-1) will initiate the polymerization of methyl methacrylate (Fig. 1-4) but the poly(methyl methacrylate) carbanion will not initiate polymerization of alpha-methylstyrene. 

Synthesis of optically active alcohols. For asymmetric synthesis of alcohols, the carbanion salt (2) is generated from optically active (R)-melhyl p-tolyl sulfoxide (1). The optically active carbanion reacts with bcnzaldehyde (3) to give a I I diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxides (4a) and (4b) in 84% yield-These can be separated by silica gel chromatography and fractional crystallizations to give (4a, oq, -t- 91.7°. 17% yield) and (4b, at, + 202.8", 15.5% yield). Raney nickel desulfurization of (4a) and (4b) gives (S)-( )-l-phcnylcthanol (5a, — 42.6 ) and... 

Aldehydes.- The reactions of phosphorus ylides, PO-activated carbanions and iminophosphoranes have been incorporated into CAMEO, an interactive computer program for the mechanistic evaluation of organic reactions.8 The chemistry and synthetic applications of phosphorus and arsenic ylides and PO-stabilised carbanions have been reviewed. ... 

Benedetti and coworkers have examined the intramolecular ring opening of epoxides by bis-activated carbanions, a process exemplifled by the rearrangement of pheny Isulfonyl epoxide 75 in a sodium ethoxide-ethanol medium to the phenylsulfony 1 cyclopentanol 76. This quantitative study on the effect of ring size in such cyclizations revealed similarities to the intramolecular radical addition onto alkenes [94JOC1518]. 

Termination Reactions The categorization of a given polymerization system as living is based on results obtained on the laboratory time scale, that is, the absence of chain termination or chain transfer reactions occurring within the normal time required to complete the polymerization and carry out any subsequent chemical reactions with the active carbanionic polymer chain ends [3, 1, 100]. In fact, the amount of spontaneous termination reactions in typical alkyllithium-initiated polymerizations of styrene and diene monomers depends on time, temperature, and whether polar additives are present [3, 101, 102]. 

The reactive chain ends are long-lived. After monomer is exhausted, polymerization begins anew on addition of more monomer because the active carbanionic centers are still present. The phrase living polymer has been used to convey this special aspect of these anionic polymerizations. They are quenched, however, by water, alcohols, or any other proton donor that is sufficiently acidic to neutralize the carbanion sites. 

The alkaline treatment of PMS with 2-phenylallyl end group also induced a depolymerization in ogranic solvent. When PAMS was mixed with n-BuLi in THF at ambient temperature, almost all of the polymer disappeared as shown in Fig. 9. A low molecular weight oligomeric mixture was observed instead of the starting polymer. This is in sharp contrast to the hydrogen-terminated polymers, which remained almost completely after the reaction with n-BuLi. (11) This result indicates that butyllithium adds to the double bond at the end of the polymer chain to form the active carbanion, followed by the depolymerization reactions. 

The reaction of active carbanionic polymer chain ends with oxygen is a complex reaction which can lead to a variety of products depending on the reaction conditions. This reaction is interesting because it offers the possibility of generating a macromolecular peroxide initiator which can be used to form new block copolymers. Recent studies of the oxidation products of poly(styryl)lithium with molecular oxygen indicate that the oxidation products include the corresponding polystyrene dimer [(PS) ] the dimeric... 

The active carbanion chain ends are contained in the polymer particle and are very accessible to the monomer which remains soluble in the pol niierization medium. 

The active carbanions can be used to form telechelic particles resulting in a variety of novel products. 

This initiator is moderately soluble in hydrocarbon solvent, thermally stable and can produce hydroxyl-terminated polybutadienes with functionalities of two. Additionally, studies on its use for polymerization of butadiene have shown predictable molecular weights and near monodisperse MWD." However, a bimodal MWD has also been reported." Quirk and co-workers have also recently investigated the use of l,3-bis(phenylethenyl)benzene as a living coupling agent (i.e. a coupling agent which retains active carbanionic centres in the coupled... 

Active carbanions are prone to substitute hydrogen of nitroaromatic compounds the substitution usually takes place either ortho or para to the nitro group. The anionic On-adduct generated as a result of nucleophilic addition can be converted into the final product by various mechanisms [167]. Most often exemplified are vicarious nucleophilic substitution (VNS) and oxidative nucleophilic substitution of hydrogen (ONSH). 

Provided inert solvents and pure reactants are used, most monomers under appropriate conditions will give rise to systems in which active carbanion end-groups are always present. The indefinite activity of the growing chains has led to the rather inappropriate term, living polymers for these materials. 

Between different forms of the active carbanion center an equilibrium state is established ... 

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