Carbanions alkynyl

Carbanions are negatively charged organic species with an even number of electrons and the charge mainly concentrated on a carbon atom. In alkyl, alkenyl, and alkynyl anions all of the... 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

McDowell and Stirling194 studied electronic effects upon the reactivity of aryl vinyl sulfones towards amines. Rate constants for t-butylamine addition in ethanol at 25 °C were well correlated by the Hammett equation, with p = 1.59. Comparison of this with p values for H-D exchange mentioned above191 suggested considerable carbanionic character in the transition state, perhaps in a concerted mechanism. Rates of addition of amines to alkenyl, allenyl and alkynyl p-tolyl sulfones have also been measured195. 

The addition of anionic heteroatom-centered nucleophiles (HO, MeO, pyr-azolate, etc.) and carbanions (CN , enolates, aUcyl or alkynyl reagents) to the cationic allenylidenes [Ru( 7 -C9H7)(=C=C=CR R )(PPh3)2][PF6] [125-128,... 

Boche and coworkers also reported that coupling of 1-alkynyl carbanions with N,N-dialkylamino groups could be achieved in preparing higher-order lithium 1-alkynylcuprates and they used 3h as the amination reagent (Scheme 25). ... 

Wipf has shown that 4,4-disubstituted cyclohexanones undergo nucleophilic attack where the facial selectivity is determined by dipolar control. Thus, compounds of the type 23 underwent nucleophilic attack anti to the electronegative substituent at C(4), whereas the fluorinated analogue, 24, underwent attack syn to the oxygen, in accordance with the inversion of the dipole moment. They found that the logarithm of the experimentally observed facial selectivity for nucleophilic attack was correlated linearly (R = 0.998) with the calculated dipole moments. The facial selectivities were also shown to depend upon the nature of the nucleophile, hydride ions and alkynyl carbanions being essentially unselective. 

Cyclobutanes can be formed by intramolecular addition of carbanions or radicals to C-C double bonds only if the latter are substituted with electron-withdrawing groups (see, e.g., Schemes 9.20 and 9.21) [81] or otherwise activated toward attack by nucleophiles. Activation by an alkynyl group or a cumulated double bond can be sufficient to promote cydobutane formation (Scheme 9.20). Unactivated alkenes, however, do not usually undergo cydization to cyclobutanes via intramolecular addition of carbanions or radicals. 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

P. J. Garratt, Alkylations of Alkynyl Carbanions, in Comprehensive Organic Synthesis (B. M. Trost, I. 

Unlike previous alkyne-aldehyde additions [23], the generation of an alkynyl carbanion is unlikely owing to the large pK, difference between the terminal acetylene and the solvent water [24]. A mechanism was proposed involving the simultaneous activation of the C-H bond of alkyne by the ruthenium catalyst and the aldehyde carbonyl by the indium ion. The ruthenium intermediate then underwent Grignard-type addition followed by an in situ hydrolysis in water to give the desired carbonyl addition product and regenerated the ruthenium and indium catalysts to catalyze further reactions (Fig. 3). 

An interesting sequence based on an intermolecular Michael addition and a subsequent transition metal-catalyzed carbocyclization was recently explored. Much of the development of this strategy relies on recent studies related to the intramolecular carbometalation reaction of stabilized carbanions bearing an unactivated alkynyl group. Several transition metal complexes such as Cu [52], Pd [53], Ti [54], Zn [55], Co [56] and Sn [57] have been reported to catalyze this reaction (Scheme 17). 

Pioneering experiments were performed on allylic alcohols [68] (Scheme 25). It is noteworthy that although several strategies have been successfully employed for the intramolecular cydization of stabilized carbanions bearing an unactivated alkynyl group, only a few examples have been reported on the similar carbocyclization onto a less reactive unactivated double bond [56,69]. 

The alkynyl (or acetylide) carbanions undergo alkylation in a similar manner. 

The alkynyl C-H is sufficiently acidic to permit the formation of synthetically useful alkyne carbanions. 

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