Allenyl Carbanions

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. 

Only a few examples exist describing the products from the allenylic/propargylic carbanion resulting from the deprotonation of 18 and reaction with other electrophiles instead of protons which lead to products analogous to 19 [48]. Thus, treating the propargyl compound 21 with tetrabutylammonium fluoride (TBAF) in the presence of benzaldehyde furnishes the C,C-connected compound 22 [41]. 

So far, the formation of cyclic alkoxyallenes bearing an exocydic or endocyclic allenyl unit has been less developed. Several examples with this structural feature are described as unstable compounds or highly reactive intermediates [32-35]. However, in the 1990s, Lavoisier-Gallo and Rodriguez demonstrated a useful one-pot protocol for the synthesis of 2-vinylidenedihydrofurans such as 29 involving a tandem C-O cycloalkylation of stabilized carbanion intermediates 28 as crucial step (Scheme 8.10) [36, 37]. 

Denmark and Harmata studied the reaction of 1,2-allenyl sulfones and 2-prope-nols under the catalysis of 5 mol% of sodium alkoxide affording 2-alloxy-2-propenyl sulfone 217, which can be converted to 2-oxo-5-alkenyl sulfone 218 by treatment with 1.5equiv. of KH in HMPA, a carbanion-accelerated Claisen rearrangement [116, 117]. 

Generation of a-sulfinyl carbanions can also occur in 1 -alkenyl sulfoxides and can be easily achieved by using such bases as lithium diisopropylamide30 32, /erf-butyllithium33 and butyl-lithium (for allenyl sulfoxides)34. 

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -allenyl/propargyl platinum complex to yield the neutral substituted-jj -TMM derivatives that undergo further [3 + 2] cycloaddition with good TT-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

Although complex 43 does not react with phenylacetylene and methane, the products resulting from the formal addition of H-C bonds of these molecules to the C -Cp and Cp-C. bonds of the allenylidene ligand of 43 can be easily obtained [23]. Because in the allenylidene ligand of 43, the C and atoms are electrophilic centers, and the Cp atom is nucleophilic, the synthetic strategy involves the initial nucleophilic attack of the corresponding carbanion at the C or atoms and the subsequent protonation of the resulting allenyl or alkynyl derivatives. 

The addition of a carbanion to an allenyl complex leads to a trimethylenemethane derivative that has been isolated, and its structure determined by X-ray crystallography (Equation (51)). ... 

The fonnation of N-ethynyl derivatives testifies that under these conditions, the allenyl carbanion nndergoes prototropic isomerization to N-propargylic (C pj-centered) carbanion. 

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