2- Propenyl anion

The 1-propenyl anions react with N20 essentially via two channels to generate the methylacetylide and (M — H) ions of diazomethane, as shown in (69) and (70). It must be noted that vinyldiazomethane anions are also generated from the OH /diazoethane complex in (69) by proton abstraction from the methyl group. 

Stereoelectronic features of vinyl anions are associated with their bent geometries and usual configurational stabilities in solution. Theory predicts a sizable inversion barrier for the parent vinyl anion (28.7kcal/ mol at the MP2/6-31+G(d)//HF/6-31 -nG(d) level). The two conformers of 1-propenyl anion can be formed stereospecifically in the gas phase (via desilylation of E- and Z-l-trimethylsilylpropene) and show different reactivity patterns. ... 

Table 1. 2-Propenylmetal Reagents for the Stereoselective Introduction of the 2-Propenyl Anion to Carbonyl Compounds...

Exercise 21-6 Use Figure 21-9 to predict the 7r-electron distribution in the 2-propenyl radical (CH2=CH—CH2-) and the 2-propenyl anion (CH2=CH—CH2 e). Show your reasoning. 

The propenyl anion. An analogous calculation (cf. Fig. 4.16, with four electrons for the anion) gives... 

The 2-propenyl anions, however, react with N20 to yield the enolate ions of acetone and the (M — H) ions of diazoethane in equal abundances, for which the routes as visualized in (71a) and (71b) have been proposed. 

The photochemical behaviour of 2-aryl-l,3-diphenyl propenyl anions has been reported. " ... 

Similarly, acetate ion is protonated at either oxygen to form acetic acid (see margin on p. 19). Analogously, the 2-propenyl anion is protonated at either terminus to furnish propene, and the corresponding cation reacts with hydroxide at either end to give the corresponding alcohol (see below). 

Thus, propene is considerably more acidic than propane (p T 50), and the formation of the propenyl anion by deprotonation appears to be unusually favored. 

The synthesis of spiro compounds from ketones and methoxyethynyl propenyl ketone exemplifies some regioselectivities of the Michael addition. The electrophilic triple bond is attacked first, next comes the 1-propenyl group. The conjugated keto group is usually least reactive. The ethynyl starting material has been obtained from the addition of the methoxyethynyl anion to the carbonyl group of crotonaldehyde (G. Stork, 1962 B, 1964A). 

These reagents are not isolated but are used directly in reactions with aldehydes, after generation of ate complexes via the addition of an alkyllithium reagent or pyridine11. 2-(2-Propenyl)-1,3,2-dioxaborolane is also metalated upon treatment with lithium tetramethylpiperidide, but mixtures of a- and y-substitution products are obtained upon treatment of this anion with alkylating agents20. Consequently, this route to a-substituted allylboron compounds appears to be rather limited in scope. 

The addition of the anion of the racemic 2-methyl-2-propenyl sulfoxides, rac-2-methyl-3-(phenyl-sulfinylpl-propene and /w-3-(rerr-butylsulfinyl)-2-methyl-l-propene to 2-cyclopentenone gives mixtures of (E)- and (Z )-y-l, 4-addition products which are a mixture of diastereomers at sulfur2. The (T )-products usually predominate, with the relative proportions of the (Z)-product increasing as the reaction temperature is increased. No asymmetric induction originating from the stereocenter at sulfur was observed when the sulfoxide substituent was phenyl however, there was a marginal improvement in the case of the (Zi)-product when the sulfoxide substituent was ferf-butyl. 

In contrast, the anion of p-tolyl (2-methyl)-2-propenyl sulphoxide 408 reacts with benzaldehyde exclusively at the y-position426 (equation 241). 

During the study of the isomerization of diallyl ether with catalysts of this type, it was observed that the nature of the complex anion (Y) has a profound influence on the relative rate of isomerization with respect to cyclization. Diallyl ether can be isomerized to a mixture consisting mainly of allyl propenyl ethers (44) with a small amount of 2-methyl-3-methylene-tetrahydrofuran (45) and dipropenyl ether (46) the catalyst for this... 

Lithiation of 2-propenyl-l,3-dithiane generates an allylic anion which reacts with active ketones at the side-chain carbon. The reaction was highly diastereoselective (96 4) and the product was obtained in 85% yield (Equation 58) <1996JOC1473>. 

It is strong evidence for Huckel s rule that 52 and 53 are not aromatic while the cyclo-propenyl cation (47) and the cyclopentadienyl anion (31) are, since simple resonance theory predicts no difference between 52 and 47 or 53 and 31 (the same number of equivalent canonical forms can be drawn for 52 as for 47 and for 53 as for 31). 

The SHM predicts the propenyl cation, radical and anion to have the same resonance energy (stabilization energy). Actually, we expect the resonance energy to decrease as we add ji electrons why should this be the case ... 

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