Propargyl anions

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

As one can notice, the monoUthiated allenic isomer seems to be preferred in almost aU cases in hexane. However, West and Gomowicz reported that the IR spectrum of monolithio-I,3-bis(trimethylsilyl)-3-phenylpropyne exhibits two bands at 2000 and 1850 cm and attributed them to the acetylenic and allenic forms ". In light of the new interpretation, Priester and coworkers ascribed the band at 2000 cm to the propargyl anion, where both phenyl and silicon stabilized the charge at carbon atom 3 . 

The following two-step operation was chosen as an example of the former case. The first step involves an allylation to generate the 1,6-enyne intermediate 1, via the reaction between a metal n -allyl complex derived from 52 and propargyl anion 51, followed by the PK-type reaction to furnish the bicyclopentenone 2 (Scheme 11.13). Since these two specific reactions have opposing electronic requirements (the first prefers Lewis basic character, while the second can be facilitated by Lewis acidic catalysts), finding the right combination of catalysts was the key to success. 

Far greater (Z)-selectivity is observed in reactions of the anion of lb, in which the (Z)/(E) ratio is 20 1. Moreover, the selectivity of this reaction is almost completely reversed by addition of HMPT. The effect of HMPT is exerted on the carbonyl addition step and not on the subsequent elimination to form the double bond. Corey and Rucker1 suggest that lithiated lb reacts mainly as a lithio allene in THF, but mainly as the propargylic anion in THF-HMPT. 

Breslow, R. Grant, J. L. 1977 Electrochemical determination of the basicities of benzyl, allyl, and propargyl anions, and a study of solvent and electrolytic effects. J. Am. chem. Soc. 99, 7745-7746. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

Alkynic intermediates serve as important functional groups in organic synthesis. Many important reactions exploiting the unique and versatile chemistry of the carbon-carbon triple bond have been devised over the last few years. A general strategy for the synthesis of substituted alkynes involves substitution and addition reactions of propargylic anion equivalents this approach is particularly well suited for the preparation of homopropargylic alcohols (Scheme 28). 

For a review concerning the chemistry of propargylic anion equivalents, see R. Epsztein, in Comprehensive Carbanion Chemistiy , ed. E. Buncel and T. Durst, Elsevier, Amsterdam, 1984,part B, p. 107. 

The importance of the propargylic anions in synthetic chemistry enhances the utility of the reaction involving teUnronium salt (d). 

According to Huisgen (1963), a 1,3-dipole is a compound isoelectronic with either propargyl anion, [CH=CH—CH2], or allyl anion, [CH2=CH—CH2] , being linear and bent species, respectively. On the other hand, the dipolarophile is an unsaturated compound. Thus, the above equation can be more explicitly written ... 

The most important of the concerted cycloaddition reactions is the Diels-Alder reaction between a diene and an alkene derivative to form a cyclohexene. The alkene reactant usually has a substiment and is called the dienophile. We discuss this reaction in detail in Section 10.2. Another important type of [2+4] cycloaddition is 1,3-dipolar cycloaddition. These reactions involve heteroatomic systems that have four rr electrons and are electronically analogous to the allyl or propargyl anions. 

Propargylic anions, although not yet frequently used, seem to be very promising candidates for new ring-closure reactions. This is illustrated by the reaction shown in Eq. (11) when l,9-bis(bromomagnesium)-2,7-nonadiyne (41) is cyclized with Me2SiCl2 to afford the silacyclodecadiyne 42 [20]. 

Alkylations and silylations. Moderately stabilized allylic and propargylic anions are readily generated by treating substituted alkenes and alkynes with BuLi. l-(Benzotriazol-l-yl)propargyl ethyl ethers are a convenient source of alkynyl ketones by virtue of their facile alkylation. 

Alkylations. -Lithiocarbonyl synthons and propargyl anions are generated from the bromides by reacting with BuTeLi and then BuLi. (Z)-l,3-Butadien-l-yllithium is available from P-(organotelluro)acroleins. 

Fig. 11.1. sp hybridized (linear dipoles like the propargyl anion). 

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