Dianions propargylic

Propargyl dianion (QF I ). This anion can be prepared by dilithiation of allene with BuLi in 1 1 ether/hexane. Use of THF (- 50°) or BuLi/TMEDA results in a mixture of propargylide and allenyl anions. The anion couples readily with alkyl and allyl halides to give terminal alkynes. The intermediate lithium acetylide can also react with various electrophiles.3 Example ... 

The present procedure, based on the controlled lithiation of aliens, produces the operational equivalent of a propargyl dianion 1 (C3H2L12), and provides a convenient single-step route to propargylated derivatives. Lithiation of allene is... 

Doubly (and even triply) charged anions appeared on the horizon of organic chemistry relatively recently, They were immediately recognized as extremely useful intermediates due to the synthetic opportunities offered by their unique pattern of reactivity. Besides the dianion 174, the dianions of carboxylic acids 178 (Scheme 2.82), propargylic dianion 179, and the dianion 180 derived from propargyl alcohol are now widely used as nucleophiles. Selectivity in electrophilic attack for these species is governed in much the same way as was described for 174 the least-stabilized anionic center is the preferential site of attack by the electrophile (marked with an asterisk in Scheme 2.82). 

Wittig rearrangement of a-allyloxycarboxylic acid dianions and allyl propargylic dianions (Section 3.11.3.3) might be expected to proceed analogously. In fact, the same high preference for ( )-pro-ducts is observed, but the diastereoselectivity is reversed (Table 17, entries 5 versus 6). A chelated bicyclo [3.3.0] transition state readily explains the anti selectivity of (Z)-allylic ethers [Scheme 12, compare (R) with (T)]. The basis for syn selectivity observed with ( )-allylic ethers (Q) versus (S) is less clear. 

Scheme 4 Synthesis of Group 6 allenylidenes from dianions of propargylic alcohols...

The silylation of propargyl alcohol dianion described here is a further... 

First, 7 is converted into its dianion 23 by two equivalents of n-butyllithium. Not only the terminal hydrogen of alkyne 7 but also the propargylic one is acidic. 23 then undergoes intramolecular nucleophilic attack of the terminal chlorine. NH4C1 work-up yields the desired cyclopropyl acetylene 25.8... 

By the action of indium, various allylic or propargylic dihalo compounds can be converted to the respective dianion equivalents. In some cases, the reaction with electrophiles proceeds stepwise, whereas in other cases novel diindium reagents are involved. Via an indium-mediated Barbier-type reaction in an aqueous medium, carbonyl compounds react with a trimethylenemethane dianion equivalent to give the corresponding diols (Equation (39)).218... 

Dilithiodibenzylacetytenebis(tmeda) (49) (68) is monomeric. Each lithium atom is bonded to a benzylic carbon and has further interactions with a phenyl C(l) and the acetylenic v system. The structure can be understood as being derived from a perpendicular 2-butyne-l,4-diyl dianion, "CH2—C=C—CH2, in which each negative charge is stabilized in its own orthogonal tt system. The lithiums bridge each of these propargyl (or allenyl) anion systems in a 1,3 manner. 

Formation of the dianion of ethyl 3-oxobutanoate 5 with two equivalents of lithium diisopropylamide (LDA) in tetrahydrofuran followed by alkylation with allyl or propargyl bromide provided jS-keto esters 6a,b in 62% and 75% yield, respectively. Condensation of these esters with methyl or phenyl hydrazine in refluxing ethanol yielded the corresponding pyrazol-3-ones 7a,b in excellent yield (99TL3535) (Scheme 2). 

A plausible mechanism for the formation of the product is shown in Scheme 21.26. The base LiNH2 picks up the hydrogen a to the chlorine atom, followed by the elimination of a P-alkoxy group to produce a vinyl chloride (3). Further reaction of 3 with LiNHj results in the formulation of a dianion (2a). This can be trapped with various electrophiles to yield alkylated propargyl alcohols (4). The yields of the products were consistently over 70%. 

The mono- and dianions of [ C2]acetylene, readily accessible by deprotonation with n-BuLi (1 equivalent) and MeLi (2.5 equivalents), respectively, can be trapped with monomeric formaldehyde to give [2,3- C2]propargyl alcohol in 40% yield and 2-[2,3- C2]butyne-l,4-diol in 72% yield. Both compounds may serve as valuable intermediates for the preparation of additional low-molecular weight building blocks and intermediates. [2,3- C2]propargyl alcohol, for example, can be reduced with aqueous CrCl2 to [2,3- C2]allyl alcohol, which was shown to be activated through tosylate formation toward nucleophilic displacement of the hydroxy function to produce radio-... 

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