Alkynes dilithiation

Regiospecific alkylation of 1,3-dilithiated acetylenes is a good method for the preparation of branched 1-alkynes dilithiation of 1-butyne, for example, followed by alkylation with RBr, gives HOCCH(R)CH3 in good yields [73]. 

A first structural characterization of a cyclobutadiene dianion was performed by Boche and coworkers, who generated the dilithium salt of the 1,2-diphenylbenzocyclobutadiene dianion (143) (by deprotonation with n-butyllithium in the presence of TMEDA) (Figure 17) . Nevertheless, the molecular structure of 143 resembles more the structures of dilithiated alkenes, synthesized by reaction of the corresponding alkynes with metallic lithium. In that class of compounds, carbon-carbon bonds, capped by two lithium centres, are the structural motif (see Section II. E). 

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 ... 

Regiospedfic Hydroxyalkyladon of a 1,3-Dilithiated Alkyne with Oxirane... 

Tandem chain extension at both the 1-alkyne and propargylic positions of 1-alkynes by sequential (1) alkylation and (2) hydroxy-alkylation (1,2-addition) of 1,3-dilithiated alkynes provides an attractive, one-pot preparation of functionally disubstituted alkynes. The required 1,3-dilithiated species are obtained by lithiation of 1-alkynes at both the 1- and 3- positions with two equivalents of n-BuLi in the presence of TMEDA (tetramethylethylenediamine). The two-stage chain extension depicted below for propyne involves initial alkylation of the dilithiated propyne with the less reactive electrophile, n-BuBr, at the more nucleophilic propargylic position. Subsequent hydroxyalkylation at the less nucleophilic alkynylide position with the more electrophilic formaldehyde furnishes the a-hydroxyalkyne. ... 

In view of the differential reactivities of the 1,3-dilithiated positions of 1-alkynes, it is possible to selectively alkylate the more reactive propargylic position. 

Within the last years, however, it has been found that even nonactivated aliphatic alkenes and alkynes can be forced to react with metallic lithium — catalized or even uncatalized — whereby dilithiated hydrocarbons are formed, although sometimes only as intermediates. 

Dilithiation of 1 - and 2-alkynes with n-butyllithium can be achieved already at room temperature by using diethyl ether as the solvent instead of hexane 3-Alkynes, however, under these conditions yield only monoanions. On the other hand up to four lithium atoms could be introduced into 1,8-cyclotetradecadiyne with n-butyllithium in THF or in the presence of TMEDA even in hexane as the solvent... 

The orthomanganated complex 153 reacts with various alkynes R OCR while under UV irradiation to give products such as 154 by insertion of the alkyne into the Mn—aryl bond. One byproduct from the PhCsCH reaction is the alkynide Mn(CsCPh)(CO)3 P(OPh)3)2. Dilithiation of the thiophene complex Cr(CO)3(Ti-C4H4S) followed by reaction with MnBr(CO)5 results in formation of 155. This gives the insertion product 156 with CO and rearranges at ambient temperature in polar solvents into 157. 03... 

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