[1070-75-3] · C2Li2 · Dilithium Acetylide · (MW 37.90)
(as dianion and as bistrimethylsilyl derivative finds use in the synthesis of variously substituted alkynes,7 alkenes,5 g-diketones,4 etc.; preferred over its sodium counterpart in reactions with enolizable carbonyls)
Alternate Name: dilithioacetylene.
Physical Data: white crystalline solid; d 1.65 g cm-3.
Solubility: insol; used as suspension in THF, Et2O, liq NH3, or a hydrocarbon.
Form Supplied in: freshly prepared sample is recommended for good results, although commercially available.
Preparative Methods: from Acetylene:2 THF (250 mL) is added to n-Butyllithium (2.0 mol) in ether (1250 mL) maintained at -20 °C and dry acetylene gas bubbled through the solution for 2-3 h to obtain a copious white precipitate. The resulting mixture is boiled under reflux for 12 h to ensure disproportionation of any Lithium Acetylide.
From Trichloroethylene:3 to 100 mL of 1:1 mixture of anhydrous ether and THF is added 0.6 mol BuLi, cooled to -78 °C, and 0.2 mol Trichloroethylene in 50 mL of ether is added dropwise over 20 min. The mixture is stirred for 2 h at rt, by which time the flask is full of white solid.
Trichloroethylene as a source is claimed to yield superior results in obtaining dilithioacetylene, as shown by silylating the dianion.
Handling, Storage, and Precautions: highly moisture sensitive; very short exposure to air under good ventilation allowed. Use of freshly prepared solution advised. Use in a fume hood.
This is a convenient reagent for preparing synthetically useful g-diketones not generally accessible by other methods. Dilithium acetylide adds to two moles of aldehyde to give alkynic glycols, which upon catalytic reduction followed by oxidation using Sarett's procedure give 1,4-diketones in moderate to good yields (eq 1).
This approach to g-diketones is superior to the complementary bis-dithiane route, especially with secondary and tertiary aldehydes.
Dilithioacetylene with trialkylboranes gives dilithium ethynylbis(trialkylborates), which on treatment with Iodine followed by hydrolysis or oxidation give alkenes or ketones. Alkenes thus obtained are a mixture of (E) and (Z) isomers. However, an alternate methodology using equimolar amounts of Cyanogen Bromide and Sodium Methoxide to treat the borate offers a convenient route to obtain (E)-alkenes exclusively. If 3 equivs of BrCN to 1 equiv of NaOMe are used, tri- and tetrasubstituted alkenes are formed (eq 2).
The yield for the disubstituted alkene drops considerably if the R group is not primary.
Therapeutically useful aryl propargylamines are obtainable in virtually quantitative yield by the reaction of dilithium acetylide with N-(methoxymethyl)aniline (eq 3). The product propargyl aniline can be further transformed by the same approach to symmetric and unsymmetric aryl diamines. This method is superior to the traditional N-alkylation method or the Mannich reaction in terms of yields and simplicity.
Although alkynide anions are excellent nucleophiles and readily undergo acylation and alkylation with appropriate electrophiles, reactions with tertiary alkyl halides are not synthetically useful because of the competing elimination reaction. However, tertiary alkylation of alkynic carbon is possible via bis-silylalkynes under Lewis acid catalysis in high yields (eq 4).
Bis(1-adamantyl)acetylene is also prepared by this route. This reaction fails with primary or secondary alkyl halides.
Bis(trimethylsilyl)acetylene finds use, inter alia, as a dienophile,8 and in making iodoethynyltrimethylsilane9 which, with arylcopper reagents, is used to make arylalkynes.10
A. V. Rama Rao
Indian Institute of Chemical Technology, Hyderabad, India