Ethynylmagnesium

A solution of about 1.1 mol of ethynylmagnesium bromide (note 1) was cooled to -50°C and 1.0 mol of trimethylchlorosilane was added in 15 min with cooling between -5 and +5°C. After the addition stirring at 0-5°C was continued for 2 h. The mixture was allowed to stand at room temperature overnight (6 h will probably be sufficient). The mixture was then poured into 1 1 of l N HCl. High-boiling light petroleum (b.p. > 180°C) (250 ml) v/as added and the mixture was shaken... 

Buchanan and co workers71 found that treatment of the 7 3 mixture of the D-altro (68) and D-alio (69) hept-l-ynitol derivatives obtained from 2,3,5-tri-O-benzyl-D-ribofuranose and ethynylmagnesium bromide with 2.2 equivalents of p-toluenesulfonyl chloride at 60° gives the 1-D-ribofuranosylethyne derivatives (70 and 71) in 52 and 13%... 

Cyclization72 of the D-allo-hept-l-ynitol derivative 72, obtained from 2,3-O-isopropylidene-D-ribofuranose and ethynylmagnesium bromide, leads in very high yield to the amorphous a-D-ribosylethyne derivative 73, which gave crystalline 1-a-D-ribofuranosylethyne (74) after acid hydrolysis. 

Iodoacetylene (prepared in situ from ethynylmagnesium bromide or tributyl (ethynyl)tin with iodine) was used as a dipolarophile in the 1,3-dipolar cycloaddition reactions with nitrile oxides to produce 2-(5-iodoisoxazol-3-yl)pyridine and 3-(4-fluorophenyl)-5-iodoisoxazole in good yield (70%-90%). Subsequently,... 

In the direct synthesis of aryl terminal alkynes via Pd-catalyzed cross-coupling of aryl halides with ethynylmetals, formation of diarylethynes is one of the potential side reactions. Indeed, the Kumada coupling of 2-iodo-5-methylthiophene (29) with ethynylmagnesium chloride gave the desired 2-ethynyl-5-methylthiophene (30) in only 35% yield, along with 24% of bis(5-methyl-2-thienyl)ethyne (31) [29], The high propensity for H-Mg exchange reaction to occur was blamed for the diarylethyne formation. 

Terminal allenes.1 A synthesis of 1,2-dienes (3) from an aldehyde or a ketone involves addition of ethynylmagnesium bromide followed by reaction of the adduct with methyl chloroformate. The product, a 3-methoxycarbonyloxy-l-alkyne (2), can be reduced to an allene by transfer hydrogenolysis with ammonium formate catalyzed by a zero-valent palladium complex of 1 and a trialkylphosphine. The choice of solvent is also important. Best results are obtained with THF at 20-30° or with DMF at 70°. 

Buynak et al. reported the synthesis of representative 7-vinylidenecephalosporine derivatives bearing an axial allene chirality (Scheme 4.5) [9]. A chiral allene 24 was prepared stereoselectively utilizing the reaction of an organocopper reagent with propargyl triflate 23, obtained by a diastereoselective ethynylation of the ketone 22 with ethynylmagnesium bromide. Terminally unsubstituted allene 26 was synthesized via bromination of the triflate 23 followed by reduction of the bromide 25 with a zinc-copper couple. 

Jin and Weinreb reported the enantioselective total synthesis of 5,11-methano-morphanthridine Amaryllidaceae alkaloids via ethynylation of a chiral aldehyde followed by allenylsilane cyclization (Scheme 4.6) [10]. Addition of ethynylmagnesium bromide to 27 produced a 2 1 mixture of (S)- and (R)-propargyl alcohols 28. Both of these isomers were separately converted into the desired same acetate 28 by acetylation or Mitsunobu inversion reaction. After the reaction of 28 with a silyl cuprate, the resulting allene 29 was then converted into (-)-coccinine 31 via an allenylsilane cyclization. 

In the same year, Hibino et al. reported a total synthesis of furostifoline (224) employing a new type of electrocyclic reaction (636). This cyclization proceeds through a 2-alkenyl-3-allenylindole intermediate, which is derived from 2-(fur-3-yl)-3-propargyUndole 1128. Compound 1128 was prepared starting from 2-chloroindole-3-carbaldehyde (891), furan-3-boronic acid (1124), and ethynylmagnesium bromide. 

The displacement of the bromine atom in the 2-position of 2,3-dibromotetrahydropyran by ethynyl proceeds with more difficulty than the analogous reaction of ethynylmagnesium bromide with chlorotetrahydropyran (exp. 29). At 20 C a relatively slow conversion takes place. It is not possible to carry out the reaction at a higher temperature, because HOCMgBr begins to disproportionate into BrMgOCMgBr and acetylene above 30 C. 

No outside cooling is employed during the preparation of ethynylmagnesium bromide. If this solution of ethynylmag-nesium bromide is cooled to 0°, a crystalline complex separates. If part of the solvent is evaporated, even at 40° under reduced pressure, there occurs disproportionation to acetylene and the bisbromomagnesium derivative. 

Methyl ethyl ketone, crotonaldehyde, and acrolein react similarly with ethynylmagnesium bromide. The respective yields of acetylenic alcohols are 69%, 84%, and 40%. 

The preparation of ethynylmagnesium bromide in ether has been described 2 3 however, the subsequent history of the compound has been controversial. The submitters have been unable to prepare cthynyl carbinols by the earlier procedures. 

Coupling, of diazotized Ji-aminoaceto-phenone with quinone, 34, 1 of diazotized 3,5-dichloro-2-aminoben-zoic acid to give 4,4, 6,6 -tetra-chlorodiphenic acid, 31, 96 of dibromomalononitrile to tetracyanoethylene, 39, 65 of diphenyldichloromethane to tetra-phenylethylene, 31,104 Creased flask, 37, 55 Creosol, 33,17 Crotonaldehyde, 33, 15 34, 29 diethyl acetal, 32, 5 reaction with ethynylmagnesium bromide, 39, 57... 

---