Sodium Triethyl-l-propynylborate

Sodium triethylhydroborate reacts with propyne (methylacetylene) with hydrogen evolution even at room temperature.4 In place of pure propyne, a hydrocarbon mixture containing ca. 30% propyne and ca. 30% allene can also be used.4 Only the propyne reacts. 

A 500-ml. three-necked flask is fitted with gas inlet tube, mechanical stirrer, and reflux condenser with a - 78° (hexane-C02) cold 

When using MAPP gas, no cold finger (— 78°) is used in order to avoid retention of the mixed gases. From 244 g. (2 moles) of sodium triethylhydroborate in 300 ml. of benzene and 280 g. of MAPP gas, the foregoing procedure gives 44.3 1. (S.T.P.) of hydrogen and 280 g. (88%) of sodium triethyl-1-propynylborate. In addition, the 500-ml. -78° cold trap contains 165 g. of colorless liquid. [Gas chromatographic analysis 13.4% propene, 33.6% propane, 39.2% allene, 2.5% isobutane, and 11.3% of 1-butene and isobutene.] 

The white crystalline sodium triethyl-1-propynylborate of m.p. 90° is completely stable at room temperature under an inert gas (N2 or Ar). The salt is sparingly soluble in pentane or in hexane, soluble in benzene, and easily soluble in diethyl ether or in tetra-hydrofuran. The compound can be recrystallized from benzene or from cyclohexane. The salt is oxidized by air and is quantitatively cleaved by water with formation of propyne. 

The proton n.m.r. spectrum in dimethyl sulfoxide-d6 solution shows a singlet propynyl CH3 at 51.53, a triplet methyl signal at 50.55 (J = 7.2 Hz.), and a quartet methylene signal at 5 — 0.22 p.p.m. in the appropriate intensity ratio of 1 3 2. 

---