Ionic additions to unsaturated systems. Part 2. Triethylammonium hydrogendichloride as reagent in the addition of hydrogen chloride to alkynes

Addition of hydrogen chloride to the carbon–carbon triple bond may easily be performed by means of triethyl-ammonium hydrogendichloride (Et3NH+HCl2–): an equimolar mixture of this reagent with an alkyne, either neat or dissolved in an aprotic medium, affords the 1 : 1 addition product in high yield, without a catalyst. This method has been applied to dimethyl but-2-ynedioate, 3-chloropropyne, phenylacetylene, 1-phenylpropyne, and 3,3-dimethyl-1-phenylbut-1-yne. The reaction proceeds essentially through anti-addition of hydrogen chloride; predominant but non-exclusive syn-addition is only observed with 3,3-dimethyl-1-phenylbut-1-yne. The results are discussed in terms of the ambivalent character of the HCl2– ion, which acts both as a precursor of hydrogen chloride and as a powerful nucleophilic reagent, and prevents to some extent the formation of vinyl cation intermediates that normally are easily generated by protonation of alkylphenylacetylenes; the results also provide new evidence of the importance of the nucleophilic character of the species involved in the C–Cl bond formation in controlling the stereochemical course of the electrophilic addition of hydrogen chloride to alkynes.