Evidence for Reversible Ylide Formation: Equilibrium between Free Alkylidenecarbenes and Ethereal Solvent−Alkylidenecarbene Complexes (Oxonium Ylides)

Abstract

Listen

Translate article

The free alkylidenecarbene, 2-butyl-1-hexenylidene 1 (R = Bu), generated by triethylamine-induced α-elimination of 2-butyl-1-hexenyliodonium tetrafluoroborate 7 in tetrahydrofuran undergoes regioselective 1,5-C-H insertions, 1,2-shifts of the butyl group, and electrophilic attack on the tertiary amine followed by protonation to give the cyclopentene 8, the alkyne 9, and the vinylammonium salt 10, respectively. In addition to these free carbene-derived products, the reaction affords the three-component coupling product, the vinyl ether 11, produced through nucleophilic attack of tetrahydrofuran on 1 (R = Bu) generating the oxonium ylide 2 (R = Bu), followed by protonation with subsequent ring-opening of the resulting cyclic oxonium salt 15 (R = Bu) by nucleophilic attack of triethylamine. Reactions were observed to be temperature dependent, as reflected in variations in the product profiles: decreasing the reaction temperature tended to decrease the yields of free alkylidenecarbene-derived products, i.e., cyclopentenes, alkynes, and vinylammonium salts, and to increase those of the vinyloxonium ylide-derived products, vinyl ethers. This temperature dependence is explained in terms of reversible oxonium ylide formation. No evidence was observed to suggest the existence of an equilibrium between the free alkylidenecarbene and the sulfonium ylide in the reaction in tetrahydrothiophene. These results are consistent with quantum calculations using the MOPAC 93 program.