EVIDENCE FOR REVERSIBLE YLIDE FORMATION - EQUILIBRIUM BETWEEN FREE ALKYLIDENECARBENES AND ETHEREAL SOLVENT-ALKYLIDENECARBENE COMPLEXES (OXONIUM YLIDES)

The free alkylidenecarbene, 2-butyl-1-hexenylidene 1 (R = Bu), generat ed by triethylamine-induced alpha-elimination of 2-butyl-1-hexenyliodo nium tetrafluoroborate 7 in tetrahydrofuran undergoes regioselective 1 ,5-C-H insertions, 1,2-shifts of the butyl group, and electrophilic at tack on the tertiary amine followed by protonation to give the cyclope ntene 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 th rough 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 nucle ophilic attack of triethylamine. Reactions were observed to be tempera ture dependent, as reflected in variations in the product profiles: de creasing the reaction temperature tended to decrease the yields of fre e alkylidenecarbene-derived products, i.e., cyclopentenes, alkynes, an d vinylammonium salts, and to increase those of the vinyloxonium ylide -derived products, vinyl ethers. This temperature dependence is explai ned in terms of reversible oxonium ylide formation. No evidence was ob served to suggest the existence of an equilibrium between the free alk ylidenecarbene and the sulfonium ylide in the reaction in tetrahydroth iophene. These results are consistent with quantum calculations using the MOPAC 93 program.