Alkene and Alkyne Reactivity over a Metal-Oxo Surface Modeled by Calix[4]arene-Tungsten(IV): Formation of 1-Metallacyclopropene and Alkylidene Complexes

This report deals with the deprotonation of [W(eta (2)-alkene)] complexes to the corresponding 1-metallacyclopropene, which can be equally well obtained via hydride addition to the corresponding [W(eta (2)-alkyne)]. The ancillary ligand supporting the metal is [p-Bu-t-calix[4]-(O)(4)],(4-) which mimics an oxo surface. The [W(eta (2)-2-methyl-2-butene)] complex 2 undergoes deprotonation to give the corresponding anionic 1-metallacyclopropene species 7. The latter can be protonated back to 2, alkylated using MeTf to give the corresponding [W(eta (2)-2,3-dimethyl-2-butene)] complex 3 or metalated at the alkylidene functionality using ClSnPh3 to give 8. The one-electron oxidation of 7 led to the homolytic cleavage of the W-C bond within the 1-metallacyclopropene unit, thus forming a free radical dimerizing to the dinuclear W-alkylidene 9. The [W(eta (2)-alkyne)] complexes 12 and 14 pro duce the 1-metallacyclopropenes 10 and 16, respectively, in the reaction with LiHBEt3. The former undergoes reversible protonation - deprotonation to the corresponding [W(eta (2)-trans-stilbene)] derivative 4, while alkylation using MeTf occurs at one of the oxygens of the calixarene tetraanion, thus forming the neutral metallacyclopropene complex 11. In the case of a strained alkene, such as acenaphthylene (complex 5), the deprotonation occurs with the concomitant, homolytic cleavage of a W-C bond and the formation of a free radical alkylidene dimerizing to the ditungsten(V)-dialkylidene species 17, which has been oxidized using [Cp2Fe](+) to the corresponding diamagnetic form 18. The electronic structure and reactivity pathway of the 1-metallacyclopropene functionality has been analyzed using the DFT (density functional theory) approach.