Genesis, Redox, and Acid-Base Relationships among W-C, W=C, and WC Functionalities over an Oxo Surface Modeled by Calix[4]arene

This report deals with the chemistry of anionic W-alkylidynes generated over the oxo surface defined by a calix[4]arene tetraanion. The exhaustive alkylation of [cis-(Cl)(2)W{p-Bu-t-calix[4]-(O)(4)}] (1), with an excess of alkylating agent led to [{p-Bu-t-calix[4]-(O)(4)}W=C-R][M] (R = Ph, M = 1/2 Mg, 2; R = Pr-n, M = Li, 3; R = SiMe3, M = Li, 4). The protonation [PyHCl] of 2 and 3 led to the corresponding alkylidenes [{p-Bu-t-calix[4]-(O)(4)}W=C(H)R] (R = Ph, 7; R = Pr-n, 8), which were reversibly deprotonated back to the starting akylidynes using LiBu. The reaction of 2-4 with AgNO3 led to dimetallic alkylidenes [{p-Bu-t-calix-[4]-(O)(4)}W=C(R)Ag)] (R = Ph, 12; R = Pr-n, 13; R = SiMe3, 14). The alkylation of 3 and 4 with MeOTf occurred at the carbon in 4 and at both carbon and oxygen in 3 leading to [{p-Bu-t-calix[4]-(O)(4)}W=C(Me)-SiMe3] (9) and to a mixture of [{p-Bu-t-calix[4]-(O)(4)}W=C(Me)Pr-n], (10) and [{p-Bu-t-calix[4]-(O)(3)-(OMe)}W=C-Pr-n] (11), respectively. The functionalization of the anionic alkylidynes was achieved by reacting 2 with electrophiles such as PhCHO leading to [{{p-Bu-t-calix[4]-(O)(4)}W=C(Ph)-C(H)(Ph)-O}(2)Mg(thf)] (15) and with Ph2C=C=O forming [{{p-Bu-t-calix[4]-(O)(4)}W=C(Ph)-C(=CPh2)-O}(2)Mg] (16). An indirect, but synthetically quite versatile, functionalization is the oxidation of 2 with I-2 producing [{p-Bu-t-calix[4]-(O)(4)}-W=C(Ph)-I] (20). The alkylidenes 7 and 8 were unexpectedly unreactive in the presence of olefins and aldehydes, e.g., PhCHO, which formed a reversible adduct binding inside the calix[4]arene cavity. However, the trans labilization of the alkylidene functionality was achieved by reacting 7 with (BuNC)-N-t, a reaction leading to cis-stilbene and the W=W dimer [({mu-p-Bu-t-calix[4] -(O)(4)}(2)W-2(CNBut)(2)] (18) The one-electron oxidation of 2 by Cp2FeBPh4 followed two different pathways via the common free-radical intermediate [{p-Bu-t-calix-[4] -(O)(4)}W=C.(Ph)], leading to either 7 (hydrogen abstraction) or [{p-Bu-t-calix[4]-(O)(4)}(2)W-2(mu(2)-eta(2):eta(2)-Ph2C2)], (19). The nature of the anionic alkylidynes, the reaction pathways with electrophiles, and the one-electron oxidation reactions were analyzed using extended Huckel calculations.