Olefin Rearrangements Assisted by a Molecular Metal−Oxo Surface: The Chemistry of Calix[4]arene Tungsten(IV)

Abstract

This report deals with the olefin rearrangements achieved on a W-calix[4]arene oxo fragment that mimicks a heterogeneous metal−oxo surface both in the results and the chemical pathways. The olefin complexation by the W(IV)-calix[4]arene fragment has been achieved, generating the [{p-But-calix[4]-(O)4}W]-d2 from the reduction of [cis-(Cl)2W{p-But-calix[4]-(O)4}], 1, in the presence of the appropriate olefin. With this method, [{p-But-calix[4]-(O)4}W(η2-C2H4)], 2, [{p-But-calix[4]-(O)4}W(η2-MeC2H3)], 3, and [{p-But-calix[4]-(O)4}W(C6H10)], 4, have been obtained. In the latter complex the very labile cyclohexene can be replaced by another olefin, such as trans-stilbene in [{p-But-calix[4]-(O)4}W(η2-Ph2C2H2)], 5. The ethylene complex, 2, undergoes deprotonation with LiBu, leading to the corresponding anionic alkylidyne [{p-But-calix[4]-(O)4}W⋮CMe]Li, 6, which can be protonated, not back to the starting olefin complex, but rather to the corresponding alkylidene [{p-But-calix[4]-(O)4}WC(H)Me], 7. Complexes 6 and 7 are interrelated by a reversible protonation−deprotonation reaction. The overall transformation of 2 into 7 is the isomerization of ethylene to alkylidene. The intermediacy of a metallacyclopropene in the deprotonation of 2 to give 6 has been proved by the isolation of [{p-But-calix[4]-(O)4}W(PhCHCPh)]Li, 8, from the deprotonation of 5. Compounds 5 and 8 are interrelated by a reversible protonation−deprotonation reaction, while the reaction of 8 with MeOTf led, rather, to the alkylation of one of the calix[4]arene oxygen atoms in [{p-But-calix[4]-(O)3(OMe)}W(PhCHCPh)], 9. The ethylene, 2, and propylene, 3, complexes undergo an electron-transfer-catalyzed reaction with ethylene and propylene to the corresponding metallacyclopentanes [{p-But-calix[4]-(O)4}W{CH2CH(R‘)CH(R)CH2}], [R = R‘ = H, 11; R = Me, R‘ = H, 12; R = R‘ = Me, 13]. Complexes 2, 3, 11, and 13 undergo a reversible one-electron reduction by Na to [{p-But-calix[4]-(O)4}W(η2-C2H3R)]Na, [R = H, 14; R = Me, 15] and to [{p-But-calix[4]-(O)4}W{CH2CH(R‘)CH(R)CH2}]Na, [R = R‘ = H, 16; R = R‘ = Me, 17]. Complexes 16 and 17 can be reoxidized by Cp2FeBPh4 back to 11 and 13, which do not undergo reversible decoupling of the olefins. The metallacyclopentane complex 11 undergoes deprotonation to [{p-But-calix[4]-(O)4}W{CH(CH2)2CH2}]Li, 18. Both 11 and 18 photochemically rearrange to the corresponding alkylidene [{p-But-calix[4]-(O)4}WC(H)Prn], 19, and alkylidyne [{p-But-calix[4]-(O)4}W⋮CPrn]Li, 20.