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

This report deals with the olefin rearrangements achieved on a W-calix[il]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 [{n-Bu-t-calix[4]-(O)(4)}W]d(2) from the reduction of [cis-(Cl)(2)W{p-Bu-t-calix[4]-(O)(4)}], 1, in the presence of the appropriate olefin. With this method, [{p-Bu-t-calix[4]-(O)(4)}W(eta(2)-C2H4)], 2, [{p-Bu-t -calix[4]-(O)(4)}W(eta(2)-MeC2H3)], 3, and [{p-Bu-t-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-Bu-t-calix[4]-(O)(4)}W(eta(2)-Ph2C2H2)], 5 The ethylene complex, 2, undergoes deprotonation with LiBu, leading to the corresponding anionic alkylidyne [{p-Bu-t-calix[4]-(O)(4)}W equivalent to CMe]Li, 6, which can be protonated, not back to the starting olefin complex, but rather to the corresponding alkylidene [{p-Bu-t-calix[4]-(O)(4)}W=C(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-Bu-t-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-Bu-t-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-Bu-t-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-Bu-t-calix[4]-(O)(4)}W(eta(2)-C2H3R)]Na, [R = H, 14; R = Me, 15] and to [{p-Bu-t-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-Bu-t-calix[4]-(O)(4)}W{CH(CH2)(2)CH2}]Li, 18. Both 11 and 18 photochemically rearrange to the corresponding alkylidene [{p-Bu-t-calix[4]-(O)(4)}W=C(H)Pr-n], 19, and alkylidyne [{p-Bu-t-calix[4]-(O)(4)}W equivalent to CPrn]Li, 20.