γ-Substituted Vinylidene, Chroman-2-ylidene, and Hexahydrochromen-2-ylidene from Ruthenium Allenylidene/Alkenylcarbyne Complexes

The activation of 1-phenyl-2-propyn-1-ol, 1-(4-methoxyphenyl)-2-propyn-1-ol, and 1-(4-fluorophenyl)prop-2-yn-1-ol by [Cp*RuCl(dippe)] (dippe = 1,2-bis(diisopropylphosphine)ethane) provides a series of
allenylidene derivatives, [Cp*Ru(dippe)(CCCHAr)][BF4] (Ar = C6H5 (1), p-C6H4(OMe) (2), p-C6H4(F) (3)), and the corresponding alkenylcarbyne complexes [Cp*Ru(dippe)(⋮C−CHCHAr)][BF4]2 (Ar
= C6H5 (4), p-C6H4(OMe) (5), p-C6H4(F) (6)) by allenylidene protonation. The dicationic alkenylcarbyne
complexes behave as a carbocationic species able to cause the aromatic electrophilic substitution of 1,3-dimethoxybenzene, furnishing γ-substituted vinylidene complexes [Cp*Ru{CCHCHAr(C6H3(OMe)2)}(dippe)][BF4] (Ar = C6H5 (7), p-C6H4(OMe) (8), p-C6H4(F) (9)). A series of bicyclic carbene complexes
[Cp*Ru(L)(dippe)][BF4] (L = 7-hydroxy-4-phenylchroman-2-ylidene (10), 4-(4-methoxyphenyl)-7-hydroxychroman-2-ylidene (11), 4-(4-fluorophenyl)-7-hydroxychroman-2-ylidene (12)) are obtained by
the direct reaction of resorcinol with alkenylcarbyne complexes. The X-ray structure of 12 shows the
formation of a chroman-2-ylidene ligand. The reaction of allenylidene complexes requires the presence
of a weak acid (NH4BF4) to perform the electrophilic aromatic substitution (step 1) and a strong acid
(HBF4) to induce the intramolecular cyclization (step 2). The γ-substituted vinylidene [Cp*Ru{CCHCHPh(C6H3(OH)2)}(dippe)][BF4] (13) has been isolated as the intermediate between steps 1 and 2.
Similar bicyclic carbon skeletons (hexahydrochromen-2-ylidene, complexes 14b−16b) are obtained by
reaction of the allenylidene complexes with cyclohexanedione. In this case, the carbene complexes are
in equilibrium with the isomeric γ-substituted vinylidenes 14a−16a. The effect of the presence of electron-donor and electron-withdrawing groups on the allenylidene/alkenylcarbyne substituents is analyzed.