Selective Activation of Alkynes through Cumulene Intermediates

This thesis is focused on attempts to stabilise ruthenium-supported cumulenes (Ru=C={C}n=CR2) which are putative intermediates in the activation of alkynes. The use of different aromatic spacer groups in the cumulene and different co-ligands at Ru is explored along the reaction chemistry of the resulting compounds.
The Markovnikov addition of these nucleophiles is explained by the initial isomerization of the vinylidene to a cumulene intermediate prior to addition to the more remote quaternary carbon. Utilising the half-sandwich fragment [Ru(dppe)Cp*]+, acetylide complexes with large terminal groups were synthesised, Ru(C≡CC6H4-4-C≡CR)(dppe)Cp*. These reacted with small electrophiles at the β carbon to form vinylidene complexes.
To try to stabilise the putative cumulene intermediate complex, the C6H4 spacer group within the organic ligand was changed for C10H6, C6H2(NSN) and C14H8. Synthesis of Ru(C≡CAr-C≡CSiMe3)(dppe)Cp* was achieved through selective lithiation of diethynyl(trimethylsilyl)aryl and addition to [Ru(dppe)Cp*]+. These reacted with smaller electrophiles at the β carbon to form vinylidenes. The bulky trityl CPh3+ reacted at the remote end of the carbon-rich ligand to give the putative cumulene complexes, followed by internal cyclization to form an indene group, [Ru(=C=C (H)-Ar-indene-3-(Ph)2)(dppe)Cp*]+. The reactions of all electrophiles, with Ru(C≡CH-C12H8-10- C≡CSiMe3) (dppe)Cp* gave multiple products that were difficult to purify.
Electrochemical measurements indicate that some complexes exhibited a reversible single electron oxidation. The resulting cations exhibit a high degree of alkynyl character and bands due to MLCT absorptions in all cases.