Cyclometallation at Pd, Mn and Co: functionalisation of C-H and C≡C bonds involving cyclisation, rearrangement and isomerisation processes

This thesis describes cyclometallation at Pd, Mn and Co: functionalisation of C H and C≡C bonds involving cyclisation, rearrangement and isomerisation processes. The use of NaNO2/NaNO3 as an oxidant in oxidative Pd-catalyzed processes has recently been reported as a complementary co-catalyst to other common oxidants (e.g. CuII/AgI salts). In view of this (Chapter two) the synthesis of a series of palladacyclic complexes containing a C^N ligand backbone, and the geometry and linkage isomerism at NO2 Pd, has been studied. The geometry about the Pd(II) centre shows the crucial role played by bulky ligands in creating hindrance and affecting phosphine dissociation. Mn-catalysed C-H bond activation is a powerful strategy for the functionalisation of organic compounds containing metal-directing groups. Chapter three of this thesis reports the characterisation of a highly reactive 7-membered MnI species which acts as anvil point between protonation and reductive elimination to deliver alkenylated and/or pyridinium products respectively. Both processes are exemplified through the reactions of a substrate containing a 2-pyridyl directing group and electron-deficient 2-pyrone motif at MnI, where C-H bond activation occurs regioselectively at C3 within the 2-pyrone. An unprecedented regioselective Diels-Alder reaction also occurs on both the pyridine and 2-pyrone ring systems. These findings provide a unique insight into MnI-mediated C-H bond activation processes, especially how relatively minor changes in substrate structure influence the product distribution. The study shows that MnI-based metallocycles warrant further study more generally in organic and organometallic chemistry The intermolecular Pauson-Khand (PK) reactions of sterically comparable (2-pyridylethynyl)-heteroaromatic compounds with norbornene, mediated by Co2(CO)8 to give cyclopentenone products, were examined in Chapter four of this thesis. The π-deficient heteroaromatic substrate, 2-pyrone, favoured the -position while the π-rich heteroaromatics such as 2-thiophene favour the β-position. The position of the nitrogen in pyridyl-containing alkyne substrates also affects the regiochemical outcome of the PK reaction. A 2-pyridyl alkyne, possessing a proximal nitrogen atom, influences the regioselectivity relative to a 4-pyridyl variant, quite dramatically, favouring the β-position in the newly formed cyclopentenone ring. Overall, the type of heteroaromatic group greatly influences PK regioselectivity. The PK cycloadducts undergo a 6π-electrocyclisation–oxidative aromatisation reaction in the presence of light, which is promoted by a LED UV-light controlled system, affording benzo[h]indeno[1,2-f]isochromene type products.