This one-pot reaction, which can be both intermolecular and intramolecular, can display regiochemical preferences, depending on the substituents on the alkene and alkyne substrates. Despite the successes of the intramolecular PKR, the intermolecular PKR suffers from less predictable regioselectivity, especially in new substrates. Here the alkene was kept constant (norbornene), as other alkenes may alter the regiochemical outcome. Compound containing 2-pyrone, were studied for their selectivity, however nitrogen containing atom at a position to the substituent compound favour the β-position exclusively. The compound containing a nitrogen atom at the meta-position to the cyclopentenone substituent shows no selectivity at all. Selectivity towards the β-position was observed for the pyrone containing 2-thiophene. However nitrogen containing in both ring system favoured the 2-pyridine at the β-position exclusively, suggesting that there may be an interaction of 2-pyridine with the cobalt complex. Further evidence is provided by 2-thiophene, where the 2-pyridine controls the regiochemical outcome, even though the α-isomer was observed. The 2-thiophene was observed controlling the regioselectivity over 4-pyridine. This suggested that the position of the nitrogen atom in pyridyl-containing system (with respect to the 2-pyrone) dramatically affects the regiochemical outcome of the PKR. Remarkably, 2-pyridine shows unique behaviour by having a significant effect on the selectivity of both the electron deficient 2-pyrone and electron rich 2-thiophene. Electrocyclisation reactions6 can occur thermally or photochemically, via two possible modes known as conrotatory and disrotatory. The simplest examples of photochemical electrocyclisation are illustrated. Compound undergoes a light-induced C3-regioselective Pi- electrocyclisation/oxidative aromatisation reaction. This was conducted in natural light and gave cyclised compounds in 4 hrs. With more controlled irradiation (focussed LED at 400 nm) led to full conversion in 3 mins (monitored by UV-Vis and NMR).

Cobalt mediated regioselectivity in Pauson-Khand reaction a possible Co-N participation and oxidative 6Pi-electrocyclization / aromatization reactions

ACS Spring Meeting 2021

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