The reaction of 1 with a series of thiophene derivatives was investigated using catalytic conditions previously established for the C-H alumination of arenes and heteroarenes. The experiments were conducted under static vacuum as extrusion of dihydrogen gas has been previously seen to help drive the equilibrium towards the products. Upon mixing the reactants, rapid evolution of gas was observed with all substrates. Immediate reactions at 25 °C were confirmed by monitoring reactions by 1 H NMR spectroscopy. After heating at 100 ºC for 2 h, a near quantitative in situ yield of 2a-d was obtained (Scheme 1). In all cases, metalation occurs at the 2-position of the Single crystals for X-ray diffraction analysis could be successfully grown for all C-H aluminated thiophenes except for 2d despite repeated attempts (Figure ). Vapour diffusion of hexamethyldisiloxane (HMDSO) into a toluene solution of 2b-c proved highly efficient and excellent quality crystals could be grown over 2-3 days. The crystal structures supported the regiochemistry predicted by the DFT calculations and assigned by NMR spectroscopic analysis. As expected, the new complexes closely resembled the crystal structures of previously reported C2-aluminated furans. The structures sit across a mirror plane that bisects the Al atoms and central C atom of the -biketiminate ligand. The C3N2Al rings in 2a-b are nearly planar. Only for 2c is there evidence of slight steric clash between the heterocycle and mesityl ligands in the solid state. Based on the calculated barriers for the ring-expansion step, higher reaction temperatures were explored. Heating of samples of 2c in C6D6 at 120 ºC with 5 mol % [Pd(PCy3)2] for 100 h led to slow consumption of the starting material, but no evidence for ring-expansion (Scheme 2, top). The reaction of 1 with benzothiophene in a flame-sealed medium-walled NMR tube at 200 ºC was performed, cumene was used as a solvent. After 1 h, formation of a single major product was observed by 1 H NMR spectroscopy. Following extensive purification, 3 was isolated in >95% purity and 29% yield (Scheme 2, bottom). The structure of 3 was confirmed by X-ray diffraction (Figure ). Although the formation of 3 occurs alongside minor unidentified side-products, again there is no evidence for C-S Insertion and ring-expansion of 2d. Monitoring reaction of 1 of benzothiophene at 100 ºC for long reaction times showed that 2d slowly converts to 3 at this temperature. While the reaction has not been conducted in the absence of the Pd-catalyst, we suggest that 3 forms from a ligand exchange process that scrambles the hydride and heteroaryl substituents i.e. 2 equiv. 2c → 3 + 1. This distortion raises the clear question as to whether strain plays a role in the different reactivity of 2methylfuran and 2-methylthiophene. Activation-strain analysis (ASA) of TS-3 was carried out by breaking the C 1 -Pd bond and decomposing the geometries into two parts (sub and cat, Figure ).