DFT Studies: A computational study on the C-H alumination of 2-methylthiophene with 1 was initiated (Figure ). A closely related pathway involving the addition of 1 to 2-methylfuran has been reported by our group and was also recalculated to ensure consistent computational methods and allow comparison. For both reactions, the active catalytic species is proposed to be the 14-electron species CAT formed during the reaction 1 and [Pd(PCy3)2]. We have previously shown that 1 can undergo dihydrogen loss in the presence of Pd, and structurally characterised a closely related species from reaction of a low-valent analogue of 1 with [Pd(PCy3)2]. Aldridge and coworkers have demonstrated complete dehydrogenation of the gallium analogue of 1 on transition metals. The reaction of 2-methylthiophene starts with the formation of a weakly bound π-complex Int-1 and then proceeds through the turnover-limiting C-H bond activation via TS-1 (Figure ). The resulting intermediate Int-2 then undergoes a cis/trans isomerisation to form Int-3. Subsequently, doublemigration of the hydride and heteroaryl ligands occurs via TS-2 to give Int-4, a product with the C-H aluminated heterocycle 2 bound to Pd centre. The overall reaction of 1 + 2-methylthiophene → 2b + H2 is slightly exergonic by Gº298K = 0.5 kcal mol -1 , and the overall free energy barrier of CAT with 2methylthiophene is G ‡ 298K = 21.7 kcal mol -1 (from Int-1 to TS-1). For the C-H alumination with 2methylfuran DFT calculations give similar results with an accessible activation energy of G ‡ 298K = 20.2 kcal mol -1 and an overall endothermic process, Gº298K = 1.8 kcal mol -1 . Therefore, the C-H alumination for both 2-methylthiophene and 2-methylfuran are energetically feasible, and can be driven to the formation of products through removal of H2. is predicted to be 48.3 kcal mol -1 (relative to Int-4, Figure ). For comparison the same pathway for 2methylfuran is calculated to have an activation barrier of 30.3 kcal mol -1 . Hence, ring-expansion of 2methylfuran is predicted to accessible with a reasonable rate constant at 100 ºC, while ring-expansion of 2-methylthiophene is not. In both cases, Nucleus Independent Chemical Shift (NICS) calculations suggest that the ring-opening step occurs with a loss of aromaticity of the heterocycle ring system. NICS(1) calculations suggest that 2-methylthiophene is more aromatic than 2-methylfuran with a more negative NICS(1) value (-9.23 versus -8.63). Both 2-methylfuran and 2-methylthiophene lose aromaticity in TS-3, with NICS(1) values decreasing for both 2-methylthiophene (-5.39) and 2methylfuran (-5.80).