We targeted the joining of 2-methyl-2-butanol (11b) and 3tert-butylphenol (1f) to investigate our hypothesis (Table ). The use of 2.5 mol% FeCl3 and 75 mol% HCl in DCE solvent afforded 72% yield of target 3fb (entry 1). In contrast to the reactions with DTBP (2), tert-alkylation does not occur with trifluoroacetic acid as the co-catalyst (entry 2), while 66% NMR yield was obtained with HBr (entry 3). Using FeCl2 instead of FeCl3 resulted in a significant drop in conversion to 15% (entry 4). FeBr3 (entry 5) and FeBr2 (entry 6) performed similarly to FeCl3 (70% yields). Increasing or decreasing the amounts of acid led to inferior 63% and 60% yields, respectively (entries 7 and 8). Exchanging the solvent for HFIP resulted in only 13% conversion (entry 9). The reaction proceeded similarly in chlorobenzene solvent (75%, entry 10). When performed in toluene, moderate levels of product formation were observed (43%, entry 11); the lower yield is attributed to toluene being reactive, which consumes a significant proportion of the alcohol (see Scheme 8). Isopropanol and THF solvents do not promote the desired alkylation (entries 12 and 13). Considering reagent cost and operation simplicity, we elected to use FeCl3, HCl, and DCE solvent as the optimal conditions to explore the substrate scope. These reactions can be set up under air. Moisture does not affect reactivity and aqueous HCl can be used as the source of Brønsted acid. The unique reactivity arising from the combination of FeCl3 and HCl previously observed in a cation-π polycyclization has been attributed to the formation of HFeCl4. The desired tert-alkylation reactions are not restricted to phenolic compounds, but also to anisolic and electron-neutral arenes, in which cases the combination of FeBr3 and HBr Table . Survey of conditions for direct Friedel-Crafts alkylation with phenolic 1f and tertiary alcohol 11b. [Fe] acid y solvent % yield 1 FeCl3 HCl(aq) 75 DCE 72 (10) 2 [a] Conditions: All reactions performed on 0.2 mmol scale, phenol (1 equiv), alcohol (1.1 equiv), 0.25 M, 50 °C, 24 h. [b] Determined by NMR analysis of the crude reaction mixture using 1,3,5trimethoxybenzene as the internal standard. [c] Without Brønsted acid.