Aryl ether and thiophene derivatives are better behaved in the dual iron(III)/HCl catalyzed tert-butylation reaction (Scheme 3). Anisole is converted to 4-tert-butylanisole (4ma) in 73% yield. 2,4-Dialkylation occurs with bromopropyl phenyl ether to afford trisubstituted arene 4naa in 48% yield, with no monoalkylation product observed. ortho-Substituted anisole precursors are site-selectively functionalized para to the methoxy group. Unlike the 2-alkylphenolic derivatives, which are poorly reactive, 2-methyl-and 2-ethylanisoles undergo tertbutylation to give 4oa and 4pa in 83% and 88% yields, respectively. Anisole derivatives with an aliphatic alcohol or bromo group at the 2-position are transformed to their corresponding tert-butylated products in moderate yields (45% for 4qa and 53% for 4ra). New C(sp 2 )-C(sp 3 ) bond formation occurs with benzodioxole, albeit less effectively than with anisole, producing 4sa in 50% yield. 3-Substituted aryl ethers are functionalized selectively to products 4ta and 4ua with alkylation at the ortho positions in 68-73% yields. Selective mono-tert-butylation proceeds with 4-tert-butylanisole to deliver 2,4-di-tert-butylanisole (4va) in 75% yield. An anisole derivative bearing a pendant ester group is accommodated and 54% of the alkylated product (4wa) is formed. In addition to anisole derivatives, thiophene derivatives react effectively. Treating 3-hexylthiophene with DTBP (2) under iron(III)/HCl catalysis favors di-tert-butylation at both the 2-and 5-positions (4xaa, 82%), whereas the analogous reaction with benzothiophene leads to selective tert-butylation at the 3position in 76% yield (4ya). In contrast, the phosphoric acidmediated direct alkylation of thiophene derivatives with tertbutanol requires 200 °C to achieve modest yields. The dual Brønsted/Lewis acid-catalyzed cross-coupling between electron-rich arenes and DTBP (2) represents an underexplored site-selective Friedel-Crafts alkylation process. However, the modest reactivity experienced by several substrates and the reliance on DTBP (2) limit synthetic practicality. We speculate side reaction pathways arising from Scheme 3. Scope of tert-butylation of phenolic, aryl ether, and thiophene derivatives. [a] Reaction conditions: arene 1 (0.2 mmol), DTBP (2, 0.2 mmol), FeCl3 (10 mol%), TFA (0.15 mmol), DCE (0.8 mL), 50 °C, 2 h. [b] Reaction conditions: arene 1 (0.2 mmol), DTBP (2, 0.2 mmol), FeCl3 (10 mol%), TFA (0.15 mmol), DCE (0.8 mL), 50 °C, 2 h. [c] FeCl3 (20 mol%), 18 h. [d] FeCl3 (1 equiv), HCl(aq) (0.15 mmol), 48 h. radical species compromise reactivity and product yields. In a proposed pathway analogous to that with hydrogen peroxide (Scheme 4a), DTBP (2) can react with iron(III) to form iron(III) tert-butylperoxide (5) and tert-butyl cation (6), the latter of which can participate in the desired electrophilic alkylation. Single electron transfer with the former would lead to iron(II) and tert-butylperoxyl radical (7), which could abstract a hydrogen atom from the solvent or substrate to give tert-butyl hydroperoxide (8). Alternatively, iron(II) produced in this manner, or through reduction of iron(III) by phenol and anisole derivatives, can reduce DTBP (2) in a Fenton-like fashion to generate iron(III) (9) and tert-butoxyl radical (10, Scheme 4b). Subsequent hydrogen atom abstraction by the oxygen-centered radical may initiate undesirable side reactions while producing tert-butanol (11a), a potential precursor to the desired Friedel-Crafts reaction. We find catalysis with FeCl2 proceeds similarly to FeCl3, which is consistent with a Fenton-initiation process. A kinetic analysis was undertaken to derive insight into optimizing the C(sp 2 )-C(sp 3 ) cross-coupling reaction. 3-tert-Butylphenol (1f) was selected as the model substrate to react with DTBP (2) because little-to-no side products form over the course of the reaction, thus simplifying the data analysis and interpretation. Initial rates for tert-butylation were then measured by varying the concentrations 1f, DTBP (2), TFA, and FeCl3 catalyst. A firstorder rate dependence on the concentration of phenolic 1f was observed (Figure ). The kinetics experiments revealed a halforder dependence with respect to the concentration of DTBP(2) (Figure ), suggestive of 2 dissociating into two active fragments and consistent with the mechanistic hypotheses presented in Scheme 4. Little change in initial rates were observed with varying TFA concentrations, which we interpret as zero-order rate dependence (Figure ). With respect to FeCl3, a relatively uncommon second order dependence on rate was observed (Figure ). Additional evidence for the catalyst order was sought by treating the reaction profile data to graphical analysis using the normalized time scale method. Rather than converting the raw data to rates, the raw concentration data of the entire data sets (i.e., [1f]) are plotted against normalized time scales, tꞏ[FeCl3] n , where t = time and n corresponds to the catalyst order when all the curves overlay on one another (Figure ). Using the data sets obtained from varying the catalyst loadings, the curves overlay when n = 2, which support a second order dependence in [FeCl3] and is consistent with the catalyst playing distinct roles in transforming DTBP (2) into the reactive alkylating agent, potentially tert-butanol (11a), and further activating it for merger with the arene coupling partner. Based on this mechanistic conjecture, DTBP (2) should be replaceable by tertalkanols. While catalytic tert-alkylations using allylic, propargylic, and benzylic alcohols are well precedented, few examples exist with unactivated tert-alkanols, especially in the context of site-selectivity. Successful tert-butylation has been reported with superstoichiometric amounts of strong acid or super acids. We envisage that the process involving a synergistic combination of Fe(III) and Brønsted acid catalysts would address the synthetic limitations imposed by using peroxides as coupling reagents, and would provide a simple approach for directly forging C(sp 2 )-C(sp 3 ) bonds with quaternary carbon centers.