The simplicity and efficiency of sp 2 -sp 2 cross-coupling technologies have driven its widespread adoption by the synthetic community, influencing synthesis strategies and the types of molecules that are most readily synthesized by the pharmaceutical industry. However, as a community, we are realizing trends that indicate enhanced developability and clinical success of organic molecules that exhibit greater degrees of saturation, which is often correlated with increasing numbers of sp 3 -hydridized carbons. This 'molecular complexity' tends to improve a compound's aqueous solubility, crystallinity, and binding specificity. All-carbon quaternary centers are frequently encountered in bioactive natural products, pharmaceuticals, and drug candidates (Figure ). In a recent analysis of modern Negishi, Suzuki, and various nickel-catalyzed photoredox cross-coupling methods for constructing C(sp 2 )-C(sp 3 ) aryl-alkyl bonds by Abbvie scientists, none were able to install a tert-butyl group. This highlights the challenges inherent in synthesizing quaternary carbon centers, as well as the limitations that exist even with state-of-the-art catalysis. As such, considerable efforts have been devoted to their catalytic synthesis with precious metals, where palladium, rhodium, and iridium demonstrate the greatest utility. Modern variations of Kumada 5 and Suzuki reactions, 6 including photoredox-mediated, reductive, and redox-active ester-mediated cross-couplings have demonstrated success in merging C(sp 2 ) and C(sp 3 ) units to forge new all-carbon quaternary centers. While the development of nickel and copper catalyses for synthesizing quaternary carbon centers have progressed in recent years, examples with other abundant transition metals such as iron, are scarce. In considering new solutions to quaternary carbon synthesis, we surmised that a Friedel-Crafts approach would effectively permit direct C-H functionalization. In this respect, Beller and coworkers reported primary and secondary, benzylic halides/acetates/alcohols coupling with arenes under iron catalysis (Scheme 1). The Cook group found that in conjunction with a silver salt, FeCl3 promotes Friedel-Crafts reactions between arenes and unactivated secondary alcohols. Herein, we disclose a Fentoninspired, synergistic Brønsted/Lewis acid-catalysis that enables aromatic alkylation with unactivated tertiary alcohols. These findings represent underexplored examples of using nonbenzylic, non-propargylic, and non-allylic alcohols for Friedel-Scheme 1. Synergistic iron/TFA-catalyzed tert-butylation of phenol using peroxide reagents with and without an acid co-catalyst.