Expansion of this method to the arylation of substituted cyclobutanes of interest for subsequent derivatization (dimethoxy ketal) or to block metabolism (gem-difluoro and gem-dimethyl) was also possible (40-47). The chemoselectivity in the case of 41, 42, and 45 is worth noting as the cyano/carbonyl functionalities would be problematic for polar bond approaches to such structures (Figure ). Azetidines, oxetanes, and spirocycles were also enlisted to deliver high-value mono-arylated structures such as 48-56. Thus, pyrimidine, pyridine and electron-poor arenes were installed successfully on both the azetidine and oxetane ring systems (48-54). Compound 51, having a chloro in ortho position was also well tolerated. The azetidines described herein have not been previously reported, although they could potentially be accessed via the Pd-enolate arylation followed by ester hydrolysis, as shown for example 48. In the case of oxetanes, such compounds have been prepared through a creative multistep sequence involving Grignard addition on 3-oxetanone, Friedel-Crafts arylation with furan, and subsequent oxidative cleavage to reveal the free acid. In contrast, the current protocol offers direct, chemoselective access to these structures thereby eliminating the need for protecting and functional group manipulations. With a library of arylated tertiary acids in substantial amounts, the scope of the second arylation step was explored. To our knowledge, none of the products synthesized in Table have been previously synthesized. Regardless of the type of four membered ring system, the second proceeded in workable to moderate yields (10-52%) with a variety of (hetero)arylbromides and iodides. Fortuitously, the reactions generally proceed in higher yields when employing medicinally relevant and traditionally challenging heterocyclic coupling partners such as pyrimidines (25, As previously mentioned, the ability to incorporate carbonyl functionalities is significant, as these groups would not be compatible with polar-bond approaches (Figure ). Additionally, the haloselectivity favoring the cross-coupling of aryl iodides and bromides over fluorides, and more notably chlorides, is advantageous for medicinal chemistry campaigns. This allows the iodide or bromide to react preferentially, followed by conventional cross-couplings (Pd/Ni/Cu/Fe) of the remaining aryl chloride or SnAr functionalization of the aryl fluoride. Tertiary redox-active esters (RAEs) bearing substituted cyclobutanes e.g., dimethoxy ketal (78-80), gem-difluoro (81 & 82) and gem-dimethyl (83), spirocycles (89 & 90), as well as azetidines (82-84) and oxetanes (87, 88) were successfully employed in the second arylation.