The　selective　functionalization　of　carboranes　has　received　increasing　research　interests　due　to　their　wild　applications　in　chemistry,　life,　and　material　sciences.　Among　various　structurally　diverse　carboranes,　the　development　of　selective　functionalization　of　the　commercially　available　o-carborane　(1,2-C2B10　H-1(2))　has　largely　focused　on　the　two　acidic　C-H　bonds.　By　contrast,　research　on　the　activation　of　the　other　ten　hydridic　cage　B-H　vertices　is　relatively　less　explored.　Of　particularly　challenging,　the　most　electron-deficient　nature　of　B(3,6)-H　bonds　render　very　few　synthetic　methods　available　for　their　functionalization.　Herein,　we　develop　a　phosphine　oxide-directed　palladium-catalyzed　highly　B(3)-H　selective　arylation　of　o-carboranes　under　very　mild　reaction　conditions　in　short　reaction　time.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.