Selective C–Cl Bond Oxidative Addition of Chloroarenes to a POP–Rhodium Complex

The C–Cl bond cis oxidative addition of 12 chloroarenes, including chlorobenzene, chlorotoluenes, chlorofluorobenzenes, and di- and trichlorobenzenes to RhH­{xant­(PiPr2)2} (1; xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) and the ability of the resulting rhodium­(III) species to undergo reductive elimination reactions are reported. Complex 1 reacts with chlorobenzene to give RhHCl­(C6H5)­{xant­(PiPr2)2} (2), which eliminates benzene to afford RhCl­{xant­(PiPr2)2} (3). On the other hand, in the presence of potassium tert-butoxide (KOtBu), it undergoes dehydrodechlorination to yield Rh­(C6H5)­{xant­(PiPr2)2} (4). The reactions of 1 with 3- and 4-chlorotoluenes lead to RhHCl­(C6H4-3-Me)­{xant­(PiPr2)2} (5) and RhHCl­(C6H4-4-Me)­{xant­(PiPr2)2} (6), respectively. Treatment of the acetone solutions of both compounds with KOtBu also results in their dehydrodechlorination to give Rh­(C6H4-3-Me)­{xant­(PiPr2)2} (7) and Rh­(C6H4-4-Me)­{xant­(PiPr2)2} (8). Chlorofluorobenzenes undergo both C–Cl oxidative addition and C–H bond activation in a competitive manner. The amount of the C–H activation product increases as fluorine and chlorine are separated. Complex 1 reacts with o-chlorofluorobenzene to afford the C–Cl oxidative addition product RhHCl­(C6H4-2-F)­{xant­(PiPr2)2} (9). The reaction of 1 with m-chlorofluorobenzene leads to RhHCl­(C6H4-3-F)­{xant­(PiPr2)2} (10; 91%) and the C–H bond activation product Rh­(C6H3-2-Cl-6-F)­{xant­(PiPr2)2} (12; 9%), whereas p-chlorofluorobenzene gives a mixture of RhHCl­(C6H4-4-F)­{xant­(PiPr2)2} (13; 61%) and Rh­(C6H3-3-Cl-6-F)­{xant­(PiPr2)2} (15; 39%). The addition of KOtBu to the acetone solutions of 9, 10, and 13 produces the HCl abstraction and the formation of Rh­(C6H4-2-F)­{xant­(PiPr2)2} (16), Rh­(C6H4-3-F)­{xant­(PiPr2)2} (17), and Rh­(C6H4-4-F)­{xant­(PiPr2)2} (18). In contrast to o-chlorofluorobenzene, 1,2-dichlorobenzene reacts with 1 to give RhHCl­(C6H4-2-Cl)­{xant­(PiPr2)2} (19; 32%), Rh­(C6H4-2-Cl)­{xant­(PiPr2)2} (20; 51%) and Rh­(C6H3-2,3-Cl2)­{xant­(PiPr2)2} (22; 17%). The reactions of 1 with 1,3- and 1,4-dichlorobenzene lead to the respective C–Cl bond oxidative addition products RhHCl­(C6H4-3-Cl)­{xant­(PiPr2)2} (23) and RhHCl­(C6H4-4-Cl)­{xant­(PiPr2)2} (24), which afford Rh­(C6H4-3-Cl)­{xant­(PiPr2)2} (25) and Rh­(C6H4-4-Cl)­{xant­(PiPr2)2} (26) by dehydrodechlorination with KOtBu in acetone. Treatment of 1 with 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzenes leads to RhHCl­(C6H3-2,3-Cl2)­{xant­(PiPr2)2} (27), RhHCl­(C6H3-3,4-Cl2)­{xant­(PiPr2)2} (28), and RhHCl­(C6H3-3,5-Cl2)­{xant­(PiPr2)2} (29). The addition of KOtBu to acetone solutions of 27-29 affords 22, Rh­(C6H3-3,4-Cl2)­{xant­(PiPr2)2} (30) and Rh­(C6H3-3,5-Cl2)­{xant­(PiPr2)2} (31).