For substrates that bear more than one halide/triflate, the ability to effect selective monofunctionalization through Suzuki cross coupling can be a powerful tool. For palladium catalysts, the general order of reactivity is the following: I > Br J OTf . Cl.
It has been established that with the Pd2(dba)3/P(t-Bu)3 catalyst system it is possible to carry out highly selective monofunctionalizations of difunctionalized arenes (Table 7). Not surprisingly, an iodo and a bromo group react in preference to a chloro group, providing the chlorinated biaryl in excellent yield (entries 1 and 2, Table 7). In the case of 4-bromophenyltriflate,it was observed very selective reaction of the bromide in the presence of the triflate (entry 3, Table 7). In view of the low reactivity of aryl triflates toward the Pd2(dba)3/P(t-Bu)3 catalyst system, it was speculated that it might even be possible to effect a selective Suzuki coupling of a chloride in the presence of an aryl triflate. As illustrated in entry 4 of Table 7, Pd2(dba)3/ P(t-Bu)3 does indeed react with the chloride of 4-chlorophenyltriflate in preference to the triflate, with excellent selectivity. In contrast, Pd(OAc)2/PCy3 exhibits the conventional pattern of reactivity (entry 5, Table 7). Thus, simply through appropriate choice of catalyst, one can selectively cross-couple an aryl chloride in the presence of an aryl triflate, or vice versa. The competition experiment illustrated in eq 2 provides further evidence for the remarkable selectivity of Pd2(dba)3/ P(t-Bu)3 for chlorides over triflates. These data indicate that kAr-Cl/kAr-OTf exceeds 20 for this reaction.
