Tri-tert-butylphosphine [P(t-Bu)3]:: An electron-rich ligand for palladium in cross-coupling reactions

(A) The Buchwald-Hartwig amination of aryl halides was realized for the first time in 1998 using P(t-Bu)3 as ligand to the palladium catalyst. Fu and co-workers extended the scope of P(t-Bu)3 as ligand for other types of palladium-catalyzed coupling reactions of aryl chlorides. These authors reported the reactions of inflates and aryl chlorides in Suzuki-Miyaura, Stille, Negishi, Hiyama and other C-C cross-coupling reactions. Recently, we found that arenesulfonyl chlorides can be used in the desulfitative Suzuki-Miyaura and Stille cross-coupling reactions. In these cases, good results were also obtained using P(t-Bu)3 as ligand to the palladium catalyst. (B) Typical reaction conditions for the Sonogashira-Hagihara cross-coupling of terminal acetylenes and aryl bromides require high temperatures and CuI as a co-catalyst. Using P(t-Bu)3 ligand, Fu and Buchwald showed that electron-rich aryl bromides can react already at room temperature. Herrmann and co-workers, as well as chemists of Merck company, showed that the use of CuI can be avoided in these reactions. Aryl chlorides can be reacted at higher temperature using the same ligand. We have shown that the desulfitative Sonogashira-Hagihara reactions with arenesulfonyl chlorides can be carried out in the presence of CuI, with 3 mol% Pd2(dba)3 and 10 mol% P(t-Bu). (C) Littke and Fu have shown the Heck-Mizoroki reactions can couple aryl chlorides with mono- or disubstituted olefins giving the corresponding arylated products with high stereoselectivity. This requires the use of P(t-Bu)3 ligand to the palladium and Cy2NMe as base. The Pd/P(t-Bu)3 system is also a good catalyst in the desulfitative Heck-Mizoroki arylation reactions with arenesulfonyl chlorides. (D) Recently, Sakei and co-workers have used 1-aryltriazenes as electrophilic partners in cross-coupling reactions. For instance, the reaction of aryltriazenes and areneboronic acids in the presence of a catalytic amount of Pd2(dba)3 and P(t-Bu)3 give the corresponding biaryls. Similarly, the 1-aryltriazenes can be cross-coupled with aryl- and alkenyltrifluorosilanes under the same conditions.