Reactivity of Aryl Triflates in Palladium Catalyzed Cross-Coupling Reaction with Organoheteroatom Stannanes

MARIANA BONATERRA; SANDRA E. MARTÍN; ROBERTO A. ROSSI

Los Cocos, Córdoba, Argentina

Conferencia; 9a Conferencia Latinoamericana de Físico-Química Orgánica; 2007

INFIQC

     The palladium catalyzed cross-coupling reaction of organic electrophiles with organostannanes is a powerful methodology for C-C1 and C-Heteroatom2 bond formation. A variety of substrates undergo the coupling reaction, the aryl halides being the most thoroughly studied.1 As organic triflates are readily available from the corresponding phenol or enolate, they have become important coupling partners in synthesis for the construction of complexes molecules.      Recently, we have developed a versatile methodology that allows C-heteroatom bond formation through a cross-coupling Pd-catalyzed reaction of different electrophiles with organoheteroatom stannanes R3SnZPhn (Z = P, As, Sb, Se).2a,b,d To extend the applications of this methodology, we have studied more extensively the Pd-catalyzed cross-coupling of these stannanes with aryl triflates (eq 1). In the present work, we report on the scope and limitations of the coupling reaction of aryl triflates with heteroatom containing stannanes.      The experiments have been performed in order to understand the effect of different additives (LiCl and CuI), ancillary ligand (Ph3P, Ph3As), solvent and catalyst, in order to establish the optimal reaction conditions, and to explain the mechanism of this cross-coupling reaction with triflates.       One of the most striking aspects of this reaction is the effect of the addition of LiCl. Another remarkable characteristic is the polarity of the solvent employed. We observed that the cross-coupling reaction with stannane 1b proceeded more efficiently in the presence of LiCl and in DMF as solvent. A dramatic improvement in the yield of the arsane 4 was observed when the cross-coupling reaction was conducted with Ph3P as ancillary ligand. In the presence of CuI as co-catalyst, the Pd-catalyzed reaction also became more efficient. Some important features of the mechanisms and particularly the effects of each additive on it will be discussed.      Similarly, we have studied the reactivity of other organohetereatom stannanes containing phosphorus (1a), antimony (1c) and selenium (2) as transferable groups in cross-coupling reactions with triflates. [1].Farina, V.; Krishnamurthy, V.; Scott, W. J. The Stille Reaction; Paquette, L. A., Ed.; Organic Reactions; John Wiley & Sons: New York, 1997; Vol. 50. 2. For some examples in C-P bond formation see: (a) Martín, S. E.; Bonaterra, M.; Rossi, R. A. J. Organomet. Chem. 2002, 664, 223. For C-As and C-Sb see: (b) Bonaterra, M.; Martín, S. E.; Rossi, R. A. Org. Lett. 2003, 15, 2731. For C-S, C-Si and C-Sn see: (c) Rossi, R. A.; Martín, S. E. Coord. Chem. Rev. 2006, 250, 575. For C-Se see: (d) Bonaterra, M.; Martín, S. E.; Rossi, R. A. Tetrahedron Lett. 2006, 47, 3511.