Sulfur Reagents in Organic Synthesis

Abstract

Pd halide and hydride complexes of a new PNP pincer ligand with a central diarylamido moiety can be prepared via N-H cleavage in a neutral amine/ diphosphine PNP ligand. The solid-state structure of (PNP)PdCl shows a meridional PNP ligand about an approximately square-planar Pd center. (PNP)PdH hy-drodehalogenates alkyl and certain aryl halides, while (PNP)PdX (X) Cl, I, H, OAc) complexes catalyze Heck coupling of ethyl acrylate with aryl halides. Utilization of tridentate PCP pincer ligands (A), pioneered by Shaw in the 1970s, 1 has been increasing in recent years, owing to their potential for supporting unusual chemical transformations on transition-metal centers. 2 These complexes have been found to be efficient catalysts for alkane dehydrogenation 3 and Heck coupling, 4-6 in addition to providing a platform for seminal mechanistic studies of CC , 7 C-N, 8 and CO 9 bond activation. 2 The realm of anionic PNP ligands has been mostly limited to the family of Fryzuk's ligands (B). 10,11 An anionic amido-PNP ligand may be viewed not only as a component of an organometallic chemist's "pincer toolbox" complementary to the PCP ligands but also as a chelate version of the ubiquitous mer,trans-Cl(R 3 P) 2 motif. A combination of soft phosphines and hard π-base amido on the same metal is often difficult to achieve without linking these into a chelate, and such a hybrid environment has been shown to lead to unusual structures and reactivity in complexes of B. 10 However, B suffers from the high oxophilicity of Si in its backbone, leading to high sensitivity to OH and other O-containing groups. 10d,11 Cyclometalation of the Si-CH 2-P group has also been described. 10f PNP ligands featuring alipha-tic linkers (C) between N and P have been reported, 12-15 yet, for these, few metal complexes utilizing a C-type ligand in its anionic form are known. 14 In addition, pincers B and C are more flexible than A and therefore do not have as strong a preference, if any, for the meridional geometry. We were attracted to the ligand construction wherein o-arylene units link the amido and phosphine sites, because it offers increased rigidity and is devoid of-hydrogens and moisture-sensitive functionalities. 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