ABSTRACT
Tetranuclear cores (M-E)2 of transition metals (M) and tetrylenes (EII=Si, Ge, Sn) are key motifs in homogeneous and heterogeneous catalysis. They exhibit a continuum of M-M and E-E bonding within the inorganic core that leads to a variety of structures for which there are no specific synthetic methods. Herein, we report a series of highly reduced [Ni0GeII]2 squares solely stabilized by bulky terphenyl (C6H3-2,6-Ar2) ligands, for which we provide complementary and high-yielding syntheses. Reactivity studies with common Lewis bases (carbene and CO) evince that the structure of the (M-E)2 core can be transformed. We have investigated this core modification by computational means, offering a rationale to better understand the continuum of bonding across these clusters.
ABSTRACT
The phosphine-borane iPr2P(o-C6H4)BFxyl2 (Fxyl = 3,5-(F3C)2C6H3) 1-Fxyl was found to promote the reductive elimination of ethane from [AuMe2(µ-Cl)]2. Nuclear magnetic resonance monitoring revealed the intermediate formation of the (1-Fxyl)AuMe2Cl complex. Density functional theory calculations identified a zwitterionic path as the lowest energy profile, with an overall activation barrier more than 10 kcal/mol lower than without borane assistance. The Lewis acid moiety first abstracts the chloride to generate a zwitterionic Au(III) complex, which then readily undergoes C(sp3)-C(sp3) coupling. The chloride is finally transferred back from boron to gold. The electronic features of this Lewis-assisted reductive elimination at gold have been deciphered by intrinsic bond orbital analyses. Sufficient Lewis acidity of boron is required for the ambiphilic ligand to trigger the C(sp3)-C(sp3) coupling, as shown by complementary studies with two other phosphine-boranes, and the addition of chlorides slows down the reductive elimination of ethane.
ABSTRACT
We recently disclosed a dehydrogenative double C-H bond activation reaction in the unusual pincer-type rhodium-germyl complex [(ArMes)2ClGeRh] (ArMes=C6H3-2,6-(C6H2-2,4,6-Me3)2). Herein we investigate the catalytic applications of this Rh/Ge system in several transformations, namely trans-semihydrogenation of internal alkynes, trans-isomerization of olefins and hydrosilylation of alkynes. We have compared the activity and selectivity of this catalyst against other common rhodium precursors, as well as related sterically hindered rhodium complexes, being the one with the germyl fragment superior in terms of selectivity towards E-isomers. To increase this selectivity, a tandem catalytic protocol that incorporates the use of a heterogeneous catalyst for the trans-semihydrogenation of internal alkynes has been devised. Kinetic mechanistic investigations provide important information regarding the individual catalytic cycles that comprise the overall trans-semihydrogenation of internal alkynes.
ABSTRACT
The phosphine-borane iPr2P(o-C6H4)BFXyl2 (Fxyl = 3,5-(F3C)2C6H3) was found to react with gold(i) alkynyl and vinyl complexes via an original 1,1-phosphaboration process. Zwitterionic complexes resulting from Au to B transmetallation have been authenticated as key intermediates. X-ray diffraction analyses show that the alkynyl-borate moiety remains pendant while the vinyl-borate is side-on coordinated to gold. According to DFT calculations, the phosphaboration then proceeds in a trans stepwise manner via decoordination of the phosphine, followed by anti nucleophilic attack to the π-CC bond activated by gold. The boron center acts as a relay and tether for the organic group.
ABSTRACT
The biological properties of trifluoromethyl compounds have led to their ubiquity in pharmaceuticals, yet their chemical properties have made their preparation a substantial challenge, necessitating innovative chemical solutions. We report the serendipitous discovery of a borane-catalyzed formal C(sp3)-CF3 reductive elimination from Au(III) that accesses these compounds by a distinct mechanism proceeding via fluoride abstraction, migratory insertion, and C-F reductive elimination to achieve a net C-C bond construction. The parent bis(trifluoromethyl)Au(III) complexes tolerate a surprising breadth of synthetic protocols, enabling the synthesis of complex organic derivatives without cleavage of the Au-C bond. This feature, combined with the "fluoride-rebound" mechanism, was translated into a protocol for the synthesis of 18F-radiolabeled aliphatic CF3-containing compounds, enabling the preparation of potential tracers for use in positron emission tomography.
