A novel 1,4-palladium migration between the o-and o'-positions of biaryls has been observed in organopalladium intermediates derived from o-halobiaryls. The organopalladium intermediates generated by this migration have been trapped either by a Heck reaction employing ethyl acrylate or by Suzuki cross-coupling using arylboronic acids. This palladium migration can be activated or deactivated by choosing the appropriate reaction conditions. Chemical and computational evidence supports the presence of an equilibrium that correlates with the C−H acidity of the available arene positions.

Nicotinate dehydrogenase (NDH) from Eubacterium barkeri is a molybdoenzyme catalyzing the hydroxylation of nicotinate to 6-hydroxynicotinate. Reactivity of NDH critically depends on the presence of labile (nonselenocysteine) selenium with an as-yetunidentified form and function. We have determined the crystal structure of NDH and analyzed its active site by multiple wavelengths anomalous dispersion methods. We show that selenium is bound as a terminal MoASe ligand to molybdenum and that it occupies the position of the terminal sulfido ligand in other molybdenum hydroxylases. The role of selenium in catalysis has been assessed by model calculations, which indicate an acceleration of the critical hydride transfer from the substrate to the selenido ligand in the course of substrate hydroxylation when compared with an active site containing a sulfido ligand. The MoO(OH)Se active site of NDH shows a novel type of utilization and reactivity of selenium in nature.S elenium is an essential component of several enzymes and has a key role in various biological redox processes. Usually selenium occurs in proteins as selenocysteine, which is cotranslationally inserted as the 21st amino acid (1) and is found in a variety of proteins in all 3 kingdoms of life (2). Selenium also finds a natural use as 5-methylaminomethyl-2-selenouridine in the ''wobble'' position of some tRNAs (3). The ionic radii and electronegativities of selenium and sulfur are similar, but selenide is a stronger reducing agent than sulfide. Because of the lower pK a value of selenols compared with thiols selenocysteine is deprotonated under physiological conditions, whereas cysteine is mostly protonated (4). The ionization state together with the better polarizability of selenium makes selenocysteine a good nucleophile. Several molybdenum-and tungsten-containing enzymes have been shown to contain selenium (5), and a recent comprehensive genomic analysis has revealed a clear relationship between selenium and molybdenum utilization across all 3 domains of life (6). Selenium is found as selenocysteine in some prokaryotic molybdenum-containing oxotransferases like formate dehydrogenase H from Escherichia coli, where it coordinates molybdenum in the oxidized state of the enzyme (7-9). In other enzymes, notably members of the molybdenum hydroxylase family (10, 11), like nicotinate dehydrogenase (NDH) from Eubacterium barkeri (12-14) and the xanthine oxidoreductases (XORs) of Clostridium purinolyticum (15), Clostridium acidiurici (16) and Eubacterium barkeri (17) and the purine hydroxylase from C. purinolyticum (15) contain a labile (nonselenocysteine) selenium in an unidentified form essential for their reactivity (18).Molybdenum hydroxylases catalyze the hydroxylation of various organic molecules following the general scheme:This hydroxylation reaction is unique in biology as it uses water as the source for the hydroxyl oxygen and not dioxygen (10). The active site of molybdenum hydroxylases contains molybdenum coordinated by the enedithiolate group of a pyrano...

The oxorhenium(V) dimer {MeReO(edt)}2 (1; where edt = 1,2-ethanedithiolate) catalyzes S atom transfer from thiiranes to triarylphosphines and triarylarsines. Despite the fact that phosphines are more nucleophilic than arsines, phosphines are less effective because they rapidly convert the dimer catalyst to the much less reactive catalyst [MeReO(edt)(PAr3)] (2). With AsAr3, which does not yield the monomer, the rate law is given by v = k [thiirane][1], independent of the arsine concentration. The values of k at 25.0 °C in CDCl3 are 5.58 ± 0.08 L mol-1 s-1 for cyclohexene sulfide and ca. 2 L mol-1 s-1 for propylene sulfide. The activation parameters for cyclohexene sulfide are ΔH⧧ = 10.0 ± 0.9 kcal mol-1 and ΔS⧧ = −21 ± 3 cal K-1mol-1. Arsine enters the catalytic cycle after the rate-controlling release of alkene, undergoing a reaction with the ReVII(O)(S) intermediate that is so rapid in comparison that it cannot be studied directly. The use of a kinetic competition method provided relative rate constants and a Hammett reaction constant, ρ = −1.0. The oxorhenium(V) dimer {MeReO(edt)} 2 (1; where edt ) 1,2-ethanedithiolate) catalyzes S atom transfer from thiiranes to triarylphosphines and triarylarsines. Despite the fact that phosphines are more nucleophilic than arsines, phosphines are less effective because they rapidly convert the dimer catalyst to the much less reactive catalyst [MeReO(edt)(PAr 3 )] (2). With AsAr 3 , which does not yield the monomer, the rate law is given by v ) k

Computational estimates have been made for the PS and AsS bond strengths in triphenylphosphine sulfide and triphenylarsine sulfide, on the basis of G3 calculations for the methyl analogues and isodesmic-exchange reactions. Also, with the performance of the G3 method level for related compounds taken into consideration, the best estimates are 82 and 68 kcal/mol, respectively. While the value for triphenylarsine sulfide is within 2 kcal/mol of the single experimental estimate, that for triphenylphosphine sulfide is lower by 6 kcal/mol. Computational estimates have been made for the PdS and AsdS bond strengths in triphenylphosphine sulfide and triphenylarsine sulfide, on the basis of G3 calculations for the methyl analogues and isodesmic-exchange reactions. Also, with the performance of the G3 method level for related compounds taken into consideration, the best estimates are 82 and 68 kcal/mol, respectively. While the value for triphenylarsine sulfide is within 2 kcal/mol of the single experimental estimate, that for triphenylphosphine sulfide is lower by 6 kcal/mol. (Capps, K. B.; Wixmerten, B.; Bauer, A.; Hoff, C. D. Inorg. Chem. 1998, 37, 2861−2864.) Despite virtually identical electronegativities of P and As, it is found that there is greater charge separation in the PdS bond. It is found that S atom transfer from thiiranes to arsines is exothermic.