When the reaction of aromatic ketones with arylboronates (arylboronic acid esters) using RuH(2)(CO)(PPh(3))(3) (3) as a catalyst was conducted in toluene, the corresponding arylation product was obtained in moderate yields. In this case, a nearly equivalent amount of a benzyl alcohol derived from a reduction of an aromatic ketone was also formed. The use of aliphatic ketones, such as pinacolone and acetone, as an additive or a solvent dramatically suppressed the reduction of the aromatic ketones and, as a result, ortho-arylation products were obtained in high yield based on the aromatic ketones. In these reactions, the aliphatic ketone functioned as a scavenger of ortho-hydrogens of the aromatic ketones and the B(OR)(2) moiety of the arylboron compound (HB species). A variety of aromatic ketones, such as acetophenones, acetonaphthones, tetralones, and benzosuberone, could also be used in this coupling reaction. Several arylboronates containing electron-donating (NMe(2), OMe, and Me) and -withdrawing (CF(3) and F) groups were also applicable to this coupling reaction. Intermolecular competitive reaction using pivalophenone-d(0)() and -d(5) and intramolecular competitive reaction using pivalophenone-d(1) were carried out using 3 as a catalyst. The k(H)/k(D) value for the intermolecular competitive reaction was substantially different, compared with intramolecular competitive reaction. This strongly suggests the production of an intermediate where the ketone carbonyl is coordinated to the ruthenium involved in this catalytic reaction. (1)H and (11)B NMR studies using 2'-methylacetophenone, phenylboronate (2), and pinacolone (6) indicate that 6 functions effectively as a scavenger of the HB species.
A carboxylesterase with a β‐lactamase fold from Arthrobacter possesses a low level of hydrolytic activity (0.023 μmol·min−1·mg−1) when acting on a 6‐aminohexanoate linear dimer byproduct of the nylon‐6 industry (Ald). G181D/H266N/D370Y triple mutations in the parental esterase increased the Ald‐hydrolytic activity 160‐fold. Kinetic studies showed that the triple mutant possesses higher affinity for the substrate Ald (Km = 2.0 mm) than the wild‐type Ald hydrolase from Arthrobacter (Km = 21 mm). In addition, the kcat/Km of the mutant (1.58 s−1·mm−1) was superior to that of the wild‐type enzyme (0.43 s−1·mm−1), demonstrating that the mutant efficiently converts the unnatural amide compounds even at low substrate concentrations, and potentially possesses an advantage for biotechnological applications. X‐ray crystallographic analyses of the G181D/H266N/D370Y enzyme and the inactive S112A‐mutant–Ald complex revealed that Ald binding induces rotation of Tyr370/His375, movement of the loop region (N167–V177), and flip‐flop of Tyr170, resulting in the transition from open to closed forms. From the comparison of the three‐dimensional structures of various mutant enzymes and site‐directed mutagenesis at positions 266 and 370, we now conclude that Asn266 makes suitable contacts with Ald and improves the electrostatic environment at the N‐terminal region of Ald cooperatively with Asp181, and that Tyr370 stabilizes Ald binding by hydrogen‐bonding/hydrophobic interactions at the C‐terminal region of Ald.
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