Ross S. Robinson

Recent developments in one-pot tandem oxidation process coupling reactions

The tandem oxidation process (TOP) constitutes a novel approach to synthetic organic chemistry. This procedure involves the oxidation of the alcohol to the aldehyde which is trapped in situ by a nucleophile leading to a variety of synthetically useful compounds. In recent times, many research groups have focused their attention on developing new approaches to the one-pot coupling process. In this review, we highlight the latest additions, over the past nine years, to the TOP realm such as the use of novel oxidants, innovative catalysts and alternate reaction conditions. In addition, the introduction of TOP to multicomponent reactions and total syntheses will also be discussed.

Green oxidations: Titanium dioxide induced tandem oxidation coupling reactions

Summary The application of titanium dioxide as an oxidant in tandem oxidation type processes is described. Under microwave irradiation, quinoxalines have been synthesized in good yields from the corresponding α-hydroxyketones.

Copper-Catalyzed Synthesis of Valuable Heterocyclic Compounds Using a Tandem Oxidation Process Approach

Abstract The synthesis of quinoxalines via a tandem oxidation process using a copper acetate monohydrate/2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) oxidative system has been described. Using this approach, a series of quinoxaline derivatives were formed in good yields within short reaction times under microwave irradiation. The oxidative system was also extended to the selective synthesis of dihydropyrazines and pyrazines. GRAPHICAL ABSTRACT

Palladium-catalysed cross-coupling reaction of ultra-stabilised 2-aryl-1,3-dihydro-1H-benzo(d)1,3,2- diazaborole compounds with aryl bromides: A direct protocol for the preparation of unsymmetrical biaryls

Summary There has been a significant interest in organoboron compounds such as arylboronic acids, arylboronate esters and potassium aryltrifluoroborate salts because they are versatile coupling partners in metal-catalysed cross-coupling reactions. On the other hand, their nitrogen analogues, namely, 1,3,2-benzodiazaborole-type compounds have been studied extensively for their intriguing absorption and fluorescence characteristics. Here we describe the first palladium-catalysed Suzuki–Miyaura cross-coupling reaction of easily accessible and ultra-stabilised 2-aryl-1,3-dihydro-1H-benzo[d]1,3,2-diazaborole derivatives with various aryl bromides. Aryl bromides bearing electron-withdrawing, electron-neutral and electron-donating substituents are reacted under the catalytic system furnishing unsymmetrical biaryl products in isolated yields of up to 96% in only 10 minutes.

Role of a 2,3-bis(pyridyl)pyrazinyl chelate bridging ligand in the reactivity of Ru(II)–Pt(II) dinuclear complexes on the substitution of chlorides by thiourea nucleophiles – a kinetic study

Chloride substitution from [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)pyrazine)Pt(II)dichloride]2+ (RuPt1), [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)quinoxaline)Pt(II)dichloride]2+ (RuPt2) and [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)benzo[g]quinoxaline)Pt(II)dichloride]2+ (RuPt3) by thiourea (TU), 1,3-dimethyl-2-thioura (DMTU) and 1,1,3,3-tetra methyl-2-thiourea (TMTU) was studied in a methanol medium (I = 0.10 M) under pseudo-first-order conditions. The rate of substitution was investigated as a function of concentration of nucleophile and temperature using the stopped-flow technique. Two consecutive substitution steps were observed. The first and fastest step was ascribed to the simultaneous substitution of the two chloride co-ligands by incoming nucleophiles according to the rate law: k1stobs = k1st2[Nu]. The subsequent step was assigned to the dechelation of the rigid 2,3-bis(pyridyl)pyrazinyl bridging ligand from the Pt(II) centres of the substituted intermediates to give Pt(Nu)42+ and (phen)2Ru(II)(2,3-bis(pyridyl)pyrazinyl) groups as products. The rate law for this step is k2ndobs = k2nd2[Nu] + k2nd−2. The second-order kinetics and large negative entropies for both steps support an associative mechanism of substitution. The rate of chloride substitution was RuPt1 ≪ RuPt2 DMTU > TMTU, in accordance with their steric bulk.

2-(3-Meth-oxy-phen-yl)-1,3-dihydro-1,3,2-benzodiaza-borole.

The title compound, C13H13BN2O, is one in a series of 1,3,2-benzodiazaboroles featuring a 2-methoxyphenyl substitution at the 2-position in the nitrogen–boron heterocyle. The dihedral angle between the mean planes of the benzodiazaborole and 2-methoxyphenyl ring systems is 21.5 (1)°. There is an intermolecular hydrogen bond between one of the NH groups and the methoxy O atom. This hydrogen bond leads to an infinite hydrogen-bonded chain colinear with the a axis.

2-[4-(Methyl-sulfan-yl)phen-yl]naphtho[1,8-de][1,3,2]diaza-borinane.

The title compound, C17H15BN2S, is one member in a series of diazaborinanes featuring substitution at the 1-, 2- and 3-positions in the nitrogen–boron heterocycle. The dihedral angle between the mean planes of the naphthalene and phenyl ring systems is 19.86 (6)°. In the crystal structure, two C—H⋯π interactions link the molecules into sheets which lie parallel to the bc plane. There is a π–π interaction between each pair of centrosymmetrically related sheets [centroid–centroid distance = 3.5922 (8) Å].

2-Phenyl-naphtho-[1,8-de][1,3,2]diaza-borinane.

The title compound, C16H13BN2, is one compound in a series of diazaborinanes featuring substitution at the 1, 2 and 3 positions in the nitrogen–boron heterocycle. The title compound is slightly distorted from planarity, with a dihedral angle of 9.0 (5)° between the mean planes of the naphthalene system and the benzene ring. The m-carbon atom of the benzene ring exhibits the greatest deviation of 0.164 (2) Å from the 19-atom mean plane defined by all non-H atoms. The two N—B—C—C torsion angles are 6.0 (3) and 5.6 (3)°. In the crystal, molecules are linked by π–π interactions into columns, with a distance of 3.92 (3) Å between the naphthalene ring centroids. Adjacent π-stacked columns, co-linear with the b-axis, are linked by C—H⋯π interactions.

2-(4-Chloro­phen­yl)naphtho­[1,8-de][1,3,2]diaza­borinane

The title compound, C16H12BClN2, is one in a series of diazaborinanes, derived from 1,8-diaminonaphthalene, featuring substitution at the 1, 2 and 3 positions in the nitrogen-boron heterocycle. The structure deviates from planarity, the torsion angle subtended by the p-chlorophenyl ring relative to the nitrogen–boron heterocycle being −44-.3(3)°. The molecules form infinite chains with strong interactions between the vacant pz orbital of the B atom and the π-system of an adjacent molecule. The distance between the B atom and the 10-atom centroid of an adjacent naphthalene ring is 3.381 (4) Å. One N-H H atom is weakly hydrogen bonded to the Cl atom of an adjacent molecule. This combination of intermolecular interactions leads to the formation of an infinite two-dimensional network perpendicular to the c axis.

Crystal structure of 1-[2-(benzyloxy)phenyl]ethanone, C15H14O2

Abstract C15H14O2, monoclinic, P21/c (no. 14), a = 10.664(2) Å, b = 14.406(3) Å, c = 7.686(1) Å, β = 94.260(3)°, V = 1177.6 Å3, Z = 4, Rgt(F) = 0.0417, wRref(F2) = 0.1036, T = 103 K.