Preparation,crystal structure and reactions of a new spin-labelling reagent,cis-3,5-dibromo-4-oxo-2,2,6,6- tetramethylpiperidin-1-yloxy : Crystal structure of a derived bis-nitroxide

Methods for the preparation of cis - 3,5 - dibromo - 4 - oxo - 2,2,6,6 - tetramethylpiperidin -1 - yloxy (2b) and its use as a convenient, selective spin-labelling reagent for amino functions, are described. The crystal and molecular structures of 2b and the bis-nitroxide 4 - [(2',2',5',5' - tetramethylpyrrolin -1 - yloxy) - 3' - carbonyloxy] - 2, 2,6,6 - tetramethyl - 3,5 - dibromo - 3,4 - dehydropiperidin - 1 - yloxy (3) [obtained on treatment of 2b with triethylamine] have been determined by conventional heavy-atom techniques. In 2b the individual molecules have mirror symmetry with the bromine atoms equatorial. The N-O bond length is 1.278(5) Å, CN?C is 125.1(3)° and the N-O bond makes an angle of 24.2° with the CN?C plane. In 3 there are two different nitroxide groups. The pyrrolinyloxy ring is virtually planar with a N-O bond length of 1.252(13) Å and CN?C of 113.1(10)°. The 6 -membered ring exists in a sofa conformation with a pseudoaxial Br atom. Its nitroxide group is tetrahedrally distorted. The N-O bond length is 1.268(15) Å, CN?C is 121.7(10)° and the N-O bond makes an angle of 16.0° with the CN?C plane. The packing of 3 is dictated by a short Br... .O secondary bond of 3.09(1) Å, 0.28 Å less than the sum of the van der Waal's radii resulting in dimeric units related by a centre of symmetry.     

Complexes of oxamic acid with Au(III) and Rh(III)

The preparation and some properties of the deprotonated complexes of oxamic acid with Au(III) and Rh(III) are reported. On the basis of analytical results, conductometric measurements, magnetic moments and spectral data (IR and UV-visible), a square planar structure is proposed for K[AuL(OH)₂] and octahedral for K₃[RhL ₃] 3H₂O (whereLH₂=oxamic acid).L ^2– acts as a bidentate, non-bridging ligand.

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Komplexe der Oxamidsäure mit Au(III) und Rh(III)

Zusammenfassung Es wird über die Darstellung und einige Eigenschaften von deprotonierten Komplexen der Oxamidsäure mit Au(III) und Rh(III) berichtet. Auf der Grundlage von analytischen Ergebnissen, Leitfähigkeitsmessungen, magnetischen Momenten und IR- und UV(vis)-spektroskopischen Daten wird für K[AuL(OH)₂] eine quadratisch planare und für K₃[RhL ₃] 3 H₂O eine oktaedrische Struktur vorgeschlagen (LH₂=Oxamidsäure).L ^2– reagiert als zweizähniger, nicht überbrückender Ligand.

     

Reversibility in the protonolysis of a beta-diketiminate stabilised calcium bis(trimethylsilyl)amide with benzylamine

Protonolysis of the beta-diketiminate calcium bis(trimethylsilyl)amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] with benzylamine is reversible and forms a quantifiable equilibrium mixture.     

The preparation and characterization of 5-substituted-4-chloro-1,2,3-dithiazolium salts and their conversion into 4-substituted-3-chloro-1,2,5-thiadiazoles

A series of monosubstituted acetonitriles were treated with disulfur dichloride at room temperature in CH2Cl2 to afford 5-substituted-4-chloro-1,2,3-dithiazolium chlorides 1.Where the 5-substituent was not a good leaving group the chloride salts were converted into the corresponding perchlorate salts 2 which were sufficiently stable and soluble to provide both (1)H- and (13)C-NMR and cyclic voltammetry data. Several of the dithiazolium chlorides were converted into their corresponding 4-substituted-3-chloro-1,2,5-thiadiazoles 13 on treatment with aqueous ammonia. Mechanisms for all reactions are proposed.     

Synthesis of the beta 2 agonist (R)-salmeterol using a sequence of supported reagents and scavenging agents

[formula: see text] The enantioselective synthesis of (R)-salmeterol has been achieved by using a sequence of supported reagents and sequestering agents. The saligenin core was installed by a regiospecific alkylation and a chiral auxilliary approach was employed to introduce the desired stereochemistry via a diastereoselective reduction.     

The Autoxidation of Triaryl Trithioarsenites, (ArS)3As: Evidence for Binding and Activation of Triplet Dioxygen by Arsenic(III)

Triaryl trithioarsenites, (ArS)₃As, are oxidized by air to As ₂ O ₃ and ArSSAr. In two cases the parent “thiol” (pyrid-2-thione and 1-hydroxypyrid-2-thione) is coproduced. The oxidation, in nonprotic solvents, is favored by electron-withdrawing groups at the para position of the phenyl group. The products obtained in nonprotic solvents were rationalized by assuming that the binding of the triplet dioxygen to arsenic(III) gives a triplet diradical, (ArS) ₃ A?─O─?, or an arsenadioxirane, (ArS) ₃ As(O ₂ ), intermediate, which decomposes after nucleophilic attack by another (ArS) ₃ As molecule. In protic solvents a zwitterion, (ArS) ₃ As⁺─O─O^?, and in the presence of moisture a hydroperoxy arsenic(V) compound, (ArS)₃As(OH)─O─OH, may be intermediates in the air oxidation of some aromatic trithioarsenites. These data tend to indicate that arsenic(III) bound to suitable groups can directly bind dioxygen, a property which may have implications in chemotherapy and carcinogenesis.     

Ferromagnetic Interactions in a 1D Alternating Linear Chain of π‐Stacked 1,3‐Diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl Radicals

X‐ray studies show that 1,3‐diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 6 ) adopts a distorted, slipped π‐stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7‐(thien‐2‐yl)benzotriazin‐4‐yl 6 show two fully reversible waves that correspond to the ?1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie–Weiss behavior in the 5–300 K region with C=0.378 emu K mol^?1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12 cm^?1, J₂=+1.28 cm^?1).     

Electron affinities of a homologous series of tertiary alkyl radicals and their C-H bond dissociation energies (BDEs)

Heats of formation have been derived from G3(MP2)//B3LYP and G3MP2B3(+) atomization energies for tert-butyl radical (6R), cubyl radical, bicyclooctyl radical (1R), and tricyclo[3.3.n.0^3,7]alk-3(7)-yl (n=0-3, 2R-5R) radicals, and their respective anions (1A-6A) and hydrocarbons (1H-6H). The electron affinity (EA) of 6R is estimated at 1.5±2 kcal/mol and tert-butyl anion (6A) is likely to be bound. In the homologous series 2R-5R the EAs range from 3.4±2 to 13.5±2 kcal/mol. The computed enthalpies of the acidities of the tricyclic hydrocarbons 1H-5H are in the range 407-411 kcal/mol. Their C-H bond dissociation energies (BDEs) are in the range 97-110 kcal/mol. The increase of the BDEs in the homologous series 2H-5H and the increase of EAs of 2A-5A is attributed to the enhanced pyramidalization induced in radicals 2R-5R by the shortening of the methylene chain connecting carbons C₃ and C₇.