Stable meso–meso-Linked 2NH-Corrole Radical Dimers as a Key Intermediate to Corrole Tape

While oxidation of 5,5′,15,15′-tetramesityl-10-10′-linked 3NH-corrole dimer with DDQ gave the corresponding triply linked 2NH-corrole tape, the use of an equimolar amount of p-chloranil as a milder oxidant resulted in the formation of a 10-10′-linked neutral 2NH-corrole radical dimer as a stable product. The stability of this peculiar product is ascribed largely to strong antiferromagnetic interaction of the two spins. Further oxidation of this diradical produced corrole tape, suggesting its involvement as a reaction intermediate to the corrole tape. Oxidation of 10-10′-linked bis-pyridine-coordinated Co^III corrole dimer with DDQ produced a cobalt corrole radical dimer and a doubly linked corrole dimer both as stable compounds bearing pyridine and cyanide axial ligands. This type of oxidative transformation involving neutral diradical intermediates is a unique reaction mechanism specific for corrole dimers.     

Cyclophane‐Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18‐Porphyrinyl Dicyanomethyl Diradicals

2,18‐Bis(dicyanomethyl)‐substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂Cl₂ at 298 K to give cyclophane‐type chlorin dimers ( 9 )₂ and ( 12 )₂ as a consequence of double recombination of biradicals 9 and 12 , respectively. Dimer ( 9 )₂ takes a syn‐conformation of two distorted Ni^II chlorins but ( 12 )₂ takes an anti‐conformation of relatively planar Zn^II chlorins. At 298 K, dimer ( 9 )₂ is stable and its ¹H NMR spectrum is sharp but becomes broad at high temperature, while the ¹H NMR spectrum of ( 12 )₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of ( 12 )₂ is much less than that of ( 9 )₂. The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer ( 16 )₂ to give homo dimers ( 9 )₂ and ( 15 )₂.     

Oxidation of some monosaccharides by hexavalent manganese in aqueous alkaline medium: A kinetic and mechanistic study

The kinetics of oxidation of some monosaccharides viz., D-ribose, D-xylose, and D-arabinose, D-glucose, D-fructose, D-galactose, 2-deoxyglucose, and α-methyl glucopyranoside by MnO₄^2? in aqueous alkaline medium have been studied. The rate of oxidation has been found to be first-order both with respect to [oxidant] and [sugar]. The rate is independent of [OH^?] under experimental conditions of [OH^?] > 0.5 M where the oxidant is stable. The effect of ionic strength is negligible on the rate. A mechanism involving the formation of a 5-membered cyclic intermediate complex between MnO₄^2? and 1,2-enediol form of the sugar is proposed. The intermediate complex decomposes to give products in the subsequent slow step. The involvement of 1,2-enediol form receives support from the reaction of α-methyl glucopyranoside, which exists in ring structure in alkaline solution reacting much slower than glucose with MnO₄^2? under similar conditions. Second-order rate constant k″ and activation parameters have been evaluated. The series of reactions exhibits a clear demonstration of applicability of isokinetic phenomenon where Arrhenius plots for all the reactions are found to intersect at a common point (295 K). © 1995 John Wiley & Sons, Inc.     

Kinetics and mechanism of protection of thymine from phosphate radical anion under anoxic conditions

The rates of photooxidation of thymine in the presence of peroxydiphosphate (PDP) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by phosphate radical anion have been determined in the presence of different concentrations of dithiothreitol (DTT). An increase in DTT is found to decrease the rate of oxidation of thymine, suggesting that DTT acts as an efficient scavenger of PO₄^·2? and protects thymine from it. Phosphate radical anion competes for thymine as well as DTT; the rate constant for the phosphate radical anion with DTT has been calculated to be 2.21 × 10⁹ dm³ mol^?1 s^?1, assuming the rate constant of phosphate radical anion reaction with thymine as 9.6 × 10⁷ dm³ mol^?1 s^?1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDP at 254 nm, the wavelength at which PDP is activated to phosphate radical anion. From the results of experimentally determined quantum yields (φ_exptl) and the quantum yields calculated (φ_cl), assuming DTT acts only as a scavenger of PO₄^·2? radicals, show that φ_exptl values are lower than φ_cl values. The φ′ values, which are experimentally found quantum yield values at each DTT concentration and corrected for PO₄^·2? scavenging by DTT, are also found to be greater than φ_exptl values. These observations suggest that the thymine radicals are repaired by DTT in addition to scavenging of phosphate radical anions. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 271–275, 2001     

N‐Heterocyclic Carbene–Phosphinidyne Transition Metal Complexes

The N‐heterocyclic carbene–phosphinidene adduct IPr?PSiMe₃ is introduced as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPr?P)ML_n] (ML_n=(η⁶‐p‐cymene)RuCl) and (η⁵‐C₅Me₅)RhCl). Their spectroscopic and structural characteristics, namely low‐field ³¹P NMR chemical shifts and short metal–phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPr?P ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi‐ and trimetallic RuAu, RhAu, Rh₂, and Rh₂Au complexes.     

Chemical investigation of the stem-bark of Cedrus deodara Isolation of a new dihydroflavonol, deodarin

A new dihydroflavonol named deodarin has been isolated from the stem-bark of Cedrus deodara. From a detailed study of its reactions and spectra, it has been tentatively assigned the constitution 3′,4′,5,6-tetrahydroxy-8-methyl dihydroflavonol.