Variety Of Pyridine Derivatives Research Articles

DFT Study on the Mechanism of 4,4'-Bipyridine-Catalyzed Nitrobenzene Reduction by Diboron(4) Compounds.

Diboron(4) compounds serve as useful reagents for borylation, diboration, and reduction in organic synthesis. A variety of pyridine derivatives have been found capable of activating diboron(4) compounds, and different reaction mechanisms have been identified. 4,4'-Bipyridine was found to activate diboron(4) to form N,N'-diboryl-4,4'-bipyridinylidene in 2015, and very recently, it has been found that this transformation is crucial in the 4,4'-bipyridine-catalyzed reduction of nitroarenes by bis(neopentylglycolato)diboron (B2nep2), which features the formation of arylnitrene intermediates. However, the mechanism of N,N'-diboryl-4,4'-bipyridinylidene formation, as well as its role in the transformation of nitroarene to arylnitrene, remains unknown. In this work, we investigated the possible pathways of this intriguing transformation and discovered several important intermediates through density functional theory (DFT) calculations. An N-boryl 4,4'-bipyridyl radical was found to be a crucial intermediate in both the formation of N,N'-diboryl-4,4'-bipyridinylidene and the reduction of nitroarene. A type of single-step reaction with three stages, including a dissociation and two migration steps, was identified in the generation of nitrosobenzene and its reduction. Arylnitrene formation was found to occur on a triplet potential energy surface, and an intersystem crossing was found to be important for achieving a reasonable activation energy barrier for nitrene formation. We anticipate our work to provide deeper insights into the nature of this reaction that could facilitate further rational design of pyridine- and bipyridine-based catalysts.

Purine N-oxides—XL: The 3-acyloxypurine 8-substitution reaction: Scope: Syntheses of 8-substituted xanthines and guanines

The 3-acyloxypurine 8-substitution reaction is a substitution-elimination reaction involving attack at C-8 by inorganic or organic nucleophiles and departure of an acyloxy group from N-3. It has been studied with 3-acetoxyxanthine, 3-hydroxyguanine and a number of related derivatives and is the method of choice for the preparation of many 8-substituted xanthines or guanines. It proceeds extremely rapidly in neutral aqueous solutions at room temperature. With water alone an 8-hydroxypurine results, and water always competes to some degree with other nucleophiles. The reaction can be carried out in dipolar aprotic solvents, in which it is also possible to prepare the acyloxy derivative in situ and to choose more effective leaving groups such as mesyloxy or tosyloxy. The reaction has been demonstrated with chloride, bromide, nitrite, and azide ions; with the thioether of methionine; a variety of pyridine derivatives, and with primary and secondary alcohols. This reaction is apparently restricted to 3-acyloxypurines which are also substituted at position-2. The behaviour of 3-acetoxy-1-methylxanthine is similar to that of 3-acetoxyxanthine, but 3-acetoxy-7-methylxanthine fails to undergo the reaction.

INDO-MO calculations of hyperfine splitting constants for pyridine anions

INDO-MO Calculations have been carried out on the anions of a variety of pyridine derivatives for which e.s.r. data have recently become available. The method gives reasonable results for proton hyperfine splitting constants but calculated nitrogen splitting constants are considerably larger than the experimental values. Some π-only calculations are included for the pyridine anion. All methods overestimate the spin density at the nitrogen atom. The orbital populations and excess charges for the pyridine molecule are not in agreement with those of an ab initio calculation.