On the importance of π–π stacking and cation–anion interactions in the construction of non-centrosymmetric networks of bromide salts of imidazolium cations bearing arene and polyfluoroarene rings

The salt 1-(2,3,5,6-tetrafluoropyridyl)-3-benzylimidazolium bromide crystallizes in the non-centrosymmetric space group Pna2₁. The structure arises from π–π stacking between the benzyl and tetrafluoropyridyl groups of the cations and cation–bromide interactions. It is the latter that gives rise to the non-centrosymmetry

Polyvinylpyrrolidone-bromine complex: An efficient polymeric reagent for selective preparation of benzyl bromides in the presence of hexamethyldisilane

Benzylic bromides were conveniently obtained in high yields via the reaction of the corresponding alcohols with crosslinked polyvinylpyrrolidone-bromine complex (PVPP-Br2)/hexamethyldisilane in chloroform at reflux condition. Selective conversion of benzyl alcohol to benzyl bromide in the presence of primary aliphatic alcohols, e.g. 2-phenylethanol was also achieved. KEY WORDS: Polyvinylpyrrolidone-bromine, Benzyl bromide, Hexamethyldisilane, Selective bromination Bull. Chem. Soc. Ethiop. 2012, 26(2), 305-309.DOI: http://dx.doi.org/10.4314/bcse.v26i2.1

Reaction of selected carbohydrate aldehydes with benzylmagnesium halides: benzyl versus o-tolyl rearrangement

The Grignard reaction of 2,3-O-isopropylidene-alpha-D-lyxo-pentodialdo-1,4-furanoside and benzylmagnesium chloride (or bromide) afforded a non-separable mixture of diastereomeric benzyl carbinols and diastereomeric o-tolyl carbinols. The latter resulted from an unexpected benzyl to o-tolyl rearrangement. The proportion of benzyl versus o-tolyl derivatives depended on the reaction conditions. Benzylmagnesium chloride afforded predominantly o-tolyl carbinols while the application of benzylmagnesium bromide led preferably to the o-tolyl carbinols only when used in excess or at higher temperatures. The structures of the benzyl and o-tolyl derivatives were confirmed unambiguously by NMR spectral data and X-ray crystallographic analysis of their 5-ketone analogues obtained by oxidation of the corresponding mixture of diastereomeric carbinols. A possible mechanism for the Grignard reaction leading to the benzyl -> o-tolyl rearrangement is also proposed

Carbohydrate-derived iminium salt organocatalysts for the asymmetric epoxidation of alkenes

A new family of carbohydrate-based dihydroisoquinolinium salts has been prepared and tested for potential as asymmetric catalysts for the epoxidation of unfunctionalized alkene substrates, providing up to 57% ee in the product epoxides

One-Pot Synthesis of Nα-Protected Amino/Peptide O-benzyl Hydroxamates and Acylaminoxy Dipeptides Employing Hydroxamic Acid

The direct conversion of simple and sterically hindered Nα-protected amino/peptide hydroxamic acids to O-benzyl hydroxamates employing K2CO3, tetrabutylammonium bromide and benzyl bromide is described. In addition, Cbz-Ala-CONHOH and Cbz-Phe-CONHOH derived acylaminoxy peptides 3j and 3k have also been prepared. The method is of importance in the view of easy availability of precursors, catalyst and reaction conditions. All the products are obtained in moderate to good yields

Synthesis and evaluation of N⁶-substituted apioadenosines as potential adenosine A₃ receptor modulators

Adenosine receptors (ARs) trigger signal transduction pathways inside the cell when activated by extracellular adenosine. Selective modulation of the A(3)AR subtype may be beneficial in controlling diseases such as colorectal cancer and rheumatoid arthritis. Here, we report the synthesis and evaluation of beta-D-apio-D-furano- and alpha-D-apio-L-furanoadenosines and derivatives thereof. Introduction of a 2-methoxy-5-chlorobenzyl group at N-6 of beta-D-apio-D-furanoadenosine afforded an A(3)AR antagonist (10c, K = 0.98 mu M), while a similar modification of an alpha-D-apio-L-furanoadenosine gave rise to a partial agonist (11c, K-i = 3.07 mu M). The structural basis for this difference was examined by docking to an A(3)AR model; the antagonist lacked a crucial interaction with Thr94

Electrogenerated N-Heterocyclic Carbene in Ionic Liquid: An Insight into the Mechanism of the Oxidative Esterification of Aromatic Aldehydes

An N-heterocyclic carbene (NHC), generated by cathodic reduction of BMImBF4, mediates the oxidative esterification of aromatic aldehydes with organic bromides in the corresponding ionic liquid as solvent. The product recovery by simple extractive work-up with diethyl ether allowed the ionic liquid to be recycled up to 9 times for subsequent electrolyses, with no significant loss in the product yield. The isolation of an intermediate, whose structure was confirmed by synthesis and transformation into the ester, provided the key for a mechanistic insight into the reaction. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Molecular Structures of the Metastable Charge-Transfer Complexes of Benzene (and Toluene) With Bromine as the Pre-Reactive Intermediates in Electrophilic Aromatic Bromination

Successful crystallization and X-ray crystallographic analyses of the highly metastable (1∶1) complexes of bromine with benzene and toluene establish the unique (localized) structure B that differs in notable ways from the long-accepted (delocalized) structure A. Furthermore, we demonstrate the (highly structured) charge-transfer complexes [C6H6,Br2] and [CH3C6H5,Br2] to be the pre-reactive intermediates that are converted (via an overall Br+ transfer) to the Wheland intermediates in electrophilic aromatic bromination. The role of the dative ion pairs [C6H6˙+ Br2˙−] and [CH3C6H5˙+ Br2˙−] in the rate-limiting activation processes is underscored