Kinetic and spectroscopic studies of the hydrolysis of bis (2,4‐dinitrophenyl ether)s of poly (ethylene glycol)

Abstract

Bis(2,4‐dinitrophenyl ether)s of the water‐soluble polymer poly(ethylene glycol) (PEG) are low‐loading models for time‐release herbicide systems and may potentially hydrolyse in basic aqueous media according to a mechanistic menu that includes elimination, aliphatic and/or aromatic displacement, as well as anchimerically assisted displacement involving O‐6 of the plyether backbone. This paper study reports kinetic (second‐order rate constnts, k2, and activation parameters) and 1H (400 MHz) NMR results (monitoring of the reaction in DMSO‐d6 and D2O) for the alkaline hydrolysis of a series of these ethers (DNPEGs). DNPEGs with varying average molecular weights (MW) of the polymer backbone (DNPEG‐59, ‐1900 and ‐3200, where the numbers refer to the PEG MW) were studied, as were DNPEGs that differed only in the degree of end‐group substitution, namely DNPEG‐1900‐59 and ‐81 (where the final numbers refer to the percentage substitution). The 1H NMR studies show significant aryl ring hydrogen–deuterium exchange in the hydroxide–DMSO‐d6 and hydroxide–D2O systems examined, as well as a poly(ethylene glycol) alkoxide (PEG‐O−) C‐1 adduct in DMSO‐d6; the PEG‐O− is displaced in an SNAr fashion from DNPEG as a result of initial hydroxide attack at C‐1 of DNPEG. The kinetic and spectroscopic results are discussed in the context of previous kinetic studies of alkaline hydrolysis of alkyl 2,4‐dinitrophenyl ethers, recent 1H NMR spectroscopic studies of electron‐deficient aryl ethers and the mechanism of hydrolysis in the DNPEG systems. The possible significance of these results for the development of time‐release herbicide systems is cosidered briefly.

Mechanistic Assessment of SNAr Displacement of Halides from 1-Halo-2,4-dinitrobenzenes by Selected Primary and Secondary Amines: Brønsted and Mayr Analyses

“…On the other hand, there has been an equivalent surge of interest in nucleophilic aromatic displacement reactions with more standard nitroaromatic electrophiles but under more exotic conditions of mediumsuch as the use of room temperature ionic liquids for the reaction medium–or of stabilizing complexants such as cavitands . Improved methods of stereoselective reaction, of incorporation of these nucleophilic aromatic substitution steps in cascade or tandem sequences, and in the preparation of electrophilic derivatives of water-soluble polymers , have been realized using S N Ar displacements.…”

Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

“…Even a cursory look at the literature shows the versatility of S N Ar reactions, which have been employed recently in at least one step of the reported syntheses of: fluorescent teraaza [8]circulenes, 8 2-amino-and 2-arylazoanulenes, 9 aromatic ring substituted porphyrins, 10 substituted amidoazopyridines from perfluorinated pyridine, 11 various potentially therapeutic substituted pyrimidines 12,13 and dipyridylazepines, 14 and benzimidazole N-oxides. 15 The reactions of water-soluble polymers end-group substituted with electron-deficient aromatics have been studied 16,17 and in post-polymerization functionalization, poly(acrylamide) bearing pendant 3,5-dichloro-2,4,6-triazinyl groups can react via S N Ar reaction to yield a wide range of modified poly(acrylamide)s. 18 Some significant potential environmental remediation protocols also rely on nucleophilic aromatic displacement. [19][20][21]…”

Fragmentable Polycationic Materials Based on Anchimeric Assistance

“…As with other reactions meeting the click chemistry standard, we look for applications in materials science. Substitution chemistry accelerated by neighboring group participation has been used intermittently for many years in polymer synthesis , and modification, − although sometimes intramolecular effects have been mistaken for true anchimeric assistance. − …”