Fiber Formation by Highly CO₂‐Soluble Bisureas Containing Peracetylated Carbohydrate Groups

Abstract

Supercritical carbon dioxide (scCO 2 ) has attracted interest as an environmentally benign solvent, [1] but its practical usage is limited by the low CO 2 -solubility of polar and high-molecularweight compounds. To enhance the CO 2 -solubility of such compounds, CO 2 -philic surfactants, [2,3] dispersants, [4,5] thickeners, [6] and polymers [5,7] have been designed. Early success was achieved by the use of polyfluoroalkyl groups: various derivatives incorporating these groups showed enhanced CO 2 -solubility. In an attempt to develop "greener" CO 2 -philes, [8][9][10][11] several investigators have explored other highly CO 2 -philic compounds composed solely of carbon, hydrogen, and oxygen. For example, the CO 2 -solubilities of poly(ether carbonate)s are comparable to that of poly(hexafluoropropylene oxide), probably owing, in part, to the concentration of negative charge density on the carbonyl oxygen atom of the carbonate units.[8] The acetate group was also recognized as having the potential to be very CO 2 -philic and has the advantage of easy introduction into polymers and surfactants. [11][12][13][14][15][16][17] Peracetylated sugars, such as sorbitol, [18] sugar amides, peracetylated cyclodextrins, [19] maltose octaacetate, [20] glucose pentaacetate, and galactose pentaacetate [21,22] also exhibit extraordinary solubility in scCO 2 . A Lewis acid-base interaction between the acetyl oxygen atom and the carbon atom of CO 2 , and weak CÀH···O hydrogen bonding between the acetyl methyl group and the oxygen atoms of CO 2 are believed to be responsible for the CO 2 -affinity of the acetyl group. [21,23] We have previously shown that self-aggregating organic compounds containing both hydrogen-bonding urea groups and fluorinated CO 2 -philic tails could modestly increase the viscosity of scCO 2 .[9] Upon depressurization, these solutions produced free-standing foams, which represent organic analogues of silicate aerogels, with submicron-sized fibers and a bulk density reduction of greater than 90 % of the parent material.[9] A critical feature of these systems is the presence of strong and directional hydrogen bonding between the carbonyl oxygen atoms and the NH units in the urea groups of adjacent molecules, which leads to the formation of twodimensional sheet-like structures (Figure 1). [24,25] The sheets likely further associate through noncovalent contacts to form viscosity-enhancing networks in solution and, subsequently, free-standing foams upon removal of the CO 2 . The objective of the present work was to design nonfluorous hydrogen-bonding molecules capable of dissolving in CO 2 and self-associating to form novel materials. An inexpensive and readily available source of multiple hydroxy groups is the family of mono-, di-, and oligosaccharides. These molecules can be readily converted into their peracetylated derivatives, which should have a density of electronegative groups similar to that of perfluoroalkanes. In particular, gluconic acid is readily available from glucose by oxidation and should be easil...

Chemoselectivity and the Curious Reactivity Preferences of Functional Groups

“…[22] In a marked improvement to synthetic efficiency, Enick, Hamilton, and co-workers showed that protection and deprotection can be circumvented by performing the reaction under acidic conditions. [23] In the presence of AcOH, the nucleophilicity of the amine is attenuated, thereby allowing the acylation of all five hydroxy groups of d-glucamine (14) to occur in one step, thus affording 15 in 72 % yield (Scheme 7).…”

A Family of Hydrogels Based on Ureido‐Linked Aminopolyol‐Derived Amphiphiles and Bolaamphiphiles: Synthesis, Gelation under Thermal and Sonochemical Stimuli, and Mesomorphic Characterization

“…The reactions of aminopolyols 1-3 with isocyanates 4-6 gave rise to a series of asymmetrically substituted monoureas (11)(12)(13)(14)(15)(16)(17)(18)(19) with different structural variations of the headand tail-group moieties (Scheme 1).…”

“…[11] This study, however, focuses on more amphiphilic substances, paying attention to unprotected derivatives at the sugar moiety and assessing the propensity for self-organization and aggregation in the search for more biodegradable materials. Such results are comprehensively shown in this article and complement current studies by other groups.…”

Chemoselektivität und die eigentümlichen Reaktivitäten funktioneller Gruppen

“…Die Synthesen polyacetylierter Zucker wie D ‐Glucamin ( 15 ) sind daher meist langwierig und beruhen auf mehrstufigen Sequenzen von Schützung/Entschützung 22. Eine signifikante Verbesserung erbrachte hier ein Ansatz von Enick und Hamilton, die zeigen konnten, dass sich Schützung und Entschützung unter sauren Reaktionsbedingungen vermeiden lassen 23. Im speziellen Fall wurde in Gegenwart von AcOH die Nucleophilie des Amins abgeschwächt, und die Acylierung aller fünf Hydroxygruppen von D ‐Glucamin ( 14 ) zur Bildung von 15 gelang in einem Schritt mit einer Ausbeute von 72 % (Schema ).…”