Reagent systems

A summary of oxidation-reduction half-reactions arranged in order of decreasing oxidation strength and useful for selecting reagent systems. 

Another approach in chemical finishing is to use reagent systems that are reactive with themselves but only to a limited extent or not at all with the fiber substrate. An example of such approaches are in situ polymer systems that form a condensed fiber system within the fiber matrix (1,2). A third type of approach may be the deposition of a polymer system on the fiber substrate. Once deposited, such systems may show a strong affinity to the fiber and may be quite durable to laundering. Polyacrjiate and polyurethane are examples of durable deposits on cotton, which last through numerous launderings (3). 

Another reagent system that has been recently employed in the Paal synthesis of thiophenes is the combination of bis(trialkyltin)- or bis(triaryltin) sulfides with boron trichloride. Known as the Steliou reagent,it has been utilized in the transformation of 1,4-diketone 11 to thiophene 12. Higher yields are obtained in shorter reaction times in contrast to the use of Lawesson s reagent. Additionally, others have noted the relative ease of the work-up procedure using the Steliou conditions, and the fact that the tributyltinchloride byproduct of the reaction is reusable. Similarly, the combination of the bis(trimethylsilyl)sulfide has been used in conjunction with trimethylsilyltriflate for the preparation of thiophenes in an analogous manner. ... 

More recent examples have employed a milder reagent system, triphenyl-phosphine and dibromotetrachloroethane to generate a bromo-oxazoline, which is subsequently dehydrohalogenated. Wipf and Lim utilized their method to transform intermediate 11 into the 2,4-disubstituted system of (+)-Hennoxazole k Subsequently, Morwick and coworkers reported a generalized approach to 2,4-disubstituted oxazoles from amino acids using a similar reagent combination, triphenylphosphine and hexachloroethane. ... 

A novel one-pot synthesis of ct-nitro ketones from alkenes has been observed 01 with trimethylsilyl nitrate-chromium trioxide or a trimeihylsilyl nitrate-DMSO reagent system fEq. 2.44. ... 

The tartramide-based reagents, 10-allyl-3,6-dibenzyl-9,ll-dioxa-3,6-diaza-10-borabicyclo[6.3.0]-undecane-2,7-diones, are significantly more enantioselective than the parent tartrate ester derivatives7lb 73. Unfortunately, they have poor solubility at — 78 =C and consequently reactions times are long and conversions are often poor. The full scope of this tartramide reagent system awaits the development of a more soluble auxiliary. 

Improved methods for the preparation of reagents such as isopinocampheyl(l-isopinocam-pheyl-2-alkenyl)borinic acids will certainly lead to a more enantioselective synthesis of anti-homoallylic alcohols, since the enantiomeric purity of the reagent is the only significant limitation to the synthetic utility of this reagent system. 

We have previously shown (ref. 1) that microporous solids are useful in the controlled bromination of aromatic substrates. In particular, we showed how a reagent system comprising V-bromosuccinimide (NBS) and silica is useful for the bromination of reactive aromatic systems such as indoles (Fig. 1) (ref. 2), carbazoles and iminodibenzyls (Fig. 2) (ref. 3). 

A number of drawbacks in the application of the 0PA/2-ME reagent system include the instability of the fluorescent isoindole derivative (5-7) the use of the noisome reagent 2-mercaptoethanol the low and solvent-dependent fluorescence efficiencies (8,9) of the isoindole and—perhaps the most limiting—the effective restriction of the OPA assay to primary aliphatic amines and to amino acids. 

The initial discoveries of the extension of the aromatic ring of the ortho-phthalaldehyde (OPA) to a naphthalene-2,3-dicarboxaldehyde (NBA) and the substitution of cyanide (CN ) for 2-ME as the nucleophile have provided the Center with a much more versatile reagent system (5,11), which maintains the sensitivity for primary aliphatic amines and amino acids, and now is known to form fluorescent products with oligopeptides, proteins, and other related analytes that possess a primary amine function (Equation 1). 

The versatility of the reagent system in the assay of small peptides is nicely illustrated in Figures 3 through 5. Neurotensin, a polypeptide composed of 13 amino acids, and three fragments from partial hydrolysis were derivatized with NDA/CN and separated by... 

In order to obtain reproducible results, near equilibrium must be established in the reagent system. This is usually accomplished by combining the material to be measured with the binding protein under fixed conditions. The time required to achieve this equilibrium may vary from several hours to days depending on the system used (3>4). 

It should be noted that the selective reduction of phenylacetylene and diphenylacetylene to either the ds-alkene or the alkane was achieved using LiAlH4 in the presence of FeCk or NiCk as a catalyst [90, 91]. However, deuterolytic workup of the reaction mixtures gave deuterium incorporations <26%, indicating that these reagent systems are not well suited for the synthesis of vinyl- or alkylaluminum compounds from alkynes. 

Suggest reagents and approximate reaction conditions that would effect the following conversions. Note any special features of the reactant that should be taken into account in choosing a reagent system. 

Enhanced stereoselectivity has been found using IBr, which reacts at a lower temperature.81 (Compare Entries 6 and 7 in Scheme 4.4.) Other reagent systems that generate electrophilic iodine, such as KI + KHSOs,82 can be used for iodocyclization. 

A reagent system that is selective for allylic, benzylic, and cyclopropyl alcohols uses iodosobenzene in conjunction with a Cr(III)(salen) complex.12... 

For determination of low hydrogen peroxide concentrations, a chemiluminescent reagent system was developed consisting of oxalyldiimidazole and an immobilized fluorophore (3-aminofluoranthene) on an acrylate polymer.[47]... 

Brines Analytical grade sodium chloride, purity 99.9% was obtained from BDH and used throughout the study. Water was purified by reverse osmosis, and deionised in a Milli-Q-Reagent system immediately prior to use. 

The results obtained by using this reagent system are comparable to those achieved using such alternative catalysts as Sc(OTf)3, Yb(OTf)3, CAN, and BF3 Et20, and even better in some cases, in terms of yield, reaction time, and anomeric selectivity. 

---