Udenfriend s reagent

Hydrogen peroxide can be replaced by molecular oxygen in vanadium-catalyzed hydroxylations of arenes, as first reported by Mukaiyama (Eq. 3) [24]. Improved versions were later described by Battistel (VCl3/ascorbate) [25] and Shul pin ((n-Bu)4NV03/pyrazine-2-carboxylic acid/ascorbic acid) [26], It is noteworthy that a molecular oxygen/sacrificial reductant (crotonaldehyde, zinc, or ascorbate) combination was used (cf. Udenfriend s reagent above) and that the mechanism is most likely to involve hydroxyl radicals. 

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

However, since Udenfriend s reagent does not cause the NIH shift,676 hydrox-ylation by it does not proceed by the same mechanism as that involved in the enzymatic oxidation. A radical process seems to be more likely. Norman and Lindsay Smith70 have suggested a mechanism without directly implicating the hydroxyl radical ... 

Udenfriend s reagent (Fe +, EDTA, O2, ascorbic acid) hydioxylates benzene to phenol evidence based on the inhibitory effects of alcohols has led to the conclusion that hydroxyl radicals are involved in this process. ... 

It is notable that flavone and its derivatives are substituted predominantly in the phenyl group at positions 3 and 4 whereas reaction with Fenton s reagent gives mainly the S-hydroxy isomer." " Similarly, comparison of the distribution of products from coumarin obtained with Udenfriend s system with that given by Fenton s reagent re-... 

Udenfriend, S., Stein, S., Bohlen, P., Dairman, W., Leimgruber, W., and Weigele, M., Fluorescamine a reagent for assay of amino acids, peptides, proteins, and primary amines in the picomole range, Science, 178, 871, 1972. 

S. Udenfriend, S. Stein, P. Bohlen, W. Dairman, W. Leimgruber, and M. Weigele, Science, 178 871-872 (1972). Fluorescamine A Reagent for Assay of Amino Acids, Peptides, Proteins and Primary- Amines in the PicoMole Range. 

The fact that Udenfriend s system does not bring about the NIH-shift [45] points to a mechanism different from that operative in enzymic hydroxylations. However, deuterated arenes with Fenton s reagent produce an NIH-shift in non-protic solvents (acetonitrile, nitromethane) [46] to an extent related to the amount of water in the system. Taking into account Groves results, supporting the formation of oxoiron intermediates from Fenton s reagent [47-50] ... 

An even more sensitive reagent for this purpose was introduced by S. Udenfriend and collaborators in 1972, M. Weigele s fluorescamine (III in Fig. 4) which at room temperature with primary amines quickly forms strongly fluorescent pyrrolinones (IV) fluorometrically measurable even in the picomole range [14]. 

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