The synthesis of 4-(4-nitrophenylazo) benzoyl chloride (T\'ABS-C1)2 and its use in the formation of esters was described3 in 1955. Since then, NABS-C1 has been employed advantageously for the derivatization of other aliphatic4 and aromatic5 alcohols, thiols,6 sugarsi7 amines,8 and amino acid methyl ester^;^ NABS-hydrazide has been prepared and used to prepare derivatives of aldehydes and ketones. lo The esters are usually solid,ll their bright red-orange color makes them highly suitable for chromatographic purif i c a t i~n ,~~'~ and their molecular weight13 may be determined from the uIt'raviolet absorption of the NABS chromophore. These properties make NABS-C1 a desirable reagent in the isolation and characterization of natural products.12f14Synthesis of NABS-C1.-In connect,ion with work on the struct'ure of ~i r e n i n , '~~ it' became necessary t'o synthesize NABS-C1 in quantity as shown in Scheme I.Our attempts to obtain the 6570% yield reported3 for the conversion of 1 into 2 on the original 5-g scale resulted in an average yield (t'en experiments) of 15%; larger scale reactions gave even lower yields. Other attempt's5" to increase the sca'le of the reaction have also rcsult'ed in lower yields of 2. A 63% yield is(1) Supported in part by the National Science Foundation.(2) Abbreviated. from its German name, 4-nitroazobenzolcarbonsaure-4-A .claimedsa using modified conditions, but these also failed in our hands. Consequently we examined this reaction in detail and now report conditions which reliably lead to 60-65% yields on up to a 40-g scale. I n particular, vigorous stirring, closely controlled temperature, and a nitrogen atmosphere are required. Also, the use of ammonium chloridels in place of the originally recommended acetic acid reduced the acidcatalyzed side reactions of the intermediate ethyl p-hydroxylaminobenzoate, which was then oxidized to 2 using a decreased quantity of ferric chloride in the cold.No major changes were required in the subsequent steps. The crude, thoroughly dried NABS ester 3a was purified by chromatography on alumina, and the most effect purification of the final SABS chloride (3c) was accomplished by vacuum sublimation.Ultraviolet Absorption of NABS Esters.-The lightactivated isomerization of substituted trans-azobenzenes to a photostationary equilibrium mixture of cis and trans isomers has been established.16 I n the presence of ordinary laboratory fluorescent light, solutions of trans-p-phenylazobenzoates (PAB esters) of aliphatic17 and aromatic5b alcohols are isomerized, and the isomers are chromatographically separable. l7Consequently, trans -4 -(4 -nitropheny1azo)benzoates (NABS esters) would be expected to behave similarly, and thus special precautions would be necessary to exclude Iight during handling of the solutions when precise chromatographic and spectroscopic determinations were being made.
M solutions of NABS ethyl ester(3a) were exposed to laboratory fluorescent light and the absorbance at 330 nm was noted as a function of time. A variety of solvents-benzene, ...
The most precisely determined parameter is the C-C bond length at 1.532 A. This is usually thought of as an average of two types of bonds, the shorter sp1 2-sp3 bonds to the carbonyl carbon and the normal C-C bond. Although these two bonds cannot be resolved it is interesting to consider their effect on the average. A commonly accepted value for a C-C bond adjacent to a carbonyl bond is 1.50 A,14 although there is some evidence in favor of a slightly longer bond.16 Any value for the C-C bond adjacent to the carbonyl bond which is less than 1.51 A would require a value of 1.538 A or more for the average C-C bond length for the other C-C bonds. This value is larger than that normally found in hydrocarbons but it is in agreement with that found for ZraMS-decalin by Davis and Hassel.16 This (14) E.
4‐Nitro‐benzoesäureester (I) wurde mit Zn/Ammonchlorid zur Nitrosover= . bindung (II) reduziert und diese mit p‐Nitranilin (III) zum Azoester (IVa) kondensiert, der über das Kaliumsalz (IVb) in das Säurechlorid (IVc) umgewandelt wurde.
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