Pyrazolin-4-ylidene

Trimethylenemethane biradicals have been proposed as intermediates in the photodecomposition of fluorine-substituted 4-methylenepyrazolines both in the gas phase and in solution.Evidence for the intermediacy of two trimethylenemethane biradicals on direct irradiation of the bi-(l-pyrazolin-4-ylidene) (29) has also been reported. Benzophenone-sensitized irradiation of the same pyrazoline, however, takes a different course and affords the allene dimer (30), presumably via the tetramethylene-ethane biradical (31). 

Only a very few 4,4 -bis(2-pyrazolin-5-ones) linked by a double bond have been prepared. The parent compound of this series, 4-(3-methyl -1 -phenyl - 5 - oxo - 2 - pyrazolin - 4-ylidene)-3-methyl-l-phenyl-2-pyrazolin-5-one (XXII-D), is known as pyrazole blue.323,807,809,816, 1090,1091 was so nameci by Knorr because of its great resemblance to indigo blue. All these compounds are strongly colored. The usual synthesis is by ferric chloride oxidation of monomeric 2-pyrazolin-5-ones (eq. 61)296,809 or by oxidation of the corresponding bis compounds... 

An ethanolic soln. of salicylaldehyde followed by coned, sulfuric acid added to an ethanolic suspension of 3-methyl-5-oxo-l-phenyl-J2.pyrazoline-4-dithiocarbox-ylic acid, shaken 3 hrs. at room temp., and allowed to stand overnight 4-o-hydroxyphenyl-2-(3-methyl-5-oxo-l-phenyl-zd2-pyrazolin-4-ylidene)-l,3-dithietane. Y 85%. F. e. and reactions s. T. Takeshima et al., Soc. Perkin I 7975, 1277. 

The linking methine chain includes an even number of methine groups (0,2,4). They are commonly named as derivatives of the ketomethylene ring, for example, 3-ethyl-5-(3-ethyl-4,5-diphenylthiazolin-2-ylidene)-rhodanine (5) and 4-[4-(3-methyl-4,5-diphenylthiazolin-2-ylidene)-2-butenylidene]-3-methyl-l-p-sulfophenyl-2-pyrazolin-5-one (6) (Scheme 4). 

Novel pyrazolin-3-ylidene type carbene complexes, in which pyrazolium salts have been oxidatively added to Pd(PPh3)4, have been synthesized 128. A molecular structure has been obtained and the preformed complexes tested in Heck coupling. 

NHC)Ru(p-cymene)] complexes (NHC = imidazol-2-ylidene, imidazolin-4-ylidene or pyrazolin-2-ylidene) were also used as catalysts for the p-alkylation of secondary alcohols in the presence of KOH, in refluxing toluene. Notably, abnormal NHC complexes were better performing than normal [(IMe)Ru-Cl2(p-cymene)] 75. For further details the reader is referred to Chapter 5. 

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