3ff-Pyrazoles

Photolysis of the sulphinyl-3ff-pyrazole 587 in ether or methylene chloride leads to the formation of a relatively stable carbene 588 that can be identified by physical methods. When the irradiation is performed in ethyl vinyl ether or in furan, the expected cyclopropanes are formed smoothly and stereospecifically (equation 374). 

A series of substituted 3ff-pyrazoles has been irradiated since this reaction is an easy entry to the class of cyclopropenes 72 76>. 

Migrations of C-linked substituents around the ring, on to carbon or nitrogen atoms, are common amongst these compounds. This is the van Alphen-Huttel rearrangement and by it 3ff-pyrazoles are converted into 1/f-pyrazoles, and 2ff-imidazoles are thermally iso-merized into IH-imidazoles. 

By Elimination of a Two-Atom Fragment 4.2.1.1. From 4,5-Dihydro-3Ff-pyrazoles... 

Diazopropane in EtjO [from acetone hydrazone (2 g) and yellow HgO (15 g) ] was added dropwise to a stirred ice-cooled solution of 3,3,5,5-tetramethyl-4,5-dihydro-3ff-pyrazole-4-thione (23, 1 g, 6.4mmol) - - ° in EtjO (25 mL) until a permanent pale-pink color was observed. The mixture was... 

Matrix irradiation of the l-pyrazoline-3,5-diones (15) resulted in fragmentation and the eventual formation of carbenes (16) by the pathway shown in Scheme 2. Vinylcarbenes are intermediates in the photoelimination of nitrogen from 3H-pyrazoles. An unusual and potentially valuable application of this photochemical decomposition has been reported in 3,3-dimethyl-5-alkynyl-3ff-pyrazole (17) which, on irradiation in the presence of cyclohex-3-en-l-one, affords the cycloadduct (18) as shown in Scheme 3. ... 

Cycloproparenes may be prepared by formation of one of the lateral cyclopropane o-bonds either via biradical closing, or via 1/3/elimination. The first reported synthesis of a benzocyclopropene derivative (see Section 1)" is an application of the former of these approaches. Upon irradiation, 3//-pyrazoles 70 loose Nj, and the intermediate biradical 71 cyclizes to 72. There is evidence that the intermediate biradical is in the triplet state, but an alternative interpretation in favor of an excited singlet state has also been presented. A variety of 1,1-disub-stituted benzocyclopropenes has been synthesized by the 3ff-indazole route, which is however limited. Cycloproparenes lacking substituents at Cl are not accessible in this way, because the required indazoles occur in the IH tautomeric form 73. 

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