Pyrazoline, photolysis

Pyrazolines (51) can be converted to cyclopropane and N2 on photolysis""" or pyroiysis. The tautomeric 2-pyrazolines (52), which are more stable than 51 also give the reaction, but in this case an acidic or basic catalyst is required, the function of which is to convert 52 to 51." In the absence of such catalysts, 52 do not react/ In a similar manner, triazolines (53) are converted to aziridines." Side reactions are frequent with both 51 and 53, and some substrates do not give the reaction at all. However, the reaction has proved synthetically useful in many cases. In general, photolysis gives better yields and fewer side reactions than pyrolysis with both 51 and 53. S/Z-Pyrazoles" " (54) are stable to heat, but in some cases can be converted to... 

Alkenyl-substituted diarylnitrilimines 325, generated by photolysis or thermolysis of corresponding tetrazoles 324, undergo a regioselective INIC reaction to yield fused 2-pyrazolines (Scheme 34) [93]. Similarly, with alkynyl derivatives... 

Photoelimination of nitrogen from 1-pyrazolines has also been employed in the synthesis of tricyclo[3.2.1.02,4]oct-6-ene,338 prismane,339 quadri-cyclane,340 snoutene ,341 and marasmic acid.342 The trimethylenemethanes 414 have been prepared by photolysis of azoalkanes 415 and characterized spectroscopically.343 Dimerization and cycloaddition to alkenes of these biradicals have been reported.344... 

An interesting preparation of aliphatic diazoalkanes (R R C = N2 R, R = alkyl) involves the photolysis of 2-alkoxy-2,5-dihydro-1,3.4-oxadiazoles (see Scheme 8.49). When the photolysis is carried out in the presence of an appropriate dipolarophUe, the diazo compounds can be intercepted (prior to their further photolysis) by a [3 + 2] cycloaddition reaction (54). As an example, 2-diazopropane was intercepted with A-phenylmaleimide (54) and norbornenes (55) to give the corresponding A -pyrazolines. 

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

The photolysis or thermolysis of certain 1,3-bis(diazomethyl)trisilanes furnished products that are derived from bicyclic pyrazolines formed by intramolecular 1,3-dipolar cycloaddition at the Si= bond of a diazosilene intermediate (343,344). 

Lack of stereospecificity, extensive formation of olefinic products, and extensive tar formation limit the thermal decomposition of pyrazolines as a route to cyclopropanes.182 263 Light-induced decomposition of stereoisomeric pyrazolines establishes a method for the formation of cyclopropanes stereospecifically.222 Photolysis of 3-carbomethoxy-cis-3,4-dimethyl-l-pyrazoline (CCLI) produced cis-l,2-dimethylcycIopropane-l-carboxylate (CCLII) and without olefinic formation. Furthermore, irradiation of 3-carbomethoxy-trans-3,4-dimethyl-l-pyrazoline (CCLIII) gave [Pg.123]

The photolysis of the 4-alkylidene-l-pyrazoline (94) gives rise to two isomeric methylene cyclopropanes (95 and 96).76 The available evidence points to the intermediacy of a trimethylenemethyl species (97) in the triplet state which can cyclize in three ways. The same species is postulated in the photolysis of a series of 4-alkylidene-l-pyrazoline-3-carboxylates.77 This appears to be a general route to derivatives of trimethylenemethyl trimethylenemethyl itself has been generated from 4-methylene-1-pyrazoline and the triplet nature of the intermediate identified by electron spin resonance (ESR) spectroscopy.78... 

A stereoselective photochemical addition of diazomethane to unsaturated uronate 40 undergoes a pyrazoline derivative 41 which, after photolysis, leads to cyclopropano-... 

Addition of diazomethane to a toluene solution of C70 generated a 12 1 2 mixture of pyrazolines. The structures of the three isomers are shown in Scheme 65. The formation of 174a and 174b in unequal amounts appears to be a well-characterized regioselective transformation at a fullerene bond. Photolysis and ther-... 

In the case of alkyl substituted pyrazolines such as (307), thermal decomposition at 40-70 °C or photolysis leads to complex products which could be explained either in terms of a diradical intermediate or of rearrangement to a diazo-compound followed by loss of nitrogen to produce a carbene. However, on brief heating to 80 °C or on... 

Photolysis of pyrazolines derived from l,2-bis-trimethylsilyl-cyclopropene-3,3-di-carboxylates has also been shown to provide an efficient route to bicyclobutanes 249). 

The photolysis of pyrazolines has attracted much attention since in this reaction 1,3-diradicals are produced, which were supposed to be related to the intermediate trimethylene diradical which would be a model for an addition of singlet... 

For easier comparison the result of the thermal reaction is included for compounds 46 and 47. As can be seen in the reaction scheme above direct photolysis of the pyrazolines 46 and 47 proceeds mainly with retention of the original stereochemistry in the cyclopropanes isolated. 48,49 and 50 however lead mainly to the inverted stereochemistry in the cyclopropanes. The singlet biradical 51 formed from 46—49 is therefore clearly not on the same energy surface as a, . possible singlet diradical in the carbene cycloaddition. However one knows today that singlet carbene cycloaddition is a concerted process, so no such diradical should be detectable. 

Scheme 3, bottom). If the energy is transferred to the enantiomeric pyrazoline moiety at different rates, appreciable CD signal may evolve in the course of the photolysis. However, the energy transfer was found to be too fast and/or too efficient to discriminate the enantiomeric configuration of the substrate moiety. 

Ground-state triplet dicarbenes in photochemical systems have also been extensively studied by ESR. Dowd and coworkers (304) examined the interesting, frequently postulated ground triplet trimethylenemethane obtained by photolysis of either 4-methylene-A -pyrazoline or a single crystal of 3-methylene-cyclobutanone. This triplet molecule is axially symmetrical and the proton hyperfine splittings of 8.9 gauss observed in single-crystal experiments indicate that all protons in the molecule are equivalent when the axis perpendicular to the plane... 

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