Pyrazolines from photolysis

The formation of 1,3,5-triphenylpyrazole by peroxy-acid oxidation of the pyrazoline (683) can be considered as a dehydration of the intermediate oxaziridine (684). The elusive thiaziridine moiety (686) has been proposed to account for the formation of benzonitrile and sulphur from photolysis of the mesoionic oxathiazolones (685). Diaziridine intermediates (688) and (690) have been suggested to account for the products in photolysis... 

This investigation of successive bimolecular reactions is analogous to that carried out on the successive unimolecular processes represented by equations (1.64a) and (1.64b). There an estimate was made of the internal excitation of cyclopropane from photolysis of 1-pyrazoline from their experiments, Endo et al concluded that the average recoil energy from (1.74) amounted to 84 1% of that reaction s exoergicity. 

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... 

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). 

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... 

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). 

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. 

The triplet benzophenone sensitized decomposition of several pyrazolines has been shown to lead to significantly different product distributions from those obtained in direct photolysis This seems to suggest that direct photolysis involves only singlet species and not concurrent singlet-triplet reactions and the benzophenone sensitized decomposition only triplet species. In agreement with this, the direct photolysis is not affected by triplet quenchers. 

The photochemical nitrogen extrusion from 3H-pyrazoles provides a major access to cyclopropenes. It has been shown that the 3H-pyrazoles are transformed into diazoalkenes in the first step of the decomposition, and the resultant diazoalkenes give the cyclopropenes by loss of nitrogen". Benzocyclopropenes have been prepared in a similar manner" The thermal or photochemical decomposition of 4-alkylidene-l-pyrazolines produces methylenecyclopropanes". The products obtained especially in the triplet sensitized photolysis are frequently derived by a methylenecyclopropane rearrangement. 

The route can be modified to make optically active amino acids.Hence addition of diazomethane to the dioxopiperazine 14 [itself prepared from (Z)-4-benzylidene-2-methyloxazolone and L-proline] gave a 70% yield of the spiro-pyrazoline 15 (ratio of diastereomers 95 5). Separation of the major diastereomer and photolysis gave a 90% yield of the cyclopropane 16, which on hydrolysis gave (-l-)-17 in 70 % yield. 

Cyclohepta-1,4-diene (3) has been obtained from the low-temperature photolysis of pyrazolines 1 and the intermediate cis- and rran5-l,2-divinylcyclopropanes (2). c -l,2-Divinylcyclopropane has been prepared by this method and shown to be stable below 0 C. ... 

Cyclopropane derivatives are the major products of photoelimination of nitrogen from 1-pyrazolines. Conversion of the fused pyrazoline (10) into the cyclopropane (11) was achieved in this way in 85% yield by irradiation in the presence of acetophenone and constitutes a valuable step in a synthesis of a diquinane alcohol. The short-lived triplet 1,3-cyclopentadiyl biradical, generated by benzophenone-sensitised irradiation of diazabicyclo-(2.2.1]hept-2-ene, has been trapped as a Jbis-alkoxyamine by a nitroxide. The diazabicycloheptenes (12) gave in a similar fashion the bicydo[2.1.0]pentanes (13) on triplet-sensitised photolysis, whereas laser/liquid jet excitation of the same compounds gave in addition the cyclopentenes (14), derived by 1,2-hydrogen shift evidence for a two-photon process is described. The results of a time-resolved spectroscopic study of the photodecomposition of 2,3-diazabicyclo[2.2.l]hept-2-ene in the vapour phase have also been reported.Photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene affords the semi-localised... 

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