Pyrazoline ring

In the substituted 2-pyrazoline ring (27) both nitrogen atoms have lone pairs of electrons available those on N-1 are no longer involved in an aromatic system, and in the two cases so far reported the salt... 

The general method for the preparation of diphenylpyrazolines is shown in Scheme 11.8, in which X is a suitable leaving group, usually chloro but sometimes dialkylamino. This reaction normally proceeds readily, although pH control may be important. Preparation of the substituted ketone and hydrazine intermediates needed for the synthesis may involve lengthy and complicated sequences. Further reactions are often required to modify the substitution in ring B after formation of the pyrazoline ring. The preparation of compound 11.26 shown in Scheme 11.9 illustrates one of the simpler instances. 

An electron-donating group in the 4-position of the pyrazoline ring has a slight hypsochromic effect, but an electron-withdrawing group has a bathochromic effect. 

The electrooxidation of substituted pyrazolines in the presence of a base (pyridine, collidine), which facilitates deprotonation from the pyrazoline ring, gives the corresponding pyrazoles (Scheme 90). In the absence of a base, an Ar-Ar coupling product is obtained [135-138]. 

Diazomethane is capable of addition reactions with double bonds. Pyrazoline rings are formed. Studebaker et al. (38) saw in this reaction an indication of quinone groups ... 

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 formation of spirocyclopropanes from the reaction of diazodiphenylmethane and ( )-8-phenylmenthyl esters of acrylic acid and methyl fumarate occurred with a modest level of diastereofacial selectivity (136). In contrast, diastereoselectivities of 90 10 were achieved in the cycloadditions of diazo(trimethylsilyl)methane with acrylamides 65 derived from camphor sultam as the chiral auxiliary (137) (Scheme 8.16). Interestingly, the initial cycloadducts 66 afforded the nonconjugated A -pyrazolines 67 on protodesilylation the latter were converted into optically active azaproline derivatives 68. In a related manner, acrylamide 69 was converted into A -pyrazolines 70a,b (138). The major diastereoisomer 70a was used to synthesize indolizidine 71. The key step in this synthesis involves the hydrogenolytic cleavage of the pyrazoline ring. 

It is evident that the pyrazol-3-one ring system is less reactive than other m-azadienophiles. This is attributed to the fact that the azo linkage in (47) is not so electron poor as those in (53) and (54). The degree of substitution in the pyrazoline ring does not appear to hinder the oxidation,... 

Traverso etal., in their analysis of NMR spectra, showed that pyrazole and pyrazoline rings in 81 are coplanar with the benzisothiazole moiety,250 probably due to resonance structures of type 7a. 

It is noteworthy that in the case of IV there is a unidirectional, regioselec-tive formation of the pyrazoline ring due to the more electrophilic character of the or carbon of the acrylate fragment than that of the /3-methyne. The regio-selectivity of this cycloaddition notwithstanding, pyrazolines usually collapse to cyclopropanes by exclusion of molecular nitrogen under the influence of moderate heat, such as in V leading to VI. 

Treatment of 0 -benzotriazolyl-Q ,/3-unsaturated ketones with monosubstituted hydrazines followed by alkylation at the 4-position of the pyrazoline ring afforded unsymmetrical l,3,5-triaryl-4-alkylpyrazolines and -pyrazoles <2001JOC6787>. One-pot reactions of a,/3-unsaturated ketones 615 with hydrazinediium dithiocyanate gave... 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

Introduction of a further phenyl ring at position 5 of the pyrazoline ring yielded products with comparable or enhanced insecticidal effect. 3,5-Bis(4-chlorophenyl)-... 

Without additional reagents Cyclopropane ring from pyrazoline ring... 

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