Pyrazoline, fragmentations

Another method for the preparation of polymers containing pyrazoline fragments was suggested in [18]. Poly (9i/-fluoren-2,7-ylene)- / -[3,5-bis(l,4-phenylene)-4,5-dihydro-1-phenylpyrazole] 53 was synthesized by the reaction of pyrazoline 51 and fluorene-2,7-bis(trimethylene boronate) 52 in toluene in the presence of a catalyst, Pd(PPh3)4, with 85% yield. 

Cyclohexadiene 45 was converted to 46 by what has proven to be a general method for preparation of the cyclohexa-2,4-dien-l-one ring system.2 Fragmentation of the aziridinyl imine in 46 at 110 °C gave an intermediate diazoalkane which underwent an intramolecular 1,3-dipolar cycloaddition to give the pyrazoline 47. At 140 °C, pyrazoline 47 expelled N2 and rearranged to the tricyclic ketone 48. The development of this and related bicyclizations29 illustrated a practical synthetic equivalence of an intramolecular diene-carbene 4-1-1 cycloaddition in the cyclohexa-2,4-dien-l-one series. 

Dimerization of pyrazolines-2 on the action of oxidants includes the formation and doubling of cation-radicals (Morkovnik and Okhlobystin 1979). The doubling process is usually characterized by the terms head and tail. The term head is applicable to the position bearing a polar fragment, whereas the term tail is adopted for an unsaturated molecular site. The reaction under consideration follows the head-to-head order. This means that the doubling cation-radical has only a single position with the maximal density of an unpaired electron (Scheme 3.11). 

Treatment of (126) with ethanolic hydrochloric acid causes fragmentation to give acetophenone and 3,5-diphenyl-2-pyrazoline (equation 17). 

The increase in energy in a molecule on absorption of UV light is sufficient to bring about bond cleavage. As a result, fragmentation and rearrangement of the molecule can occur. The effect on heterocycles is discussed in this section and, for simplicity, the transformations are classified, somewhat arbitrarily, on the basis of ring size pyrazolines are treated separately. Heterocyclic dienes and heteroaromatic compounds are also discussed separately, and the section is completed by consideration of the photochemistry of heteroaromatic A-oxides. 

It is known that pyrazolines and pyrazoles are an important class of pharmacophores [1,2,3,4, 5,6], Heterocycles containing these fragments are important targets in synthetic and medicinal chemistry since they are key moieties in numerous biologically active compounds possessing tranquilizing, muscle relaxant, psychoanaleptic, anticonvulsant and antidepressant activities [7, 8, 9,10,11,12], This includes kinesin spindle protein [12] and monoamine oxidase [13, 14] inhibitors as well as antimycobacterial [15, 16] and anti-inflammatory [17] agents. 

Complexes of type XIV are more stable in the case of diazanorbomene derivatives (37) than in the pyrazoline compounds where only one cycloadduct (34c) could be isolated. The characteristic property of XIV is the smooth transformation into the diazaferroles XV which can easily be monitored by 1H-NMR spectroscopy. This suggests that the leaving iron carbonyl fragment is converted into a diamagnetic complex. 

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. 

VI is that of II minus an HCN fragment. It is also of importance to recognize that cyclopropanation is indeed an excellent method to incorporate a methylene unit into I. Then, one is led to believe that II ought to be the cyclopropane derivative VIII, produced in all probability from pyrazoline intermediate VII (see Scheme 45.2). 

Photochemical ring contraction of various substrates, e.g. cyclobutane-1,3-diones and 1 -pyrazolin-4-ones - gives either isolable cyclopropanes or cyclopropanones as intermediates which are usually intercepted as adducts or which fragment to alkenes and carbon monoxide. The photoextrusion process most often leads to fragmentation products. 

Thermolysis of various substrates (e.g. 1-pyrazolin-4-ones) capable of expelling a neutral molecule, such as nitrogen, gives rise to cyclopropanones or fragmentation products thereof. ... 

Tandem sequences have also yielded some interesting pyrazole structures. Four-component coupling of terminal alkynes 37, hydrazines 38, carbon monoxide and aryl iodides furnished pyrazoles 39 in the presence of palladium catalyst <05OL4487>. Fully substituted 1/f-pyrazoles 42 were prepared from the condensation/fragmentation/cyclization/extrusion reactions of thietanone 40 with 1,2,4,5-tetrazines 41 <05JOC8468>. Reactions of isocyanides 43 and dialkyl acetylenedicarboxylates 44 in the presence of 1,2-diacylhydrazines 45 led to highly-functionalized pyrazolines 46 <05TL6545>. 

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. ... 

Oligophenylenevinylenes (OPV) play a prominent role in science and, owing to their optical luminescence and electronic properties, have attracted much attention from researchers in many fields [59]. The electronic properties of oligophenylenevinylenes (OPV) make them versatile photo and/or electro-active components for preparation of photochemical molecular devices [60], particularly when this fragment is bonded to the C o sphere [61]. Indeed, an oligophenylenevinylene moiety (OPV) has been attached to C o using a pyrazoline ring as a linker (Scheme 21.20) [62]. 

Experimental support for direct fragmentation of 60 was found in studies of pyrazoline oxide 65, which cannot cyclize to 67 without engendering about 22 kcal/mol of ring strain. The fact that 65 gives a 34% yield of ethylene shows that it fragments directly to 66, which implies that 60 follows the same mechanism. Isomeric radical 68 gave no ethylene because P-scission would produce 69, which is far less stabilized than 66 (see Table I and its associated discussion below). 

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