Pyrazolines, extrusion

Open-chain and cyclic compounds containing azo groups (-N2 —), such as azoalkanes, azoarenes, pyrazolines, triazolines, etc. may also eliminate N2, but these reactions are called azo-extrusions (IUPAC, 1989 a). The terms denitrogenation and nitrogen extrusion, both used by Adam et al. (1992, 1993) and by Adam and Sengelbach (1993) should not be used. They are superfluous and ambiguous. 

When l,4-dihydronaphthalen-l,4-imine (2) was first obtained via the hydrobromide (113), it was shown to react with phenyl azide to give an adduct (127). The analogous phenyl azide adduct (128) from compound 103 has been better characterized. Naphthalen-l,4-imines also add diazomethane across the 2,3-double bond, forming pyrazolines, e.g., 104 -> 129, two of which have been photolyzed to give the corresponding cyclopropane derivatives (130) with extrusion of nitrogen. ... 

In general, open structures II are energetically preferred over the closed forms I [8, 9], In the ring closed isomers I two unfavorable double bonds within five-membered rings would be required. No monoadduct with such a structure has been observed. Fulleroids such as 1-3 are usually formed via rearrangement of their pyrazoline-, triazoline- or ozonide [6,6]-precursor adducts accompanied by extrusion of N2 or O2 (see Chapter 4) [7,10-12]. 

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). 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

Thermal or photochemical extrusion of nitrogen from frawi-pyrazolines, for example, 482 gives frani-cyclopropylamino acid derivatives 483. " Photochemical decomposition of c/i-pyrazolines 476 leads to c -cyclopropylamino acid derivatives 477 642-644 thermal decomposition of 476 affords the corresponding... 

Extrusion of N2 from Pyrazolines. Pyrazoles, and Triazolines Azo-extrusion... 

Diazo compounds have been extensively used in the preparation of three-membered carbocycles either as carbene sources or as precursors for 1-pyrazolines or 3//-pyrazoles. Nitrogen extrusion from pyrazolines is particularly valuable for the synthesis of alkylcy-clopropanes, since the direct carbene route is impractical, as a matter of fact, owing to rapid intramolecular processes in alkylcarbenes. The cycloaddition of diazo compounds to unsaturated bonds to give 1-pyrazolines and 3/f-pyrazoles usually proceed in a concerted manner, and hence is stereospecific. In the subsequent nitrogen extrusion from the adducts,... 

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

Cycloaddition of diazoalkanes to 6-6 bonds was among the first reactions tested on the carbon cages41 and affords isolable fullerene-fused pyrazolines as primary adducts. Photochemically induced extrusion of N2 from the latter yields 6-6-closed methanofullerenes having the functionalized bond embedded in a cyclopropane substructure, whereas the thermal process affords 6-5 open homofullerenes in which a methylene group is bridging the open junction between a six- and a five-membered ring.15-17... 

Scheme 1.16. Thermal extrusion-rearrangement of the first isolated fullerene-fused pyrazoline substituted at C(3), affording diastereoisomeric 6-5 open homo[60]fullerene derivatives, of which the major isomer has the bulkier group located above the former pentagon.

The nitrogen extrusion from 1-pyrazolines and 3H-pyrazoles giving cyclopropanes and cyclopropenes, respectively, has been extensively reviewed The cyclopropane synthesis from 1-pyrazolines can be executed thermally as well as photochemically, but the latter method generally gives substantially better results than the former. The major side reaction observed in the thermal process is the production of olefins, which arise in the migration of a substituent from the C(4) to C(3) position. A retro-1,3-dipolar addition producing a diazoalkane and an olefin has been observed in certain cases. The decomposition of 3-acyl- or 3-alkoxycarbonyl-1-pyrazolines is catalyzed by acids, such as perchloric acid and boron trifluoride and by Ce The stereochemical course... 

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. 

Pyrazolines can also be prepared by the oxidation of pyrazolidines. 2-Pyrazolines, which are readily obtained in the reaction of a,j -unsaturated ketones with hydrazine, also undergo nitrogen extrusion at elevated temperature usually in the presence of a basic catalyst. The reaction is believed to proceed via 1-pyrazolines. Treatment of 3,3,5-trialkyl-2-pyrazolines with lead tetraacetate followed by thermolysis affords cyclopropyl acetates ". Oxidation of certain 2-pyrazolines with manganese dioxide gives 3H-pyrazoles, which in turn produce cyclopropenes in the photolysis (equation 7). ... 

We will now briefly examine y-elimination and extrusion reactions. An example of the former is the formation of a cyclopropane from 1-pyrazoline initiated either by heat or light. Suggest a mechanism for this reaction. 

An example of a y-elimination reaction is the formation of a cyclopropane from 1-pyrazoline initiated either by heat or light. The initial biradical, which is formed by the loss of the nitrogen molecule, then closes the ring to give the cyclic product. The conversion of R-CO-R to R-R and C=0 is called a Norrish Type I reaction. In this case, the C C bonds on each side of the carbonyl group cleave, and the resultant radicals then combine and so effect the extrusion of the CO molecule. 

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