Pyrazolines, nitrogen extrusion from

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,... 

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. 

Numerous unsuccessful attempts to synthesize cyclopropanethione have been reported. Thermal or photochemical generation of the C3H4S species from different sources always leads to allene episulfide. Some representative experiments include (a) in vacuo pyrolysis of the sodium salt of 2,2,4,4-tetramethylthietanone tosylhydrazone (4) into the stable tetramethylallene episulfide (S), (b) pyrolytic extrusion of nitrogen from perfluorinated thiadiazoline 6, (c) in vacuo pyrolysis of spiro compound 8 into methylenethiirane (3), (d) the flash vacuum pyrolysis-microwave spectroscopic approach applied to spiro compounds 9 and 10, (e) pyrolysis of anthracene adduct 11 and tosylhydrazide salt 12, (f) thermolytic nitrogen extrusion from pyrazoline-4-thione 13, thermolysis of tetramethylallene episulfide (5) or pyrazoline 13 in dig-lyme solution, and photolytic nitrogen extrusion from pyrazoline 13, ° (g) thionation of methylenecyclopropanone 15, and (h) reaction of donor-acceptor substituted allenes 18 with elemental sulfur. ... 

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. ... 

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... 

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. 

A theoretical study of the steric course of thermal extrusion of nitrogen from the methylene-1-pyrazoline (377) has appeared. " The deuteriated derivatives... 

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