Photochemical nitrogen extrusion

Azides undergo intramolecular cycloaddition to C—C double bonds present in rings to provide a variety of novel products. Schultz et al for example, reported thermolysis of azide (206) at 110 C to afford an isolable triazoline as a single diastereomer (Scheme 64).115 A related reaction, however, gave an isomer mixture. Photochemical nitrogen extrusion from the triazoline provided a dihydropyrrole in 97% yield. 

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. 

Transition-metal-catalyzed, thermal, or photochemical nitrogen extrusion from allyloxysilyl-substituted diazoacetates 6 leads to formation of the 2-oxa-l-silabicyclo[3.1.0]hexanes... 

As part of a study on 1,4-diradicals, various 3,4,5,6-tetrahydropyridazines were decomposed thermally to olefins and cyclobutane derivatives (see also Section III,G). The activation parameters were measured. The stereochemical paths of thermal and photochemical nitrogen extrusion from 3,4,5,6-tetrahydropyridazines have been reexamined and rationalized. Kinetic measurements of the decomposition of 3,6-dimethyl-... 

Photochemical nitrogen extrusion also is the key reaction in a cross coupling reaction between reactive sides of a polymer chains as described by H. Meier. In this case a thia-diazole linked to a polyethylene chain (prepared from 3-pentanone in five steps) serves as the chromophore and double denitrogenation leads to chain dimerization with formation of a 1,4-dithiine. 

As part of a study on 1,4-diradicals, various 3,4,5,6-tetrahydropyridazines were decomposed thermally513-518 to olefins and cyclobutane derivatives (see also Section III,G). The activation parameters were measured.519,520 The stereochemical paths of thermal and photochemical nitrogen extrusion from 3,4,5,6-tetrahydropyridazines have been reexamined and rationalized.521 Kinetic measurements of the decomposition of 3,6-dimethyl-3,6-dicyano-3,4,5,6-tetrahydropyridazine have been made.522 Pyridazines with specific functional group orientation can be used as starting material for the preparation of various dienes (Scheme 11). For example, compound 232 is quantitatively transformed into trans,frans-2,4-hexadiene (233), whereas the trans isomer 234 affords exclusively the cis-trans product (235).523,524 1,2-Dimethyl-1,2,3,6-tetrahydropyridazine is thermally decomposed to give mainly trans-azomethane, with about 10% of the cis-isomer.525... 

Pyrazolines undergo photochemical nitrogen extrusion but the efficiency of extrusion is very structure-dependent. The very low quantum yield for this process in the case of 4-oxo-3,3,5,5-tetramethyl-l-pyrazoline (6) warrants further investigation.84... 

Extrusion of Nitrogen.— Two important photochemical nitrogen extrusion reactions deserve mention. The first was in the synthesis of the highly strained prismane (188) in low yield by irradiation of the azo-precursor (187) in CD2CI2 at temperatures above 30 The second was in the conversion of the pyrazoline... 

Scheme 1 illustrates the synthesis of prismane ([3]prismane) 1 by Katz and Acton Benzvalene 5 reacts with the powerful dienophile, 4-phenyltriazohnedione 6, in diethyl ether-dioxane to give the 1 1 adduct 7 (60-70%). Refluxing 7 with KOH in MeOH/HjO, followed by treatments with acidic CuCljand aqueous NaOH, gives azo compound 8. Irradiation of 8 through Pyrex in isobutane at 35°C affords prismane 1 (1.8%) as an explosive colorless liquid. Prismane is stable at room temperature. One of the key steps of this synthesis is a photochemical nitrogen extrusion (Scheme 1). 

This contribution is preceded by an earlier chapter, which covered the main topics of the photochemistry of cyclic azoalkanes for over three decades. Herein, we address the following mechanistic aspects of cyclic azoalkane photochemistry (1) the generation of azoalkane triplet states on direct irradiation and their photochemical transformations, and (2) the double inversion of the molecular skeleton in the formation of housane during photochemical nitrogen extrusion. 

Dehnitive for triplet-state photochemistry, the a-CN-bond cleavage has been recently disclosed in the direct irradiation of azoalkanes 5. A prominent feature of the photolysis of azoalkanes 5 (Scheme 4) is the fact that the syn/anti product diastereoselectivity depends on temperature and bridgehead substitution. The temperature dependence of the direct photolysis of azoalkanes 5 stems from the competition between the singlet and triplet reaction channels (Scheme 4), the latter dominating at low temperature. Mechanistic details of this diastereoselective process is considered below, subsequent to a general discussion on the stereochemical inversion phenomenon in the photochemical nitrogen extrusion. 

Stereochemical Double Inversion in Photochemical Nitrogen Extrusion... 

Aziridinimine derivatives are also accessible by the photochemically induced extrusion of nitrogen and ring contraction of simple alkylidene tetrazolines (424) <75AG(E)428). Thus, Pyrex-filtered photolysis of (424) at -60 °C quantitatively afforded the aziridinimines (425). 

Vinylcarbenes. The use of diazoalkenes as vinylcarbene precursors is often precluded by rapid cyclization, with formation of pyrazoles. However, on photochemical generation of the diazoalkenes in situ, cyclization and nitrogen extrusion can proceed competitively. Photolysis of 1,3-diphenylpropenone to-sylhydrazone sodium salt (5) in MeOD afforded 3,5-diphenylpyrazole (9) and l,3-diphenyl-3-methoxypropene (10) in similar amounts.17 If 10 is formed by way of the 1,3-diphenylallyl cation (8), the deuterium should be distributed between C-l and C-3 of 10 (Scheme 6). The observed ratio of 10a to 10b was 66 34 the methoxy group is bound preferentially to the deuterated site, which originates from the divalent carbon of 7 (for a discussion of this effect, see below). 

Nitrogen extrusion from diazo compounds can be induced photochemieally, thermally, or by transition-metal catalysis. Complex 2f is decomposed neither by eopper(I) triflate (CuOTf) nor by Rh2(OAc)4 at room temperature. The photochemical decomposition of 2f leads to simultanous nitrogen and earbonyl extrusion, but no product could be characterized. 

Nitrogen extrusion from a-diazoketone and the 1,2-shift can occur either in a concerted manner or stepwise via a carbene intermediate known as the Wolff rearrangement (Scheme 2.58). a-Diazoketones undergo the Wolff rearrangement thermally in the range between room temperature and 750°C in gas-phase pyrolysis. Due to the formation of side products at elevated temperatures, the photochemical or silver-metal-catalyzed variants are often preferred that occur at lower reaction temperature. 

Both the thermally and the photochemically initiated extrusion of nitrogen from bicyclic diazene (1) leads to the formation of a 1,3-diradical (diyl) which is structurally related to trimethylenemethane. As shown in the cascade mechanism illustrated, two forms of the diyl can be intercepted by alkenes bearing electron-withdrawing groups (diylophiles), viz. the open form of the singlet diyl and the triplet diyl which is formed via intersystem crossing from the singlet (Scheme 1). ... 

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... 

In contrast to these results, the intramolecular cyclopropanation of (allyldiisopropylsilyl)di-azoacetate 5 occurs only photochemically to give the l-silabicyclo[2.1.0]pentane 10 in good yield. Nitrogen extrusion from 5 occurs neither thermally (180°C) nor under catalysis by rhodium(II) bis(perfluorobutanoate) dimer. 

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. ... 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

Recently Quast reported some photochemical preparations of diaziri-dinones and diaziridinimines. The three-membered ring compounds 140 and 142 are formed by nitrogen extrusion from the tetrazolines 139 and 141, respectively. 

A variety of methods are used to generate and study diyls including, to name a few, the thermal or photochemically promoted extrusion of nitrogen from a pyr-azoline, the Norrish type I photochemically initiated extrusion of carbon monoxide from a cyclic ketone, and the Bergman cyclization of enediynes [3-5]. In this chapter we focus upon the synthetic utility of diradicals, paying particular attention to the work of others as well as research originating from our laboratory. 

To determine whether enthalpic or entropic factors were responsible for controlling the product distribution, the cycloaddition was conducted at several different temperatures. In analogy with previous results, there was reason to believe that both thermally and photochemically initiated extrusion of nitrogen would lead to the same 1,3-diyl, thereby allowing one to examine the chemistry over a reasonably large temperature range [15]. In practice, the product ratio (40 sum of minor products = SMnr)... 

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