N2 extrusion

Recently, diazocyclopropane (246) was synthesized from cyclopropyl N-nitroso urea (245) and its reaction with 1 has been studied. The cycloaddition gave a mixture of the unique primary adduct 248 together with the [3]-tri-angulane (247) derived from N2 extrusion (Scheme 40) [62]. 

Phosphaalkenes that possess a A, a -phosphorus atom can be isolated when appropriately substituted (151). These systems exhibit a much more expressed dipolarophilic than dienophihc reactivity, probably as a consequence of the polarity of the P=C bond. The [3 + 2] cycloaddition of diazo compounds with phosphaalkenes 87 leads to 4,5-dihydro-3//-l,2,4-diazaphospholes 88 (Scheme 8.21) that are not always isolated. Quite often, ehmination of molecular nitrogen occurs during the cycloaddition at or below 20 °C. In other cases, N2 extrusion is achieved at... 

The cycloadduct obtained from ethyl diazoacetate and the cyclic phosphaalkene 9-ferf-butyl-1,3-diphenyl- 10-phospha-1,3-etheno- 17/-benzopyran-4(37/)-one underwent spontaneous [3-1-2] cycloreversion and produced ethyl 5-tert-butyl-1,2,4-diazaphosphole-3-carboxylate (163). Still another transformation was found for P-trimethylsilyl-substituted diazaphospholes system 94, which suffered dediazonia-tion under the cycloaddition conditions and yielded phosphaalkene 95 (162) (Scheme 8.22). It was proposed that N2 extrusion and SiMe3 migration occur in concert. On the other hand, the cycloaddition products derived from phosphaalkene 93 and 2,2-dimethyl-1-diazopropane or diazo(trimethylsilyl)methane simply underwent tautomerization to the corresponding A -phosphapyrazoline (162) (94, R = f-Bu H shift R = SiMe3 SiMe3 shift). 

With 2-diazopropane, two stereoisomers of the resulting tricyclic product 116 (cis, anti, cis and cis, syn, cis) were obtained (183). Formation of 116 can be rationalized by N2 extrusion from the cycloaddition product 114 and a subsequent [34 + 22 ] cycloaddition of the resulting 3-alkylidene-l,2,3-diazaphosphole 115 with the remaining heterophosphole 110. When an excess of 2-diazopropane was used, 115 was trapped by 1,3-dipolar cycloaddition across the exocyclic P=C bond. 

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). 

Reaction with acetylenic dipolarophiles represents an efficient method for the preparation of 2,5-dihydrothiophenes. These products can be either isolated or directly converted to thiophene derivatives by dehydration procedures. The most frequently used dipolarophile is dimethyl acetylenedicarboxylate (DMAD), which easily combines with thiocarbonyl ylides generated by the extrusion of nitrogen from 2,5-dihydro-1,3,4-thiadiazoles (8,25,28,36,41,92,94,152). Other methods involve the desilylation (31,53,129) protocol as well as the reaction with 1,3-dithiolium-4-olates and l,3-thiazolium-4-olates (153-158). Cycloaddition of (S)-methylides formed by the N2-extrusion or desilylation method leads to stable 2,5-dihydrothiophenes of type 98 and 99. In contrast, bicyclic cycloadducts of type 100 usually decompose to give thiophene (101) or pyridine derivatives (102) (Scheme 5.37). 

A diazosilene is probably also involved in the photochemical or copper-catalyzed decomposition of bis(diazoacetate) 156 in benzene (equation 36). In both cases, dia-zoketene 157 was the only identified product72. Its formation was explained by the silylcarbene-to-acylsilene-to-silylketene sequence outlined in Scheme 5. Efforts to achieve the N2 extrusion from the remaining diazo function by thermolysis in boiling toluene or by prolonged photolysis resulted only in unspecific decomposition. 

Sidewall functionalization of SWCNTs was achieved via the addition of reactive alkyloxycarbonyl nitrenes obtained from alkoxycarbonyl azides. The driving force for this reaction is the thermally-induced N2 extrusion. The nitrenes generated attack nanotube sidewalls in a [2+l]-cycloaddition forming an aziridine ring at the tubes sidewalls (Scheme 1.19). 

Diazoacetates are commonly used for the formation of aziridines from imines under Lewis or Bronsted acidic conditions - a process known as the aza-Darzens reaction. A useful twist on the reaction is achieved if the 1,2-addition intermediate undergoes deprotonation of the a proton prior to intramolecular aziridine formation with N2 extrusion. Such an interrupted aza-Darzens reaction accomplishes a... 

Kaugars and Rizzo have found that 5-alkylamino- or 5-arylamino-l,2,3,4-thiatriazoles react with isocyanates to give 3-oxo-A4-l,2,4-thiadiazolin-5-ylureas (65) (79JOC3840). The structures were verified by independent synthesis and by H and I3C NMR spectroscopy. In the presence of triethylamine the exothermic reaction went to completion in a few hours. The reaction may either take place by attack at the amino group to give (63), followed by N2 extrusion to form the reactive dipole (64 equation 31), or the reaction may be initiated by attack at the ring in the 4-position to give an intermediate thiatriazoline, which, as discussed above, reacts with heterocumulenes (see Section 4.28.2.3.l(iii)). As expected, 5-(dialkylamino)thiatriazoles were not found to react with isocyanates. 

Methyl ylide 681, produced by thermal N2 extrusion of dihydrothiadiazole 680, undergoes 1,3-dipolar cycloaddition with alkynes to give dihydrothiophenes 682 (Scheme 101) <2002HCA451>. 

On thermolysis the formation of a betaine intermediate is postulated (Scheme 11) (c/. 77CR(C)(284)509, 65CC56, 65JOC4205). This has alternative routes of stabilization by N2 extrusion, either by imine/enamine formation or by cyclization to an aziridine. 

The majority of recent research in the photochemistry of azides and the generation of nitrenes has involved aryl rather than alkyl azides. Amongst the latter, however, is an interesting study of the photodissociation of methyl azide at wavelengths from 292 to 325 nm. The photodissociation dynamics of methyl azide are apparently complex, with the predominant pathway producing CH2=NH by concerted 1,2-H shift and N2 extrusion. Triplet methylnitrene (CH3N) was also observed by emission spectroscopy, but seems to arise by a minor, spin-forbidden pathway. 

Photolysis of the spiro 2-oxo-2,5-dihydro-l,3,4-oxadiazolene derivatives prepared from (-i-)-camphor semicarbazone proceeds in cyclohexene by CO2-extrusion to yield the cyclohex-2-enylhydrazone (44%) and, through cyclodimerization, the symmetrical camphor azine (53%) with only 3% of camphor arising via the alternative CO/N2 extrusion pathway. " Pb(OAc)4 oxidation of... 

Gas (SO2, CO2, CS2, N2) extrusion in natural products total synthesis 120BC8383. 

An alternative type of diazene is typified by azobenzene (Eq. 16.70). Photolysis now does not lead to N2 extrusion, because the phenyl radicals that would be formed are too unstable. Instead, a fairly efficient cis-trans isomerization occurs. This process can be repeated m any ti mes, and since the cis and trans diazenes usually have substantially different absorption spectra, wavelengths can be chosen that strongly favor the cis or the trans form in the photostationary state. As discussed in a Connections highlight on photochromism on page 969, these factors have made azobenzene and derivatives favorites for the development of systems that are photochemically switchable between two forms. 

The three-coordinate 16-electron nickel complex (dtbpe)Ni=GPh2 was obtained by N2 extrusion from the diphenyldiazomethane complex (dtbpe)Ni(N,N N2GPh2), dtbpe= l,2-bis(di-/ r/-butylphosphino)ethane, in the presence of a catalytic amount of samarium triflate (Equation (10)). ... 

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