Showing papers on "Nitrene published in 1987"

Photochemistry of 3- and 4-nitrophenyl azides: detection and characterization of reactive intermediates

Abstract: Irradiation of nitro-substituted aromatic azides initiates a complex series of chemical reactions that lead to different products depending on the details of the experiment. There is evidence that four reactive intermediates participate in product-determining reactions. These are the singlet nitrene, the triplet nitrene, the triplet azide, and the ring-expanded dehydroazepine. The chemical and physical properties of each of these species is modified by the nitro substituent. The reactivity of the singlet nitrene with nucleophilic amines is increased, the lifetime of the dehydroazepine is shortened, and the triplet nitrene becomes a powerful single-electron acceptor. Nitro-substituted aromatic azides are widely used in photolabeling experiments. The findings of this study suggest that they may be inappropriate choices as photolabeling agents in many circumstances.

Halide promoted formation and carbonylation of .mu.3-nitrene ligands on Ru3 clusters

Abstract: Results of work reported herein show that halides dramatically promote nitrene carbonylation to form isocyanates. Halides have also been found to accelerate the formation of nitrene ligands from nitrobenzene. Reasons for the halide promotion are unknown; however, it is suggested that the halides facilitate the reaction by labilizing the Ru-N bond to form a dibridging nitrene that then undergoes nucleophilic attack on a coordinated carbonyl. 16 references, 1 figure.

Formation of imidates, amides, amines, carbamates, and ureas from the .mu.3-NPh ligands of Fe3(.mu.3-NPh)2(CO)9

Abstract: The bis(nitrene) cluster Fe3( 3-NPh)2(CO)9 (1) reacts with Li(HBEt3), MeLi, PhLi, and NaOMe to form the formyl and acyl clusters (Fe3( 3-NPh)2(CO)8C)O)R) (4, R = H; 6, R = Ph; 7, R = Me; 8, R = OMe). Formyl cluster 4 is unstable at room temperature and slowly loses CO to form the hydride cluster (HFe3( 3-NPh)2(CO)8) . The benzoyl cluster 7 reacts with EtOTf to yield the nitrene-carbene cluster Fe3( 3-NPh)2(CO)8(C)OEt)Ph)(9) which has been structurally characterized. The carbene and nitrene ligands in 9 couple to form the imidate PhN double bond C(OEt)Ph when 9 is exposed to air or allowed to stand in solution for prolonged periods under CO or N2 atmospheres. Similar nitrene-benzoyl coupling from 7 gives benzanilide, and the methoxycarbonyl and nitrene ligands in 8 couple to give methyl N-phenylcarbamate when the clusters are oxidatively degraded with (FeCp2) . The bis(phosphinidene) cluster Fe3( 3-PPh)2(CO)9 (2) also reacts with PhLi to yield a benzoyl derivative, (Fe3( 3-PPh)2(CO)8(C)O)Ph)) . However, addition of EtOTf to this species does not result in a carbene cluster analogous to 9, but instead phosphinidene-carbene coupling occurs to give Fe3( 2-PPh)( 3-PhPC)OEt)Ph)(CO)9 which has been structurally characterized. The phosphorus atom of the 3-PhPC)OEt)Ph ligand bridgesmore » two iron atoms, and the ethoxy-substituted carbon is attached to the third iron atom.« less

Are aroylnitrenes ground-state singlets? Photochemistry of .beta.-naphthoyl azide

Abstract: Both direct-irradiation and triplet-sensitized photolyses of ..beta..-naphthoyl azide (BNA) give nitrene-derived products indicative of reaction only from the singlet state of ..beta..-naphthoylnitrene (BNN). The triplet nitrene is not detected in chemical trapping or spectroscopic experiments, which readily reveal related intermediates. The results require that the energy of singlet BNN be very close to or below the energy of triplet BNN. The data are most consistent with a singlet ground state for BNN. A qualitative perturbation molecular orbital approach is used to account for this surprising finding.

Reaction of ethyl azidoformate with ketene silyl acetals

Abstract: Formation d'ethoxycarbonylamino-2- et (ethoxy N-trimethylsilyl) formimidoyloxy-2-propionate et -butyrate d'alkyle et de l'ethoxycarbonylamides acides alcoxy-2 propionique et butyrique

Studies on Diazepines. XXVIII. Syntheses of 5H-1, 3-Diazepines and 2H-1, 4-Diazepines from 3-Azidopyridines

Abstract: Photolysis of 2-unsubstituted (5a-f) and 2, 4-disubstituted (5k, l) 3-azidopyridines in the presence of sodium methoxide resulted in ring expansion to give the 4-methoxy-5H-1, 3-diazepines (8 and 18), presumably via the azirine intermediates 6 and 17 derived from the initially formed singlet 3-pyridylnitrenes by cyclization at the 2-position of the pyridine ring. On the other hand, in the photolysis of 2-substituted 3-azidopyridines (5g-j), the cyclization of the nitrenes occurred predominantly at the vacant 4-position giving rise to the 3-methoxy-2H-1, 4-diazepines (13).

Structural studies on bio-active compounds. Part 5. Synthesis and properties of 2,4-diaminopyrimidine dihydrofolate reductase inhibitors bearing lipophilic azido groups

Abstract: A series of 2,4-diamino-5-(azidoaryl)-6-alkylpyrimidines has been prepared. The azide (36)(MZP) can be reduced by thiol reagents to the corresponding amine (28) but reductive deazidation occurred when the series of azidophenyl derivatives was heated with hydrazine hydrate. Degradation of azide (36) in a trifluoroacetic acid–trifluoromethanesulphonic acid mixture at 0 °C affords a means of introducing the bulky trifluoromethylsulphonyloxy substituent into the hindered ortho-position of the 5-aryl substituent. The products formed from thermolysis and photolysis of the azide (36) and the planar analogue 2,4-diamino-6-azidoquinazoline (70) derive from the triplet nitrene reactive intermediates.The azido compounds are potent inhibitors of rat liver dihydrofolate reductase although not as active as metoprin. The azide (36), as its ethanesulphonic acid salt, was selected for clinical trial on the basis of its ease of synthesis and suitable biological and pharmaceutical properties, and has a shorter biological half-life than compounds of comparable hydrophobicity.

Vinyl azides in heterocyclic synthesis. Part 7. Synthetic studies on the cytotoxic marine alkaloid amphimedine

Abstract: In an approach to the synthesis of the pentacyclic alkaloid amphimedine (1), a key tetracyclic structure (6) is readily constructed from 2-methoxyacridine-9-carbaldehyde in three steps. Condensation of the aldehyde (2) with methyl azidoacetate gives the vinyl azide (3) which on thermolysis in xylene gives the pyridoacridines (4) and (5) in high yield (78% and 19% respectively); this represents the first cyclisation of a vinyl nitrene to a peri-position. Oxidation of the major pyridoacridine (4), or of the mixed pyridoacridines (4) and (5), with manganese dioxide gives the pyridoacridone (6), but neither (6) nor the analogous isopropyl ester (9) undergoes Diels–Alder reaction with the azadiene (7).

Photoinduced Nitrene, Carbene, and Atomic Oxygen Transfer Reactions Starting from the Corresponding Pyridinium N‐, C‐, and O‐ Ylides

Abstract: Ultraviolet irradiation of the pyridinium ylides 1,2 and 3 led to fragmentation of the exocyclic polar bonds, as well as to skeletal rearrangements. The photoinduced fragmentation processes gave the corresponding pyridines and highly reactive intermediates, i. e. Ethoxycarbonylitrene, Dicyanocarbene and atomic oxygen (oxene), respectively. Trapping of the reactive intermediates by alkanes and alkenes permitted the determination of their spin multiplicity. Ethoxycarbonylnitrene was in its triplet ground state at the moment of its formation. Dicyanocarbene occurred as a mixture of singlet and triplet, as determined by dilution experiments with variable amounts of alkenes. As to atomic oxygen, all collected data point to its formation in solution in its triplet ground state. UV-Bestrahlung der Pyridinium-Ylide 1,2 und 3 fuhrte sowohl zur Spaltung der polaren exocyclischen Bindungen, als auch zu Umlagerungen. Die Photolysen ergaben die entsprechenden Pyridine sowie die hochreaktiven Zwischenprodukte Ethoxycarbonylnitren, Dicyanocarben und atomaren Sauerstoff (Oxen). Durch Abfangreaktionen und Analyse der Produkte, konnte die Spinmultiplizitat der Zwischenstufen ermittelt werden. So entsteht zum Beispiel Ethoxycarbonylnitren, sofort nach der N-N-Spaltung, im Triplett-Grundzustand. Dicyancarben dagegen entsteht als ein Gemisch der Triplett- und Singulett-Spezies; dies konnte durch Verdunnungsexperimente mit Olefinen ermittelt werden. Schlieslich deuten alle Experimente darauf hin, das der atomare Sauerstoff im Triplett-Grundzustand auftritt.

Asymmetric induction in addition of N-nitrenes to alkenes: 2-(chiral)substituted benzimidazole-derived N-nitrene additions to alkenes

Abstract: Oxidation of the optically active 1-aminobenzimidazoles (5)–(7) with lead tetra-acetate in the presence of various prochiral alkenes yields mixtures of the diastereoisomeric aziridines with little asymmetric induction (Table). Oxidation of theracemic 1-amino-2-(1,2,2-trimethylpropyl)-1H-benzimidazole (12) in the presence of α-methylene-γ-butyrolactone (1) yields the stereoisomeric azirid (16) in a 5.5:1 ratio, and oxidation in the-presence of γ,γ-dimethyl-α-methylene-γ-butyrolactone (18) gives the aziridine (19) stereospecifically. The stereochemistry of (19) was determined by X-ray crystallography.A rationalisation for the stereospecificity of addition of the intermediate N-nitrene, produced by oxidation of (12) to the lactone (18), is given in terms of competitive reaction via two different transition states both of which lead to the same sense of chiral induction (Scheme 5).Significant asymmetric induction is also found in the addition of the nitrene derived from oxidation of (12) to (E)-but-2-ene (5.2:1 ratio-of stereoisomers) and styrene (5.6:1 ratio of stereoisomers).

Versuche zur Synthese sterisch gehinderter Thiazylarene und deren Beziehung zu Arylsulfenylnitrenen

Abstract: Attempts to Synthesize Sterically Hindered Thiazyl Arenes and Their Relationship to Arylsulfenyl Nitrenes Up till now all attempts to synthesize organic thiazyl compounds in substance failed. Stabilization leads to the corresponding disulfide 2, the thioaminyl radical 7 and the sulphur diimide 8. Formally thiazyl compounds 1 and sulfenyl nitrenes 1 were resonance structures. Via sulfenyl nitrenes 1 the formation of 2, 7, 8 can be discussed. An in situ preparation of sulfenyl nitrene 1a via the sulfenyl chloride 5a, synthesized by chlorination of the disulfide 2a, and the sulfenyl azide 6a is described in detail. 1a can also be prepared from the sulfenyl amide 4a by oxidation. The structures of the radicals 7a–f and 12f are discussed.

Intramolecular reactions of N-nitrenes: description of the transition state geometry for addition to alkenes

Abstract: Oxidation of the N-aminoquinazolones (10) and (11) generates the corresponding N-nitrenes which add intramolecularly to both double bonds. Although nitrene addition is stereospecifically cis, both faces of each double bond are attacked and consequently stereoisomers are formed. From the different selectivity of the N-nitrenes for the two double bonds in (10) and (11)[or (29)] by comparison with (15) and (24), respectively and from a consideration of the stereoisomer ratios, it is concluded that concerted addition of the N-nitrene to the double bonds in (10) and (11) obtains via a transition-state geometry resembling that shown in (34).

Azidinium-Salze. 23. Mitteilung. Photolyse heterocylischer Azidinium-Salze†

Abstract: Photolysis of Heterocyclic Azidinium Salts The photochemistry of some azidinium salts was investigated. Their photolusis led to a large variety of products which were isolated and identified. Reaction mechanisms involving singlet and triplet nitrene intermediates are discussed to explain the product formation.

Laser-induced fluorescence and vibrational relaxation of the phenyl nitrene radical

Abstract: Laser-induced fluorescence (l.i.f.) of the phenyl nitrene (PhN) radical has been observed in the wavelength region 360–370 nm following its production by infrared multiple-photon dissociation of phenyl isocyanate. The transition probes the ground 3A2 state of the species: the upper 3B1 level has a radiative lifetime of 45 ± 3 ns. Ground-state PhN is born vibrationally hot, and collisional relaxation has been studied in the presence of nine added gases, with the efficiencies of the processes depending upon long-range attractive forces between the colliding species. The dissociation process is compared with that of benzylamine, with l.i.f. detection of the 2A2–2B2 transition in the benzyl radical. An upper limit is proposed for the internal energy content of the PhN radical.

Alkoxycarbonylnitrenes as selective intramolecular CH-insertion reagents in alicyclic chemistry

Abstract: Intramolecular CH-insertion of cyclohexyl, cis- and trans-4-t-butylcyclohexyl, and cis- and trans-2-methylcyclohexyl nitrenoformates have been studied. The nitrenes were generated by vapour-phase (spray) pyrolysis or solution thermolysis of the corresponding azides, or by photolysis of tetrachlorothienyl S,N-ylides derived from the azides. The product ratios were principally (and predictably) dependent upon reaction temperatures (and thus conformational equilibria), these varying in the range 10–500°C. 17-Substituted estrone-derived nitrenoformates inserted either into the 12β(from 17β) or 14α(from 17α) CH sites while 17-derivatives with the nitrene on the end of a long inert group abstracted hydrogen from the steroid B-ring.

Novel cyclization of vinyl nitrene into 1-azaphenalene.

Abstract: Thermal reaction of 2H-azirines, bearing a methoxy or methylthio group at the neighboring position to azirine ring, was studied. In thermolysis of ethyl 2-(2-methoxynaphth-1-yl)-2H-azirine-3-carboxylate, attack of the vinyl nitrene at the peri position to form a 1-azaphenalene ring was observed. In thermal reaction of its thio analogue, 1-azaphenalene was also formed, but a naphthothiazine formed by the attack of vinyl nitrene at the sulfur atom was the major product. Mechanisms and differences of the reactions depending on O and S are discussed.