Matrix Photolysis Research Articles

Rearrangements of Nitrile Imines: Ring Expansion of Benzonitrile Imines to Cycloheptatetraenes and Ring Closure to 3-Phenyl-3 H-diazirines.

Nitrile imines are important intermediates in 1,3-dipolar cycloaddition reactions, and they are also known to undergo efficient, unimolecular rearrangements to carbodiimides via 1 H-diazirines and imidoylnitrenes under both thermal and photochemical reaction conditions. We now report a competing rearrangement, revealed by CASPT2(14,12) and B3LYP calculations, in which C-phenylnitrile imines 8 undergo ring expansion to 1-diazenyl-1,2,4,6-cycloheptatetraenes 12 akin to the phenylcarbene-cycloheptatetraene rearrangement. Amino-, hydroxy-, and thiol-groups in the meta positions of C-phenylnitrile imine lower the activation energies for this rearrangement so that it becomes potentially competitive with the cyclization to 1 H-diazirines and hence rearrange to carbodiimides. The diazenylcycloheptatetraenes 12 thus formed can evolve further to cycloheptatetraene 30 and 2-diazenyl-phenylcarbene 16 over modest activation barriers, and the latter carbenes cyclize very easily to 2 H- and 3 H-indazoles, from which 6-methylenecyclohexadienylidene, phenylcarbene, fulvenallene, and their isomers are potentially obtainable. Moreover, another new rearrangement of benzonitrile imine forms 3-phenyl-3 H-diazirine, which is a precursor of phenyldiazomethane and hence phenylcarbene. This reaction is competitive with the ring expansion. The new rearrangements predicted here should be experimentally observable, for example, under matrix photolysis or flash vacuum pyrolysis conditions.

Triplet States of Tetrazoles, Nitrenes, and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene.

Photolysis of 1- and 5-aryltetrazoles at 5-10 K using a 266 nm laser immediately generates their triplet excited states, which are characterized by their electron spin resonance (ESR) spectra with zero-field splitting parameters D = 0.12-0.13 cm-1 and E = 0.002-0.008 cm-1. Further photolysis of all of the aryltetrazoles affords arylnitrenes ( D ≅ 1 cm-1), and in the case of 5-aryltetrazoles also arylcarbenes ( D ≅ 0.5 cm-1). The formation of arylnitrenes from 5-aryltetrazoles, where no aryl-N bond is present, is explained by the photochemical rearrangement of initially formed nitrile imines ArCN+N-R to carbodiimides. The monosubstituted carbodiimide PhN═C═NH isomerizes to phenylcyanamide, PhNH-CN, and photolysis of the latter causes rapid elimination of HCN and formation of phenylnitrene. When N-methyl groups are present in the tetrazoles, methylnitrene, CH3-N, is formed too. In the case of 5-phenyltetrazole, additional hydrogen shift and fragmentation afford cyano- and isocyanonitrenes, NCN and CNN.

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry.

Carbenes and nitrenes can exist in both singlet and triplet states, sometimes equally stable and interconverting either thermally or photochemically. Many carbene and nitrene reactions proceed via tunneling at low temperatures. Numerous singlet and triplet states have been characterized spectroscopically, and a detailed understanding of the chemical and physical properties of carbenes and nitrenes is emerging. There has been significant progress in the direct observation of carbenes, nitrenes, and many other reactive intermediates in recent years through the application of matrix photolysis and flash vacuum pyrolysis linked with matrix isolation at cryogenic temperatures. Our understanding of singlet and triplet states has improved through the interplay of spectroscopy and computations. Bistable carbenes and nitrenes as well as many examples of tunneling have been discovered and numerous rearrangements and fragmentations have been documented. The correlation of the zero-field splitting parameter D with calculated spin densities on nitrenes and carbenes is discussed. This Minireview gives an overview of some of these developments.

Pyrolysis of benzotriazoles. Relationships between 1- and 2-vinylbenzotriazoles, α- and β-azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies

Contrary to a previous report based on pyrolysis-mass spectrometry, it is shown that 1-vinylbenzotriazole 4 undergoes flash vacuum pyrolysis (FVP) to N-phenylketenimine (PhNCCH2) 7 and indole 5 in parallel, not consecutive, reactions. N-Phenylketenimine does not rearrange to indole to any significant extent, but on FVP at 950°C it decomposes in a free radical reaction to yield phenylacetonitrile, acetonitrile, benzene, biphenyl and o-tolunitrile. Phenylacetonitrile itself at 1000°C yields essentially the same products. 1-(2-Methylpropeny)benzotriazole 22 pyrolyzes to C,C-dimethyl-N-phenylketenimine 23, and at higher temperature to methacrylonitrile 25 and benzene in a free radical reaction. 2-Vinylbenzotriazole 21 rearranges to 1-vinylbenzotriazole, which then affords N-phenylketenimine, phenylacetonitrile and indole at 900°C. 2-Phenyl-1-azirine 10, phenylacetonitrile and a trace of N-phenylketenimine are formed by FVP of α-styryl azide 8. 2-Phenyl-1-azirine itself pyrolyzes to phenylacetonitrile, benzonitrile and a trace of indole at 900°C. 3-Phenyl-1-azirine 12, phenylacetonitrile and indole are formed on pyrolysis of β-styryl azide 11, and matrix photolysis of 11 yields 3-phenyl-1-azirine and 1-phenylketenimine (PhCHCNH) 13. The paper emphasizes the dangers of relying solely on mass spectrometry in investigations of mechanisms of pyrolysis reaction. Observed mass spectra may on occasion correspond to very low yields of products that are not characteristic of the main reaction paths. Moreover, there are cases where major FVP reaction products have been nearly impossible to observe by online mass spectrometry.

Pyrolysis of annelated hexa- and heptamethylene-tetrazoles: Formation of 9- and 10-membered cyclic carbodiimides

A reinterpretation of the spectroscopic data for the products of flash vacuum pyrolysis (FVP) of 8- and 9-membered polymethylenetetrazoles 1, 3, 5, and 7 with annelated benzene rings indicates that these are not cyanamides 2, 4, 6, and 8, but 9- and 10-membered cyclic carbodiimides 9, 10, 11, and 12. FVP of the corresponding tetrazolo[1,5-a]azocine and tetrazolo[1,5-a]azonine derivatives 13 and 17 was investigated by isolation of the products in cryogenic matrices and by matrix photolysis and found to yield 9- and 10-membered cyclic carbodiimides 14 and 18 as transient intermediates en route to indolo[2,3-b]quinoline 15 and dibenzo[b,f]naphthyridine 19.

Ketene-ketene interconversion. 6-Carbonylcyclohexa-2,4-dienone-hepta-1,2,4,6-tetraene-1,7-dione-6-oxocyclohexa-2,4-dienylidene and Wolff rearrangement to fulven-6-one.

6-Carbonylcyclohexa-2,4-dienone (1) has been generated by flash vacuum thermolysis (FVT) with Ar-matrix isolation of methyl salicylate (7), 2-phenylbenzo-1,3-dioxan-4-one (8), phthalic peranhydride (9), and benzofuran-2,3-dione (11) and also by matrix photolysis of 9, 11, and 2-diazocyclohepta-4,6-dien-1,3-dione (12). In each case, FVT above 600 °C results in decarbonylation of 1 and Wolff rearrangement to fulven-6-one (13) either concertedly or via open-shell singlet 6-oxocyclohexa-2,4-dienylidene (18). Ketenes 1 and 13 were characterized by IR spectroscopy. Photolysis of matrix-isolated 1 at 254 nm also results in the slow formation of 13. The sequential formation of ketenes 1 and 13 from 7 has also been monitored by FVT-mass spectrometry, and 13 has been trapped with MeOH to afford methyl 1,3-cyclopentadiene-1- and -2-carboxylates 15 and 16. FVT of methyl salicylate-1-(13)C 7a revealed a deep-seated rearrangement of the (13)C-labeled 1a to hepta-1,2,4,6-tetraen-1,7-dione (17a) by means of electrocyclic ring opening followed by a facile 1,5-H shift and recyclization prior to CO-elimination and ring contraction to (13)C-labeled 13. The rearrangement mechanism is supported by M06-2X/6-311++G(d,p) calculations, which predict feasible barriers for the FVT rearrangements and confirm the observed labeling pattern in the isolated methyl salicylate 7a/7b and methyl cyclopentadienecarboxylates 20 and 21 resulting from trapping of 13 with MeOH.

Nitrile Ylides: Allenic and Propargylic Structures from Pyrazinylnitrenes. Experimental and Theoretical Characterization

Matrix photolysis of 2-pyrazinyl azides/tetrazolo[1,5-a]pyrazines generates nitrile ylides 15 via pyrazinylnitrenes 13 and triazacycloheptatetraenes 14. The nitrile ylides 15 are characterized by IR spectroscopy in conjunction with harmonic and anharmonic vibrational frequency calculations. The nitrile ylides exist in the matrices in the Z,Z-conformations in which they are born. Substitution on the nitrile carbon of nitrile ylides has a profound effect on their structure. Even different conformers of the same molecule can have differences up to 200 cm(-1) in the IR absorptions of the ylide moieties. Nitrile ylides 15a and 15b (R = H or Cl, R' = H) have allenic structures (15 Allenic). Nitrile ylide 15c (R = R' = CH3) has a distinctly propargylic structure (15 Propargylic) in the experimentally observed Z,Z-conformation.

Carbenic Nitrile Imines: Properties and Reactivity

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm(–1) have propargylic structures with a CN triple bond (RCNNSiMe3 and R2BCNNBR2), and those with IR absorptions below ca. 2150 cm(–1) are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900–2200 cm(–1) are carbenic (R–(C:)–N═N–R′). Electronegative but lone pair-donating groups NR2, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize “normal” singlet carbenes, including N-heterocyclic carbenes. NBO analyses at the CCSD(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes 21 with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1H-diazirenes of the type 24 in mildly exothermic reactions with activation energies in the range 29–38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis.

Nitrene-Carbene-Carbene Rearrangement. Photolysis and Thermolysis of Tetrazolo[5,1-a]phthalazine with Formation of 1-Phthalazinylnitrene, o-Cyanophenylcarbene, and Phenylcyanocarbene

1-Azidophthalazine 9A is generated in trace amount by mild FVT of tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm(-1) in the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring opening to generate two conformers of (o-cyanophenyl)diazomethane 11 (2079 and 2075 cm(-1)), followed by (o-cyanophenyl)carbene (3)12, cyanocycloheptatetraene 13, and finally cyano(phenyl)carbene (3)14 as evaluated by IR spectroscopy. The two carbenes (3)12 and (3)14 were observed by ESR spectroscopy (D|hc = 0.5078, E|hc = 0.0236 and D|hc = 0.6488, E|hc = 0.0195 cm(-1), respectively). The rearrangement of 12 ⇄ 13 ⇄ 14 constitutes a carbene-carbene rearrangement. 1-Phthalazinylnitrene (3)10 is observed by means of its UV-vis spectrum in Ar matrix following FVT of 9 above 550 °C. Rearrangement to cyanophenylcarbenes also takes place on FVT of 9 as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes 16-18. Thus the overall rearrangement 10 → 11 → 12 ⇄ 13 ⇄ 14 can be formulated.

Sydnone Photochemistry: Direct Observation of Earl's Bicyclic Lactone Valence Isomers (Oxadiazabicyclo[2.1.0]pentanones), Formation of Carbodiimides, Reaction Mechanism, and Photochromism

The matrix photolyses of 3-phenyl-, 3-pyridyl, and 3,4-diphenylsydnones 16, 19, and 22 were investigated by matrix-isolation infrared spectroscopy. The formation of the neutral, bicyclic lactone valence isomers postulated by Earl – the oxadiazabicyclo[2.1.0]pentanones exo-17 and exo-20 – was clearly observed in the first two cases and is also likely in the case of exo-23 (C=O absorptions in the IR at 1881, 1886, and 1874 cm–1, respectively). The efficient photodecomposition of sydnones to carbodiimides RN=C=NR′ (18, 21, and 24) and CO2 was established in all three cases. The formation of benzonitrile 27 and azacycloheptatetraene 29 in the matrix photolysis of diphenylsydnone 22 is indicative of diphenylnitrile imine PhCNNPh 26 as an intermediate (2340 cm–1). Neither bicyclic lactones nor carbodiimides have been observed previously in sydnone photochemistry. A general reaction mechanism for the formation of carbodiimides, nitrile imines, and photochromism is put forward.

Matrix-IR Spectroscopic Investigations of the Thermolysis and Photolysis of Diazoamides

Matrix photolysis of N,N-dialkyldiazoacetamides 1a-d at 7-10 K results in either the formation of C-H insertion products (in case of N,N-dimethyl and N,N-diethyl diazoamides) or almost exclusive Wolff rearrangement to ketenes (in the case of the cyclic diazoamides N-(diazoacetyl)azetidine and N-(diazoacetyl)pyrrolidine). This can be ascribed to higher activation barriers for the approach of the singlet carbene p orbital in 5 (or of the diazo carbon in an excited state of 1) to the stronger and "tied back" nature of the C-H bonds in the cyclic substituents. In contrast, flash vacuum thermolysis (FVT) of diazoamides 1a-d, in which reactions of excited states are excluded, gives rise to clean C-H insertion with only minor Wolff rearrangement to ketenes.

4-Pyridylnitrene and 2-pyrazinylcarbene

Both flash vacuum thermolysis (FVT) and matrix photolysis generate 2-diazomethylpyrazine (22) from 1,2,3-triazolo[1,5-a]pyrazine (24). FVT of 4-azidopyridine (18) as well as of 24 or 2-(5-tetrazolyl)pyrazine (23) affords the products expected from the nitrene, i.e., 4,4’-azopyridine and 2- and 3-cyanopyrroles. Matrix photolyses of both 18 and 24 result in ring expansion of 4-pyridylnitrene/2-pyrazinylcarbene to 1,5-diazacyclohepta-1,2,4,6-tetraene (20). Further photolysis causes ring opening to the ketenimine 27.

1,5-(1,7)-Biradicals and Nitrenes Formed by Ring Opening of Hetarylnitrenes

Several aromatic and heteroaromatic nitrenes and carbenes undergo photochemical and sometimes also thermal ring opening. Depending on benz-annelation, the ring-opened species may have the character of either nitrenes (for α-annelation) or 1,5-(1,7-)-biradicals (for β-annelation). Both types have been observed, and they are clearly distinguished by their characteristic electron spin resonance spectra. In addition, ring opening of hetarylnitrenes to nitrile ylides can be observed whenever there is a meta-relationship between a ring nitrogen atom and the nitrene (or carbene) centre. The factors governing the two types of ring opening have been investigated. The nitrenes and carbenes are generated by either low temperature Ar matrix photolysis or flash vacuum thermolysis of azides, tetrazoles, triazoles, or diazo compounds with matrix isolation of the products.

4‐Quinolylnitrene and 2‐quinoxalinylcarbene

Matrix photolysis of 4‐quinolyl azide 22 affords 4‐quinolylnitrene 21, identified by electron spin resonance (ESR) and infrared (IR) spectroscopy. Minor absorptions assigned to azirene 23 (1741 cm−1) were also observed. Further photolysis affords strong absorptions at 1902 and 1909 cm−1 ascribed to the cyclic ketenimine 19 as well as weaker absorptions at 2044 cm−1 ascribed to the open‐chain ketenimine 26 and at 1982 cm−1 assigned to the open‐chain nitrile ylide 25. Both 2‐(5‐tetrazolyl)quinoxaline 14 and triazolo[1,5‐a]quinoxaline 16 give rise to an absorption at 2084 cm−1 because of formation of the diazo compound 15 on mild flash vacuum thermolysis (FVT) with Ar matrix isolation of the product. Matrix photolysis of 16 affords diazo compound 15, cyclic ketenimine 19 and open‐chain ketenimine 26. 2‐Quinoxalinylcarbene 17 was also formed in the matrix photolysis and identified by its ESR spectrum. FVT of either 22 or 15/16 afforded 4‐quinolylnitrene, identified by its Ar‐matrix ESR spectrum. A second nitrene ESR signal obtained in several FVT and photolysis experiments from both 22 and 16 is ascribed to the phenylnitrene derivative 24 formed by ring opening of 2‐quinoxalinylcarbene 17. FVT of 14, 16, and 22 affords 3‐cyanoindole 27 as the major product. Minor amounts of 2‐cyanoindole 28, 4‐aminoquinoline 29 and 4,4′‐azoquinoline 30 are also formed. Preparative photolysis of 22 in solution in the presence of dialkylamines leads to trapping of the azirene 23 to afford 5‐dialkylaminobenzo[e]‐1,4‐diazepines 35. Copyright © 2011 John Wiley & Sons, Ltd.

2-Quinoxalinylnitrenes and 4-Quinazolinylnitrenes: Rearrangement to Cyclic and Acyclic Carbodiimides and Ring-Opening to Nitrile Ylides

This work was undertaken with the aim to obtain direct evidence for the interrelationships between hetarylnitrenes, their ring-expanded cyclic carbodiimide isomers, and ring-opened nitrile ylides. Tetrazolo[1,5-a]quinoxaline 11T and tetrazolo[5.1-c]quinazoline 13T undergo valence tautomerization to the corresponding azides 11A and 13A on mild flash vacuum thermolysis (FVT). Photolysis in Ar matrixes at ca. 15 K affords the triplet nitrenes 12 and 14, identified by ESR, UV, and IR spectroscopy. The nitrenes are converted photochemically to the seven-membered ring carbodiimide 15 followed by the open-chain carbodiimide 22. The 3-methoxy- and 3-chloro-2-quinoxalinylnitrenes 24 yield the ring-expanded carbodiimides 26 very cleanly on matrix photolysis, whereas FVT affords N-cyanobenzimidazoles 28. The ring-opened nitrile ylides 36 and 49 are identified as intermediates in the photolyses of 2-phenyl-4-quinazolinylnitrene 32 and 7-nitro-2-phenyl-4- quinazolinylnitrene 47. In these systems, a photochemically reversible interconversion of the seven-membered ring carbodiimides 35 and 48 and the nitrile ylides 36 and 49 is established. Recyclization of open-chain nitrile ylides is identified as an important mechanism of formation of ring contraction products (N-cyanobenzimidazoles).

ChemInform Abstract: HCN Dimers: Iminoacetonitrile and N-Cyanomethanimine.

Iminoacetonitrile (1) has been prepared by two methods: (i) thermal decomposition of the tosylhydrazone salts 13 at 200-degrees-C and (ii) Ar matrix photolysis of azidoacetonitrile (15). Ab initio calculations indicate that 1Z is of slightly lower energy than 1E, and this is confirmed by the IR spectra with use of the thermal methods. 1E/1Z undergo photochemical interconversion, giving a ca. 3:1 photostationary E:Z ratio. 1E and 1Z are fully characterized by their gas-phase, matrix, and thin-film IR spectra, which are in excellent agreement with ab initio calculations, by H-1 and C-13 NMR spectroscopy in solution, and by mass spectrometry. 1 polymerizes in solution above -40-degrees-C; pyrolysis produces HCN, and matrix photolysis produces HNC and van der Waals complexes containing HNC. N-tert-Butyliminoacetonitrile thermally fragments to tert-butyl isocyanide and HCN. N-Cyanomethanimine (3) has also been prepared by two methods: (i) pyrolysis of trimethylenetetrazole (20) at 500-800-degrees-C and (ii) pyrolysis of ditetrazolopyrazine 22 at 600-850-degrees-C. Both methods are extremely clean. 3 is fully characterized by its IR spectrum in agreement with ab initio calculations and, in conjunction with other work, by its mass and millimeter-wave spectra. 3 is thermodynamically stable in the gas phase up to ca. 800-degrees-C at low pressure and short contact times but polymerizes in the solid state above -100-degrees-C.

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides by Ring Expansion, Ring Opening, Ring Contraction, and Ring Closure: 3-Quinolylnitrene, 2-Quinoxalylcarbene, and 3-Quinolylcarbene

Photolysis of 3-azidoquinoline 6 in an Ar matrix generates 3-quinolylnitrene 7, which is characterized by its electron spin resonance (ESR), UV, and IR spectra in Ar matrices. Nitrene 7 undergoes ring opening to a nitrile ylide 19, also characterized by its UV and IR spectra. A subsequent 1,7-hydrogen shift in the ylide 19 affords 3-(2-isocyanophenyl)ketenimine 20. Matrix photolysis of 1,2,3-triazolo[1,5-c]quinoxaline 26 generates 4-diazomethylquinazoline 27, followed by 4-quinazolylcarbene 28, which is characterized by ESR and IR spectroscopy. Further photolysis of carbene 28 slowly generates ketenimine 20, thus suggesting that ylide 19 is formed initially. Flash vacuum thermolysis (FVT) of both 6 and 26 affords 3-cyanoindole 22 in high yield, thereby indicating that carbene 28 and nitrene 7 enter the same energy surface. Matrix photolysis of 3-quinolyldiazomethane 30 generates 3-quinolylcarbene 31, which on photolysis at >500 nm reacts with N2 to regenerate diazo compound 30. Photolysis of 30 in the presence of CO generates a ketene (34). 3-Quinolylcarbene 31 cyclizes on photolysis at >500 nm to 5-aza-2,3-benzobicyclo[4.1.0]hepta-2,4,7-triene 32. Both 31 and 32 are characterized by their IR and UV spectra. FVT of 30 yields a mixture of 2- and 3-cyanoindenes via a carbene–carbene–nitrene rearrangement 31 → 2-quinolylcarbene 39 → 1-naphthylnitrene 43. The reaction mechanisms are supported by density functional theory calculations of the energies and spectra of all relevant ground and transition state structures at the B3LYP/6–31G* level.

Comparative Study of the Photochemistry of the Azidopyridine 1-Oxides

The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.

Frozen matrix photolysis of Group VIII M(CO) 2Cl 2L 2 derivatives: Direct observation of CO-loss species and photochemical isomerization

Photolysis of Group VIII complexes of the form M(CO) 2X 2L 2 and chelated ruthenium compounds, Ru(CO) 2Cl 2(κ 2-L) in frozen matrices results in CO-loss. In the case of Fe(CO) 2Br 2(PMe 3) 2 evidence is presented for photochemical bromide ion elimination.

Synthesis and Photolysis of Biphenylenetetracarboxylic Dianhydride in Low-temperature Neon Matrixes

Abstract Biphenylenetetracarboxylic dianhydride was synthesized by dimerization of benzynedicarboxylic anhydride generated quantitatively by laser photolysis in low-temperature matrixes. Synthesized dianhydride was used as a precursor of matrix photolysis. It was isolated in a neon matrix and photolyzed to form biphenylynedicarboxylic anhydride, a novel aryne species.