The rearrangements of naphthylnitrenes: UV/Vis and IR spectra of azirines, cyclic ketenimines, and cyclic nitrile ylides

Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet naphthyl nitrenes, (1)()1 and (1)()2. Relaxation to the corresponding lower energy, persistent triplet nitrenes (3)()1 and (3)()2 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines 15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously, but this work provides the first experimental observation of such species. The factors which are responsible for the special stability of the ylidic species 17 and 20 are discussed.     

Photochemistry of Fluorinated Phenyl Nitrenes: Matrix Isolation of Fluorinated Azirines

2,6-Difluorophenylnitrene and pentafluorophenylnitrene were generated in solid argon at 10 K by irradiation of the corresponding phenyl azides and characterized by IR and UV spectroscopy. Selective irradiation with lambda = 444 nm results in the formation of the corresponding azirines, while ketene imines are not produced. On lambda = 366 nm irradiation the azirines rearrange back to the nitrenes. The assignment of the azirines is confirmed by ab-initio calculations.     

Photolysis of alpha-azidoacetophenones: direct detection of triplet alkyl nitrenes in solution

We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of alpha-azido acetophenone derivatives, 1. Alphazides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. (13)C and (15)N isotope labeling studies allowed us to characterize the C-N stretch of the nitrene intermediate at 1201 cm(-)(1).     

Selective formation of triplet alkyl nitrenes from photolysis of beta-azido-propiophenone and their reactivity

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.     

Role of conformation and electronic structure in the chemistry of ground and excited state o-pyrazolylphenylnitrenes

The chemistry of 2-(1-pyrazolyl)- (2a) and 2-[1-(3,5-dimethylpyrazolyl]phenylnitrene (2b) has been studied in EtOH solution at room temperature, in EtOH glasses at 90 K, and in Ar matrices at 12 K. These nitrenes were chosen as suitable models for clarifying the mechanism of intramolecular reactions because attack at the pyrazole ring can occur according to different modes and the asymmetry of the substituent gives rise to different conformations. Detailed DFT and CASSCF/CASPT2 studies on the conformation and decay paths of both spin states of the nitrenes have been carried out. Ring expansion to dehydroazepines (via benzoazirines) is calculated to be competitive in both nitrenes, but in the dimethyl derivative, 2b, attack onto the N lone pair (which is made more nucleophilic by the methyl groups) is favored. The higher barriers (by 4-8 kcal/mol) in singlet 2a cause 60-70% of this nitrene to decay by intersystem crossing to the triplet. Thus, the seemingly straightforward formation of benzo-fused heterocycles through intramolecular attack of the pyrazoline N lone pair by the singlet phenylnitrene can only overcome ring expansion and intermolecular reactions under favorable circumstances. The comparatively persistent triplet nitrenes are characterized in matrices, and the yields of photocyclization products (mainly pyrazolo[1,5-a]benzimidazole (7) from 2a and 5,6-dihydropyrazolo[1,5-a]quinoxaline (8) from 2b) are shown to depend on the preferred conformation of the starting azide and nitrene.     

Selective synthesis of substituted pyrrole-2-phosphine oxides and -phosphonates from 2H-azirines and enolates from acetyl acetates and malonates

A simple and efficient selective synthesis of 1H-pyrrole-2-phosphine oxides 3 and -phosphonates 7 by addition of enolates derived from acetyl acetates to 2H-azirinylphosphine oxide 1 and -phosphonate 6 is reported. Nucleophilic addition of enolates derived from diethyl malonate to 2H-azirines 1 and 6 led to the formation of functionalized 2-hydroxy-1H-pyrrole-5-phosphine oxide 9 and -phosphonate 10, while vinylogous α-aminoalkylphosphine oxides 14 and -phosphonate 15 may be obtained from azirines and the enolate derived from diethyl 2-phenylmalonate. Ring closure of vinylogous derivatives 14 and 15 in the presence of base led to the formation of 1,5-dihydro-3-pyrrolin-2-ones containing a phosphine oxide 17 or a phosphonate group 18.     

Ultrafast spectroscopic and matrix isolation studies of p-biphenylyl, o-biphenylyl, and 1-naphthylnitrenium cations

p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.     

Synthesis of polyhydroxylated 2H-azirines and 2-halo-2H-azirines from 3-azido-2,3-dideoxyhexopyranoses by alkoxyl radical fragmentation

The reaction of 3-azido-2,3-dideoxypyranose and 3-azido-2,3-dideoxy-2-halohexopyranose compounds with (diacetoxyiodo)benzene and iodine generated 2-azido-1,2-dideoxy-1-iodoalditols and 2-azido-1,2-dideoxy-1-halo-1-iodoalditols, respectively. These beta-iodo azides could be transformed by chemoselective dehydroiodination into 2-azido-1,2-dideoxy-4- O-formyl-pent-1-enitols and (Z, E)-2-azido-1,2-dideoxy-1-halo-4- O-formyl-pent-1-enitols in good yields. Thermolysis and photochemical studies of these vinyl azides and 1-halovinyl azides for the synthesis of polyhydroxylated 3-alkyl-2 H-azirines and the hitherto unknown 2-halo-3-alkyl-2 H-azirines have also been accomplished.     

Photolysis of ortho-Azidobenzoic Acid in Solutions and a Solid State

The photolysis of o-azidobenzoic acid in solutions, adsorbed on silica gel, and in a crystalline state was studied by IR and UV spectroscopy and thin-layer chromatography. It was found that the photolysis resulted in the formation of 2,1-benzisoxazolone (the product of intramolecular cyclization of singlet nitrenes) and anthranilic acid and o,o"-dicarboxyazobenzene (the reaction products of triplet nitrenes). The formation of 2,1-benzisoxazolone is a reversible reaction because of the secondary photolysis to singlet nitrenes, which leads to an increase in their concentration in the system.     

Fragmentation of carbohydrate anomeric alkoxyl radicals. A new synthesis of chiral beta-iodo azides, vinyl azides, and 2H-azirines

[reaction: see text] The reaction of 3-azido-2,3-dideoxy-hexopyranose compounds from the d-gluco, d-galacto, d-lacto, and l-arabino carbohydrate series, with (diacetoxyiodo)benzene and iodine, generated 2-azido-1,2-dideoxy-1-iodo-alditols with one carbon less than the starting carbohydrate. These beta-iodo azides could be transformed by dehydroiodination into vinyl azides, which in turn afforded 3-monosubstituted 2H-azirines under thermal conditions. These beta-iodo azides and 2H-azirines may be interesting chiral synthons for the preparation of more complex heterocyclic systems.     

THE PHOTOCHEMISTRY OF IODO, METHYL AND THIOMETHYL SUBSTITUTED ARYL AZIDES IN TOLUENE SOLUTION AND FROZEN POLYCRYSTALS

The photolysis of para-methyl and para-thiomethylphenylazide at 77 K produces the corresponding triplet nitrenes which can be detected by electron paramagnetic resonance (EPR) spectroscopy. Photolysis of these azides in frozen toluene at 77 K leads to insertion of the nitrene into a benzylic C-H bond of the matrix in modest yields. Photolysis of iodinated aryl azides under these conditions does not produce triplet nitrenes that can be detected by EPR spectroscopy. In contrast to the para-methyl and para-thiomethyl substituted phenyl nitrenes, photo-induced coupling of iodo-substituted phenyl nitrenes to toluene proceeds in very poor yield.     

Thermal isomerization of 5-methoxy-3-arylisoxazoles to methyl 3-aryl-2H-azirine-2-carboxylates

The thermal isomerization of 5-methoxy-3-arylisoxazoles to methyl 3-aryl-2H-azirine-2-carboxylates was subjected to a kinetic study. A correlation between the isomerization rate constants and the ⁺ constants of the substituents in the aromatic ring is observed. The enthalpies of formation of a number of isoxazoles and 2H-azirines were calculated. The effect of the nature of the substituents on the mutual isomerization of isoxazoles and azirines is discussed. The results make it possible to refute the approved and previously proposed diradical mechanism for the isomeric transformations and are in agreement with a nitrene mechanism for the formation of azirines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1053–1056, August, 1978.     

An unexpected disproportional reaction of 2H-azirines giving (1E,3Z)-2-aza-1,3-dienes and aromatic nitriles in the presence of nickel catalysts

An unprecedented nickel-catalysed disproportional reaction of 2,3-diaryl-2H-azirines forming azabutadienes and aromatic nitriles was discovered. This reaction involves the cleavage of two bonds of the 2H-azirine framework, which provides a novel type of transformation of 2H-azirines.     

The temperature dependent photochemistry of phenyl azide in diethylamine

The photochemistry of phenylazide in diethylamine changes from products consistent with electrophilic singlet intermediates to products consistent with triplet nitrenes as the temperature is lowered from 273 to 77k.     

EPR SPECTROSCOPY OF ETHIDIUM NITRENE AND PROFLAVINE NITRENE COORDINATED TO SELF-COMPLEMENTARY DINUCLEOTIDE MINI-DUPLEXES

Abstract— Photolysis of ethidium azide ( lb ) and proflavine azide ( 2b ) immobilized in glassy media at 77 K produces triplet EPR spectra of the corresponding triplet nitrenes lc and 2c . The resonance field positions of the nitrenes are shifted by the presence of self-complementary dinucleotides. Control experiments indicated that the interaction of lc and 2c with C_PG mini-duplexes is consistent with intercalation.     

Photolysis of alpha-azidoacetophenones: trapping of triplet alkyl nitrenes in solution

[reaction: see text] Selective excitation of the ketone chromophore in alpha-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group, which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo alpha-cleavage to form benzoyl and methyl azido radicals in competition with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical. This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.     

Competition between alpha-cleavage and energy transfer in alpha-azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.     

Laccase-catalyzed domino reactions between hydroquinones and cyclic 1,3-dicarbonyls for the regioselective synthesis of substituted p-benzoquinones

Highly substituted p-benzoquinones were obtained in yields ranging from 39% to 98% by laccase-catalyzed domino reactions between hydroquinones and cyclic 1,3-dicarbonyls using aerial oxygen as the oxidant. In almost all reactions bis-adducts with two adjacent 1,3-dicarbonyl substituents on the quinone moiety were formed selectively. The transformations can be regarded as domino oxidation/1,4-addition/oxidation/1,4-addition/oxidation processes. With unsubstituted hydroquinone as the substrate 2,3-disubstituted p-benzoquinones were isolated. Bis-adducts were also formed exclusively upon reaction with monosubstituted hydroquinones. In almost all cases the 2,3,5-trisubstituted p-benzoquinones were obtained. When 2,3-disubstituted hydroquinones were employed as starting materials the 2,3,5,6-tetrasubstituted p-benzoquinones were isolated. The unambiguous structure elucidation of all products has been achieved by NMR spectroscopic methods including spin pattern analysis of the long-range coupled C═O carbons and (13)C satellites analysis in (1)H NMR spectra.     

微波辐射法合成喹喔啉铱（Ⅲ）配合物及其发光性质

采用微波辐射加热方法,将2,3-二苯基喹喔啉（DPQ）与水合三氯化铱（IrCl3•H2O）反应,合成了一种新型三环喹喔啉铱配合物[Ir(DPQ)3],通过元素分析,1H NMR和质谱方法对配合物结构进行了表征,并初步研究了配合物的吸收光谱和荧光光谱。结果表明,配合物Ir(DPQ)3在387和458nm处存在单线态1MLCT（金属到配体的电荷跃迁）和三线态3MLCT的吸收;在634nm 处有较强的金属配合物三线态的磷光发射。     

2-[2-(2,3-Dihydrobenzimidazolylidene)]- and 2-[2-(2,3-Dihydropyrido[2,3-d]imidazolylidene)]-5,5-dimethyl-1,3-cyclohexanediones

The reaction of 2-aminocarbonyl-5,5-dimethyl-1,3-cyclohexanedione with 2,3-diaminopyridine, 1,2-phenylenediamine (and its 4-methyl, 4-nitro, 4-carboxy, and 4-benzoyl derivatives), and 3,3-diaminobenzidine gave the corresponding 2-[2-(2,3-dihydrobenzimidazolylidene)]- and 2-[2-(2,3-dihydropyrido[2,3-d]imidazolylidene)]-5,5-dimethyl-1,3-cyclohexanediones. Their structure was confirmed by ¹H NMR spectroscopic data and X-ray analysis.