Nitrene Adduct Research Articles

Reactions of a geminal Sc/P Lewis pair with pyridotetrazole and beyond.

Frustrated Lewis pair (FLP) chemistry has undergone remarkable growth, among which rare-earth metal-based Lewis pairs have exhibited unique reactivity in recent years. Herein, treatment of the intramolecular Sc/P Lewis pair, i.e., (ArO)2ScN(tBu)PPh2 (1, Ar = 2,6-tBu2-C6H3), with pyridotetrazole resulted in the formation of an FLP nitrene adduct with N2 elimination, offering additional insights into the mechanism of transition-metal-catalyzed denitrogenative annulation of pyridotetrazole. Reactions of complex 1 with 1,3,5-triazine and benzo[c]cinnoline generated FLP-type products via Sc/P 1,2-addition to the CN bond and the NN bond, respectively. Furthermore, treatments of 1 with phenylacetylene, diazo, and azide compounds were also investigated, leading to the formation of a variety of metallacyclic complexes displaying typical FLP behaviors.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex.

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (e.g., amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin CuI complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of CuI, is accessed following geometric rearrangement of the azide adduct from κ1-N terminal ligation to κ1-N internal ligation with subsequent expulsion of N2. For perfluorinated arylazides, stoichiometric and catalytic C-H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C-H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kinetics under relevant amination conditions with a first-order dependence on both Cu and organoazide. Activation parameters determined from Eyring analysis (ΔH‡ = 9.2(2) kcal mol-1, ΔS‡ = -42(2) cal mol-1 K-1, ΔG‡298K = 21.7(2) kcal mol-1) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C-N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C-N bond forming catalysis.

(Cobaltoceniumyl-amido)-pyridinium hexa-fluorido-phosphate.

The title compound, [Co(C5H5)(C10H9N2)]PF6, was synthesized from deproton-ated 1-amino-pyridinium iodide, followed by microwave-assisted nucleophilic aromatic substitution of iodo-cobaltocenium iodide. After anion exchange with potassium hexa-fluorido-phosphate, the title compound crystallizes as orange prisms in the space group Pc. This very stable pyridine nitrene adduct is the first example of a cobaltocenium derivative, formally containing a nitrene nitro-gen species.

Reactions of Al/P, Ga/P and P–H functionalized frustrated Lewis pairs with azides and a diazomethane – formation of adducts and capture of nitrenes

Abstract The Al- and Ga-based frustrated Lewis pairs (FLPs) Mes2P–C(MR2)=CH-R′ (1, M=Al, R= t Bu; 2, M=Al, R=CH2 t Bu; 3, M=Ga, R= t Bu) and the unique P–H functionalized FLP Mes(H)P–CH(AlR2)=C(H)- t Bu [4, R=CH(SiMe3)2] were treated with a variety of azides R′-N=N=N [R′= t Bu, SiMe3, Ph, CH2Ph, C6H4(4-Cl), C6H4(4-CF3), C6H4(4-Me), CH2C6H4(4-Cl), CH2C6H4(4- t Bu), C6H4(2-CH=CHPh)] in order to study systematically the influence of the substituents at nitrogen, phosphorus and the metal atoms on the reaction courses and the thermal stability of the products. Azide adducts (5–8) were isolated in which the terminal nitrogen atoms of the azides (Nγ γ) were bound to the phosphorus and the respective metal atoms resulting in four-membered PCMN heterocycles as the sole structural motif despite the wide range of substituents and the variation in the metal atoms of the FLPs. Thermal activation of selected azide adducts led to the elimination of N2 and the formation of the nitrene adducts 9–11 in which formally a transient, highly reactive nitrene N–R with an electron sextet nitrogen atom is trapped by the FLPs. For the first time FLPs were treated with a diazomethane, Me3Si–C(H)=N=N. Reactions with 1 and 2 afforded the adducts [Mes2P–C(AlR2)=CH-Ph](μ-N2CH–SiMe3) 12 (R= t Bu, CH2 t Bu) which had structures and spectroscopic properties similar to those of the corresponding azides. These compounds are thermally stable and do not eliminate dinitrogen upon warming or irradiation. Protonation of 12a with HCl in Et2O resulted in cleavage of the Al–N bond and formation of the zwitterionic phosphonium salt Mes2P[NH–N=C(H)–SiMe3]–C(AlCl t Bu2)=C(H)-Ph 13 with an intramolecular N–H···Cl hydrogen bond.

Electronic Structure of an Iron-Porphyrin−Nitrene Complex

Middle and late transition metal imido complexes (which may also be viewed as metal-nitrene adducts) are rather rare, especially for square-pyramidal and octahedral coordination geometries. However, an iron(II) porphyrin aminonitrene adduct, denoted here as Fe(Por)(NN), has been known for almost a quarter of a century. Unlike the corresponding S = 1 oxene and S = 0 carbene adducts, Fe(Por)(NN) exhibits an S = 2 ground state. DFT calculations reported herein provide a molecular orbital description of this unusual species as well as a rationale for its S = 2 ground state. The electronic configuration of Fe(Por)(NN) may be described as d(pi)(2)d(xy)(1)d(z(2))(1)d(x(2)-y(2))(1)d(pi')(1), where the z direction corresponds to the Fe-NN axis. The stability and double occupancy of one of the d(pi) orbitals may be attributed to a pi-backbonding interaction with the N-N pi* orbital. The weak sigma-donor ability of the aminonitrene ligand results in a relatively low-energy d(z(2)) orbital and an overall d orbital splitting pattern that engenders a high-spin ground state.

Side-wall opening of single-walled carbon nanotubes (SWCNTs) by chemical modification: a critical theoretical study.

Single-walled carbon nanotubes (SWCNTs) have unique electronic, mechanical, and structural characteristics; consequently, promising applications derived from these materials, such as chemical sensors or nanometer-scale electronic devices, can be expected. Structurally altered nanotubes with appropriate addends should facilitate their use by improving solubility, processability, and ease of dispersion, as well as by providing sites for chemical attachment to surfaces and polymer matrices. Avexing problem is ascertaining the detailed structures of nanotube derivatives after their preparation. The characterization of functionalized SWCNTs is difficult; all experimental attempts to determine the precise location and mode of addition of newly attached groups have failed. SWCNT adducts with possible three-membered rings (3MRs) that result from oxygen, methylene, and NH additions are simple but very important side-wall functionalized derivatives. Oxidation reactions are used widely to purify nanotubes, and the electrical properties of carbon nanotubes are extremely sensitive to oxygen exposure. Methylene and NH adducts are the prototypes of the recently synthesized covalently bonded dichlorocarbene and nitrene adducts. The available theoretical studies on the structures of nanotube oxide and dichlorocarbene adducts that involve either armchair or zigzag tube models, employed rather unsatisfactory methodology (see below). Owing to the large size of nanotubes, carefully chosen truncated models, appropriate for the problem being investigated, are required. One approach uses small nanotube fragments to simulate a full nanotube, but carries out computations at a relatively high level. The other approach uses the ONIOM technique, which treats part of the system at a high theoretical level but the rest of the system at a lower level. This strategy allows larger systems to be simulated at a practical computational cost. Thus, a recent ONIOM(B3LYP/6-31G*:AM1) study employed a C16 fragment (Figure 1a) for the high-level computation to

An ESR and UV Spectroscopic Study on the Photoreduction of para-Nitroaniline

The products formed in the photoreduction of para-nitroaniline were studied by the techniques of ESR spectroscopy and UV spectrophotometry. It was shown that the main radical products of the reaction in the presence of oxygen are the phenyl radical (in benzene solution) and the benzyl, phenylcarboxyl, and phenyl radicals (in toluene solution). The rate-limiting step in the formation of radical products and solvent hydroxylation products is intersystem crossing in the nitrene adduct with molecular oxygen.

Nitrene Adducts with Oxygen in the Photooxidation Reactions of Organic Azides

The photooxidation of organic azides and the photochemical reduction of nitro compounds include the formation of a common intermediate: the adduct of a nitrene with oxygen. The ground state of the adduct is singlet; the singlet–triplet gap is small and equals 80.2 or 56.7 kJ/mol for HNOO or C6H5NOO, respectively. Arguments for the involvement of singlet molecular oxygen, atomic oxygen, hydroxyl radicals, and dioxaziridines in these reactions were given.

Effect of Nitrene Multiplicity on Product Yields upon Photooxidation of Organic Azides

The effect of the spin state of intermediate species on the product yields upon photooxidation of 4,4"-diazidodiphenyl was studied. It was found that addition of triplet photosensitizers to the reaction mixture resulted in a substantial increase in the yield of nitroso compounds. Maximum sensitizing effect is observed for the photosensitizers with a triplet level energy of 250 kJ/mol or higher. Using the solvents stabilizing the nitrene adduct with oxygen RNOOSbrings about an increase in the yields of both nitro and nitroso compounds. This result indicates that the nitrene adduct is the precursor of the above-mentioned compounds.

Chemiluminescence upon Photooxidation of 4,4"-Diazidodiphenyl with Molecular Oxygen

It was found that irradiation of the solution of 4,4"-diazidodiphenyl in ethyl alcohol with UV-light at a wavelength longer than 280 nm in the presence of dissolved oxygen resulted in appearance of post-luminescence. The luminescence intensity increases with increasing the oxygen concentration; the luminescence virtually disappears after purging the solution with argon. The observed emission of light was attributed to chemiluminescence of the products of photooxidation of 4,4"-diazidodiphenyl. The rate dependence of the afterglow shows two maxima. The first is observed immediately after cessation of the UV irradiation of the solution and its transfer to a photometric cell. The intensity of the corresponding chemiluminescence decreases by an exponential law with τe= 5 s. The second maximum appears after a certain time interval after the first one. Addition of a triplet sensitizer (Michler's ketone) to the reaction mixture results in quenching of the chemiluminescence from the photooxidation products, whereas the addition of substances that stabilize electrophilic species in the singlet state leads to an increase in the chemiluminescence intensity. It was suggested that the chemiluminescence resulted from reactions involving the singlet nitrene adduct with oxygen.

Condensed pyridazines. Part II. Synthesis of pyrido[2,3‐C] pyridazines

AbstractThe synthesis of some pyrido[2,3‐c]pyridazines from 5,8‐diehloro‐3‐methyl‐2‐oxo‐2H‐pyrano‐[2,3‐d]pyridazine (I) is described. Attempted oxidation of 8‐amino‐3‐chloro‐1,6‐dimethyl‐4,7‐dioxo‐1,4,7,8‐tetrahydropyrido[2,3‐c]pyridazine (VI) with LTA led only to the deaminated compound VII. Treatment of VI with LTA A in the presence of cyclohexene gave the nitrene adduct XI.

RING-OPENING REACTION OF PALLADIACYCLOPENTADIENE COMPLEX WITH N,N-DICHLOROAMINE AND -AMIDE

Abstract Reactions of the palladiacyclopentadiene complex, Pd(C4R4)(bipy) [R=COOCH3], with N,N-dichloroamine and -amide afforded three different σ-4-chloro-1,2,3,4-tetrakis(methoxycarbonyl)buta-1,3-dienylpalladium complex [II], [III], and [IV]. No nitrene adducts were formed.

The Photolysis and Thermolysis of Ethyl Azidoformate in the Presence of Ketones

Abstract The photochemical decomposition of ethyl azidoformate (I) in an acetone solution gives the nitrene adduct, oxazirane II, whereas the thermolysis of the solution gives a C–H insertion product, III. The photolysis of I in cyclohexanone gives an α-insertion product, IX, while the thermolysis affords a β-insertion product, X, and an oxazole, XIII, in addition to IX. The multiplicity of the reactive species is examined in terms of the heavy-atom effect.

Oxidative ring-opening of aziridine-1-carboxylates with sulphoxides

The title reaction furnishes a practical preparation of α-carbalkoxyamino ketones from nitrene adducts of the corresponding olefins. The aziridines are alternatively accessible via addition ,of iodine isocyanate, alcoholysis and elimination of hydrogen iodide. DMSO-cleavage of cis and trans isomers of 1-carbethoxy-2-methyl-3-phenylaziridine proceeds regioselectively: the cis isomer reacts at 1,2-bond and 1,3-bond in a ratio of ca 1:1, whereas the trans isomer reacts at 1,3-bond almost exclusively. The 1,3-bond cleavage is predominant under acidic conditions for both isomers. Under neutral conditions, the cleavage proceeds via an S N2 type transition state.