Oxidative ortho -C-N Fusion of Aniline by OsO 4 . Isolation, Characterization of Oxo-Amido Osmium(VI) Complexes and their Catalytic Activities for Oxidative C-C Bond Cleavage of Unsaturated Hydrocarbons

In an unusual reaction


Introduction
Oragano-imido complexes including those of osmium(VIII) have been one of the most researched kinds of compounds primarily because these are potential intermediates 1-7 for organic reactions involving the formation of nitrogen-carbon bonds. However to date only a limited number of stable osmium(VIII) imido species have actually been isolated. 8 Monoimido osmium(VIII) complexes are accessible directly from OsO4 with the tertiary alkyl amines in hydrocarbon solvent or even with aqueous amine solution. [8][9][10] In contrast the corresponding reactions with aromatic amines have been confined 11 only to 2,6-disubstituted bulky arylamines. It has been reported that the similar reaction of unsubstituted aniline with OsO4 leads to degradation 12 together with the formation of unidentified azo compounds. In this paper we disclose the synthesis of a family of square pyramidal mono-oxo di-amido osmium(VI) complexes from an unprecedented reaction of OsO4 and aniline, or its substituted derivatives. In this reaction the ortho-unsubstituted anilines undergo oxidative C-N bond fusion to produce doubly deprotonated N-aryl-1,2-phenylenediamides. Our interest in these reactions originated from our previous results 13 on transition metal mediated oxidative ortho-C-N fusion of aromatic amines. The reactions occur due to ortho-C-H activation as a consequence of metal coordination. Furthermore, we have shown before 13a,14-15 that the ciscoordination of the aromatic amines is a prerequisite for the above ortho-fusion reaction. Our work in this area has so far been confined to the use of Ru(III) and Os(IV) non-oxo compounds as mediators. In this work OsO4 was chosen as a mediator since firstly, it is an exceptionally strong oxidant, and secondly, its lability towards substitution by amines is documented 8 in the literature.
These two properties are in fact essential for the above aromatic amine fusion reactions. The reactions have produced penta-coordinated mono-oxo osmium(VI) complexes of N-aryl-diamides as the only major product.
Interestingly the above osmium(VI) oxo complexes are found to catalyze oxidative cleavage of C-C bonds in unsaturated hydrocarbons like alkenes and alkynes by tbutylhydroperoxide at room temperature. Notably, since 1980s there have been exciting developments in the high valent ruthenium-oxo complexes, some of which are useful and active oxidants [16][17] for organic substrates. In comparison, osmium-oxo complexes are much weaker oxidants 17d than the corresponding ruthenium complexes and suitable osmium complex catalyst [18][19][20] for oxidative cleavage of unsaturated hydrocarbons is scarce in the literature.

Results and Discussion
Synthetic Reaction. Since we are interested in dimerization and/or polymerization of aromatic amines by ortho-C-N bond fusion, we purposefully chose aromatic amines without any orthosubstitution (like aniline and its para-substituted derivatives) for the reaction. The reactions of OsO4 with primary aromatic amines proceeded smoothly in heptane producing penta-coordinated oxo-amido osmium (VI) complexes 1a-c as shown in Scheme 1. Isolated yields of the products

Scheme 1
after TLC purification were nearly 40%; a mixture of unidentified brown products also formed along with 1. We wish to note here that a similar reaction with 2,6-dichloroaniline failed to produce any identifiable product. The organic reaction, as shown in Scheme 2, involves 2eoxidation of aromatic amines. Thus the formation of the bis-chelate, 1 (Scheme 1) Scheme 2 from 4ArNH2 and OsO4 needs a transfer of total four electrons. In comparison, the metal oxidation level in the product 1 is only two units less than that in the starting compound OsO4. The fact that the reference reaction also proceeded smoothly in an inert atmosphere producing 1 in a similar yield excluded the possibility of involvement of aerial oxygen in this oxidation reaction. Thus, it is anticipated that half of OsO4 is expended as oxidant 21 which, in turn, justifies the moderate yields of 1 (<50 %) from the above reactions. For further elucidation of the above electron accounting issue, a reaction of OsO4 and the preformed ligand, N-phenyl-1,2-phenylenediamine (o-semidine) was performed under identical reaction conditions. In this case the isolated yield of 1a was 94%. However, a higher ligand stiochiometry (OsO4: o-semidine = 1:3) was necessary for completion of the reaction. The excess ligand used herein brings about the metal reduction Os(VIII) → Os(VI) at the cost of its self oxidation [22][23] to 5-phenyl-3-phenylimino-3,5-dihydrophenazine-2-ylamine (Scheme 3). The substituted phenazine compound was isolated from the reaction mixture and characterized. A similar reaction of OsO4 with 1,2-diaminobenzene was reported 24 to give trans-[Os VIII O2(opda)2] (opda = 1,2-diaminobenzene).

Schemes 3
It is believed that the reference reaction occurs due to induced ortho-C-H activation of a coordinated aryl amido intermediate. However there may be several possibilities which remain unresolved due to the lack of chemical information of the intermediates. We wish to note here that OsO4 mediated such dimerization of aromatic amines is unknown in the literature. Mayer and coworkers, recently, have reported 21,25 a few examples of nucleophilic aromatic substitution at the para carbon of a coordinated anilido ligand in a substitutionally inert Os(IV) complex. We thus conclude that cis-disposition of the coordinated aryl-amido ligands in the intermediate complex is possibly the key for the ortho-C-N fusion reactions.
The osmium complexes 1a-c analyzed satisfactorily; their ESI-MS spectra exactly corroborated with their simulated spectra. Appearance of strong bands near 3400 and 890 cm -1 in the IR-spectra characterize the presence of N-H and Os=O fragments, respectively. The NMR spectra ( 1 H as well as 13 C) of the representative complexes are submitted as Supplementary Material ( Figure S1 and S2). Notably, the two ligands in the complexes are magnetically equivalent due to the presence of a two fold rotational symmetry axis (see below) and, hence, resonances for only one ligand were observed. For example, the complex 1b showed only two methyl proton resonances at δ 2.55 and 2.30 ppm, respectively. The amide N-H proton of 1a appeared at δ 4.5 ppm, which exchanges with D2O resulting in disappearance of the resonance.  Table 1) indicating its double bond 26 character.  Table 1 Cyclic Voltammetry. The redox behavior of the diamido complexes 1a-c was studied by cyclic voltammetry in dichloromethane (0.1 M TBAP) in the potential range between +1.5 and -1.5 V and using a platinum working electrode. The potentials are referenced to the saturated Ag/AgCl electrode. The cyclic voltammogram of 1a is displayed in Figure 2 as an illustrative example. It shows two one-electron redox processes in the above potential range. A reversible reductive response appeared near -1.0V; the second response is irreversible and occurs at an anodic potential, ca. +1.0V. The reversible response at the cathodic potential corresponds to the reduction of the metal center while the process occurring at the positive potential formally corresponds to the ligand oxidation. The potentials of the above couples all vary linearly (Supporting Figure S3) with respect to Hammett p parameter of the substitutions on the ligand. There is quite a large change in the donor character of the ligand over the above series of substituents (see Table 2).

Figure 2, Table 2
One-electron stoichiometry of the reversible process was confirmed by exhaustive electrolysis of the representative compound 1a at -1.2 V. The electrogenerated one electron reduced species [1a]was, however, EPR silent. This is possibly due to high spin orbit coupling constant of Os(V), (Os 5+ ) ca. 4500 cm -1 . Examples of EPR silent osmium(V) complexes 27 are known in the literature. These results are fully corroborated by density functional theory (DFT) calculations which show that the LUMO and HOMO of complex 1a are primarily metal and ligand centred, respectively (see below). Thus, one electron reduction of 1a is expected to create an anionic osmium(V) system with a geometry similar to one observed for the neutral species. DFT optimization carried out for [1a] -, revealed only slight elongation (0.04 Å) of the Os=O and OsN metrical parameters upon electron attachment which is in good agreement with the reduction of Os from VI to V.

UV-vis Spectra and DFT.
The compounds 1a-c exhibit rather similar low-energy absorption spectra with a weaker band near 680 nm and another near 550 nm (Table 3). Gaussian analyses of the overlapping bands are shown in Figure 3. The assignment of these bands is guided by the results from DFT calculations. Before calculation of the excitation energies using timedependent DFT (TDDFT) formalism, the molecular structure of 1a was fully optimized at the BPBE1PBE/TZVP level of theory. The calculated bond lengths and angles (see data given in Supporting Information, Table S1) can be compared to the crystallographically established metrical parameters (Table 1). Overall there is a very good agreement with the two sets of numbers as bond lengths and angles are predicted within 2 pm and 1-4 from the experimental values, respectively. Interestingly, however, the four basal nitrogen atoms in the 1,2-diamido phenelene ligands are in a distinctly non-planar orientation in the calculated structure though the X-ray analysis showed them to be in an essentially planar arrangement. This difference arises most likely from crystallographic packing of the coplanar N-phenyl groups as such interactions are not modelled in calculations.

Figure 3, Table 3
The calculated excitation energies for 1a are reported in Table 4. The low intensity tail observed in the experimental spectrum compares well with the calculated value for the HOMO → LUMO transition at around 680 nm. The two mid-intensity peaks with transition maxima close to 550 and 430 nm can be readily assigned to b(1) (HOMO-1 → LUMO) and b(2) (HOMO-2 → LUMO) transitions which are computationally predicted to appear at 560 and 436 nm, respectively. Several possible transitions were found to lie between 300 -400 nm. However, calculated oscillator strengths predict that transitions b(3), b(5) and b(6) should dominate the spectrum. In fact, the highest oscillator strength is calculated for the b(5) transition (predominately HOMO → LUMO+2 in character) at 355 nm which is in excellent agreement with the experimentally observed band maximum close to 350 nm.
An analysis of the MOs of the complex 1a reveals that the highest occupied orbitals are centred on the N-phenyl-1,2-phenelene ligands and are, thus, -type in character (see Supporting Figure S4). The lowest unoccupied MOs have significantly smaller contributions from the two ligands and consist primarily of p and d orbitals on oxygen and osmium, respectively. Hence, the transitions given in Table 4 are all predominantly L → dOs + pO type. High valent metal-oxo complexes are attractive to chemists because of their abilities to catalyze the oxidation of alkenes by formal oxygen atom transfer reaction. Taking the advantages of the stability of high oxidation state of the present osmium complexes and their coordinative unsaturated nature, we set out to explore the possibilities of their application as catalysts. A representative complex, 1a has been tested for this purpose. The reference complex indeed catalyzes oxidation of a wide variety of unsaturated hydrocarbons like alkenes, alkynes and aldehydes to the corresponding carboxylic acids in the presence of tert-butylhydroperoxide (TBHP) with remarkable efficiency at room temperature. We note here that usually the oxidative carbon-carbon bond cleavage reactions are achieved either by the cumbersome ozonolysis route 28 or by using 29-30 RuO4/NaOCl, RuO4/IO4 -, and OsO4/oxones reagents. Recently, Noyori et al.  Tables 5 and 6. Similar reaction using 70% aqueous TBHP produced the carboxylic acid as the major product; corresponding 1,2 diols, though in minor quantities (5 -20%), were also identified as by-products of these reactions. The reactions with 30% hydrogen peroxide, however, did not show any selectivity. In order to avoid the formation of 1,2-diols, the reactions were, in general carried out using TBHP in moisture free n-decane-acetonitrile mixture.

Scheme 4, Tables 5 and 6
As shown in Table 5, a number of aromatic as well as aliphatic olefins were selectively cleaved by the present protocol to yield carboxylic acids in high to excellent yields. Thus, styrene gave an 88% yield of benzoic acid (Table 5, entry1) and trans-stilbene (Table 5, entry 3) afforded two equivalent of benzoic acid within 6h. However, the α-substituted aromatic alkene, 1-methyl styrene (Table 5 entry 4) produced acetophenone in 90% yield. In a similar manner oxidation of cinnamyl alcohol and trans cinamic acid ethyl ester (Table 5, entries 5 and 6) produced benzoic acid in 82% and 78% yield, respectively. Likewise open chain α-olefins like 1-octene and 1-hexene were easily converted into the respective alkanoic acids of one less carbon in nearly 80% yield (Table 5, entries 7 and 8).
Aromatic rings having localized double bond character as in phenanthrene was also cleaved cleanly under the reaction protocol affording biphenyl dicarboxilic acid (Table 5, entry 9) in 65% yield. It may be noted here that oxidation of phenanthrene to corresponding dicarboxylic acid is usually sluggish. 16f The catalyst is found to be equally effective also for the oxidation of aromatic alkene with less double bond character. Thus α,β-unsaturated chalkones (Table 5, entries 10-13) produced carboxylic acids in high yields. Cyclic olefins like cyclohexene, cyclooctene and 1-methyl cyclohexene also were cleaved affording an 86% yield of adipic acid, an 82% yield of suberic acid and an 75% yield of 6-oxo heptanoic acids ( Table 5, entries [14][15][16] respectively.
To exclude the possibility that the complex 1a just serves as a source of OsO4 via the oxidative removal of the diamide ligands, two following control experiments were planned. First, a catalytic quantity of OsO4 was used in place of 1a for the oxidation of styrene under analogous reaction conditions as described above. The reaction is unclean and produced a mixture of products.
Working up of the mixture yielded benzyl alcohol as the major product along with minor quantities of styrene1,2-diol and benzoic acid. It also contained small quantities of several unidentified products, which could not be separated from the crude mixture. Second, to ascertain the fate of the catalyst after the reaction we have analyzed the ESI-MS spectrum of the reaction mixture that contained a catalytic quantity of 1a, styrene, and t-utylhydroperoxide. A small portion of the mixture after 7 h stirring was evaporated under vacuum, and the residue separated by washing with diethyl ether displayed a peak at m/z, 677 amu in acetonitrile. Its simulation matched with a molecular adducts ion [1a.styrene]+ (Supporting Information, Figure S6) confirming that the catalyst remained intact during the reaction. However, after a very prolonged reaction time (>24 h) the ESI-MS spectrum could not locate the existence of the catalyst indicating that the catalyst decomposes in TBHP. This is not unexpected since cyclic voltammetry of 1a also showed only an irreversible anodic wave. To confirm it further ESI-MS of a freshly prepared mixture of the catalyst and TBHP in the absence of styrene was analyzed. ESI-MS of the later mixture indicated rapid decomposition of the catalyst. In another experiment it was possible to show that the complex catalyst is stable in styrene. Thus the results of the above experiments confirm that 1a acts as a catalyst in bringing about oxidative C-C bond cleavage of olefins by TBHP.
Catalytic activities of the osmium compound (1a) toward the cleavage aromatic and aliphatic alkynes were also studied. Five alkyne substrates were subjected to oxidation and the results are summarized in Table 6. Both aromatic and aliphatic alkynes produced the corresponding carboxylic acids in high yields (70-85%). Similarly oxidation of aldehydes was also achieved in high yields (Table 7). Control experiments were also performed whereby two aldehydes, namely, benzaldehyde and n-octanal, were reacted separately with t-butylhydroperoxide without addition of the osmium catalyst. In these cases only partial oxidation of the aldehydes to the corresponding carboxylic acids was observed. In comparison, high yield oxidations (>80%) of the above aldehydes were achieved in the presence of 1a within 4 h. Similar observation was also noted by others. 33 Table 7 Conclusion In this work, we have introduced a one pot synthesis of a family of monooxo osmium(VI) complex (1)

Experimental Section
Instrumentation. UV-VIS absorption spectra were recorded on a Perkin-Elmer Lambda 950 UV/VIS spectrophotometer. 1 H-and 13 C-NMR spectra were taken on a Bruker Advance DPX 300 spectrometer. Infrared spectra were obtained using a Perkin-Elmer 783 spectrophotometer.
Cyclic voltammetry was carried out in 0.1 M Bu4NClO4 solutions using a three-electrode configuration (glassy carbon working electrode, Pt counter electrode, Ag/AgCl reference) and a PC-controlled PAR model 273A electrochemistry system. The E½ for the ferrocenium-ferrocene couple under our experimental condition was 0.39 V. A Perkin-Elmer 240C elemental analyzer was used to collect microanalytical data (C, H, N). ESI mass spectra were recorded on a micro mass Q-TOF mass spectrometer (serial no. YA 263). EPR spectra in the X-band were recorded with a Bruker System EMX spectrometer. A two-electrode capillary served to generate intermediates for the X-band EPR studies.

Materials.
The high valent metal oxide OsO4 was an Aldrich reagent and the solvents used were obtained from Qualigens and MERCK (India). N-phenyl-1,2-phenelenediamine was purchased from Aldrich. Tetrabutylammonium perchlorate (TEAP) was prepared and recrystallized as reported earlier. 34 All other chemicals were of reagent grade and used as received. CAUTION: OsO4 and perchlorate salts have to be handled with care and with appropriate safety precautions.

Synthesis. Syntheses of the complexes were carried out by the reaction of osmium(VIII) oxide
with the corresponding anilines in refluxing heptane. The complexes 1 (general formula OsOL2) were obtained as the major products along with some unidentified minor products which are not considered here.  C24H20N4OsO: C,50.46;H,3.52;N,9.79 Found: C,50.51;H,3.53;N,9.81. substituted anilines in place of aniline. Their yields and characterization data are as follows.

Reaction of OsO4 with preformed HL a .
To a solution of 0.25g (1.1mmol) N-phenyl-1,2phenelenediamine(HL a ) in 30 ml of heptane, osmiumtetroxide 0.1g (0.39 mmol) was added under dry N2 atmosphere. The resulting mixture was refluxed for 4h after which the solution was dried under reduced pressure. The crude mass, thus obtained, was loaded on a preparative alumina TLC plate for purification using toluene as the eluent. A red band of 5-Phenyl-3-phenylimino-3,5dihydro-phenazine-2-ylamine(phenz), which moved just ahead of the broad reddish brown band, was collected. It was evaporated to dryness and crystallized from CH2Cl2-C6H14 solvent mixture.
The characterization data are as follows: Yield: 16%, ESI-MS, m/z: 363[phenzH] + . Anal. Calcd. for C24H18N4: C,79.51;H,4.98;N,15.42 Found: C,79.53;H,5.01;N,15.46. The second broad brown band was also collected from the TLC plate. Subsequent evaporation of the solution followed by crystallization by slow diffusion of hexane into the solution of the compound in dichloromethane yielded a crystalline brown compound, which is identical in all respect to the structurally characterized sample, 1a. The yield of 1a from the above reaction was 94%.
Olefin oxidation. Olefin oxidation reactions were performed by using a common procedure. A representative example (oxidation of stilbene) is elaborated below. However, the duration of the reaction time varied for different substrates as noted in Tables 5-6. To an dry acetonitrile (1.5 ml) solution of stilbene (0.180g, 1 mmol), tert-butylhydroperoxide in decane (1.0 ml) was added at room temperature followed by the addition of (0.6 mol %) of the catalyst 1a. The reaction mixture was allowed to stir for 6.0 h. The reaction mixture was then extracted with ethyl acetate, washed with brine and dried over Na2SO4. Evaporation of the solvent under reduced pressure left the crude product. After column chromatography 0.224g (92%) of pure benzoic acid was isolated. The melting point and spectroscopic data (IR, 1 H and 13 C NMR) of the products were in good agreement with there of authentic sample of benzoic acid.
The above procedure was also followed for the oxidations of alkynes and aldehydes.
Crystallography. Crystallographic data of the compound 1a are collected in Table 1. Suitable Xray quality crystals of 1a were obtained by slow evaporation of a dichloromethane-hexane solution of the compound. All data were collected on a Bruker SMART APEX diffractometer, equipped with graphite monochromated MoKα radiation (λ = 0.71073 Å), and were corrected for Lorentzpolarization effects. A total of 6343 reflections were collected, out of which 2514 were unique (Rint = 0.0401), satisfying the (I > 2σ(I)) criterion, and were used in subsequent analysis.