ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 440, Part 2, pp. 294–298. © Pleiades Publishing, Ltd., 2011.Original Russian Text © G.A. Abakumov, V.K. Cherkasov, A.V. Piskunov, O.Yu. Trofimova, A.S. Shavyrin, 2011, published in Doklady Akademii Nauk, 2011, Vol. 440, No. 6,pp. 765–769.

294

Aryl�substituted o�iminobenzoquinones are widelyused in coordination chemistry as redox�active ligandsthat can change its oxidation state when coordinated [1].In a free state, they are labile compounds apt to cycliza�tion reactions resulting in different phenoxazine deriva�tives [2–7]. The chiral derivatives of 4aH�phenoxazinesupposed as intermediates in the thermal transformationof 4,6�di�tert�butyl�N�dialkylphenyl�o�iminobenzo�quinone undergo further transformations in solution onaccount of dimerization [5, 6] or oxidation [6]. In thiswork, we prepared for the first time 4aH�phenoxazine, aproduct of cyclization of 4,6�di�tert�butyl�N�2�hydroxy�5�tert�butylphenyl�o�iminobenzoquinone, and showed apossibility of its decyclization.

EPR spectra were recorded with a Bruker EMX spec�trometer. Diphenylpicrylhydrazyl (DPPH, g = 2.0037) wasused as a reference for determination of g factor. The exactvalues of hyperfine coupling (HFC) constants wereobtained by spectrum simulation with the use of Win EPRSimfonia software. NMR spectra were recorded as solu�

tions in (CD3)2SO and CDCl3 on a Bruker Avance III spec�trometer operating at 400 MHz with Me4Si as an internalreference. Electron absorption spectra were recorded on aPerkinElmer Lambda 25 spectrophotometer.

2,6,8�Tri�tert�butyl�4aH�phenoxazin�4a�ol (2)was isolated from the reaction mixture (Scheme 1) asfine yellow�green crystals in 77% yield. o�Iminoben�zoquinone (1), a product of condensation of 4�tert�butyl�o�aminophenol and 3,5�di�tert�butyl�o�benzo�quinone, can exist as E and Z isomers. The lack ofbulky substituents in the 2� and 6�positions of theN�phenyl ring facilitates the transition of the E formto the Z form for which the cyclization reaction is mostprobable. The Diels–Alder dimerization of com�pound 2 is hampered because the double bonds in the1,3�positions are shielded by tert�butyl substituent;this fact, as distinct from the results of studies [5, 6],allows one to isolate the cyclization product in an indi�vidual state. Compound 2 has a chiral center in 4a�position and forms a mixture of R and S enantiomers.

Scheme 1.

OH

NH2

O

O

O

N

OH

N

OOH

N

OOH

t�Bu

1�E

1�Z2

6

7

8

5a 54

4a 3

10a1

10

9a9

t�Bu

t�Bu t�Bu

t�Bu

t�Bu

+ MeOH

t�But�Bu

t�Bu t�Bu Bu�tBu�t2

CHEMISTRY

Cyclization–Decyclization of Sterically Hindered o�Iminobenzoquinone

Academician G. A. Abakumov, Corresponding Member of the RAS V. K. Cherkasov, A. V. Piskunov, O. Yu. Trofimova, and A. S. Shavyrin

Received May 12, 2011

DOI: 10.1134/S0012500811100090

Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, ul. Tropinina 49,Nizhni Novgorod, 603950 Russia

DOKLADY CHEMISTRY Vol. 440 Part 2 2011

CYCLIZATION–DECYCLIZATION 295

The structure of compound 2 was confirmed by thedata of IR and NMR spectroscopy. The 1H NMRspectrum shows peaks of three nonequivalent tert�butyl groups, two signals of aromatic protons, and onesignal of the OH group. The signals of C(1)H, C(3)H,and C(4)H are shifted upfield owing to the violation ofthe aromatic structure of the fragment, while theabsence of the signal at 3–5 ppm confirms that thehydroxy group occupies the 4a�position. The 13CNMR spectrum of phenoxazinol 2 shows 11 signalscorresponding to the carbon atoms at the doublebonds, one signal due to the tertiary C(4a) atom, andthe signals of three nonequivalent tert�butyl groups.

1H NMR (400 MHz, (CD3)2SO, δ, ppm, J, Hz):1.18 (s, 9H, t�Bu), 1.30 (s, 9H, t�Bu), 1.42 (s, 9H, t�Bu), 6.33 (d, 1H, C(1)H, 1.2), 6.43 (d, 1H, C(4)H,9.9), 6.63 (dd, 1H, C(3)H, 9.9, 1.2), 6.88 (s, 1H, OH),7.25 (d, 1H, C(9)H, 2.3), 7.31 (d, 1H, C(7)H, 2.3).

13C NMR (100 MHz, (CD3)2SO, δ, ppm, J, Hz):28.81, 30.24, 31.80 (С(CH3)3), 34.52, 35.05, 35.17(С(СH3)3), 84.88 (C(4a)), 118.73, 122.54, 123.24,124.42, 130.74, 134.30, 137.33, 137.37, 143.61,152.52, and 156.33 (C(1), C(2), C(3), C(4), C(5a),C(6), C(7), C(8), C(9), C(9a), and C(10)).

The IR spectrum of compound 2 displays intenseabsorption bands at 3290 and 1650 cm–1 correspond�ing to the stretching vibrations of OH and C=Ngroups, respectively.

Compound 2 in the crystalline state is stable to airoxygen and moisture, and in solutions in polar solvents(methanol, THF, dimethyl sulfoxide), it is stable forseveral hours. The electronic absorption spectrum inmethanol (λ, nm (ε, L ⋅ cm–1 ⋅ mol–1)): 359.0 (8300),749.8 (750). Solutions of compound 2 in hydrocar�bons and chloroform become purple even at 20°Cwithin a few minutes, while the NMR spectrum inCDCl3 shows a set of signals indicating the formationof a complex mixture of decomposition products ofphenoxazinol 2. When compound 2 decomposes in atoluene solution, EPR spectroscopy provides a possi�bility to observe the formation of a stable radical2,4,8�tri(tert�butyl)phenoxazin�10�yl (3) (Fig. 1). Themechanism of formation of radical 3 is unclear at

present but obviously it is not reduced to a simpleabstraction of the hydroxyl radical from compound 2.

The hyperfine structure of the EPR spectrum ofthis compound is due to HFC of unpaired electron tomagnetic nuclei 14N (99.63%, I = 1, μN = 0.4037) [8],two pairs of equivalent protons 1H (99.98%, I = 1/2,μN = 2.7928) [8], and a proton at the meta position tothe nitrogen atom.

It was found that the opening of the phenoxazinering in compound 2 proceeds in the presence of basesand metal salts. Thus, we obtained in ~85% yield adiamagnetic six�coordinated zinc complex 4 contain�ing two tridentate monoanionic ligands that are theproducts of deprotonation and ring opening of phe�noxazinol 2 (Scheme 2).

N

O

t�Bu

t�Bu

Bu�t•

3

Scheme 2.

N

O

O

Zn N

O

O

N

OOH

2

Bu�t

Bu�t

Bu�t

Bu�t

t�Bu

t�Bu

t�Bu

Bu�t

t�Bu

+ Zn(AcO)2 · 2H2OCH3CN, 2Et3N

–2[Et3NH]OAc

42

3420 3430 3440 3450 3460H, Oe

Fig. 1. Isotropic EPR spectrum of 2,4,8�tri(tert�butyl)phe�noxazin�10�yl (3) in toluene. ai(2

1H) = 0.41 mT, ai(21H) =

0.27 mT, ai(14N) = 0.76 mT, ai(

1H) = 0.09 mT, gi = 2.0036.290 K.

296

DOKLADY CHEMISTRY Vol. 440 Part 2 2011

ABAKUMOV et al.

Compound 4 was studied by 1H and 13C NMRspectroscopy in a CDCl3 solution.

1H NMR (400 MHz, CDCl3, δ, ppm, J, Hz): 1.16(s, 18H, 2 t�Bu), 1.26 (s, 36H, 4 t�Bu), 6.66 (d, 2H,CaromH, 9.4), 7.12 and 7.33 (both d, 2H each, C=CH,2.1), 7.26 (dd, 2H, CaromH, 9.4, 2.3), 7.43 (d, 2H,CaromH, 2.3).

13C NMR (100 MHz, CDCl3, δ, ppm, J, Hz):29.17, 29.59, 30.38 (С(CH3)3), 34.40, 35.13, 35.44(С(CH3)3), 116.63, 118.98, 125.10, 133.68, 135.87,136.84, 139.05, 142.09, 146.10, and 146.75 (C),177.14 (C=N), 181.42 (C=O).

The fact that the 13C NMR spectrum lacks the sig�nals that can be assigned to quaternary carbon atomsand the downfield shift of the proton signals of theaminophenol fragment is evidence of the opening of

the phenoxazine ring upon the formation of com�pound 4. The electronic absorption spectrum of thiscomplex (toluene, λ, nm (ε, L ⋅ cm–1 ⋅ mol–1)): 425.0(8500), 725.2 (28530), 793.9 (27650) shows bands typ�ical for compounds containing a similar ligand [9].

At present, one of the most widespread syntheticapproaches to the preparation of metal complexeswith tridentate O,N,O�ligand is a template synthesison the basis of 3,5�di�tert�butylcatechol, aqueousammonia solution, and appropriate metal salt [10].The authors [11] attempted to obtain tin complexeswith an unsymmetrical tridentate O,N,O�ligand byreactions shown in Scheme 3. In this case, however, asecond molecule of unsubstituted o�aminophenol isinvolved in the reaction to give diamagnetic tin com�plex with a more complicated organic ligand bound tothe metal atom.

Scheme 3.

The reaction of compound 2 with triphenyl�and trialkyltin(IV) chlorides in acetonitrile solu�tion in the presence of Et3N occurs by Scheme 4.At the first stage of the reaction, compound 2undergoes ring opening and deprotonation to givediamagnetic six�coordinated metal derivatives.The latter are unstable and undergo further trans�

formations via elimination [12] or intramolecularmigration [13] of one of hydrocarbon groupsbound to metal atom. The resultant reaction mix�tures show intense EPR spectra indicating the for�mation of five�coordinated metal complexes con�taining paramagnetic dianionic form of tridentateligand (Scheme 4).

Scheme 4.

O

NN

SnO

RR

O

OH

NH2

O

Ot�Bu

t�But�Bu

t�Bu

+ 2R2SnCl2

NH4OH, EtOH

(R = Me, Ph).

N

O

O

SnR2N

O

O

SnR3 –R•

N

OOH

2

Bu�tt�Bu

Bu�t

t�Bu

t�Bu t�Bu

t�Bu

Bu�tBu�t

+ R3SnClCH3CN, Et3N

–[Et3NH]Cl

[R = Me (5), Et (6), n�Bu (7), cyclo�Hex (8), Ph (9)].

5–9

•

DOKLADY CHEMISTRY Vol. 440 Part 2 2011

CYCLIZATION–DECYCLIZATION 297

The EPR spectra of paramagnetic tin complexes(5–9) (Fig. 2) show HFC of unpaired electron to themagnetic nuclei of two pairs of equivalent hydrogensand nitrogen atom and a satellite splitting due to themagnetic isotopes of tin (117Sn, I = 1/2, 7.75%, 119Sn,I = 1/2, 8.6%) [8], which unambiguously indicates theformation of the metal complex. We failed to observeHFC to the proton in the meta position to the nitrogenatom because the supposed magnitude of the HFCconstant (~0.05 mT) is smaller than the spectrum lin�ewidths. The parameters of the EPR spectra (table) oftin derivatives 5–9 are close to those observed for sim�ilar metal complexes containing a tetra�tert�butyl�substituted tridentate O,N,O�ligand [12, 14]. Com�plex 5 was isolated in the individual state. The elec�tronic absorpton spectrum of this compound in aceto�nitrile (λ, nm (ε, L cm–1 mol–1))—391.5 (18320),530.0 (1900), 980.4 (6787)—in comparison with theresults of [12] also confirms its structure. It should benoted that the presence of positions not shielded bytert�butyl substituents in one of the rings of the triden�tate ligand leads to the instability of its paramagnetic

form, as distinct from compounds described in [12].Solutions of compounds 5–9 even in the absence of airoxygen and moisture lose EPR activity within one day.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (projects nos. 10–03–00788–a and 11–03–97041–r_povolzh'e_a), the Council forGrants of the President of the Russian Federation forSupport of Leading Scientific Schools (grant no. NSh7065.2010.3) and Young Scientists (grant no. MK�641.2011.3), and the Federal Targeted Program “Sci�entific and Scientific�Pedagogical Personnel of theInnovative Russia in 2009–2013” (State ContractP839 of 25.05.2010).

REFERENCES

1. Poddels’ky, A.I., Cherkasov, V.K., and Abakumov, G.A.,Coord. Chem. Rev., 2009, vol. 253, no. 2, pp. 291–324.

2. Ivakhnenko, E.P., Karsanov, I.V., Khandkarova, V.S.,et al., Izv. AN. SSSR, Ser. Khim., 1986, no. 12,pp. 2755–2759.

3. Filipenko, O.S., Aldoshin, S.M., Ivakhnenko, E.P.,et al., Dokl. Chem., 2000, vol. 370, nos. 1–3, pp. 12–16[Dokl. Akad. Nauk, 2000, vol. 370, no. 3, pp. 345–349].

4. Rodriques�Morgade, S., Vazques, P., and Torres, T.,Tetrahedron, 1996, vol. 52, no. 19, pp. 6781–6794.

5. Abakumov, G.A., Druzhkov, N.O., Kurskii, Yu.A., andShavyrin, A.S., Izv. Akad. Nauk, Ser. Khim., 2003,no. 3, pp. 682–687.

6. Abakumov, G.A., Druzhkov, N.O., Kurskii, Yu.A.,et al., Izv. Akad. Nauk, Ser. Khim., 2005, no. 11,pp. 2491–2496.

7. Abakumov, G.A., Cherkasov, V.K., Druzkov, N.O.,et al., Synth. Commun., 2006, vol. 36, no. 21, pp. 3241–3247.

3400 3420 3440 3460 3480H, Oe

Fig. 2. Isotropic EPR spectrum of complex 5 in THF.290 K.

EPR parameters of solutions of paramagnetic compounds 5–9

Complexai(21H) ai(21H) ai(

14N) ai(119Sn) ai(

117Sn)gi

mT

5 0.16 0.33 0.66 5.05 4.83 2.0024

6 0.15 0.31 0.66 5.53 5.28 2.0025

7 0.15 0.33 0.65 5.47 5.23 2.0025

8 0.16 0.32 0.65 6.03 5.77 2.0022

9 0.17 0.34 0.66 4.52 4.32 2.0023

Note: THF, 290 K.

298

DOKLADY CHEMISTRY Vol. 440 Part 2 2011

ABAKUMOV et al.

8. Emsley, J., The Elements, Oxford: Clarendon, 1991.9. Chaudhuri, P., Hess, M., Hildenbrand, K., et al., Inorg.

Chem., 1999, vol. 38, no. 12, pp. 2781–2790.10. Girgis, A.I. and Balch, A.L., Inorg. Chem., 1975,

vol. 14, no. 11, pp. 2724–2727.11. Camacho�Camacho, C., Esparza�Ruiz, A., Vasquez�

Badillo, A., et al., J. Organomet. Chem., 2009, vol. 694,no. 5, pp. 726–730.

12. Piskunov, A.V, Sukhoshkina, O.Yu., and Smolyani�nov, I.V., Zh. Org. Khim., 2010, vol. 80, no. 4, pp. 629–638.

13. Camacho�Camacho, C., Mijangos, E., Castillo�Ramos, M.E., et al., J. Organomet. Chem., 2010,vol. 695, no. 6, pp. 833–840.

14. Stegmann, H.B., Chem. Ber., 1970, vol. 103, no. 4,pp. 1279–1285.