Copper-Mediated Oxygenation of Nitronate to Nitrite and Acetone in a Copper(I) Nitronate Complex

Full text of DOI:10.1021/ic980246t

Inorg. Chem. 1998, 37, 6535-6537
6535
upon oxygenation of flavoenzymes with molecular oxygen.15
Glucose oxidase and D- and L-amino acid oxidase accept
nitroalkane anions as substrates.16 The mechanism of flavo-
enzyme-catalyzed oxidation of nitroalkanes has been established
to involve an N(5)-adduct as an intermediate.16,17 Electron-
deficient flavins will also oxidize nitroalkane anions in model
reactions.18
Copper-Mediated Oxygenation of Nitronate to
Nitrite and Acetone in a Copper(I) Nitronate
Complex
EÄ va Balogh-Hergovich,1a Ga´bor Speier,*,1b
Gottfried Huttner,1c and La´szlo´ Zsolnai1c
Extracts of Neurospora crassa and pea seedlings oxidatively
degrade nitroethane and nitropropane,19 and those from the
hyphae of a nitrifying strain of Aspergillus flaVus produce nitrite
and nitrate from 3-nitropropionate.20 2-Nitropropane and some
other nitroalkanes are oxidatively metabolized by an intracellular
enzyme of Hansenula mraki.21 It has been purified and
characterized as 2-nitropropane dioxygenase presumably pos-
sessing iron ions in its active center.22 The action of this enzyme
can be best described as an intermolecular dioxygenation
reaction (eq 1b).23 In this report we describe preliminary results
obtained from similar reactions using a copper(I) aci-2-
nitropropanate complex.
Department of Organic Chemistry, University of Veszpre´m,
8201 Veszpre´m, Hungary, Department of Inorganic
Chemistry, University of Heidelberg, 69120 Heidelberg,
Germany, and Research Group for Petrochemistry of the
Hungarian Academy of Sciences, 8201 Veszpre´m, Hungary
ReceiVed March 5, 1998
Introduction
Primary and secondary but not tertiary aliphatic nitro
compounds can be transformed to aldehydes or ketones by
treatment of their conjugate bases with sulfuric acid. This is
called the Nef reaction and involves hydrolysis of the CdN
double bond (eq 1a).2,3 There are several alternative methods
for converting nitroalkanes to carbonyl compounds that give
better yields and fewer side reactions. Such methods are the
reaction of aliphatic nitro compounds with aqueous TiCl3,4
cetyltrimethylammonium permanganate,5 tin complexes and
NaHSO3,6 activated dry silica gel,7 or 30% H2O2-K2CO3,8 and
the treatment of the conjugate base of the nitro compound with
KMnO4,9 t-BuOOH and a catalyst,10 ceric ammonium nitrate,11
MoO5-pyridine-HMPA,12 ozone,13 or singlet oxygen.14
Experimental Section
Preparation of Cu((CH3)2CNO2)(PPh3)2. To a stirred solution of
2-nitropropane (356 mg, 4 mmol) in anhydrous acetonitrile (60 mL)
were added copper(I) mesityl (732 mg, 4 mmol) and triphenylphosphine
(2.096 g, 8 mmol) under argon. The mixture was stirred for 8 h, and
a white precipitate formed, which was filtered off, washed with small
amount of acetonitrile, and dried under vacuum. Recrystallization from
ether give colorless diamagnetic crystals of Cu((CH3)2CNO2)(PPh3)2
(2.32 g) in 86% yield based on initial quantities of Cu.
Characterization. Mp: 217-219 °C. IR (Nujol) [ν (cm-1)]: 1602,
1478, 1458, 1431, 1374, 1145, 1134, 1085, 1030, 1000, 938, 858, 755,
700, 502. UV-vis (CH3CN) [λmax (log ꢀ)]: 214 nm (4.68), 226 (4.61),
265 (4.08).1H NMR (CDCl3) [ppm]: 1.94 (s, 6H, CH3), 7.16-7.40
(m, 30H, ArH). 13C NMR (CDCl3) [ppm]: 133.9, 133.7, 133.5, 133.3,
133.2, 129.5, 128.5, 128.4, 23.0. 31P NMR (CDCl3) [ppm]: -2.66.
Anal. Calcd for C39H36NP2O2Cu: C, 69.27; H, 5.36; N, 2.07. Found:
C, 69.36; H, 5.41; N, 1.98.
Crystallography. Colorless crystals of Cu((CH3)2CNO2)(PPh3)2
were obtained from ether. They form in the monoclinic crystal system,
space group C2/c, in a unit cell of the dimensions listed in Table 1.
Data were collected on a Siemens (Nicolet Syntex) R3m/V diffracto-
meter, and the structure was solved by direct methods. Refinement
converged with R ) 0.0555 and Rw(F2) ) 0.155 for 3806 measured,
3216 independent (Rint ) 0.0220), and 2606 observed reflections
[I > 2σ(I)]. Positions for selected atoms of the structure are listed in
Table 2.
Reaction of Cu((CH3)2CNO2)(PPh3)2 with Dioxygen. A suspen-
sion of Cu((CH3)2CNO2)(PPh3)2 (169 mg, 0.25 mmol) in anhydrous
acetonitrile (10 mL) was stirred under dioxygen at room temperature.
The uptake of dioxygen was measured manometrically. After 8 h 3.1
mL (0.127 mmol) of dioxygen was consumed. GLC analysis of the
solvent shows the presence of acetone (9 mg, 62%). The solvent was
In biological systems, the oxygenation of nitroalkanesswith
the concomitant loss of nitritesis facilitated by species formed
(1) (a) Research Group for Petrochemistry of the Hungarian Academy of
Sciences. (b) University of Veszpre´m. (c) University of Heidelberg.
(2) Pinnick, H. W. Org. React. 1990, 38, 655.
(3) Haines, A. H. Methods for the Oxidation of Organic Compounds;
Academic Press: New York, 1988; pp 220, 416.
(4) (a) McMurry, J. E.; Melton, J. J. Org. Chem. 1973, 38, 4367. (b)
McMurry, J. E. Acc. Chem. Res. 1974, 7, 281. (c) Kirchhoff, R.
Tetrahedron Lett. 1976, 17, 2533.
(5) Vankar, P. S.; Rathore, R.; Chandrasekaran, S. Synth. Commun. 1987,
17, 195.
(6) Urpi, F.; Vilarrasa, J. Tetrahedron Lett. 1990, 31, 7499.
(7) Keinan, E.; Mazur, Y. J. Am. Chem. Soc. 1977, 99, 3861.
(8) Ola´h, G. A.; Arvanaghi, M.; Vankar, Y. D.; Prakash, G. K. S. Synthesis
1980, 662.
(9) (a) Shechter, H.; Williams, F. T. J. Org. Chem. 1962, 27, 3699. (b)
Freeman, F.; Yeramyan, A. J. Org. Chem. 1970, 35, 2061. (c) Freeman,
F.; Lin, D. K. J. Org. Chem. 1971, 36, 1335. (d) Kornblum, N.;
Erickson, A. S.; Kelly, W. J.; Henggeler, B. J. Org. Chem. 1982, 47,
4534. (e) Steliou, K.; Poupart, M.-A. J. Org. Chem. 1985, 50, 4971.
(10) Bartlett, P. A.; Green, F. R.; Webb, T. R. Tetrahedron Lett. 1977, 18,
331.
(15) Bruice, T. C. In Biomimetic Chemistry; Advances in Chemistry Series,
191; Dolphin, D., McKenna, C., Murakami, Y., Tabushi, I., Eds.;
American Chemical Society: Washington, DC, 1980; p 89.
(16) Porter, D. J. T.; Brigth, H. J. J. Biol. Chem. 1977, 252, 4361.
(17) Porter, D. J. T.; Voet, J. G.; Brigth, H. J. J. Biol. Chem. 1973, 248,
4400.
(18) Yokoe, I.; Bruice, T. C. J. Am. Chem. Soc. 1975, 97, 450.
(19) (a) Little, H. N. J. Biol. Chem. 1951, 193, 347. (b) Little, H. N. J.
Biol. Chem. 1957, 229, 231.
(11) Ola´h, G. A.; Gupta, B. G. B. Synthesis 1980, 44.
(12) Galobardes, M. R.; Pinnick, H. W. Tetrahedron Lett. 1981, 22, 5235.
(13) McMurry, J. E.; Melton, J.; Padgett, H. J. Org. Chem. 1974, 39, 259.
(14) Williams, J. R.; Unger, L. R.; Moore, R. H. J. Org. Chem. 1978, 43,
1271.
(20) Molina, J. A. E.; Alexander, M. J. Bacteriol. 1971, 105, 489, 493.
(21) Kido, T.; Yamamoto, T.; Soda, K. Arch. Microbiol. 1975, 106, 165.
(22) Kido, T.; Yamamoto, T.; Soda, K. J. Bacteriol. 1976, 126, 1261.
(23) Kido, T.; Soda, K.; Suzuki, T.; Asada, K. J. Biol. Chem. 1976, 251,
6994.
10.1021/ic980246t CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/19/1998

6536 Inorganic Chemistry, Vol. 37, No. 25, 1998
Table 1. Crystallographic Data for Cu((CH3)2CNO2)(PPh3)2
Notes
Results and Discussion
Our studies on the activation of dioxygen24 and on model
systems mimicking dioxygenases25 and the fact that nitronate
salts of nonredox metals can be converted to nitro compounds
and carbonyls with singlet oxygen inspired us to prepare
nitronato copper complexes, possessing a redox-active metal
ion, and oxygenate it with triplet dioxygen. Nitroalkanes are
known to exhibit tautomerism to yield nitronic acids and
equilibria lay well on the side of the latter.26 Deprotonation of
both tautomers leads to nitronate ions. Mesitylcopper(I)27 reacts
with 2-nitropropane and triphenylphosphine in anhydrous ac-
etonitrile under an atmosphere of argon at room temperature
(eq 2). After 8 h a white precipitate formed which was filtered,
dried, and crystallized from ether to give colorless crystals of
the copper(I) aci-2-nitropropanate complex Cu(C3H6NO2)-
(PPh3)2 (1). This complex is moderately soluble in polar organic
empirical formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
C19.50H18Cu0.50 N0.50OP
676.16
monoclinic
C2/c
15.280(8)
9.804(7)
22.783(11)
101.15(4)
3349(3)
4
190
0.710 73
1.341
0.783
0.056
â (deg)
V (Å3)
Z
T (K)
λ (Mo KR) (Å)
D
calcd (Mg m-3
)
µ (mm-1
R1
)
Rw(F2)
0.155
1.325
GOF(F2)
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for Cu((CH3)2CNO2)(PPh3)2
a
x
y
z
Ueq
Cu
P(1)
O(1)
N(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
5000
1970(1)
2862(1)
101(2)
-634(4)
-1947(5)
-2667(4)
1629(3)
1950(4)
974(4)
2500
23(1)
21(1)
33(1)
29(1)
39(1)
64(1)
26(1)
30(1)
38(1)
38(1)
35(1)
28(1)
26(1)
37(1)
47(1)
42(1)
39(1)
31(1)
24(1)
31(1)
39(1)
42(1)
42(1)
35(1)
5313(1)
4305(2)
5000
3428(1)
2571(1)
2500
5000
2500
5803(4)
5670(2)
6253(2)
6485(2)
6147(2)
5567(2)
5333(2)
6198(2)
6127(2)
6809(3)
7563(3)
7656(2)
6982(2)
4363(2)
3682(2)
2928(2)
2852(3)
3530(3)
4281(4)
2384(2)
4029(1)
4565(2)
5003(2)
4918(2)
4394(2)
3952(1)
3552(1)
3810(2)
3857(2)
3644(2)
3399(2)
3355(2)
3656(1)
3210(2)
3345(2)
3928(2)
4384(2)
4252(2)
solvents, and its solutions are not very sensitive toward oxygen
due to the facts that the PPh3 stabilizes the copper in the 1+
oxidation state and that the coordinated nitronato ligand is not
susceptible to oxidation. Characteristic NMR features of
complex 1 in CDCl3 solution include a single CH3 resonance
downfield shifted by 0.4 ppm from the free 2-nitropropane and
a multiplet resonance of the aromatic protons of the phosphine
ligands in the 1H NMR, downfield shifts of coordinated nitronato
-331(4)
-653(4)
317(3)
4129(3)
5397(4)
6339(4)
6031(4)
4776(4)
3827(4)
3690(3)
4158(3)
4752(4)
4895(4)
4451(4)
3846(4)
carbons in the 13C NMR together with a single peak in the 31
P
NMR spectrum, and intense sharp IR bands assigned to
ν(CdN) at 1602, νas(NO2) at 1145 and 1134, and νs(NO2) at
938 cm-1
. Single-crystal X-ray analysis of compound 1
revealed a distorted tetrahedral geometry around copper(I) with
a Cu-O distance of 2.139(3) Å and a Cu-P distance of
2.2510(13) Å with a 2-fold axis through Cu and N(1) (Figure
1). The atoms O(1) and O(1A) form strong bonds with the
copper atom having short bond distances and consequently the
O(1)-N-O(1A) angle is rather small. The aci-2-nitropropanate
ion is planar and the observed C-N distance is somewhat
shorter than those found by bis(propane-2-nitronato)copper(II)28
(1.34(4), 1.35(4) Å) and in R-nitro-R-aci-nitrotoluenebis(tri-
phenylphosphine)copper(I)29 (1.36(1) Å) and lie between those
for oximes (1.26-1.31 Å)30 and for nitro compounds (1.46-
1.55 Å).31 It seems likely that these bonds have appreciable
double bond character. A comparison of the N-O distance with
those of oximes (1.36-1.42 Å)30 and of nitro compounds (1.22-
a Ueq is defined as one-third of the trace of the orthogonalized Uij
tensor.
evaporated off in a vacuum, and the residue was recrystallized from
ethanol to give colorless crystals of Cu(NO2)(PPh3)2 (130 mg) in 82%
yield.
Characterization of Cu(NO2)(PPh3)2. Mp 212-214 °C. IR
(Nujol) [ν(cm-1)]: 1481, 1461, 1434, 1380, 1221, 1094, 1028, 1000,
854, 748, 693, 524, 505. UV-vis (CH3CN) [λmax (log ꢀ)]: 214 nm
(4.73), 263 (4.26). 1H NMR (CDCl3) [ppm]: 7.20-7.72 (m, ArH).
13C NMR (CDCl3) [ppm]: 133.7, 132.1, 132.0, 129.9, 128.6. 31P NMR
(CDCl3) [ppm]: -1.75. Anal. Calcd for C36H30NO2P2Cu: C, 68.18;
H, 4.76; N, 2.20. Found: C, 68.10; H, 4.64; N, 2.15.
(24) Speier, G.; Tyekla´r, Z. Transition Met. Chem. 1992, 17, 348.
(25) Speier, G. New J. Chem. 1994, 18, 143.
(26) Breuer, E.; Aurich, H. G.; Nielsen, A. In Nitrones, Nitronates and
Nitroxides. Updates from the Chemistry of Functional Groups; Patai,
S., Rappoport, Z., Eds.; John Wiley and Sons: New York, 1989; p
218.
(27) Tsuda, T.; Yazawa, T.; Watanabe, K.; Fujii, T.; Saegusa, T. J. Org.
Chem. 1981, 46, 192.
(28) Simonsen, O. Acta Crystallogr. 1973, B29, 2600.
(29) Camus, A.; Marsich, N.; Nardin, G.; Randaccio, L. J. Chem. Soc.,
Dalton Trans. 1975, 2560.
(30) (a) Andreetti, G. D.; Cavalca, L.; Manfredotti, A.; Muscatti, A. Acta
Crystallogr. 1969, B25, 288. (b) Bednowitz, A. L.; Fankuchen, I.;
Okaya, Y.; Soffer, M. D. Acta Crystallogr. 1966, 20, 100. (c) Calleri,
M.; Ferraris, G.; Viterbo, D. Acta Crystallogr. 1966, 20, 73. (d) Mootz,
D.; Berking, B. Acta Crystallogr. 1969, B25, 828. (e) Oijen, J. W. L.;
Romers, C. Acta Crystallogr. 1966, 20, 169.
18O2 Labeling Experiments. A suspension of Cu((CH3)2CNO2)-
(PPh3)2 (169 mg, 0.25 mmol) in anhydrous acetonitrile (10 mL) was
stirred under a mixture of 18O2 and 16O2 (60%:40%) at room temper-
ature. After the reactants had stirred for 8 h, GC-MS and IR analysis
of the solution showed the presence of 18O- and 16O-acetone.
Kinetic Measurements. Standard volumetric methods were used
in the kinetic measurements. Pyridine was placed under dioxygen in
a thermostated reaction vessel, connected to a thermostated gas buret
and stirred until saturated. The consumption of dioxygen was measured
after adding and dissolving the Cu((CH3)2CNO2)(PPh3)2 complex at
18.5 °C and constant O2 pressure. The log[Cu((CH3)2CNO2)(PPh3)2]
vs time plots gave straight lines with pseudo-first-order rate constants
kobs. With various O2 pressures the kobs vs [O2] plots resulted also in
straight lines.

Notes
Inorganic Chemistry, Vol. 37, No. 25, 1998 6537
used. In the IR spectra of the acetone formed peaks to ν(C16O)
(1714 cm-1) and ν(C18O) (1684 cm-1) and no new bands in
that of 2 due to ν(N18O16O) or ν(N18O18O) were found showing
that 18O2 has been incorporated only into acetone.
Kinetic measurements on the oxygenation of 1 in pyridine
solution (reasonable solubility in other solvents could not be
found) under ambient conditions (18.5 °C, 1 bar O2 pressure)
were shown to be first order with respect to 1 and dioxygen
with a second-order rate constant of k2 (M-1 s-1) ) 2.48 ×
10-2
.
3
14
The aci-2-nitropropanato anion is inactive against O2 but
the Cu(I) is redox-active in complex 1, even in the presence of
strong Cu(I)-stabilizing ligand such as PPh3.25,34 The rate-
limiting reaction step seems to be the interaction of dioxygen
with 1 leading to a superoxocopper(II) complex 3, as found in
the case of many other copper(I) complexes,35 and finally to an
intermolecular peroxidic species 4. A fast breakdown of 4 may
result in the copper nitrite complex and acetone (eq 4).
Figure 1. View showing the structure of Cu(C3H6NO2)(PPh3)2.
Important bond distances (Å) and angles (deg): Cu-O(1), 2.139(3);
Cu-O(1A), 2.139(3); Cu-P(1), 2.2510(13); Cu-P(1A), 2.2510(13);
N(1)-O(1), 1.319(3); N(1)-O(1A), 1.319(3); N(1)-C(1), 1.287(7);
C(1)-C(2), 1.483(5); O(1)-Cu-O(1A), 62.17(13); O(1)-Cu-P(1),
106.28(7); O(1)-Cu-P(1A), 112.63(7); P(1)-Cu-P(1A), 134.31(6);
O(1)-N(1)-O(1A), 113.7(4).
1.25 Å)31 indicates that the bonds between the nitrogen and
oxygen atoms in complex 1 are basically single.
Motivated by our original goal to oxygenate a nitronato redox-
metal complex we investigated the reactivity of 1 toward
dioxygen. A slurry of Cu(C3H6NO2)(PPh3)2 (1) in acetonitrile
reacts with dioxygen slowly in a 2:1 stoichiometry with the
concomitant formation of Cu(NO2)(PPh3)2 (2) and acetone (eq
3). When oxygenated 0.25 mmol of 1 took up 3.1 mL of O2 at
Labeling with 18O2 and the stoichiometry of the oxygenation
reaction gives unequivocal evidence for the incorporation of
both oxygen atoms of O2 into two substrate molecules remi-
niscent of intermolecular dioxygenase.36
On the basis of the stoichiometry, kinetics, and labeling
experiments on reaction 3, the mechanism shown in eq 4 can
be proposed. The copper-assisted intermolecular oxygenation
of the CdN double bond of the coordinated aci-2-nitropropanato
ligand with triplet dioxygen may indicate that the presence of
copper(I) is essential in the activation of triplet dioxygen, and
the reduced oxygen species is ready for oxidative splitting of
the CdN double bond.
Acknowledgment. We thank the Hungarian Research Fund
(OTKA T-016285) and COST (ERBCIPECT-926093, 12160)
for financial support and Drs. Z. De´csy, J. Mink, and G.
Szalontai for MS, IR, and NMR measurements.
23 °C (stoichiometry 2:1) during 8 h, then the reaction was
stopped to avoid unwanted oxidation of Cu(I). The presence
of acetone was proved and quantified by GC-MS (yield 62%),
and after evaporation of the solvent and recrystallization from
ethanol pure, colorless crystals of 2 in 82% yield were obtained.
The diamagnetic compound exhibits simple NMR spectroscopic
features showing only signals characteristic for PPh3 in the 1H,
13C, and 31P NMR spectra and ν(NO2) bands in IR due to
coordinated nitrito ligand. Single-crystal X-ray analysis data
of the complex 2 was identical to those found earlier.32
Supporting Information Available: Stoichiometry and kinetics of
dioxygen uptake, spectral characterization for new compounds, and
details of the X-ray structure determination (13 pages). Ordering
information is given on any current masthead page.
18O2 labeling experiments of the oxygenation of 1 were
carried out with a mixture of 18O2 and 16O2 (60%:40%). After
the reactants had stirred at 25 °C in CH3CN suspension for 8 h
GC-MS analysis of the solution showed the presence of 18O-
and 16O-acetone. The 18O-acetone gave a molecular ion at m/z
60 with a base peak at m/z 45, indicating the incorporation of
18O atom into the nitropropane moiety. The relative abundance
of m/z 45 (100) and m/z 43 (61) parallels the 18O2 enrichment
IC980246T
(33) Camus, A.; Marsich, N.; Nardin, G.; Randaccio, L. Inorg. Chim. Acta
1976, 17, L33.
(34) Speier, G.; Tyekla´r, Z.; Szabo´, L., II; To´th, P.; Pierpont, C. G.;
Hendrickson, D. N. In The ActiVation of Dioxygen and Homogeneous
Catalytic Oxidation; Barton, D. H. R., Martell, A. E., Sawyer, D. T.,
Eds.; Plenum Press: New York, 1993; p 423.
(31) (a) Cox, A. P.; Waring, S. J. Chem. Soc., Faraday Trans. 2 1972, 68,
1061. (b) Duffin, B.; Wallwork, S. C. Acta Crystallogr. 1966, 20,
210. (c) Sutor, D. J.; Calvert, L. D. Llewellyn, F. J. Acta Crystallogr.
1954, 7, 767.
(35) Karlin, K. D.; Kaderli, S.; Zuberbu¨hler, A. D. Acc. Chem Res. 1997,
30, 139.
(36) Sima´ndi, L. I. Catalytic ActiVation of Dioxygen by Metal Complexes;
Catalysis by Metal Complexes Series; Ugo, R., Ed.; Kluwer Academic
Publishers: Dordrecht, 1992; Vol. 13, p 249.
(32) Halfen, J. A.; Tolman, W. B. Acta Crystallogr. 1995, C51, 215.