Interplay between nitronates and nitriles accomplished in a PtIV-mediated reaction

Article abstract of DOI:10.1016/j.inoche.2008.12.006

Reaction between the coordinated propanenitriles in trans-[PtCl₄(EtCN)₂] and the cyclic nitronate {A figure is presented} (1) gives the N-acylated iminocomplex {A figure is presented} (2) which is unstable in wet solvents and undergo

Full text of DOI:10.1016/j.inoche.2008.12.006

Inorganic Chemistry Communications 12 (2009) 173–176
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Inorganic Chemistry Communications
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Interplay between nitronates and nitriles accomplished in a PtIV-mediated reaction
b
c
c
Nadezhda A. Bokach a, , Sema L. Ioffe , Fedor M. Dolgushin , Mikhail Yu. Antipin ,
*
Vladimir A. Tartakovskii b, Vadim Yu. Kukushkin a,d,
*
a Department of Chemistry, St. Petersburg State University, Stary Petergof, 198504 St. Petersburg, Russian Federation
b N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 47, 119991 Moscow, Russian Federation
c A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119991 Moscow, Russian Federation
d Institute of Macromolecular Compounds of the Russian Academy of Sciences, V.O. Bolshoi Pr. 31, 199004 St. Petersburg, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction between the coordinated propanenitriles in trans-[PtCl4(EtCN)2] and the cyclic nitronate
Received 22 October 2008
Accepted 7 December 2008
Available online 16 December 2008
(1)
[PtCl4{N(CEtO)=CHNOCH(C6H4OMe)CH2CMe2}2]
hydrolysis to furnish
gives
(2) which is unstable in wet solvents and undergoes
(3). The formulation of 2 and 3 was
the
N-acylated
iminocomplex
ON=CHCH(C6H4OMe)CH2CMe2O
[PtCl4{NH=CHNOCH(C6H4OMe)CH2CMe2}2]
Keywords:
Nitronates
supported by satisfactory C, H, and N elemental analyses, agreeable HRESI+-MS, IR, 1H NMR spectrosco-
pies, and single-crystal X-ray diffraction (for trans-3).
Nitriles
Platinum(IV) complexes
Ligand reactivity
Metal-mediated reaction
Ó 2008 Elsevier B.V. All rights reserved.
In view of our general interest in conversions of metal-activated
substrates, in the past decade we had focused our attention on
reactions of ligands bearing the CN triple bond, i.e. RCN and RNC
species, toward coupling [1] or cycloaddition (CA) [2] and this topic
has been surveyed by some of us [1a–d,2a–d,f–h] and others [1e–
m,2e]. Our experimental [2b–d,2f–h] and theoretical [3] results
demonstrate that the coordination of RCN to platinum centers dra-
matically enhances the reactivity of the nitriles toward dipoles of
both allyl- (e.g., open chain A and cyclic nitrones B [2b–d,2h];
Fig. 1) and propargyl/allenyl (e.g. nitrile oxides C [2f,2g]) anion
types in comparison with 1,3-dipolar cycloaddition to free RCN
molecules. In particular, nitriles coordinated to a PtIV center under-
go CA with aromatic and aliphatic nitrones [2a–c] under mild con-
ditions that are not accessible in metal-free organic syntheses.
Being interested in extension of the Pt-mediated CA reactions of
nitriles to other dipoles, we launched a project aimed to verify, by
theoretical methods, various factors affecting CA and found [3a,3b]
that the reactivity of nitronates (D; Fig. 1) toward RCN is expected
to be lower than that for the previously investigated acyclic (A)
[2b–d] and cyclic nitrones (B) [2e–h]. Supporting in a collateral
way our data [3a,b], up to now Chemical Abstracts give no even a
single reference relevant to reaction between either complexed
or uncomplexed nitrile species and nitronates.
Bearing in mind the theoretical results outlined in the previous
paragraph, we attempted to react nitronate 1 (Fig. 2; IUPAC [4]
name: 4-(4-methoxy-phenyl)-6,6-dimethyl-5,6-dihidro-4H-[1,2]
oxazin-2-oxide) with EtCN in trans-[PtCl4(EtCN)2] insofar as it
was previously demonstrated that the ligation of nitriles to a PtIV
center ought to provide even a higher activation effect upon cyclo-
addition in comparison with the introduction of such powerful
electron-acceptor group R as CF3 to RCN [3]. To our knowledge, this
report covers the first example of any reaction between a nitronate
and a nitrile and it makes known that the interaction becomes pos-
sible in a metal-mediated reaction.
The starting material, trans-[PtCl4(EtCN)2], was obtained by the
known procedure [5], and nitronate 1 (Scheme 1) was synthesized
from b-nitro-4-methoxystyrene and isobutylene by the literature
method [6]. Reaction between trans-[PtCl4(EtCN)2] and 1 proceeds
at room temperature for 12 h and gives N-acylimine complex 2 as
the major product, when the synthesis was carried out in freshly
distilled CH2Cl2 [7]. Compound 2 is found to be unstable in air
and in wet solvents and it is hydrolyzed to form imine complex
3. The latter can be alternatively synthesized from trans-
[PtCl4(EtCN)2] and 1 in moderate yield if the reaction was per-
formed in nondried solvents [8].
* Corresponding authors. Address: Department of Chemistry, St. Petersburg State
University, Stary Petergof, 198504 St. Petersburg, Russian Federation. Tel.: +7 812
4284582; fax: +7 812 4286939 (N.A. Bokach).
E-mail addresses: bokach@nb17701.spb.edu (N.A. Bokach), kukushkin@
VK2100.spb.edu (V.Yu. Kukushkin).
Compounds 2 and 3 give satisfactory C, H, N elemental analyses
and were also characterized by HRESI+-MS, IR, and 1H NMR spec-
troscopies; the structure of
crystallography.
3 was determined by X-ray
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2008.12.006

174
N.A. Bokach et al. / Inorganic Chemistry Communications 12 (2009) 173–176
A
B
D
X
Ar
R
O
N
N
O
X = CH2, O
C
O
N
N
O
Ar
O
Fig. 1. Schematic representation of open chain (A) and cyclic (B) nitrones, nitrile
oxides (C), and nitronates (D).
Complex 2 has the same brutto-formula, i.e. C32H44N4O6Cl4Pt, as
the product derived from the cycloaddition of
1 to trans-
[PtCl4(EtCN)2]. Hence, the elemental analyses and HRESI+-MS sup-
port only the composition of 2 and its structure should be verified
by physicochemical methods. The IR spectrum of 2 shows no
band(s) assignable to
two strong stretches at 1746 and 1615 cmÀ1 that were attributed
to (C@O) and
(C@N), respectively. In the 1H NMR spectrum of
m(C„N) vibrations, but the observation of
m
m
2, the ABX system [2.24 dd (12.5 and 8.1 Hz), 2.61 dd (12.5 and
9.2 Hz), and 5.30 t (8.4 Hz)] is recognized. This NMR pattern gives
one of the evidences favoring the contraction of the six-membered
ring to furnish the isoxazolidine. In 2, the former 2-H proton from
the nitronate cycle does not exhibit indirect spin–spin interactions
with the other protons, but shows the coupling from 195Pt and its
signal appears as a singlet flanked with two satellites (3JPtH
28.9 Hz). The signals from the Et group are manifested as two
broad multiplets; this broadening relates, presumably, to a slow
rotation of the C(@O)Et moiety around the NC bond. All these char-
acteristic features of the 1H NMR spectrum along with the IR data
confirm the formulation of 2 as the N-acylimine complex rather
than the cycloaddition product.
Fig. 2. Thermal ellipsoid view of
3 with atomic numbering scheme. Thermal
ellipsoids are drawn with 50% probability. One of the two forms having different
stereochemical configuration at the C6 atom is given. Selected bond lengths [Å] and
angles [°]: Pt(1)–N(1) 2.014(5), Pt(1)–Cl(1) 2.332(5), Pt(1)–Cl(2) 2.301(4), N(1)–C(1)
1.259(11), N(2)–C(1) 1.335(12), O(1)–N(2) 1.357(7); N(1)–Pt(1)–Cl(2) 91.6(6),
N(1A)1–Pt(1)–Cl(2) 88.6(5), N(1)–C(1)–N(2) 125.4(13).
(1664 cmÀ1) than in 2. In the 1H NMR spectrum, one can observe
the ABX system characteristic for the 2,3,5,5-substituted isoxazol-
idine ring; the signal of the CH proton from the amidine fragment
appears as a doublet flanked with satellites due to the coupling of
the NH proton from the amidine group with 195Pt.
Complex 3 was crystallized in non-centrosymmetrical space
group C2 and it lies in a special position at the twofold axis that
passed through the metal center perpendicular to N1–N1A line
and through the centroids between the Cl1Á Á ÁCl1A and Cl2Á Á ÁCl2A
The IR spectrum of 3, in contrast to 2, displays a medium-to-
strong band at 3380 cmÀ1 that corresponds to
m(NH) vibrations;
the C@N stretches are displayed at slightly higher wave numbers
O
N
Cl
Cl
O
(p-MeO)C6H4
Et
N
Pt
N
Et
2
Cl
Cl
1
C6H4(OMe-p)
C6H4(OMe-p)
Et
N
Cl
Pt
Cl
Cl
Pt
Cl
H
N
O
N
N
Cl
Cl
2 H2O
O
O
N
N
O
O
N
—2 EtCO2H
Cl
Cl
N
H
O
Et
(p-MeO)C6H4
(p-MeO)C6H4
20%
NMR yield 90%
2
66%
NMR yield 95%
3
Scheme 1.

N.A. Bokach et al. / Inorganic Chemistry Communications 12 (2009) 173–176
175
Et
N
Et
Pt
N
Et
O
N
O
Pt
O
Pt
N
N
O
O
N
(p-MeO)C6H4
O
(p-MeO)C6H4
(p-MeO)C6H4
Scheme 2.
of these studies and also to Drs. A.V. Lesiv and A.Yu. Sukhorukov
for both experimental assistance and valuable suggestions.
R
N
R
O
N
O
Appendix A. Supplementary material
R'
Ar
N
N
CCDC 705255 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.inoche.
2008.12.006.
R'
Ar
Scheme 3.
atoms [9]. The complex has a pseudo-center of symmetry (at the Pt
atom), which, in general, disturbed by the chiral C6 atom. Insofar
as position of the heavy atoms in the structure obeys the centro-
symmetrical law, it is not possible to determine unambiguously
an absolute configuration of the complex (Flack parameter is
0.44(2)). However, the crystallography data give evidences that
two centers (C6 and C6A in both heterocyclic ligands) display the
same configuration, i.e. RR or SS.
A plausible mechanism for the observed conversion involves CA
of 1 to platinum(IV)-activated EtCN’s followed by the ring opening
of 2,3-dihydro-1,2,4-oxadiazole ring and concomitant contraction
of the 1,2-oxazinane cycle (Scheme 2).
Some works describe the ring opening of 2,3-dihydro-1,2,4-oxa-
diazoles that proceeds via the N–O bond cleavage accompanied by
decarboxylation [10] or H-migration in free [11] or Pt-coordinated
heterocycle [12] to furnish N-acylimines. Other recently reported
transformation of 2,3-dihydro-1,2,4-oxadiazoles bearing electron-
acceptor substituents R with aryl migration onto amino-N (Scheme
3) looks rather similar to the rearrangement described in Scheme 2
[13].
References
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(b) A.J.L. Pombeiro, V. Yu. Kukushkin, Reactions of Coordinated Nitriles,
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(c) G. Wagner, M. Haukka, J.J.R. Fraústo da Silva, A.J.L. Pombeiro, V.Yu.
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Fraústo da Silva, A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2005) 3042;
(e) M.A.J. Charmier, V.Yu. Kukushkin, A.J.L. Pombeiro, Dalton Trans. (2003)
2540;
These literature data favor the CA mechanism of the interaction
between the complexed EtCN and the nitronate suggested in this
work.
We succeeded so far to achieve the interaction between one rel-
atively stable nitronate and one rather well soluble nitrile plati-
num precursor. In other cases, poor solubilities of the
[PtCl4(RCN)2] (R = Me, CH2Ph, Ph) complexes in dry dichlorometh-
ane on one hand and limited stability or low reactivity of other
(f) N.A. Bokach, A.V. Khripoun, V.Yu. Kukushkin, M. Haukka, A.J.L. Pombeiro,
Inorg. Chem. 42 (2003) 896;
(g) N.A. Bokach, V.Yu. Kukushkin, M. Haukka, A.J.L. Pombeiro, Eur. J. Inorg.
Chem. (2005) 845;
(h) A.V. Makarycheva-Mikhailova, J.A. Golenetskaya, N.A. Bokach, I.A. Balova,
M. Haukka, V.Yu. Kukushkin, Inorg. Chem. 46 (2007) 8323;
(i) K.V. Luzyanin, M. Haukka, N.A. Bokach, M.L. Kuznetsov, V.Y. Kukushkin,
A.J.L. Pombeiro, Dalton Trans. (2002) 1882;
nitronates (e.g.,
, MeCH@N(O)OSiMe ,
ON=CMeCH(C6H5)CH2CMe2O
3
(j) N.A. Bokach, V.Y. Kukushkin, M.L. Kuznetsov, D.A. Garnovskii, G. Natile,
A.J.L. Pombeiro, Inorg. Chem. 41 (2002) 2041.
[3] (a) M.L. Kuznetsov, V.Yu. Kukushkin, J. Org. Chem. 71 (2006) 582;
(b) M.L. Kuznetsov, A.A. Nazarov, L.V. Kozlova, V.Yu. Kukushkin, J. Org. Chem.
72 (2007) 4475;
EtCH@N(O)OSiMe3,) on the other hand led to unselective processes
and determined generation of broad spectra of products with no
major components. Further works on reactions between nitronates
and complexed nitriles are underway in our group.
(c) M.L. Kuznetsov, V.Yu. Kukushkin, A.I. Dement’ev, A.J.L. Pombeiro, J. Phys.
Chem. A 107 (2003) 6108;
(d) M.L. Kuznetsov, V.Yu. Kukushkin, M. Haukka, A.J.L. Pombeiro, Inorg. Chim.
Acta (Special volume dedicated to J. J. R. Fraústo da Silva) 356 (2003) 85.
[4] http://www.iupac.org/nomenclature/index.html.
Acknowledgements
[5] K.V. Luzyanin, M. Haukka, N.A. Bokach, M.L. Kuznetsov, V.Y. Kukushkin, A.J.L.
Pombeiro, J. Chem. Soc., Dalton Trans. (2002) 1882.
[6] A.A. Tishkov, A.V. Lesiv, Y.A. Khomutova, Y.A. Strelenko, I.D. Nesterov, M.Yu.
Antipin, S.L. Ioffe, S.E. Denmark, J. Org. Chem. 68 (2003) 9477.
The authors are very much obliged to the Russian Fund for Basic
Research (grant 08-03-00247-a) and to RAS Presidium Program
coordinated by acad. Nikolay T. Kuznetsov (grant 8P) for support

176
N.A. Bokach et al. / Inorganic Chemistry Communications 12 (2009) 173–176
[7] Compound 1 (131 mg, 0.560 mmol) was added to a stirred suspension of trans-
[PtCl4(EtCN)2] (100 mg, 0.224 mmol) in dry CH2Cl2 (5 mL), whereupon
selected bands, cmÀ1): 3380 m
1251 m
(C–O). 1H NMR (CDCl, d): 1.45 (s, 3H, Mea), 1.53 (s, 3H, Mee), 2.26 (dd,
m(N–H), 2973 m m(C–H), 1664 s, br m(C@N),
a
m
yellow solution was formed within 10–15 min. The reaction mixture was
stirred at room temperature for 12 h, then the volume was reduced to half of
its initial volume by vacuum evaporation at room temperature and Et2O
(10 mL) was added. Yellow powder formed was filtered off, washed with dry
Et2O (two 5-mL portions) and dried on air. Yield is 112 mg, 54%. Additional
amount of the solid complex (25 mg, 12%) can be obtained after vacuum
evaporation of the filtrate and treatment of the yellow oily residue with Et2O
(15 mL). NMR yield ca. 95%. Compound 2. Anal. Calcd for C32H44N4O6Cl4Pt Á
9.2 and 12.5 Hz, 1H), 2.63 (dd, 8.2 and 12.5 Hz, 1H)(CH2), 3.82 (s, 3H, OMe),
5.20 (t, 8.60 Hz, 1H, CH-Ar), 6.02 (d, br, JHH 14.4 Hz, 1H, NH), 6.92 (d, 8.4 Hz, 2H,
o- or m-Ph), 7.22 (d, 8.8 Hz, 2H, o- or m-Ph), 7.38 (d + satellites, 1H, JPtH 28.6 Hz,
3JHH 14.7 Hz, CH@N). 13C NMR was not measured due to low solubility of the
complex in CDCl3. Orange-yellow crystals of 3 suitable for X-ray diffraction
study were obtained by slow (1 week) evaporation of solution of 2 in nondried
CDCl3.
[9] X-ray Crystal Structure Determinations. Crystal data for 3: C26H36Cl4N4O4Pt,
0.19 Â 0.09 Â 0.06 mm3,
monoclinic,
c = 11.082(2) Å,
space
group
C2,
Z = 2,
1
4CH2Cl2: C, 41.26; H, 4.78; N, 5.97. Found C, 41.25; H, 4.90; N, 5.93 (the
M = 805.48,
solvated dichloromethane was observed in the 1H NMR spectrum). HRESI+-MS,
a = 16.123(3),
b = 10.527(2),
V = 1560.7(5) Å3,
m/z: 847.225 [MÀ2Cl+H]+ (847.231 calcd.). IR (KBr, selected bands, cmÀ1):
Dc = 1.714 g/cm3,
l
= 4.875 mmÀ1
,
F000 = 796, Bruker SMART APEX II
2977 m, 2836 m-w
m
m
(C–H), 1746 m-s, br
m
(C@O), 1615 s, br
m
(C@N), 1253 m
diffractometer, MoKa radiation, k = 0.71073 Å, T = 100(2) K, 2hmax = 56°, 5826
(C–O). 1H NMR (CDCl3, d): 1.13 (t, br, 7.73 Hz, 3H, CH3 from Et), 1,43 (s, 3H,
reflections collected, 3734 unique (Rint = 0.0403). The APEX II software [APEX II
software package, Bruker AXS Inc., 5465, East Cheryl Parkway, Madison, WI
5317, 2005] was used for collecting frames of data, indexing reflections,
determination of lattice constants, integration of intensities of reflections,
scaling and absorption correction, and SHELXTL [SHELXTL v. 5.10, Structure
Determination Software Suite, Bruker AXS, Madison, Wisconsin, USA, 1998] for
space group and structure determination, refinements, graphics, and structure
reporting. The refinement converged to wR2 = 0.0912 and GOF = 1.043
(refinement on F2 for all independent reflections, and R1 = 0.0462
Mea), 1.49 (s, 3H, Mee), 2.24 (dd, 12.5 and 9.2 Hz, 1H), 2.61 (dd, 12.5 and 8.1 Hz,
1H)(CH2), 2.93 (m br, 2 H, –CH2 from Et), 3.82 (s, 3H, OMe), 5.30 (t, 8.4 Hz, 1H,
CH–Ar), 6.92 (d, 8.7 Hz, 2H, o- or m-Ph), 7.21 (d, 8.7 Hz, 2H, o- or m-Ph), 7.67
(s + d, 1H, 3JPtH 28.9 Hz, CH@N). 13C NMR (CD2Cl2, d): 8.1 and 35.6 (Et), 24.9 and
25.3 (Mea and Mee), 48.0 (CH2), 55.2 (MeO), 66.5 (CH–Ar), 87.5 (CMe2), 114.6
(m-Ar), 127.5 (Cipso), 128.4 (o-Ar), 146.8 (CH@N), 160.4 (p-Ar), 182.1 (C@O).
[8] Complex 3 was prepared by the method described for 2 but wet solvents
(CH2Cl2 and Et2O) were used instead of the dried and freshly distilled. The
dark-brown reaction mixture after stirring for 12 h was evaporated until
dryness, and the formed brown oily residue was treated with Et2O (3 Â 3 mL).
The brown-orange solid thus obtained was recrystallized twice from a CH2Cl2/
Et2O mixture and the orange-yellow powder of 3 was obtained in ca. 20% yield.
(refinement on F for 3309 observed reflections with I > 2r(I)), 169 refined
parameters.
[10] H. Pavez, A. Marquez, P. Navarrete, S. Tzichinovsky, H. Rodríguez, Tetrahedron
43 (1987) 2223.
Complex
2
is unstable in solutions of the most common commercially
[11] R. Plate, P.H.H. Hermkens, J.M.M. Smits, R.J.F. Nivard, H.C.J. Ottenheijm, J. Org.
Chem. 52 (1987) 1047.
[12] M.A.J. Charmier, M. Haukka, A.J.L. Pombeiro, Dalton Trans. (2004)
2741.
available deuterated solvents and slowly hydrolyzed to furnish 3 with NMR
yield after 3 days at room temperature in nondried CDCl3 ca. 90%. Compound
3. Anal. Calcd for C26H36N4O4Cl4Pt: C, 38.77; H, 4.50; N, 6.96. Found C, 38.91; H,
4.62; N, 6.68. HRESI+-MS, m/z: 844.064 [MÀH+K]+ (844.065 calcd.). IR (KBr,
[13] G. Wagner, T. Garland, Tetrahedron Lett. 49 (2008) 3596.