Facile and reversible 1,3-dipolar cycloaddition of aryl ketonitrones to platinum(ii)-bound nitriles: Synthetic, structural, and theoretical studies

Article abstract of DOI:10.1021/om201026n

The reaction between trans-[PtCl₂(NCR) ₂] (R = Et 1, NMe2 2, NEt2 3, NC5H₁₀ 4) and the acyclic triaryl ketonitrones Ph2C=N(O)C6H4R-p (R = H 5, Me 6, Cl 7, OMe 8) proceeds as a facile and consecutive two-step intermolecular cycloaddition to give the mono-cycloaddition products trans-[PtCl2(NCR){Na= C(R)ON(C6H4R-p)CbPh₂}](ab) (R/R = Et/H 9, Et/Me 10, Et/Cl 11, Et/OMe 12, NMe2/H 13, NMe2/Me 14, NMe2/Cl 15, NMe2/OMe 16, NEt2/H 17, NEt2/Me 18, NEt2/Cl 19, NEt2/OMe 20, NC ₅H₁₀/H 21, NC5H10/Me 22, NC₅H₁₀/Cl 23, NC₅H₁₀/OMe 24) and then the bis-2,3-dihydro-1,2,4- oxadiazole complexes trans- [PtCl2{Na=C(R)ON(C6H4R-p)CbPh2}2](ab) (R/R = Et/H 25, Et/Me 26, Et/Cl 27, Et/OMe 28, NMe2/H 29, NMe2/Me 30, NMe2/Cl 31, NMe ₂/OMe 32, NEt2/H 33, NEt2/Me 34, NEt₂/Cl 35, NEt2/OMe 36, NC₅H₁₀/H 37, NC5H10/Me 38, NC₅H₁₀/Cl 39, NC₅H₁₀/OMe 40). The ketonitrones Ph2C=N(O)C6H4R-p were found to be unexpectedly much more reactive toward the platinum(II)-bound nitriles than the related aldonitrones p-RC6H4CH=N(O)R (R= Me, Ph; R = H, Me), and the difference in the reactivity in 1,3- dipolar cycloaddition (DCA) of the keto- and aldonitrones was interpreted by theoretical calculations and was explained in terms of the orbital arguments as a result of the increase of the HOMOnitrone energy from aldo- to ketonitrones. The first example of the reversibility in metal-mediated DCA of nitrones to nitriles was observed, and this phenomenon, as follows from the performed theoretical study, is justified by the thermodynamic instability of the PtII-bound 3,3-diaryl-2,3-dihydro-1,2,4- oxadiazoles. Metalfree C5-diphenyl-2,3-dihydro-1,2,4-oxadiazoles 42 and 43 were liberated from corresponding (oxadiazole)2PtII complexes 26 and 30 by treatment with excess NaCN, and these heterocycles were characterized by high-resolution ESI+-MS and ¹H and ¹³C{¹H} NMR spectroscopies.

Full text of DOI:10.1021/om201026n

Article
pubs.acs.org/Organometallics
Facile and Reversible 1,3-Dipolar Cycloaddition of Aryl
Ketonitrones to Platinum(II)-Bound Nitriles: Synthetic, Structural,
and Theoretical Studies
Andreii S. Kritchenkov,† Nadezhda A. Bokach,*,† Maxim L. Kuznetsov,‡ Fedor M. Dolgushin,§
Tran Q. Tung,† Alexander P. Molchanov,† and Vadim Yu. Kukushkin*,†,⊥
†Department of Chemistry, St.Petersburg State University, Universitetsky Prospect 26, 198504 Stary Petergof, Russian Federation
‡Centro de Química Estrutural, Complexo I, Instituto Superior Tec
1049-001 Lisbon, Portugal
́ ́
nico, Universidade Tecnica de Lisboa, Avenida Rovisco Pais,
§A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Street 28, 119991 Moscow,
Russian Federation
⊥Institute of Macromolecular Compounds of Russian Academy of Sciences, V.O. Bolshoii Prospect 31, 199004 St.Petersburg,
Russian Federation
S
* Supporting Information
ABSTRACT: The reaction between trans-[PtCl2(NCR)2] (R = Et 1, NMe2 2, NEt2
3, NC5H10 4) and the acyclic triaryl ketonitrones Ph2CN(O)C6H4R′-p (R′ = H 5,
Me 6, Cl 7, OMe 8) proceeds as a facile and consecutive two-step intermolecular
cycloaddition to give the mono-cycloaddition products trans-[PtCl2(NCR){Na
C(R)ON(C6H4R′-p)CbPh2}](a−b) (R/R′ = Et/H 9, Et/Me 10, Et/Cl 11, Et/OMe 12,
NMe2/H 13, NMe2/Me 14, NMe2/Cl 15, NMe2/OMe 16, NEt2/H 17, NEt2/Me 18,
NEt2/Cl 19, NEt2/OMe 20, NC5H10/H 21, NC5H10/Me 22, NC5H10/Cl 23,
NC5H10/OMe 24) and then the bis-2,3-dihydro-1,2,4-oxadiazole complexes trans-
[PtCl2{NaC(R)ON(C6H4R′-p)CbPh2}2](a−b) (R/R′ = Et/H 25, Et/Me 26, Et/Cl
27, Et/OMe 28, NMe2/H 29, NMe2/Me 30, NMe2/Cl 31, NMe2/OMe 32, NEt2/H
33, NEt2/Me 34, NEt2/Cl 35, NEt2/OMe 36, NC5H10/H 37, NC5H10/Me 38,
NC5H10/Cl 39, NC5H10/OMe 40). The ketonitrones Ph2CN(O)C6H4R′-p were
found to be unexpectedly much more reactive toward the platinum(II)-bound nitriles
than the related aldonitrones p-R‴C6H4CHN(O)R″ (R′′ = Me, Ph; R‴ = H, Me), and the difference in the reactivity in 1,3-
dipolar cycloaddition (DCA) of the keto- and aldonitrones was interpreted by theoretical calculations and was explained in terms
of the orbital arguments as a result of the increase of the HOMOnitrone energy from aldo- to ketonitrones. The first example of the
reversibility in metal-mediated DCA of nitrones to nitriles was observed, and this phenomenon, as follows from the performed
theoretical study, is justified by the thermodynamic instability of the PtII-bound 3,3-diaryl-2,3-dihydro-1,2,4-oxadiazoles. Metal-
free C5-diphenyl-2,3-dihydro-1,2,4-oxadiazoles 42 and 43 were liberated from corresponding (oxadiazole)2PtII complexes 26 and
30 by treatment with excess NaCN, and these heterocycles were characterized by high-resolution ESI+-MS and 1H and 13C{1H}
NMR spectroscopies.
HOMO of a dipole and the LUMO of a dipolarophile.17 Such
reactions can be promoted (or inhibited) by using dipoles with
INTRODUCTION
Metal-mediated cycloaddition (CA) of various dipoles to nitriles
■
electron-donor (or electron-acceptor in the case of the
inhibition) substituents, and this statement is so far well
supported by the experimental data.9,10 Therefore, the general
considerations suggest that the aldonitrones Ar(H)CN(O)R
bearing one acceptor aryl at the C atom should be more re-
active toward RCN dipolarophiles than the C-diaryl keto-
nitrones Ar2CN(O)R, and in particular, Ph(H)CN(O)Me
is expected to exhibit the higher reactivity in DCA as compared
to Ph2CN(O)Ph.
represents an efficient route to those free and/or coordinated
heterocycles that could be either difficult to obtain or even
inaccessible via metal-free protocols.1−5 In particular, nitriles bound
to platinum(IV) centers react with allyl anion dipoles, such as the
acyclic nitrones R′′R′CN(O)R, under mild conditions, and this
reaction has a general character that is applicable to a wide range of
reactants and their various combinations.1,6−12 In the past decade, a
number of experimental and theoretical works were devoted to the
1,3-dipolar cycloaddition (DCA) of nitrones to nitriles,6−16 and
they were summarized in a recent review written by us.1
In terms of orbital interactions, the nitrone−nitrile DCAs
are controlled predominantly by the interaction between the
Received: October 24, 2011
Published: December 29, 2011
© 2011 American Chemical Society
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Scheme 1. Different Reactivities of Nitrones toward Nitrile Ligands and Alkenes
Taking all these in mind, we still attempted the reaction
between the ketonitrone Ph2CN(O)Ph and RCN ligands
at a PtII center and observed unexpectedly high reactivity of
this dipole in DCA. Indeed, it appeared to be much higher
than the reactivity of the aldonitrone Ph(H)CN(O)Me
(Scheme 1-A) and comparable only with one of the most
reactive nitrones known, viz., 5,5-dimethylpyrroline N-oxide
Scheme 2. Studied Cycloadditions
CMe2CH2CH2CHN(O).
One should also notice that although DCA of aldonitrones to
nitrile ligands was repeatedly studied,6,7,11 no single example for
CA of ketonitrones to nitrile species was published to date.
However, DCA of N-aryl ketonitrones to alkenes is known, and
the reported data18 indicate a much lesser reactivity of N-aryl
ketonitrones as compared to N-aryl aldonitrones (Scheme 1-B).
All these observations, along with the literature data, pro-
mpted us to study systematically CA of the ketonitrones
Ph2CN(O)Ar to nitrile species at platinum(II) centers. Our
goals were at least fourfold: (i) to study DCA of N-aryl
ketonitrones to metal-bound nitrile species and to attempt this
CA with uncomplexed nitriles to verify the effect of the metal
center on the reaction; (ii) to compare the reactivity of N-aryl
ketonitrones and N-aryl aldonitrones in DCA to nitrile ligands
at both experimental and theoretical levels; (iii) to provide
theoretical interpretation for the opposite orders of reactivity of
aldo- and ketonitrones in CA to alkenes or nitriles (Scheme 1);
(iv) to characterize complexes bearing 2-aryl-3,3-diphenyl-2,3-
dihydro-1,2,4-oxadiazoles and to develop a route for the libera-
tion of these yet unknown heterocyclic species from their PtII
complexes.
Table 1. Compound Numbering in Scheme 1
R′ = H
R′ = Me
R′ = Cl
R′ = OMe
R
5
6
7
8
Et
1
2
3
4
9/25
10/26
14/30
18/34
22/38
11/27
15/31
19/35
23/39
12/28
16/32
20/36
24/40
NMe2
NEt2
13/29
17/33
21/37
NC5H10
these species are derived from DCA of the nitrones to one
nitrile ligand of the PtII complexes.
All reactions of 1−4 with 2 equiv of 5−7 proceed as a
consecutive DCA and lead to bis-cycloadducts (bis-CAs) 25−
27, 29−31, 33−35, and 37−39 (Scheme 2). The syntheses of
these bis-CAs were performed by three routes, depending on
solubility of the products, viz., (i) in CHCl3 solution in a 1:2
molar ratio of the nitrile complex (1 or 2) and the nitrone
(each of 5−7) accompanied by precipitation of 25−27 and
29−31; (ii) in THF solution in a 1:2 molar ratio between the
nitrile complex (3 or 4) and the nitrone (6 or 7) accompanied
by precipitation of 34, 35, 38, and 39; (iii) in CHCl3 solution in
a 1:14 molar ratio between the nitrile complex (3 or 4) and the
nitrone (5) followed by separation of 33 or 37 by column
chromatography. A detailed description of these procedures is
given in the paragraphs that follow.
RESULTS AND DISCUSSION
■
For the current work, the nitrile complexes trans-
[PtCl2(NCR)2] (R = Et 1, NMe2 2, NEt2 3, NC5H10 4) and
the N-aryl ketonitrones Ph2CN(O)C6H4R′-p (R′ = H 5, Me
6, Cl 7, OMe 8) were addressed as reactants (Scheme 2).
Synthesis and characterization of new nitrones 6−8 are given in
the Supporting Information.
The reaction between each of 1−4 and each of 5−7 (in all
possible combinations; the reactions of 8 see later) in a molar
ratio of 1:1 in CHCl3 at room temperature is complete in 5 h,
giving mono-cycloadducts (mono-CAs) 9−11, 13−15, 17−19,
and 21−23 (NMR yields ca. 100%; isolated yields ca. 80% after
column chromatography on SiO2) (Scheme 2, route A). All
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Synthetic Approach (i). The reaction of 1 or 2 with each
of 5−7 (2 equiv) at 35 °C in a chloroform solution proceeds
selectively, resulting in precipitation of bis-CA species 25−27
and 29−31. TLC monitoring of the reaction mixtures
demonstrates that the conversion of the starting complexes
into corresponding mono-CAs 9−11 and 13−15 (route A)
proceeds for ca. 2 h, whereas the total conversion into bis-CAs
25−27 and 29−31 (route B) takes ca. 5 h (isolated yields
achieved by this synthetic approach are 89−90%).
The reaction of 3 or 4 with each of 5−7 (2 equiv) in a
chloroform solution at 35 °C gave 33−35 and 37−39 (as con-
firmed by high-resolution ESI-MS and 1H NMR), but DCA was
not accompanied by their precipitation. The conversion of 3
or 4 into corresponding mono-CA species 17−19 or 21−23
proceeds for ca. 2.0−2.5 h, while only 50% conversion (based
on NMR monitoring) into bis-CAs 33−35 and 37−39 occurs
in 5 h (Scheme 3, A). The complete conversion of mono-CAs
chromatography on SiO2). Because of the high reactivity of 8,
interaction of this 1,3-dipole with each of 1−4 (molar ratio 2:1)
proceeds with low degree of selectivity at 20−25 °C, and this
method for generation of bis-CAs could not be recommended
as preparative. Bis-CA products 28, 32, 36, and 40 were
obtained by treatment of each of 1−4 with 14 equiv of 8 at −15 °C.
After 3−4 h, when the conversion of the starting complexes into
corresponding mono-CAs was completed, the reaction mixture was
heated to 35 °C and then stirred for 5 h to give 28, 32, 36, and 40
in 78−80% isolated yields after column chromatography.
Thus, in contrast to previous studies,6 we found that keto-
nitrones 5−8 undergo facile DCA to PtII-bond alkylnitriles (R = Et)
and dialkylcyanamides (R = NMe2, NEt2, NC5H10). Surpris-
ingly, the reactivity of the ketonitrones Ph2CN(O)Ar is suffi-
cient to react even with the nitrile ligand bearing the electron-
donor Et substituent. Earlier works6,7,16,19−22 give evidence of a
lower reactivity of N-alkyl aldonitrones toward DCA to RCN
ligands in comparison with 5−8. Thus, Z-ArCHN(O)Alk
does not react with the alkylcyanide ligand EtCN in their PtII
complexes and gives the cycloadducts only when they are
strongly activated by a PtIV center or in PtII complexes bearing
more dipolarophilic dialkylcyanamide ligands.11
Scheme 3. Solvent Dependence of DCA That Is Performed
at 35 °C for 5 h
In this context, the attention should be drawn to the opposite
reactivity modes of the acyclic aldo- and ketonitrones in DCA
to the CC bond,18 where the reactivity of the nitrones in
the reaction with alkylidenecyclopropanes decreases along
the series C-aryl, N-alkyl aldonitrones > C,C-diphenyl, N-aryl
ketonitrones (Scheme 1-B). The observed difference in the
reactivity modes of nitrile ligands (this work; Scheme 1-A) and
methylenecyclopropanes (Scheme 1-B) toward DCA was
rationalized in the Theoretical Studies section (see later) and
also partially presented in the Supporting Information.
Reversibility of DCA. In the previous section, we indicated
that the completeness of the conversion of mono-CAs to corre-
sponding bis-CA species depends on the nature of the employed
solvent that determines the solubility of bis-CA products. Thus,
the reaction given in route B (Scheme 3) occurs faster and with
a higher degree of conversion in those solvents, where bis-CAs
precipitate from the reaction mixtures. These experimental
observations give collateral evidence favoring the reversibility of
DCA. However, NMR monitoring of 27, taken as an example,
in CDCl3 (10 d, 20−25 °C) did not reveal the generation of
products originating from retro-DCA, and these data indicate
that if the equilibrium exists, it is so strongly shifted to bis-CAs
17−19 and 21−23 to bis-CAs 33−35 and 37−39 under these
conditions was not achieved even after 2 d, and gradual de-
gradation of mono-CA species to give a broad mixture of yet
unidentified products (7 spots on TLC) was observed.
Synthetic Approach (ii). The change of the solvent from
CHCl3 to THF in the reaction of 3 or 4 with 2 equiv of 6 or 7
(in all combinations) led to precipitation of 34, 35, 38, and 39,
respectively, furnishing bis-CAs for 5 h at 35 °C (Scheme 3, B).
After 5 h, corresponding mono-CA species were not detected
in the reaction mixture by both TLC and ESI-MS methods.
The treatment of 3 or 4 with 2 equiv of 5 at 35 °C in THF
for 5 h led to the generation of 33 or 37 (as confirmed by TLC
and ESI-MS), but the reaction was not accompanied by their
precipitation and the conversion of 3 (or 4) into 33 (or 37)
was not complete.
Synthetic Approach (iii). The complete conversion of 3
(or 4) into 33 (or 37) was achieved in a CHCl3 solution by the
use of a 7-fold excess (14 equiv) of the corresponding nitrone
for 6 h at 35 °C. Complexes 33 and 37 were isolated by column
chromatography on SiO2 in 76−79% yields.
Nitrone 8, which is the most reactive from the studied series,
reacts with 1−4 in a molar ratio of 1:1 at 20−25 °C to give,
after 2−3 min, a broad mixture of products (8 spots on TLC).
However, decreasing the temperature to −15 °C reduces the
reaction rate and improves the selectivity to accomplish good
yields of pure mono-CA complexes 12, 16, 20, and 24 after 2 d
(NMR yields ca. 100%, isolated yields ca. 80% after column
1
that the quantity of retro-CA species is below H NMR
threshold values.
To obtain data additionally supporting the reversibility, we
performed the following experiments. First (Scheme 4), a
suspension of 27 was treated with 10 equiv of 6 in CHCl3 at
20−25 °C upon continuous stirring. After 120 h, 7, 41, and 26
were detected (by TLC and ESI-MS) in the reactions mixture
along with the starting materials (27 and 6). Complex 41 was
isolated by column chromatography on SiO2 in ca. 8% yield,
and it was characterized by NMR and high-resolution ESI-MS
(see Experimental Section). Second (Scheme 5), a suspension
of 27 was stirred at 20−25 °C with 10 equiv of 2 in CHCl3.
After 120 h, 11 and 15 were isolated from the reaction mixture
and separated by column chromatography along with unreacted
27 and 2.
We also conducted a blank experiment indicating that the re-
versibility of DCA is specific for metal-bound species. Prolonged
(120 h) stirring of nitrone 6 or the nitrile NCNMe2 with free
5-ethyl-3,3-diphenyl-2-(p-tolyl)-2,3-dihydro[1,2,4]oxadiazole
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Scheme 4. Nitrone Exchange in 27
Scheme 6. Isomerization (via retrocycloaddition) or
Dipolarophile Exchange with Excess of Another Alkene
Scheme 7. Reaction of the Nitrones with trans-
[PtCl2(NCMe)2]
Scheme 5. Exchange between 27 and 2
out at the B3LYP level of theory. The calculations indicate that,
first, DCAs of all nitrones to free MeCN belong to the neutral
type II of the Sustmann classification (i.e., controlled by both
HOMOdipole/LUMOdipolarophile and LUMOdipole/HOMOdipolarophile
interacions; the reactions controlled predominantly by the
HOMOdipole/LUMOdipolarophile or HOMOdipolarophile/LUMOdipole
interactions belong to type I or type III, respectively).17,26 The
HOMOnitrone−LUMOnitrile gap is only 0.74−1.12 eV smaller than
the HOMOnitrile−LUMOnitrone gap (Figure 1-A). The coordination
of MeCN to PtII in complex I leads to an energy decrease of both
π⊥(CN) and π*⊥(CN) MOs of the nitrile. As a result, the
reactions with complex I are clearly of type I with the predominant
HOMOnitrone−LUMOnitrile type of orbital interaction (Figure 1-B).
The lowering of the LUMOnitrile energy on going from MeCN to I
explains the activation of nitriles upon coordination.9,26−30 In the
studied nitrone series, the HOMOnitrone energy slightly increases
(the HOMOnitrone−LUMOnitrile gap decreases) along the row
PhCHN(O)Me ≤ PhCHN(O)Ph < Ph2CN(O)Ph. Thus,
the simple qualitative consideration of molecular orbitals suggests
that the reactivity of nitrones should increase along the same
sequence.
(42) (or 5-(dimethylamino)-3,3-diphenyl-2-(p-tolyl)-2,3-
dihydro[1,2,4]oxadoazole, 43) (see below) at 35 °C gave no
evidence for the appearance of other heterocycles in the
mixture, and 6 or the nitrile NCNMe2 and 42 (or 43) remain
intact.
To the best of our knowledge we found the first example
of reversibility in metal-mediated DCA of nitrones to nitriles,
although one example favoring the reversibility of DCA of
nitrones to alkenes is known in metal-free organic chemistry; that
is, the alkene−nitrone cycloadduct (Scheme 6) undergoes iso-
merization (via retrocycloaddition) or dipolarophile exchange
with an excess of another alkene.23,24
Theoretical Studies. In order to interpret the experimentally
found substituent-dependent chemical behavior of various nitrones
toward DCA to nitriles, quantum-chemical calculations of DCA
of two aldonitrones in the Z-form, i.e., PhCHN(O)Me and
PhCHN(O)Ph, and one ketonitrone, Ph2CN(O)Ph, with
acetonitrilefree and coordinated to the PtII center in the com-
plex trans-[PtCl2(NCMe)2] (I) (Scheme 7)have been carried
Second, the mechanism of DCAs to free MeCN is concerted,
highly synchronous and includes the formation of a five-membered
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Figure 1. Relative energies (in eV) of the frontier MOs of nitrones and nitriles.
cyclic transition state. The synchronicity parameter, Sy, is 0.90,
Fourth, the reactions with free MeCN are only slightly endo-
0.92, and 0.87 for the reactions of PhCHN(O)Me, PhCH
N(O)Ph, and Ph2CN(O)Ph, respectively (Sy is 1 for
perfectly synchronous cycloadditions and 0 for the stepwise
mechanism; see Computational Details in the Experimental
Section). In contrast, the mechanism of the reactions with I is
asynchronous with Sy values of 0.67, 0.69, and 0.56 for the
nitrones indicated above, correspondingly.
Third, the coordination of MeCN results in a dramatic
decrease of the activation energies from 31.7−34.1 kcal/mol
(for DCAs to MeCN) to 22.5−25.9 kcal/mol (for DCAs to I)
(Table 2). Both aldonitrones PhCHN(O)Me and PhCH
ergonic, the ΔGs values being similar for all three nitrones
(0.4−1.7 kcal/mol, Table 2). In contrast, DCA of the keto-
nitrone Ph2CN(O)Ph to the Pt complex I is clearly less thermo-
dynamically favorable than the reactions of the aldonitrones
PhCHN(O)Ph and PhCHN(O)Me. These results are in
agreement with our experimental observations on the
reversibility of DCA of the ketonitrones (but not aldonitrones)
to the nitriles in the (RCN)PtII complexes. The different thermo-
dynamic behavior of the keto- and aldonitrones toward the
complexed nitriles is conceivably accounted for by a destabi-
lization of product IV due to the steric repulsion between the
phenyl groups and the metal fragment. Indeed, an introduction
of the second Ph group at the C atom of the CN bond of a
nitrone results in significant conformational changes of the Ph
groups and of the metal fragment relative to the oxadiazole ring
(Figure 2). The PtN(1)C(2) angle in IV is clearly smaller than
Table 2. Calculated Activation and Reaction Energies (in
kcal/mol) for DCAs of Nitrones to Nitriles in Gas-Phase and
CH2Cl2 Solution (in parentheses)
dipolarophile
nitrone
Ea
ΔG⧧
ΔE
ΔG
NCMe
Ph2CN(O)Ph
20.8
(26.0)
23.0
30.5
(31.7)
32.5
−9.0
(−3.1)
−9.6
(−4.4)
−7.4
(−2.7)
−10.6
(+0.1)
−18.0
(−8.0)
−17.4
(−7.9)
−0.2
(+1.7)
−1.0
PhCHN(O)Ph
PhCHN(O)Me
Ph2CN(O)Ph
PhCHN(O)Ph
PhCHN(O)Me
(27.6)
24.7
(33.2)
33.9
(+0.4)
+0.7
(28.7)
6.2
(34.1)
16.8
(+1.4)
+1.4
I
(16.1)
11.5
(22.5)
22.6
(+7.4)
−7.8
(−2.1)
−7.8
(19.2)
11.7
(25.9)
21.9
Figure 2. Equilibrium geometries of III and IV.
(19.6)
(25.4)
(−2.6)
that in III (120° and 128°, respectively). The N(1)C(3) bond
lengths are 1.524 and 1.480 Å for IV and III, correspondingly.
All this indicates noticeable steric strain in the molecule of IV.
For comparison with CAs to nitriles, the reactions of the same
N(O)Ph exhibit similar reactivity. In contrast, the ketonitrone
Ph2CN(O)Ph is more reactive than the aldonitrones by 1.5−
2.4 kcal/mol (reaction with MeCN) and 2.9−3.4 kcal/mol
(reaction with I), i.e., by a factor of 13 to 310, which is in agree-
ment with the qualitative experimental observations described
above. The calculated activation barriers (gas-phase activation
energies) correlate well with the HOMOnitrone energies (Table 2,
Figure 1). Thus, the higher reactivity of the ketonitrone versus
the aldonitrones can be interpreted in terms of the orbital
arguments as a result of the higher HOMO energy of Ph2C
N(O)Ph compared to PhCHN(O)Me and PhCHN(O)Ph.
three nitrones with methylenecyclopropane, CH2CaCH2CbH2(a−b)
,
have also been calculated. They also belong to type II; both
HOMOnitrone−LUMOalkene and HOMOalkene−LUMOnitrone gaps
are somewhat larger for the reaction with PhCHN(O)Me,
indicating that in terms of the frontier MO theory this nitrone
should be the least reactive with methylenecyclopropane. Indeed,
the gas-phase activation energy, Ea, increases along the series
of the dipoles PhCHN(O)Ph (20.1 kcal/mol) < Ph2CN(O)Ph
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Figure 3. View of 25 with the atomic numbering scheme. Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for
clarity.
Figure 4. View of 33 with the atomic numbering scheme. Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for
clarity.
1
(21.7 kcal/mol) < PhCHN(O)Me (22.1 kcal/mol) (Table S2
in Supporting Information). However, in terms of Gibbs free
energy of activation in CH2Cl2 solution, ΔGs⧧, the ketonitrone
Ph2CN(O)Ph becomes less reactive than PhCHN(O)Me,
by 1.5 kcal/mol, indicating the importance of the entropic
contribution and solvent effects in these processes. Note that
the difference of the ΔGs⧧ values, 1.5 kcal/mol, corresponds to
the ratio of the reaction rates, ca. 13, and, therefore, can explain
the experimentally observed lower reactivity of ketonitrones
toward alkylidenecyclopropanes compared to aldonitrones.18
Some additional discussion of these processes is given in the
Supporting Information.
For 9−40, signal integration in the H NMR spectra gives
evidence that the reaction between each of the coordinated
nitriles and the nitrone proceeds in a 1:1 ratio. Both H and
1
13C{1H} NMR spectra of 9−24 exhibit signals from the
oxadiazole and the nitrile ligands. The 1H NMR spectra of 13−
24 display broad (owing to the hindered rotation around the
C5−NR2 bond) singlets of the CH3 protons from C5−NMe2
(13−16; 3.75−3.82 ppm), the CH2 protons from C5−NEt2
(17−20; 4.36−4.38 ppm), and the α-CH2 protons from C5−
C5H10 (21−24; 4.38 ppm). The less broad signals from these
protons for bis-CA species are low-field shifted by 0.70−
0.80 ppm relative to the corresponding signals from mono-CAs.
The quartets due to the CH2 protons from C5−Et in the NMR
spectra of 9−12 and 25−28 appeared in the same range for
both mono- and bis-CA species (2.93 for 9 and 25; 2.81 for 10
and 26, 12 and 28; 2.82 for 11 and 27). In the 13C{1H} NMR
spectra, the peaks due to C5N (159.6−161.4) and C3 (98.0−
98.5 ppm) were recognized.
Characterization of 9−40. Complexes 9−40 were obtained
as yellow solids and characterized by elemental analyses (C, H, N),
1
high-resolution ESI+-MS, IR, and H and 13C{1H} NMR
spectroscopies and also by X-ray diffraction (for 25 and 33).
All platinum species gave satisfactory microanalyses. In the
ESI+-MS, the typical ions that were detected are [M + H]+,
[M + Na]+, and [M + K]+. A comparison of the IR spectra of
the products with those of the starting trans-[PtCl2(RCN)2]
indicated the absence of ν(CN) stretching vibrations at ca.
2300 cm−1 for 25−40, while for 9−24 these stretches
are displayed in the region from 2287 to 2305 cm−1. The
presence of intensive ν(CN) vibrations in the range
between 1636 and 1667 cm−1 was detected in the IR spectra
of all complexes.
Complexes 25 and 33 were characterized by a single-crystal
X-ray diffraction. Thus, in 25 and 33, the coordination poly-
hedra of the Pt atoms are slightly distorted square planes with
the hetetocyclic ligands in the trans-position. The Pt(1)−N(1)
(2.026(2) Å for 25 and 2.037(2) Å for 33; Table 3) bonds
lengths are typical for (imine)PtII species,31 while the Pt−Cl
bonds are specific for platinum(II) chloride species.31 The
N(1)−C(1) distances (1.282(3) Å for 25 and 1.311(3) Å for
692
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2,3-dihydro-1,2,4-oxadiazoles demonstrate all signals specific
for these heterocycles; the liberated species exhibit signals from
C3a (96.3−97.5 ppm) and CN (161.2−162.0 ppm). The
former signal is high field shifted (by ca. 1.0−1.5 ppm) and
latter low field shifted (by 1−3 ppm) relative to the corre-
sponding signals in (2,3-dihydro-1,2,4-oxadiazole)2PtII complexes
26 and 30.
Table 3. Selected Bond Lengths (Å) and Angles (deg) in 25
and 33
25
33
Pt(1)−N(1)
Pt(1)−Cl(1)
O(1)−C(1)
O(1)−N(2)
N(1)−C(1)
N(1)−C(2)
N(2)−C(2)
2.026(2)
2.3010(5)
1.336(3)
1.473(2)
1.282(3)
1.492(3)
1.519(3)
180.0
2.037(2)
2.3053(6)
1.356(3)
1.462(3)
1.311(3)
1.496(3)
1.519(3)
180.0
FINAL REMARKS
■
We found that the RCN (R = Alk, NR′2) ligands in platinum(II)
complexes 1−4 are involved in facile DCA with the acyclic
ketonitrones, and the cycloaddition proceeds rapidly under mild
conditions. This observation indicates that the ketonitrones are
unexpectedly much more reactive toward metal-bound nitriles
than the corresponding aldonitrones (e.g., in [PtCl2(EtCN)2],
the EtCN ligand easily reacts with 5−8 at 20−25 °C for 5 h,
while the N-alkyl aldonitrones ArCHN(O)R′ do not react6
with this complex even under more drastic (50 °C, 30 d) con-
ditions). The high reactivity of the ketonitrones is comparable
only with one of the most reactive nitrones known, viz., 5,5-
a
N(1A) −Pt(1)−N(1)
a
Cl(1A) −Pt(1)−Cl(1)
180.0
180.0
a
N(1A) −Pt(1)−Cl(1)
93.36(5)
86.64(5)
104.9(2)
108.7(2)
123.9(2)
126.8(1)
103.3(2)
115.5(2)
99.8(2)
87.85(6)
92.15(6)
103.1(2)
105.7(2)
127.8(2)
125.5(2)
101.2(2)
114.4(2)
97.6(2)
N(1)−Pt(1)−Cl(1)
C(1)−O(1)−N(2)
C(1)−N(1)−C(2)
C(1)−N(1)−Pt(1)
C(2)−N(1)−Pt(1)
O(1)−N(2)−C(2)
N(1)−C(1)−O(1)
N(1)−C(2)−N(2)
dimethylpyrroline N-oxide, CMe2CH2CH2CHN(O), and this
finding opens up a route for the generation of various 3,3-diaryl-
2,3-dihydro-1,2,4-oxadiazoles starting even from such inactivated
nitrile species as AlkCN.
a
Symmetry transformations used to generate equivalent atoms: −x, −y,
−z for 25 and −x+1, −y+1, −z for 33.
33) are characteristic for the NC double bond,31 while the
N(1)−C(2) and N(2)−C(2) in 25 and 33 (1.492(3) and
1.519(3) Å for 25 and 1.496(3) and 1.519(3) Å for 33) are
specific for the N−C single bonds.31
The substantially different chemical behavior of the keto-
and aldonitrones was interpreted by theoretical calculations.
The higher reactivity of ketonitrones may be explained in terms
of the orbital arguments as a result of the increase of the
HOMOnitrone energy from aldo- to ketonitrones. Such an
increase, in turn, is related to a different electronic effect
imposed by the phenyl substituents in aldo- and ketonitrones.
Indeed, in the former, the Ph group at the C atom is situated in
the CNO plane of the nitrone, it is involved in the con-
jugation, and acts as a conventional electron-acceptor sub-
stituent. In the ketonitrones, both Ph groups at the C atom are
out of the CNO plane due to steric repulsion (the dihedral
angles are 25° and 58° for Ph2CN(O)Ph). Phenyls are not
involved in the conjugation and, hence, act as electron-donor
rather than electron-acceptor substituents, providing higher
HOMOnitrone energy. In contrast to CA of ketonitrones to the
RCN ligands, in the reaction with alkylidenecyclopropanes,18
the ketonitrones are less reactive than the relevant aldonitrones.
However, in this case, the main factors determining the relative
reactivity of the dipoles are the entropic contribution and the
solvation rather than the frontier MO energies (see the
Supporting Information for details of our theretical study).
In this work, we also found the first example of the re-
versibility in metal-mediated DCA of nitrones to nitriles, and this
unusual phenomenon was explained by the thermodynamic
instability of PtII-coordinated 3,3-diaryl-2,3-dihydro-1,2,4-oxa-
diazoles. Theoretical calculations demonstrate that DCA pro-
ducts of the ketonitrones to the nitrile ligands are less thermo-
dynamically stable than those derived from CA of the aldonitrones.
Liberation of 2,3-Dihydro-1,2,4-oxadiazoles from 26 and
30. Several methods for liberation of the strongly bound imines
and nitrogen heterocycles from their PtII complexes have been
developed, and they are based on displacement with an excess
of diphosphine,32−36 monodentate and bidentate amines bear-
ing two sp3-N donor centers,10,21,37 or alkali metal cyanides.11
The latter method (initially suggested by Leung and colleagues38
for liberation of some chelated phosphine ligands strongly
bound to PtII center) could be applied, due to its high effici-
ency, for decoordination of 2,3-dihydro-1,2,4-oxadiazole ligands
bearing a strong electron-donor substituent at C5a atom and
strongly bound to a PtII center.11
To illustrate the methodology of the liberation and to
open up a route to C3-diphenyl-2,3-dihydro-1,2,4-oxadiazoles,
we addressed two complexes (26 and 30) derived from differ-
ent nitriles bearing alkyl and dialkylamino substituents, res-
pectively. Accordingly, we observed that treatment of 26 and 30
with NaCN (8 equiv, 35 °C, 3 d) leads to almost quantita-
tive formation of the free 5-ethyl-3,3-diphenyl-2-(p-tolyl)-2,3-
dihydro[1,2,4]oxadiazole (42) and 5-(dimethylamino)-3,3-
diphenyl-2-(p-tolyl)-2,3-dihydro[1,2,4]oxadiazole (43)
(Scheme 8). These metal-free species were characterized by
1
high-resolution ESI+ mass spectrometry and H and 13C{1H}
NMR spectroscopy. In the ESI+-MS, the observed peaks were
attributed to the ions [M + Na]+. 13C{1H} NMR spectra of the
Scheme 8. Liberation of the 2,3-Dihydro-1,2,4-oxadiazoles
693
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Total energies corrected for solvent effects (Es) were estimated at
the single-point calculations on the basis of gas-phase geometries at the
CPCM-B3LYP/6-311+G(d,p)//gas-B3LYP/6-31G(d) level of theory
using the polarizable continuum model in the CPCM version,47,48 with
CH2Cl2 as solvent. The UAKS model was applied for the molecular
cavity. The entropic term in CH2Cl2 solution (Ss) was calculated accord-
ing to the procedure described by Wertz49 and Cooper and Ziegler50
using eqs 1−4.
The lower stability of DCA products of ketonitrones is ac-
counted for by steric repulsions between the phenyl groups and
the metal fragment. When decoordinated, 3,3-diaryl-2,3-
dihydro-1,2,4-oxadiazoles exhibit sufficient stability due to the
absence of the steric repulsion from the metal center.
EXPERIMENTAL SECTION
■
s
Materials and Instrumentation. Solvents were obtained from
commercial sources and used as received. Complexes [PtCl2(RCN)2]
(R = Et, NMe2, NEt2, NC5H10) were synthesized in accord with the
published procedures.39,40 Nitrones 5−8 were obtained according to
the previously described protocol41 by the reaction of aryl nitroso
compounds with diphenyl diazomethane in diethyl ether at room
temperature (60−85%). Three of the four nitrones (viz., 6−8) were
not previously described, and their characterization is given in the
Supporting Information. C, H, and N elemental analyses were carried
out by the Department of Organic Chemistry of Saint-Petersburg
State University on a Hewlett-Packard 185B carbon hydrogen nitrogen
analyzer. Electrospray ionization mass spectra were obtained on a
Bruker micrOTOF spectrometer equipped with an electrospray
ionization (ESI) source. The instrument was operated in both positive
and negative ion mode using a m/z range of 50−3000. The capillary
voltage of the ion source was set at −4500 V (ESI+-MS) and the
capillary exit at (70−150) V. The nebulizer gas flow was 0.4 bar, and
drying gas flow 4.0 L/min. For ESI species were dissolved in MeOH or
MeCN, and NaBF4 was used as additional ionization agent. In the
isotopic pattern, the most intensive peak is reported. TLC was per-
formed on Merck 60 F254 SiO2 plates. Infrared spectra (4000−400 cm−1)
were recorded on a Shimadzu FTIR-8400S instrument in KBr pellets.
1H and 13C NMR spectra were measured in CDCl3 on a Bruker DPX-
300 spectrometer at ambient temperature.
ΔS = R ln V
/V
1
m,liq m,gas
(1)
(2)
s
ΔS = R ln V° /V
2
m
m,liq
,s
S°
,s
s
− (S°
+ R ln V
/V )
m,liq m,gas
liq
gas
α =
,s
s
(S°
+ R ln V
/V )
m,liq m,gas
gas
(3)
S = S + ΔS
sol
s
g
= S + [ΔS + α(S + ΔS ) + ΔS ]
g
1
g
1
2
= S + [(− 11.80 cal/mol·K) − 0.21
g
× (S − 11.80 cal/mol·K) + 5.45 cal/mol·K]
g
(4)
where Sg is the gas-phase entropy of solute, ΔSsol is the solvation
entropy, S°,sliq, S°,sgas, and V sm,liq are the standard entropies and molar
volume of the solvent in liquid or gas phases (174.5 and 270.3 J/mol·K
and 64.15 mL/mol, respectively, for CH2Cl2), Vm,gas is the molar
volume of the ideal gas at 25 °C (24 450 mL/mol), and V°m is the
molar volume of the solution corresponding to the standard conditions
(1000 mL/mol). The enthalpies and Gibbs free energies in solution
(Hs and Gs) were estimated using the expressions 5 and 6
X-ray Diffraction Study. Single-crystal X-ray diffraction experi-
ments were carried out with a Bruker SMART APEX II diffractometer
(graphite-monochromated Mo Kαα radiation, λ = 0.71073 Å, ω-scan
technique, T = 100(2) K). The APEX II software42 was used for
collecting frames of data, indexing reflections, determination of lattice
constants, integration of intensities of reflections, and scaling and
absorption correction, and SHELXTL43 for space group and structure
determination, refinements, graphics, and structure reporting. The
structures were solved by direct methods and refined by the full-matrix
least-squares technique against F2 with anisotropic thermal parameters
for all non-hydrogen atoms. The hydrogen atoms were placed geo-
metrically and included in the structure factor calculation in the riding
motion approximation. The principal experimental and crystallogra-
phic parameters are presented in Table S1 (Supporting Information).
Computational Details. The full geometry optimization of all
structures and transition states has been carried out at the DFT/HF
hybrid level of theory using Becke’s three-parameter hybrid exchange
functional in combination with the gradient-corrected correlation func-
tional of Lee, Yang, and Parr (B3LYP)44 with the help of the Gaussian-0345
program package. No symmetry operations have been applied. The
geometry optimization was carried out using a quasi-relativistic
Stuttgart pseudopotential that described 60 core electrons and the
appropriate contracted basis set (8s7p6d)/[6s5p3d]46 for the platinum
atom and the 6-31G(d) basis set for other atoms. Then, single-point
calculations were performed on the basis of the equilibrium geometries
found using the 6-311+G(d,p) basis set for nonmetal atoms.
Previously,9,26,27 this level of theory was found as a reasonable one
for the study of reactions of nitrones with nitriles and isocyanides. The
basis set superposition error was not estimated because it weakly
affects the activation and reaction energies of cycloadditions to nitriles,
as it was shown previously.28−30
H = E (6‐311+G(d, p)) + H (6‐31G(d))
s
s
g
− E (6‐311+G(d, p))
g
(5)
(6)
G = H − TS
s
s
s
where Es, Eg, and Hg are the total energies in solution and in gas phase,
and gas-phase enthalpy is calculated at the corresponding level.
The Wiberg bond indices (Bi)51 were computed by using the
natural bond orbital partitioning scheme.52 The reaction synchronicity
(Sy) was calculated using the formulas reported previously.53−56 The Sy
values vary from 0 to 1; for perfectly synchronous reactions Sy = 1; for
stepwise CAs Sy = 0.
Our previous theoretical studies14,26,28−30 indicated that both
acyclic and cyclic nitrones couple with MeCN or complex I via a
concerted mechanism, while the stepwise routes are greatly disfavored.
Hence, in this work only the concerted pathways are examined.
Synthetic Work. Synthesis of (2,3-Dihydro-1,2,4-oxadiazole)-
(nitrile)PtII Complexes (a General Procedure). The solutions of each
of nitrones 5−8 (0.1 mmol) in CHCl3 (1 mL) were added to a
solution of each of 1−4 (0.1 mmol) in CHCl3 (1 mL). The mixture
was stirred at room temperature for 5 h (at −15 °C for 2 d when
nitrone 8 was used); the progress of the reaction was monitored by
TLC. The separation of 9−24 was achieved by column chromatog-
raphy on SiO2 (the first fraction; eluent CHCl3/Me2CO, 10:1, v/v).
The solvent was evaporated in vacuo at 20−25 °C to give yellow, oily
residues. The residues were crystallized under n-hexane to form the
yellow powders of 9−24. The complexes were dried in air at 20−25 °C.
Yields: 78−81%.
9 (52.6 mg, 81%). Anal. Found: C, 46.29; H, 3.87; N, 6.47 (calcd
for C25H25N3Cl2OPt: C, 46.23; H, 3.88; N, 6.47); m/z (high-
resolution ESI+) 671.0920 ([M + Na]+, requires 671.0921); Rf = 0.37
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2934 m (C−H),
1636 s (CN); δH (300 MHz, CDCl3) 1.34 (3H, t, J = 7.6 Hz, CH3
from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 1.57 (3H, s, CH3
from Et of the nitrile ligand), 2.19 (2H, s, CH2 from Et of the nitrile
The Hessian matrix was calculated analytically for the optimized
structures in order to prove the location of correct minima (no
imaginary frequencies) or saddle points (only one negative frequency)
and to estimate the thermodynamic parameters, the latter being
calculated at 25 °C. The nature of all transition states was investigated
by the analysis of vectors associated with the imaginary frequency.
694
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ligand), 2.93 (2H, q, J = 7.6 Hz, CH2 from Et of the 2,3-dihydro-1,2,
4-oxadiazole ligand) 6.70, 7.01, 7.30, and 7.56 (15H, 4 m, Haromatic);
δC (75.5 MHz, CDCl3) 9.9 and 10.3 (CH3 from Et of the nitrile and
the 2,3-dihydro-1,2,4-oxadiazole ligands), 13.1 (CH2 from Et of the
nitrile ligand), 22.6 (CH2 from Et of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 98.2 (N−C−N), 121.2, 126.1, 126.9, 128.2, 128.8, 132.3,
138.1, and 144.5 (Caromatic), 159.6 (C(O)N); the CN carbon was
not detected.
10 (52.3 mg, 80%). Anal. Found: C, 47.07; H, 4.12; N, 6.33 (calcd
for C26H27N3Cl2OPt: C, 47.07; H, 4.10; N, 6.33); m/z (high-
resolution ESI+) 701.0819 ([M + K]+, requires 701.0816); Rf = 0.41
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2934 m (C−H),
1637 s (CN); δH (300 MHz, CDCl3): 1.29 (3H, t, J = 7.6 Hz, CH3
from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 1.57 (3H, s, CH3
from Et of the nitrile ligand), 2.14 (3H, s, CH3 from C6H4Me-p), 2.19
(2H, s, CH2 from Et of the nitrile ligand), 2.81 (2H, q, J = 7.6 Hz, CH2
from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.46 (2H, d,
o-protons from C6H4Me-p), 6.70 (2H, d, m-protons from C6H4Me-p),
7.27 and 7.50 (10H, 2 m, Haromatic); δC (75.5 MHz, CDCl3) 9.9 and
10.3 (CH3 from Et of the nitrile and the 2,3-dihydro-1,2,4-oxadiazole
ligand), 13.1 (CH2 from Et of the nitrile ligand), 21.2 (CH3 from
C6H4Me-p), 23.2 (CH2 from Et of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 99.1 (N−C−N), 121.0, 124.8, 127.5, 129.0, 129.5, 131.5,
135.0, and 141.1 (Caromatic), 162.2 (C(O)N); the CN carbon was
not detected.
11 (52.2 mg, 81%). Anal. Found: C, 43.90; H, 3.55; N, 6.16 (calcd
for C25H24N3Cl3OPt: C, 43.90; H, 3.54; N, 6.14); m/z (high-
resolution ESI+) 683.0711 ([M + H]+, requires 683.0711); Rf = 0.40
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1: 2936 m (C−H),
1636 s (CN); δH (300 MHz, CDCl3) 1.30 (3H, t, J = 7.6 Hz, CH3
from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 1.57 (3H, s, CH3
from Et of the nitrile ligand), 2.19 (2H, s, CH2 from Et of the nitrile
ligand), 2.82 (2H, q, J = 7.6 Hz, CH2 from Et of the 2,3-dihydro-1,2,
4-oxadiazole ligand), 6.49 (2H, d, o-protons from C6H4Cl-p), 6.88
(2H, d, m-protons from C6H4Cl-p), 7.32 and 7.50 (10H, 2 m, Haromatic),
δC (75.5 MHz, CDCl3) 9.9 and 10.3 (CH3 from Et of the nitrile and
the 2,3-dihydro-1,2,4-oxadiazole ligand), 13.1 (CH2 from Et of the
nitrile ligand), 22.6 (CH2 from Et of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 98.2 (N−C−N), 122.8, 127.3, 126.9, 128.5, 129.2, 132.0,
137.7, and 143.0 (Caromatic), 159.6 (C(O)N); the CN carbon was
not detected.
12 (54.1 mg, 80%). Anal. Found: C, 46.01; H, 4.03; N, 6.24 (calcd
for C26H27N3Cl2O2Pt: C, 46.96; H, 4.01; N, 6.18); m/z (high-
resolution ESI+) 717.0761 ([M + K]+, requires 717.0765); Rf = 0.41
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2934 m (C−H),
1637 s (CN); δH (300 MHz, CDCl3) 1.29 (3H, t, J = 7.6 Hz, CH3
from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 1.57 (3H, s, CH3
from Et of the nitrile ligand), 2.19 (2H, s, CH2 from Et of the nitrile
ligand), 2.81 (2H, q, J = 7.6 Hz, CH2 from Et of the 2,3-dihydro-1,2,
4-oxadiazole ligand), 3.72 (3H, s, CH3 from p-MeOC6H4), 6.62 (2H,
d, o-protons from p-MeOC6H4), 6.77 (2H, d, m-protons from p-
MeOC6H4), 7.23 and 7.53 (10H, 2 m, Haromatic); δC (75.5 MHz,
CDCl3) 9.9 and 10.2 (CH3 from Et of the nitrile and the 2,3-dihydro-
1,2,4-oxadiazole ligand), 13.3 (CH2 from Et of the nitrile ligand), 23.2
(CH2 from Et of the 2,3-dihydro-1,2,4-oxadiazole ligand), 62.0 (CH3
from p-MeOC6H4), 98.4 (N−C−N), 122.1, 127.2, 129.1, 129.2, 132.4,
136.3, 137.7, and 141.2 (Caromatic), 161.2 (C(O)N); the CN
carbon was not detected.
14 (55.9 mg, 81%). Anal. Found: C, 45.05; H, 4.21; N, 10.10 (calcd
for C26H29N5Cl2OPt: C, 45.03; H, 4.21; N, 10.10); m/z (high-
resolution ESI+) 693.1477 ([M + H]+, requires 693.1475), 716.1306
([M + Na]+, requires 716.5147); Rf = 0.39 (eluent CHCl3/Me2CO,
30:1, v/v); νmax (KBr)/cm−1 2923 m (C−H), 2301 m (CN), 1665 s
(CN); δH (300 MHz, CDCl3) 2.20 (3H, s, CH3 from C6H4Me-p),
2.86 (6H, s, NMe2 of the nitrile ligand), 3.75 (6H, br s, NMe2 of the
2,3-dihydro-1,2,4-oxadiazole ligand), 6.57 (2H, d, o-protons from
C6H4Me-p), 6.83 (2H, d, m-protons from C6H4Me-p), 7.28 and 7.56
(10H, 2 m, Haromatic); δC (75.5 MHz, CDCl3) 21.3 (CH3 from
C6H4Me-p), 40.3, 40.4 (NMe2 of the 2,3-dihydro-1,2,4-oxadiazole
ligand and of the nitrile ligand), 98.1 (N−C−N), 121.9, 127.2, 128.9,
129.0, 132.0, 136.2, 137.9, and 141.5 (Caromatic), 161.4 (C(O)N);
the CN carbon was not detected.
15 (57.5 mg, 81%). Anal. Found: C, 42.09; H, 3.67; N, 9.85 (calcd
for C25H26N5Cl3OPt: C, 42.06; H, 3.67; N, 9.81); m/z (high-
resolution ESI+) 735.0742 ([M + Na]+, requires 735.0748); Rf = 0.42
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2929 m (C−H),
2293 m (CN), 1669 s (CN); δH (300 MHz, CDCl3) 2.88 (6H, s,
NMe2 of the nitrile ligand), 3.76 (6H, br s, NMe2 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 6.63 (2H, d, o-protons from C6H4Cl-p), 6.99
(2H, d, m-protons from C6H4Cl-p), 7.32 and 7.58 (10H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 40.3, 40.4 (NMe2 of the 2,3-dihydro-
1,2,4-oxadiazole ligand and of the nitrile ligand), 98.3 (N−C−N),
122.9, 127.4, 128.5, 129.3, 131.7, 131.9, 137.5, and 142.8 (Caromatic),
161.2 (C(O)N); the CN carbon was not detected.
16 (53.3 mg, 78%). Anal. Found: C, 44.03; H, 4.12; N, 9.87 (calcd
for C26H29N5Cl2O2Pt: C, 44.01; H, 4.12; N, 9.87); m/z (high-
resolution ESI+) 731.1248 ([M + Na]+, requires 731.1244); Rf = 0.38
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2923 m (C−H),
2301 m (CN), 1665 s (CN); δH (300 MHz, CDCl3) 2.86 (6H, s,
NMe2 of the nitrile ligand), 3.70 (3H, s, CH3 from p-MeOC6H4), 3.76
(6H, br s, NMe2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.60 (2H,
d, o-protons from p-MeOC6H4), 6.79 (2H, d, m-protons from p-
MeOC6H4), 7.25 and 7.57 (10H, 2 m, Haromatic); δC (75.5 MHz,
CDCl3) 40.3, 40.4 (NMe2 of the 2,3-dihydro-1,2,4-oxadiazole ligand
and of the nitrile ligand), 61.9 (CH3 from p-MeOC6H4), 98.2 (N−C−
N), 122.1, 127.2, 128.8, 129.1, 132.1, 136.3, 137.7, and 141.5
(Caromatic), 161.3 (C(O)N); the CN carbon was not detected.
17 (59.0 mg, 80%). Anal. Found: C, 47.36; H, 4.81; N, 9.50 (calcd
for C29H35N5Cl2OPt: C, 47.35; H, 4.80; N, 9.52); m/z (high-
resolution ESI+) 757.1765 ([M + Na]+, requires 757.1764); Rf = 0.41
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2976 m (C−H),
2289 m (CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.22 (6H, t,
J = 7.4 Hz, CH3 from NEt2 of the nitrile ligand), 1.51 (6H, br s, CH3
from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 3.09 (4H, q, J =
7.4 Hz, CH2 from NEt2 of the nitrile ligand), 4.36 (4H, br s, CH2 from
NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.70, 7.01, 7.30, and
7.56 (15H, 4 m, Haromatic) δC (75.5 MHz, CDCl3): 12.9 (CH3 from
NEt2 of the nitrile ligand), 13.7 (6H, t, CH3 from NEt2 of the 2,3-
dihydro-1,2,4-oxadiazole ligand), 44.4 (CH2 from NEt2 of the nitrile
ligand), 46.2 (CH2 from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 98.2 (N−C−N), 121.2, 126.0, 126.9, 128.3, 128.8, 132.2,
138.1, and 144.4 (Caromatic), 159.6 (C(O)N); the CN carbon was
not detected.
18 (60.3 mg, 81%). Anal. Found: C, 48.14; H, 4.94; N, 9.35 (calcd
for C30H37N5Cl2OPt: C, 48.06; H, 4.97; N, 9.34); m/z (high-
resolution ESI+) 749.2101 ([M + H]+, requires 749.2101); Rf = 0.38
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2977 m (C−H),
2289 m (CN), 1648 s (CN); δH (300 MHz, CDCl3) 1.20 (6H, t,
J = 7.3 Hz, CH3 from NEt2 of the nitrile ligand), 1.49 (6H, br s, CH3
from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 2.20 (3H, s,
CH3 from C6H4Me-p), 3.06 (4H, q, J = 7.3 Hz, CH2 from NEt2 of the
nitrile ligand), 4.34 (4H, br s, CH2 from NEt2 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 6.69 (2H, d, o-protons from C6H4Me-p), 6.80 (2H,
d, m-protons from C6H4Me-p), 7.28 and 7.58 (10H, 2 m, Haromatic); δC
(75.5 MHz, CDCl3) 13.1 (CH3 from NEt2 of the nitrile ligand), 14.0
(CH3 from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 21.3
(CH3 from C6H4Me-p), 44.4 (CH2 from NEt2 of the nitrile ligand),
46.0 (CH2 from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 98.2
13 (54.8 mg, 81%). Anal. Found: C, 44.20; H, 4.05; N, 10.33 (calcd
for C25H27N5Cl2OPt: C, 44.19; H, 4.01; N, 10.31); m/z (high-
resolution ESI+) 680.1374 ([M + H]+, requires 680.1240); Rf = 0.38
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2922 m (C−H),
2305 s (CN), 1663 s (CN); δH (300 MHz, CDCl3) 2.87 (6H, s,
NMe2 of the nitrile ligand), 3.82 (6H, br s, NMe2 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 6.70, 7.01, 7.35, and 7.58 (15H, 4 m, Haromatic);
δC (75.5 MHz, CDCl3): 40.3, 40.4 (NMe2 of the 2,3-dihydro-1,2,4-
oxadiazole ligand and of the nitrile ligand), 98.3 (N−C−N), 121.6,
126.3, 127.2, 128.4, 129.1, 132.0, 137.9, and 144.2 (Caromatic), 161.2
(C(O)N); the CN carbon was not detected.
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Organometallics
Article
(N−C−N), 121.8, 127.0, 128.9, 132.2, 136.0, 138.1, and 144.7
(Caromatic), 159.9 (C(O)N); the CN carbon was not detected.
19 (62.2 mg, 81%). Anal. Found: C, 45.21; H, 4.46; N, 9.15) (calcd
for C29H34N5C3OPt: C, 45.23; H, 4.45; N, 9.09); m/z (high-resolution
ESI+) 769.1553 ([M + H]+, requires 769.1555); Rf = 0.40 (eluent
CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2979 m (C−H), 2287 m
(CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.20 (6H, t, J = 7.3
Hz, CH3 from NEt2 of the nitrile ligand), 1.49 (6H, br s, CH3 from
NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 3.06 (4H, q, J = 7.3
Hz, CH2 from NEt2 of the nitrile ligand), 4.39 (4H, br s, CH2 from
NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.67 (2H, d, o-
protons fromC6H4Cl-p), 6.97 (2H, d, m-protons fromC6H4Cl-p), 7.33
and 7.58 (10H, 2 m, Haromatic); δC (75.5 MHz, CDCl3) 13.1 (CH3 from
NEt2 of the nitrile ligand), 13.9 (CH3 from NEt2 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 44.5 (CH2 from NEt2 of the nitrile ligand),
46.3 (CH2 from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 98.2
(N−C−N), 122.8, 127.2, 126.9, 128.5, 129.1, 132.0, 137.8, and 143.0
(Caromatic), 159.9 (C(O)N); the CN carbon was not detected.
20 (60.1 mg, 79%). Anal. Found: C, 47.14; H, 4.91; N, 9.15 (calcd
for C30H37N5Cl2O2Pt: C, 47.06; H, 4.87; N, 9.15); m/z (high-
resolution ESI+) 765.2055 ([M + H]+, requires 765.2050); Rf = 0.40
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2977 m (C−H),
2289 m (CN), 1648 s (CN); δH (300 MHz, CDCl3) 1.20 (6H, t,
J = 7.3 Hz, CH3 from NEt2 of the nitrile ligand), 1.49 (6H, br s, CH3
from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 3.06 (4H, q, J =
7.3 Hz, CH2 from NEt2 of the nitrile ligand), 3.73 (3H, s, CH3 from
p-MeOC6H4), 4.34 (4H, br s, CH2 from NEt2 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 6.63 (2H, d, o-protons from p-MeOC6H4), 6.79
(2H, d, m-protons from p-MeOC6H4), 7.25 and 7.55 (10H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 13.1 (CH3 from NEt2 of the nitrile
ligand), 14.0 (CH3 from NEt2 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 44.4 (CH2 from NEt2 of the nitrile ligand), 46.0 (CH2 from
NEt2 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 62.0 (CH3 from p-
MeOC6H4), 98.2 (N−C−N), 122.2, 127.2, 128.6, 129.2, 132.4, 136.3,
137.7, and 141.1 (Caromatic), 161.3 (C(O)N); the CN carbon was
not detected.
21 (61.2 mg, 81%). Anal. Found: C, 49.08; H, 4.61; N, 9.24 (calcd
for C31H35N5Cl2OPt: C, 49.01; H, 4.64; N, 9.22); m/z (high-
resolution ESI+) 781.1767 ([M + Na]+, requires 781.1764); Rf = 0.37
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2976 m (C−H),
2289 m (CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.59 (6H, br
s, β- and γ-protons from NC5H10 of the nitrile ligand), 1.76 (6H, br s,
β- and γ-protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 3.19 (4H, q, J = 5.1 Hz, α-protons from NC5H10 of the nitrile
ligand), 4.39 (4H, br s, α-protons from NC5H10 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 6.70, 7.01, 7.30, and 7.54 (15H, 4 m,
Haromatic); δC (75.5 MHz, CDCl3) 21.3 (γ-carbons from NC5H10 of the
nitrile ligand), 22.9 (γ-carbons from NC5H10 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 24.3 (β-carbons from NC5H10 of the nitrile ligand),
22.9 (β-carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 49.3 (α-carbons from NC5H10 of the nitrile ligand), 49.9
(α-carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand
ligand), 98.2 (N−C−N), 121.1, 126.0, 126.9, 128.3, 128.6, 132.2,
138.1, and 144.4 (Caromatic), 159.6 (C(O)N); the CN carbon was
not detected.
nitrile ligand), 22.9 (β-carbons from NC5H10 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 49.2 (α-carbons from NC5H10 of the nitrile ligand),
49.8 (α-carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand ligand), 98.6 (N−C−N), 121.1, 124.8, 127.6, 129.0, 129.6, 131.5,
135.2, and 141.3 (Caromatic), 162.2 (C(O)N); the CN carbon was
not detected.
23 (64.0 mg, 81%). Anal. Found: C, 46.93; H, 4.35; N, 8.83 (calcd
for C31H34N5Cl3OPt: C, 46.89; H, 4.32; N, 8.82); m/z (high-
resolution ESI+) 815.1372 ([M + Na]+, requires 815.1374); Rf = 0.37
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2976 m (C−H),
2289 m (CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.58 (6H, br
s, β- and γ-protons from NC5H10 of the nitrile ligand), 1.76 (6H, br s,
β- and γ-protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 3.19 (4H, q, J = 5.1 Hz, α-protons from NC5H10 of the nitrile
ligand), 4.38 (4H, br s, α-protons from NC5H10 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 6.66 (2H, d, o-protons from C6H4Cl-p), 6.95
(2H, d, m-protons from C6H4Cl-p), 7.33 and 7.58 (10H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 21.2 (γ-carbons from NC5H10 of the
nitrile ligand), 22.9 (γ-carbons from NC5H10 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 24.3 (β-carbons from NC5H10 of the nitrile ligand),
22.9 (β-carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 49.2 (α-carbons from NC5H10 of the nitrile ligand), 49.8 (α-
carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand
ligand), 98.3 (N−C−N), 122.6, 127.2, 126.9, 128.5, 129.2, 132.1,
137.7, and 143.3 (Caromatic), 159.7 (C(O)N); the CN carbon was
not detected.
24 (61.7 mg, 78%). Anal. Found: C, 48.70; H, 4.70; N, 8.87 (calcd
for C32H37N5Cl2O2Pt: C, 48.67; H, 4.72; N, 8.87); m/z (high-
resolution ESI+) 811.1877 ([M + Na]+, requires 811.1870); Rf = 0.40
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2976 m (C−H),
2284 m (CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.59 (6H, br
s, β- and γ-protons from NC5H10 of the nitrile ligand), 1.76 (6H, br s,
β- and γ-protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 3.19 (4H, q, J = 5.1 Hz, α-protons from NC5H10 of the nitrile
ligand), 3.72 (3H, s, CH3 from p-MeOC6H4), 4.39 (4H, br s, α-
protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.56
(2H, d, o-protons from p-MeOC6H4), 6.74 (2H, d, m-protons from
p-MeOC6H4), 7.25 and 7.53 (10H, 2 m, Haromatic); δC (75.5 MHz,
CDCl3) 21.1 (γ-carbons from NC5H10 of the nitrile ligand), 22.9
(γ-carbons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand),
24.4 (β-carbons from NC5H10 of the nitrile ligand), 22.9 (β-carbons
from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 49.2 (α-carbons
from NC5H10 of the nitrile ligand), 49.8 (α-carbons from NC5H10 of
the 2,3-dihydro-1,2,4-oxadiazole ligand ligand), 62.1 (CH3 from
p-MeOC6H4), 98.6 (N−C−N), 122.1, 127.3, 129.1, 129.3, 132.0, 136.1,
137.5, and 141.2 (Caromatic), 161.3 (C(O)N); the CN carbon was
not detected.
Synthesis of (2,3-Dihydro-1,2,4-oxadiazole)2PtII Complexes (25−
27 and 29−31). The solutions of each of nitrones 5−7 (0.2 mmol) in
CHCl3 (1 mL) were added to a solution of the corresponding trans-
[PtCl2(NCR)2] (0.1 mmol; R = Et, NMe2) in CHCl3 (1 mL). The
mixture was stirred at 35 °C for 5 h, and the progress of the reaction
was monitored by TLC. The yellow precipitates of 25−27 and 29−31
were separated by filtration and washed by n-hexane (three 1 mL
portions). The complexes were dried in air at 20−25 °C. Yields: 89−90%.
34, 35, 38, and 39. The solutions of each of nitrones 6 and 7 (0.2
mmol) in THF (1 mL) were added to a solution of the corresponding
trans-[PtCl2(NCR)2] (0.1 mmol; R = NEt2, N(C5H10)) in THF (1 mL).
The mixture was stirred at 35 °C for 5 h, and the progress of the
reaction was monitored by TLC. The yellow precipitates of 34, 35, 38,
and 39 were separated by filtration and washed by n-hexane (three
1 mL portions). The complexes were dried in air at 20−25 °C. Yields:
88−90%.
33 and 37. The solution of nitrone 5 (1.4 mmol) in CHCl3 (1 mL)
was added to a solution of the corresponding trans-[PtCl2(NCR)2]
(0.1 mmol; R = NEt2, N(C5H10)) in CHCl3 (1 mL). The mixture was
stirred at 35 °C for 5 h, and the progress of the reaction was
monitored by TLC. The separation of 33 and 37 was achieved by
column chromatography on SiO2 (the first fraction; eluent CHCl3/
Me2CO, 10:1, v/v). The solvent was evaporated in vacuo at 20−25 °C
22 (61.9 mg, 80.1%). Anal. Found: C, 49.67; H, 4.82; N, 9.10 (calcd
for C32H37N5Cl2OPt: C, 49.68; H, 4.82; N, 9.05); m/z (high-
resolution ESI+) 795.1920 ([M + Na]+, requires 795.1921); Rf = 0.39
(eluent CHCl3/Me2CO, 30:1, v/v); νmax (KBr)/cm−1 2976 m (C−H),
2289 m (CN), 1649 s (CN); δH (300 MHz, CDCl3) 1.59 (6H, br
s, β- and γ-protons from NC5H10 of the nitrile ligand), 1.76 (6H, br s,
β- and γ-protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
ligand), 2.20 (3H, s, CH3 from C6H4Me-p), 3.19 (4H, q, J = 5.09 Hz,
α-protons from NC5H10 of the nitrile ligand), 4.39 (4H, br s, α-
protons from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand), 6.45
(2H, d, o-protons from C6H4Me-p), 6.71 (2H, d, m-protons from
C6H4Me-p), 7.27 and 7.51 (10H, 2 m, Haromatic); δC (75.5 MHz,
CDCl3) 21.1 and 21.2 (CH3 from C6H4Me-p and γ-carbons from
NC5H10 of the nitrile ligand), 22.9 (γ-carbons from NC5H10 of the 2,3-
dihydro-1,2,4-oxadiazole ligand), 24.4 (β-carbons from NC5H10 of the
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to give yellow, oily residues. The residues were crystallized under n-
hexane to form the yellow powders of 33 and 37. The complexes were
dried in air at 20−25 °C. Yields: 76−79%.
3.04 (12H, br s, NMe2), 6.34 (4H, d, o-protons from C6H4Me-p), 6.77
(4H, d, m-protons from C6H4Me-p), 7.29 and 7.42 (20H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 21.3 (CH3 from C6H4Me-p), 39.6
(NMe2), 98.3 (N−C−N), 122.2, 126.8, 128.6, 128.7, 132.8, 135.9,
138.2, and 141.5 (Caromatic), 161.2 (C(O)N).
28, 32, 36, and 40. The solution of nitrone 8 (1.4 mmol) in CHCl3
(1 mL) was added to a solution of each of 1−4 (0.1 mmol) in CHCl3
(1 mL). The mixture was stirred at −15 °C for 4 h, and the progress of
the reaction was monitored by TLC. After the total conversion of the
starting complexes into corresponding mono-CAs (3−4 h), the
reaction mixture was stirred for 5 h at 35 °C. The separation of 28, 32,
36, or 40 was achieved by column chromatography on SiO2 (the first
fraction; eluent CHCl3/Me2CO, 10:1, v/v). The solvent was
evaporated in vacuo at 20−25 °C to give yellow, oily residues. The
residues were crystallized under a layer of n-hexane to form the yellow
powders of 28, 32, 36, or 40. The complexes were dried in air at 20−
25 °C. Yields: 78−80%.
31 (90.6 mg, 90%). Anal. Found: C, 51.71; H, 3.98; N, 8.22 (calcd
for C44H40N6Cl4O2Pt: C, 51.72; H, 3.95; N, 8.23); m/z (high-
resolution ESI+) 1058.1250 ([M + K]+, requires 1058.1252); Rf = 0.58
(eluent CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2926 m (C−H),
1665 s (CN); δH (300 MHz, CD2Cl2) 3.05 (12H, br s, NMe2), 6.40
(4H, d, o-protons from C6H4Cl-p), 6.94 (4H, d, m-protons from
C6H4Cl-p), 7.29 and 7.43 (20H, 2 m, Haromatic); δC (75.5 MHz,
CDCl3) 39.6 (NMe2), 98.6 (N−C−N), 123.0, 127.2, 128.4, 128.8,
132.5, 137.6, and 142.7 (Caromatic), 166.8 (C(O)N).
32 (80.5 mg, 80%). Found: C, 54.54; H, 4.60; N, 8.28 (calcd for
C46H46N6Cl2O4Pt: C, 54.55; H, 4.58; N, 8.30); m/z (high-resolution
ESI+) 1050.2242 ([M + K]+, requires 1050.2242); Rf = 0.58 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2925 m (C−H), 1642 s
(CN); δH (300 MHz, CD2Cl2) 3.04 (12H, br s, NMe2), 3.65 (6H, s,
CH3 from p-MeOC6H4), 6.31 (4H, d, o-protons from p-MeOC6H4),
6.79 (4H, d, m-protons from p-MeOC6H4), 7.31 and 7.40 (20H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 39.6 (NMe2), 61.9 (CH3 from p-
MeOC6H4), 98.3 (N−C−N), 122.2, 126.8, 128.6, 128.7, 132.8, 135.9,
138.2, and 141.5 (Caromatic), 161.2 (C(O)N).
25 (81.7 mg, 89%). Anal. Found: C, 57.33; H, 4.38; N, 6.05 (calcd
for C44H40N4Cl2O2Pt: C, 57.27; H, 4.37; N, 6.07); m/z (high-
resolution ESI+) 960.1810 ([M + K]+, requires 960.1813); Rf = 0.57
(eluent CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2934 m (C−H),
1636 s (CN); δH (300 MHz, CD2Cl2) 1.34 (6H, t, J = 7.6, CH3
from Et), 2.93 (4H, q, J = 7.6, CH2 from Et), 6.60, 6.94, 7.32, and 7.50
(30H, 4 m, Haromatic); δC (75.5 MHz, CD2Cl2) 9.9 (CH3 from Et), 22.6
(CH2 from Et), 98.4 (N−C−N), 120.6, 126.0, 127.2, 128.3, 128.4,
129.1, 131.7, and 136.1 (Caromatic), 160.1 (C(O)N).
26 (85.2 mg, 90%). Anal. Found: 58.11; H, 4.66; N, 5.89 (calcd for
C46H44N4Cl2O2Pt: C, 58.11; H, 4.66; N, 5.89); m/z (high-resolution
ESI+) 972.2380 ([M + Na]+, requires 972.2387); Rf = 0.58 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2934, 2978 m (C−H),
1643 s (CN); δH (300 MHz, CDCl3) 1.30 (6H, t, J = 7.6 Hz, CH3
from Et), 2.14 (6H, s, CH3 from C6H4Me-p), 2.81 (4H, q, J = 7.6 Hz,
CH2 from Et), 6.46 (4H, d, o-protons from C6H4Me-p), 6.70 (4H, d,
m-protons from C6H4Me-p), 7.28 and 7.50 (20H, 2 m, Haromatic); δC
(75.5 MHz, CD2Cl2) 9.9 (CH3 from Et), 21.2 (CH3 from C6H4Me-p),
22.6 (CH2 from Et), 98.3 (N−C−N),122.1, 126.7, 128.6, 128.8, 132.8,
135.8, 138.2, and 141.3 (Caromatic), 161.1 (C(O)N).
33 (79.6 mg, 79%). Anal. Found: C, 57.21; H, 5.03; N, 8.33 (calcd
for C48H50N6Cl2O2Pt: C, 57.14; H, 5.00; N, 8.33); m/z (high-
resolution ESI+) 1046.2651 ([M + K]+, requires 1046.2657); Rf = 0.61
(eluent CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2924 m (C−H),
1667 s (CN); δH (300 MHz, CDCl3) 1.08 (12H, t, J = 6.9 Hz, CH3
from NEt2), 3.61 (8H, br s, CH2 from NEt2), 6.42, 6.93, 7.20, and 7.47
(30H, 4 m, Haromatic); δC (75.5 MHz, CDCl3) 14.1 (CH3 from NEt2),
43.7 (CH2 from NEt2), 98.5 (N−C−N), 121.6, 126.1, 127.1, 128.2,
129.0, 132.5, 137.8, and 144.1 (Caromatic), 160.5 (C(O)N).
34 (92.2 mg, 89%). Found: C, 57.91; H, 5.25; N, 8.10 (calcd for
C50H54N6Cl2O2Pt: C, 57.91; H, 5.25; N, 8.10); m/z (high-resolution
ESI+) 1035.3331 ([M + K]+, requires 1035.3333); Rf = 0.57 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2926 m (C−H), 1648 s
(CN); δH (300 MHz, CDCl3) 1.08 (12H, t, J = 6.9 Hz, CH3 from
NEt2), 2.18 (6H s, CH3 from C6H4Me-p), 3.63 (8H, br s, CH2 from
NEt2), 6.32 (4H, d, o-protons from C6H4Me-p), 6.73 (4H, d, m-protons
from C6H4Me-p), 7.24 and 7.51 (20H, 2 m, Haromatic); δC (75.5 MHz,
CD2Cl2) 14.2 (CH3 from NEt2), 21.3 (CH3 from C6H4Me-p), 43.6 (CH2
from NEt2), 98.1 (N−C−N), 122.2, 126.9, 128.6, 128.8, 132.8, 135.8,
138.2, and 141.5 (Caromatic), 161.1 (C(O)N).
27 (88.5 mg, 90%). Found: C, 53.32; H, 3.88; N, 5.70 (calcd for
C44H38N4Cl4O2Pt: C, 53.29; H, 3.86; N, 5.65); m/z (high-resolution
ESI+) 990.1471 ([M + H]+, requires 990.1475); Rf = 0.57 (eluent
CHCl3/Me2CO, 40:1, v/v); Rf = 0.60 (eluent CHCl3/Et2O, 40/1.5, v/
v); νmax (KBr)/cm−1 2934 m (C−H), 1637 s (CN); δH (300 MHz,
CDCl3) 1.30 (6H, t, J = 7.6 Hz, CH3 from Et), 2.82 (4H, q, J = 7.6 Hz,
CH2 from Et), 6.49 (4H, d, o-protons from C6H4Cl-p), 6.88 (4H, d, m-
protons from C6H4Cl-p), 7.32 and 7.50 (20H, 2 m, Haromatic), δC (75.5
MHz, CDCl3) 9.9 (CH3 from Et), 22.6 (CH2 from Et), 98.4 (N−C−
N), 123.3, 127.2, 128.3, 131.5, 132.7, 137.8, and 142.9 (Caromatic), 160.1
(C(O)N).
35 (96.6 mg, 90%). Found: C, 53.53; H, 4.49; N, 7.82 (calcd for
C48H48N6Cl4O2Pt: C, 53.49; H, 4.49; N, 7.80); m/z (high-resolution
ESI+) 1114.1877 ([M + K]+, requires 1114.1878); Rf = 0.61 (eluent
CHCl3/Me2CO, 40/1, v/v); νmax (KBr)/cm−1 2924; 2852, m (C−H),
1647, s (CN); δH (300 MHz, CDCl3) 1.07 (12H, t, J = 6.9 Hz, CH3
from NEt2), 3.62 (8H, br s, CH2 from NEt2), 6.36 (4H, d, o-protons
from C6H4Cl-p), 6.90 (4H, d, m-protons from C6H4Cl-p), 7.31 and
7.51 (20H, 2 m, Haromatic); δC (75.5 MHz, CD2Cl2) 14.1 (CH3 from
NEt2), 43.7 (CH2 from NEt2), 98.4 (N−C−N), 123.3, 127.1, 128.4,
131.5, 132.6, 137.8, and 142.8 (Caromatic), 160.9 (C(O)N).
28 (76.7 mg, 78%). Anal. Found: C, 56.18; H, 4.53; N, 5.70 (calcd
for C46H44N4Cl2O4Pt: C, 56.21; H, 4.51; N, 5.70); m/z (high-
resolution ESI+) 1004.2883 ([M + Na]+, requires 1004.2885); Rf =
0.60 (eluent CHCl3/Me2CO, 40/1, v/v); νmax (KBr)/cm−1 2934, 2978
m (C−H), 1643 s (CN); δH (300 MHz, CDCl3) 1.30 (6H, t, J = 7.6
Hz, CH3 from Et), 2.81 (4H, q, J = 7.6 Hz, CH2 from Et), 3.71 (3H, s,
CH3 from p-MeOC6H4), 6.46 (4H, d, o-protons from p-MeOC6H4),
6.70 (4H, d, m-protons from p-MeOC6H4), 7.28 and 7.50 (20H, 2 m,
Haromatic); δC (75.5 MHz, CD2Cl2) 9.9 (CH3 from Et), 22.5 (CH2 from
Et), 61.9 (CH3from p-MeOC6H4), 98.3 (N−C−N), 122.2, 127.0, 128.6,
128.7, 132.1, 136.8, 138.5, and 141.3 (Caromatic), 161.2 (C(O)N).
29 (83.2 mg, 87%). Anal. Found: C, 55.49; H, 4.40; N, 8.85 (calcd
for C44H42N6Cl2O2Pt: C, 55.46; H, 4.44; N, 8.82); m/z (high-
resolution ESI+) 990.2035 ([M + K]+, requires 990.2031); Rf = 0.57
(eluent CHCl3/Me2CO, 40/1, v/v); νmax (KBr)/cm−1 2924 m (C−H),
1667 s (CN); δH (300 MHz, CDCl3) 3.06 (12H, br s, NMe2), 6.43,
6.92, 7.22, and 7.45 (30H, 4 m, Haromatic); δC (75.5 MHz, CDCl3) 39.6
(NMe2), 98.5 (N−C−N), 121.6, 126.1, 127.0, 128.3, 128.8, 132.5,
137.9, and 144.0 (Caromatic), 160.4 (C(O)N).
36 (85.0 mg, 80%). Found: C, 56.16; H, 5.10; N, 7.88 (calcd for
C50H54N6Cl2O4Pt: C, 56.18; H, 5.09; N, 7.86); m/z (high-resolution
ESI+) 1090.3127 ([M + Na]+, requires 1090.3129); Rf = 0.58 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2926 m (C−H), 1648 s
(CN); δH (300 MHz, CDCl3) 1.08 (12H, t, J = 6.9 Hz, CH3 from
NEt2), 3.63 (8H, br s, CH2 from NEt2), 3.68 (6H, s, CH3 from p-
MeOC6H4), 6.32 (4H, d, o-protons from p-MeOC6H4), 6.77 (4H, d,
m-protons from p-MeOC6H4), 7.31 and 7.40 (20H, 2 m, Haromatic); δC
(75.5 MHz, CD2Cl2) 14.2 (CH3 from NEt2), 43.6 (CH2 from NEt2),
61.9 (CH3 from p-MeOC6H4), 98.1 (N−C−N), 122.1, 126.8, 128.6,
128.7, 132.6, 136.3, 138.2, and 141.3 (Caromatic), 161.2 (C(O)N).
37 (78.5 mg, 76%). Found: C, 58.14; H, 4.81; N, 8.15 (calcd for
C50H50N6Cl2O2Pt: C, 58.14; H, 4.88; N, 8.14); m/z (high-resolution
ESI+) 1070.2651 ([M + K]+, requires 1070.2657); Rf = 0.59 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2924 m (C−H), 1667 s
(CN); δH (300 MHz, CDCl3) 1.50 (12H, br s, β- and γ-protons
30 (87.7 mg, 90%). Found: C, 56.33; H, 4.70; N, 8.61 (calcd for
C46H46N6Cl2O2Pt: C, 56.32; H, 4.73; N, 8.57); m/z (high-resolution
ESI+) 1018.2332 ([M + K]+, requires 1018.2344); Rf = 0.60 (eluent
CHCl3/Me2CO, 40/1, v/v); νmax (KBr)/cm−1 2925 m (C−H), 1642 s
(CN); δH (300 MHz, CD2Cl2) 2.20 (6H, s, CH3 from C6H4Me-p),
697
dx.doi.org/10.1021/om201026n | Organometallics 2012, 31, 687−699

Organometallics
Article
from NC5H10), 3.54 (8H, br s, α-protons from NC5H10), 6.41, 6.93,
7.22, and 7.46 (30H, 4 m, Haromatic); δC (75.5 MHz, CDCl3): 24.2
(γ-carbons from NC5H10), 26.4 (β-carbons from NC5H10), 48.7
(α-carbons from NC5H10), 98.4 (N−C−N), 121.5, 126.1, 127.2, 128.2,
129.1, 132.5, 137.8, and 144.1 (Caromatic), 160.3 (C(O)N).
after 3 d. The metal-free heterocycles were separated from excess
NaCN, and also from NaCl and Na2[Pt(CN)4], formed in the
reaction, by evaporation of the solvent at room temperature and
treating the colorless, oily residue with CHCl3. The liquid phase was
separated by filtration, whereupon evaporation of the solvent afforded
in almost quantitative yields 42 and 43 as colorless, oily residues.
42. m/z (high-resolution ESI+) 365.1626 ([M + Na]+, requires
365.1629); δH (300 MHz, CDCl3) 1.27 (6H, t, J = 7.6 Hz, CH3 from
Et), 2.14 (6H, s, CH3 from C6H4Me-p), 2.75 (4H, q, J = 7.6 Hz, CH2
from Et), 6.45 (4H, d, o-protons from C6H4Me-p), 6.71 (4H, d,
m-protons from C6H4Me-p), 7.24 and 7.51 (20H, 2 m, Haromatic); δC
(75.5 MHz, CD2Cl2) 9.9 (CH3 from Et), 21.2 (CH3 from C6H4Me-p),
22.3 (CH2 from Et), 96.3 (N−C−N), 122.2, 126.7, 128.6, 128.7,
132.8, 134.9, 138.1, and 141.2 (Caromatic), 162.0 (C(O)N).
38 (94.2 mg, 90%). Found: C, 58.90; H, 5.14; N, 7.94 (calcd for
C52H54N6Cl2O2Pt: C, 58.86; H, 5.13; N, 7.92); m/z (high-resolution
ESI+) 1098.2976 ([M + K]+, requires 1098.2970); Rf = 0.62 (eluent
CHCl3/Me2CO, 40/1, v/v); νmax (KBr)/cm−1 2924 m (C−H), 1667 s
(CN); δH (300 MHz, CDCl3) 1.50 (12H, br s, β- and γ-protons
from NC5H10), 2.18 (6H, s, CH3 from C6H4Me-p), 3.54 (8H, br s,
α-protons from NC5H10), 6.43 (2H, d, o-protons from C6H4Me-p),
6.70 (2H, d, m-protons from C6H4Me-p), 7.27 and 7.51 (20H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 21.1 (CH3 from C6H4Me-p), 24.3
(γ-carbons from NC5H10), 26.4 (β-carbons from NC5H10), 48.8
(α-carbons from NC5H10), 98.0 (N−C−N), 122.2, 126.7, 128.6, 128.6,
132.8, 135.8, 138.1, and 141.2 (Caromatic), 161.2 (C(O)N).
43. m/z (high-resolution ESI+) 380.1731 ([M + Na]+, requires
380.1739); δH (300 MHz, CD2Cl2) 2.20 (3H, s, CH3 from C6H4Me-p),
2.97 (12H, br s, NMe2), 6.30 (4H, d, o-protons from C6H4Me-p),
6.77 (4H, d, m-protons from C6H4Me-p), 7.25 and 7.42 (20H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 21.3 (CH3 from C6H4Me-p), 39.3
(NMe2), 97.5 (N−C−N), 122.1, 126.4, 128.3, 128.8, 131.9, 135.9,
137.9, and 141.6 (Caromatic), 161.2 (C(O)N).
39 (96.5 mg, 88%). Found: C, 54.55; H, 4.39; N, 7.66 (calcd for
C50H48N6Cl4O2Pt: C, 54.50; H, 4.35; N, 7.63); m/z (high-resolution
ESI+) 1138.1875 ([M + K]+, requires 1138.1878); Rf = 0.60 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2924 m (C−H), 1667 s
(CN); δH (300 MHz, CDCl3) 1.49 (12H, br s, β- and γ-protons
from NC5H10), 3.53 (8H, br s, α-protons from NC5H10), 6.49 (4H, d,
o-protons from C6H4Cl-p), 6.88 (4H, d, m-protons from C6H4Cl-p),
7.32 and 7.50 (20H, 2 m, Haromatic); δC (75.5 MHz, CDCl3) 24.2
(γ-carbons from NC5H10), 26.4 (β-carbons from NC5H10), 48.8
(α-carbons from NC5H10), 98.4 (N−C−N), 123.2, 127.2, 128.2, 131.3,
132.8, 137.8, and 142.9 (Caromatic), 160.2 (C(O)N).
ASSOCIATED CONTENT
■
S
* Supporting Information
Full tables of crystal data, atomic coordinates and equivalent
isotropic displacement parameters, anisotropic displacement
parameters, and bond lengths and angles for 25 and 33 as
CIF files. Discussion of the theoretical results on DCA to
methylenecyclopropane; table with calculated energies. Char-
acterization of nitrones 6−8. This material is available free of
charge via the Internet at http://pubs.acs.org.
40 (85.4 mg, 78%). Found: C, 57.10; H, 5.00; N, 7.74 (calcd for
C52H54N6Cl2O4Pt: C, 57.14; H, 4.98; N, 7.69); m/z (high-resolution
ESI+) 1130.2865 ([M + K]+, requires 1130.2868); Rf = 0.60 (eluent
CHCl3/Me2CO, 40:1, v/v); νmax (KBr)/cm−1 2924 m (C−H), 1667 s
(CN); δH (300 MHz, CDCl3) 1.50 (12H, br s, β- and γ-protons
from NC5H10), 3.54 (8H, br s, α-protons from NC5H10), 3.67 (6H, s,
CH3 from p-MeOC6H4), 6.43 (4H, d, o-protons from C6H4OMe-p),
6.71 (4H, d, m-protons from p-MeOC6H4), 7.28 and 7.53 (20H, 2 m,
Haromatic); δC (75.5 MHz, CDCl3) 24.3 (γ-carbons from NC5H10), 26.4
(β-carbons from NC5H10), 48.8 (α-carbons from NC5H10), 61.7 (CH3
from p-MeOC6H4), 98.0 (N−C−N), 122.1, 127.4, 128.6, 129.4, 132.1,
136.8, 138.8, and 141.2 (Caromatic), 161.3 (C(O)N).
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +7 812 4286733. Tel: +7 812 4286890. E-mail: bokach@
nb17701.spb.edu, kukushkin@vk2100.spb.edu.
ACKNOWLEDGMENTS
Synthesis of 41. An excess of nitrone 6 (5 mmol) was added to a
suspension of 2 (0.5 mmol) and 27 in CHCl3 (6 mL), and the reaction
mixture was stirred for 120 h at 25 °C. The solution was separated by
filtration. Complex 41 was isolated from the solution by column
chromatography on SiO2 (eluent CHCl3/Et2O, 40:1.5, v/v). The
solvent was evaporated in vacuo at 20−25 °C to give a yellow, oily
residue. The residue was crystallized under n-hexane to form the yellow
powder of 41. The complex was dried in air at 20−25 °C. Yield: 8%.
41 (39.8 mg, 8%). Found: C, 55.70; H, 4.27; N, 5.75 (calcd for
C45H41N4Cl3O2Pt: C, 55.65; H, 4.25; N, 5.77); m/z (high-resolution
ESI+) 992.1841 ([M + Na]+, requires 992.1841); Rf = 0.72 (eluent
CHCl3/Et2O, 40:1.5, v/v); νmax (KBr)/cm−1 2934, 2978 m (C−H),
1643 s (CN); δH (300 MHz, CDCl) 1.30 (6H, m, CH3 from both
Et of the different 2,3-dihydro-1,2,4-oxadiazole ligands), 2.14 (3H, s,
CH3 from C6H4Me-p), 2.85 (4H, q, CH2 from Et of the different 2,3-
dihydro-1,2,4-oxadiazole ligands), 6.47, 6.73, 6.82, 7.33, and 7.51
(28H, 5 m, Haromatic); δC (75.5 MHz, CD2Cl2) 9.9 (CH3 from Et of the
different 2,3-dihydro-1,2,4-oxadiazole ligands), 21.2 (CH3 from
C6H4Me-p), 22.5 (CH2 from Et of the different 2,3-dihydro-1,2,4-
oxadiazole ligands), 98.3 (N−C−N of the different 2,3-dihydro-1,2,4-
oxadiazole ligands), 122.3, 123.3, 126.5, 127.6, 128.7, 128.9, 132.5,
135.4, 137.3, 138.2, and 141.3 (Caromatic), 161.2 (C(O)N of the
different 2,3-dihydro-1,2,4-oxadiazole ligands).
■
The authors express their gratitude to Federal Grant-in-Aid
Program “Human Capital for Science and Education in
Innovative Russia” (Governmental Contract No. P676 from
20/05/2010 and P1294 from 09/06/2010), Russian Fund of
Basic Research (grant 11-03-00262), Saint-Petersburg State
University (research grant 2011−2013). M.L.K. is grateful to
̂
the FCT and IST for a research contract within the Ciencia
2007 scientific program. A.S.K. expresses his gratitude to the
Government of Saint-Petersburg for a grant for graduate and
undergraduate students (2011).
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■
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