Cycloaddition of coordinated diazoalkanes to ethene to yield 3H-pyrazole derivatives

Article abstract of DOI:10.1021/om4004016

Diazoalkane complexes [Ru(η⁵-C₅H ₅)(N₂CAr1Ar2)(PPh₃)L]BPh₄ (1, 2; Ar1 = Ph, Ar2 = p-tolyl; Ar1Ar2 = C₁₂H₈; L = P(OMe) ₃, P(OEt)₃) were prepared b

Full text of DOI:10.1021/om4004016

Communication
pubs.acs.org/Organometallics
Cycloaddition of Coordinated Diazoalkanes to Ethene To Yield
3H‑Pyrazole Derivatives
Gabriele Albertin,*,† Stefano Antoniutti,† Daniela Baldan,† Jesus Castro,‡ and Giulia Comparin†
́
†Dipartimento di Scienze Molecolari e Nanosistemi, Universita
̀
Ca’ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italy
‡
́
́
Departamento de Quımica Inorganica, Universidade de Vigo, Facultade de Quımica, Edificio de Ciencias Experimentais, 36310 Vigo
́
(Galicia), Spain
S
* Supporting Information
ABSTRACT: Diazoalkane complexes [Ru(η5-C5H5)-
(N2CAr1Ar2)(PPh3)L]BPh4 (1, 2; Ar1 = Ph, Ar2 = p-tolyl;
Ar1Ar2 = C12H8; L = P(OMe)3, P(OEt)3) were prepared by
allowing compounds RuCl(η5-C5H5)(PPh3)L to react with
diazoalkane in ethanol. Treatment of complexes 1 and 2 with
ethylene under mild conditions (1 atm, room temperature) led
not only to the ethylene complexes [Ru(η5-C5H5)(η2-CH2
CH2)(PPh3)L]BPh4 (5, 6) but also to dipolar (3 + 2)
cycloaddition, affording the 3H-pyrazole derivatives [Ru(η5-
C5H5){η1-NNC(Ar1Ar2)CH2CH2}(PPh3)L]BPh4 (3, 4).
The propylene complexes [Ru(η5-C5H5)(η2-CH3CHCH2)(PPh3)L]BPh4 (7, 8) were also prepared. The compounds were
characterized by spectroscopy and by X-ray crystal structure determinations of 2a, 3b, and 7.
ipolar (3 + 2) cycloaddition of diazoalkane to alkene is an
important reaction which has been extensively studied, for
1959−1961 cm−1, due to νCNN of the coordinated
diazoalkane. This value also suggests an end-on η1 coordination
mode for the Ar1Ar2CN2 group, similar to that found in the solid
state for 2a, an ORTEP7 drawing of which is shown in Figure 1.
D
both fundamental and synthetic reasons.1,2 However, despite
numerous studies, no example of cycloaddition of coordinated
diazoalkane3,4 has ever been reported. We describe here the syn-
thesis of new diazoalkane complexes of ruthenium, which undergo
unprecedented (3 + 2) cycloaddition of the metal-bonded dia-
zoalkane to ethylene (CH2CH2), yielding a 4,5-dihydro-3H-
pyrazole derivative.
Mixed-ligand half-sandwich complexes5 RuCl(η5-C5H5)-
(PPh3)L (L = P(OR)3) react with an excess of Ar1Ar2CN2 in
ethanol in the presence of NaBPh4 to afford the diazoalkane
derivatives [Ru(η5-C5H5)(N2CAr1Ar2)(PPh3)L]BPh4 (1, 2) in
about 70% yields (Scheme 1).
a
Scheme 1.
Figure 1. ORTEP7 view of the cation of 2a drawn at the 30% probability
level. Hydrogen atoms, ethoxy groups at P1, and phenyl rings at P2 are
omitted for clarity.
a
L = P(OMe) (1), P(OEt)3 (2); Ar1 = Ph, Ar2 = p-tolyl (a), Ar1Ar2 =
C12H8 (b).
The cation of 2a consists of a ruthenium atom coordinated by a Cp
group, a PPh3, a P(OEt)3 ligand, and a p-tolylphenylmethylenediazo
Good analytical data were obtained for compounds 1 and 2,
which are red-orange solids and are very stable in air and in
solutions of polar organic solvents, where they behave as
1:1 electrolytes.6 IR spectra show a medium-intensity band at
Received: May 7, 2013
Published: May 29, 2013
© 2013 American Chemical Society
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Communication
a
Scheme 2.
a
L = P(OMe) (3, 5), P(OEt)3 (4, 6); Ar1 = Ph, Ar2 = p-tolyl (a), Ar1Ar2 = C12H8 (b).
a
Scheme 3.
a
L = P(OMe) (7), P(OEt)3 (8).
ligand, bound to the Ru center via the terminal nitrogen atom.
Coordination of the last ligand is expected to be linear but, in 2a,
some bending is found, especially in the N−N−Ru angle,
156.0(5)°, rather than in the N−N−C angle, 173.9(6)°. It is
noteworthy that these values are large enough for us to propose a
linear coordination to ruthenium in this compound, and similar
features have previously been reported in the literature: i.e.,
values of 158.3(3) and 170.1(3)°, respectively, for a diazofluor-
ene compound.8 It is also remarkable that, when aryldiazenide
ligands are used, only the N−N−C(aryl) part is slightly bent.9
More information about the nature of this ligand is provided
by the short N(1)−N(2) bond distance, 1.147(6) Å, which may
be viewed as being between a N−N double and triple bond. It is
only slightly longer than that found in the aforementioned
diazofluorene compound8 and similar to those found in other
aryldiazenide ruthenium(II) complexes.9 The N(2)−C(6) bond
distance, 1.299(8) Å, is also short enough to be considered as a
double bond.10 Angles around C(6) are close to 120° (from
116.3(6) to 126.1(6)°) and have a sum of 359.9°, thus con-
firming the sp2 character of this atom.
substitution with CH2CH2 was also detected, indicating that
only coordinated diazoalkane undergoes cyclization with ethyl-
ene to yield the 3H-pyrazole species. Note that the complexes
[Ru(η5-C5H5)(η2-CH2CH2)(PPh3)L]BPh4 (5, 6) were also
prepared by treating RuCl(η5-C5H5)(PPh3)L with CH2CH2
(1 atm) in the presence of NaBPh4.
N-bonded diazoalkanes are reported to give both dinitrogen
[M]−N2 and carbene [M]CAr1Ar2 complexes.4f N−N bond
cleavage11 was also observed, as well as reduction of the co-
ordinated N2CAr1Ar2 ligand.8 However, an example of the
cycloaddition reaction has been reported for a C-bonded dia-
zoalkane in Rh[η1-C(N2)SiMe3][P(OEt)3]3,12 which afforded
the triazole derivative Rh[CC(SiMe3)N2NBut](ButNC)2[P-
(OEt)3] in the reaction with ButNC. The use of the half-
sandwich fragment Ru(η5-C5H5)(PPh3)L not only allowed the
diazoalkanes to be coordinated but also led to the unprecedented
dipolar (3 + 2) cycloaddition of coordinated N2CAr1Ar2 to
ethene, affording a novel dihydrido 3H-pyrazole molecule.
Propylene was also treated with the diazoalkane complexes
[Ru(η5-C5H5)(N2CAr1Ar2)-(PPh3)L]BPh4 (1, 2) in CH2Cl2
solution, but under mild conditions (1 atm, room temperature),
no reaction took place. Under pressure (7 atm, room temperature),
substitution of the diazoalkane was observed, with the formation of
propylene complexes [Ru(η5-C5H5)(η2-CH3CHCH2)(PPh3)L]-
BPh4 (7, 8), but no evidence of a cycloaddition reaction was
detected (Scheme 3). In fact, in addition to propylene complexes,
some other unidentified products did form in the reaction under
pressure, but NMR spectra excluded the formation of 3H-pyrazole
species. Cyclization with propylene is probably very slow and
requires more drastic conditions if it is to proceed.
Diazoalkane complexes [Ru(η5-C5H5)(N2CAr1Ar2)(PPh3)-
L]BPh4 (1, 2) react with ethylene under mild conditions (1 atm,
room temperature) to give not only a small amount of the
ethylene complexes [Ru(η5-C5H5)(η2-CH2CH2)(PPh3)L]-
BPh4 (5, 6) but also the novel derivatives [Ru(η5-C5H5){η1-
NNC(Ar1Ar2)CH2CH2}(PPh3)L]BPh4 (3, 4), which contain
a 4,5-dihydro-3H-pyrazole as a ligand (Scheme 2).
The reaction proceeds with (3 + 2) cycloaddition of the ethene
to the coordinated diazoalkane, giving the 3H-pyrazole deri-
vatives 3 and 4, in which the novel heterocycle acts as a ligand.
Parallel substitution of the diazoalkane by CH2CH2 also
proceeds to a small extent, yielding ethylene complexes 5 and 6.
The new complexes 3−8 were characterized by analytical and
spectroscopic data and by X-ray crystal structure determinations
The two compounds [Ru(η5-C5H5){η1-NNC(Ar1Ar2)-
of [Ru(η5-C5H5){η1-NNC(C12H8)CH2CH2}(PPh3){P-
(OMe)3}]BPh4 (3b) and [Ru(η5-C5H5)(η2-CH3CHCH2)-
(PPh3){P(OMe)3}]BPh4 (7), the ORTEPs of which are shown
in Figures 2 and 3, respectively. The cation of 3b consists of a
ruthenium atom coordinated by a Cp group, a PPh3, a P(OMe)3,
and the new ligand 4′,5′-dihydrospiro[fluorene-9,3′-pyrazole],
CH2CH2}(PPh3)L]BPh4 (3, 4) and [Ru(η5-C5H5)(η2-CH2
CH2)(PPh3)L]BPh4 (5, 6) were recovered as solids and
separated by fractional crystallization in good yield (70−75%
for 3 and 4, 8−10% for 5 and 6) as yellow-orange crystalline
solids. In the mother liquor, free Ar1Ar2CN2 formed as a result of
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of two diastereoisomers, owing to the presence of two chiral
centers in the molecule: i.e., the ruthenium atom and the C3
1
atom of the heterocyclic ligand. H, 13C, and 31P NMR spectra
confirm the presence of the two diastereoisomers, showing two
sets of signals for the nuclei of the ligands (see the Supporting
Information). In particular, the 31P NMR spectra of 3a and 4a
show two AB multiplets, whereas the proton spectra show two
singlets for the η5-C5H5 protons and two for the methyl group of
the p-tolyl substituent at C3 of the 3H-pyrazole. In addition, two
groups of partially overlapping signals were observed for the H4
and H5 protons of the 3H-pyrazole, fitting the proposed formu-
lation for the complexes. The 31P NMR spectra of the related
complexes [Ru(η5-C5H5){η1-NNC(C12H8)CH2CH2}-
(PPh3)L]BPh4 (3b, 4b), which contain only one chiral center,
show only one AB quartet, whereas 1H and 13C spectra support
the presence of the heterocyclic ligand. In particular, a triplet at
2.23−2.20 ppm, attributed to methylene protons H4, and two
triplets at 4.89−4.84 ppm, attributed to methylene hydrogen
atoms H5, were observed in the proton spectra. The 13C spectra
of 3b show two singlets at 30.6 and 88.95 ppm. In an HMQC
experiment, the former were correlated with the triplet at 2.23
ppm and the latter with the multiplet near 4.84 ppm and
attributed respectively to the C4 and C5 carbon atoms of the 3H-
pyrazole ligand. The singlet at 98.3 ppm was attributed to C3.
At room temperature, the proton spectra of ethylene
complexes [Ru(η5-C5H5)(η2-CH2CH2)(PPh3)L]BPh4 (5, 6)
show two multiplets at 2.99−2.71 and 3.01−2.71 ppm, attributed
to the protons of the coordinated ethylene. Lowering the sample
temperature caused variations in the spectra, but even at −90 °C,
ethylene peaks were still broadened, suggesting that rotation of
CH2CH2 still occurred at this temperature. However, the
room temperature pattern can be simulated by an ABCDEF
model (E, F = 31P) with the parameters reported in the
Supporting Information, and the good fit between calculated and
experimental spectra strongly supports the proposed attributions. In
the 13C NMR spectra, the CH2CH2 carbon resonances appear as
doublets of doublets at 38.25 (5) and 38.4 ppm (6), whereas the
31P spectra are AB quartets, fitting the proposed formulation for
complexes 5 and 6.
Figure 2. ORTEP7 view of the cation of 3b drawn at the 30% probability
level. Hydrogen atoms, methoxy groups at P1, and phenyl rings at P2 are
omitted for clarity.
Figure 3. ORTEP7 view of the cation of 7 drawn at the 20% probability
level. Hydrogen atoms, methoxy groups at P1, and phenyl rings at P2 are
omitted for clarity.
bound to the Ru center via the nitrogen atom. Cp and the
phosphane ligands show their usual behavior.5
1
The H NMR spectra of the propylene derivatives [Ru(η5-
The most noteworthy parameters of the new ligand 4′,5′-
dihydrospiro[fluorene-9,3′-pyrazole] are the N−N bond dis-
tance in the 4′,5′-dihydropyrazole ring, 1.241(4) Å, which,
although longer than that found in 2a, corresponds with expected
values for a double bond,10 and the rest of the distances in the
ring, from 1.494(5) to 1.533(6) Å, as expected for a single bond.
It should be noted that (see the X-ray section in the Supporting
Information) the positions of the hydrogen atoms for this ring
were found in the density maps. The dihydropyrazole ring (rms
deviation of 0.0301 Å) is situated almost perpendicularly to the
fluorene plane, forming a dihedral angle of 85.8(2)°.
The cation of 7 consists of a ruthenium atom coordinated by a
Cp ligand, a PPh3, a P(OMe)3 ligand, and a η2-CH2CHCH3
ligand. The midpoint of the η2-coordinated propylene is situated
2.1311(5) Å from the ruthenium atom, in a slightly distorted
fashion, with a difference between the two Ru−C bonds of 0.026 Å,
the shorter being that of the CH2.13 Cp and the phosphane ligands
show the usual behavior.5
C5H5)(η2-CH3CHCH2)(PPh3)L]-BPh4 (7, 8) show two
signals for the Cp protons and two for the phosphine protons,
due to the presence of two diastereoisomers. At room temperature,
two sets of signals are also observed for the propylene hydrogen
atoms, which can be simulated with an ABCD3EF model (E, F =
31P) with the parameters reported in the Supporting Information.
The 31P NMR spectra exhibited two AB multiplets, whereas the 13C
spectra showed two sets of signals for both carbon atoms of the
ancillary ligands and those of the propylene, fitting a geometry like
those found in the solid state.
We are extending experimental work to test cyclization with
other multiple-bond systems and to study new metal fragments
for stoichiometric and catalytic cycloaddition.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra support the proposed formulations for complexes
3−8 with geometries in solution like those found in the solid state.
Text, tables, figures, and CIF files giving experimental, analytical,
and spectroscopic data for new compounds and crystallographic
data for compounds 2a, 3b, and 7. This material is available free
of charge via the Internet at http://pubs.acs.org.
Complexes [Ru(η5-C5H5){η1-NNC(p-tolyl)(Ph)-
CH2CH2}(PPh3)L]BPh4 (3a, 4a) were obtained as a mixture
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The financial support of MIUR (Rome)-PRIN 2009 is gratefully
acknowledged.
■
REFERENCES
■
(1) (a) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1963, 2, 565−632.
(b) 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New
York, 1984; Vol. 1. (c) Adam, W.; De Lucchi, O. Angew. Chem., Int. Ed.
Engl. 1980, 19, 762−779.
(2) (a) Padwa, A. In Comprehensive Organic Synthesis, Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 4, pp 1069−
1109. (b) Diederich, F.; Isaacs, L.; Philip, D. Chem. Soc. Rev. 1994, 23,
243−255. (c) Zollinger, H. Diazo Chemistry II; VCH: Weinheim,
Germany, 1995. (d) Sibi, M. P.; Stanley, L. M.; Soeta, T. Org. Lett. 2007,
9, 1553−1556 and references therein.
(3) (a) Dartiguenave, M.; Menu, M.-J.; Deydier, E.; Dartiguenave, Y.;
Siebald, H. Coord. Chem. Rev. 1998, 178−180, 623−663. (b) Mizobe, Y.;
Ishii, Y.; Hidai, M. Coord. Chem. Rev. 1995, 139, 281−311.
(4) (a) Khosle, C.; Jackson, A. B.; White, P. S.; Templeton, J. L.
Organometallics 2012, 31, 987−994. (b) Russell, S. K.; Lobkovsky, E.;
Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 36−37. (c) Albertin, G.;
Antoniutti, S.; Callegaro, F.; Castro, J. Organometallics 2009, 28, 4475−
4479. (d) Samant, R. G.; Graham, T. W.; Rowsell, B. D.; McDonald, R.;
Cowie, M. Organometallics 2008, 27, 3070−3081. (e) Nomura, M.;
Kusui, A.; Kajitani, M. Organometallics 2005, 24, 2811−2818. (f) Cohen,
R.; Rybtchinski, B.; Gandelman, M.; Rozenberg, H.; Martin, J. M. L.;
Milstein, D. J. Am. Chem. Soc. 2003, 125, 6532−6546. (g) Werner, H.;
Mahr, N.; Wolf, J.; Fries, A.; Laubender, M.; Bleuel, E.; Garde, R.;
Lahuerta, P. Organometallics 2003, 22, 3566−3576. (h) Gao, Y.;
Jennings, M. C.; Puddephatt, R. J.; Jenkins, H. A. Organometallics 2001,
20, 3500−3509.
(5) Albertin, G.; Antoniutti, S.; Bacchi, A.; Pelizzi, G.; Zanardo, G.
Organometallics 2008, 27, 4407−4418.
(6) Geary, W. J. Coord. Chem. Rev. 1971, 7, 81−122.
(7) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565−565.
(8) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Milstein, D.
Organometallics 2008, 27, 3526−3533.
(9) (a) Małecki, J. G.; Gryca, I.; Penkala, M. Polyhedron 2013, 51, 102−
110. (b) Fan, L.; Einstein, F. W. B.; Sutton, D. Organometallics 2000, 19,
684−694.
(10) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1−S19.
(11) Kaplan, A. W.; Polse, J. L.; Ball, G. E.; Andersen, R. A.; Bergman,
R. G. J. Am. Chem. Soc. 1998, 120, 21649−21662.
(12) Deydier, E.; Menu, M.-J.; Dartiguenave, M.; Dartiguenave, Y.;
Simard, M.; Beauchamp, A. L.; Brewer, J. C.; Gray, H. B. Organometallics
1996, 15, 1166−1175.
(13) (a) de Klerk-Engels, B.; Delis, J. G. P.; Vrieze, K.; Goubitz, K.;
Fraanje, J. Organometallics 1994, 13, 3269−3278. (b) Koelle, U.; Kang,
B.-S.; Spaniol, T. P.; Englert, U. Organometallics 1992, 11, 249−254.
3160
dx.doi.org/10.1021/om4004016 | Organometallics 2013, 32, 3157−3160