Cyclopropanation of Ru-diimino-pyridine ligand complexes

Article abstract of DOI:10.1039/c0dt00437e

Reaction of 2,6-bis(imino)pyridines with (Cy₃P) ₂RuCl₂(CHPh) affords the complexes C₆H ₃N(CMeNC₆H₄R)(CCH₂CHPh) NHC ₆H₅)RuCl₂(PCy₃

Full text of DOI:10.1039/c0dt00437e

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www.rsc.org/dalton | Dalton Transactions
Cyclopropanation of Ru-diimino-pyridine ligand complexes†
Christopher C. Brown and Douglas W. Stephan*
Received 6th May 2010, Accepted 20th June 2010
First published as an Advance Article on the web 1st July 2010
DOI: 10.1039/c0dt00437e
Reaction
of
2,6-bis(imino)pyridines
with
(Cy3P)2-
In a similar fashion the corresponding reaction of (Cy3P)2-
RuCl2(CHPh) with the ligand C5H3(C(Me NC6H4iPr)2)N gave
a similar product in 62% isolated yield which exhibited a 31P
resonance at 13.22 ppm and 1H NMR signals at 2.51 and 1.96 ppm
for the methine and methylene resonances. The spectral data for
1 and 2 suggests that the alkylidene-group has transferred to the
ligand generating dissymmetry (Scheme 1).
RuCl2(CHPh) affords the complexes C6H3N(CMeNC6H4R)-
(CCH2CHPh) NHC6H5)RuCl2(PCy3) (R = H 1, iPr 2) arising
from the cyclopropanation of the ligand.
Since the breakthrough discoveries in the Brookhart1 and Gibson2
laboratories in the 1990s, late transition metal complexes bearing
2,6-bis(imino)pyridyl ligands [N3] have garnered much attention.3
While much of the focus has been on the development of
olefin polymerization catalysts, the chemistry of complexes of
these ligands continues to be explored.4 For example, while the
complex [N3]RuCl2(NCMe) was shown to exhibit good activity
for the epoxidation of cyclohexene with iodosobenzene.5 The
related complex [N3]RuCl(NCMe)3(SbF6)2 has been shown to be
inactive as a catalyst for the oxidation of cyclohexane by H2O2.6
More recently, Chirik et al. have employed [N3]Fe complexes
for catalytic hydrogenation, hydrosilylation, and cycloaddition
reactions.7 Bianchini and Lee8 have described the use of [N3]Ru
complexes in the catalytic cyclopropanation of styrene with
ethyl diazoacetate. In these cases, the six-coordinate Ru(II) 2,6-
bis(imino)pyridyl carbene complexes are thought to effect carbene
transfer to the olefin via an intermolecular process, although the
authors cautioned that this may not be true in the presence of
halide scavengers. Our interest in [N3] complexes focused on
the reactivity of [N3]Ru alkylidenes. In this report we describe
the reaction of [N3] ligands with the Grubbs alkylidene species
(Cy3P)2RuCl2(CHPh).9 Herein, we demonstrate that [N3] is not
an innocent ancillary ligand in these reactions but rather is
cyclopropanated to give a dissymmetric amino-imino-pyridyl
ligand.
In an effort to generate Ru-alkylidene complexes bearing
bisiminopyridine ligands, reactions of (Cy3P)2RuCl2(CHPh) with
the ligand C5H3(C(Me NPh)2)N were undertaken. Combining
these species in a 1 : 1 ratio in benzene, the reaction was heated to
80 ◦C and monitored by 31P NMR spectroscopy over a 36 h period.
The 31P NMR spectral data for the reaction mixture revealed the
liberation of Cy3P and the generation of a new singlet resonance.
Subsequent workup provided the new Ru containing product 1
in 71% isolated yield.‡ Spectroscopic examination of compound
1 confirmed the presence of a 31P resonance at 12.81 ppm. The
corresponding 1H spectrum inferred dissymmetry in the ligand. A
resonance at 1.89 ppm accounts for one ligand methyl backbone
group. Signals at 2.50 and 1.41 ppm correspond to methine and
methylene groups. Aromatic and pyridine resonances account for
the ligand and alkylidene phenyl protons.
Scheme 1 Synthesis of 1 and 2.
The precise nature of these products was confirmed via crys-
tallographic study (Fig. 1). Crystallographic data for 1 con-
firmed the connectivity as C6H3N(CMeNC6H4R)(CCH2CHPh)
NHC6H5)RuCl2(PCy3). The Ru center adopts a distorted oc-
tahedral geometry in which the two chlorides are disposed
in trans-positions with a Cl–Ru–Cl angle of 169.97(6)◦. The
Department of Chemistry, 80 St. George St., University of Toronto, Toronto,
Ontario, Canada M5S 3H6. E-mail: dstephan@chem.utoronto.ca
† CCDC reference number 780215. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt00437e
Fig. 1 POV-ray drawing of 1, all H-atoms except the NH are omitted for
clarity C: black, Cl: green, P: orange, N: blue–green, Ru red–orange.
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˚
˚
Ru–Cl distances were found to be 2.4099(18) A and 2.4145(17) A.
The tridentate ligand is meridonally coordinated to Ru in the
plane perpendicular to the Cl–Ru–Cl vector, with the pyridine
N trans to a coordinated PCy3. The corresponding Ru–N and
Acknowledgements
Financial support from NSERC of Canada is gratefully acknowl-
edged. DWS is grateful for the award of a Killam Research
Fellowship and a Canada Research Chair.
˚
˚
Ru–P distances are 2.001(5) A and 2.4480(17) A, respectively. The
tridentate ligand is dissymmetric as one imino-fragment is reduced
to an amine with concurrent formation of a cyclopropane ring,
derived from the alkylidene fragment and the carbons alpha- and
beta- to N. The resulting Ru–N distances for the imino and amino-
Notes and references
‡
2 Synthesis of (C6H3N(CMe NC6H4R)(C(CH2CHPh)NHC6H4R)RuCl2
(PCy3) (R = H 1, iPr 2) These compounds were prepared in a similar
fashion and thus only one preparation is detailed. (Cy3P)2RuCl2(CHPh)
(164 mg, 0.20 mmol) was dissolved in benzene (5 mL) before a benzene
solution (5mL) of 2,6-bis(1-phenyliminoethyl)pyridine (65 mg, 0.20 mmol)
was added. The reaction was sealed in a Teflon capped reaction tube and
heated at 80 ◦C for 36 h. The reaction colour changes from purple to deep
green. The reaction was cooled to 25 ◦C and the benzene was removed in
vacuo. The solid was dissolved in a minimum amount of dichloromethane
(2–3 mL) to which pentane (15 mL) was added. The solution was cooled
to -35 ◦C overnight after which large green crystals formed. Subsequent
removal of the supernatant, and recrystallization of the remaining crude
product yielded more green crystals. (71%, 0.14 mmol, 121 mg) 1: 1H NMR
(CD2Cl2): 8.12 (d, 1H, Ph), 7.75 (d, 1H, Ph), 7.53 (d, 2H, Ph), 7.12–6.7 (m,
11H, Ph), 6.07 (d, 1H, Ph), 5.88 (d, 1H, Ph), 5.67 (s, 1H, Ph), 3.16-2.86
(m, 3H, Cy), 2.50 (t, 1H, Ph(CH)py), 1.89 (s, 3H, CH3), 1.8–1.5 (m, 14H,
Cy), 1.41 (d, 1H, CHCH2), 1.40 (d, 1H, CHCH2), 1.4–1.0 (m, 13H, Cy),
˚
˚
N atoms are 2.056(5) A and 2.239(5) A. This difference reflects the
lower basicity of the amino-N in comparison to the imine N. The
˚
◦
C–C bond distances within the cyclopropane ring are 1◦.515(9) A,
˚
˚
1.539(9) A and 1.485(9) A,with C–C–C angles of 58.2(4) , 60.1(4)
and 61.7(4)◦, reflecting the ring strain. An analogous structure was
confirmed by preliminary crystallographic study for 2, although
these data were not suitable for publication.
The mechanism of formation of 1 and 2 is the subject of
speculation. We note that Bianchini and coworkers have pre-
pared di-imino-pyridine-alkylidene Ru complexes of the form
[N3]RuCl2(CHCO2Et) and thus it is reasonable to suggest that
an analogous species could be formed initially upon reaction
of the ligand with the Ru-alkylidene precursor. However, it
is also noteworthy that tautomerization of such ligands to
an exocyclic enamine has been previously observed in several
systems.10 Transient generation of the enamine combined with
the more reactive alkylidene fragment results in cyclopropanation
accounting for the formation of 1 and 2. While the formation
of the cyclopropane ring generates two chiral carbon centers and
thus a mixture of diastereomeric products, it is difficult to envision
an intramolecular process. Thus, these reactions are thought to
occur in a bimolecular fashion although this aspect could not be
unambiguously confirmed. It is also reasonable to suggest that the
lesser steric congestion favors cyclopropanation of the enamine
over olefin metathesis.
To put this finding in context, we note that cyclopropana-
tion of olefins using ethyl diazoacetate has been previously
described using a variety of Ru precursors.8,11 In addition,
Dixneuf and coworkers12 demonstrated catalytic formation of
alkenylbicyclo[3.1.0]-hexanes from enynes and N2CHCO2Et or
N2CHPh using Cp*RuCl(COD) as the catalyst precursor. Suitably
modified ligand set variants have been employed to effect catalytic
asymmetric cyclopropanation.13 Despite these previous develop-
ments, to our knowledge, cyclopropanation has not been reported
using Grubbs-type alkylidene complexes and thus compounds
1 and 2 are the first examples that result from transfer of Ru-
bound alkylidene to transient enamines. The stoichiometric nature
of the present reactions may result from the “non-innocent”
nature of the [N3]Ru complexes. The participation of the enamine
tautomer in chemistry has been previously reported by Blackmore
et al.10 More recently, Berry and coworkers showed that low-
valent [N3]Ru complexes delocalize electron density from the
metal center further demonstrating the “non-innocent” electronic
nature of these ligands.14
1
0.8–0.7 (m, 3H, Cy) 31P{ H NMR (CD2Cl2) : 12.81 13C NMR (CD2Cl2)
(partial): 131.7 (Ph), 131.4 (Ph), 125 (Ph), 124.3 (Ph), 123 (Ph), 122.6
(Ph), 115.3 (Ph), 41.7 Ph(CH)py), 22.8 (CHCH2). C,H,N analysis calc. for
C46H58Cl2N3PRu C, 64.55; H, 6.83; N, 4.91. Found: C, 64.54; H, 7.04; N,
4.76. 2: (62%, 0.08 mmol, 116 mg) 1H NMR (CD2Cl2) : 8.13 (d, 1H, Ph),
7.74 (d, 1H, Ph), 7.60 (d, 2H, Ph), 7.25–6.86 (m, 8H, Ph), 6.73 (d, 1H, Ph),
6.08 (d, 1H, Ph), 5.85 (d, 1H, Ph), 5.69(d, 1H, Ph), 3.25–2.9 (m, 3H, Cy),
2.66 (m, 2H, CH(CH3)2), 2.51(t, 1H, Ph(CH)py), 2.4–2.1 (m, 3H, Cy),
1.96 (s, 3H, CH3), 1.9–1.5 (m, 18H, Cy), 1.3–1.2 (m, 6H, Cy), 0.9–0.8 (m,
3H, Cy), 1.13 (m, 12H, CH(CH3)2), 1.43 (m, 2H, CHCH2), 31P{ H} NMR
1
(CD2Cl2) : 13.22 13C NMR (CD2Cl2) (partial) : 131.8 (Ph), 131.2 (Ph), 123
(m, Ph), 115.6 (Ph), 41.9 Ph(CH)py), 22.4 (CHCH2), 18.6 (CH3). C,H,N
analysis calc. for C52H69Cl2N3PRu C, 66.44; H, 7.51; N, 4.47. Found: C,
66.54; H, 7.79; N, 4.60. Crystallographic d◦ata for 1: C46H57Cl2N3PRu(2
¯
CH2Cl2)(0.5 H20), MW = 1034.75, T = -100 C, space group triclinic, P1,
◦
˚
˚
˚
a = 12.5917(8)◦A, b = 12.8624(7) A, c = 15.5806(10) A, a = 91.817(3) ,
3
-1
˚
b = 103.746(4) , g = 93.184(3) V = 2444.7(3) A , Z = 2, m = 0.719 mm ,
measured reflections = 42719, independent reflections = 6702, parameters =
865, Rint = 0.0914, R = 0.0552, Rw = 0.1445, GOF = 1.012.
1 B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem. Soc.,
1998, 120, 4049–4050.
2 G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberley, P. J. Maddox,
S. Mastroianni, S. J. McTavish, C. Redshaw, G. A. Solan, S. Stromberg,
A. J. P. White and D. J. Williams, J. Am. Chem. Soc., 1999, 121, 8728–
8740.
3 V. C. Gibson, C. Redshaw and G. A. Solan, Chem. Rev., 2007, 107,
1745–1776.
4 (a) H. Yoo, P. J. Carroll and D. H. Berry, J. Am. Chem. Soc., 2006, 128,
6038–6039; (b) J. Scott, I. Vidyaratne, I. Korobkov, S. Gambarotta and
P. H. M. Budzelaar, Inorg. Chem., 2008, 47, 896–911.
5 B. Cetinkaya, E. C. etinkaya, M. Brookhart and P. S. White, J. Mol.
Catal. A: Chem., 1999, 142, 101–112.
6 G. J. P. Britovsek, J. England, S. K. Spitzmesser, A. J. P. White and D. J.
Williams, Dalton Trans., 2005, 945–955.
7 A. M. Archer, M. W. Bouwkamp, M.-P. Cortez, E. Lobkovsky and P. J.
Chirik, Organometallics, 2006, 25, 4269–4278.
8 C. Bianchini and H. M. Lee, Organometallics, 2000, 19, 1833–1840.
9 P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 2039–2041.
10 I. J. Blackmore, V. C. Gibson, P. B. Hitchcock, C. W. Rees, D. J. Williams
and A. J. P. White, J. Am. Chem. Soc., 2005, 127, 6012–6020.
11 (a) V. Dragutan, I. Dragutan, L. Delaude and A. Demonceau, Coord.
Chem. Rev., 2007, 251, 765–794; (b) J. A. Miller, W. Jin and S. T. Nguyen,
Angew. Chem., Int. Ed., 2002, 41, 2953–2956.
12 F. Monnier, V.-L. Bray Chloe, D. Castillo, V. Aubert, S. Derien, H.
Dixneuf Pierre, L. Toupet, A. Ienco and C. Mealli, J. Am. Chem. Soc.,
2007, 129, 6037–6049.
The present results suggest that strong donor ligand sets in
an alkylidene complex encourage reactivity. The present products
are unique examples of the cyclopropanation of the metal bound
enamine fragment. This finding prompts us to probe the reactivity
of other Ru-tridentate alkylidenes. The results of these on-going
studies will be reported in due course.
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13 (a) S. Bachmann, M. Furler and A. Mezzetti, Organometallics, 2001,
20, 2102–2108; (b) C. Bonaccorsi and A. Mezzetti, Organometallics,
2005, 24, 4953–4960; (c) C. Bonaccorsi, F. Santoro, S. Gischig and A.
Mezzetti, Organometallics, 2006, 25, 2002–2010; (d) M. Frauenkron
and A. Berkessel, Tetrahedron Lett., 1997, 38, 7175–7176; (e) M. Z.
Gao, D. Kong, A. Clearfield and R. A. Zingaro, Tetrahedron Lett.,
2004, 45, 5649–5652; (f) P. Le Maux, I. Abrunhosa, M. Berchel, G.
Simonneaux, M. Gulea and S. Masson, Tetrahedron: Asymmetry, 2004,
15, 2569–2573; (g) S.-W. Park, J.-H. Son, S.-G. Kim and K. H. Ahn,
Tetrahedron: Asymmetry, 1999, 10, 1903–1911; (h) B. Saha, T. Uchida
and T. Katsuki, Synlett, 2001, 1, 114–116; (i) M. J. Williams, H. L.
Deak and M. L. Snapper, J. Am. Chem. Soc., 2007, 129, 486–487; (j) Z.
Zheng, X. Yao, C. Li, H. Chen and X. Hu, Tetrahedron Lett., 2001, 42,
2847–2849.
14 M. Gallagher, N. L. Wieder, V. K. Dioumaev, P. J. Carroll and D. H.
Berry, Organometallics, 2010, 29, 591–603.
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