Bidentate cycloimidate palladium complexes with aliphatic and aromatic anagostic bonds

Article abstract of DOI:10.1039/c4cc01060d

Palladium(ii) complexes of bidentate cycloimidate ligand systems with a triarylmethyl moiety exhibit exceptional downfield shifts in proton NMR spectra due to rare anagostic interactions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01060d

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Bidentate cycloimidate palladium complexes with
aliphatic and aromatic anagostic bonds†
Cite this: Chem. Commun., 2014,
50, 5909
Stephan Scholer,ab Maike H. Wahl,ab Nicole I. C. Wurster,ab Arik Puls,ab
¨
Christof Hattigb and Gerald Dyker*ab
¨
Received 10th February 2014,
Accepted 6th April 2014
DOI: 10.1039/c4cc01060d
www.rsc.org/chemcomm
Palladium(II) complexes of bidentate cycloimidate ligand systems Only a few examples of iminoisobenzofurans as ligands are
with a triarylmethyl moiety exhibit exceptional downfield shifts in known.18,19
proton NMR spectra due to rare anagostic interactions.
Numerous methods are known for the synthesis of iminoiso-
benzofurans from readily accessible substrates: (1-alkynyl)-
The agostic interaction – a coordinatively-unsaturated transition benzamides can be cyclized to imidates either by synergetic
metal coordinating to a C–H group by a three-center two-electron Pd/Cu catalysis20 or by electrophilic cyclization.21,22 The
bond1 – is the topic of numerous investigations and generally fluoride-induced three component coupling of an isocyanate,
regarded to be mechanistically important for the key step in a carbonyl compound and a TMS-substituted phenyl triflate
CH-activation processes.2,3 The anagostic interaction, a C–H- was performed; an aryne is the reactive intermediate in this
transition metal arrangement with a bridging hydrogen, is far reaction.23,24
less established, nevertheless also demanding mechanistic
Our attempts to synthesize ortho-oxazolinyl-functionalized
interest as a prestage to the agostic binding situation. Both, triarylmethyl alcohols 2 from 2-bromophenyloxazolines16,25
1
agostic4 and anagostic5–9 complexes have been isolated, regularly following established procedures26 more or less surprisingly
being test objects for calculations in comparison with spectro- resulted in the isolation of the benzannulated imidoyl lactones
scopic and structural data.10–12 While for agostic complexes an 3 in excellent yields (Fig. 1).
upfield shift of the coordinated C–H-group in the proton NMR and
Compound 3 should be thermodynamically favored due to
a clearly diminished C–H coupling constant are observed,13 in diminished sterical crowdedness of its triarylmethyl moiety as
anagostic complexes the 1H NMR signal of the bridging hydrogen part of the annulated 5-membered ring system, as published
is significantly shifted downfield,10,14 regularly by about 2 to earlier by Mayers and Dordor.27,28 This argument is supported
3 ppm.5–9 Herein we report on iminoisobenzofuran palladium
chelate complexes with the rare situation of exhibiting anagostic
interactions both with aliphatic and aromatic CH-groups, resulting
in downfield shifts of up to 4.1 ppm.
Iminoisobenzofurans are important for a variety of applications
either as functionalized p-systems or as synthetic building
blocks for isobenzofurans and isoindolinones; e.g. the separa-
tion of chiral phthalides via their diastereomeric imidates,15
the synthesis of spirocyclic isobenzofurans16 and for connecting
binding components of a supramolecular ligand system.17
a
¨
Ruhr Universitat Bochum, 44801 Faculty of Chemistry, Organic Chemistry II,
¨
Universitatsstr. 150, 44801 Bochum, Germany. E-mail: gerald.dyker@rub.de
b
¨
¨
Lehrstuhl fur Theoretische Chemie, Ruhr Universitat Bochum, D-44801, Bochum,
Germany
† Electronic supplementary information (ESI) available: Experimental procedures,
full characterization including NMR, MS, IR, EA and data for all new compounds
and X-ray crystal structures of 5a,c–d, 6. CCDC 977820–977822 and 993306. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc01060d
Fig. 1 Synthesis of 5a–d (a) n-Buli, THF, ꢀ89 1C, 40 min; (b) 4,40-dimethoxy-
benzophenone (c) NaH, THF (d) 2-fluoropyridine; and (e) (MeCN)2PdCl2,
CH2Cl2, 1 h, RT.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 5909--5911 | 5909

View Article Online
Communication
ChemComm
Table 1 Chemical shifts (ppm) of the protons under the anisotropic
influence of the palladium in complexes 5a–d
by the results of Noel et al.: primary oxazoline alcohols were
obtained by reaction of sterically less crowded cyclic imidates
with aminols.29 Our spectroscopic proof of this finding is not
only based on the chemical shift of the CH2 group in the proton
NMR as described previously,28 but also on two-dimensional
R1
dH3a dH3
D(dH)H3 dH24ba dH24b D(dH)H24b
iPr
iBu
7.89
7.89
11.04 3.15
4.30
4.35
4.40
4.45
7.31
7.18
7.62
8.55
3.01
2.83
3.22
4.10
10.96 3.07
10.92 3.04
11.17 3.35
1H, 13C-HMBC spectra: 3c for instance exhibits only a single CH2Ph 7.88
Me
7.82
cross signal for the imine carbon (d = 159.4 ppm) and the
aliphatic proton at the chiral center (d = 4.37 ppm). A correlation
with the neighboring methylene group (d = 3.67 ppm) is clearly
missing, which on the other hand should be additionally registered
for structures of type 2. The sodium alcoholates of 3 undergo
a subsequent nucleophilic substitution with 2-fluoropyridine,
giving access to the bidentate ligands 4, which selectively form
Pd(II)-complexes with (MeCN)2PdCl2 in CH2Cl2 at room temperature,
while failing to complex platinum salts under various conditions.
Furthermore imidate 3a was also shown to be a suitable bidentate
ligand for Pd(II) salts, with the hydroxyl group as the coordination
site, resulting in the formation of complex 6. Single crystals were
obtained as orange needles by slow diffusion of n-hexane to
dichloromethane solutions of 5 and 6.
a
Signal of the free ligand.
Upon complexation with PdCl2 a significant change in the
proton NMR spectra was observed, since several signals were
significantly shifted downfield (Fig. 2): the signal of the proton
ortho to the pyridine nitrogen was shifted about DdH = 0.8 ppm
from d = 8.14 to d = 8.96 ppm, respectively. The signal of the
peri-proton (H3) was registered at 11.17 ppm, resembling an
astonishing downfield shift of overall 3.3 ppm. Furthermore
the signal of the methylene group H24a/b splits up into two
separate doublets. One signal was found at 8.55, the other at
4.25 ppm. Clearly, these downfield shifts are mainly caused by a
through space interaction of the protons with the palladium
Fig. 3 Thermal ellipsoid (50% probability) plot of palladium complex 5d
with a sketched plane at the palladium atom and distances d1, d2 and
angles c, y to the shifted hydrogen atoms H3 and H24b. Other protons and
the p-anisyl substituents are omitted for clarity.
ion. All complexes 5a–d exhibit these extraordinary shifts of the and H24b are then located in an approximate axial position
selected NMR signals (Table 1). above and underneath the quadratic plane which is formed by
The analysis of the solid state structures of complexes 5a,c–d, the metal atom and the ligands (Fig. 3). The distances d1 and d2
obtained from X-ray crystallography, confirms that the relevant between the palladium atom and the hydrogen atoms H3 and
geometric orientation within the compounds is identical. There- H24b as well as the angle c (Table 2) from the Pd atom to the
fore a general explanation of the proton shifts can be given: upon C–H bonds are in good agreement with the definition of an
chelatization of the palladium by the two nitrogen atoms of the anagostic interaction (d(M–H) = 2.3–2.9 Å, M–H–C = 110–1701
bidentate ligand, a 7-membered ring is formed. The protons H3 (here c)).14 The protons with shorter distances d1 and d2 to the
palladium atom should be more deshielded (Tables 1 and 2)
than those with longer distances. The dimethyl substituted
complex 5d has the shortest contacts to the metal (d1 = 2.581 Å,
d2 = 2.299 Å) and therefore indeed exhibits the most significant
downfield shifts for the protons H3 and H24b (DdH = 3.35 and
4.10 ppm). Whereas the benzyl-substituted complex 5c exhibits
the longest bond for d1 = 2.723 and the second longest for d2 =
2.447 Å. Analogously the aromatic proton H3 experiences the
smallest downfield shift of DdH = 3.04 ppm, but surprisingly only
an average one for H24b, although it shows a slightly shorter bond
for d2 (2.447 Å) than complex 5a. Clearly, other factors have to be
taken into account, such as the angle y which might function as a
control parameter for the anisotropic shift as suggested by Scherer
et al.,11 focussing on upfield shifts caused by agostic interactions.
In analogy, for anagostic interactions the anisotropic downfield
Fig. 2 Partial 1H NMR spectra (400 MHz, CDCl3) of (a) the free ligand 4d
(R1 = R2 = CH3) and (b) its complex with PdCl2 5d.
shift should be maximized with a perfect perpendicular angle
of 901 for y. The comparison of crystallographic data and NMR
5910 | Chem. Commun., 2014, 50, 5909--5911
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Table 2 Contact length and angles from the palladium atom to the shifted protons in complexes 5a,c–d and 6
R1
d1
c1
y1
d2
c2
y2
iPr 5a
2.635
2.723
2.581
2.584; 2.586
129.90
123.58
128.83
129.33; 128.75
81.18
78.36
82.55
84.67; 83.97
2.463
2.447
2.299
—
113.12
115.23
119.14
—
69.71
69.67
70.40
—
CH2Ph 5c
Me 5d
iPr 6
c1 = Pd–H3–C3 [1]; c2 = Pd–H24b–C24 [1]; y1 = N1–Pd–H3 [1]; y2 = N1–Pd–H24b [1]; d1, d2 [Å].
6 J. Ruiz, J. Lorenzo, C. Vicente, g. Lopez, J. M. Lopez-de-Luzuriaga,
M. Monge, F. X. Avile’s, D. Bautista, V. Moreno and A. Laguna, Inorg.
Chem., 2008, 47, 6990–7001.
7 C. Taubmann, K. Ofele, E. Herdtweck and W. A. Herrmann,
Organometallics, 2009, 28, 4254–4257.
8 M. S. Khalaf, S. H. Oakley, M. P. Coles and P. B. Hitchcock, Dalton
Trans., 2010, 39, 1635–1642.
9 H. V. Huynh, Y. Han, J. Hui Hui Ho and G. K. Tan, Organometallics,
2006, 25, 3267–3274.
10 Y. Zhang, J. C. Lewis, R. G. Bergman, J. A. Ellman and E. Oldfield,
Organometallics, 2006, 25, 3515–3519.
11 J. E. Barquera-Lozada, A. Obenhuber, C. Hauf and W. Scherer,
J. Phys. Chem. A, 2013, 117, 4304–4315.
12 J. C. Lewis, J. Wu, R. G. Bergman and J. A. Ellman, Organometallics,
2005, 24, 5737–5746.
13 M. Brookhart and M. L. H. Green, J. Organomet. Chem., 1983, 250,
395–408.
14 M. Brookhart, M. L. Green and G. Parkin, Proc. Natl. Acad. Sci.
U. S. A., 2007, 104, 6908–6914.
15 W. H. Pirkle and T. J. Sowin, J. Org. Chem., 1987, 52, 3011–3017.
16 A. Marxer, H. R. Rodriguez, J. M. McKenna and H. M. Tsai, J. Org.
Chem., 1975, 40, 1427–1433.
spectra of complexes 5d and 6 underlines this assumption
(Table 2). The proton NMR of complex 6 registers a 2 : 1 equilibrium
of two species in solution, both with an astonishing downfield
shift of the aryl proton H3 of more than 4.1 ppm (d = 12.06 and
12.03 ppm), interpreted as equilibrium between the chelated
complex 6 and its dimeric isomer. The latter was ascertained as
crystal structure, exhibiting an angle y of about 841, thus closer to
the perpendicular angle than in the case of 5d, but with the
distance d essentially remaining the same.
Also the results of a QTAIM analysis carried out with the
electron density of 5d from a DFT/BP86/def2-TZVP calculation
corroborate these results. The density exhibits bond paths with
bond critical points that connect Pd with H3 and H24b. At both
bond critical points the charge density is, however, only small
(1.9 ꢁ 10ꢀ2 au for Pd–H3 and 2.7 ꢁ 10ꢀ2 au for Pd–H24b) and
has a positive Laplacian (5 ꢁ 10ꢀ2 for Pd–H3 and 8 ꢁ 10ꢀ2 for
Pd–H24b). These characteristics fit well to anagostic interactions
between Pd and H3 and H24b and substantiate that in 5d the
interaction of Pd with H24b is slightly stronger than with H3, in
accordance with the experimental findings and the model
developed by Scherer et al.11
¨
17 L. Pignataro, S. Carboni, M. Civera, R. Colombo, U. Piarulli and
C. Gennari, Angew. Chem., Int. Ed., 2010, 49, 6633–6637.
¨
18 K. Bert, T. Noel, W. Kimpe, J. L. Goeman and J. Van der Eycken, Org.
Biomol. Chem., 2012, 10, 8539–8550.
19 T. Noel, K. Vandyck, K. Robeyns, L. Van Meervelt and J. Van der
Eycken, Tetrahedron, 2009, 65, 8879–8884.
20 B. Yao, C. Jaccoud, Q. Wang and J. Zhu, Chem. – Eur. J., 2012, 18,
5864–5868.
21 S. Mehta, T. Yao and R. C. Larock, J. Org. Chem., 2012, 77,
10938–10944.
22 K. Kobayashi, S. Fujita, D. Nakai, S. Fukumoto, S. Fukamachi and
H. Konishi, Helv. Chim. Acta, 2010, 93, 1274–1280.
23 K. M. Allan, C. D. Gilmore and B. M. Stoltz, Angew. Chem., Int. Ed.,
2011, 50, 4488–4491.
In summary, we have developed a bidentate ligand which
exhibits rare anagostic interactions of aromatic and aliphatic
protons upon complexation with a palladium salt. Astonishing
downfield shifts of aliphatic and aromatic protons were correlated
with geometric factors of the corresponding X-ray structures.
24 (a) H. Yoshida, H. Fukushima, T. Morishita, J. Ohshita and A. Kunai,
Tetrahedron, 2007, 63, 4793–4805; (b) H. Yoshida, H. Fukushima,
J. Ohshita and A. Kunai, Angew. Chem., Int. Ed., 2004, 43, 3935–3938.
Notes and references
1 W. Yao, O. Eisenstein and R. H. Crabtree, Inorg. Chim. Acta, 1997, 25 K. Tani, D. C. Behenna, R. M. McFadden and B. M. Stoltz, Org. Lett.,
254, 105–111. 2007, 9, 2529–2531.
2 J. L. Haller, M. J. Page, S. A. Macgregor, M. F. Mahon and 26 C. Bolm, K. Muniz-Fernandez, A. Seger, G. Raabe and K. Gu¨nther,
¨
M. K. Whittlesey, J. Am. Chem. Soc., 2009, 131, 4604–4605.
3 A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879–2932.
J. Org. Chem., 1998, 63, 7860–7867.
27 A. I. Mayer and M. A. Hanagan, Tetrahedron, 1983, 39, 1991–1999.
4 W. Scherer, V. Herz, A. Bru¨ck, C. Hauf, F. Reiner, S. Altmannshofer, 28 I. M. Dordor, J. M. Mellor and P. D. Kennewell, J. Chem. Soc., Perkin
D. Leusser and D. Stalke, Angew. Chem., Int. Ed., 2011, 50, 2845–2849. Trans. 1, 1984, 6, 1253–1258.
5 R. Angamuthu, L. L. Gelauff, M. A. Siegler, A. L. Spek and 29 T. Noel, K. Robeyns, L. Van Meervelt, E. Van der Eycken and
E. Bouwman, Chem. Commun., 2009, 2700–2702.
J. Van der Eycken, Tetrahedron: Asymmetry, 2009, 20, 1962–1968.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 5909--5911 | 5911