Reactivity of ortho-palladated phenol derivatives with unsaturated molecules. Carbodiimide insertion into a C-Pd bond and/or O-H phenol addition to a carbodiimide

Article abstract of DOI:10.1021/om0601652

The first examples of the insertion of carbodiimides into a late-transition-metal-carbon bond and the unprecedented intramolecular addition of a phenol to a carbodiimide have allowed the synthesis of O,N and O,C six-membered pallada-cycles.

Full text of DOI:10.1021/om0601652

Organometallics 2006, 25, 1851-1853
1851
Communications
Reactivity of Ortho-Palladated Phenol Derivatives with Unsaturated
Molecules. Carbodiimide Insertion into a C-Pd Bond and/or O-H
Phenol Addition to a Carbodiimide
Jose´ Vicente,* Jose´ Antonio Abad, and Mar´ıa-Jose´ Lo´pez-Sa´ez
Grupo de Qu´ımica Organometa´lica, Departamento de Qu´ımica Inorga´nica, Facultad de Qu´ımica,
UniVersidad de Murcia, Aptdo. 4021, E-30071 Murcia, Spain
Peter G. Jones†
Institut fu¨r Anorganische und Analytische Chemie der Technischen UniVersita¨t, Postfach 3329,
38023 Braunschweig, Germany
ReceiVed February 21, 2006
Summary: The first examples of the insertion of carbodiimides
into a late-transition-metal-carbon bond and the unprecedented
intramolecular addition of a phenol to a carbodiimide haVe
allowed the synthesis of O,N and O,C six-membered pallada-
cycles.
have found industrial uses.8 Other complexes have also been
reacted with carbodiimides, giving different products after
insertion of one of the CdN bonds into an M-C (M ) metal)9
or an M-H bond.10 The metals involved in the insertion of
carbodiimides into an M-C (M ) metal) bond are main group,
lanthanide, and early transition elements. Although carbodiimide
insertion into the M-C bond of a late-transition-metal complex
has been proposed as a mechanistic step, unambiguous evidence
for such reactions has up until now not been available.11 Herein
we report for the first time two types of carbodiimine reactivity
toward a metal complex. The first involves insertion of one of
the CdN groups into the M-C bond and protonation of the
other N. This process is related to one that we have just reported
between a related palladium complex and nitriles,12 except that
protonation in the present case occurs at the uncoordinated
nitrogen and that an additional different reaction also occurs.
This involves addition of an OH to one of the CdN groups
and coordination of the other N to the metal. This process is
similar to that in the reaction of [PdX2(κ1N-RNdCdNR′)2] (X
) Cl, Br, R ) tBu, R′ ) tBu, Me) with MeOH, giving [PdX2-
(RNH-C(OMe)dNR′)2], although this is an intermolecular
process and the mode of bonding of the isourea ligand was not
unambiguously established.13
Carbodiimides are important intermediates in industrial-scale
syntheses. The incorporation of carbodiimides into polymeric
materials accounts for the bulk of their world production. Other
applications include the synthesis of nucleotides and peptides,
heterocycle synthesis, biological modifications, and cycloaddi-
tion reactions.1 The reaction products of metal complexes and
carbodiimides are also of great interest. Thus, carbodiimides
react with alkyl (mainly Me) or amido complexes, giving the
corresponding amidininato2-4 or guanidinato derivatives,4-6
respectively; some of them have been used as catalysts2,5,7 or
* To whom correspondence should be addressed. E-mail: jvs1@um.es.
WWW: http://www.um.es/gqo/.
† E-mail: p.jones@tu-bs.de.
(1) Muthyala, R. Functions with at Least One Nitrogen and No
Chalcogens. In Synthesis: Carbon with Two Attached Heteroatoms with
at Least One Carbon-to-Heteroatom Multiple Link; Moody, C. J., Ed.,
Elsevier Science: Oxford, U.K., 1995; Vol. 5, p 1061. Williams, A.;
Ibrahim, I. T. Chem. ReV. 1981, 81, 589.
(2) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G.
Organometallics 1997, 51833.
Equimolecular amounts of [Pd(C6H4OH-2)I(bpy)] (bpy )
2,2′-bipyridine),14 ToNdCdNTo (To ) p-tolyl), and TlTfO
(TfO ) CF3SO3) react to give after 3 h at room temperature
[Pd{κ2O,N-O{C6H4{C(NHTo)dNTo}-2}}(bpy)]OTf (1a; Scheme
1) in 86% yield. When CyNdCdNCy (Cy ) cyclohexyl) was
used, a mixture of the corresponding insertion, [Pd{κ2O,N-
(3) (a) Klose, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.;
Re, N. J. Am. Chem. Soc. 1994, 116, 9123. (b) Coles, M. P.; Swenson, D.
C.; Jordan, R. F.; Young, V. G. Organometallics 1998, 17, 4042. Kissounko,
D. A.; Zhang, Y. H.; Harney, M. B.; Sita, L. R. AdV. Synth. Catal. 2005,
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DiLabio, G. A.; Barry, S. T. Inorg. Chem. 2005, 44, 1983. Srinivas, B.;
Chang, C. C.; Chen, C. H.; Chiang, M. Y.; Chen, I. T.; Wang, Y.; Lee, G.
H. J. Chem. Soc., Dalton Trans. 1997, 957. Zhang, J.; Ruan, R. Y.; Shao,
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253. Terry, M. R.; Mercando, L. A.; Kelley, C.; Geoffroy, G. L.; Nombel,
P.; Lugan, N.; Mathieu, R.; Ostrander, R. L.; Owenswaltermire, B. E.;
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C. J.; Gately, D. A.; Norton, J. R. Organometallics 2001, 20, 254.
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10.1021/om0601652 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/18/2006

1852 Organometallics, Vol. 25, No. 8, 2006
Communications
Scheme 1. Synthesis of Complexes
Scheme 2. Proposed Reaction Pathway for the Synthesis of
Complexes
O{C6H4{C(dNCy)NHCy}-2}}(bpy)]OTf (1b), and addition
products, [Pd{κ2C,N-{C6H4{OC(dNCy)}NHCy-2}}(bpy)]OTf
(2), was obtained. A study of the influence of the reaction
conditions (molar ratio of reagents, reaction time, and temper-
ature) allowed us to isolate pure 1b or 2. The best results were
obtained by refluxing the reaction mixture in 1,2-dichloroethane
or by using a large excess of the carbodiimide (20:1) at room
temperature, respectively.
It can be postulated that the first steps in these processes are
the replacement of iodine by triflate and then of the triflato
ligand by the carbodiimide, which, in agreement with the
structure of [PdCl2{N(tBu)dCdNtBu}2],15 should be N-coor-
dinated (A in Scheme 2). This step seems to be important,
because in the absence of TlTfO, ToNdCdNTo gives a
complex mixture of products and CyNdCdNCy does not react.
In the reaction between the related N,N,N′,N′-tetramethyleth-
ylenediamine (tmeda) complex and various nitriles, the subse-
quent step was the protonation of the coordinated nitrogen atom
of the nitrile.12 In the present case, the protonation also occurs,
but on the more basic uncoordinated nitrogen. The electronic
structure of this intermediate can be formulated using the two
extreme resonance forms B and B′, as proposed for the insertion
of nitriles.12 The partial negative charge at the phenolic oxygen
atom can explain the nucleophilic attack at the carbodiimide
central carbon atom to give the addition product 2. This attack
was not observed in the reaction with nitriles.12 Because
carbodiimides add phenols only at relatively high temperatures,16
the formation of 2 at room temperature proves that it is an
intramolecular and metal-assisted process. On the other hand,
the positively charged metal center and the electron-releasing
nature of the negatively charged oxygen atom favor the location
of a partial negative charge on the ipso carbon atom of the aryl
ligand (form B′). This can explain the formation of the insertion
products through nucleophilic attack at the central carbon atom
of the carbodiimide by the ipso carbon to give the four-
membered metallacycle C. Cleavage of the C-Pd bond and
rotation of the aryl ligand around the newly formed C-C bond
allows the coordination of the oxygen atom to give 1a or 1b.
However, while the 1:1:1 (Pd:RNdCdNR:TlOTf) room-tem-
perature reaction gives only the insertion product for R ) To
(1a), a 1:1 mixture of the insertion (1b) and addition (2) products
was obtained for R ) Cy. The more basic character of CyNd
CdNCy could be responsible for a decrease in the formal charge
at palladium, reducing the contribution of the resonance form
B′. This would disfavor the insertion and allow the addition
process. Consistently, a large excess of this ligand (20:1) that
can lead to a pentacoordinated species, reducing the formal
charge at palladium even further, fully suppresses the insertion
at room temperature, giving only the addition product 2.
However, heating the 1:1:1 reaction mixture (refluxing in 1,2-
dichloroethane) leads to an increase in the proportion of the
insertion product (1b:2 ) 3:1), probably because the last step
C f 1b, requiring Pd-C bond cleavage, is favored on heating.
In this reaction, thermal conversion of complex 2 into 1b must
be ruled out, because 2 remains unaltered after refluxing in 1,2-
dichloroethane. The less basic character of nitriles compared
to carbodiimides could also explain why they only insert into
the C-Pd bond of the related tmeda complex.12 A room-
temperature reaction with a large excess (20:1) of ToNdCd
NTo leads to a complex mixture of products. Preliminary results
with other aryl palladium complexes and carbodiimides show
that the above results are general.
The crystal structures of complexes 1a and 2 have been solved
by X-ray diffraction studies (Figures 1 and 2).17 They show a
distorted-square-planar geometry at palladium, with normal bond
distances and angles. For 1a the expected classical N-H‚‚‚O
contact from cation to anion is surprisingly long (H‚‚‚O ) 2.57-
(2) Å) and is accompanied by four C-H‚‚‚O contacts that could
be classified as hydrogen bonds. For 2, the N-H‚‚‚O contact
is of normal length (H‚‚‚O ) 2.22(2) Å) but the anion is slightly
disordered (minor site ca. 11% occupied); there are also several
C-H‚‚‚O contacts (for details, see the Supporting Information).
The structure of 1b was unequivocally determined by
comparing the chemical shifts of the aryl carbon nuclei with
those of 1a and 2. The phenolate aryl group in the insertion
(15) Anderson, R. A.; Einstein, F. W. B. Acta Crystallogr., Sect. B 1978,
34, 271.
(16) Vowinkel, E. Chem. Ber. 1963, 96, 1702.

Communications
Organometallics, Vol. 25, No. 8, 2006 1853
Figure 2. Thermal ellipsoid plot (50% probability) of the cation
of 2.
Our results represent (i) the first isolation of a product of
carbodiimide insertion into a C-M bond, where M is a late
transition element, and (ii) a rare insertion of a carbodiimide
into an aryl-M bond.3a,18 In addition to the insertion product,
the more basic CyNdCdNCy gives the product of OH addition
to the CdN bond, N being the noncoordinated nitrogen. Heating
or using a large excess of the ligand favors the insertion or the
addition product, respectively. This is the first report of such a
dual behavior of a cumulene.
Figure 1. Thermal ellipsoid plot (50% probability) of the cation
of 1a.
products 1a and 1b gives resonances (from C(1) to C(6), values
(1 ppm) at 123 (not observed in 1b), 167, 121, 134, 117, and
131 ppm, while 2 shows these resonances at 136, 153, 115,
125, 126, and 134 ppm.
In conclusion, we have reported the insertion of carbodiimides
RNdCdNR (R ) To, Cy) into a Pd-C bond. The process is
assisted by three factors: the protonation of the nonbonded
nitrogen by the ortho hydroxyl group, the positive charge of
the metal center, and the +M effect of the phenolate oxygen.
Acknowledgment. This work was supported by the Minis-
terio de Educacio´n y Ciencia (MEC, Spain) and FEDER (EU)
by Contract No. CTQ2004-05396. M.-J.L.-S. is grateful to the
MEC for a grant, and we gratefully acknowledge Dr. E.
Mart´ınez-Viviente for some NMR studies.
(17) CCDC-287822 (1a) and CCDC-287823 (2) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.ccdc-
.cam.ac.uk/data_request/cif. Data were recorded at -140 °C on a Bruker
SMART 1000 CCD diffractometer in ω and æ scan modes to 2ϑmax ) 60°,
using monochromated Mo KR radiation (λ ) 0.710 73 Å). Absorption
corrections were based on indexed faces (2) or multiple scans (1a). Structures
were refined on F2 using the program SHELXL-97 (G. M. Sheldrick,
University of Go¨ttingen). Hydrogen atoms were refined using rigid methyl
groups or a riding model, except for NH hydrogens, which were refined
freely. The triflate group of 1a is disordered over two positions in the ratio
8:1. Crystal data for 1a: C32H27F3N4O4PdS, monoclinic, a ) 13.8541(11)
Å, b ) 12.0079(11) Å, c ) 18.6470(14) Å, â ) 100.035(4)°, U ) 3054.6
Å3, space group P21/n, Z ) 4, µ(Mo KR) ) 0.74 mm-1, 56 753 reflections
measured, 8924 unique (Rint ) 0.055). Refinement proceeded to Rw(F2) )
0.084 (all data) and R(F) ) 0.034 for 412 parameters; maximum ∆F 1.1 e
Å-3. Crystal data for 2: C30H35F3N4O4PdS, monoclinic, a ) 25.677(2) Å,
b ) 13.7723(11) Å, c ) 20.4054(16) Å, â ) 122.774(4)°, U ) 6067.4 Å3,
space group C2/c, Z ) 8, µ(Mo KR) ) 0.74 mm-1, 57 320 reflections
measured, 8890 unique (Rint ) 0.047). Refinement proceeded to Rw(F2) )
Supporting Information Available: Text giving experimental
details for the preparation of complexes and spectroscopic and
analytical data and listings of all refined and calculated atomic
coordinates, anisotropic thermal parameters, and bond lengths and
angles for complexes 1a and 2; crystallographic data are also given
as CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM0601652
(18) Hill, N. J.; Moore, J. A.; Findlater, M.; Cowley, A. H. Chem.
Commun. 2005, 5462. Lee, J. P.; Pittard, K. A.; DeYonker, N. J.; Cundari,
T. R.; Gunnoe, T. B.; Petersen, J. L. Organometallics 2006, 25, 1500.
0.075 (all data) and R(F) 0.029; maximum ∆F 1.0 e Å-3
.