X-ray, 13C NMR, and DFT studies on a ruthenium(IV) allyl complex. explanation for the observed control of regioselectivity in allylic alkylation chemistry

Article abstract of DOI:10.1021/om049057a

X-ray, ¹³C NMR, and DFT studies on the cationic Ru(IV) allyl complex Ru(Cp*)Cl(CH₃CN)(η³PhCHCHCH ₂), as a PF₆ salt, have revealed a marked asymmetry in the bonding of the allyl ligand, which can be interpreted as arising from differences in π-bonding from the metal center to the two terminal allyl carbons. This asymmetry in the bonding is offered as an explanation for the observed control of regioselectivity in the Rucatalyzed allylic alkylation reaction.

Full text of DOI:10.1021/om049057a

Organometallics 2005, 24, 1809-1812
1809
13
X-ray, C NMR, and DFT Studies on a Ruthenium(IV)
Allyl Complex. Explanation for the Observed Control of
Regioselectivity in Allylic Alkylation Chemistry
Rene´ Hermatschweiler, Ignacio Ferna´ndez, Paul S. Pregosin,* and
Eric J. Watson
Laboratory of Inorganic Chemistry, ETH HCI, Ho¨nggerberg CH-8093 Zu¨rich, Switzerland
Alberto Albinati* and Silvia Rizzato
Department of Structural Chemistry (DCSSI), University of Milan, 20133 Milan, Italy
Luis F. Veiros*
Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais 1,
1049-001 Lisbon, Portugal
Maria Jose´ Calhorda
ITQB, Av. da Repu´blica, EAN, Apart. 127, 2781-901 Oeiras, Portugal, and
Departamento de Quı´mica e Bioqu´ımica, Faculdade de Cieˆncias, Universidade de Lisboa,
1749-016 Lisbon, Portugal
Received December 1, 2004
Summary: X-ray, 13C NMR, and DFT studies on the
cationic Ru(IV) allyl complex Ru(Cp*)Cl(CH3CN)(η3-
PhCHCHCH2), as a PF6 salt, have revealed a marked
asymmetry in the bonding of the allyl ligand, which can
be interpreted as arising from differences in π-bonding
from the metal center to the two terminal allyl carbons.
This asymmetry in the bonding is offered as an explana-
tion for the observed control of regioselectivity in the Ru-
catalyzed allylic alkylation reaction.
the presumed mechanistic steps.1 It is not immediately
clear why the branched isomer is formed, although, in
contrast to related Pd(II) chemistry,4 the product is
clearly not formed under steric control. We offer here
an explanation for this selectivity based on X-ray, NMR,
and computational studies.
Since this allylation reaction is thought to proceed via
a Ru(IV) allyl intermediate,2 we have prepared the allyl
complex 3 in 94% yield by a stoichiometric reaction of
2 (X ) Cl) with 1.5 Complex 3 reacts quantitatively with
The Ru-catalyzed allylic alkylation reaction has at-
tracted significant interest due to its recognized regi-
oselectivity in favor of branched products.1 The most
commonly used catalyst precursor contains a Cp or Cp*
ligand.2,3 Trost and co-workers1 have reported that
[Ru(Cp*)(CH3CN)3](PF6) (1) is an excellent catalyst for
this reaction and, specifically, that reaction of the allyl
substrate PhCHdCHCH2X (2; X ) halogen or carbon-
ate) with either a carbon or nitrogen nucleophile, Nu-,
preferentially affords the branched product PhCH(N)-
CHdCH2 (see eq 1). When the branched starting mate-
morpholine after extraction of chloride with AgPF6 to
afford both the branched and linear isomeric organic
products in a 95:5 ratio.
PhCHdCHCH2X + Nu- [Ru(Cp*)(CH3CN)3](PF6)8
DMF or acetone
PhCH(Nu)CHdCH2 + X- (1)
(4) Trost, B. M.; van Vranken, D. L. Chem. Rev. 1996, 96, 395.
Hayashi, T.; Kawatsura, M.; Uozumi, Y. J. Am. Chem. Soc. 1998, 120,
1681-1687. Pfaltz, A.; Pre´toˆt, R., A. Angew. Chem. 1998, 110, 337-
339 and references therein.
rial PhCH(X)CHdCH2 is used as substrate, the reaction
is thought to proceed with retention of configuration at
the methine carbon atom, i.e., with inversion in both of
(5) [RuCp*Cl(η3-phenylallyl)CH3CN]PF6 (3): trans-cinnamyl chlo-
ride (136 µL, 0.963 mmol, 1.2 equiv) was added to a stirred solution of
[RuCp*(CH3CN)3]PF6 (405 mg, 0.803 mmol) in 5 mL of CH2Cl2, and
the resulting clear solution was stirred at room temperature overnight.
The solution volume was reduced under vacuum to 1 mL, and diethyl
ether was added, precipitating a red powder. The solid was washed
with diethyl ether (2 × 5 mL) and dried under vacuum to yield 434
mg (94%) of product. Crystals suitable for an X-ray structure deter-
mination were obtained by layering Et2O in a CH2Cl2 solution of 3.
Anal. Calcd for C21H27NF6PClRu: C, 43.87; H, 4.73; N, 2.44. Found:
C, 43.01; H, 4.53; N, 1.85. HR-MALDI MS: m/z 389.1, 355.1, 315.1.
There is precedence for this type of reaction; see: Matsushima, Y.;
Onitsuka, K.; Takahashi, S. Organometallics 2004, 23, 3763-3765.
(1) (a) Trost, B. M. J. Org. Chem. 2004, 69, 5813-5837. Trost, B.
M.; Crawley, M. L. Chem. Rev. 2003, 103, 2921-2943. (b) Trost, B.
M.; Rudd, M. T. Org. Lett. 2003, 5, 1467-1470. (c) Trost, B. M.; Fraisse,
P. L.; Ball, Z. T. Angew. Chem., Int. Ed. 2003, 41, 1059-1061. (d) Trost,
B. M.; Older, C. M. Organometallics 2002, 21, 2544-2546.
(2) Morisaki, Y.; Kondo, T.; Mitsudo, T. Organometallics 1999, 18,
4742-4746. Kondo, H.; Yamaguchi, Y.; Nagashima, H. Chem. Com-
mun. 2000, 1075-1076.
(3) Kundig, E. P.; Monnier, F. R. Adv. Synth. Catal. 2004, 346, 901-
904.
10.1021/om049057a CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/15/2005

1810 Organometallics, Vol. 24, No. 8, 2005
Communications
In the solid state the molecule exists as the isomer
shown; however, an acetone solution prepared from the
recrystallized material reveals three species in the ratio
78:19:3 (see Chart 1). A solution prepared from the
product as a powder clearly reveals the same three
species (in the ratio 55:14:31), but the minor component
is now more abundant. An acetone solution containing
1 equiv each of 1 and 2 (X ) Cl) showed four allyl
species.8 We believe these three complexes are due to
the presence of exo and endo isomers, i.e., the central
CH bond can point toward or away from the Cp* and
there is an interchange of the allyl relative to the
positions of the complexed acetonitrile and chloride
ligands.8
1
The 13C and H chemical shifts for the allyl carbons
of all three isomers are shown in Chart 1 and have been
determined using a mixture of two-dimensional NMR
methods. The major isomer (endo) is that found in the
solid state on the basis of NOE experiments. The
resonances for the methylene carbon C1 are all found
at rather normal positions7a,b for a Ru-allyl complex:
ca. 59-68 ppm. However, the resonance positions for
the methine carbon C3 are found at much higher
frequency: ca. 91-103 ppm. These chemical shift data
suggest much less π back-bonding from the metal center
to this terminal carbon and thus a relatively more
electrophilic carbon center.
Figure 1. ORTEP view of the cation of salt 3 showing
50% probability ellipsoids. Selected bond lengths (Å) and
bond angles (deg): Ru-N(1A), 2.065(2); Ru-Cl, 2.3998(7);
Ru-C(1L), 2.192(3); Ru-C(2L), 2.162(3); Ru-C(3L),
2.351(2); Ru-C(1), 2.181(3); Ru-C(2), 2.221(3); Ru-C(3),
2.248(3); Ru-C(4), 2.263(3); Ru-C(5), 2.191(3);
N(1A)-Ru-Cl, 82.25(6).
We considered the possibility that a resonance struc-
ture such as 5 might be a contributor. This would be a
The solid-state structure for 3 was determined by
X-ray diffraction methods,6 and an ORTEP view of this
cation is shown in Figure 1, with selected geometrical
parameters given in the caption. The immediate coor-
dination sphere consists of the Ru atom surrounded by
the Cp*, the η3-allyl ligand, one complexed acetonitrile,
and the coordinated chloride ion. The most interesting
aspect of this structure involves the two markedly
different terminal Ru-C bond distances, Ru-C1L and
Ru-C3L, at 2.192(3) and 2.351(2) Å, respectively. Obvi-
ously the two terminal C atoms, the possible candidates
for attack by the nucleophile, Nu-, are quite different,
with the Ru-C3L bond being especially long. Routine
Ru-C allyl separations are on the order of 2.1-2.2 Å.7
We note that Bruneau and co-workers7d have found
similar values in their structure of the Ru(Cp*)-
(phenanthroline) dicationic complex 4.
Ru(II) (and not Ru(IV)) cation with the charge localized
on carbon C3. It would fit the observed Ru-C1L and
Ru-C2L bond lengths.7 However, one might expect
some delocalization of the positive charge into the
phenyl ring if C3 was strongly positive. For all three
isomers the para carbon signals appear at ca. 129.5-
130.5 ppm, i.e., in routine positions, so that we do not
(6) (a) Crystallographic data for 3: C21H27ClF6NPRu, fw ) 574.93,
monoclinic, space group P21/c (No. 14), a ) 11.8085(13) Å, b )
15.1804(17) Å, c ) 13.4054(16) Å, â ) 104.605(4)°, V ) 2325.4(5) Å3, Z
) 4, Dc ) 1.642 g cm-3, Mo KR radiation, λ ) 0.710 73 Å, µ ) 9.14
cm-1, T ) 120(2) K. A total of 20 762 reflections were collected on a
Bruker SMART CCD diffractometer in the range 2.98 e θ e 26.01°. A
total of 4556 unique reflections, after corrections for Lp and absorption,
were used for the solution and refinement (based on Fo2) of the
structure. All non-H atoms were refined anisotropically. Hydrogen
atoms, in their calculated positions, were included in the refinement
using a riding model. The final R factor is 0.0272 for 3712 observed
reflections and 0.0398 for all data.
(7) (a) Castro, A.; Turner, M. L.; Maitlis, P. M. J. Organomet. Chem.
2003, 674, 45-49. (b) Becker, E.; Slugovc, C.; Ruba, E.; Standfest-
Hauser, C.; Mereiter, K.; Schmid, R.; Kirchner, K. J. Organomet. Chem.
2002, 649, 55-63. (c) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard,
O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989,
S1-S83. (d) Mbaye, M. D.; Demerseman, B.; Renaud, J. L.; Toupet,
L.; Bruneau, C. Angew. Chem., Int. Ed. 2003, 42, 5066-5068. (e)
Mbaye, M. D.; Demerseman, B.; Renaud, J. L.; Toupet, L.; Bruneau,
C. Adv. Synth. Catal. 2004, 346, 835-841.
In 3 the relatively slender acetonitrile is proximate
to the phenyl group of the allyl ligand, although there
are no especially close contacts between these two. On
the basis of the structure of 3 this allyl complex is open
to attack by a nucleophile from the back side.
(8) There is an additional 2 equiv of acetonitrile and/or acetone
which may be involved as ligands for the fourth species.

Communications
Organometallics, Vol. 24, No. 8, 2005 1811
1
Chart 1. 13C and H Allyl Chemical Shifts for the Three Isomers of 3 in Acetone Solutiona
a Species I corresponds to the X-ray structure. Species II is thought to have the allyl phenyl proximate to the chloride ligand
and an endo allyl structure.
favor a structure with significant carbonium-ion char-
acter.
molecule, 3, by the replacement of the Cp* by the
smaller Cp. The optimized structure of 3′ is depicted in
Figure 2a. It compares reasonably well with the ex-
perimental X-ray structure for 3, although the Ru-X
distances in the model are a bit on the long side
(mean and maximum absolute deviations of 0.05 and
0.14 Å, respectively). The overall geometry of the
model reproduces the experimental structure, as
shown by the Cp-Ru-L angles, with the mean and
maximum absolute deviations between optimized and
experimental Cp-Ru-L angles of 1 and 4°, respec-
tively.
In earlier X-ray and 13C NMR studies9 involving
the palladium cation Pd(1,3-diphenylallyl)(Binap or
MeO-Biphep)+ (6), we suggested that an allyl reso-
nance structure of the type shown might be an impor-
tant contributor to the ground-state structure. In 3, we
The calculations reproduce the geometric distortion
of the allyl ligand observed in the X-ray structure of
3 reasonably, namely, the bond length Ru-C3L is 0.26
Å longer than Ru-C1L in the optimized model, 3′. The
electronic nature of this difference is confirmed by the
corresponding Wiberg indices,11 which are well-known
bond strength indicators and are more reliable than
distances. The calculated values for these indices (Fig-
ure 2b) show a stronger bond between the metal
and C1L (0.459), compared to Ru-C3L (0.319). The
relevance of the Wiberg indices for evaluating bond
strengths is demonstrated by the value for the Ru-C2L
bond (0.243). Despite the similar bond distances
Ru-C1L and Ru-C2L, the latter is the weakest of the
cannot completely exclude a contribution from an “ene-
yl” structure, i.e., 7, since the observed allyl 13C chem-
ical shifts would fit this suggestion; however, we
note that (1) the separations C1L-C2L and C2L-C3L
at ca. 1.415(4) and 1.412(4) Å, respectively, are iden-
tical within the experimental error (see the Sup-
porting Information) and (2) the relatively short
Ru-C2L bond length, 2.162(3) Å, which is even shorter
than the Ru-C1L distance, 2.192(3) Å, is not in keeping
with 7.
The simplest structural solution remains that of a
distorted Ru(IV) π-allyl ligand in which the bonding is
rather asymmetric, and to test this, we have carried out
DFT calculations10 on a model complex, [Ru(η5-Cp)Cl-
(CH3CN)(η3-PhCHCHCH2)] (3′), differing from the real
(9) Pregosin, P. S.; Salzmann, R. Coord. Chem. Rev. 1996, 155, 35-
68. Pregosin, P. S.; Trabesinger, G. J. Chem. Soc., Dalton Trans. 1998,
727-734. Barbaro, P.; Pregosin, P. S.; Salzmann, R.; Albinati, A.; Kunz,
R. W. Organometallics 1995, 14, 5160-5170. Pregosin, P. S.; Ruegger,
H.; Salzmann, R.; Albinati, A.; Lianza, F.; Kunz, R. W. Organometallics
1994, 13, 83-90. Barbaro, P.; Pregosin, P. S.; Salzmann, R.; Albinati,
A.; Kunz, R. W. Organometallics 1995, 14, 5160.
(10) DFT calculations were performed with the Gaussian 98 soft-
ware package using the B3LYP hybrid functional with a LanL2DZ
basis set augmented with a polarization function for Ru and Cl and
a 4-31G(d) basis set for the other atoms. Computational details and
the corresponding list of references are given as Supporting Informa-
tion.
(11) Wiberg, K. B. Tetrahedron 1968, 24, 1083;

1812 Organometallics, Vol. 24, No. 8, 2005
Communications
Figure 2. (a) Optimized geometry (B3LYP) of [Ru(η5-Cp)-
Cl(CH3CN)(η3-PhCHCHCH2)]. Ru and the allylic carbon
atoms are shaded. (b) Bond distances (Å), Wiberg indices
(in italics), and NPA charges (in boldface) relevant for the
allylic coordination to Ru.
Figure 3. (a) View of the HOMO-6 of cation 3′ showing
the differences in bonding between the Ru-C bonds
involving the two terminal carbons of the allyl ligand. (b)
Representation of the LUMO of 3′, revealing the impor-
tance of the contribution on C3L, the preferred location of
nucleophilic attack.
three Ru-C(allyl) bonds. This is expected, given the
nodal characteristics of the orbitals of a coordinated
allyl, specifically π2, with no contribution from the
central carbon. The differences between the Ru-C1L
and Ru-C3L terminal bonds in the allylic ligand can
be assigned to less efficient back-donation from the
metal to C3L and are reflected in the charges calculated
by means of a natural population analysis (Figure 2b).
They show an electron poorer C3L, in comparison with
either C1L or C2L. This result corroborates the 13C
NMR data, discussed above.
The difference between the bond strengths of the two
terminal bonds (Ru-C3L and Ru-C1L) in 3′, i.e., the
distortion on the coordination of the allyl ligand, can
also be related to the orbital interactions between the
metal and the ligand. This is illustrated by the HOMO-6
of the molecule (Figure 3a), showing a much stronger
bonding interaction between the metal and C1L relative
to the metal and C3L.
sistent with attack by a nucleophile at the less negative
(but not positive) C3L allyl carbon atom.
In conclusion, on the basis of crystallography, NMR
and calculations, we find that a geometrically and
electronically distorted allyl ligand is the source of the
observed control of regioselectivity in the Ru(II)-
catalyzed allylic alkylation reaction.
Acknowledgment. P.S.P. thanks the Swiss Na-
tional Science Foundation, the “Bundesamt fu¨r Bildung
und Wissenschaft”, and the ETHZ for financial support.
A.A. thanks the MURST for a grant (PRIN 2002). I.F.
thanks the Junta de Andalucia for a research contract.
We also thank Johnson Matthey for the loan of precious
metals. We acknowledge the COST working group
D24/0014/02.
Supporting Information Available: Text and a table
giving computational details and a CIF file giving crystal-
lographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
Further, the implications from the discussion above,
with respect to the position of the nucleophilic attack,
are reinforced by the nature of the LUMO (Figure 3b),
which demonstrates a relatively large contribution at
C3L. The orbital picture is, therefore, completely con-
OM049057A