Novel P-stereogenic PCP pincer-aryl ruthenium(II) complexes and their use in the asymmetric hydrogen transfer reaction of acetophenone

Article abstract of DOI:10.1002/hlca.200590048

Achiral P-donor pincer-aryl ruthenium complexes ([RuCl(PCP)(PPh ₃)]) 4c,d were synthesized via transcyclometalation reactions by mixing equivalent amounts of [1,3-phenylenebis(methylene)] bis[diisopropylphosphine] (2c) or [1,3-phenylenebis(methylene)] bis[diphenylphosphine] (2d) and the N-donor pincer-aryl complex [RuCl{2,6-(Me₂NCH₂)₂C₆H ³}(PPh₃)], (3; Scheme 2). The same synthetic procedure was successfully applied for the preparation of novel chiral P-donor pincer-aryl ruthenium complexes [RuCl(P*CP*)(PPh₃)] 4a,b by reacting P-stereogenic pincer-arenes (S,S)-[1,3-phenylenebis(methylene)]bis[(alkyl) (phenyl)phosphines] 2a,b (alkyl = ⁱPr or ^tBu, P*CHP*) and the complex [RuCl{2,6-(Me₂NCH ₂)₂C₆H₃}(PPh₃)], (3; Scheme 3). The crystal structures of achiral [RuCl(^iPr,iPrPCP) (PPh₃)] 4c and of chiral (S,S)-[RuCl-(^tBu,PhPCP)(PPh ₃)] 4a were determined by X-ray diffraction (Fig. 3). Achiral [RuCl(PCP)(PPh₃)] complexes and chiral [RuCl(P*CP*) (PPh₃)] complexes were tested as catalyst in the H-transfer reduction of acetophenone with propan-2-ol. With the chiral complexes, a modest enantioselectivity was obtained.

Full text of DOI:10.1002/hlca.200590048

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Helvetica Chimica Acta ± Vol. 88 (2005)
Novel P-Stereogenic PCP Pincer-Aryl Ruthenium(II) Complexes and
Their Use in the Asymmetric Hydrogen Transfer Reaction of Acetophenone
by Serenella Medicia), Marcella Gagliardoa), Scott B. Williamsa), Preston A. Chasea), Serafino Gladialib),
Martin Lutzc), Anthony L. Spekc)1), Gerard P. M. van Klinka), and Gerard van Koten*a)
a) Debye Institute, Department of Metal-Mediated Synthesis, Utrecht University, Padualaan 8,
NL-3584 CH Utrecht (phone ‡ 31-30-2533120; fax ‡ 31-30-2523615; e-mail g.vankoten@chem.uu.nl)
b
Á
) Dipartimento di Chimica, Universita di Sassari, Via Vienna 2, I-07100 Sassari
c) Bijvoet Center for Biomolecular Research, Department of Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, NL-3584 CH Utrecht
Â
Dedicated to Professor Andre Merbach on the occasion of his 65th birthday, for his excellent contributions to the
field of inorganic chemistry and for his support and friendship
Achiral P-donor pincer-aryl ruthenium complexes ([RuCl(PCP)(PPh3)]) 4c,d were synthesized via
transcyclometalation reactions by mixing equivalent amounts of [1,3-phenylenebis(methylene)]bis[diisopro-
pylphosphine] (2c) or [1,3-phenylenebis(methylene)]bis[diphenylphosphine] (2d) and the N-donor pincer-aryl
complex [RuCl{2,6-(Me2NCH2)2C6H3}(PPh3)], (3; Scheme 2). The same synthetic procedure was successfully
applied for the preparation of novel chiral P-donor pincer-aryl ruthenium complexes [RuCl(P*CP*)(PPh3)]
4a,b by reacting P-stereogenic pincer-arenes (S,S)-[1,3-phenylenebis(methylene)]bis[(alkyl)(phenyl)phos-
phines] 2a,b (alkyl ˆ iPr or tBu, P*CHP*) and the complex [RuCl{2,6-(Me2NCH2)2C6H3}(PPh3)], (3;
i
Scheme 3). The crystal structures of achiral [RuCl(iPr, PrPCP)(PPh3)] 4c and of chiral (S,S)-[RuCl-
t
(
Bu,PhPCP)(PPh3)] 4a were determined by X-ray diffraction (Fig. 3). Achiral [RuCl(PCP)(PPh3)] complexes
and chiral [RuCl(P*CP*)(PPh3)] complexes were tested as catalyst in the H-transfer reduction of acetophenone
with propan-2-ol. With the chiral complexes, a modest enantioselectivity was obtained.
Introduction. ± The interaction of multidentate phosphine ligands with late
transition metals has been extensively studied since these ligands can be modularly
designed to ꢀtuneꢁ the stereochemical and electronic properties of the corresponding
metal species. Complexes of the platinum-group metals with monoanionic P-donor
pincer ligands of the type [2,6-(R2PCH2)2C6H3]À (R ˆ Me, iPr, tBu, cyclohexyl, Ph) [1 ±
5] have proven to be effective homogeneous catalysts in a number of organic
transformations [6 ± 12]. Recently, the C-chiral analogs {2,6-[R2PC*H(R')]2C6H3}À
have been prepared and were shown to be catalytically active, e.g., in the asymmetric
condensation of aldehydes with methyl isocyanoacetate, but exhibit only mild
stereochemical induction [13]. These chiral versions of PCP pincers feature two
stereogenic benzylic C-atoms as the unique chiral element.
To achieve high enantiomer excesses, the chiral information should be situated in
close vicinity to the reactive site, in this case, the metal center [14]. Therefore, a new
chiral P*CP* pincer ligand 2a was synthesized bearing the chiral information on the
1
)
Corresponding author for crystallographic data (phone ‡ 31-30-2532538; fax ‡ 31-30-2523940; e-mail
a.1.spek@chem.uu.nl).
ꢂ 2005 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta ± Vol. 88 (2005)
695
P-atom, and the corresponding platinum and palladium complexes were isolated [15].
The latter complex, along with the iridium adduct, was independently reported by other
authors [16]. In both cases, however, the stereoselectivities recorded in various
catalytic reactions with these complexes were low.
The high activity displayed by the RuII complexes containing the achiral,
monoanionic pincer ligand [2,6-(R2PCH2)2C6H3]À 2d (R ˆ Ph) in the catalytic H-
transfer reduction of ketones [9] prompted us to investigate the chemistry of the P-
stereogenic P*CP* ligands in this reaction.
Herein, we report the synthesis of a new P-stereogenic P*CP* pincer ligand 2b
(Scheme 1) and the preparation of the novel chiral RuII complexes 4a and 4b
iPr
(Scheme 3). Complex 4c, containing the achiral PCP ligand 2c, was also prepared to
compare its behavior with that of its chiral counterparts, as well as the achiral PhPCP
ligand 2d. Preliminary results on the catalytic activity of the prepared complexes in the
H-transfer reaction of acetophenone with propan-2-ol are also presented.
Results and Discussion. ± P-Stereogenic P*CP* Ligands. The novel chiral ligand 2b
(Scheme 1) was prepared by following the recently reported synthetic procedure
applied for the synthesis of the chiral ligand 2a (Scheme 1) [15]. In the first step, the
borane adduct of racemic alkyl(phenyl)monophosphine 1b was prepared in high yield
by reaction of PhPCl2 with iPrMgCl, followed by reduction with LiAlH4 and protection
of the secondary phosphine with BH3 ´ SMe2. Deprotonation of the phosphine ± borane
adduct was achieved by reaction with BuLi in Et2O in the presence of (À)-sparteine at
À 788. The resulting reaction mixture was stirred for 1 h at 308 and then allowed to
react at À 788 with 1,3-bis(bromomethyl)benzene.
Scheme 1. Synthesis of P-Stereogenic Ligands 2a and 2b
t
i
a) 1) BuLi (for 1a) or PrMgCl (for 1b), Et2O, À788; 2) LiAlH4, filtration; 3) BH3 ´ SMe2. b) 1) BuLi,
(À)-sparteine, Et2O, À788; 2) 308, 1 h; 3) 1,3-bis(bromomethyl)benzene, À788.
Chiral ligand 2b was obtained as a mixture of the racemate ((S,S)- and (R,R)-
enantiomers) and the meso-diastereoisomer. Ligand (S,S)-2b was isolated in 31% yield
in 77% e.e. (determined by HPLC) after fractional crystallization from Et2O/hexane,
which left the contaminating meso-isomer in solution. The achiral ligands [2,6-
(R2PCH2)2C6H3]À (ˆ RPCP; R ˆ iPr for 2c and Ph for 2d; (see Scheme 2) were
synthesized based on reported protocols [2c][3b].
Ruthenium Complexes by the Transcyclometalation Reaction. The key step of the
synthesis of the achiral complexes [RuCl(PCP)(PPh3)] 4c,d is the transcyclometalation
(TCM) reaction [17]. This alternative synthetic route, which represents an elegant

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methodology for the creation of a metal ± C bond under relatively mild conditions,
proceeds selectively and in quantitative yield. The reaction pathway (Scheme 2), which
is similar to an electrophilic aromatic substitution reaction, involves the formal
exchange of the tridentate monoanionic 2,6-bis(aminomethyl)phenyl ligand [2,6-
(Me2NCH2)2C6H3]À of the complex [RuCl{2,6-(Me2NCH2)2C6H3}(PPh3)] (3;
Scheme 2) by a corresponding tridentate 2,6-bis(phosphinomethyl)phenyl ligand [2,6-
(R2PCH2)2C6H3]À 2 (R ˆ iPr, Ph). Therefore, the benzenedimethanamine is the only
co-product. Its high solubility in apolar solvents allows for facile separation from the
desired product 4.
Scheme 2. Synthesis of Achiral [RuCl(PCP)(PPh3)] Complexes 4c and 4d via the Transcyclometalation
Procedure
The synthesis of 4c and 4d was achieved by means of the TCM methodology by
reaction of a 1:1 molar mixture of the bis[phosphines] 2,6-(iPR2PCH2)2C6H4 (2c) [2c]
and 2,6-(Ph2PCH2)2C6H4] (2d) [5d], respectively, with complex [RuCl{2,6-
(Me2NCH2)2C6H3}(PPh3)] (3) in refluxing benzene.
Both complexes were isolated in good yield as green, air-sensitive solids.
Spectroscopic characterization indicated that both 4c and 4d have square-pyramidal
geometry with the PPh3 ligand occupying the apical position, in accordance with the
results published by Jia [5b] and Milstein [2c]. The X-ray crystal-structure determi-
nation of single crystals of 4c, grown from a CH2Cl2 solution into which hexane vapor
was allowed to diffuse slowly, confirms the features observed by 1H-, 13C-, and
31P-NMR spectroscopy in solution. A molecular drawing of complex 4c is depicted in
Fig. 3 (vide infra), and a selection of bond lengths and angles and torsion angles is
summarized in Table 1 (vide infra).
Previously, kinetic studies have been performed to gain insight into the mechanism
of the TCM reaction [17b]. However, from the data obtained, it was impossible to draw
definite conclusions on the reaction mechanism. The postulated intermediates 5 and 6
(Fig. 1) [18] formed during the course of the reaction after a fast, irreversible
cyclometalation step, were observed in the present study, for the first time, by
monitoring aliquots of a mixture containing equivalent amounts of ligand 2d and
complex 3 in refluxing benzene by 31P-NMR spectroscopy. The chemical shift and
multiplicity of the 31P-NMR signals provide an excellent probe, which allowed for the
assignment of three species: the product [RuCl{2,6-(Ph2PCH2)2C6H3]}(PPh3)] (4d) and
two intermediates. These intermediates, depicted in Fig. 1, are the monomer 5 and the
dimer 6, which both contain kP,kC,kP'-bonded PCP and kP-bonded PCHP ligands at a
RuII metal center.

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Fig. 1. Intermediates formed during the TCM reaction
The 31P-NMR spectrum of the reaction mixture 2d/3 after 2 h refluxing (Fig. 2),
recorded at room temperature, shows a d at d(P) 37.9 (PCP P-nuclei, 2J(P,P) ˆ 31.5 Hz)
2
and a t at d(P) 83.2 (PPh3, J(P,P) ˆ 31.8 Hz).
These signals are characteristic for a complex in which an apical phosphine is in a cis
arrangement to two magnetically equivalent P-atoms of the cyclometalated PCP ligand
in 4d [2c][5b][19]. The monomer 5, present as minor compound, contains both a kP-
coordinated PCHP ligand (t at d(P) 85.6, 2J(P,P) ˆ 31.5 Hz) and an kP,kC,kP'-
2
coordinated PCP ligand (d at d(P) 42.3, J(P,P) ˆ 33.9 Hz) coordinated to the same
RuII metal center. The s at d(P) À 9.7 can be ascribed to a dangling PPh2 moiety. The
dimeric RuII species 6 contains two kP,kC,kP'-bonded RuII(PCP) units (d at d(P) 41.9,
2J(P,P) ˆ 31.5 Hz) connected through a m-kP,kP'-coordinated PCHP ligand acting as a
bridge (t at d(P) 85.6, 2J(P,P) ˆ 31.53 Hz). Therefore, the relatively broad t at d(P) 85.6
has to be regarded as two superimposed t. The free PPh3 present in the reaction mixture
(s at d(P) À3.8 ppm) slowly displaces the coordinated PCHP arene ligands to form
complex 4d. This is evidenced by the slow disappearance of the peak corresponding to
free PPh3 and the concomitant enhanced intensity of the signals of 4d. After 14 h, the
formation of 4d is complete.
Apparently, the rate-limiting step in the TCM reaction is not the RuÀC bond-
formation or cleavage but rather the displacement of the kP-coordinated PCHP ligand
Fig. 2. Selected portions of the 31P-NMR spectrum (benzene, r.t.) of the TCM reaction mixture containing
equivalent amounts of ligand 2d and complex 3 after 2 h reflux

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by the free PPh3. When an excess of PCHP ligand is added to a solution of 4d in
benzene at reflux temperature, the presence of intermediates 5 and 6 in the reaction
mixture is observed in the 31P-NMR spectrum. This is in concert with our hypothesis
and establishes that these intermediates are in equilibrium with the desired product
during the TCM reaction. Recent results demonstrated that the TCM reaction is also a
superior synthetic method over existing metalation procedures to introduce ruthenium
metal centers in shape-persistent nanosize multi(metal ± pincer) complexes [20].
The TCM procedure used for the preparation of achiral [RuCl(PCP)(PPh3)] was
slightly modified to make it amenable for the preparation of ruthenium complexes 4a
and 4b with chiral pincer ligands (Scheme 3). In a direct one-pot synthesis, the
deprotection of the prepared phosphine ± boranes 2a or 2b and the subsequent TCM
reaction to form the chiral [RuII(P*CP*)] complexes was performed. Deprotection of
the phosphine ± boranes was easily accomplished by overnight heating of a benzene
solution of 2a or 2b in the presence of an excess of Et2NH at 458. Williams et al. showed
that these mild conditions are necessary to prevent inversion of the stereogenic P-
centers in ligand 2a, though epimerization becomes rapid at higher temperature or over
longer times [15]. [RuCl(NCN)(PPh3)] 3 was then added, and the obtained mixture
stirred for 24 h at 458 (Scheme 3). The presence of Et2NH, which could be subsequently
removed by evaporation, did not affect the course of the TCM reaction.
Scheme 3. Synthesis of [RuCl(P*CP*)(PPh3)] Complexes 4a and 4b via the Transcyclometalation Procedure
Complex 4a was isolated as a relatively air-stable, ink-blue solid, which does not
require an inert atmosphere for workup. However, for long-term storage, an inert
atmosphere is required. Its purification could be conveniently accomplished via column
chromatography, with only a minor amount of decomposition. The 1H-NMR spectrum
of 4a in CD2Cl2 at room temperature shows two different tBu groups on the P-centers
and the AB portion of two ABX patterns for the diastereotopic benzylic protons, due to
the coupling with the geminal proton and with the vicinal P-atom. In the aromatic
region, broad resonances indicate dynamic behavior, which was investigated by
variable-temperature NMR spectroscopy (213 ± 363 K, CD2Cl2). The dynamic process
is the result of rotation of the Ph rings bonded to the P-centers of the PCP ligand, and
each ring rotates at a slightly different rate. By line-shape analysis of the signals
assigned to the ortho-protons of these rings [21], an Eyring plot of both rotations could
³
be constructed. The more rapidly rotating ring, distal to the PPh3 ligand, has a DH of

Helvetica Chimica Acta ± Vol. 88 (2005)
699
³
9.0 Æ 0.1 kcal/mol and a DS of À 13 Æ 1 e.u., while the more slowly rotating ring,
³
³
proximal to the PPh3 ligand, exhibits a DH of 10.2 Æ 0.4 kcal/mol and a DS of À 15 Æ 2
e.u. The rather sizable negative entropies of activation are somewhat surprising and
suggest that the molecule must rearrange considerably for rotation to occur. However,
³
the low DH suggests that the PÀRu bonds are still intact throughout the dynamic
process. The ABX pattern observed in the 31P-NMR spectrum of 4a, recorded in
CD2Cl2 at room temperature, clearly shows the nonequivalence of the P-atoms of the
P*CP* ligand, which couple with the coordinated PPh3 ligand. The resonance for the
PPh3 ligand appears at d(P) 70.3 as a ꢀddꢀ (J(A,X) ˆ 35.6 Hz, J(B,X) ˆ 23.7 Hz) due to
inequivalent cis-couplings with the two P-centers of the P*CP* ligand, while the signals
assigned to the P*CP* moiety are centered at d(P) 40.3 (2 ꢀddꢁ, J(A,B) ˆ 254.1 Hz, in
agreement with their trans-disposition around the metal). Single crystals of 4a, suitable
for an X-ray crystal-structure determination, were grown by slow diffusion of hexane
into a Et2O solution of the complex at room temperature. A plot of the molecular
structure is drawn in Fig. 3, and a selection of bond lengths and angles and torsion
angles is summarized in Table 1 (vide infra). The obtained molecular structure of 4a
confirms that the structural features observed in solution are preserved also in the solid
state.
Complex 4b was obtained pure in low yield after several purification steps as an air-
sensitive, deep green, crystalline solid. Unfortunately, due to its extreme air sensitivity,
satisfactory elemental and mass analyses were not obtained; the complete character-
1
ization of 4b is thus based mostly on H- and 31P-NMR spectroscopic data. The
1
broadened resonances observed in the aromatic region of the H-NMR spectrum
(CD2Cl2, room temperature) are an indication of dynamic behavior very similar to that
observed for 4a. Further investigations in this direction were hampered by the low
stability of 4b under the required experimental conditions. The 31P-NMR spectrum
Fig. 3. Displacement ellipsoid plot (50% probability) of molecules 4a and 4c in the crystal

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Table 1. Selected Bond Lengths and Angles and Torsion Angles for 4a, 4c, and 4d
4a
4c
4d [5b]
2.459(1)
Bond lengths [Š]
Ru(1)ÀCl
2.5036(5)
2.082(2)
2.4384(4)
2.0575(17)
2.3243(5)
2.3541(5)
2.2044(5)
91.641(16)
93.294(15)
123.639(17)
146.56(6)
Ru(1)ÀC(1)
2.070(4)
2.297(1)
2.284(1)
2.196(1)
96.1(1)
Ru(1)ÀP(1)
2.3442(5)
2.3043(5)
2.2096(5)
93.248(17)
101.736(18)
100.649(18)
169.36(6)
168.09(7)
Ru(1)ÀP(2)
Ru(1)ÀP(3)
Bond angles [8]
Cl(1)ÀRuÀP(1)
Cl(1)ÀRuÀP(2)
Cl(1)ÀRu(1)ÀP(3)
Cl(1)ÀRu(1)ÀC(1)
Cl(1)ÀRu(1)ÀP(1)ÀC(7)
Cl(1)ÀRu(1)ÀP(2)ÀC(8)
95.0(1)
113.6(1)
161.6(1)
À 175.1a)
À 172.4a)
Torsion angles [8]
À 159.66(6)
À 171.84(8)
167.76(7)
a) Calculated from coordinates of [5b].
(CD2Cl2, room temperature) of 4b shows the expected ABX pattern observed also for
complex 4a. In this case, however, the signal assigned to the PPh3 ligand, centered at
d(P) 86.19 appears as a virtual t due to the similar values for the couplings of the PPh3
ligand with the two P-atoms of the P*CP* ligand (J(A,X) ˆ 31.7 Hz, J(B,X) ˆ 31.1 Hz,
resp.). Signals for the P*CP* portion still appear as 2 ꢀddꢁ centered at d(P) 44.5
(J(A,B) ˆ 265.0 Hz).
In Table 1, the geometric data of complexes 4a and 4c (Fig. 3) in the solid state are
compared to the data of complex 4d obtained by Jia et al. [5b]. All three complexes
show the RuII centers in a distorted square-pyramidal environment embedded in the
coordination pocket of the tridentate PCP ligands. The PPh3 ligands occupy the apical
positions while the other two P-centers, together with Cl and the Cipso atom, form the
base of the square-pyramidal configuration. In the enantiomerically pure complex 4a,
viewed along the ClÀRu bond, two diagonally opposite quadrants are occupied by tBu
groups, while the other two contain the Ph groups. In the racemic complex 4c, all four
quadrants are occupied by iPr groups.
The RuÀP distances of the tridentate PCP-pincer ligand vary significantly, more
than three standard uncertainties, within one complex as well as between the three
complexes 4a, 4c, and 4d [5b]. The RuÀC(1) distances are quite similar in all three
complexes and consistent with the distances found in a series of five-coordinate
ruthenium(II) complexes containing the PCP- or NCN-pincer ligand fragment [5b].
Although the molecular geometry of 4a, 4c, and 4d [5b] are similar, the bulkiness of the
substituents on the P-atoms of the pincer ligands seem to influence the bond angles
ClÀRuÀP(1), ClÀRuÀP(2), ClÀRuÀP(3), and ClÀRuÀC(1) and the torsion angles
ClÀRu(1)ÀP(1)ÀC(7) and ClÀRu(1)ÀP(1)ÀC(8) (see Table 1).
Catalytic Activity. Ruthenium complexes containing pincer-aryl ligands (NCN and
PCP) have been shown to be catalyst precursors of outstanding activity in the reduction
of ketones via the H-transfer reaction with propan-2-ol (cf. Scheme 4). For example,
high conversions of cyclohexanone and turnover frequencies (TOFs) up to 27000 hÀ1
were observed with the triflato (ˆtrifluoromethanesulfonato) analog of complex 4d as
catalyst precursor [9]. For this reason, the novel chiral complexes 4a and 4b, and achiral
4c were tested as catalyst precursors in the conversion of acetophenone (Scheme 4).

Helvetica Chimica Acta ± Vol. 88 (2005)
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Scheme 4. Hydrogen Transfer Reaction of Acetophenone by Ruthenium Complexes 4a ± d
Table 2. Hydrogen-Transfer Reduction of Acetophenone. Conditions: RuII (0.01 mmol), acetophenone
(1 mmol), KOH (0.2 mmol), iPrOH (10 ml), reflux temperature.
a
Entry
Complex
Time [h]
Conv. [%]
e.e. [%]
)
1
2
3
4
4a
4b
17
15
5
40
40
> 95
100
18
12
±
4c
4db)
0.8
±
a) All the e.e.ꢁs recorded were in favor of the (R)-enantiomer (HPLC). b) See [22].
The preliminary results are summarized in Table 2, in which the values obtained with 4d
[22] are added for comparison.
Catalytic experiments were performed by addition of the substrate to a suspension
of the ruthenium complexes and KOH in propan-2-ol, previously heated to reflux
temperature under N2. The reaction mixtures were maintained at 828 during the course
of the reaction. Both chiral precursors 4a and 4b having mixed aliphatic and aromatic
substituents on the P-atoms were catalytically less active than the symmetric 4c, which
contains aliphatic substituents. Thus, whereas 95% conversion was obtained with 4c
after 5 h (Entry 3), only 40% conversion was reached with either 4a or 4b after ca. 15 h
(Entries 1 and 2). The activities of precursors 4a and 4b at low conversion of
acetophenone were comparable. Precursor 4d with four identical aromatic substituents
was considerably more active in H-transfer catalysis than 4c (Entry 4). The results
shown in Table 2 point to the existence of a subtle balance between electronic and steric
effects in the rates of H-transfer catalyzed by 4a ± d. Electron-rich (Entry 3) and
relatively electron-poor (Entry 4) ruthenium ± PCP complexes both showed high
activity in H-transfer from propan-2-ol to acetophenone. Fogg and co-workers recently
also demonstrated high transfer rates with a flexible electron-rich ruthenium ± cyPCP
complex (cy ˆ cyclohexyl) [9b].
At low acetophenone conversion, precursors 4a and 4b induced a modest chirality
transfer. As shown in Entries 1 and 2 of Table 2, e.e.s of up to 18% were observed.
Apparently, the chiral pocket of the catalyst precursors allows for transfer of
dihydrogen to both faces of the substrate equally well. Moreover, epimerization at
the stereogenic P-centers under the conditions applied during the catalytic experiments
cannot be excluded [15].
However, after a prolonged reaction period (4 days), a complete loss of optical
activity was observed with both catalyst precursors 4a and 4b. It must be noted that
racemization of chiral alcohols is known to occur in ruthenium-catalyzed H-transfer
processes [23]. Also in our case, the degradation of chiral product into a racemic
mixture of products under the reaction conditions may explain the observed loss of
optical activity. Further investigations are underway in our laboratory to gain more

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insight into the effects governing H-transfer rates and chirality transfer in reactions
catalyzed by [RuX(PCP)(PPh3)] complexes.
Conclusions. ± The P-stereogenic pincer ligands 2a and 2b were used, as well as the
achiral PCP ligands 2c and 2d, in the transcyclometalation reaction to obtain the chiral
[RuCl(P*CP*)(PPh3)] complexes 4a and 4b and the achiral [RuCl(PCP)(PPh3)]
complexes 4c and 4d, respectively. The results show that it is also possible to prepare P-
chiral [RuII (P*CP*)] complexes via this route with retention of initial chirality at the
P-atoms of the P*CHP* ligand. The chiral complexes were tested as catalysts in the H-
transfer reaction of acetophenone with propan-2-ol. Preliminary results showed that
although the novel chiral ruthenium complexes are moderately active, the chiral
induction is lost upon prolonged reaction.
Dr. H. P. Dijkstra and H. Kleijn are kindly thanked for helpful discussions, and J. de Pater for NMR
experimental assistance. This work was supported in part (M. L., A. L. S.) by The Netherlands Foundation for
Chemical Sciences (CW) with financial aid from the Netherlands Organization for Scientific Research (NWO)
and (P. A. C. ) the Natural Sciences and Engineering Research Council of Canada.
Experimental Part
General. [RuCl(NCN)(PPh3)] 3 [24], [RuCl(PhPCP) (PPh3)], 4d [17b], 2a [15], iPrPCP [2c], and PhPCP [3b]
were prepared according to literature procedures. All enantiomer excesses were determined by chiral HPLC
analysis (Daicel Chiralcel-OD column; flow rate 1.0 ml/min; hexane/iPrOH/AcOEt 96 :2 :2). All reactions were
carried out under dry N2 by using standard Schlenk techniques unless specified otherwise. Purchased chemicals
(Acros and Aldrich) were used without further purification. Solvents were dried by standard procedures,
distilled, and stored under N2. Et2NH was degassed before use by the freeze-pump-thaw technique. 1H-, 13C- and
31P-NMR Spectra: Bruker AC200 or Varian Unity-INOVA-300 NMR spectrometer; chemical shifts d in ppm
referenced to the residual solvent signal (1H and 13C) and a capillary containing 85% H3PO4 31P); coupling
(
constants J in Hz.
Trihydro[isopropyl(phenyl)phosphine]boron (1b). PhPCl2 (5 g, 28 mmol) was dissolved in dry Et2O
(70 ml) and stirred at À788 for 15 min. Subsequently, 2m iPrMgCl Et2O (15.4 ml, 30.8 mmol) was added
dropwise via syringe within 15 min. The obtained suspension was stirred at À788 for 0.5 h, then allowed to warm
to r.t., and stirred for an additional 1.5 h. The mixture was filtered through a sintered-glass filter and was added
within 45 min to a suspension of LiAlH4 (1.17 g, 30.8 mmol) in Et2O (30 ml) cooled at À788. After stirring for
1 h, the mixture was allowed to warm to r.t., quenched carefully with degassed H2O, and filtered through a glass
filter. BH3 ´ SMe2 (94%; 10.6 ml, 140 mmol) was added by syringe at 08, and the soln. was stirred at r.t. for 0.5 h
before being carefully poured into a mixture of ice (100 g) and 4m HCl (50 ml). The aq. layer was washed twice
with Et2O (2 Â 50 ml), the combined org. phase washed with H2O (50 ml) and brine (50 ml), dried (MgSO4),
and evaporated, and the residue dissolved in CH2Cl2 and filtered through silica gel. Evaporation gave 1b (3.0 g,
1
1
64%). Dense oil. H-NMR (300 MHz, CDCl3): 0.74 (br. q, J(H,B) ˆ 96, 4 H, BH3); 1.15 (dq, J (H,H) ˆ 7.2, J
(H,P) ˆ 9.0, 3 H, Me2CH); 1.18 (dq, J(H,P) ˆ 7.4, J(H,P) ˆ 9.6, 3 H, Me2CH); 2.23 (m, Me2CH); 5.25 (ddq,
3J(H,H) ˆ 4.0, 2J(H,B) ˆ 6.6, 1J(H,P) ˆ 365.5, HP); 7.40 ± 7.57 (m, 3 arom. H); 7.62 ± 7.71 (m, 2 arom. H).
13C-NMR (300 MHz, CDCl3): 18.04 (d, 2J(C,P) ˆ 41.8, Me2C); 29.71 (d, 1J(C,P) ˆ 34.6, Me2CH); 125.05 (d,
1J(C,P) ˆ 53.4, Cipso); 129.11 (d, 3J(C,P) ˆ 9.7, 2 Cm); 131.90 (d, 4J(C,P) ˆ 2.4, Cp); 133.65 (d, 2J(C,P) ˆ 8.8, Co).
31P-NMR (200 MHz, CDCl3): 28.85 (br. m). Anal. calc. for C9H16BP (166.01): C 65.12, H 9.72, P 18.66; found
C 65.06, H 9.64, P 18.49.
Hexahydro{m-{[P(S),P'(S)]-[1,3-phenylenebis(methylene)]bis[isopropyl(phenyl)phosphine-kP]}}diboron
(2b). BuLi (1.6m in hexane; 18.3 ml, 11.5 mmol) was added dropwise via syringe to a soln. of 1b (2.0 g,
12.05 mmol) in dry Et2O (50 ml), cooled at À788. The mixture was stirred at low temp. for 15 min. Then (À)-
sparteine (3.67 g, 15.7 mmol) was added via syringe, the cooling bath removed, and the mixture allowed to rise
to r.t. within 1 h. The mixture was placed in a warm-water bath (ca. 308) for 1 h, after which the temp. was
lowered to À788 and 1,3-bis(bromomethyl)benzene (1.43 g, 5.4 mmol) added as soln. in Et2O (20 ml). The
mixture was stirred for 3 h while the temp. was slowly raised to À208 and subsequently placed in the freezer

Helvetica Chimica Acta ± Vol. 88 (2005)
703
(À 308) overnight. Then, 5% H2SO4 soln. (100 ml) was slowly added, the aq. layer washed with CH2Cl2 (3 Â
50 ml), the combined org. layer washed with aq. Na2CO3 soln. (100 ml) and brine (100 ml) and evaporated, and
the residue washed with hexanes/Et2O 3 :1: 2b (730 mg, 31%). White solid as fine powder. HPLC: (S,S)-
1
enantiomer in 77% e.e. [a]2D1 ˆ ‡8 (c ˆ 1, CHCl3). H-NMR (300 MHz, CDCl3): 0.05 ± 1.80 (br. q, BH3); 0.99
(dd, 3J(H,H) ˆ 6.9, 3J(P,H) ˆ 15.6, 6 H, Me2CH); 1.31 (dd, 3J(H,H) ˆ 7.2, 3J(P,H) ˆ 15.9, 6 H, Me2CH); 2.33
2
1
2
2
(sept., J(H,H) ˆ 6.9, 2 Me2CH); 3.17 (2 ꢀddꢁ, AB of ABX J(Ha,Hb) ˆ 14.2, J(Ha,P) ˆ 9.3, J(Hb,P) ˆ 12.0, 2
CH2); 6.54 (dd, J ˆ 2.0, J ˆ 7.5, HÀC(4), HÀC(6)); 6.80 (d, J ˆ 2, HÀC(5)); 6.85 (s, HÀC(2)); 7.36 (dd, J ˆ 1.8,
7.5, 4 Ho (PhP)); 7.44 ± 7.50 (m, 4 Hm (PhP), 2 Hp (PhP)). 13C-NMR (300 MHz, CDCl3): 17.01 (s, 2 Me2C); 22.15
(d, 1J(C,P) ˆ 34.8, 2 Me2CH); 32.07 (d, 1J(C,P) ˆ 30.5, 2 CH2); 126.70 (d, 1J(C,P) ˆ 50.1, 2 arom. C (quat.));
128.34 (dt, 2J(C,P) ˆ 47.2, 4J(C,P) ˆ 2.4, C(1), C(3)); 128.60 (d, 3J(C,P) ˆ 9.1, 4 Cm (PhP)); 128.70 (s, C(5));
131.30 (t, 3J(C,P) ˆ 3.8, C(2)); 131.51 (d, 4J(C,P) ˆ 2.4, 2 Cp (PhP)); 132.93 (dd, J ˆ 2.4, 6.1, C(4), C(5), C(6));
133.35 (d, 2J(C,P) ˆ 8.0, 4 Co (PhP)). 31P-NMR (200 MHz, CDCl3): 28.03 (br. m). Anal. calc. for C26H38B2P2
(434.16): C 71.93, H 8.82, P 14.27; found: C 71.86, H 8.80, P 14.34.
{2,6-Bis{{[P(S)]-(tert-butyl)phenylphosphino-kP}methyl}phenyl-kC}chloro(triphenylphosphine)rutheni-
um (4a). Phosphine ± borane 2a (173 mg, 0.375 mmol) was heated overnight (458) in a mixture of Et2NH and
benzene. Complex [RuCl(NCN)(PPh3)] 3 (246 mg, 0.417 mmol) was subsequently added, and the heating was
continued for 48 h (deep purple ! deep blue). The volatiles were removed under high vacuum and the obtained
solid residue washed with cold pentane, dissolved in Et2O and purified by column chromatography: 4a (200 mg,
t
60%). Ink-blue air-stable solid. 1H-NMR (300 MHz, (D8)-toluene): 0.48 (d, 3J(H,P) ˆ 12.3, 1 Bu); 1.35 (d,
3J(H,P) ˆ 13.3, 1 tBu); 1.96 (ꢀddꢁ, A of ABX, 2J(H,H) ˆ 16.0, 2J(H,P) ˆ 3.6, 1 H, CH2); 2.94 (ꢀddꢁ, A' of A'B'X',
2J(H,H) ˆ 15.9, 2J(H,P) ˆ 8.7, 1 H, CH2); 3.29 (ꢀddꢁ, B of ABX, 2J(H,H) ˆ 16.0, 2J(H,P) ˆ 14.0, 1 H, CH2); 3.35
(ꢀtꢁ, B' of A'B'X', 2J(H,H) ˆ 2J(H,P) ˆ 15.9, 1 H, CH2); 6.48 (br., 1 Ho (PhP)); 6.80 (dt, J ˆ 7.2, 2.1, 4 Hm (PhP), 2
Hp (PhP)); 6.94 (td, J ˆ 7.2, 1.3, HÀC(3), HÀC(4), HÀC(5)); 6.99 ± 7.10 (m, 6 Hm (Ph3P), 3 Hp (Ph3P)); 7.56 (td,
J ˆ 11.3, 3.3, 6 Ho (Ph3P)); 8.30 (br., 2 Ho (PhP)); 9.43 (br. 1 Ho (PhP)). 13C-NMR (300 MHz, CDCl3): 27.55 (d,
J ˆ 4.2, 1 Me3C); 27.95 (d, J ˆ 4.3, 1 Me3C); 31.63 (d, J ˆ 17.6, 1 C(quat.)); 34.36 (dd, J ˆ 11.5, 7.3, 1 C(quat.));
35.03 (d, J ˆ 25.5, 1 CH2); 36.43 (d, J ˆ 24.9, 1 CH2); 120.80 (d, J ˆ 16.5, C(3) or C(5)); 121.65 (s, C(4)); 123.26
(d, J ˆ 14.6, C(3) or C(5)); 126.48 (d, J ˆ 9.7, 5 Cm (Ph3P)); 127.64 (d, J ˆ 7.3, 4 Cm (PhP)); 128.51 (s, 2 Cp (PhP));
128.93 (s, 3 Cp (Ph3P)); 129.35 (dd, J ˆ 10.3, 1.8, 4 Co (PhP)); 134.77 (d, J ˆ 9.7, 6 Co (Ph3P)); 134.85 (d, J ˆ 13.0, 2
C
ipso (PhP)); 136.42 (d ꢁtꢁ, J ˆ 49.7, 2.4, 3 Cipso (Ph3P)); 150.35 (d, J ˆ 9.1, C(2) or C(6)); 152.67 (dd, J ˆ 15.75, 2.7,
C(2) or C(6)); 173.92 (d, J ˆ 17.6, C(1)). 31P-NMR (200 MHz, (D6)benzene): 40.28 (2 ꢀddꢁ, AB of ABX,
J(A,X) ˆ 35.6, J(B,X) ˆ 23.7, J(A,B) ˆ 254.1); 70.33 (ꢀddꢁ, X of ABX, J(A,X) ˆ 35.6, J(B,X) ˆ 23.7). Anal. calc.
for C46H50ClP3Ru (832.38): C 66.38, H 6.05, P 11.16; found: C 66.42, H 6.12, P 11.21.
{2,6-Bis{{[P(S)]-isopropyl(phenyl)phosphino-kP}methyl}phenyl-kC}chloro(triphenylphosphine)rutheni-
um (4b). As described for 4a, with 2b (260 mg, 0.6 mmol) and 3 (350 mg, 0.6 mmol) (deep purple ! deep
green). The volatiles were removed in vacuo to give an air-sensitive green solid, which was dissolved in Et2O.
The soln. was filtered through a sintered-glass filter and evaporated: 80 mg (16%) of 4b. No further purification
was possible since 4b is soluble in all standard org. solvents. 1H-NMR (300 MHz, (D6)benzene): 0.17 (dd,
3J(H,P) ˆ 13.8, 3J(H,H) ˆ 7.2, 3 H, Me2CH); 0.52 (dd, 3J(H,P) ˆ 11.4, 3J(H,H) ˆ 7.2, 3 H, Me2CH); 0.71 (dd,
3J(H,P) ˆ 13.8, 3J(H,H) ˆ 7.2, 3 H, Me2CH); 1.67 (dd, 3J(H,P) ˆ 11.4, 3J(H,H) ˆ 7.2, J ˆ 1,1, 3 H, Me2CH); 1.91
3
2
(sept., J(H,H) ˆ 7.2, 1 Me2CH); 2.02 (ꢀddꢁ, A of ABX, 1J(H,H) ˆ 16.5, J(H,P) ˆ 4.2, 1 H, CH2); 2.18 (dsept,
3J(H,H) ˆ 7.2, 2J(H,P) ˆ 2.4, 1 Me2CH); 2.39 (ꢀddꢁ, A' of A'B'X', 1J(H,H) ˆ 16.5, 2J(H,P) ˆ 7.2, 1 H, CH2); 3.01
(ꢀddꢁ, B' of A'B'X', 1J(H,H) ˆ 16.5, 2J(H,P) ˆ 13.2, 1 H, CH2); 3.37 (ꢀddꢁ, B of ABX, 1J(H,H) ˆ 16.5, 2J(H,P) ˆ
13.2, 1 H, CH2); 6.73 (dt, J ˆ 2.3, J ˆ 7.5, 3 arom. H); 6.81 (td, J ˆ 1.8, J ˆ 7.2, 1 arom. H); 6.93 ± 7.01 (series of m,
12 arom. H); 7.35 (br., 2 arom. H); 7.48 (m, 3 arom. H); 7.71 (ddd, J ˆ 1.5, 8.0, 11.7, 3 arom. H); 7.79 (dt, J ˆ 1.8,
8.0, 1 arom. H); 8.22 (dt, J ˆ 1.5, 8.0, 1 arom. H). 31P-NMR (200 MHz, (D6)benzene): 44.49 (2 ꢀddꢁ; AB of ABX,
J(A,X) ˆ 31.67, J(B,X) ˆ 31.07, J(A,B) ˆ 265.0); 86.19 (ꢀtꢁ, X of ABX, J(A,X) ˆ J(B,X) ˆ 31.50).
{2,6-Bis[(diisopropylphosphino-kP)methyl]phenyl-kC}chloro(triphenylphosphine)ruthenium (4c). To a
i
soln. of [RuCl(NCN)(PPh3)] 3 (425 mg, 0.72 mmol) in benzene (15 ml) was added a soln. of PrPCP (258 mg,
0.76 mmol) in benzene (15 ml). The mixture was stirred for 15 h under reflux and then evaporated to give an air-
sensitive green solid. The solid was washed with Et2O (3 Â 5 ml) and then dissolved in benzene, and the soln.
filtered through a sintered-glass filter and evaporated: pure 4c (300 mg, 57%). Green powder. NMR: in
agreement with values previously reported [2d].
Crystal-Structure Determinations. X-Ray intensities were measured on a Nonius KappaCCD diffractometer
with rotating anode (Mo-Ka, l 0.71073 Š) at 150 K. The structures were solved with automated Patterson
methods [25] (4a) or direct methods [26] (4c) and refined with SHELXL97 [27] against F2 of all reflections.

704
Helvetica Chimica Acta ± Vol. 88 (2005)
Structure calculations, drawings and checking for higher symmetry was performed with the PLATON package
[28].
Data of 4a: C46H50ClP3Ru, Mr ˆ 832.29; black block, 0.30  0.30  0.15 mm; tetragonal, P41 (No. 76), a ˆ
b ˆ 11.9965(1), c ˆ 28.7858(2) Š, Vˆ 4142.74(6) Š3, Z ˆ 4; F(000) ˆ 1728, Dc ˆ 1.334 g cmÀ3; 52092 measured
reflections, 9482 unique reflections (Rint ˆ 0.038); absorption correction based on multiple measured reflections
(PLATON [22], routine MULABS, m ˆ 0.59 mmÀ1, correction range 0.86 ± 0.90); 540 refined parameters, 1
restraint; Flack parameter [29] x ˆ À0.008(13); R(I > 2s(I)): R1 ˆ 0.0230, wR2 ˆ 0.0560. R(all data): R1 ˆ
0.0254, wR2 ˆ 0.0571; S ˆ 1.018; residual electron density (min/max) ˆ À0.39/0.70 e/Š3.
3
Å
Data of 4c: C38H50ClP3Ru, Mr ˆ 736.21; dark red block, 0.33  0.21  0.15 mm ; triclinic, P1 (No. 2); a ˆ
10.4440(1), b ˆ 10.9182(1), c ˆ 17.6424(2) Š, a ˆ 72.8243(4), b ˆ 81.0784(4), g ˆ 66.7457(5)8, Vˆ 1764.20(3) Š3,
Z ˆ 2; F(000) ˆ 768, Dc ˆ 1.386 g cmÀ3; 28439 measured reflections, 8037 unique reflections (Rint ˆ 0.042);
absorption correction based on multiple measured reflections (PLATON [22], routine MULABS, m ˆ
0.68 mmÀ1, correction range 0.84 ± 0.88); 484 refined parameters, 0 restraints; R(I > 2s(I)): R1 ˆ 0.0265,
wR2 ˆ 0.0608; R(all data): R1 ˆ 0.0329; wR2 ˆ 0.0635; S ˆ 1.046; residual electron density (min/max) ˆ À0.80/
1.16 e/Š3.
CCDC-253863 (4a) and 253864 (4c) contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union
Road, Cambridge CB21EZ, UK; fax: ‡ 441223336033; e-mail: deposit@ccdc.cam.ac.uk).
Hydrogen-Transfer Reaction: General Procedure. A suspension of complexes 4a, 4b, or 4c (0.01 mmol) and
0.1m KOH in iPrOH (2 ml) in degassed iPrOH (8 ml) was heated at 828 under N2 for 1 h. To the resulting soln.
was added acetophenone (120 ml, 1 mmol) via a syringe. The temp. of the mixture was maintained constant at
828 during the reaction, which was monitored in time via HPLC analysis (Daicel Chiralcel-OD column; flow rate
i
1.0 ml/min; hexane, PrOH 96 :4).
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Received December 22, 2004