Cationic ruthenium-cyclopentadienyl-diphosphine complexes as catalysts for the allylation of phenols with allyl alcohol; Relation between structure and catalytic performance in O-vs. C-allylation

Article abstract of DOI:10.1002/adsc.200900085

A new catalytic method has been investigated to obtain either O-or C-allylated phenolic products using allyl alcohol or diallyl ether as the allyl donor. With the use of new cationic ruthenium(II) complexes as catalyst, both reactions can be performed with good selectivity. Active cationic Ru(II) complexes, having cyclopentadienyl and bidentate phosphine ligands are generated from the corresponding Ru(II) chloride complexes with a silver salt. The structures of three novel (diphosphine)Ru(II)CpCl catalyst precursor complexes are reported. It appears that the structure of the bidentate ligand has a major influence on catalytic activity as well as chemoselectivity. In addition, a strong cocatalytic effect of small amounts of acid is revealed. Model experiments are described that have been used to build a reaction network that explains the origin and evolution in time of both O-allylated and C-allylated phenolic products. Some mechanistic implications of the observed structure vs. performance relation of the [(diphosphine)RuCp]⁺ complexes and the cocatalytic role of added protons are discussed.

Full text of DOI:10.1002/adsc.200900085

FULL PAPERS
DOI: 10.1002/adsc.200900085
Cationic Ruthenium-Cyclopentadienyl-Diphosphine Complexes
as Catalysts for the Allylation of Phenols with Allyl Alcohol;
Relation between Structure and Catalytic Performance in
O- vs. C-Allylation
Jimmy A. van Rijn,a Martin Lutz,b Lars S. von Chrzanowski,b Anthony L. Spek,b
Elisabeth Bouwman,a,* and Eite Drenta
a
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 Leiden, The Netherlands
Fax : (+31)-71527-4451; e-mail: bouwman@chem.leidenuniv.nl
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH
b
Utrecht, The Netherlands
Received: February 9, 2009; Revised: May 6, 2009; Published online: June 16, 2009
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.20090085.
Abstract: A new catalytic method has been investi- as well as chemoselectivity. In addition, a strong co-
gated to obtain either O- or C-allylated phenolic catalytic effect of small amounts of acid is revealed.
products using allyl alcohol or diallyl ether as the Model experiments are described that have been
allyl donor. With the use of new cationic rutheni- used to build a reaction network that explains the
ACHTUNGTRENNUNGum(II) complexes as catalyst, both reactions can be origin and evolution in time of both O-allylated and
performed with good selectivity. Active cationic C-allylated phenolic products. Some mechanistic im-
Ru(II) complexes, having cyclopentadienyl and bi- plications of the observed structure vs. performance
dentate phosphine ligands are generated from the relation of the [(diphosphine)RuCp]+ complexes and
corresponding Ru(II) chloride complexes with a the cocatalytic role of added protons are discussed.
silver salt. The structures of three novel (diphos-
ACHTUNGTRENNUNGphine)Ru(II)CpCl catalyst precursor complexes are
reported. It appears that the structure of the biden- Keywords: allylation; allylic alcohols; phenols; phos-
tate ligand has a major influence on catalytic activity phines; ruthenium
Introduction
However, the Pd systems for phenol allylation all use
stoichiometric amounts of base [Ti(O-i-Pr)4] to induce
AHCTUNGTRENNUNG
For catalytic allylation reactions, allyl substrates with O-allylation,[5] or Lewis acid (BEt3) to promote C-al-
good leaving groups like carboxylate and halide have lylation.[6] With such Pd systems, electron-rich phenols
been employed in combination with ruthenium and such as 3,5-dimethoxyphenol and 1-naphthol were se-
palladium catalysts. In these reactions generally a lectively C-allylated with allyl alcohol into 4-allyl-3,5-
stoichiometric amount of base is added to induce O- dimethoxyphenol and 2-allyl-1-naphthol, respectively,
allylation, thereby producing inorganic salts as side- of which the latter does not require the use of a stoi-
products.[1–3] In the context of the development of an chiometric amount of base.[5,7] In one example, C-ally-
environmentally benign catalytic route to epoxy lation of phenols in an aqueous environment is in-
resins, the O-allylation of phenols is desired.[4] Allyl duced by adding large amounts of base to the
phenyl ethers can be epoxidized to obtain glycidyl system.[8] The yields in this system are very low. Using
ethers, which are currently produced on a multi-hun- a Pd/TiACTHNUTRGNEG(NU O-i-Pr)4 system, also aniline can be allylated
dred kilotonne/year scale in the epoxy resin industry with allyl alcohol.[9] Although O-allylation of aliphatic
via the conventional epichlorohydrin route with the alcohols was previously reported,[10] phenols appeared
coproduction of stoichiometric amounts of chloride to be much less reactive.
salt waste. From an environmental and atom-efficien-
Only a few examples report catalytic allylation of
cy point of view it would be attractive to use allyl al- phenols using allyl alcohol without the need of stoi-
cohol as the allyl donor, as only water is coproduced. chiometric additives, which are based on ruthenium
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Jimmy A. van Rijn et al.
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Table 1. The ruthenium complexes and phosphine ligands
used in this study with their abbreviations.
precursor by addition of AgOTs (in the absence of
AgOTs hardly any activity is observed), is decisively
controlled by the diphosphine ligand. In particular, it
will be shown that the steric properties of the diphos-
phine ligand are also critically important for the Ru-
catalyzed conversion of the O-allylated product into
thermodynamically more favourable C-allylated prod-
ucts. In addition, evidence will be given of a strong
cocatalytic effect of small amounts of added acid not
only on the rate but surprisingly also on the course of
the allylation reactions.
X
R
Abbreviation
methylene
R1
R1
R1
R1
R2
R2
R2
R3
R4
dppm
dppe
dppp
dppb
o-MeOdppm
o-MeOdppe
o-MeOdppp
o-EtOdppe
dcpe
Results and Discussion
1,2-ethylene
1,3-propylene
1,4-butylene
methylene
1,2-ethylene
1,3-propylene
1,2-ethylene
1,2-ethylene
Synthesis
The new ruthenium complexes were successfully syn-
thesized by displacement of the monodentate triphe-
nylphosphine in [RuCpClACHTNUGTRNEUNG(PPh3)2] with bidentate
phosphine ligands, with complex yields in the range of
80–85%. Reaction temperatures below 1008C were
used in the synthesis of the complexes with the li-
gands bearing ortho-substituents on the phenyl rings.
catalysts.[11,12] Pregosin and co-workers have described When higher temperatures were used, methyl chlo-
a system that is able to C-allylate phenols.[11] Finally, a ride or ethyl chloride was eliminated from the com-
CpRu(diphosphine) system was reported earlier with plex, resulting in a chelating phenolate group.[13] For
which both O-and C-allylated products were formed. the unsubstituted ligands, a slight excess of ligand was
It was proposed that under the applied reaction con- used to ensure complete conversion of the starting
ditions a thermal uncatalyzed Claisen rearrangement material. The excess of ligand and the displaced tri-
of O-allylated product occurred resulting in a mixture phenylphosphine could be easily separated from the
of O- and C-allylated products.[12]
product by flushing the reaction mixture over a small
In the present research, we explored the influence silica gel column with toluene. The orange product
of the diphosphine ligand (Table 1) in cationic band is immobile when toluene is used as the eluents
CpRu(II) complexes on their performance in the cat- and it can be eluted with ethyl acetate. Exactly one
alytic allylation of 4-tert-butylphenol with allyl alcohol equivalent of the ortho-substituted ligands compared
(Scheme 1). It will be shown that chemoselectivity of to ruthenium was used, as it proved to be difficult to
cationic CpRu(diphosphine) complexes, in situ pro- remove excess of ligand from the mixture, either by
duced from the corresponding neutral Ru chloride flash silica gel chromatography or crystallization. All
Scheme 1. Model reaction of 4-tert-butylphenol 1 and allyl alcohol 2 catalyzed by ruthenium complexes showing the multiple
of phenol-derived products 3–6.
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Cationic Ruthenium-Cyclopentadienyl-Diphosphine Complexes as Catalysts
FULL PAPERS
Figure 1. Displacement ellipsoid plots of (a) [RuCpCl(o-EtOdppe)], (b) [RuCpCl(o-MeOdppm)] and (c) [RuCpCl(o-
MeOdppp)] with the adopted atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity. The (disordered) solvent molecules in the structure of [RuCpCl(o-MeOdppm)] are not shown.
complexes have been characterized with 1H- and structures to be considered as an interaction, how-
31P NMR spectroscopy and elemental analysis.
ACHTUNGTRENeNUNG ver, they are close enough to sterically block the
metal centre. The two other oxygen atoms, O(27) and
O(37), are relatively far away from the ruthenium
atom [5.1764(15)–5.3428(13) ꢁ]. The P···O distances
are smaller than the sum of the van-der-Waals radii.
Crystal Structures
The molecular structures of [RuCpCl(o-EtOdppe)], The bite angles of the phosphine ligands very clearly
[RuCpCl(o-MeOdppm)] and [RuCpCl(o-MeOdppp)] reflect the increase of the carbon chain bridge in
are shown in Figure 1. Selected bond distances and these three complexes, going from 72 via 83 to 938 for
angles are listed in Table 2.
the methylene, ethylene and propylene bridges, re-
The binding of the ruthenium ion to the bidentate spectively.
phosphine ligand is slightly asymmetrical in
ACHTUNGTRENNUNG[RuCpCl(o-EtOdppe)] and [RuCpCl(o-MeOdppp)],
À
while in [RuCpCl(o-MeOdppm)] the Ru P distances Catalytic Allylation
are equal. The distances of the ruthenium centre to
the cyclopentadienyl group and the chloride anion are The complexes were tested in the reaction shown in
similar for all three compounds and are comparable Scheme 1. A low catalyst loading of 0.1 mol% was
to those of related ruthenium complexes.[14–17] The dis- used at a temperature of 1008C. Apart from the O-al-
tance of the oxygen atoms O(17) and O(47) to Ru is lylated product 3, the C-allylated products 4–6 were
too large [3.7932(17)–4.2351(14) ꢁ] for all of the observed (Scheme 1). The products were isolated, and
characterized by NMR and GC/MS. The evolution of
phenol-derived products in a typical experiment,
using [RuCp(o-EtOdppe)]ACTHNURGTNE(UNG OTs) as catalyst precursor,
Table 2. Selected bond lengths and angles for the complexes
[RuCpCl(o-EtOdppe)] (a), [RuCpCl(o-MeOdppm)] (b) and
[RuCpCl(o-MeOdppp)] (c).
is shown in Figure 2. From this it can be seen that in
the first 30 min the formation of O-allylated product
3 is very rapid, however, after about one hour the
amount of 3 starts to decrease in time. A similar (but
less pronounced) concentration profile is observed for
5. The concentration of C-allylphenols 4 and 6 steadi-
ly increases in time. After long reaction times (>12 h)
almost only the C-allylated products 4 and 6 are ob-
served. These concentration profiles thus indicate that
the growth of 4 and 6 occurs at the cost of respective-
ly 3 and 5 by a consecutive reaction. Apparently, 4
and 6 are the thermodynamic end products.[18] After
the first hour, the conversion of phenol hardly
changes, however, the product composition changes in
a major way. It thus appeared that the selectivity of
the phenol allylation reaction to the respective prod-
ucts (3–6) is reaction time-dependent, but the product
composition also strongly depends on the structure of
the Ru catalyst used. The results for a series of Ru
(a)
(b)
(c)
Bond distances (ꢀ)
À
Ru(1) Cl(2)
Ru(1) P(3)
2.4456(13)
2.2711(16)
2.2958(14)
1.8395(5)
4.094(4)
3.927(4)
5.260(4)
5.298(3)
2.967(5)
3.443(6)
4.174(5)
2.4363(5)
2.2947(4)
2.2926(4)
1.8386(2)
3.7975(14)
3.7932(17)
5.2606(16)
5.1764(15)
2.9073(13)
4.649(2)
2.4653(4)
2.2967(4)
2.2644(4)
1.8449(1)
3.8416(13)
5.2343(15)
5.3428(13)
4.2351(14)
2.9365(13)
3.7891(19)
3.176(2)
À
À
Ru(1) P(7)
À
Ru(1) Cp
Ru(1)···O(17)
Ru(1)···O(47)
Ru(1)···O(27)
Ru(1)···O(37)
P(3)···O(17)
O(17)···O(27)
O(37)···O(47)
Angles (8)
4.760(2)
À
À
P(3) Ru(1) P(7)
83.43(6)
86.10(5)
91.55(5)
72.145(16)
90.147(16)
90.131(16)
92.591(15)
85.646(15)
86.753(15)
À
À
P(3) Ru(1) Cl(2)
À
À
P(7) Ru(1) Cl(2)
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Jimmy A. van Rijn et al.
FULL PAPERS
trast, with a 1,4-butylene bridge (bite angle 948,
entry 4), all activity for allylation is lost. Instead, the
complex appears active in the isomerization of allyl
alcohol into propanal. The complex with two mono-
dentate phosphine ligands (entry 5) is a very active
catalyst in this isomerization reaction, in agreement
with a prior report,[20] and shows no activity at all to-
wards allylation. When the phenyl groups of the thus
far most active dppe complex are replaced by cyclo-
hexyl groups, the initial activity drops considerably
(from k=1.19 hÀ1 to k=0.08 hÀ1), but the selectivity
for O-allylation increases significantly from 40% to
76% (entry 6).
Figure 2. Formation of phenol-derived products in the reac-
tion of 4-tert-butylphenol with allyl alcohol in time in a typi-
~
&
^
cal experiment; ( ) 3 ( ) 4 ( ) 5 (ꢀ) 6. Reaction conditions:
ratio 4-tert-butylphenol/allyl alcohol/[RuCpCl(o-EtOdppe)]/
AHCTUNGTRENNUNG
AgOTs=1000/1000/1/2. toluene, 1008C. 5 mmol of phenol
was used
When the phenyl groups of the phosphines are sub-
stituted with ortho-methoxy groups, a similar reactivi-
ty pattern is observed. [RuCp(o-MeOdppm)] (OTs)
phosphine complexes are listed in Table 3; some clear (entry 7) is, like its unsubstituted analogue, very selec-
trends can be observed. Product development graphs tive in the formation of C-allylated product 4. There
like Figure 2 for a selection of these complexes are in- is again an optimum in activity with an ethylene
cluded in the Supporting Information.
bridging group (entries 8 and 9). The complexes with
Looking at the phosphines with unsubstituted a methylene or 1,2-ethylene bridge in the ligand
phenyl groups, there is an optimum in activity with a (entry 7–9) show a similar or higher activity compared
1,2-ethylene bridging group in the ligand. to their unsubstituted analogues (entry 1 and 2). The
A
ACHTUNGTRENNUNG(dppm)] (OTs) (entry 1) is more than an order [RuCp(o-MeOdppp)] (OTs) complex, however
A
R
ACHTUNGTRENNUNG
the high selectivity for the C-allylated product 4 with tive for the formation of 3. Having a very similar ac-
the small bite-angle (~728) dppm ligand is remark- tivity as the catalyst based on dppm (entry 1), the
able. This suggests that the formation of the C-allylat- contrast in chemoselectivity is remarkable. Finally,
ed product requires a relatively large free coordina- not only allyl alcohol can be used as allylating agent,
tion space at Ru for this reaction to occur. Increasing but also diallyl ether (entry 11). In all reactions diallyl
the bridge length from 1,2-ethylene to 1,3-propylene ether is formed initially from allylation of allyl alco-
(bite angles respectively ~838 and 928) leads to a de- hol itself, forming water in the process. The diallyl
crease in activity (entry 3, k=0.55 hÀ1), but with a sig- ether further reacts with 1 to form 3–6. It appears
nificant increase in selectivity for O-allylation. In con- that the activity and selectivity of the catalyst are
Table 3. Reaction of 4-tert-butylphenol (1) with allyl alcohol (2) catalyzed by different [RuCp(PP)]+ complexes.[a]
Entry
PP in RuCp(PP)
Conversion of 1 [%]
k[c] [hÀ1]
Selectivity [%]
3
4
3
4
3
4
5
6
0.5 h
1 h
3 h
0.5 h[e]
1 h[e]
3 h
1
2
3
4
dppm
dppe
3
7
21
72
53
0
0.06
1.19
0.55
0
8
92
47
20
–
3
89
53
30
–
2
4
44
–
–
93
56
47
–
2
12
6
–
–
3
28
3
45
24
0
56
42
0
47
80
–
38
70
–
dppp
dppb[b]
–
[b]
5
PPh3
0
0
0
0
–
–
–
–
–
6
7
8
9
10
11
dcpe
4
6
44
40
0
10
15
53
63
7
34
39
60
73
21
87
0.08
0.12
1.15
1.02
0.07
3.22
81
13
58
67
–
19
84
42
33
–
76
7
39
53
99
81
18
90
51
38
1
76
4
12
23
87
53
22
90
74
54
13
20
1
0
5
11
0
1
6
9
12
0
o-MeOdppm
o-MeOdppe
o-EtOdppe
o-MeOdppp
o-EtOdppe[d]
80
84
92
8
13
18
9
[a]
Reaction conditions: ratio 4-tert-butylphenol/allyl alcohol/[Ru]/AgOTs=1000/1000/1/2, toluene, 1008C.
Active in the isomerization of allyl alcohol into propanal.
After 0.5 h.
With diallyl ether instead of allyl alcohol.
In cases 3 and 4 do not add up to 100%, the remainder is product 5 and 6.
[b]
[c]
[d]
[e]
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Cationic Ruthenium-Cyclopentadienyl-Diphosphine Complexes as Catalysts
FULL PAPERS
highly enhanced if diallyl ether is used as the allyl induction period of several hours. However, when a
source, possibly because less water is formed.
stoichiometric amount of p-cresol is added, a conver-
sion of 3 of 92% is observed after only two hours resi-
dence time; not only products 4–6, but also the analo-
gous p-cresol-derived allylation products are formed
as well as 4-tert-butylphenol by transetherification. At
Reactivity of Allyl Ethers
An intriguing aspect of the evolution of products in this stage we hypothesized that the added p-cresol –
the course of the reaction shown in Figure 2, is the or likewise the 4-tert-butylphenol generated by the
question by which mechanism the concentration of in- hydrolysis of 3, – would function as an acidic cocata-
itially formed O-allylated products 3 (and 5) decrease lyst, and have a devastating effect on the selectivity
at longer reaction times, while in parallel C-allylated for O-allylation to give C-allylation instead. We there-
products 4 (and 6) steadily increase. As we observed fore supplemented the activated Ru catalyst with a
that diallyl ether can also be used as allylating agent catalytic amount of hydrated p-toluenesulfonic acid
as a substitute for allyl alcohol, it was suspected that (HOTs·xH2O) and observed that 3 was now reacted
the product ether 3 itself would also show reactivity to form 1 and products 4–6 with a conversion of 86%
towards the catalyst. Thus, phenyl allyl ether 3 was in only two hours. When this reaction is performed
exposed to several conditions and additives to investi- with a catalytic amount of camphorsulfonic acid
gate its reactivity both under thermal conditions in under strictly anhydrous conditions (entry 5), a con-
the absence of the ruthenium catalyst as well as under version of 3 of 49% after two hours is achieved to
prevailing catalytic conditions. The results are given give again a mixture of products 1–6. It should be
in Table 4. When 3 is exposed to allylation reaction noted that the combined amount of 5 and 6 is equal
conditions with any of the reagents present in a typi- to the amount of 1, since the total number of allyl
cal reaction mixture, but in the absence of a catalyst, moieties remains constant.
no reaction is observed, thus excluding the possibility
of a thermal Claisen-type rearrangement. Adding water, hydrolysis of 3 to phenol 1 and allyl alcohol 2
only [RuCp(o-EtOdppe)](OTs) to 3 in toluene also can take place, showing that O-allylation is a reversi-
In summary, in the presence of Ru catalyst and
AHCTUNGTRENNUNG
gives no conversion (entry 1). In contrast, when both ble reaction that is thermodynamically limited. The
the Ru catalyst and a stoichiometric quantity of water conversion to O-allylated product in a batch allylation
with respect to 3 are present, compound 3 is fully con- process will thus be limited by the concentration of
verted in 18 h to give 4-tert-butylphenol, allyl alcohol, water in the reaction medium at reaction tempera-
and C-allylated products 4–6 (entry 2). Under these ture. However, the utilization of an apolar solvent
conditions conversion of 3 takes place only after an such as toluene allows the reaction to proceed beyond
its thermodynamic equilibrium as the solubility of
water in the reaction medium is low and water will
form a separate phase. In order to prevent the re-
Table 4. Reaction of
3 in the presence of [RuCp(o-
EtOdppe)]
(OTs) and different additives.[a]
versed reaction of 3 in a batch process, an experiment
was executed in which water was removed from the
system by means of a Dean–Stark trap. A conversion
of 85% with a selectivity towards O-allylated product
3 of 80% was observed after 30 min. An initial cata-
lyst TOF of about 1700 hÀ1 was thus achieved.
For the reaction of the phenol 1 and allyl alcohol 2
with [RuCp(o-EtOdppe)]ACTHNUTRGNEUGN(OTs) as catalyst, it was
Entry Additives
Conversion Selectivity [%]
of 3 [%]
found that adding catalytic amounts of a Brønsted
acid affects both the selectivity and the rate of the re-
action. When a catalytic amount (2 equivalents on
[Ru]) of strong acid in the form of HOTs is added the
rate of the reaction increases in a dramatic way to an
initial turnover frequency of 6200 hÀ1 in the first
5 min, while for this catalytic system the selectivity
completely shifts towards C-allylation. For the acidic
system in the absence of Ru complex, no activity is
observed, excluding the possibility of an acid-cata-
lyzed reaction.
1
4
5
6
1
2
3
4
5
–
0
0
0
0
3
2
3
0
22
4
24
7
H2O[b]
p-cresol[c]
HOTs[d]
99
92
86
20 55
40 54
33 40
camphorsulfonic acid 49
28 44 21
[a]
Reagents and conditions: ratio 3/ACHTUNTRGNEUNG[RuCpCl(o-EtOdppe)]/
AgOTs=1000/1/2, 1008C, 2 h. If added, additives in ratio
3/H2O/cresol/HX=1000/1000/1000/2.
After 18 h.
Yields are total of both tert-butylphenol and p-cresol de-
rived products.
[b]
[c]
[d]
[e]
HOTs=p-toluenesulfonic acid.
Determined after 18 h.
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Jimmy A. van Rijn et al.
FULL PAPERS
Table 5. Reaction of 4-tert-butylphenol (1) with allyl alcohol (2) catalyzed by different [RuCp(PP)]+ complexes in the pres-
ence of added HOTs.[a]
Entry
PP in RuCp(PP)
Conversion of 1 [%]
k [hÀ1]
Selectivity [%]
3
4
3
4
3
4
5
6
0.5 h
1 h
3 h
0.5 h[b]
1 h[b]
3 h
1
2
3
4
dppm
dppe
dppp
dppb
26
65
70
19
47
65
74
44
64
66
80
58
0.60
2.10
2.41
0.42
100
0
35
0
80
0
24
20
85
52
0
44
0
2
48
84
69
13
4
0
4
4
4
85
47
0
16
25
0
100
100
83
[a]
Reaction conditions: ratio 4-tert-butylphenol/allyl alcohol/[Ru]/AgOTs/HOTs=1000/1000/1/2/2, toluene, 1008C (reaction
shown in Scheme 1).
In the cases that 3 and 4 do not add up to 100, the remainder is product 5 and 6
[b]
Catalyst Performance vs. Catalyst Structure
tion of 3 with p-cresol, but is only producing O-ally-
lated products. Apparently, for this catalyst C-allyla-
The effect of acid addition was also studied for
CpRu(PP) complexes in the direct allylation of 1 with
2 (Table 5; to be compared to Table 3). The product
development graphs for the reactions given in Table 5
are given in the Supporting Information.
Table 6. Reactivity of 3 in the presence of various catalysts
[RuCp (PP)]ACTHNUTRGNEUNG
(OTs) and added HOTs.[a]
The rate of the allylation reaction for all four cata-
lysts is strongly enhanced by the addition of two
equivalents of HOTs (Table 5). The effect of acid on
the selectivity of the reaction is not the same for all
Entry PP in RuCp(PP) Conversion
Selectivity [%][b]
catalysts. For [RuCp
(OTs) the selectivity shifts towards C-allylation. Un-
expectedly, for [RuCp(dppm)] (OTs), O-allylation be-
comes favored, with an initial selectivity of 100% for
3 after 30 min. Surprisingly, also [RuCp(dppb)] (OTs)
ACHUTNGTRENNUG(dppe)] (OTs) and [RuCpACHTUNTGREN(NUGN dppp)]
of 3 [%]
1 h
3 h
1
4
5
6
AHCTUNGTRENNUNG
1
2
3
4
dppm
dppe
dppp
dppb
7
17
88
97
2
28 36 21 15
79
86
2
26 42
26 45
6
3
0
26
26
0
AHCTUNGTRENNUNG
becomes active for O-allylation. The isomerization of
allyl alcohol into propanal, observed with this catalyst
under neutral conditions, is apparently blocked by ad-
dition of acid. This latter observation could be ration-
alized by the fact that the formation of a Ru(II)-allyl
alcoholate intermediate, being the first step in the iso-
merization reaction,[12] is suppressed in acidic condi-
tions.
1
99
[a]
Reagents and conditions: ratio 3/[Ru]/AgOTs/HOTs=
1000/1/2/2, 1008C.
After 3 h.
[b]
Table 7. Transallylation of 3 with p-cresol in the presence of
various catalysts [RuCp(PP)]
(OTs).[a]
AHCTUNGTRENNUNG
The reaction of 3 into 1–6 with these four catalysts
in the presence of added HOTs was also investigated
(Table 6). [RuCp
version of 3 into C-allylated products, while [RuCp-
(dppb)] (OTs) appears to be hardly active at all. In
contrast, [RuCp(dppe)] (OTs) and [RuCp(dppp)]
(OTs) are very active for the conversion of 3 to C-al-
lylated products.
ACHTUNGTRENNUNG(dppm)] (OTs) shows only low con-
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
Entry PP in RuCp(PP) Conversion
Selectivity [%][b]
of 3 [%]
1 h
3 h
1
4
5
6
The apparent unreactivity of 3 with [RuCp
(OTs) and [RuCp(dppb)] (OTs), prompted us to in-
vestigate the transallylation of 3 with p-cresol in the
absence of additional acid (Table 7). For
[RuCp(dppm)] (OTs) the reactivity is extremely low.
As expected, [RuCp(dppe)] (OTs) and [RuCp(dppp)]
ACHTUNGTRNE(NUNG dppm)]
AHCTUNGTRENNUNG
1
2
3
4
dppm
dppe
dppp
dppb
0
0
–
–
–
0
0
0
–
0
0
0
99
80
19
99
91
49
50
50
49[c]
50
50
0
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[a]
[b]
[c]
Reagents and conditions: ratio 3/p-cresol/[Ru]/AgOTs=
1000/1000/1/2, 1008C.
After 3 h; yields are total of both tert-butylphenol and p-
cresol derived products.
(OTs) behave very similar to [RuCp(o-EtOdppe)]
(OTs) and rapidly produce a large quantity of C-ally-
lated products. Surprisingly, [RuCp
ACHTUNGTREN(NGNU dppb)] (OTs) in
the presence of acid is quite active in the transallyla-
Remaining 51%=t-butyl phenyl and p-cresyl allyl ethers.
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Cationic Ruthenium-Cyclopentadienyl-Diphosphine Complexes as Catalysts
FULL PAPERS
tion is blocked; only after longer reaction times (6– [(PP)CpRu(IV)
ACHTUNGTRENNUNG
18 h) do C-allylated products slowly appear. Appa- s- to p-allyl rearrangement occurs, to prevent a 20-
rently the result from Table 6, entry 4, should be inter- electron species either a p-allyl dicationic complex
preted such that this catalyst is reactive with 3, but (C) is formed, or a phosphine must dissociate (D).
rapidly regenerates it again in the absence of other Displacement of the hydroxide anion by phenolate in
phenol moieties.
an acid-base reaction from species C will be fast and
results in the species [(PP)CpRu(IV)(p-allyl)] (pheno-
AHCTUNGTRENNUNG
late) (OTs) (C’, R’=Ph) while H2O is coproduced.
Due to the coordinating nature of the phenolate
anion, strongly attracted to the positively charged
Mechanistic Considerations
It is well-known that the activation of allyl-X (X= Ru(IV), either species B’ or D’ will be formed. After
halide, carboxylates, alkoxide, etc) substrates by reductive elimination, Ru(II)-bound product is ob-
L2CpRu(II)X complexes in allylation reactions pro- tained (A’) or C-allylated products are irreversibly
ceeds via oxidative addition to give intermediate formed.
LnCpRu(IV)
G
Complexes of type B, C and D must play a central
0 for p-allyl).[21] The initially formed product of oxida- role in the reaction network of all possible allylation
tive addition appeared to be a s-allyl species which reactions. Four possible reaction pathways can be fol-
only after dissociation of a phosphine or CO ligand lowed, the first being the reverse reaction towards
converted into the thermodynamically more stable p- phenol 1 and allyl alcohol 2 in the presence of water.
allyl species. Reasoning along these lines with allyl al- The second is the reversible formation of diallyl
cohol as substrate in the present study, its oxidative ether, caused by displacement of the hydroxy or phe-
addition in [(PP)CpRu(II)(allyl alcohol)] OTs nolate anion by an allyl alcoholate anion, while form-
(Scheme 2, A, R=H) will initially produce ing water or phenol in the process. Note that diallyl
Scheme 2. Proposed catalytic cycle for allylation. B, C, D (R=H, allyl) and B’, C’, D’ (R=phenyl) are isomeric 18-electron
species formed after oxidative addition.
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Jimmy A. van Rijn et al.
FULL PAPERS
ether can also be used as the allylation agent, which p-allyl moiety of these strongly electrophilic com-
has been confirmed experimentally (Table 3). A third plexes at phenol.[11]
possibility is reductive elimination of the phenyl allyl
Based upon the observed dependence of the allyla-
ether 3 from B’, C’ or D’ (R’=Ph), a step that is also tion selectivity on the structure of the applied phos-
reversible. The final option is the irreversible forma- phine bidentate ligand and its chelating strength, we
tion of the C-allylated product 4. Likewise, the prod- favour a hypothesis that C-allylation also requires ac-
ucts 5 and 6 can be formed starting from 4 and allyl tivation of the phenol via its O atom at ruthenium
alcohol. When 4 is present in a significant concentra- (Scheme 3). If the isomeric p-allyl species C’ and D’
tion, exchange of phenolates could occur in inter- are more abundantly present, due to weak chelation
mediates B’, C’ and D’. The occurrence of exchange or space around ruthenium, reductive elimination of
also rationalizes the observations in Table 4 and O-allylated product will be relatively slow, as this pro-
Table 6 that reaction products 5+6 are produced in ceeds via the s-allyl intermediate B’. It is proposed
stoichiometric amounts on 1; these products can only that C-allylated products are formed starting from in-
be formed when phenolate 1 is exchanged for pheno- termediate D’ (Scheme 3). The mechanism could pro-
late 4, regenerating 1 in the process. Since the O-ally- ceed via an intramolecular Friedel–Crafts reaction, in
lation steps all are reversible while formation of the which the phenolate ring has to reorientate in the
C-allylated products is irreversible, all initially formed same plane as the allyl group, and for which sufficient
phenyl ether products can thus eventually be convert- space around ruthenium is needed, forming species E
ed, if a sufficient amount of 2 is available, into fully (Scheme 3). After tautomerization the C-allylated
C-allylated product 6 as is observed after long reac- product is formed (G). Another mechanism could
tion times (e.g., 18 h).
s-p allyl
proceed via an ortho-metallation-type reaction, for
A
interconversion
of
the which space around ruthenium is also required in
À
(PP)CpRu(IV)allyl phenolate complex (Scheme 2, B’, order to activate the C H bond, forming species F;
for R’=Ph) will be dependent on the chelating after reductive elimination species G is then formed.
strength of PP, which is expected to decrease in the
order dppp>dppe>dppm. O-Allylated product 3 will
be formed from B’ (for R’=Ph) in a microscopic Effect of the Acid
analogous reverse of the oxidative addition of allyl al-
cohol 2 to complex A’ (for R’=Ph). However, C-ally- We thus summarize our working hypothesis: more
lation is generally thought to proceed via p-allyl spe- free coordination space at Ru is required for C-allyla-
cies. For instance, C-allylation catalyzed by tion. Therefore weakly chelating and/or small bite
(p-allyl)]2+ complexes was pro- angle phosphine ligands give more selective catalysts
ACHTUNGTRENNUNG[Cp*Ru(IV)ACHTUNGTRENNUNG(CH3CN)2ACHTGUNTRENNUGN
posed to proceed via Friedel–Crafts-type attack of the for C-allylation, whereas stronger chelating, and/or
larger bite angle ligands give catalysts which show
higher selectivity for O-allylation.
The rate-determining step in the allylation reaction
with allyl alcohol is generally proposed to involve the
oxidative addition of allyl alcohol, as OH is consid-
ered to be a poor leaving group.[11] Often intermedate
protonation of allyl alcohol is implicated to increase
the reactivity of allyl alcohol for allylation reac-
tions.[10,11,22,23] The strong increase in allylation reaction
rate observed with addition of a catalytic quantity of a
strong acid such as HOTs (Table 5), can thus be ex-
plained by protonation of the allyl alcohol, making
H2O the better leaving group.
By the same reasoning, the protonation of the allyl
ether 3 at the phenoxy group, makes the phenol a
good leaving group, thus rationalizing the promoting
effect of acid on the rate of activation of allyl ether 3
(Table 6).
[(PP)CpRu(IV)
A
strongly electrophilic, dicationic
AHCTUNGTRENNUNG
and phosphine dissociation is not necessary to form
the stable, resting state p-allyl species. Phenol may
assist in the oxidative addition, perhaps as a proton
donor or otherwise in hydrogen bonding and a con-
certed mechanism, as is clear from the transallylation
Scheme 3. Proposed mechanism for C-allylated products via
either Friedel–Crafts or ortho-metallation.
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Cationic Ruthenium-Cyclopentadienyl-Diphosphine Complexes as Catalysts
FULL PAPERS
results shown in Table 4. However, the oxidative addi- tions. Thus, C-allylated products can ultimately be
tion also occurs in the presence of much weaker acids, catalytically produced in high yield by allylation of
such as 1-octanol.[24] In both cases, the addition of a phenols with allyl alcohol.
strong acid results in a strongly increased rate of reac-
tion. The dramatic change in the reactivity of the cat- however, strongly dependent on the structural charac-
alyst [CpRu
(dppb)]+ upon addition of acid is indica- teristics of the Ru complex as appears from the re-
The efficiency of the consecutive C-allylation is,
ACHTUNGTRENNUNG
tive of a change in the formed intermediate. It has sults given in Table 3. It appears that restricted coor-
been reported that oxidative addition of allyl alcohol dination space at the ruthenium centre favours the
in the absence of acid does take place, the reaction formation of the O-allylated product, while sufficient
medium becoming more basic,[25] which supports our space, either due to weak chelation and/or small bite
theory on initial hydroxyl anion formation.
angle, favours C-allylation. The addition of catalytic
The microscopic reverse of the acid-catalyzed oxi- amounts of acid strongly promotes the rate of the re-
dative addition of phenyl allyl ether, that is, the for- action. The chelate strength plays a less dominant
mation of 3 by coordination of phenol to the Ru(IV) role in the acidic system, because the bidentate phos-
centre via (concerted) deprotonation and reductive phine is less likely to dissociate and thus the bite
elimination of 3 is expected to be more facile and angle seems to be determining selectivity. Further
thus occurs with higher rate in the presence of acid. studies will be devoted to further optimization of the
As the rates of the reactions shown in Scheme 3 to- catalyst structure aimed at totally selective O-allyla-
wards either species E or F are expected to be rela- tion, and fully blocking the C-allylation pathway.
tively insensitive to acid, it can be rationalized that These studies will include DFT calculations of pro-
even with small bite angle dppm as well as the large posed steps to obtain a more detailed characterization
bite angle ligand dppb an increased selectivity for O- and understanding of the individual selectivity- and
allylation is observed in the presence of a catalytic rate-determining steps.
quantity of acid. However, the cause of this increased
selectivity is different for the two catalysts. While
[RuCp
tion of allyl alcohol with a slightly higher rate than
that of 3 (still being slow for both), [RuCp(dppb)] General Remarks
(dppm)] (OTs) undergoes the oxidative addi- Experimental Section
AHCTUNGTRENNUNG
(OTs) performs the oxidative addition of 3 in a
higher rate, however, it is hardly able to produce C-al-
All reactions were performed under an argon atmosphere
using standard Schlenk techniques. Solvents were dried and
distilled by standard procedures and stored under argon.
The phosphine ligands dppm, dppe, dppp, dppb, dcpe, and
triphenylphosphine were commercially available and used as
received. The synthesis of the substituted phosphine ligands
lylated products. The complexes [RuCp
ACHTUNGTREN(NUNG dppe)] (OTs)
and [RuCp(dppp)] (OTs) react in such a high rate,
ACHTUNGTRENNUNG
that whereas oxidative addition of 3 as well as the re-
ductive elimination to 3 has become more facile, the
high rate of C-allylation with these catalysts makes
that the reaction rapidly proceeds to the thermody-
namic sink of C-allylated products.
o-MeOdppm,[26]
o-MeOdppe,[27]
o-MeOdppp,[28]
and
o-EtOdppe[29] has been previously reported in literature.
RuCl3·3H2O (Johnson & Matthey) was used as received.
[RuCpCl
[RuCpCl
[RuCpClACTHUNGTRENNNUG
(PPh3)2],[14] [RuCpCl(dcpe)],[30]
ACHUTGTNRENNUG CAHTUNGTRENNUGN
A
G
E
(dppe)],[16] [RuCpCl
ACHUTGTNRNEUNG ACHTUNGTRENNUNG(dppp)],
Conclusions
prepared according to literature procedures. C,H,N,S analy-
ses were performed on a Perkin–Elmer 2400 Series II ana-
lyzer.
In summary, a catalytic system has been developed
that can catalyze both O- as well as C-allylation of
phenols, without the need of any stoichiometric
amounts of additives. It was shown that the O-allylat-
ed products are reversibly formed, while C-allylated
products are produced irreversibly. Small quantities of
an acid can fulfill a strong rate-promoting role. It is
proposed that protons strongly reduce the activation
barriers for oxidative addition and reductive elimina-
NMR Experiments
1H NMR spectra (300 MHz), and 31P{1H} NMR spectra
(121.4 MHz) were measured on a Bruker DPX-300. Chemi-
cal shifts are reported in ppm. Proton chemical shifts are
relative to TMS, and phosphorus chemical shifts are relative
to 85% aqueous H3PO4. The spectra were taken at room
temperature.
tion at the RuCpACTHNUTRGNE(NUG P–P) centre, for both of the sub-
strate allyl alcohol as well as product allyl ether com-
pounds.
General Procedure for the Eynthesis of RuCpCl(PP)
It has been unambiguously demonstrated that a
Ru-catalyzed conversion of O-allylated products to
the thermodynamically more favourable C-allylated
A solution of RuCpClACHTNUTRGNEUNG(PPh3)2 (72 mg, 0.1 mmol) and the bi-
dentate phosphine ligand (0.1 mmol) in 5 mL toluene was
stirred for 16 h at 908C. The solution was cooled to room
products may readily occur under allylation condi- temperature and flushed over a column of silica gel (3 g, d=
Adv. Synth. Catal. 2009, 351, 1637 – 1647
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asc.wiley-vch.de
1645

Jimmy A. van Rijn et al.
FULL PAPERS
1 cm) with 15 mL of toluene to remove the triphenylphos-
phine. Finally, the orange product was eluted with ethyl ace-
tate until the eluents was colourless. The solution was then
concentrated under vacuum to approximately 1 mL and the
product precipitated with petroleum ether.
RuCpCl(o-EtOdppe): Obtained as a yellow/orange solid;
yield: 63 mg (81%). Crystals suitable for X-ray diffraction
were obtained by slow diffusion of n-hexane into a solution
of the complex in toluene. Anal. calcd. for
X-Ray Crystal Structure Determinations
X-ray intensities were measured on a Nonius Kappa CCD
diffractometer with rotating anode (graphite monochroma-
tor, l=0.71073 ꢁ) up to a resolution of (sin q/l)max
=
0.65 ꢁÀ1 at a temperature of 150 K. The structures were
solved with automated Patterson methods (program
DIRDIF-99[33]). Refinement was performed with SHELXL-
97[34] against F2 of all reflections. Geometry calculations, il-
lustrations, and checking for higher symmetry was per-
formed with the PLATON program.[35]
C39H45ClO4P2Ru·0.5ACHTNUGRTENUNG(toluene): C 62.07, H 6.01; found: C
61.76, H 5.99. 1H NMR (CDCl3): d=8.20–8.11 (m, 2H,
ArH), 7.24–7.17 (m, 4H, ArH), 6.99 (t, 2H, J=7 Hz, ArH),
6.96 (bs, 2H, ArH), 6.69–6.61 (m, 6H, ArH), 4.56 (s, 5H,
Cp), 3.69–3.56 (m, 8H, OCH2), 2.63 (m, 4H, CH2), 0.85 (t,
6H, J=7 Hz, CH3), 0.77 (t, 6H, J=7 Hz, CH3); 31P{1H}-
NMR (CDCl3): d=68.1.
The crystal of [RuCpCl(o-EtOdppe)] was cracked into
two fragments. The orientation matrices of both fragments
were taken into account during intensity integration with
the program EvalCCD.[36] Refinement was performed on a
HKLF5 file.[37] Hydrogen atoms were introduced in calculat-
ed positions and refined with a riding model. One ethyl
group was refined with a disorder model.
The crystal of [RuCpCl(o-MeOdppm)] contained large
voids (211.6 ꢁ3/unit cell) filled with disordered solvent mol-
ecules. Their contribution to the structure factors was se-
cured by back-Fourier transformation using the SQUEEZE
routine of the program PLATON,[35] resulting in 22 elec-
trons/unit cell. Hydrogen atoms were introduced in calculat-
ed positions. The hydrogen atoms of the Cp ligand were re-
fined freely with isotropic displacement parameters; all
other hydrogen atoms were refined with a riding model.
In [RuCpCl(o-MeOdppp)] hydrogen atoms were intro-
duced in calculated positions. The hydrogen atoms of the Cp
ligand were refined freely with isotropic displacement pa-
rameters; all other hydrogen atoms were refined with a
riding model.
RuCpCl(o-MeOdppm): Obtained as an orange solid;
yield: 58 mg (82%). Crystals suitable for X-ray diffraction
were obtained by slow diffusion of n-hexane into a solution
of the complex in toluene. Anal. calcd. for
C34H35ClO4P2Ru·1.5ACTHUNGTRENNG(U water): C 53.94, H 5.06; found: C
1
54.09, H, 4.96. H NMR (CDCl3): d=8.05 (d, 2H, J=5 Hz,
ArH), 7.60–7.54 (m, 4H, ArH), 7.28–7.14 (m, 4H, ArH),
6.96–6.75 (m, 6H, ArH), 4.50 (s, 5H, Cp), 3.65 (s, 6H,
OMe), 3.62 (s, 6H, OMe), 2.77 (s, 2H, CH2); 31P{1H} NMR
(acetone-d6): d=4.1.
RuCpCl(o-MeOdppp): Obtained as a yellow solid; yield:
52 mg (71%). Crystals suitable for X-ray diffraction were
obtained by slow diffusion of n-hexane into a solution of the
complex in toluene. Anal. calcd. for C36H39ClO4P2Ru·1.33
1
N
(CDCl3): d=7.36–7.29 (m, 4H, ArH), 7.04–6.99 (m, 4H,
ArH), 6.82 (t, 4H, J=7 Hz, ArH), 6.65–6.61 (m, 4H, ArH),
4.28 (s, 5H, Cp), 3.36 (s, 6H, OMe), 3.27 (s, 6H, OMe), 2.8–
2.4 (br m, 6H, CH2); 31P{1H} NMR (acetone-d6): d=40.2.
CCDC 678457 [for RuCpCl(o-EtOdppe)], CCDC 678458
[for RuCpCl(o-Meodppm)] and CCDC 678459 [for
RuCpCl(o-MeOdppp)] contain the supplementary crystallo-
graphic 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. Further de-
tails about the crystal structure determinations are given in
the Supporting Information.
General Procedure for Catalytic Reactions
5 mmol of 4-tert-butylphenol (or in some experiments 4-tert-
butylphenyl allyl ether), 0.005 mmol of the ruthenium com-
plex, 0.01 mmol of AgOTs and, if indicated, 0.01 mmol of
additive were charged into the reaction vessel and flushed
with argon. Degassed and dried toluene was added (5 mL)
and the mixture was stirred for five minutes. Allyl alcohol
(or diallyl ether) was added (5–10 mmol) and the reaction
mixture was stirred for 3 h at 1008C. Samples were taken at
certain time intervals with an airtight syringe and analyzed
by gas chromatography. To isolate and characterize com-
pounds 3–6, preparative HPLC purification was performed
for selected experiments; the isolated yields corresponded
with the yields found by GC. The NMR and mass spectra of
the products 3–6 were in agreement with the data found in
the literature.[32]
Acknowledgements
This work was supported by the Technology Foundation
STW and in part (ML, ALS) by the Council for the Chemical
Sciences of the Netherlands Organization for Scientific Re-
search (CW-NWO). We would like to thank Shell Global sol-
utions International BV, in particular Dr. K. Almeida Lenero
and Mr. L. Schoon for supplying phosphine ligands. Dr. R.
Postma (Hexion) and Dr. J.K. Buijink (Shell Global Solu-
tions International BV) are acknowledged for fruitful discus-
sions.
GLC Method
Quantitative gas liquid chromatography analyses were car- References
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VF-1 ms (25 mꢂ0.25 mm) column with decane as internal
standard. The temperature gradient used was: isothermal
for 5 min at 408C, heating 108C/minute to 2508C and finally
isothermal for 5 min at 2508C.
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Adv. Synth. Catal. 2009, 351, 1637 – 1647
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