The lewis acidic ruthenium-complex-catalyzed addition of β-diketones to alcohols and styrenes is in fact brensted acid catalyzed

Article abstract of DOI:10.1002/chem.200700705

The Perchlorate salt of the dicationic bipy-ruthenium complex cis-[Ru(6,6′-Cl₂bipy)₂(H₂O)2] ²⁺ effectively catalyzes addition of β-diketones to secondary alcohols and styrenes to yield the α-alkylated β-diketones. In a catalytic addition reaction of acetylacetone to 1-phenylethanol, the κ²-acetylacetonate complex [Ru(6,6′-Cl ₂bipy)₂(κ²-acac)]ClO₄ was isolated after the catalysis; this complex is readily synthesized by reacting cis-[Ru(6,6′-Cl₂bipy)₂-(H₂O) ₂] (ClO₄)₂ with acetylacetone. [Ru(6,6′-Cl₂bipy)₂(κ-acac)]ClO₄ is unreactive toward 1-phenylethanol in the presence of HClO₄; it also fails to catalyze the addition of acetylacetone to 1-phenylethanol. On the basis of these observations, it is proposed and confirmed by independent experiments that the catalytic addition of β-diketones to the secondary alcohols is in fact catalyzed by the Bronsted acid HClO₄, which is generated by the reaction of cis-[Ru(6,6′-Cl₂bipy)₂(H ₂O)₂]-(ClO₄)₂ with the β-diketone.

Full text of DOI:10.1002/chem.200700705

DOI: 10.1002/chem.200700705
The Lewis Acidic Ruthenium-Complex-Catalyzed Addition of b-Diketones
to Alcohols and Styrenes Is in Fact Brønsted Acid Catalyzed
Pei Nian Liu, Zhong Yuan Zhou, and Chak Po Lau*[a]
Abstract: The perchlorate salt of the
dicationic bipy–ruthenium complex cis-
by
reacting
cis-[Ru(6,6’-Cl2bipy)2-
phenylethanol. On the basis of these
observations, it is proposed and con-
firmed by independent experiments
that the catalytic addition of b-dike-
tones to the secondary alcohols is in
fact catalyzed by the Brønsted acid
HClO4, which is generated by the reac-
A
ACHTREUNG
[Ru(6,6’-Cl2bipy)2
A
[Ru(6,6’-Cl2bipy)2(k2-acac)]ClO4 is un-
reactive toward 1-phenylethanol in the
presence of HClO4; it also fails to cata-
lyze the addition of acetylacetone to 1-
catalyzes addition of b-diketones to
secondary alcohols and styrenes to
yield the a-alkylated b-diketones. In a
catalytic addition reaction of acetylace-
tone to 1-phenylethanol, the k2-acetyla-
cetonate complex [Ru(6,6’-Cl2bipy)2(k2-
acac)]ClO4 was isolated after the catal-
ysis; this complex is readily synthesized
tion of cis-[Ru(6,6’-Cl2bipy)2
A
Keywords:
addition reaction
Brønsted acids · ruthenium
beta-diketones
·
·
AHCTREUNG
·
alcohols
Introduction
Li and coworkers have employed silver salts or combina-
tions of silver and gold salts as catalysts for the addition of
b-dicarbonyl compounds to styrenes, dienes, trienes, and
cyclic enol ethers;[6] coinage metals have not been common
catalysts of choice in synthesis. More recently, the same
group has developed a new route to fused lactones, the syn-
thesis of which are based on a cascade addition of b-ketoest-
ers to a cyclic diene followed by an in situ lactonization; the
reactions are catalyzed by a combination of Lewis acid and
Alkylation of b-dicarbonyl compounds is one of the most
À
common methodologies for C C bond construction; the ad-
dition of b-dicarbonyl compounds to olefins, which yields al-
kylated dicarbonyls and is a highly atom-economical pro-
cess,[1] has attracted much attention in recent years. Widen-
hoefer and co-workers reported several cases of Pd-cata-
lyzed intramolecular addition of b-diketones and b-ketoest-
ers to olefins to form six-membered ring compounds.[2]
Similar Pd-catalyzed cyclization reactions of alkenyl b-keto
esters and amides to give six-, seven-, or eight-membered-
ring carbocycles were found to be promoted by lanthanide
triflate.[3] Intermolecular addition of b-dicarbonyl com-
pounds to ethylene and propylene catalyzed by Pd and Pt
complexes has recently been published.[4] Moreover, Pd-cat-
Brønsted acid, Ga
(OTf)3/HOTf.[7]
A
Most of the recent studies of the catalytic addition of b-di-
carbonyl compounds to olefins or polyenes are based on pal-
ladium, coinage metals, such as silver and gold, and gallium;
however, the catalytic activities of other transition metals
for this reaction have scarcely been investigated. Reported
here is our work on the ruthenium-catalyzed addition of b-
diketones to alcohols and styrenes. This study, however, de-
monstates that the addition reactions are in fact catalyzed
by the Brønsted acid HClO4 generated by the reaction of
the Lewis acidic ruthenium complex with the b-diketone;
the ruthenium complex merely acts as a precatalyst.
À
alyzed intermolecular addition of the a-C H bond of mono-
carbonyl and b-dicarbonyl compounds to dienes has also
been developed.[5]
[a] Dr. P. N. Liu, Prof. Dr. Z. Y. Zhou, Prof. Dr. C. P. Lau
Department of Applied Biology & Chemical Technology
The Hong Kong Polytechnic University, Hung Hom, Kowloon
Hong Kong (China)
Results and Discussion
Fax : (+852)2364-9932
E-mail: bccplau@polyu.edu.hk
We studied the catalytic activities of several dicationic
ruthenium complexes containing bipyridine (bipy) ligands.
We hoped that by virtue of the complexes being able to pro-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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Chem. Eur. J. 2007, 13, 8610 – 8619

FULL PAPER
vide two cis vacant sites readily
and the high Lewis acidity of the
metal centers, dual activation of
the two substrates, that is, the
olefin and the b-dicarbonyl com-
catalytic addition of acetylacetone to 1-phenylethanol (5a)
instead of styrene; we anticipated that this reaction might
yield the same product of the addition of 2a to styrene. It
has been reported that benzylation of active methylenes,
such as b-diketones and b-keto esters, with benzhydryl alco-
hols are promoted by a stoichiometric amount of Lewis
acid, BF3·OEt2.[9] In a more recent communication, Bi-
pound, might be possible. The
design and synthesis of electrophil-
ic transition-metal complexes has
G
been an important area in organo-
metallic chemistry catalysis.[8]
It can be seen from Table 1 that
the dicationic bipy–ruthenium com-
plexes are active for the catalytic
addition reaction of acetylacetone (2a) to styrene to afford
4a with similar activity. The systems are efficient as a cata-
Table 2. Catalytic addition of acetylacetone (2a) to 1-phenylethanol (5a)
Table 1. Catalytic addition of acetylacetone (2a) to styrene (3a) with
with bipy–ruthenium complexes.[a]
bipy–ruthenium complexes.[a]
Yield [%][b]
Entry
Catayst
Solvent
Yield [%][b]
Entry
Catalyst
Solvent
1
2
3
4
1a
1b[c]
1c
1d
1d
1d
1d
1d
1d
neat
neat
neat
neat
71
66
59
86
61
72
52
0
1
1a
1a
1a
1a
1a
1b[d]
1c
1d
1d
1d
neat
47
46
47
34
0
46
39
49
37
34
2[c]
3[c]
4[c]
5
DCE
CHCl3
CH2Cl2
dioxane
neat
neat
neat
neat
neat
5[d]
6
neat
DCE
CHCl3
dioxane
neat
6
7
8
7
8
9[e]
21
9[e]
10[f]
[a] Reaction conditions: catalyst (0.004 mmol), 2a (1.5 mmol), 5a
(1.0 mmol), 808C, 6 h, 2.0 mL of solvent was added if noted. [b] Based on
the amount of 3a used, which was determined by 1H NMR spectroscopy
with CH3NO2 as the internal standard. [c] Compound 1b contains traces
of an impurity (see the Experimental Section). [d] Reaction temperature:
708C. [e] Reaction conditions: catalyst (0.002 mmol), 2a (3.0 mmol), 5a
(2.0 mmol), 808C, 17 h.
[a] Reaction conditions: catalyst (0.004 mmol), 2a (1.5 mmol), 3a
(1.0 mmol), 908C, 8 h, 2.0 mL of solvent was added if noted. [b] Based on
the amount of 3a used, which was determined by 1H NMR spectroscopy
with CH3NO2 as the internal standard. [c] In reflux. [d] Compound 1b
contains traces of an impurity (see the Experimental Section). [e] Re-
action temperature 808C. [f] Reaction conditions: catalyst (0.002 mmol),
2a (3.0 mmol), 3a (2.0 mmol), 1008C, 16 h.
are effective for the catalytic addition of acetylacetone to
neat 1-phenylethanol (5a) to give 4a; the yield of product in
each case is higher than that of the corresponding addition
reaction to styrene. Among the complexes containing the
triflate anion, the one containing the 6,6’-dichloro-2,2’-bipyr-
idine (6,6-Cl2bipy) ligand gave the highest yields (entries 1–
3, Table 2). Changing the anion of the (6,6’-Cl2bipy)-com-
lyst loading of only 0.4 mol% is needed, while 10 mol% of
AgOTf or 5 mol% AuCl3/15 mol% AgOTf was needed in
an earlier report.[6] The reaction can be performed in air and
no solvent is needed; addition of solvent does not seem to
be beneficial (entries 2–4). No desired product was yielded
when 1,4-dioxane was used as solvent and a complex mix-
ture of products were formed. Yao and Li also reported that
in their study of the AgOTf-catalyzed addition of acetylace-
tone to styrenes, no addition product was obtained when
using dioxane.[6a] Lowering the temperature (to 808C) re-
sulted in a lower yield (entry 9). The catalytic system was
still efficient even when the substrate/catalyst (S/C) ratio
was increased to 1000, a 34% yield could be obtained after
16 h (entry 10).
À
plex to ClO4 further increases the catalytic activity of the
dicationic complex (entry 4, Table 2). The complex cis-
[Ru(6,6’-Cl2bipy)2
throughout the study. It is, however, strange to find that the
tetrafluoroborate complex cis-[Ru(6,6’-Cl2bipy)2(H2O)2]-
(BF4)2 (1e) is totally inactive for the addition reaction. Yao
A
N
ACHTREUNG
AHCTREUNG
and Li found that while AgOTf is an active catalyst for the
addition of acetylacetone to styrene, AgBF4 is totally inacti-
ve.[6c] The same authors also learned that changing the coun-
terion of the silver salt in the bimetallic system AuCl3/
At this stage, we were also interested to see if the dicat-
ionic bipy–ruthenium complexes were also active for the
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C. P. Lau et al.
AgOTf, which is an active catalyst for the addition of acety-
Subsequently, the addition reaction was examined on
more substrates by using 1d as the catalyst. b-Diketones
were added to various alcohols at 808C (Table 3). Benzyla-
tion of b-diketones 2a–c with benzylic alcohols 5a–f and 5j
are highly effective (entries 1–6, 10–14). The allylation reac-
tions of 2a and 2c with allyl alcohols 5g and 5h are also ef-
fective, the yields of allyl product 4g, 4h, 4o, and 4p are
comparable to those of the reaction with benzyl alcohols
(entries 7, 8, 15, and 16). It is noteworthy that the reaction
of 2a with the heterocycle alcohol 5i gives the functional-
ized heterocycle product 4i, although the yield is relatively
À
lacetone to styrene, from OTfÀ to BF4 , resulted in a very
substantial loss of catalytic activity in the system; no explan-
ation was provided for this phenomenon.[6b] A lower yield
was obtained at lower temperature (entry 5, Table 2). The
reactions performed in 1,2-dichloroethane (DCE) and
CHCl3 could also afford the product 4a, with slightly lower
yields; however, the reaction in dioxane gave no yield of 4a
(entries 6–8). It is noteworthy that when the S/C ratio was
increased to 1000, a product yield of 21% was obtained
after 17 h (entry 9).
Table 3. Catalytic addition of b-diketones to alcohols with cis-[Ru(6,6’-Cl2bipy)2
A
ACHTREUNG
Entry
b-Diketone
Alcohol
Yield [%][b]
1
82
2
82
90
86
72
81
3
4
5
6[c]
7[c]
86
8
9
73
41
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Addition of b-Diketones to Alcohols and Styrenes
FULL PAPER
Table 3. (Continued)
Entry
b-Diketone
Alcohol
Product
Yield [%][b]
10[c]
92 (1:0.8)[d]
11[c]
12[c]
13[c]
14[c]
92
91
97
86[e]
15[c]
61
16[c]
71
17[c]
65[f]
[a] Reaction conditions: catalyst (0.004 mmol), b-diketone (1.5 mmol), alcohol (1.0 mmol), 808C, 6–17 h, no solvent was used unless noted. [b] Based on
the amount of alcohol used (isolated yield). [c] DCE added (2.0 mL). [d] The ratio of diastereomers was determined by 1H NMR spectroscopy. [e] Re-
action time: 20 h. [f] Reaction time: 42 h.
low (entry 9). The reaction of 2c with norbornyl alcohol 5k
affords a moderate yield and a much longer reaction time is
needed (entry 17). The 1d-catalyzed addition of b-keto
esters and b-diester to the secondary alcohols that we used
in this study was, unfortunately, not successful under the
present reaction conditions; neither was the addition of b-di-
ketones to primary alcohols.
For comparison purposes, 1d-catalyzed addition reactions
of b-diketones to styrene and styrene derivatives have also
been performed (Table 4); it was found that the yields of
the addition products are, in general, lower than those of
the reactions with the corresponding alcohols. In each cata-
lytic reaction, the styrene dimer 1,3-diarylbut-1-ene and
higher oligomers of the styrene are formed.
It is known that Lewis acids could act as promoters for
the conversion of alcohols to symmetrical ethers;[12] we also
À À
found that the symmetrical ether PhMeHC O CHMePh
(6) was present in a 1d-catalyzed addition of acetylacetone
(2a) to 1-phenylethanol (5a) to yield 4a. The following 5a/
6/4a ratios were found at different times of the catalytic re-
action: 0.37:1.73:1 (1 h), 0.09:0.42:1 (2 h), 0:0:1 (3 h); these
results show that the symmetrical ether 6 was formed and
later consumed in the course of the catalysis.
After the conclusion of the 1d-catalyzed reaction of 2a
with 5a, in which larger sample sizes of reactants and com-
plex were used, diethyl ether was added to precipitate a
deep-red solid; the solid was filtered and washed several
1
times with Et2O. The solid was characterized by H NMR,
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C. P. Lau et al.
Table 4. Catalytic addition of b-diketones to alkenes with cis-[Ru(6,6’-Cl2bipy)2
A
ACHTREUNG
Entry
b-Diketone
Alkene
Yield [%][b]
1
46
2
3
35
52
4[c]
5[c]
6[c]
37 (1:0.8)[d]
52
72
7[c]
90
[a] Reaction conditions: catalyst (0.004 mmol), b-diketone (1.5 mmol), styrene (1.0 mmol), 908C, 15 h, no solvent was used unless noted. [b] Based on
1
the amount of styrene used, isolated yield. [c] DCE added (2.0 mL). [d] Ratio of diastereomers was determined by H NMR spectroscopy.
13C{1H} NMR, IR, and mass spectroscopic analyses. After
crystal data and refinement details are given in Table 5. The
these analyses, the solid was identified as the k2-acetylaceto-
coordination geometry of 7+ is essentially octahedral al-
nate (k2-acac) complex [Ru(6,6’-Cl2bipy)2
A
1
In the H NMR spectrum of 7, the acac group gives rise to a
methyl singlet and a methylene singlet at 1.64 and 5.16 ppm,
respectively. The 13C{1H} NMR spectrum shows one singlet
each for the two equivalent methyl groups and the methyl-
ene carbon, respectively, at d=27.3 and 98.7 ppm, while the
acac C=O resonance appears as a singlet at d=187.4 ppm.
The IR spectrum of 7 shows two strong bands, characteristic
of bidentate O-bonded acac, at n˜ =1592 and 1556 cmÀ1.[13]
The mass spectrum shows the parent peak of cation cis-
[Ru(6,6’-Cl2bipy)2
(k2-acac)]+ (7+) with m/z equal to
N
648.8417 (calcd: 648.9300).
Complex 7 was independently prepared by reacting 1d
with 2a at 908C for 1 h. Single crystals of 7 suitable for X-
ray crystallographic study were readily obtained by vapor
diffusion of Et2O into an acetone solution of 7 kept in a
sealed vessel. Figure 1 shows the X-ray structure of 7. The
Figure 1. ORTEP diagram of [Ru(6,6’-Cl2bipy)2
(k2-acac)]ClO4.
A
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Addition of b-Diketones to Alcohols and Styrenes
Table 5. Crystal data and structure refinement for 7.
FULL PAPER
tion of b-diketones (using acetylacetone and 1-phenyletha-
nol as examples) to secondary alcohols. It is seen that the
addition reaction is in fact a Brønsted acid catalyzed reac-
tion.[15] The catalytic process begins with 1d reacting with
acetylacetone (2a) to form 7 and concomitantly yield an
equivalent of H+, which then protonates the alcohol and
generates the carbocation. The carbocation could attack an
alcohol molecule to give the symmetric ether 6 or it can un-
dergo electrophilic attack at the a-carbon center of the b-di-
ketone (probably the enol form) to give the addition prod-
uct with regeneration of a proton. It has been reported that
the symmetrical ether 6 is formed as an intermediate in the
Lewis acid catalyzed benzylation reaction of active methyl-
enes with benzyl alcohol.[9a] The perchlorate anion might
also play a role in the catalysis, probably in stabilizing the
carbocation; the analogous Cl2bipy–Ru complex containing
tetrafluoroborate anion, 1e, is totally inactive for the addi-
tion reaction. The fact that the 1d-catalyzed benzylation of
2a with 5a was quenched by base, namely triethyl amine or
K2CO3, is a strong indication that the addition reaction is
Brønsted acid catalyzed. The fact that benzylation of 2c
with (S)-1-phenylethanol leads to the isolation of completely
racemized product (Scheme 2) lends support to the pro-
posed mechanism involving the intermediacy of the carbo-
cation.
7
Empirical formula
Ru(C5H7O2)(C10H6N2Cl2)2ClO4
A
Fw
749.76
T [K]
294(2)
l [Š]
0.71073
crystal system
space group
rhombohedral
R3
¯
unit cell dimensions
a [Š]
b [Š]
c [Š]
a [8]
b [8]
g [8]
V [Š3]
Z
34.201(3)
34.201(3)
15.425(3)
90
90
120
15626(4)
18
1.434
0.876
6732
0.32”0.24”0.12
2.25–27.57
À44ꢀhꢀ42
À44ꢀkꢀ43
À20ꢀlꢀ19
48089
1calcd [MgmÀ3
m [mmÀ1
(000)
]
]
F
A
crystal size [mm3]
q range [8]
index ranges
no. of reflns collected
no. of independent reflns
completeness to
7947 (Rint =0.0342)
98.9
q=27.578 [%]
absorption correction
max. and min. transmission
refinement method
data/restraints/parameters
GOF on F2
semi-empirical from equivalents
1.000 and 0.817
Hartwig et al. have suggested that addition of a broad
full-matrix least-squares on F2
7947:56:363
À
À
range of O H and N H bond donors to unactivated alkenes
catalyzed by metal triflates might not be truly metal-cata-
lyzed reactions; the metal might simply generate a protic
acid which is the real catalytic species.[16] It has also been re-
ported that in the Lewis acidic metal complex catalyzed
hetero-Michael addition reactions, the proton, rather than
the metal ion is the true catalyst.[17] On the other hand, He
et al. reported that the gold-mediated addition reactions of
phenols, carboxylic acids, and tosylamides to simple olefins
are truly metal-catalyzed; they do not support the possibility
of these reactions being catalyzed by acid generated from
the reaction of the gold(I) catalyst with the nucleophile.[18]
For comparison purposes, we carried out the HClO4-cata-
lyzed addition reactions of acetyacetone to two of the alco-
hols and styrene. The results collected in Table 7 show that
the yields of the addition products are basically identical to
those of the corresponding 1d-catalyzed reactions. We have
carried out a more comprehensive study on the acid-cata-
lyzed addition reactions of b-diketones to 1-phenylethanols
and styrenes, the results of which will be published else-
where.
1.010
final R indices [I>2s(I)]
R indices (all data)
R1 =0.0838, wR2 =0.2533
R1 =0.1489, wR2 =0.3300
1.091 and À0.914
largest diff peak and
À3
hole [eŠ
]
though the two bipyridines sustain relatively small angles at
the metal center due to ligand constraints. The Ru O (acac)
À
bond lengths (2.04 and 2.05 Š) are similar to those of other
RuII–acac complexes.[14] The structure of 7 as a whole is
quite unremarkable; selected bond lengths and bond angles
are shown in Table 6. Complex 7 did not react with 1-phe-
nylethanol (5a), in the absence or presence of HClO4; it
also did not catalyze the addition of 2a to 5a.
On the basis of these observations, we are prompted to
propose the following mechanism (Scheme 1) for the addi-
Table 6. Selected bond lengths [Š] and angles [8] in [Ru(6,6’-Cl2bipy)2-
A
In two separate addition reactions of 2a to 5a under iden-
tical reaction conditions, one of which is catalyzed by
0.4 mol% of 1d and the other by the same concentration of
HClO4, the yields of the addition products with times were
measured. Figure 2 shows that the yields versus time graphs
of the two catalytic processes are basically identical, there-
fore strongly implicating that the two reactions might have a
common catalytic species, which is probably the protic acid.
A catalytic self-condensation reaction of 1-phenylethanol
(5a) to form the symmetrical ether 6 with 1d/2a was moni-
À
À
Ru(1) O(1)
2.0531(19)
1.241(4)
1.380(5)
2.043(3)
2.058(3)
Ru(1) O(2)
2.043(2)
1.254(4)
1.453(6)
2.071(2)
2.047(3)
À
À
O(1) C(23)
O(2) C(21)
À
À
C(21) C(22)
C(22) C(23)
À
À
Ru(1) N(1)
Ru(1) N(2)
À
À
Ru(1) N(3)
Ru(1) N(4)
O(1)-Ru(1)-O(2)
N(3)-Ru(1)-N(4)
O(1)-Ru(1)-N(2)
N(1)-Ru(1)-N(3)
O(2)-Ru(1)-N(2)
91.00(8)
78.36(11)
173.17(9)
177.60(8)
82.57(9)
N(1)-Ru(1)-N(2)
N(4)-Ru(1)-N(2)
O(2)-Ru(1)-N(4)
O(1)-Ru(1)-N(4)
78.35(9)
101.36(9)
175.63(9)
85.14(9)
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C. P. Lau et al.
sically identical, again indicat-
ing a common catalytic species
(the proton) in both reactions.
Finally, we have demonstrat-
ed that reaction of 2a with the
symmetrical ether 6 instead of
1-phenylethanol (5a) in the
presence of a catalytic amount
of 1d yields the addition prod-
uct 4a. Figure 4 shows that the
results of the HClO4-catalyzed
reaction are identical to that of
the 1d-catalyzed one.
The postulate (Scheme 1)
that the addition product is fi-
nally generated by attack of
the carbocation on the free b-
diketone rather than the biden-
tate acac ligand of 7 is support-
ed by the fact that 7 fails to
react with 1-phenylethanol
(5a) or the ether 6 in the pres-
ence of HClO4, which proto-
nates 5a or 6 and generates
the benzyl carbocation in situ.
Scheme 1. Proposed mechanism for the catalytic addition of b-diketone to 1-phenylethanol with 1d.
Scheme 2. Benzylation of 2c with (S)-1-phenylethanol.
tored and comparison with an identical reaction with HClO4
was made. As shown in Figure 3, the plots of percent con-
version to the ether with times for the reactions are also ba-
Table 7. Catalytic addition of acetylacetone (2a) to alcohols and styrene
with HClO4 and 1d.
Figure 2. Conversion (to 4a, based on 5a) versus time plots of the cata-
Entry Addition reaction Product Yield with HClO4 Yield with 1d
~
( ,
&
lytic addition of 2a to 5a with 1d ( , 0.4 mol%) and HClO4
[%][a]
[%][a]
0.4 mol%).
A mechanism very similar to that depicted in Scheme 1 is
believed to be operative for the 1d-catalyzed addition of b-
diketones to styrenes (Scheme 3, with 2a and styrene as ex-
amples). Again, the crucial species is the carbocation; it at-
tacks the b-diketone to yield the addition product or it
reacts with styrene to yield the dimer and higher oligomers
as byproducts.
1
4a[b]
87
82
2
3
4h[b]
70
42
73
46
4a[c]
Conclusion
[a] Based on alcohol or stryrene used (isolated yield). [b] Reaction condi-
tions were same as those in Table 3. [c] Reaction conditions were same as
those in Table 4.
In Lewis acidic metal catalysis, one has to caution against
the true catalytic species not being the metal complex but
8616
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Chem. Eur. J. 2007, 13, 8610 – 8619

Addition of b-Diketones to Alcohols and Styrenes
FULL PAPER
Figure 3. Conversion versus time plots for catalytic self-condensation of
&
5a to the symmetrical ether 6 with 1d/2a ( , 0.4/0.8 mol%) and HClO4
Figure 4. Conversion to 4a (based on 6) versus time plots for the catalytic
~
(
, 0.4 mol%).
~
&
reactions between 2a and 6 with 1d ( , 0.4 mol%) and HClO4
( ,
0.4 mol%).
the proton generated by the reaction of the metal with the
substrate. Our work on the dicationic Cl2bipy–Ru-complex-
catalyzed addition of b-diketones to alcohols and styrenes
constitutes a good example which unequivocally illustrates
this phenomenon. Addition of b-diketones to 1-phenylethyl
alcohols generally gives higher yields of benzylated b-dike-
tones than addition to the corresponding styrenes because
the former does not generate styrene dimer and oligomer
byproducts.
synthesized by using the same procedure as for the preparation of 1a,
except that 6,6’-Me2bipy was used in place of 6,6’-Cl2bipy; H NMR spec-
trum of 1c was found to be identical to that of the perchlorate salt cis-
1
[Ru(6,6’-Me2bipy)2
N
N
ylphenyl)ethanol (5c), 1-(4-fluororophenyl)ethanol (5d), 1-(4-chlorophen-
yl)ethanol (5e), and 1,3-diphenyl-2-propen-1-ol (5g) were prepared by
reduction of the corresponding ketone precursors with NaBH4 in meth-
anol. The other reagents were obtained from Aldrich and used without
further purification. 1H NMR spectra were obtained from
a Bruker
DPX-400 spectrometer at 400 MHz; chemical shifts (d, ppm) were re-
ported by using TMS as the internal standard. 13C NMR spectra were re-
corded with a Bruker DPX-400 spectrometer at 100 MHz; chemical shifts
were internally referenced to CDCl3 (d=77.0 ppm). IR spectra were ob-
tained from a Bruker Vector 22 FTIR spectrophotometer. Mass spec-
trometry was carried out with a Finnigan MAT 95S mass spectrometer
with the samples dissolved in dichloromethane or acetone. HRMS was
Experimental Section
General: Complexes 1a and 1d were prepared according to the literature
method.[19] The complex cis-[Ru(6,6’-Me2bipy)2
N
N
Scheme 3. Proposed mechanism for the catalytic addition of b-diketone to styrene with 1d.
Chem. Eur. J. 2007, 13, 8610 – 8619
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8617

C. P. Lau et al.
+
carried out with Waters Micromass Q-Tof-2. Elemental analyses were
performed by M-H-W Laboratories, Phoenix, AZ (USA). Melting points
were determined on a Barnstead Electrothermal 9100 apparatus and
were uncorrected.
[M+Na]+ (100); HRMS (+ESI): m/z: calcd for C13H15FNaO2
245.0948; found: 245.0972 [M+Na]+.
:
1,3-Diphenyl-2-[1-(4-fluorophenyl)ethyl]propane-1,3-dione (4l): M.p.
110–1128C; 1H NMR (400 MHz, CDCl3): d=1.34 (d, ArCHCH3, J=
7.0 Hz, 3H), 4.06–4.14 (m, ArCHCH3, 1H), 5.66 (d, J=10.0 Hz, 1H;
PhC(O)CHC(O)Ph), 6.81–6.86 (m, 2H), 7.23–7.28 (m, 4H), 7.39–7.43 (m,
3H), 7.51–7.53 (m, 1H), 7.77 (d, J=7.3 Hz, 2H), 8.06 ppm (d, J=7.6 Hz,
2H); 13C NMR (100 MHz, CDCl3): d=20.2 (ArCHCH3), 40.4
(ArCHCH3), 64.4 (PhC(O)CHC(O)Ph), 114.8, 115.1, 128.3, 128.4, 128.6,
128.7, 129.1, 129.2, 133.1, 133.5, 136.6, 136.9, 139.3, 160.0, 162.5, 194.4
(CH3C(O)CHC(O)CH3), 194.6 ppm (CH3C(O)CHC(O)CH3); IR (KBr):
cis-[Ru(6,6’-Br2bipy)2
A
G
N
added to RuCl3·3H2O (99 mg, 0.38 mmol), 6,6’-Br2bipy (250 mg,
0.80 mmol), and LiCl (1.0 g, 23 mmol). The mixture was heated at 1408C
for 4 h under N2. After the mixture had been cooled to room tempera-
ture, H2O (3 mL) was added. The precipitated dark solid was collected
by filtration; it was then washed twice with water (2”5 mL). The solid
was dissolved in water (16 mL) and AgOTf (0.16 g, 0.62 mmol) was
added. The resulting mixture was heated at 608C for 30 min, followed by
filtration. The volume of the filtrate was reduced to 3 mL under reduced
pressure and trific acid (0.5 mL) was added. The solution was placed in
the refrigerator for 1 day, after which time, a thick oil separated. Chloro-
form (1 mL) was added to the solution and the mixture was stirred vigo-
rously to enable the solidification of the oily substance. The mixture was
filtered and a tacky brown solid was collected and recrystallized with
THF/CHCl3. The product, which remained a tacky brown solid, was
dried in vacuo for 5 h. Yield: 0.05 g (13%). 1H NMR spectrum of the
product shows two triplets and four doublets in the d=7.72–8.50 ppm
n˜ =3065, 2970, 1687, 1595, 1510, 1447, 1265, 1219, 971, 838, 684, 604,
+
544 cmÀ1
;
HRMS (+ESI): m/z: calcd for C23H19FNaO2
: 369.1261;
found: 369.0547 [M+Na]+.
Comparison of the catalytic addition of acetylacetone (2a) to 1-phenyle-
thanol (5a) with HClO4 and 1d: HClO4 (60% solution, 0.4 mL,
0.004 mmol) or 1d (3.2 mg, 0.004 mmol) was added to a mixture of 2a
(154 mL, 1.5 mmol) and 5a (122 mg, 1.0 mmol), the resulting solution was
stirred at 808C; samples removed by the use of a microsyringe at differ-
ent time intervals were analyzed by 1H NMR spectroscopy to give the
percentage conversions.
1
Comparison of catalytic self-condensation of 1-phenylethanol (5a) to the
symmetrical ether 6 with 1d/2a and HClO4: HClO4 (60% solution,
0.4 mL, 0.004 mmol) or 1d/2a (3.2 mg, 0.004 mmol/0.8 mL, 0.008 mmol)
was added to a sample of 5a (122 mg, 1.00 mmol), the resulting solution
was stirred at 808C. Samples removed by the use of a microsyringe at dif-
ferent time intervals were analyzed by 1H NMR spectroscopy to give the
percentage conversions.
region; this pattern is very similar to those found in the H NMR spectra
of 1a and 1c. Some very small multiplets at d=7.36–7.41 and 7.93–
8.04 ppm indicate that minute amounts of impurities are present. It was
found to be very difficult to completely remove these impurities from the
product due to the tacky nature of 1b. 1H NMR (400 MHz, D2O, 25 8C):
d=7.72 (d, J=7.7 Hz, 2H; dbbp-5,5’-H), 7.85 (t, J=7.9 Hz, 2H; dbbp-
4,4’-H), 7.97 (d, J=7.5 Hz, 2H; dbbp-5,5’-H), 8.04 (t, J=7.9 Hz, 2H;
dbbp-4,4’-H), 8.38 (d, J=8.0 Hz, 2H; dbbp-3,3’-H), 8.50 ppm (d, J=
8.2 Hz, 2H; dbbp-3,3’-H); MS (+ESI): m/z: 382.8 [M]2+ (23), 364.7
[MÀ2H2O]2+ (89).
Comparison of the catalytic reactions between acetylacetone (2a) and
ether 6 with 1d and HClO4: HClO4 (60% solution, 0.4 mL, 0.004 mmol)
or 1d (3.2 mg, 0.004 mmol) was added to a mixture of 2a (154 mL,
1.50 mmol) and ether 6 (113 mg, 0.5 mmol), the resulting solution was
stirred at 808C. Samples removed by the use of a microsyringe at differ-
ent time intervals were analyzed by 1H NMR spectroscopy to give the
percentage conversions.
cis-[Ru(6,6’-Cl2bipy)2
(k2-acac)]ClO4 (7): Complex 1d (150 mg, 2.0 mmol)
T
was added to acetylacetone 2a (2 mL) and the solution was stirred under
N2 for 1 h at 908C. After cooling to room temperature, the volatile mate-
rial was removed under vacuum to afford a deep-red solid. The residue
was washed with diethyl ether (3”10 mL) and hexane (10 mL) and then
dried under vacuum for 5 h. 1H NMR (400 MHz, CDCl3, 258C): d=1.64
(s, 6H; CH3C(O)CHC(O)CH3), 5.16 (s, 1H; CH3C(O)CHC(O)CH3),
7.62–7.67 (m, 4H), 8.10–8.14 (m, 4H), 8.63–8.68 ppm (m, 4H); 13C NMR
(100 MHz, CDCl3, 258C): d=27.3 (CH3C(O)CHC(O)CH3), 98.7
(CH3C(O)CHC(O)CH3), 187.4 (CH3C(O)CHC(O)CH3), other signals
due to the bipyridine ligands: 122.6, 123.4, 125.5, 125.7, 138.7, 139.8,
158.7, 160.0, 160.9, 162.3 ppm; IR (KBr): n˜ =3096, 1592 (C=O of acacÀ),
1556 (C=O of acacÀ), 1455, 1435, 1410, 1368, 1240, 1187, 1144, 1113,
Crystallographic structure analysis of [Ru(6,6’-Cl2bipy)2
(7): Deep-red crystals were obtained by slow infusion of diethyl ether
into an acetone solution of [Ru(6,6’-Cl2bipy)2
(k2-acac)]ClO4 kept in a
(k2-acac)]ClO4
AHCTREUNG
sealed vessel. A suitable crystal with dimensions of 0.32”0.24”0.12 mm3
was mounted on a Bruker CCD area detector diffractometer by using
MoKa radiation (l=0.71073 Š) from a generator operating at 50 kV,
30 mA condition. The intensity data were collected and corrected for
SADABS (Sheldrick, 1996) program. The structure was solved by direct
methods, expended by difference Fourier syntheses and refined by full-
matrix least squares on F2 by using the Bruker Smart and Bruker
SHELXT1 program packages. All non-hydrogen atoms were refined ani-
sotropically. Hydrogen atoms were placed in ideal positions and refined
as riding atoms. The final cycle of the full-matrix least-squares refinement
based on 7947 observed reflections (I>2a(I)) and 363 parameters con-
verged to the R and Rw values of 0.0838 and 0.2533 for [Ru(6,6’-
1090, 856, 785, 741, 626 cmÀ1
;
HRMS (+ESI): calcd for
C25H19Cl4N4O2Ru+: 648.9300; found: 648.8417 [M]+; elemental analysis
calcd (%) for C25H19Cl5N4O6Ru: C 40.05, H 2.55, N 7.47; found: C 40.25,
H 2.38, N 7.21.
Typical procedure for the catalytic reaction of b-diketone with alcohol
(or styrene): To a mixture of b-diketone (1.5 mmol) and alcohol (or sty-
rene, 1.0 mmol) was added the catalyst 1d (0.004 mmol). The resulting
solution was stirred at 808C (908C for styrene) and monitored by TLC or
1H NMR spectroscopy. When the maximum conversion had been
reached, the desired product was isolated by flash column chromatogra-
phy on silica gel. (4d and 4l are new compounds and the other products
4a–c, 4e–k, 4m–q, and 6 are known compounds).
Cl2bipy)2
(k2-acac)]ClO4. Further details of the data collection are sum-
U
marized in Table 5, and selected bond distances and angles are listed in
Table 6. CCDC-646248 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
3-[1-(4-Fluorophenyl)ethyl]pentane-2,4-dione (4d): M.p. 57–588C;
1H NMR (400 MHz, CDCl3) d=1.20 (d, J=6.9 Hz, 3H, ArCHCH3), 1.86
(s, 3H; CH3C(O)CHC(O)CH3), 2.26 (s, 3H; CH3C(O)CHC(O)CH3),
3.58–3.63 (m, 1H; ArCHCH3), 4.01 (d, J=11.3 Hz, 1H;
CH3C(O)CHC(O)CH3), 6.96–7.00 (m, 2H; ArH), 7.15–7.19 ppm (m, 2H;
ArH); 13C NMR (100 MHz, CDCl3) d=20.7 (ArCHCH3), 29.5
Acknowledgements
P.N.L. thanks the Hong Kong Polytechnic University for a postdoctoral
fellowship.
(CH3C(O)CHC(O)CH3),
29.7
(CH3C(O)CHC(O)CH3),
39.5
(ArCHCH3), 76.6 (CH3C(O)CHC(O)CH3), 115.4, 115.6, 128.6, 128.7,
À
138.7, 160.3, 162.7 (Ar C), 202.9 (CH3C(O)CHC(O)CH3), 203.1 ppm
(CH3C(O)CHC(O)CH3); IR (KBr): n˜ =2968, 2934, 1724, 1699, 1511,
1363, 1292, 1225, 1161, 1148, 835, 542 cmÀ1; MS (+ESI): m/z (%): 245.05
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8618
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Chem. Eur. J. 2007, 13, 8610 – 8619

Addition of b-Diketones to Alcohols and Styrenes
FULL PAPER
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Received: May 9, 2007
Published online: July 19, 2007
Chem. Eur. J. 2007, 13, 8610 – 8619
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8619