Ruthenium-mediated selective head-to-tail dimerization of acrylic and α,β-unsaturated carbonyl compounds: Generation of an acrylate-hydride complex C5Me5Ru(PCy3)(CH2=CHCO 2Et)H

Article abstract of DOI:10.1016/S0022-328X(97)00591-3

The ruthenium-hydride complex C₅Me₅Ru(PCy₃)H₃ (1a) was found to be a selective catalyst precursor for the head-to-tail dimerization of acrylic and α,β-unsaturated carbonyl compounds to produce bifunctional 1,5-dicarbonyl compounds. A new ruthenium species C₅Me₅Ru(PCy₃)(CH₂=CHCO ₂Et)H (6a) was independently generated from the substitution reaction of 1a with ethyl acrylate. The exclusive formation of the head-to-tail dimers suggested that, the tertiary phosphine, generated from the substitution reaction of 6a with an olefin, was the active species for the dimerization reaction.

Full text of DOI:10.1016/S0022-328X(97)00591-3

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.
Journal of Organometallic Chemistry 553 1998 157–161
Ruthenium-mediated selective head-to-tail dimerization of acrylic and
a,b-unsaturated carbonyl compounds: generation of an acrylate-hydride
ž
/ž
/
complex C5Me5Ru PCy3 CH2 5CHCO2 Et H
)
Chae S. Yi , Nianhong Liu
Department of Chemistry, Marquette UniÕersity, Milwaukee, WI 53201-1881, USA
Received 20 May 1997; received in revised form 21 July 1997
Abstract
Ž
.
Ž .
The ruthenium-hydride complex C5Me5Ru PCy3 H3 1a was found to be a selective catalyst precursor for the head-to-tail
dimerization of acrylic and a,b-unsaturated carbonyl compounds to produce bifunctional 1,5-dicarbonyl compounds. A new ruthenium
Ž
.Ž
.
Ž .
species C5Me5Ru PCy3 CH2 5CHCO2Et H 6a was independently generated from the substitution reaction of 1a with ethyl acrylate.
The exclusive formation of the head-to-tail dimers suggested that, the tertiary phosphine, generated from the substitution reaction of 6a
with an olefin, was the active species for the dimerization reaction. q 1998 Elsevier Science S.A.
Keywords: Head-to-tail dimerization of acrylic compounds; Ruthenium-hydride complexes
1. Introduction
pounds would in principle produce bifunctional 1,5-di-
carbonyl compounds, a useful class of substrates in
aldol-type condensation and other annulation reactions
The transition metal-catalyzed dimerization of acrylic
compounds has been extensively studied as an alternate
way of forming commercially important hexendioates
and adiponitriles 1–9 . Currently, Brookhart’s cationic
rhodium catalyst is the most active system for the
selective tail-to-tail dimerization of acrylates, and the
mechanism of this dimerization reaction has been well-
w
x
14,15 . Herein, we report the ruthenium-mediated se-
lective head-to-tail dimerization of acrylic and a,b-un-
saturated carbonyl compounds to produce 1,5-di-
carbonyl compounds and the formation of ruthenium
acrylate-hydride intermediate species.
1
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x
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x
established 10 . In contrast to the tail-to-tail dimeriza-
tion, however, the synthetic utility for the head-to-tail
dimerization of acrylic compounds has been limited due
to low selectivity and yields on the dimeric products
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x
5–9 . While tertiary phosphines are known to catalyze
the head-to-tail dimerization of acrylates, the reactions
typically require elevated temperatures and often yield
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x
mixtures of dimeric and oligomeric products 4–9 .
Selective head-to-tail dimerization of acrylic com-
2. Results and discussion
We recently reported that the ruthenium-hydride
Ž .
Ž
Ž
.
Ž
.
complexes C5Me5 L RuH3 L5PCy3 1a , PPh3 1b ,
)
Corresponding author.
1 For leading examples on the metal-catalyzed tail-to-tail dimeriza-
tion reactions, see Refs. 10–13 .
Ž
..
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x
PMe3 1c 16–18 were effective catalysts for the
dimerization of terminal alkynes 19 . The same ruthe-
w
x
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x
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

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158
C.S. Yi, N. LiurJournal of Organometallic Chemistry 553 1998 157–161
Table 1
The metal-catalyzed tail-to-tail dimerization of acrolein
The ruthenium complex 1a-catalyzed dimerization of acrylic and
w
x
has recently been reported 13 .
a,b-unsaturated carbonyl compoundsa
Ž
.
Entry
Olefin substrate
Product
Yield %
1
2
3
4
5
CH2 5CHCO2 Me
CH2 5CHCO2 Et
CH2 5CHCOMe
CH2 5CHCHOb
CH2 5CHCN
2a
2b
3
91
86
88
57d
73
4c
5c
aReaction conditions: THF 5 ml ; 4–8 mmol of an alkene; 3–5
mol% of the catalyst 1a; 808C; 24 h. The isolated product yield after
a simple silica gel column chromatography.
Ž
.
b The commercial olefin substrate contained ;3% of water and 0.1%
of hydroquinone.
The hydrogen ligand in ruthenium-polyhydride com-
plexes are well known to interconvert between the
h2-H2 and the classical metal-hydride coordination
cApproximately 10–20% of white polymeric products was formed.
d The yield was determined by the 1H NMR.
w
x
modes 19–23 , and we thought that the hydride ligand
of 1a could be substituted by an acrylate ligand. The
Ž
.
treatment of 1a 50 mg, 0.096 mmol and 5 equivalents
of ethyl acrylate in C6 D6 at 808C for 4 h cleanly
produced the new metal-hydride species along with the
nium-hydride complex 1a was found to be a selective
catalyst precursor for the dimerization of acrylic com-
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.
hydrogenated ethyl propionate ;10% . The new
Ž
.
pounds. In a typical catalytic reaction, a THF 5 ml
Ž
ruthenium-hydride signal at d y10.31 d, JPyH s36.8
Ž
.
solution of 1a 10 mg, 0.019 mmol and an excess of
methyl acrylate 0.7 ml, 7.8 mmol was stirred at 808C
for 24 h. The branched dimer 2a 91% was isolated
.
Hz and the diastereotopic OCH2 proton resonances at
Ž
.
1
Ž
.
d 4.34 and 4.07 dq, Js11.4, 7.4 Hz by H NMR
Ž
.
were consistent with the acrylate-hydride complex 6a
after a simple column chromatography on a silica gel
Ž
.
81% yield by NMR , but the olefinic proton peaks
Ž
.
3: 1 hexanesrEt2O . The structure of 2a was estab-
were completely masked by the cyclohexyl group. To
establish the coordinated olefin resonances, PPh3-sub-
stituted 6b was prepared from an analogous reaction of
lished from the observation of two vinyl proton reso-
1
Ž .
Jgem ;1 Hz by the H
nances at d 6.04 and 5.31
NMR and the detection of Mq mrzs172 by the
Ž
.
1
1b with ethyl acrylate. In this case, the H NMR of 6b
2
GC-MS. An analogous dimerization reaction of ethyl
clearly exhibited the coordinated olefinic resonances at
acrylate by 1a also yielded 2b in 86% yield. No other
dimeric or oligomeric products has been detected during
an NMR tube reaction of 1a and methyl acrylate in
C6 D6 as monitored by 1H NMR in the temperature
range of 25–758C. The catalysts 1b and 1c were inac-
tive toward the catalytic dimerization of methyl acrylate
under similar reaction conditions, giving mostly recov-
ered starting materials after 3 days.
Some functional group tolerance have been observed
toward the dimerization of other olefins. The dimeriza-
tion reactions of methyl vinyl ketone, acrolein, and
acrylonitrile by 1a exclusively produced the branched
Ž
.
Ž
.
d 1.76 d, Js10.0 Hz , 1.68 br t, Js7.1 Hz and
Ž
.
1.26 dd, Js10.0, 7.1 Hz along with the Ru-H reso-
Ž
.
nance at d y10.15 d, JPH s36.0 Hz . Both 6a and 6b
were stable in solid state at room temperature under N2
atmosphere, but the complexes slowly decomposed in
Ž
.
C6 D6 solution t1r2 ;2 days .
The independently formed 6a was subsequently
shown to be active toward the catalytic dimerization of
Ž
acrylic compounds. The treatment of 6a 50 mg, 0.081
.
Ž
mmol with an excess of ethyl acrylate 0.3 ml, 4.2
mmol in 5 ml of C6 H6 solution at 808C produced the
dimer 2b in a similar rate as before )95% conversion
after 24 h . Initially, a new ruthenium species was
.
Ž
dimers 3, 4 and 5, respectively, with good to high yields
.
2
Ž
.
Table 1, . None of the head-to-head dimers or higher
oligomers was detected by the 1H NMR, but 10–20% of
the polymeric products was formed during the dimeriza-
tion of acrolein and acrylonitrile entries 4 and 5 . The
dimerization reactions of the substituted acrylates such
as methyl methacrylate and trans-ethyl crotonate, methyl
butenoate, and N,N-dimethylacrylamide by 1a failed to
give dimeric products under similar reaction conditions.
formed along with free PCy3 at the expense of the
resonances due to 6a. The spectroscopic data of the new
3
ruthenium species suggested of the olefin complex 7,
Ž
.
but its structure could not be unambiguously assigned
due to the masked resonances by PCy3 group and the
1
3 Selected spectroscopic data for 7: H NMR C6 D6, 300 MHz d
Ž
.
Ž
.
Ž
.
3.96 q, Js7.2 Hz, CO2C H2CH3 , 1.49 s, C5Me5 olefinic pro-
tons were masked by cyclohexyl group.
MHz d 173.9 CO2 Et , 86.3 C5Me5 , 60.1 CO2CH2CH3 , 9.7
CO2CH2CH3 , 9.2 C5 Me5 , olefinic carbons were overlapped with
13
Ä1
4 Ž
C H NMR C6 D6, 75
.
Ž
.
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.
Ž
.
2 The spectroscopic data for some of these compounds have been
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.
Ž
.
w
x
previously reported in Refs. 5–9 .
cyclohexyl group.

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C.S. Yi, N. LiurJournal of Organometallic Chemistry 553 1998 157–161
159
dimer 2b. Several attempts to isolate this species thus
far have been unsuccessful. Similar dimerization reac-
tion of ethyl acrylate by 6b failed to yield any dimeric
products under a similar reaction condition, although
the dimerization of methyl vinyl ketone by 1b yielded
approximately 10% of the dimeric product 3 after 3
days.
and the 1,3-hydrogen migration and the elimination of
PCy3 from 9 would yield the head-to-tail dimers.
The reasons for higher activity and selectivity of 1a
compared to the phosphine-catalyzed reactions toward
the dimerization of acrylic compounds are not clear.
One possible explanation is that the ruthenium complex
might be involved in controlling the phosphine concen-
tration during the reaction. Initially, the equilibrium
between the acrylate-hydride 6a and 7 would be shifted
toward the generation of free PCy3 when the monomer
concentration is high. Toward end of the reaction, the
acrylate concentration would become low as most of the
monomer was converted to the dimer, and the equilib-
rium would be shifted back toward 6a, thereby minimiz-
ing the secondary and other side reactions catalyzed by
free PCy3. The PCy3-substituted 1a was much better
catalyst than 1b because sterically demanding and nu-
cleophilic PCy3 ligand would be readily dissociated
from 6a under relatively mild reaction conditions.
In summary, the bifunctional 1,5-dicarbonyl com-
pounds 2–5 have been selectively obtained from the
ruthenium-mediated head-to-tail dimerization of acrylic
and a,b-unsaturated carbonyl compounds. Catalytically
active acrylate-hydride complex 6a was successfully
generated from the substitution reaction of 1a with an
acrylate compound. The exclusive formation of the
head-to-tail dimers suggested that the free PCy3 was the
active species during the dimerization reaction. Further
studies on improving the catalytic activity and on under-
standing a detailed reaction mechanism are currently in
progress.
A series of control experiments was conducted using
PCy3 as a catalyst to compare the activity with that of
1a. In general, the catalytic activity of PCy3 toward the
dimerization reaction of acrylic compounds was consid-
erably lower than 1a. For example, the dimerization of
Ž
.
methyl acrylate catalyzed by PCy3 6 mg, 0.02 mmol
under otherwise same reaction condition resulted in
49% of the product 2a after 24 h. Much less dimeric
Ž
.
products ;10% were isolated in the PCy3-catalyzed
reactions of methyl vinyl ketone, acrolein and acryloni-
trile, as in these cases, gave mostly insoluble polymeric
products.
Both the formation of the head-to-tail dimers 2–5
and the generation of the acrylate-hydride 6 suggest the
mechanism as illustrated in Scheme 1. It is well-estab-
lished that the tertiary phosphines selectively catalyzed
the head-to-tail dimerization of acrylic compounds
w
x
3,5–9 . In this case, the exclusive formation of the
head-to-tail dimers 2–5 suggests that the dimerization
reaction was probably catalyzed by the free phosphine
PCy3 generated during the reaction. The substitution of
another olefin substrate from the acrylate complex 6a
would generate free PCy3 and the ruthenium complex 7.
Sterically demanding and soft PCy3 ligand should favor
the 1,4-addition to the olefin substrate to form the
zwitterionic intermediate 8. The similar mechanism in-
volving zwitterionic phophonium intermediate has been
commonly proposed in phosphine-catalyzed dimeriza-
3. Experimental
w x
tion reactions 3 . Addition of another acrylate molecule
3.1. General
All materials were manipulated in an inert-atmo-
sphere glove box or by standard high vacuum and
Schlenk line techniques unless otherwise mentioned.
Tetrahydrofuran and benzene were distilled from purple
solutions of sodium and benzophenone immediately
prior to use. C6 D6 was dried from activated molecular
˚
Ž
.
Ž .
sieves 4 A . The ruthenium complexes C5Me5Ru L H3
Ž
Ž
.
Ž
.
Ž
..
L5PCy3 1a , PPh3 1b , PMe3 1c were prepared
w
x
according to literature procedures 16–18 . The olefin
compounds were received from the commercial sources
and used without further purification. The H and 13C
1
NMR spectra were recorded on a GE GN-Omega 300
MHz FT-NMR spectrometer. Mass Spectra were
recorded on a Hewlett-Packard HP 5970 GCrMS spec-
Ž
trometer or on a FAB-MS spectrometer Center of Mass
.
Spectrometry, Washington University . Elemental anal-
yses were performed at the Midwest Microlab, Indi-
anapolis, IN.
Scheme 1. A plausible mechanism of the ruthenium-mediated head-
to-tail dimerization of acrylic compounds.

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)
160
C.S. Yi, N. LiurJournal of Organometallic Chemistry 553 1998 157–161
Ž
.
Ž
.
3.2. General experimental procedure for the catalytic
dimerization reaction of acrylic compounds
29.8 5CCH2 , 15.4 CH2CN . GC-MS mrzs106
q
Ž
.
.
M
3 . 3 .
G e n e r a l
( )(
p r o c e d u r e
f o r
In a 25 ml Teflon-joint Schlenk tube equipped with a
magnetic stirring bar, the ruthenium complex 1a 10
)
(
( )
C5 Me5 Ru L CH2 5CHCO2 Et H LsPCy3 6a , PPh3
Ž
( ))
6b
.
Ž
.
mg, 0.019 mmol and C6 Me6 internal standard, 10 mg
were dissolved in 5 ml of dried THF. Excess alkene
In a 25 ml Teflon-joint Schlenk tube equipped with a
magnetic stirring bar, the ruthenium-hydride complex 1
Ž
.
monomer 7–8 mmol was added via a syringe to the
solution. The reaction mixture was stirred for 24 h in an
oil bath at 808C under a closed system. After the
solution was cooled to room temperature, a small sam-
ple was drawn out from the solution for GC-MS analy-
sis. Volatiles were removed under a vacuum. The
dimeric organic product was isolated by the column
Ž
Ž
.
.
1.63 mmol and 10 equiv. of an acrylate compound
16.3 mmol were mixed in 10 ml of C6 H6. The
reaction mixture was stirred at 808C for 12 h under a
closed vessel. The solvent was removed under a high
vacuum. The resulting brown residue was triturated
with ;5 ml of Et2O, and the precipitate was washed
several times with a small amount of Et2O. Recrystal-
Ž
.
chromatography on a silica gel hexanes: Et2Os3: 1 .
CH2 5C CO2 Me CH2CH2CO2 Me 2a . 1H NMR
Ž
.
Ž
.
Ž
.
lization from Et2O ;5 ml at y788C gave 6 as an
Ž
.
Ž
.
Ž
C6 D6, 300 MHz : d 6.04 s, C H H5 , 5.31 s,
CH H5 , 3.38 s, 5CCO2CH3 , 3.34 s, CH2CO2C H3 ,
Ž
.
analytically pure pale-yellow solid 52–60% yield .
.
Ž
.
Ž
.
1
Ž
.
Ž
For 6a: H NMR C6 D6, 300 MHz : d 4.34 dq,
Ž
.
Ž
2.31 t, Js7.4 Hz, 5CCH2 , 2.26 t, Js7.4 Hz,
.
Ž
Js11.4, 7.4 Hz, OC H HCH3 , 4.07 dq, Js11.4, 7.4
13
.
Ä1
4
Ž
.
C H2CO2CH3 . C H NMR C6 D6, 75 MHz : d
.
Ž
.
Ž
Ž
.
s
Hz, OCH HCH3 , 2.20–1.00 m, Cy , 1.86 s, C5Me5 ,
Ž
.
Ž
.
172.4 5CCO2 Me , 166.7 CH2 CO2 Me , 139.5
CH2 5C , 125.4 CH2 5C , 51.4 5CCO2CH3 , 51.0
CH2CO2CH3 , 32.9 CH2CO2CH3 , 27.6 5CCH2 .
Ž
.
0.98 t, Js7.4 Hz, OCH2C H3 , y10.31 d, JPyH
Ž
Ž
.
Ž
.
Ž
.
.
36.8 Hz, Ru–H , olefinic protons were masked by Cy
.
Ž
.
Ž
.
13
Ä1
4
Ž
.
group. C H NMR C6 D6, 75 MHz : d 178.5
q
Ž
.
.
GC-MS mrzs172 M
Ž
.
Ž
.
Ž
.
Ž
CO2 Et , 94.0 C5Me5 , 59.9 CO2CH2CH3 , 38.9 d,
CH2 5C CO2 Et CH2CH2CO2 Et 2b . 1H NMR
Ž
.
Ž
.
.
Ž
.
Js22.0 Hz, Cy , 30.6 CH2 5CH , 30.3, 28.2, and
Ž
.
Ž
.
Ž
C6 D6, 300 MHz : d 6.09 s, C H H5 , 5.31 s,
Ž
.
.
Ž
.
Ž
.
27.1 Cy , 26.4 CH2 5CH , 15.1 CO2CH2CH3 , 11.0
.
Ž
.
CH H5 , 3.93 q, Js7.4 Hz, 5CCO2C H2CH3 , 3.90
31
Ž
Ä1
4
Ž
.
C5 Me5 ; P H NMR C6 D6, 121.6 MHz . d 63.8.
Ž
.
Ž
q, Js7.4 Hz, CH2CO2C H2CH3 , 2.58 t, Js7.4 Hz,
q
Ž
.
FAB-MS mrz s 618
M
.
Anal. Calcd. for
.
Ž
.
5CCH2 , 2.34 t, Js7.4 Hz, C H2CO2CH2CH3 , 0.94
C33 H57O2 PRu: C, 64.15; H, 9.30. Found C, 65.06; H,
13
Ž
.
Ä1
4
and 0.93 t, Js7.4 Hz, CO2CH2C H3 . C H NMR
9.05.
Ž
Ž
Ž
Ž
Ž
.
Ž .
172.0 5CCO2 Et , 166.2
C6 D6 , 75 MHz :
d
1
Ž
.
For 6b: H NMR CD2Cl2 , 300 MHz : d 7.70–7.30
.
Ž
.
Ž
.
CH2CO2 Et , 139.8 CH2 5C , 125.1 CH2 5C , 60.5
5CCO2CH2CH3 , 60.1 CH2CO2CH2CH3 , 33.2
CH 2CO2 Et , 27.5 5CCH 2 ,
Ž
Ž
.
Ž
.
m, Ph , 4.07 dq, Js11.1, 7.4 Hz, OC H HCH3 , 3.95
.
Ž
.
.
Ž
dq, Js11.1, 7.4 Hz, OCH HCH3 ,1.76 d, Js10.0
.
Ž
.
14.2 and 14.1
.
Ž
Hz, CH H5CH , 1.68 br t, J5 JPyH s7.1 Hz,
q
.
Ž
.
.
CO2CH2CH3 . GC-MS mrzs200 M
.
Ž
.
Ž
C H H5CH , 1.52 s, C5Me5 , 1.26 dd, Js10.0, 7.1
CH2 5C COCH3 CH2CH2COCH3 3 . 1H NMR
Ž
.
Ž .
.
Ž
.
Hz, CH2 5C H , 1.21 t, Js7.4 Hz, OCH2C H3 ,
Ž
.
Ž
.
Ž
C6 D6, 300 MHz : d 5.45 s, C H H5 , 5.35 s,
13
Ž
.
Ä1
4
y10.15 d, JHyP s36.0 Hz, Ru-H . C H NMR
.
Ž
.
Ž
CH H5 , 2.47 t, Js7.4 Hz, C H2COCH3 , 2.15 t,
Ž
.
Ž
.
C6 D6, 75 MHz : d 177.4 CO2 Et , 136.0, 135.4,
.
Ž
Ž
.
Ž
Js7.4 Hz, 5CCH2 , 1.89 s, 5CCOCH3 , 1.62 s,
Ž
Ž
.
Ž
.
134.4 and 129.1 Ph , 94.9 C5 Me5 , 58.8
13
.
Ä1
4
.
CH2COC H3 . C H NMR C6 D6, 75 MHz : d 206.0
Ž
.
.
Ž
.
CO2CH2CH3 , 33.6 CH2 5CH , 28.5 CH2 5CH ,
Ž
.
Ž
.
Ž
Ž
.
.
5CCOMe , 198.4 CH2COMe , 148.0 CH2 5C ,
31
Ž
.
.
Ž
.
Ä1
4
15.0 CO2CH2CH3 , 10.1 C5 Me5 . P H NMR
Ž
Ž
Ž
.
Ž
.
125.3 CH2 5C , 42.1 CH2COCH3 , 29.2 5CCH2 ,
C6qD6, 121.6 MHz : d 74.8. FAB-MS mrzs600
Ž
.
Ž
.
25.4 5CCOCH3 , 25.3 CH2COCH3 . GC-MS mrz
.
M
. Anal. Calcd. for C33 H39O2 PRu: C, 66.09; H,
q
Ž
.
.
s140 M
6.55. Found C, 65.90; H, 6.71.
1
Ž
.
Ž .
Ž
CH2 5C CHO CH2CH2CHO 4 . H NMR C6 D6,
300 MHz : d 9.12 s, 5CCHO , 9.08 CH2C HO , 5.49
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
s, C H H5 , 5.16 s, CH H5 , 2.20 t, Js7.4 Hz,
Acknowledgements
13
.
Ž
.
Ä1
4
C H2CHO , 1.88 t, Js7.4 Hz, 5CCH2 . C H
Ž
.
Ž
.
NMR C6 D6, 75 MHz : d 199.4 5CCHO , 193.1
Financial support from Marquette University Com-
mittee-on-Research is gratefully acknowledged.
Ž
Ž
.
Ž
.
Ž
.
CH2CHO , 133.5 CH2 5C , 127.6 CH2 5C , 41.5
q
.
Ž
.
Ž
.
CH2CHO , 20.8 5CCH2 . GC-MS mrzs112 M .
1
Ž
.
Ž
Ž .
Ž
CH2 5C CN CH2CH2CN 5 . H NMR C6 D6, 300
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.
.
Ž
.
Ž
MHz : d 5.27 s, C H H5 , 5.00 s, CH H5 , 1.67 t,
.
Ž
.
Js5.2 Hz, 5CC H2 , 1.63 t, Js5.2 Hz, CH2CN .
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13
Ä1
4
Ž
Ž
.
Ž
.
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.
Ž
.
Ž
.
w x
2
Ž .
119.4 CH2 5C , 117.7 5CCN , 117.3 CH2CN ,
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C.S. Yi, N. LiurJournal of Organometallic Chemistry 553 1998 157–161
161
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