Cobalt-catalysed asymmetric hydrovinylation of 1,3-dienes

Article abstract of DOI:10.1039/c5sc00929d

In the presence of bidentate 1,n-bis-diphenylphosphinoalkane-CoCl₂ complexes {Cl₂Co[P ～ P]} and Me₃Al or methylaluminoxane, acyclic (E)-1,3-dienes react with ethylene (1 atmosphere) to give excellent yields of hydrovinylat

Full text of DOI:10.1039/c5sc00929d

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Page 1 of 17
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DOI: 10.1039/C5SC00929D
EDGE ARTICLE
Cobalt-Catalysed Asymmetric Hydrovinylation of
1,3-Dienes
Cite this: DOI: 10.1039/x0xx00000x
Yam N. Timsina, Rakesh K. Sharma,a, T. V. RajanBabu*
In the presence of bidentate 1,n-bis-diphenylphosphinoalkane-CoCl2 complexes {Cl2Co[P~P]} and Me3Al
or methylaluminoxane, acyclic (E)-1,3-dienes react with ethylene (1 atmosphere) to give excellent yields
of hydrovinylation products. The regioselectivity (1,4- or 1,2-addition) and the alkene configuration (E-
or Z-) of the resulting product depend on the nature of the ligand and temperature at which the reaction is
carried out. Cobalt(II)-complexes of 1,1-diphenylphosphinomethane and similar ligands with narrow bite
angles give mostly 1,2-addition, retaining the E-geometry of the original diene. Complexes of most other
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
o
ligands at low temperature (– 40 C) give almost exclusively a single branched product, (Z)-3-alkylhexa-
1,4-diene, which arises from a 1,4-hydrovinylation reaction. A minor product is the linear adduct, a 5-
alkyl-hexa-1,4-diene, also arising from a 1,4-addition of ethylene. As the temperature is increased, a
higher proportion of the major 1,4-adduct appears as the (E)-isomer. The unexpectedly high selectivity
seen in the Co-catalysed reaction as compared to the corresponding Ni-catalysed reaction can be
rationalized by invoking the intermediacy of an η4-[(diene)[P~P]CoH]+-complex and its subsequent
reactions. The enhanced reactivity of terminal E-1,3-dienes over the corresponding Z-dienes can also be
explained on the basis of the ease of formation of this η4-complex in the former case. The lack of
reactivity of the X2Co(dppb) (X = Cl, Br) complexes in the presence of Zn/ZnI2 makes the Me3Al-
mediated reaction different from the previously reported hydroalkenylation of dienes. Electron-rich
phospholanes, bis-oxazolines and N-heterocyclic carbenes appear to be poor ligands for the Co(II)-
catalysed hydrovinylation of 1,3-dienes. An extensive survey of chiral ligands reveals that complexes of
DIOP, BDDP and Josiphos ligands are quite effective for these reactions even at – 45 oC and
enantioselectivities in the range of 90-99 % ee can be realized for a variety of 1,3-dienes. Cobalt(II)-
complex of an electron-deficient Josiphos ligand is especially active, requiring only <1 mol% catalyst to
effect the reactions
.
R
[L*]Ni(II)-catalyst
R'
R
*
+
H
(1)
R'
Introduction
vinylarenes, 1,3-dienes
strained bicyclics
Heterodimerization reactions involving ethylene and other
alkenes catalysed by late transition metals including iron,1-3
cobalt,4-8 nickel,9-13 rhodium,14 palladium15 and ruthenium14a,16
had a long history17 even before the more recent discoveries of
applications of related chemistry in olefin polymerization
H
OMe
OMe
H
R'
H
H
Ar
Ar
H
reactions emerged.18-21 Among these, nickel and cobalt, and to
22-29
2 R' = alkyl
1 Ar = aryl, heteroaryl
a limited extent, ruthenium catalysed reactions
have
3
attracted the most attention in the stereoselective synthesis of
small molecules. Through optimizations of ligands, promoters
and reaction conditions, the Ni-catalysed asymmetric
hydrovinylations (eqn 1) of vinylarenes,30-36 selected dienes,37-
H
H
*
20S (or R)
H
39
and strained alkenes40,41 have been accomplished with
H
excellent regio- and stereoselectivities giving highly valuable
intermediates. Prototypical examples of the products of these
reactions (1-6) are shown in Figure 1. In a related reaction, Ho
et al reported42 dimerization of vinylarenes and 1-alkenes
catalysed by N-heterocyclic carbene complexes of Ni(II)-salts
to give uncommon tail-to-tail dimerization products (7, eqn 2).
H
H
0,1
Z
BnO
5
6
4
Figure 1. Examples of products of Ni(II)-catalyzed asymmetric
hydrovinylation
In sharp contrast, stereoselective dimerization reactions
catalysed by cobalt-complexes are much less developed,43,44
even though a high pressure reaction between 1,3-butadiene and
ethylene in the presence of Cl2Co(dppe)2 [dppe = 1,2-bis-
diphenylphosphinoethane] and Et3Al to give hexadiene isomers
(Eq 3) has been known since 1967.45
This journal is © The Royal Society of Chemistry 2013
Chem. Sci. 2015, 00, 1-3 | 1

Chemical Science
Page 2 of 17
ARTICLE
ChVeiemwiAcraticlleSOcinelinnece
DOI: 10.1039/C5SC00929D
P
P
CoCl2 (0.13 mol%)
Ar
N
H
Et2AlCl (5 Eq.)
OTf
H
Ni
(4)
0 oC, 90 min, CH2Cl2/hexane
N
+
R
(5 mol%)
Ar
dppm, dppp yield 99%
Trost Ligand 74% conv., 47% ee
ethylene (30 atm)
(2)
P
P
H
+
toluene, rt
Ar
Ar
R
(major)
vinylarene:alkene (1:3)
7 (R = alkyl)
tail-to-tail dimerization
O
O
Cl2Co(dppe)2/Et3Al
toluene, 80 oC
NH PPh2
NH PPh2
10 (Trost ligand)
(3)
+
OR
H
[(ethylene)2RhCl]2
EtOH
(Z) + (E)
(25 kg/cm2)
(Z)- major with Co(II)
In 2001 Hilt et al reported46-48 a remarkable hydroalkenylation
reaction between 2,3-dimethylbuta-1,3-diene and a terminal
alkene in the presence of Br2Co(dppe)/Zn/ZnI2 to give 1,4-
addition products in very high yields and selectivities.
Expansion of the scope of this reaction and several applications
of this and related heterodimerization reactions have since been
reported.49-52 A notable result in this area relevant to the present
discussion is the ligand control of regio- and stereoselectivity in
the reactions of substituted butadienes with terminal alkenes to
give either the branched (8) or linear (9) adduct (Scheme 1).48,53
LCoCl2, 0 oC, 20 min
(5)
methylaluminoxane
toluene
+
ethylene (4 atm)
(96%)
N
Pri
LCoCl2
Cl Co
Cl
N
Pri
•
No reaction with styrene
11
P
P
CoCl2, Me3Al
1
H
ethylene (1 atm)
Br2Co(dppe) [cat.]
(6)
R
(5 mol% Co, Al/Co = 3)
CH2Cl2:toluene (4:1)
4
*
Zn, ZnI2, CH2Cl2, rt
R
8 (99%)
R
branched regioisomer
12 (R = alkyl)
13 (>90% y.; >95% ee)
(1,4-HV major)
+
Br2Co(SchmalzPhos) [cat.]
R
O
O
PPh2
PPh2
PPh2
PPh2
Zn, ZnI2, CH2Cl2, rt
R
P
P
PPh2
PPh2
9 (99%)
Ph Ph
O
BDPPB
(SS)-BDPP
(SS)-DIOP
O
O
Ph
linear regioisomer
SchmalzPhos:
O P
O
In 2010 we reported a novel protocol for the cobalt-
catalysed asymmetric hydrovinylation of unactivated linear 1,3-
dienes (12) under one atmosphere of ethylene and temperatures
Ph
Ph
PPh2
in the range of – 45 C to 25 C (eqn 6).57a This class of
substrates gave unacceptable results in all previously reported
HV reactions using iron,3 ruthenium,25 and nickel.13,58 We have
since found that ligands and promoters play significant and,
perhaps more importantly, predictable roles in the control of
o
o
Scheme 1.
Hydroalkenylation of dienes catalysed by
Br2Co(P~P)/Zn/ZnI2. Ligand effects
Expanded scope of substrates in the cobalt-catalysed
hydrovinylation (HV, addition of ethylene) of alkenes and
related reactions have been the subject of several recent
publications. In 2006 Vogt reported the first example of a Co-
catalysed asymmetric hydrovinylation of styrene using a
Co(II)-complex of the Trost ligand 10 giving modest yield and
selectivity (eqn 4).54,55 Similarly norbornene has been reported
regio- and stereoselectivities of these reactions.
Some
expansion of the scope of this reaction57b and remarkable
reactivity differences between (Z)- and (E)-terminal 1,3-dienes
have also been reported.57c With some modifications, the new
protocols for the cobalt-catalysed reactions are more broadly
applicable and more substrates than originally reported undergo
the reaction under moderate conditions. The full details of
these studies are reported in this paper.
to undergo
a highly efficient alkylation-hydrovinylation
catalysed by a pyridine-imine cobalt complex 11 (eqn 5).56
Incidentally, styrene does not undergo HV under these
conditions.
Results
Hydrovinylation of Linear 1,3-Dienes. 1,3-Dienes are among the
most readily available59 yet under-utilized precursors in
enantioselective carbon-carbon bond-forming reactions. Thanks
2 | Chem. Sci. 2015, 00, 1-3
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Page 3 of 17
Chemical Science
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View ArticAleROTnlIinCeLE
DOI: 10.1039/C5SC00929D
to advances in Wittig reactions and its alternatives, elimination, expanded coordination sites could have other advantages such as
reducing the conformational mobility of a 1,3-diene via an η4-
cross-coupling and cross-metathesis reactions, many structural
and stereochemical variations in the dienes are possible.
However, except for Diels-Alder reactions,60 few useful
asymmetric-catalysed intermolecular C-C bond-forming
reactions involving these substrates are known. In these
instances synthetically acceptable enantioselectivities have been
achieved only for limited reaction types [e.g.,
cyclopropanation,61,62 and hydroformylation,63,64] and that too for
a limited set of substrates. Ni(II)-33,37 and Ru(II)25-catalysed
hydrovinylation has been moderately successful in 1-
arylbutadiene and methyl 2,4-pentadienoate, yet the
enantioselctivities for these substrates remain unacceptably low
(Scheme 2). Attempts to carry out the Ni-catalysed asymmetric
hydrovinylation of an unactivated 1,3-diene (R = alkyl in 12,
Scheme 2) such as (E)-1,3-octadiene under a variety of
conditions led to a mixture of several products including 1,2- and
1,4-HV products.
coordination at critical stages in the catalytic cycle (vide infra),
and if so, could this contribute to increased selectivity in the
reactions of the 1,3-dienes.
Synthesis of the X2Co(P~P). The bis-phosphine complexes were
readily prepared by modification of a procedure described in the
literature.70,72 We57 and others55 have previously described the
full characterization of the several Cl2Co(P~P) complexes
including their solid-state structures. Several more have been
characterized recently.73 Anhydrous CoCl2 dissolved in freshly
distilled THF (12 mL per mmol) was treated with 1.05
equivalents of the bis-phosphine for 15 minutes under argon to
get a blue solution to which was added 12 mL/mmol of degassed
ether to form a turbid solution. The mixture was stirred for 12-
24 h and excess degassed hexane was added to precipitate the
complex, which was collected and washed with 1:1 ether hexane.
This solid was used directly for the subsequent hydrovinylation
reactions.
Ph
*
Optimization of
a
Protocol for Co-Catalysed
R
(77% ee)
R = Ph
(12)
Hydrovinylation. Effect of Ligands. For our initial studies
we chose (E)-1,3-nonadiene (12a) as a prototypical, unactivated
1,3-diene substrate. After an initial series of experiments in
which the reaction parameters such temperature, time, solvents
and sequence of addition of various reagents were
systematically varied, we settled on a general protocol that is
outlined in eqn 7.
+
H
*
ethylene (1 atm)
R = CO2Me
MeO2C
(<10% ee)
[(allyl)Ni[(L)X] (cat.)
H
mixture of (E)- and (Z)-1,2
R = alkyl
and 1,4-HV products
ethylene (1 atm.)
1
Scheme 2. Limitations of Ni(II)-catalysed asymmetric HV of
1,3-dienes
Cl2Co(dppe) (x equiv.)
4
additive (y equiv.)
A
careful review of the extensive literature on the
CH2Cl2/toluene (4:1)
R
codimerization of butadiene and ethylene 1,5,10,14,65,66 revealed that
while most reactions were carried out at higher temperatures
(typically > 60 oC) and pressures (> 20 kg/cm3), remarkable
selectivity for the formation of the Z- or E-1,4-hexadiene can be
achieved by selection of the appropriate metal/ligand
combinations. While highest selectivity for the E-1,4-hexadiene
was obtained with [(ethylene)2RhCl]2 in alcoholic medium,
12 (12a R = C5H11
)
H
H
+
+
+
(7)
R
R
R
R
H
H
13 1,4-HV (Z )
16 1,4-HV
(linear)
14 1,4-HV (E )
15 1,2-HV (E )
(branched)
Cl2Co(II)(P~P) [(P~P)
=
1,2-bis-diphenylphosphinoethane)
provided the best selectivity for the Z-1,4-hexadiene (Eq 3). The
most selective catalysts appeared to be those based on Fe(III)65
and Co(II)5,7 and we decided to concentrate our efforts on these
two metals in our initial explorations.
The reaction conditions and the distribution of products
obtained are shown in Table 1. The results for the HV of (E)-
1,3-nonandiene using Cl2Co(dppe) and trimethylaluminum as an
While we have convincingly demonstrated that the Ni(II)- additive (Al:Co = 3) at various temperatures are shown in entries
o
catalysed hydrovinylation of vinylarenes was completely 1-4.74 Thus using 10 mol% of the cobalt complex at – 40 C a
inhibited by chelating bis-phosphines,24,67 many experiments in clean reaction ensues giving one major (13a) and two minor (14a
the literature suggested that chelating ligands can be used in the and 16a) products. These products, arising from 1,4-addition
cobalt- or iron-mediated reactions since these metals are capable across the 1,3-diene, were identified by comparison of gas
of higher coordination numbers.54 Our investigations started
chromatographic retention times and spectroscopic data with
with an examination of 1,n-bis-diphenylphosphinoalkane- those of authentic samples, whose structures were rigorously
established earlier.57,74 As the temperature is raised, the
complexes of cobalt(II) X2Co[(Ph2P-(CH2)n-PPh2)] as catalysts in
the presence of various additives in the reactions of ethylene with proportion of the 1,4-linear adduct, 16a, remains constant, but
prototypical 1,3-dienes. Based on our work on the mechanism of
increasing amounts of a new geometric isomer of the major
the Ni(II)-catalysed HV,68 we set out to generate a cobalt-hydride branched product, viz., 14a, is formed (entries 2-4). The
or an equivalent species in solution (more on this later) which
reactivity of the substrate and the distribution of products are
could serve as a catalyst in these reactions. Several cobalt also dependent on the bite angle of the ligand75,76 employed. The
hydride species carrying bisphosphines (e.g. (dppe)2CoH,69,70
Cl2Co(dppp) complex is slightly better than the corresponding
[HCo(dppe)2(CH3CN)]+)71], which could serve such a role have dppe complex in both reactivity and selectivity (compare entries
been fully characterized. Further we wondered whether the
1 and 5). The most reactive and selective ligand in this series
This journal is © The Royal Society of Chemistry 2015
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ARTICLE
ChVeiemwiAcraticlleSOcinelinnece
DOI: 10.1039/C5SC00929D
bite angle (β = 122o) also gives the 1,4-Z-adduct 13a as the
major (65%) product, but with up to 34% of the 1,2-adduct 15a.
Finally reaction of Cl2Co(Ph3P)2 in the presence of Me3Al gave
mostly polymeric materials (entry 15).
was identified as 1,4-bis-diphenylphosphinobutane (dppb), which
yielded the (Z)-1,4-HV product 13a in 96% isomeric purity
(entry 9) in less than 0.5 h at 0 oC. Most notably, the (E)-isomer
of the 1,4-HV (14a) was not detected by GC (limits of detection
<0.5%). The reaction can be done equally effectively using
Br2Co(dppb) complex using Me3Al as a promoter (entry 10); but
the reaction fails completely when Zn/ZnI2 is used (entry 11),
clearly establishing this procedure as distinctly different from the
more reducing Hilt protocol (Scheme 1). Yet another remarkable
observation, as shown in entries 12 and 13, is the effect of the
narrow bite-angle (β = 72) ligand bis-diphenylphosphinomethane
(dppm), which yielded a 1,2-HV product 15a as the major
product with very little of the 1,4-Z adduct (13a) or the 1,4-linear
adduct (16a). The minor component in this reaction was
identified as the 1,4-E adduct, 14a. Catalyst derived from dppm
is also very reactive, requiring only 3 mol% catalyst to complete
Identification of the Hydrovinylation Products.
For an
understanding of the mechanism of the reaction and for any
further applications of the products, rigorous identification of
all of the isomers of the hydrovinylation is critical. Fortunately,
the remarkable ligand effects seen in these reactions enable
preparation of several of the products in nearly pure state,
which make their identification, and, the identification of minor
products sometimes formed along with these compounds, fairly
straight forward. We have also established conditions for gas
chromatographic separation of all compounds including those
of the enantiomers on appropriate columns.74 All estimates of
isomeric ratios derived from proton NMR data have been
corroborated by GC analysis.
o
the reaction in less than 12 h at –20 C (entry 13). BISBI (2,2’-
(diphenylphospinomethyl)-1,1’-biphenyl), a ligand with a large
Table 1. Hydrovinylation of 12a (R= C5H11) catalysed by Cl2Co(P~P). Effect of ligands and temperature a
Product, yield (%) b
Temp (oC)
Time (h)
Conversion
Bite
angle
Cat.(mol%)
Al/Co
P~P
16a
Entry
13a
(1,4-Z)
14a
15a
(1,4-linear)
(1,4-E)
(1,2-E)
15
12
11
12
15
4
85
85
85
85
91
91
91
98
98
98
98
72
72
10/3
10/3
10/3
10/3
10/5
10/5
10/5
10/5
10/5
10/5
--f
dppe
dppe
–40/14/93c
–20/28/>99
–12/15/>99
0/6/>99
78
64
66
73
85
76
86
93
96
94
0
0
19
22
15
0
0
0
1.
2.
dppe
0
3.
dppe
0
4.
5.d
6.d
7.d
8.d
9.d
10.d,e
11.d,e,f
12.d
13.d
14.
15.
dppp
dppp
dppp
dppb
dppb
dppb
dppb
dppm
dppm
BISBIg
2 Ph3P
–40/8/>99
13/5/>99
rt/1/>99
0
20
8
0
5
0
0
–10/8/>99
0/0.5/>99
-20/20/>99
rt/0/0
7
0
4
0
0
5
0
0
0
0
0
0
10/3
3/5
rt/2/>99
3
30
33
0
67
64
34
0
0
–20/12/>99
-12/6/100
–10/12/- h
2
0
122
-
100/10
3/5
65
0
0
0
a
See eqn 7 and Supporting Information for details. b Estimated by GC and NMR. c The rest is starting material. d Entries 5-12 in neat
e
f
g
CH2Cl2.
Using Br2Co(dppb).
Use of 20 mol% Zn, 20 mol% ZnI2 at 0 oC-rt for 4 h returns starting material.
2,2’-
(Diphenylphospinomethyl)-1,1’-biphenyl. h Polymerisation.
4 | Chem. Sci. 2015, 00, 1-3
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DOI: 10.1039/C5SC00929D
Chemical Science
ARTICLE
H11
H11
Promoters. Even though methyl-aluminum reagents such as
methylaluminoxane have been widely used for the activation
of cobalt-halide complexes in polymerisation reactions,19, 20, 80
the use of trimethylaluminum in our reactions is novel and
noteworthy, since it has been reported that in cobalt-catalysed
hydrovinylation of styrene this additive is ineffective.55 Our
protocol is also distinctly different from the hydroalkenylation
of 1,3-dienes reported by Hilt in which a bromide complex
Br2Co(P~P) [(P~P) = dppm, dppe, dppp] is used along with Zn
and ZnI2 as additives (Scheme 1) to get high yields of the
adducts (8). While we find that Br2Co(dppb) complex works
just as well as the corresponding chloride complex in our
protocol using Me3Al (eqn 8a), this bromide complex does not
catalyse the HV reaction in the presence of Zn and ZnI2 (Eq
H10
CH1
H10
H2
H12
H12
3
H4
H4
CH9
H2
CH9
CH1
3
3
3
H3
H3
H3
H5
H5
H5
H5
13a
14a
H3
CH11
H5
3 H1t
H6
H6
H1c
H310C
H3
H2
H4
15a
H6
H6
16a
Figure 2. Labeling of protons in 1H NMR of HV products
81
8b, Hilt conditions, also entry 17, Table 2).
The products 13a, 14a, 15a and 16a derived from
hydrovinylation of (E)-1,3-nonadiene (eqn 7) are typical. The
assignments of signals described below have been confirmed
by COSY, NOESY and appropriate decoupling experiments.
The major product seen in most hydrovinylation reaction, 13a,
ethylene (1 atm)
H
H
Br2Co(P~P), Me3Al
+
(8a)
R
(10 mol% Co, Al/Co = 5)
CH2Cl2, -20 oC, 20 h
(P~P)
R
is characterized by, among other signals, the vinyl-CH3
-
hydrogens (H1) which appear at δ 1.622 (dd, 6.5 Hz, 2.0 Hz)
and the bis-allylic hydrogen (H4) at δ 3.011 (m, NOE with H1).
The 13C signal for the methyl carbon linked to this cis-double
bond appears at δ 14.07 ppm. In 14a, the corresponding trans-
1,4-HV adduct, this methyl carbon appears at δ 17.96, in
keeping with the generally observed trend of lower field for
16a
13a
dppp
dppb
70%
94%
13%
5%
C5H11
ethylene (1 atm)
12a
Br2Co(dppb) (10 mol%)
(8b)
no reaction
1
trans alkene-methyls.77a This is also reflected in the H NMR
Zn/ZnI2 (40 mol% each)
rt, 72 h
(8b)
signals of the CH3 groups of the cis- and trans compounds.
The trans CH3 signals are slightly lower field compared to the
1
cis-CH3 signals. In the present case the H NMR the methyl
In light of the observations documented in the previous
paragraph, it is of interest to note that several of the commonly
used promoters (hydride reagents, alkylating agents, Lewis
acids) also do not promote the hydrovinylation reactions in the
presence of Cl2Co(P~P) complexes (Table 2, entries 1-15).
Combination of Zn/ZnI2 does affect the reaction giving
moderate yields of 13a and 16a if Cl2Co(dppe) is used instead
of Cl2Co(dppp) (entries 15 versus 16, Table 2). The
corresponding complexes with CoBr2 catalyze the reaction,
but with significant proportion of the linear 1,4-adduct 16
(entries 19 and 20). This stark difference between various bis-
diphenylphosphine ligands (dppm, dppe vs dppb) has been
noted earlier by Hilt in his hydroalkenylation studies (entry 18,
Table 2).81
signal in the trans-compound 14a appears at δ 1.669 (dd, 6.0
Hz, 0.5 Hz) and of the cis-compound (13a) at δ 1.622.77b The
bis allylic hydrogen H4 in 14a appears at δ 2.619 (tdd, 7.5 Hz,
7.5 Hz, 7.5 Hz). This signal, corresponding to the bis-allylic
hydrogens, is distinctly different for all 4 isomers and is highly
diagnostic. The 1,2-HV-adduct 15a is characterized by an up-
field methyl doublet at δ 1.075 (d, J = 6.5 Hz, H11) and a bis-
allylic hydrogen (H3) signal at δ 2.807 (ddq, 6.0 Hz, 6.0 Hz,
6.0, Hz). The (E)-geometry of the alkene is obvious from the
large J4,5 value in 15a (15.5 Hz). Closely related compounds
have been described in the literature.78,79 The linear adduct
16a is characterized by bis-allylic H’s (H3) at δ 2.778 (t, 2H)
and allylic H’s (H6) at δ 2.019 (q, 2H). The GC retention time
of this isomer (16) in every case is significantly longer than
the other three isomers on a methylsilicone column. In
addition, 16a is the only compound among the adducts that is
achiral, and thus showing no resolution on the chiral stationary
phase (CSP) GC columns.
polymethylsiloxane column at 80 C the following retention
On
a
typical 30
m
o
times are observed for the various adducts: 13a: 21.169 min.;
o
14a: 21.93min.; 15a: 22.77 min.; at 75 C: 13a: 28.22 min,
16a: 40.77 min. CSP GC (Cyclodex B) 60 oC 13a: RT = 26.07
min. (S), 27.25 (R); 16a: 44.79 min. The assignment of the
absolute configuration is based on the observation that the
product from Fe-catalysed asymmetric hydrovinylation of 1,3-
pentadiene3 gave the laevorotatory product (S) as the major
one and the same product was obtained in the asymmetric HV
of 1,3-pentadiene using Cl2Co[(R,R)(–)-DIOP]. Accordingly,
the levorotatory enantiomer was assumed to have the (S)-
configurations. Configurations of all other 1,4-adducts were
assigned by analogy.
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Table 2. Effect of promoters in the X2Co(P~P)-catalysed hydrovinylation of 1,3-dienes.a
Temp (oC)
Conversion
[products]
Additive
(mol%)
P~P
Entry
Time (h)
0
0
no additive
Me3B (100)
Et3B (100)
dppp
dppp
dppp
dppp
dppp
dppp
dppb
dppe
dppe
dppe
dppe
rt (2)
rt (2)
1.
2.
0
rt (2)
3.
0
Ph3B (100)
rt (2)
4.
0
i-BuAlH (100)
LiEt3BH (100)
Zn/ZnI2(5, 5)
PhMgBr (400)
Mn (100)
rt (2)
5.
0
Rt (2)
rt, (16)
rt (7)
6.
0
7.
0
8.
0
rt (14h)
-10 - rt (10)
rt (2)
9.
0
InI3 (100)
10.
11.
<2%
0
Et2AlOEt (100)
MeMgBr,
AgOTf
dppm
0 - rt (4)
12.
(100, 100)
Et2AlCl (50)
0 b
0 b
dppb
dppb
-10 (2)
-10 (2)
13.
14.
EtAlCl2 (50)
Zn/ZnI2 (20,
20)
0
dppp
0 - rt (5)
15.
72 [56% 1,4-Z
(13a); 11% 1,4-
Z-lin (16a)] c
Zn/ZnI2 (20,
20)
dppe
0 –rt (5)
16.
Br2Co
(dppb)
Br2Co
(dppb)
Zn/ZnI2 (20,
20)
Zn/ZnI2(10,
10)
0
0
0 -rt (4)
rt (16)
17.
18. d
100 [85% 1,4-Z
(13a); 15% 1,4-
Z-lin (16a)]
Br2Co
(dppe)
Zn/ZnI2 (20,
20)
0 -rt (4)
0 –rt (4)
19.
20.
100 [79% 1,4-Z
(13a); 21% 1,4-
Z-lin (16a)]
Br2Co
(dppp)
Zn/ZnI2 (20,
20)
a
See eqn 7 and Supporting Information for details. All using Cl2Co(P~P) unless indicated otherwise. Entries 13-20 using (E)-C8H17-
CH=CH-CH=CH2. b No volatile products (polymers?). c 2% Each 2 other isomers. d Ref. 81 (in hydroalkenylation of isoprene).
R
amines and N-heterocyclic carbenes (Figure 3) for this
reaction. Cobalt (II)-complexes of these ligands failed to
R
N
N
N
yield any HV products in reactions of (E)-1,3-nonadiene
under conditions similar to what has been prescribed for the
bis-phosphines in eqn 7.
N
O
O
bis-Oxazolines
R = t-Bu, Ph
Diamines
Scope of 1,3-diene substrates. The full scope of the new
protocol for the Co-catalysed hydrovinylation reaction was
examined with the help of the substrates shown in Table 3.
The general optimized procedure used for the reactions is
shown in eqn 9. In this procedure, a round- bottomed flask
with a side-arm is charged with the X2Co(P~P) complex (5-
(–)-Sparteine
N
N
••
••
N
N
But
tBu
••
N
N
o
N
N
10 mol%) in degassed methylene chloride at 0 C under
••
••
argon. When methylaluminoxane (MAO) is used as the
promoter, it is charged to the reaction flask right after the
addition of the cobalt-complex. If Me3Al is used, a
solution of Me3Al in toluene is added to the Co-complex
and the reaction vessel is carefully evacuated and refilled
with ethylene from a balloon, which is kept in place until
the reaction is quenched. It is subsequently cooled to the
prescribed temperature and the substrate is added and the
mixture stirred for the periods shown in the table. Progress
of the reaction maybe followed by gas chromatography,
N
N
N-Heterocyclic Carbenes
Figure 3. Assorted ligands found unsuitable for Co(II)-
catalysed hydrovinylation
Ligands. Having identified a chelating bis-phosphine and
Me3Al as a viable ligand/promoter combination for an
efficient cobalt-catalysed HV, we also examined three other
popular classes of ligands, bis-oxazolines, chelating bis-
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reactions,1 under cobalt-mediated reactions reported here
there is much less discrimination between the (E)- and (Z)-
isomers. Nearly quantitative yields of the expected products
are formed upon reaction of (E/Z)-mixtures of dienes 12d,
12e, 12g and 12j (entries 5, 6, 11, 16) with ethylene using
Cl2Co(dppb) complex and Me3Al or MAO. However, as
will be described later, chiral ligands (S,S)-DIOP and to
some extent (S,S)-BDPP, react much faster with the (E)-
isomers of 12d and 12e, leaving behind most of the Z-isomer
unreacted (vide infra).
where baseline separation of the various possible adducts is
observed. Excess methanol is added to quench the reaction
and the products are extracted with pentane/ether. The
conversions generally are quantitative under conditions
shown in the table and, the yield, as judged by estimated
weight of the product and its purity (GC and NMR) is
>95% in most cases. The lower yields of some of the
products result from the volatility of the hydrocarbon
products. Attempts to remove last traces of the solvent
(especially on small scales) result in some loss of materials.
Details of the characterization of the isomeric products
obtained from prototypical substrates (eqn 7) and the
diagnostic peaks in the 1H NMR that characterize the
various isomers are documented in the Supporting
Information.
Cobalt(II)-Catalysed Heterodimerization between Nona-1,3-
diene and Propylene. Upon reaction with [(allyl)NiBr]2 and
Ph3P, propylene and various vinylarenes undergo efficient
Ni-catalysed heterodimerization reactions.82
Various
attempts to carry out this reaction under conditions that
enabled the addition of ethylene to (E)-non-1,3-diene using
Cl2Co(P~P)/Me3Al gave no conversion of the starting
material (eqn 10).
ethylene (1 atm)
H
Cl2Co(P~P) (5-10 mol%)
+
(9)
Me3Al or MAO (Al:Co = 3-20)
(CH2Cl2, toluene, T oC)
R
H
R
R
propylene (1 atm)
16
13
Cl2Co(P~P) (10 mol%)
no reaction (10)
Me3Al (Al:Co = 3)
(CH2Cl2, 0 oC - rt)
C5H11
12a
As shown in eqn 9, the cobalt-catalysed hydrovinylation
of 1,3-dienes is most useful to prepare two types of adducts,
1,4-adducts 13 and 16 depending on the nature of the
substituent R and the chelating bis-phosphine that is used. In
general, alkyl- and substituted alkyl-1,3-dienes give
excellent yields of the 1,4-(Z)-adduct 13 (entries 1-7) with
the Cl2Co(dppb) complex with either Me3Al or MAO as the
activating agent. The ester bearing substrate 12h (entries 12-
14) is uniquely different in its reactivity in that neither the
commonly used dppp complex (entry 12) nor the dppb
complex (entry 13) is able to effect the hydrovinylation of
this substrate. Yet a cobalt complex with a bis-phosphine
ligand with a narrow bite-angle, dppm gives an excellent
yield of the product 13h (entry 14). Recall that dppm-
complex with the substrate with a simple alkyl substituent
(12a) gave a 1,4-E-adduct 14a and a 1,2-E-adduct 15a
(Table 1, entries 12 and 13).
[(P~P) = dppm, dppe]
Iron(III)-catalysed codimerization of ethylene and (E)-nona-
1,3-diene. Iron(III)-catalysed codimerisation of ethylene and
1,3-butadiene (60 kg/cm2, 30-80 oC) in the presence of
alkylaluminum reagents is among the earliest reported
examples of these reactions.1,2 Other related reactions
involving Fe-catalysis include codimerization of 1,3-
pentadiene and ethylene,3 dimerization of α-olefins,83 and
1,4-addition of α-olefins to dienes.84 In a brief exploration
to establish a direct comparison of reactivity of (dppe)FeCl3
with Cl2Co(dppe) revealed that the former is much less
reactive, with no particular advantages in terms of selectivity
of products obtained, even though 1,4-Z-adduct 13a is the
major product in both cases. One of the moderately
successful experiments is depicted in eqn 11.
1,3-Dienes with aromatic ring in conjugation with the
diene belong to
a class of substrates that behaves
dramatically differently (12g: entries 8-11, 12k: entry 17).
These substrates give linear 1,4-adducts 16g and 16k (Fig 4)
with a (Z)-geometry in the HV reactions catalysed by
Cl2Co(dppb) and Cl2Co(dppp) complexes (entries 8, 9, 17).
The Cl2Co(dppm) complex, in sharp contrast, gives
exclusively a 1,2-(E)-adduct, 15g (entry 10, Fig 4). Most
notably, there is no contamination from the isomeric
impurities in either of these reactions. Likewise (E)-2-
methyl-1-phenyl-1,3-butadiene (12l) gives only a 1,2-HV
product (15l, Fig 4) with both the dppm and dppp complexes
(entries 18 and 19).
ethylene (1 atm)
H
Cl3Fe(dppe) (20 mol%)
+
(11)
Me3Al (Al:Fe = 2.5)
(CH2Cl2, -10 oC to 15 oC )
(~ 40% conv.)
H
C5H11
C5H11
13a (30%)
C5H11
16a (10%, 1,4-linear)
12a
Cl3Fe(dppm) 0 oC, 8 h
Cl3Fe(Ph3P)2 10 oC, 9 h
<5%
0
3-Substituted 1,3-butadienes, β-myrcene (12m) and
isoprene (2-methyl-1,3-butadiene, 12n) gave very high
yields of 1,4-linear adducts 16m and 16n (Fig 4) in which
the hydrogen adds to the less substituted double bond
(entries 20 - 22).
Hydrovinylation of (Z)- vs (E)-1,3-Dienes. While the
increased reactivity of 1,3-dienes which can readily assume
s-cis conformation (for example E- vs Z-1,3-pentadiene) has
been noted in the Fe(III)-catalysed hydrovinylation
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Table 3. Scope of substrates in the Co-catalysed hydrovinylation of linear 1,3-dienesa
Chemical Science
Yield
Diene (12)
R in 12 Eq 9
Cl2Co(P~P)
(P~P), mol%
Entry
Al/Co
Conditions
temp (oC)/time (h)
0/0.5
16
13
dppb (10)
dppb (5)
dppb (5)
dppb (5)
dppb (10)
5
3
C5H11 (12a)
C6H13 (12b)
C7H15 (12c)
C8H17 (12d)
C8H17 (12d)
(E:Z 54:46)
>95
>95
>95
>95
1
2
3
4
–10/6 b
–10/6 b
–10/6 b
–10/8 c
3
3
20
82
5
cyclohexyl (12e)
(E:Z 45:55)
dppb (10)
dppb (5)
20
3
–10/8 c
–10/6b
90
6
7
CH3 (12f)
>95
>99
(86)
dppp (10)
dppb (10)
3
3
–20/4
–10/6
0
8
>99
(87)
0 d
Ph (12g)
0
0
0
9
dppm (10)
dppb (10)
3
–20/7
10
11
Ph (12g)
(E:Z 40:60)
20
–10/8 c
>99
0
0
dppp (10)
dppb (10)
dppm (10)
dppb (10)
10
10
10
20
5/11
5/11
0
0
12
13
14
15
CH2CO2Et (12h)
0/15
–15/14 c
84
>97
<2
79
CH2CH2OBn (12i)
CH2CH2Ph (12j)
(E:Z 53:47)
4-Me2N-C6H4
(12k)
dppb (10)
dppb (10)
20
20
–10/8 c
0/13 c
84
0
16
17
Other Dienes
0 e
0 f
>95 g
dppm(5)
3
-10/6 b
0
0
18
19
20
21
22
Ph
Me
(12l)
dppp
3
3
3
10/6
0/6
dppb (10)
dppp (10)
0
0
β-myrcene (12m)
83 g
rt/1
>99
Isoprene (12n)
dppp (10)
10
–20/4
0
b
d
e
a See eqn 9 for procedure. Solvent CH2Cl2:tolune 4:1. c MAO as ‘Al-Me’ source. 1,2-linear product 15g (99%). 1,2-linear product
15l, 97%. f Product 15l 62%. g Co:Al 1:3 (Z)-2-Methyl-6-(3-propenyl)octa-2,6-diene (16m). See Figure 4 for structures of 15g, 15l, 16g,
16k, 16m and 16n.
H
The reactivities of the chelating chiral ligands roughly
parallel those of the corresponding achiral ligands with
Ph
Ar
R
H
similar bite angles. In general, bis-diarylphosphino-ligands
with bite angles comparable to dppe, dppp and dppb are the
most reactive (Table 4, entries 1-3 and 7-9, 17, 18; ligands
L1, L2, L3, L7, L8, L9, L17, L18). As expected, ligands
with narrow bite-angles, (S,S)-chiraphos (L1) and (R)-
prophos (L2) give, as the major product the 1,2-HV product
15a, along with minor amounts of 13a, the 1,4-(Z)-adduct
(entries 1 and 2). The enantioselectivities for these products
are low, with the ee for 13a almost always higher than that
of the any other adducts in these reactions. Two other
phosphines containing 2-carbon chains in the backbone, L3
and L4, give 13a as the major product with very low
enantioselectivities (entries 3 and 4). Two chiral analogs of
dppm, miniphos L5 and trichickenfootphos L6 are electron-
rich phosphines,85 which are poor ligands for this reaction
(entries 5 and 6).
H
16g Ar = Ph
16k Ar = 4-Me2NC6H4
H
15g R = H
15l R = Me
16n
16m
Figure 4. Major Products from 12g, 12k, 12l, 12m, 12n
Asymmetric Hydrovinylation of 1,3-Dienes
Effect of Chiral Ligands. The remarkable facility with
which chelating bis-phosphine complexes of Co(II) catalyze
the chemo- and regioselective hydrovinylation of 1,3-dienes
prompted further investigations into an asymmetric version
of this reaction. Hydrovinylation of a prototypical substrate
12a (R = C5H11) was examined under conditions shown in
eqn 12 using Co(II)-complexes of several commercially
available enantiopure chiral ligands. The structures of these
ligands are shown in Figure 5. The distribution of products
and the enantioselectivities of the chiral products obtained
are listed in Table 4.
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*
*
+
ethylene (1 atm.)
R
1
R
Cl2Co(P~P) (10 mol%)
13 1,4-HV (Z )
14 1,4-HV (E )
(12)
TMA in tol. 30 mol%
or MAO, CH2Cl2, 6 h
temp -45 oC or rt
4
R
*
+
(E)-12
R
R
16 1,4-HV (Z )
15 1,2-HV (E )
also (E + Z)-12
Table 4. Enantioselective HV of (E)-nona-1,3-diene. Effect of chiral ligands a
Products (%ratio /%ee) b
Ligandc
Conversion
Entry
*
*
*
R
R
R
R
14a
13a
16a
0
15a
L1
L2
99
99
99
66
41/78(S)
4/-
55/16
1.
2.
2
18/44
15/-
62/1
9
L3
90/8
0
0
3.
12
L4
54/1
0
0
4.
40 d
0 d
99
99
99
0
L5
7/-
-
31/8
5.
-
<1
<1
0
L6
-
-
-
6.
L7
96/97(R)
0
0
7.
L8
95/95(S)
0
0
8.
L9
86/64(S)
11/-
2/-
9.
-
L10
L11
L12
L13
L14
L15
L16
L17
L18
L19
L20
-
-
-
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
-
0
-
66/8
63/14
39/95(S)
6/-
-
-
7
88
95
80
~6e
99
87d
95
0
0
0
4
0
11/31
40
0
0
0
0
0
0
0
28/20
59/12
8
79/10
95/87(S)
-
0
0
-
0
0
-
4
-
-
0
-
-
-
d
e
a See eqn 12 for typical procedure (R = C5H11). b Determined by CSP GC. c See Figure 5 for structures of ligands. At -45 oC /8 h.
Rest starting material.
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MeO
PPh2
PPh2
P
Ph2P
Ph2P
P
PPh2
PPh2
Ph
OMe
Ph
L2 (R)-Prophos L3 Norphos
L1 (S,S)-Chiraphos
L4 (R,R )-DIPAMP
But
But
Me
Me
But
P
P
P
P
But
But
Me
L5 miniphos
L6 trichickenfootphos
Ph2P
O
O
PPh2
PPh2
Ph2P
PPh2
N
But
PPh2
O
O
L7 (S,S )-BDPP
L8 (R,R )-DIOP
L9 BPPM
O
O
O
PPh2
PPh2
PPh2
PPh2
PPh2
PPh2
MeO
MeO
PPh2
PPh2
O
L13 (S )-Segphos
L10 BINAP
L12 (R )-MeO-Biphe
L11 H8-BINAP
H
H
P
P
H
P
P
tBu
tBu
H
tBu
tBu
L14 (1S,1'S,2R,2'R)-Tangphos L15 (1R,1'R,2S,2'S)-Duanphos
CF3
P
Ph2P
P
F3C
F3C
P
Fe
P
PPh2
Fe
Fe
CF3
L16 Walphos
L18 Josiphos-2
L17 Josiphos-1
PPh2
PPh2
Ph2P
N
PPh2
Ph
L19 Phanephos L20 N-benzyl-3,4-diphenyl-
phosphnopyrrolidine
Figure 5. Ligands for cobalt(II)-catalysed asymmetric hydrovinylation of 1,3-dienes
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The dppp analog (S,S)-BDPP (L7) and dppb analog (R,R)-
DIOP (L8) gave the best yields (99%) and highest
enantioselectivities (97% and 95% respectively) for the
hydrovinylation of 1,3-nonadiene (entries 7 and 8). In these
cases less than 1% of the isomeric linear product (16a) is
formed as the only contaminant. A constrained analog of
dppb, L9, derived from proline also gives 13a as the major
product, albeit in relatively low ee (entry 9).
that gave none of the most commonly observed major
product 13a (entry 16). This complex gave the (E)-1,4 and
(E)-1,2- adducts 14a and 15a as the major products, both in
only modest selectivities (entry 16). Two other related
ligands Josiphos-1 (L17) and Josiphos-2 (L18) are among
the best ligands, the latter giving up to 87% ee for the
branched product 13a, with <4 % of the linear adduct 16a as
a contaminant (entry 18). A remarkable electronic effect
was observed in going from complex of L17, a ligand with
diphenylphosphino-substituent, to one with a more electron-
withdrawing di-(bis-1,3-CF3-phenylphosphino)-ligand L18.
The enhancement of reactivity (yield: improvement from
87% to >95%) and selectivity (from 10% ee to 87% ee)
observed in this ligand change is quite uncommon, and is
reminiscent of similar effects seen in Ni-catalysed
asymmetric hydrocyanation of vinylarenes.86 Electronic
tuning of ligands might offer an attractive option to enhance
enantioselectivity in the asymmetric hydrovinylation of 1,3-
dienes. In addition the complex from ligand Josiphos 2
(L18) effects these reactions at 0.01 equivalent loading level
(See Table 5, entries 4, 7, 9).
2,2’-Diphenylphosphino-1,1-biaryls are among the most
widely studied ligands in asymmetric catalysis, and in the
present case only the biaryl ligands that gave any products
are (R)-MeO-Biphe (L12) and (S)-Segphos (L13). Only
low ee’s (entries 12 and 13) were observed in these cases.
Co(II)-complexes of BINAP (L10) and H8-BINAP (L11)
failed to affect the reaction (entries 10 and 11).
Cobalt(II) complexes of electron-rich phospholanes L14
and L15 are poor catalysts, with the latter giving <6 %
conversion (entries 14 and 15). It is not unreasonable to
assume that in the cases of ligands L6 (entry 6) and L15
(entry 15) the reactions might be retarded by both electronic
and steric properties of these ligands. Yet, Tangphos (L14),
an electron-rich bis-phospholane is among the best ligands
(95% ee) for the formation of the 1,4-Z-HV product 13a,
even though it gave up to 40% of the linear achiral product
16a (entry 14).
Finally two other ligands, Phanephos (L19) and the bis-
phosphinopyrrolidine L20 are not competent for this reaction
under the standard conditions (entries 19 and 20).
The complex derived from ferrocene-based ligand
Walphos (L16) is the only ligand among the many we tested
Table 5. Co-Catalysed asymmetric HV of linear 1,3-diene a
Diene (12)
R in 12 eqn 12
(P~P) in
Cl2Co(P~P)
L8 (R,R)-DIOP
Conditions
temp (oC)/time (h)
-45 /6
Yield
(%)
13 (%ee) b
16 (%)
Al/Co
Entry
3
3
95.0(S)
97.1(R)
>95
97.1
39
1
2
3
4
5
6
7
8
9
L7 (S,S)-BDPP
L14 Tangphos
L18 Josiphos 2c
L8 (R,R)-DIOP
L8 (R,R)-DIOP
-45/6
C5H11 (12a-E)
40
<4
3
3
–10/8
–20/14
-45 /6
95.0(S)
87.0(S)
95.3(S)
>95
>95
>98
>95
95
3
3
C6H13 (12b-E)
C7H15 (12c-E)
-45 /6
95.4(S)
L18 Josiphos 2c
<3
3
3
–20/14
-45 /6
87.0(S)
96.1(S)
L8 (R,R)-DIOP
C8H17 (12d-E)
L18 Josiphos 2c
L8 (S,S)-DIOP
3
5
–20/14
-45 /1
88
86.0(S)
74.0(R)
<3
5
C8H17 (12d)
(E:Z = 54:46)
cyclohexyl (12e)
(E:Z = 47:53)
CH3 (12f-E)
Ph (12g-E)
53d
10
11
49e
84.0(R)
90.1(S)
--g
5
L8 (S,S)-DIOP
L8 (R,R)-DIOP
3
3
–10/8
-45 /6
>95f
46
12
13
14
15
16
17
18
55
x
L8 (S,S)-DIOP
L7 (S,S)-BDPP
L18 Josiphos 2h
L8 (R,R)-DIOP
L8 (S,S)-DIOPj
L7 (S,S)-BDPPj
3
0/5
10
0/15
92(R)
84
CH2CO2Et (12h-E)
0
3
3
3
3
10/8
-20 /6
–10/6
–10 /9
0
40i
0
0
99.0(S)
94.0(R)
92.0(R)
6
CH2CH2OBn (12i-E)
99
0
>99
a See eqn 12 for procedure. b Determined by CSP GC. Configurations are tentative and are based on the known product of HV of 1-
c
d
methylbuta-1,3-diene.3 See Supporting Information for details. 1 mol% Co.
Reaction stopped after most of the (E)-isomer was
e
converted, recovered starting material (E:Z = 1:17).
Reaction stopped after most of the (E)-isomer is converted, recovered starting
material (E:Z = 1:49). Estimated by GC, volatile products. g 1,2 –Adduct, 15g (43% yield; 52% ee), rest linear 16g.
5 mol%, rest
f
h
i
j
starting material. Using MAO.
Scope of Substrates in the Asymmetric Hydrovinylation of
1,3-Dienes. A close examination of the results presented in
Table 4 suggests that under optimized conditions the (Z)-1,4-
HV product 13a can be prepared in synthetically useful yield
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and enantioselectivity by asymmetric hydrovinylation of (E)-
1,3-dienes. The best ligands for this process have been
identified as BDPP (L7), DIOP (L8) and Josiphos 2 (L18)
narrow bite angles such as dppm give predominantly the 1,2-
adduct (15, Eq 7) whereas complexes of almost all other
ligands lead to 1,4-HV leading to the major product 13 in
which the initial hydride addition takes place at the terminal
carbon and the vinyl group at the C4-carbon. The major
contamination in these reactions results from a 1,4-addition
with the hydride adding to the C4-carbon and the vinyl group
to C1, leading to linear adducts (16). When the reaction is
carried out at low temperature, the configuration of the
internal double bond is almost exclusively (Z). When the
diene is conjugated with an aromatic nucleus this linear
product is formed almost exclusively.
and they appear to have much broader applications.
A
variety of simple and functionalized 1,3-dienes undergo the
reaction giving excellent yields and enantioselectivities and
the results are documented in Table 5. For simple 1,3-dienes
such as 12a-g (entries 1-13) DIOP and BDPP gave the best
results. In most cases, under optimized conditions, nearly
quantitative yield of the product (13) is obtained in >95% ee
with very little if any of contamination by an isomeric
product, which is usually the linear adduct 16. Complexes of
Josiphos 2 (L18, Figure 4) are much more active as
compared to BDPP (L7, Figure 4) and DIOP (L8, Figure 4),
even though the ee’s are slightly lower (1 mol% catalyst
under otherwise identical conditions entry 6 vs 7 and 8 vs 9).
Only in the case of Tangphos ligand L14 the linear product
16 is formed in significant amount (entry 3). As expected
from studies described earlier, this product is also the major
one in the cases of 1,3-dienes in conjugation with an
aromatic nucleus, e. g., 12g (entry 13). In this case, up to
54% of linear adduct 16g is formed with the DIOP
complexes. The major chiral product is the 1,2-E-adduct
15g, which is formed in a modest ee of 52%.
The remarkable selectivity seen in the HV of 1,3-dienes
catalysed by cobalt complexes of dppp- and dppb-analogs
(Table 3) stands in stark contrast to the corresponding Ni-
catalysed reaction, which leads to an intractable mixture of
products. We ascribe the poor results in the Ni-catalysed
HV to the inability of this metal (small, limited coordination
possibilities) to control the conformational mobility (s-cis/s-
trans) of the 1,3-dienes. Based on several anecdotal
5,20,56,71,80
observations in the literature
and analogy to our
own previous work on the mechanism of Ni-catalysed
hydrovinylation of vinylarenes,68 we propose a mechanism
for this reaction which is shown in Scheme 3. In this
mechanism we hypothesize that a cationic cobalt hydride 23
is the true catalyst in the reaction (panel A). This species is
formed by a sequence of reactions which start with the
reaction of Cl2Co(P~P) (17) with Me3Al to form 18.
Abstraction of a halide by the in situ generated Lewis acid
(Me2AlCl) would give the coordinately unsaturated complex
19, which facilitates an alkene (ethylene or the 1,3-diene)
insertion into the Co-Me bond to generate 22. A β-hydride
elimination from this species would produce the putative
catalyst 23.
The reaction is compatible with an ester group (12h) on
the diene as shown in entries 14 and 15. Unexpectedly,
cobalt complex of the Josiphos 2 ligand (L18) failed to affect
the reaction of this substrate even with higher catalyst
loading (entry 15). The diene containing a benzyl ether (12i)
undergoes the reaction giving excellent yield and selectivity
in the presence of CoCl2 complexes of DIOP and BDPP
(entries 16-18). The DIOP complex, especially at low
temperature is excellent for this reaction (>99% ee).
During these studies we also noticed significant difference
in the rates of reactions of (E)- and (Z)-1,3-dienes. While
this difference is hardly perceptible with the achiral
Cl2Co(dppb) complex (entries 5, 6, 11 and 16 in Table 3),
with the Cl2Co(DIOP) the (E)-isomers react significantly
faster, leaving behind essentially unreacted (Z)-isomer near
the end of the reaction (entries 10, 11, Table 5). In the case
of substrate 12e (entry 11, E:Z = 47:53), the unreacted
starting material left behind at the end of the HV reaction (-
The catalyst 23 upon coordination of a diene would form
24 (panel B). We believe it is this η4-complex that is
responsible for the high selectivity of this reaction.
Isoelectronic metal complexes containing two chelating
phosphines and a hydride are known in the literature71 and
this is not an unreasonable intermediate for this process.
Such a structure would limit the conformation of the
coordinated diene to the s-cis arrangement, a possibility that
does not exist in any corresponding Ni(II)-species. Two
modes of addition of hydride to the intermediate 24 are
possible. Addition of hydride to the terminal position (C1)
would lead to an η3-allyl complex 25(syn-anti), which after
incorporation of ethylene at C4 leads to 27, and from there,
to 13 with regeneration of the catalyst 23 by β-hydride
elimination. Instead, if the ethylene addition takes space at
C2 of 26, 31 will result, which after β-hydride elimination of
23 would lead to the 1,2-HV product 15. In the second
mode, hydride addition takes place at C4 of 24 to give 28,
which upon addition of ethylene at C1 would lead to the 1,4-
linear adduct, 16.
o
10 C, 8 h) using the complex Cl2Co(DIOP) is essentially
pure (Z)-isomer (E:Z = 1:49). Similar behavior is also seen
with substrate 12d (entry 10).
isomerically pure (E)-12d leads to 96.1 %ee (entry 8), the
E/Z mixture gives only 74% ee (entry 10), suggesting that
the E- and Z- isomers of a given diene lead to different
proportion of the enantiomers in the asymmetric HV.
Note that while the
Discussion
There are several key features that differentiates this
reaction from the more well-studied (P)Ni(allyl)Br-catalysed
hydrovinylation of 1,3-dienes87 and the hydroalkenylation
reaction of 1,3-dienes catalysed by Br2Co(P~P) /Zn/ZnI2.88
We expect the initially formed adduct 25(syn-anti) to
be configurationally stable at low temperature and this would
account for the Z-geometry seen in the major product 13 and the
E-geometry seen in 15. However, if the syn-anti isomer of 25
undergoes isomerization to the more stable 25(syn-syn), for
example at higher temperature, the 1,4-adduct 14 with an E-
configuration of the double bond will result.66,90,91 This indeed
has been observed (compare entries 1-3, or entries 5, 6 in Table
1). Further support for the intermediacy of the η4-complex also
This journal is © The Royal Society of Chemistry 2012
Whereas the nickel-catalysed reaction is completely
inhibited by chelating ligands,67,
the cobalt-catalysed
89
reaction works well with a wide range of chelating ligands,
and, by appropriate tuning of these ligands it is possible to
obtain synthetically useful yields and selectivities of specific
isomers of these hydrovinylation products. For example,
cobalt complexes of chelating bis-phosphine ligands with
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ARTICLE
comes from the enhanced reactivity of the E-isomer in a mixture
of (Z)- and (E)-terminal 1,3-dienes (entries 10 and 11 in Table
5). For steric reasons, the formation of the η4-complex should
be significantly favored for the (E)-isomer of the diene, which
can adapt an s-cis conformation much more easily as compared
to the corresponding (Z)-diene. This differential reactivity is
especially striking in the case of a highly discriminating ligand
such as DIOP.
Scheme 3. Possible Mechanism of Co(II)-Catalysed Hydrovinylation of 1,3-Dienes
A. Generation of a Cationic Co(II)-Hydride
R
+
+
Cl
Cl
P
P
Me
Me3Al
Me
Me2AlCl
P
P
P
P
P
P
Co
Co
Co Me
Co
CH2Cl2
Cl
diene or
ethylene
(– Me2AlCl)
18
19
17
R
20
+
+
+
Me
β−hydride
elimination
Me
R
P
P
P
catalyst?
23
P
H
Co
Co
Co
P
P
22
21
R
B. A Possible Hypothesis on the Role of a Co(II)-Hydride Catalyst
16 (1,4-HV)
H
R
*
H
R
H
+
H
P
P
+
Co
H
14 (1,4-HV)
13 (1,4-HV)
P
*
P
Co
R
Co
H
P
23 (catalyst?)
H
P
[ – 23]
H
R
1
+
R
H
30
H
+
+
29
P
P
P
P
P
P
2
Co
Co
4
*
H
Co
R
+
4
P
4
*
15 (1,2-HV)
*
R
Co
H
24
H
R
P
H
R
– [23]
27-Z
low temp.
27-E
R
28
H
H
H
H
+
H
H
+
+
+
2
+
2
> 20 oC
P
P
P
P
Co
P
Co
*
Co
P
P
Co
P
2
P
Co
P
4
4
R
R
R
R
R
26-E
[ethylene]
25 (syn-syn)
25 (syn-anti)
26-Z
31
Table 2). It is reasonable to assume that under the highly
reducing conditions, the reactions probably involve Co(I)-
intermediates, where as under our reaction conditions, viz.,
using [Cl2Co(P~P)/Me3Al], a cationic cobalt hydride is the
active catalyst. All attempts to obtain discernable NMR
spectra of any of the putative intermediates under our
conditions have been unsuccessful, suggesting involvement
The differences between the hydrovinylation reactions
catalysed by Cl2Co(P~P)/Me3Al and the corresponding
reactions carried out under the reducing conditions carried
out by Hilt et al [Br2Co(P~P)/Zn/Zn/ZnI2]46,88 are also
noteworthy.
The former reaction proceeds in nearly
quantitative yields with a broad selection of chelating bis-
phosphines from dppm (bite angle 72) to BISBI (bite angle
122) [Table 1]. The reaction under reducing conditions
(Zn/ZnI2) is limited to cobalt complexes of dppm, dppe and
dppp.81 We have confirmed these ligand effects in additions
of ethylene to 1,3-nonadiene (See Table 2, entries 17, 19,
20). Further, while the more easily reduced complex Br2Co
(dppp) is catalytically competent upon treatment with
Zn/ZnI2, the corresponding Cl2Co(dppp) returned no
products under identical conditions (entry 15 vs entry 20 in
of paramagnetic intermediates.
On the other hand,
(dppe)2CoH, an isolable diamagnetic complex (1H –16 ppm,
31P 69 ppm), described in literature69,70 is competent to
effect the hydrovinylation of (E)-1,3-dodecadiene (12d),
o
albeit in relatively low yield (10 mol%, CH2Cl2, 0 C to rt, 7
h, 43% conversion to 13d and 16d). This experiment
provides indirect support to the idea of a Co(I) hydride in
some of the HV reactions. Other observations in the
literature, for example, Et2AlCl is a better activator than
Et3Al for HV reaction of 1,3-butadiene in the presence of
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isolated (dppe)2CoH,92 is not also inconsistent with the
viability of a cobalt hydride in these reactions. Further
studies to clarify the mechanisms of these reactions and to
expand their scope will be reported in due course.
(6) Pu, L.; Yamamoto, A.; Ikeda, S. J. Am. Chem. Soc. 1968, 90,
7170.
(7) Kagawa, T.; Inoue, Y.; Hashimoto, H. Bull. Chem. Soc. Jpn
1970, 43, 1250.
(8) Yamamoto, A.; Kitazume, S.; Pu, L. S.; Ikeda, S. J. Am. Chem.
Soc. 1971, 93, 371.
(9) Wilke, G.; Bogdanovi´c, B; Hardt, P.; Heimbach, P.; Keim, W.;
Kroner, M.; Oberkirch, W.; Tanaka, K.; Steinrucke, E.; Walter,
D.; Zimmermann, H. Angew. Chem. Int. Ed. 1966, 5, 151.
(10) Miller, R. G.; Kealy, T. J.; Barney, A. L. J. Am. Chem. Soc.
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(11) Maruya, K.; Mizoroki, T.; Ozaki, A. Bull. Chem. Soc. Jpn.
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(12) Bogdanovi´c, B.; Henc, B.; Meister, B.; Pauling, H.; Wilke, G.
Angew. Chem. Int. Ed. 1972, 11, 1023. Among metal-catalysed
asymmetric carbon-carbon bond-forming reactions, only Nozaki's
Conclusions
In the presence of bidentate 1,n-bis-diphenylphos-
phinoalkane-CoCl2 complexes {Cl2Co[P~P]} and Me3Al or
methylaluminoxane, acyclic (E)-1,3-dienes react with
ethylene (1 atmosphere) to give excellent yields of
hydrovinylation products. The regioselectivity (1,4- or 1,2-
addition) and the configuration (E- or Z- internal alkene) of
the product depend on the nature of the ligand and
temperature at which the reaction is carried out. Cobalt(II)-
complexes of 1,1-dipheylphosphinomethane and similar
ligands with narrow bite angles give mostly 1,2-addition,
retaining the E-geometry of the original diene. Complexes
of most other ligands at low temperature (– 40 oC) give
almost exclusively the branched product, (Z)-3-alkylhexa-
1,4-diene, which arises from a 1,4-hydrovinylation reaction.
A minor product is the linear adduct, 6-alkyl-hexa-1,4-diene,
Cu(II)-catalysed cyclopropanation of styrene
with ethyl
diazoacetate predates this discovery. See: Nozaki, H.; Moriuti,
S.; Takaya, H.; Noyori, R. Tetrahedron Lett. 1966, 7, 5239.
(13) Denis, P.; Jean, A.; Crcizy, J. F.; Mortreux, A.; Petit, F. J. Am.
Chem. Soc. 1990, 112, 1292.
(14) (a) Alderson, T.; Jenner, E. L.; Lindsey, R. V., Jr. J. Am.
Chem. Soc. 1965, 87, 5638. (b) Cramer, R. J. Am. Chem. Soc.
1967, 89, 1633
(15) (a) Britovsek, G. J. P.; Cavell, K. J.; Keim, W. J. Mol. Catal.
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Jastrzebski, J. T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; van
Koten, G. Angew. Chem. Int. Ed. Engl. 1999, 38, 1655. (d)
Englert, U.; Haerter, R.; Vasen, D.; Salzer, A.; Eggeling, E. B.;
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also arising from a 1,4-addition of ethylene.
As the
temperature is increased, a higher proportion of the major
1,4-adduct appears as the (E)-isomer. The unexpectedly
high selectivity seen in the Co-catalysed reaction as
compared to the corresponding Ni-catalysed reaction can be
explained by invoking an η4-[(diene)[P~P]CoH]+-complex
and its subsequent reactions. The enhanced reactivity of
terminal E-1,3-dienes over the corresponding Z-dienes can
also be explained on the basis of the ease of formation of the
η4-complex in the former case. The complete lack of
reactivity of the X2Co(dppb) (X = Cl, Br) complexes in the
presence of Zn/ZnI2 sets the Me3Al-mediated reaction apart
from the previously reported hydroalkenylation of dienes.
Electron-rich phospholanes, bis-oxazolines and N-
heterocyclic carbenes appear to be poor ligands for the
(17) For reviews, see, (a) Muller, H.; Wittenberg, D.; Seibt, H.;
Scharf, E. Angew. Chem., Int. Ed. Engl. 1965, 4, 327. (b) Su, A.
C. L. "Codimerization of ethylene and butadiene", Adv.
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"Dimerization and Codimerization", in Applied Homogeneous
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Herrmann, W. A., Eds.; VCH: New York, 1996; Vol. 1, pp. 258-
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Homogeneous Catalysis with Organometallic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: New York, 1996;
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RajanBabu, T. V.; Cox, G. A.; Lim, H. J.; Nomura, N.; Sharma,
R. K.; Smith, C. R.; Zhang, A. "Hydrovinylation Reactions in
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Co(II)-catalysed hydrovinylation of 1,3-dienes.
An
extensive survey of chiral ligands reveals that complexes of
DIOP, BDDP and Josiphos ligands are quite effective for
o
these reactions even at at – 45 C and enantioselectivities in
the range of 90-99 % ee can be realized for a variety of 1,3-
dienes.
Acknowledgements
Financial assistance for this research provided by US National
Science Foundation CHE-1362095 and National Institutes of
Health (General Medical Sciences R01 GM075107).
a
Department of Chemistry and Biochemistry, The Ohio State
University, 100 West 18th Avenue, Columbus, Ohio 43210, USA. E-
Mail: rajanbabu.1@osu.edu; FAX +1 614 292-1685; Tel: + 1 614-688-
3543.
(18) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem.
Soc. 1995, 117, 6414.
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b
Current address. Centre for Energy, IIT Jodhpur, Old Residency
Road, Jodhpur-342011, Rajasthan, India (rks@iitj.ac.in
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S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.;
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DOI: 10.1039/C5SC00929D
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ARTICLE
For Table of Contents Only
H
ethylene (1 atm.)
[P~P]CoCl2, MAO (cat.)
R
*
CH2Cl2 – 45 oC to rt
R
(90% yield
90-99% ee)
[(S,S)-BDPP]CoCl2
R = alkyl, CH2CO2Et, CH2OBn
Excellent selectivity with complexes of DIOP, BDPP and Josiphos with
Z-1,3-dienes reacting faster than the E-isomers at low temperatures
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DOI: 10.1039/C5SC00929D
EDGE ARTICLE
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