Chemoselective reactions of (E)-1,3-dienes: Cobalt-mediated isomerization to (z)-1,3-dienes and reactions with ethylene

Article abstract of DOI:10.1021/ja501979g

In the asymmetric hydrovinylation (HV) of an E/Z-mixture of a prototypical 1,3-diene with (S,S)-(DIOP)CoCl₂ or (S,S)-(BDPP)CoCl₂ catalyst in the presence of Me₃Al, the (E)-isomer reacts significantly faster, leaving behind the Z-isomer intact at the end of the reaction. The presumed [LCo-H]⁺-intermediate, especially when L is a large-bite angle ligand, similar to DIOP and BDPP, promote an unusual isomerization of (E/Z)-mixtures of 1,3-dienes to isomerically pure Z-isomers. A mechanism that involves an intramolecular hydride addition via an [η⁴-(diene)(LCo-H)]⁺ complex, followed by p-s-p isomerization of the intermediate Co(allyl) species, is proposed for this reaction.

Full text of DOI:10.1021/ja501979g

Communication
pubs.acs.org/JACS
Chemoselective Reactions of (E)‑1,3-Dienes: Cobalt-Mediated
Isomerization to (Z)‑1,3-Dienes and Reactions with Ethylene
Yam N. Timsina, Souvagya Biswas, and T. V. RajanBabu*
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210,
United States
S
* Supporting Information
Scheme 2. Working Hypothesis on the Mechanism of
Co(II)-Catalyzed Hydrovinylation
ABSTRACT: In the asymmetric hydrovinylation (HV) of
an E/Z-mixture of a prototypical 1,3-diene with (S,S)-
(DIOP)CoCl2 or (S,S)-(BDPP)CoCl2 catalyst in the
presence of Me3Al, the (E)-isomer reacts significantly
faster, leaving behind the Z-isomer intact at the end of the
reaction. The presumed [LCo−H]+-intermediate, espe-
cially when L is a large-bite angle ligand, similar to DIOP
and BDPP, promote an unusual isomerization of (E/Z)-
mixtures of 1,3-dienes to isomerically pure Z-isomers. A
mechanism that involves an intramolecular hydride
addition via an [η4-(diene)(LCo−H)]+ complex, followed
by π−σ−π isomerization of the intermediate Co(allyl)
species, is proposed for this reaction.
e recently reported a new protocol for a highly enantio-
Wselective Co(II)-catalyzed asymmetric hydrovinylation
(HV) of unactivated 1,3-dienes that involves the use of a
[1,n-bis-diphenylphosphinoalkane]CoCl2 and Me3Al (Scheme 1).1,2
are also known to be capable of isomerization of alkenes, we
wondered if conditions can be found to maximize the Z-isomer
from an E/Z-mixture under kinetic conditions.5,6 The results of
these studies are described in this paper.
Scheme 1. Ni(II)- and Co(II)-Catalyzed Hydrovinylation of
1,3-Dienes
The enantioselectivity in the asymmetric HV of a mixture of
(E)-8 and (Z)-8 (Z:E = 53:47) using [(S,S)-DIOP]CoCl2/
Me3Al (TMA) was found to depend on the conversion, with
the (E)-isomer reacting at a significantly faster rate (eq 1, Table 1).6,7
In order to explain the improved selectivity in the Co-catalyzed
reactions as compared to the corresponding Ni-catalyzed
reactions3 (Scheme 1), we invoked1a an η4-cobalt-hydride
complex 4 that restricts the conformations of the reactive
intermediates in the former (Scheme 2). Complex 4 sub-
sequently forms an allyl-cobalt intermediate 5, which undergoes
ethylene insertion and β-hydride elimination to regenerate the
presumed [LCo−H]+ catalyst 34 to complete the catalytic
cycle. When an E/Z-mixture of a prototypical 1,3-diene was
subjected to our standard HV conditions (except for the
presence of ethylene), the terminal (Z)-1,3-diene was found to
be mostly unreactive at low temperature. The attendant
implication is that the (Z)-isomer is unreactive toward
[LCoH]+ or the equivalent catalyst. Since such hydride species
At low conversions (entries 1−3, Table 1), only E-8 undergoes
hydrovinylation giving a maximum of ∼83% ee. As conversion
increases, the proportion of Z-8 increases, leaving behind, at 23 h,
essentially pure (Z)-8 (Z:E = 49:1, entry 5). A minor product
(<5%), tentatively identified as a linear hydrovinylation product
(10), is also formed at higher conversions. A similar behavior is
Received: February 25, 2014
Published: April 8, 2014
© 2014 American Chemical Society
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Table 1. Chemoselectivity in the Asymmetric HV of
(Z/E)-8
Scheme 3. Co(I)-Catalyzed Isomerization Reactions
a
b
b
entry
time (h)
% (9)
ee % (9)
% (8)
Z:E (8)
10
c
(S,S)-DIOP
1
2
3
4
5
0.5
1.2
3
16
23
26
31
49
80
81
83
82
84
84
77
71
65
46
2.1:1.0
2.7:1.0
3.4:1.0
4.4:1.0
49:1
−
−
3
6
4
the [bis-phosphine]CoCl2/Me3Al-mediated reactions described
in this paper is highlighted by the total absence of the 1,5-
hydrogen migration in the latter and the higher yields of
Z-isomerization products obtained.
The initial scouting experiments were carried out on a
mixture of (Z)- and (E)-8 using CoCl2 complexes of chelat-
ing bis-diphenylphosphinoalkane ligands under conditions
similar to the HV except for the presence of ethylene (eq 2).
23
d
5
(S,S)-BDPP
6
1
21
30
42
47
61
85
85
82
77
73
79
70
49
43
25
1.9:1.0
2.6:1.0
7.1:1.0
13.3:1.0
49:1
<1
<1
9
7
2
8
5.2
8
9
10
13
10
23
a
b
See eq 1 for procedure. Determined by GC. See Supporting
Information for chromatograms. At −45 °C. (S)-9 major. At −15 °C.
(R)-9 major.
c
d
observed with (S,S)-2,4-BDPP ligand, except a notable decrease in
enantioselectivity (from 85% ee at 21% conversion to 73% ee at
61% conversion, entries 6−10) results. These results are most
readily rationalized if one assumes that the (Z)-isomer is a
reluctant partner in the HV reaction, while the (E)-isomer
undergoes a fast reaction giving the (S)-9 [with (S,S)-DIOP] as
the major product. With the more reactive BDPP complex, at
higher temperatures (−15 °C), upon complete conversion of the
(E)-isomer, the (Z)-isomer undergoes the reaction, giving
enantiomeric product, along with 10. The (DIOP)CoCl2 complex
shows much more discrimination in the reactions of the (Z)- and
(E)-isomers of the starting diene, leaving behind almost all of the
(Z)-isomer unreacted (46%, Z:E = 49:1, entry 5) at the end of the
reaction (entry 5, compared to 25% in entry 10 using 2,4-BDPP).
The lower enantioselectivity at higher conversions using the
BDPP ligand (entries 9 and 10) also suggests that the 1,4-HV
of the (Z)-8 gives predominantly the opposite enantiomer.6
Implicit in these observations is also the striking absence of
the isomerization of the (Z)-(8) into the thermodynamically
favored (E)-isomer in the presence of the presumed Co(II)-
hydride intermediates. Since (E)-8 appears to readily engage
the [LCo−H]+ in the facile hydrovinylation, in the absence
of ethylene, under proper conditions, kinetic control might
prevail in the (E)−(Z)-isomerization of 8, and if so, using
ligand effects could one make the (Z)-isomer?5 These expec-
tations have indeed been borne out, and here we report the
details of these investigations. While there are excellent
methods for the formation of the (E)-1,3-diene isomer,8 there
is a dearth of methods for the synthesis of isomerically pure
(Z)-1,3-dienes,9 and in many biologically important com-
pounds this configuration is important.10
The composition of isomers and identification of the
product(s) were determined by 1H and 13C NMR spectroscopy
and gas chromatography.6 The results are presented in Table 2.
Table 2. Isomerization of 1,3-Diene (Z/E)-8: Ligand
a
Effects
start mat
(Z):(E)-8
bite
angle
temp (°C)/ product (Z):
entry
ligand
time (h)
(E)- 8
1
2
3
4
5
6
7
33:67
33:67
33:67
33:67
33:67
33:67
33:67
[DPPM]
[DPPE]
[DPPP]
72
85
91
91
98
−
−15/14
−10/14
−16/22
−4/16
−15/14
−15/14
37:63
74:26
82:18
33:67
>99:<1
>99:<1
b
[DPPP]
[DPPB]
c
[DPPpent]
d
(S,S)-DIOP
98
−10/12
c
100:0
a
b
See eq 2 for procedure. Using CoBr2. Bis-1,5-diphenyphos-
phinopentane. Isomeric product not seen in GC or NMR.
d
As shown in Table 2, the Z/E composition of the products is
highly dependent on the ligand. A Co-complex containing
ligand with a small bite angle,11 1,1-bis-diphenylphosphino-
methane (DPPM, bite angle β = 72), showed little tendency to
effect the isomerization (entry 1), whereas complexes of large
bite angle ligands, 1,4-bis-diphenylphosphinobutane (entry 5,
β = 98), 1,5-bis-diphenylphosphinopentane (entry 6), and
DIOP (entry 7, β = 98), gave quantitative conversion to the
(Z)-isomer at low temperature. Bis-diphenylphosphinopropane
(DPPP, bite angle 91) gave up to 82% of the (Z)-isomer at
−15 °C (entry 3). The reaction is specific for the chloride
complex; as shown in the entry 4, the corresponding bromide
complex is ineffective for the isomerization reaction.
We have examined the isomerizations of the mixtures of a
number of 1,3-dienes under the optimized conditions (eq 2),
and the most significant results are listed in Table 3. An
expanded list of complexes and their effect on the isomerization
of each of the dienes is included in the Supporting Information.
As can be seen in entries 1−9, the isomerization reaction is
broadly applicable giving excellent yields of the (Z)-isomers of
most dienes. As inferred from the scouting studies (Table 2),
ligands with relatively large bite angles, DPPB and DPPPent,
Before we describe our results, attention should be drawn to
a related paper by Hilt et al., who documented a distinctly
different protocol for the E/Z isomerization of 1,3-dienes.5d In
this report (Scheme 3), a Co(I) complex prepared under
reducing conditions from a tridentate pyridine-imine ligand
(11), CoBr2, Zn, and ZnI2 gave moderate yields of (Z)-1,3-
dienes (14) from a 1:1 mixture of (Z)- and (E)-dienes (13).
Behavior of the 1,3-diene was found to be highly dependent
on the ligand, with a bis-phosphine-CoBr2 under identical
conditions (i.e., with Zn/ZnI2) giving modest yields of a
product (15), arising via a 1,5-hydrogen migration. The
difference between Hilt’s [bis-phosphine]Co-chemistry and
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Table 3. Co(II)-Catalyzed Isomerization of (Z)- and (E)-1,3-
a
Dienes
Scheme 4. Plausible Mechanism for the E- to
Z-Isomerization of a 1,3-Diene
conformation, precluding any further η4-complexation with the
Co(II)-catalyst.15
a
See eq 2 for procedure. For an expanded list of ligand effects for each
b
In summary, attempts to effect asymmetric hydrovinylation
of a mixture of (Z)- and (E)-1,3-dienes using (P ∼ P)CoCl2/
Me3Al reveal that there is a significant difference in the relative
rates of ethylene incorporation, with the (E)-isomer reacting signi-
ficantly faster. In the absence of ethylene, under otherwise
identical conditions, this Co-catalyst promotes an unusual isomeri-
zation of an (E)/(Z)-mixture of 1,3-dienes almost exclu-
sively to the (Z)-isomer. This result is strikingly different from
the related reaction mediated by the reagent combination
[(P ∼ P)CoBr2/Zn/ZnI2), where a product of 1,5-hydrogen
shift is the major.5d,16 A mechanism that involves an intra-
molecular hydride addition via an η4-complex and subsequent
π−σ−π isomerization of the intermediate Co(allyl) species is
proposed for this reaction.
substrate, see Supporting Information. Yield of volatile products were
estimated from NMR and GC. Product contains an unidentified
impurity from the starting material.
c
were found to be the most generally applicable for this reaction.
For substrates where the diene is conjugated to an aromatic
moiety (entries 5, 6, and 7), DPPPent is the ligand of choice,
giving excellent conversion to the expected (Z)-isomer. DPPB
leads to slightly lower selectivities. The isomerization reaction
gives satisfactory results even in substrates that contain Lewis
basic centers (entries 6 and 7). A preparative scale experiment
(3 mmol) using 16 as the starting material gave 91% isolated
yield of the expected product.6
The isomerization reaction appears to be limited to ter-
minal 1,3-dienes as illustrated by the examples shown in eqs 3 and
4. Substrates 24 and 25 failed to undergo the reaction under a
variety of conditions using the ligands discussed previously. The
starting material was recovered virtually unchanged.
The experiments listed in previous sections suggest that one
reason for the poor reactivity/selectivity of the Z-substrates
might be their reluctance to form an η4- complex.12 A plausible
explanation for the observed results, based on the assumption
that the initial [LCo−H]+ addition to the 1,3-diene is revers-
ible, is shown in Scheme 4. An intramolecular hydride delivery
via an η4-complex 4E gives the syn-anti-Co(allyl)-complex 5as.13
This species could undergo the familiar π−σ−π isomerization14
to give, among others, an anti−anti complex (5aa). Hydride
elimination from this species would generate a diene complex 4Z,
which for steric reasons, might dissociate to give the (Z)-diene.
The (Z)-diene, once formed, will most likely exist in the (s)-trans
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org
■
S
AUTHOR INFORMATION
Corresponding Author
rajanbabu.1@osu.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial assistance for this research provided by US National
Science Foundation (CHE-1057818) and National Institutes of
■
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Health (General Medical Sciences, R01 GM075107) is
gratefully acknowledged. We are grateful to Mr. William
Coldren and Professor Chris Hadad for help with the DFT
calculations.
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