Regioselective cobalt-catalysed hydrovinylation for the synthesis of non-conjugated enones and 1,4-diketones

Article abstract of DOI:10.1002/adsc.201100800

The highly regioselective cobalt-catalysed 1,4-hydrovinylation of terminal alkenes with 2-trimethylsilyloxy-1,3-butadiene generates in a stereospecific fashion unsaturated E-configured silyl enol ether intermediates that are suitable for diastereoselective Mukaiyama-aldol reactions with bulky aliphatic aldehydes. The acidic hydrolysis of the enol ethers to γ,δ- unsaturated ketones followed by ozonolysis can be used for the synthesis of various 1,4-diketones and polycarbonyl derivatives. The 1,4-diketones and polycarbonyl derivatives were successfully tested for the synthesis of some mono- and bis-pyrrole derivatives. The γ,δ-unsaturated ketones are useful building blocks (e.g., in natural product synthesis) and can be generated in a one-pot procedure. Copyright

Full text of DOI:10.1002/adsc.201100800

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DOI: 10.1002/adsc.201100800
Regioselective Cobalt-Catalysed Hydrovinylation for the
Synthesis of Non-Conjugated Enones and 1,4-Diketones
Laura Kerstena and Gerhard Hilta,*
a
Fachbereich Chemie, Philipps-Universitꢀt Marburg, Hans-Meerwein-Straße, 35043 Marburg, Germany
Fax : (+49)-6421-282-5677; e-mail: hilt@chemie.uni-marburg.de
Received: October 17, 2012; Published online: February 28, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100800.
Abstract: The highly regioselective cobalt-catalysed polycarbonyl derivatives. The 1,4-diketones and poly-
1,4-hydrovinylation of terminal alkenes with 2-trime- carbonyl derivatives were successfully tested for the
thylsilyloxy-1,3-butadiene generates in a stereospecif- synthesis of some mono- and bis-pyrrole derivatives.
ic fashion unsaturated E-configured silyl enol ether The g,d-unsaturated ketones are useful building
intermediates that are suitable for diastereoselective blocks (e.g., in natural product synthesis) and can be
Mukaiyama-aldol reactions with bulky aliphatic alde- generated in a one-pot procedure.
hydes. The acidic hydrolysis of the enol ethers to g,d-
unsaturated ketones followed by ozonolysis can be Keywords: catalysis; cobalt; diketones; 1,4-hydrovi-
used for the synthesis of various 1,4-diketones and nylation; g,d -unsaturated ketones
Introduction
application of unsymmetrical 1,3-dienes the control of
the regioselectivity of the 1,4-hydrovinylation process
The synthesis of acyclic compounds with unsaturated is of great interest and the problem has only partially
functional groups at distinct positions and with a pre- been solved.[6] Moreover, the control of the regiose-
dictable geometry has steadily gained importance in lectivity concerning the carbon-carbon bond forma-
modern organic synthesis. Especially atom economic tion at the terminal alkene either toward linear prod-
carbon-carbon bond forming reactions are of increas- ucts (1/3) or branched products (2/4)[7] was addressed
ing interest.[1] Among these transformations, the 1,2- recently with a new ligand design with excellent re-
hydrovinylation of alkenes[2] and the 1,4-hydrovinyla- sults (Scheme 1).[8]
tion of substituted 1,3-butadienes with terminal al-
Herein, we specifically report the regioselective
kenes proved to be relevant for the construction of cobalt-catalysed 1,4-hydrovinylation utilizing 2-trime-
linear carbon skeletons.[3] More recently, the applica- thylsilyloxy-1,3-butadiene (5) as diene component
tion of the 1,4-hydrovinylation reaction in the synthe- which led to the exclusive generation of the branched
sis of 1,4-dienes and 1,3-dicarbonyl derivatives[4] has silyloxy-functionalized 1,4-diene product of type 2
been investigated in more detail thus far.[5] For the (Scheme 1).
Results and Discussion
Synthesis of Non-Conjugated Enones
The 1,4-hydrovinylation of 5 catalysed by a catalyst
system comprising CoBr2ACTHNUTRGNE(UNG dppe), zinc powder and zinc
iodide in dichloromethane at ambient temperatures is
different from earlier reported hydrovinylation reac-
tions in several distinctive aspects.
First, the process is highly regioselective for the ex-
clusive formation of intermediates of type 6 where
Scheme 1. 1,4-Hydrovinylation products utilising unsymmet-
rical components.
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Laura Kersten and Gerhard Hilt
FULL PAPERS
termediates[9] dictates the exclusive formation of the
E-geometry of the silyl enol ether subunit in 6 (see
Scheme S1, Supporting Information), which is of high
synthetic value for follow-up transformations (vide
infra). Third, the in situ hydrolysis of 6 led to the se-
lective formation of the g,d-unsaturated ketone 7 in
good to excellent yields without any isomerisation of
the methylene double bond toward a,b-unsaturated
methyl ketones.
The scope of this 1,4-hydrovinylation reaction uti-
lizing the specific diene 5 was investigated by apply-
ing a wide range of terminal alkenes. The in situ hy-
drolysis of the corresponding intermediates of type 6
led to the desired products (7) in a straightforward
one-pot procedure. The results of this investigation
Scheme 2. Regioselective cobalt-catalysed 1,4-hydrovinyla-
tion of 5 with terminal alkenes.
the carbon-carbon bond formation took place at the are summarized in Table 1.
internal carbon of the terminal alkene (C-b) and at
The scope of the reaction sequence is relatively
C-4 of 5 (Scheme 2). The high degree of regioselectiv- broad based on the rather mild reaction conditions of
ity is based on the steric demand of the trimethylsil- the cobalt-catalysed transformation tolerating several
ACHTUNGTRENNUNGyloxy substituent as well as the enhanced electron functional groups. Aliphatic, aromatic, heteroaromat-
density of the 1,3-diene caused by this substituent ic, ester, ether and phthalimide functional groups as
which favours the formation of products of type 2. well as an unprotected secondary alcohol and a,w-
Second, the reaction mechanism via cobaltacyclic in- dienes are well accepted for the alkene component.
Table 1. Results of the cobalt-catalysed 1,4-hydrovinylation of 5 with terminal alkenes.[a]
Entry
Product 7
Yield [%]
99
1
7a
7b
2
99
3
4
7c
50
68
7d
5
6
7e
7f
82
77
7
8
7g
7h
74
60
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Regioselective Cobalt-Catalysed Hydrovinylation for the Synthesis of Non-Conjugated Enones
Table 1. (Continued)
Entry
Product 7
Yield [%]
84
9
7i
7j
10
95
11
12
13
14
7k
7l
74
77
7m
7n
77
67[b]
15
8
66
[a]
Reaction conditions: 1) CoBr2ACHTNUTRGNE(NUG dppe) (10 mol%), zinc powder (20 mol%), zinc iodide (20 mol%), terminal alkene
(1.0 equiv.), 5 (1.2–1.5 equiv.) 13–24 h; 2) HCl (c=2 mol/L).
Deprotection of the TMS group with KF/TBABr.
[b]
Notably, in all cases the 1,4-hydrovinylation products nent in order to obtain a crystalline product suitable
of type 2 (=6 in Scheme 2) were obtained as the ex- for X-ray analysis.
clusive regioisomer. Interestingly, the hydrolysis of in-
termediate 6n under acidic conditions led to the het-
erocycle 8 in good yield whereas the desired unsatu-
rated ketone 7n was generated when KF/TBABr was
used for the deprotection of 6n.
Mukaiyama-Aldol Reaction
Inspired by the encouraging results reported by
Heathcock,[10] the prototype silyl enol ether 6b was
then applied in situ in Mukaiyama-aldol reactions
with various aldehydes initiated by BF3·OEt2 as Lewis
acid additive (Scheme 3).
While the application of aliphatic aldehydes gave
good yields and good syn-diastereoselectivities were
obtained for the formation of 9a–9d, the correspond-
ing reaction with an aromatic aldehyde, such as 3-ni-
trobenzaldehyde for the formation of 9e, was far less
selective. Product 9e was isolated as a syn:anti=66:34
mixture in 72% yield. In this case, instead of 6b 1-(al-
lyloxy)-4-bromobenzene was used as alkene compo- enol ether 6b with various aldehydes.
Scheme 3. BF3-initiated Mukaiyama-aldol reaction of silyl
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Table 2. Results of the ozonolysis of selected unsaturated ke-
tones 7 and diketone 10 to 1,4-diketones 11 and higher polycar-
bonyl compounds.[a]
Entry
Product 11
Yield
[%]
1
2
3
4
11a
11b
11c
11d
65
91
97
92
Scheme 4. Copper(II)-initiated dimerisation of the enolate
of methyl ketone 7b.
Copper(II)-Initiated Dimerisation
In addition to the Mukaiyama-aldol transformation of
the enol ether, we were also interested in the applica-
tion of the methyl ketones in an oxidative dimerisa-
tion reaction. In a proof-of-principle transformation,
adopting the protocol of Saegusa,[11] the unsaturated
ketone 7b was deprotonated with LDA at low temper-
ature and addition of anhydrous CuCl2/DMF led to
the oxidative dimerisation to the desired 1,4-diketone
10 which was isolated in 59% yield (Scheme 4).
5
6
7
11e
11f
11g
76
86
77
Synthesis of 1,4-Diketones
Then, the application of the g,d-unsaturated ketones
for the synthesis of 1,4-diketones and higher poly-
ketones was investigated. In light of our previous in-
vestigations for the synthesis of 1,3-diketones (12) via
a cobalt-catalysed 1,4-hydrovinylation reaction of 2,3-
dimethyl-1,3-butadiene[4a] (Scheme 5), the herein pre-
sented protocol adds another dimension for the di-
verse synthesis of poly-functionalised building blocks.
Applying diene 5 in the cobalt-catalysed 1,4-hydro-
vinylation reaction leads via hydrolysis and ozonolysis
to the corresponding 1,4-diketone products 11. Nota-
bly, the key step for both approaches is the cobalt-cat-
8
11h
11i
11j
87
85
75
9
10
11
11k
69[b]
[a]
Reaction conditions: O3, PPh3 (2.0 equiv.).
Generated from 10.
[b]
alysed 1,4-hydrovinylation with simple 1,3-butadiene
derivatives. Accordingly, the same cobalt-catalysed re-
action leads to both types of diketones 11 and 12
based on the choice of the 1,3-diene.
The results for the straightforward synthesis of 1,4-
diketones of type 11 and higher polycarbonyl com-
Scheme 5. Cobalt-catalysed 1,4-hydrovinylation for the syn-
thesis of 1,3- as well as 1,4-diketone derivatives.
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Adv. Synth. Catal. 2012, 354, 863 – 869

Regioselective Cobalt-Catalysed Hydrovinylation for the Synthesis of Non-Conjugated Enones
pounds by ozonolysis of the g,d-unsaturated ketones 7 62% yield whereas the mono-pyrrole adduct 15 was
are summarised in Table 2.
obtained in 32% yield.
The scope of this approach is mostly limited by the
restrictions of the ozonolysis reaction. Electron-rich
aromatic substituents are not compatible with the re- Conclusions
action conditions. Noteworthy, product 11a is a direct
precursor of dihydrojasmone which is reported to be We have demonstrated that the cobalt-catalysed 1,4-
an ingredient of perfumes.[12] Nevertheless, we were hydrovinylation reaction of 2-trimethylsilyloxy-1,3-bu-
able to generate a number of very interesting 1,4-di- tadiene is a highly regioselective carbon-carbon bond
ketones which are currently under investigation for formation process leading to well-defined silyl enol
the synthesis of pyrrole derivatives and other follow- ether intermediates. The follow-up reactions of these
up transformations.
silyl enol ether intermediates with aldehydes in a Mu-
Finally, the simple 1,4-dicarbonyl derivative 11b kaiyama-type aldol reaction as well as the oxidative
was converted with allylamine to the corresponding copper-induced coupling of the corresponding eno-
pyrrole derivative 13 which was isolated in excellent lates to bis-unsaturated diketones were briefly investi-
92% yield (Scheme 6).[13] Especially the tetraketone gated. The ozonolysis of the g,d-unsaturated ketones
derivatives generated in this study are of interest for toward 1,4-diketones and polyketones opens the way
the synthesis of bis-pyrrole derivatives with variable toward the synthesis of more complex structures in
linker units in between the two pyrrole rings (11h– the future such as bis-pyrrole adducts which are cur-
11k). Probably, the most interesting starting material rently under investigation.
in this series is 11k (and 11h) because it could lead to
a bis-pyrrole adduct 14 as well as to a mono-pyrrole
adduct 15 in the case where the inner two carbonyl Experimental Section
groups would form the heterocycle. Accordingly, 11k
was reacted with five equivalents of allylamine and General Remarks
the desired bis-pyrrole product 14 was isolated in
NMR spectra were recorded on a Bruker Avance 300 or
DRX 500 (1H: 300 MHz or 500 MHz, 13C: 75 MHz or
125 MHz) spectrometer using residual CHCl3 as internal
standard (d=7.26 ppm). Mass and GC/mass spectra were
measured on a Hewlett Packard 6890 GC-System including
a Hewlett Packard 5973 Mass Selective Detector. For (high
resolution) mass spectra a Finnigan MAT 95S and a Finnigan
LTQ (ESI, HR-MS) spectrometer were used. Analytical
thin layer chromatography was performed on Merck silica
gel 60 F254. For column chromatography Merck silica gel 60
(230–400 mesh) was used. All reactions were carried out
under an inert atmosphere (nitrogen or argon) using stan-
dard Schlenk techniques. Dichloromethane was dried over
phosphorus pentoxide, tetrahydrofuran and diethyl ether
over sodium. The pyrrole derivatives 13, 14 and 15 were syn-
thesised adopting a procedure described by Trost.[13]
1,4-Hydrovinylation with 2-Trimethylsilyloxy-1,3-
butadiene and Deprotection with HCl
In a flame-dried Schlenk tube fitted with a Teflon screw cap
CoBr2ACTHNURGTNE(NUG dppe) (0.1 mmol, 10 mol%), anhydrous zinc iodide
(0.2 mmol, 20 mol%) and zinc powder (0.2 mmol, 20 mol%)
were suspended in 1 mL of anhydrous dichloromethane
under an argon atmosphere. Then 1.0 equivalent of the ter-
minal alkene (1.0 mmol) and 2-trimethylsilyloxy-1,3-buta-
diene (1.2–1.5 mmol) were added and the mixture was
stirred at room temperature until complete conversion of
the starting materials was monitored by GC/MS analysis
(13–24 h). Then 5 mL of pentane were added to the suspen-
sion and the reaction mixture was filtered over a short pad
of silica gel (diethyl ether). The solvent was removed under
reduced pressure. The residue was redissolved in 5 mL di-
ethyl ether, 10 mL aqueous HCl (c=2 mol/L) were added
Scheme 6. Synthesis of pyrroles from selected di- and tetra-
carbonyl derivatives.
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Laura Kersten and Gerhard Hilt
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and the solution was strongly stirred until complete depro-
tection was monitored by GC/MS analysis (3–24 h). The
aqueous layer was separated and extracted with diethyl
ether (2ꢂ10 mL). The combined organic layers were washed
with brine and dried over MgSO4. The solvent was removed
under reduced pressure and the residue was purified by
flash column chromatography on silica gel.
duced pressure. The residue was purified by flash column
chromatography on silica gel (pentane:diethyl ether=4:1).
The product 10 was obtained as a yellow oil; yield: 501 mg
(1.34 mmol, 59%).
Ozonolysis
A 10-mL Schlenk tube, fitted with a glass tube to admit
ozone, was charged with 1.0 equivalent of the enone and 3–
6 mL of dichloromethane. The reaction mixture was cooled
to À788C and ozone in a stream of oxygen was bubbled
through the solution. The ozone addition was stopped when
the solution turned blue. The excess of ozone was removed
by passing oxygen through the solution. Then 2.0 equiva-
lents of triphenylphosphine were added and the solution
was allowed to warm to room temperature. After stirring
for 1 h the solvent was removed under reduced pressure.
The residue was absorbed on silica gel and purified by flash
column chromatography.
Mukaiyama-Aldol Reaction
In a flame-dried Schlenk tube fitted with a Teflon screw cap
CoBr2ACHTUNGTRENNUNG(dppe) (0.12 mmol, 10 mol%), anhydrous zinc iodide
(0.24 mmol, 20 mol%) and zinc powder (0.24 mmol,
20 mol%) were suspended in 1 mL of anhydrous dichloro-
methane under an argon atmosphere. Then 1.2 equivalents
of the terminal alkene (1.2 mmol) and 2-trimethylsilyloxy-
1,3-butadiene (1.4–1.8 mmol) were added and the mixture
was stirred at room temperature until complete conversion
of the starting materials was monitored by GC/MS (13–
24 h). Then 5 mL of pentane were added to the suspension
and the reaction mixture was filtered over a short pad of
silica gel (pentane:diethyl ether=1:1). The solvent was re-
moved under reduced pressure and the silyl enol ether was
added to a stirring solution of 1.0 equivalent of freshly dis-
tilled aldehyde (1.0 mmol) in 4 mL of anhydrous dichloro-
methane according to a procedure described by Heath-
cock.[10] The solution was cooled to À788C and 1.0 equiva-
lent of BF3·OEt2 (1.0 mmol) was added dropwise. After 4 h
the reaction was quenched by injecting 2.5 mL of saturated
aqueous NaHCO3. The reaction mixture was allowed to
warm to room temperature and stirred overnight. The aque-
ous layer was separated and extracted with diethyl ether
(3ꢂ30 mL). The combined organic layers were dried over
MgSO4 and the solvent was removed under reduced pres-
sure.
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1
The ratio of isomers was determined by H NMR of the
crude product. If this was not possible due to high complexi-
1
ty of the spectrum, the ratio was determined by H NMR
after purification by column chromatography on silica gel.
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isomer) could be isolated. The stereochemical assignment
1
(syn or anti) was made by H NMR, using the well-accepted
J
anti >Jsyn relationship.[14] The coupling constants were deter-
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constants the NMR probe was treated with D2O.
Cu(II)-Initiated Dimerisation
According to a procedure reported by Saegusa[11a] a solution
of 1.1 equivalents of diisopropylamine (5.0 mmol) in 5 mL
anhydrous tetrahydrofuran was treated with 1.1 equivalents
of n-butyllithium (5.0 mmol) at À788C. After 20 min
1.0 equivalent of 5-benzylhex-5-en-2-one (7b) (4.5 mmol)
was added dropwise. After additional stirring for 30 min 1.5
equivalents of anhydrous CuCl2 (7.0 mmol) in anhydrous di-
methylformamide (7 mL) were added in one batch. The
dark green solution was stirred for 30 min at À788C and
then allowed to warm to room temperature. The reaction
was quenched by addition of an aqueous solution of NH4Cl
(5 mL). The aqueous layer was separated and extracted with
diethyl ether (3ꢂ30 mL). The combined organic layers were
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carbon bond formation at the internal carbon of the
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