meta-directing cobalt-catalyzed diels-alder reactions

Article abstract of DOI:10.1002/anie.200601974

(Chemical Equation Presented) Overcoming the ortho/para rule? The regioselectivity of Diels-Alder reactions with neutral electron demand between 1,3-dienes with alkynes can be controlled by simple cobalt diimine complexes so that the meta-substituted cycloadducts are generated in good yields and excellent regioselectivity (see scheme; DDQ=2,3-dichloro-5,6-dicyano-1,4- benzoquinone).

Full text of DOI:10.1002/anie.200601974

Communications
Diels–Alder Reaction
and isolated as side products in transition-metal-catalyzed
cycloaddition reactions.[4] In these transition-metal-catalyzed
reactions the para-substituted product 3 is formed as the
major product.
DOI: 10.1002/anie.200601974
meta-Directing Cobalt-Catalyzed Diels–Alder
Reactions**
In our investigations of cobalt-catalyzed cyclizations we
reported the use of a simple cobalt catalyst system consisting
of [CoBr2dppe] (dppe = 1,2-bis(diphenylphosphino)ethane),
ZnI2,Zn,or Bu 4NBH4 as reducing agent for the Diels–Alder
reaction of unactivated starting materials under mild con-
ditions for the production of the para-substituted Diels–Alder
adducts 3.[5] Recently,we also identified a simple cobalt
catalyst system consisting of [CoBr2(diimine)] (5 Mol%),
ZnI2,Zn,and Fe powder (each 10 Mol%) which performs the
cobalt catalyzed Diels–Alder reaction between phenyl acet-
ylene and isoprene,gives very good yields,and forms the
meta-substituted isomer 5 in excellent regioselectivity. The
corresponding aromatic product 6 was then generated by
oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and its identity determined by NMR spectroscopy.
Among several ligands tested the best chemo- and regiose-
lectivity was obtained for cobalt complexes with pyridine
donor ligands,such as 7 (Scheme 2) or 2,2’-bipyridine (8,
Gerhard Hilt,* Judith Janikowski, and Wilfried Hess
The Diels–Alder reaction has belonged to the arsenal of
synthetic organic chemists for decades as can be seen in the
large number of applications in natural-product synthesis and
of biologically active compounds.[1] The usefulness of the
Diels–Alder reaction is based on the cyclic transition state
which allows the transfer of the stereochemical information in
the starting material E- or Z-selectively into cis- or trans-
configured products. Furthermore,the regiochemistry of the
intermolecular Diels–Alder reaction can be predicted by the
Woodward–Hoffmann rules and the therein implied ortho/
para rules.[2] While the ortho/para-selectivity of the thermal
Diels–Alder reaction allows access to ortho- or para-substi-
tuted products (such as 1,Scheme 1) the intermolecular
reaction is limited to the generation of para-substituted
products when starting from isoprene,the meta-substituted
products of type 2 (Scheme 1) can not be accessed. Only the
Scheme 2. meta-Selective cobalt-catalyzed Diels–Alder reaction.
Scheme 3) or 1,10-phenanthroline.[6] Also symmetrical ali-
phatic diimine ligands (9) as well as mixed pyridine imine
ligands (10 and 11) gave good results. In addition,by using a
combination of zinc- and iron powder as the reducing agent,
the formation of the [2+2+2]-cyclotrimerization product
from the alkyne was completely suppressed. The meta-
Scheme 1. para- and meta-Diels–Alder reaction products. EWG=elec-
tron-withdrawing group.
thermal Diels–Alder reaction of alkenyl boron compounds,
such as the vinyl-9-borabicyclo[3.3.1]nonane,with unsym-
metrical 13, -dienes,such as isoprene,poses an exception. In
this case the regioselective meta-directing Diels–Alder reac-
tion generates the product of type 2 in a ratio of meta:para =
90:10.[3]
To our knowledge an analogous meta-directing Diels–
Alder reaction to produce 4 starting from a 1,3-diene and an
alkyne has not been described. Nevertheless,the meta-
substituted dihydroaromatic systems 4 have been detected
[*] Prof. Dr. G. Hilt, J. Janikowski, W. Hess
Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Strasse, 35043 Marburg (Germany)
Fax: (+49)6421-282-5677
E-mail: hilt@chemie.uni-marburg.de
Scheme 3. Preparative application of the meta-selective cobalt-cata-
lyzed Diels–Alder reaction. Mes=2,4,6-trimethylphenyl, R1, R2: see
Table 1
[**] This work was supported by the DFG(Deutsche Forschungsge-
meinschaft).
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Angewandte
Chemie
substituted product 6 could be isolated in an acceptable yield
of 45% and in an almost perfect regioselectivity of 99:1. We
recognized that the yield was enhanced in dichloromethane
compared to that in acetone. The substitution of the
cyclohexyl substituent for aromatic substituents (phenyl- or
mesityl-, 10 and 11) also improved the yield whereas the
regioselectivity under these conditions was slightly lower.
In a series of experiments with four selected cobalt
complexes aromatic-,aliphatic-,olefinic-,and silyl-function-
alized terminal alkynes were converted with isoprene,myr-
cene and 2-trimethylsilyloxy-1,3-butadiene in dichlorome-
thane (Scheme 3).
Besides aryl-,alkenyl- and alkyl-substituted alkynes also
silyl- and sulfone functionalized terminal alkynes,as well as
the non-protected propargylic alcohol,could be successfully
converted. The reaction times of the Diels–Alder reactions of
the cobalt diimine type ligands are somewhat longer com-
pared to the para-directing Diels–Alder reaction with the
{Co(dppe)} complex. As the results in Table 1 illustrate the
meta-substituted products are formed with a high level of
regioselectivity and in good to excellent yields. While a single
perfect ligand for all reactions was not identified among the
four selected ligands,at least one diimine ligand was
identified which gave good results for both yield and
regioselectivity.
For this new catalyst system we propose that the
preformed cobalt(II) complexes are reduced in situ to the
corresponding cobalt(I) complexes (neither the zinc powder
nor the iron powder are sufficient reducing agents for a
further reduction of the cobalt(I) species[7]). We can not
rationalize the effect of the iron powder in preventing the
formation of the [2+2+2]-cyclotrimerization side product.[8]
When one component of the catalyst system is omitted,
reduced chemoselectivities or reduced conversions are
encountered. In some cases the formation of the 1,3,6-
cyclooctatriene side product is observed. Nevertheless,the
amount of the [4+2+2]-cycloaddition adduct is barely higher
than 10% (GC-analysis) and this by-product can easily be
separated by column chromatography.[9]
Surprisingly the reaction of the unprotected propargylic
alcohol (Scheme 4) with isoprene occurs in relatively good
Table 1: Cobalt-catalyzed Diels–Alder reactions with ligands 8–11(see
Scheme 3).[a]
Entry R1
R2
Product (12)
Yield (meta/para)
1
2
3
4
8: 77% (98:2)
9: 90% (97:3)
10: 98% (80:20)
11: 80% (95:5)
Me
Ph
5
6
7
8
8: 85% (92:8)
9: 81% (73:27)
10: 90% (80:20)
11: 85% (82:18)
Me
Me
Me
nBu
9
8: 80% (83:17)
9: 66% (57:43)
10: 80% (93:7)
11: 93% (85:15)
10
11
12
SiMe3
13
14
15
16
8: 47% (95:5)
9: 57% (90:10)
10: 61% (69:31)
11: 63% (95:5)
CH2SO2Ph
17
18
19
20
8: 67% (97:3)
9: 75% (96:4)
10: 95% (96:4)
11: 53% (95:5)
Ph
Scheme 4. meta-Selective Diels–Alder reaction of a higher substituted
1,3-diene and of an internal alkyne; Phth=phthaloyl.
21
22
23
24
8: 88% (82:18)[c]
9: 88% (85:15)[c]
10: 80% (87:13)[c]
11: 81% (90:10)[c]
Me
chemo- and regioselectivity. In contrast,the similar reaction
with the [CoBr2(dppe)] catalyst could not be realized to date.
Nevertheless,the subsequent DDQ oxidation generates the
corresponding aldehyde so that in this conversion 2.2 equiv-
alents of DDQ were used to obtain the product 13 in
reasonable yields.[10]
25
26
27
28
8: 22% (98:2)
9: 80% (99:1)
10: 78% (97:3)
11: 41% (94:6)
OSiMe3[b] tBu
Using the cobalt catalysts system [CoBr2(dppe)],ZnI ,
2
[a] The best results in terms of yield and regioselectivity are highlighted
in bold text. [b] For the work-up the silica gel should be deactivated by
addition of triethylamine to the eluent. [c] The DDQ oxidation leads to
decomposition of the product so that the dihydroaromatic compound
was isolated in approximately 95% purity.
Zn,the propargylic phthalimide 14 reacts with 1,3-dienes
giving the cycloaddition products in rather low yields,[11] with
the new cobalt diimine system (Scheme 4) 15 could be
obtained in yields of up to 55%,a significant increase. The
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Communications
2002; E. J. Corey, Angew. Chem. 2002, 114,1724; Angew. Chem.
Int. Ed. 2002, 41,1650; K. C. Nicolaou,S. A. Snyder,T.
Montagnon,G. E. Vassilikogiannakis, Angew. Chem. 2002, 114,
1743; Angew. Chem. Int. Ed. 2002, 41,1668.
regioselectivity in this case is diminished and a meta/para =
ratio of 71:29 was observed.
To further illustrate the complementary regioselectivity of
the new cobalt catalyst system to that of the para-selective
[CoBr2(dppe)] system the unsymmetrical 1,3-diene 16 was
converted with phenyl acetylene under the new reaction
conditions to give the corresponding regioisomeric meta-
product 17 in good yield and excellent regioselectivity (93:7).
Because the reactivity of the higher substituted 1,3-diene 16 is
reduced the alkyne trimerization product is isolated in 45%
yield. In addition,the cobalt diimine catalyst system also
accepts internal unsymmetrical alkynes,such as the 1-phenyl-
1-propyne (18),and the corresponding aromatic cycloaddi-
tion product (19) could be obtained in excellent yield (88%)
and regioselectivity (86:14) after DDQ oxidation.
Herein we described the first broad application of the
meta-selective Diels–Alder reaction of non-activated starting
materials catalyzed by a new cobalt diimine catalyst system.
The dihydroaromatic intermediates can be isolated,but for
the determination of the regioisomers a DDQ oxidation was
performed to obtain the aromatic products. As we could
demonstrate by a variation of the ligands and the reaction
conditions regioisomeric Diels–Alder products can be
obtained from identical starting materials in good yields and
excellent regioselectivities either in favor of the para-sub-
stituted product ({Co(dppe)} complex) or of the meta-
substituted product ({Co(diimine)} complex).
[2] K. A. Jørgensen in Cycloaddition Reactions in Organic Synthesis
(Eds.: S. Kobayashi,K. A. Jørgensen),Wiley-VCH,Weinheim,
2002,chap. 8,pp. 301 – 327; I. Fleming
Frontier Orbitals and
Organic Chemical Reactions,Wiley,London, 1977; R. B. Wood-
ward,R. Hoffmann, The Conservation of Orbital Symmetry,
VCH,Weinheim, 1970.
[3] M. A. Silva,S. C. Pellegrinet,J. M. Goodman, J. Org. Chem.
2003, 68,4095; M. A. Silva,S. C. Pellegrinet,J. M. Goodman,
Arkivoc 2003,556; S. C. Pellegrinet,M. A. Silva,J. M. Good-
man, J. Am. Chem. Soc. 2001, 123,8832; Y.-K. Lee,D. A.
Singleton, J. Org. Chem. 1997, 62,2255; D. A. Singleton,K. Kim,
J. P. Martinez, Tetrahedron Lett. 1993, 34,3071.
[4] Selected references: R. Z. Dolor,P. Vogel, J. Mol. Catal. 1990,
60,59; S.-J. Paik,S. U. Son,Y. K. Chung, Org. Lett. 1999, 1,2045.
[5] Selected references: G. Hilt,K. I. Smolko, Angew. Chem. 2003,
115,2901; Angew. Chem. Int. Ed. 2003, 42,2795; G. Hilt,T. J.
Korn, Tetrahedron Lett. 2001, 42,2783; G. Hilt,S. Lüers,K.
Polborn, Isr. J. Chem. 2001, 41,317; G. Hilt,F.-X. du Mesnil,
Tetrahedron Lett. 2000, 41,6757.
[6] Cobalt complexes of the vitamin B12-type,such as salen-,
glyoximato-,or phthalocyanin cobalt complexes are not reactive
in the Diels–Alder reaction.
[7] The redox potential of the [CoBr2(10)] complex was determined
in an electroanalytical investigation. The differential pulse
voltamogramms showed a single-electron reduction [CoII/CoI;
E = À560 mV; Ag/AgCl] within the potential window of the
solvent (CH2Cl2; 0.1m Bu4NClO4; E > À1.60 V). Therefore,a
further reduction of the proposed cobalt(I) species seems
improbable with the mild reducing agents zinc and iron powder.
[8] G. Hilt,T. Vogler,W. Hess,F. Galbiati, Chem. Commun. 2005,
Experimental Section
1474; G. Hilt,W. Hess,T. Vogler,C. Hengst,
J. Organomet.
Representative procedure for the meta-selective cobalt-catalyzed
Diels–Alder reaction (Table 1,entry 2): A solution of [CoBr 2(9)]
(39 mg,0.1 mmol,5.0 Mol%),zinc iodide (64 mg,0.2 mmol,
10.0 Mol%) zinc powder (13 mg,0.2 mmol,10.0 Mol%),and iron
powder (11 mg,0.2 mmol,10.0 Mol%) were briefly heated to boiling
in dry dichloromethane (1.0 mL) under nitrogen atmosphere. Then
isoprene (136 mg,2.0 mmol) and phenyl acetylene (204 mg,
2.0 mmol) were added and the suspension was stirred until the
starting materials were completely consumed (GC control) at room
temperature. Then the suspension was filtered over a small amount of
silica gel (eluent: diethyl ether),the solvent was removed in vaccuo,
taken up in benzene,and the dihydroaromatic product was oxidized
by DDQ (545 mg,2.4 mmol,1.2 equiv). After 2 h at room temper-
ature the solution was diluted with diethyl ether (50 mL) and washed
with an aqueous sodium hydroxide (10%)/sodium thiosulfate (10%)
solution. The solvent was removed in vaccuo and the residue was
purified by column chromatography on silica gel (eluent: penta-
ne:CH2Cl2 = 100:1).The product was obtained as colorless oil
(305 mg,0.18 mmol,90%). The analytical data are in accordance
with the literature.[12] The ratios of regioisomers were determined by
integration of the GC and NMR signals.
Chem. 2005,690,5170.
[9] The results for the optimization of the cobalt-catalyzed [4+2+2]-
cycloaddition will be reported elsewhere.
[10] H. D. Becker,A. Bjoerk,E. Adler, J. Org. Chem. 1980, 45,1596.
[11] The [CoBr2(dppe)]-catalyzed reaction of the propargylic phtha-
limide 8 with 2,3-dimethyl-1,3-butadiene generated the cyclo-
addition products in 36% yield; see: G. Hilt,F. Galbiati, Synlett
2005,829.
[12] We thank Dr. L. Ackermann,LMU München,for providing the
analytical data; see: L. Ackermann,R. Born, Angew. Chem.
2005, 117,2497; Angew. Chem. Int. Ed. 2005, 44,2444.
Received: May 18,2006
Published online: July 7,2006
Keywords: 1,3-dienes · alkynes · cobalt · cycloaddition ·
.
regioselectivity
[1] W. Carruthers Cycloaddition Reactions in Organic Synthesis,
Pergamon,Oxford, 1990; S. Kobayashi,K. A. Jørgensen Cyclo-
addition Reactions in Organic Synthesis,Wiley-VCH,Weinheim,
5206 www.angewandte.org
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