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Page 1 of 26 Linear ACSCross-Dimerization Catalysis Catalytic 1 2 3 4 5 6 7
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Recent Advances in the Catalytic Linear Cross-Dimerizations Masafumi Hirano*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-2416 Nakacho, Koganei, Tokyo 184-8588, Japan Supporting Information
ABSTRACT: Catalytic cross-dimerization is one of the powerful synthetic methods to produce linear molecules with high atom and step economy. Because this process involves carbon-carbon bond forming reaction, the enantioselective reactions have also been achieved. The most well reviewed area in this field is probably hydrovinylation using ethylene, but the linear cross-dimerizations using substituted alkenes and alkynes have also been extensively developed. Not only the difference of the products depending on these substrates, but they mostly differ from hydrovinylation in the mechanism. This Perspective presents a comprehensive summary on the basis of these substrates, including their brief historical background, mechanism, applications to the synthesis of biologically active compounds or contributions to the total synthesis, and the state-of-the-art advancements. The controlling factors in the chemo- and regioselectivities are also discussed.
KYEWORDS: cross-dimerization, co-dimerization, hydrovinylation, linear molecules, enantioselective coupling, biologically active compounds
1.
accepted for such 1,4- and 1,2-hydrovinylations of conjugated compounds. Ethylene is in fact an excellent coupling partner and the coupling reactions with a conjugated compound, vinyl arenes and strained alkenes are also performed. If substituted alkenes can be used for this coupling reaction, diversity of the coupling products would greatly increase. In fact, Wittenberg also reported the coupling reaction between butadiene and methyl acrylate giving methyl hepta-4,6-dienoate in the same report.2 Although the term “hydrovinylation” is often overinterpreted for such reactions using substituted alkenes, these reactions are much more diverse from the synthetic point of view, and the mechanism and the catalyst are often largely different from the pure hydrovinylation. Actually, most of hydrovinylation catalysts cannot apply to the cross-dimerization using substituted alkenes. In this Perspective, we therefore classify the linear cross-dimerizations under 3 topics based on the coupling partners, “ethylene”, “substituted alkenes” and “alkynes”. The scope of this Perspective is to highlight major recent advances, mostly in the past decade, in the cross-dimerizations giving linear molecules, with their brief historical background, and applications to biologically active compounds as one of their major outreaches. In this
INTRODUCTION
Catalytic linear cross-dimerization is a reliable and promising method for production of linear organic molecules with perfect atom and step economy.1 This reaction extends back to the first brief report by Wittenberg in 1963, which documented the cross-dimerization of butadiene with ethylene giving hexa-1,3-diene catalyzed by a cobalt-diene complex.2 However, because this article was a kind of abstract of an annual meeting, the detailed information on the catalysis was not described. At nearly the same time, Hata gave a minute report on production of hexa-1,4-diene (3) from reaction of butadiene (1) with ethylene (2) catalyzed by a Ziegler type system, [Fe(acac)3]/AlEt3 (eq 1).3 [Fe(acac)3] (0.5 mol %) [AlEt3] (2 mol %)
+ 1
2
30 ˚C, 1.5 h
(1) 3 35%
Ethylene is used as a coupling partner in these reactions and nowadays the term “hydrovinylation” is widely
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ACS Catalysis
Perspective, only addition reaction-type cross-dimerizations are reviewed and the related C-C bond forming reactions involving the substitution reaction of a leaving group, and olefin metathesis reactions are not covered. 2.
However, the functional group tolerance is limited for such a Ziegler-type system. The cationic Ni(II) system with a weakly coordinating anion such as BArF- works well without use of alkylaluminum under mild conditions (vide infra).15 The (h3-allyl)nickel(II) system with monodentate phosphines catalyzes the reaction and this feature leads to the asymmetric hydrovinylation using a chiral phosphine ligand from the early period. However, the Ni(II) system is deactivated with addition of a bidentate phosphine and chiral diphosphines cannot be used for the enantioselective reaction. This is a sharp contrast with the Co(II) system. In fact, the Ni(II) and Co(II) systems mostly show high branch selectivity, which is why these complexes are widely used for the enantioselective hydrovinylation. On the other hand, the Pd(II) system is prone to give a linear hydrovinylation product and the catalytic activity is generally low. The Ru(0) system also gives a linear product. A 5-coordinate Ru(II) complex, [RuHCl(CO)L2] is a promising catalyst precursor for hydrovinylation and gives a branched product. However, these regioselectivities strongly depend on the coupling partners, and such a trend and understanding are described later. 2-2. Mechanism. Most of hydrovinylation mechanism has been recognized to involve an insertion of ethylene into h3-allylmetal species. Scheme 1 shows a typical mechanism of hydrovinylation of styrene (4) catalyzed by an in situ formed hydridonickel(II) species.4 This mechanism is generally called the hydride-insertion mechanism. In this mechanism, 4 inserts in to the Ni-H bond in a 2,1-fashion to give a (h3-allylic)nickel(II) intermediate. Ethylene (2) inserts into the Ni-C bond and subsequent b-hydride elimination releases the hydrovinylation product with re-generation of the hydridonickel(II)
CROSS-DIMERIZATIONS USING ETHYLENE
2-1. Brief Historical Overview. Because hydrovinylation is concisely reviewed by RajanBabu and co-workers,4 a very brief historical overview of the cross-dimerization using ethylene is described in this Perspective. Followed by the early reports on the cross-dimerizations catalyzed by Co2 and [Fe(acac)3]/[AlEt3]3 complexes described above, Alderson and co-workers reported a detailed study on cross-dimerization of ethylene with conjugated dienes, styrene or methyl methacrylate catalyzed by [RhCl3·H2O] and [RuCl3·3H2O] in 1965.5 This reaction proceeded in a 1/1 ratio of ethylene with conjugated diene, but the reaction with styrene needed high ethylene pressure (100 MPa), although both reactions underwent at 50 ˚C. EtOH are MeOH were found to be efficient solvents for these reactions. Wilke and co-workers discovered an efficient (h3-allyl)nickel(II) complex as the catalyst in 1966.6 It is interesting to note that even now, the (h3-allyl)nickel(II) system is one of the most powerful catalytic systems for hydrovinylation. Table 1 summarizes selected catalytic systems for hydrovinylation.3,5-19 Table 1. Selected Catalyst System for Hydrovinylation. entry catalyst precursor
ligand/addtive
ref.
1
[Fe(acac)3]
AlEt3
3
2
[FeCl3]
AlEt3/dppe
7
3
[RuHCl(CO)(PCy3)2]
HBF4·OEt2
8
4
[RuHCl(CO)(diphosphine)] AgSbF6
9
5
[Ru(h -cot)(h -dmfm)2]
10
6
[ppn][Ru3(PhNpy)(CO)10]
7
[CoCl2(dppe)]
AlEt3
12
*
8
[CoCl2(dppm)]
AlEt2Cl
13
5
9
[RhCl3·H2O]
10
[Ni(h3-C3H5)Br]2
11
[Ni(h3-C3H5)2]
12
[Ni(h3-C3H5)Cl]2
AlEt3/dimenthylmethyl- 16 phosphine
13
[NiBr(mesityl)(PPh3)2]
BF3·OEt2
14
[PdCl2]
18
15
[PdCl2(NCPh)2]
18
16
[PdCl2(styrene)2]2
19
6
2
1-octanol
11
β-hydride elimination
5,14 NaBArF/mop
4 P Ni H
15
P Ni
*
6
H
P Ni
migrately insertion
insertion
17
P Ni
Many di- and trivalent late transition-metal complexes are reported to catalyze hydrovinylation. Among the catalyst precursors, (h3-allyl)(halido)nickel(II) and dihalido(diphosphine)cobalt(II) catalyze hydrovinylation with high activity in the presence of alkylaluminum.
P Ni
2
Scheme 1. Typical Mechanism for Hydrovinylation (The Hydride-Insertion Mechanism)
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8 catalyzed by [RuClCp*(h4-1,5-cod)] under 0.1 MPa of ethylene, is believed to proceed by this mechanism (Scheme 3).20 Although no direct evidence for supporting this mechanism in this case, there are sufficient collateral evidence for the oxidative coupling mechanism for the cross-dimerizations catalyzed by RuCp and RuCp* complexes (vide infra). 2-3. Enantioselective Reaction. One of the interesting features on hydrovinylation is that the enantioselective reaction has already been found in early 1970s by the combination of a (h3-allyl)nickel(II) complex with suitable chiral phosphine ligands. In 1972, Wilke and coworkers briefly reported the enantioselective cross-dimerization of 1,3-cyclooctadiene with ethylene catalyzed by [Ni(h3-C3H5)Cl]2/Al2Et3Cl3 in the presence of a dimenthylmethylphosphine (eq 2).16 Since the first report on homogeneous asymmetric hydrogenation, and the first metal-catalyzed C-C bond forming reaction by the cyclopropanation appeared in 196821a,b and 1966,21c respectively, the enantioselective hydrovinylation has almost the same history.
species. The valency of the nickel remains unchanged through this catalysis. The key step is an insertion of 2 into the (h3-allylic)nickel(II) species. The enantioselective hydrovinylation is therefore regarded as one of the asymmetric allylations. A similar h3-allyl mechanism is generally proposed for hydrovinylation of conjugated dienes. The hydride-insertion mechanism is also proposed for non-conjugated alkenes such as norbornene (6), although a h3-allyl intermediate is not formed during the catalysis (Scheme 2).15b,16b X
* 7
-
Ni H
P
β-hydride
L
+
migrately insertion
elimination X
-
-
X
+
Ni
P
P
X
insertion
+
Ni
L
[Ni(η3-C3H5)Cl]2 L Al2Et3Cl3
2 +
-
*
CH2Cl2, -75 ˚C
+
Ni
P
Page 4 of 26
2 11 yield was not reported 53% ee
L 10
L=
6
P Me
Scheme 2. Hydrovinylation of Norbornene
Improvement of the chiral ligand was achieved by the same group to give a higher ee for hydrovinylation (eq 3).16,22 Although the chiral azaphospholene ligand has two tertiary phosphines, it acts as a monodentate ligand to form a dinuclear Ni complex.
As described above, most of hydrovinylations proceed by the hydride-insertion mechanism. However, hydrovinylation by the oxidative coupling mechanism, which involves a metallacycle intermediate, is also proposed in some cases. As an example, hydrovinylation of ynamide Ts
N
Me Cp* Ru
9
Cl
+
Me N Ts
4
oxidative coupling
Ph
Me Ts
P N
L=
N 8
H
Cp*
(3)
R-5
Ph
2 Ph
*
CH2Cl2 97%, 93% ee
reductive elimination
+
[Ni(η3-C3H5)Cl]2 L Al2Et3Cl3
2
Ph
(2)
H
Ph N P
Cp*
Ru
Ru
Cl Ph
H
Cl
N Me Ts
Ph
N Ts
Me
However, these Ziegler-type catalyst systems using alkylaluminum has low functional group tolerance. In 1998, RajanBabu and co-workers discovered a combination of the cationic h3-allylnickel(II) species with a weakly coordinating anion such as OTf- or BArF- and a monodentate
β-hydride elimination
Scheme 3. Hydrovinylation by the Oxidative Coupling Mechanism
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ACS Catalysis
Vogt and co-worker documented the reaction catalyzed by an in situ formed cationic (h3-allyl)palladium(II) species with a P-chiral center monodentate phosphine [Pd(h3-C3H5){P(menthyl-O)PhtBu}]+, where a weakly coordinating counter anion such as BF4-, PF6- or SbF6gave the better yield and ee (up to 87% ee).26 A P-chiral monodentate phosphine ligand PBzCyPh is also effective for the cationic Pd-catalyzed reaction (up to 85% ee).27 2-4. Applications to Natural and Biologically Active Compounds. RajanBabu and co-workers reported an efficient enantioselective synthesis of ibuprofen (19), a non-steroidal anti-inflammatory drug, by the cross-dimerization of vinyl arene (16) with ethylene (2) using a phosphoramidite ligand (Scheme 4).28
[Ni(η3-C
3H5)Br]2 (0.7 mol %) L (1.4 mol %) NaBArF (1.7 mol %)
2 (0.1 MPa) +
*
CH2Cl2, -70 ˚C, 7 h MeO
(4)
13 93%, 80% ee
MeO 12 OBn PPh2
L=
mop
chiral phosphine leads to the enantioselective hydrovinylation under 1 atmosphere of ethylene (eq 4).15a,23 Because bidentate phosphines do not work well for the Ni(II)catalyzed system, a monodentate phosphine such as mop was employed to give up to 80% ee. Although the hemilabile nature of mop was of advantage, Leitner and coworkers found chiral phosphoramidite to be an excellent ligand for the enantioselective hydrovinylations (up to 95% ee) (Chart 1).24
[Ni(η3-C3H5)Br]2 (0.7 mol %) L (1.4 mol %)
+
NaBArF, -78 ˚C, 1 h
2 (0.1 MPa)
iBu
16 OH
H
1. O3
*
O
*
*
iBu 17 98%, > 96% ee
O
Bu
2. Me2S
O
O
P N
P N
iBu S-19 (S)-ibuprofen
iBu 18
O MeO
O
Chart 1. Chiral Phosphoramidite Ligands O
Moving away from the Ni(II)-catalyzed hydrovinylation, Vogt and co-workers reported the Co-catalyzed reaction. They used a bidentate chiral bisphosphineamide ligand based on trans-1,2-diaminocyclohexane, and [CoCl2] was reduced by [AlEt2Cl]. This is the first Cocatalyzed hydrovinylation reaction using styrene and ethylene, although the enantioselectivity remained at 47% ee.13 RajanBabu and co-workers have succeeded the enantioselective hydrovinylation of linear conjugated diene, that used to be difficult by the Ni(II) system, by the [CoCl2L]/AlMe3 system with excellent yield and ee (eq 5).25 This Co-system is proposed to proceed by an in situ formed cationic hydridocobalt(II) species [CoH{(S,S)bdpp}]+ by the hydride-insertion mechanism. The bidentate chiral phosphine ligand effectively provides the chiral scaffold on the Co catalyst. 2 (0.1 MPa) + C5H11 14
[CoCl2{(S,S)-bdpp}] (10 mol %) AlMe3 (30 mol %) CH2Cl2, -45 ˚C, 6 h (S,S)-bdpp: Ph2P
C5H11
PPh2
L=
P N O
Scheme 4. Synthesis of Ibuprofen Using Enantioselective Hydrovinylation A natural anti-bacterial compound, trikentrin A (23), was prepared by the reactions involving the enantioselective hydrovinylation catalyzed by a Ni(II) complex (Scheme 5).29 The second introduction of the chiral center was achieved by the asymmetric hydrogenation of the trisubstituted alkene substructure in 22 catalyzed by the Crabtree’s catalyst [Ir(h4-1,5-cod)(PCy3)(py)]PF6, which shows high catalytic activity toward the hydrogenation of trisubstituted alkenes. Recently, RajanBabu and co-workers documented an elegant application of the enantioselective hydrovinylation to the total synthesis of diterpene compounds, having potent pharmacological activities ranging from anti-inflammatory to antituberculosis.30 For example, 2 Ni-catalyzed enantioselective
(5)
* R-15 97%, 97% ee
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2 (0.1 MPa) +
In this reaction, although introduction of the isobutenyl fragment in 28 by the combination of hydrovinylation and olefin methanesis failed, at least a new efficient approach to diterpenes using an enantioseletive hydrovinylation is demonstrated. As described above, [RuHCl(CO)(PCy3)2]/HBF4·OEt2 is also a promising catalyst for hydrovinylation. Yi and co-workers demonstrated to prepare a steroid 31 with a non-natural configuration by hydrovinylation as a single diastereomer (Scheme 7).31
[Ni(η3-C3H5)Br]2 (4 mol %) L (8 mol %), NaBArF, N H
CH2Cl2, -78 ˚C, 3 h * N H
21 99%, 96% ee
20
5 steps
2 steps N H
Page 6 of 26
N H
*
2
* 23 (+)-cis-trikentrin A
* 22
H H
H 29
O
3 steps
2 (0.1 MPa) +
+
*
CH2Cl2, -80 ˚C, 2 h
OMe
ethylene (0.1 MPa) [Ni(η3-C3H5)(L2)][BArF] OMe (2.5 mol %)
* 7 steps
OMe
31 14%
*
2 (0.1 MPa) +
OMe
CH2Cl2, 0 ˚C, 4 h
OMe 27 >90%, 84% ee
H
*
* * *
CH2Cl2, -70 ˚C, 4 h
32
(6) Ph * 33 95%, >95% ee
O L=
OH
P N O
OH 28
P N O
The major difficulties in this reaction are isomerizations of the starting exo olefin to the endo compound. However, careful setup of the reaction conditions reduces these side reactions. Lim and RajanBabu demonstrated new approaches to bioactive compounds such as anticholenergic agents, having a chiral quaternary carbon, by use of the enantioselective hydrovinylation.34 When N-acetyl-a-substituted enamine is used for hydrovinylation, a quaternary carbon center with N-acetyl amides is produced (eq 7).35 Although the trials to prepare the chiral product were premature (up to 14% ee with
O
O L1 =
NHAc
[Ni(η3-C3H5)Br]2 (0.5 mol %) L (1 mol %), NaBArF
Ph
12 steps
(7) Ph
2-5. Recent Advances. Catalytic asymmetric synthesis of all-carbon quaternary centers is undoubtedly a subject of considerable topical interest. By hydrovinylation of 1,1-disubstituted alkenes with ethylene, asymmetric quaternary carbon centers were produced (eq 6).33
*
26
O
Enantioselective synthesis of steroids by use of hydrovinylation is also documented.32
99%, >99% ee
24
H
[RuHCl(CO)(PCy BnO 3)2] (5 mol %) AgOTf/MS 4A (5 mol %)
29
25
OMe
H H
Scheme 7. Synthesis of a Steroid Using30DiastereNHAcHydrovinylation Ph 95% oselective
OMe
OMe
O O
dichloroethane, 65 ˚C, 2 h
hydrovinylations are used for construction of chiral centers of psudopterosin aglycone 28 (Scheme 6).
[Ni(η3-C3H5)(L1)][BArF] (1.4 mol %)
H
30 88% single diastereomer
H
P N
2 (0.1 MPa)
H
BnO
O
Scheme 5. Synthesis of Trikentrin A Using Enantioselective Hydrovinylation
H
benzene, 75 ˚C, 7 h
BnO
L=
H
[RuHCl(CO)(PCy3)2] HBF4 OEt2 (2 mol %)
+
L2 =
P N O
Scheme 6. Synthesis of Pseudopterosin G-J Aglycone Using 2 Enantioselective Hydrovinylations.
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ACS Catalysis
pentadiene)] (39) reacted with 1,3-pentadiene in the presence of AgOTf to give a cationic supine,prone-bis(allylic)ruthenium(IV) compound 40, which was confirmed by the X-ray analysis (eq 10).
P(menthyl)Cy2), such a quaternary center bearing an amino group is an important substructure in many pharmaceutical compounds. 2 (0.1 MPa) +
[RuHCl(CO)(PCy3)2] (5 mol %) AgOTf/MS 4A (5 mol %)
(7) 35 95%
NHAc
Ph
AgOTf
NHAc
Ph
dichloroethane, 65 ˚C, 2 h
Cl
An interesting enantioselective hydrovinylation by the combination of an achiral Ru(II) complex with a chiral counter anion is reported (eq 8).36 Although the ee of this product 13 remained at a low level so far (~46% ee), the enantioselectivity was induced by the chiral counter anion. Probably a weak interaction between [RuH(CO)(PCy3)2]+[chiral anion]- affords a chiral reaction site. 2 (0.1 MPa) +
[RuHCl(CO)(PCy3)2] (2.0 mol %) Ag salt (2.2 mol %)
*
benzene, r.t., 12 h MeO
12
(8)
O
O
O Ag salt =
P O
40 92%
When the butadiene complex was used for this reaction, an octatriene complex of Ru(II) was obtained, which isomerized to a cyclooctadiene complex. Since the oxidative addition to Ru(II) is relatively uncommon, this is a highly suggestive study for the C-C bond forming reactions catalyzed by RuCp* and RuCp species via oxidative coupling mechanism through Ru(IV), and subsequent hydrogen migration reaction and the reductive elimination. Fujiwhara and co-workers applied [RuCp*Cl(h4-1,5cod)] to the linear cross-dimerization of conjugated dienes with substituted alkenes (eq 11).39 The scope of this catalysis is limited to 2-substituted-1,3-dienes such as isoprene and vinyl esters as the best coupling partner. The homo-dimerization of isoprene is the major side reaction.
13 99%, 46% ee
Ar
MeO
O 41 ＋
OAg
Ar
3-1. Brief Historical Overview. Mitsudo and coworkers found that [Ru(h4-1,5-cod)(h6-1,3,5-cot)] catalyzed the cross-dimerization of methyl acrylate with butadiene dominantly giving methyl (3E,5Z)-hepta-3,5dienoate (37) (eq 9).37 Because both reactions using 2-buten-1-yl and 3-buten-2-yl carbonates also produced the same product, this reaction is proposed to involve a (h3allylic)ruthenium(II) intermediate.
43 95%
O 44 +
[RuCpCl(η4-1,5-cod)] (10 mol %) CeCl3 7H2O (15 mol %) 3-hexyn-1-ol (10 mol%) AcO dmf, 60 ˚C, 6 h
37 41% +
N-methylpiperidine 80 ˚C, 9 h
(12) 46 81%
CO2Me
1
(11)
Trost and co-workers reported the cross-dimerization of conjugated ketone 44 with allene 45 to give a 1,3-diene product 46 catalyzed by [RuCpCl(h4-1,5-cod)] (eq 12).40 Because allenes have low ability to remove the 1,5-cod ligand, addition of catalytic amount of an internal alkyne such as 3-hexyn-1-ol promotes the catalysis as an activator.
3. CROSS-DIMERIZATIONS USING SUBSTITUTED ALKENES
[Ru(η4-1,5-cod)(η6-1,3,5-cot)] (4 mol %)
O
MeOH, 100 ˚C, 14 h 42
CO2Me
O
[RuCp*Cl(η4-1,5-cod)] (0.7 mol %)
Ar = 2,4,6-iPr3C6H2
36 +
(10)
CH2Cl2, r.t., 20 h
39
34
OTf-
+
Ru
Ru
(9)
45
O
OAc
A cross-dimerization of 2,3-dimethylbutadiene (47) with substituted alkenes such as styrene (4) was catalyzed by a titanacyclopentadiene complex (eq 13).41 Such crossdimerization catalyzed by an early transition-metal complex is rare. However, the scope of this reaction was limited, and when isoprene was employed in this reaction, the dominant products were a mixture of the homo-cycloaddition products of isoprene with small amount of a mixture of the cross-dimers.
CO2Me 38 10%
Itoh and co-workers discovered a cationic Ru(II) complex, [RuCp*(h4-diene)]+ showed catalytic activity toward cyclodimerizations of conjugated dienes.38 In this chemistry, a Ru(II) complex, [RuCp*Cl(h4-cisoid-1,3-
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Page 8 of 26 pin B
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Et ArO Ph
ArO
52
Ti Et
4 +
Ph
Et
48 78% +
Et
+
EtO2C
+
OHC
53
54
[CoBr2(dppe)] Zn EtO2C ZnI2
(13)
55
OH
benzene, 65 ˚C
56 61%, dr = 95:5
Ph 47
+
49 22% ArO = 2,6-diisopropyloxyphenoxide
[CoBr2(dppe)] Zn ZnI2 EtO2C 52 + 53
CO2Et pin B
H 54
O
with isoprene Ph
Ph
11%
58
OH 11%
55
EtO2C
Ph
Ph
Scheme 9. Co-initiated 4-Components Coupling Reaction
3%
3%
the regioselectivity dramatically altered to give the linear product 61 when SchmalzPhos was employed as the supporting ligand (Scheme 10).44
The cross-dimerization of conjugated dienes with substituted alkenes was catalyzed by [CoBr2(dppe)]/Bu4NBH4/ZnI2 system. In these reactions, the regioselectivity is an important issue. A detailed study of the regioselectivity catalyzed by the Co system was performed. Reaction using acrylates gives the linear coupling product 50, while the reaction using functionalized alkenes exclusively yields the blanched product 51 (Scheme 8).42 It is notable that both Bu4NBH4 and Zn can be used as a reductant for the Co(II) system. [CoBr2(dppe)] CO2Bu (1-5 mol %) BuO2C Bu4NBH4 (1-3 mol %) CH2Cl2, 25 ˚C,16 h ZnI2 (3-9 mol %)
56
59
+
+
R2
57
+
O B O
[CoBr2(dppe)] (5 mol %) Bu4NBH4 (1-3 mol %) ZnI2 (3-9 mol %) C 3H 7 CH2Cl2, 25 ˚C,16 h
R +
[CoBr2(SchmalzPhos)] (5 mol %) Zn (10 mol %) ZnI2 (10 mol %) C5H11 CH2Cl2, r.t., 14-30 h
47
Ph Ph 50 92 %
60 95%
SchmalzPhos =
61 97 % with branched isomer (2%) Ph
O O P O
O O
PPh2 Ph Ph 47
Scheme 10. Co-Catalyzed Ligand-Dependent Regiodivergent Cross-Dimerizations
CO2Bu BuO2C CH2Cl2, 40 ˚C, 16 h
51 97 %
In the case of cross-dimerization between conjugated dienes, a linear product 63 was dominantly obtained (eq 14).45 Although the scope of this reaction is limited for 1arylbutadiene 62 with 2,3-dimethylbutadiene (47), such a linear cross-dimerization between conjugated dienes is a
Scheme 8. Co-Catalyzed Substrate-Dependent Regiodivergent Cross-Dimerizations By use of these substrate-dependent regioselectivities, Hilt and Erver succeeded a fabulous Co-initiated one-pot reaction of 4 components (Scheme 9).43 In this reaction, acrylate 52 initially reacts with the borylated isoprene 53 to give a linear cross-dimer 57. The subsequent allylboration gives the anti-homoallyl alcohol 59, and the final alkene/diene coupling produces the branched coupling product 56. As shown above, the [CoBr2(dppe)]/Zn(or Bu4NBH3)/ZnI2 system produced the branched product 60 in the reaction using substituted alkenes. Interestingly,
Ph Ph 57 +
[CoBr2(dppe)] (10 mol %) Zn (20 mol %) ZnI2 (20 mol is)
58 78% +
CH2Cl2, r.t. Ph
47
7 ACS Paragon Plus Environment
59 13%
(14)
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ACS Catalysis
rare example. When [CoBr2(SchmalzPhos)]/Zn/ZnI2 system was used for this reaction, the linear coupling product 63 was exclusively formed, although the product yield remained moderate (65%). A cross-dimerization of 2,3-dihydrofuran (65) with trimethylvinylsilane (66) was briefly noted as an unpublished result by Wakatsuki and Yamazaki (eq 15).46
Scheme 11. The Oxidative Coupling Reaction of Ru Methyl Methacrylate on Ru(0) D
D
Ru
oxidative coupling
Ph
+
D fast
O 65
thf, 100 ˚C
66
SiMe3
O
(15)
Ru
52 O 68
O
H
O
MeO
Ph
7065
O
O
Ru
OMe H D
71
71 67% 73
D D
D
D
Ph D
47
reductive elimination
Ph
MeO2C H MeCN Ru 74 MeCN
Ru
49-1,5-d2
+
+ MeCN
D
Ph
H CO2Me
4-d2
Scheme 11. The Oxidative Coupling Mechanism 73 35%
1,5-cod)(NCMe)] by the treatment of [Ru(h6-naphthalene)(h4-1,5-cod)] with conjugated diene in the presence of MeCN.49 Using this (h4-cisoid-butadiene)ruthenium(0) complex, an intermediate [Ru(h4-2,3-dimethylbutadiene)(h2-styrene)(h4-1,5-cod)] (71) was observed in toluene-d8 at -80 ˚C, which released the coupling product 49 upon warming to room temperature.48a The following b-hydride elimination occurs from the Hendo in the diastereotopic methylene proton in 72 to give the (E)-C=C bond. The DFT calculations and the kinetic isotope effect support this mechanism.48a The highest transition state in energy is the b-hydride elimination step but the transition state with the largest DDG¹ is the oxidative coupling step in the case of the reaction of 2,3-dimethylbutadiene (47) with styrene (4). However, because the transition states for the oxidative coupling, the b-hydride elimination, and the reductive elimination are close to each other, the ratelimiting step probably changes depending on the substrates. The final skipped diene (1,4-diene) complex 74 is a stable intermediate and a large substituent in the cyclic diene ligand would effectively sweep the product away from the Ru center.50 As described above, the pioneer discovery of a Ru(0)-catalyzed cross-dimerization was documented by Mitsudo and co-workers.37 They used [Ru(h4-1,5-cod)(h6-1,3,5-cot)] as the catalyst precursor and it easily dissociates the 1,5-cod ligand rather than the 1,3,5-cot ligand.51 On the other hand, the naphthalene ligand is easily removed to leave the 1,5-cod ligand on the Ru(0) center for [Ru(h6-naphthalene)(h4-1,5-cod)].49,52 Moreover, [Ru(h4-1,5-cod)(h6-1,3,5-cot)] readily converts into a Ru(II) complex, [Ru(h5-cyclooctadienyl)2] by
(3 mol %) dma, 160 ˚C, 20 h
1h
D
[Ru(η4-1,5-cod)(η6-1,3,5-cot)] CO2Et
D hexane, r.t., Ru
elimination
H H Ph O Ru Ru OMe D
O
Ru
67
Similar reactions of 2,3-dihydrofuran (65) with styrene (4) and with ethyl acrylate (52) proceeded with a different regioselectivity of alkene. The cross-dimerizations of 2,3- and 2,5-dihydrofurans (65 and 68) with ethyl acrylate (52) were catalyzed by [Ru(h4-1,5-cod)(h6-1,3,5cod)] to give the same regioisomer, 2-alkylidenetetrahydrofurans 69, under harsh conditions (eq 16).47 The furans react with acrylate only at the C2-position. According to the DFT calculations and the deuterium labeling experiments, this reaction is proposed to proceed by the hydride-insertion mechanism. In fact, [Ru(h4-1,5-cod)(h61,3,5-cod)] easily forms a hydride species by heat. O 65 or +
MeO
OMe
[RuHCl(CO)(PPh3)3] SiMe3
72 88%β-hydride
Ph
72
+
CO2Me
D 2C MeO
75
CO2Et (16)
69 70% from 65 62% from 68
3-2. Mechanism. The Oxidative Coupling Mechanism: One of the most well-defined catalytic systems for the cross-dimerization is the Ru(0) system and the mechanism is illustrated in Scheme 11, which goes along with the oxidative coupling mechanism.48 A Ru(0) complex [Ru(h6-naphthalene)(h4-1,5-cod)] (70) readily loses the naphthalene ligand. The naphthalene ligand (10p) in 70 coordinates to the “Ru(h4-1,5cod)” fragment as a 6p donor to satisfy the 18-electron rule. Meanwhile the uncoordinated ring in the naphthalene ligand is forced to be a 4p ring, suggesting loss of the aromaticity. Recovery of the aromaticity in this 4p ring encourages facile dissociation of the naphthalene ligand. Naphthalene ligand is therefore much more labile ligand than the other arenes. Accordingly, a conjugated compound and alkene coordinates to the resulting “Ru(h4-1,5cod)” fragment as 4p and 2p donors to the formal 6p vacant site, respectively. These coordination numbers determine the substrate selectivity. Bennett and our group documented a series of [Ru(h4-cisoid-conjugated diene)(h4-
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heating,53 via a hydridoruthenium intermediate.54 Therefore [Ru(h4-1,5-cod)(h6-1,3,5-cot)] and [Ru(h6-naphthalene)(h4-1,5-cod)] behave differently to form a catalytically active species. On the other hand, both systems have in common nature for rapid b-hydride elimination. The DFT calculations suggest that an interaction of the cleaving hydrogen in a ruthenacycle with a sp2 carbon in the cyclic diene ligand reduces the activation energy of the bhydride elimination step.48a In the homo-dimerization of methyl acrylate catalyzed by [Ru(h6-naphthalene)(h4-1,5-cod)] (70), a catalytically active ruthenacyclopentane 76 was isolated and the molecular structure was revealed by X-ray analysis (Scheme 12).55 The stereochemistry of the methoxycarbonyl groups at the 2- and 5-positions in the ruthenacycle is trans, suggesting the two methyl acrylate molecules to coordinate to the Ru(0) center through the same prostereogenic face, re and re, or si and si faces. The dimeric complex 76 was extremely sensitive to air, but it was converted into the more stable mononuclear species 78 by addition of MeCN into a thf solution. The molecular structure of 78 was also confirmed by the X-ray analysis.
Page 10 of 26
benzene, r.t., 30 min
70
79 78%
Reaction at -60 ˚C in toluene-d8, a (h4-cisoid-butadiene)(h2-transoid-butadiene)ruthenium(0) intermediate 82 was observed by 1H NMR and COSY. Treatment of [Ru(h4-cisoid-2,3-dimethylbutadiene)(h4-1,5cod)(NCMe)] (80) with butadiene exclusively gave supine,prone-(h3:h3-2,3-dimethylocta-2,6-diene-1,8diyl)ruthenium(II) 81 (Scheme 13).49d This also provides evidence for the oxidative coupling of h4-cisoid-diene with h2-transoid-diene at Ru(0) center.
Ru NCMe
Ru
benzene, r.t., 20 h
81 75%
80
CO2Me
MeO2C
(17)
Ru
Ru
Ru
77 88% + OMe Ru
70
MeO
Ru
H H O Ru
O hexane, r.t., 1 h
O
O
H
O
MeO
82
OMe H
Scheme 13. Stereoselective Oxidative Coupling between Conjugated Dienes on Ru(0) Complex
OMe
In cross-dimerization of conjugated dienes with substituted alkenes, our group isolated a h4-methyl hepta2,4-dienoate complex of Ru(0) at 6 ˚C in benzene (Scheme 14).56
76 67% + MeCN
OMe
MeO2C H MeCN Ru MeCN
O
H CO2Me
CO2Me
Ru NCMe
Ru NCMe benzene, 6 ˚C, 3 h
78 35%
84 97% isolated
83
Scheme 12. The Oxidative Coupling Reaction of Methyl Methacrylate on Ru(0) Ru
CO2Me
toluene-d8 -60 ˚C
The other support for the oxidative coupling mechanism is provided by similar dimerization of conjugated dienes at a Ru(0) center. Reaction of [Ru(h6-naphthalene)(h4-1,5-cod)] (70) with excess butadiene in benzene exclusively produced supine,prone-(h3:h3-octa-2,6-dien1,8-diyl)ruthenium(II) complex 79, where the two h3-allylic fragments locate in the supine (a concave orientation with respect to the Ru center) and prone (a convex orientation) configuration (eq 17).49b
85 observed
Scheme 14. Stoichiometric Reaction of Coordinated Butadiene with Methyl Acrylate When this reaction was conducted at -60 ˚C in toluene, a new species, which is assignable to a (h4-cisoid-butadiene)(h2-methyl methacrylate)ruthenium(0) 85, was observed. Because it converted into the h4-methyl hepta2,4-dienoate complex of Ru(0) 84 upon warming to room
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ACS Catalysis
temperature, the new Ru(0) species is therefore regarded as an intermediate in this reaction. These Ru(0) complexes catalyzed the cross-dimerization of conjugated dienes with acrylates. As the other example in evidence of the oxidative coupling mechanism was the reaction of coordinated butadiene with vinyl acetate to give a h1:h3:k1-O-hex-4-en-1,6-diyl acetate complex 86 (eq 18).56 The h1:h3-ruthenium(II) species is trapped by coordination of the acetate group.
Cy3P H 2O Cy3P Pd H
O
CO2Me
92
CO2Me
coordination
91
(18)
Ru O
benzene, 6 ˚C, 3 h 83
1,2-insertion
+
O
dma, 130 ˚C, 3 h
MeO2C
Pd Cy3P
O
OMe Cy3P
+
Pd MeO2C
O
OMe
91 followed by the 1,2-insertion of 36 to give a 5-memberd intermediate 93. This insertion process is generally reversible and the 2,1-insertion product is kinetically preferred. However, the 2,1-insertion product eventually converts into thermodynamically more stable 1,2-insertion product giving a 5-membered chelate 93 instantly. The molecular structure of 93 was revealed by X-ray as the aqua adduct trans-94·H2O. If the second 36 occupies the same coordination site for the aqua ligand in trans94·H2O, trans-95 would be formed. However, because the cis conformation between the alkyl and h2-methyl acrylate requires for the subsequent insertion, cis-95 would be responsible for the tail-to-tail dimerization giving 77. This is a non-redox cycle. The C-H Bond Activation Mechanism: This mechanism extends back to the Murai’s catalytic C-H bond activation of aromatic ketones catalyzed by cis[RuH2(CO)(PPh3)3].60 Trost and co-workers documented the carbonyl-directed cross-dimerization of benzyl 1-cyclohexenecarboxylate 97 with vinylsilane (100).61 97 coordinates to a putative Ru(0) species 96 through the carbonyl group. As the result, the C-H bond at the 3-position is forced in proximity to the Ru center. The subsequent C-H bond oxidative addition gives a hydride intermediate 99. Vinylsilane inserts into the Ru-H bond to give 101, and the reductive elimination releases the cross-dimer 102 (Scheme 17). Although this proposed mechanism largely relies on the Murai’s catalytic cycle, the 1H NMR study shows disappearance of the hydride signals and GC analysis shows the formation of the hydrogenated olefin, the catalytic cycle starts from a Ru(0) species 96. The related cross-dimerizations of acyclic a,b-unsaturated ketones with styrene and vinylsilane are also reported.62 As a collateral
CO2Et
Ru
OC O
36 +
trans-95
87 CO
coordination
Scheme 16. The Hydrido-Insertion Mechanism for Homo-Dimerization of Methyl Acrylate
89 19%
OC
OMe
93
cis-95
CO2Et
N N
CO2Me 77
88 53% +
O
O
2,1-migrately insertion and β-hydride elimination
CO2Et
N 52 +
Cy3P Pd
MeO2C
O RuCl3 nH2O (5 mol %) Zn-Cu (50 mol %) EtOH (30 mol %)
+ H2O - H2O
CO2Me
86 68%
This oxidative coupling mechanism is a common process catalyzed by a Ru(0) species. Ura and co-workers also documented a ruthenacycle 90, supporting this mechanism (Scheme 15).57
CO2Et
OMe
trans-94 H2O
36
+
OCOMe
O
+
Cy3P Pd H Ru NCMe
+
Pd
N
90 isolated
Scheme 15. Ru-Catalyzed Cross-Dimerization of Vinylacetoamide with Ethyl Acrylate Contrary to the mechanism shown above, the same group reported the coupling reactions using the same substrates 52 and 87 catalyzed by a Ru(0) complex [Ru(h61,3,5-cot)(h2-dmfm)2] to proceed by the hydride-insertion mechanism based on the deuterium labeling experiments.58 One of the possibilities is that the mechanism varies according to the supporting ligand. Cross-dimerizations catalyzed by Ru(II) complexes such as [RuClCp*(h4-1,5-cod)] and [RuCp*(NCMe)3]+ are also proposed to proceed by the oxidative coupling mechanism. The isolation of a Ru(IV)Cp complex would support this mechanism (eq 10). The Hydride-Insertion Mechanism: The hydride complex also catalyzed the cross-dimerization by the hydride-insertion mechanism. Brookhart and co-workers reported detailed mechanism on the homo-dimerization of methyl acrylate (36) by this mechanism (Scheme 16).59 This mechanism starts with a cationic hydride complex
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PhCH2O
tetrahydrofuran 109 in 74% yield with 80% ee. By use of the C1-symmetric (S,S)-2-methylbicyclo[3.3.1]nona-2,6diene (Me-bnd*) ligand, the R-product of 109 was produced (eq 21).
OCH2Ph
O
O C-H oxidative addition
RuL3
coordination
98
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97 PhCH2O RuL3
[RuH2(CO)(PPh3)3]
RuL3
96
PhCH2O
insertion
PhCH2O
O
H 99
Si(OEt)3 100
RuL3 101
102
The cross-dimerization of conjugated dienes with acrylates was also achieved by a Ru(0) complex having Ph-bod* ligand (eq 22).66 However, the activity of the catalyst and enantioselectivity of the product remained moderate.
Si(OEt)3
Scheme 17. C-H Bond Activation Mechanism for Cross-Dimerization of Benzyl 1-Cyclohexene Carboxylate with Vinylsilane.
CO2tBu [Ru(naphthalene)(η4-Ph-bod*)]
support of this mechanism, a stoichiometric reaction using cis-[RuH2(PPh3)4] with methyl methacrylate (103) gave a ruthenacycle 104 (eq 19).63 The molecular structure of the butyl methacrylate analogue was determined by X-ray analysis.
110 +
103
H 104 73%
[Ni(η4-1,5-cod)2] (10 mol %) 1-octene (15 mol %) NHC (10 mol %) p-anisaldehyde (10 mol %) TESOTf (22 mol %) tBuO NEt3 (60 mol %)
113 +
toluene, r.t., 40 h OtBu 114
Cy NHC =
OTBS CHO
Ph 105 +
106
Ph Ph (10 mol %)
F
CHO
Ph 107 59% +
Ph N
Ph
* (23) 115
85% (with isomers) 94% ee
Cy
N Cy
A chiral-at-Rh(III) complex catalyzed the cross-dimerization of ethene-1,1-diylbis(4-N,N-dimethylaniline) (116) with a,b-unsaturated 2-acylimidazole 117 in high yield and ee (eq 24).68 As a similar reaction, which is limited to ethene-1,1diylbis(4-N,N-aniline) (116) as a coupling partner of enone 119, a quinine-derived organocatalyst catalyzed the reaction (eq 25).69
(20)
MeOH, r.t., 20 h
Ph
Ph
A Ni-catalyzed enantio- and branch-selective crossdimerization is reported by Ho and co-workers (eq 23).67 By use of a chiral NHC ligand, a high ee was achieved in this reaction. The DFT calculations suggested the mechanism involving a 2,1-insertion of vinyl arene to hydridonickel followed by insertion of allylbenzene. The insertion step determines the stereochemistry.67b
(19)
3-3. Enantioselective Reactions. The enantioselective cross-dimerization using substituted alkenes is undoubtedly much more divergent than hydrovinylation. However, such reactions are far less reported than the enantioselective hydrovinylation. To our best knowledge, the first intermolecular enantioselective coupling reaction was achieved by an organocatalyst, diphenylprolinol silyl ether (eq 20).64 This is a formal Alder ene reaction using cyclopentadiene (106) with enal 105, via iminium intermediate formed by prolinol with enal.
N H
112 31% (E/Z = 93/7) 58% ee
(S,R,R)
Ru(PPh3)3
CO2tBu (22)
*
toluene, 50 ˚C, 4 h OMe
111
O neat, r.t., 1 day
(10 mol %)
Ph-bod* =
OMe
CO2Me
74% (E/Z = 93/7) E-form: 80% ee Z-form: 86% ee
(S,S)
reductive elimination
(21)
R-109
Me-bnd* =
O 68
CO2Me
* O
neat, 20 ˚C, 7 d
O Si(OEt)3
[RuH2(PPh3)4] +
[Ru(naphthalene)(η4-Me-bnd*)] (10 mol %)
CO2Me 103 +
O
CHO
108 25% 92% ee as a mixture
We have succeeded the first transition-metal catalyzed enantioselective cross-dimerization between substituted alkenes.65 A C1-symmetric cyclic diene complex of Ru(0) catalyzed the cross-dimerization of 2,5-dihydrofuran (68) with methyl methacrylate (103) to give the C3-substituted
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ACS Catalysis
ArF
Note that the stereochemistry of the 5-position in 123 is E in this reaction.
tBu
O N +
Rh
NCMe
[Pd2(dba)3] (0.2 mol %) HBF4·OEt2/PBu3
BF4+
NCMe
CO2Me
CO2Me
neat, 80 ˚C, 5 h
36
1
123 83%
N Ar
ArF Ar
tBu
O
ArF = 3,5-C6H3(CF3)2 (1 mol %)
116 + O
OH
5 steps
N
dce, 30 ˚C, 2 h
CO2Me NH2 ·2HCl
Ar (24) N
NH2
Ar
O
124 iPr
118 95%, 93% ee
N N
iPr
OH
NH N H
5 steps R1
117
NH R2
Ar = 4-C6H4NMe2
NH2
125
O
Sperabillin B: R1 = Me, R2 = H Sperabillin D: R1 = H, R2 = Me
OMe
Scheme 18. Synthesis of Sperabillins B and D
NH2 N
Ar
The 5-membered heterocyclic compounds with a substituent at the C3 position are widely found in natural products, pharmaceutical molecules and anti-biotics. We have reported the Ru(0)-catalyzed C3 selective coupling of unsaturated 5-membered heterocycles with conjugated carbonyl compounds. In this reaction the bicyclo[3.3.1]nonadiene complex of Ru(0) is an efficient catalyst (Table 2).72 2-Methyl-3-(tetrahydrofuran-3-yl)pro-
N
Ar
(20 mol %) TsOH (20 mol %)
116 + O
CHCl3, 65 ˚C, 20 h Ph
MS 4A
O
Ph
Ar Ar
(25)
120 85%, 90% ee
119 Ar = 4-C6H4NMe2
Table 2. C3-Selective Cross-Dimerizations of Unsaturated 5-Membered Heterocyclic Compounds with Acrylate and Acrylamide
Recently, an excellent catalytic system for cross-dimerization of conjugated dienes with acrylates was reported by RajanBabu and co-workers (eq 26).70 This system requires an in-situ reduction of Co(II) with Zn and an oxidative coupling mechanism by a cationic Co(I) species is proposed. As described above, a bidentate phosphine ligand can be used for the Co system and this chiral scaffold effectively works to produce chiral products like 122.
Ru
CO2Me 36 +
NaBArF (20 mol %) Zn (100 mol %) CH2Cl2, 23 ˚C, 12 h
C6H13
Y
R1 Z
(1 mol%)
Y
+
Ph2 P Br Co Br P Ph2 (10 mol %)
121
NH2 O
O
C6H13
128 99%
3-4. Applications to Natural and Biologically Active Compounds. One of the applications of the crossdimerizations is synthesis of natural and biologically active compounds. Davies and co-workers reported the total syntheses of antibiotics, sperabillins B and D (125), in which a skipped dienyl ester, methyl (2E,5E)-hepta-2,5dienoate (123) was prepared by the cross-dimerization of butadiene (1) with methyl acrylate (36) (Scheme 18).71
131 0%
127 0% O
129 82%
130 0% OMe
S
OMe
TsN
O
O O
O NH2
TsN
O OMe
TsN
NH2
O
126 75% OMe
TsN
O NH2
O
O
CO2Me 122 89%, 98% ee
O OMe
109 65%
(26)
R1
r.t
R2
O
R2
Z
TsN
132 82%
133 94% O
TsN 134 93%
135* 88%
*[Ru(naphthalene)(cod)] (10 mol%) was used as catalyst.
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panamide was prepared by the hydrogenation of 126 starting from commercially available 2,5-dihydrofuran and methacryl amide in 62% total yield. This compound is a pesticide lead patented by Novartis for green rice leafhoppers.73 According to this patent, this compound was prepared by a 5-step reaction from 3-chloromethyltetrahydrofuran. The compound 134 is an analogue of pyrrolostatin, a lipid peroxidation inhibitor. 2,5-Dihydrofuran 68 also reacts with b-myrcene to give a congener of dendrolasin 135, a marine franoterpenoid.74 Unfortunately, simple acrylate and acrylamide did not react with heterocyclic 5-membered rings. 3-5. Recent Advances. One of the recent advances in this field is the cross-dimerization using substituted alkenes catalyzed by Fe complexes. Iron is the most earthabundant transition-metal element and has the higher pharmaceutical contamination allowance than the other metals. The Fe(II) precursor is reduced by Mg to give a Fe(0) species and the oxidative coupling mechanism is proposed (eq 27). 75,76
In the reaction of isoprene (42) with 1-hexene (136), the branched cross-dimers 137 and 138 are obtained. This is a sharp contrast from the in situ reduction catalysis of Fe(II) with Mg, that gives the linear coupling products (eq 27). This is probably arising from the participation of the redox active supporting ligand, diimine. The oxidative coupling mechanism with Fe(I)/Fe(II) redox cycle is proposed for this reaction. Transformation of tetrafluoroethylene (tfe) is also a current topic in this field. Transformation of tfe is an important issue in the fluorochemical industry because tfe has zero global warming potential but is used almost only for production of Teflon.78 Parshall and Jones reported several tfe complexes such as [Rh(acac)(h2CF2=CF2)(h2-CH2=CH2)], [IrCl(h2CF2=CF2)(CO)(PPh3)2] and [Ni(h2-CF2=CF2)(PPh3)3], and they pointed the tfe bound to the metal center tightly and approached the form of a metallacyclopropane.79 A versatile octafluoronickelacyclopentane, the oxidative coupling product of tfe, [Ni(C4F8)(h4-1,5-cod)] was also prepared.80 They are stable and the metallacycle remained attached during the ligand exchange reaction of the ancillary ligand. Ogoshi, Ohashi and co-workers reported brilliant transformations of tfe. Treatment of [Ni(h4-1,5cod)2] with tfe in the presence of PPh3 produced very stable octafluoronickelacyclopentane. However, in the presence of PCy3, a new reactive Ni(0) complex [Ni(h2CF2=CF2)(PCy3)2] was obtained in 64% yield, suggesting importance of the PCy3 ligand in this reaction.81 This breakthrough opens the chemical transformations of tfe. Reaction of tfe (139) with ethylene (2) was catalyzed by [Ni(h4-1,5-cod)2]/PCy3 and 5,5,6,6-tetrafluoro-1-hexene (140) was obtained by the oxidative coupling mechanism (eq 28).
Ph N
Cl Fe Cl
N
(1 mol %) Mg (4 mol %)
Ph 4 +
(27)
Ph
Et2O, 23 ˚C, 6 h 49 94% 47
This mechanism is consistent with the deuterium-labeling experiment and the Fe(0)/Fe(II) redox cycle mechanism is proposed. A diimine complex of Fe(0) is also an active catalyst for the cross-dimerization of conjugated dienes with aliphatic alkenes (Scheme 19).77
F2C CF2 139 (0.5 MPa) +
N N C 4H 9 136 +
Fe
(1 mol %)
2
C 4H 9
N N
C 4H 9
N
I
Fe
N
toluene, 40 ˚C, 2 h
F
(28)
F 140 TON = 13
Baker and co-workers reported the detailed study on the reaction of Ni(0)/PR3 with trifluoroethylene and the treatment of the resulting hexafluoronickelacyclopentane with Lewis acids.82 In the reaction of [Ni(h4-1,5-cod)2] with trifluoroethylene in the presence of tertiary phosphine, the cis-head-to-tail nickelacycle 141 was dominantly obtained with PPh3, and trans-head-to-head nickelacycle 142 dominated when PMe3 was used (eq 29).
138 4% b/l = >98:2
42
[Ni(η4-cod)2] (1.00 mmol) F F PCy3 (2.00 mmol) H
2 (2.5 MPa)
137 93% +
C6D6 or C6D12, 23 ˚C, 4 h
Page 14 of 26
II
Fe
F F2C CFH [Ni(η4-cod)2]/PR3
C 4H 9
R 3P
Ni
R 3P F F 141
Scheme 19. Cross-Dimerization of Diene with Alkene Initiated by Fe(0)-Diamine Complex
F F
F F F
+
R 3P R 3P
F F 142
PPh3: 141/142 = 53%/17% PMe3: 141/142 = 38%/49%
13 ACS Paragon Plus Environment
F
Ni
(29) F
Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
The b-hydride elimination from the nickelacycle seems to be difficult, but treatment of the nickelacycle with Lewis acid initiated the a-fluoride abstraction to give tetrafluorocyclobutene or 1,1,3,4-tetrafluorobutadiene, depending on the Lewis acid used. Cross-dimerization by organocatalysts is a topic to be covered. In 2011, two reports were independently published for the tail-to-tail homo-dimerization of methyl methacrylate by N-heterocyclic carbene (NHC) as the catalyst.83,84 This reaction involves the Michael reaction of NHC with methyl methacrylate (103), and subsequent nucleophilic attack of the resulting Michael product to the second 103 with concomitant liberation of NHC. Recently, the cross-dimerization of 103 with tert-butyl acrylate (110) catalyzed by NHC was reported (Scheme 20).85 In order to reduce the formation of homo-dimer of 110, a slow dropwise addition of 110 is required for this crossdimerization. The second key for this cross-dimerization is that reaction of the deoxy-Breslow intermediate 144 with 110 is faster than with 103. N
CO2tBu 110
N Ph
Ph N
MeO2C Ph
Co
CO2Me
Ru (1-2 mol %)
152 +
(10 mol %)
MeO2C
toluene, reflux, 6 h
143 70%
CO2tBu
N
Ph N+
CO2Me -
N Ph 145
Ph
CO2tBu
[Ru(η4-1,5-cod)(η6-1,3,5-cot)] (4 mol %)
Ph
A regioselective coupling of an internal silylalkyne 157 with terminal alkene 158 giving a skipped diene 159 is also reported (eq 34).93
CN PPh3
157 +
NC Ph
Ph 148 +
NC
[RuCp(NCMe)3]PF6 (10 mol %) acetone, r.t., 2 h
(34)
Me3Si 159
CO2Me
158
(30)
benzene, 70 ˚C Ph
SiMe3 CO2Me
CN
CONMe2 (33) 156 87%
155
4-1. Brief Historical Overview. As a very early work in relation to this reaction, Wakatsuki and co-workers found formation of cross-trimers 148 and 149 from diphenylacetylene (146) with acrylonitrile (147) catalyzed by [CoCp(PhCºCPh)(PPh3)] in 1974 (eq 30).86
Ph
Ph neat, 80 ˚C, 1 h
CONMe2
4. CROSS-DIMERIZATIONS USING ALKYNES
CN 147
153
by [RuH2(PBu3)4]: 100%
154 +
Ph
(32)
benzene, 60 ˚C 4-8 h
These interesting applications involve the enantioselective conjugate hydroalkynylation by [Cu(OAc)2·H2O],88 and [Cu(NCMe)4]PF6.89 One of the problems for the hydroalkynylation is that the cross-dimerization between a terminal alkyne with conjugated compound competes with the homo-dimerization between terminal alkynes. However, bulky silylalkynes effectively promotes the cross-dimerization.90,91 The cross-dimerization of an internal alkyne 154 with dimethyl acrylamide 155 is catalyzed by [Ru(h4-1,5cod)(h6-1,3,5-cot)] (eq 33).92
CO2tBu
deoxy-Breslow Intermediate
Ph
CO2Me 151 30%
Mitsudo and co-workers pioneeringly documented the cross-dimerization of terminal acetylene 152 with butadiene (1) giving 1,3-enynes 153 catalyzed by [RuH2(PBu3)4] or [Ru(h4-1,5-cod)(h6-1,3,5-cot)] (eq 32).87
Scheme 20. NHC-Catalyzed Cross-Dimerization
146 +
(31)
[Ru(η4-1,5-cod)(η6-1,3,5-cot)]/2PBu3: 74%
N Ph 144
Ph
CO2Me
MeO2C
1
Co
Co
benzene, r.t.
150
Ph N
CO2Me
CO2Me Ph
PPh3
N Ph
Ph CO2Me 103 +
Although [CoCp(PhCºCCO2Me)(PPh3)] (150) did not show any catalytic activity, the stoichiometric reaction of 150 with dimethyl maleate yielded the corresponding cobaltacyclopentene 151, whose molecular structure was revealed (eq 31). This provides strong evidence for the coupling using 148 and 149 by the oxidative coupling mechanism.
The Co-catalyzed reductive alkyne/acrylate cross-dimerization was achieved by [CoI2(PPh3)2]/Zn/PPh3 system in wet MeCN (eq 35).94 The regioselectivity was the same as the Ru(0) system and the final step would be the
Ph
149 E/Z = 1/1 560%/Co
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Page 16 of 26
protonation of alkenylcobalt by water, and mono-ene product was therefore obtained. [CoI2(PPh3)2] (5 mol %) Zn (3 equiv) PPh3 (15 mol %)
Ph 154 +
Ph
NCMe/H2O, 80 ˚C 12 h
CO2Bu 160
(35)
CO2Bu 161 96%
Tanaka and co-workers reported an alkyne/acrylate coupling catalyzed by cationic Rh(I)/H8-binap system. Electron-deficient internal alkynes are required for the efficient catalysis (eq 36).95 Ph
CO2Et 162 + CO2Bu
[Rh(η4-1,5-cod)2]BF4 (5 mol %) H2-binap (5 mol %) EtO2C CH2Cl2, 80 ˚C, 4-19 h
Scheme 21. Regioselectivity for Cross-Dimerizations Using Unsymmetrical Alkynes
Ph
Hilt and co-workers reported an interesting comparison concerning the regioselectivity of these two catalytic systems using the same substrates. Treatment of the silylprotected 1,3-butadiyne 168 with 1-octene (169) gives a sharp difference in the regioselectivity by these two catalytic systems (Scheme 22).100 Although the reaction occurs at the CºC triple bond close to the anisyl group, the C-C bond forming reaction occurs at the C3 position by the Ru(II) system, while the reaction occurs at the C4 position by the Co(II) system.
CO2Bu (36) 163 91%
160
The same group documented the amide-directed cross-dimerization of alkynes to give a (Z,Z)-1,3-dienyl amide 165 (eq 37).96 This reaction involves the C-H bond activation directed by the amide group. Ph Ph
Ph 146 + O
Ph O
[Rh(η4-1,5-cod)
2]BF4 (5 mol %) biphep (17 mol %)
(37)
N
MeO
CH2Cl2, 25 ˚C, 72 h
N 164
C5H11
Ph 146 + O
[RuCp(NCMe)3]PF6 (10 mol %)
thf, 100 ˚C, 16 h
Ph O
[CoBr2(dppp)] (10 mol %) Zn (20 mol %) ZnI2 (20 mol %)
Ph 166
C5H11 170 65% SiMe3 MeO
CH2Cl2, r.t., 18 h
(38)
N Ph
N
MeO
acetone, r.t., 3 h
Ph [RuCl2(p-cymene)]2 (3 mol %) AgSbF6 (12 mol %)
SiMe3
169
In the Rh(I) system, introduction of a substituent at the a-position in acrylic amide shut down the reaction. However, [RuCl2(h6-p-cymene)]2 catalyzed the reaction using a-substituted acrylic amide 166 (eq 38).97 In this reaction, the kinetic isotope effect (kH/kD = 1.9) is consistent with the C-H bond activation mechanism. Ph
SiMe3 168 +
165 94% (selectivity 88%)
171 65%
167 92%
C5H11
Scheme 22. Control of Regioselectivity by Ru and Co Complexes
For unsymmetrically substituted internal alkynes, the regioselectivity in the alkyne/alkene cross-dimerization is an important issue. There is a sharp contrast between the and typical catalysts, [RuCp(NCMe)3]PF6 [CoBr2(dppp)]/Zn/ZnI2. The characteristic feature is observed in the coupling reactions using unsymmetrically substituted propargyl esters. The Ru(II) system is prone to form the C-C bond at the a-carbon in propargyl esters.98 On the other hand, the Co(II) system dominantly forms the C-C bond at the a-carbon in propargyl esters.99 The C-C bond formation occurs at the sterically morehindered side for the Ru(II) system, while the Co(II) system favors to form the C-C bond at the less-hindered side (Scheme 21).
According to the regioselectivity observed in the Ru(II)-catalyzed cross-dimerizations, reaction of a terminal alkyne with alkene 172 catalyzed by [RuCp(NCMe)3]PF6 dominantly gives a branched coupling product 173. However, when a propargyl alcohol is employed in this reaction, the linear product 174 dominated (Scheme 23).101 The choice of solvent also affects the regioselectivity. Dimethylformamide encourages to give the branched product and acetone promotes the formation of the linear product.
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ACS Catalysis C7H14CO2Me
M
M
+
172
183 560˚
C7H14CO2Me
O
OH
C7H14CO2Me
HO
174 major product: linear 91% (l/b = 32/1)
Scheme 23. Regioselectivity-Control by Substrate 4-2. Mechanism. Although there is no direct evidence for the mechanism of the [RuCp(NCMe)3]PF6 system, it is widely accepted this system to proceed by the oxidative coupling mechanism102,103 which is close to the catalytic cycle shown in Schemes 3 and 10. In the ruthenacyclopentene bearing a substituent at the a-position on the sp2 carbon, 175 is disfavored because of the steric hindrance (Scheme 24). 177 is therefore preferred to give a branched product 178. O +
O
R
R2
176
R1
175 disfavored
R2
M
518˚
M
R3
R2 R1
186
M
R3
187 (40)
R1
R3
M
R2
M
R
Ru
R
O
R2 178
R1,R2 =
aryl, alkyl, alkenyl R3 = COR, CO2R, CN, alkyl M = Ru, Rh, Co, Ni, Pd
177 favored
The situation changed in the case of propargyl alcohol. In this case, the hydroxy group interacts with the coordinatively unsaturated cationic Ru(II) center 179 and the linear product 180 dominated (Scheme 25). +
Ru
R
HO R
188
R3
R1
189
R3
In general, the electron-withdrawing group prefers to come to the electron-rich a-position. The metallacyclopentenes 186 and 187 are therefore favored. If the R1 is larger in size or more electron-withdrawing than R2, 186 is the most favored. Now the product is obtained by the subsequent b-hydride elimination and reductive elimination. The b-hydride elimination from the metallacycle ring or a-alkyl group in 190 leads to the formation of a conjugated diene 191 and skipped diene 192, respectively (Scheme 26).
Scheme 24. Ruthenacycles Derived from Terminal Alkynes
HO
(39)
+
+
O
M
185
184 539˚
R1
R
Ru
+
Then, let us consider about how the metallacyclopentene is selectively formed from an alkyne and alkene. Cheng and Jeganmahan discussed about this issue based on the stability of metallacycles (eq 39).103 In the cross-dimerization of alkyne with alkene by the oxidative coupling mechanism, a metallacyclopentene is a possible intermediate. Without chemoselectivity, 3 metallacyclopentenes 183-185 are possible. According to the ordinary bond angles for sp2 (120˚) and sp3 (109.5˚) carbons and for a mean carbon-metal-carbon (80˚), the total sum of the interior angles is estimated to be 560˚ (183), 539˚ (184) and 518˚ (185), respectively. Regardless of length of sides, the sum of interior angles for a pentagon is 540˚(= 180˚× (5–2)). 183 and 185 are therefore distorted nonplanar structures and 184 is the ideal and planer 5-membered ring. For the coupling reactions using unsymmetrical internal dienes with electron deficient alkenes, 4 different metallacyclopentenes 186-189 can be formed (eq 40).
O 173 major product: branched 86% (l/b = 1/5)
[RuCp(NCMe)3]PF6
M
+
180
179 favored
R2 R
HO
+
Ru
R
1. β-hydride elimination from ring-methylene R1
182
HO
R2 181 disfavored
M
H
H R
H
H
190
Scheme 25. Ruthenacycles Derived from Propargyl Alcohol
2. reductive elimination 1. β-hydride elimination from α-alkyl 2. reductive elimination
R1
R
H 191 conjugate diene R2 R1
R H 192 skipped diene
Scheme 26. Position of b-Hydride Elimination
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Page 18 of 26
4-3. Enantioselective Reactions. One of the most established reactions in this field is the alkyne addition to alkene. The first catalytic enantioselective reaction has been achieved by the Cu-catalyzed reaction by Carreira and co-workers (eq 42).88
The following is a good instance for switching around the position of the b-hydride elimination. The cross-dimerization of internal alkyne 193 with methyl methacrylate (103) dominantly produced the conjugated diene product 194 with concomitant formation of the skipped diene 195. When acetonitrile was added in this catalysis, the skipped diene 195 dominated (eq 41).104
Ph
Et Et
Et 193 +
Et 194 + Et
benzene, r.t., 24 h
72%, 194/195 = 58/42 w/o NCMe: w/ NCMe (50 mol %): 96%, 194/195 = 2/98
H
Et Et
OMe
194
i
198
199
205 +
Et
Ru
OMe
1,4-dioxane, 80 ˚C, 12 h
196
Ru
O (43) Ph 207 99%, 91% ee
O 206
OMe Et
PPh2
Ph
Et O
204 79%, 97% ee
[Rh(µ-OAc)(η2-C2H4)2]2 (2.5 mol %/Rh) L (5.5 mol %)
Pr3Si
O Et
Et
PINAP
O Ru
H
O
The problem of this type conjugate addition reaction is that the ordinary terminal alkyne is more active than enones. However, Hayashi, Nishimura and co-workers discovered that bulky terminal silylalkynes are excellent substrates for this reaction and a high ee was obtained by [Rh(µ-OAc)(h2-C2H4)2]2 (eq 43).90
OMe
Et
O
Et OH
N N
MeO
Et
Ru H O
Ph
L=
195
No interconversions between 194 and 195 was observed under these conditions. The DFT calculations of this reaction suggests the key for this regioselectivity is the b-hydride elimination step (Scheme 27).
(42)
Ph
O
CO2Me
103
O
O
O
(41)
O
H2O, 0 ˚C, 51 h Na-(+)-ascorbate (2 mol %)
203
Et
CO2Me
O
CO2Me
[Ru(naphthalene)(η4-1,5-cod)] (10 mol %)
[Cu(OAc)2·H2O] (1 mol %) L (1 mol %)
202 +
O
L=
PAr2 PAr2
O
O
SiiPr3
O Ar = C6H2(tBu2-3,5)(OMe-4)
197
(R)-DTBM-segphos + MeCN
Et Et
H CH2 Ru
OMe
H
Et
O NCMe
Et
The reaction can be applied for conjugated dienes but the Ru-catalyzed achiral reactions are reported to give a mixture of regioisomers.106 However, a Ni(0) catalyst, [Ni(h2-1,5-cod)2] catalyzed the selective 1,2-addition reaction of terminal silylated alkynes to conjugated dienes in the presence of a chiral ligand (eq 44).107
OMe O 195
Ru
-MeCN 200
201
Scheme 27. Regioselectivity-Control by Added MeCN
iPr
iPr
[Ni(η4-cod)2] (10 mol %) L (11 mol %)
PhMe2SiO 208 +
Coordination of the a-carbonyl group in ruthenacycle 197 compensates a part of the coordinative unsaturation, but this coordination leads the a-methyl group away from the Ru(II) center. The similar example for coordination of an a-carbonyl group in a ruthenacyclopentene is also reported.105 Subsequent b-hydride elimination therefore occurs from the ruthenacycle ring 198. The subsequent reaction releases 194. On the other hand, MeCN coordinates to 197 in the presence of MeCN. As the result, the a-methyl group can approach to the metal center. A skipped diene 195 is eventually obtained from 201.
Ar1
thf, r.t., 82-90 h
Ar1 209 Ar1 = C6H4OMe-3
2 Ar2 Ar O P NPh2 O O Ar2 Ar2
O
L=
Ar2 = 3,5-xylyl
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*
(44) OSiMe2Ph
iPr iPr 210 68%, 93% ee
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ACS Catalysis
The 1,6-addition of a terminal silylalkyne was also reported by use of dienyl esters and amides catalyzed by a Co system (eq 45).91b Addition of tert-amyl alcohol improved yield in this system. [Co(OAc)2 4H2O] (5 mol %) O L (5 mol %) Zn (50 mol %) EtO
i Pr Si 3
O
205 +
dmso, t-amyl alcohol 80 ˚C, 20 h
OSiiPr3
[RuCp(NCMe)3]PF6 (10 mol %)
218 +
O O
acetone, r.t., 13 h OH
(45)
* 212 81%, 96% ee
SiiPr3
O
219
O
EtO 211
Et
Et L=
OH
P
OSiiPr3
220 75 %
P
Et Et (S,S)-Et-duphos
Most of the reaction of terminal alkynes with oxa- or azabenzonorbornadienes involve the ring-opening reaction,108 but Tenaglia and co-workers reported the reaction without the ring-opening.109 The first enantioselective addition reaction was reported by the Co system using (R,R)-QuinoxP* as the chiral scaffold (eq 46).110 i
[Co(OAc)2 4H2O] (5 mol %) L (5 mol %) Zn (10 mol %)
Pr3Si 205 + O
OH
P
O
Scheme 28. Total Synthesis of Amphidinolide P A similar alkyne/alkene coupling reaction was also applied to the synthesis of lasonolide A (Scheme 29).113 In this case, the linear regioselective coupling product was obtained in around 2:1 ratio by use of [RuCp(NCMe)3]PF6 catalyst. Further improvement in the regioselectivity is the future problem.
214 91%, 99% ee
P L=
O
221 amphidinolide P
(46)
O
tBu
213
O
SiiPr3
dmso, 10 ˚C, 20 h
O
4 steps
OH
tBu
O O
O
R,R-QuinoxP*
Bulky terminal silylalkynes are also effective for this reaction, while use of simple phenylacetylene or 1-octyne shut down the reaction. In relation to this reaction, a onepot combination reaction of enyne cyclization and the enantioselective addition to azabenzonorbornadiene was also reported. This reaction is catalyzed by [Rh(h4-1,5cod)2]BF4/(R)-An-SDP (eq 47).111
222 +
O
[RuCp(NCMe)3]PF6 (10 mol %)
HO OH
and 3 steps
O
MeO
OH 223 OH
TsN 215 +
[Rh(η4-1,5-cod)2]BF4 (5 mol %) L (6.5 mol %) dce, 40 ˚C, 36 h
X
O NTs
X
(47)
L=
O HO O
217 84%, 99% ee
216 X = NBoc
O
O
PAr2 PAr2
O OH
224 lasonolide A
Scheme 29. Total Synthesis of Lasonolide A Ar = 4-MeOC6H4
The Ru-catalyzed alkyne/alkene cross-dimerization was used twice in the total synthesis of piericidin A (Scheme 30).114 The first cross-dimerization of the internal alkyne with propylene gave the target product in quantitatively. However, the second cross-dimerization be-
4-4. Applications to Natural and Biologically Active Compounds. The alkyne/alkene cross-dimerization is applied to a total synthesis of amphidinolide P (Scheme 28).112 The initial idea for the key reaction was a branch selective coupling of an internal silylalkyne with terminal alkene described above. However, use of a terminal alkyne was more effective for this reaction.
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Page 20 of 26 F2C CF2
(condensed)
139 +
[RuCp(NCMe3)3]PF6 (10 mol %) OSiBu3 225
2
acetone, -78 ˚C to r.t 30 min
226 OSiBu3 >98%
N
OTeoc
233 >99% β-hydride elimination and reductive elimination
MeO
OSiBu3 F F
N 227 60% with a branch regioisomer l/b = 4/1 OTeoc
MeO
thf, 50 ˚C, 15 h
F
oxidative coupling of tetrafluoroethylene with ethyelne
[RuCp(NCMe3)3]PF6 MeO (10 mol %)
TBAF
F
F F
232
MeO
acetone, r.t., 8 h
toluene, 40 ˚C, 0.5 h
2 +
OTeoc
MeO
[Ni(η4-1,5-cod)2] (10 mol %) PCy3 (40 mol %) H
MeO
F
Ni
F F insertion F F of alkyne
Ni 235
234
F F insertion Ni F F of ethylene 236
Scheme 32. 4-Component Reaction using Tetrafluoroethylene
OH N
oxidative coupling product of 2 equiv of 139 on Ni(0), is a very stable unreactive compound. Thereby coexistence of 2 is important in this catalysis. As we saw in the case of Scheme 31, the Ni-CF2CF2R bond is insensitive to insert an unsaturated compound. Then, a rational explanation may be difficult, but further insertion of 232 and 2 occurs in this order. Although the b-hydride elimination from a nickelacycle is normally slow, the ring distortion of the resulting 9-membered nickelacycle is considered to facilitate the b-hydride elimination. The 4-component reaction involving tetrafluoroethylene/styrene/4-octyne/ethylene is also possible. We recently found an interesting coupling reaction between conjugated dienes with internal alkynes to give conjugate trienes 238 (eq 48).117 Because conjugated polyenes are conventionally prepared by the successive stoichiometric reactions involving Wittig reaction or HornerWadsworth-Emmons reaction of enal with ylid ester followed by reduction to a primary alcohol and partial oxidation to give dienyl aldehyde. This is reliable but one needs 3-step reactions to expand just one C=C double bond by this method. The present reaction has a relatively wide scope with high tolerance both in alkynes and conjugated dienes, but the reaction is limited to internal alkynes.
228 piericidin A 87%
Scheme 30. Total Synthesis of Piericidin A tween pyridylmethyl alkyne with the triene gave the preferred linear coupling product with concomitant formation of the branched product in 11%. The regioselectivity is still fraught with challenges. 4-5. Recent Advances. Transformations of fluorinated compounds are also current topics in this field. As an important finding in past, Bennett and co-workers documented the stoichiometric oxidative coupling reaction of tfe with naphthalyne on nickel(0) 229 and further insertion of dimethyl acetylene dicarboxylate into the nickelacycle 230 to give 231 (Scheme 31).115 Ni(dcype)
F
F2C CF2 thf, -60 ˚C, 16 h
229
F
F F Ni F (dcype) 330 75%
MeO2CC CCO2Me thf, reflux, 16 h
F F
MeO2C MeO2C
F Ni (dcype)F
[Ru(naphthalene)(η4-1,5-cod)] (10 mol %)
193 +
231
Scheme 31. Reactions of Tetrafluoroethylene and Dimethylacetylene Dicarbocylate
(48) CO2Me
benzene, r.t. 10 min CO2Me
238
237
When butadiene is used for this reaction, 2 equiv of alkyne reacts with butadiene to give conjugated tetraenes regio- and stereoselectively (eq 49).118 According to a deuterium labeling experiment using CD2=CHCH=CD2
Ogoshi, Ohashi and co-workers reported interesting 4component coupling reactions using tetrafluoroethylene catalyzed by [Ni(h4-1,5-cod)]/PCy3 (Scheme 32).116 The key of this chemoselectivity is the selective formation of a nickelacyclopentane 234 from tfe (139) and ethylene (2). As described above, octafluoronickelacyclopentane, the
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ACS Catalysis
(1-d4), these reactions are regarded as a formal syn insertion of an internal alkyne into the terminal C-H(D) bond in butadiene.
forming reaction. Because an alkynyl group is easily attached to an aromatic compound by the Sonogashira coupling reaction, this new approach will provide an efficient method for the marriage between aromatic compounds and p-conjugation groups.
R
R [Ru(naphthalene)(η4-1,5-cod)] R D D 239 (10 mol %) R + D R (49) benzene, r.t. 10 min D D D R D R = isopentyl D 240-d4 1-d4
The stoichiometric reaction starting from [Ru(h4-butadiene)(h4-1,5-cod)(NCMe)] (83) with 1 equiv of 3hexyne at room temperature instantly produced a mixture involving a new species which is probably assignable to [Ru(h4-butadiene)(h2-3-hexyne)(h4-1,5-cod)] (241), and these mixtures finally converged into a h4-triene complex 242 within 5 min at room temperature (Scheme 33). Et Ru
[Ru(naphthalene)(η4-1,5-cod)] (10 mol %)
247 +
benzene, 50 ˚C, 20 h CO2Me CO2Me
237
Et
Et
+ MeCN Ru
NCMe
- MeCN
Et 241
83 Et Et
Et Ru NCMe
Et Et
(50) MeO2C
Et
3 NOE 6
4 5
8
NOE
7 NOE
10
9
Et
Et
248
Ru NCMe
242
72%
243 (98 %)
5. SUMMARY AND OUTLOOK Catalytic cross-dimerization is a prominent method to produce linear molecules with perfect atom and step economy. In the past decade, not only ethylene, substituted alkenes became possible to use as the substrate for the enantioselective reactions. Some of these chiral and achiral reactions were started to apply to the total synthesis of natural products, pharmaceutical compounds and complex molecules. For this purpose, [RuCp(NCMe)3]PF6 (Schemes 28-30), [CoX2(chiral diphosphine)]/NaBArF/Zn (eq 26), [CoBr2(dppe)]/Zn/ZnI2 (Scheme 9), and [Ni(h3-allyl)X2]/NaBArF/chiral phosphoramidite ligands (Schemes 4-6) are one of the most successful systems in hydrovinylation but the enantioselective reactions, particularly with substituted alkenes, have just started, and the other systems involving Fe(0), Fe(I), Ru(0), Ru(II), Rh(III) and Pd(0) complexes are also promising candidates. These progresses surely change the methodology for the synthesis of biologically active molecules. The cross-dimerization also opens the way for achiral new molecules. Catalytic transformations of tetrafluoroethylene are featured in this Perspective, and such efforts should contribute to the fluorinated pharmaceuticals and material sciences (eq 28 and Scheme 32). Because crossdimerization is also an excellent method to produce skipped dienes and conjugated polyenes, building blocks for macrolide antibiotics would be important outreaches.
Et Et
Et +
CO2Me
Ru
Et 244 (92 %) 245 (92 %)
Scheme 33. Stoichiometric Reaction of Coordinated Butadiene with 3-Hexyne Further treatment of the resulting h4-triene complex 242 with 3-hexyne gave a h4-cisoid,transoid,transoid-teteraene complex 243. Tetraene 244 was released by exposure to butadiene along with the formation of the oxidative coupling product of butadiene on Ru(II) 245. These observations, and the DFT calculations, support the oxidative coupling mechanism. When unsymmetrical internal alkynes are employed in this reaction, the sp-carbon bearing an electron donating substituent is susceptible to react with a conjugated diene. This is consistent with the stability of metallacyclopentene discussed in eq 40. The highly regio- and stereoselectivities of this reaction enable to prepare a benzene-fused nonaene product 248 in a one-pot starting from 1,3,5-triynylbenzene 247 (eq 50).119 This molecular “triskelion”, a symbol of Sicily, has a perfect C3 symmetry, suggesting that the Ru catalyst reliably distinguishes the propyl and aryl groups in 247 in the C-C bond
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I hope those who read this Perspective will become interested in this field, know the comprehensive understanding of the brief historical background and current frontiers, and exploit the latest advances in the cross-dimerization.
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(1) We and many groups term “cross-dimerization” (or “co-dimerization”) for these reactions because this reaction extends back to the early selective oligomerization by the Ziegler-type catalytic system as described in this Perspective. We hesitate to use the term “cross-coupling” because this term reminds someone of a catalytic process involving a transmetallation step. (2) Wittenberg, D. Über Neuartige Dienylierungsreacktionen. Angew. Chem. 1963, 75, 1124-1124. (3) Hata, G. Stereospecific Synthesis of 1,4-Dienes. J. Am. Chem. Soc. 1964, 86, 3903-3903. (4) (a) RajanBabu, T. V. Asymmetric Hydrovinylation Reaction. Chem. Rev. 2003, 103, 2845-2860. (b) RajanBabu, T. V. In Pursuit of an Ideal Carbon-Carbon Bond-Forming Reaction: Development and Applications of hydrovinylation of Olefins. Synlett 2009, 853-885. (c) RajanBabu, T. V.; Cox, G. A.; Lim, H. J.; Nomura, N.; Shama, R. K.; Smith, C. R.; Zhang, A. “Hydrovinylation Reactions in Organic Synthesis” In Comprehensive Organic Synthesis, 2nd ed.; Knochel, P.; Molander, G. A. Eds.; Elsevier: Amsterdam, 2014, Vol. 5, pp 15821620. (5) Alderson, T.; Jenner, E. L.; Lindsey, Jr., R, V. Olefin-to-Olefin Addition Reactions. J. Am. Chem. Soc. 1965, 87, 5638-5645. (6) Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.; Kröner, M.; Oberkirch, W.; Tanaka, K.; Steinrücke, E.; Walter, D.; Zimmerman, H. Allyl-transition Metal Systems. Angew. Chem. 1966, 5, 151-164. (7) Iwamoto, M.; Yuguchi, S. Reaction of Butadiene with Ethylene. II. New Catalytic Systems in Synthesis of 1,4-Hexadiene. J. Org. Chem. 1966, 31, 4290-4291. (8) Yi, C. S.; He, Z.; Lee, D. W. Hydrovinylation of Alkenes Catalyzed by the Ruthenium-Hydride Complex Formed in situ from (PCy3)2(CO)RuHCl and HBF4·OEt3. Organometallics 2001, 20, 802804. (9) Sanchez, Jr., R. P.; Connell, B. T. A Ruthenium-Based Catalyst System for Hydrovinylation at Room Temperature. Organometallics 2008, 27, 2902-2904. (10) Kondo, T.; Takagi, D.; Tsujita, H.; Ura, Y.; Wada, K.; Mitsudo, T. Highly Selective Dimerization of Styrenes and Linear Co-dimerization of Styrenes with Ethylene Catalyzed by a Ruthenium Complex. Angew. Chem. Int. Ed. 2007, 46, 5958-5961. (11) Lavigne, G.; Lugan, N.; Rivomanana, S.; Mulla, F.; Soulié, Kalck, P. Base-Promoted Ruthenium Carbonyl Cluster Complexes: From Fundamental Reactions to Catalysis. J. Cluster Sci. 1993, 4, 4958. (12) Iwamoto, M.; Tani, K.; Igaki, H.; Yuguchi, S. Preparation of 3Methyl-1,4-heptadiene, 3-Ethyl-1,4-hexadiene, and Methyl-1,4-hexadienes. J. Org. Chem. 1967, 32, 4149-4149. (13) Grutters, M. M. P.; Müller, C.; Vogt, D. Highly Selective Cobalt-Catalyzed Hydrovinylation of Styrene. J. Am. Chem. Soc. 2006, 128, 7414-7415. (14) Umezaki, H.; Fujiwara, Y.; Sawara, K.; Teranishi, S. Catalytic Codimerization of Styrene with Lower Olefins by Rhodium and Ruthenium Catalyst. Bull. Chem. Soc. Jpn. 1973, 46, 2230-2231. (15) (a) Nomura, N.; Jin, J.; Park, H.; RajanBabu, T. V. The Hydrovinylation Reaction: A New Highly Selective Protocol Amenable to Asymmetric Catalysis. J. Am. Chem. Soc. 1998, 120, 459-460. (b) Kumareswaran, R.; Nandi, M.; RajanBabu, T. V. Hydrovinylation of Norbornene. Org. Lett. 2003, 5, 4345-4348. (16) (a) Bogdanovic, V. B.; Henc, B.; Meister, B.; Pauling, H.; Wilke, G. A Catalyzed Asymmetric Synthesis. Angew. Chem. Int. Ed. 1972, 11, 1023-1024. (b) Bogdanovic, von B.; Henc, B.; Lösler, A.; Meister, B.; Pauling, H.; Wilke, G. Asymmetric Syntheses with the Aid od Homogeneous Transition Metal Catalysts. Angew. Chem. Int. Ed. 1973, 12, 954-964. (17) Kawata, N.; Maruya, K.; Mizoroki, T.; Ozaki, A. Codimerization of Ethylene and Styrene Catalyzed by Bis(triphenylphosphine)saryl Nickel(II) Halide-Trifluoroboron Etherate. Bull. Chem. Soc. Jpn. 1971, 44, 3217-3217. (18) Barlow, M. G.; Bryant, M. J.; Haszeldine, R. N.; Mackie, A. G. Organic Reactions Involving Transition Metals III. Palladium(II)Catalysed Dimerization of Olefinic Compounds. J. Organomet. Chem. 1970, 21, 215-226.
■ AUTHOR INFORMATION
Corresponding Authors *M. Hirano. Email:
[email protected]. ORCID Masafumi Hirano: 0000-0001-7835-1044 Notes The author declares no competing financial interest. ■ ACKNOWLEDGMENTS
M. H. is grateful to the current students and alumni, Dr. N. Komine and Dr. S. Kiyota in his research team, whose names can be found in the references of our contributions. He also thanks Profs. Emer. S. Komiya (Tokyo University of A & T) and M. A. Bennett (Australian National University) for useful advice in this chemistry. Financial support from Japan Science and Technology Agency (JST), ACT-C (JPMJCR12Z2), and Grant-in-Aid for Scientific Research (B) 17H03051. A part of this research work was also supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas “3D Active-Site Science” (26105003). ■ ABBREVIATIONS
acac: acetylacetonate (C5H7O2). BArF: tetrakis[3,5bis(trifluoromethyl)phenyl]borate (C32H12BF24). bdpp: 1,4-bis(diphenylphosphino)pentane (C29H30P2). Bz: benzyl (C7H7). cod: cyclooctadiene (C8H12). cot: cyclooctatriene (C8H10). dce: dichloroethane (C2H4Cl2). dcype: 1,2bis(dicyclohexylphosphino)ethane (C26H48P2). dma: N,Ndimethylacetamide (C4H9ON). dmf: N,N-dimethylformamide (C3H7ON). dmfm: dimethyl fumarate (C6H8O4). dmso: dimethyl sulfoxide (C2H6OS). Cp: cyclopentadienyl (C5H5). Cp*: pentamethylcyclopentadienyl (C10H15). dppe: 1,2-bis(diphenylphosphino)ethane (C26H24P2). dppm: 1,2-bis(diphenylphosphino)methane (C25H22P2). dppp: 1,3-bis(diphenylphosphino)propane (C27H26P2). OTf: trifluorosulfonate (CF3SO3). ppn: bis(triphenylphosphine)iminium (C36H30NP2). py: pyridine (C5H4N). tbaf: tetrabutylammonium fluoride (C16H36NF). tbs: tributylsilyl (C12H27Si). S: 2-(trimethylsilyl)ethoxycarbonyl (C6H13O2Si). tfe: tetrafluoroethylene (C2F4). thf: tetrahydrofuran (C4H8O). Ts: p-toluenesulfonyl (C7H7O2S). ■ REFERENCES
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Catalytic Linear Cross-Dimerization
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