Tetrahedron Letters 52 (2011) 6869–6872

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Tetrahedron Letters

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Cobalt oxide supported gold nanoparticles as a stable and readily-preparedprecursor for the in situ generation of cobalt carbonyl like species

Akiyuki Hamasaki a,b, Akiko Muto a,b, Shingo Haraguchi a,b, Xiaohao Liu a,b, Takanori Sakakibara a,b,Takushi Yokoyama a,b, Makoto Tokunaga a,b,c,⇑a Department of Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japanb JST (Japan Science and Technology Corporation), CREST, Japanc International Research Center for Molecular Systems (IRCMS), Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan

a r t i c l e i n f o

Article history:Received 21 July 2011Revised 10 September 2011Accepted 15 September 2011Available online 21 September 2011

Keywords:Cobalt carbonylsPauson–Khand reactionAlkoxycarbonylationAlternative catalyst

0040-4039/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.tetlet.2011.09.067

⇑ Corresponding author. Tel./fax: +81 92 642 7528.E-mail address: mtok@chem.kyushu-univ.jp (M. T

a b s t r a c t

A treatment of cobalt oxide supported gold nanoparticles (Au/Co3O4) under syngas atmosphere effec-tively generated a cobalt carbonyl-like active species in the reaction vessel. The preparation of Au/Co3O4 was quite simple and the in situ generated cobalt species could be used as a stable and easy han-dling alternative for dicobalt octacarbonyl without bothersome purification prior to use. The reactions,which are sensitive to the purity of the dicobalt octacarbonyl, such as the alkoxycarbonylation of epox-ides and the Pauson–Khand reaction, smoothly progressed with Au/Co3O4.

� 2011 Elsevier Ltd. All rights reserved.

Cobalt carbonyls are one of the most versatile reagents amongmetal carbonyls. Dicobalt octacarbonyl1 (Co2(CO)8) in particularis broadly used as a catalyst for hydroformylation,2 carbonylationreactions,3 and amidocarbonylation.4 It is also employed as a stoi-chiometric reagent for the Nicholas reaction5 and the protection ofalkynes.6 However, Co2(CO)8 often requires sublimation and/orrecrystallization prior to use to obtain good catalytic activity andrepeatability because of decomposition during storage even below0 �C under an inert gas atmosphere due to the dissociation of CO.Furthermore, a less obvious problem is that a preparation ofCo2(CO)8 from Co(II) salt requires a harsh condition, for exampleCO/H2 pressure of 3500 psi (ca. 24 MPa), and a temperature of150 �C.7,8

Recently, we have reported a novel function of cobalt oxide sup-ported gold nanoparticles (Au/Co3O4) to provide Co2(CO)8-like spe-cies under a CO/H2 atmosphere.9a–c The active species were formedby the reduction of the support metal and by subsequent bindingto CO. The existence of Co(0) after the treatment of Au/Co3O4 underH2 was clearly observed using an XRD and a XANES spectra.9d Aunanoparticles played an important role for the first reduction stepto generate spillover hydrogen, therefore Co3O4 without Au did notshow any catalytic activity for the hydroformylation of olefins.9a,c

Au/Co3O4 can be readily prepared from commercial HAuCl4�4H2Oand Co(NO3)2�6H2O even on a large scale by simple mixing of a me-

ll rights reserved.

okunaga).

tal solution and a base solution at rt (co-precipitation method).9b,c

Since the obtained solid consisted of gold(0) and cobalt oxide, itwas fairly stable to air and moisture at least for several months.Au/Co3O4 worked as a recyclable heterogeneous catalyst in nonpo-lar solvents such as heptane.9a,c However, our initial investigationsproved that this heterogeneous system showed poor activity forthe alkoxycarbonylation of epoxides and the Pauson–Khand reac-tions (PKR). Therefore, we have attempted to use Au/Co3O4 as aprecursor of homogeneous Co2(CO)8-like species by employingresoluble polar solvents. This catalyst system was able to generatefresh active species continuously during the reaction, thus it wasconsummately ideal for the reactions sensitive to the purity ofCo2(CO)8.

O

3

Co2(CO)8 (5 mol%)3-hydroxypyridine(10 mol%)CO (4 MPa)

MeOH, THF 3

OH

OMe

O

1a 2afresh Co2(CO)8: 91% conv., 81% yieldaged Co2(CO)8: 70% conv., 37% yield

ð1Þ

Co2(CO)8 catalyzes the alkoxycarbonylation of epoxides to giveb-hydroxyesters. This was first reported in 196110a and several ex-tended works were found in recent years.10b–h,11 Although specificmention about sensitivity to the purity of Co2(CO)8 did not appear

Table 2Au/Co3O4 catalyzed alkoxycarbonylation using various epoxides and alcoholsa,b

RO

R

OH

OR'

O

11.4 wt% Au/Co3O4(0.6 mol% Au, 11 mol% Co)Pyrazole (3 mol%)CO (4 MPa)

1 2R'OH/THF (1:4, 5 mL)65 °C, 40 h

Entry Substrate R0OH Product Yieldc (%)

1 O MeOH 2b 58

2O

5MeOH 2c 79

3O

9MeOH 2d 45

4 O MeOH 2e 86

5

OMeOH 2f 5

6O

3EtOH 2g 82

7O

32-PrOH 2h 33

a The catalyst was pretreated under CO/H2 (3:1, 2 MPa) at 120 �C for 3 h in thereaction solvent prior to the reaction.

b The reaction was conducted on a 5 mmol scale.c GC yield.

6870 A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872

in the literatures, this reaction seemed to be greatly dependent onthe fineness of Co2(CO)8. We investigated the difference of catalyticactivity between a freshly sublimed Co2(CO)8 and an aged one byfollowing Jacobsen’s procedure10d (Eq. 1). A fresh catalyst gavethe result of 91% conversion and 81% yield. On the other hand,the catalytic activity decreased to 70% conversion and 37% yieldafter storage for several weeks at 4 �C under a N2 atmosphere.The result of the initial survey of the reaction conditions includingsolvent, additive, syngas ratio, temperature, and reaction time isshown in Table 1. Under the conditions of Table 1, entry 1, only2% of the desired b-hydroxyester was obtained along with a largeamount of nucleophilic ring opening ether products. Whereas anaddition of 3-hydroxypyridine was reported in the literature to im-prove the reaction efficiency,10c it was found to be ineffective forthe Au/Co3O4 catalyst system with a negligible improvement ofthe product yield (Table 1, entry 2). Recently, pyrazole was also re-ported as an additive,10f and it was employed for the reaction be-cause it worked better than 3-hydroxypyridine (Table 1, entry 3).A low yield of the desired product was caused by the formationof several by-products including ketones, acetals, and ethers.10f

While hydroformylation of epoxides could take place under a CO/H2 atmosphere,12 b-hydroxyaldehyde or reduced 1,3-diol productswere not detected in the reaction mixture. Since the most signifi-cant side reaction among them was an ether formation by a nucle-ophilic attack of MeOH, the amount of MeOH was restricted. As aresult, the decrease in MeOH suppressed the formation of byprod-ucts and gave a better yield of b-hydroxyesters (Table 1, entries 4and 5). Additionally, increasing the reaction time at a milder reac-tion temperature (65 �C) and a higher CO partial pressure effi-ciently improved the results (Table 1, entries 6–8). Pretreatingthe catalyst prior to each reaction was important for the efficientgeneration of the active species. The CO/H2 atmosphere seemedto be preferable to H2 since the active species exist at the initialstage of the reaction which avoids the side reactions irrespectiveof the catalysis (Table 1, entries 1–4 vs 5–8).

Next, we turned our attention to the substrate scope of the reac-tion. Although a slight difference of reactivity according to thechain length was observed, alkyl epoxides were smoothly reactedto give the desired b-hydroxyesters in moderate to good yields(Table 2, entries 1–4). Styrene oxide showed a low reactivity(Table 2, entry 5). As for alcohol nucleophiles, EtOH showed a

Table 1Optimization of reaction conditions for Au/Co3O4 catalyzed alkoxycarbonylation of epoxid

11.4 wt% Au/Co3O4(0.6 mol % Au, 11 moadditive (3 mol%)CO/H2 (4 MPa)

1a

O

3 65 °C, 40 h

Entry Pretreatmentb Additive

1c,d A None2c,d A 3-Hydroxypyridine (10 mol%)3c,d A Pyrazole4c,d A Pyrazole5c,e B Pyrazole6e B Pyrazole7e B Pyrazole8e B Pyrazole

a GC yield.b Pretreatment method: (A) The catalyst was pretreated under H2 (2 MPa) at 120 �C f

under CO/H2 (3:1, 2 MPa) at 120 �C for 3 h in the reaction solvent prior to the reaction.c The reaction was carried out at 80 �C for 20 h.d The reaction was conducted on a 2 mmol scale in 2 mL of the solvent.e The reaction was conducted on a 5 mmol scale in 5 mL of the solvent.

reactivity similar to MeOH (Table 2, entry 6). However, 2-PrOHgave a lower yield probably due to the steric problem (Table 2,entry 7).

The PKR is a formal [2+2+1] cyclization for providing cyclo-pentenone derivatives. Since this reaction is quite useful for con-structing complex molecular skeletons, it is incorporated in manynatural product syntheses.13 This reaction was originally reportedas a stoichiometric reaction using Co2(CO)8,14 and catalytic ver-sions were developed by employing various additives15 or highly

es

l% Co)

2a

OH

OMe

O

3

Solvent CO:H2 Yielda (%)

MeOH 3:1 2MeOH 3:1 3MeOH 3:1 17MeOH/THF (1:1) 3:1 26MeOH/THF (1:4) 3:1 33MeOH/THF (1:4) 3:1 48MeOH/THF (1:4) 7:1 59MeOH/THF (1:4) CO only 68

or 2 h in the reaction solvent prior to the reaction; (B) The catalyst was pretreated

Table 3Au/Co3O4 catalyzed intramolecular PKR of various substratesa

YR1

Y O

R111.4 wt% Au/Co3O4(0.6 mol% Au, 11 mol% Co)CO/H2 (3:1, 4 MPa)

DME (2 mL)100 °C, 60 h

3 (0.5 mmol) 4

R2

R2

Entry Substrate Product Yieldb (%)

1

PhEtO2C

EtO2C 3a EtO2C

EtO2CO

Ph

4a 81

2EtO2C

EtO2C3b

EtO2C

EtO2CO 4b 91

3

MeEtO2C

EtO2C 3c EtO2C

EtO2CO

Me

4c 81

4

PhEtO2C

EtO2CMe

3dEtO2C

EtO2CO

Ph

Me

4d 89

5 TsNPh

3eTsN O

Ph4e 96

6c

OPh

3fO O

Ph4f

15d

7e 30

a The catalyst was pretreated under H2 (2 MPa) at 100 �C for 3 h in DME prior to each run.b Isolated yield.c The reaction was conducted on a 1 mmol scale at 100 �C for 65 h.d The starting material 3f was recovered at 79%.e The reaction was conducted at 160 �C for 20 h.

A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872 6871

pure Co2(CO)816 from the middle of the 1990s. Other homogeneous

Co sources also provided effective catalyst systems.17 Alongsidethe development of homogeneous catalysts, heterogeneouscatalysts including Co on silica,18a Co nanoparticles,18b,c Ru/Coheterobimetallic nanoparticles,18d Rh/Co heterobimetallic nano-particles,18e,f and Pd/Co heterobimetallic nanoparticles18g havebeen actively investigated by Chung in recent years. Under thesesituations, we believed Au/Co3O4 would catalyze the PKR basedon a unique mechanism which was different from the above cata-lyst systems.

As expected, several 1,6-enyne substrates were effectivelytransformed into cyclopentenone derivatives under the optimalcondition (Table 3). A substitution of the substrates on alkyneand alkene moieties generally decreases the reactivity due to a ste-ric reason.15b A non-substituted substrate 3b indeed gave a slightlybetter yield among malonate-tethered substrates (Table 3, entry 2),but other compounds with methyl and/or phenyl substituents alsoshowed satisfactory reactivities as well (Table 3, entries 1, 3 and 4).A tether like malonate or N-tosylate was preferable to locate tworeaction sites closer together (Table 3, entry 5). An ether tetherwas also expected to have a similar character, but the reactionwas very sluggish at 100 �C and the desired product 4f was ob-tained in 15% with 79% of intact 3f (Table 3, entry 6). Under aharsher reaction temperature, only slight improvement of the yieldwas observed while all the substrate was consumed in 20 h (Table3, entry 7). An explanation for this result is that the in situ forma-tion of Co-hydride species led to cleavage of the ether moiety byhydrogenolysis, especially under high temperature.19

An intermolecular PKR employing a reactive alkene and termi-nal alkynes was attempted. An equimolar of norbornene (5) and1-hexyne (6a) were treated with Au/Co3O4 (0.6 mol % Au and11 mol % Co) and 4 MPa of CO/H2 (3:1) at 100 �C for 61 h to afford7a in 62% isolated yield (Eq. 2). The reaction using phenylacetylene(6b) in place of 6a resulted in a steep decrease in the yield.

R

+

11.4 wt% Au/Co3O4(0.6 mol% Au, 11 mol% Co)CO/H2 (3:1, 4 MPa)

DME (2 mL)100 °C

7a : R = n-Bu7b : R = Ph

62% (61 h)27% (60 h)

50.5 mmol

6a : R = n-Bu6b : R = Ph

0.5 mmol

O

R

ð2Þ

The two reactions in this study required dissolution of the ac-tive species for a smooth progress of the reactions. It was a com-pletely different concept from a common heterogeneous catalysissince Au/Co3O4 was used as a precursor of the actual active species.A treatment of the catalyst in MeOH/THF under H2 at 120 �C for 3 hgave a colorless suspension (Fig. S1, b). On the other hand, thesame treatment under CO/H2 resulted in a dark brown solutionindicating the formation of a CO-bound active species (Fig. S1, c).A polar solvent was preferable for these reactions and this solventeffect might be explained by the solubility of the cobalt species.Interestingly, Co2(CO)8 is soluble in common organic solvents,

6872 A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872

including nonpolar solvents.1 This fact indicated the possibilitythat the in situ generated species and Co2(CO)8 were not exactlyidentical. An IR analysis of the dark brown solution after the pre-treatment under CO/H2 showed a characteristic peak at 1965 cm�1 indicating the presence of CO-bound species. However, the peakdid not correspond to Co2(CO)8, HCo(CO)4, or Co4(CO)12.20 Anatomic absorption spectrophotometry of the post-reaction mixtureof the PKR revealed that 38% of the Co atoms were leached fromAu/Co3O4. This result implied that it would be difficult to recyclethe catalyst since a spontaneous reassembly of the proper Au/Co3O4 structure from the solution state is difficult to achieve.

In summary, the alkoxycarbonylation of epoxides and the Pau-son–Khand reaction effectively proceeded using Au/Co3O4 as a pre-cursor of the Co active species. Au/Co3O4 can be readily obtained bysimple mixing of the two solutions and is fairly stable to air andmoisture even at ambient temperature. The largest feature of thiscatalyst system was the continuous supply of fresh active speciesduring the reaction. Thus, it is quite effective for the reactions thatare sensitive to the purity of Co2(CO)8. Additional studies to utilizethis catalyst system are currently underway.

Acknowledgments

We gratefully acknowledged to the Institute for MaterialsChemistry and Engineering (IMCE), Kyushu University for the MSspectrum measurements. This work was supported by a Grant-in-Aid for the Global-COE program, ‘Science for Future MolecularSystems’, Scientific Research (A) (No. 20245014) (to M.T.), and Re-search Activity Start-up (No. 21850023) (to A.H.) from the Ministryof Education, Culture, Science, Sports and Technology of Japan.

Supplementary data

Supplementary data (experimental procedures and copies of 1Hand 13C NMR spectra for all products) associated with this Lettercan be found, in the online version, at doi:10.1016/j.tetlet.2011.09.067.

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