Carbonylative cyclization of alkynes using cobalt carbonyl species prepared via reduction of CoBr2 with Zn under a carbon monoxide atmosphere

Article abstract of DOI:10.1021/om990534c

Alkyne-Co₂(CO)₆ complexes prepared in situ using CoBr₂, Zn, and CO give the corresponding cycloentenones or cyclopentadienones on heating in toluene or in CH₂Cl₂ at 25°C in the presence of DMSO or amines.

Full text of DOI:10.1021/om990534c

Organometallics 1999, 18, 5709-5712
5709
Notes
Ca r bon yla tive Cycliza tion of Alk yn es Usin g Coba lt
Ca r bon yl Sp ecies P r ep a r ed via Red u ction of CoBr 2 w ith
Zn u n d er a Ca r bon Mon oxid e Atm osp h er e
Thotapally Rajesh and Mariappan Periasamy*
School of Chemistry, University of Hyderabad, Hyderabad-500 046, India
Received J uly 12, 1999
Sch em e 1
Summary: Alkyne-Co2(CO)6 complexes prepared in situ
using CoBr2, Zn, and CO give the corresponding cyclo-
pentenones or cyclopentadienones on heating in toluene
or in CH2Cl2 at 25 °C in the presence of DMSO or
amines.
In recent years, metal-mediated cyclization reactions
have been increasingly used in organic synthesis.1 One
of the most widely used organometallic reactions is the
Pauson-Khand reaction of alkyne-Co2(CO)6 com-
plexes.2 Previously, we reported the generation of
alkyne-Co2(CO)6 complexes in situ using CoBr2, Zn, and
CO in THF, CH2Cl2/t-BuOH, and toluene/t-BuOH sol-
vent systems for applications in Pauson-Khand reac-
tions.3,4 Herein, we report the results of carbonylative
cyclization of the alkynes without using externally
added olefins.
Sch em e 2
We have observed that the alkyne-Co2(CO)6 com-
plexes, prepared in situ in toluene/t-BuOH, upon heat-
ing at 110 °C give the corresponding substituted cyclo-
pentenones in moderate yields (Scheme 1). The results
are summarized in Table 1. Two regioisomers were
obtained. The regiochemistry of the major product is in
line with that of cyclopentenones obtained in the Pau-
son-Khand reaction. However, whereas the 3-substi-
tuted cyclopentenones are not formed in the Pauson-
Khand reaction, they are obtained in minor amounts
here (Table 1).
in minor amounts (5%). The formation of cyclopenten-
ones may be rationalized by the reaction of alkyne-Co2-
(CO)6 complexes with the olefinic intermediates, which
might have formed through the cleavage of the alkyne
moiety by the HCo(CO)4 species formed in situ in the
medium. Previously, similar results were observed in
the reaction of alkyne-Co2(CO)6 complexes with CF3-
COOH in THF at 80-90 °C for 24 h.4b
We have also examined the reactivity of alkynes
containing trimethylsilyl substituents. Such trimethyl-
silyl-substituted alkynes have been used previously to
change the regioselectivity in Pauson-Khand reac-
tions.5 The formation of corresponding substituted cy-
clopentadienones is observed under the present condi-
tions (Scheme 2).
In the case of the reaction with PhCtCH, 2,5-
diphenylcyclopentenone was obtained as a major prod-
uct; the trimerized product of PhCtCH was also formed
(1) (a) Schore, N. E. Chem. Rev. 1988, 88, 1081. (b) Seebach, D.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1320. (c) Lautens, M.; Klute,
W.; Tam, W. Chem. Rev. 1996, 96, 49. (d) Fruhauf, H. W. Chem. Rev.
1997, 97, 523.
(2) (a) Pauson, P. L.; Khand, I. U. Ann. N. Y. Acad. Sci. 1977, 295,
2. (b) Pauson, P. L. Tetrahedron 1985, 41, 5855. (c) Pauson, P. L. In
Organometallics in Organic Synthesis; de Meijiere, A., tom Dieck, H.,
Eds.; Springer-Verlag: Berlin, 1988; p 233. (d) Schore, N. E. Chem.
Rev. 1988, 88, 1085. (e) Geis, O.; Schmalz, H. G. Angew. Chem., Int.
Ed. 1998, 37, 911.
(3) (a) Devasagayaraj, A.; Periasamy, M. Tetrahedron Lett. 1989,
30, 595. (b) Devasagayaraj, A.; Achyutha Rao, S.; Periasamy, M. J .
Organomet. Chem. 1991, 403, 387. (c) Rajesh, T.; Periasamy, M.
Tetrahedron Lett. 1998, 39, 117. (d) Rajesh, T.; Periasamy, M.
Tetrahedron Lett. 1999, 40, 817.
(4) (a) Periasamy, M.; Reddy, M. R.; Devasagayaraj, A. Tetrahedron
1994, 50, 6955. (b) Rao, M. L. N.; Periasamy, M. Organometallics 1996,
15, 442. (c) Rao, M. L. N.; Periasamy, M. J . Organomet. Chem. 1997,
532, 143. (d) Periasamy, M.; Rao, M. L. N.; Rajesh, T. J . Organomet.
Chem. 1998, 571, 183.
The formation of cyclopentadienones was reported in
the reaction of alkynes with CpCo(CO)2 and Fe(CO)5.7
6
Recently, a direct synthesis of cyclopentadienones was
reported using Co2(CO)8 through reaction of cobalt-
complexed alkynylsilanes with free alkynylsilanes.8 The
(5) Dehmlow, E. V.; Winterfeldt, A.; Pickardt, J . J . Organomet.
Chem. 1989, 363, 223.
(6) (a) Gesing, E. R. F.; Tane, J . P.; Vollhardt, K. P. C. Angew. Chem.
1980, 92, 1057. (b) Gesing, E. R. F.; Tane, J . P.; Vollhardt, K. P. C.
Angew. Chem., Int. Ed. Engl. 1980, 19, 1023.
(7) Knolker, H. J .; Heber, J .; Mahler, C. H. Synlett 1992, 1002 and
references therein.
(8) Shibata, T.; Ohta, T.; Soai, K. Tetrahedron Lett. 1998, 39, 5785.
10.1021/om990534c CCC: $18.00 © 1999 American Chemical Society
Publication on Web 12/02/1999

5710 Organometallics, Vol. 18, No. 26, 1999
Notes
Ta ble 2. Rea ction s of (RCtCSiMe3)Co2(CO)6
Ta ble 1. Rea ction s of (RCtCR′)Co2(CO)6
Com p lexes w ith t-Bu OH
Com p lexes in Tolu en e
a
All reactions were carried out using CoBr2 (20 mmol), Zn (20
b
mmol), and alkyne (10 mmol). The products were identified by
IR, H NMR, and 13C NMR spectral data and elemental analysis.
1
c Yields reported here are of products separated from chromatog-
raphy on a silica gel column using hexane/ethyl acetate as eluent.
a
All reactions were carried out using CoBr2 (20 mmol), Zn (20
b
mmol), and alkyne (10 mmol). The products were identified by
IR, 1H NMR, and 13C NMR spectral data reported in the literature.
c Yields reported here are of products separated from chromatog-
raphy on a silica gel column using hexane/ethyl acetate as eluent.
present method (Scheme 2) using the simple benchtop
chemicals CoBr2, Zn, and CO in toluene/t-BuOH is
advantageous for such synthetic applications. The re-
sults are summarized in Table 2. We have obtained
symmetrical cyclopentadienones as major products in
all cases except in the case of phenyl-substituted alky-
nylsilane. In this case, both 2,5-diphenyl- and 3,5-
diphenyl-substituted isomers were obtained. The posi-
tions of the phenyl and silyl groups of the 3,5-diphenyl-
substituted cyclopentadienone 6a were also confirmed
by X-ray crystal structure analysis (Figure 1).
F igu r e 1. Ortep diagram of 6a .
of a trimethylsilyl-substituted alkyne-Co2(CO)6 com-
plex to produce the corresponding alkenylsilane inter-
mediate is expected to be difficult due to steric reasons.
However, it may be of interest to note that the
(RCtCSiMe3)Co2(CO)6 complexes are readily cleaved to
give alkenylsilanes on heating with CF3COOH.4b
The cyclopentenones and cyclopentadienones (Tables
1 and 2) were obtained from the alkyne-Co2(CO)6
complexes under heating conditions. Previously, we
have reported the use of certain amines and amides as
promoters for carrying out the Pauson-Khand reaction
Presumably, in the case of trimethylsilyl-substituted
alkynes, the cyclopentadienones are obtained instead
of cyclopentenones (Scheme 1), as the reductive cleavage

Notes
Organometallics, Vol. 18, No. 26, 1999 5711
in the hot-air oven at 150 °C for 5-6 h and further dried at
150 °C for 4 h under vacuum. Zn dust was activated by treating
commercial Zn dust with 1% H2SO4, washing with water and
acetone, and drying at 150 °C for 4 h under vacuum. Carbon
monoxide was generated by dropwise addition of formic acid
(98%) to concentrated H2SO4 (96%) at 90 °C using an ap-
paratus recommended for use in the carbonylation of orga-
noboranes.13
Sch em e 3
Gen er a l Meth od s for Syn th esis of Cyclop en ten on es
a n d Cyclop en ta d ien on es. The cobalt carbonyl species was
prepared by reducing CoBr2 (4.36 g, 20 mmol) with Zn (1.43
g, 20 mmol) and alkyne/alkynylsilane (10 mmol) in toluene
(50 mL)/t-BuOH (1.5 mL) while bubbling CO with stirring for
5 h at 25 °C. An additional amount of t-BuOH (2 mL) was
added, and the contents were stirred at 110 °C for 10 h. The
cobalt carbonyl species completely decomposed during this
time. The contents were brought to room temperature. Diethyl
ether (25 mL) was added and the mixture washed successively
with water (20 mL) and brine solution (10 mL). The combined
organic extracts were dried over anhydrous MgSO4. The
solvent was removed, and the residue was subjected to
chromatography on silica gel using hexane/ethyl acetate as
eluent. The structural assignments are based on IR, 1H NMR,
13C NMR, and mass spectral data and elemental analysis.
Spectral data obtained for the cyclopentadienones are sum-
marized below.
at room temperature.3c Pauson and co-workers have also
reported the use of DMSO for similar purposes.9 Ac-
cordingly, we have examined the effect of these promot-
ers on the reaction of alkyne-Co2(CO)6 complexes
without using an added olefin. In all cases, the dicyclo-
pentadienones 11-13 were obtained, along with uni-
dentified cobalt carbonyl complexes. The results are
summarized in Scheme 3. Previously, the formation of
dicyclopentadienones was reported in the reaction of
Co2(CO)8 in DME with a 1/1 mixture of acetylene and
CO at 65 °C for 5 days under 1 atm pressure.10
The formation of dicyclopentadienones (Scheme 3)
would most probably occur through the corresponding
cyclopentadienone intermediates. However, efforts to
trap the cyclopentadienone intermediate by carrying out
the reaction in the presence of dienophiles such as
maleic anhydride, crotonaldehyde, and dimethyl fuma-
rate were unsuccessful. In all cases, only the dicyclo-
pentadienones were isolated.
6a : yield 30% (1.12 g); IR (KBr) ν 1687, 1440 cm-1; 1H NMR
(CDCl3) δ -0.3 (s, 9H), 0.0 (s, 9H) 7.2-7.4 (m, 10H); 13C NMR
(CDCl3) δ -0.20, 0.39, 125.2, 127.2, 127.7, 128.0, 128.1, 128.3,
129.9, 133.7, 139.6, 145.1, 155.0, 176.6, 206.6.
6b: yield 15% (0.56 g); IR (KBr) ν 1685, 1440 cm-1; 1H NMR
(CDCl3) δ 0.3 (s, 9H), 7.2-7.4 (m, 5H); 13C NMR (CDCl3) δ
-0.11, 127.4, 128.0, 128.2, 135.7, 171.1, 209.9.
7: yield 40% (1.45 g); IR (neat) ν 1684, 1466 cm-1; 1H NMR
(CDCl3) δ 0.2 (s, 9H), 1-2.5 (m, 11H); 13C NMR (CDCl3) δ
0.060, 13.9, 22.4, 28.3, 30.4, 32.3, 128.2, 172.8, 210.8; MS m/z
366. Anal. Calcd for C21H40Si2O: C, 69.23; H, 10.98. Found:
C, 69.28; H, 11.00.
8: yield 38% (1.48 g); IR (neat) ν 1684, 1466 cm-1; 1H NMR
(CDCl3) δ 0.2 (s, 9H), 1-2.5 (m, 13H); 13C NMR (CDCl3) δ
0.015, 13.9, 22.5, 28.3, 29.7, 30.6, 31.5, 128.2, 172.6, 210.7; MS
m/z 392. Anal. Calcd for C23H44Si2O: C, 70.40; H, 11.20.
Found: C, 70.45; H, 11.26.
9: yield 38% (1.70 g); IR (neat) ν 1684, 1466 cm-1; 1H NMR
(CDCl3) δ 0.2 (s, 9H), 1-2.5 (m, 17H); 13C NMR (CDCl3) δ
0.059, 14.0, 22.6, 28.4, 29.1, 29.3, 30.1, 30.7, 31.8, 125.3, 172.3,
210; MS m/z 449. Anal. Calcd for C27H52Si2O: C, 72.30; 11.60.
Found: C, 72.35; H, 11.65.
10: yield 35% (1.76 g); IR (neat) ν 1684, 1466 cm-1; 1H NMR
(CDCl3) δ 0.2 (s, 9H), 1-2.5 (m, 21H); 13C NMR (CDCl3) δ
0.112, 14.0, 22.7, 28.4, 28.8, 29.1, 29.3, 29.5, 30.1, 30.8, 31.9,
125.3, 172.7, 210.9.
Rea ction of Alk yn e-Coba lt Ca r bon yl Com p lexes in
th e P r esen ce of DMSO, DMF , a n d TMEDA in CH2Cl2/t-
Bu OH. The cobalt carbonyl species was prepared by reducing
CoBr2 (4.36 g, 20 mmol) with Zn (1.43 g, 20 mmol) and alkyne
(10 mmol) in CH2Cl2 (50 mL)/t-BuOH (1.5 mL) while bubbling
CO with stirring for 5 h at 25 °C. The promoter (3 equiv) was
added, and the contents were stirred for 5 h at 25 °C. The
mixture was washed successively with dilute HCl (20 mL),
water (2 × 20 mL), and brine solution (10 mL). The organic
extracts were dried over anhydrous MgSO4. The solvent was
removed, and the residue was subjected to chromatography
on a silica gel column using hexane/ethyl acetate as eluent.
The structural assignments are based on IR, 1H NMR, 13C
NMR, DEPT experiments, mass spectral data, and elemental
analysis. Spectral data obtained for the dicyclopentadienones
are summarized below.
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were carried out under an
atmosphere of predried nitrogen. All transfers and manipula-
tion of compounds were carried out under a nitrogen atmo-
sphere. Toluene was distilled over sodium-benzophenone
ketyl. All alkynes and the silyl derivatives were prepared by
following reported procedures.12 1H and 13C NMR spectra were
recorded on a Bruker AC-200 spectrometer with chloroform-d
as solvent and TMS as reference (δ 0 ppm). All IR spectra were
recorded on a J ASCO FT-5300 instrument with polystyrene
as reference. Elemental analysis was carried out on a Perkin-
Elmer 240C elemental analyzer. Column chromatography was
carried out using Aceme silica gel (100-200 mesh). Anhydrous
CoBr2 was prepared from the hydrated complex by keeping it
(9) Chung, Y. K.; Lee, B. Y.; J eong, N.; Hudecek, M.; Pauson, P. L.
Organometallics 1993, 12, 220.
(10) Schore, N. E.; La Belle, B. E.; Knudsen, M. J .; Xu, X.-J . J .
Organomet. Chem. 1984, 272, 435.
(11) All reactions were carried out using CoBr2 (20 mmol), Zn (20
mmol), and alkyne (10 mmol). The products were identified by IR, 1H
NMR, and 13C NMR spectral data and elemental analysis. Yields
reported here are for products separated from chromatography on silica
gel column using hexane/ethyl acetate as eluent. In all reactions, some
amounts of unreacted cobalt carbonyl complexes were recovered.
(12) Cetini, G.; Gambino, O.; Rosetti, R.; Sappa, E. J . Organomet.
Chem. 1967, 8, 147.
(13) Sternberg, H. W.; Greenfield, H.; Friedel, R. A.; Wotiz, J .;
Markby, R.; Wender, I. J . Am. Chem. Soc. 1954, 76, 1457.

5712 Organometallics, Vol. 18, No. 26, 1999
Notes
11: yield 20% (0.88 g); IR (neat) ν 1774, 1701, 1464 cm-1
;
30.3, 31.8, 51.0, 55.6, 58.2, 61.8, 132.1, 133.0, 153.1, 154.6,
204.8, 208.6. Anal. Calcd for C42H72O2: C, 82.8; H, 11.8.
Found: C, 83.2; H, 11.8.
1H NMR (CDCl3) δ 0.5-2.5 (m), 3.1 (s, 1H), 5.9 (d, J ) 20 Hz,
1H), 6.1 (d, J ) 20 Hz, 1H), 6.9 (s, 1H); 13C NMR (CDCl3) δ
13.9, 22.4, 23.0, 24.9, 25.2, 25.4, 25.5, 27.1, 27.5, 30.2, 31.4,
32.3, 32.5, 51.0, 55.6, 58.2, 61.8, 132.1, 133.0, 153.2, 154.6,
204.8, 208.6; MS m/z 412 (-CO).
Ack n ow led gm en t. We are grateful to the UGC and
DST (New Delhi, India) for financial support. Also, we
thank the UGC, New Delhi, India, for support under
the Special Assistance Program. X-ray analysis was
carried out using the National Single Crystal X-ray
Facility, School of Chemistry, University of Hyderabad,
funded by the DST, New Delhi, India.
12: yield 18% (0.89 g); IR (neat) ν 1772, 1701, 1464 cm-1
;
1H NMR (CDCl3) δ 0.5-2.5 (m), 3.1 (s, 1H), 5.9 (d, J ) 20 Hz,
1H), 6.1 (d, J ) 20 Hz, 1H), 6.9 (s, 1H); 13C NMR (CDCl3) δ
14.03 (-CH3), 22.5, 23.1, 24.9, 25.5, 25.7, 25.8, 27.2, 27.8, 28.9,
29.8, 30.0, 30.3, 31.6 (-CH2), 51.0 (-CH), 55.6, 58.3, 61.9
(quaternary) 132.1, 133.0, 153.2 (-CH), 154.6 (quaternary),
204.9, 208.7 (CO); MS m/z 469 (-CO).
Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectrum
of the compounds 1-13. This material is available free of
charge via the Internet at http://pubs.acs.org.
13: yield 16% (0.97 g); IR (neat) ν 1772, 1703, 1464 cm-1
;
1H NMR (CDCl3) δ 0.5-2.5 (m), 3.1 (s, 1H), 5.9 (d, J ) 20 Hz,
1H), 6.1 (d, J ) 20 Hz, 1H), 6.9 (s, 1H); 13C NMR (CDCl3) δ
14.0, 22.6, 23.0, 24.9, 25.5, 25.7, 25.8, 27.2, 27.8, 29.2, 30.1,
OM990534C