A silicon-tethered allenic Pauson-Khand reaction

Article abstract of DOI:10.1016/S0040-4039(02)00633-0

This manuscript describes a silicon-tethered Pauson-Khand reaction and subsequent cleavage of the resulting vinyl silane to afford functionalized alkylidene cyclopentenones. Altering the reaction conditions affords a selective reaction with either π-bond

Full text of DOI:10.1016/S0040-4039(02)00633-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3735–3738
A silicon-tethered allenic Pauson–Khand reaction
Kay M. Brummond,a,b,* Peter C. Sill,a,b Brenden Rickardsb and Steven J. Geiba,†
aDepartment of Chemistry, University of Pittsburgh, Pittsburgh, PA 15206, USA
bDepartment of Chemistry, West Virginia University, Morgantown, WV 26506, USA
Received 15 August 2001; revised 5 March 2002; accepted 21 March 2002
Abstract—This manuscript describes a silicon-tethered Pauson–Khand reaction and subsequent cleavage of the resulting vinyl
silane to afford functionalized alkylidene cyclopentenones. Altering the reaction conditions affords a selective reaction with either
p-bond of the allene. © 2002 Published by Elsevier Science Ltd.
The Pauson–Khand reaction is a formal [2+2+1]
cycloaddition involving an alkene, an alkyne and car-
bon monoxide to afford a cyclopentenone.1 Despite
many advances made in the Pauson–Khand reaction in
the past three decades, the intermolecular Pauson–
Khand reaction is still largely restricted to the use of
strained olefins and is not considered regioselective with
respect to the alkene.2 Therefore, using this method to
access monocyclic, cyclopentenone natural products is
limiting.3 In one notable exception, Krafft has insight-
fully directed an intermolecular Pauson–Khand reac-
tion by using a coordinating heteroatom tethered to the
alkene.4 Unstrained olefins have been used successfully
in the intramolecular version of the Pauson–Khand
reaction and react with high regioselectivity. One strat-
egy to access monocyclic compounds involves the
For example, researchers have found that treatment of
the silyl ether 1 to dicobaltoctacarbonyl and N-mor-
pholine N-oxide gave none of the expected cycloadduct
2 but afforded cyclooctadiene analog 3 in low to mod-
erate yields based upon the substitution pattern (Eq.
(1)).8
(1)
Whitby has shown that 1,6- and 1,7-enynes incorporat-
ing a silicon–nitrogen and silicon–oxygen tethers,
respectively, can be cyclized using the Negishi cycliza-
tion protocol.9 The enynes possessing silicon–nitrogen
tethers cyclized to afford cyclopentenones, but subse-
quent reactions resulted in decomposition. The enynes
possessing silicon–oxygen tethers formed zirconacycles
but CO insertion could not be effected.
removal or ring opening of
a tether once the
intramolecular cycloaddition has been effected. To
date, the tethers have heteroatoms such as sulfur, nitro-
gen or oxygen. Smit was the first to show that the
cobalt complex of an allylpropargyl ether undergoes
cyclization to afford a hydrogenolyzed product under
‘dry’ alumina conditions.5
As part of our studies on transition metal mediated
allenic Pauson–Khand reactions10 we too have explored
the feasibility of the silicon–tethered Pauson–Khand
reaction (Eq. (2)). Initially, a dimethylsilyl ether (R=
CH3) was examined due to the ease in which the tether
could be hydrolytically cleaved after cyclization. The
preparation of these differentially substituted silyl ether
Pauson–Khand reaction precursors is straightforward.11
Addition of dichlorodimethylsilane to bis(dimethyl-
amino)dimethyl silane in THF at room temperature
affords the (dimethylamino)chlorodimethyl silane.12
This compound is not isolated but sequentially treated
with ethynyl magnesium bromide then 3-butenol to
afford the desired Pauson–Khand reaction precursor.
We are interested in developing a silicon-tethered Pau-
son–Khand reaction based upon the premise that the
tether could be easily cleaved and the resulting cycload-
ducts
used
to
synthesize
cyclopentenone
prostaglandins.6 Silicon tethers are certainly not a new
idea7 and application of this omnipresent strategy has
even been attempted with the Pauson–Khand reaction.
* Corresponding author. Tel.: (412) 624-1955; fax: (412) 624-8611;
e-mail: kbrummon@pitt.edu
† Crystal structure availability. Tel.: (412) 624-1131; e-mail: geib+
@pitt.edu
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)00633-0

3736
K. M. Brummond et al. / Tetrahedron Letters 43 (2002) 3735–3738
R=Ph, 14) also provides good yields of the corre-
sponding cycloadducts 21, 22, and 23. A longer alkyl
chain on allene 16 affords a 55% yield of 24.
[Rh(CO)2Cl]2, unlike Mo(CO)6 appears to be tolerant
of substitution on the terminus of the alkyne.
O
X
O
O
Si
Si
(2)
H
R
R
R
R
H
4
5
We are very interested in applying this new method to
the synthesis of the cyclopentenone prostaglandin, 15-
deoxy-D12,14 PGJ2. Conditions to cleave the resulting
vinyl silane were investigated (Scheme 1). All attempts
to cleave the vinyl silane of enone 9-Z to give cyclopen-
tenone 25 led to nucleophilic addition to the endocyclic
olefin of the enone to give either 26 or 27 or decompo-
sition. Thus, direct desilylation of enone 9-Z was
judged as untenable. Moderation of the electrophilicity
of 9-Z could be accomplished by reduction of the
carbonyl. Alcohol 28 was synthesized from 9-Z as a
single diastereomer using the Luche protocol. Treat-
ment of 28 with tetra-n-butyl ammonium fluoride
afforded the desired ring opened product 29 in 70%
yield.18 To demonstrate versatility and show this
method provides a suitable handle for the ultimate
conversion to the desired prostaglandin skeleton,
silanol 29 was subjected to a Tamao–Kumada oxida-
tion protocol to afford the acid labile diol 30 in 78%
yield.19 Chromatography of compound 30 was effected
using silica gel treated with triethylamine prior to pack-
ing. Triethylamine was also used in the elution mixture.
If this precaution was not taken diol 30 was converted
quantitatively to a bicyclic ether via a SN2% addition of
the primary alcohol onto the protonated allylic alcohol.
All attempts to effect a silicon tethered Pauson–Khand
reaction with the cobalt complex of this silyl ether 4
gave either decomplexation or decomposition. Replace-
ment of the two methyl groups on the silane with
phenyl or tert-butyl groups gave similar results. Treat-
ment of these substrates with Mo(CO)6 gave either no
reaction or decomposition products.13 Replacement of
the alkene moiety with an allene did give the Pauson–
Khand cycloadduct (6“7) but in consistently low yields
(Eq. (3)).
CH3
CH3
Mo(CO)6, DMSO
(3)
O
O
O
15-25%
Si
Si
H
Ph
Ph
H
Ph
Ph
6
7
A solution to the instability problem was found by
replacing the more labile silyl ether tether with a silyl
carbon tether (attempts to effect the Pauson–Khand
cyclization of a silicon-tethered alkynyl alkene substrate
were still unsuccessful). Replacing the alkene with an
allene group did provide the desired cycloadduct, pre-
sumably due to the increased reactivity of the allene.
Treatment of compound 814 with molybdenum hexacar-
bonyl and DMSO in refluxing toluene affords only 9 in
64% yield as a 1:2 mixture of E:Z isomers (Eq. (4)).15
Table 1. Silicon-tethered Pauson–Khand reaction
Substrate
Product
Conditions
Yield
R1
CH3
H
CH3
O
R1
O
H
•
Mo(CO)6, DMSO
64%
(4)
Si
Si
Ph
Si
Ph
Si
Ph
Ph
R
Ph
Ph
8
Ph
R
Ph
9
9, R=H, R1=CH3
8, R=H, R1=CH3
A
A
A
A
A
64%
0%
0%
0%
36%
11, R=Bu, R1=CH3
13, R=TMS, R1=CH3
15, R=Ph, R1=CH3
17, R=H, R1=C5H11
We have shown that the E:Z ratio can be increased
marginally (1:4) by using mesitylene molybdenum tri-
carbonyl.16 Both of these reaction conditions give E:Z
selectivity opposite of that observed in the all carbon
tether, where the E-isomer predominates.10 Moreover,
if the hydrogen on the terminus of the alkyne is
replaced with a butyl 10, trimethylsilyl 12 or phenyl 14
group, no Pauson–Khand cycloadduct is observed and
only starting material was recovered when treated with
Mo(CO)6 (Table 1). Substituting a longer alkyl chain
on allene 16 gives only a 36% yield of 17 in 1:2.5, E:Z
ratio. Alkynyl allene 18 was prepared and readily
underwent the allenic Pauson–Khand cycloaddition to
afford the a-methylene cyclopentenone 19 in 48% yield
using Mo(CO)6. Conditions to effect the Pauson–
Khand reaction in higher yields are currently being
investigated. These investigation led to the discovery
that treatment of 8 with 5 mol% [Rh(CO)2Cl]2 led to
exclusive formation of the 4-alkylidene cyclopentenone
20 in 64% yield.17 Changing the substituent on the
terminus of the alkyne (R=n-Bu, 10, R=TMS, 12, and
10, R=Bu, R1=CH3
12, R=TMS, R1=CH3
14, R=Ph, R1=CH3
16, R=H, R1=C5H11
O
Si
Ph
Si
Ph
Ph
H
H
Ph
48%
A
18
19
R1
R1
O
Si
Si
Ph
Ph
Ph
R
R
Ph
20, R=H, R1=CH3
21, R=Bu, R1=CH3
22, R=TMS, R1=CH3
23, R=Ph, R1=CH3
24, R=H, R1=C5H11
8, R=H, R1=CH3
B
64%
75%
74%
51%
55%
10, R=Bu, R1=CH3
12, R=TMS, R1=CH3
14, R=Ph, R1=CH3
16, R=H, R1=C5H11
B
B
B
B
Conditions A: 1. 2 equiv. Mo(CO)6, DMSO, toluene, 90 °C;
Conditions B: 5 mol% [Rh(CO)2Cl]2, CO (1 atm), toluene, 90 °C

K. M. Brummond et al. / Tetrahedron Letters 43 (2002) 3735–3738
3737
References
H
H
CH3
O
CH3
O
1. For reviews, see: (a) Brummond, K. M.; Kent, J. L.
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D. Synlett 2000, 1415; (b) Sugihara, T.; Yamada, M.;
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X
Ph
Si
Si
Ph
X
Ph
Ph
9Z
25
a
H
CH3
O
H
CH3
Si
Ph
Ph
OH
X
Si
Ph
Ph
26, X=F
27, X=OH
28
b
H
H
CH3
CH3
c
HO
OH
Ph
OH
Si
Ph
OH
30
29
Scheme 1. Cleavage of the silicon tether. (a) NaBH4, CeCl3,
CH3OH, 0°C, 62%; (b) nBu4NF, THF, HMPA, 1 h, 0°C,
70%; (c) KF, NaHCO3, H2O2, THF, CH3OH, 80%.
To test the applicability of this cyclization strategy for
the synthesis of 15-deoxy-D12,14-PGJ2, compound 31
was prepared and subjected to the molybdenum-medi-
ated conditions (Eq. (5)). Compound 32 was obtained
in 40% yield with an E:Z ratio of 1:3. Treatment of this
mixture of isomers to light, gives 100% conversion to a
1:1, E:Z isomeric mixture.
4. Krafft, M. E.; Scott, I. L.; Romero, R. H.; Feibelmann,
S.; Van Pelt, C. E. J. Am. Chem. Soc. 1993, 115, 7199–
7207.
5. Smit, V. A.; Simonyan, S. O.; Shashkov, A. S.; Mamyan,
S. S.; Tarasov, V. A.; Ibragimov, I. I. Izv. Akad. Nauk.
SSSR, Ser. Khim. 1987, 234.
nC5H11
nC5H11
C5H11
6. Strauss, D. S.; Glass, C. K. Med. Res. Rev. 2001, 21,
185–210.
7. For reviews on silicon-tethered reactions, see: (a) Fenster-
bank, L.; Malacria, M.; Sieburth, S. McN. Synthesis
1997, 813; (b) Bois, M.; Skrydstrup, T. Chem. Rev. 1995,
95, 1253.
R
H
1. Mo(CO)6
DMSO 40%
•
(5)
O
O
2. hν 100%
Si
R=(CH2)3CO2H
Ph
Ph
Ph
Ph
15-deoxy-∆12,14-PGJ2
31
32
8. Kagoshima, H.; Hayashi, M.; Yukihiko, H.; Saigo, K.
Organometallics 1996, 15, 5439.
9. Probert, G. D.; Harding, R.; Whitby, R. J.; Coote, S. J.
In summary, we have successfully demonstrated a sili-
con-tethered allenic Pauson–Khand reaction and selec-
tive cleavage of the carbonꢀsilicon bond of the vinyl
silane.20 Subsequent conversion of the ring-opened
silanol to a hydroxyl moiety makes this a very useful
method for the preparation of cross-conjugated a-
Synlett 1997, 1371.
10. (a) Kent, J. L.; Wan, H.; Brummond, K. M. Tetrahedron
Lett. 1995, 36, 2407; (b) Brummond, K. M.; Wan, H.
Tetrahedron Lett. 1998, 39, 931; (c) Brummond, K. M.;
Wan, H.; Kent, J. L. J. Org. Chem. 1998, 63, 6535; (d)
Brummond, K. M. In Advances in Cycloaddition; Har-
mata, M., Ed.; JAI Press Inc: Stamford, Connecticut,
1999; Vol. 6, p. 211.
11. Stork, G.; Kaitz, P. F. Tetrahedron Lett. 1989, 30, 6981.
12. Washburne, S. S.; Peterson, W. R., Jr. J. Organomet.
Chem. 1970, 21, 59.
alkylidene cyclopentenones, such as 15-deoxy-D12,14
-
PGJ2.
The
Mo(CO)6
silicon-tethered
allenic
Pauson–Khand reaction does not appear to be tolerant
of functionality other than a hydrogen on the terminus
of the alkyne. [Rh(CO)2Cl]2 gives selective reaction with
the distal p-bond of the allene to afford 4-alkylidene
cyclopentenones. Further delineation of the scope and
limitations of this reversal in regiochemistry will be
published shortly.
13. For similar studies by Pagenkopf, B. et al., see Reich-
wein, J. F.; Iacono, S. T.; Patel, M. C.; Pagenkopf, B. L.
Tetrahedron Lett. 2002, 43, 3739.
14. Compound 8 can be prepared in large quantities (13 g)
and in good yields (85%) by the sequential treatment of
6-iodo-2,3-hexadiene with tert-butyllithium, dichloro-
diphenylsilane, then ethynyl magnesium bromide.
15. We thank Dr. Junquan Wang for demonstrating the
feasibility of this reaction.
Acknowledgements
We are grateful to the National Institutes of Health
(GM-54161) for generous financial support.
16. Nicholls, B.; Whiting, M. C. J. Chem. Soc. 1959, 551.

3738
K. M. Brummond et al. / Tetrahedron Letters 43 (2002) 3735–3738
17. Koga, Y.; Kobayashi, T.; Narasaka, K. Chem. Lett.
1998, 249–250. While applying these conditions to
catalyst and TLC showed no further reaction. The reac-
tion mixture was cooled to rt and filtered through a plug
of silica gel and the filter cake washed with EtOAc (100
mL). The organic layer was then washed with 50 mL of
water, followed by brine (2×25 mL), and dried over
Na2SO4. The compound was purified by silica gel chro-
matography eluting with 5:95“10:90 EtOAc:hexanes to
alkynyl allenes, Narasaka published
a manuscript
describing the scope and limitations of [Rh(CO)2Cl]2. He
describes the reaction of a single alkynyl allene, affording
a 4-alkylidene cyclopentenone: Kobayashi, T.; Koga, Y.;
Naraska, K. J. Organomet. 2001, 624, 73.
18. Oda, H.; Sato, M.; Morizawa, Y.; Oshima, K.; Nozaki,
afford 64% yield of a 2:1 mixture of Z and E
1
diastereomers: 9Z H NMR (270 MHz, CDCl3) l 7.64–
7.61 (m, 2H), 7.58–7.54 (m, 2H), 7.48–7.37 (m, 6H), 6.72
(d, 1H, J=2.7 Hz), 6.21 (q, 1H, J=7.4 Hz), 3.58–3.48
(m, 1H), 2.62–2.49 (m, 1H), 2.29 (d, 3H, J=7.4 Hz),
1.68–1.25 (m, 3H); 13C NMR (67.9 MHz, CDCl3) l
199.6, 178.3, 142.8, 139.4, 135.1, 135.0, 134.5, 132.9,
132.8, 130.4, 130.3, 128.4, 128.3, 55.5, 29.7, 13.8, 13.2; IR
(neat) 3048, 3010, 2924, 2855, 1690, 1643, 1428, 1114,
1026, 974 cm−1; MS (GC/MS) m/e 316 (M+), 301, 238,
H. Tetrahedron 1985, 41, 3257.
19. Tamao, K.; Ishida, N.; Kumada, M. J. Org. Chem. 1983,
48, 2120. For a review on the oxidation of the car-
bonꢀsilicon bond, see: Jones, G. R.; Landais, Y. Tetra-
hedron 1996, 52, 7599.
20. Representative experimental procedure for the prepara-
tion of 8, 10, 12, 14, 16, 18 and 31. Ethynyl-hexa-3,4-
dienyl-diphenyl-silane (8): To a stirred solution of the
6-iodo-2,3-hexadiene (11.05 g, 53.1 mmol) in 200 mL of
dry Et2O at −78°C was added tert-butyllithium (50.0 mL,
1.7 M in pentane, 85.0 mmol) dropwise over 10 min via
an addition funnel and the reaction mixture stirred at
−78°C for an additional 15 min. To the reaction mixture
was then added diphenyldichlorosilane (10.6 mL, 50.4
mmol) and the mixture allowed to gradually warm to rt
and stirred an additional 3 h during which time the
reaction mixture became cloudy with precipitated lithium
chloride. The reaction mixture was then recooled to
−78°C and the addition funnel charged with ethynylmag-
nesium bromide (220 mL, 0.5 M in THF, 110 mmol) via
cannula. The Grignard reagent was added dropwise over
20 min and the reaction mixture allowed to warm slowly
to rt. After 2.5 h TLC showed complete consumption of
starting material and the reaction mixture was quenched
with 50 mL of satd NH4Cl. The resulting reaction mix-
ture was filtered through a plug of Celite® and the
aqueous layer extracted with pentane (4×30 mL). The
organic layers were combined and dried over Na2SO4 and
filtered. The organic layers were concentrated under
reduced pressure and the product purified by silica gel
chromatography eluting with hexanes to afford 13.01 g
1
212, 181, 105. 9E H NMR (270 MHz, CDCl3): l 7.64–
7.61 (m, 2H), 7.56–7.53 (m, 2H), 7.48–7.35 (m, 6H), 6.73
(d, 1H, J=2.5 Hz), 6.68 (dq, 1H, J=7.4, 1.9 Hz), 3.69–
3.58 (m, 1H), 2.82–2.72 (m, 1H), 1.96 (dd, 3H, J=7.4, 0.6
Hz), 1.68–1.28, (m, 3H); 13C NMR (67.9 MHz, CDCl3)
l: 197.8, 179.9, 141.5, 140.4, 135.2, 135.1, 132.8, 132.7,
131.3, 130.5, 128.5, 128.4, 53.2, 29.1, 15.6, 13.0; IR (neat)
3056, 2933, 2851, 1692, 1646, 1428, 1112, 738 cm−1; MS
(GC/MS) m/e 316 (M+), 288, 260, 238, 207, 181, 105, 79,
53. Representative experimental procedure for the
[Rh(CO)2Cl]2 catalyzed Pauson–Khand reaction. 3-Butyl-
1-methyl-4,4-diphenyl-1,4,5,6-tetrahydro-4-sila-inden-2-one
(21): Allenyne 10 (50 mg, 0.14 mmol) was azeotroped
with benzene (3×1 mL) and then sealed with a septa and
parafilm then evacuated and purged with nitrogen three
times. A flame dried 2-neck flask containing a stir bar
and capped with
a condensor was charged with
[Rh(CO)2Cl]2 (4.2 mg, 0.01 mmol) and flushed with a
stream of nitrogen for 10 min. Allenyne 10 was diluted
with 8.0 mL of dry toluene and added via cannula to
[Rh(CO)2Cl]2. The entire system was then evacuated
(mechanical pump) and purged with CO(g) three times,
and left under a balloon of CO(g) after the final purge.
The reaction mixture was then immersed in a 90°C sand
bath and the reaction was followed via TLC. After 1.5 h
TLC showed complete consumption of starting material
and the reaction was cooled to rt and concentrated under
reduced pressure. The products were purified by silica gel
chromatography eluting with 10:90 EtOAc:hexanes to
afford 41.1 mg (76%) of 21 as a brown crystalline solid.
1H NMR (300 MHz, CDCl3): l 7.59–7.52 (m, 4H),
7.50–7.37 (m, 6H), 6.46 (td, 1H, J=4.7, 0.6 Hz), 2.79 (qd,
1H, J=7.5, 1.0 Hz), 2.50 (q, 2H, J=6.2 Hz), 2.04–2.00
(m, 2H), 1.43 (t, 2H, J=6.6 Hz), 1.26 (d, 3H, J=7.5 Hz),
1.05–0.83 (m, 4H), 0.58 (t, 3H, J=7.1 Hz); 13C NMR (75
MHz, CDCl3): l 209.8, 158.0, 153.3, 146.9, 135.6, 132.9,
130.4, 128.5, 125.6, 44.2, 31.2, 26.5, 23.4, 23.1, 15.3, 13.8,
10.4; IR (neat) 3068, 3048, 2927, 2870, 1694, 1428, 1198,
1111, 973, 711 cm−1; MS (GC/MS) m/e 372 (M+), 357,
315, 294, 252, 199, 183, 161, 105, 78; HRMS calcd for
C25H28OSi: 372.190944; found: 372.190808. X-Ray crys-
tallographic data have been obtained for compounds 21,
22, 23.
1
(89%) of the allenyne 8 as a pale green oil: H NMR (270
MHz, CDCl3) l 7.71–7.66 (m, 4H), 7.45–7.37 (m, 6H),
5.21–5.04 (m, 2H), 2.71 (s, 1H), 2.24–2.13 (m, 2H), 1.66
(dd, 3H, J=6.9, 3.2 Hz), 1.35–1.26 (m, 2H); 13C NMR
(67.9 MHz, CDCl3) l 204.3, 134.9, 133.6, 130.1, 128.3,
97.4, 92.9, 86.9, 85.6, 23.2, 14.8, 13.5; IR (neat) 3200,
3068, 2919, 2853, 2361, 2035, 1961, 1428, 1370, 1113, 907,
737 cm−1; MS (GC/MS) m/e 288 (M+), 260, 210, 181,
105. Representative experimental procedure for the
Mo(CO)6 reactions. Ethylidine-1,1-diphenyl-2,3,3a,4-tetra-
hydro-1H-1-sila-pentalen-5-one (9): Allenyne 8 (2.0 g, 6.93
mmol) was azeotroped under vacuum with toluene (5×2
mL). To a flame dried three neck flask containing a stir
bar and capped with a condenser was added the allenyne
8, 30 mL of dry toluene and 4.5 mL of dry DMSO and
then degassed with a stream of argon for 20 min. Next
molybdenum hexacarbonyl (2.69 g, 10.2 mmol) was
added in one portion. The reaction mixture was
immersed into a preheated sand bath (110°C) and stirred
for 5 h under an argon atmosphere, at which time visual
inspection showed decomposition of the molybdenum