Allylsilyl propargyl ethers as substrates for intramolecular Pauson-Khand reactions

Article abstract of DOI:10.1080/00397910500406120

Silyl ethers that are synthesized by coupling a propargylic alcohol with an allylsilyl chloride are shown to undergo sulfide-promoted Pauson-Khand reactions, affording bicyclic enones. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500406120

Synthetic Communicationsw, 36: 547–557, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500406120
Allylsilyl Propargyl Ethers as Substrates for
Intramolecular Pauson–Khand Reactions
Salma Ishaq and Michael J. Porter
Department of Chemistry, University College London, London, UK
Abstract: Silyl ethers that are synthesized by coupling a propargylic alcohol with an
allylsilyl chloride are shown to undergo sulfide-promoted Pauson–Khand reactions,
affording bicyclic enones.
Keywords: Cobalt, cyclization, Pauson–Khand, silicon tether, sulfide
INTRODUCTION
The Pauson–Khand reaction, discovered in 1973,[1] is the reaction of a
dicobalt hexacarbonyl-complexed alkyne with an alkene to give a cyclopente-
none in a formal [2 þ 2 þ 1] cycloaddition.[2] Recent years have shown a
resurgence of interest in this reaction with numerous methods being
developed to extend the scope of the reaction or to render it catalytic in
cobalt. Despite these advances, intermolecular Pauson–Khand reactions are
still restricted, with a few exceptions,[3] to strained olefins. Moreover, regios-
electivity tends to be poor when asymmetrical alkenes are employed. For these
reasons, intermolecular Pauson–Khand reactions with simple unstrained
olefins are frequently of limited synthetic use.
One widely used method that can be employed to circumvent problems of
reactivity and regioselectivity in a variety of settings is the use of a cleavable
silane or silyl ether tether, a tactic that turns an intermolecular reaction into an
Received in Poland July 15, 2005
Address correspondence to Michael J. Porter, Department of Chemistry, University
College London, Christopher Ingold Building, 20 Gordon Street, London WC1H 0AJ,
UK. Tel.: þ44 20 7679 4710; Fax: þ44 20 7679 7463; E-mail: m.j.porter@ucl.ac.uk
547

548
S. Ishaq and M. J. Porter
Scheme 1. Reagents and conditions: (i) Co2(CO)8, CH2Cl2, rt; (ii) NMO, CH2Cl2, rt,
46%.
intramolecular one.[4] Indeed, this strategy has previously been investigated in
the context of Pauson–Khand and related reactions.[5]
Kagoshima and coworkers reported in 1996 that treatment of the dicobalt
hexacarbonyl complex of enyne 1 with N-methylmorpholine N-oxide (NMO)
led not to the expected bicyclic enone but to a monocyclic diene, 2, in 46%
yield (Scheme 1).[6] Under the same conditions, four other related enynes
were cyclized to analogous products in 22–30% yield.
More recently, Reichwein et al. reported the reaction of vinylsilyl ethers
such as 3 with dicobalt octacarbonyl (Scheme 2).[7,8] Once again, none of the
expected bicyclic enone was obtained; in this case, the major product was a
cyclopentenone 4 in which reductive removal of the silyl ether tether had
taken place. Use of acetonitrile that contained a small amount of water was
found to be essential for the obtention of good yields; under other conditions,
metal decomplexation, hydrolysis of the silyl ether, and/or decomposition
occurred.
The only examples in which “normal” cyclopentenone products have
been obtained from silicon-tethered substrates were reported by Brummond
et al.[9] Allene-containing substrates such as 5 underwent Pauson–Khand-
type cyclizations to give bicycles 6 and 7, with the selectivity between the
two products dependent on the conditions used (Scheme 3). Cyclization
using stoichiometric molybdenum hexacarbonyl gave the 5,5-fused product
6, whereas use of 5 mol% of a rhodium(I) complex under an atmosphere of
carbon monoxide gave exclusively the 6,5-fused ring system 7. Markedly
lower yields were obtained when silyl ether substrates were employed in
these reactions.
In this communication we report the successful utilization of silyl ethers
bearing an allyl group on the silicon atom as substrates in Pauson–Khand
reactions.
Scheme 2. Reagents and conditions: Co2(CO)8, MeCN, 1% H2O, reflux, 74%.

Silicon-Tethered Pauson–Khand Reactions
549
Scheme 3. Reagents and conditions: (i) Mo(CO)6, DMSO, toluene, 908C, 64%; (ii)
5 mol% [Rh(CO)2Cl]2, CO, toluene, 908C, 64%.
RESULTS AND DISCUSSION
Initial studies focused on the silyl ether 8a, which could be obtained in 78%
yield from 3-phenylprop-2-yn-1-ol and allyldimethylsilyl chloride, and the
corresponding diphenylsilyl compound 8b, obtained in 50% yield over two
steps from allyltrichlorosilane. (Allyldiphenylsilyl chloride is not commer-
cially available; it was generated in situ by treatment of allyltrichlorosilane
with 2 equivalents of phenylmagnesium bromide, and the resulting crude
silyl chloride were treated directly with the appropriate alcohol and triethyl-
amine.[10]) These alkynes were converted to their dicobalt hexacarbonyl
complexes and then subjected to a range of typical Pauson–Khand conditions
(Scheme 4, Table 1).
Cyclization of the cobalt complexes of 8a or 8b under purely thermal con-
ditions, using either toluene or acetonitrile as solvent, gave poor to moderate
yields of the desired bicycles 9a and 9b respectively (entries 1, 2, 5, and 6).
Addition of a small amount of water, as reported by Reichwein et al. for the
cyclization of vinylsilane substrates (Scheme 2),[7] did not greatly affect the
yields obtained (entries 3 and 7).
Attempts to promote the cyclization by the use of molecular sieves[11]
(entry 8) or NMO[12] (entry 9) did not lead to any improvement in the cycliza-
tion yield. However, the use of n-butyl methyl sulfide as a promoter[3b] in 1,2-
dichloroethane as solvent afforded the bicycles 9a and 9b in yields of 72% and
70% respectively, and these conditions were adopted for subsequent studies.
The more robust diphenylsilyl ethers (corresponding to 8b) were selected
for a study of the scope of this Pauson–Khand cyclization reaction. Thus a
Scheme 4. Reagents and conditions: (i) Co2(CO)8, CH2Cl2, rt, 89% (8a); 93% (8b);
(ii) see Table 1.

550
S. Ishaq and M. J. Porter
Table 1. Cyclization of enynes 8a and 8b to bicycles 9a and 9b
Yield,
%
Entry
Substrate
Conditionsa
Toluene, reflux, 2 h
Acetonitrile, reflux, o/n
Acetonitrile, H2O (1% v/v), reflux, o/n
n-BuSMe (3.5 equiv.), ClCH2CH2Cl, reflux, o/n
Toluene, reflux, o/n
1
2
8a
8a
8a
8a
8b
8b
8b
8b
8b
8b
40
39
33
72
28
35
48
15
45
70
3
4
5
6
Acetonitrile, reflux, o/n
Acetonitrile, H2O (1% v/v), reflux, o/n
˚
Toluene, 4A molecular sieves, rt, 1 h
7
8
9
10
NMO (6 equiv.), CH2Cl2, rt, o/n
n-BuSMe (3.5 equiv.), ClCH2CH2Cl, reflux, o/n
aAbbreviations: NMO ¼ N-methylmorpholine-N-oxide; o/n ¼ overnight.
range of allyldiphenylsilyl propargyl ethers, with a variety of substitution
patterns on both the alkyne and alkene portions, were synthesized and
subjected to the Pauson–Khand cyclization conditions (Scheme 5, Table 2).
Few of the compounds synthesized proved to be effective substrates for
the Pauson–Khand reaction. A silyl ether derived from propargyl alcohol
(8c, entry 1) underwent only decomposition, whereas alkynes bearing a
terminal methyl or trimethylsilyl group (entries 2 and 3) afforded the
corresponding bicycles 9d and 9e in poor yields.
Use of secondary propargylic alcohols for the synthesis of the silyl ethers
was investigated next. Compound 8f, bearing a propargylic phenyl group, did
not give any of the desired product, possibly because of the ready formation of a
benzylic cobalt-stabilized cation by heterolysis of the C–O bond. A compound
with a propargylic methyl group, 8g, was also synthesized; this underwent
cyclization in very poor yield and with low diastereoselectivity (entry 5).
The tolerance of the reaction to substitution on the alkene double bond was
also studied; of the three substrates prepared, only that derived from crotyl
alcohol (8j, entry 8) was transformed to the corresponding bicycle, in 33% yield.
Scheme 5. Reagents and conditions: (i) Co2(CO)8, ClCH2CH2Cl, rt; (ii) n-BuSMe
(3.5 equiv.), ClCH2CH2Cl, reflux.

Silicon-Tethered Pauson–Khand Reactions
551
Table 2. Cyclization of a range of allylsilyl propargyl ethers
Yield, %
(RSM)a
Entry
Substrate
R1
R2
R3
R4
R5
1
2
3
4
5
6
7
8
8c
8d
8e
8f
H
H
H
H
H
—
38
Me
Me3Si
Ph
H
H
H
H
H
H
H
H
14
—(23)
9b
Ph
Me
H
H
H
H
8g
8h
8i
Ph
Ph
H
H
H
Me
Me
H
H
H
—(36)
—(28)
33 (13)
Ph
Ph
H
H
Me
Me
H
8j
H
aA dash indicates that extensive decomposition of the substrate was observed.
Figures in parentheses indicate recovered starting material.
bThe product was obtained as a 1.5 : 1 mixture of diastereomers.
In conclusion, we have shown that tethering enynes through an allylic
silyl ether linkage affords viable substrates for Pauson–Khand reactions,
although the substrate scope is somewhat limited under the conditions inves-
tigated. Subsequent transformations of the resulting bicycles are currently
under investigation.
EXPERIMENTAL
Allyldimethyl(3-phenylprop-2-ynyloxy)silane (8a). Allylchlorodimethylsilane
(0.34 mL, 2.2 mmol) was added dropwise to a solution of 3-phenyl-2-
propyn-1-ol (0.31 g, 2.4 mmol) and triethylamine (0.62 mL, 4.5 mmol) in
dichloromethane (8 mL) under nitrogen at 08C. After stirring at 08C for
10 min and at rt overnight, the reaction mixture was quenched with
saturated aqueous NH4Cl (8 mL). The organic layer was removed, and the
aqueous layer extracted with dichloromethane (3 ꢀ 10 mL). The combined
organic extracts were washed successively with water (20 mL) and brine
(20 mL), dried (Na2SO4), and concentrated in vacuo to obtain the crude
product. Flash chromatography (SiO2, Et2O:petrol 40–60 2:98) afforded
silyl ether 8a (0.14 g, 78%) as a yellow oil: nmax (film)/cm21 3077 m,
2959 m, 2859 m, 1630 s, 1599 m, 1490 s; dH (400 MHz; CDCl3) 7.46–7.43
(2H, m), 7.33–7.30 (3H, m), 5.85 (1H, ddt, J 16.8, 10.0, 8.1), 4.97–4.89
(2H, m), 4.55 (2H, s), 1.74 (2H, d, J 8.1), 0.23 (6H, s); dC (75 MHz;
CDCl3) 133.7, 131.6, 128.32, 128.26, 122.8, 114.0, 87.4, 85.1, 51.9, 24.5,
–2.3; m/z (CI pos) 230 (Mþ, 41%), 229 (62), 189 (100), 115 (79).
Allyldiphenyl(3-phenylprop-2-ynyloxy)silane (8b). A solution of phenylmag-
nesium bromide (1.0M in THF, 2.3 mL, 2.3 mmol) was added dropwise to a

552
S. Ishaq and M. J. Porter
stirred solution of allyltrichlorosilane (0.17 mL, 1.1 mmol) at 2788C. The
reaction mixture was stirred at 2788C for 30 min, warmed to rt, and then
heated under reflux for 2 h to obtain allylchlorodiphenylsilane in solution.
This solution was cooled to rt and added dropwise to a solution of
3-phenyl-2-propyn-1-ol (0.15 g, 1.1 mmol) and triethylamine (0.16 mL,
1.1 mmol) in dichloromethane (8 mL) at 08C. The mixture was stirred at
08C for 40 min then at rt overnight. The mixture was washed with sat. aq.
NH4Cl (15 mL), and the aqueous layer extracted with dichloromethane
(3 ꢀ 50 mL). The combined organic extracts were washed with brine
(100 mL), dried (Na2SO4), and concentrated in vacuo to obtain a crude
product. Flash chromatography (SiO2, Et3N–hexane 1:99) afforded silyl
ether 8b (0.20 g, 50%) as a colorless oil: nmax (CHCl3 cast)/cm21 3071 s,
2972 m, 2862 m, 2150 w, 1630 s, 1589 m, 1489 s; dH (500 MHz; CDCl3)
7.68–7.66 (4H, m), 7.44–7.29 (11H, m), 5.90 (1H, ddt, J 18.0, 10.1, 7.9),
4.99 (1H, ddt, J 17.0, 1.9, 1.5), 4.92 (1H, ddt, J 10.1, 1.9, 1.0), 4.63 (2H, s),
2.30 (2H, dt, J 7.9, 1.2); dC (75 MHz; CDCl3) 134.9, 133.9, 132.7, 131.6,
130.1, 128.3, 128.2, 127.9, 122.8, 115.4, 87.3, 85.6, 52.8, 22.0; m/z (FAB
pos) 377 (MNaþ, 2%), 313 (35), 283 (100) 199 (41); HRMS calculated for
C24H22OSiNa (MNaþ) 377.1338; found 377.1334.
Allyldiphenyl(prop-2-ynyloxy)silane (8c). Prepared by the procedure
described for 8b. Colorless oil; nmax (film)/cm21 3292 s, 3025 s, 2999 m,
2866 m, 2100 w, 1630 s, 1589 m, 1487 w; dH (500 MHz; CDCl3) 7.65–7.63
(4H, m), 7.47–7.38 (6H, m), 5.87 (1H, ddt, J 17.0, 10.1, 7.9), 4.99 (1H,
ddt, J 17.0, 1.9, 1.5), 4.93 (1H, ddt, J 10.1, 1.9, 1.1), 4.39 (2H, d, J 2.4),
2.41 (1H, t, J 2.4), 2.26 (2H, dt, J 7.9, 1.3); dC (100 MHz; CDCl3) 134.8,
133.5, 132.6, 130.2, 127.9, 115.4, 81.7, 73.5, 52.0, 21.8; m/z (FAB pos)
360 (100%), 301 (MNaþ, 10), 237 (28), 207 (75); HRMS calculated for
C18H18OSiNa (MNaþ) 301.1025; found 301.1020.
Allyl(but-2-ynyloxy)diphenylsilane (8d). Prepared by the procedure described
for 8b. Colorless oil; nmax (film)/cm21 3071 s, 2999 m, 2866 m, 2235 w,
1630 s, 1589 m, 1487 w; dH (500 MHz; CDCl3) 7.65–7.63 (4H, m), 7.45–
7.38 (6H, m), 5.88 (1H, ddt, J 17.0, 10.1, 7.9), 4.98 (1H, ddt, J 17.0, 1.9,
1.5), 4.92 (1H, ddt, J 10.1, 2.0, 1.1), 4.37 (2H, q, J 2.4), 2.26 (2H, dt, J 7.9,
1.3), 1.80 (3H, t, J 2.4); dC (100 MHz; CDCl3) 134.8, 133.9, 132.8, 130.0,
127.8, 115.2, 81.8, 77.2, 52.5, 21.9, 3.6; m/z (FAB pos) 315 (MNaþ, 29%),
251 (35), 221 (100); HRMS calculated for C19H20OSiNa (MNaþ) 315.1181;
found 315.1182.
Allyldiphenyl(3-trimethylsilanylprop-2-ynyloxy)silane (8e). Prepared by the
procedure described for 8b. Pale yellow oil; nmax (film)/cm21 3071 s,
2961 s, 2860 m, 2179 m, 1630s, 1589 m, 1489 m; dH (500 MHz; CDCl3)
7.67–7.65 (4H, m), 7.46–7.38 (6H, m), 5.89 (1H, ddt, J 17.0, 10.1, 7.9),
4.99 (1H, ddt, J 17.0, 2.0, 1.5), 4.93 (1H, ddt, J 10.1, 2.0, 1.1), 4.41 (2H, s),

Silicon-Tethered Pauson–Khand Reactions
553
2.28 (2H, dt, J 7.8, 1.3), 0.17 (9H, s); dC (125 MHz; CDCl3) 134.9, 133.8,
132.8, 130.1, 127.8, 115.3, 103.7, 90.5, 52.8, 22.0, 20.3; m/z (FAB pos)
373 (MNaþ, 30%), 209 (15), 279 (100); HRMS calculated for
C21H26OSi2Na (MNaþ) 373.1420; found 373.1424.
Allyldiphenyl(1,3-diphenylprop-2-ynyloxy)silane (8f). Prepared by the
procedure described for 8b. Viscous yellow oil; nmax (film)/cm21 3070 s,
3027 s, 2200 w, 1685 m, 1630 m, 1599 m, 1490 m; dH (500 MHz; CDCl3)
7.74–7.70 (4H, m), 7.59–7.57 (2H, m), 7.48–7.44 (2H, m), 7.42–7.38
(6H, m), 7.36–7.28 (6H, m), 5.92 (1H, ddt, J 17.0, 10.1, 8.0), 5.81 (1H, s),
4.99 (1H, ddt, J 17.0, 2.0, 1.4), 4.93 (1H, ddt, J 10.1, 2.1, 1.0), 2.39 (1H,
ddt, J 14.1, 7.7, 1.3), 2.35 (1H, ddt, J 14.1, 8.1, 1.3); dC (125 MHz; CDCl3)
141.1, 135.05, 135.00, 134.2, 134.1, 132.9, 131.6, 130.04, 130.00, 128.4,
128.3, 128.1, 127.9, 127.8, 127.7, 126.5, 122.6, 115.3, 89.5, 86.5, 65.9,
22.3; m/z (FAB pos) 453 (MNaþ, 1%), 389 (15), 283 (35), 191 (100);
HRMS calculated for C30H26OSiNa (MNaþ) 453.1651; found 453.1661.
Allyldiphenyl(4-phenylbut-3-yn-2-yloxy)silane (8g). Prepared by the
procedure described for 8b. Light yellow oil; nmax (film)/cm21 3070 m,
2999 m, 2200 w, 1630 m, 1590 w, 1490 s; dH (500 MHz; CDCl3) 7.75–7.73
(4H, m), 7.49–7.41 (6H, m), 7.35–7.30 (5H, m), 5.97 (1H, ddt, J 17.0,
10.1, 7.9), 5.04 (1H, ddt, J 17.0, 2.0, 1.5), 4.97 (1H, ddt, J 10.1, 2.0, 1.0),
4.89 (1H, q, J 6.5), 2.37–2.36 (2H, m), 1.61 (3H, d, J 6.5); dC (125 MHz;
CDCl3) 134.93, 134.91, 134.40, 134.38, 133.0, 131.5, 129.94, 129.91,
128.1, 127.8, 127.7, 122.8, 115.2, 91.2, 84.1, 60.2, 25.2, 22.3; m/z (FAB
pos) 369 (MHþ, 4%), 327 (100), 291 (70), 267 (60), 129 (90); HRMS calcu-
lated for C25H25OSi (MHþ) 369.1675; found 369.1672.
Methallyldiphenyl(3-phenylprop-2-ynyloxy)silane (8h). Prepared by the
procedure described for 8b, using trichloro(methallyl)silane.[13] Colorless
oil; nmax (film)/cm21 3071 s, 2914 s, 2860 m, 2260 w, 1639 s, 1589 m,
1489 s; dH (500 MHz; CDCl3) 7.70–7.68 (4H, m), 7.46–7.29 (11H, m),
4.67–4.66 (1H, m), 4.62–4.61 (1H, m), 4.61 (2H, s), 2.29 (2H, d, J 1.0),
1.66 (3H, dd, J 1.3, 0.8); dC (125 MHz; CDCl3) 141.5, 134.9, 134.0, 131.6,
130.0, 128.3, 128.2, 127.8, 122.8, 110.9, 87.3, 85.5, 52.8, 25.8, 25.4; m/z
(FAB pos) 391 (MNaþ, 41%), 283 (100), 199 (50), 176 (75); HRMS calcu-
lated for C25H24OSiNa (MNaþ) 391.1494; found 391.1498.
(Z)-(2,3-Dimethylallyl)diphenyl(3-phenylprop-2-ynyloxy)silane (8i). Prepared
by the procedure described for 8b,using(Z)-trichloro(2,3-dimethylallyl)silane.[14]
Colorless oil; nmax (film)/cm21 3069 m, 2914 m, 2858 m, 2220 w, 1600 m,
1490 s; dH (500 MHz; CDCl3) 7.73–7.70 (4H, m), 7.49–7.30 (11H, m),
5.14 (1H, br q, J 6.7), 4.61 (2H, s), 2.25 (2H, br s), 1.70 (3H, dt, J 2.9, 1.4),
1.34 (3H, br d J 6.7); dC (125 MHz; CDCl3) 134.9, 134.2, 131.6, 131.4,
130.0, 128.21, 128.16, 127.8, 122.8, 118.2, 87.3, 85.4, 52.7, 26.3, 20.8,

554
S. Ishaq and M. J. Porter
13.8; m/z (CI pos) 383 (MHþ, 40%), 330 (100), 313 (54), 216 (75); HRMS
calculated for C26H27OSi (MHþ) 383.1831; found 383.1822.
Crotyldiphenyl(3-phenylprop-2-ynyloxy)silane (8j). Prepared by the pro-
cedure described for 8b, using trichloro(crotyl)silane.[13] Colorless oil; nmax
(film)/cm21 3069 s, 2916 s, 2855 s, 2270 w, 1610 m, 1589 m, 1489 s; dH
(500 MHz; CDCl3) 7.69–7.67 (4H, m), 7.46–7.29 (11H, m), 5.52 (1H, dtq,
J 15.1, 7.6, 1.5), 5.41 (1H, dqt, J 15.1, 6.3, 1.3), 4.63 (2H, s), 2.21 (2H,
ddq, J 7.6, 1.3, 1.3), 1.61 (3H, ddt, J 6.3, 1.4, 1.3); dC (125 MHz; CDCl3)
134.9, 134.2, 131.6, 129.0, 128.24, 128.17, 127.8, 125.9, 124.4, 122.8,
87.3, 85.4, 52.8, 20.0, 18.1; m/z (CI pos) 369 (MHþ, 9%), 313 (100), 283
(100), 199 (45); HRMS calculated for C25H25OSi (MHþ) 369.1675; found
369.1664.
3,3-Dimethyl-7-phenyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one (9a).
A
solution of silyl ether 8a (0.10 g, 0.4 mmol) in dichloromethane (1.5 mL)
was added to a solution of dicobalt octacarbonyl (0.39 g, 1.1 mmol) in dichlor-
omethane (2 mL), under nitrogen at rt. The resulting reaction mixture was
stirred at rt for 1 h then concentrated in vacuo. The residue was purified by
flash chromatography (Florisilw, petrol 40–60) to afford allyldimethyl(3-phe-
nylprop-2-ynyloxy)silane dicobalt hexacarbonyl complex (0.20 g, 89%) as a
deep red oil: nmax (CHCl3 cast)/cm21 2960 w, 2918 m, 2091 s, 2051 s,
1650 m, 1475 m; dH (300 MHz; CDCl3) 7.54–7.52 (2H, m), 7.34–7.32
(3H, m), 5.92–5.78 (1H, m), 5.01 (2H, br s), 4.97–4.88 (2H, m), 1.72 (2H,
br d, J 7.9), 0.21 (6H, s); dC [75 MHz; CDCl3 þ Cr(acac)3] 199.5, 137.8,
133.7, 129.7, 128.8, 127.8, 114.0, 97.1, 89.7, 63.5, 24.3, –2.8; m/z (ES
pos) 413 (37%), 349 (29), 305 (45), 301 (39), 261 (100).
A solution of this complex (0.20 g, 0.4 mmol) and n-butyl methyl sulfide
(0.17 mL, 1.4 mmol) in 1,2-dichloroethane (3.9 mL) under nitrogen was
heated to reflux overnight. The reaction mixture was cooled and concentrated
in vacuo. The residue was purified by flash chromatography (Florisilw,
gradient elution; Et2O:petrol 40–60 0 : 100–50 : 50) to afford bicycle 9a
(72 mg, 72%) as a yellow oil; nmax (film)/cm21 2955 m, 2922 m, 1699 s,
1630 w, 1599 w, 1495 m; dH (400 MHz; CDCl3) 7.42–7.25 (5H, m), 4.95
(1H, d, J 16.3), 4.80 (1H, d, J 16.3), 3.25–3.19 (1H, m), 2.93 (1H, dd, J
18.9, 6.4), 2.25 (1H, dd, J 18.2, 1.6), 1.35 (1H, dd, J 14.3, 5.1), 0.76 (1H, t,
J 13.9), 0.31 (3H, s), 0.21 (3H, s); dC (75 MHz; CDCl3) 206.0, 173.0, 138.0,
130.7, 128.9, 128.3, 128.1, 62.3, 45.8, 35.4, 21.6, –0.9, –2.0; m/z (CI pos)
259 (MHþ, 98%), 185 (25), 40 (100); HRMS calculated for C15H19O2Si
(MHþ) 259.1154; found 259.1150.
3,3,7-Triphenyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one (9b). Prepared
from 8b using the procedure described for 9a. White foam, mp: 598C; nmax
(KBr)/cm21 2924 s, 2853 m, 1701 s, 1429 m; dH (500 MHz; CDCl3) 7.73–
7.71 (2H, m), 7.54–7.52 (2H, m), 7.47–7.46 (3H, m), 7.41–7.35 (6H, m),

Silicon-Tethered Pauson–Khand Reactions
555
7.25–7.23 (2H, m), 5.15 (1H, d, J 16.2), 4.98 (1H, d, J 16.2), 3.34–3.31 (1H,
m), 2.94 (1H, dd, J 18.9, 6.5), 2.34 (1H, dd, J 18.9, 2.1), 1.90 (1H, dd, J 14.6,
4.9), 1.24 (1H, dd, J 14.5, 13.7); dC (125 MHz; CDCl3) 205.7, 172.1, 138.5,
134.3, 134.2, 133.8, 133.5, 130.6, 130.54, 130.49, 129.0, 128.4, 128.3,
128.2, 128.1, 63.2, 45.8, 35.6, 19.2; m/z (FAB pos) 383 (MHþ, 15%), 338
(100), 307 (30); HRMS calculated for C25H23O2Si (MHþ) 383.1467; found
383.1472.
7-Methyl-3,3-diphenyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one (9d). Prepared
from 8d using the procedure described for 9a. Yellow oil; nmax (film)/cm21
2922 m, 2853 w, 1701 s, 1647 s, 1589 m; dH (500 MHz; CDCl3) 7.72–7.70
(2H, m), 7.55–7.54 (2H, m), 7.49–7.43 (4H, m), 7.41–7.36 (2H, m), 5.07
(1H, d, J 16.2), 4.86 (1H, d, J 16.2), 3.15–3.09 (1H, m), 2.79 (1H, dd, J
18.8, 6.3), 2.16 (1H, dd, J 18.8, 1.5), 1.81 (1H, dd, J 14.5, 4.8), 1.70 (3H,
br s), 1.21 (1H, t, J 14.3); dC (125 MHz; CDCl3) 207.9, 170.6, 134.5, 134.2,
134.1, 134.0, 133.6, 130.5, 130.4, 128.2, 128.1, 62.8, 45.1, 35.3, 18.8, 7.7;
m/z (FAB pos) 321 (MHþ, 25%), 307 (40), 289 (18), 154 (100); HRMS
calculated for C20H21O2Si (MHþ) 321.1311; found 321.1303.
3,3-Diphenyl-7-trimethylsilanyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one (9e).
Prepared from 8e using the procedure described for 9a. Yellow oil; nmax
(film)/cm21 2955 m, 2899 m, 1690 s, 1589 s; dH (500 MHz; CDCl3) 7.69–
7.68 (2H, m), 7.57–7.56 (2H, m), 7.49–7.35 (6H, m), 5.12 (1H, d, J 17.1),
4.98 (1H, d, J 17.1), 3.30–3.23 (1H, m), 2.75 (1H, dd, J 18.5, 6.0), 2.13
(1H, dd, J 18.6, 2.5), 1.80 (1H, dd, J 14.7, 4.9), 1.19 (1H, t, J 14.4), 0.21
(9H, s); dC (125 MHz; CDCl3) 211.7, 187.1, 138.1, 134.22, 134.21, 134.0,
133.7, 130.6, 130.5, 128.2, 128.1, 64.6, 46.2, 38.5, 18.4, –0.6; m/z (CI pos)
379 (MHþ, 11%), 363 (15), 309 (70), 273 (85), 199 (100), 163 (80); HRMS
calculated for C22H27O2Si2 (MHþ) 379.1550; found 379.1560.
5-Methyl-3,3,7-triphenyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one (9g). Pre-
pared from 8 g using the procedure described for 9a. Isolated as an inseparable
mixture of diastereoisomers in the ratio of 1 : 1.5; nmax (film)/cm21 2925 s,
1710 s, 1699 s, 1682 m, 1428 s; dH (500 MHz; CDCl3) major isomer: 7.74–
7.72 (2H, m), 7.53–7.51 (2H, m), 7.48–7.30 (9H, m), 7.20–7.18 (2H, m),
5.42 (1H, q, J 6.6), 3.46–3.42 (1H, m), 2.95 (1H, dd, J 18.9, 6.5), 2.35 (1H,
dd, J 18.9, 1.7), 1.98 (1H, dd, J 14.7, 5.2), 1.40 (3H, d, J 6.7), 1.22 (1H, dd,
J 14.7, 13.3); minor isomer: 7.68–7.66 (2H, m), 7.62–7.60 (2H, m), 7.48–
7.30 (9H, m), 7.22–7.21 (2H, m), 5.63 (1H, qt, J 6.9, 1.1), 3.46–3.42 (1H,
m), 2.96 (1H, ddd, J 18.9, 6.5, 1.2), 2.34 (1H, dd, J 18.8, 2.8), 1.77 (1H, dd,
J 14.7, 4.0), 1.36 (1H, t, J 14.5), 1.13 (3H, d, J 6.7); dC (125 MHz; CDCl3)
206.3, 205.7, 179.0, 176.8, 138.5, 138.3, 135.7, 135.0, 134.9, 134.6, 134.3,
134.21, 134.18, 134.0, 131.9, 131.0, 130.4, 130.32, 130.30, 128.8, 128.62,
128.6, 128.5, 128.11, 128.08, 128.05, 128.03, 127.99, 70.9, 69.1, 45.7, 45.3,
35.7, 32.7, 23.9, 23.4, 20.3, 16.6; m/z (CI pos) 397 (MHþ, 100%), 319

556
S. Ishaq and M. J. Porter
(38), 199 (35); HRMS calculated for C26H25O2Si (MHþ) 397.1624; found
397.1631.
(1Rꢁ,9Rꢁ)-9-Methyl-3,3,7-triphenyl-4-oxa-3-silabicyclo[4.3.0]non-6-en-8-one
(9j). Prepared from 8j using the procedure described for 9a. Pale yellow oil;
nmax (film)/cm21 2926 m, 2852 m, 1705 s, 1589 m, 1495 m; dH (500 MHz;
CDCl3) 7.75–7.73 (2H, m), 7.58–7.56 (2H, m), 7.52–7.47 (2H, m), 7.44–
7.33 (7H, m), 7.28–7.26 (2H, m), 5.19 (1H, d, J 16.6), 5.00 (1H, dd, J 16.5,
1.3), 2.90–2.84 (1H, m), 2.29 (1H, qd, J 7.3, 2.5), 1.97 (1H, dd, J 14.6,
5.0), 1.30 (3H, d, J 7.4), 1.26 (1H, dd, J 14.3, 13.5); dC (125 MHz; CDCl3)
207.8, 169.9, 137.2, 134.3, 134.2, 134.0, 133.5, 130.7, 130.54, 130.53,
129.0, 128.34, 128.30, 128.11, 128.08, 63.1, 51.7, 44.3, 17.5, 14.6; m/z (CI
pos) 397 (MHþ, 100%), 319 (55), 199 (15); HRMS calculated for
C26H25O2Si (MHþ) 397.1624; found 397.1631.
ACKNOWLEDGMENT
We thank the Engineering and Physical Sciences Research Council for the
award of a QUOTA studentship to S. I., and we thank Philip Garner (Case
Western Reserve University) for a procedure for the preparation of allyldiphe-
nylsilyl chloride.
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