Coupling of propargylsilanes with Fischer carbene chromium complexes: A new synthesis of conjugated dienes

Article abstract of DOI:10.1016/j.jorganchem.2004.07.051

The reaction of propargylsilanes with Fischer carbene complexes has been examined. If the silane-containing carbon is secondary the predominant pathway involves formation of conjugated dienes through a 1,2-silicon shift process of the initially formed vin

Full text of DOI:10.1016/j.jorganchem.2004.07.051

Journal of Organometallic Chemistry 689 (2004) 3379–3387
www.elsevier.com/locate/jorganchem
Coupling of propargylsilanes with Fischer carbene
chromium complexes: a new synthesis of conjugated dienes
a
Paren P. Patel , Yixin Zhu , Lei Zhang , James W. Herndon
a
b
b,
*
a
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA
b
Department of Chemistry and Biochemistry, New Mexico State University, MSC 3C, Las Cruces, New Mexico 88003, USA
Received 17 June 2004; accepted 27 July 2004
Available online 28 August 2004
Abstract
The reaction of propargylsilanes with Fischer carbene complexes has been examined. If the silane-containing carbon is secondary
the predominant pathway involves formation of conjugated dienes through a 1,2-silicon shift process of the initially formed vinyl-
carbene complex intermediate. If a primary propargylsilane is employed, the silicon does not shift and normal alkyne-Fischer car-
bene coupling processes are observed. The process is moderately stereoselective, resulting in the E enol ether and Z alkenylsilane.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Carbene complexes; Chromium; Alkynes; Silicon migration; Dienes
1. Introduction
undergo annulation processes, then silylated products
useful for further transformations would be obtained.
For example, the Do¨tz benzannulation reaction [3] using
The instability of Fischer carbene complexes that
contain an a-silyl group is well-known [1]. Deliberate at-
tempts to prepare this class of compounds often result in
their conversion to 1-alkoxy-1-silylalkenes. Most of the
stable a-silylcarbene complexes feature additional stabi-
lizing influences such as a tungsten metal or amino
substitution at the carbene carbon. In one case, the
room-temperature conversion of an a-silylcarbene com-
plex to an alkenylsilane was observed and measured
kinetically. Based on these observations, a program to
examine the reaction between propargylsilanes and
Fischer carbene complexes was initiated (Scheme 1)
[2]. This reaction would afford an intermediate carbene
complex (3) that features an a-trimethylsilyl group. A
conjugated diene-enol ether (4) would result if these
complexes undergo the silyl migration process in refer-
ence 1. Alternatively, if propargylsilanes successfully
a
propargylsilane and
a
phenylcarbene complex
would afford a benzylsilane (e.g. 5). Alternatively, the
cyclopentannulation reaction of a cyclopropylcarbene
complex and a propargylsilane would lead to a cyclo-
pentadienone 6 [4], which would then likely rearrange
to fulvene 7 after a 1,7-silicon shift [5]. The conversion
of 6 to 7 (R1 = H) is a highly exothermic process accord-
ing to DFT calculations (DH = ꢀ7.9 kcal/mol) [6] (see
Fig. 1).
2. Results
General methods for preparation of propargylsilanes
and carbene complexes are depicted in Scheme 2. Pro-
pargylsilanes were prepared through the double-depro-
tonation of terminal alkynes followed by reaction with
one mole of trimethylsilyl chloride [7]. The most gener-
ally efficient procedure involves the treatment of the
*
Corresponding author. Tel.: +1 5056462487; fax: +1 5056462649.
E-mail address: jherndon@nmsu.edu (J.W. Herndon).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.07.051

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P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
R1
SiMe3
R1
Cr(CO)5
R2
SiMe3
R1
+
MeO
Cr(CO)4
SiMe3
2
1
R2
MeO
R2
3
MeO
4
R2 =
c-C3H5
R2 = Ph
R1
R1
SiMe3
R1
SiMe3
OH
OSiMe3
O
MeO
MeO
7
6
MeO
5
Scheme 1.
R1 = Ph
ucts (e.g. 6a or 7a) were isolated from this reaction.
The coupling of the propargylsilane derived from
1-hexyne (1b) with cyclopropylcarbene complex 2a
was also examined. A similar process was observed,
resulting in the formation of an isomeric mixture
(76:15:9 ratio) of diene 4e (Entry E). Various carbene
complexes were tested in their reaction with proparg-
ylsilanes. In most of the cases, diene formation was
the exclusive reaction process. Other processes are
not competitive with the diene synthesis. As noted in
Entries C and G, benzannulation hypothetically result-
ing in naphthols (5a or 5b) does not occur. A more
severe test, triene synthesis from alkenylcarbene com-
plexes (Entries H–J) was also successful. Competing
benzannulation was not observed in any of the reaction
in Entries H–J. Intramolecular cyclopropanation was
also non-competitive with the diene synthesis (Entry
D). Failure of the diene synthesis was noted in the
reaction process involving simple propargylsilane 1c
(Entry K and Scheme 3). This reaction afforded exclu-
sively the Do¨tz benzannulation product 5c and none of
the expected diene derivative 4k.
a
b
c
R1 = n-C3H7
R1 = H
Fig. 1. Substituent letter codes for compounds 1, 5–7.
SiMe3
1. 2 eq n-BuLi / Et2O / reflux
2. 1 eq. Me3SiCl
R1
R1
1
8
Cr(CO)5
R2
1. Cr(CO)6
R2-Li
R3O
2. MeOSO2CF3 (if R3 = Me) or
AcCl, then 4-penten-1-ol (2d)
9
2
Scheme 2.
terminal alkyne with two equivalents of n-butyllithium
in refluxing ether. Carbene complexes were prepared
from the corresponding organolithium reagents accord-
ing to the method of Fischer (see Fig. 2).
Initially the coupling of cyclopropylcarbene complex
2a and propargylsilane 1a was examined (Table 1, en-
try A). This coupling afforded an isomeric mixture
(83:17 ratio) of diene 4a. No cyclopentannulation prod-
3. Discussion
The major isomers of the diene adducts were assigned
as E enol ethers based on well-documented trends in the
chromium carbene–alkyne coupling [9] and based on the
chemical shifts of the enol ether carbon atoms in the ma-
jor isomers [10] (Table 2). Note that in the cases where a
direct comparison can be made (Entries A, C, and G),
the chemical shift of asterisked carbon for the major
diene stereoisomer is about 10 ppm less than the analo-
gous carbon one of the minor stereoisomers. Based on
these chemical shift trends, the minor isomer in Entry
A has been assigned as the E, Z isomer [8]. In Entries
C and G, one of the minor isomers has been assigned
a
b
c
d
e
f
R2 = c-C3H5, R3 = Me
R2 = Me, R3 = Me
R2 = Ph, R3 = Me
R2 = Me, R3 = -(CH2)3CH=CH2
R2 = -CH=CHCH3, R3 = Me
R2 = -C(CH3)=CH2, R3 = Me
R2 = (5,6-dihydropyran)-2-yl, R3 = Me
g
O
Fig. 2. Substituent letter codes for compound 2.

P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
3381
Table 1
Synthesis of dienes through coupling of propargylsilanes with Fischer carbene complexes
R3
O
Cr(CO)5
R1
SiMe3
R1
R2
Me3Si
+
R2
OR3
4
1
2
Entrya (Reactants)
R1
R2
R3
Yieldb (%)
Isomer ratioc,d,e
A (1a + 2a)
B (1a + 2b)
C (1a + 2c)
D (1a + 2d)
E ( 1b + 2a)
F (1b + 2b)
G (1b + 2c)
H (1b + 2e)
I (1b + 2f)
J (1b + 2g)
K (1c + 2c)
a
Phenyl
Phenyl
Phenyl
Phenyl
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
H
c-C3H5
Methyl
Phenyl
Methyl
c-C3H5
Methyl
Phenyl
Methyl
Methyl
Methyl
80
78
82
90
90
85
75
70
72
80
0
83:17
63:19:18
63:20:17
77:23
–(CH2)3 CH@CH2
Methyl
Methyl
76:15:9
76:15:9
56:25:19
72:14:14
69:25:6
84:16
Methyl
Methyl
trans-CH@CHCH3
–C(CH3)@CH2
(5,6-dihydropyran)-2-yl
Methyl
Methyl
Methyl
f
–
Phenyl
Entry letters correlate with R1–R3 substituents for compound 4 and intermediates 3, 12, 15, and 16.
The yield refers to the combined yield of all stereoisomers; complete separation was impossible in most cases.
The major isomer was the E, E isomer [8].
b
c
d
e
f
The minor isomer(s) was assumed to differ from the major isomer in the configuration at one (not both) double bonds.
See Tables 2 and 3 and the experimental for specific discussions on the stereochemical assignments of minor isomers in Entries A, C, and G.
See Scheme 3.
OMe
MeO
Cr(CO)5
OMe
SiMe3
+
Ph
Me3Si
SiMe3
Ph
1c
2c
4k
5c
OH
Not
Observed
Scheme 3.
as the E, Z isomer, and the other has been assigned as
the Z, Z isomer.
Assignment of stereochemistry to the alkenylsilane
functionality was more difficult. Protiodesilylation of
the isomeric mixture from Entry C led to a mixture of
three dienes (structure 10, Table 3) in an identical ratio
to those obtained in the carbene coupling. The observed
couplings are depicted in Table 3. Only Minor Isomer 1
affords a product consistent with trans coupling between
H0B and H0C (J = 15.6 Hz). This coupling in the product
derived from the other isomers is consistent with cis cou-
pling (J = 11.2 and 11.6 Hz). These values suggest that
the E alkenylsilane is the major isomer produced as a re-
sult of the carbene coupling. This stereochemistry is con-
sistent with that observed in previous conversion of a-
silylcarbene complexes to alkenylsilanes [1] and is con-
sistent with other reaction processes that transform car-
bene complexes into alkenes [11]. In the other cases
where three isomers were obtained, the two minor iso-
mers were assumed to differ in the configuration at
one and not both of the alkene units.
Table 2
NMR data for isomer comparisons
R2
MeO
R1
*
*
R1
vs
R2
Me3Si
MeO
Me3Si
4 (Z enol ether)
4 (E enol ether)
Compound
*C d
(major isomer)
*C d
(2nd isomer)
105.4
*C d
(3rd isomer)
Entry A (4a)
Entry B (4b)
Entry C (4c)
Entry D (4d)
Entry E (4e)
Entry F (4f)
Entry G (4g)
96.1
96.7
102.1
97.6
95.5
106.0
114.0
105.8
The mechanism for the formation of dienes is depicted
in Scheme 4. After formation of the vinylcarbene complex
3, migration of silicon occurs to afford the diene deriva-
tive. The Do¨tz benzannulation reaction, which requires
96.4
105.6
111.3

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P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
Table 3
Observed couplings in the protiodesilylation of isomeric compounds 4c
MeO
Ph
HA'
MeO
HA
KF
Ph
Ph
HB
Ph
Me3Si
4c
DMSO
HB'
HC'
10
Isomera
dHA
dHB
dH0A
dH0B
dH0C
Major (E, E)
Minor 1 (E, Z)
Minor 2 (Z, E)
a
5.61
5.90
6.16
6.69
6.80
6.96
6.14 (d, J = 11.2)
5.75 (d, J = 10.6)
6.47 (d, J = 11.6)
6.20 (d, J = 11.2)
6.49 (d, J = 15.6)
6.51 (d, J = 11.6)
6.30 (t, J = 11.2)
6.89 (dd, J = 15.6, 10.6)
6.77 (t, J = 11.6)
The second letter refers to the alkenylsilane stereochemistry in 4c, the letter would be opposite for diene 10.
CO insertion to form a ketene (12), is not competitive
with diene formation since the silicon migration proc-
ess occurs prior to any CO insertion. The cyclopentan-
nulation likely has a similar requirement [4]. The diene
synthesis was also favored over intramolecular cyclo-
propanation. Apparently this process is easily intercepted
by coordination of the alkyne to a coordinatively unsatu-
rated carbene complex intermediate [12].
Synthesis of a diene (4k) was not observed in the cou-
pling of simple propargylsilane 1c with phenylcarbene
complex 2c. Only the benzannulation product 5c was
isolated from this reaction. The diene synthesis likely
fails in this case because the silicon shift process is less
favorable. In the coupling of 1c and 2c, silyl migration
from the alkenylcarbene complex intermediate (3k) gen-
erates a primary-like carbocation intermediate (15k) A
similar observation was noted in [3 + 2]-cycloadditions
of allenylsilanes. In these cases, the cycloaddition proc-
ess failed for allenylsilanes devoid of an alkyl group al-
pha to the silane group [13].
H3Si
Cr(CO)5
Cr(CO)5
18
OH
OH
Cr(CO)5
OH
∆G = + 0.9 kcal/mol
H2C
20
H3Si
SiH3
17
Cr(CO)5
H2C
19
OH
SiH3
Cr(CO)5
H
Cr(CO)5
∆G = -20.3 kcal/mol
H3Si
H2C
H
SiH3
21
22
Scheme 5.
CH3
11
O
R3O
R2
Cr(CO)3
X
R3O
R1
R1
Cr(CO)5
SiMe3
R1
R2
+
R2
OR3
SiMe3
Cr
(CO)4
SiMe3
1
2
O
12
3
R3O
R1
SiMe3
R3O
R2
R3O
R3O
R2
R1
R1
R1
R2
Me3Si
OH
R3O
Me3Si
Cr
5
Cr(CO)4
R1
SiMe3
(CO)4
4
16
15
R3O
SiMe3
13
R1
SiMe3
O
14
CH3
OH
Scheme 4.

P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
3383
SiR3
SiR3
Cr(CO)4
alkenyl
R1
SiR3
H
R1
alkenyl
H
alkenyl
Cr(CO)4
R1
3 E,E*
H
Cr(CO)4
23
15
R1
SiMe3
MeO
R2
Cr(CO)4
SiR3
alkenyl
R1
16
SiMe3
4 E,E
R2
MeO
R2
3
H
R1
H
Cr(CO)4
MeO
SiR3
SiMe3
alkenyl
H
(CO)4Cr
R1
3 E,Z*
4 E,Z
H
R1
SiR3
Cr(CO)4
Cr(CO)4
SiR3
R2
Cr(CO)4
SiR3
MeO
or
alkenyl
MeO
R1
3 E,E*
H
R1
H
R2
R1
H
Scheme 6.
Mechanistically, both a concerted 1,2-migration of
silicon and a 1,3-shift of silicon followed by reductive
elimination have been proposed as likely mechanisms
for transformation of a-silylcarbene complexes to alken-
ylsilane–metal complexes. Evaluation of this process
was attempted computationally using DFT calculations
on the simple systems 17 and 21 (see Scheme 5). All at-
tempts to effect energy minimization on the intermedi-
ates 18 and 19 resulted in structures identical to 20.
Energy minimizations performed on other seven-coordi-
nate pentacarbonylchromium complexes analogous to
18 also failed; an attempt to minimize cis-Me2Cr(CO)5
resulted in Cr(CO)5 coordinated to a C–H bond of
ethane [14]. Unsuccessful minimization of 19 is not sur-
prising since it can be viewed as an extreme resonance
form of 20 [15]. The 1,2-shift mechanism is favored
due to the unique failure of the diene synthesis employ-
ing the simple propargylsilane 1c and the instability of
R2Cr(CO)5 species. Transformation of the a-silylcar-
bene complex to an alkenylsilane-Cr(CO)5 complex
was slightly endothermic for the hydroxycarbene com-
plex 17 but considerably exothermic for the non-oxygen-
ated carbene complex 21. In both carbene complexes 17
and 21, a nearly perfect overlap of the carbon–silicon
bond and the p-orbital at the carbene carbon was noted
in the energy-minimized structures.
and the R1 substituent [16]. Silicon migration from the
more stable conformer 3 E,E* would initially provide
zwitterionic intermediate 15. Hyperconjugation from
the more electropositive element chromium can be
achieved through a 60° rotation of the chromium-bear-
ing carbon leading to intermediate 23 [17], which can
be viewed as an extreme resonance form of alkene
chromium complex 16. Decomplexation of chromium
provides the E,E isomer of diene 4. If the minor con-
former 3 E,Z* proceeds through the same reaction se-
quence, the E,Z isomer of diene 4 would be produced.
4. Conclusion
The coupling of propargylsilanes and carbene com-
plexes readily affords diene derivatives featuring enol
ether and alkenylsilane functionalities. A mechanism
involving alkyne insertion followed by rapid 1,2-migra-
tion of silicon and decomplexation of chromium has
been proposed and supported through analysis of the
scope of the reaction and through theoretical calcula-
tions. The only failure noted was the coupling of carbene
complexes with the unsubstituted propargylsilane 1c.
The stereochemistry of alkenylsilane formation is
rationalized in Scheme 6. Two conformers featuring
overlap of the carbene p-orbital with the carbon–silicon
bond can be envisaged; these have been labeled 3 E,E*
and 3 E,Z*. Conformer 3 E.Z* is predicted to be less
stable due to steric interaction between the Cr(CO)4 unit
5. Experimental [18]
5.1. Starting materials
The following were prepared according to literature
procedures: 3-phenyl-3-trimethylsilyl-1-propyne (1a)

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P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
[7], 3-trimethylsilyl-1-hexyne (1b) [7], cyclopropylcar-
bene complex 2a [19], methylcarbene complex 2b [20],
phenylcarbene complex 2c [21], pentenyloxycarbene
complex 2d [22], 1-propenylcarbene complex 2e [23], 2-
propenylcarbene complex 2f [24], and pyranylcarbene
complex 2g [25]. The simple propargylsilane 3-trim-
ethylsilylpropyne is commercially available.
(M, 6), 243 (100), 215 (3), 159 (5), 141 (3), 115 (7), 73
(42); HRMS: Calc. for C16H22OSi 258.1439. Found
258.1442. The more polar fraction was assigned as the
1
E, E isomer. H NMR (CDCl3): d 7.57–7.11 (m, 5H),
6.67 (d, 1H, J = 2.0 Hz), 5.10 (d, 1H, J = 2.0 Hz), 3.53
(s, 3H), 1.40 (m, 1H), 0.59–0.50 (m, 2H), 0.30–0.20 (m,
2H), 0.16 (s, 9H); 13C NMR (CDCl3): d 153.9, 140.9,
138.9, 137.4, 129.1, 128.7, 127.9, 126.8, 96.1, 54.6,
5.2. General procedure for the coupling of carbene
complexes with propargylsilanes
11.6, 5.2, 4.2, 0.2, ꢀ1.5; IR (CH2Cl2): 1631 (s) cmꢀ1
;
Mass Spec. (EI): 272 (M, 37), 229 (22), 225 (5), 167
(6), 141 (7), 105 (11), 73 (100); HRMS: Calc. for
C17H24OSi 272.1596. Found 272.1586.
A solution of carbene complex (1.0 eq) and proparg-
ylsilane (1.5 eq) in dioxane or THF was heated at reflux
until a routine TLC indicated complete consumption of
the starting carbene complex (typically 2–3 h). The reac-
tion mixture was allowed to cool to room temperature
and hexane (10 mL) was added. The green solution
was filtered through a bed of Celite and washed with
4:1 hexane: ethyl acetate. Removal of the solvent on a
rotary evaporator afforded the crude diene product. At
this point the isomer ratio was determined through
examination of peaks in the region d 5–7 of the 1H
NMR spectrum. Final purification was achieved by
Flash chromatography on triethylamine-treated silica
gel [26] using pure hexane as the eluent. Due to difficulty
in separation, spectral data are reported for the mixture
of isomers. The exceptions are Entries A, D, and J,
where the E and Z isomers were separable.
O
TMS
24
5.4. Coupling of carbene complex 2b with propargylsilane
1a (Table 1, Entry B)
The general procedure was followed using methylcar-
bene complex 2b (0.050 g, 0.20 mmol) and propargylsi-
lane 1a (0.056 g, 0.30 mmol) in dioxane (10 mL). Final
purification using hexane as the eluent afforded a single
fraction identified as a 63:19:18 mixture of dienes 4b
(0.038 g, 78%). 1H NMR (CDCl3): major (E, E) isomer:
d 6.62 (s, 1H), 5.10 (s, 1H), 3.60 (s, 3H), minor isomers: d
6.62 (s, 1H), 6.58 (s, 1H), 5.36 (s, 1H), 5.22 (s, 1H), 3.53
(s, 3H), 3.49 (s, 3H); the following peaks are overlapping
in all three isomers: d 7.49–7.41 (m, 2H), 7.28–7.15 (m,
3H), 1.45 (s, 3H), 0.12 (s, 9H); 13C NMR (CDCl3): d
151.8, 141.2, 138.8, 137.2, 128.7, 128.0, 127.7, 126.9,
126.7, 125.2, 104.2, 96.7, 54.5, 17.0, ꢀ1.8, ꢀ3.4; IR
(CH2Cl2): 1644 (m); Mass Spec. (EI): 246 (M, 7), 217
(47), 189 (13), 143 (8), 105 (29), 73 (100); HRMS: Calc.
for C15H22OSi 246.1440. Found 246.1439.
5.3. Coupling of carbene complex 2a with propargylsilane
1a (Table 1, Entry A)
The general procedure was followed using cyclopro-
pylcarbene complex 2a (0.055 g, 0.20 mmol) and pro-
pargylsilane 1a (0.056 g, 0.30 mmol) in dioxane (10
mL). Final purification using hexane as the eluent af-
forded a single fraction identified as an 83:17 mixture
of dienes 4a (0.043 g, 80%). A small portion of this sam-
ple (0.020 g) was applied to a preparative TLC plate and
could be separated into two bands. The minor (less po-
lar) fraction was tentatively assigned as the Z, E isomer
based on the relatively large difference in the d of the
enol ether proton (5.33) vs. the same proton in the major
(E, E) isomer (5.10). 1H NMR (CDCl3): d 7.42–7.16 (m,
5H), 6.58 (s, 1H), 5.33 (s, 1H), 3.66 (s, 3H), 1.47 (m, 1H),
0.78–0.69 (m, 2H), 0.59–0.54 (m, 2H), 0.09 (s, 9H); 13C
NMR (CDCl3): d 156.2, 139.8, 138.9, 136.6, 129.3,
127.8, 126.6, 105.4, 54.9, 14.1, 12.1, 5.7, 1.0, ꢀ0.5; IR
(CH2Cl2): 1631 (m) cmꢀ1. This product was not further
characterized, but was hydrolyzed using aqueous con-
centrated hydrochloric acid to afford ketone 24. 1H
NMR (CDCl3): d 7.42–7.23 (m, 5H), 7.07 (s, 1H), 3.65
(s, 2H), 2.05 (m, 1H), 1.12–1.04 (m, 2H), 0.95–0.86 (m,
2H), 0.20 (s, 9H); 13C NMR (CDCl3): d 209.3, 141.2,
137.8, 128.3, 128.2, 127.8, 127.0, 45.4, 20.2, 11.0, 0.2,
ꢀ1.4; IR (CH2Cl2): 1700 cmꢀ1; Mass Spec. (EI): 258
5.5. Coupling of carbene complex 2c with propargylsilane
1a (Table 1, Entry C)
The general procedure was followed using phenylcar-
bene complex 2c (0.062 g, 0.20 mmol) and propargylsi-
lane 1a (0.055 g, 0.30 mmol) in dioxane (10 mL). Final
purification using hexane as the eluent afforded a single
fraction identified as a 63:20:17 mixture of dienes 4c
(0.050 g, 82%). 1H NMR (CDCl3): major (E, E) isomer:
d 6.69 (d, 1 H, J = 2.4 Hz), 5.61 (d, 1H, J = 2.4 Hz), 3.80
(s, 3H), E, Z isomer: d 6.80 (d, 1H, J = 1.9 Hz), 5.90 (d,
1H, J = 1.9 Hz), 3.51 (s, 3H); Z, E isomer: d 6.96 (d, 1H,
J = 1.8 Hz), 6.16 (d, 1H, J = 1.8 Hz), 3.75 (s, 3H); the
following peaks are overlapping in all three isomers: d
7.67–7.11 (m, 10H), 0.14 (s, 9H); 13C NMR (CDCl3):
d 155.3, 154.2, 153.6, 145.7, 141.3, 140.4, 139.6, 139.0,

P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
3385
138.8, 138.6, 136.9, 136.6, 136.3, 129.5, 129.4, 128.8,
128.4, 128.2, 127.7, 127.0, 126.8, 126.1, 125.0, 114.0,
106.0, 102.1, 58.5, 55.9, 55.7, 0.0, ꢀ0.7, ꢀ3.0; IR
(CH2Cl2): 1624 (m); Mass Spec. (EI): 308 (M, 29), 293
(100), 234 (24), 203 (15), 145 (8), 102 (8) 73 (73); HRMS:
Calc. for C20H24OSi 308.1596. Found 308.1582.
5.8. Coupling of carbene complex 2b with propargylsilane
1b (Table 1, Entry F)
The general procedure was followed using methylcar-
bene complex 2b (0.050 g, 0.20 mmol) and propargylsi-
lane 1b (0.046 g, 0.30 mmol) in dioxane (10 mL). Final
purification using hexane as the eluent afforded a single
fraction identified as a 76:15:9 mixture of dienes 4f
(0.037 g, 85%). 1H NMR (CDCl3): major (E, E) isomer:
d 5.73 (dt, 1H, J = 6.7, 1.7 Hz), 4.80 (br s, 1H), 3.54 (s,
3H), E, Z isomer: d 5.88 (dt, 1H, J = 6.8, 1.6 Hz), 5.01
(br s, 1H), 3.50 (s, 3H), Z, E isomer: 5.65 (dt, 1H,
J = 6.5, 1.5 Hz), 5.10 (br s, 1H), 3.52 (s, 3H), the follow-
ing peaks are overlapping in all isomers: d 1.95 (m, 2H),
1.58 (s, 3H), 1.45–1.27 (m, 2H), 0.90 (t, 3H, J = 7.8 Hz),
0.05 (s, 9H); 13C NMR (CDCl3): d 152.4, 147.6, 144.9,
140.9, 138.3, 102.6, 96.4,54.3, 54.0, 33.5, 32.2, 23.2,
22.3, 17.0, 14.0, 13.8, 0.2, ꢀ1.5, ꢀ2.5; IR (CH2Cl2):
1636 (m) cmꢀ1; Mass Spec. (CI): 213 (M + 1, 17), 73
(100); HRMS: Calc. for C12H25OSi 213.1675. Found
213.1681.
5.6. Coupling of carbene complex 2d with propargylsilane
1a (Table 1, Entry D)
The general procedure was followed using pente-
nyloxycarbene complex 2d (0.066 g, 0.20 mmol) and
propargylsilane 1a (0.056 g, 0.30 mmol) in dioxane
(10 mL). Final purification using hexane as the eluent
afforded a single fraction identified as a 77:23 mixture
of dienes 4d (0.054 g, 90%). A sample of the pure ma-
jor (E, E) isomer was obtained via preparative TLC.
1H NMR (CDCl3): d 7.45 (d, 2 H, J = 8.0 Hz),
7.28–7.10 (m, 3H), 6.60 (d, 1H, J = 1.9 Hz), 5.91
(ddt, 1H. J = 16.9, 10.2, 6.9 Hz), 5.08 (d, 1H,
J = 1.9 Hz), 5.05 (br d, 1H, J = 16.9 Hz), 4.99 (br d,
1H, J = 10.2 Hz), 3.73 (t, 2H, J = 6.5 Hz), 2.17 (br
q, 2H, J = 6.5 Hz), 1.78 (quintet, 2H, J = 6.5 Hz),
1.42 (s, 3H), 0.12 (s, 9H); 13C NMR (CDCl3): d
151.3, 141.4, 139.0, 138.0, 137.1, 128.8, 128.0, 126.7,
115.0, 97.6, 66.0, 30.3, 28.2, 17.2, ꢀ1.7; IR (CH2Cl2):
1640 cmꢀ1; Mass Spec. (EI): 300 (M, 32), 231 (4), 216
(12), 141 (5), 115 (7), 73 (100); HRMS: Calc. for
C19H28OSi 300.1909. Found 300.1918. The stereo-
chemistry of the minor isomer could not be reliably
assigned due to overlapping peaks in the alkene region
5.9. Coupling of carbene complex 2c with propargylsilane
1b (Table 1, Entry G)
The general procedure was followed using phenylcar-
bene complex 2c (0.062 g, 0.20 mmol) and propargylsi-
lane 1b (0.046 g, 0.30 mmol) in dioxane (10 mL). Final
purification using hexane as the eluent afforded a single
fraction identified as a 56:25:19 mixture of dienes 4g
(0.041 g, 75%). 1H NMR (CDCl3): major (E E ) isomer:
d 5.80 (dt, 1H, J = 7.7, 1.9 Hz), 5.40 (br s, 1H), 3.65 (s,
3H), E, Z isomer: d 5.80 (overlaps with E, E), 5.25 (br s,
1H), 3.50 (s, 3H), Z, E isomer: 5.65 (dt, 1H, J = 6.7, 2.0
Hz), 5.40 (overlaps with E, E ), 3.62 (s, 3H), the follow-
ing peaks are overlapping in all isomers: d 7.37–7.20 (m,
5H), 1.98 (m, 2 H), 1.58 (s, 3H), 1.14 (m, 2H), 0.71 (t,
3H, J = 7.2 Hz), 0.10 (s, 9 H); 13C NMR (CDCl3): d
154.1, 153.8, 146.9, 142.7, 136.9, 134.7, 129.4, 128.3,
127.7, 127.6, 127.1, 126.3, 125.8, 111.3, 105.8, 105.6,
55.5, 55.2, 34.3, 22.6, 13.9, 13.7, 1.0, 0.3, ꢀ0.7, ꢀ1.1;
IR (CH2Cl2): 1630 (m) cmꢀ1; Mass Spec. (CI): 274
(M, 3), 259 (27), 201 (7), 171 (6), 141 (14), 89 (24), 73
(100); HRMS: Calc. for C17H26OSi 274.1753. Found
274.1736.
1
of the H NMR spectrum.
5.7. Coupling of carbene complex 2a with propargylsilane
1b (Table 1, Entry E)
The general procedure was followed using cyclopro-
pylcarbene complex 2a (0.055 g, 0.20 mmol) and pro-
pargylsilane 1b (0.046 g, 0.30 mmol) in dioxane (10
mL). Final purification using hexane as the eluent af-
forded a single fraction identified as a 76:15:9 mixture
1
of dienes 4e (0.043 g, 90%). H NMR (CDCl3): major
(E, E ) isomer: d 5.78 (td, 1H, J = 6.7, 1.7 Hz), 4.82
(br s, 1H), 3.48 (s, 3 H), E, Z isomer: d 6.09 (td,
1H, J = 7.6, 1.8 Hz), 5.03 (br s, 1H), 3.62 (s, 3H),
Z, E isomer: 5.67 (dt, 1H, J = 6.8, 1.5 Hz), 5.06 (br
s, 1H), 3.43 (s, 3H), the following peaks are overlap-
ping in all isomers: d 2.10 (m, 2H), 1.48–1.20 (m,
3H), 0.88 (t, 3H, J = 7.2 Hz), 0.72–0.63 (m, 2H),
0.44–0.49 (m, 2H), 0.04 (s, 9H); 13C NMR (CDCl3):
d 155.9, 154.2, 153.8, 149.5, 141.3, 138.0, 95.5, 55.4,
32.3, 31.2, 22.4, 19.6, 13.9, 11.3, 4.0, 0.0, ꢀ1.5, ꢀ1.7;
IR (CH2Cl2): 1696 (m), 1640 (m) cmꢀ1; Mass Spec.
(EI): 238 (M, 7), 181 (17), 155 (11), 107 (29), 73
(100); HRMS: Calc. for C14H26OSi 238.1753. Found
238.1764.
5.10. Coupling of carbene complex 2e with propargylsi-
lane 1b (Table 1, Entry H)
The general procedure was followed using 1-prope-
nylcarbene complex 2e (0.276 g, 1.0 mmol) and pro-
pargylsilane 1b (0.231 g, 1.50 mmol) in THF (100
mL). Final purification using hexane as the eluent af-
forded a single fraction identified as a 72:14:14 mix-
1
ture of trienes 4h (0.165 g, 70%). H NMR (CDCl3):
major (E, E, E) (enol ether, alkenylsilane, original

3386
P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
alkene) isomer: d 5.95 (dq, 1H, J = 15.6, 6.7 Hz), 5.83 (d,
1H, J = 15.6 Hz), 5.80 (td, 1H, J = 6.7, 2.0 Hz), 4.97
(br s, 1H), 3.62 (s, 3H), (E, Z, E) isomer: d 5.31 (br s,
1H), 3.45 (s, 3H) (all other alkene HÕs obscured by major
isomer), (Z, E, E) isomer: d 3.46 (s, 3H) (all alkene HÕs
obscured by peaks for the major isomer), the following
peaks are overlapping in all isomers: d 2.15–1.89 (m,
2H), 1.77 (d, 3 H, J = 6.7 Hz), 1.42 (sextet, 2H, J = 7.0
Hz), 0.85 (t, 3H, J = 7.0 Hz), 0.03 (s, 9H); 13C NMR
(M), 265, 251, 237, 207; HRMS: Calc. for C16H28O2Si
280.1859. Found 280.1869.
5.13. Coupling of carbene complex 2c with propargylsi-
lane 2c (Table 1, Entry K)
The general procedure was followed using carbene
complex 2c (0.375 g, 1.20 mmol) and propargyltrimeth-
ylsilane (1c) (0.113 g, 1.00 mmol) in THF (10 mL). Final
purification by flash chromatography on untreated silica
gel using 9:1 hexane:ethyl acetate as the eluent afforded
a single fraction identified as naphthol 5c (0.162 g, 62%).
1H NMR (CDCl3): d 8.15 (d, 1H, J = 8.1 Hz), 7.99 (d,
1H, J = 8.1Hz); 7.42 (m, 2H), 6.43 (s, 1H), 4.45 (s,
1H), 3.92 (s, 3H), 2.17 (s, 2H), 0.05 (s, 9 H) 13C NMR
(CDCl3): d 149.3, 140.3, 125.8, 125.7, 124.1, 124.0,
121.9, 120.3, 119.9, 106.9, 55.7, 21.4, ꢀ1.5; MS (EI):
260 (M, 12), 244 (4), 243 (40), 229 (8), 171 (9), 73
(100); HRMS: Calc. for C15H12O2Si 260.1234. Found
260.1236.
(CDCl3):
d
151.2, 144.1, 143.8, 142.0, 141.7,
137.9,134.2, 133.5, 131.0, 124.9, 124.6, 100.0, 56.5,
54.7, 51.0, 49.0, 45.1, 42.0, 33.5, 31.5, 22.3, 22.2, 19.2,
18.0, 13.8, ꢀ1.5, ꢀ1.7; IR (CH2Cl2): 1681 (m), 1630
(m) cmꢀ1; Mass Spec. (EI): 238 (M, 4), 223 (7), 209
(3), 195 (4), 181 (6), 165 (11), 73 (100); HRMS (CI):
Calc. for C14H27OSi 239.1831. Found 239.1847.
5.11. Coupling of carbene complex 2f with propargylsilane
1b (Table 1, Entry I)
The general procedure was followed using 1-prope-
nylcarbene complex 2f (0.276 g, 1.00 mmol) and pro-
pargylsilane 1b (0.231 g, 1.50 mmol) in THF (100
mL). Final purification using hexane as the eluent af-
forded a single fraction identified as a 69:25:6 mixture
5.14. Fluoride-induced protiodesilylation and hydrolysis of
4c
Desilylation was effected using a literature procedure
[27]. A solution of diene 4c (0.100 g, 0.324 mmol,
63:20:17 mixture of stereoisomers) in DMSO (5 mL)
was heated to 40–45 °C under nitrogen using an oil bath.
Potassium fluoride dihydrate (0.092 g, 0.972 mmol) was
added and the mixture was stirred at this temperature un-
til disappearance of the starting compound was verified
by TLC analysis (about 6 h). The mixture was poured
into a mixture of water and ether in a separatory funnel.
The aqueous layer was extracted two times with ether.
The combined ether layers were washed once with satu-
rated aqueous ammonium chloride solution and dried
over sodium sulfate. After evaporation of the solvent, a
63:20:17 mixture of diene stereoisomers was obtained.
1
of trienes 4i (0.172 g, 72%). H NMR (CDCl3): d 5.80
(td, 1H, J = 6.7, 1.4 Hz), 5.61 (br s, 1H), 5.26 (br d,
1H, J = 1.4 Hz), 4.91 (br s, 1H), 3.47 (s, 3H), 2.06 (t,
2H, J = 6.7 Hz), 1.87 (br s, 3H), 1.37 (sextet, 2 H,
J = 6.7 Hz), 0.87 (t, 3H, J = 6.7 Hz), 0.09 (s, 9H); the
following peaks can be attributed to minor isomers: d
3.47 (s, 3H, 2nd most isomer), 3.44 (s, 3H, minor iso-
mer) 13C NMR (CDCl3): d 142.6, 140.7, 138.5, 132.6,
127.3, 124.0, 113.1, 112.1, 58.9, 41.8, 33.0, 29.7, 22.2,
20.1, 18.6, 13.9, ꢀ1.6, ꢀ2.4; IR (CH2Cl2): 1578 (m)
cmꢀ1; Mass Spec. (EI): 239 (M + 1, 6), 224 (18), 106
(14), 73 (100); HRMS: Calc. for C14H27OSi 239.1831.
Found 239.1817.
1
The crude reaction mixture was analyzed by H NMR
5.12. Coupling of carbene complex 2g with propargylsi-
lane 1b (Table 1, Entry J)
to determine the stereochemistry of the process. 1H
NMR (CDCl3): major (E, Z) [28] isomer: d 6.30 (t, 1H,
J = 11.2 Hz), 6.20 (d, 1 H, J = 11.2 Hz), 6.14 (d, 1H,
J = 11.2 Hz), 2nd most abundant (E, E) isomer: 6.89
(dd, 1 H, J = 15.6, 10.6 Hz), 6.49 (d, 1H, J = 15.6 Hz),
5.75 (d, 1H, J = 10.6 Hz), minor (Z, E) isomer: d 6.77
(t, 1H, J = 11.6 Hz), 6.51 (d, 1H, J = 11.6 Hz), 6.47 (d,
1H, J = 11.6 Hz). After the NMR analysis, the mixture
was dissolved in dichloromethane (25 mL) and concen-
trated aqueous hydrochloric acid (0.5 mL) was added.
The reaction mixture was stirred at 25 °C until all starting
material had been consumed according to TLC analysis
(about 1 h). Water (20 mL) was added and the mixture
was extracted with dichloromethane. The organic layer
was washed once with saturated aqueous sodium bicar-
bonate solution, dried over sodium sulfate, and the sol-
vent was removed on a rotary evaporator. Final
The General Procedure was followed using carbene
complex 2g (0.302 g, 1.00 mmol) and propargylsilane
1b (0.231 g, 1.50 mmol) in THF (100 mL). Final purifi-
cation using hexane as the eluent afforded a single frac-
tion identified as a 84:16 mixture of trienes 4j (0.220 g,
80%). Further purification using preparative TLC and
partial band cutting provided a pure sample of the ma-
1
jor (E, E) stereoisomer. H NMR (CDCl3): d 5.65 (dt,
1H, J = 6.7, 2.0 Hz), 5.13 (br s, 1H), 4.90 (t, 1H,
J = 3.9 Hz), 3.96 (t, 2H, J = 5.1 Hz), 3.58 (s, 3H), 2.05
(m, 2H), 1.77 (m, 2H), 1.75 (m, 4H), 1.37 (m, 2H),
0.88 (t, 3H, J = 7.2 Hz), 0.03 (s, 9H); 13C NMR
(CDCl3): d 149.4, 148.3, 142.6, 136.4, 110.1, 98.3, 68.1,
59.0, 32.9, 22.4, 22.3, 20.6, 13.9, ꢀ0.9; MS (EI): 280

P.P. Patel et al. / Journal of Organometallic Chemistry 689 (2004) 3379–3387
3387
[9] This issue is most elegantly addressed in the following reference:
J.S. McCallum, F.A. Kunng, S.R. Gilbertson, W.D. Wulff,
Organometallics 7 (1988) 2346–2360.
purification was achieved by flash chromatography on
silica gel using 9:1 hexane:ethyl acetate as eluent. A single
fraction was isolated and identified as 1,4-diphenyl-3-bu-
ten-2-one (0,055 g, 76%). M.p. 89.5–90.7 °C (lit 92.0–93.0
[10] P. Strobel, C.G. Andrieu, D. Paquer, M. Vazeaux, C.C. Pham,
New J. Chem. 44 (1980) 101–107.
1
°C); H NMR (CDCl3): d 8.17–7.85 (m, 2H), 7.65–7.12
[11] (a) D.F. Harvey, D.A. Neil, Tetrahedron 49 (1993) 2145–2150;
(b) B.C. Soderberg, M.J. Turbeville, Organometallics 10 (1991)
3951–3953;
(m, 3H), 6.60–6.41 (m, 2H), 3.89 (d, 2H, J = 5.2 Hz); irra-
diate as d 3.89: d 6.55 (d, J = 16.0 Hz), 6.45 (d, 1H,
(c) K. Fuchibe, N. Iwasawa, Tetrahedron 56 (2000) 4907–4915.
[12] Similarly successful carbene-alkyne couplings have been reported
for pentenyloxycarbene complexes. J.W. Herndon, P.P. Patel,
Tetrahedron Lett. 38 (1997) 59–62.
J = 16.0 Hz); IR (CH2Cl2): 1684 (s), 1598 (m) cmꢀ1
.
The spectral data were in agreement with those previ-
ously reported for this compound [29].
[13] R.L. Danheiser, D.J. Carini, A. Basak, J. Am. Chem. Soc. 103
(1981) 1604–1606.
[14] (a) A similar failure in geometry optimization of this complex was
previously documented. Successful optimization was only effected
starting from a non-cis arrangement of methyl groups X. Wang,
E. Weitz, J. Phys. Chem. A 106 (2002) 11782–11790;
(b) A similar phenomenon has been reported for H2Cr(CO)5,
which is unstable relative to (H2)Cr(CO)5 J. Tomas, A. Lledos,
Y. Jean, Organometallics 17 (1998) 4932–4939.
Acknowledgements
We thank the Petroleum Research Fund, Adminis-
tered by the American Chemical Society, and the
SCORE Program of NIH for financial support.
[15] For a discussion of deviation of alkene-metal complexes from
totally symmetrical coordination see
O. Eisenstein, R.
Hoffmann, J. Am. Chem. Soc. 103 (1981) 4308–4320.
[16] For a general discussion of the steric bulk of the Cr(CO)5 unit, see
L.S. Hegedus ‘‘Transition Metals in the Synthesis of Complex
Organic Molecules, second ed, University Science Books, Sausa-
lito, CA, 1999, pp 143–167. Due to the wall-like shape of the cis
CO ligands at chromium, the steric bulk of the Cr(CO)4 unit
should exceed that of the alkene substituent..
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[8] The first letter refers to the stereochemistry of the enol ether. The
second letter refers to the stereochemistry of the alkenylsilane.