Alkylation of acylmetalates with alkyl halides to prepare Fischer carbene complexes: An improved protocol

Article abstract of DOI:10.1016/j.tet.2010.11.016

Alkoxy Fischer carbene complexes have been synthesized by alkylation of lithium acylmetalates with alkyl halides in the presence of catalytic amount (5-10 mol %) of n-tetrabutylammonium bromide (n-Bu₄NBr) restricting the temperature below 55 °C to minimize decomposition of the product. The reaction occurs in a biphasic condition involving water and alkyl halide. The effect of cesium on this alkylation reaction has been studied. The presence of a radical quencher, di-tert-butyl phenol, neither affects the yield nor leads to the formation of dimer of di-tert-butyl phenol, which rules out the possibility of radical pathway mechanism. The kinetic study and the ¹H NMR spectra of products suggest an S_N2 pathway particularly involving alkyl halides.

Full text of DOI:10.1016/j.tet.2010.11.016

Tetrahedron 67 (2011) 106e113
Contents lists available at ScienceDirect
Tetrahedron
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Alkylation of acylmetalates with alkyl halides to prepare Fischer carbene
complexes: an improved protocol
*
Bappaditya Nandi, Surajit Sinha
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Alkoxy Fischer carbene complexes have been synthesized by alkylation of lithium acylmetalates with
alkyl halides in the presence of catalytic amount (5e10 mol %) of n-tetrabutylammonium bromide (n-
Bu4NBr) restricting the temperature below 55 ꢀC to minimize decomposition of the product. The reaction
occurs in a biphasic condition involving water and alkyl halide. The effect of cesium on this alkylation
reaction has been studied. The presence of a radical quencher, di-tert-butyl phenol, neither affects the
yield nor leads to the formation of dimer of di-tert-butyl phenol, which rules out the possibility of radical
pathway mechanism. The kinetic study and the 1H NMR spectra of products suggest an SN2 pathway
particularly involving alkyl halides.
Received in revised form 2 November 2010
Accepted 2 November 2010
Available online 6 November 2010
Keywords:
Fischer carbene complex
Alkylation
Cesium carbonate
Mechanistic study
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Fischer carbene complexes have proven to be useful in the
synthesis of a range of interesting and useful organic compounds1
and suitably substituted alkoxy Fischer carbene complexes of
chromium, tungsten or molybdenum are normally used as syn-
thons. These complexes are usually prepared by the treatment of M
(CO)6 with organolithium reagents followed by alkylation. Simple
alkyl groups such as methyl and ethyl are introduced using Meer-
wein oxonium salts2 or methyl fluorosulfate or methyl triflate3 but
these methods are not useful for the incorporation of more complex
alkyl groups. Fischer carbene complexes with complex alkoxy
groups are synthesized by treating the tetraalkylammonium acyl-
metalate with different acyl halides like acetyl chloride, acetyl
bromide or pivaloyl chloride at low temperature, followed by ad-
dition of alcohol.4 Chromium Fischer carbene complexes are
obtained in good yield, but tungsten carbene complexes give
moderate yield in this procedure. Direct incorporation of alkyl
groups using alkyl halides have been reported earlier5 but this
method is limited to the preparation of only chromium carbene
complexes. Moreover the reaction was done in large scale
(20 mmol) and variable yields were reported.5c In this direction,
Matsuyama’s method6 is more general and has covered a wide
range of substrates.
2.1. Preparation of Fischer carbene complexes
During the course of our work on the application of metal-
locarbenes in the synthesis of organic compounds, we wanted to
make the Fischer carbene complex 1 using Hoye’s5a method as it
appeared to be simple and straightforward. Unfortunately, we
isolated the desired complex 1 in very poor yield (20%) using
methyl iodide as an alkylating agent. However, the reaction was
performed in 2 mmol scale, which was much lower than the
reported scale (30 mmol). The reaction was even tried at elevated
temperature (65e70 ꢀC) in aqueous medium according to their
procedure, yet the yield was same. Moreover, methyl iodide es-
caped during heating. Carrying out the reaction in various solvents
like DCM, DMF or THF, did not improve the yield. When the reaction
was performed in these organic solvents, we observed that the
reaction mixture turned green and significant amount of un-
identified byproducts were formed. Based on our extensive in-
vestigation and guided by earlier reports,5a,c we found the optimum
condition which involved (a) filtering the aqueous solution of the
lithio salt of the transition metalecarbene complex for removing
decomposed acylmetalate impurity and unreacted metal hex-
acarbonyl prior to the addition of the alkyl halides, (b) restricting
the temperature below 55 ꢀC to minimize decomposition of the
product, (c) performing the reaction in biphasic condition of neat
alkyl halides (no organic solvent for dilution) and water to make the
reaction faster and move to forward direction.
* Corresponding author. Tel.: þ91 33 2473 4971; fax: þ91 33 2473 2805; e-mail
address: ocss5@iacs.res.in (S. Sinha).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.11.016

B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
107
Table 1
Synthesis of alkoxy Fischer carbene complexes
Entry
1
Metal (M)
Cr
R
R0X (equiv)
Rxn. condition temp/time
25 ꢀC/4 h
Product
Yielda (%)
CH3
CH3I (25)
EtI (5)
62
65
61
2
3
Cr
Cr
CH3
Ph
50 ꢀC/2 h
25 ꢀC/7 h
CH3I (25)
4
Cr
Ph
i-PrI (5)
55 ꢀC/10 h
58
5
Cr
W
W
W
W
W
W
CH3
CH3
CH3
CH3
CH3
CH3
CH3
25 ꢀC/1.5 h
25 ꢀC/8 h
55
60
57
68
60
72
70
6
CH3I (25)
EtI (5)
7
55 ꢀC/6 h
8
i-PrI (5)
55 ꢀC/6.5 h
55 ꢀC/10 h
25 ꢀC/2.5 h
25 ꢀC/6 h
9
n-BuI (5)
PhCH2Br (5)
10
11
12
13
14
15
16
W
W
W
W
Mo
n-Bu
n-Bu
Ph
CH3I (25)
i-PrI (5)
25 ꢀC/9 h
55 ꢀC/8 h
25 ꢀC/7 h
55 ꢀC/12 h
25 ꢀC/1.5 h
48
65
61
50
CH3I (25)
i-PrI (5)
Ph
Ph
CH3I (25)
45
(continued on next page)

108
B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
Table 1 (continued )
Entry
Metal (M)
R
R0X (equiv)
Rxn. condition temp/time
25 ꢀC/1.5 h
Product
Yielda (%)
17
Mo
Mo
n-Bu
CH3I (25)
35
18
n-Bu
i-PrI (5)
40 ꢀC/2 h
40
a
Yields refer to the isolated pure compounds.
Under this optimized condition, we have not only prepared
alkoxy Fischer carbene complexes of Cr but several W- and Mo-
complexes were also prepared (Table 1). Notably, it was not pos-
sible to prepare the W- and Mo-complexes earlier by the direct
alkylation method.5a All these reactions were repeated 3e4 times
in order to ensure reproducibility. In addition, we have done pre-
liminary mechanistic studies of this alkylation reaction.
The complex 1 was prepared in the presence of excess methyl
iodide (25 equiv) and 5e10 mol % of n-tetrabutylammonium bro-
mide in aqueous medium at room temperature to obtain 62% yield
after column purification (entry 1, Table 1). Interestingly, the re-
action worked well even in 2 mmol scale. The excess methyl iodide
promoted a rapid reaction and minimized decomposition of ther-
mally sensitive product. The completion of the reaction was evident
from the disappearance of the yellow color of water layer and ap-
pearance of yellow or red color in organic layer. The allylation was
done at room temperature with allyl bromide and complex 5 was
isolated in 55% yield (entry 5, Table 1). It is worth to mention here
that the same complex 5 was prepared previously5a using allyl io-
dide whereas allyl bromide did not work. The alkylation of
acylmetalates with unactivated alkyl halides like ethyl, iso-propyl,
and butyl iodides gave poor yield at room temperature. We found
that the reaction proceeded smoothly at 55 ꢀC and gave good yields
(entries 2 and 4, Table 1). This is the optimum temperature, above
which decomposition started. Moreover, the preparation of tung-
sten carbene complexes was not reported earlier5a,c except only one
example where the yield was very poor. Under the modified con-
ditions, we isolated several tungsten complexes in very good to
excellent yields (entries 6e15, Table 1). Again the allyl bromide
worked well and the allylated complex 11 was isolated in 70% yield
after column purification (entry 11, Table 1).
Next our aim was to prepare alkoxy carbene complexes of mo-
lybdenum as methodology for the preparation of alkoxy carbene
complexes of molybdenum is still very limited and Hoye’s condition
was also not effective. Using the modified conditions, we isolated
the products with various alkyl halides in reasonably good yields
(entries 16e18, Table 1).
Some of the Fischer carbene complexes studied were crystalline
solids. Fig. 1 shows the single crystal X-ray structure of compounds
8 and 10.
2.2. Cesium effect
It is known in the literature7 that cesium increases the nucleo-
philic character of cesium carboxylate in esterification reaction of
N-protected amino acids. Therefore, we were interested to see the
‘effect of cesium’ in our alkylation reaction. The reaction was re-
peated in the presence of 1e2 equiv of Cs2CO3 in aqueous medium.
As we have already discussed, at room temperature the reaction
gave incomplete conversion with unactivated alkyl halides (Table 2)
but interestingly, we observed that cesium has a positive effect in
the alkylation reaction at room temperature to increase the yields
by 4e13% in every case except for allyl complex 5. Attempts to
improve it by heating the reaction mixture failed to give better
results because decomposition became dominant reaction. It was
tried in different solvents such as DMF or DCM or THF but in
all these cases the yield was very poor and
carbene was formed as a side product.
a-C-alkylated Fischer
2.3. Mechanistic study
The plausible mechanism of the alkylation reaction has been
previously suggested by Hoye et al.5a According to them, the re-
action perhaps takes place via SN2 pathway or a radical transfer
process but no in depth study of the reaction was carried out.
We established that the reaction was not taking place via radical
pathway because the yield of the Fischer carbene complex 6 was not
affected when the reaction was done in the presence of excess
(8 equiv) radical quencher 2,4-di-tert-butyl phenol. Moreover, there
was no indication of the formation of coupling product 4,40,6,60-
tetra-tert-butyl-2,20-biphenol (Scheme 1, b), which is known to form
if phenoxyl radical is generated in the reaction medium (Scheme 1,a).
Fig. 1. ORTEP plot of the solid-state molecular structure of 8 and 10. Hydrogen atoms
are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level.

B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
109
Table 2
the kinetics in the homogenous phase with alkyl halide. The ad-
vantage is that the starting material, 21 and the product, 3 show
lmax at 350 nm and 405 nm, respectively, in the UVevisible ab-
sorption spectra (Fig. 2).
Effect of cesium in alkylation reaction
Carbene complexes
Time (h) No Cs2CO3 [yieldb (%)] Cs2CO3 [yieldb (%)]
Scheme 2. Methylation of n-tetrabutylammonium salt 21.
4
2
3
62
10
7
66
20
15
1.5
8
55
14
22
53
27
32
12
Fig. 2. UVevis absorption spectra of 21 (concn¼1.75ꢁ10ꢂ4 M) and
(concn¼1.1ꢁ10ꢂ4 M).
3
12
14
23
Compound 21 (0.71 mM) was taken in deoxygenated dichloro-
methane in a cuvette (3 mL and 1 cm). To that an excess of methyl
iodide in DCM was added to obtain an overall concentration of
0.72 M and optical spectral changes were recorded at 25 ꢀC. The
formation of the product was monitored by following the absor-
bance at 405 nm. Initially, a lag phase was observed for w4 min and
thereafter the reaction shows a pseudo first order conversion of 21
to 3 with an isobestic point at 375 nm (Fig. 3). This is consistent
aMeI of 25 equiv or 5 equiv of other halides were used.
b
Yields refer to the isolated pure compounds.
Scheme 1. (a) Dimerization of di-tert-butyl phenol in presence of radicals. (b) Alkyl-
ation reaction in the presence of radical quencher.
2.4. Kinetics
We have studied the kinetics of alkylation of 21 (an intermediate
of the alkylation reaction) with alkyl halide by monitoring the re-
action with optical spectroscopy (Scheme 2). It was difficult to
monitor the kinetics of the alkylation reaction of lithium acylme-
talate under biphasic conditions due to opacity, so we have studied
Fig. 3. Spectral changes observed upon mixing of DCM solution of 21 with large excess
methyl iodide at 25 ꢀC over 11 h. Concentration of solutions 21 0.71 mM; CH3I, 0.72 M.
lmax corresponding to 21 starts to decay, lmax corresponding to product, 3 starts to
accumulate. Inset: single-wavelength registration (at 405 nm) of the formation of 3
overlayed with a first-order fit; kobs 7.36ꢁ10ꢂ5 sꢂ1
.

110
B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
with the observed reaction rate dependence on the concentration
of methyl iodide. Therefore the following rate equation may be
suggested for this reaction, rate¼kobsꢁ[21]; where kobs¼kꢁ[CH3I] (k
is actual rate constant).
(þ)-neomenthol, 24, [
a
]
27 þ15.9 (c 6.3, CHCl3) (route 3, Scheme 3),
D
had spectral features clearly different from that of complex 23
(Fig. 4).
The (þ)-neomenthol 24 was synthesized from (ꢂ)-menthol in
optically active form via the formation of p-nitro-benzoyl ester
using DEAD/PPh3, followed by hydrolysis with K2CO3 in MeOH/THF
(1:1) (see Supplementary data).
2.5. NMR study
The opposite stereochemical configuration at C1 of the two
optically active isomers 23 and 25 was further confirmed by CD
spectra (Fig. 5).
The menthyloxy Fischer carbene complex 23 was synthesized
from (þ)-neomenthyl iodide 22 (route 1, Scheme 3). The complex
23 was isolated in 15% yield as a single product. The specific rota-
Scheme 3. Synthesis of optically active Fischer carbene complexes.
tion and 1H NMR spectrum of the compound 23 indicated that the
reaction proceeded with complete inversion of configuration at the
reaction center. The neomenthyl iodide 22 (see Supplementary
data for spectral data) was synthesized from commercially avail-
26
able optically active (ꢂ)-menthol, [
a
]
ꢂ45 (c 4.1, CHCl3) using
D
reported procedure;8 however, no spectral data were included in
that paper. The 1H NMR spectrum of complex 23 was compared
with the sample synthesized directly from (ꢂ)-menthol by the
known method (route 2, Scheme 3).
The optical rotation and the 1H NMR spectra of both the samples
were same and particularly, the proton attached at CeO showed
a clear td at 4.90 ppm with J values 10.4 and 3.7 Hz, respectively.
The corresponding complex 25, synthesized by acetylation of tetra-
ethylammonium salt of acylmetalate followed by addition of
Fig. 5. CD spectra of complex 23 (10ꢂ2
M in CHCl3, solid line) and complex 25
(4.6ꢁ10ꢂ3 M in CHCl3, dashed line, data plotted after multiplication by three).
3. Conclusions
In conclusion, we have found an optimum condition for the
preparation of Fischer carbene complexes of Cr, W, and Mo by
direct alkylation of acylmetalates with activated and unactivated
alkyl halides. Though this method did not work well (5e10%
yield) in case of functionalized alkyl halides (BrCH2CO2Et,
methoxyethoxymethyl chloride) but it could be useful for the
large scale preparation of Fischer carbene complexes as alkyl
halides are easily available. The effect of cesium ion on the al-
kylation reaction has been studied. The kinetics and 1H NMR
studies suggest that the alkylation reaction takes place pre-
dominantly via SN2 pathway particularly in case of alkyl halides
as was proposed earlier by Hoye.5a
Fig. 4. 1H NMR spectra (CDCl3, 500 MHz) of (ꢂ)-menthyloxy Fischer carbene complex
23 and (þ)-neomenthyloxy Fischer carbene complex 25.

B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
111
4. Experimental section
the reaction proceeded, the yellow aqueous layer became faint and
an orange or red layer formed to the bottom of the round-bottomed
flask. The reaction mixture was extracted with petroleum ether
(40e60 ꢀC) (3ꢁ20 mL). The combined organic extracts were
washed with ice-cold water (25 mL), dried over sodium sulfate. The
solvent was removed under reduced pressure without heating.
Column purification was done under argon or N2 pressure using
petroleum ether (40e60 ꢀC) as eluent.
4.1. General considerations
All reagents were purchased from commercial sources and used
without further purification, unless otherwise stated. Petroleum
ether (PE) refers to the fraction of petroleum boiling between 40 and
60 ꢀC. DMF, DCM, and THF are the abbreviation of dimethylforma-
mide, dichloromethane, and tetrahydrofuran, respectively. All re-
actions were carried out in oven-dried glassware under an argon
atmosphere using anhydrous solvents, standard syringe, and sep-
tum techniques unless otherwise indicated. The aqueous solution of
acylmetalates was filtered through a syringe filter, purchased from
SigmaeAldrich. Organic extracts were dried over anhydrous Na2SO4
and then filtered prior to removal of all volatiles under reduced
pressure on rotary evaporation. Chromatographic purification of
products was accomplished using column chromatography on silica
gels (mesh 100e200). Thin-layer chromatography (TLC) was carried
out on aluminum sheets, Silica Gel 60 F254 (Merck; layer thickness
0.25 mm). Visualization of the developed chromatogram was per-
formed by UV light and/or vanillin stains.
4.4.1. Pentacarbonyl(1-methoxyethylidene)chromium(0)
(1)5a. Yellow crystalline solid (62%); IR (neat):
n 2064, 1917 (s),
1456, 1261 cmꢂ1; 1H NMR (CDCl3, 300 MHz):
d
4.71 (br s, 3H, OCH3),
360.6 (C]Cr), 223.5
2.96 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz):
d
(COtrans), 216.5 (COcis), 67.4 (br, OCH3), 49.2 (br, CH3). Anal. Calcd for
C8H6O6Cr: C, 38.41; H, 2.42. Found: C, 38.71; H, 2.60.
4.4.2. Pentacarbonyl(1-ethoxyethylidene)chromium(0) (2). Orange
oil (65%); IR (neat):
300 MHz):
5.01 (br s, 2H, OCH2), 2.95 (s, 3H, CH3), 1.65 (t, J¼7.0 Hz,
3H, OCH2CH3); 13C NMR (CDCl3, 75 MHz):
357.6 (C]Cr), 223.5
n ;
2064, 1917 (s), 1260 cmꢂ1 1H NMR (CDCl3,
d
d
(COtrans), 216.6 (COcis), 77.2 (OCH2CH3, merged with CDCl3) [78.7,
75.3, 73.4, br s, OCH2; observed in acetone-d6], 50.1, 49.6 (br, CH3),
15.1 (OCH2CH3). Anal. Calcd for C9H8O6Cr C, 40.92; H, 3.05. Found:
C, 40.56; H, 3.11.
4.2. Physical measurements
Infrared spectra were recorded using an FTIR instrument (Shi-
madzu FTIR-8300), the frequencies are reported in wave number
(cmꢂ1) and intensities of the peaks are denoted as s (strong). 1H and
13C NMR spectra were recorded on a Bruker DPX-300 and DPX-500
spectrometers, respectively. Chemical shifts are reported in parts
per million (ppm). Coupling constants are reported wherever it is
necessary in hertz (Hz). Elemental analyses were performed on
a PerkineElmer 2400 series II CHN analyzer. Solution electronic
spectra were measured on an Agilent 8453 diode array spectro-
photometer. CD spectra were recorded on a JASCO J-815 CD spec-
trometer and optical rotations were measured on JASCO P-1020.
Optical rotation values reported are uncorrected.
4.4.3. Pentacarbonyl(methoxy(phenyl)methylene)chromium(0)
(3)6b. Red crystalline solid (61%); IR (neat):
n
2062, 1925 (s), 1449,
7.41 (s, 3H, ArH), 7.30 (s,
2H, ArH), 4.70 (s, 3H, OCH3); 13C NMR (CDCl3, 75 MHz):
351.3 (C]
1231 cmꢂ1 1H NMR (CDCl3, 300 MHz):
; d
d
Cr), 224.3 (COtrans), 216.3 (COcis), 153.9 (ArC), 130.5 (ArC), 128.3
(ArC), 123.1 (ArC), 67.3 (OCH3). Anal. Calcd for C13H8O6Cr: C, 50.01;
H, 2.58. Found: C, 50.27; H, 2.60.
4.4.4. Pentacarbonyl(isopropoxy(phenyl)methylene)chromium(0)
(4)6b. Red crystalline solid (58%); IR (neat):
n
2062, 1925 (s),
7.41e7.44 (m, 3H, ArH),
7.22e7.24 (m, 2H, ArH), 5.66 (br s, 1H, OCH), 1.60 [d, J¼5.8 Hz, 6H,
OC(CH3)2]; 13C NMR (CDCl3, 75 MHz):
326.4 (C]Cr), 224.5
1244 cmꢂ1 1H NMR (CDCl3, 300 MHz):
; d
4.3. X-ray crystal structure determination
d
(COtrans), 216.4 (COcis), 154.0 (ArC), 129.9 (ArC), 128.3 (ArC), 122.5
(ArC), 85.9 (OCH), 22.8 [OCH(CH3)2]. Anal. Calcd for C15H12O6Cr: C,
52.95; H, 3.55. Found: C, 52.75; H, 3.63.
Crystallographic data for 8 and 10 are provided in Table S1
(Supplementary data). X-ray single crystal data were collected at
ꢀ
150 K using MoK
a
(
l
¼0.7107 A) radiation on a SMART APEX dif-
fractometer equipped with CCD area detector. Data collection and
reduction and structure solution/refinement were carried out using
the software package of APEX II.9 Fischer carbene complex 10 was
solved by direct method, complex 8 was solved by Patterson
method and refined in a routine manner. The non-hydrogen atoms
were treated anisotropically. All hydrogen atoms were geo-
metrically fixed. Three hydrogen atoms in methyl group of complex
8 came in six positions due to symmetry of the molecule.
4.4.5. Pentacarbonyl(1-(allyloxy)ethylidene)chromium(0) (5)5a. Red
oily liquid (poor stability) (55%); IR (neat):
1258 cmꢂ1; 1H NMR (CDCl3, 500 MHz):
6.17 (br s, 1H, OCH2CH]),
5.46e5.54 (m, 4H, OCH2C]CH2), 2.99 (s, 3H, CH3); 13C NMR (CDCl3,
125 MHz): 358.8 (C]Cr), 223.4 (COtrans), 216.5 (COcis), 130.8
n 2064, 1917 (s),
d
d
(CH]), 120.5 (]CH2), 81.3 (br, OCH2), 49.6 (br, CH3).
4.4.6. Pentacarbonyl(1-methoxyethylidene)tungsten(0)
(6)6b. Yellow crystalline solid (60%); IR (neat):
n
2070, 1929 (s),
4.61 (s, 3H, OCH3),
333.4 (C]W), 203.6
4.4. General procedure
1451, 1263 cmꢂ1
; d
1H NMR (CDCl3, 300 MHz):
2.91 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz):
d
To a stirred suspension of metal hexacarbonyl (2 mmol) in
freshly dried ether (8 mL) was added slowly alkyl or phenyl lithium
(1e1.6 M, 1.98 mmol, n-BuLi in hexane, MeLi and PhLi in ether) at
20 ꢀC under argon atmosphere. The reaction mixture was protected
from light and was stirred for 20 min at room temperature. The
slurry became a homogeneous yellowish brown or red solution.
The solvent was removed under reduced pressure without heating,
argon purged ice-cold water (5 mL) was added quickly under argon
atmosphere. The aqueous solution was then filtered through a sy-
ringe filter to a 10-mL round-bottomed flask. Tetrabutylammonium
bromide (5e10 mol %) and organic halide (amount according to
Table 1) were added and the biphasic solution was stirred vigor-
ously under argon atmosphere (condition according to Table 1). As
(COtrans), 197.4 (COcis), 70.4 (OCH3), 52.3 (CH3). Anal. Calcd for
C8H6O6W: C, 25.16; H, 1.58. Found: C, 25.56; H, 1.74.
4.4.7. Pentacarbonyl(1-ethoxyethylidene)tungsten(0)
(7). Yellow
1H
crystalline solid (57%); IR (neat):
n
2071, 1912 (s), 1258 cmꢂ1
;
NMR (CDCl3, 300 MHz):
d
4.87 (q, 2H, J¼6.8 Hz, OCH2), 2.89 (s, 3H,
CH3), 1.62 (t, 3H, J¼7.1 Hz, OCH2CH3); 13C NMR (CDCl3, 75 MHz):
d
330.4 (C]W), 203.6 (COtrans), 197.4 (COcis), 80.7 (OCH2), 52.5
(CH3), 14.9 (OCH2CH3). Anal. Calcd for C9H8O6W: C, 27.30; H, 2.04.
Found: C, 27.02; H, 1.92.
4.4.8. Pentacarbonyl(1-isopropoxyethylidene)tungsten(0)
(8)6b. Yellow crystalline solid (68%); IR (neat):
n 2072, 1910 (s),

112
B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
1283 cmꢂ1
;
1H NMR (CDCl3, 300 MHz):
d
5.63e5.66 (m, 1H, OCH),
W), 203.7 (COtrans), 197.2 (COcis), 155.5 (ArC), 131.4 (ArC), 128.0
(ArC), 126.0 (ArC), 88.8 (OCH), 22.4 [OCH(CH3)2]. Anal. Calcd for
C15H12O6W: C, 38.16; H, 2.56. Found: C, 38.52; H, 2.45.
2.86 (s, 3H, CH3), 1.53 [d, 6H, J¼6.1 Hz, OCH(CH3)2]; 13C NMR (CDCl3,
75 MHz):
d 325.6 (C]W), 203.6 (COtrans), 197.3 (COcis), 90.0 (OCH),
52.6 (CH3), 22.3 [OCH(CH3)2]. Anal. Calcd for C10H10O6W: C, 29.29;
H, 2.46. Found: C, 29.52; H, 2.44.
4.4.16. Pentacarbonyl(methoxy(phenyl)methylene)molybdenum(0)
(16). Orange solid (poor stability) (45%); IR (neat):
1449, 1234 cmꢂ1 1H NMR (CDCl3, 500 MHz):
7.57 (m, 2H, ArH),
7.40e7.47 (m, 3H, ArH), 4.86 (s, 3H, OCH3); 13C NMR (CDCl3,
125 MHz): 339.8 (C]Mo), 213.7 (COtrans), 205.9 (COcis), 153.8
n 2068, 1925 (s),
4.4.9. Pentacarbonyl(1-butoxyethylidene)tungsten(0) (9). Yellow oil
;
d
(60%); IR (neat):
300 MHz):
4.81(t, J¼5.9 Hz, 2H,OCH2e), 2.88(s, 3H,CH3),1.91e2.00
n ;
2070, 1917 (s), 1265 cmꢂ1 1H NMR (CDCl3,
d
d
(m, 2H, OCH2CH2e), 1.51e1.58 (m, 2H, OCH2CH2CH2e), 1.03 (t,
(ArC), 132.0 (ArC), 128.2 (ArC), 126.2 (ArC), 69.4 (OCH3).
J¼7.4 Hz, 3H, OCH2CH2CH2CH3); 13C NMR (CDCl3, 75 MHz):
d 330.5
(C]W), 203.7 (COtrans), 197.5 (COcis), 84.7 (OCH2e), 52.3 (CH3), 31.2
(OCH2CH2e), 19.3 (OCH2CH2CH2e), 13.7 (OCH2CH2CH2CH3). Anal.
Calcd for C11H12O6W: C, 31.16; H, 2.85. Found: C, 31.06; H, 3.05.
4.4.17. Pentacarbonyl(1-methoxypentylidene)molybdenum(0)
(17). Yellow oil (35%); IR (neat):
n ;
2070, 1921 (s), 1452, 1258 cmꢂ1
1H NMR (CDCl3, 500 MHz):
d
4.68 (s, 3H, OCH3), 3.24 (t, J¼7.5 Hz,
2H, CH2e), 1.45e1.50 (m, 2H, CH2CH2e), 1.31e1.35 (m, 2H,
CH2CH2CH2e), 0.91 (t, J¼7.2 Hz, 3H, CH2CH2CH2CH3); 13C NMR
4.4.10. Pentacarbonyl(1-(benzyloxy)ethylidene)tungsten(0)
(10). Yellow crystalline solid (72%); IR (neat):
1260 cmꢂ1; 1H NMR (CDCl3, 300 MHz):
7.45 (m, 5H, ArH), 5.81 (s,
2H, OCH2), 2.93 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz):
331.1 (C]
n
2066, 1937, 1904,
(CDCl3, 125 MHz): d 355.9 (C]Mo), 213.2 (COtrans), 205.8 (COcis),
d
69.4 (OCH3), 62.9 (CH2e), 28.1 (CH2CH2e), 22.4 (CH2CH2CH2e), 13.8
(CH2CH2CH2CH3).
d
W), 203.5 (COtrans), 197.4 (COcis), 134.0 (ArC), 129.5 (ArC), 129.1
(ArC), 128.7 (ArC), 86.4 (OCH2), 52.5 (CH3). Anal. Calcd for
C14H10O6W: C, 36.71; H, 2.20. Found: C, 36.61; H, 2.12.
4.4.18. Pentacarbonyl(1-isopropoxypentylidene)
(18). Yellow oil (40%); IR (neat):
2068, 1921 (s), 1283 cmꢂ1
NMR (CDCl3, 500 MHz):
molybdenum(0)
n
;
1H
d
5.67 (heptet, J¼6.2 Hz, 1H, OCH), 3.18 (t,
4.4.11. Pentacarbonyl(1-(allyloxy)ethylidene)tungsten(0)
J¼7.8 Hz, 2H, CH2e), 1.52 (d, J¼6 Hz, 6H, OCH(CH3)2), 1.42e1.48 (m,
2H, CH2CH2e), 1.30e1.37 (m, 2H, CH2CH2CH2e), 0.91(t, J¼7.2 Hz,
(11). Yellowish orange oil (70%); IR (neat):
n
2072, 1915 (s),
1260 cmꢂ1 1H NMR (CDCl3, 300 MHz):
;
d
6.17 (ddt, J¼16.3, 10.5,
3H, CH2CH2CH2CH3); 13C NMR (CDCl3, 125 MHz):
d 347.2 (C]Mo),
5.8 Hz, 1H, OCH2CH]), 5.51 (ddd, J¼17.0, 2.6, 1.2 Hz, 1H, OCH2CH]
CHcis), 5.46 (ddd, J¼10.5, 2.1, 1.0 Hz, OCH2CH]CHtrans), 5.31 (d,
J¼5.5 Hz, 2H, OCH2e), 2.92 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz):
213.5 (COtrans), 205.8 (COcis), 89.1 (OCH), 63.0 (CH2e), 28.0
(CH2CH2e), 22.5, 22.2 [CH2CH2CH2e and OCH(CH3)2], 14.0
(CH2CH2CH2CH3).
d
331.3 (C]W), 203.5 (COtrans), 197.3 (COcis), 130.8 (CH]), 120.7 (]
CH2), 84.8 (OCH2), 52.4 (CH3).
4.4.19. Pentacarbonyl(1-((1R
*
,2S
*
,5R
*
)-2-isopropyl-5-methyl-
cyclohexyl)propan-2-ylidene)tungsten(0) (23). Yellow oil; Rf (hex-
25
4.4.12. Pentacarbonyl(1-methoxypentylidene)tungsten(0)
ane) 0.71; [
1275 cmꢂ1
a
]
D
ꢂ75.4 (c 1.4, CHCl3); IR (neat):
n 2070, 1915 (s),
4.90 (td, J¼10.4, 3.7 Hz,
(12). Orange oil (48%); IR (neat):
n
2070, 1915 (s), 1451, 1258 cmꢂ1
;
;
1H NMR (CDCl3, 500 MHz):
d
1H NMR (CDCl3, 500 MHz):
d
4.60 (s, 3H, OCH3), 3.21 (t, J¼7.5 Hz,
1H, OCH), 2.92 (s, 3H, W]CCH3), 2.10 (br d, J¼12 Hz, 1H), 1.76e1.89
2H, CH2e), 1.47e1.52 (m, 2H, CH2CH2e), 1.33e1.37 (m, 2H,
CH2CH2CH2e), 0.92 (t, J¼7.2 Hz, 3H, CH2CH2CH2CH3); 13C NMR
(m, 4H), 1.65 (br s, 1H), 1.18e1.30 (m, 2H), 0.92e1.04 (m, 7H), 0.84
(d, J¼7 Hz, 3H); 13C NMR (CDCl3, 125 MHz):
d 327.2 (C]W), 203.0
(CDCl3, 125 MHz):
d
337.9 (C]W), 203.4 (COtrans), 197.5 (COcis), 70.5
(COtrans), 197.6 (COcis), 95.8 (OCH), 53.6 (W]CCH3), 47.9, 41.9, 34.2,
31.3, 26.9, 24.3, 22.0, 21.4, 17.3. Anal. Calcd for C17H22O6W: C, 40.34;
H, 4.38. Found: C, 40.52; H, 4.07.
(OCH3), 64.9 (CH2e), 28.7 (CH2CH2e), 22.5 (CH2CH2CH2e), 14.0
(CH2CH2CH2CH3). Anal. Calcd for C11H12O6W: C, 31.16; H, 2.85.
Found: C, 31.49; H, 2.91.
4.4.20. Pentacarbonyl(1-((1S
*
,2S
*
,5R )-2-isopropyl-5-methyl-
*
4.4.13. Pentacarbonyl(1-isopropoxypentylidene)tungsten(0)
cyclohexyloxy)ethylidene)tungsten(0) (25). Yellow oil; Rf (hexane)
26
(13). Yellow crystalline solid (65%); IR (neat):
n
2068, 1913 (s),
0.66; [
a
]
þ40.6ꢀ (c 1.3, CHCl3); IR (neat):
n 2070, 1915 (s),
D
1263 cmꢂ1. 1H NMR (CDCl3, 300 MHz):
d
5.64 (heptet, J¼6.1 Hz, 1H,
1263 cmꢂ1; 1H NMR (CDCl3, 500 MHz):
d 5.42 (br s, 1H, OCH), 2.94
OCH), 3.16 (t, J¼7.5 Hz, 2H, CH2e), 1.52 [d, J¼6.2 Hz, 6H, OCH(CH3)2],
1.41e1.56 (m, 2H, CH2CH2e), 1.35 (sextet, J¼7.2 Hz, 2H,
CH2CH2CH2e), 0.92 (t, J¼7.2 Hz, 3H, CH2CH2CH2CH3); 13C NMR
(s, 3H, W]CCH3), 2.07 (br d, J¼14.5 Hz, 1H), 1.82e1.89 (m, 2H),
1.54e1.62 (m, 3H), 1.36e1.44 (m, 2H), 1.04e1.09 (m, 1H), 0.92e0.98
(m, 6H), 0.87 (d, J¼6.5 Hz, 3H); 13C NMR (CDCl3, 125 MHz):
d 327.6
(CDCl3, 75 MHz):
d
329.9 (C]W), 203.6 (COtrans), 197.4 (COcis), 89.8
(C]W), 203.1 (COtrans), 197.6 (COcis), 93.7 (OCH), 53.6 (W]CCH3),
47.0, 41.0, 34.7, 29.8, 27.2, 24.4, 22.4, 21.8, 20.0. Anal. Calcd for
C17H22O6W: C, 40.34; H, 4.38. Found: C, 40.53; H, 4.01.
(OCH), 64.9 (CH2), 28.5 (CH2CH2e), 22.5, 22.3 [CH2CH2CH2e and
OCH(CH3)2], 14.1 (CH2CH2CH2CH3). Anal. Calcd for C13H16O6W: C,
34.54; H, 3.57. Found: C, 34.92; H, 3.65.
Acknowledgements
4.4.14. Pentacarbonyl(methoxy(phenyl)methylene)tungsten(0)
(14). Orange crystalline solid (61%); IR (KBr):
1219 cmꢂ1 1H NMR (CDCl3, 500 MHz):
7.56e7.58 (m, 2H, ArH),
7.46e7.50 (m, 1H, ArH), 7.40e7.43 (m, 2H, ArH), 4.77 (s, 3H, CH3);
13C NMR (CDCl3, 125 MHz):
322.2 (C]W), 203.6 (COtrans), 197.3
(COcis), 155.3 (ArC), 131.8 (ArC), 128.2 (ArC), 126.3 (ArC), 70.1 (OCH3).
Anal. Calcd for C13H8O6W: C, 35.16; H, 1.82. Found: C, 35.51; H, 1.60.
n 2070, 1929 (s),
S.S. thanks DST, India, for financial support by a grant [SR/S1/OC-
38/2007]. B.N. is thankful to CSIR for his fellowship. We thank
Professor Amitabha Sarkar for helpful discussion in the manuscript
preparation. Crystal structure determination was performed at the
DST-funded National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, IACS.
;
d
d
4.4.15. Pentacarbonyl(isopropoxy(phenyl)methylene)tungsten(0)
Supplementary data
(15). Orange crystalline solid (50%); IR (neat):
n
2068, 1991, 1977,
7.50e7.53 (m, 2H,
1904 (s),1244 cmꢂ1; 1H NMR (CDCl3, 300 MHz):
d
1H and 13C spectra for all Fischer carbene complexes. CCDC
787311 and 787312 contain crystallographic data for 8 and 10. This
material can be obtained free of charge from the Cambridge
ArH), 7.41e7.48 (m, 3H, ArH), 5.92 (heptet, J¼6.0 Hz, 1H, OCH), 1.66
(d, J¼6.2 Hz, 6H, OC(CH3)2); 13C NMR (CDCl3, 75 MHz):
d 316.1 (C]

B. Nandi, S. Sinha / Tetrahedron 67 (2011) 106e113
113
Chem., Int. Ed. 2000, 39, 3964e4002; (n) Barluenga, J. Pure Appl. Chem. 2002, 74,
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif (fax: þ44 1223 336033 or e-mail: depos-
it@ccdc.cam.ac.uk). Supplementary data associated with this article
can be found in the online version, at doi:10.1016/j.tet.2010.11.016.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
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