(Cyclopentadienone)iron shvo complexes: Synthesis and applications to hydrogen transfer reactions

Article abstract of DOI:10.1021/om101101r

A series of (cyclopendienone)iron tricarbonyl complexes were prepared using an intramolecular cyclization strategy. These were applied to the catalysis of the oxidation of alcohols to aldehydes and ketones. When paraformaldehyde was used as the hydrogen acceptor, formate esters were obtained as coproducts and, in several cases, the major products.

Full text of DOI:10.1021/om101101r

ARTICLE
pubs.acs.org/Organometallics
(Cyclopentadienone)iron Shvo Complexes: Synthesis and
Applications to Hydrogen Transfer Reactions
Tarn C. Johnson, Guy J. Clarkson, and Martin Wills*
Department of Chemistry, The University of Warwick, Coventry CV4 7AL, U.K.
S Supporting Information
b
ABSTRACT: A series of (cyclopendienone)iron tricarbonyl
complexes were prepared using an intramolecular cyclization
strategy. These were applied to the catalysis of the oxidation of
alcohols to aldehydes and ketones. When paraformaldehyde
was used as the hydrogen acceptor, formate esters were
obtained as coproducts and, in several cases, the major products.
’ INTRODUCTION
The ruthenium dimer 1 is widely employed as a reagent for the
transfer of pairs of hydrogen atoms between alcohols and
ketones/aldehydes.1-4 Catalyst 1 splits into two monometallic
complexes, 2 and 3, which are oxidized and reduced versions of
each other; complex 2 removes two hydrogen atoms from an
Figure 1. Mechanism of hydrogen transfer to CdO bonds: concerted
alcohol via a cyclic transition state (Figure 1), while complex 3
transfers two hydrogen atoms to a ketone or aldehyde via the
same mechanism.4 Coupling this process to an enantioselective
esterification process has been employed in efficient dynamic
kinetic resolution of alcohols and amines.2,3 Dimer 1 is prepared
from the tricarbonylruthenium complex 4, by refluxing in
isopropyl alcohol,1 and can be converted fully to 3 using
hydrogen gas or by transfer hydrogenation, e.g. from formic
acid, and thus act as a ketone or imine reduction catalyst.
“outer sphere” process.
biomass residues (e.g., glycerol from biodiesel production, carbo-
hydrates from starch and cellulose, etc.). The use of precious-
metal complexes for this purpose is well established but is prob-
lematic due to their high cost and toxic properties.5 For these
reasons we have recently investigated the use of iron-based com-
plexes for organic transformations and, in particular, (cyclope-
ntadienone)iron complexes6-10 for hydrogen transfer processes.
Several examples of the synthesis and applications of such
complexes to alcohol oxidation10 and ketone reduction6 have
been disclosed in the recent literature. The use of a number of
other iron complexes for reduction of ketones, including asym-
metric variants, has also recently been reported.11
In previously reported work in this area, Casey and Guan
reported on the synthesis and applications of the related iron
hydride complex 5 to ketone hydrogenation and transfer hydro-
genation.6a-c Hydride 5 was formed from the tricarbonyliron
precursor 6, using a process reported by Kn€olker.6d (Cyclopen-
tadienone)iron complexes of this type have been known for some
time,7 having been prepared by the reaction of iron carbonyl
complexes Fe2(CO)9 and Fe3(CO)12 with diphenylacetylene in
1959 by Schrauzer.7a The intramolecular variation of this cycliza-
tion was used in the synthesis of 68 by Pearson et al., who also
We are interested in the development of catalysts for the
transfer of hydrogen atoms between organic molecules, in order
to produce a convenient liquid fuel from alcohols available in
Received: November 23, 2010
Published: March 09, 2011
r
2011 American Chemical Society
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Organometallics
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Scheme 1. Synthesis of (Cyclopentadienone)iron Complexesa
a Reagents and conditions: (i) BrCH2CCH, NaH, THF, 0 °C. (ii) for 13c-e, nBuLi, THF, -78 °C then R3SiCl, -78 °C to room temperature, for 13a,b,
PhI, PdCl2(PPh3)2, CuI, NEt3, 72 h; (iii) Fe(CO)5, toluene, 130 °C, 24 h.
noted that it was an effective method for the formation of
derivatives 7 containing a chiral center (diastereomeric ratio
1.8:1).8 Iron hydride complexes similar to 5 have been
reported and studied by Baird et al.,9 and recently both
Guan10a and Funk10b reported on the use of 5 in the oxidation
reactions of alcohols, using acetone as an acceptor, while
Williams reported a similar application of the iron derivatives
8 and 9.10c In this paper, we describe the synthesis, and
applications to transfer hydrogenation, of a series of (cyclope-
ntadienone)iron complexes.
Figure 2. X-ray crystallographic structure of the minor isomer of
complex 10d,12 showing one of two crystallographically independent
but chemically identical enantiomeric molecules in the X-ray crystal-
lographic structure.
alkylated using propargyl bromide to the diynes 12a,b, respec-
tively. In the next step, either a trialkylsilyl or a phenyl group was
introduced. The resulting diynes that were prepared were then
cyclized using Fe(CO)5 to give the complexes shown. In the case
of 10b-d an unequal mixture of two separable diastereoisomers
was formed. The structure of the minor diastereoisomer of
complex 10d was obtained by X-ray crystallography (Figure 2)12
and proved to be that in which the methyl group on the
dihydrofuran ring was trans to the iron tricarbonyl group. The
relative stereochemistry in 10b,c has been assigned by analogy
’ RESULTS AND DISCUSSION
with that found for 10d. If Fe3(CO)12 was used in the com-
Earlier reports on the use of (cyclopentadienone)iron com-
plexes for hydrogen transfer reactions suggested that a higher
catalyst loading was generally needed in comparison to the case
for the analogous ruthenium catalysts. We therefore selected a
catalyst design (10) which would permit the relatively simple
introduction of variable groups at three positions, providing
scope to adjust the steric hindrance and electronic properties
of the complexes. The approach to the catalysts is summarized in
Scheme 1 and began from the alcohols 11a,b, which were first
plexation, a quantity of an unwanted diiron complex was also
formed; this class of product has previously been identified and
characterized in diyne cyclizations with iron carbonyl reagents.7
The separated diastereoisomers, where appropriate, were tested
separately in the subsequent hydrogen transfer reactions.
In addition, the nitrogen-bridged derivative 14 was prepared
by a similar intramolecular cyclization of 15 (25% yield). An
attempt was made to form complexes in which R2 = H, by
cyclization of 12a,b; however, these were formed in low yields
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Table 1. Hydrogen Transfer from 1-Phenylethanol to Acetone: Initial Testsa
b
entry
complex
cat. (mol %)
[ketone] (mol dm-3
)
added H2Oc
T (°C)
time (days)
conversn to ketone (%)
1
6
6
8
8
8
8
8
8
8
8
8
10
10
10
5
0.24
0.59
0.19
0.38
0.19
0.38
0.19
0.38
0.19
0.38
0.19
0.19
0.17
0.59
0.21
0.59
0.59
yes
yes
no
60
80
60
60
80
80
60
60
80
80
60
60
60
80
60
80
80
2
2
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
trace
trace
29
2
3
4
no
29
5
10
5
no
63
6
no
45
7
10
5
yes
yes
yes
yes
10 mol %
yes
yes
yes
yes
yes
yes
82
8
67
9
10
5
95
10
11
12
13
14
15
16
17
92
10
10
10
10
10
10
10
85
14
trace
trace
trace
trace
trace
trace
10a
10b (major)
10c (major)
10c (major)
14
a Reactions were followed by 1H NMR. b Acetone was used as solvent. c Added water refers to addition of ca. 35 mg of water to the reaction, except for
entry 11.
and were contaminated by side products; therefore, these could
not be tested in hydrogen transfer reactions.
ketone could be obtained using the tetraphenyl-substituted
“iron-Shvo” catalyst 8, only traces of product were obtained with
the other catalysts, even at higher concentrations and after
heating for several days.
In view of the low conversions, efforts were made to synthesize
hydroxycyclopentadienyl iron hydrides;6 the hydride derived
from 6 has been shown to be a very effective alcohol oxidation
catalyst by Guan et al.10a The methods previously discussed for
complex 6 involving base hydrolysis6 were, however, found to be
unsuccessful in our hands when applied to complex 8, although
an impure iron hydride complex could be observed by 1H NMR
when 6 was used as the starting material (see the Supporting
Information). Guan et al. have reported10a that attempts to
isolate iron hydride derivatives of closely analogous complexes
bearing phenyl rings adjacent to the OH group on the cyclo-
pentadienyl ring resulted in decomposition, which they specu-
lated to proceed via a dimeric complex. In contrast, hydride 5
appears to be more stable due to the steric effects of the bulky
trimethylsilyl substituents, which prevent a detrimental dimer
formation.10a There is precedent for the use of KBEt3H to
produce a ruthenium formyl complex from a tolyl analogue of
8 which converted to the hydride upon raising the temperature.4a
Attempts in our hands to reproduce the procedure on complex 8,
however, failed to produce any observable hydride or formyl
proton signals in the 1H NMR spectrum.
For comparative purposes, samples of the tetraphenyl com-
plex 8 and the n-butyl-bridged complex 6 were also prepared,
following literature methods. Complex 6 was prepared by
cyclization of the diyne precursor with iron pentacarbonyl
(67%),6 while 8 was made by direct complexation of 2,3,4,5-
tetraphenylcyclopentadienone with triiron dodecacarbonyl in
91% yield.
The new catalysts were tested in the oxidation of 1-pheny-
lethanol using a series of ketones and aldehydes as the hydrogen
acceptors. Initial tests with acetone were conducted without prior
formation or isolation of the corresponding iron hydride com-
plex: i.e., the objective was to form this in situ. These reactions
1
were initially followed by H NMR or by gas chromatography
(GC); however, the 1H NMR method was prone to errors due to
the volatility of the reagents, and hence GC analysis represents
the preferred technique and was used throughout the rest of our
studies. The results for the acetone-promoted oxidation are
shown in Table 1. Adding a small amount of water to the system
gave higher conversions, perhaps serving to hydrolyze one of the
CO ligands to form the active species, in agreement with results
published by Williams.10c While good conversions of alcohol to
A similar approach by analogy with a communication by
Ogoshi13 involved using borane to donate a hydride to one of
the CO ligands or directly to the metal center via a ring-slip
mechanism. This method enjoyed limited success using the
ruthenium analogue of 8, i.e. 4; weak hydride signals were
observed in the 1H NMR spectrum at -9.86 and -18.37 ppm,
indicating the presence of monomeric and dimeric hydride
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Table 2. Oxidation of 1-Phenylethanol and Derivatives Catalyzed by Iron Complexes Activated in Situ by TMANOa
entry
R
X
R1
R2
cat.
conversn (%)
1
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
6
61
99
2
H
8
3
H
10a
15
4
H
10b (major)
14
5
H
10b (minor)
2
6
H
10c (major)
11
7
H
10c (minor)
34
8
H
10d (major)
11
9
H
10d (minor)
63
10
11
12
13
14
15
16
17
18
19
20
21
H
14
8
8
8
8
8
8
8
8
8
8
8
17
OMe
OMe
Cl
100 (6 h)
88 (5 h)b
48
Me
c-C6H11CH(OH)Me
86
43b
24b
34 b
63b
15b
27b
22b
Me
Me
Me
Me
H
H
H
H
H
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
H
OMe
OMe
Cl
H
H
Me
H
c-C6H11COMe
H
a When acetone was the oxidant, it was used as the solvent. When an aldehyde was the oxidant, 5 equiv was used and toluene was employed as solvent. In
all cases, [ketone] = 0.2 M. b Trace or no formation of ester.
complexes, respectively.4a Using this method with the iron
complex 8, a broad signal was observed at 13.81 ppm, falling
near the expected range for metal formyl protons,14 which could
indicate the presence of an iron formyl complex.
complexes 10b-d, there was a pronounced difference in reac-
tivity between diastereoisomers of the same complex. An elec-
tron-rich substrate was more readily oxidized than an electron-
poor one, and a corresponding primary alcohol proved to be
more resistant to oxidation, giving a product in lower conversion
in agreement with related published results.10 Acetylcyclohexane
formation (entry 14) could be achieved in good conversion
under the standard conditions, indicating that the reaction is not
limited to the preparation of acetophenone derivatives.
Some relatively volatile aldehydes were tested as acceptors for
the reaction—in this case using a 5-fold excess of aldehyde in
toluene solution. While the results were positive, the conversions
remained below those obtained using acetone. Although there is
potential for formation of esters under these conditions, these
were not observed.
When the complexes were tested using paraformaldehyde as
an acceptor with toluene as the solvent, an unexpected observa-
tion was made: the formation of acetophenone was achieved but
the major product of the reaction in most cases was 1-pheny-
lethyl formate (Table 3). Although complex 8 again gave the
most consistently high conversions of 1-phenylethanol, both
isomers of 10c and the nonchiral 10e also gave products in good
conversions under the standard conditions listed. Several com-
plexes showed increased selectivity for 1-phenylethyl formate
over the ketone product. The use of more paraformaldehyde
resulted in increased levels of formation of the formate, although
Following unsuccessful attempts to form hydroxycyclopenta-
dienyl hydride complexes, and with a view to develop a practical
process, our efforts were instead focused on in situ activation.
Trimethylamine N-oxide (TMANO) is a known reagent for the
decarbonylation of metal carbonyl complexes15 which has been
used to mediate ligand substitution reactions of cyclopentadie-
none carbonyl complexes16 and demetalation to form the free
cyclopentadienone.17 Since we started this project, Funk et al.
reported the use of this method for activation of complex 6
toward the alcohol oxidation process and disclosed extensive
applications and mechanistic details.10b It was found that a
vented vessel was required for best results: i.e., to release the
trimethylamine and carbon dioxide which is likely formed upon
reaction of Me3NO with a carbonyl ligand of the complex,
thereby rendering the decarbonylation irreversible.
Using 1 molar equiv of TMANO per mole of complex,
improved in situ activation of iron cyclopentadienone complexes
toward hydrogen transfer was achieved using standard conditions
of heating at 60 °C for 24 h in the presence of an excess of the
acceptor (Table 2). When acetone was used as the acceptor,
complex 8 again gave the highest conversion (99%) out of the
catalysts tested, followed by complex 10d (minor) (63%). Using
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Table 3. Reaction of 1-Phenylethanol in the Presence of Paraformaldehyde with Iron Complexesa
selectivity
entry
complexb
X
R
n
total conversn (%) (time (h))c
ketone
formate
1
6
8
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
5
67 (6)
88 (6)
30 (6)
24 (6)
7 (6)
26
56
26
29
39
52
22
41
22
32
42
19
15
14
70
55
65
50
19
24
74
44
74
71
61
48
78
59
78
68
58
81
85
86
30
45
35
50
81
76
2
H
5
5
3
10a
H
4
10b (major)
H
5
5
10b (minor)
H
5
6
10c (major)
H
5
98 (6)
85 (6)
34 (6)
71 (6)
96 (5)
78 (6)
94 (5)
99 (4)
80 (6)
93 (6)
97 (3)
94 (3)
96 (6)
99 (3)
91 (6)
7
10c (minor)
H
5
8
10d (major)
H
5
9
10d (minor)
H
5
10
11
12
13
14
15
16
17
18
19
20
10e
14
H
5
H
5
10e
10e
10e
8
H
10
15
25
5
H
H
OMe
OMe
Cl
8
5
8
Me
Me
H
5
10e
10e
10e
OMe
OMe
Cl
5
5
Me
5
a In all cases, [ketone] = 0.2 M; in cases where the reaction time is 24 h, a further 5 equiv of paraformaldehyde was added after 4 h. b A control reaction
with no catalyst resulted in no formation of product. c Unless otherwise stated, the reaction time was 24 h.
the ratio appeared to remain unchanged at ca. 15:85 (entries 12-
14), even when a large excess was used. At these high loadings of
paraformaldehyde, the conversion decreased, possibly due to
catalyst inhibition. The promising results obtained with 8 and
10e prompted us to conduct further tests on extended substrates
(entries 14-19). Similar results were obtained to those observed
with acetone, with electron-rich substrates being more quickly
oxidized in higher conversions. We are not aware of a similar
transformation using an iron-based catalyst.
The formate may be formed by trapping the alcohol with a
molecule of formaldehyde and subsequent hydride transfer
(Scheme 2). The hydride transfer step would be required to
take place via a η5-η3 slippage of the cyclopentadienyl ring, as
has been proposed for related systems.4 Alternatively, a hemi-
acetal may be lost and subsequently oxidized through a
Tishchenko-type mechanism, catalyzed by the complex.18a
Subsequent to the completion of this series of experiments, a
report on the formylation of amines with paraformaldehyde
using iridium complexes was published,18b the mechanism of
which may have features in common with that shown in
Scheme 2.
that were examined, under conditions of in situ activation, the
(tetraphenylcyclopentadienone)iron catalyst 8 proved to be the
most active for oxidation using acetone as an acceptor, although
several catalysts exhibited a similar activity for hydrogen transfer
with paraformaldehyde as an acceptor, resulting in an unexpected
competing formylation reaction. To our knowledge, the paraf-
ormaldehyde-formate conversion has not previously been re-
ported using any iron-based catalyst and may have some value as
a potential “green” transformation given the relatively low
toxicity of iron compared to that of more commonly used
precious-metal catalysts.
’ EXPERIMENTAL SECTION
Solvents and reagents for the synthesis of complexes and catalytic
reactions were degassed prior to use, and all reactions were carried out
under either a nitrogen or argon atmosphere. All heated experiments
were conducted using thermostatically controlled oil baths. Reactions
were monitored by TLC using aluminum-backed silica gel 60 (F254)
plates and visualized using UV 254 nm or phosphomolybdic acid
(PMA), ninhydrin, potassium permanganate, and vanillin dips as
appropriate. Flash column chromatography was carried out routinely
using 60 Å silica gel (Merck). Reagents were used as received from
commercial sources unless otherwise stated. 1H NMR spectra were recor-
ded on a Bruker DPX (300 or 400 MHz) spectrometer. Chemical shifts are
reported in δ units, parts per million relative to the singlet at 7.26 ppm for
In summary, a series of novel (cyclopentadienyl)iron tricar-
bonyl complexes were prepared and tested, alongside closely
related but known complexes, as catalysts for the oxidation of
alcohols by a transfer hydrogenation mechanism. Of the series
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Scheme 2. Proposed Mechanism for Formation of Formate
was added. After 17 h the reaction was quenched with H2O (10 mL), the
THF was removed under reduced pressure, and the product was extracted
into Et2O (3 ꢀ 20 mL). The combined organic phase was dried over
Na2SO4 andfiltered, andthesolventwasremovedunderreducedpressureto
give the product 13c as a brown oil (1.385 g, 5.40 mmol, 99%). ESI MS: m/z
found Mþ þ Na 279.1182, calcd for C16H20NaOSi 279.1176. IR: νmax 1489,
1
1443, 1330, 1250, 1094, 1067, 990, 839, 754, 689 cm-1. H NMR (400
MHz, CDCl3): δ 7.42-7.46 (m, 2H, Ar), 7.30-7.33 (m, 3H, Ar), 4.60 (q,
J= 6.5 Hz, 1H, CCH(CH3)O), 4.41(d, J= 15.6 Hz, 1H, CCH2O), 4.31 (d, J
= 15.6 Hz, 1H, CCH2O), 1.56 (d, J = 6.5 Hz, 3H, CCH(CH3)O), 0.19 (s,
9H, Si(CH3)3). 13C NMR (75 MHz, CDCl3): δ 131.73, 128.39, 128.27,
122.54, 101.27, 91.31, 88.12, 85.54, 64.68, 56.62, 22.05, -0.18. ESMSþ: m/
z 279 [M þ Na]þ.
4-Phenyl-3-butyn-2-yloxy(3-(tert-butyldimethylsilyl)prop-2-
yne) (13d). This compound was synthesized by the same procedure as for
13c using 4-phenyl-3-butyn-2-yloxy(prop-2-yne) (12b; 0.350 g, 1.90
mmol), n-butyllithium in hexanes (1.6 M, 1.40 mL, 6.53 mmol), and tert-
butyldimethylsilylchloride (0.373 g, 2.48 mmol) and was purified by
column chromatography on silica with a gradient elution from 100% hexane
to 80/20 hexane/ethyl acetate to give the product 13d as a yellow oil (0.421
g, 1.41 mmol, 74%). ESI MS: m/z found Mþ þ Na 321.1637, calcd for
C19H26NaOSi 321.1645. IR: νmax 2953, 2930, 2856, 1490, 1463, 1471,
1443, 1330, 1251, 1094, 1068, 990, 836, 824, 810, 775, 754, 689 cm-1. 1H
NMR (300 MHz, CDCl3): δ 7.42-7.46 (m, 2H, Ar), 7.28-7.34 (m, 3H,
Ar), 4.64 (q, J = 6.8 Hz, 1H, CCH(CH3)O), 4.41 (d, J = 15.8 Hz, 1H,
CCH2O), 4.33 (d, J = 15.8 Hz, 1H, CCH2O), 1.55 (d, J = 6.8 Hz, 3H,
(CCH(CH3)O), 0.95 (s, 9H, Si(CH3)2C(CH3)3) 0.12 (s, 6H, Si-
(CH3)2C(CH3)3). 13C NMR (75 MHz, CDCl3): δ 131.77, 128.43,
128.23, 122.54, 101.95, 89.68, 88.18, 87.25, 85.89, 64.36, 56.58, 26.05,
22.02, -4.68. ESMSþ: m/z 321 [M þ Na]þ.
chloroform. Coupling constants (J) are measured in hertz. IR spectra were
recorded on a Perkin-Elmer Spectrum One FT-IR Golden Gate instrument.
Mass spectra were recorded on a Bruker Esquire2000 or a Bruker
MicroTOF mass spectrometer. Melting points were recorded on a Stuart
Scientific SMP 1 instrument and are uncorrected. GC analysis was
performed using a Hewlett-Packard 5890 instrument. Dry solvents were
purchased and used as received. The following compounds are known and
have been fully characterized: N-tert-butoxycarbonyldipropargylamine,19
1,8-bis(trimethylsilyl)-1,7-octadiyne,20 4-phenyl-3-butyn-2-yloxy(prop-2-yne)
(12b),21 3-phenyl-2-propyn-1-yloxy(prop-2-yne) (12a),22 3-phenyl-2-
propyn-1-yloxy(3-phenylprop-2-yne) (13a),23 4-phenyl-3-butyn-2-ylo-
xy(3-phenylprop-2-yne) (13b),21 and tricarbonyl(2,4-bis(trimethyls-
ilyl)bicyclo[4.3.0]nona-1,4-dien-3-one)iron (6).8
3-Phenyl-2-propyn-1-yloxy(3-(trimethylsilyl)prop-2-yne)23b
(13e). This compound was synthesized by the same procedure as for 13c
using 3-phenyl-2-propyn-1-yloxy(prop-2-yne) (1.00 g, 5.88 mmol), n-
butyllithium in hexanes (1.6 M, 4.38 mL, 7.01 mmol), and chlorotrimethyl-
silane (0.96 mL, 7.56 mmol) was added. The product was isolated as an
orange oil (1.249 g, 5.15 mmol, 88%). ESI MS: m/z found Mþ þ Na
265.1018, calcd for C15H18NaOSi 265.1019. IR: νmax 2957, 2899, 1489,
1344, 1249, 1077, 998, 839, 755, 690 cm-1. 1H NMR (300 MHz, CDCl3):
δ 7.42-7.49 (m, 2H, Ar), 7.28-7.35 (m, 3H, Ar), 4.47 (s, 2H, CH2), 4.32
(s, 2H, CH2), 0.19 (s, 9H, Si(CH3)3). 13C NMR (75 MHz, CDCl3):
δ 131.8, 128.5, 128.3, 122.5, 100.7, 92.0, 86.7, 84.3, 57.4, 57.4, -0.2).
ESMSþ: m/z 265 [M þ Na]þ.
1,7-Bis(trimethylsilyl)-N-tert-butoxycarbonyldipropargy-
lamine (15). N-tert-Butoxycarbonyldipropargylamine19 (1.50 g, 7.78
mmol) was dissolved in dry THF (50 mL) and cooled to -78 °C.
n-Butyllithium in hexanes (1.6 M, 10.0 mL, 16.0 mmol) was added
dropwise, and the mixture was stirred for 2 h, after which time
chlorotrimethylsilane (2.00 mL, 15.6 mmol) was added and the solution
was warmed to room temperature. The reaction was quenched after 45 h
with saturated NH4Cl solution (50 mL), and the product was extracted
into Et2O (3 ꢀ 50 mL), dried over MgSO4, and filtered, and the solvent
was removed in vacuo. The product 15 was purified by column
chromatography on silica with a gradient elution from 100% hexane
to 80/20 hexane/ethyl acetate to give a pale yellow liquid (1.34 g, 3.97
mmol, 51%). ESI MS: m/z found Mþ þ Na 360.1802, calcd for
C17H31NNaO2Si2 360.1791. IR: νmax 1703, 1444, 1400, 1365, 1240,
1162, 1006, 837, 758 cm-1. 1H NMR (400 MHz, CDCl3): δ 4.14 (broad
Tricarbonyl(2,4-bis(trimethylsilyl)-7-N-tert-butoxycarbo-
nylaminebicyclo[3.3.0]hepta-1,4-dien-3-one)iron (14). Fe-
(CO)5 (1.56 mL, 11.9 mmol) and 1,7-bis(trimethylsilyl)-N-tert-butox-
ycarbonyldipropargylamine (15; 0.499 g, 1.48 mmol) were dissolved in
dry toluene (10 mL) and heated at 130 °C in a sealed pressure tube for
24 h. The solution was cooled to room temperature before the pressure
was released. Hot filtration and removal of the solvent under reduced
pressure gave a brown solid (0.886 g). The product was purified by
column chromatography on silica with a gradient elution from 98/2
hexane/ethyl acetate to 85/15 hexane/ethyl acetate to give the product
14 as a yellow solid (0.189 g, 0.374 mmol, 25%). Mp: 166-167 °C. ESI
MS: m/z found Mþ þ H 506.1122, calcd for C21H32FeNO6Si2
506.1112. IR: νmax 2070, 2016, 1994, 1695, 1620, 1415, 1363, 1243,
1165, 1109, 840, 766 cm-1. 1H NMR (400 MHz, CDCl3): δ 4.32-4.52
(broad m, 4H, CH2), 1.51 (s, 9H, (CH3)3CCO2N), 0.26 (s, 18H,
Si(CH3)3). 13C NMR (100 MHz, CDCl3): δ 207.80, 181.58, 154.56,
112.19, 111.76, 81.01, 69.55, 69.25, 47.57, 28.39, -1.04. ESMSþ: m/z
506 [M þ H]þ.
s, 4H, CH2), 1.47 (s, 9H, (CH3)3CCO2N), 0.16 (s, 18H, Si(CH3)3). 13
C
NMR (100 MHz, CDCl3): δ 165.49, 154.40, 100.86, 80.77, 36.00, 28.29,
-0.11. ESMSþ: m/z 360 [M þ Na]þ.
4-Phenyl-3-butyn-2-yloxy(3-(trimethylsilyl)prop-2-yne)
(13c). 4-Phenyl-3-butyn-2-yloxy(prop-2-yne) (12b; 1.00 g, 5.45 mmol)
was dissolved in dry THF (15 mL) and cooled to -78 °C. n-Butyllithium in
hexanes (2.5 M, 2.61 mL, 6.53 mmol) was added dropwise, and the mixture
was stirred for 1 h, after which chlorotrimethylsilane (0.90 mL, 7.09 mmol)
Tricarbonyl(tetraphenylcyclopentadienone)iron (8).7a
Fe3(CO)12 (0.362 g, 0.653 mmol) and tetraphenylcyclopentadienone
(0.250 g, 0.650 mmol) were dissolved in dry toluene (3 mL) and heated
at 80 °C in a sealed pressure tube for 20 h, after which the solution was
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cooled to room temperature and the solvent was removed under
reduced pressure. The black solid was dissolved in ethyl acetate, the
solution was filtered through Celite, and the solvent was removed under
reduced pressure to give the product 8 as a yellow solid (0.311 g, 0.593
mmol, 91%). Mp: 174-175 °C dec. ESI MS: m/z found Mþ þ Na
547.0604, calcd for C32H20FeNaO4 547.0604. IR: νmax 2061, 1987,
from 100% hexane to 40/60 hexane/ethyl acetate to give two diaster-
eomers (2.7:1) of product, which were separated as brown oils. Minor
diastereomer (0.060 g, 0.141 mmol, 12%): ESI MS m/z found Mþ þ H
425.0497, calcd for C20H21FeO5Si 425.0502; IR νmax 2065, 2010, 1992,
1633, 1249, 1056, 842, 768, 695 cm-1; 1H NMR (300 MHz, CDCl3) δ
7.99-8.03 (m, 2H, Ar), 7.29-7.40 (m, 3H, Ar), 5.57 (q, J = 6.4 Hz, 1H,
CCH(CH3)O), 4.81 (d, J = 12.8 Hz, 1H, CH2), 4.71 (d, J = 12.8 Hz, 1H,
CH2), 1.52 (d, J = 6.4 Hz, 3H, CH3), 0.33 (s, 9H, Si(CH3)3); 13C NMR
(75 MHz, CDCl3) δ 207.86, 177.16, 131.85, 128.89, 128.23, 126.88,
108.46, 107.89, 77.25, 75.87, 66.08, 65.70, 18.98, -0.87; ESMSþ m/z
425 [M þ H]þ. Major diastereomer (0.166 g, 3.91 mmol, 33%): ESI MS
m/z found Mþ þ H 425.0501, calcd for C20H21FeO5Si 425.0502; IR
νmax 2064, 1998, 1635, 1250, 842 cm-1; 1H NMR (300 MHz, CDCl3) δ
7.48-7.52 (m, 2H, Ar), 7.30-7.40 (m, 3H, Ar), 5.36 (q, J = 6.4 Hz, 1H,
CCH(CH3)O), 4.79 (d, J = 13.2 Hz, 1H, CH2), 4.71 (d, J = 13.2 Hz, 1H,
CH2), 1.65 (d, J = 6.4 Hz, 3H, CH3), 0.31 (s, 9H, Si(CH3)3); 13C NMR
(75 MHz, CDCl3) δ 207.88, 174.92, 129.73, 129.40, 128.40, 128.24,
113.20, 108.68, 81.58, 74.90, 66.72, 64.77, 21.67, -01.00; ESMSþ m/z
425 [M þ H]þ.
1639, 1498, 1444, 752, 695 cm-1 1H NMR (300 MHz, CDCl3):
.
δ 7.55-7.61 (broad m, 4H, para H) 7.20-7.28 (broad m, 8H, meta
H), 7.16 (broad d, J = 4.5, 8H, ortho H). 13C NMR (75 MHz, CDCl3):
δ 208.48, 169.73, 131.73, 130.74, 130.24, 129.82, 128.64, 127.98, 127.97,
127.82, 103.97, 82.42. ESMSþ: m/z 525 [M þ H]þ.
Tricarbonyl(2,4-bis(phenyl)-7-oxybicyclo[3.3.0]hepta-1,4-
dien-3-one)iron (10a). Compound 13a (0.300 g, 1.22 mmol) and
Fe(CO)5 (0.48 mL, 3.65 mmol) were dissolved in dry toluene (3 mL)
and heated at 130 °C for 24 h, after which the solution was cooled to
room temperature and the solvent was removed under reduced pressure.
The brown residue was filtered through Celite using a 9/1 hexane/ethyl
acetate mixture to give an orange residue. The product was purified by
column chromatography on silica with a gradient elution from 100%
hexane to 80/20 hexane/ethyl acetate to give the product 10a as a
yellow-brown solid (0.196 g, 0.473 mmol, 39%). Mp: 218-220 °C dec.
ESI MS: m/z found Mþ þ Na 437.0076, calcd for C22H14FeNaO5
Tricarbonyl(2-(tert-butyldimethylsilyl)-4-phenyl-6-methyl-
7-oxybicyclo[3.3.0]hepta-1,4-dien-3-one)iron (10d). These
complexes (two diastereomers) were synthesized by the same procedure
as for 10a, using 13d (0.300 g, 1.01 mmol) and Fe(CO)5 (0.40 mL, 3.04
mmol), and were purified by column chromatography on silica with a
gradient elution from 100% hexane to 60/40 hexane/ethyl acetate to
give two diastereomers (3.0:1) of product, which were separated. Minor
diastereomer: yellow solid (0.066 g, 0.142 mmol, 14%); mp 124-
126 °C; ESI MS m/z found Mþ þ H, 467.0974, calcd for C23H26FeO5Si
467.0972; IR νmax 2064, 1991, 1635, 1250, 1056, 826, 770, 694 cm-1; 1H
NMR (300 MHz, CDCl3) δ 7.99-8.05 (m, 2H, Ar), 7.29-7.39 (m, 3H,
Ar), 5.56 (q, J = 6.8 Hz, 1H, CCH(CH3)O), 4.81 (d, J = 13.2 Hz, 1H,
CH2), 4.71 (d, J = 13.2 Hz, 1H, CH2), 1.53 (d, J = 6.8 Hz, 3H, CH3), 1.01
(s, 9H, SiC(CH3)3) 0.47 (s, 3H, Si(CH3)2C(CH3)3), 0.08 (s, 3H,
Si(CH3)2C(CH3)3); 13C NMR (75 MHz, CDCl3) δ 207.79, 176.89,
131.83, 128.91, 128.27, 126.96, 109.31, 108.14, 76.51, 75.84, 66.54,
65.86, 27.19, 18.96, 18.64, -5.01, -5.32; ESMSþ m/z 467 [M þ H]þ.
Major diastereomer: brown oil (0.181 g, 0.388 mmol, 39%); ESI MS m/
z found Mþ þ H 467.0974, calcd for C23H26FeO5Si 467.0972; IR νmax
2063, 1993, 1634, 1249, 1053, 825, 763, 694 cm-1; 1H NMR (300 MHz,
CDCl3) δ 7.47-7.53 (m, 2H, Ar), 7.29-7.41 (m, 3H, Ar), 5.38 (q, J =
6.0 Hz, 1H, CCH(CH3)O), 4.79 (d, J = 13.2 Hz, 1H, CH2), 4.73 (d, J =
13.2 Hz, 1H, CH2), 1.65 (d, J = 6.0 Hz, 3H, CH3), 0.97 (s, 9H,
SiC(CH3)3) 0.51 (s, 3H, Si(CH3)2C(CH3)3), 0.06 (s, 3H, Si(CH3)2C-
(CH3)3); 13C NMR (75 MHz, CDCl3) δ 207.91, 174.68, 129.68,
129.55, 128.53, 128.38, 114.96, 108.03, 81.17, 74.98, 67.16, 65.36,
27.08, 21.82, 18.76, -5.16; ESMSþ m/z 467 [M þ H]þ.
1
437.0083. IR: νmax 2064, 2004, 1634, 1055, 766, 693 cm-1. H NMR
(400 MHz, CDCl3): δ 7.89 (d, J = 7.0, 4H, Ar), 7.33-7.44 (m, 6H,
phenyl), 5.21-5.27 (d, J = 12.1 Hz, 2H, CH2), 5.08-5.13 (d, J = 12.1
Hz, 2H, CH2). 13C NMR (75 MHz, CDCl3): δ 207.63, 169.68, 131.46,
129.05, 128.57, 127.32, 100.55, 68.33, 65.82. ESMSþ: m/z 415 [M þ
H]þ. A small, broad resonance from 6.8 to 7.8 ppm and a smaller broad
resonance at 5.0 ppm in the 1H NMR spectrum have not been assigned;
these may be due to paramagnetic impurities.
Tricarbonyl(2,4-bis(phenyl)-6-methyl-7-oxybicyclo[3.3.0]-
hepta-1,4-dien-3-one)iron (10b). These complexes (two diaster-
eomers) were synthesized by the same procedure as for 10a, using 13b
(0.300 g, 1.15 mmol) and Fe(CO)5 (0.46 mL, 3.50 mmol), and were
purified by column chromatography on silica with a gradient elution
from 100% hexane to 60/40 hexane/ethyl acetate to give two diaster-
eomers (1.2:1) of the product, which were separated. Minor diaster-
eomer: brown powder (0.050 g, 0.117 mmol, 10%); mp 102-104 °C
dec; ESI MS m/z found Mþ þ Na 451.0235, calcd for C23H16FeNaO5
451.0239; IR νmax 2066, 1995, 1712, 1645, 1444, 1069, 752, 697 cm-1
;
1H NMR (300 MHz, CDCl3) δ 8.06-8.11 (m, 2H, Ar), 7.86-7.93
(m, 2H, Ar), 7.32-7.45 (m, 6H, Ar), 5.64 (q, J = 6.4 Hz, 1H,
(CCH(CH3)O), 5.17 (s, 2H, CH2), 1.54 (d, J = 6.4 Hz, 3H,
(CCH(CH3)O); 13C NMR (75 MHz, CDCl3) δ 207.81, 171.75,
131.73, 131.46, 128.98, 128.95, 128.51, 128.31, 127.34, 126.99, 75.94,
66.31, 19.21; ESMSþ m/z 451 [M þ Na]þ. A broad resonance from 6.5
to 7.6 ppm in the 1H NMR spectrum has not been assigned; this may be
due to paramagnetic impurities. Major diastereomer: brown powder
(0.065 g, 1.52 mmol, 13%); mp 130-132 °C dec; ESI MS m/z found
Tricarbonyl(2-(phenyl)-4-trimethylsilyl-7-oxybicyclo[3.3.0]-
hepta-1,4-dien-3-one)iron (10e). This compound was synthesized
by the same procedure as for 10a, using 13e (0.300 g, 1.24 mmol) and
Fe(CO)5 (0.49 mL, 3.73 mmol), and was purified by column chroma-
tography on silica with a gradient elution from 100% hexane to 60/40
hexane/ethyl acetate to give the product as a yellow solid (0.253 g, 0.617
mmol, 50%). Mp: 129-133 °C;. ESI MS: m/z found Mþ þ H 411.0365,
calcd for C19H19FeO5Si 411.0346. IR: νmax 2058, 1993, 1627, 1246, 843,
761, 691 cm-1. 1H NMR (400 MHz, CDCl3): δ 7.80-7.83 (m, 2H, Ar),
7.31-7.38 (m, 3H, Ar), 5.16-5.20 (d, J = 12.6 Hz, 1H, CH), 5.02-5.07
(d, J = 12.6 Hz, 1H, CH), 4.78-4.82 (d, J = 12.6 Hz, 1H, CH), 4.73-
4.77 (d, J = 12.6 Hz, 1H, CH). 13C NMR (100 MHz, CDCl3): δ 207.8,
176.0, 131.5, 129.0, 128.4, 127.2, 108.9, 104.4, 79.0, 68.3, 67.7, 65.8, -
1.0. ESMSþ: m/z 411 [M þ H]þ.
M
þ þ Na 451.0240, calcd for C23H16FeNaO5 451.0239; IR νmax 2064,
2003, 1718, 1638, 1449, 1054, 768, 694 cm-1; H NMR (300 MHz,
CDCl3) δ 7.90-7.96 (m, 2H, Ar), 7.53-7.59 (m, 2H, Ar), 7.32-7.45
(m, 6H, Ar), 5.40 (q, J = 6.0 Hz, 1H, (CCH(CH3)O), 5.25 (d, J = 13.2
Hz, 1H, CH2), 5.03 (d, J = 13.2 Hz, 1H, CH2) 1.67 (d, J = 6.0 Hz, 3H,
(CCH(CH3)O); 13C NMR (75 MHz, CDCl3) δ 207.91, 131.34,
129.71, 129.04, 128.63, 128.56, 128.45, 127.26, 104.71, 104.56, 79.15,
75.04, 67.33, 30.90, 21.83; ESMSþ m/z 451 [M þ Na]þ. A broad
resonance from 6.6 to 7.8 ppm in the 1H NMR spectrum has not been
assigned; this may be due to paramagnetic impurities.
1
Tricarbonyl(2-(trimethylsilyl)-4-phenyl-6-methyl-7-oxybicyclo-
[3.3.0]hepta-1,4-dien-3-one)iron (10c). These complexes (two
diastereomers) were synthesized by the same procedure as for 10a, using
13c (0.300 g, 1.17 mmol) and Fe(CO)5 (0.46 mL, 3.50 mmol), and were
purified by column chromatography on silica with a gradient elution
Oxidation of 1-Phenylethanol using Iron Catalysts: Table 1.
Complex 8 (10.0 mg, 19.1 μmol) and 1-phenylethanol (23.0 mg, 0.188
mmol) were dissolved in acetone (1 mL) and heated at 60 °C in a sealed
pressure tube for 4 days, after which the solution was cooled to room
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temperature and the solvent was removed under reduced pressure. The
conversions were calculated from the integrations of the methyl peaks in the
1H NMR spectra. The above procedure was repeated for the other
complexes, and the conditions are shown in Table 1. Reactions with a 5
mol % catalyst loading were performed by doubling the quantity of
1-phenylethanol (46.0 mg, 0.377 mmol) without changing any other
conditions.
Oxidation of 1-Phenylethanol using Iron Catalysts: Table 2.
Complex 8 (10.0 mg, 19.1 μmol), trimethylamine N-oxide (2.10 mg, 18.9
μmol), and 1-phenylethanol (23.0 mg, 0.188 mmol) were dissolved in
acetone (1 mL) and heated at 60 °C for 24 h. The reaction was monitored
over time by GC (BP20 PEG column, T = 130 °C, inj T = 220 °C, det T =
220 °C, 15 psi He carrier gas): RT for acetophenone 4.7 min and for
1-phenylethanol 8.1 min. The above procedure was repeated for other
complexes and substrates.
GC conditions: 1-(4-methoxyphenyl)ethanol (BP20 PEG column,
T = 150 °C, inj T = 220 °C, det T = 220 °C, 15 psi He carrier gas), RT for
40-methoxyacetophenone 13.4 min and for 1-(4-methoxyphenyl)ethanol
17.8 min; (anisyl)methanol (BP20 PEG column, T = 150 °C, inj T =
220 °C, det T = 220 °C, 15 psi He carrier gas,. RT for 40-methoxyben-
zaldehyde 9.3 min and for anisylmethanol 22.5 min; 1-(4-chlorophe-
nyl)ethanol (BP20 PEG column, T = 150 °C, inj T = 220 °C, det T =
220 °C, 15 psi He carrier gas), RT for 40-chloroacetophenone 6.0 min
and for 1-(4-chlorophenyl)ethanol 13.7 min; Cyclohexylmethyl alcohol
(BP20 PEG column, T = 110 °C, inj T = 220 °C, det T = 220 °C, 15 psi
He carrier gas), RT for cyclohexyl methyl ketone 3.2 min and for
cyclohexylmethyl alcohol: 5.2 min.
Oxidation of 1-Phenylethanol using Iron Catalysts and
Paraformaldehyde: Table 3. Complex 8 (10.0 mg, 19.1 μmol),
trimethylamine N-oxide (2.1 mg, 18.9 μmol), 1-phenylethanol (23 mg,
0.188 mmol), and paraformaldehyde (29.0 mg, 0.966 mmol) were
dissolved in toluene (1 mL) and heated at 60 °C for 24 h. After 4 h
more paraformaldehyde (29.0 mg, 0.966 mmol) was added. The
reaction was monitored over time by GC (BP20 PEG column, T =
130 °C, inj T = 220 °C, det T = 220 °C, 15 psi He carrier gas): RT for
acetophenone 4.7 min, for 1-phenylethyl formate 5.0 min, and for
1-phenylethanol 8.1 min. Formates were independently synthesized,
and standards were prepared in order to compare GC response
factors.
found Mþ - CO2H, 105.0705, calcd for C8H9 105.0699). IR: νmax 2982,
2931, 1717, 1496, 1452, 1375, 1165, 1059, 1029, 992, 759, 697 cm-1. 1H
NMR (300 MHz, CDCl3): δ 8.10 (s, 1H, OC(O)H), 7.28-7.41 (m,
5H, Ar), 6.03 (q, J = 6.6 Hz, 1H, PhCH), 1.60 (d, J = 6.6 Hz, 3H, CH3).
13C NMR (75 MHz, CDCl3): δ 160.29, 140.83, 128.52, 128.09, 126.09,
72.14, 22.06. ESMSþ: m/z 105 [M - CO2H]þ.
1-(4-Methoxyphenyl)ethyl Formate.24c This compound was
synthesized by the same procedure as for 1-phenylethyl formate using
1-(4-methoxyphenyl)ethanol (0.150 g, 0.986 mmol) and formic acid
(5 mL) and was purified by column chromatography on silica (90/10
hexane/ethyl acetate) to give the product as a colorless oil (0.089 g,
0.494 mmol, 50%). ESI MS: m/z found Mþ þ Na 203.0682, calcd for
C10H12NaO3 203.0679. IR: νmax 2933, 2837, 1718, 1613, 1514, 1459,
1297, 1247, 1169, 1033, 830 cm-1. 1H NMR (300 MHz, CDCl3): δ 8.07
(s, 1H, OC(O)H), 7.28-7.34 (m, 2H, Ar), 6.86-6.92 (m, 2H, Ar), 5.98
(q, J = 6.6 Hz, 1H, PhCH), 3.81 (s, 3H, OCH3), 1.58 (d, J = 6.6 Hz, 3H,
CH3). 13C NMR (75 MHz, CDCl3): δ 160.43, 159.45, 132.91, 127.67,
113.88, 71.93, 55.26, 21.83. ESMSþ: m/z 135 [M - CO2H]þ.
(4-Anisyl)methyl Formate.24 This compound was synthesized by
the same procedure as for 1-phenylethyl formate using (4-ani-
syl)methanol (0.070 g, 0.507 mmol) and formic acid (5 mL) and was
purified by column chromatography on silica (90/10 hexane/ethyl
acetate) to give the product as a colorless oil (0.037 g, 0.223 mmol,
44%). ESI MS: m/z found Mþ þ Na 189.0526, calcd for C9H10NaO3
189.0522. IR: νmax 2936, 2837, 1716, 1612, 1514, 1461, 1303, 1246,
1
1150, 1031, 820 cm-1. H NMR (400 MHz, CDCl3): δ 8.11 (s, 1H,
OC(O)H), 7.29-7.33 (m, 2H, Ar), 6.88-6.92 (m, 2H, Ar), 5.14 (s, 2H,
PhCH2), 3.81 (s, 3H, OCH3). 13C NMR (100 MHz, CDCl3): δ 160.87,
159.80, 130.24, 127.29, 113.99, 65.51, 55.27. ESMSþ: m/z 121 [M -
CO2H]þ.
1-(4-Chlorophenyl)ethyl Formate.24d This compound was
synthesized by the same procedure as for 1-phenylethyl formate using
1-(4-chlorophenyl)ethanol (0.150 g, 0.958 mmol) and formic acid
(5 mL) and was purified by column chromatography on silica (90/10
hexane/ethyl acetate) to give the product as a colorless oil (0.102 g,
0.553 mmol, 58%). ESI MS: m/z found Mþ - CO2H 139.0310, calcd
for C8H8Cl 139.0309. IR: νmax 2984, 2930, 1719, 1494, 1452, 1409,
1
1375, 1342, 1162, 1091, 1058, 1014, 996, 823 cm-1. H NMR (400
MHz, CDCl3): δ 8.07 (s, 1H, OC(O)H), 7.28-7.35 (m, 4H, Ar), 5.97
(q, J 6.5 Hz, 1H, PhCH), 1.56 (d, J 6.5 Hz, 3H, CH3). 13C NMR (100
MHz, CDCl3): δ 160.14, 139.37, 133.85, 128.71, 127.53, 71.38, 22.01.
ESMSþ: m/z 139 [M - CO2H]þ.
GC conditions: 1-phenylethanol (Chrompac cyclodextrin-β-236 M
50 M column, T = 130 °C, inj T = 220 °C, det T = 220 °C, 15 psi He
carrier gas), RT for acetophenone 13.4 min, for 1-phenylethyl formate
15.1 and 15.5 min, and for 1-phenylethanol 17.4 and 18.0 min; 1-(4-
methoxyphenyl)ethanol (Chrompac cyclodextrin-β-236 M 50 M col-
umn, T = 130 °C, inj T = 220 °C, det T = 220 °C, 15 psi H2 carrier gas),
RT for 40-methoxyacetophenone 24.1 min, for 1-(4-methoxyphe-
nyl)ethyl formate 23.7 and 24.8 min, and for 1-(4-methoxyphe-
nyl)ethanol 25.5 and 26.4 min; (4-anisyl)methanol (Chrompac
cyclodextrin-β-236 M 50 M column, T = 150 °C, inj T = 220 °C, det
T = 220 °C, 15 psi H2 carrier gas), RT for 40-methoxybenzaldehyde 8.6
min, for (4-methoxy)benzyl formate 10.4 min, and for (4-ani-
syl)methanol 11.7 min; 1-(4-chlorophenyl)ethanol (Chrompac cyclo-
dextrin-β-236 M 50 M column, T = 150 °C, inj T = 220 °C, det T =
220 °C, 15 psi H2 carrier gas), RT for 40-chloroacetophenone 7.3 min, for
1-(4-chlorophenyl)ethyl formate 8.8 and 9.1 min, and for 1-(4-chlor-
ophenyl)ethanol 10.8 and 11.1 min.
Procedures for Attempted Hydroxycyclopentadienyl Hy-
dride Complex Formation. CO Hydrolysis and Hydride Forma-
tion Using NaOH. Aqueous 1 M NaOH solution (0.96 mL) was added to
a solution of 8 (40.0 mg, 95.6 μmol) in dry THF (4 mL). After 2.5 h a
solution of 85% H3PO4 (0.03 mL) in H2O (1 mL) was added and the
product was extracted into Et2O (3 ꢀ 5 mL), the extracts were dried
over Na2SO4 and filtered, and the solvent was removed in vacuo. No
1
hydride signals were observed in the H NMR spectrum. The above
procedure was repeated for complex 6, and a signal at -12.07
1
ppm attributable to an iron hydride was observed in the H NMR
spectrum (see the Supporting Information).
Loss of CO and Hydride Formation Using BH3. BH3 Me2S (2 M in
3
THF, 0.02 mL, 40.0 μmol) was added to a solution of 4 (0.010 g, 17.6
μmol) in dry THF (5 mL) cooled to -78 °C. After 1 h H2O (0.1 mL) was
added and the solution was warmed to room temperature, after which the
solvent was removed in vacuo. Resonances at -9.86 and -18.37 ppm in
1-Phenylethyl Formate.24a-c 1-Phenylethanol (0.150 g, 1.23
mmol) was dissolved in formic acid (5 mL) with 3 Å molecular sieves,
and the mixture was stirred for 18 h, after which H2O (5 mL) was added.
The product was extracted into Et2O (2 ꢀ 10 mL), the extracts were
washed with H2O (3 ꢀ 20 mL) and dried over MgSO4, and the solvent
was removed under reduced pressure. The product was purified by
column chromatography on silica (90/10 hexane/ethyl acetate) to give
the product as a colorless oil (0.112 g, 0.746 mmol, 61%). ESI MS: m/z
1
the H NMR spectrum indicate the presence of small quantities of the
monomeric and dimeric hydride complexes, respectively. The same
procedure was attempted with 8 and resulted in a broad peak at δ 13.81
in the 1H NMR spectrum, which could indicate the presence of an iron
formyl complex.
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Supporting Information. Text, figures, and a CIF file
b
giving 1H and 13C NMR spectra of novel compounds and X-ray
crystallographic data for 10d (minor isomer). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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Corresponding Author
*Tel: þ44 24 7652 3260. Fax: þ44 24 7652 4112. E-mail:
m.wills@warwick.ac.uk.
’ ACKNOWLEDGMENT
We thank the EPSRC for funding of TCJ through the
“Hydrogen Delivery” Supergen 14 consortium project (EP/
G01244X/1). Dr. B. Stein and colleagues of the EPSRC National
MS service (Swansea) are thanked for HRMS analyses. We
acknowledge the use of the EPSRC Chemical Database Service.25
The Oxford Diffraction Gemini instrument was obtained
through the Science City Project with support from the Advan-
tage West Midlands (AWM) Advanced Materials Project and
partially funded by the European Regional Development Fund
(ERDF).
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