Oxidative radical cyclizations of malonate enolates induced by the ferrocenium ion - A remarkable influence of enolate counterion and additives

Article abstract of DOI:10.1039/b102211n

To generalize oxidative radical cyclizations of malonate enolates induced by recyclable single electron transfer (SET) oxidant ferrocenium hexafluorophosphate 1, the reactivity of a series of enolates 3 that differ only in their counterions was studied. O

Full text of DOI:10.1039/b102211n

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Oxidative radical cyclizations of malonate enolates induced by the
ferrocenium ion – a remarkable influence of enolate counterion
and additives
1
Ullrich Jahn* and Philip Hartmann
Institut für Organische Chemie, Technische Universität Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
Received (in Cambridge, UK) 7th March 2001, Accepted 20th July 2001
First published as an Advance Article on the web 24th August 2001
To generalize oxidative radical cyclizations of malonate enolates induced by recyclable single electron transfer
(
SET) oxidant ferrocenium hexafluorophosphate 1, the reactivity of a series of enolates 3 that differ only in their
counterions was studied. Only lithium enolates were oxidized by 1, irrespective of their generation method. The
presence of amines in stoichiometric or substoichiometric amounts is necessary for SET oxidation–bicyclization
of Na, K, Mg, Zn, Si, or Ti enolates 3. The nucleophilicity of 3, the amine, and the presence of halide ions
determine the bicyclization yield of lactone 7.
Introduction
The reactivity of carbanionic intermediates is critically depend-
ent on the metal ion used. In particular, the reactivity of enolates
1
is modulated by the counterion as manifested in aldol additions,
2
3
conjugate additions or enolate alkylations. Another well-
known synthetic application of enolates is their SET oxidation–
dimerization to 1,4-dicarbonyl compounds. Most commonly,
lithium enolates were applied with different oxidants such as
4a,d,j,k
4
5
FeCl , Cu() salts or I ; other metal ions, such as Na, K,
3
2
6
7
8
Si, Ti, or Sn were less frequently used. Apart from dimer-
ization reactions, the potential of enolates as precursors for
radical or radical cation reactions has been explored much less.
Schmittel et al. studied the reactivity of dimesitylmethyl ketone
9
10
11
enolates in cyclizations to benzofurans. Kende and Whiting
reported radical cyclizations of equilibrium-generated enolates
onto phenolates induced by K [Fe(CN) ]; however, these
3
6
cyclizations are limited to the most acidic carbonyl precursors
such as 1,3-diketones or nitro compounds. Deprotonated nitro
6d,12
compounds can be oxidatively cyclized by CAN.
Some
cyclizations of silyl enol ethers as covalent enolate equivalents,
13
occurring in the radical cation or radical cyclization mode,
have been reported; these were induced by photoelectron
transfer (PET) oxidation with dicyanoanthracene (DCA) or
dicyanonaphthalene (DCN) that give only moderate yields of
14
15
6b,c,16
cyclized products, by anodic oxidation or by oxidants.
Our interest in enolate oxidation was fueled by the idea of
combining the often complementary reactivities of inter-
mediates of different oxidation states such as enolates, radicals
and carbocations in reaction sequences [eqn. (1)].
Scheme 1
1Ϫ
2Ϫ
3
ؒ
4
ؒ
5ϩ
6ϩ
M
[R
R
R
R
R
R ]
P
(1)
19
reactions. Thus, the inherent limitations of oxidative radical
reactions of neutral carbonyl compounds, mediated by Mn-
In these reaction sequences, intermediates are umpoled
2
Ϫ
20
21
(
R
nucleophilic enolates R to electrophilic α-carbonyl radicals
(OAc)3 and CAN which are most efficient for 1,3-dicarbonyl
compounds, can now be overcome since our method allows
oxidative reactions of all acidic carbonyl substrates such as
3
ؒ
4
ؒ
or nucleophilic alkyl radicals R to electrophilic carbenium
5ϩ
ions R ) thus offering a solution to reactivity limitations of
these constitutionally identical but electronically different
intermediates. Recently, we discovered that ferrocenium hexa-
fluorophosphate 1 is a very convenient reagent for the predict-
able SET oxidation of lithium enolates and certain radicals.
From these studies, it became clear that enolates are very useful
22
esters, nitriles, ketones, etc. However, to develop this field
further it is of critical importance to test the general applicability
of metal enolates or their covalent equivalents in SET oxid-
ations by 1 since the efficiency of many anionic processes is
determined by the metal counter ion. In this report, results of
a first systematic study on the overall oxidative bicyclization
efficiency of malonate enolates 3 by 1 are presented (Scheme 1).
We address the remarkable influence of the metal counterion
17
18
precursors for radical cyclizations and radical combinations
Scheme 1). Based on these results, we were then able to develop
(
oxidative tandem processes that combine anionic and radical
DOI: 10.1039/b102211n
J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282
2277
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Table 1 Oxidative cyclizations of metal 2-(5,5-diphenylpent-4-enyl)malonate enolates 3
Entry
MB (equiv.)
1 (equiv.)
2 (%)
7 (%)
8 (%)
9 (%)
10 (%)
a
1
2
3
4
5
6
7
8
LDA (1.3)
2.5
2.4
3.0
2.5
2.2
1.5
2.9
2.6
—
—
13
60
24
63
92
90
64
57
54
Trace
52
26
—
—
12
6
—
—
—
—
—
3
—
—
—
—
10
—
—
—
a
LiHMDS (1.1)
BuLi (1.0)
—
—
—
—
—
—
NaH (2.8)
b
t-BuMgCl (1.3)
BuLi–ZnCl (1.3, 2)
NaH–TMSCl (1.4, 1.5)
NaH–ClTi(Oi-Pr) (1.4, 1.5)
2
—
—
3
a
b
Compare ref. 17. In the presence of 1.7 equivalents of HMPA to solubilize t-BuMgCl.
and amines on the oxidizability of the enolates 3 to α-carbonyl
yield of 7 dropped to 45% while 13% of 8 and 29% of 2 were
isolated. Even tertiary amines such as Hünig’s base or the
essentially non-basic tris(p-bromophenyl)amine proved to be
equally useful for the induction of oxidative cyclizations of 3Na
(Table 2 entries 4,5). Similar yields and product distributions of
7–10 were also observed when 3K was cyclized with 1 (Table 2
radical 4. While radical 5-exo cyclization 4
oxidation 5 6 are apparently not influenced by the reaction
conditions, we show that stabilization of 6 to the final products
–10 is again dependent on the properties of 3 and the presence
5 and radical
7
of amines and halides.
entry 6). Better yields of 7 were achieved when i-Pr NMgCl was
2
used instead of t-BuMgCl although 10% of 9 was formed in
addition to a 13% yield of alkene 10 (Table 2 entry 7, compare
Table 1, entry 5). The amine effect is also clearly seen in the
Results and discussion
In our initial report on the oxidative cyclizations of 3, we
17
employed LDA or LiHMDS as bases for deprotonation of
cyclization of the zinc enolate 3Zn when i-Pr NH was present,
23
2
malonate 2 in DME and isolated 64% or 57%, respectively, of
bicyclic lactone 7 accompanied by minor amounts of 2-
ethylmalonate 8 (Table 1, entries 1,2). To gain more insight into
this oxidative cyclization, we applied other common bases for
malonate deprotonation followed by oxidative cyclization
induced by 1. While oxidative cyclization after deprotonation
with n-BuLi provided an almost identical result as that
obtained with amide bases (Table 1 entry 3), application of
giving 7 in the highest yield of 83% of all systems studied (Table
2
entry 8). Finally, in the presence of i-Pr NH even 3Si and 3Ti
2
underwent a slow cyclization to yield 7 in moderate yield after
1
8 hours (Table 2 entries 9, 10). We were, however, not able to
drive the reaction to completion even by increasing the
amounts of amine to 2 equiv., that of 1 to 4 equiv. and extend-
ing the reaction times. Application of LDA as the base and
quenching the lithium enolate 3Li with TMSCl prior to SET
oxidation facilitated the cyclization somewhat compared to
NaH, but the product distribution remained very similar. To
gain further insight into the role of amines in the oxidative
cyclizations of 3, we performed an experiment in which the
not directly oxidizable enolate 3Na obtained from deproton-
ation of 2 with NaH was treated with the stable SET oxidant
3
Na, generated by standard treatment of 2 with NaH under
otherwise identical reaction conditions, surprisingly failed
completely (Table 1 entry 4). Only 60% of starting material 2
was recovered. t-BuMgCl as base precipitated in DME and
deprotonation of 2 did not occur. To solubilize the Grignard
reagent, 1.7 equivalents of HMPA had to be added prior to
addition of 2. Oxidative cyclization of 3Mg thus generated
induced by 1 provided 52% yield of 7 and a considerable 10% of
24
tris(p-bromophenyl)aminium hexachloroantimonate 11 [eqn.
(
2)]. Lactone 7 was formed as the major product in 30% yield.
1
3
0 (Table 1 entry 5). Oxidative cyclization of the zinc enolate
Zn, generated by BuLi deprotonation–Li–Zn transmetalation,
(
2)
was not very effective, affording besides 63% recovered 2 only
6% of 7 and traces of alcohol 9 (entry 6). Oxidative cycliz-
2
ations of covalent enolate equivalents, such as the silyl ketene
acetal 3Si or the titanium enolate 3Ti, generated by deproton-
ation with NaH–Na–Si or Na–Ti transmetalation, were investi-
gated for comparison, but both enolates did not cyclize after
treatment with 1 (>90% recovery of 2, entries 7, 8).
These rather surprising failures of all enolates other than 3Li
or 3Mg to undergo oxidative cyclization induced by 1 neces-
sitated a closer inspection of the influence of additives on the
In addition, an inseparable mixture of 24% of chloromalonate
12, 2% of 8 and 10% of 2 was isolated, showing clearly that 3Na
was oxidized by stable aminium salt 11.
overall transformation 2
7. The most promising candi-
dates to influence the oxidative cyclizations were the amines
generated during deprotonation of 2.
This proved to be the case. In the presence of amines, all
compounds 3 undergo oxidative cyclizations induced by 1
To investigate the issue of conversion for 3Si, the sodium
enolate and silyl ketene acetal generation was followed by
NMR spectroscopy in THF-D . While the sodium enolate 3Na
8
was formed quantitatively from 2 and NaH, addition of care-
fully dried TMSCl to the sodium enolate solution under inert
conditions led reproducibly (triple run) to a 57 : 43 mixture of
starting 2 and 3Si as a 1 : 1 E/Z-mixture after 90 min according
(
Table 2). After deprotonation of 2 with NaHMDS instead of
NaH, a smooth cyclization of 3Na occurred to give 52% of 7
and 22% of 8 (Table 2 entry 1). More convincingly, oxidative
cyclizations of 3Na, generated by deprotonation with NaH
under identical conditions as described above (compare Table 1,
entry 4), proved to be effective when an amine was added
immediately prior to oxidative cyclization by 1 (Table 2 entries
1
13
to H and C NMR spectroscopy. 3Si then remained stable
1
for at least 12 hours as indicated by the H NMR spectra. To
simulate the oxidative cyclization conditions more closely, we
added 1.0 equivalents of dried i-Pr NHؒHCl to the THF-D8
2
2
–5). Diisopropylamine served well as an additive when applied
solution of 3Si, since we speculated that amine hydrochloride
might contribute to the recovery of 2. Indeed, a slow conversion
of 3Si back to 2 was observed. The ratio of 2/3Si amounted to
70 : 30 after 12 h and the conversion to 2 was complete after
48 h. On the other hand, the reverse reaction – the conversion
in stoichiometric amounts (Table 2 entry 2) or even in a sub-
stoichiometric amount of 0.5 equivalents (entry 3) providing
identical yields of 7 but increasing amounts of 8. When the
amount of i-Pr NH was further reduced to 0.25 equivalents, the
2
2
278
J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282

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Table 2 Oxidative cyclizations of enolates 3 in the presence of amines
Entry
MB (equiv.)
Additive (equiv.)
1 (equiv.)
2 (%)
7 (%)
8 (%)
9 (%)
10 (%)
1
2
3
4
5
6
7
8
9
NaHMDS (1.4)
NaH (1.4)
NaH (1.4)
NaH (1.4)
NaH (1.4)
—
2.5
2.4
2.5
4.0
2.5
2.5
2.7
2.9
2.9
2.6
Trace
—
13
6
5
2
7
3
32
56
52
54
54
54
50
52
65
83
48
37
22
19
28
36
28
11
—
2
—
—
—
—
—
5
10
—
—
trace
—
—
—
—
—
2
13
—
16
—
i-Pr NH (1.4)
i-Pr NH (0.5)
i-Pr NEt (1.4)
(p-BrPh) N (1.4)
2
2
2
3
KHMDS (1.4)
—
i-Pr NMgCl (1.3)
HMPA (1.7)
—
i-Pr NH (1.0)
i-Pr NH (1.0)
2
LDA–ZnCl (1.3, 2)
2
NaH–TMSCl (1.4, 1.5)
NaH–ClTi(Oi-Pr) (1.4, 1.5)
—
—
2
1
0
3
2
of 2 to 3Si – with TMSCl–Hünig’s base or diisopropylamine
with or without added DMAP did not proceed at all in 24 hours.
Moreover, the formation of compound 10 was puzzling, since
it was not observed in cyclizations of alkali enolates (Table 1,
entries 1–3, Table 2, entries 1–5), but forms in almost all cycliz-
ation experiments where chloride ions were involved during
generation of 3 (Table 1, entry 5, Table 2, entries 7, 9). To test
the influence of halide ions, the oxidative cyclization of 3Li
generated by deprotonation with LDA was performed in the
presence of 8 equivalents of anhydrous LiBr,† followed by add-
ition of DBU prior to workup [eqn. (3)]. Lactone 7 was isolated
(
3)
Scheme 2
cyclization giving 12, thus limiting the applicability of this
oxidant. 2) Substoichiometric amounts of the amine suffice to
promote the oxidative cyclization in comparable yield (Table 2,
entry 3 vs. entry 2).
in 67% yield, comparable to the experiment in the absence of
LiBr (entry 1), but significantly, no 8 was isolated and 10 was
formed instead in 20% yield!
The structures of 7–9 and 12 were assigned on the basis of
their NMR spectra. The structure of minor product 10 could be
confirmed by a X-ray crystal structure analysis, which could
unfortunately not be completely refined because of disorder.
Significant trends for the application of enolates as radical
reaction precursors emerge from these results.
A similar aminium radical cation-catalyzed process was pro-
25
posed very recently for the epoxidation of alkenes. Although
rate constants for the SET oxidation of amines by 1 are
unknown to the best of our knowledge,§ they should be reason-
ably fast, since Nelsen et al. showed that hydrazines are oxidized
4
5
Ϫ1
by 1 with rate constants ranging from 1.4 × 10 –8.1 × 10 M
Ϫ1 26
s . An alternative amine coordination to the metal, leading
to a modification of 3 easier to oxidize, seems to be less
likely, since at least (p-BrPh) N exhibits only low basicity/
3
Enolate oxidation
nucleophilicity. Moreover, Collum and coworkers showed
recently that diisopropylamine coordination can not compete
with diethyl ether solvent coordination to the lithium ion of
Enolate 3Li is the only anion oxidized smoothly by 1 irrespect-
ive of the presence or absence of an amine (Table 1, entries
27
1
–3).‡ All other compounds 3 are oxidized slowly (M = Zn) or
enolates.
not at all in the absence of amine (M = Na, Si, Ti). For 3Mg, the
situation can currently not be decided with certainty since the
presence of HMPA might influence the oxidation course even in
the absence of amine. The results are a clear demonstration that
the SET oxidation rate is strongly dependent on the enolate
counterion under identical experimental conditions. However,
The reactivity of covalent 3Si is different from that of the
ionic enolates. In oxidative cyclizations to 7, considerable
amounts of starting 2 were always recovered (Table 2, entry 9).
This may be due to an unidentified proton transfer reaction, as
seen in the NMR experiments (vide supra), prior to SET oxid-
ation of 3Si by 1, but we do not believe this to be the major
28
in the presence of the secondary amines i-Pr NH, HMDS or
reason, since the cyclization yields of 3Si to 7 and 10 in the
preparative experiments in DME were consistently higher than
the amount of 3Si formed in the NMR experiments (64% vs.
43%). More significantly, because of the higher oxidation
potential of the silyl ketene acetal compared to the ionic
enolates, oxidation of 3Si is much slower.¶ In the last step of
the sequence, 6 will most probably be fragmented to 7 and
i-Pr NH PF (Scheme 3), which should irreversibly desilylate
2
tertiary amines, such as Hünig’s base or (p-BrPh) N the SET
3
oxidation of 3 is strongly accelerated, although it remains
rather slow for covalent 3Si or 3Ti. This amine effect is
unprecedented in enolate oxidation and must probably be
ascribed to the action of the amine as the actual SET oxidant
(
Scheme 2). Thus, initial SET oxidation of the amine by 1 gives
an aminium radical cation 11, which abstracts the electron from
enolate 3 to give malonyl radical 4 and re-forms the amine. This
pathway is supported by the following facts: 1) stable aminium
salt 11 promoted SET oxidation of 3Na [eqn. (2)] followed by
cyclization. However, ligand transfer from the hexachloro-
antimonate anion of 11 to 4 competes effectively with radical
2
2
6
3Si to 2 in competition to SET oxidation in analogy to
i-Pr NHؒHCl in the NMR experiments. The same arguments
2
should hold for the cyclization of the titanium enolate.
§
In one study, it was established that 1 undergoes fast reduction to
ferrocene in the presence of amines (ref. 30).
†
Lithium chloride as chloride source was not useful due to its very
poor solubility in DME.
The lithium methylmalonate enolate has an oxidation potential
¶ Oxidation potentials of simple silyl ketene acetals vary between ϩ0.83
and ϩ1.3 V vs. SCE. Although not known exactly, the oxidation poten-
tial of 3Si is expected to be found at the upper limit of this range.
Known electron transfer rate constants between simple silyl ketene
acetals and 1 support the very slow oxidation of 3Si by 1 (ref. 31).
‡
ϩ
of ϩ0.03 V vs. the Fc/Fc redox couple as determined by cyclic
voltammetry (ref. 29).
J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282
2279

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points are uncorrected. IR spectra were taken on a Nicolet DX-
3
20 FT-IR spectrometer. UV–VIS spectra were taken in CH3-
CN, on a Hewlett Packard 8452 diode array spectrometer. NMR
spectra were, unless otherwise noted, recorded in CDCl with
3
1
13
reference to Me Si for H spectra and CDCl for C spectra
4
3
on Bruker DRX 400 or AC 200 spectrometers. The coupling
constants are given in Hz. EI-mass spectra were recorded on
Finnigan MAT 8430 or MAT 8400 spectrometers at 70 eV.
Combustion analyses were performed at the Microanalytical
Laboratories of the Technical University of Braunschweig.
Deprotonation of 2 with amide bases or BuLi–oxidative
cyclization (general procedure)
At Ϫ78 ЊC, 250 mg (0.657 mmol) of 2 was added to a solution
of the lithium, sodium or potassium amide or BuLi (1.6 M in n-
hexane) in 13 ml dry DME (for the amount of base, see Table 1,
entries 1–3; Table 2, entries 1,6). The solution was stirred for 30
min between Ϫ78 and Ϫ60 ЊC. Solid 1 was added in portions at
Scheme 3
0
ЊC until a blue–green color of the reaction mixture persisted
Stabilization of 6 to 7–10
for 30 min. The mixture was stirred at 0 ЊC for 2 h. The mixture
The product distribution in the oxidative cyclizations is also
dependent on the enolate properties (Scheme 3). Apparently,
SET oxidation of 3 to 4 is slower than radical cyclization
was quenched with four drops of a saturated NH Cl solution
4
and warmed to room temperature. The reaction mixture was
diluted with 20 ml diethyl ether and filtered through a pad of
silica gel. The solvent was evaporated and the inhomogeneous
residue was preadsorbed on silica gel. Crude flash chrom-
atography (50 : 1 gradient to 10 : 1) gave >90% ferrocene
followed by 8, 9, 10, 2 and 7, respectively. The individual
compounds were further purified by flash chromatography as
indicated in the characterization section.
4
5–SET oxidation 5
6, thus 3 is still present when 6 is
formed. Two opportunities for the deethylation of 6 to 7 can be
envisaged. The more nucleophilic alkali enolates of 3 (M = Li,
Na, K) in part deethylate 6 or its bicyclic congener 6Ј forming
2
-ethylmalonate 8 in addition to 7, and are bases leading to
some starting malonate 2 and ethylene. The less nucleophilic
magnesium or zinc enolates 3 cannot undergo alkylation by
6
6
and consequently do not form 8. Therefore, stabilization of
must occur by β-deprotonation at the ethyl group. The actual
Deprotonation of 2 with NaH
At 0 ЊC, 250 mg (0.657 mmol) of 2 was added to a suspension
of NaH (80% in mineral oil) in 13 ml dry DME. After stirring
the solution for 30 min at 0 ЊC, i-Pr NH, i-Pr NEt or (p-
base for deprotonation of 6 to 7 can be either 3 (Table 1, entries
, 6) or if present i-Pr NH, since almost no 2 was recovered in
5
2
2
2
these cyclization reactions and the yields were the highest of the
series (Table 2, entries 7, 8). Thus, enolates of intermediate
nucleophilicities/basicities should be best suited to promote
reaction sequences involving carbenium ions.
It must be emphasized that product distribution is also gov-
erned by halide ions. In the absence of halides, stabilization of 6
occurs by lactonization to 7 and not by deprotonation of 6 to
BrPh) N was added (for the amount of NaH, and amine, see
3
Table 2, entries 2–5). After stirring for another 60 min at 0 ЊC, 1
was added and workup was conducted according to the general
procedure.
Deprotonation of 2 with magnesium bases
1
0. For the formation of 10, halide ions should trap 6, forming
At 0 ЊC, 250 mg (0.657 mmol) of 2 was added to a solution of
0.427 ml (0.854 mmol) t-BuMgCl (2 M in diethyl ether) or
activated diphenylmethyl halides 13 from which alkene 10 can
be formed easily via dehydrohalogenation under the basic reac-
tion conditions while the small amounts of alcohol 9 may result
from solvolysis of 13 during workup.
i-Pr
i-Pr
NMgCl (prepared by addition of 0.120 ml (0.854 mmol)
2
NH to a solution of 0.427 ml (0.854 mmol) t-BuMgCl) in
2
0.2 ml HMPA and 13 ml DME at 0 ЊC. The solution was stirred
for 60 min at 0 ЊC. Addition of 1 and workup were conducted
according to the general procedure.
In conclusion, we have shown that most ionic malonate
enolates 3 are oxidized easily by the ferrocenium ion 1, provided
a secondary or tertiary amine is present as the actual SET
oxidant. Covalent enolate equivalents represented by the silyl
ketene acetal or a titanium enolate are only slowly oxidized by 1
even in the presence of amines. The final product distribution is
determined by the nucleophilicity/basicity of the enolate and
the presence of amines or halide ions in the reaction mixtures.
Enolates of moderate nucleophilicity/basicity provide the best
yields of bicyclic lactone 7 while halides compete for carbenium
ion 6 leading to alkene 10.
Transmetalation of 3Li with ZnCl2
At Ϫ78 ЊC, 250 mg (0.657 mmol) of 2 was added to a solution
of 0.854 mmol LDA or BuLi in 4 ml dry DME and the resulting
solution was stirred for 30 min between Ϫ78 and 0 ЊC. This
solution was transferred via syringe to a suspension of 180 mg
(
1.321 mmol) anhydrous ZnCl in 9 ml dry DME and stirred for
2
4
0 min at room temperature. Addition of 1 and workup were
conducted at 0 ЊC according to the general procedure.
Oxidative cyclization of 3Si
Experimental
At 0 ЊC, 250 mg (0.657 mmol) of 2 was added to 28 mg (0.920
mmol) NaH (80% in mineral oil) in 13 ml dry DME. After
stirring for 45 min at 0 ЊC, 0.125 ml (0.986 mmol) TMSCl was
added (colorless precipitate of NaCl) and the mixture was
General
All reactions were conducted in flame dried glassware under a
nitrogen atmosphere. DME, HMPA, diisopropylamine and
hexamethyldisilazane were dried with standard methods. LiBr
stirred at room temperature for 60 min. i-Pr NH (0.092 ml,
2
and ZnCl were dried in vacuum (160 ЊC, 0.8 mbar) for 8 h.
0.657 mmol) was added and the solution was stirred for 5 min
before addition of 631 mg (1.9 mmol) 1 in one portion at room
temperature. The blue–green inhomogeneous reaction mix-
ture was stirred at room temperature for 18 h and worked up
according to the general procedure.
2
TLC plates POLYGRAM SIL G/UV254 (Macherey-Nagel) were
used for monitoring reactions. Chromatographic separations
were performed on silica gel 60 (Fluka, 230–400 mesh) with
n-hexane–ethyl acetate as eluent in the given ratio. Melting
2
280
J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282

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Reaction of 3Si with i-Pr NHؒHCl (procedure A)
warmed to room temperature. The reaction mixture was diluted
2
with 20 ml diethyl ether and filtered through a pad of silica gel.
The solvent was evaporated and the residue was preadsorbed
on silica gel. Crude flash chromatography (50 : 1 gradient to
In a NMR tube, 50 mg (0.131 mmol) of 2 was added to 6 mg
(
0.2 mmol) NaH (80% in mineral oil) in 1.3 ml dry THF-D at
8
room temperature. After shaking for 120 min at room temper-
1
: 1) gave 8, 2, 12 and 7 contaminated with considerable
1
13
ature, H and C NMR spectra showed complete formation of
Na. On addition of 0.025 ml (0.197 mmol) TMSCl a colorless
amounts of Sb residues. The individual components were dis-
solved in 2 ml n-hexane and filtered through a pad of silica gel
before they were subjected to another flash chromatography.
3
precipitate of NaCl was formed. The mixture was shaken at
room temperature for 90 min, and the reaction mixture was
analyzed by NMR spectroscopy. The reaction mixture con-
sisted of a mixture of 3Si (M = TMS, E/Z ≈ 1 : 1) and 2 in a
ratio of 43 : 57, which did not significantly change during 12 h
at room temperature. Addition of 18 mg (0.131 mmol)
Oxidative cyclization of 2 in the presence of LiBr
At Ϫ78 ЊC, 250 mg (0.657 mmol) of 2 was added to a mixture
of 456 mg (5.256 mmol) anhydrous LiBr and 0.854 mmol LDA
in 13 ml dry DME. The mixture was stirred for 30 min between
Ϫ78 and Ϫ60 ЊC. Solid 1 696 mg (2.1 mmol) was added in three
portions at 0 ЊC before a persistent blue–green color of the
reaction mixture was observed. After 10 min, the color of the
mixture changed slowly to brown. After stirring at 0 ЊC for
i-Pr NHؒHCl changed the 3Si–2 ratio to 30 : 70 during 12 h.
2
After 48 h, conversion to 2 was complete.
Attempted generation of 3Si (procedure B)
In a NMR tube, 50 mg (0.131 mmol) of 2 in 1.3 ml dry CDCl3
1
5 min, 5 drops of DBU were added. The mixture was stirred
was mixed with 0.039 ml (0.223 mmol) i-Pr NEt and 0.025 ml
2
at 0 ЊC for an additional 10 min, quenched with six drops of
a 2 M HCl solution, stirred for 10 min and warmed to room
temperature. Workup was conducted according to the general
procedure.
(
0.197 mmol) TMSCl at room temperature. NMR spectro-
scopic analysis of the reaction mixture after 90 min, 14 h and 24
h showed no conversion. Similarly, no conversion was observed
in the presence of DMAP or Et N as the base.
3
Ethyl 3-oxo-1,1-diphenyltetrahydrocyclopenta[c]furan-3a-
carboxylate (7)
3
Na. δ (400 MHz, THF-D ) 1.12 (6 H, t, J 7.0, OCH CH ),
H 8 2 3
1
.50 (2 H, quint, J 7.4, CH CH CH ), 2.08 (2 H, q, J 7.4,
2 2 2
᎐
᎐
CHCH ), 2.24 (2 H, t, J 7.3, ᎐CCH ), 3.90 (4 H, q, J 7.1,
[R (2 : 1) 0.71], mp 88 ЊC (colorless blocks from pentane)
f
2
2
OCH ), 6.17 (1 H, t, J 7.4, ᎐CH) and 7.09–7.32 (10 H, m, Ph);
(Found C, 75.1; H, 6.4. C H O requires C, 75.4; H, 6.3%);
2
22 22
Ϫ1
4
Ϫ1
3
δ (100 MHz, THF-D ) 15.7 (q, OCH CH ), 26.7 (t), 30.8 (t),
λmax/nm 194 (ε/dm mol cm 46700), 204 (44300), 222
C
8
2
3
Ϫ1
3
3.0 (t), 57.0 (t, OCH ), 74.5 (s, C᎐CO ), 127.1 (d, Ph), 127.3 (d,
(39400) and 260 (21200); νmax(KBr)/cm 1764, 1725, 762 and
2
2
Ph), 128.1 (d, Ph), 128.6 (d, Ph), 128.7 (d, Ph), 130.9 (d, Ph),
702; δ (400 MHz) 0.78 (3 H, t, J 7.2, OCH CH ), 1.30 (1 H, m,
H
2
3
1
32.9 (s, ᎐CO ), 141.4 (s, Ph), 141.7 (s, Ph), 144.5 (s, C᎐CH) and
CHCH ), 1.73 (3 H, m, CHCH CH ), 2.34 (2 H, m,
᎐
2
2
2
2
1
72.2 (s, -CO ).
CH CCO ), 3.47 (1 H, dq, J 10.7 and 7.2, OCH ), 3.76 (1 H,
2 2 2
2
dq, J 10.7 and 7.2, OCH ), 4.03 (1 H, t, J 8.4, CH CH), 7.17–
2
2
(
E/Z )-3Si. δ (400 MHz, THF-D ) 0.23 (9 H, s, SiCH ), 0.27
7.32 (6 H, m, Ph), 7.40 (2 H, m, Ph) and 7.55 (2 H, m, Ph);
δ (100 MHz) 13.3 (q, OCH CH ), 26.2 (t), 31.1 (t), 36.5 (t), 55.9
H
8
3
(
9 H, s, SiCH ), 1.13–1.26 (12 H, m, OCH CH ), 1.54 (4 H,
3
2
3
C
2
3
quint, J 7.6, CH CH CH ), 2.05–2.28 (8 H, m, CH CH CH ),
(d, CHCH ), 61.8 (t, OCH ), 63.2 (s, CCO ), 89.7 (s, CPh ),
2
2
2
2
2
2
2 2 2 2
3
7
0
.94–4.19 (8 H, m, OCH ), 6.09 (2 H, t, J 7.4, ᎐CH) and 7.13–
124.7 (d, Ph), 125.5 (d, Ph), 127.3 (d, Ph), 127.7 (d, Ph), 128.3
(d, Ph), 128.4 (d, Ph), 141.4 (s, Ph), 143.4 (s, Ph), 170.9 (s,
2
.37 (20 H, m, Ph); δ (100 MHz, THF-D ) 0.22 (q, SiCH ),
C
8
3
ϩ
.86 (q, SiCH ), 14.7 (q, OCH CH ), 15.0 (q, OCH CH ), 15.1
CO CH ) and 175.2 (s, CO CPh ); m/z (EI): 350 (M , 46%), 183
3
2
3
2
3
2
2
2
2
(
q, OCH CH ), 26.2 (t), 27.8 (t), 30.37 (t), 30.41 (t), 30.5 (t),
(100), 140 (42) and 105 (41).
2
3
3
1.1 (t), 59.5 (t, OCH ), 61.0 (t, OCH ), 90.2 (s, C᎐CO ), 95.4
2 2 2
(
s, C᎐CO ), 127.28 (d, Ph), 127.30 (d, Ph), 127.5 (d, Ph), 127.6
Diethyl 2-(5,5-diphenylpent-4-enyl)-2-ethylmalonate (8)
᎐
2
(
d, Ph), 127.88 (d, Ph), 127.91 (d, Ph), 128.6 (d, Ph), 128.78 (d,
Flash chromatography (50 : 1) gave 8 [R (5 : 1) = 0.56] as a
f
Ph), 128.81 (d, Ph), 129.6 (d, Ph), 130.7 (d, Ph), 141.29 (s),
41.34 (s), 142.4 (s), 142.6 (s), 143.0 (s), 143.9 (s), 162.7 (s, -CO2)
colorless oil (Found C, 76.3; H, 7.9. C H O requires C, 76.4;
2
6
32
4
1
3
Ϫ1
Ϫ1
H, 7.9%); λmax/nm 194 (ε/dm mol cm 46 400), 212 (42300),
and 167.6 (s, -CO ) (The two expected C᎐CO resonances could
not be observed with certainty.)
2
2
2
18 (40800), 226 (39500), 252 (38500), 264 (36900), 274
Ϫ1
(
33400) and 282 (30400); νmax(film)/cm 2977, 1731, 1445,
028 and 702; δ (200 MHz) 0.72 (3 H, t, J 7.5, CCH CH ), 1.13
1
H
2
3
Oxidative cyclization of 3Ti
(
6 H, t, J 7.1, OCH CH ), 1.22 (2 H, m, ᎐CHCH CH ), 1.78 (2
2
3
2
2
At 0 ЊC, 250 mg (0.657 mmol) of 2 was added to 28 mg (0.920
mmol) NaH (80% in mineral oil) in 13 ml dry DME. After
H, m, CCH CH ), 1.84 (2 H, q, J 7.5, CCH CH ), 2.04 (2 H, q,
J 7.4, ᎐CHCH ), 4.08 (4 H, q, J 7.1, OCH ), 5.97 (1 H, t, J 7.4,
᎐
2 2
2
2
2
3
stirring for 45 min at 0 ЊC, 0.986 ml (0.986 mmol) (i-PrO) TiCl
᎐CH) and 7.06–7.33 (10 H, m, Ph); δ (50 MHz) 8.4 (q,
3
C
(
1 M in n-hexane) was added and the solution was stirred for an
CCH CH ), 14.1 (q, OCH CH ), 24.4 (t, CH CH ), 25.2 (t,
2
3
2
3
2
3
additional 60 min. i-Pr NH 0.092 ml (0.657 mmol) was added
CH CH C), 29.9 (t), 31.3 (t), 57.9 (s, CCH ), 60.9 (t, OCH ),
2
2
2
2
2
and the solution was stirred for 5 min before addition of 560 mg
126.8 (d, Ph), 126.9 (d, Ph), 127.2 (d, Ph), 128.0 (d, Ph), 128.1
(
1.692 mmol) 1 in one portion at room temperature. The blue–
(d, Ph), 129.1 (d, ᎐CH), 129.8 (d, Ph), 140.1 (s), 142.1 (s), 142.6
᎐
ϩ
green inhomogeneous reaction mixture was stirred at room
temperature for 18 h and worked up according to the general
procedure.
(s) and 171.7 (s, CO ); m/z (EI): 408.2292 (M , C H O
2
26 32
4
requires 408.2301, 58%), 362 (42), 289 (30), 288 (80), 206 (100),
205 (23), 191 (24), 188 (24), 178 (20), 115 (27) and 91 (27).
Deprotonation of 2 with NaH and oxidative cyclization induced
Diethyl 2-(diphenylhydroxymethyl)cyclopentane-1,1-
dicarboxylate (9)
by ( p-BrPh) NSbCl (11)
3
6
At 0 ЊC, 250 mg (0.657 mmol) of 2 was added to a suspension
of 28 mg (0.933 mmol) NaH (80% in mineral oil) in 13 ml dry
DME. After stirring the solution for 40 min at 0 ЊC, 1.56 g
Flash chromatography (50 : 1) gave 9 as a colorless oil [R
(5 : 1) = 0.52]. λmax/nm 198 (ε/dm mol cm 45800), 218
f
3
Ϫ1
Ϫ1
(41400), 224 (40600), 230 (39500), 252 (38700) and 276
Ϫ1
(
1.911 mmol) 11 was added in two portions. The green reaction
(32300); νmax(film)/cm 3459, 2978, 1730, 1242, 1177, 1086 and
mixture was stirred at 0 ЊC for 90 min. The mixture was
702; δ (200 MHz) 0.89 (3 H, t, J 7.2, OCH CH ), 1.18 (1 H, m),
H
2
3
quenched with four drops of a saturated NH Cl solution and
1.24 (3 H, t, J 7.1, OCH CH ), 1.44 (1 H, m), 1.82 (2 H, m),
4
2 3
J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282
2281

View Article Online
2
.24 (1 H, dt, J 12.7 and 5.7), 2.45 (1 H, dd, J 12.2 and 5.2), 2.69
(j) R. H. Frazier Jr. and R. L. Harlow, J. Org. Chem., 1980, 45, 5408;
(
k) C. Alvarez-Ibarra, A. G. Csaky, B. Colmenero and M. L.
(
1 H, dq, J 10.8 and 7.1, OCH ), 3.54 (1 H, dq, J 10.8 and 7.1,
2
Quiroga, J. Org. Chem., 1997, 62, 2478; (l ) T. J. Brocksom, N.
Petragnani, R. Rodrigues and H. La Scala Texeira, Synthesis, 1975,
OCH ), 4.15 (2 H, q, J 7.1, OCH ), 4.47 (1 H, t, J 8.0,
CHCH ), 4.72 (1 H, s, OH) and 7.00–7.72 (10 H, m, Ph); δ (50
MHz) 13.3 (q, OCH CH ), 14.0 (q, OCH CH ), 24.4 (t), 27.8
2
2
2
C
3
96; (m) M. Tokuda, T. Shigei and M. Itoh, Chem. Lett., 1975, 621.
2
3
2
3
5 (a) J. W. Kim, J.-J. Lee, S.-H. Lee and K.-H. Ahn, Synth. Commun.,
1998, 28, 1287; (b) T. K. Chakraborty and S. Dutta, Synth.
Commun., 1997, 27, 4163; (c) T. Kauffmann, G. Beiβner, H. Berg, E.
Köppelmann, J. Legler and M. Schönfelder, Angew. Chem., 1968,
(
t), 39.0 (t), 51.8 (d, CH CH), 61.7 (t, OCH ), 62.0 (t, OCH ),
2
2
2
6
1
4.3 (s, CCH ), 79.4 (s, COH), 125.2 (d, Ph), 125.8 (d, Ph),
2
26.0 (d, Ph), 126.4 (d, Ph), 127.85 (d, Ph), 127.93 (d, Ph), 147.0
8
0, 565; (d ) E. M. Kaiser, J. Am. Chem. Soc., 1967, 89, 3659.
(
s, Ph), 147.1 (s, Ph), 172.3 (s, CO ) and 174.9 (s, CO ); m/z (EI):
2
2
6
(a) M. Schmittel, A. Burghart, H. Werner, M. Laubender and
R. Söllner, J. Org. Chem., 1999, 64, 3077 and references therein; (b)
K. Ryter and T. Livinghouse, J. Am. Chem. Soc., 1998, 120, 2658; (c)
P. Langer and V. Köhler, Chem. Commun., 2000, 1653; (d ) T. Hirao,
T. Fujii and Y. Ohshiro, Tetrahedron, 1994, 50, 10207 and references
therein; (e) A. B. Paolobelli, D. Latini and R. Ruzziconi, Tetrahedron
Lett., 1993, 34, 721; ( f ) T. Fujii, T. Hirao and Y. Ohshiro,
Tetrahedron Lett., 1992, 33, 5823; (g) E. Baciocchi, A. Casu and R.
Ruzziconi, Tetrahedron Lett., 1989, 30, 3707; (h) G. E. Totten, G.
Wenke and Y. E. Rhodes, Synth. Commun., 1985, 15, 291; (i) Y. Ito,
T. Konoike and T. Saegusa, J. Am. Chem. Soc., 1975, 97, 649.
ϩ
3
96 (M , 1%), 379 (8), 260 (10), 232 (9), 214 (21), 183 (52), 182
(
51), 168 (22), 105 (100) and 77 (46).
Diethyl 2-(diphenylmethylene)cyclopentane-1,1-dicarboxylate
10)
(
[R (5 : 1) = 0.47]; mp 61–62 ЊC (colorless blocks from CHCl )
f
3
(
Found C, 76.1; H, 7.0. C H O requires C, 76.2; H, 6.9%);
22 26 4
3 Ϫ1 Ϫ1
λmax/nm 196 (ε/dm mol cm 45400), 202 (45100), 206
(
(
44600), 220 (41000), 238 (40100), 258 (38200) and 272
33600); νmax(film)/cm 2981, 1732, 1264, 1175 and 703; δ (200
Ϫ1
7 (a) N. Kise, T. Ueda, K. Kumada, Y. Terao and N. Ueda, J. Org.
Chem., 2000, 65, 464; (b) N. Kise, K. Kumada, Y. Terao and N.
Ueda, Tetrahedron, 1998, 54, 2697; (c) E. N. Jacobsen, G. E. Totten,
G. Wenke, A. C. Karydas and Y. E. Rhodes, Synth. Commun., 1985,
15, 301; (d ) S.-i. Inaba and I. Ojima, Tetrahedron Lett., 1977, 2009.
H
MHz) 1.09 (6 H, t, J 7.1, OCH CH ), 1.60 (2 H, tt, J 7.4 and
2
3
7
.0, CCH CH ), 2.41 (2 H, t, J 7.4, CCH ), 2.43 (2 H, t, J 6.9,
2
2
2
CCH ), 3.69 (2 H, dq, J 10.7 and 7.1, OCH ), 3.88 (2 H, dq,
J 10.7 and 7.1, OCH ) and 7.09–7.36 (10 H, m, Ph); δ (50
MHz) 13.8 (q, OCH CH ), 23.1 (t), 32.7 (t), 40.0 (t), 61.3 (t,
OCH ), 65.0 (s, CCO ), 126.4 (d, Ph), 126.6 (d, Ph), 127.5 (d,
2
2
8
(a) Y. Kohno and K. Narasaka, Bull. Chem. Soc. Jpn., 1995, 68, 322;
b) Y. Kohno and K. Narasaka, Chem. Lett., 1993, 1689.
(a) M. Schmittel, J.-P. Steffen and A. Burghart, Acta Chem. Scand.,
999, 53, 781 and references therein; (b) M. Schmittel and A.
2
C
(
2
3
9
2
2
1
Ph), 128.1 (d, Ph), 128.3 (d, Ph), 129.2 (d, Ph), 138.96 (s),
39.02 (s), 141.0 (s), 144.1 (s), 169.4 (s, CO ) and 171.0 (s, CO );
Langels, J. Org. Chem., 1998, 63, 7328 and references therein; (c) M.
Schmittel and R. Söllner, Chem. Commun., 1998, 565 and references
therein; (d ) Review: M. Schmittel, Top. Curr. Chem., 1994, 169, 183.
1
2
2
ϩ
m/z (EI): 378.1823 (M , C H O requires 378.1831, 32%), 260
24
26
4
1
1
1
0 A. S. Kende, K. Koch and C. A. Smith, J. Am. Chem. Soc., 1988,
10, 2210.
(
2
33), 259 (42), 232 (33), 231 (100), 216 (33), 215 (40), 203 (31),
02 (42), 184 (20) and 165 (26).
1
1 D. HobbsMallyon and D. A. Whiting, J. Chem. Soc., Chem.
Commun., 1991, 899.
Diethyl 2-(5,5-diphenylpent-4-enyl)-2-chloromalonate (12)
2 (a) A.-C. Durand, J. Rodriguez and J.-P. Dulcere, Synlett, 2000, 731;
(
b) A.-C. Durand, E. Dumez, J. Rodriguez and J.-P. Dulcere, Chem.
Flash chromatography (50 : 1) gave an inseparable 5 : 1 : 12
Commun., 1999, 2437; (c) N. Arai and K. Narasaka, Bull. Chem.
Soc. Jpn., 1997, 70, 2525; (d ) N. Arai and K. Narasaka, Chem. Lett.,
1
mixture of 2, 8 and 12 as a colorless oil [R (5 : 1) = 0.52]. δ (400
f
H
MHz) 1.24 (6 H, t, J 7.1, OCH CH ), 1.52 (2 H, m), 2.04–2.23
2
3
995, 987.
(
4 H, m), 4.22 (4 H, q, J 7.1, OCH ), 6.05 (1 H, t, J 7.4,
2
13 Review: S. Hintz, A. Heidbreder and J. Mattay, Top. Curr. Chem.,
1996, 177, 77.
᎐
᎐
CHCH ) and 7.10–7.35 (10 H, m, Ph); δ (100 MHz) 13.9 (q,
2 C
OCH CH ), 24.4 (t), 29.2 (t), 37.0 (t), 62.9 (t, OCH ), 70.9 (s,
CCl), 126.9 (d, Ph), 127.0 (d, Ph), 127.2 (d, Ph), 128.1 (d, Ph),
14 L. Ackermann, A. Heidbreder, F. Wurche, F.-G. Klärner and
J. Mattay, J. Chem. Soc., Perkin Trans. 2, 1999, 863 and references
therein.
2
3
2
1
1
28.2 (d, Ph), 128.5 (d, ᎐CH), 129.8 (d, Ph), 140.0 (s), 142.46 (s),
᎐
1
5 Reviews: (a) K. D. Moeller, Tetrahedron, 2000, 56, 9527; (b) K. D.
Moeller, Top. Curr. Chem., 1997, 185, 49; (c) H. J. Schäfer, Angew.
Chem., 1981, 93, 978.
ϩ
42.55 (s) and 166.7 (s, CO ); m/z (EI): 414/416 (M , 0.9/0.3%)
2
and 379 (1.7).
1
6 (a) B. B. Snider and T. Kwon, J. Org. Chem., 1992, 57, 2399; (b) B. B.
Snider and T. Kwon, J. Org. Chem., 1990, 55, 4786.
7 U. Jahn and P. Hartmann, Chem. Commun., 1998, 209.
8 U. Jahn, J. Org. Chem., 1998, 63, 7130.
Acknowledgements
We thank DFG and the Fonds der Chemischen Industrie
for generous funding. Professor H. Hopf’s interest and
encouragement is gratefully acknowledged.
1
1
1
9 U. Jahn, M. Müller and S. Aussieker, J. Am. Chem. Soc., 2000, 122,
5
212.
2
0 (a) G. G. Melikyan, Org. React., 1997, 49, 427; (b) B. B. Snider,
Chem. Rev., 1996, 96, 339.
21 V. Nair, J. Mathew and J. Prabhakaran, Chem. Soc. Rev., 1997, 26, 127.
2 U. Jahn and P. Hartmann, unpublished work.
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4
(
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J. Chem. Soc., Perkin Trans. 1, 2001, 2277–2282