Beyond the Tebbe Olefination: Direct Transformation of Esters into Ketones or Alkenes

Article abstract of DOI:10.1055/s-0039-1691594

A direct, effective, and operationally simple transformation of esters into ketones or alkenes by the exclusive action of Tebbe's reagent has been developed. The transformation utilizes the dual character of Tebbe's reagent as both a methylenation agent and a rearrangement catalyst in the reaction of a wide range of substituted vinyl ethers. The resulting transformation involves sequential methylenation and rearrangement reactions and it offers a high degree of selectivity toward the synthesis of ketones or alkenes. The scope and limitations of the developed methods have been also examined.

Full text of DOI:10.1055/s-0039-1691594

SYNLETT
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-
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6
© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–G
letter
A
en
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
Beyond the Tebbe Olefination: Direct Transformation of Esters
into Ketones or Alkenes
Anna M. Domżalska-Pieczykolan
Direct transformation of esters into olefins
using Tebbe's reagent only
Bartłomiej Furman*
0
0
0
0-0
0
0
1-5
4
5
9-
8
0
2
6
R1
O
R1
CH2
R3
R1
O
3 steps
2 steps
Institute of Organic Chemistry Polish Academy of Sciences,
Kasprzaka 44/52, 01-224 Warsaw, Poland
bartlomiej.furman@icho.edu.pl
R2
C
R3
R2
C
R2
O
R3
up to 84%
up to 72%
H2
H2
benzyl esters,
benzhydryl alkohol esters
R3
R3
R2 R1
R2 R1
3 steps
2 steps
R1
O
up to 72%
up to 90%
C
O
C
CH2
R2
O
R3
H2
H2
allyl esters
Direct transformation of esters into ketones
using Tebbe's reagent only
Received: 23.12.2019
Accepted after revision: 24.01.2020
Published online: 06.02.2019
a. The Claisen rearrangement
2
R3
R2
O
2
1
O
R3
3
1
R3
R2
3
3
DOI: 10.1055/s-0039-1691594; Art ID: st-2019-v0687-l
O
R1
R1
R2
Claisen rearrangement
R1
1
1
Abstract A direct, effective, and operationally simple transformation
of esters into ketones or alkenes by the exclusive action of Tebbe’s re-
agent has been developed. The transformation utilizes the dual charac-
ter of Tebbe’s reagent as both a methylenation agent and a rearrange-
ment catalyst in the reaction of a wide range of substituted vinyl ethers.
The resulting transformation involves sequential methylenation and re-
arrangement reactions and it offers a high degree of selectivity toward
the synthesis of ketones or alkenes. The scope and limitations of the de-
veloped methods have been also examined.
3
2
2
b. The [1,3] O-to-C rearrangement
2
2
3
R3
R3
3
1
1
O
1
O
LA
R3
O
R2
R2
R1
R1
1
R1
R2
Scheme 1 The rearrangement reactions
Key words vinyl ethers, rearrangement, Tebbe’s reagent, methylena-
tion, alkenes, ketones
The potential of the [1,3]-rearrangement of vinyl ethers
has already been proven by its successful implementation
in the total synthesis of biologically active compounds.5
Nonetheless, it still remains in the shadow of the Claisen re-
arrangement,3 due to difficulties in controlling the reac-
tions of vinyl ethers under acidic conditions and the forma-
tion of undesirable polymeric products.6 On the other hand,
vinyl ethers have been postulated to act as good intermedi-
ates in domino or sequential reactions, which generally
show good atom economy and high overall yields.7 These
reactions permit the construction of complex molecules in
relatively few synthetic steps, and are often characterized
by freedom from unwanted byproducts, making them envi-
ronmentally friendly and commercially more-viable alter-
natives to conventional reaction pathways.8
Our group recently discovered that substituted vinyl
ethers and unsymmetrical ketene acetals undergo smooth
conversions into chain-extended ketones or esters in the
presence of a catalytic amount of trimethylsilyl triflate.9 We
also found that O-1,3-alkoxydienes 1 undergo a smooth vi-
nylogous Ferrier–Petasis-type reaction when treated with a
catalytic amount of a carefully selected Lewis acid (Scheme 2).10
Methods for the construction of new C–C bond are
of fundamental importance in organic synthesis.1 Especially
valuable are reactions that proceed in a highly regio- and
chemoselective manner, under mild reaction conditions.
There are many strategies that meet these requirements,
but of special significance are rearrangement reactions.1,2
One notable example of this class of reactions is the Claisen
rearrangement, which permits the transformation of a vari-
ety of molecules into complex carbonyl compounds
through a [3,3]-sigmatropic rearrangement. In this trans-
formation, an allyl vinyl or allyl aryl ether reacts to produce
,-unsaturated carbonyl compounds (Scheme 1a).3
On the other hand, vinyl ethers undergo a lesser-known
[1,3]-rearrangement reaction that is also powerful in terms
of building complexity. In general, this transformation
is believed to proceed through ionic cleavage to form an ion
pair (an enolate and a carbocationic species), followed by
the recombination of the nascent stabilized carbocation
with the enolate anion equivalent to generate a new C–C
bond (Scheme 1b).4
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
nyl ether (8a) as a model compound that would permit the
study of the rearrangement–methylenation reaction se-
quence independently of the first methylenation reaction
(Table 1). Surprisingly, the rearrangement reaction did not
proceed in THF (Table 1, entries 1 and 2), probably due to
the influence of this solvent on the acidic properties of
Tebbe’s reagent.14 Consequently, we chose toluene as the sol-
vent. Next, the dependence of the reaction on the tempera-
ture was investigated (entries 3–7). At lower temperatures,
the reaction could be limited to produce the ketone 9a ex-
clusively (entry 5), whereas raising the temperature result-
ed in higher yields of the alkene 10a (entry 6). Solvents
used to quench the reaction also had a significant impact on
the selectivity of the process (entries 3 and 4). The use of a
cold mixture of NaOH and toluene stopped the reaction at
the ketone stage (entry 3), whereas, the use of cooled pen-
tane with a small amount of pyridine (2 equiv) led to a
higher yield of the alkene, despite the fact that the reaction
was carried out at –70 °C (entry 4). We then found that in-
creasing the reaction time had a slight positive effect on the
olefination reaction and led to the alkene, even without ad-
dition of pyridine (entry 6). However, when an equimolar
amount of base was added after the starting vinyl ether 8a
had been consumed, the reaction time was considerably
shortened but still gave a high yield (entry 8). Finally, other
additives were examined (entries 10 and 11); however,
these did not have a crucial influence on the reaction yield.
Finally, the optimal amount of Tebbe’s reagent was deter-
mined (entries 4–12), and it was found that 2.5 equivalents
of Tebbe’s reagent in toluene resulted in full conversion of
the starting material (entry 9). However, an excess of three
equivalents proved to be essential to convert ketone 9a into
R
Ar
O
Ar
TiCl4
[1,5]
O
R
1
2
Ar: Tol, PMP, Ph;
R: H, Pr, Tol, PMP, 3-BrC6H4, 4-FC6H4, 4-F3CC6H4, 4-NCC6H4, 3-NCC6H4
Scheme 2 The Ferrier–Petasis-type rearrangement reaction of O-1,3-
alkoxydienes
This rearrangement reaction proceeds with good chem-
ical yield and regioselectivity to afford the corresponding
cyclohexene carbaldehydes 2, which are important struc-
tural synthons for syntheses of many biologically active
molecules.10
During the course of our studies, we noticed that the
Tebbe olefination of aldehyde 3 resulted in the formation of
a complex mixture of products. Careful analysis showed
that along with expected ether 4, small amounts of alde-
hyde 5 and alkene 6 were also formed (Scheme 3).
Considering our observations that Tebbe’s reagent
[-chlorobis(η5-cyclopentadienyl)(dimethylaluminum)--
methylenetitanium] not only acts as an olefination reagent,
but also exhibits Lewis acid properties (dual character of
Tebbe’s reagent),11,12 we decided to use it in attempts at de-
veloping a transformation involving sequential methylena-
tion and rearrangement reactions. We chose to start from
the simple esters 7 and to develop a process for the selective
preparation of ketones 9 or olefins 10 (Scheme 4).
Because the results of [1,3]-rearrangements can be
roughly predicted from the electrophilicity of the R+ ions
generated from vinyl ethers,13 we chose diphenylmethyl vi-
H
H
C
N
(H3C)2Al
TiCp2
H2C TiCp2
Cl
H
H
C
H2
C
H2
C
Cp2Ti
Al(CH3)2
Ar
+
Ar
O
Ar
O
Ar
Cl
+
PhMe, pyridine,
–40 °C –20 °C
O
O
CH2
CH2
3
5
6
4
Ar: PMP, Tol
Scheme 3 Side reactions in Tebbe’s olefination of O-1,3-alkoxycarbaldehydes
direct transformation of esters into ketones
(2 steps)
R2
O
R2
CH2
R3
R2
O
R2
CH2
R3
Cp2Ti CH2
Cp2Ti CH2
O
C
methylenation
methylenation
R1
O
R3
R1
O
R1
C
R3
R1
C
H2
H2
7
8
9
10
(3 steps)
direct transformation of esters into alkenes
Scheme 4 Direct transformation of esters into ketones or alkenes
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G

C
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
Table 1 Optimization of the Rearrangement–Methylenation Reaction Sequence of Vinyl Ether 8a
Ph
O
Ph
CH2
Cp2Ti CH2
O
CH2
C
Ph
methylenation
Ph
C
Ph
C
Ph
O
8a
H2
H2
10a
9a
(2 steps)
direct transformation of vinyl ethers into alkenes
Temp (°C) Time (h) Quench
Entry Tebbe reagent Additivea (equiv)
(equiv)
Solvent
Yield (%) of Yield (%) Yieldb (%)
8a
of 9a
of 10a
1
3
–
THF
–70
rt
17
17
2
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
Pentane + py (2 equiv)
no reaction
2
3
–
THF
no reaction
3
3
–
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
–70
–70
–70
rt
21
19
–
69
33
72
-
–
4
3
–
2
37
–
5
3
–
17
17
2.5
2.5
2.5
2.5
2.5
3
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
NaOH (10% aq, 5 equiv) + PhMe
6
3
–
–
82
56
80
52
81
42
56
7
3
–
rt
–
34
–
8
3
py (3)
py (2.5)
rt
–
9
2.5
3
rt
–
21
–
10
11
12
py (3); MeOH (1)
py (3); SiO2
py (5)
rt
–
3
rt
–
17
–
5
rt
–
a The additives were added after consumption of the vinyl ether.
b Yield over two steps.
Table 2 Optimization of the Direct Transformation of Ester 7a into Alkene 10a
Ph
O
Ph
CH2
Cp2Ti CH2
Cp2Ti CH2
Ph
O
Ph
CH2
O
C
methylenation
methylenation
Ph
C
Ph
C
Ph
O
Ph
O
H2
9a
H2
10a
(3 steps)
7a
8a
direct transformation of esters into alkenes
Entry
Tebbe reagent (equiv)
Additivea (equiv)
Solvent
Temp (°C)
Time (h)
8a (%)
10ab (%)
1
4
py (6)
PhMe
THF
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
–40 to rt
1
81
80
82
90
92
89
77
–
–
2
2
py (4)
2
–
3
2
py (4)
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
2
–
4
1.5
1.4
1.4
1.3
3.4
3.4
4.4
6.4
6.4
py (4)
2.5
3
–
5
py (4)
–
6
py (1.4)
3
–
7
py (4)
3
–
8
py (1.4 + 2)
py (1.4 + 2); MeOH (1)
py (1.4 + 4)
py (1.4 + 5)
py (1.4 + 5)
17
3.5
15
3.5
14
19
14
traces
32
29
9
–
10
11
12
–
–
–
a The second portion of additive was added after consumption of the vinyl ether.
b Yields over three steps.
the corresponding alkene 10a (entries 6, 8, and 10). The use
of a greater excess of Tebbe’s reagent (entry 12) was prepar-
atively not attractive and gave lower yields, probably due to
degradation of the product.
Having established the optimal reaction conditions and
with knowledge of how to control the process, we attempt-
ed a direct transformation of starting ester 7a into alkene
10a (Table 2). Unfortunately, unlike the stepwise process,
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G

D
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
the direct transformation proceeded with low yields, even
when a second portion of pyridine was added after con-
sumption of the vinyl ether 8a (Table 2, entries 8–12).
After analyzing the results, and taking into account the
reaction mechanism of the Tebbe olefination, we ascribed
the lower yields to the different decomposition pathways
following quenching of the titanium complex formed
during the reaction. Therefore, we modified the method for
conducting the reaction (Table 3). Upon consumption of the
starting ester 7a, as monitored by TLC chromatography, the
reaction was stopped by the addition of 10% aqueous NaOH.
The raw vinyl ether 8a was then used in a subsequent
transformation into ketone 9a or ester 10a by treatment
with Tebbe’s reagent under the previously optimized condi-
tions. As expected, this small change led to a substantial
improvement in the yield.
In the next stage of our investigations, we explored the
scope of the direct three-step transformation of esters 7 by
reaction with Tebbe’s reagent (Scheme 5a). As ascertained
from an examination of the separate transformations, the
structural character of the substrate affected the reaction
time; therefore, all the reactions were monitored by TLC
chromatography. The second portion of pyridine was al-
ways added after consumption of the vinyl ether 8. We be-
gan our exploration of the scope of this reaction by study-
ing the influence of the aliphatic substituent R in the ben-
zhydryl alcohol esters 7a–d. We found that increasing the
carbon chain length in 7a–c resulted in a reduction in
yield.15 In the next stage of our investigations, we explored
the influence of substituents on the aromatic ring in the
benzhydryl alcohol esters 7e–l. Benzhydryl benzoates with
electron-donating functional groups such as methoxy,
methyl, or phenyl functionalities, 7e–g, gave the desired
products in good total yields (more than 90% per step). Con-
versely, a suppressed reactivity was observed when elec-
tron-withdrawing groups were present on the aryl ring in
7i–l. Moreover, the effect of the position of the substituent
on the aryl ring was also examined. The reaction of the
ortho-isomer 7j and the meta-isomer 7k as substrates both
led to moderate total yields of the corresponding products.
These comparable yields suggested that the position of a
substituent on the aryl ring does not significantly affect the
course of our transformation. We next focused our atten-
tion on the benzylic acid esters 7m–u, and we found that
benzylic alcohol esters containing electron-donating
groups 7n–q gave good total yields (~70–80% per step).
When an additional methoxy group was present in the
meta position in 7m, a reduction in the yield was observed
(about 65% per step); this was attributed to the destabiliz-
ing effect of this substituent on the resulting carbocation. In
turn, comparable results were observed for the silyl-stabi-
lized carbocation 7r, the oxocarbenium ion 7s, and the tertia-
ry aliphatic carbocation 7t. However, the secondary alcohol
derivative 7u did not lead to the expected product 10u. Fi-
nally, a chiral ester 7v bearing an -stereocenter was cho-
sen as a challenging substrate for this transformation, as the
stereocenter should permit facile racemization. Treatment
of the (R)-naproxen ester 7v with Tebbe’s reagent under our
developed conditions gave the alkene 10v in 16% yield
(~55% per step) with no detectable racemization of the
product. These experiments highlight the effectiveness of
these reaction conditions with regard to sensitive sub-
strates.
A further exploration of the scope of our reaction
showed that allyl esters 11a–d derived from terpene alco-
hols [geraniol (11a and 11c), myrtenol (11b), or retinol
(11d)] exhibited a different reactivity in exhaustive deoxy-
genative methylenation, resulting in the formation of the
corresponding dienes 12a–d. The presence of an allyl group
resulted in a methylenation/Claisen/methylenation reaction
sequence (Scheme 5b). Note that the Claisen rearrangement
takes place under mild conditions at room temperature.16
As mentioned previously, this process permits the con-
trolled synthesis of alkenes 10 or ketones 9 through manip-
ulation of the reaction temperature and the method used
for quenching the reaction. Examples of direct syntheses of
ketones 9 from the corresponding esters 7 are shown in
Scheme 6a. Similarly, the reaction of allylic esters 11 could
also be controlled, permitting the synthesis of ketones 14
(Scheme 6b), the formation of which also provided confir-
mation of the mechanism of the entire process. Finally, it
was recognized that this chemistry could provide new ave-
nues to biologically active compounds and hence,
an alternative pathway for the synthesis of loureirin A (13)
was proposed.17
Table 3 Modified Process for the Direct Transformation of Ester 7a into Alkene 10a
Entry
Tebbe
reagent (equiv)
Additivea (equiv)
Solvent
Temp (°C)
Time (h)
Yieldb of
10a (%)
1
2
3
1.4 + 3
1.4 + 2
1.4 + 1.5
py (1.4 + 3)
py (1.4 + 2)
py (1.4 + 1.5)
PhMe
PhMe
PhMe
–40 to rt
–40 to rt
–40 to rt
4
68
56
40
2.5
4
a The second portion of the additive was added after consumption of the vinyl ether.
b Yield over three steps.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G

E
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
a. Scope of the methylenation/[1,3] O-to-C rearrangement/methylenation sequence
1. Tebbe (0.5 M in PhMe, 1.4 equiv)
pyridine (1.4 equiv), PhMe, –40 °C to rt
O
2. Tebbe (0.5 M in PhMe, 3 equiv)
PhMe, –40 °C to rt
O
R
R
3. pyridine (3 equiv) , 0 °C to rt
7a–l
10a–l
R:
7a 10a
3 h, 68%
7b 10b
6 h, 28%
7c 10c
7 h, 37%
7d 10d
24 h, 38%
I
Ph
OMe
Cl
I
CF3
7e 10e
5 h, 67%
7f 10f
12 h, 71%
7g 10g
18 h, 81%
7h 10h
3.5 h, 84%
7i 10i
17 h, 30%
7j 10j
46 h, 28%
7k 10k
46 h, 23%
7l 10l
46 h, 17%
O
1. Tebbe (0.5 M in PhMe, 1.4 equiv)
pyridine (1.4 equiv), PhMe, –40 °C to rt
R
R
O
2. Tebbe (0.5 M in PhMe, 3 equiv)
PhMe, –40 °C to rt
3. pyridine (3 equiv), 0 °C to rt
7m–w
10m–w
R:
MeO
MeO
MeO
7m 10m
40 h, 33%
7n 10n
26 h, 49%
7o 10o
17 h, 38%
7p 10p
22 h, 54%
7q 10q
27 h, 51%
O
TMS
7r 10r
18 h, 27%
7s 10s
49 h, 21%
7t 10t
17 h, 30%
7u 10u
19 h, 0%
OMe
OMe
O
Tebbe
O
MeO
MeO
10v
7v
24 h, 16%
ee 99%
b. Scope of the methylenation/Claisen rearrangment/methylenation seqence
1. Tebbe (0.5 M in PhMe, 1.4 equiv)
R1
O
R3
R2 R1
pyridine (1.4 equiv), PhMe, –40 °C to rt
R2
O
R3
C
CH2
2. Tebbe (0.5 M in PhMe, 3 equiv)
PhMe, –40 °C to rt
H2
11
12
3. pyridine (3 equiv), 0 °C to rt
Esters type 11
O
O
O
O
O
Ph
O
Ph
O
Et
O
Et
11a
11b
11c
11d
Methylenation/Claisen/methylenation reaction sequence products 12
CH2
CH2
Et
CH2
Ph
CH2
Ph
C
C
Et
C
C
H2
H2
H2
H2
12a
19 h, 72%
12b
21 h, 48%
12c
21 h, 37%
12d
5 h, 34%
Scheme 5 The scope of the direct transformation esters into alkenes
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G

F
A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
a. Synthesis of ketones 9
1. Tebbe (0.5 M in PhMe, 1.4 equiv)
pyridine (1.4 equiv), PhMe, –40 °C to rt
O
O
R1
R1
R2
2. Tebbe (0.5 M in PhMe, 3 equiv)
PhMe, –70 °C
R2
O
7
9
O
O
O
O
O
MeO
MeO
7h 9h
7v 9v
7a 9a
7n 9n
23 h, 72%**
16 h, 18%
4 h, 69%
5 h, 50%
23 h, 33%*
OMe
O
OMe
O
Bu4NF, THF
2 h, 99%
7w 9w
6 h, 52%
13
MeO
OTBDMS
MeO
OH
loureirin A
*the reaction was quenched by the addition of cooled pentane containing a small amount of pyridine (2%)
**the reaction was quenched by the addition of a cold mixture of NaOH(10% aq) and toluene (5:95)
b. Synthesis of ketone 14a from the corresponding geranyl ester 11a
O
O
1. Tebbe (0.5 M in PhMe, 1.4 equiv)
pyridine (1.4 equiv), PhMe, –40 °C to rt
O
Ph
C
Ph
H2
2. Tebbe (0.5 M in PhMe, 3 equiv)
PhMe, –70 °C
11a
14a
5 h, 90%
Scheme 6 Direct synthesis of ketones from the corresponding esters
In summary, we have developed an innovative process
based on the use of Tebbe’s reagent for the direct transfor-
mation of esters into ketones or olefins.18 This procedure
has been optimized to limit the number of synthetic and
hence chromatographic purification steps needed, which
makes this transformation more economical and more user
friendly. The developed methodology is appropriate for es-
ters that contain a fragment that can form a stabilized car-
bocation with respect to the requirements for the rear-
rangement reaction. To achieve the expected results repro-
ducibly, the entire process must be monitored by using
simple TLC. The general applicability of the developed
methodology is illustrated by the synthesis of loureirin A, a
potential anticancer drug.
References and Notes
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(2) Molecular Rearrangements in Organic Synthesis; Rojas, C. M., Ed.;
Wiley: Hoboken, 2015, DOI: 10.1002/9781118939901.
(3) (a) Martín Castro, A. M. Chem. Rev. 2004, 104, 2939. (b) Ilardi, E.
A.; Stivalaa, C. E.; Zakarian, A. Chem. Soc. Rev. 2009, 38, 3133.
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Soc. 2002, 124, 9720. (b) Nasveschuk, C. G.; Rovis, T. Angew.
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(7) Padwa, A.; Bur, S. K. Tetrahedron 2007, 63, 5341.
(8) Tietze, L. F.; Beifuss, U. Angew. Chem. Int. Ed. 1993, 32, 131.
(9) Maziarz, E.; Furman, B. Tetrahedron 2014, 70, 1651.
(10) Domżalska, A.; Maziarz, E.; Furman, B. Tetrahedron 2017, 73,
7030.
Funding Information
The authors are grateful to Narodowe Centrum Nauki (National Sci-
ence Centre of Poland) for financial support of the project (research
grant PRELUDIUM No. UMO-2017/25/N/ST5/00209 and OPUS No.
UMO-2013/11/B/ST5/01188).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1691594.
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© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G

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A. M. Domżalska-Pieczykolan, B. Furman
Letter
Synlett
(15) Daub, G. W.; McCoy, M. A.; Sanchez, M. G.; Carter, J. S. J. Org.
Chem. 1983, 48, 3876.
(16) Stevenson, J. W. S.; Bryson, T. A. Tetrahedron 1982, 23, 3143.
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(18) Direct Formation of Alkenes 10; General Procedure
A 0.5 M solution of the Tebbe reagent in PhMe (1.4 equiv) and
pyridine (1.4 equiv) were successively added to a cooled solu-
tion of the appropriate ester 7 in PhMe (4 mL/mmol) at –40 °C.
The resulting mixture was allowed to warm to rt and stirred
until the ester was completely consumed (TLC; 2–12h). The
mixture was then poured into a cold 1:1 mixture of 10% aq
NaOH and toluene at 0 °C and immediately filtered. The organic
phase was washed with the dil aq NaOH and H2O, dried
(Na2SO4), concentrated, redissolved in the PhMe (6 mL/mmol),
cooled to –40 °C, and treated with a second portion of Tebbe
reagent (3.0 equiv). The resulting mixture was allowed to warm
to rt and stirred until the vinyl ether disappeared (TLC). Pyri-
dine (3.0 equiv) was added and the resulting mixture was
stirred at rt for 2–4 h until the ketone was completely con-
verted. The mixture was then poured into a cold 1:1 mixture of
10% aq NaOH and toluene at 0 °C. The precipitate that formed
was filtered off and the organic phase was washed with the dil
aq NaOH and H2O (×2), then dried (Na2SO4) and concentrated.
The residue was purified by flash column chromatography with
an appropriate solvent system.
1,1′-(2-Methylbut-1-ene-4,4-diyl)dibenzene (10a)15
Synthesized according to the general procedure, starting from
ester 7a (226.1 mg, 1 mmol), as a colorless oil; yield: 128.6 mg
(57%); Rf = 0.88 (20% EtOAc–hexane). 1H NMR (400 MHz, C6D6):
 = 7.16–7.03 (m, 8 H), 7.02–6.92 (m, 2 H), 4.74–4.52 (m, 2 H),
4.06 (t, J = 7.9 Hz, 1 H), 2.64 (d, J = 7.9 Hz, 2 H), 1.52 (s, 3 H). 13
C
NMR (101 MHz, C6D6):  = 144.8, 143.0, 128.2, 128.0, 126.0,
112.6, 49.4, 43.8, 22.3.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–G