Direct organocatalytic oxo-metathesis, a trans-selective carbocation-catalyzed olefination of aldehydes

Article abstract of DOI:10.1002/ejoc.201403651

A direct organocatalytic carbonyl/olefin oxo-metathesis has been developed. The reaction is catalyzed by trityl tetrafluoroborate (TrBF₄) and utilizes unactivated alkenes for the olefination of aromatic aldehydes to give trans β-alkylstyrenes in yields of 44-85% with only acetone as the byproduct. The pronounced Lewis acidity of the carbocation results in unusual reactivity that is proposed to catalyze a stepwise [2+2] cycloaddition to give an oxetane intermediate. Fragmentation of the latter in a formal retro [2+2] reaction gives the oxo-metathesis product.

Full text of DOI:10.1002/ejoc.201403651

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FULL PAPER
DOI: 10.1002/ejoc.201403651
Direct Organocatalytic Oxo-Metathesis, a trans-Selective
Carbocation-Catalyzed Olefination of Aldehydes
[a]
[a]
[a]
Veluru Ramesh Naidu, Juho Bah, and Johan Franzén*
Keywords: Organocatalysis / Metathesis / Carbocations / Ketones / Alkenes
A direct organocatalytic carbonyl/olefin oxo-metathesis has
The pronounced Lewis acidity of the carbocation results in
been developed. The reaction is catalyzed by trityl tetra- unusual reactivity that is proposed to catalyze a stepwise
fluoroborate (TrBF
olefination of aromatic aldehydes to give trans β-alkylstyr-
enes in yields of 44–85% with only acetone as the byproduct.
4
) and utilizes unactivated alkenes for the
[2+2] cycloaddition to give an oxetane intermediate. Frag-
mentation of the latter in a formal retro [2+2] reaction gives
the oxo-metathesis product.
Introduction
Alkenes are essential functional groups in organic
chemistry, both as feedstock for further chemical transfor-
mations and as synthetic targets in their own right. Thus,
chemical transformations leading to carbon–carbon double
bond formation are of central importance in organic
chemistry and carbonyl/ylide metathesis[ (e.g., Wittig,
Julia olefination) and olefin/olefin metathesis[ reactions
are examples of commonly used synthetic methods both in
academia and industry. On the other hand, direct catalytic
oxo-metathesis between an alkene and a carbonyl is a highly
powerful complement to the strategies known today, and
will open up new synthetic opportunities in particular
with respect to atom economy and substrate scope
1]
2]
Scheme 1. Double bond metathesis reactions.
Lambert and co-workers developed an elegant organo-
catalytic ring-opening oxo-metathesis based on a [2+3]/
retro[2+3] cycloaddition sequence [Scheme 2, Equa-
[
3a]
[
3–5]
tion (b)].
Unfortunately, the alkene was restricted to
(Scheme 1).
Despite the undisputable potential of this
cyclopropene derivatives and opening of the three-mem-
bered ring was proposed to be the major driving force of
process, the carbonyl/olefin oxo-metathesis has rarely been
reported in the literature and the development of a general
catalytic and operationally simple strategy still remains a
[
4]
this reaction. Twenty years earlier, Bickelhaupt and co-
workers reported two isolated examples of the direct ole-
[
6]
challenging task.
fination of benzaldehyde in the presence of a heterogeneous
The few reports on carbonyl/olefin oxo-metathesis
known so far describe photochemically promoted [2+2]/
zinc(II) catalyst, EPZ-10 [Scheme 2, Equation (c)].[
5,10]
Un-
[
7]
der these conditions, the oxo-metathesis products, styrene
and β-methylstyrene, were obtained in low yields.
retro[2+2] cycloadditions [Scheme 2, Equation (a)] and
[8]
the use of stoichiometric amounts of Lewis acid or a tran-
sition-metal complex,[9] which strongly limits the general
Herein we present the further development of Bickel-
haupt’s strategy towards an operationally simple organo-
catalytic carbonyl/olefin oxo-metathesis. The reaction was
applied to simple aromatic aldehydes and unactivated alk-
enes (amylene analogues) and is catalyzed by trityl tetra-
applicability of the procedure in synthesis. Furthermore,
attempts to establish catalytic oxo-metathesis processes have
met with limited success and, to the best of our knowledge,
there are only two known examples reported. In 2012,
fluoroborate (TrBF ) under mild reaction conditions.
4
[
a] Department of Chemistry, Organic Chemistry, Royal Institute
of Technology (KTH),
Results and Discussion
Teknikringen 30, 10044 Stockholm, Sweden
E-mail: jfranze@kth.se
Optimization of the Reaction Conditions
https://www.kth.se/en/che/divisions/orgkem/research/
JohanFranzen
Recently our group has shown that trityl tetrafluoro-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403651.
borate (TrBF ) is an efficient Lewis acid catalyst for several
4
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V. R. Naidu, J. Bah, J. Franzén
FULL PAPER
Scheme 2. Previous efforts toward carbonyl/olefin oxo-metathesis.
organic transformations (e.g., Diels–Alder, aza-Diels–Alder, Table 1. Screening of the reaction conditions for the TrBF
conjugate addition, halogenation) and an interesting com-
4
-cata-
lyzed oxo-metathesis.[
a]
plement to traditional metal- or metalloid-catalyzed proto-
cols.[
11–16]
In fact, the application of carbocations as Lewis
acid catalysts is poorly studied and we reasoned that the
carbocation might have properties different to those of
metal- or metalloid-based Lewis acid catalysts. With this in
mind we set out to evaluate the trityl cation in the reaction
reported by Bickelhaupt and co-workers.[5] In our initial re-
action, a 1:1 ratio of benzaldehyde (1a) and amylene (2a)
in the presence of 7 mol-% trityl tetrafluoroborate (TrBF4)
gave the desired β-methylstyrene (3a) in 20% yield (Table 1,
entry 1). Under these conditions the reaction stopped at
Yield[b] [%]
Entry
Ratio
TrBF
4
Solvent
t [h]
Conv.
[%]
[1a/2a] [mol-%]
1
2
3
4
5
6
7
8
9
1:1
1:2
1:5
2:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
7
7
7
7
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
100
100
100
100
100
31
31
100
31
31
31
31
31
77
99
100
85
100
100
100
100
100
100
100
0
20
18
23
34
43
7
10
20
20
40
20
20
20
20
20
20
57 (50)
77% conversion with both unreacted aldehyde and olefin
68 (60)
left in the reaction mixture. To push the reaction to comple-
tion we varied the ratio of the substrates. Increasing the
amount of amylene (2a) resulted in the full conversion of
benzaldehyde (1a) together with increasing amounts of un-
73[c]
75 (59)
1
0
53
55
–
–
–
1
1
CH
CH
3
NO
CN
2
1
2
3
identified byproducts and the yield of the product 3a re-
13
THF
0
0
0
maining low (Table 1, entries 2 and 3).[ On the other 14
17]
Et O
2
31
31
15
PhMe
–
hand, increasing the amount of benzaldehyde (1a) led to an
increase in both the conversion and yield. Thus, using a 5:1 [a] The catalyst was added to a solution (0.3 m) of benzaldehyde
1
ratio of aldehyde 1a to olefin 2a gave the product 3a in 43% (1a) and amylene (2a). [b] Determined by H NMR analysis by
using 1-methylnaphthalene as the internal standard. The yields
yield (Table 1, entries 4 and 5).
given in parentheses are isolated yields. [c] Reaction performed at
The catalyst loading had a pronounced effect on the yield
–78 °C.
of the reaction and increasing the amount of TrBF from 7
4
to 10 mol-% resulted in an increase in yield from 43 to 57%
The time course of the oxo-metathesis of amylene (2a)
(Table 1, entries 5 and 6). The use of a 20 mol-% catalyst
and benzaldehyde (1a) catalyzed by TrBF reveals the fast
4
loading gave β-methylstyrene (3a) in 68% yield. Further in-
creases in the catalyst loading had a less pronounced effect
on the yield, with a 40 mol-% catalyst loading giving a yield
of 75% (Table 1, entry 9). We compromised between cata-
lyst loading and yield and settled on a 20 mol-% catalyst
loading for further optimization.
consumption of the olefin with full conversion within 3 h
(Figure 1). Surprisingly, after full conversion of amylene
The trityl cation is relatively unstable and even at gently
elevated reaction temperatures (DCM at reflux) both the
catalyst and amylene decomposed and no product form-
1
ation could be observed by H NMR spectroscopy of the
crude reaction mixture. However, lowering the reaction
temperature to –78 °C had a positive effect on the yield and
β-methylstyrene (3a) was formed in 73% yield. However, a
disadvantage was the increased reaction time from 31 h to
more than 4 d before the full conversion of amylene (2a)
was observed (Table 1, entry 8).
Figure 1. Reaction profile for the TrBF
4
(20 mol-%)-catalyzed oxo-
metathesis of amylene (2a) and benzaldehyde (1a; 1:5 ratio of 2a/
1
1
a). The yields were determined by H NMR spectroscopy using
1-methylnaphthalene as the internal standard.
2
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Direct Organocatalytic Oxo-Metathesis
(
4
4
2a), the yield of β-methylstyrene (3a; and acetone) is only Table 2. Screening of Lewis acids for the TrBF -catalyzed oxo-
0% (cf. Table 1, entry 7). Interestingly, H NMR analysis
metathesis.[
a]
1
of the crude reaction mixtures over time revealed the build-
up of two or more unidentified intermediates.[ However,
a prolonged reaction time led to a decrease of these inter-
mediates and a slow increase of the product to give a yield
of 68% over 30 h. It is also interesting to point out that the
yield of acetone never deviates far from the yield of the
product, which indicates that acetone does not undergo any
aldol condensation reactions under these conditions (see
18]
[
5]
Figure 1). The high selectivity for the formation of the
trans double bond as the only observable isomer should
also be emphasized.
Entry
Catalyst (amount [mol-%])
Conversion [%]
Yield[b] [%]
The solvent had a strong impact on the reactivity and
1
2
3
4
5
6
7
8
9
TrBF
ZnCl
Zn(OTf)
4
(20)
(20)
100
0
0
68
–
–
–
trace
7
41
44
21
45
2
coordinating solvents (CH CN and ethers) completely sup-
2
3
2
(20)
pressed the reactivity, most likely due to blocking of the
Lewis acidic site of the carbocation, and only starting mate-
rial could be recovered (Table 1, entries 12–14). Nitrometh-
ane gave rather a high yield but a sluggish reaction with
SnCl
InCl
4
(20)
(20)
0
3
Ͻ5
78
100
100
75
100
18
49
AlCl (20)
3
3
BF ·OEt
2
(25)
1
HBF ·OEt (30)
4
2
several unidentified byproducts observed in the crude H
4 (20)
NMR spectrum (Table 1, entry 11). TrBF is poorly soluble
0[c]
4
1
4 (20)
5 (20)
5 (20)
in toluene, which most likely inhibits any reaction, and only
1
1
[c]
starting material was recovered under these reaction condi- 12
tions (Table 1, entry 15).
10
[
a] The catalyst was added to a solution (0.3 m) of benzaldehyde
1
After establishing the optimal reaction conditions for the (1a; 5 equiv.) and amylene (2a; 1.0 equiv.). [b] Determined by H
NMR analysis using 1-methylnaphthalene as the internal standard.
oxo-metathesis catalyzed by TrBF , we evaluated different
4
[c] 100 h reaction time.
Lewis acids. BF ·OEt has been used before in intramolecu-
3
2
lar stoichiometric oxo-metathesis reactions[ and Bickel-
8]
haupt and co-workers used a zinc catalyst immobilized on electron-withdrawing groups, such as halogenated benz-
[
5]
clay. Interestingly, zinc(II) salts as well as SnCl were com- aldehydes (Cl, F, Br), gave the trans product 3 as the only
4
pletely inactive as catalysts under our conditions and both observable isomer in yields of 54–59% (Table 3, entries 5,
benzaldehyde (1a) and amylene (2a) remained unreacted 6, and 8).
(
Table 2, entries 2–4), whereas the use of InCl gave traces
Weakly electron-donating groups, for example, o-, m-,
3
of product (Table 2, entry 5). On the other hand, AlCl led and p-tolualdehyde as well as 2-naphthaldehyde, worked
3
to 78% conversion of amylene (2a) over 30 h, but again best in this transformation and gave the corresponding
only traces of product were observed, most likely due to trans alkenes in yields of 55–85% (Table 3, entries 3 and 12–
Lewis acid initiated polymerization (Table 2, entry 6). 14). As was observed for the reaction of benzaldehyde (1a)
BF ·OEt and HBF ·OEt were found to be potent and amylene (2a; Table 3, cf. entries 1 and 2), we also ob-
3
2
4
2
catalysts, although less efficient than the trityl ion, and gave served an improvement in yield for the reactions of p-tolu-
the product in yields of 41 and 44%, respectively (Table 2, aldehyde and p-fluorobenzaldehyde with amylene (2a) from
entries 7 and 8).
55 to 74% and from 59 to 66%, respectively, by decreasing
The trityl cation is a rather unique carbon-centered the reaction temperature to –78 °C (Table 3, cf. entries 3
Lewis acid as its acidity can be readily tuned by variation and 4, and 6 and 7). Unfortunately this was not a general
[
12,19]
of the electronic properties of the aromatic groups.
As trend and lowering the reaction temperature did not im-
expected, the oxo-metathesis became slower as the electron prove the yields of the reactions of o- or m-tolualdehyde or
density of one of the aromatic rings increased (i.e., decreas- p-bromo- or p-chlorobenzaldehyde.
ing the Lewis acidity of the carbocation) on going from
trityl to a mono-methyl- (4) to mono-methoxy-substituted and in some cases volatile making them hard to isolate such
5) trityl cation (Table 2, entries 9–12). Unfortunately, that the isolated yields were in all cases substantially lower
decreasing the Lewis acidity of the trityl ion did not prevent than the yields determined against internal standard.
In most cases the β-alkylstyrenes are relatively unstable
(
any side-reactions and had a negative effect on the yield.
Interestingly, for the strongly electron-donating substitu-
ent p-anisaldehyde, only traces of product could be ob-
1
served by H NMR analysis of the crude reaction mixture
(
Table 3, entry 9). However, full conversion of amylene and
Scope and Limitations
the formation of acetone corresponding to a yield of 60%
Following this we investigated the substrate scope of the was observed. This indicates that the corresponding oxo-
oxo-metathesis reaction. This protocol tolerates different metathesis product is indeed formed but is not stable under
benzaldehydes although with some restrictions. Weakly the reaction conditions and decomposes, most likely by
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V. R. Naidu, J. Bah, J. Franzén
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Table 3. Scope of the TrBF
4
-catalyzed oxo-metathesis.[a]
acid-catalyzed polymerization. By lowering the temperature
to –78 °C, the decomposition of the product was somewhat
diminished and p-methoxystyrene was obtained in 15%
[
20]
yield (Table 3, entry 10).
On the other hand, with the
strongly electron-withdrawing p-nitrobenzaldehyde, the full
conversion of amylene was observed to give a mixture of
1
unidentified and unstable products (observed by H NMR
analysis of the crude reaction mixture) that decomposed
upon attempted isolation (Table 3, entry 11). For this reac-
tion, the formation of acetone could not be observed in
the reaction mixture, which indicates that no oxo-metathesis
product was formed.
Unfortunately, screening of different olefins turned out
to be less productive. The oxo-metathesis of 2-methyl-2-hex-
ene (2b) and citronellyl acetate (2c) with different benz-
aldehydes at room temperature gave the trans products with
approximately one catalyst turnover, for example, yields of
24–35% were obtained. Both 2-methyl-2-hexene (2b) and
citronellyl acetate (2c) turned out to be substantially less
reactive than amylene (2a), and no reaction was observed
at –78 °C. After extensive optimization the best reaction
conditions for olefins 2b and 2c were found to be –20 °C
for 135 h. Under these conditions the corresponding trans
olefins were obtained in yields of 36–51% (Table 3, en-
tries 15–20).
Surprisingly, the use of a tetrasubstituted olefin [e.g., 2,3-
dimethyl-2-butene (6)] gave no reaction under the standard
reaction conditions at room temperature and only starting
materials could be recovered. On the other hand, aliphatic
aldehydes, for example, octanal (7), decomposed under the
same conditions and only amylene (2a) was recovered with-
out any observable formation of the corresponding oxo-
metathesis product (Scheme 3).
4
Scheme 3. Ineffective substrates for the TrBF -catalyzed oxo-
metathesis.
Mechanistic Proposal
The mechanism for the oxo-metathesis, first proposed by
[
5]
Bickelhaupt and co-workers, is believed to involve LUMO
activation of aldehyde 1 with the formation of oxonium ion
A (Scheme 4).
Nucleophilic attack of alkene 2 on the oxonium ion A
gives tertiary carbocation B, which is analogous to the
[
21]
initial step of the Prins reaction.
The following step is
believed to deviate from a normal Prins reaction and carbo-
cation B is proposed to rapidly cyclize to give the oxetane
intermediate C in an overall stepwise [2+2] cycloaddition.
Opening of the oxetane ring generates the benzylic carbo-
[
a] TrBF was added to a solution (0.3 m) of aldehyde 1 (5 equiv.)
4
1
and alkene 2 (1.0 equiv.). [b] Determined by H NMR analysis
using 1-methylnaphthalene as the internal standard. The yields
given in parentheses are isolated yields. [c] Product decomposition.
[
1
d] Complex mixture, no product observed by crude H NMR spec- cation D, which undergoes a Lewis acid catalyzed fragmen-
tation reaction to give acetone and the trans alkene.[
22]
troscopy.
4
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Direct Organocatalytic Oxo-Metathesis
Scheme 4. Plausible mechanism for the trityl-catalyzed oxo-metathesis.
dried with MgSO
The residue was purified by silica gel column chromatography
4
and the solvent was distilled at ambient pressure.
Conclusions
We have developed a mild organocatalytic carbonyl/ (pentane 100%) to give the trans β-alkylstyrenes 3.
olefin oxo-metathesis between readily available benz-
aldehyde derivatives and simple unactivated olefins. The re-
action is operationally simple and uses cheap and readily
Acknowledgments
available trityl tetrafluoroborate (TrBF ) as a Lewis acid
4
This work was made possible by grants from the Swedish Research
catalyst. Despite a relatively high catalyst loading and Council (VR). J. F. thanks the Stenbäcks Foundation, The Royal
narrow substrate scope, the catalytic carbonyl/olefin oxo- Swedish Academy of Sciences, and the Lars Hierta Memorial
metathesis is an important proof-of-concept for a reaction Foundation for grants. V. R. N. thanks the Wenner-Gren Founda-
tion for a grant.
that predominantly relies on stoichiometric transition-metal
reagents or photochemistry. In addition, we have demon-
strated novel catalytic activity for the cheap and readily [1] T. Takeda (Ed.), Modern Carbonyl Olefination, Wiley-VCH,
Weinheim, Germany, 2004.
available tritylium ion. Although a highly interesting Lewis
acid, this carbocation has received very little attention as a
catalyst for organic transformations and the unusual reac-
[
2] K. Grela (Ed.), “Cross Metathesis”, in Olefin Metathesis:
Theory and Practice, John Wiley & Sons, Inc., Hoboken, NJ,
USA, 2014.
tivity demonstrated here shows the future potential of the [3] a) A. K. Griffith, C. M. Vanos, T. H. Lambert, J. Am. Chem.
Soc. 2012, 134, 18581; b) A.-L. Lee, Angew. Chem. Int. Ed.
carbocation as an environmentally friendly complement to
2013, 52, 4524.
traditional metal- and metalloid-based Lewis acid catalysts.
We are continuing to explore the carbonyl/olefin oxo-
metathesis reaction to obtain a deeper understanding of the
[
4] a) X. Hong, Y. Liang, A. K. Griffith, T. H. Lambert, K. N.
Houk, Chem. Sci. 2014, 5, 471; b) X. Zhang, Comput. Theor.
Chem. 2014, 1027, 179.
mechanism and further extend the reaction scope towards [5] H.-P. van Schaik, R.-J. Vijn, F. Bickelhaupt, Angew. Chem. Int.
Ed. Engl. 1994, 33, 1611; Angew. Chem. 1994, 106, 1703.
a more general methodology. We are also continuing to
[
6] There are several reports in the literature on Lewis acid and
Brønsted acid catalyzed carbonyl/alkyne metathesis. For some
recent examples, see: a) C. Trujillo, G. Sanchez-Sanz, I. Karpa-
viciene, U. Jahn, I. Cikotiene, L. Rulísek, Chem. Eur. J. 2014,
explore the scope and limitations of carbocation catalysis
to gain a greater understanding of its ability as a Lewis acid
catalyst. These results will be reported in due course.
20, 10360; b) S. Jalal, K. Bera, S. Sarkar, K. Paul, U. Jana,
Org. Biomol. Chem. 2014, 12, 1759; c) S. Maiti, P. Biswas, J.
Ghosh, M. G. B. Drew, C. Bandyopadhyay, Tetrahedron 2014,
Experimental Section
7
0, 334; d) I. Karpaviciene, I. Cikotiene, Org. Lett. 2013, 15,
2
24; e) I. R. Siddiqui, Rahila, S. Shamim, P. Rai, Shireen,
General Procedure for the TrBF
Metathesis: Aldehyde 1 (5 equiv.) and alkene 2 (1 equiv.) were
added to a solution of TrBF (20.0 mol-%) in dichloromethane (c
0.3 m with respect to the alkene) at the indicated temperature.
After the indicated reaction time, the reaction mixture was
quenched with saturated aq. NaHCO and the resulting mixture
was extracted with DCM (ϫ3). The combined organic phases were
4
-Catalyzed Carbonyl/Olefin Oxo-
M. A. Waseem, A. A. H. Abumhdi, Tetrahedron Lett. 2013, 54,
6991; f) K. Bera, S. Sarkar, S. Biswas, S. Maiti, U. Jana, J. Org.
4
Chem. 2011, 76, 3539; g) K. Bera, S. Sarkar, S. Jala, U. Jana,
J. Org. Chem. 2012, 77, 8780.
=
[
7] a) R. A. Valiulin, T. M. Arisco, A. G. Kutateladze, J. Org.
Chem. 2011, 76, 1319; b) M. DЈAuria, R. Racioppi, L. Viggi-
ani, Photochem. Photobiol. Sci. 2010, 9, 1134; c) R. A. Valiulin,
3
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V. R. Naidu, J. Bah, J. Franzén
FULL PAPER
1997, 119, 11341; m) C.-T. Chen, S.-D. Chao, J. Org. Chem.
1999, 64, 1090; n) T. Mukaiyama, H. Nagaoka, M. Murakami,
M. Ohshima, Chem. Lett. 1985, 997; o) S. Kobayashi, M. Mu-
rakami, T. Mukaiyama, Chem. Lett. 1985, 953; p) A. Brunner,
S. Taudien, O. Riant, H. B. Kagan, Chirality 1997, 9, 478; q)
S. Taudien, O. Riant, H. B. Kagan, Tetrahedron Lett. 1995, 36,
3513; r) H. Latham, T. Sammakia, Tetrahedron Lett. 1995, 36,
6867.
[
[
[15]
For mechanistic studies, see: a) C.-T. Chen, S. Denmark, Tetra-
hedron Lett. 1994, 35, 4327; b) T. K. Hollis, B. Bosnich, J. Am.
Chem. Soc. 1995, 117, 4570.
[
16] a) O. Sereda, N. Clemens, T. Heckel, R. Wilhelm, Beilstein J.
Org. Chem. 2012, 8, 1798; b) V. Jur cˇ ík, R. Wilhelm, Org. Bio-
mol. Chem. 2005, 3, 239; c) C. Nicolas, J. Lacour, Org. Lett.
2006, 8, 4343.
[
17] It was found that TrBF itself reacts with amylene (1:5 ratio,
4
DCM, room temp.) to give a complex mixture of unidentified
compounds.
18] Several unsuccessful attempts to isolate and characterize these
intermediates were made.
[
Jossifov, Makromol. Chem. Rapid Commun. 1983, 4, 659.
10] Envirocat EPZ-I0 is zinc(II) chloride immobilized on clay man-
ufactured by Contract Chemicals, Prescot, England.
[20] All attempts to optimize this reaction using less Lewis acidic
carbocations failed.
[
[21] For a recent review, see: I. M. Pastor, M. Yus, Curr. Org. Chem.
2012, 16, 1277.
[
11] J. Bah, J. Franzén, Chem. Eur. J. 2014, 20, 1066.
[
5]
[
12] J. Bah, V. R. Naidu, J. Teske, J. Franzén, Adv. Synth. Catal.
[22] An alternative mechanism, via the Prins reaction product,
2
015, 375, 148.
involves addition of a second benzaldehyde 1 to intermediate
B leading to the 1,3-dioxane intermediate F. Decomposition of
the latter leads to the formation of 3, A, and acetone.
[
13] For a review see: O. Sereda, S. Tabassum, R. Wilhelm, Top.
Curr. Chem. 2010, 291, 349.
[
14] a) S. Kobayashi, S.-L. Shoda, T. A. Mukaiyama, Chem. Lett.
1984, 13, 907; b) T. A. Mukaiyama, S. Kobayashi, S.-I. Shoda,
Chem. Lett. 1984, 1529; c) M. Ohshima, M. Murakami, T. A.
Mukaiyama, Chem. Lett. 1985, 1871; d) M. Yanagisawa, T. A.
Mukaiyama, Chem. Lett. 2001, 224; e) T. Mukaiyama, S. Ko-
bayashi, M. Murakami, Chem. Lett. 1984, 13, 1759; f) S. Ko-
bayashi, S. Murakami, T. A. Mukaiyama, Chem. Lett. 1985,
14, 447; g) S. Kobayashi, M. Murakami, T. Mukaiyama, Chem.
Lett. 1985, 14, 1535; h) S. Kobayashi, S. Matsui, T. A. Mukai-
yama, Chem. Lett. 1988, 17, 1491; i) J. S. Han, H. Akamatsu,
T. A. Mukaiyama, Chem. Lett. 1990, 19, 889; j) M. Yanagis-
awa, T. Shimamura, D. Iida, J.-I. Matsuo, T. A. Mukaiyama,
Chem. Pharm. Bull. 2000, 48, 1838; k) S.-D. Chao, K.-C. Yen,
C.-T. Chen, Synlett 1998, 924; l) S.-D. Chao, K.-C. Yen, C.-H.
Chen, I.-C. Chou, S.-W. Hon, C.-T. Chen, J. Am. Chem. Soc.
Received: December 18, 2014
Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O43651
/KAP1
Date: 04-02-15 12:54:08
Pages: 7
Direct Organocatalytic Oxo-Metathesis
Oxo-Metathesis
V. R. Naidu, J. Bah, J. Franzén* ....... 1–7
Direct Organocatalytic Oxo-Metathesis, a
trans-Selective
Carbocation-Catalyzed
Olefination of Aldehydes
A new partner in the metathesis dance:
Trityl tetrafluoroborate (TrBF ) catalyzes
the direct oxo-metathesis of aldehydes and
derivatives and acetone through an unusual
metal-free formal [2+2]/retro [2+2] reaction
sequence.
4
Keywords: Organocatalysis / Metathesis /
Carbocations / Ketones / Alkenes
unactivated olefins to give β-alkylstyrene
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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