Aryl-substituted cyclopropyl acetylenes as sensitive mechanistic probes in the gold-catalyzed hydration of alkynes. comparison to the Ag(I)-, Hg(II)-, and Fe(III)-catalyzed processes

Article abstract of DOI:10.1021/jo401325t

The gold-catalyzed hydration of 2-phenyl- or 2,2-diphenylcyclopropyl acetylene, sensitive probes to trace the formation of vinyl carbocations, provides exclusively the corresponding cyclopropyl methyl ketones. On the other hand, in the Ag(I)- or Fe(III)-c

Full text of DOI:10.1021/jo401325t

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Aryl-Substituted Cyclopropyl Acetylenes as Sensitive
Mechanistic Probes in the Gold-Catalyzed Hydration of Alkynes.
Comparison to the Ag(I), Hg(II) and Fe(III)-Catalyzed Processes
Georgia Velegraki, and Manolis Stratakis
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo401325t • Publication Date (Web): 29 Jul 2013
Downloaded from http://pubs.acs.org on August 8, 2013
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Aryl-Substituted Cyclopropyl Acetylenes as Sensitive Mechanistic
Probes in the Gold-Catalyzed Hydration of Alkynes. Comparison to
the Ag(I), Hg(II) and Fe(III)-Catalyzed Processes
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Georgia Velegraki and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes 71003 Iraklion, Greece
E-mail: stratakis@chemistry.uoc.gr
Graphical Abstract for the Table of Contents
Au+
Au
Ph
Ph
X
H2O
H2O
Ph
Ph
Ph
Ph
O
Fe2+
Fe3+
Ph
Me
Ph
Ph
Ph
Me
Ph
ABSTRACT: The gold-catalyzed hydration of 2-phenyl or 2,2-diphenylcyclopropyl
acetylene, sensitive probes to trace the formation of vinyl carbocations, provide
exclusively the corresponding cyclopropyl methyl ketones. On the other hand, in the
Ag(I) or Fe(III)-catalyzed hydration, a profound vinyl carbocationic character appears
in the initially formed metal-alkyne complexes, as judged by the partial (Ag+) or
exclusive (Fe3+) formation of allene-type rearrangement products. These findings
provide clear evidence for subtle electronic differences in metal-alkyne complexes
including Au(I or III), Ag(I), Fe(III) and Hg(II).
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Homogeneous gold(I or III) activation of alkynes is nowadays one of the most
active topics in synthetic organic chemistry.1 In the same context, the interest in the
activation of alkynes by gold nanoparticles under heterogeneous conditions is
constantly growing.2 The fascinating catalytic properties of gold are attributed to a
relativistic effect which stabilizes the outermost 6s2 electron pair, thus the reactivity and
catalytic efficiency are governed by its high energy 5d orbitals.3 Alkyne activation is
promoted by coordination of Au(I) to alkynes which enhances their electrophilicity
towards intra or intermolecular nucleophilic attack, providing thus a vast array of
reaction pathways via unprecedented skeletal rearrangements, especially when the
nucleophile is a π bond. Among the various reaction motifs in organogold chemistry
studied so far, the Au-catalyzed hydration of alkynes has been proven as an extremely
practical method for their transformation into carbonyl compounds (Scheme 1).4-5
Terminal alkynes provide exclusively Markovnikov selectivity yielding methyl ketones.
On the other hand, internal alkynes exhibit moderate regioselectivity. Surprisingly if
one of the substituents is a phenyl group the selectivity is peculiar and depends on the
catalyst. This is exemplified in the Ph3PAuNTf2-catalyzed hydration of PhC≡CC6H13,6
where the two regioisomeric ketones a and b (Scheme 1) are formed in equimolar
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Scheme 1. The gold-catalyzed hydration of alkynes.
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amounts, and in the (IPr)Au(I)-catalyzed hydration of PhC≡CCH2CH3,7 where a formal
anti-Markovnikov selectivity was observed (Scheme 1). In general, the gold-catalyzed
hydration of alkynes can be achieved in the presence of Au(I),6-12 Au(III),13-16 Au(I) in
combination with acids,17-21 or by “type II Au(I)-Ag(I) bimetallic” systems.22
Mechanistic studies were also reported regarding this transformation which emphasize
the importance of gem-diaurated intermediates via a dual activation mechanism,23 and
the pivotal role of protic solvents (e.g. methanol) into the energy reaction profile.24-25
On the basis of the lack of an apparent trend of Markovnikov selectivity in the case of
phenyl-alkyl substituted internal alkynes (e.g. the examples shown in Scheme 1), the
>99% Markovnikov selectivity in the gold-catalyzed hydration of terminal alkynes
might be seen as rather surprizing. It is reasonable from the first point of view, that the
regioselectivity is controlled by the electrophilic nature of the sp-carbons of the triple
bond upon interaction to Au(I or III). The interaction of ionic gold to a terminal alkyne
(e.g. propyne) has been computed to be slightly unsymmetrical. The bonding interaction
of the metal with the internal sp-carbon atom is longer than the analogous distance to
the terminal one.16,26 This rationalizing concept provides tentative clues for the higher
electrophilicity of the internal sp-C atom in the hydration process. Yet, these arguments
contradict the cases of phenyl-alkyl substituted internal alkynes, where acetophenones
should primarily or exclusively be anticipated.
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In this manuscript we explore through the gold-catalyzed hydration of terminal
alkynes the nature of the ionic gold interaction to a C-C triple bond, and more
specifically, whether a “loose” complex (symmetrical I; unsymmetrical II), or a formal
vinyl carbocation III are formed (Scheme 2). The existence of vinyl gold carbocations
such as III, could be traced by using aryl-substituted cyclopropyl alkynes as sensitive
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mechanistic probes. Such substrates have been tested as probes in the past to distinguish
among polar or radical pathways27-30 in addition reactions to alkynes. For instance,
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Scheme 2. Possible structures resulting from the interaction of Au+ to a terminal alkyne
(L=ligand).
trans-(2-phenyl)cyclopropan-1-yl alkyne 1 (Scheme 3) has been used by Baines and co-
workers31 to study the mechanism of its hydration catalyzed by sulphuric acid. It was
found that vinyl carbocation 1-H+ (from protonation of 1) rearranges into allenyl
carbocation 1a-H+ at a high rate constant of approximately kR ~ 2x1012 s-1.
Subsequently 1a-H+ undergoes nucleophilic attack from a H2O molecule to yield allenyl
alcohol 1a. Alkyne 1 could therefore be considered as a quite appropriate probe to trace
Scheme 3. The acid-catalyzed hydration of cyclopropyl alkyne 1.31
the existence of vinyl carbocations during the activation of alkynes by ionic gold. Thus,
we undertook the examination of the gold-catalyzed hydration of 1, as well as, its gem-
diphenyl analogue 2, which is anticipated to be an even more sensitive probe. The
synthesis of 1 and 2 was accomplished based on known literature protocols.28,31 For
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comparison purposes, the hydration of 1 and 2 was studied under known Ag(I),32
Fe(III)33 and Hg(II)34-catalyzed protocols and provided fruitful information regarding
the nature of the intermediates under these conditions.
9
To study the Au-catalyzed hydration of cyclopropyl alkyne 1 we adopted the
protocol by Leyva and Corma6 using the Au(I)-based Ph3PAuNTf2 (1 mol%) as catalyst
and methanol as solvent containing 4 equiv of H2O. For the Au(III)-catalyzed hydration,
AuCl3 (1 mol%) in methanol containing 4 equiv of H2O was utilized as catalyst. We
wish to report herein that replacing acetonitrile as the solvent of an existing AuCl3-
catalyzed hydration protocol16 with methanol, resulted to an approximately 5-10 fold
enhancement of the reaction rate. It was found that under both catalytic conditions, 1 is
cleanly transformed after 14 h (Ph3PAuNTf2) or ~1 h (AuCl3), respectively, into methyl
ketone 3 in almost quantitative yields (Scheme 4). No rearrangement products were
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detected. The hydration of 1 under catalysis by AgSbF6 (10 mol%) in refluxing
MeOH/H2O=10/1 for 24 h, provides initially a mixture of 3 and the rearranged
methanol-captured allene 435 in a ~70/30 relative ratio and 75-80% yield. Methyl ketone
3 gradually reacts with methanol under the reaction conditions through a profound Ag+-
catalyzed activation of the carbonyl moiety, leading in part to rearranged methoxy
ketone 536. This side-pathway was verified by the independent treatment of 3 with
33
AgSbF6 in MeOH. In the presence of in situ generated Fe(NTf2)3 (15 mol%) only the
allene bearing rearrangement products 1a31 and 637 were seen as an almost equimolar
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mixture (1,4-dioxane as solvent; 4 equiv H2O; 80 C, 8h, 69% yield), without any
ketone 3 being formed. This outcome resembles the H2SO4-catalyzed hydration of 1
which yields 1a.31 The relative ratio of 1a/6 depends on reaction time, as conjugated
allenene 6 is a secondary product obtained via dehydration of the initially formed
allenyl alcohol 1a under the reaction conditions. On prolonged reaction time (24 h), 1a
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completely dehydrates into relatively labile 6. Finally, the Hg(TfO)2(TMU)2-catalyzed34
(TMU: 1,1,3,3-tetramethylurea) hydration of 1 in methanol (5 mol% catalyst loading, 4
equiv of H2O), afforded exclusively ketone 3 (92% isolated yield), just as under gold
catalysis conditions.
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Scheme 4. The hydration of cyclopropyl-substituted alkyne 1 catalyzed by Au(I or III),
Ag(I), Fe(III) and Hg(II).
We next focused on studying the hydration of gem-diphenyl analogue 2 under
the same catalytic conditions. Alkyne 2 is foreseen as an even more sensitive
mechanistic probe relative to 1. The products from the hydration of alkyne 2 are
presented in Scheme 5. Its Ph3PAuNTf2-catalyzed hydration cleanly afforded methyl
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ketone 738 as the only reaction product (~90% yield). Cyclopropyl methyl ketone 7 was
also exclusively formed in the presence of Hg(TfO)2(TMU)2 in >90% isolated yield. On
contrary, the AgSbF6-catalyzed hydration results primarily to a mixture of rearranged
methoxy allene 8 and ketone 7 in a relative ratio 9/7~7/3. During the progress of the
reaction, 7 gradually decomposes forming methanol adducts, while minor amounts
(~10% in total) of allenene 10 and naphthalene 11 were detected after reaction
completion, which probably derive via elimination of methanol from 8. Finally, the
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Me
O
Hg(TfO)2(TMU)2
(5%)
Ph3PAuNTf2 (1%)
2
Ph
Ph
MeOH, 25 oC
H2O (4 equiv)
90%
MeOH, 25 oC
H2O (4 equiv)
92%
Ph
Ph
2
2
7 (exclusively)
CH2
AgSbF6 (10%)
Ph
7 (30%)
+
MeO
MeOH/H2O (10/1)
70 oC, 68%
reaction
conditions
Ph
8 (70%)
slowly
MeOH addition
products
10+11
reaction
conditions
CH2
Fe(NTf2)3 (15%)
Ph
HO
2
dioxane, 80 oC
H2O (4 equiv)
-H2O
Ph
9 (non isolable)
Me
CH2
reaction
conditions
11
(48% isolated yield)
10
Ph
Ph
Ph
Scheme 5. Hydration of alkyne 2 catalyzed by Au(I), Ag(I), Fe(III) and Hg(II).
Fe(NTf2)3-catalyzed hydration of 2 forms at the initial stages of reaction allenene 10,
apparently via dehydration of the anticipated highly unstable allenyl alcohol 9.
Although compound 9 was not detected during the progress of the reaction, it is
reasonably considered as a reaction intermediate. Allenene 10 gradually undergoes
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Friedel-Crafts-type intramolecular cycloisomerization under the reaction conditions into
1-methyl-4-phenylnaphthalene (11)39, which eventually becomes the only reaction
product after 16 h (48% isolated yield). The low isolated yield of 11 is associated with
its tendency to form side oxidation products at the methyl group under the reaction
conditions.
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The absence of allene-type side products during the Au-catalyzed hydration of 1
or 2, is in sharp contrast to the known Bronsted acid-catalyzed procedure31; the latter
undoubtedly involves the intermediacy of vinyl carbocations such as 1-H+ shown in
Scheme 3. Thus, we envision a loose coordination of gold on the triple bond of 1 or 2
(see the case of alkyne 2 in Scheme 6) which does not generate a vinyl carbocation such
as 2-II, followed by nucleophilic attack (syn or anti) on the internal sp-C atom. The
Ph
Ph
2
Au+
Au
Au+
Ph
Ph
Ph
Ph
Ph
Ph
X
2-III
Au
2-II
2-I
H2O
-Au+ H2O
Ph
HO
Ph
CH2
H
O
Au+
Ph
Ph
H
Ph
O
Ph
not formed
Au
7
Me
2-IV
AuL
H2O
R
AuL
LAu
R
OH2
H2O
H2O
R
VI
V
Scheme 6. Mechanistic considerations in the Au+-catalyzed hydration of cyclopropyl
alkyne 2.
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reasons of the Markovnikov selectivity might be attributed to the higher electrophilic
character of the internal sp-C upon interacting to gold (presumably a slight δ+ charge
not capable of causing skeletal rearrangement to an allene). In addition to this argument,
we tentatively propose that steric factors may also play an important role and contribute
significantly to the regioselectivity of hydration of terminal alkynes. Thus, nucleophilic
attack on the terminal acetylenic carbon atom induces repulsive non-bonding
interactions among the ligated Au-C bond and the R group of the reacting terminal
alkyne (RC≡CH), as shown in the intermediate V at the bottom part of Scheme 6. On
the other hand, nucleophilic attack on the internal acetylenic carbon atom (intermediate
VI) leads to less destabilizing non-bonding interactions. The Ag(I)-catalyzed hydration
of 1 and 2 yields significant amounts of allenes (4 and 8, respectively), which implies
that the internal Csp carbon atom of the alkyne has a pronounced electrophilic character,
resembling an open vinyl carbocation after complexation to Ag+.40 The Fe(III)-
catalyzed hydration reaction of 1 and 2 provides exclusively allene rearrangement
products, in accordance with a pure carbocationic character of the intermediate adduct
among alkyne and the metal. This trend is also reflected in the Fe(NTf2)3-catalyzed
hydration of internal phenyl-alkyl substituted alkynes which exclusively yields
acetophenones33 (>99% Markovnikov selectivity). Finally, mercury(II) provides
idendical to Au(I)-catalysis results. In general, the similarities between the relativistic
Au(I) and Hg(II) in catalysis have been pointed out.41
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A further example which shows the reluctance of a Au+-bound alkyne, such as 1
or 2, to undergo cyclopropyl rearrangement into an allene, was shown in the
Ph3PAuNTf2-catalyzed isomerization of the trans-(2-phenyl)cyclopropan-1-yl internal
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alkynol 12 in methanol, which produced after 2h at 25 C cyclopropyl enone 1342
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(Scheme 7) in 82% isolated yield via a Meyer-Schuster rearrangement.43 No allene side
products were detected in the crude reaction mixture.
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Scheme 7. The Au(I)-catalyzed Meyer-Schuster rearrangement of internal alkynol 12.
In conclusion, we have shown herein that the Au(I or III) activation of alkynes
does not generate vinyl carbocations as intermediates supporting loose metal – π
complexes (such as I or II, Scheme 2), as judged by the lack of allene rearranged
products in the hydration or Meyer-Schuster rearrangement of aryl-substituted
cyclopropyl alkynes (hypersensitive probes to trace vinyl carbocations). Our findings
are in agreement with reported examples of crystal structures of complexes between
alkynes and Au+ where the triple bond acts as a weak electron donor.44-45 On the other
hand, in the Ag(I) or Fe(III) interaction to alkynes a profound vinyl carbocationic
character appears in the metal-alkyne complexes as judged by the partial (Ag+) or
exclusive (Fe3+) formation of allene-type rearrangement products from the aryl-
substituted cyclopropyl alkynes 1 and 2.
EXPERIMENTAL SECTION
General: The reactions were monitored by thin-layer chromatography (TLC) carried
out on silica gel plates (60F-254) with UV light as the visualizing method and an acidic
mixture of phosphomolybdic acid/cerium(IV) sulfate accompanied by heating of the
plate as a developing system. Flash column chromatography was carried out on SiO2
(silica gel 60, particle size 0.040–0.063 mm) with the specified eluent. NMR spectra
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were recorded on a Bruker DPX-300 instrument. Electrospray ionization (ESI) mass
spectrometry (MS) experiments were performed with a GC–MS QP 5050 Shimadzu
single-quadrupole mass spectrometer. High resolution mass spectra (HRMS) were
recorded on ESI-Orbitrap mass spectrometer.
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Synthesis of cyclopropyl alkynes: Alkynes 1 and 2 were prepared according to known
literature protocols (see a schematic presentation in Supporting Information). Internal
alkyne 12 was prepared in 67% isolated yield by reacting 1 (0.1 g, 0.75 mmol) with 1.2
equiv of n-BuLi (1.6M in hexanes, 0.52 mL) in dry THF at -78 oC for 20 min, followed
o
by quench with 50% molar excess of paraformaldehyde at 0 C for 1 h. (trans-2-
1
Ethynylcyclopropyl)benzene (1)31: H NMR (300 MHz, CDCl3): 7.27-7.16 (m, 3H),
7.09 (dd, J1 = 7.0 Hz, J2 = 1.5 Hz, 2H), 2.29-2.25 (m, 1H), 1.91 (d, J = 2.0 Hz, 1H),
1.54-1.48 (m, 1H), 1.37-1.30 (m, 1H), 1.28-1.21 (m, 1H); 13C NMR (75 MHz, CDCl3):
140.5, 128.4, 126.3, 126.0, 86.2, 64.8, 26.1, 17.4, 10.8. (2-Ethynylcyclopropane-1,1-
diyl)dibenzene (2)28: 1H NMR (300 MHz, CDCl3): 7.43 (d, J = 7.5 Hz, 2H), 7.35-7.15
(m, 8H), 2.22-2.16 (m, 1H), 1.88 (d, J = 2.0 Hz, 1H), 1.72 (dd, J1 = 5.0 Hz, J2 = 5.0 Hz,
1H), 1.64 (dd, J1 = 7.0 Hz, J2 = 5.0 Hz, 1H); 13C NMR (75 MHz, CDCl3): 140.5, 128.4,
126.3, 126.0, 86.2, 64.8, 26.1, 17.4, 10.8. trans-3-(2-Phenylcyclopropyl)prop-2-yn-1-
ol (12): 1H NMR (300 MHz, CDCl3): 7.30-7.07 (m, 5H), 4.27 (d, J = 1.5 Hz, 2H), 2.30-
2.23 (m, 1H), 1.57-1.50 (m, 1H), 1.55 (br s, 1H, -OH), 1.35-1.23 (m, 2H); 13C NMR (75
MHz, CDCl3): 140.4, 128.3, 126.1, 125.8, 87.8, 75.0, 51.0, 26.0, 17.4, 11.1; HRMS
(ESI-Orbit trap) m/z: [M + H]+ Calcd for C12H12O+H, 173.0966; Found 173.0960.
Hydration of cyclopropyl alkynes: The hydration of alkynes 1 and 2 as well as the
Meyer-Schuster rearrangement of 12 were performed on a 0.1-0.3 mmol scale following
known literature protocols.6,16,32,33,34,43 The products were purified by flash column
chromatography using petroleum ether or hexane/ethyl acetate gradients.
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Products from the metal-catalyzed hydration of alkynes
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trans-1-(2-Phenylcyclopropyl)ethanone (3)46: Colorless oil (0.015 g from a 0.1 mmol
scale reaction of 1 catalyzed by Ph3PAuNTf2, 94%). Rf = 0.73 (hexane/EtOAc = 2:1);
1H NMR (300 MHz, CDCl3): 7.29-7.20 (m, 3H), 7.10 (dd, J1 = 7.0 Hz, J2 = 1.5 Hz, 2H),
2.58-2.50 (m, 1H), 2.30 (s, 3H), 2.27-2.19 (m, 1H), 1.71-1.63 (m, 1H), 1.41-1.35 (m,
1H); 13C NMR (75 MHz, CDCl3): 206.8, 140.3, 128.5, 126.5, 126.0, 32.8, 30.8, 30.0,
19.1.
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(1-Methoxypenta-3,4-dien-1-yl)benzene (4)35: Colorless oil (0.011 g from a 0.3 mmol
1
scale reaction of 1 catalyzed by AgSbF6, 22%). Rf = 0.65 (hexane/EtOAc = 6:1); H
NMR (300 MHz, CDCl3): 7.39-7.28 (m, 5H), 5.08 (m, 1H), 4.64 (m, 2H), 4.19 (dd, J1 =
7.5 Hz, J2 = 5.5 Hz, 1H), 3.24 (s, 3H), 2.56-2.43 (m, 1H), 2.41-2.30 (m, 1H); 13C NMR
(75 MHz, CDCl3): 209.2, 141.5, 128.3, 127.6, 126.7, 86.3, 83.5, 74.6, 56.7, 37.1.
5-Methoxy-5-phenylpentan-2-one (5)36: Colorless oil (the isolated yield of 5 depends
on the reaction time since it is a secondary product deriving from 3). Rf = 0.41
1
(hexane/EtOAc = 6:1); H NMR (300 MHz, CDCl3): 7.37-7.26 (m, 5H), 4.13 (dd, J1 =
7.5 Hz, J2 = 5.5 Hz, 1H), 3.20 (s, 3H), 2.50 (t, J = 7.0 Hz, 2H), 2.11 (s, 3H), 2.06-1.88
(m, 2H); 13C NMR (75 MHz, CDCl3): 208.5, 141.7, 128.4, 127.6, 126.5, 82.8, 56.6,
39.7, 32.0, 29.9.
1-Phenylpenta-3,4-dien-1-ol (1a)31: Colorless oil (the isolated yield of 1a depends on
the reaction time since it gradually dehydrates to 6 under the reaction conditions). Rf =
1
0.25 (hexane/EtOAc = 6:1); H NMR (300 MHz, CDCl3): 7.37-7.26 (m, 5H), 5.12 (m,
1H), 4.76 (t, J = 6.5 Hz, 1H), 4.73 (m, 2H), 2.51-2.43 (m, 2H), 2.11 (br s, 1H –OH); 13C
NMR (75 MHz, CDCl3): 209.3, 143.4, 128.2, 127.4, 125.6, 85.9, 83.5, 74.9, 73.4, 38.3.
(E)-Penta-1,3,4-trien-1-ylbenzene (6)37: Colorless oil (the isolated yield of 6 depends
on the reaction time since it is a secondary product resulting from the dehydration of 1a
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under the reaction conditions). Rf = 0.70 (hexane/EtOAc = 6:1); H NMR (300 MHz,
CDCl3): 7.40-7.07 (m, 5H), 6.60 (dd, J1 = 16.0 Hz, J2 = 10.0 Hz, 1H), 6.49 (d, J = 16.0
Hz, 1H), 6.01 (td, J1 = 10.0 Hz, J2 = 6.5 Hz, 1H), 5.01 (d, J = 6.5 Hz, 2H); 13C NMR
(75 MHz, CDCl3): 212.6, 137.0, 130.1, 128.4, 127.2, 126.0, 123.9, 93.8, 76.4.
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1-(2,2-Diphenylcyclopropyl)ethanone (7)38: Colorless oil (0.021 g from a 0.1 mmol
scale reaction of 2 catalyzed by Ph3PAuNTf2, 90%). Rf = 0.43 (hexane/EtOAc = 6:1);
1H NMR (300 MHz, CDCl3): 7.32-7.16 (m, 10H), 2.83 (dd, J1 = 8.0 Hz, J2 = 6.0 Hz,
1H), 2.29 (dd, J1 = 6.0 Hz, J2 = 4.5 Hz, 1H), 2.16 (s, 3H), 1.60 (dd, J1 = 8.0 Hz, J2 = 4.5
13
Hz, 1H); C NMR (75 MHz, CDCl3): 203.9, 145.0, 139.4, 129.9, 128.5, 128.4, 127.5,
127.1, 126.6, 42.7, 37.2, 31.3, 21.1.
(1-Methoxypenta-3,4-diene-1,1-diyl)dibenzene (8): Colorless oil (the isolated yield of
8 depends on the reaction time and varies from 35-45% since it gradually decomposes
into 10 and 11 during the progress of the reaction). Rf = 0.60 (hexane/EtOAc = 10:1);
1H NMR (300 MHz, CDCl3): 7.50-7.25 (m, 10H), 4.82 (m, 1H), 4.55 (td, J1 = 7.0 Hz, J2
= 1.5 Hz, 2H), 3.10 (s, 3H), 3.09 (td, J1 = 7.0 Hz, J2 = 1.5 Hz, 2H); 13C NMR (75 MHz,
CDCl3): 209.4, 135.0, 132.2, 127.7, 126.3, 84.4, 83.5, 73.9, 50.0, 35.2; HRMS (ESI-
Orbit trap) m/z: [M + H]+ Calcd for C18H18O+H, 251.1436; Found 251.1431.
Penta-1,3,4-triene-1,1-diyldibenzene (10): 1H NMR (300 MHz, CDCl3): 7.45-7.25 (m,
10H), 6.55 (dd, J1 = 11.0 Hz, J2 = 1.0 Hz, 1H), 5.95 (td, J1 = 11.0 Hz, J2 = 6.5 Hz, 1H),
4.96 (dd, J1 = 6.5 Hz, J2 = 1.0 Hz, 2H); 13C NMR (75 MHz, CDCl3): 213.7, 142.0,
141.6, 139.3, 130.3, 128.3, 128.2, 127.4, 127.4, 127.3, 122.6, 91.9, 76.1. This
compound is an intermediate product in the Fe(III)-catalyzed isomerization of 2 to 11,
and during the progress of the reaction coexists either with 2 (early stages) or with 11
(late stages). As it is isomeric to 2 to 11 and chromatographically inseparable from
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them, no HRMS could be recorded. It was however, detected by GC-MS. MS (EI): 218
(M+, 100%), 203 (63%), 107 (30%), 101 (32%), 94 (27%).
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1-Methyl-4-phenylnaphthalene (11)39: Colorless oil (0.010 g from a 0.1 mmol scale
reaction of 2 catalyzed by Fe(NTf2)3, 48%). Rf = 0.71 (hexane/EtOAc = 10:1); 1H NMR
(300 MHz, CDCl3): 8.07 (d, J = 9.0 Hz, 1H), 7.57-7.31 (m, 9H), 2.75 (s, 3H); 13C NMR
(75 MHz, CDCl3): 141.0, 138.7, 135.2, 133.8, 132.8, 131.7, 130.2, 128.2, 127.0, 126.7,
126.6, 126.2, 125.6, 124.4, 19.6.
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trans-1-(2-Phenylcyclopropyl)prop-2-en-1-one (13)41: Colorless oil (0.014 g from a
0.1 mmol scale reaction of 12 catalyzed by Ph3PAuNTf2, 83%). Rf = 0.76
1
(hexane/EtOAc = 2:1); H NMR (300 MHz, CDCl3): 7.34-7.11 (m, 5H), 6.51 (dd, J1 =
17.5 Hz, J2 = 11.0 Hz, 1H), 6.28 (dd, J1 = 17.5 Hz, J2 = 1.0 Hz, 1H), 5.84 (dd, J1 = 11.0
Hz, J2 = 1.0 Hz, 1H), 2.62-2.56 (m, 1H), 2.47-2.41 (m, 1H), 1.80-1.74 (m, 1H), 1.49-
1.43 (m, 1H); 13C NMR (75 MHz, CDCl3): 198.5, 140.4, 136.8, 128.5, 128.1, 126.6,
126.1, 30.2, 29.6, 19.2.
Acknowledgments: This was supported in part by the ELKE, University of Crete (KA
3823). We thank ProFI (ITE, Heraklion, Greece) for obtaining the HRMS spectra of
unknown compounds.
Dedication: Dedicated to professor Michael Orfanopoulos on the occasion of his 65th
birthday.
1
Supporting Information Available: Copies of H and 13C NMR of all compounds.
This material is available free of charge via the Internet at http://pubs.acs.org.
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