Gold(I) carbenes by retro-buchner reaction: Generation and fate

Article abstract of DOI:10.1021/ja411626v

The fate of the aryl gold(I) carbenes generated by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent on the constitution of the ortho substituent. Indenes and fluorenes are obtained by intramolecular reaction of highly electrophilic gold(I) carbenes with alkenes and arenes. According to density functional theory calculations, the gold-catalyzed retro-Buchner process occurs stepwise, although the two carbon-carbon cleavages occur on a rather flat potential energy surface.

Full text of DOI:10.1021/ja411626v

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Gold(I) Carbenes by Retro-Buchner Reaction: Generation and Fate
Yahui Wang,† Paul R. McGonigal,† Bart Herle,† Maria Besora,† and Antonio M. Echavarren*,†,‡
́
†Institute of Chemical Research of Catalonia (ICIQ), Av. Paísos Catalans 16, 43007 Tarragona, Spain
‡Departament de Química Analítica i Química Organ
̀
ica, Universitat Rovira i Virgili, C/Marcel·li Domingo s/n, 43007 Tarragona,
Spain
S
* Supporting Information
ABSTRACT: The fate of the aryl gold(I) carbenes generated
by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-
cycloheptatrienes is dependent on the constitution of the
ortho substituent. Indenes and fluorenes are obtained by
intramolecular reaction of highly electrophilic gold(I) carbenes
with alkenes and arenes. According to density functional
theory calculations, the gold-catalyzed retro-Buchner process
occurs stepwise, although the two carbon−carbon cleavages
occur on a rather flat potential energy surface.
ently with gold(I) or gold(III) catalysts to form indenes by a
complex cycloisomerization that proceeds through gold-
stabilized barbaralyl cations.14
1. INTRODUCTION
The Buchner reaction of metal carbenes generated from
diazocarboxylates and related substrates has been observed
under catalytic or stoichiometric conditions using copper(I),1−3
silver(I),4,5 gold(I),3,6 rhodium(II),7 and ruthenium com-
plexes.8 Chloro(tetraphenylporphyrinato)iron has also been
reported as a catalyst for the Buchner reaction.9 The copper-
and rhodium-catalyzed processes have often been used in total
synthesis.10,11 In contrast, we recently found that 7-substituted
1,3,5-cycloheptatrienes 1 react with cationic gold(I) complexes
under catalytic conditions through their norcaradiene tauto-
mers 2 to generate gold(I) carbenes [LAuCHR]+ (3) in an
overall retro-Buchner process (a form of retro-cyclopropana-
tion) (Scheme 1).12 Our discovery of this reaction was
prompted by the observation of a gold(I)-promoted retro-
cyclopropanation in the context of a synthesis of 1,3-
disubstituted naphthalenes.13 The retro-Buchner reaction
proceeds readily with 7-aryl- and 7-alkenyl-1,3,5-cyclohepta-
trienes, whereas 7-alkynyl-1,3,5-cycloheptatrienes react differ-
Other related gold(I)-promoted retro-cyclopropanations
have also been shown to occur in the gas phase by cleavage
of 1-ethoxy-2-methoxycyclopropane with [AuIMes]+ [IMes =
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene].15,16 The
gold(I)-catalyzed retro-Buchner reaction obviates the use of
diazo derivatives to generate reactive gold(I) carbenes. Indeed,
some diazo compounds are shock-sensitive.17 Although in
certain cases the use of tosylhydrazone salts offers a safer
alternative for handling diazo compounds,18 the generation of
the carbenes requires the use of a strong base, which is not
compatible with Lewis acidic [LAuL′]X complexes.
Gold(I) carbenes 3 undergo inter- or intramolecular
reactions with alkenes to give cyclopropanes, as shown in the
formation of 5. Insertion into a C(sp3)−H bond was also
observed in the retro-Buchner reaction of 4b to form 2-
phenylindane (6), although with very low efficiency.12
Here we report new results that demonstrate the highly
electrophilic character of gold(I) carbenes 3 by their reactions
with alkenes and arenes. In these reactions, indenes and
fluorenes are obtained by novel annulation processes. The
formation of indenes involves an electrophilic addition of the
gold(I) carbene to the alkene, which can be followed by an
unprecedented 1,4-metallotropic shift. Significantly, the inter-
mediates generated by the retro-Buchner reaction behave more
like simple carbenes than benzylic carbocations, as demon-
strated in the fluorene annulation.
Scheme 1
The mechanism of the Buchner reaction of silver(I)5 and
ruthenium(II)19 carbenes has been recently examined computa-
tionally. Here we also report a theoretical study of the
generation of donor gold(I) carbenes 3 by retro-Buchner
Received: November 14, 2013
Published: December 20, 2013
© 2013 American Chemical Society
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reactions of 7-substituted 1,3,5-cycloheptatrienes 1 showing
that the retro-cyclopropanation most probably occurs in a
stepwise manner, although the barrier for the second C−C
cleavage is small.
Scheme 2
2. RESULTS AND DISCUSSION
Annulation of o-Styrene Derivatives. (E)-7-(2-
Styrylphenyl)cyclohepta-1,3,5-triene (7a) reacted with cationic
gold(I) complexes to give 2-phenyl-1H-indene (8a) (Table 1).
Table 1. Gold(I)-Catalyzed Synthesis of Indene 8a via Retro-
a
Buchner Reaction
b
entry
[Au]
yield (%)
c
1
2
3
4
5
A
B
C
D
E
86 (74 )
86
67
8
a
Reaction with catalyst E (5 mol %), 12 h.
85
a
Scheme 3
Reaction at 120 °C (0.1 M in 1,2-dichloroethane), catalyst (5 mol %),
b
c
1
3 h. Yields determined by H NMR analysis. Isolated yield.
Nearly identical good yields were obtained using cationic
phosphine−gold(I) complexes A and B and carbene complex E
(Table 1, entries 1, 2, and 5), whereas catalyst D led to poor
results (entry 4). We identified JohnPhos−gold(I) complex A
as the catalyst of choice because of its ready availability and
robustness for the synthesis of 2-substituted indenes from o-
alkenyl cycloheptatrienes (Scheme 2). This reaction proceeds
in a rather general manner with substrates substituted with
alkyl, alkenyl, or aryl substituents at C2 to give indenes 8a−l in
moderate to good yields.
Other cationic gold(I) complexes B−E were also active as
catalysts but usually led to lower yields, although for the
synthesis of indene 8j IPr−gold(I) catalyst E was used. In this
case, only the closest double bond reacted with the carbene
intermediate. The cyclopropyl group in 7k did not suffer ring
expansion. 1,4-Bis(1H-inden-2-yl)benzene (8l) could also be
obtained in moderate yield in a double annulation reaction.
The annulation of o-cyclohexenyl derivative 7m under the
standard conditions gave exclusively 2,3,4,4a-tetrahydro-1H-
fluorene (8m), which has not been reported before (Scheme
3). The known and more stable isomer20 2,3,4,9-tetrahydro-
1H-fluorene (8m′), which would have been formed by C−H
insertion, was not observed. The symmetrical derivative 7n
reacted by a double retro-Buchner reaction via 7o to give 4b,5-
dihydroindeno[2,1-a]indene (8n), a member of a class of
compounds that have attracted certain attention lately (Scheme
3).21 This method is thus complementary to the synthesis of
indenes by annulation of 2-alkenyl-2-(trifluoromethyl)benzenes
with NbCl5 and LiAlH4, since 2-arylindene 9 leads exclusively
to 5,10-dihydroindeno[2,1-a]indene (8n′).22
Although at first glance indenes 8a−l appear to have been
formed by a C(sp2)−H insertion of a gold(I) carbene
intermediate, the formation of 8m and 8n strongly suggests
that electrophilic addition of a gold(I) carbene to the alkene
operates in these systems.
The exclusive formation of 8m and 8n from 7m and 7n
contrasts with the observed reactivity of substrates 7p−r
(Scheme 4). The reactions of substrates 7p and 7q bearing a p-
Me substituent led to 1:1 mixtures of indenes 8p/8p′ and 8q/
8q′, respectively. A 1:1 regioisomeric ratio was also obtained
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Scheme 4
Scheme 6. DFT Calculations on the Cyclization of Ia and Ib
(Free Energies in kcal mol−1)
when the reaction was performed at 100 °C for 3 h (57%
1
conversion as determined by H NMR analysis). Even more
surprising was the result obtained in the cyclization of 7r
bearing a p-MeO substituent, which led to a 1:2 mixture of 8r
and 8r′.
Regioisomeric indenes 8p′, 8q′, and 8r′ could have arisen by
isomerization of 8p, 8q, and 8r by two consecutive [1,5]-H
sigmatropic migrations.23 However, the reaction of 7a-d1 with
catalyst A led exclusively to 8a-d1 with the deuterium label at
the methylene, which is not consistent with an isomerization via
[1,5]-H sigmatropic migrations, which would have also formed
8a′-d1 (Scheme 5). Additionally, no deuterium incorporation
Scheme 5
was observed when the reaction of 7a was performed in 1,2-
dichloroethane saturated with D2O. Finally, no kinetic isotope
effect was observed in the reaction of a 1:1 mixture of 7a and
7a-d1 with gold(I) complex A at 100 °C.24
proposed in a metathesis-type process occurring in competition
with the cyclopropanation of enol ethers by gold(I) carbenes
generated in the gas phase.15c Whereas for intermediate IIa this
migration is degenerate, the p-OMe-substituted substrate II′b is
4.2 kcal mol−1 more stable than IIb as a result of the
stabilization of the benzylic carbocation by the p-MeO group
(Scheme 6). The transition states TSII−II′ for the 1,4-
metallotropic migration show an η4-(2H-indene)Au(I) struc-
ture, with shorter distances from the metal center to the
internal carbons C4a−C7a (2.51−2.54 Å) than to C1 and C3
(2.86−2.88 Å).
The 1,4-migration of AuPMe3 in IIb requires almost the
same activation energy (12.4 vs 12.5 kcal mol−1) as the
competitive 1,2-H shift to form IIIb (Scheme 7). The
difference in activation energies is higher for the unsusbtituted
system IIa/IIIa (2.5 kcal mol−1). The computed tendencies are
in close agreement with those found experimentally in the
cyclizations of 7p−r, which led to 1:1 mixtures in the first two
cases (p-Me as the donor substituent), whereas substrate 7r (p-
MeO substituent) gave 8r′ preferentially as a result of a more
favorable migration of IIb to II′b.30
A closer mechanistic inspection of these puzzling results
suggested that an unexpected 1,4-gold(I) migration could be
involved in the formation of mixtures of regioisomers in some
of the indene syntheses. First, we studied computationally the
formation of model indenes 8s and 8t from the corresponding
gold(I) carbenes using density functional theory (DFT) at the
M06 level including solvent effects for 1,2-dichloroethane
(Scheme 6). After the retro-Buchner reaction, highly electro-
philic gold(I) carbenes Ia and Ib (L = PMe3) react
intramolecularly with the alkene through TSIa−IIa and TSIb−IIb
in highly exothermic processes to form benzylic carbocations
IIa and IIb, respectively. Despite being formally a 5-endo-trig
cyclization from the perspective of the alkene,25 this process is
rendered kinetically and thermodynamically favorable by the
high electrophilicity of the gold(I) carbenes. As expected, the p-
OMe group in Ib attenuates the electrophilicity of the gold(I)
carbene, raising the energy of TSIb−IIb and resulting in a less
exothermic cyclization. Carbocationic intermediates IIa and IIb
can then evolve by a 1,2-H shift to form directly the indenes η2-
coordinated to AuPMe3 (IIIa and IIIb, respectively).26,27
According to DFT calculations, the formation of 2,3,4,4a-
tetrahydro-1H-fluorene (8m) followed a similar pathway
involving intramolecular electrophilic attack of the gold(I)
carbene on the alkene in Ic to form intermediate IIc, which
undergoes 1,2-H shift to give (η2-alkene)Au(I) complex IIIc
(Scheme 8). In this case, however, the transition state required
for the suprafacial 1,4-metallotropic migration (IIc to II′c) was
found to lie 3.9 kcal mol−1 higher than that of the 1,2-H shift,
and moreover, II′c is destabilized with respect to IIc, which
+
Mechanistically, the cyclization of Ia/Ib to IIa/IIb is formally
related to the intramolecular reaction of rhodium nitrenes with
alkenes in the rhodium-catalyzed synthesis of indoles from o-
alkenylaryl azides.28
The DFT calculations show that intermediates II can evolve
into II′ by a formal metal migration from C1 to C3, which
actually corresponds to a suprafacial 1,4-metallotropic migra-
tion.29 Another type of gold(I) migration has also been
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explains the selective formation of 8m over the more stable 8m′
from 7m (Scheme 3).
Surprisingly, cis-7a did not behave in the same manner as its
Scheme 7. Details of 1,4-Metallotropic Migration versus
Formation of (η2-Indene)gold(I) Complexes in the
Formation of IIIb/IIIb′
trans isomer in the presence of catalyst A (Scheme 9).
Scheme 9
Presumably as a result of the proximity of the phenyl ring to the
gold carbene in intermediate 13, an intramolecular Buchner
reaction takes place to form 14, which then undergoes
disrotatory norcaradiene-to-cycloheptatriene opening followed
by a 1,5-H shift to give 12. A similar transformation had been
observed in the thermal decomposition of diazo compounds to
form naphtho[a]cycloheptenes of type 12.31
An even more elaborate cascade of retro-cyclopropanation/
cyclopropanation reactions was observed starting from diene 7u
(Scheme 10). The initially formed intermediate 16, resulting
Scheme 10
Scheme 8. DFT Calculations on the Formation of 8m (Free
Energies in kcal mol−1)
from a gold-catalyzed retro-Buchner reaction followed by
intramolecular cyclopropanation, evolves by a second retro-
cyclopropanation to form naphthalene with the concomitant
generation of the reactive gold carbene [LAuCHPh]+,13
which reacts with 16 to give 15.
In our previous study,12 we found that indenes are suitable
substrates for intermolecular cyclopropanation by gold
carbenes generated in a retro-Buchner reaction. An efficient
tandem transformation can be achieved by combining indene
formation and intermolecular cyclopropanation with [LAu
CHPh]+ by reacting 7a with 1a to give 5a and 5a′ in a 4:1 ratio
(67% yield) (Scheme 11).
Annulation of o-Biphenyl Derivatives. Free o-biphenyl-
carbene has been demonstrated to undergo cyclization to form
fluorene,32,33,40c whereas o-phenylbenzyl carbocation failed to
undergo a similar cyclization.34 o-Biphenyl gold(I) carbenes
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Scheme 11
Scheme 13
aromatic framework and their high rigidity, which stems from
the methylene bridging unit of their biphenyl core.43 By
extending our new method to a double annulation, we prepared
indeno[1,2-b]fluorene (18k) as a mixture with indeno[2,1-
a]fluorene (18k′) in 53% isolated yield from 17k (Scheme 14).
generated by retro-Buchner reactions behave like free carbenes,
leading to fluorenes. Thus, reaction of 2-cycloheptatrienyl
biphenyls 17a−i with catalyst A or E gave fluorenes 18a−i in
moderate to good yields by a Friedel−Crafts-type methylena-
tion reaction (Scheme 12). The annulation proceeded
Scheme 14
Scheme 12
During our attempts to form 4,8-dihydrocyclopenta[def ]-
fluorene by generating two gold(I) carbenes on the same
aromatic ring, we found that the rigidity of the fluorene
backbone makes the second annulation unfavorable. Instead,
intermolecular dimerization to give 19 occurred, with no
indication of other diastereoisomers (Scheme 15). The reaction
Scheme 15
a
Reaction with catalyst E (5 mol %), 3 h.
satisfactorily with substituents at different positions, although
fluorene 18g was obtained in low yield. As expected, in the case
of m-ClC6H4 derivative 17f, a mixture of two regioisomers 18f
and 18f′ was obtained.
This new fluorene synthesis complements other recently
developed methods based on transition-metal-catalyzed annu-
lations.35−37 Interestingly, related rhodium- or copper-catalyzed
annulation of biaryl diazoacetates leads to fluorene carbox-
ylates,38 which emphasizes the similar reactivity of rhodium,
copper, and gold carbenes.
With this method, 2-binaphthylcycloheptatriene (17j),
prepared in one step from commercially available 2-bromo-
1,1′-binaphthalene,39 was converted into nonplanar 7H-
dibenzo[c,g]fluorene (18j) (Scheme 13).40 This is the shortest
synthesis of 18j, whose anion, dibenzo[c,g]fluorenide, has
attracted recent interest for its particular aromatic character41
and as a six-π-electron donor ligand in organometallic
chemistry.42
proceeds by retro-Buchner reaction of 17l to form 20 followed
by a second retro-Buchner reaction of its tautomer 21 to form a
fluorenyl gold(I) carbene intermediate that cyclopropanates
one of the double bonds of 21.
For the reactions leading to fluorenes by retro-Buchner
reactions of 2-cycloheptatrienyl biphenyls 17, DFT calculations
(M06 level, 1,2-dichloroethane) are consistent with a Friedel−
Crafts-type reaction of gold(I) carbene IV (L = PMe3) through
transition state TSIV−V in a moderately exothermic process to
form Wheland intermediate V (Scheme 16). Intermediate V
evolves by a 1,2-H shift via transition state TSV−VI to form (η1-
Indenofluorenes (IFs) have found various applications in
organic electronics on account of the conjugation along their
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Scheme 16. DFT Calculations on Intermediates IV−VI (Free
Scheme 17
Energies in kcal mol−1)
electrophiles such as TeCl4 lead to the cleavage of a single
C−C bond to form benzylic chlorides.49 The reaction of 7-
ethynylcyclohepta-1,3,5-triene with trifluoroacetic acid leads to
phenylallene by protonation of the alkyne in the norcaradiene
tautomer followed by cyclopropane cleavage to form the
arenium cation.50 The reaction of 7-ethoxycarbonyl-1,3,5-
cycloheptatriene with an equimolecular amount of Pd(OAc)2
at 80 °C in MeCN had been reported to give ethyl 2- and 4-
formylbenzoate (8% each) by cleavage of one cyclopropane C−
C bond of the corresponding norcaradienes.51 In addition,
diethyl maleate (14% yield) was formed, presumably by a
dimetization of a Pd(II) carbene formed by a retro-Buchner
process.52 Formation of benzyl-Rh(II) from cycloheptatrienyl-
Rh(I) occurs by a metalloradical process. Cycloheptatriene has
also been found to react on the metal carbide W(100)-(5 × 1)-
C surface to generate benzene at 600 K,53 whereas
(tricarbonyl)(tropylium)chromium perchlorate forms benzene-
(tricarbonyl)chromium by reaction with sodium cyclopenta-
dienide or diethyl sodium malonates by an unknown
mechanism.54 Carbenes have been generated by photochemical
cleavage of a cyclopropanated phenanthrene.55
Our DFT computational study of the gold-catalyzed retro-
Buchner reaction (M06 level, 1,2-dichloroethane) was
complicated by the likely existence of several η2-coordinated
gold(I) species in solution. Thus, in addition to η2-coordinated
cycloheptatriene (VIIa) and norcaradiene (VIIb), the (η1-
arene)gold(I) complex VIIc was also found as a local minimum
(Scheme 18). An intermediate norcaradiene in which gold(I) is
η2-coordinated to the cyclopropane C−C bond (VIId) was also
found at a free energy 12.7 kcal mol−1 higher than that of VIIb.
Previously, on the basis of related DFT calculations, we
proposed the formation of related edge- or corner-metalated
cyclopropanes as products in intra- and intermolecular gold(I)-
catalyzed cyclopropanations of alkenes with 1,6-enynes56 and
intramolecular cyclopropanation of 1,5-enynes.57
fluorene)gold(I) complex VI instead of VI′ with gold(I)
coordinated to the C1−C9a bond. All of the calculations led
directly to VI as the global minimum by a further shift of
[Au(PMe3)]+ toward C2, which suggests that a very low barrier
exists for the isomerization of VI′ into V. In this process, the
[Au(PMe3)]+ fragment migrates toward the aryl ring in a
concerted but highly asynchronous diatropic-type transforma-
tion. The calculated shortest Au−C bond distance of 2.30 Å
with C2 in VI is very similar to the shortest contacts
determined by X-ray diffraction in complexes [LAu(ArH)]SbF6
(ArH = toluene, p-xylene).44
Aryl carbenes generated by pyrolysis of aryl diazomethanes
undergo formal insertion into the adjacent ortho position of the
XC6H5 ring to form dihydroanthracenes (X = CH2) and
dihydroacridines (X = NH), whereas substrates with X = O or
S lead to products of the Buchner reaction.45 In our case, the
reaction of cycloheptatrienyl derivative 22a gave 9H-xanthene
(23a), the product of a formal insertion into the ortho position
of the phenyl group, albeit in low yield (Scheme 17). Despite
the steric hindrance introduced by the ortho OR substituents,
the ortho-substituted aryl gold(I) carbenes generated from 22a
and 22b could be trapped efficiently with trans-stilbene to form
the corresponding cyclopropanes 24a and 24b. Similarly, 25a
could be obtained by trapping with 1-phenylbutane-1,3-dione.
Mechanism of the Retro-Buchner Reaction. Cleavage of
cyclopropanes to form metal carbenes had only been achieved
electrophilically by using a mixture of highly electrophilic
PhWCl3 and RAlCl2 (R = Et, Cl).46 On the other hand, highly
strained bicyclo[1.1.0]butanes undergo ring opening by
oxidative addition to Ni(0) or Rh(I) complexes.47
A transition state (TSVIId−VIII) was found for the electrophilic
cleavage of intermediate VIId (Scheme 19), which lies 23.3 kcal
mol−1 higher than the most stable initial complex VIIb. This
value for the activation energy of the retro-Buchner reaction is
consistent with the range of temperatures required for these
reactions (100−120 °C). Transition state TSVIId−VIII leads to
Wheland-type intermediate VIII, which is in a shallow
minimum that smoothly evolves through TSVIII−IX to form
phenyl gold(I) carbene IX and benzene. Although the overall
process of the retro-Buchner reaction is moderately endother-
Prior to our initial report,12 the metal-catalyzed retro-Bucher
reaction was unknown, although the reverse process, formation
of cycloheptatrienes, is a minor reaction in the gold(I)-
catalyzed reaction between ethyl diazoacetate and arenes.48 In
contrast to the cleavage of substituted cycloheptatriene
derivatives observed in the presence of Au(I), strong
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Scheme 18. DFT Calculations on η2-Coordinated Gold(I)
the cationic intermediates generated by the retro-Buchner
reaction is more similar to that of metal carbenes of rhodium or
copper or even free carbenes than that of carbocations.
Closer scrutiny of the mechanisms of these reactions has
revealed some intriguing details. Thus, in the indene synthesis,
we have found that a novel 1,4-metallotropic migration
competes with the primary pathway for the formation of the
(η2-indene)gold(I) complexes by a concerted 1,2-H migration/
gold(I) elimination. The formation of fluorenes involves a
diatropic-type process in the formation of an (η1-fluorene)-
gold(I) complex. Finally, the mechanism of the gold(I)-
catalyzed retro-Buchner reaction of substituted cyclohepta-
trienes proceeds by electrophilic cleavage of two C−C bonds of
the norcaradiene tautomers. Although the cleavage occurs
stepwise, the potential energy surface connecting the two
transition states is rather shallow.
Species (Free Energies in kcal mol−1)
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional data, experimental details, and characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Scheme 19. DFT Calculations on the Mechanism (Free
Energies in kcal mol−1)
AUTHOR INFORMATION
■
Corresponding Author
aechavarren@iciq.es
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MEC (Project CTQ2010-16088/BQU), the Euro-
pean Research Council (Advanced Grant 321066), EU Project
ICT (ATMOL, Contract FP7-270028), the AGAUR (Project
2009 SGR 47), and the ICIQ Foundation for financial support.
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In summary, gold(I) carbenes generated by the retro-Buchner
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complexes can be trapped intramolecularly by alkenes or arenes
to form indenes or fluorenes. This methodology provides a new
synthetic approach to indenes and fluorenes and may be
applied to the synthesis of indenofluorenes used in organic
electronics. These reactions proceed via intramolecular
Friedel−Crafts-type attack of the highly electrophilic gold(I)
carbenes to the alkenes and arenes. The reactivity displayed by
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