Reactivity of stabilized vinyl diazo derivatives toward unsaturated hydrocarbons: Regioselective gold-catalyzed carbon-carbon bond formation

Article abstract of DOI:10.1002/chem.201203217

They struck gold! New reaction modes were observed in the gold-catalyzed reaction of alkenyldiazo compounds and unsaturated substrates. The process represents a new C-C bond-formation reaction in which alkenes, alkynes, and arenes are active reagents for the Cγ-allylation, -allenylation, and -arylation, respectively, of alkenyldiazo substrates (see scheme). The reactivity pattern is likely to rely on the formation of a highly electrophilic alkenylgold carbenoid, which may be involved in a carbocation-type mechanism. Copyright

Full text of DOI:10.1002/chem.201203217

COMMUNICATION
DOI: 10.1002/chem.201203217
Reactivity of Stabilized Vinyl Diazo Derivatives toward Unsaturated
Hydrocarbons: Regioselective Gold-Catalyzed Carbon–Carbon Bond
Formation
Josꢀ Barluenga,* Giacomo Lonzi, Miguel Tomꢁs, and Luis A. Lꢂpez*[a]
The transition-metal-catalyzed decomposition of simple
and unsaturated a-diazo carbonyl derivatives and the subse-
quent transfer of the carbene unit to saturated and unsatu-
rated substrates is one of the most popular and powerful
tools in organic synthesis.[1] In particular, the [2+1] cycload-
dition of alkenes and alkynes is recognized as a straightfor-
ward and useful approach for the construction of three
membered carbocycles. Extensive studies have led to the
achievement of high levels of chemo-, diastereo-, and enan-
Figure 1. Regioselective preparation of g-substituted, a,b-unsaturated car-
bonyl compounds.
tioselectivity by employing mostly complexes of copper and
À
rhodium. In spite of the potential of gold catalysis in C C
bond formation,[2] the first example of gold-catalyzed car-
bene transfer from ethyl diazoacetate to unsaturated sub-
strates was reported in 2005 by Nolan, Dꢀaz-Requejo, Pꢁrez
et al.[3] After this seminal contribution, few gold-catalyzed
transformations involving simple diazo substrates have been
reported.[4] On the other hand, the synthetic potential of
vinyl diazo derivatives has been well established, particular-
ly in the cycloaddition area, and the reactivity of the car-
bene carbon versus the vinylogous carbon found to be de-
pendent, at least in part, on the metal catalyst.[5]
carbon bond-forming process to alkynes and arenes are also
reported.
For our exploratory studies, we investigated ethyl 2-diazo-
but-3-enoate (1a) and 1-hexene (2a, 4 equiv) as substrates
and a series of gold catalysts (5 mol%) in dichloromethane
at room temperature. To our delight, we found that the use
of IPrAuNTf2 gave (2E)-ethyl deca-2,6-dienoate 3a in mod-
erate yield (67%) as a nonseparable 1:1 mixture of 2E,6E/
2E,6Z diastereoisomers (Scheme 1). This unprecedented
Continuing with our interest in the development of new
transition-metal-catalyzed transformations of stabilized vinyl
diazo derivatives,[6] as well as in gold catalysis,[7] we herein
report the first studies of the gold-catalyzed reaction of al-
kenyldiazo compounds and neutral alkenes[8] that results in
À
the room temperature selective C C coupling between the
Scheme 1. Initial finding for the gold-catalyzed reaction of alkenyl diazo-
acetate 1a and 1-hexene (2a).
Csp2
(alkene) and the Cg(alkenyldiazo) atoms. From a syn-
thetic point of view, the alkenyl diazo compound behaves as
a ethoxycarbonyl allyl cation in this process, allowing the re-
gioselective preparation of g-substituted, a,b-unsaturated
carbonyl compounds (Figure 1).[9] This reactivity pattern
contrasts with that observed when other common sources of
alkenylgold carbenoids, such as propargylic esters or cyclo-
propenes, are used (2+1 cycloaddition).[10] Preliminary stud-
ies on the extension of this new gold-catalyzed carbon–
process comprises two significant features: 1) in contrast to
the results found in the reactions of alkenylgold carbenoids
generated from other precursors,[10] neither cyclopropanes
À
nor products resulting from an allylic C H insertion reac-
tion were detected, and 2) only the vinylogous position of
the vinyldiazoacetate was involved in the reaction.[11]
On the basis of this promising result, we then focused on
whether this reaction could be extended to a range of al-
kenes 2 and vinyldiazoacetates 1. As depicted in Scheme 2,
the process was found to tolerate a wide range of olefin sub-
stitution, including mono-, 1,1-di-, 1,2-di-, tri-, and even tet-
rasubstituted alkenes, affording the 2,6-diene derivatives 3
in moderate to high yields. Even simple petrochemical raw
materials such as propylene (2b) and isobutylene (2c) were
able to participate in this process, affording the coupling
products 3b and 3c, respectively, in good yields. In general,
[a] Prof. J. Barluenga, G. Lonzi, Prof. M. Tomꢂs, Dr. L. A. Lꢃpez
Instituto Universitario de Quꢀmica Organometꢂlica “Enrique Moles”
Universidad de Oviedo, c/Juliꢂn Claverꢀa 8, 33007-Oviedo (Spain)
Fax: (+34)989-510-3450
E-mail: barluenga@uniovi.es
lalg@uniovi.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203217.
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Other alkenes were tested to gain insight into the cou-
pling reaction. Thus, we found that 1,1-diphenylethene (2j)
reacts with vinyldiazoacetate 1a under the standard condi-
tions to afford (E)-ethyl 6,6-diphenylhexa-2,5-dienoate (4)
in moderate yield after chromatographic purification, as
well as minor amounts (<10%) of the cyclopropanation
product [Eq. (1)]. On the other hand, in the case of b-
pinene (2k), the corresponding coupling was accompanied
by rearrangement, giving rise to the limonene derivative 5
[Eq. (2)].
A mechanistic proposal that rationalizes the obtained re-
sults is given in Scheme 3. The initial reaction of the alkenyl
diazo compound with the gold complex would afford the
gold alkenylcarbenoid intermediate I, which can be de-
AHCTUNGTRENNUNG
scribed as an allyl gold cation.[14] Next, regioselective nucle-
Scheme 2. Gold-catalyzed reaction of alkenyl diazoacetates 1 and alkenes
2. Reaction conditions: (0.5 mmol), (2.0 mmol), IPrAuNTf2
1
2
ophilic attack of the alkene to the C3 allyl cation would
form the most stable carbocation intermediate II. Then, its
deprotonation and protodemetalation would account for the
formation of the products 3 and 4. In the case of b-pinene,
the initially formed tertiary carbocation would evolve
through ring-opening of the cyclobutane unit followed by
deprotonation/demetallation on the new carbocation to pro-
vide the observed cyclohexene derivative 5.[15,16]
(5 mol%), CH2Cl2 (0.1m), RT. Values in parenthesis are the yields of the
isolated products. [a] A 4:1 mixture of (E)-ethyl 6-methylhepta-2,6-dien-
oate and (E)-ethyl 6-methylhepta-2,5-dienoate was isolated. [b] A 4:1
mixture of (E)-ethyl 5,6-dimethylhepta-2,6-dienoate and (E)-ethyl 5,6-di-
methylhepta-2,5-dienoate was isolated. [c] A 1.5:1:1 mixture of isomers
was isolated. [d] A 1:1 mixture of E and Z isomers was isolated. [e] The
product arising from a cyclopropanation reaction was also formed in
35% yield.
Our attention then turned towards determining whether
closely related systems, such as alkynes, would be suitable
substrates in this gold-catalyzed process. The preliminary ex-
no products resulting from either [2+1] or [3+2] cycloaddi-
tion reactions were observed.[12] Moreover, in all cases, the
alkene coupled to the diazo substrate with complete regiose-
lectivity towards the less substituted alkene carbon atom. In-
terestingly, in most cases the reactions are highly diastereo-
selective, affording exclusively the E isomer at the C2
double bond. In relation to the structure of the diazo com-
ponent 1, some points are remarkable: 1) the reaction works
well for substrates with alkyl substitution at the Ca atom
(diazoacetate 1b; R1 =Me, R2 =H), affording the expected
product 3j in moderate yield; 2) the presence of a substitu-
ent at the Cb position (diazoacetate 1c; R1 =H, R2 =Et)
seems to exert a noticeable influence on the regiochemical
outcome of the reaction as demonstrated by the formation
of a 1:1 mixture of the expected product 3k and the corre-
sponding cyclopropane from the reaction with 2,3-dimethyl-
but-2-ene; 3) diazo derivatives in which the electron-with-
drawing group (EWG) is an acetyl are also amenable to this
coupling reaction, affording compound 3l in moderate yield
and with complete regio- and stereoselectivity. In general,
the reaction showed a marked preference for the formation
of the 2,6- rather than the 2,5-diene isomer.[13]
Scheme 3. Proposed mechanism for the gold-catalyzed reaction of alkenyl
diazoacetates 1 and alkenes 2.
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running title
COMMUNICATION
periments for dialkylacetylenes, such as 3-hexyne (6a) and
4-octyne (6b), demonstrated that they behave in a similar
way to alkenes when exposed to vinyldiazoacetates 1a and
1b under the same reaction conditions.[17] Thus, the syntheti-
cally useful allylallenes 7a–c were isolated in moderate yield
and with high regio- and E-stereoselectivity [Eq. (3)].
viding the corresponding alkylated derivatives 9a–e in mod-
erate to excellent yields (40–95%) after chromatographic
purification. The less-activated 1,3,5-trimethylbenzene (8b)
provided the corresponding arylation product 9 f in 78%
yield. On the other hand, arenes with a single alkyl activat-
ing group, such as cumene (8c) and toluene (8d), gave the
expected products in rather low isolated yield (35 and 47%,
respectively) as mixtures of the ortho and para isomers. In-
À
terestingly, it was found that the presence of the N H func-
tionality did not affect the course of the reaction.[17] Thus,
the reaction of diazoacetate 1a with acetanilide (8e) pro-
ceeded smoothly to provide 9i as a 1:2 ortho/para mixture
in 72% yield. This gold-catalyzed coupling reaction also
proved to be amenable for electron-rich heteroarenes. Thus,
the treatment of furan (8 f), N-phenylpyrrol (8g), and ben-
zo[b]thiophene (8h) with the diazo compound 1a under the
above reaction conditions afforded the compounds 9j–l in
good yield. Notably, N-phenylpyrrol underwent the reaction
selectively at the electron-rich pyrrol moiety.[18]
According to the mechanistic picture (see Scheme 3), we
searched for other neutral nucleophiles that were amenable
À
to C C bond formation. As shown in Scheme 4, a number
of activated arenes 8 were also prone to react at room tem-
perature to afford the corresponding Friedel–Crafts prod-
ucts 9 (substituted 4-aryl-2-butenoate esters) with variable
efficiency. First, the highly activated arene 1,3,5-trimethoxy-
benzene (8a) reacted under the standard conditions with un-
substituted and substituted alkenyldiazo derivatives 1, pro-
In conclusion, we have discovered new reaction modes in
the gold-catalyzed reaction of vinyldiazoacetates and un
ACHTUNGTRENNUNGsat-
AHCTUNGTREGuNNUN rated substrates and shown that the product outcomes
differ significantly from those obtained using alternative
sources of alkenylgold carbenoids. It is likely that the ob-
served reactivity pattern relies on the formation of a highly
electrophilic alkenylgold carbenoid, which may be involved
in a carbocation-type mechanism. From a synthetic point of
À
view, the process described represents a new C C bond-for-
mation reaction in which alkenes, alkynes, and arenes are
active reagents for the Cg-allylation, -allenylation, and -ar
ACHTUNGTRENNUNGyl-
AHCTUNGTRENNUNG
crease the reaction scope further and to gain insight into the
reaction mechanism are ongoing.
Experimental Section
Representative procedure (3i): IPrAuNTf2 (21.6 mg, 0.025 mmol,
5 mol%) was added to
a solution of vinyldiazoacetate (1a; 70 mg,
0.5 mmol) and 2,3-dimethylbut-2-ene (2i; 168 mg, 2.0 mmol, 4.0 equiv) in
CH2Cl2 (5 mL). The mixture was stirred at room temperature until the
disappearance of 1a (monitored by TLC; 2 h). The solvent was removed
under reduced pressure and the resulting residue was purified by flash
chromatography (SiO2, n-hexane/EtOAc 50:1) to yield (E)-ethyl 5,5,6-tri-
methylhepta-2,6-dienoate (3i) (75 mg, 76%) as a pale yellow oil.
Acknowledgements
We are grateful to the Ministerio de Economꢀa y Competitividad of
Spain (MINECO) (Grant no. CTQ2010–20517-C02–01). G.L. thanks the
MINECO and European Union (Fondo Social Europeo) for a predoctor-
al grant.
Scheme 4. Gold-catalyzed reaction of alkenyl diazoacetates 1 and arenes
8. Reaction conditions:
1 (0.5 mmol), 8 (2.0 mmol), IPrAuNTf2
(5 mol%), CH2Cl2 (0.1m), RT. Values in parenthesis are the yields of the
isolated products. [a] A 1:1 mixture of E and Z isomers was isolated.
[b] A 4:1 mixture of Z and E isomers was isolated.
Keywords: alkenes · alkynes · arenes · diazo compounds ·
gold
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www.chemeurj.org
1575

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À
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[15] For a similar carbocationic rearrangement, see reference [13].
[16] It should be mentioned that, although the cationic mechanism de-
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tive concerted mechanism, which is analogous to that reported by
Davies et al. (reference [11]), in some related rhodium-catalyzed re-
actions cannot be strictly eliminated.
[17] The formation of compounds 7 can be explained by assuming a simi-
lar mechanism to that proposed for the reaction involving alkenes.
In addition to compounds 7, small amounts of cyclopentadiene de-
rivatives resulting from a competing formal [3+2] cycloaddition re-
action were also obtained as side products.
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[18] For the addition of nitrogen nucleophiles to the vinylogous position
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[19] Unfortunately, non-activated arenes such as benzene or chloroben-
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[8] An example dealing with gold-catalyzed decomposition of vinyl
ACHTUNGERTNdNUNG i-
ACHTUNGTRENNUNG
provide quinolone oxides has just been reported, see: V. V. Pagar,
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recently, the same group reported the gold-catalyzed reaction of ar-
omatic imines and alkenyldiazocarbonyl compounds, see: A. M.
Received: September 10, 2012
Published online: December 19, 2012
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Chem. Eur. J. 2013, 19, 1573 – 1576