Iodoarylation reactions of allenes: Inter- and intramolecular processes

Article abstract of DOI:10.1002/chem.200901663

A study was conducted to demonstrate the use of iodnium ions on the trigger for a β iodoallylation of a C-H bond of an arene by reaction with an allene. It was demonstrated that the iodonium approach complemented the requirements of the powerful carbometa

Full text of DOI:10.1002/chem.200901663

DOI: 10.1002/chem.200901663
Iodoarylation Reactions of Allenes: Inter- and Intramolecular Processes
Josꢀ Barluenga,* Esther Campos-Gꢁmez, Ana Minatti, David Rodrꢂguez, and
Josꢀ M. Gonzꢃlez[a]
Polar addition reactions to allenes are well established
and various processes involving this mechanistic pathway
are known.[1,2] The reaction of arenes with electrophiles is a
À
conceptually powerful method to functionalize C H
bonds.[3] Therefore, the feasibility of using carbon-based
electrophiles, in reactions with arenes, was soon recognized
À
as an entry for C C bond formation. The Friedel–Crafts al-
kylation of arenes with alkenes is a relevant example.[4] Nev-
ertheless, the related alkenylation reaction involving alkynes
was more sluggishly developed and only recent advances in-
volving metal-catalyzed transformations have made this
target practicable.[5,6] The reaction of arenes with the transi-
tory species generated upon interaction of iodonium ions
Scheme 1. Arene addition reactions across allenes.
with alkenes[7] or alkynes[8] represents a powerful route for
À
preparing cyclic compounds involving related C C bond-
forming processes. Moreover, examples of Friedel–Crafts ar-
ylation reactions of allenes are rare.[9] Recent advances in-
volve formally related processes using precious metal cata-
lysts and are mainly intramolecular transformations.[10]
Herein, we report the first solid evidence on the use of io-
donium ions as the trigger for an unprecedented b iodoallyl-
lene core, an elusive and valuable building block for cross-
coupling chemistry that will also be highlighted.
The iodonium-promoted carbocyclization was first tested
on 2,3-butadienyl benzene (1a), a demanding model sub-
strate considering that the participation of an electronically
activated arene is mandatory in most allene hydroarylation
reactions.[10] Several iodinating reagents were tested in an
effort to synthesize 2-iodo-1,4-dihydronaphthalene (2a)
through a straightforward iodoarylation strategy. Some key
experiments are outlined in Table 1.
À
ation of a C H bond of an arene by reaction with an allene
(Scheme 1). Both, intra- and intermolecular processes are
presented. The iodonium approach would complement the
requirements of the powerful carbometallation reaction of
allenes.[11] The intermolecular process requires a substituted
arene, whereas in the intramolecular process the assembled
skeleton is endowed with further functionality.[12] Interest-
ingly, the application of this technology to ring elaboration
enables a direct entry into the 2-iodo-1,4-dihydronaphtha-
Table 1. Iodoarylation of 1a: screening for useful iodonium donors.[a]
Entry
Iodinating reagent
T [8C]
t [min]
Yield 2a[b] [%]
[c]
1
2
3
4
ICl
I2
À80
RT
25
20
30
20
–
–
–
[a] Prof. Dr. J. Barluenga, Dr. E. Campos-Gꢀmez, Dr. A. Minatti,
Dr. D. Rodrꢁguez, Prof. Dr. J. M. Gonzꢂlez
Instituto Universitario de Quꢁmica Organometꢂlica
“Enrique Moles”-Unidad Asociada al C.S.I.C.
C/Juliꢂn Claverꢁa 8, 33006 Oviedo (Spain)
Fax : (+34)985-103450
[d]
[e]
NIS/TfOH
IPy2BF4/HBF4
À80
À90
70[f]
[a] 1a (0.1m, 1 mmol scale); 1a/iodinating agent (1:1). [b] Yield of the
isolated products. [c] ICl–allene adducts. [d] I2 addition to the terminal
allene bond. [e] Acetone used as solvent, 1a/TfOH (1:1); 50% of 1a re-
E-mail: barluenga@uniovi.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901663.
covered from a complex mixture. [f] HBF4 added (1 equiv, 54% in
Et2O); monitoring by TLC showed instantaneous disappearance of 1a.
8946
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COMMUNICATION
The IPy2BF4 reagent (Py=pyridine) acted as a genuine
iodonium donor to achieve the desired ring-closing step.[13]
This cyclization requires prior treatment of the iodinating
agent with HBF4 (1 equiv) to preclude pyridine incorpora-
tion. Performing the reaction at low temperature avoids
competitive side processes and enables a fast cyclization.[14]
Other iodinating agents did not promote the cyclization of
1a. Instead, the products of facile addition reactions were
observed. These adducts are formed upon capture of the in-
termediate iodonium ions by an external nucleophile, result-
ing from activation of 1a by a true source of electrophilic
iodine. Formation of the adducts could not be prevented,
even when conducting the cyclization at lower temperatures.
Nonetheless, the influence that the iodonium source may
have over the reaction outcome was further tested in cycli-
zation reactions employing 1b and 1c. The cyclizations ac-
complished by using IPy2BF4 are depicted in Scheme 2. The
Interestingly, other partially hydrogenated scaffolds de-
rived from related aromatic hydrocarbons can be accessed
by this methodology. Thus, the intramolecular allene iodoar-
ylation strategy also enables the easy construction of the
elusive 2-iodo-1,4-dihydrophenanthrene framework featured
in 2i (Scheme 3).[19]
Scheme 3. Access to 2-iodo-1,4-dihydrophenanthrene derivatives by ap-
plying an iodoarylation reaction.
By design, all of the assembled carbocycles feature an al-
kenyl iodide functionality[20] that can be exploited for the
fast introduction of molecular complexity. Significant trans-
formations of 2a by Sonogashira or Negishi cross-coupling
reactions, leading to 3a and 4a are depicted in Scheme 4.[21]
These simple derivatives are not accessible by using a Birch
reduction of the parent arenes.
Scheme 2. Iodoarylation of 3-substituted-2,3-butadienylbenzenes for in-
stant entry to derivatives of the 2-iodo-1,4-dihydronaphthalene core.
reaction of 1b with ICl gave rise to a complex mixture,
which included 2b. Interestingly, N-iodosuccinimide (NIS)
selectively gave the cyclization product 2b in 85% yield.[15]
In contrast, several attempts to cyclize 1c by using both re-
agents were ineffective. In general, IPy2BF4 provided a
robust method for the synthesis of the target 1,4-dihydro-
naphthalene core and was systematically used in related cyc-
lizations (Scheme 2).[16]
As depicted, allenes with various C substituents yielded
the desired iodinated dihydronaphthalene scaffold. The
lower isolated yield for the substrate with the phenyl sub-
stituent, 2e, reflects its lack of stability to the reaction con-
ditions and, particularly, to the purification step. The results
outlined in Scheme 2 were obtained from reactions routinely
conducted by employing 1 mmol of 1; nonetheless, some re-
actions were also scaled up. As a result, compound 2a was
prepared in comparable yield when 30 mmol of 1a were
used.[17] All of the reported cyclizations led selectively to the
formation of a six-membered ring. The reaction of 1g pro-
vides a striking example of this feature. In this case, upon
activation of the allene at the central carbon, the reaction
might progress by ring-closing processes leading to the for-
mation of either a six- or five-membered ring. Only the
former pathway was observed. The regiochemistry of the io-
donium-promoted cyclization is complementary to that of
the carbopalladation of 1,2-dienes.[18]
Scheme 4. Rapid molecular diversification from allenes: iodonium-pro-
moted cyclization/metal-catalyzed cross-coupling strategy.
The Kumada cross-coupling reaction with vinylmagnesium
bromide gave 2-vinyl-1,4-dihydronaphthalene (5a) in 85%
yield as a new compound that was further elaborated to give
the polycyclic skeleton of 6a upon heating with dimethyl
acetylenedicarboxylate (DMAD). The overall sequence
gave practical access to
(Scheme 5).
a
rare unsaturated motif
Scheme 5. Construction of polycyclic systems by allene iodoarylation.
The cyclization leading to the formation of 2 from 1 could
be reasonably explained by assuming an initial interaction
of the allene with the iodonium ion. Bonding the electro-
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J. Barluenga et al.
phile to the central carbon atom of the allene would lead to
the formation of an electron-deficient species stabilized by
the neighboring iodine atom.[22] This would render the ter-
minal carbon atom of the allene electrophilic, allowing a
subsequent Friedel–Crafts-like process to take place. The
electrophilic nature of this ring-closing step would favor the
formation of a six-membered ring, as is observed. For 1g,
the lack of formation of the five-membered ring product
supports this view, as opposed to a radical pathway.
The intermolecular hydroarylation reaction of allenes is a
demanding process and has scarcely been reported. Recent
studies focusing on the usefulness of different catalytic sys-
tems report timely and synthetically valuable alternatives.[23]
We decided to try to develop another alternative based on
an intermolecular iodoarylation reaction of allenes. Our
study began by exploring the dependence of the process on
the electronic activation of the arene and on the degree of
substitution of the allene. Regarding the latter characteristic,
the process was found to tolerate a wide set of simple model
allenes, ranging from mono- up to trisubstituted allenes (7–
14).
Scheme 6. Intermolecular allene iodoarylation: exploratory studies.
doarylation approach with the allene substitution pattern is
slightly superior to that found in alternative electrophilic ad-
dition reactions of arenes to allenes promoted by cationic
À
À
gold species. Interestingly, as a rule, this C C/C I bond-
forming approach gives the addition products as single iso-
mers.[25]
Moderately activated arenes are needed to furnish the de-
sired adducts. Benzene itself fails to add efficiently across 1-
phenyl-1,2-propadiene (7). However, arenes, such as 4-tert-
butylbenzene (15) or 1,3-dimethylbenzene (16) successfully
reacted with the monosubstituted allene 7 to give com-
pounds 21 and 22, respectively, in moderate yields
(Scheme 6). Two sets of conditions (method A or B) were
established in an attempt to influence the process.[24] Thus,
by using appropriate experimental conditions, more elec-
tron-rich arenes, such as 1,2-dimethoxybenzene (18) or pen-
tamethylbenzene (20), could take part in this process (see
compounds 29 and 31). Unfortunately 1H-indole failed to
provide adducts under related conditions.
For all compounds depicted in Scheme 6, the reactions
were conducted by using an excess of the arene with respect
to the allene (3:1 molar ratio). Conventional TLC is useful
for monitoring the reaction progress. It shows the disappear-
ance of the parent allene, typically in the range of 30–
90 min, with the slower reaction corresponding to the trisub-
stituted allene 11. For the intermolecular reaction, two syn-
thetic constraints were identified. Thus, for the assayed 1,3-
disubstituted allenes (9 and 10), 1,2-iodofluorination occurs
as a competitive process, hampering the yield of the corre-
sponding iodoarylation product. Depending on the nature of
the allene, aromatic iodination might also become a limiting
process. As for the intermolecular hydroarylation reaction
of allenes, the iodoarylation process also requires moderate-
ly activated arenes. However, the compatibility of the io-
À
This iodonium-promoted C C bond-forming process can
be rationalized in terms of an electrophilic aromatic substi-
tution reaction. Although mechanistically unrelated, the io-
doarylation of allenes reported herein could be formally
viewed as a method to synthesize the products that result
À
from an arene C H activation, followed by insertion to the
allene and trapping with iodine. Overall, this iodonium ap-
proach lacks precedent and gives b-iodoallylated arenes in a
straightforward manner. As a result, this methodology could
be a key step in a direct approach to the preparation of a
class of tetrasubstituted alkenes featuring a distinctive C I
bond available for subsequent and selective functionaliza-
tion (Scheme 7).
À
In summary, intra- and intermolecular pathways enabling
a new arylation reaction of allenes have been presented.
Scheme 7. Straightforward assembly of hybrid cycloalkane–arene scaf-
folds using a novel C-H arene functionalization approach.
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Iodoarylation of Allenes
COMMUNICATION
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Experimental Section
Typical procedure (synthesis of 2a): A 54% solution of HBF4 in diethyl
ether (1 equiv, 0.14 mL; commercially available) was added to a stirred
solution of IPy2BF4 (1 equiv, 0.372 g) in CH2Cl2 (8 mL) at À308C. After
stirring for 10 min, the mixture was cooled to À908C and a solution of 1a
(1 mmol, 0.130 g) in CH2Cl2 (2 mL) was added. Once the starting allene
had disappeared (typically after a few minutes as monitored by TLC),
the reaction was quenched by addition of an aqueous solution of Na2S2O3
(5%) and warmed to room temperature. After extraction with CH2Cl2
the mixture was consecutively washed with a 5% aqueous solution of
Na2S2O3 and a 1n aqueous solution of HCl, the organic layer was dried
over anhydrous Na2SO4 and concentrated to dryness. Purification of the
residue by column chromatography on silica gel (hexane as eluent) af-
forded 2a as a colorless oil (0.179 g, 70%).
Acknowledgements
Funding for this research from the Spanish MICINN (CTQ2007-61048)
and Principado de Asturias, PRINCAST (IB08-088) is gratefully ac-
knowledged. E.C.-G. and D.R. thank the Spanish MEC, and A.M. thanks
the German DAAD for fellowships.
Keywords: allenes
· arenes · cyclization · electrophilic
substitution · iodination
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J. Barluenga et al.
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fer, J. Organomet. Chem. 2009, 694, 571–575.
[24] Method A (synthesis of 21–27): allene (1 equiv), IPy2BF4 (1 equiv),
HFB4 (0.8 equiv), CH2Cl2, À408C. Method B (synthesis of 28–33
and 36–37): allene (1 equiv), IPy2BF4 (1 equiv), HFB4 (3 equiv),
CH2Cl2, À908C.
[25] NMR spectroscopy analysis of crude reaction mixtures showed the
formation of the addition products as a single stereoisomer, together
with an excess of the arene and some iodoarene. Exceptions to this
trend (not depicted in Scheme 6): 1,2-dimethylbenzene and 1,1-di-
phenyl-1,2-propadiene gave (method B) an inseparable mixture of
1,2,4- and 1,2,3-iodoarylated isomers, (41%, 2:1 ratio, respectively).
Iodonium-promoted addition of 1,3,5-trimethylbenzene to 1-phenyl-
1,2-propadiene occurs (method A) with low selectivity (51% com-
bined yield). Addition to the terminal double bond mainly takes
place to give a 5:1 mixture of Z and E isomers, respectively. Trace
amounts of addition to the internal bond were detected in the crude
mixture.
Received: June 17, 2009
Published online: August 4, 2009
8950
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Chem. Eur. J. 2009, 15, 8946 – 8950