Direct ortho iodination of β- and γ-aryl alkylamine derivatives

Article abstract of DOI:10.1002/anie.200603631

Two in one! A trifluoroacetamide protecting group not only masks an amine functionality, but also mimics peptidyl directing effects as a controlling unit in an efficient ortho iodination of a variety of biologically relevant small molecules (see example). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200603631

Angewandte
Chemie
DOI: 10.1002/anie.200603631
Electrophilic Substitution
Direct ortho Iodination of b- and g-Aryl Alkylamine Derivatives**
JosØ Barluenga,* Julia M. lvarez-GutiØrrez, Alfredo Ballesteros, and JosØ M. Gonzµlez
À
Selective C H functionalization reactions allow the rapid
modification of available molecular scaffolds. Organometallic
and electrophilic processes serve as synthetic tools to this end.
Selective electrophilic aromatic substitution at the ortho
position remains elusive in the case of monosubstituted
arenes. Therefore, ortho-substituted derivatives are prepared
typically by indirect strategies. Arynes,[1] or the ortho-
metalated species obtained upon the treatment of arenes
with appropriate bases,[2] offer feasible alternatives. The
Table 1: Substrates and conditions for the o-iodination of a-phenyl-
alanine derivatives.[a]
Entry
X
1
t
[h]
CH2Cl2/TFA
[mL/mL]
2 [%][b]
2 (o/p)[b]
1[c]
2[c]
3
H
1a
1b
1c
1c
1c
1c
1c
1c
1c
1c
5
4
2
6
0.5
4
24
0.1
2.5
4
100/10
100/10
100/10
100/10
10/1
100/1
150/15
–/10
53
32
>95
>95
>95
50
57
>95
91
2a (1:1)
2b (2:1)
2c (10:1)
2c (10:1)
2c (5:1)
2c (5:1)
2c (12.5:1)
2c (4:1)
2c (1:1)
2c (1:1)
À
metal-catalyzed activation of the ortho C H bond is also
COCH3
COCF3
COCF3
COCF3
COCF3
COCF3
COCF3
COCF3
COCF3
useful in this context, and a number of approaches for the
preparation of ortho-halogenated derivatives have recently
been disclosed.[3] The recognition of controlling units that
assist the delivery of the electrophile to the desired ortho
position through standard electrophilic processes is crucial,
and should lead to interesting opportunities.[4]
4[d]
5
6
7
8
9[e]
10
10/1
10/–
Herein, results on the controlled ortho iodination of
phenylalanine and related b- and g-aryl alkylamines are
reported. The bonding of the amine to a small organic
fragment able to weakly interact with the iodonium species
was a key to this study from the outset.[5] The use of a
trifluoroacetamide as an amine surrogate was investigated.[6]
Our hypothesis was validated with the O-methyl ester
derivatives of phenylalanine for the iodination of free
amine, N-acetyl, and N-trifluoroacetyl substrates. The effect
of the experimental conditions was also tested. Representa-
tive results are outlined in Table 1. Entries 1–3 show that the
conversion of the amine into an amide modifies the regiose-
lectivity of the reaction.[7] The use of the trifluoroacetamide
derivative of the amine led to an unusual ortho iodination of
the phenylalanine derivative.[5] Both the use of a mixture of
CH2Cl2 and trifluoroacetic acid (TFA) as the solvent (Table 1,
60
[a] 0.5 mmol of 1; ratio 1/IPy2BF4/HBF4 (1:1.5:3.1). [b] The conversion
and the o/p ratio of 2 were determined by GC and/or 1H NMR
spectroscopic analysis of the crude reaction mixture. [c] The reaction
was carried out with a 1/IPy2BF4/HBF4 ratio of 1:1.05:2.1. [d] The reaction
was carried out at À208C. [e] The reaction was carried out without HBF4.
entries 3, 8, and 10) and the addition of HBF4 (Table 1,
entries 5 and 9) affect the selectivity in favor of the ortho-
substituted isomer. The dependence of the regioselectivity on
the concentration was tested. Thus, with a CH2Cl2/TFA ratio
of 10:1, higher selectivity was observed for the iodination
performed at 4.5 ” 10À3 m than for the reaction in the more
concentrated solution (5 ” 10À2 m). Dilution to 3 ” 10À3
m
improved the selectivity, but the iodination did not proceed
to conclusion, even after a longer reaction time (Table 1,
entries 3, 5, and 7).
The conditions outlined in Table 1, entry 3 were then used
routinely to investigate the generality of this ortho iodination.
By this protocol, pure o-2c[8] was isolated in 80% yield, and p-
2c in 5% yield. The iodination of trifluoroacetamides
prepared from simpler amines showed that this method
gives the desired selectivity for derivatives of b- and g-aryl
alkylamines (Scheme 1). In all cases, the conversion was
higher than 95%, and the reaction furnished iodinated
compounds as the only products.[9] The length of the tether
between the arene ring and the trifluoroacetamide moiety
drastically affects the selectivity. Excellent results were
observed for the iodination when the spacer contained two
or three methylene groups, for both secondary and tertiary
amides. Compounds in which one or four methylene groups
link the two active units were iodinated with 1:1 o/p
selectivity. As expected, the reaction of the amide derived
from aniline gave only the p-iodo regioisomer. Branching in
the aliphatic spacer at the b position to the nitrogen atom led
to poorer selectivity. This result probably reflects constraints
[*] Prof. Dr. J. Barluenga, Dr. A. Ballesteros, Dr. J. M. Gonzµlez
Instituto Universitario de Química Organometµlica “Enrique
Moles”
Unidad Asociada al C.S.I.C., Universidad de Oviedo
Juliµn Clavería, 8; 33006-Oviedo (Spain)
Fax: (+34)985-103-450
E-mail: barluenga@uniovi.es
Dr. J. M. lvarez-GutiØrrez
Departamento de Química, rea de Química Orgµnica
Facultad de Ciencias, Universidad de Burgos
Plaza Missael Baæuelos s/n; 09001 Burgos (Spain)
[**] Financial support from the Ministerio de Educación y Ciencia of
Spain (Grant CTQ 2004-08077-C02-01) and the Consejería de
Educación y Cultura Principado de Asturias (IB05-136) is acknowl-
edged. J.M.A.-G. thanks the Spanish M.E.C. for a Ramón y Cajal
contract. We thank Ana-BelØn García for assistance with HPLC
analysis.
Supporting information for this article, including experimental
procedures and spectral data for 2c–m, 3, 4, 5, and 6, is available on
the WWW under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 1281 –1283
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Communications
Scheme 3. Model compounds.
The amide 1m prepared from b-phenylethylamine con-
tains a strong coordinating fragment based on pyridine. This
substrate was transformed under the standard conditions into
the iodinated compound 2m, with total conversion after 2 h.
Analysis of the crude reaction mixture by 1H NMR spectros-
copy indicated a 23:1 ratio for the o/p selectivity; o-2m was
isolated in 90% yield. On the other hand, the iodination of 1n
led to a complex mixture. Thus, a certain degree of freedom in
the tethering chain seems to be necessary to allow for a
selective ortho iodination. These results and the observed
influence of the length of the tethering chain suggest that the
ortho iodination might arise from an initial close contact
between the electrophile and the tri-
fluoroacetamide moiety. This interac-
tion would ultimately precede an intra-
molecular delivery of the electrophile
to the ortho position. The fluorine
Scheme 1. Regioselectivity in the iodination of N-trifluoroacetylamides.
The o/p ratios were determined by GC and/or 1H NMR spectroscopic
analysis of the crude reaction mixtures; the yields of the individual
isomers after isolation by chromatographic separation are given.
atoms should play a key role in this
model, as their characteristics would
in the adoption of a conformation favoring ortho iodination.
Interestingly, substitution in the a position to the nitrogen
atom does not preclude the desired control over the
regioselectivity, as evidenced in the iodination of compound
1l (Scheme 2). This reaction is a significant example of a
chemo- and regioselective iodination of the phenetylamine
skeleton.[10]
The iodination of 1d led with remarkable control to the
formation of the target regioisomer. Full conversion into 2d
and a 25:1 ratio of the ortho- to the para-substituted
regioisomer were observed after 2 h. The uniqueness of
these reaction conditions for the successful ortho iodination
was checked: In the absence of an acid, HBF4 and/or TFA, a
major drop in the o/p ratio was observed, and upon replacing
TFA with acetic acid a 1:1 mixture of iodinated regioisomers
was obtained.
make possible such a contact.[11]
A
tentative model for this interaction is
proposed in Scheme 4.
Scheme 4. Proposed
interaction that might
control the ortho iodi-
nation of amides
derived from aryl-sub-
stituted alkylamines.
We recognized the utility of our
new method for the straightforward
preparation of o-iodo derivatives of
phenetylamines and phenylalanine.
These o-iodo compounds were further
elaborated into biaryl scaffolds by palladium-catalyzed cross-
coupling reactions. This two-step sequence offers a conven-
ient preparation of aryl-substituted constrained phenylala-
nine derivatives[12] in a synthetic scenario in which the
À
potential of the metal-mediated arylation of the C H bond
has not yet been exploited.[13] The results of the Suzuki–
Miyaura coupling of boronic acids with 2c are summarized in
Scheme 5.[14,9] Overall, this synthesis of constrained phenyl-
alanine derivatives starting from the building block 1c is a
demanding option.[15]
A hypothesis was formulated to explain these results,
although no compelling evidence had yet been gathered. Two
model compounds were designed as substrates to provide
more information about the reaction and were subjected to
the iodination (Scheme 3). The additional results supported
the proposed working model.
In summary, an aromatic electrophilic substitution of
monosubstituted arenes with the selective incorporation of
iodine at the ortho position has been reported. The products
are valuable reagents for the rapid generation of diversity by
metal-mediated reactions. Further synthetic work and mech-
anistic studies should provide insight into the origin of this
unusual reactivity. The transformation offers a promising
alternative to the use of transition metals or strong bases to
force the difficult iodination of ortho carbon atoms. It should
prompt efforts to explore the reactivity of other electrophiles
and starting scaffolds, and to search for alternative directing
groups for electrophilic transformations.
Scheme 2. Chemo- and regioselective iodination of 1l.
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[6] We speculated that lipophilic groups capable of establishing
dipolar contacts with iodonium ions could favor selective ortho
iodination. Also, the incorporation and removal of this control-
ling unit must be simple. For a recent review on organofluorine
compounds, see: M. Shimizu, T. Hiyama, Angew. Chem. 2005,
117, 218 – 234; Angew. Chem. Int. Ed. 2005, 44, 214 – 231.
[7] This selectivity is in sharp contrast to that observed in previous
aromatic iodinations with this reagent: a) T. Shimada, M. Suda,
T. Nagano, K. Kakiuchi, J. Org. Chem. 2005, 70, 10178 – 10181;
b) T. L. Hudgens, K. D. Turnbull, Tetrahedron Lett. 1999, 40,
2719 – 2722. The usual para selectivity was observed in the
iodination of the methyl ester of phenylalanine by using IPy2BF4
and CF3SO3H: c) J. Barluenga, M. A. García-Martín, J. M.
Gonzµlez, P. ClapØs, G. Valencia, Chem. Commun. 1996, 1505 –
1506.
Scheme 5. Synthesis of ortho-substituted biaryls derived from phenyl-
alanine by selective iodination followed by palladium-catalyzed cross-
coupling.
Experimental Section
Typical procedure: Compound 1c (137 mg, 0.5 mmol) was dissolved
in a mixture of anhydrous CH2Cl2 (100 mL) and TFA (10 mL). HBF4
(54 wt% in diethyl ether; 0.21 mL, 1.5 mmol) was then added,
followed by IPy2BF4 (0.28 g, 0.75 mmol; Py = pyridine), whereupon
the solution turned pink. The mixture was stirred at room temper-
ature for 2 h, then quenched with cold water. The organic layer was
washed twice with water, once with 5% aqueous sodium thiosulfate,
and again with water, then dried over anhydrous sodium sulfate and
concentrated under reduced pressure. The crude product was purified
by column chromatography (hexane/ethyl acetate 3:1; Rf = 0.57) to
afford 2c (160 mg, 80%).
[8] Compound o-2c was formed as single enantiomer by the
iodination of (S)-1c. Its stereochemical integrity was confirmed
by HPLC on a chiral phase, by comparison with a sample of
racemic o-2c.
[9] For reaction times, experimental details, and characterization
data, see the Supporting Information.
2
À
[10] For metal-catalyzed N C(sp ) cyclization reactions of these
compounds, see, for example: a) A. Klapars, S. Parris, K. W.
Anderson, S. L. Buchwald, J. Am. Chem. Soc. 2004, 126, 3529 –
3533; b) R. Omar-Amrane, A. Thomas, E. Brenner, R.
Schneider, Y. Fort, Org. Lett. 2003, 5, 2311 – 2314; c) K.
Yamada, T. Kubo, H. Tokuyama, T. Fukuyama, Synlett 2002,
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Chem. Commun. 2000, 583 – 584; e) B. H. Yang, S. L. Buchwald,
Org. Lett. 1999, 1, 35 – 37; for a biological-activity profile of
related compounds, see: f) R. H. P. Porter, J. T. Greenamyre, J.
Neurochem. 1995, 64, 614 – 623; g) W. S. Sun, Y. S. Park, J. Yoo,
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[11] For examples of cooperative weak interactions based on close
contacts, see (CF···CH): a) S. C. F. Kui, N. Zhu, M. C. W. Chan,
Angew. Chem. 2003, 115, 1666 – 1670; Angew. Chem. Int. Ed.
2003, 42, 1628 – 1632; (CF···Si): b) S.-Y. Kim, A. Saxena, G.
Kwak, M. Fujiki, Y. Kawakami, Chem. Commun. 2004, 538 –
539; for the notion of halogen bonding, see: c) P. Metrangolo, H.
Neukirch, T. Pilati, G. Resnati, Acc. Chem. Res. 2005, 38, 386 –
395; d) P. Auffinger, F. A. Hays, E. Westhof, P. S. Ho, Proc. Natl.
Acad. Sci. USA 2004, 101, 16789 – 16794.
Received: September 5, 2006
Revised: October 23, 2006
Published online: January 5, 2007
Keywords: amino acids · cross-coupling · electrophilic aromatic
.
substitution · iodine · neighboring-group effects
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À
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[15] HPLC analysis of 3 on a chiral phase proved that it was formed
as single enantiomer (> 98% ee). Compound 6 was formed as a
mixture of two diastereomers, each with > 97% ee.
Angew. Chem. Int. Ed. 2007, 46, 1281 –1283
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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