Iridium-catalyzed regioselective silylation of aromatic and benzylic C-H bonds directed by a secondary amine

Article abstract of DOI:10.1002/anie.201404620

Reported herein is an iridium-catalyzed, regioselective silylation of the aromatic C-H bonds of benzylamines and the benzylic C-H bonds of 2,N-dialkylanilines. In this process, (hydrido)silyl amines, generated in situ by dehydrogenative coupling of benzylamine or aniline with diethylsilane, undergo selective silylation at the C-H bond γ to the amino group. The products of this silylation are suitable for subsequent oxidation, halogenation, and cross-coupling reactions to deliver benzylamine and arylamine derivatives.

Full text of DOI:10.1002/anie.201404620

Angewandte
Chemie
DOI: 10.1002/anie.201404620
À
C H Activation
Iridium-Catalyzed Regioselective Silylation of Aromatic and Benzylic
À
C H Bonds Directed by a Secondary Amine**
Qian Li, Matthias Driess, and John F. Hartwig*
À
Abstract: Reported herein is an iridium-catalyzed, regioselec-
would significantly increase the synthetic utility of the C H
silylation reaction.
À
tive silylation of the aromatic C H bonds of benzylamines and
À
the benzylic C H bonds of 2,N-dialkylanilines. In this process,
Recently, we reported a hydroxy-directed silylation of
aromatic[6d] and aliphatic[10] C H bonds by generation of
À
(hydrido)silyl amines, generated in situ by dehydrogenative
coupling of benzylamine or aniline with diethylsilane, undergo
diethyl(hydrido)silyl ethers in situ. These reactions generate
oxasilolanes, which can undergo oxidation and cross-coupling
reactions to give diols and biphenyl products. Based on these
studies, we reasoned that a secondary amine might serve as
a directing group for the silylation of aromatic and aliphatic
À
selective silylation at the C H bond g to the amino group. The
products of this silylation are suitable for subsequent oxidation,
halogenation, and cross-coupling reactions to deliver benzyl-
amine and arylamine derivatives.
À
C H bonds. An amine-directed silylation would generate
À
T
he direct silylation of C H bonds catalyzed by transition-
azasilolanes in which the nitrogen atom on the silicon would
activate the organosilane products towards further trans-
formation.
metal complexes provides a straightforward method to
synthesize organosilanes.[1] The organosilane products are
valuable synthetic intermediates,[2,3] which can be converted
into useful compounds through a number of transformations,
such as halogenation, oxidation, and Hiyama coupling.[4]
However, several challenges face the translation of the
silylation reactions directed by an alcohol to a silylation
reaction directed by an amine. For example, the strong
binding ability of a basic nitrogen atom might inhibit the
catalytic activity of the transition-metal catalyst. In addition,
[5–7]
À
Thus, a catalytic silylation of C H bonds,
subsequent transformations of organosilanes, creates a con-
coupled with
À
À
venient strategy to convert C H bonds into common func-
tional groups.
the N Si bond is significantly weaker and more sensitive to
[11]
À
À
Brønsted acids than an O Si bond. Therefore, the Si N
linkage might not be stable to the conditions of the silylation
reaction.
In recent years, a variety of directing groups have been
[8,9]
À
used to control the regioselectivity of C H bond silylation.
For example, imine,[8a,e] oxazoline,[8c,i] pyridine,[8d,f,i,j,9a,b] pyra-
zole,[8d,g] and tertiary amine[8d] functionalities have been used
as directing groups for the silylation of aromatic and benzylic
Herein we demonstrate that the complex formed from
[{Ir(OMe)(cod)}2] and 3,4,7,8-tetramethyl-1,10-phenanthro-
line (Me4Phen) catalyzes the regioselective silylation of the
À
À
C H bonds with trialkylsilanes. However, these directing
aromatic C H bonds in benzylamines and the primary and
À
groups are usually undesired in the final product, and multiple
chemical transformations are needed to install and remove
them. In addition, these reactions usually generate a mixture
of mono- and disilylated products from substrates containing
secondary benzylic C H bonds of 2,N-alkylanilines. In these
reactions, disilylation processes do not occur because the
C H functionalization step is intramolecular. The silylation
À
product obtained can undergo further functionalizations, such
as oxidation, halogenation, and cross-coupling reactions.
To identify an active catalyst for the silylation of
secondary amines, we first studied the reaction of N-
methylbenzylamine (1a). The dehydrogenative coupling of
1a with diethylsilane catalyzed by 0.5 mol% [{Ir(OMe)-
(cod)}2] furnished the (hydrido)silyl amine 2a in 78% yield at
room temperature in THF after 20 hours (see Table 1). When
the reaction was conducted under neat conditions, 2a was
formed quantitatively after 20 hours. With 2a in hand, we
studied the dehydrogenative silylation step (Table 1). In the
presence of an iridium catalyst and 4,4’-di-tert-butylbipyridine
as ligand, the intramolecular cyclization at 808C occurred to
give the azasilolane product 3a in 88% yield (entry 4). The
electronic properties of the ligands had a strong influence on
the yield. When 4,7-dichlorophenanthroline was used as
a ligand, a lower yield (19%) of 3a was obtained (entry 1).
In contrast, when the more strongly electron-donating
Me4Phen was used, full conversion of 2a was observed, thus
affording the product in high yield (95%, entry 3).
À
two C H bonds that are available for silylation. Furthermore,
À
the silylation of a C H bond with trialkylsilanes produces
tetraorganosilanes, which are less suitable for subsequent
oxidation and cross-coupling reactions than analogous prod-
ucts containing a silicon–heteroatom bond. Therefore, the
identification of directing groups that circumvent these issues
[*] Dr. Q. Li, Prof. Dr. J. F. Hartwig
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
E-mail: jhartwig@berkeley.edu
Dr. Q. Li, Prof. Dr. M. Driess
Technische Universitꢀt Berlin, Institut fꢁr Chemie
Metallorganische Chemie und Anorganische Materialien, Sekr. C2
Strasse des 17. Juni 135, 10623 Berlin (Germany)
[**] We gratefully acknowledged financial support by the Einstein
Foundation Berlin (J.F.H.), the NSF (CHE-01213409) (J.F.H.), and
the Cluster of Excellence UniCat (financed by the DFG and
administered by the TU Berlin to M.D.) for research support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404620.
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Table 1: Effect of the ligand on the silylation of benzylic silylamines.[a]
Table 2: Silylation of benzylmethylamine.[a,b]
Entry
Ligand
Yield [%][b]
1
2
3
4
4,7-dichloro-1,10-phenanthroline
1,10-phenanthroline
3,4,7,8-tetramethyl-1,10-phenanthroline
4,7-di-tert-butyl-2,2’-bipyridine
19
87
95
88
[a] Reaction conditions: 1a (1.0 mmol), SiH2Et2 (2.0 mmol), [{Ir(cod)-
OMe}2] (0.5 mol%), neat, RT, 24 h; evaporation of excess silane,
[{Ir(cod)OMe}2] (1.5 mol%), Me4phen (4.5 mol%), nbe (1.2 equiv),
THF, 808C, 20 h. [b] Yield determined by GC using isododecane as an
internal standard. cod=1,5-cyclooctadiene, nbe=norbornene, nba=
norbornane, THF=tetrahydrofuran.
[a] Reaction conditions: 1 (1.0 mmol), SiH2Et2 (2.0 mmol), [{Ir(cod)-
OMe}2] (0.5 mol%), neat, RT, 24 h; evaporation of excess silane,
[{Ir(cod)OMe}2] (1.5 mol%), Me4phen (4.5 mol%), nbe (1.2 equiv),
THF, 808C, 20 h. [b] Yield of product isolated from bulb-to-bulb
distillation. [c] [{Ir(cod)OMe}2] (2.5 mol%), Me4phen (7.5 mol%).
[d] There is a small amount of an inseparable impurity in the product.
These impurities can be separated after oxidation. [e] Obtained as an
inseparable mixture of constitutional isomers in a 1.0:1.7 ratio, as
Having identified an efficient catalyst for the silylation of
benzylamine, we studied the scope of this reaction (Table 2).
Benzylamines containing a substituent at the ortho or para
position underwent the silylation in good yields (3b,c). The
reaction of benzyl silylamines bearing a meta substituent
occurred with high regioselectivity for the more sterically
1
determined by H NMR spectroscopy. [f] 1008C, 40 h.
Yet, a simple increase of the reaction temperature to
1008C led to complete consumption of the starting material
and high yield of the cyclization product 6a, as shown in
À
accessible C H bond. For example, silylation of benzylamines
À
substituted in the meta-position with methyl, chloro, bromo,
and methoxy substituents proceeded in high yields with
greater than 20:1 regioselectivity (3d–g). The reaction of the
3-fluoro derivative was less regioselective, presumably
because of the smaller size of fluorine relative to the other
substituents (3h). The reaction of benzylamine containing
a methyl group at the position a to the nitrogen atom also
proceeded to completion (3i); however, a higher catalyst
loading and temperature were required. This result contrasts
with the silylation of benzyl alcohols,[6d] for which secondary
alcohols were more reactive than primary alcohols.[12]
Table 3. This silylation of the benzylic C H bonds is not
sensitive to the electronic properties of substituents on the
arene. The silylation of 2-methylanilines bearing electron-
donating groups occurred in yields similar to those of the
silylation of 2-methylanilines bearing electron-withdrawing
groups (6b–e).
À
The amine-directed silylation of benzylic C H bonds also
À
occurred at secondary benzylic C H bonds. As a result of the
À
steric hindrance, the reactivity of secondary C H bonds is
À
typically lower than that of primary C H bonds when an
organometallic intermediate is generated, and the examples
À
The high activity of the iridium catalyst for the amine-
of the high-yielding silylation of secondary C H bonds are
directed silylation of aromatic C H bonds suggested the
rare.[6h,9c,e,11c] Nevertheless, the amine-directed silylation of
À
potential of the process to occur at sp3 C H bonds. The
secondary benzylic C H bonds occurred in good to excellent
À
À
dehydrogenative coupling of 2,5-dimethyl-N-methylaniline
(4a) with diethylsilane catalyzed by 0.5 mol% [{Ir(OMe)-
(cod)}2] produced the (hydrido)silyl aniline 5a [Eq. (1)].
However, the same reaction conditions that we used for the
silylation of benzylamines did not provide significant amounts
yields (6 f–h and 7i–l, Table 3). In some cases, longer reaction
times (7i) or higher temperatures (7j–l) were required
(Table 3). The reaction tolerated various functional groups
on the alkyl chain, including bromide, methoxy, siloxy, and
acetal groups (6 f–h, 7i–l). The silylation products were
isolated by either bulb-to-bulb distillation or column chro-
matography on silica gel. When the silylation product was
purified by column chromatography, the azasilolane ring
opened to afford the corresponding silanol (7i–l),[13] which
was suitable for further transformations (see below).
À
of product from the silylation of the benzylic C H bond of the
arylamine.
À
For these C H bond functionalizations to be synthetically
valuable, reaction conditions to transform the silylation
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Table 3: Silylation of N,2-alkylanilines.[a]
Table 4: Oxidation of the silanol products to the corresponding amino-
alcohols.[a,b]
[a] Reaction conditions: 30% H2O2 (8.0 equiv), KHF2 (8.0 equiv), KF
(2.0 equiv), KHCO3 (8.0 equiv), THF/MeOH (v/v=1:1), RT, 16 h.
[b] Yield of product isolated from column chromatography on silica gel.
good yield with various azasilolanes, containing substituents
on the arene or position a to nitrogen atom, to give the
products 10a–e. In addition to oxidation, 9 underwent
halogenation. Treatment of 9a with iodine monochloride
gave the ortho-iodination product 11a in 52% yield, as shown
in Scheme 1. Thus, silylation and subsequent derivatization
provides methods for the synthesis of ortho-hydroxylated and
halogenated benzylamines.
Finally, we evaluated the potential of the azasilolanes or
derivatives of them to undergo cross-coupling. Direct Hiyama
cross-coupling of 3 or 9 with aryl halides under various
reaction conditions did not provide the biaryl product.
However, biaryl products were obtained by an alternative
route: conversion of the phenol, which was derived from the
[a] Reaction conditions: 4 (1.0 mmol), SiH2Et2 (2.0 mmol), [{Ir(cod)-
OMe}2] (0.5 mol%), neat, RT, 24 h; evaporation of excess silane, then
[{Ir(cod)OMe}2] (1.5 mol%), Me4phen (4.5 mol%), nbe (1.2 equiv),
THF, 1008C, 20 h. [b] Yield of isolated product from bulb-to-bulb
distillation. [c] Yield of product isolated from column chromatography on
silica gel. [d] 1008C, 40 h. [e] 1208C, 30 h.
Table 5: Oxidation of the azasilolanes to the corresponding amino-
phenols.[a]
products must be developed. To do so, we subjected the
À
products of the amine-directed C H bond silylation to
Fleming–Tamao oxidation conditions. The oxidation of both
the primary and secondary benzylic silanols with hydrogen
peroxide proceeded under mild reaction conditions to
generate the benzyl alcohols 8a–d in high yields
(Table 4).[14] Under these mild reaction conditions, the free
amino group was not oxidized. This strategy provides
a method for the regioselective hydroxylation of primary
À
and secondary benzylic C H bonds directed by an unpro-
tected amine.
[a] Reaction conditions: air, THF, RT, 2 h, then Ac2O (1.2 equiv), NEt3
(1.5 equiv), DMAP (1.0 mol%), CH2Cl2, RT, 2 h; removal of volatiles,
30% H2O2 (8.0 equiv), KHF2 (8.0 equiv), KF (2.0 equiv), KHCO3
(8.0 equiv), THF/MeOH (v:v=1:1), RT, 16 h. Yield of isolated products
are reported. [b] 8 h. [c] 30% H2O2 (16 equiv).
À
Oxidation of the Si C bonds in the products from the
À
silylation of aromatic C H bonds was more challenging
because benzylamines are more susceptible to oxidation than
are anilines. Direct oxidation of these azasilolanes under
various reaction conditions afforded the phenol products in
low yield (< 20%) because of the competing oxidation of the
amino group. Therefore, we sought to protect the amine
before oxidizing the silane. Exposure of the silylation product
3 to air for 2 hours provided the ring-opened product, which
was then acylated to form the amide 9 (Table 5). Subsequent
oxidation of 9 afforded the phenol product 10 in good yield.
The sequence of amine protection and oxidation occurred in
Scheme 1. Iodination of 3a. Reaction conditions: air, THF, RT, 2 h;
then Ac2O (1.2 equiv), NEt3 (1.5 equiv), DMAP (2 mol%), CH2Cl2, RT,
2 h; removal of volatiles, ICl (2.0 equiv), CH2Cl2, RT, 12 h.
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DMF = N,N,-dimethylformamide, Tf = trifluoromethanesul-
fonyl.
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In summary, we have developed an iridium-catalyzed
À
silylation of the ortho-aryl C H bonds of benzylamines and
À
the benzylic C H bonds of 2,N-alkylanilines. The secondary
À
amino group directs the iridium-catalyzed C H bond activa-
tion by generation of diethyl(hydrido)silyl amines in situ and
subsequent dehydrogenative cyclization. The silylation prod-
À
ucts contain a silicon–heteroatom bond, and this Si X bond
allows subsequent transformations, such as oxidation, and
À
maintains the ability of the Si C bonds to undergo halogen-
ation. Further studies to expand the substrate scope and gain
insight into the reaction mechanism are currently in progress.
Received: April 23, 2014
Published online: && &&, &&&&
À
Keywords: arenes · C H activation · iridium · oxidation · silane
.
Angew. Chem. Int. Ed. 2014, 53, 3168.
3
À
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À
C H Activation
Q. Li, M. Driess,
J. F. Hartwig*
&&&& — &&&&
Iridium-Catalyzed Regioselective
À
Silylation of Aromatic and Benzylic C H
Bonds Directed by a Secondary Amine
A ’SiN’ch: In the title reaction of benzyl-
amines or anilines, (hydrido)silyl amines
are generated in situ which undergo
can be further functionalized through
oxidation, halogenation, and cross-cou-
pling reactions. cod=1,5-cyclooctadiene,
nbe=norbornene.
À
selective silylation at the C H bond g to
the amino group. The silylation products
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