Directed Amination of Aryl Methyl Ethers Mediated by Ti(NMe2)4 at Room Temperature

Article abstract of DOI:10.1021/acs.orglett.5b01229

An efficient C-O amination of aryl methyl ethers has been achieved. This transformation proceeds via imine-directed Ti(IV)-mediated cross-coupling reactions between aryl methyl ethers and Ti(NR₂)₄ at room temperature, straightforwardly leading to a series of arylamines. This protocol features a wide substrate scope, exclusive regioselectivity, and mild reaction conditions.

Full text of DOI:10.1021/acs.orglett.5b01229

Letter
pubs.acs.org/OrgLett
Directed Amination of Aryl Methyl Ethers Mediated by Ti(NMe2)4 at
Room Temperature
Zhou Chen,† Jinna Liu,† Hao Pei,† Wei Liu,† Yanmei Chen,† Jian Wu,† Wu Li,‡ and Yahong Li*,†
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215006, China
‡Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Qinghai 810008, China
S
* Supporting Information
ABSTRACT: An efficient C−O amination of aryl methyl
ethers has been achieved. This transformation proceeds via
imine-directed Ti(IV)-mediated cross-coupling reactions
between aryl methyl ethers and Ti(NR2)4 at room temper-
ature, straightforwardly leading to a series of arylamines. This protocol features a wide substrate scope, exclusive regioselectivity,
and mild reaction conditions.
romatic amines are a valuable class of compounds that have
numerous applications in pharmaceutical, agrochemical,
aromatic ethers and N-heteroaryl methyl ethers as electrophiles
(Scheme 1a).
A
dye, and polymer industrial.1 Traditionally, arylamines are
prepared by metal-catalyzed hydrogenation of aromatic nitro
compounds.2 Since the pioneering work by Migita and co-
workers,3 who established an effective palladium-catalyzed
synthesis of arylamines via coupling reactions, spectacular
advancements have been made in the efficient constructions of
Caryl−N bonds by metal-catalyzed coupling reactions. The
important methodologies include: (i) Buchwald−Hartwig4 or
Ullmann-type,5 and Chan−Lam6 coupling reactions by employ-
ing palladium,7 nickel,8 copper,9 and cobalt10 as catalysts
(Scheme 1a); (ii) metal-catalyzed amination of organometallic
The above-mentioned precedents can provide an efficient and
facile route to the arylamines. However, these coupling reactions
were accomplished at a price. They rely on both noble palladium
or other late transition metal catalysts, and specific phosphine-,
carbene-, or phenolic ligands. Moreover, most of organometallic
compounds are air and moisture sensitive. For metal-free
reactions, drastic conditions are usually required to achieve
high conversions. Hence, the development of a new protocol that
provides amines without using any ligands and under mild
conditions is in high demand and also poses an actual challenge.
Herein, we demonstrate a new strategy that involves imine-
directed, Ti(IV)-mediated amination of aryl methyl ether by
Ti(NR2)4, achieving arylamines at room temperature without
using ligands and noble or late transition metals (Scheme 1e).
This is the first time that an early transition metal was employed
to mediate the amination reaction.
Scheme 1. Synthesis of Arylamines via Coupling Reactions
Recently, we disclosed that the NMe2 group of Ti(NMe2)4
could activate the C−H bond.26 We reasoned that the C−O
bond of a phenol derivative might be activated by Ti(NR2)4.
Encouraged by our previous success and also inspired by the
literature reports that phenol derivatives could be aminated by
the cleavage of aryl C−O bonds,25 we postulated that readily
available Ti(NR2)4 might be used both as a C−O bond activator
and as an amination partner with phenol derivatives, giving
arylamines straightforwardly via coupling reactions. Thus, we
began our investigation by testing the reaction of Ti(NMe2)4 and
a Schiff base N-(2-methoxybenzylidene)-1-(1H-pyrrol-2-yl)-
methanamine (3a), which was formed by the condensation
reaction between (1H-pyrrol-2-yl)methanamine (1a) and 2-
methoxybenz-aldehyde (2a). In 3a, an imine group is attached to
the ortho position of a methoxy group as a directing group since
imines have been previously employed as directing groups for
amination reactions.27 To our delight, an aminated product 2-
reagents (e.g., B, Zn, Mg, etc.)11 (Scheme 1b); (iii) direct C−H
amination of aromatic compounds12 (Scheme 1c). More
recently, transition-metal-free amination of aryl boronic acids
and derivatives has emerged as an attractive strategy due to being
devoid of metal catalysts (Scheme 1d).13
Due to the high bond dissociation energy of C−O bond, the
phenol derivatives have been previously considered poor
electrophiles in cross-coupling reactions. However, Wenkert,14
Dankwardt,15 Shi,16 Chatani,17 Kwong,18 Garg,19 Itami,20
Martin,21 Cook,22 Han,23 and other groups24 reported that
phenol derivatives could be applied as the electrophiles to the
Kumada−Tamao−Corriu, Suzuki−Miyaura, Negishi, and other
coupling reactions. Chatani25 reported that Buchwald−Hartwig
type amination could be accomplished by employing fused
Received: April 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01229
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
(dimethylamino)benzaldehyde (4a) (Table 1, entry 1) is
obtained in high yield (92%) after hydrolysis. Intrigued by this
Table 2. Ti(IV)-Mediated Amination of Aryl Methyl Ether by
Ti(NMe2)4
Table 1. Discovery and Evaluation of Reaction Conditions
a
Reaction conditions: 3 (0.5 mmol), solvent (5 mL), Ti(NMe2)4 (0.5
b
or 0.25 mmol), room temperature, 2 h. Amines used for the synthesis
of the directing imine groups of 3. The loadings of 1 and 2 and the
detailed preparations of 3 are provided in the Supporting Information.
c
Yield of the isolated product.
harvest observation, we endeavored to investigate this directed
amination more thoroughly by performing extensive screening of
the directing groups, solvents, and loadings of Ti(NMe2)4.
After the amine employed to prepare the directing group was
switched to 2-(aminomethyl)phenol (1b), the desired product
4a was obtained in good (Table 1, entries 4 and 6) and relatively
lower (Table 1, entries 3 and 5) yields, showing 1a and 1b are
good amine substrates for the directing group. Other amines are
also tested, and none of them showed better yields than 1a and
1b (Table 1, entries 7−15). 2-Aminoethanol (1c) afforded 4a in
low yield, 2-aminophenol (1d) gave trace amount of the product,
and no products were detected for 1e, 1f, and 1g. Solvents did
not show the remarkable effect on the yields. The yields of 4a in
THF and DCM were almost the same. We found that the
loadings of Ti(NMe2)4 were crucial to the yields of 4a. When 1a
was used as the directing amine group, excellent yield of 4a was
afforded for 1 equiv of Ti(NMe2)4. Nevertheless, 0.5 equiv of
Ti(NMe2)4 was the better choice as 1b was used for constructing
the directing group.
With these optimized reaction conditions in hand, we
endeavored to expand the scope and generality of this directed
amination reaction (Table 2). To simplify the reaction, the
starting substrates (3) were changed to the substituted
methoxybezealdehydes (2) and amines (1a or 1b) since the
directing imine groups could be generated in situ via the
condensation reaction of 2 and amines (1a or 1b). When 2-
methoxybezealdehyde (2a) was used as the substrate, 87% yield
of the amination product was afforded, which is slightly lower
a
Reaction conditions: 1a (0.5 mmol), 2 (0.5 mmol), solvent (5 mL),
b
c
Ti(NMe2)4 (0.5 mmol), room temperature, 2 h. Isolated yield. 1a
and 1b were employed to synthesize the directing imine group.
d
e
Ti(NEt2)4 was used as amination partner. Ti(HNPh)4 was used as
coupling partner. A mixture of 4q and 2-methoxy-N-phenyl-6-
((phenylimino)methyl)aniline (4q-1) was obtained. See Supporting
Information for the details.
than that of straightly employing imine as the directing group
(Table 1, entry 1). Electron-rich methoxybenzaldehydes with
another one methoxy group at either the para or ortho position,
or other two methoxy groups at the meta position, of the OMe
group of methoxybenzaldehydes were proved to be good
substrates for this transformation, producing the corresponding
aminated products in good yields (Table 2, 2a−2e). 4-
(Diethylamino)-2-methoxybenzaldehyde (2f), albeit also effec-
tive, furnished the product in good yield. Having an electron-
B
DOI: 10.1021/acs.orglett.5b01229
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
withdrawing NO2 substituent at the para position, methox-
ybenzaldehyde 2g gave excellent coupling product. Remarkably,
the fluoro, chloro, and bromo moieties in methoxybenzaldehydes
2h−2k were all tolerated under this novel transformation and
afforded the targeted products in moderate to good yields,
making further elaborations of the corresponding aminated
products possible. 2-Methoxy-1-naphthaldehyde (2l) was found
to couple with Ti(NMe2)4 efficiently and afforded the desired
product in good yield. The bulky substituents on the phenyl ring
of the methoxybenzaldehyde derivative 2m affected the
efficiency of the coupling reaction, and trace amount of the
aminated product was obtained. When Ti(NEt2)4 was used as the
coupling partner (Table 2, entries 14−16), good yields of the
targeted products (4n−4p) were also obtained.
dene)-1-(1H-pyrrol-2-yl)methane amine (3ac) in THF at
room temperature (Figure 1). X-ray quality crystals of 5ac
Figure 1. Solid state structure of 5ac with thermal ellipsoids drawn at
50%. Hydrogen atoms have been omitted for clarity.
Importantly, exclusive regioselectivity was observed in all
cases, and the methoxy group at the ortho position was aminated
only. With two ortho positions of the aldehyde group being
occupied by methoxy groups, doubly aminated products were
produced (4e and 4p).
were grown from solvent mixture of toluene and hexane. The
titanium center of 5ac displays distorted octahedron geometry
and is chelated by two aminated ligand sets, suggesting the
amination occurs via an imine-directed Ti(IV)-mediated process.
In light of the observations that one equivalent of Ti(NMe2)4
gave lower yields of 4a while 0.5 equiv of Ti(NMe2)4 resulted in
good yields of 4a (Table 1, entries 3−6) when 1b was used as the
amine source for the directing group, we attempted to explore if
the substrate 2-(((2-methoxybenzylidene)amino) methyl)-
phenol (3ba), which was generated by the condensation reaction
between 1b and 2a, was consumed by some unexpected side
reactions. To this end, we tried to isolate the intermediate of the
reaction between 3ba and Ti(NMe2)4. A titanium compound
Ti2[2-((2-(2-(dimethylamino)benzylamino)-1-(2-(dimethyl-
amino)phenyl)-2-(2-hydroxyphenyl)ethylimino)methyl)-
phenol](NMe2)4 (OMe) (6ba) was generated and its crystal
structure was determined (Figure S2). Complex 6ba is dinuclear
and is chelated by an in situ formed molecule 2-((2-(2-
(dimethylamino)benzyl-amino)-1-(2-(dimethylamino)pheny)-
2-(2-hydroxyphenyl)ethyl-imino)ethyl) phenol. It is obvious
that C−C coupling28 and C−O amination reaction occurred
simultaneously, as the bulky molecule 2-((2-(2-(dimethyl-
amino)benzylamino)-1-(2-(dimethylamino)pheny)-2-(2-
hydroxyphenyl)ethylimino)ethyl)phenol was produced by C−C
coupling of 2-((2-(dimethylamino)benzyl-amino)methyl)-
phenol and 2-((2-(dimethylamino)benzylimino)methyl)phenol.
The formation of 6ba suggests that some of Ti(NMe2)4 reagents
were consumed by the side reaction, giving the target products
with lower yields.
The generality of this process was expanded by conducting
coupling reaction between 2c and Ti(HNPh)4. It was found that
Ti(HNPh)4 could be employed in this transformation. Two
products 3-methoxy-2-(phenylamino)benzaldehyde (4q) and 2-
methoxy-N-phenyl-6-((phenylimino)methyl)aniline (4q-1)
were determined. For the convenience of purification, 4q-1
was further reduced to 2-methoxy-N-phenyl-6-((phenylamino)-
methyl)aniline (4q-2). Compounds 4q and 4q-2 were fully
characterized. Unfortunately, −HtNBu and −NPh2 could not be
introduced into the products by using this methodology.
A proposed mechanism for the amination process is outlined
in Scheme 2. Initially, the condensation reaction between 1a and
Scheme 2. Postulated Mechanism for the Formation of the
Aminated Product
2 generates the imine substrate 3. Next, the reaction of
Ti(NMe2)4 with 3 gives titanium complex IN1 in which the
metal center may adopt a pseudo octahedral geometry. The
nitrogen atom of a NMe2 group, which is cis to the methoxy
oxygen atom, might be quite close to the carbon atom bearing the
methoxy group. After dearomatization process, the interactions
of C, N, O, and Ti atoms may give a four-membered ring
transition state TS1, which subsequently undergoes a C−O
bond-cleavage and a C−N bond-forming process to give
complex 5. Hydrolysis of 5 affords the aminated product 4. For
the methoxy group being meta to the imine directing group,
greatly enlarged Ti−O and C−N distances are resulted. Thus,
the expected four-membered Ti−N−C−O ring could not be
generated, and no amination occurs for the meta C−O bond,
elucidating the regioselectivity of the reactions.
In summary, we have developed a general, mild and
experimentally simple method for the amination of aryl methyl
ethers. This transformation proceeds via imine-directed Ti(IV)-
mediated cross-coupling reactions between aryl methyl ethers
and Ti(NR2)4 at room temperature. A variety of aryl methyl
ethers can participate in the process with good yields. The room
temperature reaction coupled with a broad substrate scope
render this method particularly attractive for the preparation of
arylamines.
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of the synthesis of the complexes, the characterizations of
the compounds, and X-ray crystallographic data of 5ac and 6ba.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01229.
One of the titanium complexes [Ti(C15H18N3O)2(OMe)-
(NMe2)](hex)0.125(tol)(H2O)0.25 (5ac) was isolated from the
reaction between Ti(NMe2)4 and N-(2,3-dimethoxybenzyli-
C
DOI: 10.1021/acs.orglett.5b01229
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Organic Letters
Letter
(13) (a) Coeffard, V.; Moreau, X.; Thomassigny, C.; Greck, C. Angew.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: liyahong@suda.edu.cn.
Notes
(d) Zhu, C.; Li, G.; Ess, D. H.; Falck, J. R.; Kurti, L. J. Am. Chem. Soc.
̈
2012, 134, 18253.
(14) (a) Wenkert, E.; Michelotti, E. L.; Swindell, C. S. J. Am. Chem. Soc.
1979, 101, 2246. (b) Wenkert, E.; Michelotti, E. L.; Swindell, C. S.;
Tingoli, M. J. Org. Chem. 1984, 49, 4894.
The authors declare no competing financial interest.
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■
The authors appreciate the financial support from Natural
Science Foundation of China (21272167 and 21201127), A
Project Funded by the Priority Academic Program Development
of Jiangsu Higher Education Institution, and KLSLRC
(KLSLRC-KF-13-HX-1).
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