Hydroaminoalkylation of sterically hindered alkenes with: N, N -dimethyl anilines using a scandium catalyst

Article abstract of DOI:10.1039/c8ob02657b

An atom- and step-economical C(sp³)-H addition of N,N-dimethyl anilines to various sterically demanding 1,1- and 1,2-disubstituted alkenes has been achieved by using a simple β-diketiminato ligand supported scandium dialkyl complex in combinati

Full text of DOI:10.1039/c8ob02657b

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Hydroaminoalkylation of sterically hindered
alkenes with N,N-dimethyl anilines using a
scandium catalyst†
Cite this: DOI: 10.1039/c8ob02657b
Jianhong Su, Yiqun Zhou and Xin Xu
*
An atom- and step-economical C(sp3)–H addition of N,N-dimethyl anilines to various sterically demand-
ing 1,1- and 1,2-disubstituted alkenes has been achieved by using a simple β-diketiminato ligand
supported scandium dialkyl complex in combination with a borate compound. The corresponding
C(sp3)–C(sp3) bond forming reaction occurs with excellent regioselectivities to give a variety of tertiary
aromatic amines.
Received 26th October 2018,
Accepted 10th December 2018
DOI: 10.1039/c8ob02657b
rsc.li/obc
enabled hydroaminoalkylation of aromatic tertiary amines
with terminal alkenes affording branched products for the
Introduction
The development of efficient and selective methods to syn- first time (Scheme 1c).13 Mechanistic studies revealed that a
thesize amines plays a leading role in synthetic organic chem- switch of ortho-C(sp2)–H activation to α-C(sp3)–H activation of
istry because of the great importance of nitrogen-containing N,N-dimethyl aniline occurred during the reaction, which led
compounds in the fine chemical, pharmaceutical, and agro- to this anti-intuitive result.
chemical industries.1 Among various synthesis approaches,2–4
Although intermolecular hydroaminoalkylation of tertiary
transition-metal catalyzed direct addition of an α-C(sp3)–H aniline was realized using a scandium catalyst, only mono-sub-
bond of an amine across a CvC double bond of an alkene, stituted terminal alkenes could be used as substrates, which
namely hydroaminoalkylation, has attracted considerable greatly restricted its synthesis utility. We propose that the steri-
interest because it offers a route to substituted alkylamines cally demanding β-diketiminato ancillary ligand may hinder
with 100% atom economy.5 Catalysts based on early transition the coordination and/or insertion of crowded alkene sub-
metal complexes including Ti,6 Zr,7 and Ta8 have made sub- strates, thus resulting in a limited substrate scope. With this
stantial progress in this area over the last decade. However, in mind, we decided to investigate whether the substrate
the substrate scope is still limited in primary and scope could be expanded by modifying the ligand framework
secondary amines because the presence of an N–H bond in the of the scandium catalyst. We herein report the first
starting material is essential to form catalytically active examples of hydroaminoalkylation of tertiary amines with
species.
sterically hindered alkenes, such as 1,1-disubstituted alkenes
With the assistance of suitable directing groups,9–11 α-C and internal alkenes, using a scandium catalyst supported by
(sp3)–H alkylation of tertiary amines with terminal alkenes an easily accessible and less sterically encumbered
could also be achieved by using Ru or Ir catalysts to give linear β-diketiminato ligand with substrate-dependent regioselectivity
addition products (Scheme 1a). The first example of scan- (Scheme 1d).
dium-catalyzed hydroaminoalkylation of aliphatic tertiary
amines with terminal or activated alkenes was recently
reported by Hou and co-workers (Scheme 1b).12 We sub-
sequently showed that a scandium complex based on a
Results and discussion
β-diketiminato ligand with
a pendant phosphine group
Considering both the steric and electronic features of the
ligand, which are crucial to stabilize rare-earth alkyl com-
plexes,14 we decided to synthesize a less sterically demanding
β-diketiminato ligand precursor HL 15 and prepare the corres-
ponding scandium dialkyl complex 1 through the well-estab-
lished alkane elimination reaction (Scheme 2). Complex 1 was
solvent-free, isolated in 78% yield, and fully characterized by
multinuclear NMR spectroscopy, elemental analysis, and X-ray
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: xinxu@suda.edu.cn
†Electronic supplementary information (ESI) available. CCDC 1875283. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ob02657b
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Scheme 1 Intermolecular hydroaminoalkylation of tertiary amines with alkenes.
Scheme 2 Synthesis of scandium alkyl complex 1.
diffraction. It is notable that complex 1 is thermally stable, with that of rare-earth metal-catalyzed C–H functionalization
with no decomposition observed over 12 h at 80 °C in C6D6 of styrenic substrates.12,16 Neither neutral complex 1 nor
solution.
We initially examined the newly prepared scandium dialkyl wise identical conditions.
complex together with one equivalent of [PhNMe2H] Based on this result, we subsequently investigated hydro-
[PhNMe2H][B(C6F5)4] alone showed any activity under other-
1
[B(C6F5)4] for the hydroaminoalkylation of N,N-dimethyl aniline aminoalkylation reactions of N,N-dimethyl aniline (2a) with a
(2a) with 1,1-disubstituted alkene α-methylstyrene, which was variety of 1,1-disubstituted alkenes catalyzed by the scandium
not tolerated by our previous catalytic system.13 Excitingly, the complex 1/[PhNMe2H][B(C6F5)4] system at 120 °C in toluene
expected α-C(sp3)–H alkylation reaction took place, exclusively and the results are summarized in Table 1. With 10 mol%
affording the olefin functionalized product 4a in 91% isolated catalyst loading,
a range of meta- and para-substituted
yield within 48 h under the same conditions as those in our α-methylstyrenes were functionalized to generate the corres-
previous work (10 mol% catalyst loading, amine/alkene molar ponding linear amination products 4b–4g in good to excellent
ratio = 1.5 : 1, 120 °C, toluene as the solvent, Table 1). Notably, isolated yields. When using 1,3- or 1,4-diisopropenylbenzene
the hydroaminoalkylation reaction was highly regioselective as the olefinic substrate, the dialkylation product 4h or 4i
with the formation of a linear product which is consistent could be selectively obtained by increasing the amount of the
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Table 1 Scandium-catalyzed hydroaminoalkylation of N,N-dimethyl aniline with 1,1-disubstituted alkenesa
a Reaction conditions: 2a (0.81 mmol), 3 (0.54 mmol), toluene (2.0 mL), isolated yield. b 2a (1.62 mmol), 3 (0.54 mmol). c The reaction was per-
formed at 150 °C.
starting aniline. Even with more crowded α-substituted
Encouraged by the excellent performance of the scandium-
styrenes, the reactions still occurred, providing access to the based catalyst system in the hydroaminoalkylation of sterically
expected addition products 4j–4l although with a prolonged demanding 1,1-disubstituted alkenes with N,N-dimethyl
reaction time. Bulky 1,1-diphenylethylene, which is quite a aniline, we next investigated a variety of linear and cyclic
challenging alkene substrate, was converted to the functionali- internal alkenes as coupling partners under analogous
zation product 4m by increasing the reaction temperature conditions (Table 2). Initially, treatment of cis-3-hexene with
from 120 °C to 150 °C. Furthermore, two gem-dialkyl substi- N,N-dimethyl aniline afforded the expected C(sp3)–H alkylation
tuted
alkenes,
methylenecyclopentane
and
methyl- product 6a in 65% isolated yield in 48 h without other regio-
enecyclohexane, were also tested as substrates. Remarkably, isomers being observed (Table 2, entry 1). In contrast, the
the reactions showed completely different regioselectivities catalytic activity of the system was much lower when using
from those of the styrenic substrates. The C(sp3)–C(sp3) bond trans-3-hexene as the alkene substrate, producing compound
forming reactions took place exclusively at a more substituted 6a in only 21% yield even with a prolonged reaction time of
carbon to generate branched products 5a and 5b with 72 h (Table 2, entry 2). Hydroaminoalkylation of unactivated
newly formed β-quaternary carbon centers, albeit requiring an cyclic alkenes was also successfully achieved giving the
elevated temperature (150 °C) and a prolonged reaction corresponding addition products 6b–6d in moderate yields
time (96 h). Attempts to use halogen- or oxygen-containing (Table 2, entries 3–5). Norbornene was highly active in
alkenes, such as 4-bromo-α-methylstyrene or 4-methoxy- the reaction, exhibiting complete conversion within 12 h
α-methylstyrene, in the hydroaminoalkylation reaction failed (Table 2, entry 6).
under the standard conditions, which is probably caused by
Reactions of
a
variety of tertiary anilines with
the intrinsic affinity of the rare-earth metal ion for halogens α-methylstyrene promoted by the scandium complex 1/
and oxygen.
[PhNMe2H][B(C6F5)4] system were subsequently performed
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Table 2 Scandium-catalyzed hydroaminoalkylation of N,N-dimethyl aniline with internal alkenesa
Entry
1
Alkene
Product
Time (h)
48
Yield
65%
2
3
72
48
21%
52%
4
5
48
48
63%
71%
6
12
71%
a Reaction conditions: 2a (0.81 mmol), 3 (0.54 mmol), toluene (2.0 mL), isolated yield.
under our typical conditions and the results are listed in formal α-C(sp3)–H alkylation product exclusively. We thus pro-
Table 3. A number of meta- and para-substituted N,N-dimethyl posed a conversion of ortho-C(sp2)–H metalation to α-C(sp3)–H
anilines containing methyl, phenyl, and ring-fused groups metalation in the mechanistic framework, which was probably
were efficiently alkylated at the α-N-methyl position with steri- induced by a sterically encumbered ligand.13 To examine
cally hindered α-methylstyrene, exclusively yielding linear whether a similar reaction course might be followed in this
alkylation products 7a–7f in 75–87% isolated yields. Polar highly regioselective scandium-catalyzed hydroaminoalkyl-
functional groups such as –NMe2 were found to be tolerated in ation, we first reacted N,N-dimethyl aniline with the less
this case, giving the monoalkylation product 7g without hindered terminal alkene 1-octene promoted by the
noticeably changing the catalytic activity. A substituent at the 1/[PhNMe2H][B(C6F5)4] system under our typical conditions
ortho-position of N,N-dimethyl aniline was not tolerated even (Scheme 3a). After workup, this reaction gave a mixture of the
with increased catalyst loading, highlighting the importance of branched C(sp3)–H alkylation product 8 in 30% yield and the
the presence of an ortho-C(sp2)–H bond in this C(sp3)–H arene ortho-C(sp2)–H alkylation product 9 in 60% yield.
alkylation.13
Second, we performed a deuterium labeling experiment to
It is generally believed that ortho-C(sp2)–H activation will obtain more information about this reaction. The reaction of
most probably occur when the stoichiometric treatment of a N,N-(dimethyl-d6)-p-toluidine with α-methylstyrene catalyzed
cationic rare-earth alkyl species with aromatic heteroatom-con- by 1/[PhNMe2H][B(C6F5)4] gave the hydroaminoalkylation
taining compounds e.g. pyridines,17 anisoles,18 and tertiary product 7a-d6, which contained 18% deuterium incorporation
anilines is performed,19 resulting in highly regioselective ortho at the ortho position of the arene (Scheme 3b). These results,
functionalization reactions. Although the same situation together with the finding that ortho-substituted N,N-dimethyl
occurred initially for our previous system, it finally gave a anilines were not tolerated by this catalytic reaction, indicated
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Table 3 Scandium-catalyzed hydroaminoalkylation of tertiary anilines with α-methylstyrenea
a Reaction conditions: 2 (0.81 mmol), 3a (0.54 mmol), toluene (2.0 mL), isolated yield. b 1 (20 mol%), [PhNHMe2][B(C6F5)4] (20 mol%).
Scheme 3 (a) Scandium-catalyzed reaction of N,N-dimethyl aniline with 1-octene; (b) deuterium labeling experiment for the α-C(sp3)–H alkylation
reaction.
that ortho-C(sp2)–H activation might be still involved in this hindered alkene to the scandium center to give B 21 rather
α-C(sp3)–H alkylation reaction.
than direct Sc–C(sp2) insertion. Subsequent 2,1-insertion of
Finally, a plausible pathway for the formation of the C(sp3)– the styrenic substrate into the Sc–C(sp3) bond leads to the
H alkylation product was proposed based on the above obser- formation of five-membered azametallacyclic complex C,22
vations and a previous report, and is depicted in Scheme 4. followed by the ortho C(sp2)–H activation of another molecule
The reaction of the Sc complex 1 with [PhNMe2H][B(C6F5)4] of the amine substrate with the release of the alkylation
gives cationic Sc monoalkyl species which then reacts product. Alternatively, the reaction of C with the amine sub-
with N,N-dimethyl aniline to afford ortho-C(sp2)–H activation strate could also take place in N-methyl C(sp3)–H activation to
intermediate A.20 Complex A may undergo intramolecular give D with the release of the final alkylation product, which
proton migration upon the coordination of
a
sterically may give rise to a lower deuterium incorporation at the ortho
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Preparation of complex 1 24
A toluene solution (1.5 mL) of HL (334 mg, 1.0 mmol) was
added to solution of Sc(CH2SiMe3)3(THF)2 (448 mg,
a
1.0 mmol) in toluene (1.5 mL) at room temperature. The reac-
tion mixture was stirred for 8 h, and then the volatiles were
removed under vacuum. The resulting residue was washed
with n-hexane (2 mL*3) to finally give 1 as a pale yellow solid
(434 mg, 78%). Crystals suitable for X-ray single crystal struc-
ture analysis were grown at −30 °C from a mixture of toluene
and hexane (v/v
= 1 : 1). Elemental analysis: calcd for
C31H51N2ScSi2: C, 67.34; H, 9.30; N, 5.07; found: C, 67.77; H,
9.02; N, 4.91. 1H NMR (600 MHz, C6D6, 298 K): δ = 7.20 (m,
4H, o-Ph, m-Ph), 7.10 (m, 3H, m-PhDipp, p-PhDipp), 6.99 (m, 1H,
p-Ph), 5.00 (s, 1H, N=CCH), 3.15 (m, 2H, CHMe2), 1.72 (s, 3H,
NCMe), 1.56 (s, 3H, NCMe), 1.35 (d, 3JHH = 6.9 Hz, 6H, CHMe2),
3
1.15 (d, JHH = 6.9 Hz, 6H, CHMe2), 0.12 (overlapped, 4H,
CH2SiMe3), 0.11 (s, 18H, SiMe3). 13C{1H} NMR (151 MHz, C6D6,
298 K): δ = 168.2 (NvC), 165.4 (NvC), 145.7 (i-Ph), 142.8
(o-PhDipp), 140.7 (i-PhDipp), 130.2 (m-Ph), 127.3 (p-PhDipp), 126.1
(p-Ph), 125.1 (o-Ph), 124.6 (m-PhDipp), 98.7 (NvCCH), 45.0
(ScCH2Si), 28.7 (CHMe2), 25.4 (CHMe2), 24.2 (CHMe2), 23.4
(NCMe), 23.1 (NCMe), 3.4 (CH2SiMe3).
General procedure for the hydroaminoalkylation
Complex 1 (30 mg, 0.054 mmol) in 0.5 mL of toluene was
added to
0.054 mmol) in 0.5 mL of toluene. After stirring at room tem-
perature for h, amine 2 (0.81 mmol) and alkene
a solution of [PhNHMe2][B(C6F5)4] (43 mg,
1
3
(0.54 mmol) were added and the reaction mixture was heated
and maintained at 120 °C. After completion of the reaction
(monitored by H NMR), the mixture was cooled to room tem-
Scheme 4 Plausible pathway for the Sc-catalyzed hydroaminoalkyl-
ation reaction.
1
perature and the volatiles were removed under vacuum. The
residue was purified by silica gel column chromatography
(petroleum ether/ethyl acetate: 100/1, silica gel, a small
amount of Et3N if necessary) to afford the alkylation product.
position as expected in the above deuterium labeling
experiment.
Conclusions
In summary, we developed an efficient and highly regio-
selective catalytic hydroaminoalkylation of various sterically
hindered alkenes with tertiary N,N-dimethyl anilines by using
an easily accessible β-diketiminato ligand supported scandium
dialkyl complex in the presence of [PhNMe2H][B(C6F5)4]. The
experimental results revealed that an ortho-C(sp2)–H activation
of the aniline substrate was involved in this formal α-C(sp3)–H
alkylation reaction. Compared with that of a previous report,13
the substrate scope was expanded to 1,1- and 1,2-disubstituted
alkenes by simply modifying the ligand framework of the
scandium precatalyst, which may provide new insights into the
rare-earth catalyst design and development.23 Future work will
focus on mechanistic investigations and enantioselective
hydroaminoalkylation by rare-earth catalysis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from the National Natural Science
Foundation of China (Grant No. 21871204 and 21502132), the
1000-Youth Talents Plan, the Jiangsu Specially-Appointed
Professor Plan, PAPD, and the Project of Scientific and
Technologic Infrastructure of Suzhou (SZS201708).
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