Hydroaminomethylation of Olefins with Aminomethyltrifluoroborate by Photoredox Catalysis

Article abstract of DOI:10.1002/adsc.201400556

A photocatalytic hydroaminomethylation of olefins with N-protected aminomethyltrifluoroborates has been developed. This methodology provides a new strategy for the introduction of a primary aminomethyl group onto electron-deficient C=C bonds. This reaction constitutes a facile entry into synthetically useful γ-aminobutyric acid (GABA) derivatives such as baclofen.

Full text of DOI:10.1002/adsc.201400556

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DOI: 10.1002/adsc.201400556
Hydroaminomethylation of Olefins with Aminomethyltrifluoro-
borate by Photoredox Catalysis
ACHTUNGTRENNUNG
Kazuki Miyazawa,a Takashi Koike,a,* and Munetaka Akitaa,*
a
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
Fax : (+81)-45-924-5230; phone: (+81)-45-924-5230; e-mail: koike.t.ad@m.titech.ac.jp or makita@res.titech.ac.jp
Received: June 7, 2014; Revised: July 11, 2014; Published online: September 3, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400556.
Abstract: A photocatalytic hydroaminomethylation
of olefins with N-protected aminomethyltrifluoro-
borates has been developed. This methodology pro-
vides a new strategy for the introduction of a pri-
mary aminomethyl group onto electron-deficient
C=C bonds. This reaction constitutes a facile entry
into synthetically useful g-aminobutyric acid
(GABA) derivatives such as baclofen.
erful tool for oxidative transformation of electron-
rich amines such as tertiary amines and enamines
under visible light irradiation.[2–6] Reactive chemical
species, such as radicals, iminium ions, and azome-
thine ylides, can be generated through photoredox-
catalyzed single electron transfer (SET) processes and
result in diverse nitrogen-containing products
(Scheme 2, a). While previous reports mainly describe
the synthesis of tertiary or secondary amines, an
access to useful primary amines has not been well
documented.[7]
Keywords: amino acids; aminomethylation; elec-
tron transfer; photocatalysis; photochemistry
Recently, our group reported that organoborates
can serve as an electron donor and an organic radical
precursor by the action of Ir photoredox catalysts,
[Ir(dF
G
E
U
(1a)
[Ir(dF-
Aminoalkyl structures are present in many biological-
G
E
U
ly active compounds (Scheme 1).[1] In particular, pri- fluoromethyl)-2-(2,4-difluorophenyl)pyridine, bpy=
mary aminoalkyl motifs are frequently found in 2,2’-bipyridine, dtbbpy=4,4’-di-tert-butyl-2,2’-bipyri-
amino acids. Therefore, a facile and selective con- dine],[8] of which the photoexcited species have redox
struction of the primary aminoalkyl component is de- potentials sufficient to oxidize organoborates.[2e,9] N-
sirable to access bioactive amino acid derivatives and Protected aminomethyltrifluoroborates have been de-
pharmaceuticals bearing aminoalkyl moieties.
veloped and are known as easy-to.-handle organome-
For the past few years, photoredox catalysis with tallic reagents.[10] Thus, we envisioned that photoredox
ruthenium polypyridine derivatives and the relevant catalysis could induce aminomethylation through gen-
iridium cyclometalated complexes has become a pow- eration of aminomethyl radicals from the correspond-
ing N-protected aminomethyltrifluoroborates, leading
to the primary aminoalkylated product after depro-
tection (Scheme 2, b). Herein, we report on hydroa-
minomethylation of electron-deficient olefins with
Boc-protected aminomethyltrifluoroborate (Boc=
tert-butoxycarbonyl) by visible light-driven photore-
dox catalysis. Furthermore, we extended this photoca-
talytic aminomethylation to synthesis of a bioactive g-
aminobutyric acid (GABA), baclofen motif.[1e,11] This
is a new strategy for access to molecules bearing the
primary aminomethyl moiety.
We initially examined the photocatalytic reaction of
Boc-protected aminomethyltrifluoroborate (2a) with
methyl acrylate (3a) in the presence of the Ir photore-
dox catalyst 1a (2 mol%) in the mixed solvent system
Scheme 1. Examples for bioactive molecules containing the
primary aminoalkyl moiety.
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Scheme 2. Photoredox-catalyzed synthesis of nitrogen-containing molecules.
[D6]acetone/CD3OD (1/1) under visible light irradia- and 2b are even smaller than the redox potential of
tion (blue LEDs: lmax =425 nm) for 1 h. As a result, the photoexcited 1a (E1/2 = +0.91 V vs. [Cp2Fe] in
the corresponding aminomethylated product 4aa was MeCN),[9] indicating that they can be easily oxidized
obtained in 92% NMR yield (entry 1 in Table 1). by the photoexcited 1a, compared to 2c and 2d. In ad-
Every entry accompanied by alteration of solvent re- dition, under the above-mentioned reaction condi-
sulted in formation of the desired product 4aa (en- tions, 2c and 2d were not completely soluble. These
tries 2–6), but the mixture of [D6]acetone and factors might cause a significant decline in efficiency
CD3OD turned out to be the best. It should be noted of the reactions of 2c and 2d. Thus, potassium Boc-
that the typical photoredox catalyst [Ru
ACHTUNGTREN(NUG bpy)3]ACHTUGNTERN(NUGN PF6)2 protected aminomethyltrifluoroborate 2a proved to
(1c) and fac-[Ir(ppy)3] (1d) (ppy=2-phenylpyridine), be the best aminomethylborate reagent in terms of
ACHTUNGTRENNUNG
did not enable the present reaction at all (entries 7 yield and efficiency.
and 8).[12] Notably, the product 4aa was not obtained
The scope of the present photocatalytic aminome-
either in the dark or in the absence of the photocata- thylation of olefins with Boc-protected aminomethyl-
lyst (entries 9 and 10), strongly supporting that the trifluoroborate 2a is summarized in Table 2. Typical
photoexcited species of the photoredox catalyst plays electron-deficient alkenes such as, methyl acrylate
a key role in the reaction.
(3a), methyl methacrylate (3b), methyl vinyl ketone
Next, protective groups on the nitrogen atom were (3c), phenyl vinyl sulfone (3d), and acrylonitrile (3e)
investigated as shown in Scheme 3. The reaction of produced the corresponding aminomethylated prod-
aminomethyltrifluoroborate protected by para-toly- ucts 4aa–4ae in good yields (54–85% isolated yields,
loxycarbonyl group (2b) with methyl acrylate (3a) 4–6 h). The reaction of N,N-dimethyl acrylamide (3f)
under the above-mentioned optimal conditions af- gave the product 4af in a lower yield (31%). Cyclic
forded the corresponding aminomethylated product olefins, 2-cyclopentenone (3g) and 2,5-dihydro-2-fura-
4ba in 87% NMR yield, but this reaction required none (3h), reacted slowly, but the products 4ag and
a longer time (3 h) than the reaction of 2a (1 h). Yield 4ah were obtained in moderate yields (4ag: 54%,
and efficiency of the reaction of the Ts-protected ami- 10 h, 4ah: 33%, 24 h) probably due to the steric hin-
nomethylborate (2c) (Ts= para-toluenesulfonyl) were drance on the reaction site.
significantly deteriorated (4ca: 58% NMR yield,
To expand this scope, alkenes bearing two electron-
24 h). Furthermore, the reaction of potassium phthali- withdrawing groups were examined. Ethylidenema-
midomethyltrifluoroborate (2d) gave only a trace lonic acid diethyl ester (3i) readily reacted with 2a
amount of the coupling product (4da). Cyclic voltam- (4 h) to give the aminomethylated product 4ai in 85%
mograms for N-protected aminomethylborates (2a–d) isolated yield. Furthermore, to our delight, the alkene
exhibited broad irreversible oxidation waves at bearing a more bulky substituent at the reaction site,
+0.74 V, +0.76 V, +0.98 V, and +0.90 V (vs. [Cp2Fe] benzylidenemalonic acid diethyl ester (3j), was also
in MeCN), respectively. Oxidation potentials of 2a a suitable substrate for the present photocatalytic
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Hydroaminomethylation of Olefins with Aminomethyltrifluoroborate
Table 1. Optimization of the photocatalytic aminomethylation of methyl acrylate (3a) with potassium Boc-protected amino-
methyltrifluoroborate (2a).[a]
Entry
Photocatalyst
Solvent
Yield of 4aa [%][b]
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1a
1c
1d
1a
–
[D6]acetone/CD3OD
[D6]acetone/D2O
[D6]DMSO/CD3OD
CD3CN/CD3OD
[D6]acetone
92
43
59
42
55
72
0
0
0
0
CD3OD
[D6]acetone/CD3OD
[D6]acetone/CD3OD
[D6]acetone/CD3OD
[D6]acetone/CD3OD
8
9[c]
10
[a]
For detailed reaction conditions, see the Experimental Section and the Supporting Information.
Yields were determined by H NMR spectroscopy using Si(Et)4 as an internal standard.
In the dark. LED=light-emitting diode.
[b]
[c]
1
Scheme 3. Examination of protective groups on the nitrogen atom in aminomethyltrifluoroborates.
transformation. The reaction required longer reaction yields (60–88%, 24 h). In contrast, the reaction of sty-
time (48 h) and higher catalyst loading (5 mol%), but rene (3o) resulted in a low yield (5%, 24 h) because
afforded the product 4aj in an excellent yield (91% a side reaction of 3o itself significantly proceeded.[13]
isolated yield). As further extension of the olefin sub- These results show that electron-deficient olefins with
strate, enamides and aromatic alkenes were also ex- various functional groups, such as ester, ketone, sul-
amined. The reaction of 2-acetylaminoacrylic acid fone, nitrile, amide, pyridyl, and perfluoroaryl groups
methyl ester (3k) smoothly proceeded to give the cor- can be applied to the present photocatalytic amino-
À
responding C C coupled product 4ak in 68% yield. 2- methylation.
Vinylpyridine (3l), 4-vinylpyridine (3m) and 2,3,4,5,6-
Encouraged by the aminomethylation of 3j, we ex-
pentafluorostyrene (3n) afforded the corresponding tended this transformation to the reaction of a benzyli-
aminomethylated products (4al, 4am, 4an) in high denemalonic acid ester derivative (3p) (Scheme 4).
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Table 2. Scope of the photocatalytic aminomethylation.[a,b]
[a]
[b]
[c]
For detailed reaction conditions, see the Experimental Section and the Supporting Information.
Yields of isolated products.
1a (5 mol%) was used.
The aminomethylation of 3p gave the corresponding which is well-known muscle relaxer and antispastic
coupling product 4ap in 89% yield. Subsequent acid- agent. This result suggests that we can easily access g-
mediated decarboxylation and deprotection produced aminobutyric acid (GABA) derivatives by the consec-
racemic baclofen hydrochloric acid salt (98% yield), utive processes consisting of photocatalytic aminome-
thylation and deprotection under acidic conditions. It
is noteworthy that this sequence is regarded as a syn-
thetic equivalent of primary aminomethylation.
Finally, the present photocatalytic system can har-
ness ambient sunlight as the light source instead of
blue LED lamps. Sunlight induced the reaction in
a manner similar to irradiation with blue LED lamps
[Eq. (1)].
On the basis of the experimental results, a plausible
reaction mechanism is shown in Scheme 5. First, irra-
diation of visible light excites the Ir catalyst 1a (IrIII)
to *IrIII. Luminescence quenching experiments sup-
port the occurrence of an SET process between the
resultant *IrIII and aminomethylborate 2a (see the
Supporting Information). The first SET event causes
1e-oxidation of 2a to generate aminomethyl radical
2a’ together with the highly reduced IrII species. Radi-
cal addition of 2a’ to olefin 3 provides radical inter-
Scheme 4. Application to the synthesis of the baclofen
motif.
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Hydroaminomethylation of Olefins with Aminomethyltrifluoroborate
Scheme 5. A plausible reaction mechanism.
and [Ir(dFACHTUNGTNERN(NGU CF3)ppy)2AHCTUNRTEGNGUN(N bpy)]ACTHNUGTREN(UNNG PF6) (1a) (8.0 mmol) were added
mediate 4’, which is reduced via the second SET
event to be converted to carbanioinic intermediate 4À.
Subsequent protonation from solvent, MeOH, produ-
ces the aminomethylated product 4.
In conclusion, we have developed the photocatalyt-
ic aminomethylation of olefins with Boc-protected
aminomethyltrifluoroborates. This reaction is regard-
ed as a formal net primary aminomethylation when
combined with the deprotection. The present protocol
provides us with a new strategy for the synthesis of
biologically active g-aminobutyric acid (GABA) de-
rivatives. Further development of photocatalytic
methods towards synthetically valuable compounds is
a continuing effort in our group.
under N2. The mixture was degassed by three freeze-pump-
thaw cycles. The tube was placed at a distance of 2–3 cm
from blue LED lamp (hn=425Æ15 nm). The resulting mix-
ture was stirred at room temperature (water bath) under
visible light irradiation. Then, the reaction mixture was con-
centrated under vacuum. Products were extracted with
CH2Cl2, dried (Na2SO4) and filtered. The filtrate was con-
centrated under vacuum and the residue was purified by
column chromatography on aluminium oxide (treated with
7.0 wt% water) (hexane/EtOAc=10:1).
Procedure for Photocatalytic Reaction with Sunlight
A cylindrical vessel (diameter: 4.5 cm, height: 1.5 cm) was
used for the photoreaction under sunlight (see the Support-
ing Information). To an acetone (2.0 mL)-MeOH (2.0 mL)
solution of methyl acrylate (3a) (0.40 mmol) in the vessel,
potassium N-Boc-protected aminomethyltrifluoroborate
Experimental Section
(2a) (0.48 mmol) and [Ir(dFACTHUNGTNER(NGUN CF3)ppy)2ACHTUNTRGEG(NNUN bpy)]ACHTNUGTERN(NUGN PF6) (1a)
(4.0 mmol) were added under N2. The mixture was degassed
by three freeze-pump-thaw cycles. The reaction mixture was
exposed to sunlight for 4 h below 208C (March 6th, 2014).
Then, the reaction mixture was concentrated under vacuum.
Products were extracted with CH2Cl2, and combined organic
layers were dried (Na2SO4), and filtered. The filtrate was
evaporated and the residue was purified by flash column
chromatography on aluminum oxide (treated with 7.0 wt%
water) to afford 4aa; yield: 70%.
Typical NMR Experimental Procedure
To methyl acrylate (3a) (50.0 mmol) dissolved in an appro-
priate solvent (0.50 mL) in an NMR tube, potassium N-Boc-
protected aminomethyltrifluoroborate (2a) (60.0 mmol),
photocatalyst (1.0 mmol) and tetraethylsilane (an internal
standard) were added under N2. The mixture was degassed
by three freeze-pump-thaw cycles. The reaction was carried
out at room temperature (water bath) under irradiation of
visible light (placed at a distance of 2–3 cm from blue LED
lamp: hn=425Æ15 nm).
Application to the Synthesis of Baclofen
To an acetone (2.0 mL)-MeOH (2.0 mL) solution of diethyl
(4-chlorobenzylidene)malonate (3p) (0.40 mmol) in a 20-mL
Schlenk tube, potassium N-Boc-protected aminomethyltri-
General Procedure for the Photocatalytic Reaction of
Aminomethyltrifluoroborates with Olefins
To an acetone (2.0 mL)-MeOH (2.0 mL) solution of olefin
(3X) (0.40 mmol) in a 20-mL Schlenk tube, potassium N-
Boc-protected aminomethyltrifluoroborate (2a) (0.48 mmol)
fluoroborate (2a) (0.48 mmol) and [Ir(dFACHTNGUTRENNU(G CF3)ppy)2ACHTUNGTRENNUNG(bpy)]
(PF6) (1a) (20 mmol) were added under N2. The mixture was
degassed by freeze-pump-thaw cycles. The tube was placed
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at a distance of 2–3 cm from blue LED lamp (hn=425Æ
15 nm). The resulting mixture was stirred at room tempera-
ture (water bath) under visible light irradiation for 48 h.
Then, the reaction mixture was concentrated under vacuum.
The reaction mixture was extracted with CH2Cl2, dried
(Na2SO4) and filtered. The filtrate was concentrated under
vacuum and the residue was purified by column chromatog-
raphy on aluminium oxide (treated with 7.0 wt% water)
(hexane/EtOAc=10:1) to afford 4ap; yield: 89%.
The aminomethylated product 4ap and 6N aqueous HCl
solution were mixed in a 20-mL Schlenk tube. The mixture
was refluxed at 1008C for 24 h. Then, the reaction mixture
was filtered, and the volatiles were removed under reduced
pressure to give baclofen hydrochloride as a white solid;
yield: 98%.
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Acknowledgements
This work was supported by a Grant-in-Aid from the Minis-
try of Education, Culture, Sports, Science of the Japanese
Government (No 26288045).
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Adv. Synth. Catal. 2014, 356, 2749 – 2755
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