Rhodium-Catalyzed N-tert-Butoxycarbonyl (Boc) Amination by Directed C H Bond Activation

Article abstract of DOI:10.1002/adsc.201600410

N-tert-Butoxycarbonyl azide (BocN₃) was shown to be an efficient and economic source for the directed introduction of N-Boc protected amino groups into the thiophene and benzene nucleus. Yields for the amination of 2-pyridin-2-ylthiophenes (10 examples) were 52–88%. For the amination of the respective benzenes (10 examples) yields between 54% and 99% were recorded with an improved reactivity observed for substrates that bear an electron-withdrawing group. The reaction was applied to short total syntheses of the indoloquinoline alkaloids quindoline and cryptolepine. The facile removal of the Boc protecting group was the key to the success of the syntheses. The scope of the reaction was extended to a C(sp³) H bond amination and to the amination of 2-phenyloxazoline. For the amination of 2-pyridin-2-ylbenzene a kinetic deuterium isotope effect of 2.0 was determined. (Figure presented.) .

Full text of DOI:10.1002/adsc.201600410

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DOI: 10.1002/adsc.201600410
Rhodium-Catalyzed N-tert-Butoxycarbonyl (Boc) Amination by
ꢀ
Directed C H Bond Activation
Julian Wippich,a Nadina Truchan,a and Thorsten Bacha,*
a
Department Chemie and Catalysis Research Center (CRC), Technische Universitꢀt Mꢁnchen, 85747 Garching, Germany
Fax : (+49)-89-289-13315; phone: (+49)-89-289-13330; e-mail: thorsten.bach@ch.tum.de
Received: April 18, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600410.
Abstract: N-tert-Butoxycarbonyl azide (BocN3) was
shown to be an efficient and economic source for
the directed introduction of N-Boc protected amino
groups into the thiophene and benzene nucleus.
Yields for the amination of 2-pyridin-2-ylthiophenes
(10 examples) were 52–88%. For the amination of
the respective benzenes (10 examples) yields be-
tween 54% and 99% were recorded with an im-
proved reactivity observed for substrates that bear
an electron-withdrawing group. The reaction was
applied to short total syntheses of the indoloquino-
line alkaloids quindoline and cryptolepine. The
facile removal of the Boc protecting group was the
spective azides as the nitrogen source.[4] These reac-
tions have been typically performed with amino pre-
cursors bearing protecting groups, the further manipu-
lation of which is inferior to the Boc group.
Some time ago, we described tert-butoxycarbonyl
azide (BocN3) as a favorable reagent for the iron-cat-
alyzed imination of sulfides and sulfoxides.[5] Based
on this experience we wondered whether the reagent
would be equally suitable for the introduction of
a Boc-protected amino group into arenes and alkanes
ꢀ
by C H activation chemistry thus avoiding the pro-
duction of undesired by-products generated by other
reagents.[3] In this manuscript we describe our prelimi-
nary work, which led to the discovery of a broadly ap-
plicable amination reaction employing a pyridinyl-
type directing group and a rhodium catalyst to ach-
key to the success of the syntheses. The scope of
3
ꢀ
the reaction was extended to a C(sp ) H bond ami-
nation and to the amination of 2-phenyloxazoline.
For the amination of 2-pyridin-2-ylbenzene a kinetic
deuterium isotope effect of 2.0 was determined.
ꢀ
ieve the C H activation step. To illustrate the superi-
or properties of the Boc group for preparative pur-
poses, the protocol was applied to the total synthesis
of the Cryptolepis alkaloids quindoline and cryptole-
pine.[6]
ꢀ
Keywords: amination; C H activation; heterocy-
cles; homogeneous catalysis; rhodium
From recent work on the oxidative cross-coupling
of thiophenes,[7] we had a variety of pyridin-2-yl sub-
stituted thiophenes at hand, which was the reason
why we started optimization reactions (Table 1; for
Over many decades, the tert-butoxycarbonyl (Boc) a complete set of the reactions, see the Supporting In-
group has established itself as the most frequently formation) with 2-pyridin-2-ylthiophene (1a). We
used protecting group for amines and anilines.[1] Its were pleased to note that unlike related ruthenium
lipophilicity, its stability under a broad variety of con- (entry 1) and iridium (entry 3) complexes, the rhodi-
ditions and its facile removal have made it extremely um complex (Cp*RhCl2)2 (Cp*=pentamethylcyclo-
popular among synthetic chemists. There are literally pentadienyl)[8] was a competent catalyst for the de-
hundreds of total syntheses, in which the Boc group sired amination (entry 2). After a reaction time of
has served to protect a nitrogen atom through 14 h, conversion was incomplete and attempts to in-
a number of steps. While the protecting group is nor- crease the conversion by increasing the temperature
mally attached to a preexisting amine or aniline, (entry 4) were unsuccessful. A change of solvent from
ꢀ
recent advances in C H activation chemistry enable DCE to either a less polar (entry 5) or a more polar
the introduction of an appropriately protected amino (entries 6, 7) medium turned out not to be beneficial.
unit directly into an arene or alkane scaffold.[2,3] The addition of alkali salts (one equiv.) had a positive
ꢀ
Recent work on aromatic C H amination has re- effect on the conversion with K3PO4 being superior to
vealed that it is possible to introduce protected amino other potassium salts (entries 8–10). While an increase
groups by transition metal catalysis employing the re- of the BocN3 equivalents had no effect on the conver-
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Table 1. Selected optimization conditions for the amination of 2-pyridin-2-ylthiophene (1a) with BocN3.
Entry
BocN3 [equiv.]
Catalyst[a]
Additive
Temperature [8C]
Solvent
Conversion[b] [%]
Yield[b] [%]
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3.0
1.5
(pCyRuCl2)2
(Cp*RhCl2)2
(Cp*IrCl2)2
–
–
–
–
–
–
–
80
80
80
100
80
80
80
80
80
80
80
80
DCE[c]
DCE
DCE
11
59
11
51
5
15
12
60
59
75
70
81
11
39
6
41
5
15
11
44
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
DCE
PhMe
DMA[d]
t-AmOH
DCE
DCE
DCE
K2CO3
KOAc
K3PO4
K3PO4
K3PO4
9[e]
10
11
12[e]
40
65
DCE
DCE
63
71 (81)[f]
[a]
Reaction conditions: 1a (1.0 equiv., c=0.1M), t=14 h.
Conversion and yield as determined by NMR analysis using 1,3,5-trimethoxybenzene as internal standard.
1,2-Dichloroethane.
N,N-Dimethylacetamide.
0.5 equivalents of the additive were used.
Yield of isolated product in brackets.
[b]
[c]
[d]
[e]
[f]
sion (entry 11), it was shown that the reaction could nificant functional group tolerance for various thio-
be promoted even with 0.5 equiv. K3PO4. In a prepara- phene substituents (alkoxycarbonyl, alkoxy, chloro,
tive run on a larger scale, the conversion was further acyl, bromo). BocN3, which is known to be a potential
improved and product 2a could be isolated in 81% explosive, was employed in crude form without prior
yield.
Without any further modification, related thio-
distillation.[9,10]
Under identical conditions, 3-pyridin-2-ylthio-
phenes were subjected to the amination conditions phenes were subjected to the amination reaction. If
ꢀ
(Scheme 1). Yields were good to moderate with a sig- the 2-position was available for C H activation
a facile amination was observed, which delivered the
Boc-protected 2-aminothiophenes 3a and 3b in high
yields (Figure 1). If the 2-position was blocked amina-
tion was directed to position C-4 and amination prod-
uct 3c was obtained in 55% yield.
When turning to benzenes as potential substrates
we were pleased to note that they undergo the amina-
tion reaction even more readily than thiophenes. The
reaction was highly chemoselective and side reactions
were not notable. In all cases shown in Scheme 2,
yields exceeded 75% if based on conversion. If lower
yields of isolated product were recorded it was be-
Scheme 1. Rhodium-catalyzed amination of various substi-
tuted 2-pyridin-2-ylthiophenes 1 with BocN3.
Figure 1. Products 3 obtained from the rhodium-catalyzed
amination of 3-pyridin-2-ylthiophenes.
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ꢀ
planar position relative to the adjacent C H bond,
the conversion is lower. If the C H bond is locked in
ꢀ
the required planar arrangement (e.g., 4i!5i) the
yield is high. (ii) Electron-withdrawing groups facili-
tate the amination reaction while electron-donating
groups retard it. This is in line with the observation
that substituted benzenes react more readily than
comparable thiophenes (e.g., 1c!2c vs. 4e!5e or
1f!2f vs. 4g!5g).
The fact that our protocol allows the introduction
of an amino group in the phenyl ortho-position of 2-
phenylquinolines invited an application to the synthe-
sis of indoloquinoline alkaloids (Scheme 3). Com-
pound 5j was readily converted via amine 6 to azide
7. Despite an encouraging literature precedent[11] it
was, however, not possible to transform azide 7 suc-
cessfully into an indoloquinoline.
As an alternative route, readily available 3-fluoro-
2-phenylquinoline (4k)[12] was aminated to yield the
N-Boc protected product 5k in 90% yield based on
conversion. This compound underwent a facile ring
closure to quindoline (8) when heated in pyridinium
hydrochloride.[13] Quindoline in turn was successfully
converted into cryptolepine (9) following a reported
procedure.[14]
For an extension of the directed N-Boc amination
method, other substrates were explored. Notably, the
method could be successfully applied to the function-
Scheme 2. Rhodium-catalyzed amination of various substi-
tuted pyridin-2-ylbenzenes 4 with BocN3.
3
ꢀ
alization of a C(sp ) H bond (Scheme 4). When 8-
cause the reaction was not complete after 14 h. In the methylquinoline (10) was treated with BocN3 under
reaction of ortho-substituted pyridin-2-ylbenzenes 4b the conditions previously established for C(sp ) H
and 4c, for example, the substrates were recovered in amination a clean conversion into the respective N-
2
ꢀ
yields of 31% and 20%.
Boc protected aminomethyl derivative 11 was ob-
Generally, there are two reactivity trends to be served. The product was obtained in 97% yield based
noted: (i) If the pyridine ring cannot adopt a synperi- on conversion. Moreover, it could be established that
Scheme 3. Synthesis of a potential precursor 7 to indoloquinoline alkaloids and total synthesis of quindoline (8) and cryptole-
pine (9).
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Figure 2. Putative intermediates 14 (L=ligand), 15, and 16
in the rhodium-catalyzed amination reaction 4a!5a.
other azides[4h,15] – by dediazotization of intermediate
15 and formation of an intermediate Rh(V) nitrene
ꢀ
complex 16 which undergoes insertion into the Rh C
bond. The catalytic cycle is closed by protonation of
this species by another substrate molecule, which
leads to formation of compound 14 and liberation of
the product.
Scheme 4. Introduction of an N-Boc protected amino group
3
ꢀ
into a C(sp ) H bond (10!11) and into the ortho-position
of 2-phenyloxazoline (12) by rhodium-catalyzed amination.
In summary, a new method for the directed amina-
ꢀ
other directing groups facilitate the amination reac- tion of aromatic and aliphatic C H bonds has been
tion in the same fashion as the pyridin-2-yl group. 2- developed. The readily available reagent BocN3 was
Phenyloxazoline (12) was transformed into the de- employed as the nitrogen source and allows for the
sired amination product 13. No further optimization immediate introduction of an N-Boc protected amino
ꢀ
was required to enable the desired C N bond forma- group. In contrast to other methods for the introduc-
tion, only the reaction temperature was increased to tion of N-alkoxycarbonyl protected amino groups[3]
908C.
the only by-product formed in this transformation is
Preliminary mechanistic work was performed with nitrogen. The reaction sets the stage for a versatile
deuterated compound 4a-d5, which could be readily amination protocol as manifested by the successful
prepared from commercially available bromoben- total synthesis of two indoloquinoline alkaloids.
zene-d5 (for the synthesis, see the Supporting Infor- Mechanistic studies suggest the involvement of the
ꢀ
mation). When subjected to the reaction conditions of C H activation in the rate-determining step but fur-
the amination reaction, the respective deuterated ana- ther experiments are required to provide a more de-
logue of product 5a, compound 5a-d4, was observed. tailed understanding of the reaction. In addition, an
After appropriate calibration, initial rates could be extension of the method and additional synthetic ap-
determined for this reaction and for the analogous re- plications are currently being studied in our laborato-
action of the undeuterated substrate 4a!5a. The ki- ry.
netic isotope effect (KIE) from these measurements
was determined as kH/kD =2.0 (Scheme 5). Intermo-
lecular competition experiments in the same reaction
vessel were found to be not suitable for KIE determi-
Experimental Section
nation because an H/D scrambling was observed
under the reaction conditions.
ꢀ
General Procedure for Rhodium-Catalyzed C H
Amination with BocN3
The preliminary data seem consistent with a rate-
A 30-mL Schlenk vial with a screw cap was charged with
thiophene 1a (64.5 mg, 400 mmol, 1.0 equiv.) and K3PO4
(42.5 mg, 200 mmol, 0.5 equiv.) in air. The screw cap was
tightly closed and the tube was evacuated and refilled with
argon three times. Under a positive pressure of argon con-
secutively [Cp*RhCl2]2 (12.4 mg, 20.0 mmol, 5 mol%),
AgSbF6 (27.5 mg, 80.0 mmol, 20 mol%) and a solution of
BocN3 (85.9 mg, 600 mmol, 1.5 equiv.) in 1,2-dichloroethane
(DCE, 4 mL) were added. The resulting suspension was
stirred at 808C for 14 h, cooled to room temperature and di-
luted with CH2Cl2 (10 mL). The suspension was filtered
through a plug of Celiteꢃ, the plug was washed with addi-
tional CH2Cl2 (3ꢄ20 mL). The filtrate was concentrated
under vacuum, loaded onto silica (1.5 g) and subjected to
flash chromatography (pentane/Et2O 100:3!100:5). The de-
sired product 2a was obtained as a colorless solid; yield:
89.7 mg (325 mmol, 81%); TLC: Rf =0.72 (pentane/Et2O
4:1) [UV]; mp 89–908C; IR (ATR): n=3221 (br), 2977 (w),
ꢀ
limiting C H activation step which leads to a primary
species 14 (Figure 2), to which BocN3 adds in ex-
change for a weak ligand (L). The further course of
the reaction can be interpreted – as suggested for
Scheme 5. Rhodium-catalyzed amination of deuterated sub-
strate 4a-d5 that was employed to determine the kinetic iso-
tope effect relative to 4a in parallel experiments.
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1719 (m), 1578 (s), 1475 (m), 781 cmꢀ1 (m); 1H NMR
(500 MHz, CDCl3, 300 K): d=1.55 (s, 9H), 7.08 (ddd, J=
Shin, H. Kim, S. Chang, Acc. Chem. Res. 2015, 48,
1040–1052; h) M. A. Ali, X. Yao, H. Sun, H. Lu, Org.
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17200–17204.
3
4
3
7.5, 5.0 Hz, J=1.1 Hz, 1H), 7.23 (d, J=5.5 Hz, 1H), 7.44
3
4
5
3
(virt. dt, J=8.1 Hz, J ꢁ J=1.1 Hz, 1H) 7.66 (virt. td, J ꢁ
4
3
7.8 Hz, J=1.8 Hz, 1H), 7.94 (s, 1H), 8.55 (ddd, J=5.0 Hz,
4J=1.8 Hz, 5J=1.0 Hz, 1H), 11.24 (s, 1H, NH); 13C NMR
(126 MHz, CDCl3, 300 K): d=28.6 (q), 80.3 (s), 117.8 (s),
120.2 (d), 120.3 (d), 123.1 (d), 124.7 (d), 137.1 (d), 139.3 (s),
148.1 (d), 153.1 (s), 154.4 (s); HR-MS (ESI): m/z=277.1003,
calcd. for C14H16N2O2S [M+H]+: 277.1005.
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Acknowledgements
This project was supported by the Deutsche Forschungsge-
meinschaft (Ba 1372-19/1), by the Elitenetzwerk Bayern
(scholarship to J.W.), and by the TUM Graduate School. Dr.
S. Breitenlechner is acknowledged for his help with the KIE
measurements.
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6 Rhodium-Catalyzed N-tert-Butoxycarbonyl (Boc)
ꢀ
Amination by Directed C H Bond Activation
Adv. Synth. Catal. 2016, 358, 1 – 6
Julian Wippich, Nadina Truchan, Thorsten Bach*
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