Anti -Selective aminofluorination of alkenes with amidines mediated by hypervalent iodine(III) reagents

Article abstract of DOI:10.1039/c5ob01854d

anti-Selective aminofluorination of alkenes with amidines was enabled by hypervalent iodine(iii) reagents, affording 4-fluoroalkyl-2-imidazolines. Further reductive ring-opening of the 2-imidazoline moiety could deliver highly functionalized 3-fluoropropane-1,2-diamine derivatives.

Full text of DOI:10.1039/c5ob01854d

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ARTICLE TYPE
Anti-Selective Aminofluorination of Alkenes with Amidines Mediated by
Hypervalent Iodine(III) Reagents
Hui Chena Atsushi Kagaa and Shunsuke Chibaa*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
diamines.7
hypervalent
In this context, we have recently disclosed
iodine(III)ꢀmediated (transitionꢀmetal free)
antiꢀselective aminoacetoxylatio/formalꢀ
Anti-selective aminofluorination of alkenes with amidines
were enabled by hypervalent iodine(III) reagents, affording 4-
fluoroalkyl-2-imidazolines. Further reductive ring-opening of
the 2-imidazoline moiety could deliver highly functionalized
diastereoselective
aminohydroxylation, and diamination of alkenes with amidines
45 (Scheme 3).8 The process likely involves concerted alkeneꢀ
aziridination with a putative amidineꢀI(III) intermediate and
subsequent nucleophilic ringꢀopening of the aziridine moiety
caused by the corresponding counter Oꢀ or Nꢀions, which
therefore results in antiꢀselective aminoꢀfunctionalization of
50 alkenes.
10 3-fluoropropane-1,2-diamine derivatives.
Difunctionalization of alkenes is one of the most powerful
processes for chemical transformations in organic synthesis. A
variety of synthetic methods enabling stereoꢀ and chemoselective
difunctionalization of alkenes, including the Sharpless
15 dihydroxylation1 and aminohydroxylation2 as representative
examples, have been exploited to generate diverse molecular
complexity.3 A major concern in the methodology development
of hetero difunctionalization of alkenes such as aminoꢀ
functionalization is regioselectivity and stereoselectivity (antiꢀ or
20 synꢀ) (Scheme 1).
Scheme 2. Intramolecular aminoꢀdifunctionalization of alkenyl amides
for azaheterocycle synthesis.
Scheme 1. Regioꢀ and stereoselectivtiy in aminoꢀdifunctionalization of
alkenes.
Among
the
known
difunctionalization
reactions,
25 intramolecular aminoꢀdifunctionalization of alkenes4 provides a
convenient access to various nitrogenꢀcontaining heterocycles
(azaheterocycles), which were prevalent scaffolds in potent
pharmaceutical drugs.5,6 Within this arena, a mainstream strategy
is to use alkenyl amides/sulfonamides under various oxidative
30 reaction conditions with/without transition metal catalysis
(Scheme 2).
In comparison with these alkenyl amides, alkenyl amidines are
expected to exhibit somewhat different reactivity trends in the
oxidative aminoꢀdifunctionalization for synthesis of nitrogenꢀ
35 containing molecules, because (1) the electronꢀrich nature of
amidines might provide a unique and unprecedented mode of
reaction in alkene difunctionalization; (2) azaheterocyclic
products, cyclic amidines such as 2ꢀimidazolines, include 1,3ꢀ
diamino functionality, and further reductive ringꢀopening of
40 cyclic amidines enables construction of highly functionalized 1,2ꢀ
55
Scheme 3. Metalꢀfree diastereoselective aminoꢀdifinctionalization of
alkenes with amidines by hypervalent I(III) reagents.
To explore further potential of this hypervalent iodine(III)
reagentsꢀmediated transitionꢀmetal free strategy for alkene
60 aminoꢀfunctionalization with amidines,9 we wondered if the
transient aziridinium ions could be attacked by external fluoride
nucleophiles
for
aminofluorination
of
alkenes.10
Aminofluorination of alkenyl sulfonamides were recently
reported independently by Nevado10f and Meng/Li10i using
65 difluoroiodoarenes (ArIF2) and a PhI(OPiv)2ꢀHF•pyridine system,
respectively (Scheme 4ꢀa for Nevado’s work). However, both of
these reactions resulted in the formation of endoꢀselective
aminofluorination products, and substrates amenable to these
methods were limited to terminal alkenes. We describe herein
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DOI: 10.1039/C5OB01854D
antiꢀselective aminofluorination of Nꢀallylamidines by the
combined use of iodobenzene dicarboxylates and Et3N•3HF for
construction of diastereochemically pure 4ꢀfluoroalkylꢀ2ꢀ
afforded 2ꢀimidazolines 2c in 64% yield, demonstrating unique
chemoselectivity of the present aminofluorination of alkenes that
prefers internal alkenes. By varying substituents R2 on the Eꢀ
imidazolines divergently from internal Eꢀ and Zꢀalkenes (Scheme 40 alkene part, it was shown that 4ꢀchloroꢀ (for 1d) and 4ꢀ
5
4ꢀb).11 Furthermore, facile reductive ringꢀopening of the 4ꢀ
fluoroalkylꢀ2ꢀimidazoline moiety has also been executed to
synthesize highly functionalized 3ꢀfluoropropaneꢀ1,2ꢀdiamine
derivatives.12,13
methoxyphenyl (for 1e) groups were well tolerated, providing 2d
and 3e in good yields. Aminofluorination of the dienyl moiety
also worked efficiently to give 2ꢀimidazoline 2f in 62% yield,
while the reaction resulted in moderate diastereoselectivity (2:1).
45 The reaction of amidine 1g with a methyl group as R4 afforded 2ꢀ
imidazoline 2g bearing three successive stereogenic centers in
good diastereoselectivity. In this process, the N–C bond could be
constructed from the opposite side to the methyl group.
50 Table 1. Substrate scope on aminofluorination of disubstituted alkenes
using Et3N•3HFa,b
R2
H
R2
H
Me
Me
PhI(OCOR)2 (1.4 equiv)
Et3N•3HF (5 equiv)
NH
H (R3)
Me
Me
Me
F
R4
N
R =
Ar
N
R4
N
Ar
CH2Cl2, rt
R1
R1
1
2
10
Scheme 4. Aminofluorination mediated by hypervalent iodine(III)
reagents.
Cl
H
Ph
H
H
Ph
The initial study commenced with the reactions of Nꢀ
allylamidine 1a (Scheme 5). It is known that the reactions of
iodobenzene dicarboxylates with fluoride salts generate the
H
F
N
H
F
N
H
N
Ph
F
N
N
Ph
15 corresponding difluoroiodanes,14 which might be used for
aminofluorination with amidine 1a. As expected, the reaction of
amidine 1a with 1.3 equiv of PhI(OAc)2 and 5 equiv of Et3N•3HF
in CH2Cl2 delivered 4ꢀfluoromethylꢀ2ꢀimidazoline 2a in 59%
yield along with aminoacetoxylation product 3a in 18% yield
20 (Scheme 5). To improve the yield of 2a with reduction of the
amount of undesired aminocarboxylation product, more bulky
carboxylates on hypervalent iodine reagents were examined. Use
of iodobenzene dipivalate [PhI(OPiv)2] slightly improved the
yield of 2a to 69% yield, and the yield of the corresponding
25 aminopivalation product 4a was 13%. Use of 2ꢀisopropylꢀ2,3ꢀ
dimethylbutanoate counter ion could enhance the yield of 2a to
73% yield with 8% yield of carboxylation product 5a.
N
Ph
Ph
Ph
2b 70%
2c 64%
2d 71%
OMe
H
Ph
H
Ph
F
N
H
H
H
H
Me
N
F
Ph
F
F
N
N
N
N
Ph
Ph
Ph
Ph
F
2e 62% (dr = 7.1:1)
2f 62% (dr =2.0:1)
2g 69% (dr = 7.2:1)
a The reactions were carried out using 0.3 mmol of amidines 1. Unless
otherwise noted, the reactions provided diastereomerically pure products
55 2. b Aminocarboxylation compounds 5 were formed as a minor product in
8ꢀ21% yields. See the Electronic Supplementary Information (ESI) for
more details.
We next investigated the reactions of 3,3ꢀdisubstituted allyl
60 amidines 1h-k (Table 2). It was found that use of PhI(OPiv)2 was
optimal for aminofluorination of these triꢀsubstituted alkenes,
undergoing aminofluorination selectively to give 2ꢀimidazolines
2h-k in good yields.
Similarly with the previous
aminoacetoxylation and diamination,8 diastereoꢀdivergency could
65 be observed for the aminofluorination of amidine 1l with Eꢀ
alkene and 1m with Zꢀalkene, giving 2l and 2m, respectively, in
good yields.15
The process also enabled antiꢀselective
Scheme 5. Aminofluorination of 1a using Et3N•3HF salt. a Isolated as an
aminofluorination of 2,3ꢀdisubstituted allylamidines 1nꢀ1p as
well as aminofluorination of tetraꢀsubstituted alkene 1q
70 exclusively in exoꢀselective cyclization mode, giving the
corresponding 2ꢀimidazolines 2nꢀq as the sole products..
30
inseparable mixture with 4a.
With the reaction conditions using PhI[OCOC(iꢀPr)2Me]2 (1.4
equiv) and Et3N•3HF (5 equiv), we examined substrate scope for
aminofluorination of disubstituted Eꢀalkenes (where R3 = H)
(Table 1). As for R1 on the nitrogen, a benzyl (for 1b) group was
35 well tolerated and provided 2b in 70% yield. The reaction of
amidine 1c having an allyl functionality as R1 selectively
2
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Table 2. Substrate scope on aminofluorination of triꢀ and tetraꢀ
Table 3. Reductive ringꢀopening of 2
substituted alkenes using Et3N•3HFa
R2
R3 R2
R4
R4
PhI(OCOt-Bu)2 (1.3 equiv)
Et3N•3HF (2 equiv)
NH
R3
F
N
Ph
N
N
CH2Cl2, rt
Ph
Ph
Ph
1a
2
Ph
Me
Ph
Me
F
F
F
F
N
N
N
N
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
N
Ph
Ph
2k 95%
2h 83%
2i 73%
2j 87% (84%)b
Me Ph
Me
Ph
H
Ph
H
Me
H
F
F
F
N
N
N
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
2l 71%
(dr = >20:1)
2m 68%
2n 63%
Me
H
H
F
N
N
N
F
F
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
2p 90%c
2o 56%c
2q 95%
aUnless otherwise noted, the reactions were carried out using 0.3 mmol of
amidines 1 and afforded diastereomerically pure products 2. bThe yield of
1j in the reaction using 4 mmol of 1j. cThe reaction was carried out using
5
1.4 equiv of PhI[OCO(iꢀPr)2Me]2 and
5 equiv of Et3N•3HF salt.
Aminocarboxylation compounds 5o and 5p were formed as a minor
product in 3% and 7% yields, respectively. See the ESI for more details.
10
It has already been reported that reductive ringꢀopening of 2ꢀ
imidazoline derivatives could be mediated by AlH3 to afford 1,2ꢀ
diamines.7 However, treatment of 4ꢀfluoroalkylꢀ2ꢀimidazolines 2
with AlH3 gave a complex mixture of unidentified compounds
a15 equiv allylꢀBr was used.
15 instead of desired diamines. We thus sought an alternative milder
ringꢀopening method using 2j and found that a stepwise
procedure including 1) formation of amidinium salts with alkyl
halides (MeI, BnBr, and allylꢀBr); 2) NaBH4 reduction; 3) acidic
solvolysis gave the corresponding ringꢀopened diamines 6jꢀ8j in
20 good yields (Scheme 6). Using this 3ꢀsteps procedure with allyl
bromide, a variety of 3ꢀfluoropropaneꢀ1,2ꢀdiamines 8 were
synthesized from the corresponding 2ꢀimidazolines 2 as shown in
Table 3.
30 In summary, we have developed methods for diastereoselective
aminofluorination of Nꢀallylamidines that utilize hypervalent
iodine(III) reagents. The resulting 4ꢀfluoroalkylꢀ2ꢀimidazolines
could be concisely converted into various 3ꢀfluoropropaneꢀ1,2ꢀ
diamine derivatives. We anticipate that this strategy is capable of
35 supplying various fluoroꢀcontaining polyamine compounds useful
for medicinal, materials, and catalysis applications.
This work was supported by funding from Nanyang
Technological University (NTU) and Singapore Ministry of
Education (Academic Research Fund Tier 2: MOE2012ꢀT2ꢀ1ꢀ
40 014).
F
X–
F
NaBH4 (5 equiv)
MeOH, rt
R-X (10 equiv)
CHCl3, reflux
N
R
N
N
Ph
Notes and references
Ph
N
Ph
Ph
2j
a Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore
637371, Singapore. Fax: +65-67911961; E-mail: shunsuke@ntu.edu.sg
45
4M HCl in dioxane
F
Ph
(5 equiv)
R
† Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization data of products, and copies of 1H and 13C
NMR spectra. See DOI: 10.1039/b000000x/
N
F
N
H
MeOH, rt
HN
R
N
Ph
Ph
6j (R = Me) 74% from MeI
7j (R = Bn) 71% from BnBra
8j (R = allyl) 73% from allyl-Br
50
1
For leading reviews, see: (a) H. C. Kolb, M. S. VanNieuwenhze and
K. B. Sharpless, Chem. Rev. 1994, 94, 2483. (b) A. B. Zaitsev and H.
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25 Scheme 6. Reductive ringꢀopening of 2j. a4 equiv of BnBr was used.
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