Regioselective Radical Alkene Amination Strategies by Using Phosphite-Mediated Deoxygenation

Article abstract of DOI:10.1055/s-0037-1611911

Nitrogen-containing compounds are an essential motif in all disciplines of chemistry and the efficient synthesis of these frameworks is a highly sought-after goal. Presented here is a summary of recent efforts conducted by our group to develop radical-mediated amination strategies for the formal synthesis of primary amines from alkenes with exclusive anti-Markovnikov regioselectivity. We have found that N-hydroxyphthalimide is an effective reagent capable of supplying both the N and H atoms for alkene hydroamination in a group transfer radical addition-type mechanism. Furthermore, allyl-oxyphthalimide derivatives are similarly capable of radical group transfers and allow for the aminoallylation of an external alkene.

Full text of DOI:10.1055/s-0037-1611911

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S. W. Lardy, V. A. Schmidt
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Regioselective Radical Alkene Amination Strategies by Using
Phosphite-Mediated Deoxygenation
Samuel W. Lardy
O
G
P(OR)3
G
[N]
Valerie A. Schmidt*
[N]
+
R1
R1
Department of Chemistry and Biochemistry, University of Cali-
fornia, San Diego, La Jolla, California 92093, USA
vschmidt@ucsd.edu
OP(OR)3
40+ examples
G = H or allyl
R1 = alkyl, O-alkyl, S-alkyl, terminal, geminal and
1,2-disubstituted, and trisubstituted alkenes
Received: 08.07.2019
Accepted after revision: 30.07.2019
Published online: 09.08.2019
DOI: 10.1055/s-0037-1611911; Art ID: st-2019-p0357-sp
Abstract Nitrogen-containing compounds are an essential motif in all
disciplines of chemistry and the efficient synthesis of these frameworks
is a highly sought-after goal. Presented here is a summary of recent ef-
forts conducted by our group to develop radical-mediated amination
strategies for the formal synthesis of primary amines from alkenes with
exclusive anti-Markovnikov regioselectivity. We have found that N-hy-
droxyphthalimide is an effective reagent capable of supplying both the
N and H atoms for alkene hydroamination in a group transfer radical ad-
dition-type mechanism. Furthermore, allyl-oxyphthalimide derivatives
are similarly capable of radical group transfers and allow for the amino-
allylation of an external alkene.
Samuel W. Lardy grew up in Edina, Minnesota, USA and obtained a
Bachelor’s degree in chemistry from Loyola Marymount University in
2016. He performed research under the supervision of Dr. Jeremy Mc-
Callum as an undergraduate and started his graduate studies at the Uni-
versity of California, San Diego in 2017 under the leadership of Prof.
Valerie A. Schmidt. His research focus is on phosphorous(III)-mediated
atom- and group-transfer methodology development.
Key words radicals, polarity effects, aminoallylation, hydroamination,
phosphite, O-atom transfer
Valerie A. Schmidt earned her undergraduate degree in chemistry
from Towson University. She then pursued graduate studies under the
supervision of Prof. Erik J. Alexanian at the University of North Carolina
at Chapel Hill as a Burroughs-Wellcome Fellow. She then performed
postdoctoral studies with Prof. Paul Chirik at Princeton University as a
Ruth L. Kirschstein National Institutes of Health Postdoctroal Fellow de-
veloping Fe- and Co-based catalytic systems for hydrocarbon function-
alization enabled by redox-active chelates. In 2016, she began her
independent career at the University of California, San Diego. Her main
interests are the development of new chemical processes by harnessing
the unique reactivities of open-shell intermediates.
Introduction
The use of radical-mediated processes to selectively in-
troduce carbon- and heteroatom-based functionality is ro-
bust and continues to inspire new developments in syn-
thetic methodology. Atom transfer radical additions (ATRA),
also known as the Kharasch reaction, were first reported in
the 1940s where halogenated methanes added across car-
bon–carbon double bonds, initiated thermally or with light
1
(
(
Scheme 1A). Introduction of transition-metal complexes
Kamigata developed a desulfonylative ATRA transforma-
tion using trifluoromethanesulfonyl chloride and a Ru cata-
lyst that delivers trifluoromethyl and chloro groups across
an alkene, demonstrating that the atoms/groups delivered
need not be directly bonded to one another to participate in
alkene addition (Scheme 1B).6
based on Cu, Fe, Ru, Ni) as catalysts has greatly expanded
the scope and efficiency of ATRA. The development of
atom transfer radical polymerization (ATRP) by Sawamoto
2
3
4
and Matyjszewski further cemented the utility of this ap-
proach for macromolecule synthesis.5
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E
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Synlett
S. W. Lardy, V. A. Schmidt
Synpacts
A. The Kharasch reaction (1945)a
Phthalimidyl radicals generated from N–X homolytic
bond cleavage have been used previously for the amination
X
R
R
R
X
thermal init. or light
+
11
X3C
or cat. [M]
of arenes as well as for the aminohalogenation of alkenes
X3C
(
Scheme 1C).12 Installation of a phthalimidyl group is of
B. Desulfonylative group transferb
great interest as it can easily be hydrolyzed to furnish the
Cl
R
Cl
O
O
RuCl2(PPh3)3
S
+
corresponding primary amine, thereby serving as an am-
CF3
13
–
SO2
monia surrogate. While aminohalogenation using PhthN
·
F3C
C. Aminative group transferc
precursors proceeds through an ATRA pathway, regioselec-
tivity is difficult to control because of the simultaneous for-
R
X
R
X
thermal init. or light
+
PhthN
mation of PhthN
·
and X through homolysis. Additionally,
·
PhthN
D. Hydroamination and aminoallylatione
current PhthN precursors preclude an ATRA-type hydro-
·
G
R
G
R
amination because of the lack of transferable H atoms.
While this challenge could be overcome by the addition of
an external H-atom source, this opens up chemoselectivity
concerns which may detract from desired reactivity.
P(OEt)3
– OP(OEt)3
O
+
PhthN
G = H or allyl
PhthN
Scheme 1 Historical context of radical-mediated atom and group
transfer. See ref 1. See ref 6. See ref 12. d See refs 16 and 19.
a
b
c
N-Hydroxyphthalimide (NHPI) has well-established
uses in numerous oxidative radical methodologies, serving
1
4
as the precursor to PhthNO
·
.
NHPI is an excellent reagent
These methods are excellent ways to add halogens and
halogenated alkyl groups across olefinic bonds. The instal-
lation of other heteroatomic functionality is similarly of
great interest with high potential synthetic utility.
because of its commercial availability, low cost, and long-
term stability towards air and moisture. Owing to the weak
–1
–1
O–H (89 kcal mol ) and N–O (~65 kcal mol ) bonds of
NHPI,15 we envisioned that under deoxygenative conditions
Nitrogen atom functionalities are crucial components of
biopolymers (DNA and proteins), bioactive small mole-
cules, natural products, materials, and agrochemicals. In-
stallation of N-based groups by using open-shell radical in-
termediates is particularly promising because of the typi-
cally high compatibility of radicals with various polar
functional groups. Generating N-centered radicals has tra-
ditionally been accomplished through homolytic cleavage
of weak N–X bonds which can be achieved photochemically
or thermally, such as in the Hoffman–Löffler–Freytag reac-
it could be used as a source of PhthN
·
as well as H for the
·
development of a formal ammonia ATRA-mediated alkene
hydroamination with high fidelity for anti-Markovnikov re-
gioisomers. Similarly, we hypothesized that this strategy
could achieve alkene difunctionalization by using O-func-
tionalized NHPI derivatives to transfer groups beyond a hy-
drogen atom (Scheme 1D).16
Reaction Development
7
tion. Similarly, amines covalently attached to redox-active
groups can, upon single electron oxidation or reduction,
cleave the N–leaving group bond to form N-centered radi-
cals to achieve aryl and alkene functionalizations. The ad-
dition of N-centered radicals to olefins is an intriguing
pathway to synthesize amines because it would provide ac-
cess to the more elusive anti-Markovnikov regioisomeric
products.
We began by attempting to perform the hydroamina-
tion of n-butyl vinyl ether (NBVE) using NHPI in the pres-
ence of a thermal radical initiator and triethyl phosphite as
an O-atom acceptor.
8
Phosphorus(III) reagents such as triethyl phosphite have
been shown to reduce N–O functionalities17 and we envi-
sioned that the thermodynamic driving force for the ex-
–1 18
Considerable efforts have led to the development of a
variety of alkene hydroamination methods that achieve
high levels of selectivity for the anti-Markovnikov regioiso-
change of a weak N–O bond (~55–65 kcal mol ) for a
strong phosphoranyl unit (148 kcal mol–1 for OP(OEt)3)
would allow for the generation of the desired phthalimidyl
radical.
9
meric products. Despite these advances, the use of ammo-
nia as an aminating reagent, while maintaining high levels
of regioselectivity, remains an unmet challenge. The direct
use of ammonia as a N-atom source for alkene hydroamina-
tions is hindered by several practical and design consider-
ations: (a) ammonia itself is a corrosive gas and its use re-
quires specific safety and handling procedures; (b) direct,
polar, or acid-promoted pathways are sluggish and would
favor the Markovnikov addition isomer, and (c) transition-
metal-catalyzed strategies are hampered by the propensity
to form catalytically inactive Werner-type complexes with
Our first attempts involved heating a mixture of NHPI (1
equiv), NBVE (10 equiv), 2,2′-azobis(2-methylpropionitrile)
(AIBN) (0.25 equiv), and triethyl phosphite (1.5 equiv) in
benzene to 90 °C. After 12 hours, NHPI had been fully con-
sumed, with the major product isolated arising from the
polar O addition of NHPI to NBVE (30% yield) and the de-
sired hydroamination product only detected in small
amounts (<10% yield). In an attempt to improve reaction ef-
ficiency and decrease the undesired production of acetal, a
variety of radical initiators and solvents were screened.
While common initiators such as tert-butylhydroperoxide,
10
ammonia.
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S. W. Lardy, V. A. Schmidt
Synpacts
di-tert-butyl peroxide, and benzoyl peroxide resulted in
very poor reaction performance, dilauroyl peroxide [(unde-
cyl(CO ) ] at 90 °C and di-tert-butyl hyponitrite [(t-BuON) ]
at 35 °C proved particularly effective at promoting exclu-
sively the desired hydroamination process. Similarly, reac-
tion efficiency was found to be optimal when using 1,2-di-
chloroethane (DCE), whereas many other common solvents
such as benzene, toluene, or ethyl acetate resulted in poor
reaction efficiency.
cause of reaction scale. Furthermore, the phthalimidyl
group was easily removed by using aqueous hydrazine in
methanol to reveal the corresponding primary amine in
only 0.5 hours (Scheme 3B).
2
2
2
A. Reaction scalability
P(OEt)3 (1.5 equiv)
AIBN (0.25 equiv)
H
PhthNO
H
+
PhthN
DCE, 90 °C, 12 h
5 equiv
80% yield
1.2 g
Scheme 2 depicts a selection of alkenes we found
amenable to this hydroamination procedure. Electronically
rich vinyl ethers, silanes, and sulfides were well tolerated,
as were unactivated alkenes such as 1-hexene. Internal
alkenes similarly underwent this transformation in good
yields and exclusive anti-Markovnikov regioselectivity. No-
tably, a primary bromide was retained indicating that hy-
droamination outperformed potential Arbuzov-type side
reactivity. Compounds containing multiple alkenes capable
of cyclization such as diallyl ether and 1,5-cyclooctadiene
also took part in this reaction. While it was observed that
diallyl ether exclusively underwent a 5-exo cyclization, the
reaction with 1,5-cyclooctadiene resulted in a separable
mixture of bicyclic and monocyclic hydroamination prod-
ucts. Notably, 1-octyne underwent hydroamination in
modest yield to deliver exclusively the trans-vinyl-
phthalimide.
8
0:20 exo:endo
B. Phthalimide deprotection
H2NNH2·H2O (5 equiv)
MeOH, 60 °C, 0.5 h
H2N
PhthN
9
5% yield
8:22 exo:endo
7
Scheme 3 Hydroamination scalability and utility
Mechanistic Investigation
In order to determine whether or not this radical-medi-
ated hydroamination was truly an ATRA process in which
NHPI supplied both the N and the H atoms for the overall
hydroamination, we prepared d -NHPI and subjected it to
1
our standard reaction conditions with tert-butyl ethylene
(Scheme 4). The corresponding 3,3-dimethylbutan-2-d-1-
phthalimide was isolated in 58% yield with deuteration ob-
served exclusively at the C2 position. This supports our pro-
posed ATRA-mediated pathway of hydroamination.
H
R
H
R
P(OEt)3, initiator
0.04 M DCE
O
+
PhthN
PhthN
n
n
nBu
tBu
P(OEt)3 (1.5 equiv)
(tBuON)2 (0.10 equiv)
tBu
H
O Bu
H
SiMe3
H
S Bu
H
D
D
O
+
PhthN
DCE, 50 °C, 12 h
PhthN
PhthN
PhthN
PhthN
PhthN
10 equiv
2% yielda
56% yieldb
43% yieldc,d
46% yieldb
58% yield
8
H
H
nPr
Scheme 4 Isotopic labeling
H
O
H
3
Br
PhthN
6% yieldb
PhthN
nPr
PhthN
PhthN
55% yieldb
Expansion to GTRA
7
60% yieldd
46% yieldd
PhthN
H
PhthN
H
nHex
H
H
Having demonstrated NHPI was able to carry-out deox-
ygenative ATRA alkene hydroamination, we hypothesized
that O-modified NHPI derivatives could be used as reagents
for alkene difunctionalization. Previous work from Tanko
and co-workers showed that O-allyl oxyphthalimides can
undergo -fragmentation in the presence of nucleophilic
benzylic radicals, serving as an allylation reagent and pro-
ducing PhthNO as a by-product. With this in mind, we
turned our attention to O-allyl oxyphthalimides as poten-
tial GTRA reagents for alkene aminoallylation.
+
PhthN
PhthN
O
H
H
27% yieldd
95:5 E:Z
14% yieldd
2
3% yieldb
9:41 cis:trans
21% yieldd
>
5
>95:5 dr
Scheme 2 Selected hydroamination substrate scope. Reagents and con-
ditions: All reactions carried out by using NHPI (1 equiv) and alkene (10
equiv) with the specified initiator and temperature. Carried out by us-
ing (t-BuON) (0.25 equiv) at 35 °C. Carried out by using (undecyl-
CO ) (1 equiv) at 110 °C. 5 equiv of alkene used. Carried out by using
a
•
b
2
c
d
2
2
(
t-BuON) (0.25 equiv) at 50 °C.
2
We began by subjecting an allyl oxypthalimide deriva-
tive bearing a cyano group to the standard reaction condi-
tions used for the hydroamination procedure and 10 equiv-
alents of NBVE.19 Under these conditions, the correspond-
ing aminoallylation product was obtained in 34% yield with
the main product being N-(O-ethyl)hydroxypthalimide re-
The scalability of this method was demonstrated by the
hydroamination of norbornene on a gram scale (Scheme
A). The desired phthalimidyl product was easily obtained
in 80% yield with no significant decrease in efficiency be-
3
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S. W. Lardy, V. A. Schmidt
Synpacts
sulting from a polar S 2′ Arbuzov-type reaction. Through a
OP(OEt)3
N
series of exhaustive optimization studies we were able to
improve reaction efficiency to 45% by switching to trimeth-
yl phosphite as this decreased the Arbuzov side reactivity.
While pursuing the scope of allyl oxyphthalimide sub-
strates amenable to this transformation, we observed sig-
nificant electronic effects (Scheme 5). For instance, we
found that electron-withdrawing groups such as nitrile,
ethyl ester, and fluorinated electron-deficient arenes were
tolerated vinyl substituents partnering with NBVE. Howev-
er, less electronically biased substrates such as the unsub-
stituted O-allyl parent, or derivatives bearing a vinyl sul-
fide, amide, sulfoxide, or phenyl group did not result in
aminoallylation. Similarly, electronic effects were crucial
with regard to the external alkene component used. Pro-
ductive reactivity was observed only when the external
alkene contained an ether substituent. Less biased alkenes
such as vinyl silanes, sulfides, and acetates were not tolerat-
ed and lead solely to O-allyl oxyphthalimide reagent de-
composition.
PhthN O
PhthN
PhthN
O
G
P(OR)3
radical
initiation
PhthN
G
group
transfer
R
R
PhthN
R
Scheme 6 Mechanistic proposal
Typically, these effects manifest as relative rates of reac-
tivity. For instance, nucleophilic radicals add to electron-
deficient alkenes at much faster rates than electron-rich
alkenes, with the opposite being true for electrophilic radi-
cals (Scheme 7A). These preferences can be understood
through frontier molecular orbital (FMO) interactions
(Scheme 7B) in which the singly occupied molecular orbital
(SOMO) of a nucleophilic radical preferentially engages the
low-lying lowest-unoccupied-molecular-orbital (LUMO) of
an electron-deficient alkene over the relatively high energy
highest-occupied-molecular-orbital (HOMO) of an elec-
tron-rich alkene so as to minimize the energy of the subse-
quent intermediate.
EWG
EWG
P(OMe)3
undecylCO2)2
(
+
OR
0 equiv
PhthNO
PhthN
1
DCE, 90 °C, 12 h
OR
n
CF3
R = O Bu
CF3
O
N
A. Trends in radical additions
EWG =
OEt
25% yield
CF3
EWG
R
2
6% yield
35% yield
37% yield
fast - polarity matched
EWG
EDG
EWG = 4-CF3(C6H4)
R
e- rich
R =
Cl
EDG
R
slow - polarity mismatched
24% yield
62% yield
26% yield
36% yield
46% yield
B. Frontier molecular orbital considerations
Scheme 5 Aminoallylation scope. Reagents and conditions: All reactions
carried out by using the corresponding allyl oxyphthalimide (1 equiv),
trimethyl phosphite (1.5 equiv), and undecyl(CO ) (0.1 equiv) in 1,2-
LUMO
2
2
dichloroethane (0.40 M) at 90 °C for 12 h.
EWG
LUMO
A mechanistic hypothesis consistent with our data is
outlined in Scheme 6. Radical initiation generates an elec-
trophilic imidoxyl radical which undergoes O-atom transfer
with the trialkylphosphite, exchanging the weak N–O bond
EDG
R
SOMO
R
R
HOMO
EDG
•
for a strong P–O bond, to generate PhthN . Phthalimidyl rad-
HOMO
EWG
ical is then poised to add to the external, electron-rich ole-
fin preferentially generating a nucleophilic C-centered radi-
cal intermediate. Intermolecular group transfer from an-
other equivalent of either NHPI or an allyl derivative
produces the functionalized alkene product and regener-
Scheme 7 Polarity matching effects
•
ates PhthNO .
In our system, the electrophilic phthalimidyl radical
generated from deoxygenation with phosphite preferential-
ly adds to the electron-rich external alkene to furnish a
new, nucleophilic C-centered radical that selectively adds to
the electronically deficient alkene of the O-allyl oxyph-
thalimide. The FMO energy considerations of each radical
We postulate that the stark electronic effects, most pro-
nounced in our aminoallylation studies, are a result of
proper radical polarity matching. Significant effort has been
expended to understand the polarity effects of radical reac-
20
tions.
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S. W. Lardy, V. A. Schmidt
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intermediate formed throughout this process explains why
the process is so well controlled, and also rationalizes the
inability of unbiased alkenes to participate.
(8) (a) Kärkäs, M. D. ACS Catal. 2017, 7, 4999. (b) Davies, J.; Morcillo,
S. P.; Douglas, J. J.; Leonori, D. Chem. Eur. J. 2018, 24, 12154.
(9) For recent examples, please see: (a) Ickes, A. R.; Ensign, S. C.;
Gupta, A. K.; Hull, K. L. J. Am. Chem. Soc. 2014, 136, 11256.
(b) Klinkenberg, J. L.; Hartwig, J. F. Angew. Chem. Int. Ed. 2011,
5
0, 86.
Future Outlook
(
10) Werner, A. Z. Anorg. Allg. Chem. 1893, 3, 267.
(
11) (a) Kim, H.; Kim, T.; Lee, D. G.; Roh, S. W.; Lee, C. Chem.
Commun. 2014, 50, 9273. (b) Allen, L. J.; Cabrera, P. J.; Lee, M.;
Sanford, M. S. J. Am. Chem. Soc. 2014, 136, 5607.
We have developed a deoxygenative strategy to achieve
alkene aminations using N-hydroxyphthalimide derivatives
and trialkylphosphites. Current efforts in our group involve
expanding the scope of N-hydroxy-containing functional-
ities capable of serving as N-centered radical precursors to
engage in a wider range of synthetically useful processes.
(
12) (a) Day, J. C.; Katsaros, M. G.; Kocher, W. D.; Scott, A. E.; Skell, P.
S. J. Am. Chem. Soc. 1978, 100, 1950. (b) Kirsch, A.; Lüring, U. J.
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13) Wuts, P. G. M.; Greene, T. W. Greene’s Protective Groups in
Organic Synthesis, 4th ed; Wiley: Hoboken, 2007, 790–793.
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Funding Information
This work was supported by start-up funds provided by the Universi-
ty of California, San Diego.()
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Compounds; CRC Press: Boca Raton, 2003.
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2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E