Iridium-catalyzed synthesis of primary allylic amines from allylic alcohols: Sulfamic acid as ammonia equivalent

Article abstract of DOI:10.1002/anie.200700159

(Chemical Equation Presented) Two for the price of one: Sulfamic acid serves not only as a nitrogen source but also as an in situ activator of hydroxy groups in the first direct iridium-catalyzed synthesis of primary allylic amines from allylic alcohols (see scheme; cod = cycloocta-1,5-diene). The reaction is catalyzed by a commercially available iridium complex and a phosphoramidite-based bidentate phosphorus-olefin ligand.

Full text of DOI:10.1002/anie.200700159

Angewandte
Chemie
DOI: 10.1002/anie.200700159
Allylic Amination
Iridium-Catalyzed Synthesis of Primary Allylic Amines from Allylic
Alcohols: Sulfamic Acid as Ammonia Equivalent**
Christian Defieber, Martin A. Ariger, Patricia Moriel, and Erick M. Carreira*
A large range of building blocks are accessible thanks to the
impressive advances in homogeneous catalysis. Many reac-
tions require the use of preactivated reactants with the
exception of simple atom-transfer processes, such as epox-
idation or hydrogenation. Additionally, the protected prod-
ucts must be subjected to further chemical steps prior to their
actual application. The rapid and direct access to building
blocks without additional processing being required is scarce
despite the fact that such processes would be of great use.[1]
We have been interested in these types of transformations for
some time;[2,3] we have focused our attention on the develop-
ment of nitrogen nucleophiles that would give access to
unprotected primary amines.[4]
Herein we report the use of sulfamic acid (H2NSO3H) as
an inexpensive, commercially available ammonia equivalent
in allylic substitutions to afford directly allylic amines in
excellent regioselectivity and synthetically useful yields.[5] The
process employs a commercially available iridium catalyst
precursor as well as a novel phosphoramidite–olefin ligand,[6]
which can be synthesized in one step. A welcome side effect is
the fact that allylic alcohols can be directly employed as the
starting materials for the transformation without the need for
prior activation [Eq. (1); cod = cycloocta-l,5-diene].
amines can be accessed in excellent yields and stereoselec-
tivities, primary amines can only be prepared through the use
of protected forms of ammonia.[11] In this respect, Helmchen
and co-workers has recently reported the use of o-nosyl-
amides and N,N-diacylamides in enantioselective Ir-catalyzed
allylic amination reactions.[12]
To increase the practicality and atom economy[13] of the Ir-
catalyzed allylic amination reaction to give primary amines,
we have investigated the chemistry of sulfamic acid and
specifically its capacity to serve as an ammonia equivalent.
Sulfamic acid is a crystalline, inexpensive solid that has found
limited applications in organic synthesis,[14] primarily as an
acid catalyst.[15] However, to the best of our knowledge,
sulfamic acid has never been employed as a nitrogen source in
a process, despite its many potential advantages (low toxicity,
non-corrosive, non-odorous, low molecular weight).
At the outset of our studies, we examined the reaction of
tert-butyl cinnamyl carbonate (1) as a test substrate with
sulfamic acid in the presence of a catalyst derived from the
Feringa phosphoramidite ligand L1 and IrI (Scheme 1).[3,16]
One of the most convenient methods for the synthesis of
allylic amines is the Ir-catalyzed allylic substitution pioneered
by the research groups of Takeuchi,[7] Hartwig,[8] Helmchen[9]
and Alexakis.[10] However, whereas secondary and tertiary
Scheme 1. Test substrates and ligands for the Ir-catalyzed allylic
amination reaction with sulfamic acid.
[*] C. Defieber, M. A. Ariger, P. Moriel, Prof. Dr. E. M. Carreira
Laboratorium für Organische Chemie
ETH Zürich, HCI H335
8093 Zürich (Switzerland)
Fax: (+41)44632-1328
E-mail: carreira@org.chem.ethz.ch
However, all experiments conducted in THF, CH2Cl2,
CH3CN, EtOH, MeOH, or acetone were hampered by the
low solubility of sulfamic acid, which led to recovery of
starting material 1. Although sulfamic acid is soluble in
dipolar aprotic solvents such as N,N-dimethylformamide
(DMF), dimethylacetamide (DMA), or dimethyl sulfoxide
(DMSO), no conversion into the desired amine product or its
derivatives (that is, secondary sulfamate) was observed.
[**] This research was supported by the ETH Zürich and the Swiss
National Science Foundation. P.M. wishes to thank MEC of Spain
for a predoctoral fellowship. The authors acknowledge the English
translation of Ref. [26b] by Dr. Nicka Chinkov.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Investigation of the substrate scope.
The first set of promising results were achieved with
branched tert-butyl carbonate 2 as the substrate in DMF
(15% conversion). Of greater significance, however, was the
subsequent observation that alcohol 3 could be employed
directly under otherwise identical conditions (15% conver-
sion). This finding was key to the further optimization of the
process. In transition-metal catalyzed allylic substitution
reactions, activated allylic substrates (for example, halides,
esters, carbonates, or carbamates) are usually required to
facilitate the formation of the crucial allyl–metal intermedi-
ate.[17] Alternatively, reagents such as Et3B or metal catalysts
(BiIII) can provide the in situ activation of a free alcohol.[18]
The discovery of a process that involved the direct use of
allylic alcohols was an unexpected bonus. Moreover, to the
best of our knowledge, no transition-metal-mediated process
in which a single reagent serves as both the ammonia source
and in situ activator of a hydroxy group has been previously
reported.
We further investigated the influence of various ligands.
The nitrogen-based pyridylbis(oxazoline) (pybox) ligand only
displayed decreased reactivity (5% conversion).[19] No ami-
nation was observed with the use of PPh3 and P(NMe2)3
(<5% conversion) but we obtained 25% conversion with
P(OPh)3. Using 1.5 mol% of [{Ir(cod)Cl}2] and 3 mol% of the
achiral phosphoramidite ligand L2, we achieved 30% con-
version of 3 (DMF, 24 h, 238C).
Entry
Substrate
Product
Yield [%][a]
82[b]
1
2
3
4
73[c]
71[d]
71[e]
5
6
78[b]
75[b]
7
8
71[b]
75[b]
Based on our interest in alkenes as ligands in transition-
metal catalysis,[20] we investigated the olefin ligand L3 in the
substitution process. Ligand L3 can easily be synthesized from
[a] Yield of product isolated after purification by chromatography.
Regioselectivity was >99:1 as shown and determined by 1H NMR
spectroscopy of the unpurified reaction mixtures. [b] Isolated as the
hydrochloride salt by treatment of the purified amine with 2m HCl in
diethyl ether. [c] Treatment of the unpurified reaction mixture with Et3N,
BzCl. [d] Treatment of the unpurified reaction mixture with 0.5m aq
NaOH, Boc2O. [e] Treatment of the unpurified reaction mixture with
K2CO3, (CF3CO)2O. For further experimental details, see the Experimen-
tal Section and the Supporting Information.
inexpensive
2,2’-biphenol,
PCl3,
and
5H-dibenzo-
[b,f]azepine.[21] The reaction with this ligand was remarkably
clean (> 99% conversion, DMF, 24 h, 238C). When we used
the saturated analogue, ligand L4, only 20% conversion of
alcohol 3 was achieved.[22,23]
It is worth noting that the Ir-catalyzed transformation
proceeds with complete regioselectivity.[24] Furthermore,
neither di- nor triallylated amines could be observed in the
unpurified reaction mixture. With the optimal conditions in
hand, we investigated the scope of the reaction [Eq. (2),
substituents are permitted in the substrate including phenyl-
(entry 5), cylohexyl- (entry 6), as well as benzyloxymethyl-
substituted allylic amines (entry 7, Bn = benzyl). Noteworthy
is also the isolation of hexa-1,5-dien-3-amine hydrochloride
(entry 8) in 75% yield without any isomerization of the
double bond.
To get further insight in this interesting process, the course
1
of the reaction was followed using time-dependent H NMR
spectroscopy in [D7]DMF, in which the signal corresponding
to H-3 of our test alcohol 3 was monitored.[25] In analogy to
the reaction on preparative scale, the NMR experiments
showed the clean transformation of 3 (d(H-3) = 4.04 ppm)
into the corresponding amine 4 (d(H-3) = 3.88 ppm) within
three hours (Scheme 2). In a separate experiment, we
investigated the influence of an excess of sulfamic acid
(2 equiv). The desired amine 4 was generated after two hours
as shown by the appearance of the corresponding signal of the
amine. When the reaction was allowed to continue over 8 h,
amine 4 is observed to undergo partial conversion into
another product with a characteristic signal at d = 4.46 ppm.
To assign a structure for this product, we conducted additional
experiments in [D7]DMF: Treatment of alcohol 3 with two
equivalents of sulfamic acid after eight hours gave quantita-
Table 1]. A convenient way to isolate the amine products is
through the precipitation of their hydrochloride salts. How-
ever, a salient feature of the process results from the use of an
ammonia equivalent, in which the amine can be subsequently
protected in situ as the benzamide (entry 2, Bz = benzoyl),
the Boc-carbamate (entry 3, Boc = tert-butoxycarbonyl), or
the trifluoroacetamide (entry 4). In fact, the protection can
facilitate the isolation of the product for certain low-
molecular-weight substances produced on small scale. Various
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allylic alcohol to form 8.[28] This activated species then can
participate in an oxidative addition reaction with the iridium
complex.[29] The resulting iridium–allyl species 9 is sufficiently
electrophilic to undergo a nucleophilic attack by ammonia,
thus liberating the primary protonated allylic amine product.
Our aim was to apply the unique aspects of sulfamic acid
chemistry to a catalytic asymmetric process. The modular
structure of phosphoramidite ligand L3 allows the incorpo-
ration of a wide range of chiral diol backbones. For example,
the integration of (S)-binol in place of 2,2’-biphenol resulted
in the chiral ligand L5, which, in combination with IrI, can be
used in the reaction of 1-cyclohexylprop-2-en-1-ol to provide
(S)-1-cyclohexylprop-2-en-1-amine hydrochloride in 70%
yield and 70% ee.[30] [Eq. (3); coe = cyclooctene] Although
Scheme 2. Selected results from the spectroscopic experiments in
[D7]DMF.
tive sulfate ester 5 (d(H-3) = 4.72 ppm) which is consistent to
what is known in the literature.[5] The more potent commer-
cially available sulfating agent sulfur trioxide-N,N-dimethyl-
formamide performs sulfation in an analogous way to deliver
5 already after 1 h. When amine 4 is treated under the same
conditions, sulfamate 6 could be obtained along with side
products after nine hours.
These spectroscopic observations allow us to make the
following conclusions:
1) When an excess of sulfamic acid was employed in the Ir-
catalyzed process, the amine 4 initially produced under-
went partial sulfamation with the second equivalent of
sulfamic acid to form 6. This sulfamation only began after
3 had been entirely transformed into 4.
2) Throughout the course of the Ir-catalyzed process, no
signals corresponding to the sulfate ester 5 were observed.
Given the long reaction time required to perform the
sulfation with sulfamic acid (eight hours), it is unlikely that
the sulfate ester 5 serves as activated intermediate in the
Ir-catalyzed reaction.
3) The absence of signals corresponding to 6 leads us to
suspect that sulfamic acid does not act as a nucleophile in
the catalytic process.
this result is far from optimal, it represents the first example
of direct generation of a primary enantiomerically enriched
allylic amine from an allylic alcohol.[31]
In conclusion, we demonstrate for the first time the direct
Ir-catalyzed conversion of an allylic alcohol into an allylic
amine with the use of sulfamic acid. This method does not
require either separate prior activation or protecting group
operations and thus the process is attractive both econom-
ically and ecologically. Also, promising preliminary results
towards the development of an asymmetric process have been
achieved. Further mechanistic investigations as well as the
fine-tuning of ligands are underway. The fact that sulfamic
acid can serve as an ammonia equivalent is intriguing and may
have an application in other transformations that invlove the
conversion of OH into NH2 groups.
Based on these observations, we developed a working
model shown in Scheme 3. It has been suggested in the
literature that N,N-dimethylformamide undergoes a conden-
sation reaction with sulfamic acid[26] to form a Vilsmeier-like
intermediate 7.[27] We speculate that this reactive intermedi-
ate is generated here, and that it subsequently reacts with the
Experimental Section
Representative procedure: A Schlenk flask under
argon was charged with [{Ir(cod)Cl}2] (10.1 mg,
15 mmol, 1.5 mol%) and L3 (12.2 mg, 30 mmol,
3 mol%). DMF (2 mL) was added and the reac-
tion mixture was stirred at 238C for 15 min. The
allylic alcohol (1.00 mmol, 1 equiv) was added by
syringe, followed by solid sulfamic acid (97 mg,
1.00 mmol, 1 equiv). The resulting reaction mix-
ture was heated to 508C. After completion of the
reaction (usually 3–4 h, as monitored by TLC), the
solvent was evaporated at high vacuum. The
resulting brown residue was dissolved in CH2Cl2
(10 mL) and sat. aq NaHCO3 solution (10 mL) and
stirred for 10 min. The aqueous layer was
extracted with CH2Cl2 (3 ” 15 mL). The combined
organic layers were dried (Na2SO4) and concen-
trated under reduced pressure to afford the crude
allylic amine. The ratio of regioisomers was
Scheme 3. Proposed working model. L=ligand.
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Communications
determined by 1H NMR analysis of the unpurified sample. The
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desired amine was obtained after purification of the residue by flash
chromatography on basic or neutral alumina using CH2Cl2/MeOH as
eluent. As some amines proved to be unstable and/or volatile, they
were obtained through precipitation of their more-stable hydrochlo-
ride salts by addition of 2m HCl in Et2O.
Received: January 12, 2007
Published online: March 12, 2007
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Keywords: alcohols · allylation · allylic compounds · amines ·
.
iridium
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Supporting Information.
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107, 2058 – 2066.
[30] The resulting amine was trapped in situ with benzoyl chloride
and triethylamine to obtain the UV-active benzamide. The
enantioselectivity was determined by HPLC on a chiral sta-
tionary phase. The ee value was not dependent on the type of
starting material used (carbonate or alcohol).
[31] Cinnamyl alcohol did not serve as substrate under our current
reaction conditions, even when the Hartwig catalyst-activation
protocol (IrI and L1) was employed; see Ref. [8d].
Angew. Chem. Int. Ed. 2007, 46, 3139 –3143
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3143