Asymmetric cascade reaction to allylic sulfonamides from allylic alcohols by palladium(II)/base-catalyzed rearrangement of allylic carbamates

Article abstract of DOI:10.1002/anie.201403090

A regio- and enantioselective tandem reaction is reported capable of directly transforming readily accessible achiral allylic alcohols into chiral sulfonyl-protected allylic amines. The reaction is catalyzed by the cooperative action of a chiral ferrocene palladacycle and a tertiary amine base and combines high step-economy with operational simplicity (e.g. no need for inert-gas atmosphere or catalyst activation). Mechanistic studies support a Pd ^II-catalyzed [3,3] rearrangement of allylic carbamates - generated in situ from the allylic alcohol and an isocyanate - as the key step, which is followed by a decarboxylation.

Full text of DOI:10.1002/anie.201403090

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Angewandte
Communications
DOI: 10.1002/anie.201403090
Cascade Reactions
Asymmetric Cascade Reaction to Allylic Sulfonamides from Allylic
Alcohols by Palladium(II)/Base-Catalyzed Rearrangement of Allylic
Carbamates**
Johannes Moritz Bauer, Wolfgang Frey, and Renꢀ Peters*
Abstract: A regio- and enantioselective tandem reaction is
reported capable of directly transforming readily accessible
achiral allylic alcohols into chiral sulfonyl-protected allylic
amines. The reaction is catalyzed by the cooperative action of
a chiral ferrocene palladacycle and a tertiary amine base and
combines high step-economy with operational simplicity (e.g.
no need for inert-gas atmosphere or catalyst activation).
Mechanistic studies support a PdII-catalyzed [3,3] rearrange-
ment of allylic carbamates—generated in situ from the allylic
alcohol and an isocyanate—as the key step, which is followed
by a decarboxylation.
In our initial studies we examined the possibility of
a decarboxylative asymmetric rearrangement with the allylic
N-tosyl carbamate 1a (Table 1). The pentaphenylferrocene
oxazoline palladacycle [PPFOP-Cl]2—in the initial studies
activated by treatment with AgNO3 for chloride exchange—
Table 1: Development of the enantioselective allylic carbamate rear-
rangement.
C
hiral a-branched allylic amines are valuable synthetic
building blocks and their catalytic enantioselective synthesis
has been intensively studied. Two of the most versatile
strategies to form allylic amines with high levels of enantio-
and regioselectivity are the Ir-catalyzed allylic amination[1–3]
and the PdII-catalyzed Overman (aza-Claisen) rearrangement
of allylic imidates.[4,5] For the latter method the undesired
regioisomer is often not detected as a consequence of
a rearrangement mechanism via a six-membered cyclic
intermediate.[6] The aza-Claisen rearrangement generates
carboxamide-protected amines. Unfortunately, only for tri-
chloro- and trifluoroacetamide products the protecting
groups could be readily removed.[7,8] Preparation of the
trifluoroacetimidate substrates is relatively tedious and
expensive, requires the use of CCl4, and produces large
amounts of PPh3-based waste.[9] In addition, the isolation and
storage of trifluoroacetimidates is often hampered by their
sensitivity towards hydrolysis.
Entry
X
YÀ
Z
Base
–
T [8C] Yield [%][a]
ee [%][b]
2a
3
2a
À
1
2
3
4
5
6
7
8
5
3
3
3
2
2
2
2
1
1
1
NO3À
NO3À
NO3À
NO3À
NO3
0
60
60
60
60
60
60
60
60
40
40
85
17
6
3
5
6
14
14
4
3
4
10
87
96
91
87
75
70
95
95
22
88
9
89
88
86
86
92
91
87
88
93
92
100 iPr2NEt
100 PS
10 PS
20 PS
20 PS
20 PS
20 PS
20 PS
20 PS
25 PS
À
PF6
TfOÀ
À
F3CCO2À
9
F3CCO2
Herein, we report an alternative, operationally simple
tandem reaction, which transforms linear achiral allylic
alcohols with high regio- and enantioselectivity into branched
chiral sulfonyl protected allylic amines.[10] This reaction
proceeds via allylic carbamate intermediates, which undergo
a PdII-catalyzed asymmetric [3,3] rearrangement followed by
a decarboxylation step.[11,12]
[c]
10
–
11[d]
–
6
[c]
1
[a] Yield determined by H NMR analysis using an internal standard.
[b] Enantiomeric excess determined by HPLC. [c] No silver salt was used
for catalyst activation. [d] CHCl3 was used as solvent.
produced small amounts of rearrangement product 2a after
18 h in a sealed tube in CH2Cl2 at 608C (entry 1), yet in almost
racemic form. To increase the reactivity of the carbamate N-
center, different bases were studied as stoichiometric addi-
tives. O-bases resulted in traces of 2a, at most, but sterically
demanding tertiary amines like iPr2NEt and proton sponge
[*] Dipl.-Chem. J. M. Bauer, Dr. W. Frey, Prof. Dr. R. Peters
Institut fꢀr Organische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
E-mail: rene.peters@oc.uni-stuttgart.de
Homepage: http://www.peters.oc.uni-stuttgart.de
[**] This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG, PE 818/4-1). The Fonds der Chemischen
Industrie (FCI) and the Carl-Zeiss-Stiftung are acknowledged for
Ph.D. fellowships to J.M.B.
(PS, 1,8-bis(N,N-dimethylamino)naphthalene) caused
a
strong increase in reactivity (entries 2 and 3). The product
was formed in high yields and with good enantioselectivity,
also even when only catalytic amounts of PS were used
(entry 4).[13]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403090.
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Table 2: Application of the cascade title reaction.
Various silver salts AgY were studied for the catalyst
activation.[7e,8,14] Nitrate (entry 5), and F3CCO2À (entry 8) and
a number of other anionic ligands YÀ (not shown) resulted in
similar yields and ee values, employing 2 mol% of precatalyst.
With weakly or noncoordinating anions like triflate (entry 7)
À
and PF6 (entry 6) more substrate decomposition towards 3
was noticed.[15] With trifluoroacetate the rearrangement was
still efficient at 408C and with 1 mol% of the precatalyst
(entry 9). When the precatalyst was not activated by a silver
salt, the reactivity was low under these conditions in agree-
ment with our previous investigations (entry 10).[7e,8h,14,16] For
the above-mentioned aza-Claisen rearrangement using the
same precatalyst, it was even necessary to use an excess of
silver salt in order to generate a paramagnetic PdIII species,
which offered significantly higher catalytic activity.[8i] This
type of catalyst oxidation is not required in the allylic
carbamate rearrangement. Also the chloride exchange can
be avoided, if the rearrangement is conducted at a higher
temperature. At 858C in CHCl3 using the non-activated
[PPFOP-Cl]2, 2a was formed in 88% yield and with 92% ee
(entry 11).
Entry
4/2
R
Yield [%][a]
RS [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
g
h
i
nPr
Et
Me
80
88
90
85
86
80
83
90
52
79
20:1
16:1
40:1
22:1
25:1
26:1
20:1
20:1
7:1
90
91
72
93
92
90
98
98
92
89
nPent
(CH2)2Ph
(CH2)2CO2Me
CH2OTBS
CH2OBn
iPr
10
j
iBu
10:1
[a] Yield of isolated product. [b] Regioselectivity determined by 1H NMR
analysis of the crude product. [c] Enantiomeric excess determined by
HPLC.
To further improve the operational simplicity, the option
of a one-pot procedure was explored, in which the allylic
alcohol is used as the substrate (Scheme 1, top).[17] The latter
conditions and provided high enantioselectivity, albeit with
reduced reactivity in the first case. In addition, several
functional groups were compatible with the cascade reaction
conditions as shown in Table 2 for an ether moiety (entry 8),
a silyl ether (entry 7), and an ester residue (entry 6), thus
providing rapid access to protected chiral b-amino alcohol
and g-amino acid derivatives possessing an olefin moiety for
further synthetic manipulations. In contrast, aromatic resi-
dues R were not well tolerated. The observed substrate
preference seems to be complementary to that of the Ir-
catalyzed allylic aminations, for which aromatic residues R
often led to higher regioselectivities than aliphatic ones.[1]
Like the PdII-catalyzed allylic imidate rearrangement, the
title reaction is slower for Z-configured substrates. Under the
conditions listed in Table 2, the product 2a was formed in only
29% yield starting from (Z)-4a and with opposite absolute
configuration (75% ee, not shown in Table 2). The allylic
alcohols should thus be nearly geometrically pure for the best
possible enantioselectivity (see also the Supporting Informa-
tion).
Scheme 1. Step-economic approaches towards 2a.
was stirred with 1 equivalent of pTsNCO in CHCl3 for 30 min
at room temperature before [PPFOP-Cl]2 (3 mol%) and PS
(20 mol%) were added.[18] At 858C in a sealed tube, the
product was again formed in high yield and with high
enantioselectivity. This prompted us to inspect a tandem
version, in which all reaction components were directly added
without separate preformation of the allylic carbamate.[19]
This simple procedure led to an almost identical reaction
outcome, even in the presence of air (Scheme 1, bottom).
This cascade reaction was investigated for different
substrates (Table 2). When the olefin substituent R was an
aliphatic group, the product was usually formed in good to
high yields and with high regio- and enantioselectivity. The
most difficult example in terms of enantioselection, in which
R = Me, gave the product with an ee of 72% (entry 3),
whereas for the other examples the ee values ranged from 89–
98%. Substrates with a- and b-branched alkyl residues
(entries 9 and 10) could also be utilized under the standard
The scalability of the cascade reaction has been examined
for substrate 4d on a gram scale. Repeating the reaction of
Table 2, entry 4 with 5.24 mmol of substrate provided 1.366 g
of 2d (92% yield) with 91% ee.
Tosyl protecting groups on allylic amines can often be
removed in good yields under reductive conditions, even in
large-scale industrial processes.[20] To showcase the utility of
the products, 2d was deprotected under standard condi-
tions[20c] in high yield and with no loss of optical purity
(Scheme 2). The decarboxylative allylic carbamate rearrange-
ment also offers the opportunity of altering the N-protecting
group by the formation of different allylic carbamates. This
has been demonstrated for carbamates 5 carrying a dimethyl-
aminosulfonyl protecting group (Scheme 2). In this case the
rearrangement proceeded with high yields and regioselectiv-
ities (6a/d: 24:1; 6e: 39:1) and gave the sulfonamides 6 with ee
values of 94–98%. Deprotection of 6 under standard con-
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Scheme 2. Application of a dimethylaminosulfonyl protecting group.
ditions[21] using 1,3-diaminopropane furnished the free amines
in high yields. For substrate 5e we have also studied the use of
the non-activated catalyst [PPFOP-Cl]2 (1.0 mol%) in the
presence of PS (20 mol%). Gratifyingly, in CHCl3 the product
was again formed in good yield (93%) with a high ee value
(96%) and good regioselectivity (24:1) after 24 h at 808C (not
shown).[22]
Scheme 4. Possible simplified mechanism of the PdII/base-catalyzed
decarboxylative allylic carbamate rearrangement.
Crossover experiments have been performed to deter-
mine whether the sulfonamide formation proceeds by a rear-
rangement mechanism. A 1:1 mixture of allylic carbamates 1c
and 8, which differ in both the N-sulfonyl moiety and the
olefin substituent, was treated with the palladacycle catalyst
(Scheme 3). The two products that are expected for an
the ground-state conformation in which the allylic C-1 meth-
ylene moiety with its relatively low steric demand points
towards the massive C5Ph5 ligand, while the sulfonyl moiety
avoids the ferrocene core to minimize repulsive interactions
(Scheme 4). Coordination of the enantiotopic olefin face is
assumed to be less favorable for steric reasons. In one possible
scenario, the deprotonated carbamate N atom could bind to
the Pd atom (cis to the oxazoline N) and could attack the
olefin within this chelate 11’ by an inner-sphere mechanism.
However, since olefin insertions of this type are usually
concerted processes, the reactive conformation would require
the olefin double bond to be in the PdII square plane, thus
entailing a rotation of the coordinated olefin by roughly
908.[25] This should cause considerable repulsion between the
C5Ph5 ligand and an olefin substituent. In the alternative case
of an outer-sphere mechanism via olefin complex 11 the
anionic N center would attack from the face remote to the Pd
center. Inner- and outer-sphere mechanisms are thus expected
to provide different enantiomers. For the proposed face
selectivity of the substrate coordination only the suggested
outer-sphere attack would be in agreement with the absolute
configuration of the major product enantiomers.
Scheme 3. Crossover experiment confirming an intramolecular reaction
pathway.
intramolecular pathway were formed in equally high quanti-
ties, whereas crossover products were present in only trace
quantities (< 1% in GC–MS). For that reason the reaction
most likely occurs by means of an intramolecular carbamate
rearrangement, and not by an allylic substitution pathway.
Based on the result of these crossover experiments and
the absolute configuration of the products[23] we suggest the
mechanism depicted in Scheme 4. We propose—like in the
allylic imidate rearrangement[7e]—a face-selective coordina-
tion of the olefin moiety to the PdII center in 11. Based on the
typical coordination properties of ferrocene palladacyc-
les,[7e,24] the neutral olefin is expected to coordinate trans to
the oxazoline N atom. A stereoelectronically preferred
orientation of the olefin part parallel to the ferrocene axis,
that is, perpendicular to the Pd square plane,[25] is likely for
1
Kinetic investigations by H NMR spectroscopy showed
a nearly linear relationship of the product yield and the
reaction time for yields up to about 80% (see the Supporting
Information) indicating a pseudo-zero-order kinetic depend-
ence on the carbamate. This suggests 1) a substrate saturation
À
and thus a high tendency of the substrate to coordinate, 2) C
N bond formation to be the probable rate-limiting step, and
3) that catalyst deactivation or decomposition play only
a minor role. The high affinity for the substrate coordination,
which is not observed in the aza-Claisen rearrangement for
the same catalyst,[7e] might be explained by an initial
formation of chelate complex 11’, in which both the olefin
and the anionic carbamate moiety bind to the metal center.
This would be in agreement with the observation that the type
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React. 2005, 66, 1; d) R. Peters, D. F. Fischer, S. Jautze, Top.
Organomet. Chem. 2011, 33, 139.
of anionic ligand in the activated catalysts is less critical than
usually found and that even the often inert chloride-bridged
dimer [PPFOP-Cl]2 is a competent catalyst at more higher
temperatures, as YÀ or chloride would be replaced by the
deprotonated substrate. The suggested outer-sphere mecha-
nism would require a dissociation of the anionic carbamate
moiety prior to the rearrangement.
To confirm substrate saturation, the reaction mixture was
examined by ESI-HRMS after 0.5 h at 358C using 5 mol% of
[PPFOP-O2CCF3]2. In the dominating ferrocene palladacycle
detected (carrying not only the C,N ligand), one substrate
molecule is bound to a monomeric palladacycle (see the
Supporting Information). Heavier species, in which, for
example, two substrate molecules bind to one palladacycle,
provided smaller peaks. 19F NMR spectroscopy confirmed
that nearly none (< 2%) of the initially generated activated
catalyst species [PPFOP-O2CCF3]2 is still present under the
reaction conditions. The large majority of ÀO2CCF3 seems not
to bind any more to the catalyst (broad signals, see the
Supporting Information).
In conclusion, we have described a step-economic cata-
lytic asymmetric methodology capable of transforming achi-
ral allylic alcohols in a single step and with high enantio- and
regioselectivity into sulfonyl-protected chiral allylic amines.
These reactions have been demonstrated to proceed through
a decarboxylative rearrangement of allylic carbamates, which
explains the preference for the branched allylic product
regioisomers. The allylic carbamates can be generated in situ
by addition of the corresponding allylic alcohol to an
isocyanate. The title reaction offers the additional practical
advantages that it can be performed under air and that
catalyst activation by a silver salt, which is necessary for many
other chiral palladacycle-catalyzed asymmetric reactions, is
not required in the present case.
[5] Another very versatile method employs alkynes and imines as
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[10] For an excellent recent review about cascades involving catalysis
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Received: March 7, 2014
Published online: June 4, 2014
Keywords: Brønsted bases · domino reactions · Lewis acids ·
.
palladacycles · sulfonamides
[12] For a PdII-catalyzed asymmetric synthesis of oxazolidinones see:
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[13] A solvent screening revealed that CH2Cl2 and other non- or
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reactivity and enantioselectivity, whereas Lewis basic solvents
such as acetonitrile and DMF considerably slow the process.
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[23] The absolute configuration of 2a was determined by X-ray
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the absolute configurations have already been assigned (see the
Supporting Information).
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[22] Under identical conditions a p-nosyl-protected substrate carry-
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Angew. Chem. Int. Ed. 2014, 53, 7634 –7638