Asymmetric Palladium-Catalyzed Alkene Carboamination Reactions for the Synthesis of Cyclic Sulfamides

Article abstract of DOI:10.1002/chem.201600887

The synthesis of cyclic sulfamides by enantioselective Pd-catalyzed alkene carboamination reactions between N-allylsulfamides and aryl or alkenyl bromides is described. High levels of asymmetric induction (up to 95:5 e.r.) are achieved using a catalyst composed of [Pd₂(dba)₃] and (S)-Siphos-PE. Deuterium-labelling studies indicate the reactions proceed through syn-aminopalladation of the alkene and suggest that the control of syn- versus anti-aminopalladation pathways is important for asymmetric induction. Aminate the double bond! The enantioselective Pd-catalyzed coupling of aryl or alkenyl bromides with N-allylsulfamides affords substituted cyclic sulfamides in good yield with high levels of asymmetric induction (see scheme). The reactions proceed through stereoselective syn-aminopalladation of the alkene, which is important for high enantioselectivity.

Full text of DOI:10.1002/chem.201600887

DOI: 10.1002/chem.201600887
Communication
&
Amination
Asymmetric Palladium-Catalyzed Alkene Carboamination
Reactions for the Synthesis of Cyclic Sulfamides
Zachary J. Garlets, Kaia R. Parenti, and John P. Wolfe*[a]
that afford pyrrolidines,[11] cyclic ureas,[12] or benzo-fused six-
Abstract: The synthesis of cyclic sulfamides by enantiose-
lective Pd-catalyzed alkene carboamination reactions be-
tween N-allylsulfamides and aryl or alkenyl bromides is de-
scribed. High levels of asymmetric induction (up to 95:5
e.r.) are achieved using a catalyst composed of [Pd2(dba)3]
and (S)-Siphos-PE. Deuterium-labelling studies indicate the
reactions proceed through syn-aminopalladation of the
alkene and suggest that the control of syn- versus anti-
aminopalladation pathways is important for asymmetric
induction.
membered nitrogen heterocycles.[13] All of these transforma-
tions were demonstrated to proceed through syn-aminopalla-
dation of the alkene. As such, we elected to focus on reaction
conditions that would promote syn-aminopalladation over
anti-aminopalladation: use of aryl bromide electrophiles,
xylene as a nonpolar solvent, and NaOtBu as a base. In our pre-
vious studies on asymmetric carboamination reactions of
ureas, we discovered that the nature of the protecting group
on the cyclizing nitrogen atom has a large influence on enan-
tioselectivity.[12] The best results were obtained with substrates
bearing electron-poor aryl groups (e.g., p-nitrophenyl or p-
chlorophenyl) on the cyclizing nitrogen atom. Thus, we initially
examined the reactivity of substrates 1a and 1b bearing an N-
aryl group on the cyclizing nitrogen atom and a tert-butyl
group on the other nitrogen atom. Unfortunately, these reac-
tions failed to provide satisfactory results; products 2a and 2b
were generated in low yield and modest enantiomeric ratio
(e.r.; Table 1, entries 1 and 2). However, we subsequently dis-
covered that substrates 1c–e with an N-benzyl group (or sub-
stituted N-benzyl group) on the cyclizing nitrogen atom were
transformed to the desired products 2c–e in good yields with
useful levels of enantioselectivity (entries 3–5). Further explora-
tion of substrate structure indicated the presence of the tert-
butyl group on the non-cyclizing nitrogen atom was essential
Cyclic sulfamides are present in a wide variety of natural prod-
ucts and pharmaceuticals.[1] In addition, the sulfonyl group can
easily be cleaved from these compounds to afford synthetically
useful 1,2-diamines.[2,3] Although a number of approaches have
been developed for the synthesis of racemic cyclic sulf-
amides,[4] few methods exist for the enantioselective genera-
tion of these compounds. Enantiomerically enriched cyclic sulf-
amides are often prepared from amino acids through multistep
routes,[5] and only one asymmetric metal-catalyzed reaction for
the synthesis of cyclic sulfamides has been previously report-
ed.[6]
Our group recently developed a new method for the synthe-
sis of racemic cyclic sulfamides by Pd-catalyzed alkene carbo-
amination reactions between N-allylsulfamides and aryl halides
or triflates.[7,8] We reasoned that an asymmetric variant of this
transformation could provide straightforward access to enan-
tiomerically enriched cyclic sulfamides. However, our prior
studies had illustrated that the sulfamide-forming carboamina-
tion reactions may proceed by either syn- or anti-aminopalla-
dation of the alkene depending on the catalyst structure and
reaction conditions.[7] Because the two different CÀN bond-
forming (and potentially enantiodetermining) aminopalladation
pathways proceed through very different transition states, it
seemed likely that achieving high selectivity for one pathway
over the other would be critical for asymmetric induction.[9,10]
We have previously found that palladium catalysts support-
ed by the chiral ligand (S)-Siphos-PE provide good to excellent
results in Pd-catalyzed asymmetric carboamination reactions
Table 1. Optimization of protecting groups.[a]
Entry
R1
R2 (substrate)
Yield [%][b]
e.r.[c]
1
2
3
4
5
6
7
8
9
tBu
tBu
tBu
tBu
tBu
Bn
C6H4-p-OMe
C6H4-p-Cl
Ph2CH
C6H4-p-OMe (1a)
C6H4-p-Cl (1b)
Bn (1c)
p-MeO-Bn (1d)
m-MeO-Bn (1e)
Bn (1 f)
Bn (1g)
Bn (1h)
Bn (1i)
47 (2a)
11 (2b)
70 (2c)
77 (2d)
60 (2e)
0 (2 f)
0 (2g)
0 (2h)
0 (2i)
78:22
73:27
93:7
91:9
91:9
–
–
–
–
[a] Z. J. Garlets, K. R. Parenti, Prof. Dr. J. P. Wolfe
Department of Chemistry, University of Michigan
930. N. University Ave., Ann Arbor, MI 48109-1055 (USA)
E-mail: jpwolfe@umich.edu
[a] Conditions:
1
(1.0 equiv), 4-bromo-tert-butylbenzene (2.0 equiv),
NaOtBu (2.0 equiv), [Pd2(dba)3] (1 mol%), (S)-Siphos-PE (5 mol%), xylenes
(0.125m), 1208C, 18 h; reactions were conducted on a 0.30 mmol scale.
[b] Isolated yield. [c] Enantiomeric ratios were determined by chiral HPLC
analysis.
Supporting information and ORCID from the author for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201600887.
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Communication
for the reactivity, as substrates 1 f–i were not transformed to
the desired cyclic sulfamides (entries 6–9).[14]
Table 2. Enantioselective synthesis of cyclic sulfamides.[a]
Having identified satisfactory reaction conditions and a set
of nitrogen-protecting groups that provided acceptable enan-
tioselectivities, we set out to explore the scope of this transfor-
mation. Thus, substrate 1c was treated with a range of aryl
and alkenyl halides using the optimized conditions (Table 2). In
general, transformations of electron-neutral or -rich aryl bro-
mides provided the highest levels of asymmetric induction (en-
tries 1–7), whereas use of electron-poor substrates resulted in
slightly lower selectivity (entries 8–10). Transformations of sub-
strates bearing meta or para substituents proceeded smoothly,
but ortho-substituted aryl bromides were converted to the de-
sired products with dramatically lower enantioselectivities and
yields (entries 12 and 13). Reactions of alkenyl bromides pro-
ceeded with high enantioselectivity (>93:7 e.r.), but low yields
were obtained due to competing Heck alkenylation of the N-al-
lylsulfamide starting material (entries 14 and 15).
Entry
RÀX
Product
Yield [%][b]
e.r.[c]
1a
1b[d]
51
96
93:7
93:7
3a
2a
2b[d]
62
95
94:6
94:6
3b
3b
3c
3a
3b[d]
46
77
81:19
75:25
4a
4b[d]
48
85
94:6
95:5
5a
5b[d]
70
89
94:6
94:6
In our previous investigations of asymmetric Pd-catalyzed
carboamination reactions of ureas, we discovered that the ad-
dition of two equivalents of water improved enantioselectivi-
ties in transformations of electron-poor aryl bromides.[12a] For
example, the use of 4-bromobenzotrifluoride as an electrophile
in the asymmetric carboamination of 1-allyl-1-methyl-3-(4-ni-
trophenylurea) under standard conditions resulted in the for-
mation of the desired cyclic urea product with only 77% ee.
However, when this reaction was conducted in the presence of
water the enantioselectivity improved to 95% ee.[12a] In our
present studies on transformations of N-allylsulfamides, we
found that addition of two equivalents of water led to slight
improvements in enantioselectivity with some electron-defi-
cient electrophiles (Table 2, entries 8, 10, and 13), but did not
have an impact on asymmetric induction with most electron-
neutral or -rich aryl or alkenyl halides. However, we were sur-
prised to discover that in almost all cases, water exhibited a sig-
nificant positive influence on the chemical yield of these reac-
tions (an increase of 10–20% for most electrophiles and a 45%
increase with bromobenzene). As observed with urea sub-
strates, use of an aryl iodide rather than an aryl bromide led to
a significant decrease in both yield and enantioselectivity
(entry 3). The addition of water to this reaction improved the
yield of the product, but interestingly the enantioselectivity
was diminished. Substrates 4a and 4b that contain substitu-
ents at the internal alkene position proved to be unreactive
even when the reaction temperature was increased to 1358C
[Eq. (1)].
3d
6a
67
75
94:6
94:6
6b[d]
3e
3 f
7a
7b[d]
50
74
91:9
92:8
8a
8b[d]
65
74
87:13
90:10
3g
9a
9b[d]
62
77
89:11
88:12
3h
3i
10a
10b[d]
53
69
88:12
90:10
11a
11b[d]
60
72
90:10
92:8
3j
12a
12b[d]
46
58
76:24
68:32
3k
3l
13a
13b[d]
22
21
57:43
62:38
14a
14b[d]
56
44
94:6
94:6
3m
3n
15a
15b[d]
44
34
93:7
95:5
[a] Conditions: 1c (1.0 equiv), RÀX (2.0 equiv), NaOtBu (2.0 equiv),
[Pd2(dba)3] (1 mol%), (S)-Siphos-PE (5 mol%), xylenes (0.125m), 1208C, 18 h;
reactions were conducted on a 0.30 mmol scale. [b] Isolated yield (average
of two or more reactions). [c] Enantiomeric ratios were determined by chiral
HPLC analysis. [d] The reaction was conducted with 2.0 equiv of water
added.
group is easily and selectively cleaved by trifluoroacetic acid[15]
to afford 6 in near quantitative yield with no loss of optical
purity. In contrast, subjection of 3b to more strongly acidic
conditions[2] (HBr/phenol) leads to formation of chiral 1,2-di-
amine 7 (in high yield and e.r.) by removal of both the tert-
To further illustrate the synthetic utility of this method, we
examined the deprotection and desulfonylation of products
3a and 3b. As shown in Equations (2) and (3), the tert-butyl
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Communication
butyl and SO2 groups. Thus, overall this method provides
straightforward access to enantiomerically enriched chiral dia-
mines, which are useful building blocks for organic synthesis.[3]
actions of electron-poor aryl halides.[17] Addition of water to
the reaction between 8 and 4-bromobenzotrifluoride led to
a significant improvement in yield, but little change to diaste-
reoselectivity or enantioselectivity.[18,19]
In conclusion, we have developed an enantioselective Pd-
catalyzed alkene carboamination between N-allylsulfamides
and aryl or alkenyl bromides that affords cyclic sulfamide prod-
ucts in good yield with high enantioselectivity. The sulfamide
starting material is easily prepared in one step from the corre-
sponding allylic amine. Deprotection and/or desulfonylation of
the products can be achieved in good yield, thus providing
access to enantiomerically enriched substituted 1,2-diamines.
These transformations constitute rare examples of asymmetric
metal-catalyzed reactions that afford cyclic sulfamides. These
studies also illustrate the importance of controlling syn- versus
anti-alkene aminopalladation pathways in asymmetric Pd-cata-
lyzed alkene carboamination reactions.
We had initially postulated the mechanism of these transfor-
mations should proceed through syn-aminopalladation under
our chosen reaction conditions. In order to directly probe the
alkene addition stereochemistry and test this hypothesis, we
prepared deuterated substrate 8 and investigated its coupling
with several different aryl bromide electrophiles under our
standard conditions (Table 3). In all cases examined, the major
Acknowledgements
The authors acknowledge the NIH-NIGMS (GM098314) for fi-
nancial support of this work. The authors thank Mr. Ryan M.
Fornwald for conducting preliminary experiments in this area.
Table 3. Deuterium-labelling studies.[a]
Keywords: asymmetric
catalysis
·
enantioselective
·
heterocycles · palladium · sulfamides
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R
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d.r.[c]
e.r.[d]
1
2
3
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tBu
H
Ph
CF3
58 (9a)
67 (9b)
65 (9c)
58 (9d)
78 (9d)
17:1
8:1
9:1
7:1
94:6
95:5
92:8
88:12
89:11
5:1
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8
(1.0 equiv), ArÀX (2.0 equiv), NaOtBu (2.0 equiv),
[Pd2(dba)3] (1 mol%), (S)-Siphos-PE (5 mol%), xylenes (0.125m), 1208C,
18 h; reactions were conducted on a 0.25 mmol scale. [b] Isolated yield
(result of a single experiment). [c] Diastereomeric ratios were determined
by 1H NMR integration [d] Enantiomeric ratios were determined by chiral
HPLC analysis. [e] The reaction was conducted with 2.0 equiv of water
added.
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product 9a in 17:1 d.r. and 94:6 e.r. (entry 1), whereas the anal-
ogous reaction with 4-bromobenzotrifluoride provided product
9d in 7:1 d.r. and 88:12 e.r. (entry 4). The diminished diastereo-
selectivity likely results from the competing anti-aminopallada-
tion mechanistic pathway,[16] and this competing pathway also
is likely responsible for the decreased enantioselectivity in re-
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ˇ
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Received: February 25, 2016
Published online on March 10, 2016
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