Palladium-catalyzed picolinamide-directed acetoxylation of unactivated γ-C(sp3)-H bonds of alkylamines

Article abstract of DOI:10.1002/adsc.201400121

We report a new protocol for palladium-catalyzed acetoxylation of the γ-C(sp³)-H bonds of N-alkylpicolinamide substrates using PhI(OAc)₂ oxidant. These reactions involve the use of substoichiometric amounts of Li₂CO₃ additive, which effectively suppresses the competing intramolecular C-H amination process. Under these conditions, N-propylpicolinamides bearing α substituents can be cleanly converted to γ-acetoxylated amine products in excellent yield. This C-H acetoxylation can also be used in concert with other Pd-catalyzed picolinamide-directed C-H functionalization reactions for rapid scaffold diversification.

Full text of DOI:10.1002/adsc.201400121

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DOI: 10.1002/adsc.201400121
Palladium-Catalyzed Picolinamide-Directed Acetoxylation of
3
À
Unactivated g-C
N
Qiong Li,a Shu-Yu Zhang,a Gang He,a William A. Nack,a and Gong Chena,*
a
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
Fax : (+1)-814-863-5319; e-mail: guc11@psu.edu
Received: January 31, 2014; Published online: April 30, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400121.
Abstract: We report a new protocol for palladium-
icant advances, further development is needed to
3
3
À
À
catalyzed acetoxylation of the g-C
N
broaden the utility of this Pd-catalyzed C
G
N-alkylpicolinamide substrates using PhI
N
genation chemistry in organic synthesis.[8] Herein, we
report a new protocol for Pd-catalyzed picolinamide-
idant. These reactions involve the use of substoi-
chiometric amounts of Li2CO3 additive, which effec-
directed PhIACHTNUTRGNEUNG(OAc)2-mediated acetoxylation of unacti-
3
À
À
tively suppresses the competing intramolecular C
vated CACTHNGUTRENNU(G sp ) H bonds at the g-positions of alkyla-
H amination process. Under these conditions, N-
propylpicolinamides bearing a substituents can be
cleanly converted to g-acetoxylated amine products
mine substrates, featuring the use of Li2CO3 additive.
Our laboratory has extensively investigated the ap-
plications of the picolinamide (PA) moiety as a direct-
À
À
in excellent yield. This C H acetoxylation can also
be used in concert with other Pd-catalyzed picolin-
amide-directed C H functionalization reactions for
ing group in Pd-catalyzed C H functionalization reac-
tions.[9] The PA group, first introduced by the Daugu-
N
lis laboratory,[10a] enables a range of C C bond form-
À
À
rapid scaffold diversification.
ing transformations.[10,11] Recently, we discovered that
PA-coupled alkylamine substrates (e.g., valine ester
1) underwent intramolecular amination at the g-posi-
tion, forming 4-membered azetidine products (e.g., 3)
along with a small amount of acetoxylated side prod-
À
Keywords: C H acetoxylation; palladium; picolin-
ACHTUNGTRENNUNGamides
uct (e.g., 4) under Pd-catalyzed PhIACTHNUTRGENUG(N OAc)2-mediated
conditions [Eq. (1), Scheme 1]. Under the same reac-
tion conditions substrates lacking b-substituents (e.g.,
Selective carbon-hydrogen (C H) bond oxygenation 5) favor the formation of acetoxylated product (e.g.,
À
is a common strategy used by nature for both biosyn- 7) [Eq. (2)]. It is likely that this PA-directed intramo-
3
thesis and post-synthetic modification.[1] C H oxygen- lecular C
G
À
À
ation reactions have also received much attention in membered Pd(II) metallacycle intermediate, which is
organic synthesis due to their transformational sim- oxidized by PhI
ACHTUNGRTEN(NUNG OAc)2 to form a Pd(IV) species (e.g.,
plicity and the accessibility of starting materials.[2,3] In 2).[12,13] This resulting Pd(IV) intermediate can then
À
À
particular, palladium-catalyzed oxygenation of C H undergo either C N reductive elimination to give a cy-
bonds, including unactivated sp3 hybridized C H clized product, or C O reductive elimination to give
À
À
bonds, has been quickly advanced over the past the acetoxylated product.[14]
decade.[4–7] Several protocols employing various N-
Encouraged by these results, we sought to better
based directing groups have been developed for the control these Pd-catalyzed PA-directed PhI(OAc)2-
(sp ) H bonds under oxida- mediated C H functionalization reactions to develop
AHCTUNGTRENNUNG
3
À
À
selective oxygenation of CACHTGNUTRENNUNG
3
À
tive conditions. In a seminal work, Sanford and co- a generally applicable method for CACTHNUTRGNE(NUG sp ) H acetoxy-
workers reported
a
Pd-catalyzed oxime-directed lation. Our investigation commenced with the acetox-
(OAc)2 as oxi- ylation of simple aliphatic picolinamide 8 [Eq. (3),
3
À
C
G
E
dant.[5a] Yu and co-workers have elegantly demon- Table 1]. Reaction optimization quickly revealed that
strated Pd-catalyzed oxazoline-directed diastereose- PhI
(OAc)2 was the most effective oxidant; oxidants
3
À
lective C
as oxi
rected C
has also been reported by Sahoo. Despite these signif- tion of 8 and a mixture of acetoxylated and cyclized
A
N
(sp ) H acetoxylation at room temperature
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Palladium-Catalyzed Picolinamide-Directed Acetoxylation
Table 2. Screening of salt additives.
Entry
Additive (equiv.)
Yield [%][a]
8
9
10
1
2
3
4
5
6
7
8
NaCl (0.2)
LiCl (0.2)
CsF (0.2)
29
22
6
5
<2
5
5
55
7
4
17
56
60
68
77
83
6
7
16
10
<3
11
3
K2CO3 (0.2)
Li2CO3 (0.2)
Cs2CO3 (0.2)
Na2CO3 (0.2)
Ag2CO3 (0.2)
NaOAc (0.2)
Li2CO3 (0.3)
Li2CO3 (0.5)
Li2CO3 (1.0)
>95 (91)[b]
82
89
33
79
94
80
38
6
9
13
<2
<2
<2
10
11
12
[a]
Scheme 1. Pd-catalyzed PA-directed functionalization of g-
Screening reactions were carried on a 0.2-mmol scale;
yields are based on H NMR analysis of the crude reac-
tion mixture.
Isolated yield.
3
1
À
C
G
N
[b]
products 9 and 10. In our previous study of intramo-
lecular amination, we found that the selectivity of
À
acetoxylation and C N cyclization can be influenced
by the addition of small amount of AcOH. We were tion of both cation and anion species often had a nota-
In our previous studies, we observed that the addi-
delighted to find that the yield of 9 was further im- ble impact on the reactivity and selectivity of our Pd-
À
proved to 82% using a mixed solvent of AcOH and catalyzed PA-directed C H functionalization systems.
xylene (1:2) (entry 9).[15]
Thus, we proceeded to investigate the effect of vari-
ous salt additives (Table 2). As shown in entries 1–3,
while halogen anions seem to be slightly inhibitory,
carbonate salts show some beneficial effect. To our
[16]
Table 1. Optimization of the reaction conditions.
delight, use of 20–30 mol% of Li2CO3
effectively
suppressed the formation of 10 and afforded 9 in 91%
isolated yield (entry 5). Interestingly, use of 1 equiv.
of Li2CO3 resulted in a lower conversion of 8
(entry 12).
With the optimized conditions in hand, we next ex-
amined the substrate scope of this Pd-catalyzed C H
Entry Reagents
Solvent[b]
Yield [%][a]
À
9
10
acetoxylation reaction (Figure 1). Ester, t-Bu ether
and OTBS functional groups were tolerated under the
standard reaction conditions (see 11, 13, 16). The pri-
1
2
3
4
5
6
7
8
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
A
A
A
A
T
X
<2
<2
<2
66
<2
<2
<2
<2
52
À
mary g-C H bonds of propylpicolinamides bearing a-
E
R
E
N
G
G
A
substituents can be acetoxylated in good to excellent
yield (see 11, 14, 19). In contrast, unsubstituted pro-
pylpicolinamide only gave a 37% yield of acetoxylat-
ed product 15, along with 55% unreacted starting ma-
terial. Additionally, even under our improved condi-
tions, the acetoxylation of substrates bearing b-sub-
stituents remains challenging. For instance, a t-Bu-
protected threonine substrate gave a mixture of ace-
toxylated product 16 (42%) and cyclized azetidine
product 16c (54%). In comparison, cyclized product
16c was formed in 85% yield under the original condi-
tions [see the conditions of Eq. (1), Scheme 1]. Acet-
46
41
53
A/T (1:1)
77
13
A/X (1:1) 76
A/X (1:2) 82
A/X (1:4) 72
A/X (1:2) 78
10
16
24
16
9
10
11
12
PdCl2, PhI
Pd
R
(OAc)2 A/X (1:2) 74
22
[a]
Screening reactions were carried on a 0.2-mmol scale;
1
yields are based on H NMR analysis of the crude reac-
tion mixture.
A=AcOH, T=toluene, X=xylene.
[b]
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Qiong Li et al.
3
À
Figure 2. Acetoxylation of benzylic C
CACTHNUTRGNE(UNG sp ) H bonds at the g-position.
(sp ) H and ortho
2
À
À
AHCTUNGTREGoNNUN xylation of methylene C H bonds generally failed to
proceed in meaningful yield (<5%) for most sub-
strates, with the exception of 17. As shown in Fig-
À
ure 1B, this Pd-catalyzed PA-directed C H acetoxyla-
tion reaction can be used in concert with other PA-di-
À
rected C H functionalizations to transform simple
amine precursors into complex products. For example,
the g-methyl group of 3-pentylpicolinamide 20 can be
first monoalkylated,[9g] monoarylated,[9a] or monome-
thoxylated[9d] to give intermediates 21, 23, and 25
under our previously reported Pd-catalyzed condi-
tions. The remaining g-methyl group of these inter-
mediates can then be acetoxylated to give 22, 24 and
26 in good yield and selectivity. Similarly, 1-phenyl-
2
À
propylpicolinamide 27 underwent C
G
thoxylation at the ortho-position to give 28, which
was then acetoxylated at the g-methyl position to give
29 in good yield under the standard conditions.
3
À
As shown in Figure 2, C
(sp ) H acetoxylation of
the methyl group of PA-coupled o-toluidines[17] and
2
À
C
(sp ) H acetoxylation of the ortho-position of ben-
zylamines[18] also proceeded in good yield under Pd-
catalyzed conditions. In both cases, addition of
Li2CO3 additive was not required to suppress the C
N cyclization process.
Figure 1. Substrate scope of PA-directed acetoxylation of
The mechanistic details of this Pd-catalyzed PA-di-
3
À
unactivated g C
(sp ) H bonds.
3
À
rected PhI
G
N
are uncertain. We can only speculate on the possible
role of Li+ cation.[19,20] This reaction likely proceeds
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Palladium-Catalyzed Picolinamide-Directed Acetoxylation
bonds of N-alkylpicolinamide substrates using PhI-
AHCTUNGRTEN(NGUN OAc)2 oxidant. These reactions involve the use of
substoichiometric amounts of Li2CO3 additive, which
can effectively suppress the competing intramolecular
À
C H amination process. N-Propylpicolinamide sub-
strates bearing a substituents can be cleanly convert-
ed to g-acetoxylated amine products in excellent
yield. This C H acetoxylation can be used in concert
Figure 3. Plausible functional role of the Li+ cation.
À
À
with other PA-directed C H functionalizations for
À
through C,N,N-pincer type Pd(IV) intermediates scaffold diversification. The PA-directed C H acetox-
(Figure 3). In principle, the deprotonated amide ylation of o-toluidines and benzylamines also pro-
group of PA can adopt two different coordination ceeded smoothly under similar reaction conditions.
modes: as an X-type amidate ligand (as in 36) or as
an L-type carboximidate ligand (as in 37). The bind-
ing modes of the PA group, which are influenced both Experimental Section
by the structure of the substrate (e.g., the ring strain
placed on the palladacycle and torsional strain) and Standard Procedure for Pd-Catalyzed C
reaction conditions (e.g., ligand and additive effect), Acetoxylation of Alkylpicolinamides
could exert a significant effect on the reactivity of the
(sp3)-H
ACHTUNGTRENNUNG
To an oven-dried 10-mL vial was added compound 8 (36 mg,
0.2 mmol), PdACHTNUGTRENNUG(OAc)2 (98%, 4.4 mg, 0.02 mmol), PhIACHTUNGTRENNUNG(OAc)2
Pd complex. We hypothesize that the X-type coordi-
À
nation mode could be associated with C N reductive
(161 mg, 0.5 mmol), AcOH (glacial, 0.65 mL), Li2CO3 (3 mg,
20 mol%) and xylene (1.35 mL). The reaction vial was
purged with argon for 1 min and sealed with a PTFE-lined
cap. The reaction mixture was heated at 1108C for 20 h. The
reaction mixture was cooled to room temperature, and con-
centrated under vacuum. The resulting residue was purified
by flash chromatography using hexanes/ethyl acetate (3/1)
to afford the desired product 9 as a white solid; yield: 43 mg
(91%).
elimination to give cyclized products and the L-type
À
coordination mode could be associated with C O re-
ductive elimination, forming acetoxylated products. If
so, the effect of Li+ cation could be attributed to its
interaction with O-imidate (see 37), facilitating it to
À
adopt an L-type coordination mode and favoring C
H acetoxylation. The contribution of other factors
such as the concentration of Li+ and AcOH co-sol-
vent remain poorly understood and necessitate future
mechanistic investigations.
The amide-linked PA group of acetoxylated prod-
ucts can be removed by hydrolysis (Figure 4). For in-
stance, the OAc group of 33 was first removed by
NaOH/MeOH to give 38. Boc activation of 38, fol-
lowed by amide cleavage with LiOH/H2O2 at room
temperature, gave benzylamine 39. The PA group of
30 was removed by NaOH/EtOH at 708C to give 40.
In summary, we have developed a new protocol for
Acknowledgements
We thank The Pennsylvania State University and the NSF
(CAREER CHE-1055795) for financial support of this work.
G.C. is an Amgen Young Investigator.
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