Palladium-catalyzed picolinamide-directed alkylation of unactivated C(sp3)-H bonds with alkyl iodides

Article abstract of DOI:10.1021/ja312277g

We report an efficient method for the alkylation of γ-C(sp ³)-H bonds of picolinamide-protected aliphatic amine substrates with primary alkyl iodides via palladium catalysis. Ag₂CO₃ and dibenzyl phosphate, (BnO)₂PO₂H, are critical promoters of this reaction. These reactions provide a convenient and straightforward method for the preparation of high-value N-containing products from readily available amine and alkyl iodide precursors.

Full text of DOI:10.1021/ja312277g

Communication
pubs.acs.org/JACS
Palladium-Catalyzed Picolinamide-Directed Alkylation of
Unactivated C(sp3)−H Bonds with Alkyl Iodides
Shu-Yu Zhang, Gang He, William A. Nack, Yingsheng Zhao, Qiong Li, and Gong Chen*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
S
* Supporting Information
inert γ-C(sp3)−H bonds of PA-protected aliphatic amine
substrates could be alkylated in a similar fashion.
Our initial attempt with aliphatic picolinamide substrate 3
and EtI failed to generate any desired product 4 under the
original C(sp2)−H alkylation conditions (Table 1, entry 1).
ABSTRACT: We report an efficient method for the
alkylation of γ-C(sp3)−H bonds of picolinamide-protected
aliphatic amine substrates with primary alkyl iodides via
palladium catalysis. Ag2CO3 and dibenzyl phosphate,
(BnO)2PO2H, are critical promoters of this reaction.
These reactions provide a convenient and straightforward
method for the preparation of high-value N-containing
products from readily available amine and alkyl iodide
precursors.
Table 1. Alkylation of γ-C(sp3)−H Bonds with EtI
yield of 4
he metal-catalyzed coupling of unactivated sp3-hybridized
a
entry
additives (equiv)
solvent
(%)
T
C−H bonds with alkyl halides remains one of the most
difficult challenges in the C−H functionalization field.1,2
Advances in this area could offer greatly simplified methods
for the construction of C(sp3)−C(sp3) bonds from a large pool
of readily accessible and economical starting materials.3
Uniquely, palladium complexes have demonstrated the
versatility both to facilitate the selective cleavage of C(sp3)−
H bonds and to effect cross-coupling with alkyl halides.1
However, despite the progress made on Pd-catalyzed C(sp2)−
H alkylation reactions over the past decade,4,5 alkylation of
unactivated C(sp3)−H bonds with alkyl halides has been much
less advanced, and protocols with synthetic relevance are even
rarer.6,7 Herein we report our latest developments on Pd-
catalyzed, picolinamide (PA)-directed alkylation of unactivated
γ-C(sp3)−H bonds of aliphatic amine substrates with primary
alkyl iodides.
1
2
K2CO3 (2), NaOTf (3), O2
K2CO3 (2), air
t-AmylOH
toluene
<2
7
3
AgOAc (2), air
toluene
<2
16
12
6
4
Ag2CO3 (1), air
toluene
b
5
Ag2CO3 (1), PdCl2 (0.1) , air
toluene
6
Ag2CO3 (1), PivOH (0.2), air
t-AmylOH
c
7
Ag2CO3 (1), BINA-PO2H (0.2), air t-AmylOH
Ag2CO3 (1), (PhO)2PO2H (0.2), air t-AmylOH
Ag2CO3 (1), (BnO)2PO2H (0.2), air t-AmylOH
18
19
23
26
8
9
10
Ag2CO3 (1), (BnO)2PO2H (0.2), air 9:1 toluene/
t-AmylOH
11
12
13
14
15
16
17
18
Ag2CO3 (1), (BnO)2PO2H (0.2), O2 9:1 toluene/
23
40
41
<2
t-AmylOH
Ag2CO3 (1), (BnO)2PO2H (0.2), Ar 9:1 toluene/
t-AmylOH
Ag2CO3 (1), (BnO)2PO2H (1), Ar
9:1 toluene/
t-AmylOH
Over the past three years, our laboratory has pursued Pd-
catalyzed C−H functionalization reactions directed by the PA
group,8 first introduced by the Daugulis laboratory in 2005.9
Last year, we reported that the ortho γ-C(sp2)−H bonds of PA-
protected benzylamine substrates (e.g., 1) can be alkylated with
alkyl halides (e.g., n-PrI) under the catalysis of Pd(OAc)2 (eq
1).8b In the course of our investigation, we found that
Ag2CO3 (1), (BnO)2PO2H (0.2),
NaOTf (0.3), Ar
9:1 toluene/
t-AmylOH
d
Ag2CO3 (1), (BnO)2PO2H (0.2),
NaI (0.3), Ar
9:1 toluene/
t-AmylOH
65 (60 )
Ag2CO3 (1), (BnO)2PO2H (0.2),
LiCl (0.3), Ar
9:1 toluene/
t-AmylOH
48
61
29
Ag2CO3 (1), (BnO)2PO2H (0.2), KI 9:1 toluene/
(1), Ar
t-AmylOH
Ag2CO3 (1), NaI (0.3), Ar
9:1 toluene/
t-AmylOH
19
20
Ag2CO3 (2), (BnO)2PO2H (0.2), Ar t-AmylOH
12
38
Ag2CO3 (2), (BnO)2PO2H (0.2),
CuCl2 (0.3), Ar
t-AmylOH
All of the screening reactions were carried out in a 10 mL glass vial on
a 0.2 mmol scale. See the SI for more extensive screening results.
nucleophilic carboxylate ligands (e.g., OAc) quickly react with
alkyl halide electrophiles to form ester side products, causing
the premature termination of catalytic C−H alkylation.4e
Gratifyingly, the desired alkylation reactions could be restored
effectively with the application of K2CO3 as a base and NaOTf
as an additive.10 Encouraged by these results on ortho C(sp2)−
H alkylation, we proceeded to investigate whether the more
a
b
Based on GC−MS analysis of the reaction mixtures. Pd(OAc)2 was
c
replaced with PdCl2. (S)-(+)-1,1′-Binaphthyl-2,2′-diyl hydrogen
d
phosphate. Isolated yield; ∼25% of 3 was recovered.
Received: December 19, 2012
Published: January 27, 2013
© 2013 American Chemical Society
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Interestingly, using simply 2 equiv of K2CO3 in toluene
provided us with a promising starting point (entry 2). Despite
the low yield, this result clearly demonstrated the feasibility of
the desired C(sp3)−H alkylation transformation. We next
included Ag+ salts, hoping that their I− scavenging ability could
improve the catalytic turnover and thus increase the conversion
of 3. Ag+ ions might also facilitate the oxidative addition (OA)
of alkyl iodides if an SN2-type OA mechanism is operative.11
Our subsequent survey revealed that Ag+ salts do influence the
reaction, and Ag2CO3 provided the best yield (16%; entry 4)
[see the Supporting Information (SI) for more extensive
screening results]. Interestingly, we found that the alkylation
reaction could proceed, albeit to a smaller extent, when the
Pd(OAc)2 catalyst was replaced with PdCl2, indicating that the
carbonate ion from Ag2CO3 could also facilitate the PA-
directed palladation of γ-C(sp3)−H bonds (entry 8).12 While
Ag+ ions could enhance the alkylation reaction, high
concentrations of free Ag+ ion might cause significant
decomposition of the electrophile, presumably through an E2
elimination pathway.
Table 2. Evaluation of Alkyl Halides
To suppress such decomposition, we sought better control of
the concentration of Ag+ species in solution. In a recent report
from the Toste laboratory,13 organic phosphoric acids were
applied as solid-to-solution phase-transfer catalysts (PTCs) for
Ag2CO3.14 Inspired by this study, we surveyed organic
phosphate additives and found that use of a catalytic amount
of organic phosphate (∼20 mol %) did promote the C−H
alkylation reaction. Simple dibenzyl phosphate (BnO)2PO2H,
commercially available at low cost, was most effective (entry 9).
In contrast with our previous C(sp2)−H alkylation system, we
found that O2 has an inhibitory effect on the reaction and that
an atmosphere of Ar provides better results (entries 10−12).
The addition of NaOTf, which promotes C(sp2)−H alkylation,
instead shut down the reaction, whereas the addition of 30 mol
% NaI or 1 equiv of KI improved the yield of the reaction by
∼20% (entries 14−20; see the SI for more conditions).15
Finally, a 60% isolated yield of 4 was obtained under the
following conditions: 10 mol % Pd(OAc)2, 20 mol %
(BnO)2PO2H, and 30 mol % NaI in 9:1 toluene/t-AmylOH
at 110 °C for 20 h (entry 15).
With the optimized conditions in hand, we then probed the
scope of alkyl halides with substrate 3 (Table 2). A number of
linear primary alkyl iodides, such as 2-chloroethyl iodide (7)
and OBn-substituted ethyl iodide 17, provided alkylated
products in moderate to good yields under the standard
conditions (A). Moreover, MeI and α-iodoacetic ester 11 were
identified as two superior alkylating reagents that afforded the
corresponding products in high yield. Methylation of 3 with
MeI in the absence of the NaI additive (condition B; see 5)
gave a comparable yield. Efficient alkylation with 11 could also
be achieved in the absence of NaI in t-AmylOH solvent
(condition C; see 12). Interestingly, the alkylation of 3 with
other less effective α-iodoacetic esters (e.g., tert-butyl ester 13
and allyl ester 15) could be notably improved with the
application of 30 mol % CuCl2 additive (condition D).16 In
general, we found that NaI is beneficial to alkylations with
simple alkyl iodides in toluene/t-AmylOH, while CuCl2
improves alkylations with α-iodoacetic esters in t-AmylOH.17
The clean transformation of PA starting materials was observed
in all of the above reactions; no N-alkylation product was
formed, and unreacted PAs could be largely recovered.
Reactions were run on a 0.2 mmol scale. Isolated yields are shown.
a
Condition B is similar to condition A except for the omission of 0.3
b
equiv of NaI (see Table 1, entry 12). Condition C is similar to
condition B except that 2 equiv of Ag2CO3 and t-AmylOH solvent are
c
used (see Table 1, entry 19). Condition D is similar to condition C
except for the addition of 0.3 equiv of CuCl2 (see Table 1, entry 20).
amine substrates, including protected threonine, alloisoleucine,
and β-homothreonine, can be alkylated with MeI and α-
iodoacetic esters in good to excellent yields under the standard
conditions (see 21, 27, 29, and 30). The alkylation of threonine
substrates could provide a convenient method for the synthesis
of a variety of β-hydroxylated amino acids, which are found in
many complex peptide natural products.18 Methylation using
inexpensive isotope-enriched 13CH3I and CD3I could also
provide a simple method for site-selective isotopic labeling of
various amino acids, which is challenging by other means (e.g.,
21 and 24).19 3-Pentylamine picolinamide 31 bearing two
equivalent γ-methyl groups could be bisalkylated with α-
iodoacetic ester 11 to give 28; 31 could also be monoalkylated
with 17 to form 32, and the remaining primary γ-C(sp3)−H
bond subsequently could be alkylated with 11 to give 33.
Furthermore, we found that ortho C(sp2)−H bonds of
benzylamine (see 34) and even β-arylethylamine substrates
(see 35 and 36) could be alkylated in good yields. Under our
previously reported Ag-free conditions, alkylation of the more
remote δ-C(sp2)−H bonds of 35 and 36 was unsuccessful.8b
In general, methylene 2° γ-C(sp3)−H bonds are much less
reactive than the 1° γ-C(sp3)−H bonds of methyl groups in this
reaction system; most of the substrates tested above were
selectively monoalkylated at the γ-methyl position. However,
we were surprised to observe that a 2° γ-C(sp3)−H bond of
exo-norbornene substrate 37 can be cleanly substituted with an
isopropyl group to form 40 in 77% yield under the typical
methylation conditions B with 5 equiv of MeI (Table 4).20 We
postulate that a 2° γ-C(sp3)−H bond of 37 was first methylated
to form 38, which was then methylated twice at the δ-C(sp3)−
H position, providing the isopropyl product. A similar result
was obtained using endo-norbornene substrate 41. Moreover,
substrate 45 could be selectively alkylated at the γ-methylene
position to give either methyl- or ethyl-substituted product 46
The scope of PA substrates was examined next (Table 3).
The primary γ-C(sp3)−H bonds of a variety of PA-protected
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Table 3. Substrate Scope of the C−H Alkylation Reaction
Scheme 1. Mechanistic Hypothesis
tentatively propose that palladacycle intermediate 49, presum-
ably generated from 48 via a concerted palladation/
deprotonation mechanism,22 undergoes OA with the alkyl
iodide via an SN2 mechanism, although a radical mechanism or
PdIII pathway23 cannot be ruled out. Dibenzyl phosphate might
work as a PTC, slowly bringing Ag+ ions into the solution phase
to activate the alkyl iodide.24 The functional roles of the NaI
and CuCl2 additives are not known at present.15,16
A more easily removable PA auxiliary (53)8a can be
employed in this C−H alkylation reaction system (Scheme
2). For example, threonine methyl ester 52 was equipped with
Scheme 2. Facile Synthesis of 2-Piperidinone
auxiliary 53 by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC)-mediated amide coupling. The resulting substrate 54
was then alkylated with 55 under standard conditions (C) in
excellent yield. The auxiliary group of 56 was then removed in
HCl(aq)/MeOH solution to give a free amine intermediate,
which cyclized in CH2Cl2 at room temperature to form 5,6-
disubstituted piperidinone 57.25
In summary, we have developed a new set of readily
applicable Pd-catalyzed reactions to alkylate unactivated,
remote C(sp3)−H bonds of picolinamide-protected aliphatic
amines with primary alkyl iodides. The reactions require
Ag2CO3 and a newly identified organic phosphate promoter,
(BnO)2PO2H. In particular, the use of MeI and α-iodoacetic
esters provides an efficient and straightforward method for
preparing high-value N-containing products from readily
available precursors.
Reactions were run on a 0.2 mmol scale. Isolated yields are shown.
Conditions A and B use 9:1 toluene/t-AmylOH solvent ( NaI);
a
conditions C and D use t-AmylOH solvent (∓CuCl2). ∼38% of the
starting material 31 was recovered.
Table 4. Sequential C(sp3)−H Methylation
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
a
Reactions were run on a 0.2 mmol scale. Isolated yields are shown. 5
b
equiv of MeI and 2 equiv of Ag2CO3 were used. 3 equiv of MeI was
c
used. 5 equiv of MeI was used.
AUTHOR INFORMATION
■
or 47, respectively, as the major product, depending on the
reaction conditions (Table 4).
This Pd-catalyzed PA-directed C−H alkylation reaction likely
proceeds through a C−H palladation/coupling sequence, and a
PdII/IV manifold might be operative (Scheme 1).21 We
Corresponding Author
guc11@psu.edu
Notes
The authors declare no competing financial interest.
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(12) For a recent example of carbonate-promoted Pd-catalyzed
C(sp3)−H functionalization, see: Chaumontet, M.; Piccardi, R.; Audic,
N.; Hitce, J.; Peglion, J. L.; Clot, E.; Baudoin, O. J. Am. Chem. Soc.
2008, 130, 15157.
(13) Hamilton, G. L.; Kanai, T.; Toste, F. D. J. Am. Chem. Soc. 2008,
130, 14984.
ACKNOWLEDGMENTS
■
We thank The Pennsylvania State University, NSF (CAREER
CHE-1055795), and ACS-PRF (51705-DNI1) for financial
support of this work.
(14) For two recent applications of organic phosphate ligands in Pd-
catalyzed reactions, see: (a) Jiang, G.; Halder, R.; Fang, Y.; List, B.
Angew. Chem., Int. Ed. 2011, 50, 9752. (b) Chai, Z.; Rainey, T. J. J. Am.
Chem. Soc. 2012, 134, 3615.
REFERENCES
■
(1) For selected reviews of C−H alkylations, see: (a) Ackermann, L.
Chem. Commun. 2010, 46, 4866. (b) Chen, X.; Engle, K. M.; Wang, D.-
H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (c) Lyons, T. W.;
Sanford, M. S. Chem. Rev. 2010, 110, 1147. (d) Colby, D. A.; Tsai, A.
S.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2012, 45, 814.
(2) For selected reviews of C(sp3)−H functionalizations, see:
(a) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083. (b) Jazzar,
R.; Hitce, J.; Renaudat, A.; Sofack-Kreutzer, J.; Baudoin, O. Chem.
Eur. J. 2010, 16, 2654. (c) Hartwig, J. F. Chem. Soc. Rev. 2011, 40,
1992.
(15) Cations (e.g., Na+, K+, and even H+) might play a useful role in
this PA-directed C−H functionalization reaction. The relatively high
solubility of NaI and KI in toluene/t-AmylOH could allow the metal
cations to be readily introduced into the reaction system. We suspect
that the cations might interact with the O atom of the PA group, which
might adopt an iminol type of binding mode with PdII. Similar cation
promoting effects have been observed in several Pd-catalyzed amide-
directed C−H functionalization reactions. For an elegant mechanistic
investigation on related reaction systems, see: (a) Zhang, X.-G.; Dai,
H.-X.; Wasa, M.; Yu, J.-Q. J. Am. Chem. Soc. 2012, 134, 11948. For a
strong promoting effect from LiCl, see: (b) Ano, Y.; Tobisu, M.;
Chatani, N. J. Am. Chem. Soc. 2011, 133, 12984.
(3) For selected reviews of cross-coupling of alkyl halides, see:
(a) Netherton, M. R.; Fu, G. C. Adv. Synth. Catal. 2004, 346, 1525.
(b) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674.
(c) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656.
(4) For selected examples of Pd-catalyzed alkylation of C(sp2)−H
bonds, see: (a) Tremont, S. J.; Rahmen, H. U. J. Am. Chem. Soc. 1984,
106, 5759. (b) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 119. (c) Hennessy, E.; Buchwald, S. L. J. Am.
Chem. Soc. 2003, 125, 12084. (d) Bressy, C.; Alberico, D.; Lautens, M.
J. Am. Chem. Soc. 2005, 127, 13148. (e) Zhang, Y.-H.; Yu, J.-Q. Angew.
Chem., Int. Ed. 2009, 48, 6097. (f) Lapointe, D.; Fagnou, K. Org. Lett.
2009, 11, 4160. (g) Mukai, T.; Hirano, K.; Satoh, T.; Miura, M. Org.
Lett. 2010, 12, 1360. (h) Jiao, L.; Bach, T. J. Am. Chem. Soc. 2011, 133,
12990.
(16) The functional role of the CuCl2 additive is unclear. Besides a
possible interaction with the PA directing group, its weak Lewis acidity
could also activate the α-iodoester electrophile.
(17) The effect of the CuCl2 additive appears to be more substrate-
dependent (see 27, 29, and 30).
(18) Boger, D. L. Med. Res. Rev. 2001, 21, 356.
(19) Gruhler, S.; Kratchmarova, I. Methods Mol. Biol. 2008, 424, 101.
(20) A small amount of further methylated product was observed
when the NaI additive was used under condition A.
(21) For selected reviews of PdIV chemistry, see: (a) Sehnal, P.;
Taylor, R. J.; Fairlamb, I. J. S. Chem. Rev. 2010, 110, 824. (b) Hickman,
A. J.; Sanford, M. S. Nature 2012, 484, 177.
(5) For selected examples of C(sp2)−H alkylation catalyzed by other
metals, see: (a) Murai, S.; Kakiuchi, S.; Sekine, S.; Tanaka, A.;
Kamatani, M.; Chatani, N. Nature 1993, 366, 529. (b) Ackermann, L.;
(22) Lapointe, D.; Fagnou, K. Chem. Lett. 2010, 39, 1118.
(23) (a) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T.
J. Am. Chem. Soc. 2009, 131, 17050. (b) Deprez, N. R.; Sanford, M. S.
J. Am. Chem. Soc. 2009, 131, 11234.
(24) The phosphate ion might play an active role during the OA
step; for instance, it might act as a weakly coordinating anion for Pd
intermediates (e.g., 49 or 51; see ref 14a for an isolated PdII complex
containing a phosphate ligand).
́
Novak, P.; Vicente, R.; Hofmann, N. Angew. Chem., Int. Ed. 2009, 48,
6045. (c) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc.
2007, 129, 5332. (d) Vechorkin, O.; Proust, V.; Hu, X. Angew. Chem.,
Int. Ed. 2010, 49, 3061. (e) Dieu, L.; Daugulis, O. Org. Lett. 2010, 12,
4277. (f) Chen, Q.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2011, 133,
428.
(6) For a recent report on an elegant C(sp3)−H alkylation reaction,
see: Shabashov, D.; Daugulis, O. J. Am. Chem. Soc. 2010, 132, 3965.
(7) (a) Chen, X.; Li, J.-J.; Hao, X.-S.; Goodhue, C. E.; Yu, J.-Q. J. Am.
Chem. Soc. 2006, 128, 78. (b) Chen, X.; Goodhue, C. E.; Yu, J.-Q. J.
Am. Chem. Soc. 2006, 128, 12634. (c) Wang, D.-H.; Wasa, M.; Giri, R.;
Yu, J.-Q. J. Am. Chem. Soc. 2008, 130, 7190. (d) Young, A. J.; White,
M. S. J. Am. Chem. Soc. 2008, 130, 14090. (e) Feng, Y.; Chen, G. Org.
Lett. 2010, 49, 958.
(25) Weintraub, P. M.; Sabol, J. S.; Kane, J. M.; Borcherding, D. R.
Tetrahedron 2003, 59, 2953.
(8) (a) He, G.; Chen, G. Angew. Chem., Int. Ed. 2011, 50, 5192.
(b) Zhao, Y.; Chen, G. Org. Lett. 2011, 13, 4850. (c) He, G.; Zhao, Y.;
Zhang, S.; Lu, C.; Chen, G. J. Am. Chem. Soc. 2012, 134, 3. (d) Zhang,
S.-Y.; He, G.; Zhao, Y.-S.; Wright, K.; Nack, W. A.; Chen, G. J. Am.
Chem. Soc. 2012, 134, 7313. (e) He, G.; Lu, C.-X.; Zhao, Y.-S.; Nack,
W. A.; Chen, G. Org. Lett. 2012, 14, 2944. (f) Zhao, Y.-S.; He, G.;
Nack, W. A.; Chen, G. Org. Lett. 2012, 14, 2948.
(9) (a) Zaitsev, V. G.; Shabashov, D.; Daugulis, O. J. Am. Chem. Soc.
2005, 127, 13154. (b) Nadres, E. T.; Daugulis, O. J. Am. Chem. Soc.
2011, 133, 7.
(10) The addition of NaOTf is necessary to obtain high yields; its
functional role remains elusive. Both Na+ and OTf− ions could play an
active role in the reaction system.
(11) (a) Reference 4a. (b) Canty, A. J. Acc. Chem. Res. 1992, 25, 83.
(c) Stille, J. K.; Lau, K. S. Y. Acc. Chem. Res. 1977, 10, 434. For
discussion of SN2-type OAs of alkyl halides to other metal complexes,
see: (d) Labinger, J. A.; Braus, R. J.; Dolphin, D.; Osborn, J. A. Chem.
Commun. 1970, 612. (e) Ellis, P. R.; Pearson, J. M.; Haynes, A.;
Adams, H.; Bailey, N. A.; Maitlis, P. M. Organometallics 1994, 13,
3215.
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