Photoinduced, copper-catalyzed alkylation of amides with unactivated secondary alkyl halides at room temperature

Article abstract of DOI:10.1021/ja4126609

The development of a mild and general method for the alkylation of amides with relatively unreactive alkyl halides (i.e., poor substrates for S _N2 reactions) is an ongoing challenge in organic synthesis. We describe herein a versatile transition-metal-catalyzed approach: in particular, a photoinduced, copper-catalyzed monoalkylation of primary amides. A broad array of alkyl and aryl amides (as well as a lactam and a 2-oxazolidinone) couple with unactivated secondary (and hindered primary) alkyl bromides and iodides using a single set of comparatively simple and mild conditions: inexpensive CuI as the catalyst, no separate added ligand, and C-N bond formation at room temperature. The method is compatible with a variety of functional groups, such as an olefin, a carbamate, a thiophene, and a pyridine, and it has been applied to the synthesis of an opioid receptor antagonist. A range of mechanistic observations, including reactivity and stereochemical studies, are consistent with a coupling pathway that includes photoexcitation of a copper-amidate complex, followed by electron transfer to form an alkyl radical.

Full text of DOI:10.1021/ja4126609

Article
pubs.acs.org/JACS
Photoinduced, Copper-Catalyzed Alkylation of Amides with
Unactivated Secondary Alkyl Halides at Room Temperature
Hien-Quang Do, Shoshana Bachman, Alex C. Bissember, Jonas C. Peters,* and Gregory C. Fu*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: The development of a mild and general method for the
alkylation of amides with relatively unreactive alkyl halides (i.e., poor substrates
for SN2 reactions) is an ongoing challenge in organic synthesis. We describe
herein a versatile transition-metal-catalyzed approach: in particular, a photo-
induced, copper-catalyzed monoalkylation of primary amides. A broad array of
alkyl and aryl amides (as well as a lactam and a 2-oxazolidinone) couple with unactivated secondary (and hindered primary) alkyl
bromides and iodides using a single set of comparatively simple and mild conditions: inexpensive CuI as the catalyst, no separate
added ligand, and C−N bond formation at room temperature. The method is compatible with a variety of functional groups,
such as an olefin, a carbamate, a thiophene, and a pyridine, and it has been applied to the synthesis of an opioid receptor
antagonist. A range of mechanistic observations, including reactivity and stereochemical studies, are consistent with a coupling
pathway that includes photoexcitation of a copper−amidate complex, followed by electron transfer to form an alkyl radical.
analogous to that observed with a copper−carbazolide complex.
Indeed, all of our previous photoinduced, copper-catalyzed
reactions of alkyl or of aryl electrophiles had employed
nucleophiles in which the nucleophilic site either was part of an
aromatic ring (carbazoles, indoles, or imidazoles)12,13a,c or was
directly attached to an aromatic ring (thiophenols).13b Never-
theless, as described in this report, we have now established that
the photoinduced coupling of an amide with an alkyl halide can in
fact be achieved: specifically, that C−N bond formation proceeds
with a wide range of coupling partners with a simple catalyst under
mild conditions (room temperature; eq 1).
INTRODUCTION
■
Because a wide array of important compounds (such as natural
products, pharmaceuticals, and functional polymers/materials)
contain nitrogen,1 the development of methods for the efficient
formation of C−N bonds is a central challenge in organic
chemistry.2,3 The formation of a Csp −N bond through a sub-
3
stitution reaction (SN2) between a nitrogen nucleophile and an
alkyl halide is a classic method4 that is widely used, despite its limita-
tions. In the case of the N-alkylation of an amide, the coupling can
proceed in good yield with electrophiles that are generally useful in
SN2 processes (e.g., primary halides), but the yield can be poor with
less reactive alkyl halides.5 Consequently, the development of a mild
and general method for amide N-alkylation with electrophiles such
as unactivated secondary halides is a worthwhile objective.
In recent years, significant advances have been described in
the discovery of transition-metal-catalyzed approaches to C−N
6,7
2
bond formation (e.g., Csp −N: Buchwald−Hartwig couplings;
Csp −N: reductive amination and olefin hydroamination9);
8
3
RESULTS AND DISCUSSION
■
however, to the best of our knowledge, there are few reports of
metal-catalyzed N-alkylations of amines with alkyl halides.10
Building on our earlier studies of the photophysics of discrete
copper−carbazolide complexes,11 we recently reported that photo-
induced, copper-catalyzed couplings of carbazoles with unactivated
secondary alkyl bromides and iodides can be achieved under mild
conditions.12 Whereas that investigation provided proof of principle
for such transition-metal-catalyzed N-alkylation reactions, we sought
to demonstrate that this photoinduced, copper-catalyzed approach
has broader generality and can be applied to more ubiquitous
nitrogen nucleophiles, for example, an amide.
In view of our hypothesis that a copper−nucleophile
complex undergoes excitation in these and related photo-
induced processes,12,13 it was unclear at the outset that a
copper−amidate complex would have the necessary absorption/
reactivity profile to engage in a copper-catalyzed N-alkylation
In a preliminary study, we determined that the conditions that we
had developed for photoinduced, copper-catalyzed N-alkylations of
carbazoles with alkyl bromides12 were ineffective when cyclo-
hexanecarboxamide was employed as the nucleophile (<2% yield;
eq 2).14 However, optimization of the various reaction parameters,
with the most critical one being the wavelength of light used, led to
N-alkylation of the amide in good yield (Table 1, entry 1).
Received: December 12, 2013
© XXXX American Chemical Society
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Control reactions established that, in the absence of CuI,
light, or LiO-t-Bu, essentially no C−N bond formation was
observed (Table 1, entries 2−4). Copper(I) complexes other
Table 2. Copper-Catalyzed N-Alkylation of an Amide at
Room Temperature: Scope with Respect to the Secondary
Alkyl Bromide
Table 1. Photoinduced, Copper-Catalyzed N-Alkylation of
an Amide by an Unactivated Secondary Alkyl Halide: Effect
of Reaction Parameters
a
entry
change from the “standard conditions”
none
yield (%)
1
90
<2
<2
<2
82
78
38
10
9
2
no CuI
3
no hν
4
no LiO-t-Bu
5
CuBr, instead of CuI
CuCl, instead of CuI
NaO-t-Bu, instead of LiO-t-Bu
KO-t-Bu, instead of LiO-t-Bu
Cs2CO3, instead of LiO-t-Bu
K3PO4, instead of LiO-t-Bu
no DMF
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
4
83
40
87
83
69
30
10
<2
90
33
85
1.2 equiv CyBr and 1.2 equiv LiO-t-Bu
5.0% CuI
2.5% CuI
1.0% CuI
0.5% CuI
hν (300 nm)
hν (100 W Hg lamp)
Honeywell 36 W UVC air-treatment lamp
Honeywell 36 W UVC air-treatment lamp, 0 °C
1.0 equiv H2O
a
a
Determined through GC analysis (average of two experiments).
Yield of purified product (average of two experiments).
secondary alkyl iodide (eq 3; for a flow reaction, see eq 419,20).21
Furthermore, we have established for the first time that an alkyl
chloride (neophyl chloride) can serve as an effective electrophile in a
photoinduced, copper-catalyzed N-alkylation (eq 3).
than CuI could be utilized with a small loss in yield (entries 5
and 6), whereas the use of several other Brønsted bases resulted
in a substantial drop in yield (entries 7−10). The omission of
DMF as a cosolvent was deleterious (entry 11),15 as was the use
of a smaller excess of electrophile/LiO-t-Bu (entry 12). A
catalyst loading as low as 2.5% could be employed with only a
minor impact on N-alkylation (entries 13−16).16 Little C−N
coupling was observed when the reaction mixture was irradiated
with longer-wavelength light (entries 17 and 18). Furthermore,
a household Honeywell 36 W UVC air-treatment lamp17 could
be used as the light source, affording the N-alkylation product
in good yield at room temperature (entry 19), whereas bond
formation was slow at 0 °C (entry 20). Finally, this method is
not highly water-sensitive (entry 21).
This photoinduced, copper-catalyzed C−N bond-forming
process can be applied to the room-temperature alkylation of an
amide with a wide range of unactivated secondary alkyl bromides
(Table 2). Thus, both acyclic and cyclic electrophiles couple with
cyclohexanecarboxamide in generally good yield. Functional groups
such as an ether, nitrile, and carbamate are compatible with the
reaction conditions, although we observe a modest yield for the
ester-containing electrophile illustrated in entry 6.
The scope of this transition-metal-catalyzed method for
N-alkylation is also fairly broad with respect to the amide
nucleophile (Table 3). Thus, a range of aliphatic primary
amides (including sterically demanding ones: entries 2−4)
couple with unactivated secondary alkyl bromides at room
Under the same conditions, cyclohexanecarboxamide also under-
goes N-alkylation with neopentyl bromide, a primary electrophile that
is typically a relatively poor substrate for SN2 reactions,18 as well as a
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Table 3. Copper-Catalyzed N-Alkylation of Aliphatic Amides
at Room Temperature
Table 4. Copper-Catalyzed N-Alkylation of Aromatic Amides
at Room Temperature
a
Yield of purified product (average of two experiments).
reaction conditions; in the case of aromatic amides, the C−N
couplings were conducted with alkyl iodides, since the relatively
low solubility of many of these amides in CH3CN/DMF led to
lower yields with alkyl bromides.23 Electron-rich, electron-poor,
and naphthyl amides are suitable reaction partners with unactivated
cyclic and acyclic secondary iodides (entries 3−9). An array of
heteroaromatic primary amides can be utilized as nucleophiles,
including furyl-, thienyl-, and pyridyl-containing substrates
(entries 10−13). These examples also illustrate the compatibility
of the coupling conditions with an aryl amine, ether, fluoride, and
nitrile.
a
Yield of purified product (average of two experiments).
temperature. An array of functional groups, including an olefin,
silyl ether, acetal, ester, and carbamate, are compatible with the
C−N bond-forming conditions.
For the photoinduced couplings illustrated in Tables 2 and 3,
we have not observed a significant amount of double-alkylation
of the primary amide to form a tertiary amide (<2%). Never-
theless, a lactam and a 2-oxazolidinone can serve as useful
nucleophiles under the same conditions, coupling with cyclic
and acyclic unactivated secondary alkyl bromides at room
temperature (eqs 5 and 6).22
Blanco-Pillado has reported that amide 2, which was
prepared in six steps, serves as an opioid receptor antagonist
(Figure 1).24 Using photoinduced, copper-catalyzed N-alkylation of
The N-alkylation of an α,β-unsaturated amide (eq 7) and of
aromatic amides (Table 4) can also be achieved under identical
Figure 1. Application of photoinduced, copper-catalyzed N-alkylation
to the synthesis of an opioid receptor antagonist (2).
C
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an unactivated secondary alkyl halide as a key step, we have
synthesized amide 2 in four steps from commercially available
pyridyl amide 1.25
We have begun to investigate the mechanism of photoinduced,
copper-catalyzed N-alkylations of amides; an outline of one possible
pathway (more complex mechanisms are also under consideration)
is depicted in Figure 2.26,27 We postulate that a copper(I)−amidate
Figure 2. Outline of one of the possible pathways for photoinduced,
copper-catalyzed N-alkylations of amides.
complex, [LnCu(Nu)] (3),28 may be a critical intermediate in the
catalytic cycle, undergoing excitation and then electron transfer.
Although we have not been able to crystallographically characterize
a relevant copper complex of a primary amide, we were able to
generate X-ray-quality crystals of a Cu(I)−oxazolidinyl tetramer (4)
through treatment of mesitylcopper(I) with 2-oxazolidinone (Figure 3).
Through competition experiments between alkyl halides, we
have established that selective N-alkylation can be achieved
according to the following reactivity order: I > Br > Cl (eq 11).
This preference is consistent with the expectations for a process
in which selectivity is determined by the relative propensity of
the alkyl halide to participate in an electron-transfer reaction.31
Finally, we have established that a photoredox catalyst does
not effect amide N-alkylation under a variety of conditions
(eq 12).32 As suggested in Figure 2, we postulate that, for the
Figure 3. Synthesis and crystal structure of copper complex 4 (thermal
ellipsoids drawn at 50% probability and hydrogens omitted for clarity).
Each oxazolidinyl group bridges two copper centers through a nitrogen
and an oxygen, affording a structure with D2 symmetry.
Copper complex 4 can be used in place of CuI in a
photoinduced, copper-catalyzed N-alkylation of an amide (eq 8).29
Furthermore, whereas tetramer 4 does not react with cyclohexyl
iodide or bromide at room temperature in the absence of light,
C−N bond formation proceeds upon irradiation (eq 9). Thus,
a copper−oxazolidinyl adduct is chemically competent in an
N-alkylation reaction with an alkyl halide.
To gain insight into the possible intermediacy of an alkyl
radical in these photoinduced couplings, electrophiles that bear
pendant olefins were subjected to the standard N-alkylation
conditions, resulting in the formation of a cis-fused [3.3.0]
bicyclic product in good yield, via cyclization and then C−N
bond formation (eq 10). The endo:exo ratio of each coupling
product is similar to that observed for the respective radical
cyclizations of these alkyl bromides,30 as would be expected if
there is a common intermediate in the ring-forming step.
photoinduced process described herein, it is not only a copper
complex that undergoes excitation but also a copper complex that
participates in the key bond-forming step of the catalytic cycle.
CONCLUSION
■
We have developed, to our knowledge, the first method for the
transition-metal-catalyzed N-alkylation of amides with unac-
tivated secondary alkyl halides: specifically, a photoinduced,
D
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copper-catalyzed coupling of primary amides with secondary
(and hindered primary) alkyl bromides and iodides. Under a
single set of relatively simple, mild conditions, a wide variety of
aliphatic and aromatic primary amides, as well as a lactam and a
2-oxazolidinone, are alkylated in generally good yield with a
range of alkyl halides. An array of functional groups, including
an acetal, olefin, carbamate, thiophene, and pyridine, are
compatible with the method. We hypothesize that the reaction
mechanism includes photoexcitation of a copper−amidate
complex (demonstrating for the first time that the nucleophile
need not include an aromatic ring), followed by electron transfer
to form an alkyl radical. We have crystallographically characterized
a tetrameric copper−oxazolidinyl complex and established that,
upon irradiation, it reacts with alkyl electrophiles to generate a new
C−N bond. Stereochemical and relative reactivity studies are
consistent with the proposed electron-transfer/radical pathway.
Our current efforts are directed at expanding the scope of these
photoinduced, copper-catalyzed processes and improving our
understanding of the reaction mechanism.
REFERENCES
■
(1) For example, see: (a) The Alkaloids: Chemistry and Biology;
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Organic Chemistry, 5th ed.; Roberts & Company: Greenwood Village,
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1575.
EXPERIMENTAL SECTION
■
General Procedure for the Photoinduced, Copper-Catalyzed
N-Alkylation of Amides. CuI (19.5 mg, 0.10 mmol), the amide
(1.00 mmol), and LiO-t-Bu (160 mg, 2.00 mmol) were added to an
oven-dried 10 mL quartz test tube that contained a stir bar (3 × 13 mm).
The test tube was fitted with a rubber septum, the joint was wrapped with
electrical tape, and the test tube was evacuated and back-filled with
nitrogen (3 cycles). A solution of the alkyl bromide (2.00 mmol) in
CH3CN (5.4 mL) and then DMF (0.8 mL) were added in turn by
syringe. The test tube was detached from the nitrogen line, and the
puncture holes of the septum were covered with vacuum grease. The
resulting mixture was stirred vigorously for 5 min, and then the quartz test
tube was placed in a Luzchem photoreactor. The stirred mixture was
irradiated with UVC lamps centered at 254 nm for 24 h (during the first
12 h, the reaction tube was shaken vertically every ∼2−3 h to ensure good
mixing of the entire reaction mixture). Next, the reaction mixture was
purified by flash column chromatography.
(9) For reviews, see: (a) Reznichenko, A. L.; Hultzsch, K. C. Top.
Organomet. Chem. 2013, 43, 51−114. (b) Nishina, N.; Yamamoto, Y.
Top. Organomet. Chem. 2013, 43, 115−144.
ASSOCIATED CONTENT
■
(10) A few publications have described N-alkylations of amines with
unactivated alkyl halides in the presence of a substoichiometric
amount of a transition metal. For example, see: (a) Aydin, A.; Kaya, I.
Electrochim. Acta 2012, 65, 104−114 (165 °C; primary alkyl bromide).
(b) Tu, X.; Fu, X.; Jiang, Q.; Liu, Z.; Chen, G. Dyes Pigm. 2011, 88,
39−43 (80 °C; primary alkyl bromide). (c) Kozuka, M.; Tsuchida, T.;
Mitani, M. Tetrahedron Lett. 2005, 46, 4527−4530 (83 °C; primary
alkyl bromide).
S
* Supporting Information
Text, figures, tables, and a CIF file giving experimental
procedures, compound characterization data, and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
(11) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690−3692.
(12) Bissember, A. C.; Lundgren, R. J.; Creutz, S. E.; Peters, J. C.; Fu,
G. C. Angew. Chem., Int. Ed. 2013, 52, 5129−5133.
(13) (a) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C. Science
2012, 338, 647−651. (b) Uyeda, C.; Tan, Y.; Fu, G. C.; Peters, J. C. J.
Am. Chem. Soc. 2013, 135, 9548−9552. (c) Ziegler, D. T.; Choi, J.;
Munoz-Molina, J. M.; Bissember, A. C.; Peters, J. C.; Fu, G. C. J. Am.
Chem. Soc. 2013, 135, 13107−13112.
(14) The conditions that were reported for the alkylation of
carbazoles with alkyl iodides12 were also ineffective (<2% yield).
(15) The beneficial effect of DMF may be due in part to its solvation
properties.
(16) The advantage of using 10% CuI is greater in the case of more
hindered coupling partners. Because copper is an earth-abundant
metal, CuI is comparatively inexpensive.
■
Corresponding Author
jpeters@caltech.edu; gcfu@caltech.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Gordon and Betty Moore
Foundation. We acknowledge Prof. Rick L. Danheiser (MIT)
for helpful discussions, and we thank Trixia M. Buscagan,
Sidney E. Creutz, Gregory P. Harlow, Dr. Nathan D. Schley,
Dr. David VanderVelde (Caltech NMR Facility), and Dr. Scott
C. Virgil (Caltech Center for Catalysis and Chemical Synthesis,
supported by the Gordon and Betty Moore Foundation) for
experimental assistance.
(17) See: http://www.amazon.com/Honeywell-UV100A1059-
Surface-Treatment-System/dp/B004I5CSCQ/ref=pd_sim_sbs_
indust_3 (accessed December 1 2013). Price: $155.
E
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(18) For example, see: Lowry, T. H.; Richardson, K. S. Mechanism
and Theory in Organic Chemistry; HarperCollins: New York, 1987; pp
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(21) (a) Under our standard conditions, an acyclic unactivated
tertiary alkyl bromide was not a suitable coupling partner. (b)
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bromide and iodide can be achieved in high yield.
(22) (a) This is not yet a general method. For example, under our
standard conditions, a six-membered lactam undergoes N-alkylation
with cyclohexyl bromide in relatively low yield (∼25%). (b)
A
preliminary attempt to alkylate a relatively unhindered (N-methyl)
secondary amide was not successful.
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nm) for 1 day at room temperature, a secondary alkyl iodide is slowly
consumed.
(24) Blanco-Pillado, M.-J.; Chappell, M. D.; De la Torre, M. G.; Diaz
Buezo, N.; Fritz, J. E.; Holloway, W. G.; Matt, J. E., Jr.; Mitch, C. H.;
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iodopyridine is formed; the mixture of chlorinated and iodinated (Cl:I
= 2.5:1) secondary amides is used in the next step of the synthesis,
since both aryl halides react with the cesium phenolate to generate the
desired diaryl ether.
(26) For the sake of simplicity, several of the copper complexes in the
catalytic cycle are illustrated as neutral species (Figure 2), although we
have established the chemical competence of anionic Cu−nucleophile
adducts in photoinduced C−X bond-forming processes (e.g., refs 12
and 13b).
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of the N-alkylation.
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