Asymmetric alkylation of remote C(sp3)-H bonds by combining proton-coupled electron transfer with chiral Lewis acid catalysis

Article abstract of DOI:10.1039/c7cc04941b

The catalytic asymmetric alkylation of the remote, unactivated δ-position of N-alkyl amides was enabled by the combination of visible-light-induced proton-coupled electron transfer, 1,5-hydrogen atom transfer, and chiral Lewis acid catalysis in up to 82% yield and up to 97% ee.

Full text of DOI:10.1039/c7cc04941b

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Asymmetric alkylation of remote C(sp3)–H bonds
by combining proton-coupled electron transfer
with chiral Lewis acid catalysis†
ab
Cite this:DOI: 10.1039/c7cc04941b
Received 26th June 2017,
Accepted 20th July 2017
Wei Yuan,a Zijun Zhou,a Lei Gong *a and Eric Meggers
*
DOI: 10.1039/c7cc04941b
rsc.li/chemcomm
The catalytic asymmetric alkylation of the remote, unactivated alkyl C–H bonds involving nitrogen radical formation through
d-position of N-alkyl amides was enabled by the combination of proton-coupled electron transfer (PCET),14,15 1,5-hydrogen
visible-light-induced proton-coupled electron transfer, 1,5-hydrogen atom transfer (HAT) to generate a carbon radical, followed by
atom transfer, and chiral Lewis acid catalysis in up to 82% yield and up trapping with electron-deficient alkenes.
to 97% ee.
Despite these impressive advances, the combination of such
heteroatom-radical-enabled remote C(sp3)–H activations with
The direct functionalization of C(sp3)–H bonds provides one of asymmetric catalysis is still a formidable task due to the necessary
the most desirable synthetic approaches to carbon–carbon and compatibility of the individual catalytic processes and reactivity of
carbon–heteroatom bond formation.1 Although remarkable the involved radicals.13 Herein we wish to report a catalytic
efforts have been made, it remains challenging to selectively asymmetric alkylation of unactivated d-C(sp3)–H bonds of N-alkyl
functionalize unactivated alkyl C–H bonds owing to their amides by merging PCET, 1,5-HAT, and chiral Lewis acid catalysis
thermodynamic and kinetic stability. Their bond dissociation (Fig. 1).‡
energies (BDE) are significantly higher than those for C–H
We were inspired by the amide-directed C–H functionalization
bonds adjacent to a heteroatom or a p-system. For instance, the recently reported by Knowles11 and Rovis12 and thought to com-
BDE for the secondary C–H of n-propane is 98.6 kcal molꢀ1, while bine it with our methodology of rhodium-catalyzed asymmetric
the C–H BDE for the methyl of toluene is only 89.7 kcal molꢀ1 2,3
.
radical chemistry (Table 1).16–18 Initially, the reaction of 4-methoxy-
Recently, heteroatom-radical-mediated hydrogen atom transfer N-(4-methylpentyl)-benzamide 1a11 with the a,b-unsaturated 2-acyl
(HAT)4 induced by visible light has emerged as a promising imidazole 2a was chosen as a model system. In the presence of
strategy for the functionalization of remote C(sp3)–H bonds, our previously developed chiral-at-rhodium Lewis acid catalyst
considering the mild reaction conditions, high site-selectivity, L-RhO19 (8 mol%), the photoredox mediator [Ir(dF(CF3)ppy)2-
and functional group tolerance.5–13 Qin and Yu recently reported (5,50-dCF3bpy)](PF6) (PC1, 2 mol%),20 the weak phosphate base
a visible-light-promoted C(sp3)–H amidation and chlorination.5
˜
Muniz developed N-radical-mediated aminations of remote
C(sp3)–H bonds by utilizing photoinduced iodine catalysis.6 The
Chen group reported a selective allylation and alkenylation of
inert alkyl C–H bonds employing an O-centered radical which
was initiated by visible-light-driven photoredox catalysis.9 Glorius
developed a regioselective trifluoromethylthiolation of substrates
containing several alkyl C(sp3)–H bonds, which proceeds through
hydrogen abstraction by a benzoyloxy radical.10 Knowles11 and
Rovis12 independently reported the catalytic alkylation of unactivated
a College of Chemistry and Chemical Engineering, Xiamen University, Xiamen,
361005, People’s Republic of China. E-mail: gongl@xmu.edu.cn
b
¨
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse 4,
35043, Marburg, Germany. E-mail: meggers@chemie.uni-marburg.de
† Electronic supplementary information (ESI) available: Experimental details and
analytical data. CCDC 1554166. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7cc04941b
Fig. 1 Concept of this study by combining PCET, 1,5-HAT, and chiral
Lewis acid catalysis. PMP = p-methoxyphenyl.
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Table 1 Initial experimentsa
Table 2 Scope of a,b-unsaturated imidazolesa
Entry R1
R2
Product t (h) Yieldb (%) eec (%)
1
2
3
4
5
6
7
8
C6H5
CH3
i-C3H7 C6H5
C6H5
C6H5
3a
3b
3c
3d
3e
3f
3g
3h
3i
38
41
46
48
48
41
45
48
48
48
38
30
38
80
62
50
62
55
68
65
41
51
40
0
94
97
95
93
94
93
92
95
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-F-C6H4
4-Cl-C6H4
4-Br-C6H4
4-(CF3)-C6H4
4-(CH3)-C6H4
3-(CH3)-C6H4
2-(CH3)-C6H4
4-(CH3O)-C6H4 3k
Me
Rh cat.
Entry (mol%)
PC
Base
Yieldb eec
9
95
(mol%) (mol%) Solvent t (h) (%)
(%)
10
11
12
13
3j
90
n.a.d
n.d.d
n.a.d
1
2
3
4
5
6
7
8
L-RhO (8) PC1 (2) B1 (10) CH2Cl2 42
L-RhO (8) PC1 (4) B1 (10) CH2Cl2 38
72
74
80
57
28
22
91
94
94
90
75
95
3l
3m
Trace
0
n-Pr
L-RhO (8) PC1 (4) B1 (8)
L-RhO (8) PC1 (4) B1 (8)
L-RhO (8) PC1 (4) B1 (8)
L-RhO (8) PC1 (4) B2 (8)
L-RhO (8) PC1 (4) B3 (8)
L-RhO (8) PC2 (4) B1 (8)
L-RhO (8) PC3 (4) B1 (8)
CH2Cl2 38
PhCF3
CHCl3
38
20
a
Reaction conditions: 1a (0.30 mmol), 2a–m (0.20 mmol), L-RhO
(8 mol%), PC1 (4 mol%), B1 (5 or 8 mol%) and 4 Å MS (200 mg),
CH2Cl2 38
CH2Cl2 44
CH2Cl2 38
CH2Cl2 38
CH2Cl2 38
CH2Cl2 38
CH2Cl2 38
CH2Cl2 38
b
CH2Cl2 (1.0 mL), 27 1C, argon, two 24 W blue LED lamps. Isolated
Trace n.d.d
Trace n.d.d
Trace n.d.d
c
d
yield. Determined by chiral HPLC. n.d. = not determined, n.a. = not
applicable.
9
10e
11
12
13f
None
L-RhO (8) None
PC1 (4) B1 (8)
B1 (8)
8
5
0
0
n.d.d
n.d.d
n.a.d
n.a.d
the electron-donating 4-methoxylphenyl group (3k, entry 11) nor
aliphatic substituents provided the desired product in significant
amounts (3l and 3m, entries 12 and 13).
As revealed in Fig. 2, an electron donating methoxy group at
the para-position of the phenyl ring of the benzamide sub-
strates was highly beneficial for obtaining high isolated yields
and enantioselectivities, most likely by rendering the amide
more electron rich and facilitating its oxidation under PCET
L-RhO (8) PC1 (4) None
L-RhO (8) PC1 (4) B1 (8)
a
Reaction conditions: 1a (0.15 mmol), 2a (0.10 mmol), L-RhO (0 or 8 mol%),
PC1–3 (0–4 mol%), B1–3 (0–10 mol%), 4 Å MS (100 mg), CH2Cl2 (0.50 mL),
27 1C, and blue LED lamps (2 ꢁ 24 W) under argon. b Isolated yield.
c Determined by chiral HPLC. d n.d. = not determined, n.a. = not applicable.
e [2+2] cycloaddition product of 2a was isolated in 26% yield. f In the dark.
NBu4P(O)(OBu)2 (B1, 10 mol%)11 under irradiation with two conditions (compare 3n–3p). Moreover, we evaluated the influence
24 W blue LEDs, the desired C(sp3)–H activation product 3a was of the steric environment around the remote inert C(sp3)–H bonds
obtained in 72% yield and with an enantioselectivity of 91% ee on the reaction outcomes. Products 3q–3x contain all-carbon
(entry 1). Molecular sieves was added to reproducibly deplete quaternary carbons next to the introduced stereocenter and were
any water content. The reaction could be further improved by obtained in 51–82% yield and with 89–97% ee (28 : 1 dr for 3t).
adjusting the amount of the photoredox mediator and phos- However, some limitations were observed. First, in cases where a
phate base, and CH2Cl2 appeared to be the solvent of choice second stereocenter was implemented (at the d-position of the
(entries 2–5). Under optimized conditions, the amide directed benzamide nitrogen), the diastereoselectivities were low (products
d-C–H functionalization was provided in 80% yield with 94% ee 3ya–3za). Second, the functionalization of methyne over methylene
(entry 3). Other photoredox mediators such as [Ir(dF(CF3)ppy)2(4,40- C(sp3)–H bonds (products 3ya and 3yb) is clearly favored.
dtbbpy)]PF6 (PC2) and [Ir(dF(CF3)ppy)2bpy] PF6 (PC3) as well as
The proposed mechanism merges visible-light-induced PCET,
phosphate bases B2 and B3 were less beneficial for the transforma- 1,5-HAT, and asymmetric Lewis acid catalysis (Fig. 3). Accordingly,
tion 1a + 2a - 3a (entries 6–9). Control experiments demonstrate the carbon radical formation follows the mechanism previously
that this reaction relies on the combination out of rhodium catalyst, proposed for the remote C–H functionalization of amides.11,12
photoredox mediator, phosphate base, and blue LEDs (entries Visible-light-induced PCET converts the amide N–H into an amidyl
10–13). Furthermore, replacing the rhodium catalyst with radical which then undergoes an intramolecular 1,5-HAT, thereby
nickel(II) or copper(II) bis-oxazoline complexes did not provide converting the d-C–H group into a carbon-centered radical. Such
any product 3a (see ESI† for details).
a radical pathway is supported by control experiments in the
Next, we evaluated the substrate scope. A variety of presence of air or the radical inhibitor BHT which suppressed
a,b-unsaturated 2-acyl imidazoles containing different b- or yields and enantioselectivities. We could also trap the proposed
N-substituents were tested under our optimized standard conditions intermediate carbon-centered radical with an acceptor substituted
(Table 2). As a result, different N-imidazole substituents (R1) were alkene in analogy to the recent work by Knowles11 and Rovis12 (see
well tolerated (entries 1–3) as well as a variety of substituents in the ESI† for more details). In our initial proposal we envisioned that
b-phenyl group (R2) (entries 4–10), affording the products 3a–3j in this carbon-centered radical (intermediate B) would engage in a
40–80% yield and with 90–97% ee. However, as a limitation, neither stereocontrolled conjugate addition to the rhodium-coordinated
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additions.17a,b,22 We therefore propose a somewhat modified
reaction scheme for the here introduced reaction, namely that
the Rh-coordinated a,b-unsaturated 2-acyl imidazole (inter-
mediate C) is first reduced to a radical anion (intermediate D),23
thereby regenerating the photoredox mediator (PCꢂꢀ-PC). This
rhodium-stabilized and most likely persistent24 enolate radical
anion then combines with the carbon-radical B to form a new
C–C bond in an asymmetric fashion.25 The enolate intermediate E
is subsequently protonated to rhodium-coordinated product (inter-
mediate F) and upon release of the product and coordination of
new substrate a new catalytic cycle can be initiated. For this
mechanism to operate, the single electron reduction of the
rhodium-coordinated a,b-unsaturated 2-acyl imidazole is a key
step and must outcompete with the direct conjugate addition of
the radical intermediate B. This is apparently only the case
when the formed rhodium-enolate radical anion (intermediate C)
is stabilized by a b-aryl group and which does not contain strongly
electron-donating groups, thus rationalizing the observed sub-
strate scope. We recently realized an enantioselective catalytic
b-amination of such a,b-unsaturated 2-acyl imidazoles in which
we also propose a rhodium-coordinated enolate radical anion
as the key intermediate.17d However, for the here reported
reaction, we cannot exclude that both mechanisms, the direct
conjugate radical addition and the here proposed radical–
radical recombination, coexist and a substrate dependence
cannot be excluded either. At the end, these two mechanistic
scenarios only differ by the order of two steps, the back electron
transfer and C–C coupling step. Regardless, the asymmetric
induction for both mechanistic scenarios is supposed to be
similar, providing S-configured products with L-configured
rhodium catalyst (see ESI†).
In summary, we have developed a visible-light-induced stereo-
selective alkylation of remote, unactivated C(sp3)–H bonds by
employing a combination of visible-light-induced proton-coupled
electron transfer, hydrogen atom transfer, and chiral Lewis acid
catalysis. As a potentially useful approach for selective functio-
nalization of inert C(sp3)–H bonds, the method further demon-
strates that this chiral-at-rhodium Lewis acid is well compatible
with other catalysts and additives in such a complicated system.
We are grateful for financial support from the National
Natural Science Foundation of China (grant no. 21572184 and
21472154), the Natural Science Foundation of Fujian Province of
China (grant no. 2017J06006), and the Fundamental Research
Funds for the Central Universities (grant no. 20720160027).
Fig. 2 Scope of benzamide substrates.
Notes and references
‡ Representative catalytic asymmetric C–H activation: a dried 10 mL
Schlenk tube was charged with [Ir(dF(CF3)ppy)2(5,50-dCF3bpy)](PF6)
(PC1, 9.17 mg, 0.0080 mmol), L-RhO (13.28 mg, 0.0160 mmol), tetra-
butylammonium dibutyl phosphate (B1, 7.22 mg, 0.016 mmol), 4 Å
molecular sieves (200 mg), benzamide 1a (70.5 mg, 0.30 mmol) and
a,b-unsaturated 2-acyl imidazole 2a (54.8 mg, 0.20 mmol). The reaction
mixture was degassed and backfilled with argon for three cycles.
Degassed CH2Cl2 (1.0 mL) was added under argon. The Schlenk tube
was sealed and positioned approximately 3 cm away from two 24 W blue
LED lamps. After being stirred at 27 1C for 38 h, the reaction mixture
was concentrated and then purified by flash chromatography on silica
Fig. 3 Proposed catalytic cycle. See ESI† for a variation of this mechanism
in which radical addition occurs directly to the rhodium-coordinated
a,b-unsaturated 2-acyl imidazole.
a,b-unsaturated 2-acyl imidazole (intermediate C).21 However,
the substrate scope of this work is remarkably distinguished
from our previous work on rhodium-catalyzed conjugate radical gel (acetone/n-hexane = 1 : 5 to 1 : 4) to afford product 3a as a white solid
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