Identification of Bond-Weakening Spirosilane Catalyst for Photoredox α-C?H Alkylation of Alcohols

Article abstract of DOI:10.1002/adsc.201901253

The development of catalyst-controlled site-selective C(sp³)?H functionalization is a current major challenge in organic synthesis. This paper describes DFT-guided identification of pentavalent silicate species as a novel bond-weakening catalyst for the α-C?H bonds of alcohols together with a photoredox catalyst and a hydrogen atom transfer catalyst. Specifically, Martin's spirosilane accelerated α-C?H alkylation of alcohols. (Figure presented.).

Full text of DOI:10.1002/adsc.201901253

Title: Identification of Bond-Weakening Spirosilane Catalyst for
Photoredox α-C–H Alkylation of Alcohols
Authors: Kentaro Sakai, Kounosuke Oisaki, and Motomu Kanai
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Identification of Bond-Weakening Spirosilane Catalyst for
Photoredox -C–H Alkylation of Alcohols
Kentaro Sakai,a Kounosuke Oisaki,a* and Motomu Kanaia*
a
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan
Fax: (+81)-3-5684-5206; Tel: (+81)-3-5841-4830; e-mail: oisaki@mol.f.u-tokyo.ac.jp, kanai@mol.f.u-tokyo.ac.jp
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. The development of catalyst-controlled site-
selective C(sp3)–H functionalization is a current major
challenge in organic synthesis. This paper describes DFT-
guided identification of pentavalent silicate species as a
novel bond-weakening catalyst for the α-C–H bonds of
alcohols together with a photoredox catalyst and a
hydrogen atom transfer catalyst. Specifically, Martin’s
spirosilane accelerated -CH alkylation of alcohols.
Keywords: alcohols; bond-weakening; C–H activation;
photoredox catalysis; regioselectivity; silicates
CH functionalization reactions facilitate the
discovery and lead optimizations of functional
materials and drugs by realizing efficient production
of structurally intractable compounds and late-stage
Scheme 1. Bond-weakening Strategy toward Catalyst-
controlled Site-selective C(sp3)-H Functionalization
diversification.[1]
functionalizations are desired for drug discovery due
to the tendency toward achieving higher success rates
with C(sp3)-rich molecules.[2,3]
Particularly,
C(sp3)H
catalysis in organic synthesis.[8] A coordination-
induced bond-weakening strategy using low-valent
metals, however, would be problematic in
combination with the strongly oxidative PC-HAT
hybrid catalysis for targeting CH bonds.
Many groups, including ours, are studying
C(sp3)H functionalizations promoted by the
combination of a photoredox catalyst (PC) and
hydrogen atom transfer (HAT) catalyst.[4,5] Despite
the great bond dissociation energies (BDEs) of
C(sp3)H bonds, the reactions generally proceed
under mild conditions and exhibit high functional
group tolerance. In most cases, however, C(sp3)H
bonds with the smallest BDE or the most hydridic
CH bonds in a substrate are preferentially converted
(Scheme 1a). Realizing catalyst-controlled site-
selectivity in PC-HAT hybrid catalysis thus remains a
challenge.
One potential strategy to furnish catalyst-
controlled site-selectivity is the cooperative use of a
bond-weakening catalyst (Scheme 1b). Coordination-
induced bond-weakening of NH and OH bonds by
low-valent metals have been studied in the field of
inorganic chemistry.[6] Application of the bond-
weakening phenomenon to organic synthesis,
however, is limited.[7,8] Knowles and co-workers
reported pioneering work on NH bond-weakening
Alternatively,
hydrogenphosphate[9a,b]
and
diphenylborinic acid[10] act as bond-weakening
additives or catalysts to accelerate -C–H alkylation
of alcohols promoted by PC-HAT hybrid catalysis by
increasing the electron density of the oxygen atom
through hydrogen bonding or borate formation and
facilitating the HAT process from the -C–H bonds.
The development of a structurally novel bond-
weakening catalyst will provide great potential for
exploring unprecedented reactivity, selectivity, and
substrate generality in C(sp3)H functionalizations.
Here, on the basis of DFT calculation-guided
screening, we identified a structurally tunable
pentavalent silicate species as a bond-weakening
catalyst. This silicate species accelerated -C–H
alkylation of alcohols promoted by PC-HAT hybrid
catalysis.
Silicon is an oxophilic element. Nevertheless,
alkoxy-ligands on a silicon atom are exchangeable
1
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
through the formation of hypervalent silicate
species.[11] We conducted DFT calculations to
estimate the BDEs of -C–H bonds in various silyl
Table 1. Optimization of Reaction Conditions[a]
ethyl ethers as model compounds. When
a
pentavalent anionic silicate was formed, the BDE of
the -C–H bond was weakened by 3–7 kcal/mol,
whereas neutral silyl ethers never underwent such
bond-weakening (Scheme 2).[12] From these results,
we hypothesized that silicon compounds, which
easily form pentavalent silicates, could potentially be
utilized as an alcohol-selective bond-weakening
catalyst by accelerating the HAT process at the -C–
H bond of alcohols.
Entry
PC
HAT
[Si]
Yield/%[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20[d]
none
6a
Si(OEt)4
Si(OEt)3F
24
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4a
4a
4a
4a
none
4a
4a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
81 (94)[c]
20
30
27
39
37
38
31
5
<1
63
0
5
<1
0
Me3SiC6F5
6b
6c
6d
6e
6f
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
Scheme 2. Bond-weakening Effects by Silicate Formation
Supported by DFT Calculation
5d
5e
5a
None
5a
0
0
0
0
According to this hypothesis, we screened silicon
compounds that would accelerate -C–H alkylation
of alcohols catalyzed by PC-HAT hybrid systems
(Table 1). Benzalmalononitrile (1a), ethanol (2a),
photoredox catalyst 4a,[13] and HAT catalyst 5a[14]
were selected as the initial components. In the
absence of any silicon source under visible-light
irradiation, the desired C–H alkylation products were
obtained as a mixture of 3aa and 3aa' in only 24%
yield (Table 1, Entry 1).[15] Next, a catalytic amount
of various silanes was added to this reaction system
(Entries 2–11). Only the addition of Martin’s
spirosilane (6a)[16] drastically improved the yield to
81% (Entry 2). When the reaction concentration was
changed to 0.05 M, the yield further improved to 94%.
Simple silanes did not improved the yield (Entries 3–
5), probably due to the inefficient formation of
hypervalent silicate species with 2a. Spirosilane 6b,
which forms pentavalent silicate species,[17] improved
the yield by only 15% (Entry 6) compared with the
absence of any silicone source. Stable and isolatable
pentavalent silicates (6c, 6d, 6e)[17] prepared from 6a
improved the yield to only a limited extent (Entries
7–9). Lewis acidic bis(perfluorocatecholato)silane[18]
(6f) inhibited this reaction (Entry 10).
We next tried other PCs (Entries 11–13). When
using a PC with either a lower oxidation potential
(4b) or a much higher oxidation potential (4d), the
desired products 3aa and 3aa' were not obtained
(Entries 11 and 13). When 4c[13] was used, the
product was obtained in 63% yield (Entry 12). The
oxidation potential of PC 4c was almost identical to
that of 4a (E(IrIII*/IrII) = +1.68 V for 4c and +1.65 V
for 4a vs SCE in CH3CN), but the slightly weaker
[a] General conditions: 1a (1 equiv, 0.10 mmol), 2a (3
equiv), PC (1 mol%), HAT (15 mol%), Si source ([Si]: 10
mol%), CH3CN (0.1 M to 1a), blue LED was irradiated at
room temperature for 14 h. [b] Combined yield (3aa +
3aa') was determined by 1H NMR analysis in the presence
of 1,1,2,2-tetrachloroethane as an internal standard. [c]
CH3CN (0.05 M to 1a). [d] No light irradiation.
2
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
reduction potential (E1/2(IrII/IrIII) = –0.69 V for 4b and
–0.79 V for 4a vs SCE in CH3CN) perhaps rendered
the catalyst regeneration step more difficult. When
Table 2. Substrate Scope of Acceptors
other HAT catalysts (5d[19] and 5e[5b]
)
and
9b, 20]
quinuclidine derivatives (5b[9a,
and 5c) were
used instead of 5a, the CH alkylation product was
obtained in only low yield (Entries 14–17). 5a might
function as not only a HAT catalyst but also a base to
form a silicate.[21] When we applied previously
reported reaction conditions of -C–H alkylation of
alcohols[9a, 9b, 10] to 1a and 2a, products 3aa + 3aa'
were not obtained.[12] In the absence of any elemental
factors, PC, HAT, or blue LED irradiation, the
products were not obtained (Entries 18–20).
Entry
Acceptor
Product
Yield/%[a]
1
2
3
94[c]
81[c]
75[c]
Having determined the optimal conditions (Table 1,
Entry 2), we next investigated the substrate scope
(Tables 2 and 3). In many cases, C–H alkylated
products were isolated after cyclization (Table 2,
Entries 1–6 and Table 3). Using ethanol (2a) as an
alcohol substrate, arylidene malononitriles 1a–1c led
to products 3aa–3ca in excellent yield regardless of
the presence of an electron-donating or -withdrawing
group (Table 2, Entries 1–3). The reaction with
sterically hindered dialkylidene malononitrile 1d also
proceeded in good yield (3da, Entry 4). A range of
acrylates was applicable as acceptors (Entries 5–7).
N-Phenylacrylamide (1h), phenylvinylketone (1i) and
1,1-bis(phenylsulfonyl)ethylene (1j) were also
competent (Entries 8-10).
We then investigated CH alkylation between
various alcohols and 1e or 1f (Table 3). The reaction
with methanol (2b) produced corresponding lactone
3fb in moderate yield due to instability of the primary
carbon radical (Table 3, Entry 1). Despite increased
steric hindrance at the -position, 3fc was produced
in excellent yield (Entry 2). Long-chain alcohol 2d
could be used to produce lactone 3ed in high yield
(Entry 3). The reaction proceeded efficiently even
when using alcohol 2e having an electron-
withdrawing group, although the isolated yield of
lactone 3ee was low due to its volatility (Entry 4).
The presence of a p-toluenesulfonylamide functional
group acting as a hydrogen-bonding donor was not
problematic for the reaction, and product 3ef was
obtained in good yield (Entry 5). For mono-protected
diol 2g, CH alkylation occurred selectively at the -
position of the unprotected hydroxy group to produce
3eg in excellent yield (Entry 5). Alcohols containing
benzylic CH bonds (2h), -C–H bonds of a cyclic
ether (2i), and -C–H bonds of an N-heterocycle (2j)
afforded the corresponding products. Although these
CH bonds are generally more reactive than the -C–
H bonds of alcohols in the HAT process, the desired
lactones 3eh–3ej were selectively obtained in good
yield (Entries 7–9).
4
61
5
52 (80)[d]
6
7
8
92[e]
94[f]
45
9
42
44
10
[a] Isolated yields. 1H NMR yields (1,1,2,2-
tetrachloroethane was used as an internal standard) are
described in the parenthesis. [b] After blue LED irradiation,
acidic work-up (with Amberlyst-15 (100 mg) for 3 h at 50
degrees) was conducted. [c] d.r. = 1:1.0 ~ 1:1.5. See SI for
details. [d] Low isolated yield was due to volatility of the
product. [e] cis/trans = 2/1. [f] cis/trans = 1.5/1.
3
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
Table 3. Substrate Scope of Alcohols
acidic work-up (with Amberlyst®-15 (100 mg) for 3 h at 50
C) was conducted. [c] cis/trans = 2/1. [d] Low isolated
yield was due to volatility of the product.
Entry
1
Alcohol
Product
Yield/%[a]
38[b]
2
84[b][c]
3
4
82
Scheme 3. Proposed Catalytic Cycle
25 (89)[d]
Our working hypothesis for the catalytic cycle of
this reaction is shown in Scheme 3. First, PC 4a is
excited by visible light irradiation. The excited PC
(E(IrIII*/IrII) = +1.65 V vs SCE)[13] oxidizes the HAT
catalyst 5a (E1/2 = +1.41 V vs SCE)[14] to generate
radical cation 7 and Ir(Ⅱ) species. Silane catalyst 6a
forms an anionic pentavalent silicate 8 through a
reaction with alcohol 2, leading to the lowered BDE
of the -C–H bond by 2–3 kcal/mol (supported by
DFT calculations[12]). Quinuclidinium radical 7
abstracts the -C–H bond of silicate 8 to produce
carbon radical 9 and regenerate 5a after releasing a
proton. Improvement of the yield might also partially
originated from electrostatic interation between the
anionic silicates and the quinulidiunm radical cation
during this HAT process.[22] The thus-generated
carbon radical 9 reacts with acceptors 1, forming
stabilized radical 10. After reducing 10 by the Ir(Ⅱ)
5
6
7
75
85
75
Ⅱ
Ⅲ
species (E1/2 (Ir /Ir ) = –0.79 V vs SCE)[13]
,
protonation, and alcohol exchange at the silane center,
product 3 is generated.
In conclusion, we demonstrated that Martin’s
spirosilane 6a functioned as a novel bond-weakening
catalyst for the -C–H bond of alcohols by forming
silicate species. In cooperation with a PC-HAT
hybrid system, -C–H alkylation of alcohols was
realized using a range of substrates. Further
applications of this bond-weakening catalyst to other
reactions are ongoing in our laboratory.
8
62
9
69
Experimental Section
[a] Isolated yields. 1H NMR yields (1,1,2,2-
tetrachloroethane was used as an internal standard) are
described in the parenthesis. [b] After blue LED irradiation,
Typical Procedure for Photocatalytic -C–H Alkylation
of Alcohols Catalyzed by Silicon-Photoredox-HAT
Hybrid System (represented by the synthesis of 3aa)
4
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
Ir[dF(CF3)ppy]2[4,4’-d(CF3)bpy]PF6 (4a, 1.2 mg, 0.001
mmol, 1 mol%), HAT catalyst 5a (4.0 mg, 0.015 mmol, 15
mol%), Martin’s spirosilane (6a, 5.1 mg, 0.01 mmol, 10
mol%), and benzalmalononitrile (1a, 15.4 mg, 0.1 mmol, 1
eq) were added to a dried screw-cap vial. Ethanol (2a, 17.5
L, 0.3 mmol, 3 equiv) and degassed CH3CN (2 mL) were
added to the vial under argon atmosphere in a glovebox.
The vial was sealed with the screw cap. The resulting
mixture was then placed near a light source[23] and
irradiated with blue LED lights while stirring at ambient
temperature for 14 h. After evaporation, the residue was
purified by flash column chromatography to afford 3aa
(18.8 mg, 94 % yield, white solid).
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This work was supported by a JSPS fellowship Grant Number
JP19J23157 (K.S.), JSPS KAKENHI Grant Number JP16H04239
in Precisely Designed Catalysts with Customized Scaffolding and
JSPS KAKENHI Grant Number JP18K06545 in Scientific
Research C (K.O.), JSPS KAKENHI Grant Number JP17H06442
in Hybrid Catalysis (M.K.).
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5
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Advanced Synthesis & Catalysis
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With a strong fan, we controlled the temperature to 25–33
ºC.
[23] A Valore VBP-L24-C2 with 38W LED lamp (VBL-
SE150-BBB(430)) was used as the 430-nm light source.
6
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10.1002/adsc.201901253
Advanced Synthesis & Catalysis
COMMUNICATION
Identification of Bond-Weakening Spirosilane
Catalyst for Photoredox -C–H Alkylation of
Alcohols
Adv. Synth. Catal. Year, Volume, Page – Page
Kentaro Sakai, Kounosuke Oisaki*, Motomu
Kanai*
7
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