Synthesis of functionalized organoboron/silicon compounds by copper-catalyzed coupling of alkylsilyl peroxides and diboron/silylborane reagents

Article abstract of DOI:10.1021/acs.orglett.9b00874

The facile synthesis of functionalized organoboron/silicon compounds by copper-catalyzed coupling of alkylsilyl peroxides and diboron/silylborane reagents is reported. The reactions proceed smoothly under mild, neutral conditions in short reaction times t

Full text of DOI:10.1021/acs.orglett.9b00874

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis of Functionalized Organoboron/Silicon Compounds by
Copper-Catalyzed Coupling of Alkylsilyl Peroxides and Diboron/
Silylborane Reagents
Takumi Seihara, Shunya Sakurai, Terumasa Kato, Ryu Sakamoto,* and Keiji Maruoka*,†,‡
†
†
†
,†
†
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
‡
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
*
S Supporting Information
ABSTRACT: The facile synthesis of functionalized organoboron/silicon
compounds by copper-catalyzed coupling of alkylsilyl peroxides and diboron/
silylborane reagents is reported. The reactions proceed smoothly under mild,
neutral conditions in short reaction times to generate organoboron/silicon
compounds bearing a ketone moiety, which are useful synthetic intermediates
that are otherwise difficult to access. The results of mechanistic investigations
suggest the radical-mediated formation of carbon−boron and carbon−silicon
bonds via β-scission of alkoxy radicals.
rganoboron compounds are structurally important
Scheme 1. General Synthetic Routes to Alkylboronic Esters
O
molecules in synthetic chemistry due to their wide
1
applications as synthetic building blocks. In particular,
functionalized alkylboronic esters are highly valuable inter-
mediates for the introduction of an alkyl chain with the
2
corresponding functional group. Although considerable
research efforts have been devoted to the development of
synthetic routes to alkylboronic esters, the limited functional-
group tolerance observed in most cases still requires improve-
ment. The transmetalation of organolithium/magnesium
reagents and the hydroboration of olefins, which represent
the most common and reliable means of synthesizing
alkylboronic esters, often suffer from the incompatibility of
several functional groups such as ketones or aldehydes
3−5
(Scheme 1a,b).
Although the transition metal-catalyzed
borylation of alkyl halides, the so-called Miyaura-type
borylation, has recently emerged as a promising synthetic
approach to alkylboronic esters, it usually requires strong bases,
6
which often limits the substrate scope (Scheme 1c).
Therefore, the development of novel synthetic routes to
functionalized alkylboronic esters is highly desirable. In this
report, we provide an alternative synthetic approach via a
radical mechanism for alkylboronic esters bearing a ketone
group, which are otherwise difficult to access, by the copper-
catalyzed coupling of alkylsilyl peroxides and diboron reagents
promising approach for the synthesis of functionalized ketones
3
3
7
through the cleavage of C(sp )−C(sp ) bonds. Although
under mild, neutral conditions (Scheme 1d). Moreover, we
various types of radical reactions have been applied to the β-
report that this approach can also be applied to silylation
reactions that generate organosilicon compounds bearing a
ketone moiety by simply switching the reaction partner from a
diboron to a silylborane reagent.
10,11
scission of alkoxy radicals,
a radical-mediated formation of
carbon−boron bonds through β-scission of alkoxy radicals has
not yet been developed. This is probably due to the
incompatibility of organoboron compounds/agents with the
strong oxidative conditions usually required for the generation
The selective cleavage of inert carbon−carbon bonds for the
generation of alkyl radicals is an important research topic in
modern organic synthesis because it can offer a new synthetic
8
,9
approach to organic molecules. In this field, β-scission of
Received: March 11, 2019
alkoxy radicals generated from cycloalkanols has emerged as a
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00874
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
12
a
of alkoxy radicals from alcohols. On the other hand, our
group has recently reported a novel reductive β-scission
strategy using alkylsilyl peroxides 1, which are readily prepared
Table 1. Optimization of Reaction Conditions
13
from alcohols or alkenes. As alkylsilyl peroxides 1 can easily
generate alkoxy radicals under mild and reductive conditions
with a copper catalyst, we envisioned that our reductive β-
scission strategy using alkylsilyl peroxides would be suitable for
the radical-mediated formation of carbon−boron bonds via the
β-scission of alkoxy radicals, thus providing an alternative
synthetic strategy for organoboron compounds bearing a
ketone moiety. Scheme 2 shows the catalytic cycle that we
b
entry
[M]
CuI
CuI
CuI
CuI
[B] (equiv)
yield of 6 (%)
c
1
B pin (1.3 equiv)
38 (6a)
95 (6a)
32 (6a)
2
2
2
B pin (1.3 equiv)
2 2
d
3
e
B pin (1.3 equiv)
2
2
4
B pin (1.3 equiv)
71 (6a)
2
2
5
6
7
8
CuOAc
CuCl2
Cu(acac)2
FeCl2
CuI
B pin (1.3 equiv)
53 (6a)
64 (6a)
74 (6a)
not determined
97 (6a)
70 (6a′)
70 (6a″)
2
2
Scheme 2. Proposed Reaction Mechanism for the Copper-
Catalyzed Borylation Reaction of Alkylsilyl Peroxides with
Bis(pinacolato)diboron
B pin (1.3 equiv)
2
2
B pin (1.3 equiv)
2
2
B pin (1.3 equiv)
2
2
f
9
B pin (1.0 equiv)
2 2
f
1
1
0
1
CuI
CuI
B cat (1.0 equiv)
2
2
f
B nep (1.0 equiv)
2 2
a
Unless otherwise specified, the reactions were carried out in the
presence of 1a (0.2 mmol), B pin (1.3 equiv), a metal catalyst (5 mol
2
2
%
), and 1,10-Phen (5 mol %) in MeCN for 2 h under an argon
b
1
atmosphere. The yield was determined by H NMR spectroscopy
using 1,1,2,2-tetrachloroethane as the internal standard. The reaction
was conducted instead of 1,10-Phen. Benzene was used as the
solvent. THF was used as the solvent. The reaction time was 0.5 h.
c
d
e
f
membered (1b) and seven-membered (1c) cyclic peroxides
furnished the corresponding products (6b and 6c, respectively)
in good yields. Different substituents on the aryl moiety of the
peroxides (1d−f) were well tolerated, affording the respective
products (6d−f) in moderate to high yields. The reaction with
1g, which bears an ether moiety, afforded 6g in 58% yield.
When ethyl-substituted cyclopentyl peroxide 1h was used, the
ring opening of the cyclopentyl moiety occurred selectively to
furnish δ-boryl ketone 6h in an acceptable yield. Peroxide 1i
derived from 1-indanone generated 6i. Acyclic alkylsilyl
peroxides 1j and 1k were also used to give 6j and 6k,
respectively. Furthermore, acyclic peroxide 1l, which is derived
from pentoxifylline, also underwent this transformation to
afford highly functionalized alkylboronic ester 6l.
propose for the copper-catalyzed borylation of alkylsilyl
9d,14
peroxide 1 with bis(pinacolato)diboron (B pin ).
Initially,
2
2
single-electron transfer (SET) from a copper catalyst to 1
should lead to the generation of an alkoxy radical (2) and a
copper−silanoxide complex (3). Subsequent β-scission of 2
could generate the corresponding alkyl radical (4), while
transmetalation between 3 and B pin2 would generate a
2
copper−boron complex (5). Finally, coupling of the alkyl
radical (4) and the copper−boron complex (5) should lead to
the formation of a carbon−boron bond, thus generating an
15
organoboron product (6).
We next envisioned that the use of a silylborane instead of a
diboron reagent for the reaction with alkylsilyl peroxides
should lead to a radical-mediated formation of carbon−silicon
bonds via a process analogous to that depicted in Scheme
Initially, we examined the reaction between cyclic alkylsilyl
peroxide 1a and B pin , which should furnish δ-boryl ketone
2
2
6
a (Table 1). When the reaction was conducted using 5 mol %
17,18
copper iodide in acetonitrile, the desired product (6a) was
obtained in 38% yield (entry 1). The addition of 1,10-
phenanthroline (1,10-Phen) improved the yield significantly
2.
Similar to the case of organoboron compounds, access
to organosilicon compounds bearing a ketone moiety remains
19,20
nontrivial.
After extensive screening of the conditions, we
(
95%, entry 2). Solvents other than acetonitrile and copper
discovered that the reaction of 1a and (dimethylphenylsilyl)-
boronic acid pinacol ester (1.3 equiv) in the presence of CuI
(5 mol %) and 1,10-Phen (5 mol %) at 130 °C furnishes δ-silyl
sources other than copper iodide were less effective (entries
3
chloride did not promote the reaction (entry 8). After
additional slight modifications of the conditions, we found
that the amount of B pin and the reaction time could be
reduced (from 1.3 to 1.0 equiv, from 2 to 0.5 h) without
affecting the product yield (entry 9). Bis(catecholato)diboron
(
−7), while the use of other metal salts such as iron(II)
21
ketone 7a in 78% yield (Scheme 4). The reactions with
various cyclic and acyclic alkylsilyl peroxides also proceeded
smoothly to afford 7b−m in moderate to high yields.
2
2
A series of control experiments for the borylation reaction
22
were then carried out (Scheme 5). When the reactions were
performed using cycloalkanol 8 or alkyl hydroperoxide 9
instead of alkylsilyl peroxide 1a under standard conditions, the
formation of 6a was not observed (Scheme 5a). On the other
hand, attempted reactions of 8 with B pin under oxidative
B cat ) or bis(neopentyl glycolato)diboron (B nep ) also
2 2 2 2
furnished the corresponding product (6a′ or 6a″, respectively),
albeit with a yield slightly lower than that of B pin (entries 10
2
2
and 11).
2
2
With the optimized conditions in hand, we subsequently
examined the scope of this reaction with respect to the
alkylsilyl peroxide substrates (1) (Scheme 3). The use of six-
conditions were not successful (Scheme 5b, conditions a or
11a,d
b).
These results demonstrate that the reductive β-scission
16
of alkylsilyl peroxides is crucial for this transformation. The
B
DOI: 10.1021/acs.orglett.9b00874
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
,c
a,b,c
Scheme 3. Scope of Alkylsilyl Peroxides 1
Scheme 4. Scope of the Silylation of 1
a
Unless otherwise specified, the reactions were carried out in the
presence of 1 (0.2 mmol), B pin (1.0 equiv), CuI (5 mol %), and
2
2
1
,10-Phen (5 mol %) in MeCN for 0.5 h under an argon atmosphere.
b
c d
1
Isolated yield. H NMR yield given in parentheses. On a 0.5 mmol
scale. The reaction time was 1 h. The temperature was 80 °C.
e
f
a
Unless otherwise specified, the reactions were carried out in the
presence of 1 (0.1 mmol), (dimethylphenylsilyl)boronic acid pinacol
ester (1.3 equiv), CuI (5 mol %), and 1,10-Phen (5 mol %) in
chlorobenzene (0.3 M) for 0.5 h under an argon atmosphere.
hypothesis that alkyl radical intermediates are formed in this
reaction was supported by the results of the following
experiments. The addition of the radical scavenger 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) under standard con-
ditions inhibited the reaction, affording radical adduct 10
b
c
1
d
Isolated yield. H NMR yield given in parentheses. The reaction
e
time was 2 h. (Dimethylphenylsilyl)boronic acid pinacol ester (1.0
f
equiv) was used. (Dimethylphenylsilyl)boronic acid pinacol ester
(1.5 equiv) was used.
(
Scheme 5c). Furthermore, we tested the radical clock reaction
using alkylsilyl peroxide 11 with B pin2 under standard
conditions, which resulted in the formation of ring-opened
adduct 12 instead of 13 (Scheme 5d).
In conclusion, a copper-catalyzed ring-opening borylation of
alkylsilyl peroxides with diboron reagents via cleavage of
C(sp )−C(sp ) bonds has been developed. The reaction
proceeds smoothly under mild conditions in short reaction
times to generate functionalized alkylboronic esters. Addition-
ally, this approach allows the synthesis of organosilicon
compounds bearing a ketone moiety by using silylborane
instead of diboron reagents. Further investigations of the
applications of such a reductive β-scission strategy for alkylsilyl
peroxides for the formation of carbon−carbon or carbon−
heteroatom bonds are currently in progress in our laboratory.
2
3
3
C
DOI: 10.1021/acs.orglett.9b00874
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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Preparation and Applications in Organic Synthesis Medicine and
Materials, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2011.
Scheme 5. Control Experiments
(
2) Kubota, K. Synthesis of Functionalized Organoboron Compounds
through Copper(I) Catalysis; Springer, 2017.
3) (a) Brown, H. C. Organic Synthesis via Organoboranes; Wiley-
(
Interscience: New York, 1975. (b) Brown, H. C.; Cole, T. E.
Organoboranes. 31. A Simple Preparation of Boronic Esters from
Organolithium Reagents and Selected Trialkoxyboranes. Organo-
metallics 1983, 2, 1316−1319.
(4) (a) Brown, H. C. Hydroboration; W. A. Benjamin Inc.: New
York, 1962. (b) Fu, G. C. Transition Metals for Organic Synthesis:
Building Blocks and Fine Chemicals; Beller, M., Bolm, C., Eds.; Wiley-
VCH Verlag GmbH: Weinheim, Germany, 2008; p 193.
(
5) (a) Man
Rhodium Complexes. Angew. Chem., Int. Ed. Engl. 1985, 24, 878−
79. A few examples of the chemoselective hydroboration of alkenes
nig, D.; Noth, H. Catalytic Hydroboration with
̈ ̈
8
in the presence of ketones or aldehydes have recently been reported.
However, these often require precious transition metal catalysts and/
or specific ligands. See: (b) Obligacion, J. V.; Chirik, P. J.
Bis(imino)pyridine Cobalt-Catalyzed Alkene Isomerization−Hydro-
boration: A Strategy for Remote Hydrofunctionalization with
Terminal Selectivity. J. Am. Chem. Soc. 2013, 135, 19107−19110.
(c) Zhang, L.; Zuo, Z.; Leng, X.; Huang, Z. A Cobalt-Catalyzed
Alkene Hydroboration with Pinacolborane. Angew. Chem., Int. Ed.
2
014, 53, 2696−2700. (d) Docherty, J. H.; Peng, J.; Dominey, A. P.;
Thomas, S. P. Activation and Discovery of Earth-abundant Metal
Catalysts using Sodium tert-Butoxide. Nat. Chem. 2017, 9, 595−600.
(e) Ibrahim, A. D.; Entsminger, S. W.; Fout, A. R. Insights into a
Chemoselective Cobalt Catalyst for the Hydroboration of Alkenes
and Nitriles. ACS Catal. 2017, 7, 3730−3734.
(
6) (a) Ito, H.; Kubota, K. Copper(I)-Catalyzed Boryl Substitution
ASSOCIATED CONTENT
Supporting Information
■
of Unactivated Alkyl Halides. Org. Lett. 2012, 14, 890−893.
(b) Dudnik, A. S.; Fu, G. C. Nickel-Catalyzed Coupling Reactions
of Alkyl Electrophiles, Including Unactivated Tertiary Halides, To
Generate Carbon−Boron Bonds. J. Am. Chem. Soc. 2012, 134,
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00874.
10693−10697. (c) Bose, S. K.; Fucke, K.; Liu, L.; Steel, P. G.; Marder,
T. B. Zinc-Catalyzed Borylation of Primary, Secondary and Tertiary
Alkyl Halides with Alkoxy Diboron Reagents at Room Temperature.
Angew. Chem., Int. Ed. 2014, 53, 1799−1803. (d) Atack, T. C.; Lecker,
R. M.; Cook, S. P. Iron-Catalyzed Borylation of Alkyl Electrophiles. J.
Am. Chem. Soc. 2014, 136, 9521−9523. (e) Atack, T. C.; Cook, S. P.
Manganese-Catalyzed Borylation of Unactivated Alkyl Chlorides. J.
Am. Chem. Soc. 2016, 138, 6139−6142. (f) Bose, S. K.; Brand, S.;
Omoregie, H. O.; Haehnel, M.; Maier, J.; Bringmann, G.; Marder, T.
B. Highly Efficient Synthesis of Alkylboronate Esters via Cu(II)-
Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides in
Air. ACS Catal. 2016, 6, 8332−8335.
Experimental procedures and characterization data for
all compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*
*
E-mail: ryu-s@kuchem.kyoto-u.ac.jp.
E-mail: maruoka@kuchem.kyoto-u.ac.jp.
ORCID
Shunya Sakurai: 0000-0003-1331-2794
Ryu Sakamoto: 0000-0001-8350-2636
Keiji Maruoka: 0000-0002-0044-6411
Notes
(7) The conjugate borylation of α,β-unsaturated carbonyl com-
pounds represents a facile route to β-boryl carbonyls. For a review,
see: Hartmann, E.; Vyas, D. J.; Oestreich, M. Enantioselective Formal
Hydration of α,β-Unsaturated Acceptors: Asymmetric Conjugate
Addition of Silicon and Boron Nucleophiles. Chem. Commun. 2011,
47, 7917−7932.
The authors declare no competing financial interest.
(8) (a) Murakami, M.; Ito, Y. Activation of Unreactive Bonds and
ACKNOWLEDGMENTS
Organic Synthesis; Springer, 2001; pp 97−129. (b) Jun, C.-H.
Transition Metal-Catalyzed Carbon−Carbon Bond Activation.
Chem. Soc. Rev. 2004, 33, 610−618. (c) Marek, I.; Masarwa, A.;
Delaye, P.-O.; Leibeling, M. Selective Carbon−Carbon Bond
Cleavage for the Stereoselective Synthesis of Acyclic Systems.
Angew. Chem., Int. Ed. 2015, 54, 414−429. (d) Souillart, L.;
Cramer, N. Catalytic C−C Bond Activations via Oxidative Addition
to Transition Metals. Chem. Rev. 2015, 115, 9410−9464.
(e) Fumagalli, G.; Stanton, S.; Bower, J. F. Recent Methodologies
That Exploit C−C Single-Bond Cleavage of Strained Ring Systems by
Transition Metal Complexes. Chem. Rev. 2017, 117, 9404−9432.
(9) Recently, the decarboxylative borylation reactions of esters via a
radical mechanism emerged as a promising synthetic method for the
generation of boronic esters. However, they also still suffer from some
synthetic drawbacks, such as the requirement of photoirradiation and
■
This work was supported by JSPS KAKENHI Grants
JP26220803 and JP17H06450 (Hybrid Catalysis). S.S. thanks
the Japan Society for the Promotion of Science for Young
Scientists for a Research Fellowship.
REFERENCES
■
(
1) (a) Jana, R.; Pathak, T. P.; Sigman, M. S. Advances in Transition
Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-
organometallics as Reaction Partners. Chem. Rev. 2011, 111, 1417−
492. (b) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. The B-Alkyl
Suzuki-Miyaura Cross-Coupling Reaction: Development, Mechanistic
Study, and Applications in Natural Product Synthesis. Angew. Chem.,
Int. Ed. 2001, 40, 4544−4568. (c) Hall, D. G. Boronic Acids:
1
D
DOI: 10.1021/acs.orglett.9b00874
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
the use of an excess of boron reagents or additives. See: (a) Li, C.;
Wang, J.; Barton, L. M.; Yu, S.; Tian, M.; Peters, D. S.; Kumar, M.;
Yu, A. W.; Johnson, K. A.; Chatterjee, A. K.; Yan, M.; Baran, P. S.
Decarboxylative Borylation. Science 2017, 356, eaam7355. (b) Hu, D.;
Wang, L.; Li, P. Decarboxylative Borylation of Aliphatic Esters under
Visible-Light Photoredox Conditions. Org. Lett. 2017, 19, 2770−
(13) (a) Sakamoto, R.; Sakurai, S.; Maruoka, K. Alkylsilyl Peroxides
as Alkylating Agents in the Copper-Catalyzed Selective Mono-N-
Alkylation of Primary Amides and Arylamines. Chem. - Eur. J. 2017,
23, 9030−9033. (b) Sakamoto, R.; Kato, T.; Sakurai, S.; Maruoka, K.
3
Copper-Catalyzed C(sp)−C(sp ) Coupling of Terminal Alkynes with
Alkylsilyl Peroxides via a Radical Mechanism. Org. Lett. 2018, 20,
1400−1403. (c) Sakurai, S.; Kato, T.; Sakamoto, R.; Maruoka, K.
Generation of Alkyl Radicals from Alkylsilyl Peroxides and Their
Applications to C−N or C−O Bond Formations. Tetrahedron 2019,
2
773. (c) Fawcett, A.; Pradeilles, J.; Wang, Y.; Mutsuga, T.; Myers, E.
L.; Aggarwal, V. K. Photoinduced Decarboxylative Borylation of
Carboxylic Acids. Science 2017, 357, 283−286. (d) Wang, J.; Shang,
M.; Lundberg, H.; Feu, K. S.; Hecker, S. J.; Qin, T.; Blackmond, D.
G.; Baran, P. S. Cu-Catalyzed Decarboxylative Borylation. ACS Catal.
7
5, 172−179. See also: Sakamoto, R.; Sakurai, S.; Maruoka, K.
Bis(trialkylsilyl) Peroxides as Alkylating Agents in the Copper-
Catalyzed Selective Mono-N-Alkylation of Primary Amides. Chem.
Commun. 2017, 53, 6484−6487.
2018, 8, 9537−9542. See also the recent examples on radical
borylation reactions (e) Chen, K.; Zhang, S.; He, P.; Li, P. Efficient
Metal-free Photochemical Borylation of Aryl Halides under Batch and
Continuous-flow Conditions. Chem. Sci. 2016, 7, 3676−3680. (f) Yan,
G.; Huang, D.; Wu, X. Recent Advances in C−B Bond Formation
through a Free Radical Pathway. Adv. Synth. Catal. 2018, 360, 1040−
(
14) (a) Iwamoto, H.; Akiyama, S.; Hayama, K.; Ito, H. Copper(I)-
Catalyzed Stereo- and Chemoselective Borylative Radical Cyclization
of Alkyl Halides Bearing an Alkene Moiety. Org. Lett. 2017, 19,
2
614−2617. (b) Cui, J.; Wang, H.; Song, J.; Chi, X.; Meng, L.; Liu,
Q.; Zhang, D.; Dong, Y.; Liu, H. Copper(I)-Catalyzed 5-exo-trig
Radical Cyclization/Borylation of Alkyl Halides: Access to Function-
alized Pyrrolidine Derivatives. Org. Biomol. Chem. 2017, 15, 8508−
1
053. (g) Cheng, Y.; Mu
Radical Borylation of Alkyl and Aryl Iodides. Angew. Chem., Int. Ed.
018, 57, 16832−16836. (h) Mfuh, A. M.; Doyle, J. D.; Chhetri, B.;
̈
ck-Lichtenfeld, C.; Studer, A. Metal-Free
2
8
512.
Arman, H. D.; Larionov, O. V. Scalable, Metal- and Additive-Free,
Photoinduced Borylation of Haloarenes and Quaternary Arylammo-
nium Salts. J. Am. Chem. Soc. 2016, 138, 2985−2988. (i) Zhang, L.;
Jiao, L. Pyridine-Catalyzed Radical Borylation of Aryl Halides. J. Am.
Chem. Soc. 2017, 139, 607−610. (j) Wu, J.; He, L.; Noble, A.;
Aggarwal, V. K. Photoinduced Deaminative Borylation of Alkyl-
amines. J. Am. Chem. Soc. 2018, 140, 10700−10704.
(
15) Alternatively, the reaction between the alkyl radical (4) and the
3
2
silanoxide-activated sp −sp diboron species should directly generate
the borylated product (6). See refs 9b and 9c.
(
16) The isolation of several borylated products was problematic
1
due to their instability on silica gel. Thus, we provide both H NMR
spectra and isolated yields.
(17) (a) Sakaguchi, H.; Ohashi, M.; Ogoshi, S. Fluorinated
(
10) (a) Beckwith, A. L. J.; O’Shea, D. M.; Westwood, S. W.
Vinylsilanes from the Copper-Catalyzed Defluorosilylation of
Rearrangement of Suitably Constituted Aryl, Alkyl, or Vinyl Radicals
by Acyl or Cyano Group Migration. J. Am. Chem. Soc. 1988, 110,
Fluoroalkene Feedstocks. Angew. Chem., Int. Ed. 2018, 57, 328−
3
32. (b) Meng, F.-F.; Xie, J.-H.; Xu, Y.-H.; Loh, T.-P. Catalytically
2
565−2575. (b) Salamone, M.; Bietti, M. Reaction Pathways of
Asymmetric Synthesis of 1,3-Bis(silyl)propenes via Copper-Catalyzed
Alkoxyl Radicals. The Role of Solvent Effects on C−C Bond
Fragmentation and Hydrogen Atom Transfer Reactions. Synlett 2014,
Double Proto-Silylations of Polar Enynes. ACS Catal. 2018, 8, 5306−
5
(
312.
2
5, 1803−1816. (c) Murakami, M.; Ishida, N. β-Scission of Alkoxy
Radicals in Synthetic Transformations. Chem. Lett. 2017, 46, 1692−
700.
11) (a) Wang, S.; Guo, L.-N.; Wang, H.; Duan, X.-H. Alkynylation
18) (a) Xue, W.; Qu, Z.-W.; Grimme, S.; Oestreich, M. Copper-
Catalyzed Cross-Coupling of Silicon Pronucleophiles with Unac-
tivated Alkyl Electrophiles Coupled with Radical Cyclization. J. Am.
Chem. Soc. 2016, 138, 14222−14225. (b) Xue, W.; Oestreich, M.
Copper-Catalyzed Decarboxylative Radical Silylation of Redox-Active
Aliphatic Carboxylic Acid Derivatives. Angew. Chem., Int. Ed. 2017,
56, 11649−11652.
1
(
of Tertiary Cycloalkanols via Radical C−C Bond Cleavage: A Route
to Distal Alkynylated Ketones. Org. Lett. 2015, 17, 4798−4801.
(
b) Ren, R.; Zhao, H.; Huan, L.; Zhu, C. Manganese-Catalyzed
Oxidative Azidation of Cyclobutanols: Regiospecific Synthesis of
Alkyl Azides by C−C Bond Cleavage. Angew. Chem., Int. Ed. 2015, 54,
(19) (a) Marciniec, B. Hydrosilylation: A Comprehensive Reviews on
Recent Advances; Springer: Berlin, 2009. (b) Brook, M. A. Silicon in
Organic, Organometallic and Polymer Chemistry; Wiley: New York,
2000. (c) Marciniec, B. Catalysis by Transition Metal Complexes of
Alkene Silylation−Recent Progress and Mechanistic Implications.
Coord. Chem. Rev. 2005, 249, 2374−2390.
1
2692−12696. (c) Jia, K.; Zhang, F.; Huang, H.; Chen, Y. Visible-
3
Light-Induced Alkoxyl Radical Generation Enables Selective C(sp )−
C(sp ) Bond Cleavage and Functionalizations. J. Am. Chem. Soc.
3
2016, 138, 1514−1517. (d) Ren, R.; Wu, Z.; Xu, Y.; Zhu, C. C−C
Bond-Forming Strategy by Manganese-Catalyzed Oxidative Ring-
Opening Cyanation and Ethynylation of Cyclobutanol Derivatives.
Angew. Chem., Int. Ed. 2016, 55, 2866−2869. (e) Wang, D.; Ren, R.;
Zhu, C. Manganese-Promoted Ring-Opening Hydrazination of
Cyclobutanols: Synthesis of Alkyl Hydrazines. J. Org. Chem. 2016,
(20) (a) Buslov, I.; Becouse, J.; Mazza, S.; Montandon-Clerc, M.;
Hu, X. Chemoselective Alkene Hydrosilylation Catalyzed by Nickel
Pincer Complexes. Angew. Chem., Int. Ed. 2015, 54, 14523−14526.
(
b) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. A Highly
Chemoselective Cobalt Catalyst for the Hydrosilylation of Alkenes
using Tertiary Silanes and Hydrosiloxanes. ACS Catal. 2016, 6, 3589−
8
1, 8043−8049. (f) Li, Z.-L.; Li, X.-H.; Wang, N.; Yang, N.-Y.; Liu,
3
593.
X.-Y. Radical-Mediated 1,2-Formyl/Carbonyl Functionalization of
Alkenes and Application to the Construction of Medium-Sized Rings.
Angew. Chem., Int. Ed. 2016, 55, 15100−15104. (g) Guo, J.-J.; Hu, A.;
Chen, Y.; Sun, J.; Tang, H.; Zuo, Z. Photocatalytic C−C Bond
Cleavage and Amination of Cycloalkanols by Cerium(III) Chloride
Complex. Angew. Chem., Int. Ed. 2016, 55, 15319−15322. (h) Davis,
D. C.; Haskins, C. W.; Dai, M. Radical Cyclopropanol Ring Opening
Initiated Tandem Cyclizations for Efficient Synthesis of Phenan-
thridines and Oxindoles. Synlett 2017, 28, 913−918.
(
21) For details of the optimizations, see the Supporting
Information.
22) We also conducted similar experiments for the silylation
reactions. For details, see the Supporting Information.
(
(
12) (a) Sorin, G.; Martinez Mallorquin, R.; Contie, Y.; Baralle, A.;
Malacria, M.; Goddard, J.-P.; Fensterbank, L. Oxidation of Alkyl
Trifluoroborates: An Opportunity for Tin-Free Radical Chemistry.
Angew. Chem., Int. Ed. 2010, 49, 8721−8723. (b) Huang, H.; Zhang,
G.; Gong, L.; Zhang, S.; Chen, Y. Visible-Light-Induced Chemo-
selective Deboronative Alkynylation under Biomolecule-Compatible
Conditions. J. Am. Chem. Soc. 2014, 136, 2280−2283.
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DOI: 10.1021/acs.orglett.9b00874
Org. Lett. XXXX, XXX, XXX−XXX