Metal-Free and Redox-Neutral Conversion of Organotrifluoroborates into Radicals Enabled by Visible Light

Article abstract of DOI:10.1002/anie.201807181

Converting organoboron compounds into the corresponding radicals has broad synthetic applications in organic chemistry. To achieve these transformations, various strong oxidants such as Mn(OAc)₃, AgNO₃/K₂S₂O₈, and Cu(OAc)₂, in stoichiometric amounts are required, proceeding by a single-electron transfer mechanism. Established herein is a distinct strategy for generating both aryl and alkyl radicals from organotrifluoroborates through an S_H2 process. This strategy is enabled by using water as the solvent, visible light as the energy input, and diacetyl as the promoter in the absence of any metal catalyst or redox reagent, thereby eliminating metal waste. To demonstrate its synthetic utility, an efficient acetylation to prepare valuable aryl (alkyl) methyl ketones is described and applications to construct C?C, C?I, C?Br, and C?S bonds are also feasible. Experimental evidence suggests that triplet diacetyl serves as the key intermediate in this process.

Full text of DOI:10.1002/anie.201807181

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Accepted Article
Title: Metal-free and Redox-neutral Conversion of
Organotrifluoroborates to Radicals Enabled by Visible Light
Authors: Wenbo Liu, Peng Liu, Leiyang Lv, and Chao-Jun Li
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10.1002/anie.201807181
Angewandte Chemie International Edition
COMMUNICATION
Metal-free and Redox-neutral Conversion of
Organotrifluoroborates to Radicals Enabled by Visible Light
Wenbo Liu,[a] Peng Liu,[a], [b] Leiyang Lv,[a] and Chao-Jun Li*[a]
Abstract: Converting organoboron compounds to corresponding
radicals has broad synthetic applications in organic chemistry. To
achieve these transformations, various strong oxidants such as
Mn(OAc)3, AgNO3/K2S2O8 and Cu(OAc)2 in stoichiometric amount are
required through single electron transfer mechanism. Herein, we
establish a distinct strategy to generate both aryl and alkyl radicals
with organotrifluoroborates as the precursors via the SH2 process.
This strategy is enabled by using water as the solvent and visible light
as the energy input and diacetyl as the promoter in the absence of
any metal catalyst or redox reagent, thereby eliminating the metal
waste. To demonstrate its synthetic utility, an efficient acetylation to
prepare valuable aryl (alkyl) methyl ketones is developed and
applications to construct other C-C, C-I, C-Br, C-S bonds are also
feasible. Experimental evidences suggest that triplet diacetyl serves
as the key intermediate in this process.
Organoboron compounds are versatile in organic synthesis.
Early boron chemistry mainly focused on organoboranes, which
carry some limitations including: (1) they are pyrophoric, which
Figure 1. Traditional approaches to convert boron compounds to radicals and
makes them difficult to handle; (2) in most cases, only one
substituent in the boranes is delivered into the products, which
reduces the atom economy especially when the organoboranes
are valuable. In this regard, organoboronic acids, esters and
trifluoroborates are more advantageous because of their stability,
low toxicity, and ease of preparation, storage and handling.[1]
Many elegant and powerful reactions involving these boron
compounds have been established including Suzuki cross
coupling,[2] Petasis reaction,[3] Chan-Lam coupling[4] and the
Hayashi-Miyaura reaction,[5] in which the boron compounds serve
as the nucleophiles. Besides the nucleophiles, organoboron
compounds can also be the precursors of radicals.[6]
Radicals are important intermediates in organic synthesis.[7]
Approaches that generate radicals from readily available
precursors are highly desirable. Benefited from the Suzuki
coupling and other boron-based reactions, numerous
organoboronic acids and trifluoroborates have been
commercialized and readily accessible. Thus, developing efficient
this work.
methods that convert these boron compounds to radicals is
significant. Hitherto, the main strategy to convert these boron
compounds to radicals is through the single electron transfer
(SET) mechanism with transition metals as the oxidants. For
example, Demir reported that aryl boronic acids were converted
to aryl radicals in the presence of Mn(OAc)3 at high temperature.[8]
Baran showed that aryl boronic acids and trifluoroborates were
oxidized to aryl radicals by AgNO3/K2S2O8 at room temperature.[9]
In addition to aryl boronic compounds, alkyl counterparts can also
be oxidized to the corresponding alkyl radicals with suitable
oxidants. For instance, Fensterbank and Molander demonstrated
that alkyl trifluoroborates were oxidized to alkyl radicals with
stoichiometric amount of Cu(OAc)2 (Figure 1a top).[10]
Nonetheless, the engagement of stoichiometric strong metal-
based oxidants not only significantly limits the functional group
compatibility but also generates large amounts of waste, which
provokes serious environmental concerns. Upon these issues,
Akita, Molander and others introduced photo-redox catalysts to
oxidize alkyl trifluoroborates to alkyl radicals, thereby eliminating
the generation of metal wastes (Figure 1a bottom).[11] However,
due to higher oxidative potential, these benign photo-redox
protocols fail to transform aryl trifluoroborates jnto radicals
although the more reactive aryl (triol)borate can be oxidized under
photo-redox conditions as demonstrated by Akita.[11c] Moreover,
through using the Lewis base catalysis strategy, Ley has reported
that electron-rich aryl boronic acids can be converted into the aryl
radicals under photo-redox conditions.[12] In this context, we wish
[a]
Dr. W. Liu#, Dr. P. Liu#, Dr. L. Lv, and Prof. Dr. C.-J. Li*
Department of Chemistry and FRQNT Centre for Green Chemistry
and Catalysis, McGill University, 801 Sherbrooke St. W., Montreal,
Quebec H3A 0B8, Canada
E-mail: cj.li@mcgill.ca
#W. Liu and P. Liu contributed equally.
[b]
Dr. P.Liu
Ministry of Education Key laboratory of Combinatorial Biosynthesis
and Drug Discovery, Hubei Provincial Engineering and Technology
Research Center for Fluorinated Pharmaceuticals, School of
Pharmaceutical Sciences, Wuhan University, Wuhan, 430071,
China
to report
a general approach to convert both aryl/alkyl
trifluoroborates to the aryl/alkyl radicals promoted by visible light
without any photo-redox catalyst or strong oxidants. This
approach via the bimolecular homolytic substitution (SH2)
mechanism both acquires practical advantages and provides a
complementary mechanistic alternative to SET. The success of
Supporting information for this article is given via a link at the end of
the document.
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this protocol hinges upon using water as the solvent and diacetyl
as the visible light responsive promoter (Figure 1b).
Our work was inspired by the well-established organoborane
chemistry, which engages the SH2 at the boron center initiated by
an oxygen radical (Figure 2a).[13] However, this SH2 process is
only applicable to the highly reactive pyrophoric organoboranes
and the specialized catechol boronic esters. Regular boronic
acids and esters, particularly aryl boronic acids and esters, are
not suitable [6, 14] because: (1) the conjugation of two oxygen lone
pairs with the empty p orbital of boron atom decreases the Lewis
acidity of aryl boronic acid (and ester); (2) a high energy barrier
needs to overcome to generate the highly reactive aryl radical
(Figure 2b). Upon these challenges, we hypothesized that ArBF2,
which can be generated from ArBF3 salt in situ,[15] might be
sufficiently reactive towards oxygen radical to undergo the SH2
process due to its stronger Lewis acidity (Figure 2c). With these
concerns in mind and motivated by Davies’ seminal investigation
on triplet ketones,[16] we envisioned that organotrifluoroborate
might interact with the triplet diacetyl to produce the radicals via
the SH2 process.
Scheme 1. Radical trapping experiments with TEMPO.
Table 1. Synthesis of aryl (alkyl) methyl ketone from
trifluoroborate and diacetyla
Figure 2. Research design. (a) Oxygen radical reacting with organoborane via
SH2 process to form radical, (b) challenges to convert aryl boronic compounds
via SH2 process, (c) research proposal to convert aryl trifluoroborates to aryl
radicals via triplet diacetyl promoted by visible light.
To evaluate this hypothesis shown in Figure 2c, we selected
diacetyl as the photo-sensitizer because: (1) it is visible light
sensitive; (2) it is cheap and readily available and (3) it is non-
toxic since it is widely exploited as the food additive. In addition,
water was chosen as the solvent because: (1) water is not a
hydrogen donor for radical and (2) the hydrophobic effect[17] would
favor the interaction between the triplet ketone and ArBF2. Based
on the persistent radical effect [18], TEMPO was added to trap the
radical intermediate, thereby serving as a convenient and reliable
indicator of the radical intermediate via forming the
TEMPO/radical adduct. The compact fluorescence lamp (CFL)
was chosen as the light source and the reaction temperature was
selected as 4 °C to alleviate the potential protodeboronation.[19]
When 4-biphenyltrifluoroborate was subjected to the test
conditions, it was found that the biphenyl/TEMPO adduct can be
detected in 56% yield (Scheme 1). Control experiments showed
that no radical adduct was formed without diacetyl (Scheme 1).
These two experiments suggest the generation of aryl radical from
aryl trifluoroborate under the light irradiation assisted by diacetyl.
In contrast, other common boronic compounds including biphenyl
boronic acid, pinacol ester and MIDA ester cannot form the
corresponding radical under these conditions, thereby signifying
[a] Reactions were performed at 0.1 mmol scale unless otherwise specified, with
1.5 equivalent of AcOH, 1 mL water at 4 °C under the irradiation of 2 × 34 Watts
household CFL lamp for 24 hours unless otherwise specified. All the yields refer
to the isolated average yields of two parallel runs. [b] Under Condition B (See
SI)
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10.1002/anie.201807181
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the importance of F atom in this triplet diacetyl engaged SH2
process. Moreover, other water-insoluble diketones such as 2,3-
hexanedione and 3,4-hexanedione cannot promote the
generation of radical, which implies the unique reactivity of
diacetyl in this process (Scheme 1).
It is fully expected that this acetylation would further upgrade
the versatility of organotrifluoroborates even though they have
already found extensive applications in organic synthesis.[15b-d, 27]
To elucidate this argument, we have prepared a wide array of
important synthons from organotrifluoroborate with the acetylation
as the key steps (please see SI). For example, 4-
biphenyltrifluoroborate was converted into the ester via the
combination of acetylation and oxidation. Moreover, a new
To evaluate the synthetic utility of this approach generating
radicals, we turned attention to the radical addition of diacetyl
because this will represent an interesting reaction, in which
diacetyl serves both as radical promoter and reactant.[20]
Therefore, it can be envisioned that at least two equivalents of
diacetyl are needed since one equivalent acts as the radical
promoter and the other as the acylation reagent. More
significantly, the prepared aryl (alkyl) methyl ketones from the
radical addition are highly useful intermediates to heterocycles,
fragrances, resins and a wide range of drug candidates.[21]
Previous approaches towards these compounds are comprised of
cross couplings, Wacker oxidation and alkyne hydration, which
require either transition metal catalysts or sensitive or toxic
reagents.[22] Therefore, an efficient approach to prepare these
methyl ketones with environmentally benign trifluoroborates and
diacetyl would be more advantageous. Towards this goal and
following extensive optimization, we found that 4-
biphenyltrifluoroborate, as the model substrate, can be acetylated
in 86% yield under visible light irradiation in water [23] (Please see
SI for details).
With the optimized conditions identified, we subsequently
examined the scope of this mild acetylation protocol. As shown in
Table 1, a wide range of aryl and alkyl trifluoroborates can be
acetylated under the standard conditions in good to excellent
yields (35% - 90%). Various functional groups, including (-OPh, -
OBn, -Cl, -Br, -F, -CF3 and ketone) are also compatible with the
conditions. Remarkably, aryl trifluoroborates, benzylic and non-
activated primary as well as the secondary alkyl trifluoroborates
can all be employed in this acetylation. The broad scope is
complementary to the related elegant photo-redox approach
disclosed by Molander and co-workers.[24] To highlight the
practicality of this method, the gram scale synthesis was
performed (entries 8 & 13 in Table 1). It is noteworthy that during
the scope investigation, trace amount of a by-product (< 5%) was
always observed along with the desired methyl ketones as shown
in Table 1.[25]
aromatization reaction was developed through
a cascade
acetylation and aldol condensation. Furthermore, the
trifluoroborate can be transformed into the carboxylic acid by this
acetylation, followed by oxidation. Finally, vinyl triflate, as a
versatile intermediate in various cross-couplings, was prepared
from the trifluoroborate. These examples suggest the unique roles
of this acetylation in diversifying organotrifluoroborate.
To explore further preparative potential of this approach
forming radicals in different contexts, we found that other bonds
including C-C, C-S, C-I, C-Br can be constructed in presence of
the radical trapping reagents in synthetically useful yields without
optimization (Scheme 2). Control experiments suggest that light
irradiation and diacetyl are indispensable for all these
transformations, which rule out the ionic pathway. These
examples corroborate the synthetic potential of this new method
to convert trifluoroborates to radicals and further optimization are
undergoing. Preliminary optimization results suggest that upon
removing the acetic acid additive, increasing the temperature to rt
and the radical trapping reagents to
2 equivalents, 4-
bromobiphenyl and 4-iodobiphenyl can be obtained in 84% and
90% yield, respectively.
To further investigate the functional group tolerance of this
reaction, we also performed the parallel reactions in the presence
of a wide variety of additives (see SI), known as “robustness
screen”.[26] These results demonstrate that the presence of an
alkene, alkyne, alcohol, ketone, aldehyde, ester, amide and aryl
iodide has little or no impact on this reaction since the desired
product can be obtained in comparable yields and the additives
can be recovered in > 76% yields. Notably, considering that this
reaction employs water as the solvent and visible light as the
energy input in the absence of any toxic transition metal catalysts
at low temperature, we envisioned that this approach may find
potential application in chemical biology. As a preliminary study,
we found three types of important molecules (i.e. amino acid,
glucose and uridine), which comprise three fundamental bio-
macromolecules (i.e. protein, sugar and nucleic acids), can all be
tolerated under the standard conditions. However, among all
additives, an exception is nitrobenzene, which completely inhibits
the reaction.
Scheme 2. Synthetic application of this approach to generate radicals in other
contexts (all the yields are not optimized).
Based on our hypothesis in Figure 2c, the triplet diacetyl is the
key initiator for the SH2 process. To collect evidences of this
hypothesis, spectroscopic investigations and control experiments
were performed. It was found that in contrast to diacetyl, diethyl
oxalate and ethyl pyruvate cannot initiate this process, which is
consistent with the UV-Vis study because only diacetyl displays
notably absorption in the visible light region ( > 405 nm) (Figure
3a). The intensive absorption of visible light would excite diacetyl
to its triplet state rather than diethyl oxalate and ethyl pyruvate. In
addition, as known triplet quenchers of diacetyl,[28] pyrene
significantly retards this reaction and oxygen completely inhibits
it, both of which imply the engagement of the triplet diacetyl
(Figure 3b & 3c).
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[7]
a) M. Yan, J. C. Lo, J. T. Edwards, P. S. Baran, J. Am. Chem. Soc.
2016, 138, 12692-12714; b) M. R. Heinrich, Chem. Eur. J. 2009, 15,
820-833; c) W. Liu, X. Yang, Y. Gao, C.-J. Li, J. Am. Chem. Soc.
2017, 139, 8621-8627.
[8]
[9]
a) A. S. Demir, Ö. Reis, M. Emrullahoglu, J. Org. Chem. 2003, 68,
578-580; b) A. Dickschat, A. Studer, Org. Lett. 2010, 12, 3972-3974.
a) I. B. Seiple, S. Su, R. A. Rodriguez, R. Gianatassio, Y. Fujiwara,
A. L. Sobel, P. S. Baran, J. Am. Chem. Soc. 2010, 132, 13194-
13196; b) J. W. Lockner, D. D. Dixon, R. Risgaard, P. S. Baran, Org.
Lett. 2011, 13, 5628-5631.
[10]
[11]
a) G. Sorin, R. Martinez Mallorquin, Y. Contie, A. Baralle, M.
Malacria, J.-P. Goddard, L. Fensterbank, Angew. Chem. Int. Ed.
2010, 49, 8721-8723; b) G. A. Molander, V. Colombel, V. A. Braz,
Org. Lett. 2011, 13, 1852-1855; c) M. Presset, N. Fleury-Brégeot, D.
Oehlrich, F. Rombouts, G. A. Molander, J. Org. Chem. 2013, 78,
4615-4619.
a) J. K. Matsui, D. N. Primer, G. A. Molander, Chem. Sci. 2017, 8,
3512-3522; b) D. R. Heitz, K. Rizwan, G. A. Molander, J. Org. Chem.
2016, 81, 7308-7313; c) Y. Yasu, T. Koike, M. Akita, Adv. Synth.
Catal. 2012, 354, 3414-3420; d) H. Huang, G. Zhang, L. Gong, S.
Zhang, Y. Chen, J. Am. Chem. Soc. 2014, 136, 2280-2283; e) T.
Koike, M. Akita, Synlett 2013, 24, 2492-2505; f) K. Miyazawa, Y.
Yasu, T. Koike, M. Akita, Chem. Commun. 2013, 49, 7249-7251; g)
M. Kazuki, K. Takashi, A. Munetaka, Adv. Synth. Catal. 2014, 356,
2749-2755; h) Y. Li, K. Miyazawa, T. Koike, M. Akita, Org. Chem.
Front. 2015, 2, 319-323;
[12]
[13]
F. Lima, U. K. Sharma, L. Grunenberg, D. Saha, S. Johannsen, J.
Sedelmeier, E. V. Van der Eycken, S. V. Ley, Angew. Chem. Int. Ed.
2017, 56, 15136-15140.
a) A. G. Davies, B. P. Roberts, Acc. Chem. Res. 1972, 5, 387-392;
b) P. J. Krusic, J. K. Kochi, J. Am. Chem. Soc. 1969, 91, 3942-3944;
c) A. G. Davies, B. P. Roberts, Nature 1971, 229, 221-223; d) C.
Carra, J. C. Scaiano, Eur. J. Org. Chem. 2008, 2008, 4454-4459.
a) V. Darmency, P. Renaud, in Top. Curr. Chem. (Ed.: A. Gansäuer),
Springer Berlin Heidelberg, Berlin, Heidelberg, 2006, pp. 71-106; b)
J. A. Baban, N. J. Goodchild, B. P. Roberts, J. Chem. Soc., Perkin
Trans. 2 1986, 157-161.
a) E. Vedejs, R. W. Chapman, S. C. Fields, S. Lin, M. R. Schrimpf,
J. Org. Chem. 1995, 60, 3020-3027; b) G. A. Molander, N. Ellis, Acc.
Chem. Res. 2007, 40, 275-286; c) S. Darses, J.-P. Genet, Eur. J.
Org. Chem. 2003, 2003, 4313-4327; d) S. Darses, J.-P. Genet,
Chem. Rev. 2008, 108, 288-325; e) A. J. J. Lennox, G. C. Lloyd-
Jones, J. Am. Chem. Soc. 2012, 134, 7431-7441.
[14]
[15]
Figure 3. Mechanistic investigation. (a) UV-Vis spectroscopic analysis, (b)
triplet quencher pyrene retards the reaction, (c) triplet quencher oxygen
completely inhibits the reaction.
In summary, a general and convenient approach to produce
radicals from organotrifluoroborates promoted by visible light has
been established. This approach utilizes the triplet diacetyl to
activate organotrifluoroborate and proceeds under mild conditions
without strong redox-reagents, thereby avoiding the
stoichiometric metal waste. To demonstrate its synthetic utility, an
acetylation of organotrifluoroborates has been developed.
Moreover, the application in other contexts to synthesize carbon-
carbon and carbon-heteroatom bonds is also viable. This mild and
environmentally friendly approach to generate radicals is
anticipated to find applications in other radical engaged reactions.
[16]
A. G. Davies, D. Griller, B. P. Roberts, J. C. Scaiano, J. Chem. Soc.
D 1971, 196-197.
D. C. Rideout, R. Breslow, J. Am. Chem. Soc. 1980, 102, 7816-7817.
a) H. Fischer, Chem. Rev. 2001, 101, 3581-3610; b) T. Vogler, A.
Studer, Synthesis 2008, 1979-1993.
[17]
[18]
[19]
[20]
P. A. Cox, A. G. Leach, A. D. Campbell, G. C. Lloyd-Jones, J. Am.
Chem. Soc. 2016, 138, 9145-9157.
a) W. G. Bentrude, K. R. Darnall, J. Am. Chem. Soc. 1968, 90, 3588-
3589; b) I. Tabushi, S. Kojo, K. Fukunishi, J. Org. Chem. 1978, 43,
2370-2374; c) A. Citterio, M. Serravalle, E. Vismara, Tetrahedron
Lett. 1982, 23, 1831-1834; d) S. Kim, Adv. Synth. Catal. 2004, 346,
19-32.
[21]
[22]
H. Siegel., M. Eggersdorfer, in Ullmann's Encyclopedia of Industrial
Chemistry, Wiley-VCH Verlag GmbH: Germany, 2000, pp. 187-207.
a) S. D. Ramgren, N. K. Garg, Org. Lett. 2014, 16, 824-827; b) C. E.
Russell, L. S. Hegedus, J. Am. Chem. Soc. 1983, 105, 943-949; c)
A. Hallberg, L. Westfelt, B. Holm, J. Org. Chem. 1981, 46, 5414-
5415; d) H. B. Kwon, B. H. McKee, J. K. Stille, J. Org. Chem. 1990,
55, 3114-3118; e) M. Tanaka, Synthesis 1981, 1981, 47-48; f) B. W.
Michel, A. M. Camelio, C. N. Cornell, M. S. Sigman, J. Am. Chem.
Soc. 2009, 131, 6076-6077; g) T. Mitsudome, T. Umetani, N.
Nosaka, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda, Angew. Chem.
Int. Ed. 2006, 45, 481-485; h) T. Tachinami, T. Nishimura, R.
Ushimaru, R. Noyori, H. Naka, J. Am. Chem. Soc. 2013, 135, 50-
53; i) S. Liang, J. Jasinski, G. B. Hammond, B. Xu, Org. Lett. 2015,
17, 162-165.
The beneficial effect of acetic acid in this acetylation may be
attributed to the fact that protonation of diacetyl can increase its
electrophilicity since aryl and alkyl radicals are nucleophilic.
J. Amani, E. Sodagar, G. A. Molander, Org. Lett. 2016, 18, 732-735.
Please see SI for the possible mechanism to generate this by-
product.
Acknowledgements
We are grateful to the Canada Research Chair Foundation, the
CFI, FRQNT Center for Green Chemistry and Catalysis, NSERC,
and McGill University for supporting our research. W. L. is grateful
to McGill Chemistry department and P. L. is grateful to China
Scholarship Council for fellowship support. L. L is grateful for the
support from National Postdoctoral Program for Innovative
Talents (No. BX201700110) and China Postdoctoral Science
Foundation Funded Project (No. 2017M623270).
[23]
[24]
[25]
Keywords: organotrifluoroborate • radical • visible light • diacetyl
• SH2
[26]
[27]
a) K. D. Collins, F. Glorius, Acc. Chem. Res. 2015, 48, 619-627; b)
K. D. Collins, F. Glorius, Nat. Chem. 2013, 5, 597-601;
a) G. A. Molander, D. E. Petrillo, J. Am. Chem. Soc. 2006, 128,
9634-9635; b) G. A. Molander, D. J. Cooper, J. Org. Chem. 2007,
72, 3558-3560; c) G. A. Molander, L. N. Cavalcanti, J. Org. Chem.
2011, 76, 623-630; d) G. A. Molander, L. N. Cavalcanti, J. Org.
Chem. 2011, 76, 7195-7203;
[1]
D. G. Hall, in Boronic Acids, Wiley-VCH Verlag GmbH & Co. KGaA,
2011, pp. 1-133.
[2]
[3]
N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457-2483.
N. R. Candeias, F. Montalbano, P. M. S. D. Cal, P. M. P. Gois, Chem.
Rev. 2010, 110, 6169-6193.
[28]
H. L. J. Bäckström, K. Sandros, Acta Chem. Scand. 1958, 12, 823-
832.
[4]
[5]
[6]
J. X. Qiao, P. Y. S. Lam, Synthesis 2011, 829-856.
T. Hayashi, K. Yamasaki, Chem. Rev. 2003, 103, 2829-2844.
C. Ollivier, P. Renaud, Chem. Rev. 2001, 101, 3415-3434.
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Entry for the Table of Contents (Please choose one layout)
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Wenbo Liu, Peng Liu, Leiyang Lv and
Chao-Jun Li*
Page No. – Page No.
Metal-free and Redox-neutral
Conversion of Organotrifluoroborates
to Radicals Enabled by Visible Light
Text for Table of Contents
Converting organoboron compounds to corresponding radicals has broad synthetic applications in
organic chemistry. To achieve these transformations, various strong oxidants such as Mn(OAc)3,
AgNO3/K2S2O8 and Cu(OAc)2 in stoichiometric amount are required through single electron transfer
mechanism. Herein, we establish a distinct strategy to generate both aryl and alkyl radicals with
organotrifluoroborates as the precursors via the SH2 process. This strategy is enabled by using
water as the solvent and visible light as the energy input and diacetyl as the promoter in the absence
of any metal catalyst or redox reagent, thereby eliminating the metal waste. To demonstrate its
synthetic utility, an efficient acetylation to prepare valuable aryl (alkyl) methyl ketones is developed
and applications to construct other C-C, C-I, C-Br, C-S bonds are also feasible. Experimental
evidences suggest that triplet diacetyl serves as the key intermediate in this process.
This article is protected by copyright. All rights reserved.