Merging Photoredox with 1,2-Metallate Rearrangements: The Photochemical Alkylation of Vinyl Boronate Complexes

Article abstract of DOI:10.1021/jacs.7b02569

Vinyl boronates react with electron-deficient alkyl iodides in the presence of visible light to give boronic esters in which two new C-C bonds have been created. The reaction occurs by radical addition of an electron-deficient alkyl radical to the vinyl boronate followed by electron transfer with another molecule of alkyl iodide, continuing the chain, and triggering a 1,2-metalate rearrangement. In a number of cases, the use of a photoredox catalyst enhances yields significantly. The scope of the radical precursor includes α-iodo ketones, esters, nitriles, primary amides, α-fluorinated halo-acetates and perfluoroalkyl iodides.

Full text of DOI:10.1021/jacs.7b02569

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Communication
Merging Photoredox with 1,2-Metallate Rearrangements:
The Photochemical Alkylation of Vinyl Boronate Complexes
Mattia Silvi, Christopher Sandford, and Varinder K. Aggarwal
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b02569 • Publication Date (Web): 12 Apr 2017
Downloaded from http://pubs.acs.org on April 12, 2017
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Merging Photoredox with 1,2-Metallate Rearrangements: The Pho-
tochemical Alkylation of Vinyl Boronate Complexes
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Mattia Silvi, Christopher Sandford, and Varinder K. Aggarwal*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom.
Supporting Information Placeholder
Scheme 1. a) 1,2-Metallate rearrangements. b) Photoredox
radical-polar crossover reactions. c) This work: merging
photoredox with 1,2-metallate rearrangements.
ABSTRACT: Vinyl boronates react with electron-deficient alkyl
iodides in the presence of visible light to give boronic esters in
which two new C–C bonds have been created. The reaction occurs
by radical addition of an electron-deficient alkyl radical to the vinyl
boronate followed by electron transfer with another alkyl iodide,
continuing the chain, and triggering a 1,2-metallate rearrangement.
In a number of cases, the use of a photoredox catalyst enhances
yields significantly. The scope of the radical precursor includes α-
iodo ketones, esters, nitriles, primary amides, α-fluorinated halo-
acetates and perfluoroalkyl iodides.
Since its discovery over 50 years ago, the 1,2-metallate rear-
rangement of boronate complexes possessing αleaving groups
(Scheme 1a, Matteson reaction) has been developed and refined
into a powerful and versatile, substrate-controlled, asymmetric
methodology, which has been widely used in synthesis.1 In the last
decade, we have established a suite of related reagent-controlled
homologations of boronic esters.2 This methodology allowed us to
develop iterative processes that enable the conversion of simple
starting materials into complex molecules without intermediate pu-
rification, a strategy that was applied to the synthesis of natural3
and non-natural products.4 Over the same period, photoredox catal-
ysis has emerged as a tremendously versatile platform for organic
synthesis, enabling reactive radical centers to be generated at spe-
cific positions in organic molecules under exceptionally mild con-
ditions.5,6 One of the key features of photoredox catalysis is the po-
tential to crossover from a radical to a polar pathway7 during the
overall catalytic process. For example, in photoredox olefin func-
tionalizations with electrophilic radicals,8 single-electron transfer
(SET) from radical 4 generates cation 5, which can be trapped by
nucleophiles to afford adducts 6 (Scheme 1b).9 We considered the
possibility of merging the features of photoredox reactions with
1,2-metallate rearrangements, two contemporary synthetic strate-
gies that are currently unconnected. Specifically, we reasoned that
related reactions of vinyl boronates 7 with electrophilic radicals 8
would lead to a new radical anion 9 that might undergo single-elec-
tron oxidation and trigger a 1,2-metallate rearrangement, generat-
ing diversely functionalized boronic esters 10 (Scheme 1c).10 This
strategy, which was ultimately successful, provides a novel three-
component coupling reaction where two new C-C bonds are formed
in one step, rapidly building up molecular complexity while retain-
ing the highly versatile boronic ester moiety. During the prepara-
tion of this manuscript, Studer and co-workers reported a related
radical three-component coupling reaction of alkyl iodides and vi-
nyl boronate complexes, utilizing triethylborane/air for radical gen-
eration.11
We began our studies by exploring the reaction between vinyl boro-
nates and phenacyl halides and tested various solvents and metal
catalysts under visible light irradiation (Table 1). Phenacyl halides
were selected as radical precursors owing to their well-established
tendency to readily undergo one-electron reduction generating sta-
bilized, electrophilic α-carbonyl radicals,12 and because they have
been used in photoredox-mediated reactions.5a For our optimization
studies, boronate complex 15a was generated in situ by treating the
boronic ester 14a with n-butyllithium in Et2O. After solvent re-
moval, a solution of phenacyl halide and the photocatalyst were
added to the vessel and the mixture irradiated with blue light.
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31) were tolerated, although the unsubstituted vinyl boronate
Table 1. Optimization Studies
(R1=H, product 28) required the use of a photocatalyst to achieve
high yields (37% yield in the absence of Ru(bpy)3Cl2·6H2O). The
scope of the phenacyl iodide radical precursors was also explored:
electron-rich and electron-deficient aromatic groups (products 32–
33), as well as an N-H indole moiety (product 27), were success-
fully introduced.
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Scheme 2. Scope of boronates and phenacyl iodidesa
8
9
Entrya
Solvent
X
Photocatalyst
17 (%)b
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1
2
DMSO
DMSO
DMSO
DMSO
THF
Cl
Br
Br
I
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ir(ppy)3
traces
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3
57
4
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
none
80
5
I
38
6
DMI
I
85
7c
8c
9c,d
DMI
I
86 (85)
87 (88)
15
DMI
I
DMI
I
none
aUnless otherwise noted, all the reactions were carried out under a nitrogen
atmosphere, using Schlenk techniques, on a 0.15 mmol scale. After boro-
nate complex formation, solvent exchange to the selected solvent was per-
formed. b1H NMR yield using 1,3,5-trimethoxybenzene as an internal stand-
ard; yield of isolated product based on a 0.3 mmol scale reaction is given in
parentheses. cReaction carried out by adding a DMI solution of iodoaceto-
phenone (0.1 M) to the ethereal suspension of boronate complex. dReaction
carried out in the dark.
Whilst phenacyl chloride was not a good substrate for our reaction
(entry 1), we were pleased to find that subjecting the corresponding
bromide to our reaction conditions in the presence of
Ru(bpy)3Cl2·6H2O afforded boronic ester 17 in 57% yield (entry
2). The use of the more reducing photocatalyst Ir(ppy)3 gave similar
results (entry 3). Gratifyingly, simply by replacing phenacyl bro-
mide with the corresponding iodide, the yield improved to 80% (en-
try 4). The improvement of yield going from Cl to Br to I follows
the increasing ease of one-electron reduction along the series.12,13
Solvent effects were also explored and further improvements were
observed by switching from DMSO to 1,3-dimethyl-2-imidazoli-
dinone (DMI, entry 6). We also found that Et2O-DMI mixtures
could be used (entry 7), simplifying the reaction set up: after for-
mation of the boronate complex in ether, a DMI solution of the rad-
ical precursor was added to the boronate complex, avoiding solvent
exchange. However, and much to our surprise, a control experiment
without the photocatalyst and just irradiation with blue LEDs af-
forded the product in a similar yield (88%, entry 8).14 A further
control experiment (entry 9) showed that light was essential as little
product was obtained in the dark.
aReactions were carried out under a nitrogen atmosphere, using Schlenk
techniques on a 0.3 mmol scale; yields after chromatographic purification.
bYield of the corresponding alcohol following H2O2/NaOH oxidation.
c1 mol% of Ru(bpy)3Cl2·6H2O was used.
We then tested a more diverse range of alkyl halide radical pre-
cursors (Scheme 3). We were delighted to find that subjecting ali-
phatic α-iodo ketones (preformed in the reaction medium by treat-
ment of the corresponding bromides or chlorides with stoichio-
metric NaI, see SI and vide infra) to our reaction conditions af-
forded the desired compounds in high yield (compounds 35–36).
Other functionalities such as esters, nitriles and, remarkably, pri-
mary amides could also be employed affording the desired products
in good to excellent yields (products 37–39). α-Fluorinated halo-
acetates and perfluoroalkyl iodides,8,15,16 including CF3I,17 were
also successfully employed, affording the corresponding fluori-
nated tertiary boronic esters in good to excellent yields (products
40–43), although the use of a photocatalyst was found to be neces-
sary in most cases in order to achieve good reaction efficiency. Di-
ethyl bromomalonate, an efficient reagent for atom-transfer radical
additions (ATRA) to olefins,8 was also a competent radical precur-
sor furnishing, in the presence of a ruthenium photocatalyst, bo-
ronic ester 44 in high yield.
With optimum reaction conditions established, we tested a range
of different boronate complexes (Scheme 2). Boronate complexes
obtained by treating boronic ester 14a with n-butyllithium, iso-
propyllithium or tert-butyllithium all reacted efficiently (Scheme 2,
17–19), demonstrating that the level of steric hindrance around the
boron center does not significantly impact on the radical addition,
subsequent oxidation or 1,2-migration steps. A range of aryllithi-
ums of different steric and electronic properties worked well (20–
24). Organolithiums generated from heteroaromatics could also be
used: the 5-indolyl-substituted compound 26 was obtained with
high yield, although the 3-pyridyl compound 25 was obtained in
lower yield. Variation of the α-substituent (R1) of the vinyl boro-
nate (products 28–30) and a fully substituted vinyl moiety (product
The products from these reactions are noteworthy. For example,
whilst ample methods are available for the synthesis of -carbonyl
boronic esters (e.g. by conjugate addition to Michael acceptors),18
there are few examples for the synthesis of γ-carbonyl boronic es-
ters19 which are now easily accessible using the current methodol-
ogy. Furthermore, it is not possible to create compounds like 40 by
existing metallate rearrangements since intermediate boronate
complexes with -leaving groups undergo elimination instead of
1,2-migration.20
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Scheme 3. Scope of radical precursorsa
radical chain is likely to be inefficient, stemming from slow initia-
1
2
3
tion or slow electron/atom transfer. However, in these particular
cases, a ruthenium photocatalyst is able to mediate these steps,
leading to high yields.
4
Scheme 5. Proposed mechanism
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aReactions were carried out under a nitrogen atmosphere, using Schlenk
techniques, on a 0.3 mmol scale; yields after chromatographic purification.
bReaction carried out starting from the corresponding bromide or chloride,
Finally, since alkyl iodides are often not commercially available,
are light- and air sensitive, and are severe lachrymators, we sought
to generate them in situ from the corresponding bromides with NaI.
Subjecting phenacyl bromide to our reaction conditions in the pres-
ence of sodium iodide (20 mol%), we were delighted to observe the
formation of the desired compound in 78% yield, further improving
the efficiency and sustainability of our approach (Scheme 6). Thus,
two pathways are available from the bromoketone: use of 1 mol%
of the Ru photosensitizer or 20 mol% of NaI.
c
premixed with NaI prior to addition (see SI). 5 equiv. of CF3I·2DMSO
d
e
f
were used. 1 mol% of Ru(bpy)3Cl2·6H2O was added. 2:1 d.r. Reaction
g
carried out starting from the corresponding bromide. Boronate complex
derived from phenyllithium.
Next, we conducted further experiments to probe the reaction
mechanism. Carrying out the reaction in the presence of a nitroxyl
radical (TEMPO) completely shut down the reaction, suggesting
that a radical mechanism is operative (Scheme 4). Furthermore,
when the reaction was carried out in the presence of a styryl radical
trap (1,1-diphenylethylene),21 addition to the boronate was sup-
pressed and adduct 45 was observed in the crude reaction mixture.22
This product could arise from addition of the phenacyl radical to
the radical trap23 followed by either oxidation of the stabilized ben-
zylic radical generated, or hydrolysis of the benzyl iodide atom-
transfer product upon workup.
Scheme 6. Using NaI for bromoacetophenone activation.
Scheme 4. Control experiments
In conclusion, after over 50 years of 1,2-metallate rearrange-
ments involving leaving groups, we have discovered a novel
trigger mechanism involving oxidation of an -boronate radical.
This radical species can be generated from a halide and a vinyl
boronate by photoredox, visible light-induced activation or by Stu-
der’s Et3B/air protocol.11 The collective methods cover vast sub-
strate scope providing a novel, efficient and atom-economic three-
component coupling reaction. Furthermore, many of the products
are simply not accessible through traditional 1,2-metallate rear-
rangements, thereby opening up new chemical space.
To gain insight into the mechanism of the photoinitiation event,
we conducted UV/Vis absorption analysis of both the boronate
complex and iodoacetophenone separately and then combined;
however, no new bands in the visible region were observed, indi-
cating that a ground-state charge-transfer complex was not formed
in solution.14a,b We did observe very low residual visible light ab-
sorption of the iodoacetophenone (see SI for further details), which
we believe is the photoinitiation event. Thus, the mechanism pro-
posed involves homolytic cleavage of the alkyl iodide by visible
light, generating the electrophilic alkyl radical 46,23,24 which reacts
with the electron-rich vinyl boronate complex 15a to form boronate
radical 47 (Scheme 5). This electron-rich radical anion then under-
goes facile single-electron oxidation (or iodine atom transfer, not
shown)25 with another molecule of alkyl iodide,11,26 thus triggering
rapid (or concerted)27 1,2-metallate rearrangement to afford the
product 17. The process also releases a new α-carbonyl radical 46,
thereby feeding a radical chain propagation pathway. We have
found that for the reactions affording compounds 27, 28 and 41–
44, the addition of a Ru photocatalyst was necessary to achieve high
yields. In these cases, in the absence of photocatalyst, the innate
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Detailed experimental procedure and characterization data for
new compounds (PDF).
AUTHOR INFORMATION
Corresponding Author
v.aggarwal@bristol.ac.uk
Notes
The authors declare no competing financial interests.
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Am. Chem. Soc. 1997, 119, 2420.
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We thank the University of Bristol for financial support. MS thanks
the EU for H-2020 Marie Skłodowska-Curie Fellowship (grant no.
744242). We thank Dr. Eddie Myers for useful discussions.
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