Angewandte
Communications
Chemie
Arylboronic Acid Synthesis
Rhodium-Catalyzed Decarbonylative Borylation of Aromatic
Thioesters for Facile Diversification of Aromatic Carboxylic Acids
Abstract: Transformation of aromatic thioesters into aryl-
boronic esters was achieved efficiently using a rhodium
catalyst. The broad functional-group tolerance and mild
conditions of the method have allowed for the two-step
decarboxylative borylation of a wide range of aromatic
carboxylic acids, including commercially available drugs.
confirmed the validity of this approach. Herein, we report
a decarbonylative borylation of aromatic thioesters that
enabled two-step decarboxylative borylation of aromatic
carboxylic acids.
The challenge of this transformation was to cleave a stable
À
C(aromatic) C(carbonyl) bond while forming an easily trans-
À
formable C B bond. Although various transition-metal-
T
he carboxylic acid is a fundamental functional group often
catalyzed decarboxylative transformations of aromatic
carboxylic acids have been reported,[2] these transformations
were achieved at elevated temperature (typically > 1508C),
which greatly limits the scope of applicable substrates.[11] This
is probably because a large amount of energy is required to
found in a broad range of organic molecules, including natural
products, pharmaceuticals, agrochemicals, and functional
materials. The availability of carboxylic acids has been further
increased by recent advances in carboxylation reactions.[1] In
line with the increasing availability of carboxylic acids, their
direct decarboxylative functionalization, which significantly
expands the diversity of synthesizable molecules, has also
been attracting considerable interest.[2] One of the most
straightforward approaches for facile diversification is to
transform carboxylic acids into multitransformable inter-
mediates such as organoboron compounds, which serve as
versatile synthetic intermediates demonstrating a wide spec-
trum of reactivities (Scheme 1A).[3] Indeed, recent stud-
À
À
cleave the stable C C bond. To achieve the C C bond
cleavage under milder conditions, we focused on transition-
metal-mediated decarbonylation by acylmetal species
I
(Scheme 1B).[12] We anticipated that reverse insertion of
a carbonyl group in I could smoothly occur by the cleavage of
À
the C C bond under mild conditions to afford arylmetal
species II,[2a,13] which could react with a diboron compound to
furnish the borylated product 3.[10a] Recently, Shi et al. and
Rueping et al. independently reported nickel-catalyzed
decarbonylative borylations based on this approach using
esters and amides as precursors for the acylmetal species.
However, these reactions still required high temperature
(> 1508C) with moderate substrate scope.[14] We envisioned
that thioester 2, which is a stable carboxylic acid derivative,[15]
would be a better precursor of the acylmetal species I because
ies,[4–10] including ours,[9b,10a] on catalytic borylative trans-
formations by cleavage of stable bonds, such as C H,
[5]
À
[6]
[7]
[8]
[9]
[10]
À
À
À
À
À
C O, C N, C CN, C F, and C S
bonds, have
À
highly chemoselective cleavage of a C(carbonyl) S bond in
thioester 2 by oxidative addition to a low-valent transition
metal was anticipated to proceed under mild conditions.[16]
After extensive screening of the reaction conditions using
S-ethyl 4-phenylbenzothioate (2a, 0.200 mmol), we found
that a rhodium complex in the presence of a phosphine ligand
and a base efficiently catalyzed the desired decarbonylative
borylation under mild conditions (Table 1). Heating a mixture
of 2a, bis(pinacolato)diboron (4a, (Bpin)2, 2 equiv),
[Rh(OH)(cod)]2
(5 mol%;
cod = 1,5-cyclooctadiene),
P(nBu)3 (50 mol%), and KOAc (20 mol%) in cyclopentyl
methyl ether (CPME) at 808C for 24 h afforded the desired
arylboronate 3a in high yield (Table 1, entry 1). Reactions
using a ligand other than P(nBu)3 and PEt3, as well as the
ligandless reaction, provided poor results with recovery of 2a
(Table 1, entries 2–5; Supporting Information, Table S1).
Whereas the amount of P(nBu)3 could be reduced to
10 mol% (Rh:P = 1:1) to afford 3a in a reasonable yield
(Supporting Information, Table S2) and highly reproducible
result was obtained using 50 mol% of the ligand.[17] The
complexes [RhCl(cod)]2 and [RhCl(CO)2]2 also catalyzed this
reaction, although prolongation of the reaction time from
24 h to 120 h was needed to achieve efficient transformation
Scheme 1. Proposed strategy.
[*] Dr. H. Ochiai, Dr. Y. Uetake, Dr. T. Niwa, Prof. Dr. T. Hosoya
Chemical Biology Team, Division of Bio-Function Imaging, RIKEN
Center for Life Science Technologies
6-7-3 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047 (Japan)
E-mail: takashi.niwa@riken.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!