Undirected ortho-selectivity in C–H borylation of arenes catalyzed by NHC platinum(0) complexes

Article abstract of DOI:10.1016/j.mencom.2020.09.005

Borylation of arenes with bis(pinacolato)diboron catalyzed by sterically encumbered NHC platinum complexes proceeds predominantly at ortho-position even in the absence of a directing group (o: m: p ratio up to 10: 3: 1). The similar borylation with pinacolborane would proceed less selectively.

Full text of DOI:10.1016/j.mencom.2020.09.005

Mendeleev
Communications
Mendeleev Commun., 2020, 30, 569–571
Undirected ortho-selectivity in C–H borylation of arenes
catalyzed by NHC platinum(0) complexes
Sergey A. Rzhevskiy,a Maxim A. Topchiy,a Yulia D. Golenko,a Pavel S. Gribanov,b
Grigorii K. Sterligov,a,c Nikita Yu. Kirilenko,a,c Alexandra A. Ageshina,a Maxim V. Bermeshev,a
Mikhail S. Nechaev*,a,c and Andrey F. Asachenko*,a,b
a A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Moscow, Russian
Federation. E-mail: aasachenko@ips.ac.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
c Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
E-mail: m.s.nechaev@org.chem.msu.ru
DOI: 10.1016/j.mencom.2020.09.005
Me
Me
Borylation of arenes with bis(pinacolato)diboron catalyzed
by sterically encumbered NHC platinum complexes proceeds
predominantly at ortho-position even in the absence of
a directing group (o:m:p ratio up to 10:3:1). The similar
borylation with pinacolborane would proceed less selectively.
2 molꢁ ꢂNꢃCꢄPtꢂdvtmsꢄ
ꢅ20 ꢆCꢇ 20 h
ꢀ
B2Pin2
BPin
oꢈmꢈp uꢉ to ꢅ0 ꢈ ꢊ ꢈ ꢅ
Keywords: N-heterocyclic carbenes, platinum(0) complexes, borylation, arenes, organoboron compounds, regioselectivity, catalysis.
Platinum complexes are efficient catalysts of hydrosilylation,1
hydroboration and diboration,2 examples of catalytic C–H functi-
onalization of aromatic compounds employing platinum catalysts3
are also known. To the best of our knowledge, just a few works
on C–H borylation of arenes mediated by platinum complexes
have been published at the time when the manuscript was being
prepared. In 2015, Furukawa and co-authors were the first to
describe borylation of arenes catalyzed by platinum(0) carbene
complexes.3(b) At the same time, the group of Iwasawa and
Takaya independently reported on catalytic C–H borylation of
polyfluorinated aromatic compounds using phosphine pincer
complexes of platinum.3(a) As of today, borylation of arenes
mediated by platinum complexes bearing carbene ligands has
been the most successful due to high stability of these complexes
as well as wide variability of the carbene component. However, one
of the most important issues associated with selectivity of such
borylation of monosubstituted benzenes, e.g. toluene, remains
open. Moreover, the mechanism of this reaction is poorly studied,
and the nature of key intermediates is not known. The question
whether pinacolborane (HBPin) released in the course of the
reaction would participate in borylation is yet to be answered.
Our ongoing interest in application of N-heterocyclic carbene
(NHC) complexes of palladium,4 gold,5 and copper6 in fine organic
synthesis triggered our study of catalytic activity of such platinum
complexesinborylationofarenes. Here, wereportoncomparative
study of catalytic activity of a series of NHC platinum complexes
1a-o (Scheme 1, for the structural details see Online
Supplementary Materials, Scheme S1). When studying the effect
of carbene ligand structure on selectivity of the toluene
borylation, we have unexpectedly discovered a new phenomenon
of high ortho-selectivity among C–H functionalization methods
that do not rely on a directing group. Also, we demonstrated the
possibility of HBPin application for the borylation of arenes.
Notably, selectivity of the reaction of HBPin with toluene differs
cardinally from that of bis(pinacolato)diboron (B2Pin2).
Catalytic properties of the obtained platinum(0) complexes
1a-o were compared in the reaction of borylation of toluene with
diboropinacolate (see Scheme 1). The initial series of experiments
was aimed to reproduce the results reported by Furukawa3(b) using
toluene as the substrate (Table 1, entry 1). First set of catalytic tests
were performed for complexes bearing imidazolium carbenes with
varied steric bulk (entries 1-5). In all cases borylation proceeded
at all three (ortho-, meta- and para-) positions of toluene. In case
of catalyst 1a (entry 1), total 94% yield was achieved and the
isomer ratio was close to statistical with a slight predominance
of the meta-isomer. Similar results were obtained previously by
Chatani.3(b),(c) On moving to catalysts 1b,c with bulkier ligands
Me
Me
Me
Me
B2Pin2
or
BPin
i
ꢃ
ꢃ
ꢃ
cat.
BPin
O
ꢁBPin
BPin
pꢄ2
oꢄ2
mꢄ2
Me
Me
Me
Me
Me
O
O
Me
B2Pin2 ꢂ
B
B
ꢁBPin ꢂ ꢁB
Me
Me
Me
Me
Me
Me
O
O
O
R
Ar
ꢀ
ꢀ
SiMe2
O
SiMe2
N
N
SiMe2
O
SiMe2
N
Pt
N
or
Pt
n
R
Ar
ꢅNꢁCꢆPtꢅdvtmsꢆ
1a−o ꢅcat.ꢆ
Scheme 1 Reagents and conditions: i, (NHC)Pt(dvtms) 1a-o (2 mol%),
120°C, 20 h.
© 2020 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2020, 30, 569–571
Table 1 Dehydroboration of toluene with B2Pin2 or HBPin catalyzed by
Me
Me
(NHC)Pt(dvtms) complexes 1a-o.
BPin
i or ii
Total yield (%) (o-2:m-2:p-2 ratio)
Entry Catalyst
with B2Pin2
with HBPin
Me
Me
1
2
ICyPt(dvtms) (1a)
IButPt(dvtms) (1b)
IPrPt(dvtms) (1c)
94 (1.7:2.8:1.0)
61 (3.6:2.1:1.0)
69 (4.9:2.2:1.0)
3 (1.5:3.0:1.0)
8 (0.6:1.8:1.0)
28 (0.8:1.9:1.0)
p-3
4
iꢀ ꢁꢂꢃꢄ iiꢀ 2ꢅꢃ
Me
Me
Me
3
Me
Me
BPin
IPr*Pt(dvtms) (1d)
IPr*OMePt(dvtms) (1e)
IPrClPt(dvtms) (1f)
IPrBrPt(dvtms) (1g)
IPrMePt(dvtms) (1h)
IMesMePt(dvtms) (1i)
SIMesPt(dvtms) (1j)
SIPrPt(dvtms) (1k)
(6-Mes)Pt(dvtms) (1l)
151 (0.8:1.8:1.0)a 99 (0.3:1.8:1.0)
Me
4
ꢆ
i or ii
5
73 (1.2:1.9:1.0)
92 (3.0:2.1:1.0)
13 (5.5:2.4:1.0)
96 (0.4:1.8:1.0)
18 (0.6:1.8:1.0)
15 (0.6:1.7:1.0)
BPin
iꢀ ꢇꢈꢃ ꢉ5ꢊ5' ꢋ ꢈ ꢊ ꢌꢍꢄ iiꢀ 2ꢁꢃ ꢉ5ꢊ5' ꢋ ꢌ ꢊ 2.2ꢍ
6
7
o-3
5
5
'
8
130 (2.2:2.1:1.0)a 23 (0.6:1.9:1.0)
9
71 (2.8:2.1:1.0)
28 (2.6:2.6:1.0)
52 (4.1:2.2:1.0)
39 (4.1:2.1:1.0)
5 (1.4:2.4:1.0)
4 (0.7:2.5:1.0)
14 (0.5:1.8:1.0)
14 (0.8:2.8:1.0)
R
R
R
R
10
11
12
13
14
15
BPin
i or ii
ꢆ
ꢆ
(6-Dipp)Pt(dvtms) (1m) 129 (5.3:2.6:1.0)a 54 (1.2:2.3:1.0)
BPin
BPin
(7-Mes)Pt(dvtms) (1n)
(7-Dipp)Pt(dvtms) (1o)
64 (5.4:2.2:1.0)
130 (9.6:2.8:1.0)a 62 (1.5:2.4:1.0)
14 (1.0:2.6:1.0)
o-6a−c
m-6a−c
pꢐ6a−c
a R ꢋ Butꢊ iꢀ ꢎꢎꢃ ꢉoꢊmꢊp ꢋ 0 ꢊ ꢌ.ꢎ ꢊ ꢌꢍꢄ iiꢀ 0ꢃ
b R ꢋ Phꢊ iꢀ ꢌꢈꢎꢃ ꢉoꢊmꢊp ꢋ 0 ꢊ ꢌ.ꢅ ꢊ ꢌꢍꢄ
iiꢀ 2ꢌꢃ ꢉoꢊmꢊp ꢋ 0 ꢊ ꢌ.ꢅ ꢊ ꢌꢍ
a Relative yields were calculated based on stoichiometric amounts of the
limiting reactant (B2Pin2).
c R ꢋ Clꢊ iꢀ ꢅꢌꢃ ꢉoꢊmꢊp ꢋ ꢇ.2 ꢊ ꢈ.ꢁ ꢊ ꢌꢍꢄ
iiꢀ ꢌ2ꢃ ꢉoꢊmꢊp ꢋ ꢈ ꢊ 2.ꢏ ꢊ ꢌꢍ ꢆ 2ꢌꢃ PhBPin
(entries 2 and 3), yields drop to 61 and 69%, respectively.
Simultaneously, the increase in ortho-selectivity was observed.
However, further increase in steric bulk of carbene ligand (1d
and 1e, see Table 1, entries 4 and 5) leads to a drop of ortho-
selectivity. Notably, in case of bulky catalyst 1d, the total yield
of products formally exceeded 100% (entry 4). One may suppose
that in this case not only B2Pin2 but also HBPin (which is formed
in the course of the reaction) can serve as borylating reagent.
Analogously to complex 1d, in case of catalyst 1h, the yield of
products exceeded 100% (entry 8).
Me
ꢑ
Me
Me
BPin
i or ii
ꢆ
BPin
ꢑ
ꢑ
7
7'
iꢀ ꢏꢇꢃ ꢉ7ꢊ7' ꢋ ꢌ.ꢁ ꢊ ꢌꢍꢄ iiꢀ ꢏꢈꢃ ꢉ7ꢊ7' ꢋ ꢌ.ꢇ ꢊ ꢌꢍ
Next, we tested complexes 1j-o with saturated five-, six- and
seven-membered ring carbene backbone7 bearing bulky Mes and
Dipp substituents at the nitrogen atoms. In this series of experi-
ments the following trend can be clearly traced: upon transition
from less bulky Mes-substituted carbenes to bulkier Dipp-
substituted ones, drastic increase in yield and significant growth of
ortho-selectivity take place (see Table 1, entry 10 vs. 11, entry 12
vs. 13, entry 14 vs. 15). Analogously, upon expansion of the ring,
and corresponding increase in steric bulk of a carbene ligand, the
yields and ortho-selectivity grow simultaneously (entries 10, 12,
14 vs. entries 11, 13, 15). As a result, in this series the highest yield
(130%) and the best ortho-selectivity (o:m:p = 9.6:2.8:1.0)
were observed for complex bearing seven-membered ring Dipp-
substituted carbene 1o (entry 15).
Attempted borylation of toluene in air with the use of catalyst
1c gave only traces of products. When the reaction was performed
in a sealed vessel which was not pre-flushed with dry argon, the
yield turned to be 18% (cf. 69% under argon, see Table 1, entry 3).
Apparently, catalytically active species and/or intermediates are
air-sensitive.
Then we studied borylation of various arenes mediated by
complexes 1o and 1d, which demonstrated the highest activity
and selectivity in toluene borylation (Scheme 2). In most cases,
catalyst 1o was more active than 1d. Interestingly, borylation of
o-xylene catalyzed by 1o gave mostly ortho-isomer 5 while in
case of 1d there was virtually no selectivity. Reaction of biphenyl
mediated by 1o led to products m/p-6b in excessive (> 100%)
yield, which was probably due to participation of HBPin in the
whole process. The absence of ortho-isomer is presumably
caused by steric bulk of the phenyl substituent.
Me
Me
Me
BPin
i or ii
ꢆ
ꢑ
ꢑ
ꢑ
BPin
8
8'
iꢀ ꢇꢇꢃ ꢉ8ꢊ8' ꢋ ꢌ ꢊ ꢌ.ꢌꢍꢄ iiꢀ ꢈꢅꢃ ꢉ8ꢊ8' ꢋ ꢌ ꢊ ꢌ.ꢈꢍ
Scheme 2 Reagents and conditions: i, B2Pin2, (NHC)Pt(dvtms) 1o (2 mol%),
120°C, 20 h; ii, B2Pin2, (NHC)Pt(dvtms) 1d (2 mol%), 120°C, 20 h.
Both catalysts have shown virtually the same activity in borylation
of p- and m-fluorotoluenes (products 7,7′ and 8,8′, respectively,
see Scheme 2). Although fluorine acts as a directing group in
many C-H functionalization reactions,8 in these two cases the
ratio of isomers is close to statistical.
Previously, Furukawa has shown that HBPin was unreactive
under platinum-catalyzed conditions with benzene.3(b) In this
study as indicated above (see Table 1, entries 4, 8, 13, 15), we
accounted yields higher than 100% for participation of HBPin in
the course ofborylation.Therefore, weconductedcontrolborylation
of toluene with HBPin under the same conditions as for B2Pin2.
In all cases the amount of ortho-substitution product decreased
considerably compared to the case with B2Pin2, while the ratio
between the para- and meta- isomers varied about the statistical
distributionof1to2.Themostactivecatalystsintheseexperiments
are complexes bearing highly sterically hindered imidazolium
NHC ligands 1e and 1d (see Table 1, entries 4 and 5). Markedly,
these complexes ensure the poorest ortho-selectivity. In the series
of experiments (entries 11–15) using complexes 1k-o bearing 5-,
6- and 7-membered ring carbenes, analogously to the reaction
with B2Pin2, some regularities can be traced, namely, expansion
of the ring size as well as the increase in steric bulk of the aryl
Borylation of chlorobenzene mediated by 1o proceeds effici-
ently while, in case of 1d, the total yield of products o/m/p-6c is
low (12%), the side product, PhBPin, is formed in > 20% yield.
– 570 –

Mendeleev Commun., 2020, 30, 569–571
group (Dipp vs. Mes), which leads to increase in the product
yields. However, contrary to borylation with B2Pin2, predominant
formation of meta-isomers occurred.
2 (a) T. Ishiyama and N. Miyaura, Chem. Rec., 2004, 3, 271; (b) E. C. Neeve,
S. J. Geier, I.A. Mkhalid, S.A. Westcott and T. B. Marder, Chem. Rev., 2016,
116, 9091; (c) J. Takaya and N. Iwasawa, ACS Catal., 2012, 2, 1993;
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In conclusion, a series of fifteen (NHC)Pt(dvtms) complexes
was tested as catalysts in borylation of toluene with B2Pin2. The
nature of a carbene ligand significantly affect the catalytic
activity and selectivity of the reaction. Most surprising specific
feature of the platinum NHC complexes is the unusual ortho-
selectivity of C–H activation in monosubstituted benzenes: o/m/p
ratio amounts to ~ 10:3:1 in borylation of toluene under catalysis
with (7-Dipp)Pt(dvtms). Further mechanistic studies to rationalize
this effect are needed. We have found that the borylation could be
also performed with HBPin being the secondary product in the
process with B2Pin2; this was the reason why in some cases with
B2Pin2 the product yields would exceed 100%.
3 (a) J. Takaya, S. Ito, H. Nomoto, N. Saito, N. Kirai and N. Iwasawa,
Chem. Commun., 2015, 51, 17662; (b) T. Furukawa, M. Tobisu and
N. Chatani, J. Am. Chem. Soc., 2015, 137, 12211; (c) T. Furukawa,
M. Tobisu and N. Chatani, Bull. Chem. Soc. Jpn., 2017, 90, 332.
4 (a) P. B. Dzhevakov, A. F. Asachenko, A. N. Kashin, I. P. Beletskaya and
M. S. Nechaev, Russ. Chem. Bull., Int. Ed., 2014, 63, 890 (Izv. Akad. Nauk,
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V. S. Kochurov, D. Y. Paraschuk and M. S. Nechaev, Adv. Synth. Catal.,
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O. S. Morozov, A. F. Asachenko and M. S. Nechaev, Eur. J. Org. Chem.,
2016, 1908; (d) A. A. Ageshina, G. K. Sterligov, S. A. Rzhevskiy,
M. A. Topchiy, G. A. Chesnokov, P. S. Gribanov, E. K. Melnikova,
M. S. Nechaev, A. F. Asachenko and M. V. Bermeshev, Dalton Trans.,
2019, 48, 3447.
M. A. Topchiy, G. K. Sterligov, N. Yu. Kirilenko and
A. A. Ageshina are grateful to the Russian Science Foundation
for financial support (project no. 19-73-10185) [synthesis of
(NHC)Pt(dvtms) complexes]. M. S. Nechaev and A. F. Asachenko
are thankful to the Russian Foundation for Basic Research
(project no. 16-29-10706 ofi_m) for financial support. Part of
this work [catalytic activity studies of five-membered (NHC)Pt
complexes] was carried out by S. A. Rzhevskiy in the framework
of A. V. Topchiev Institute of Petrochemical Synthesis Russian
Academy of Sciences State Program. Authors are grateful to the
Moscow State University (Russia) for the opportunity to use the
NMR facilities of the Center for Magnetic Tomography and
Spectroscopy. The contribution of the Center for Molecular
Composition Studies of INEOS RAS is also gratefully
acknowledged.
5 O. S. Morozov, P. S. Gribanov, A. F. Asachenko, P. V. Dorovatovskii,
V. N. Khrustalev, V. B. Rybakov and M. S. Nechaev, Adv. Synth. Catal.,
2016, 358, 1463.
6 (a) G. A. Chesnokov, M. A. Topchiy, P. B. Dzhevakov, P. S. Gribanov,
A. A. Tukov, V. N. Khrustalev, A. F. Asachenko and M. S. Nechaev, Dalton
Trans., 2017, 46, 4331; (b) M.A. Topchiy,A.A.Ageshina, G.A. Chesnokov,
G. K. Sterligov, S.A. Rzhevskiy, P. S. Gribanov, S. N. Osipov, M. S. Nechaev
and A. F. Asachenko, ChemistrySelect, 2019, 4, 7470; (c) M. A. Topchiy,
A.A.Ageshina, P. S. Gribanov, S. M. Masoud, T. R.Akmalov, S. E. Nefedov,
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M. A. Belaya, A. A. Ageshina, M. V. Bermeshev, M. S. Nechaev and
A. F. Asachenko, J. Organomet. Chem., 2020, 912, 121140.
8 J. Choi and A. S. Goldman, in Iridium Catalysis, ed. P. G. Andersson,
Springer, Berlin, Heidelberg, 2011, pp. 139-167.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.09.005.
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Received: 15th April 2020; Com 20/6196
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