Ruthenium catalysed C-H bond borylation

Article abstract of DOI:10.1039/c4cc02096k

An easily prepared series of phenylindenyldihydridosilyl ruthenium complexes (2a-2d) was obtained by reaction of tertiary silanes with the commercially-available [RuCl(3-phenylindenyl)(PPh₃)₂] (1). The [RuH₂(3-phenylindeny

Full text of DOI:10.1039/c4cc02096k

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Ruthenium catalysed C–H bond borylation†
Jose A. Fernandez-Salas,‡ Simone Manzini,‡ Lorenzo Piola, Alexandra M. Z. Slawin
and Steven P. Nolan*
´
´
Cite this: Chem. Commun., 2014,
50, 6782
Received 20th March 2014,
Accepted 12th May 2014
DOI: 10.1039/c4cc02096k
www.rsc.org/chemcomm
An easily prepared series of phenylindenyldihydridosilyl ruthenium the nature of the silane used,3a–g especially the lack of a synthetic
complexes (2a–2d) was obtained by reaction of tertiary silanes with route using the very common tertiary silanes.
the commercially-available [RuCl(3-phenylindenyl)(PPh3)2] (1). The
In this context and taking into account the lack of simple
[RuH2(3-phenylindenyl)(SiEt3)] (2a) complex was shown to be highly synthetic protocols leading to hydridosilyl ruthenium complexes,
efficient (1.5 mol%) in the ortho-selective borylation of pyridyl substrates, we were pleased to discover that [RuCl(PPh3)2(3-phenylindenyl)]4
with yields of up to 90%. A novel ruthenium(IV)-catalysed C–H activation (1) when simply heated in toluene in the presence of tertiary
borylation/functionalization reaction using a remarkably low catalyst silanes (2a–2e), allowed for easy access to a new family of
loadings is described.
3-phenylindenyl dihydridosilyl ruthenium complexes in good
yields (Scheme 1).
Organosilicon ruthenium complexes have attracted significant
The complexes were fully characterized by spectroscopic
attention due to their potential use as catalysts enabling the techniques and elemental analysis confirms bulk purity of 2.
transformation of silane reagents in processes such as hydro- Of note, the 1H-NMR spectra of 2 reveal various patterns in the
silylation, dehydrogenative polymerization and silane redistribution hydride region as a function of the silyl group. This can be
reactions.1 For example, Nikonov has reported the hydrosilylation of attributed to conformational interchanges of the hydrides
carbonyl moieties using [RuCp(PiPr3)(NCMe)(Z2-HSiR3)].1d
about the ruthenium centre associated with silane electronic
A plethora of silicon ruthenium complexes can be found in and steric properties.3c To unequivocally establish the atom
the literature. Most relevant to the work presented here, in 1988 connectivity in 2, the molecular structure of 2a was elucidated
Tilley described the synthesis of [RuCp*H2(PhSiH2)(PiPr3)]2 by X-ray5 diffraction analysis of a single crystal (see ESI†).
using PhSiH3. Subsequently, the synthesis and catalytic applications
The mechanism of formation of 2, presumably occurs via
of half-sandwich hydridosilyl ruthenium complexes was widely oxidative addition of the Si–H bond of the silane to ruthenium
developed.3 In 2011, Tilley and coworkers following on seminal associated with a PPh3 ligand displacement. The potential
work of Caulton3d described a family of dihydridopentamethyl- reversible binding of the silane, via reductive elimination,
cyclopentadienyl ruthenium complexes [RuCp*(H)2(SiRR0OTf)(PiPr3)], prompted us to investigate whether complexes 2 could be active
as intermediates in the synthesis of silylene ruthenium species.3h in C–H activation reactions, more specifically in the arene C–H
This procedure required the synthesis of a key Ru-OTf bearing bond borylation reactions.
complex to access the desired hydridosilyl ruthenium complexes,
The borylation of arenes through direct C–H activation has
as limited reactivity was exhibited by [RuClCp*(PiPr3)] with primary become a very straightforward and attractive route for the
silanes.
Despite the interest and the significant potential of these
preparation of boron-containing reagents, excluding the need
hydridosilyl ruthenium complexes, methods for their preparation
have remained limited. Their isolation requires usually multistep
synthetic approaches,3a,c,d,f,h that suffer from limitations regarding
EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST,
UK. E-mail: snolan@st-andrews.ac.uk
† Electronic supplementary information (ESI) available. CCDC 976897. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02096k
Scheme 1 Synthesis of [RuH2(SiR3)(PPh3)(3-phenylindenyl)] (2a–e).
‡ These authors contributed equally.
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for the corresponding halides as intermediates.6 In 1999 and starting material was recovered unchanged. Ruthenium complexes
2000, Smith and co-workers7 described the direct arene borylation 2b–e showed much lower reactivity in the C–H activation reaction,
catalysed by rhodium and iridium catalysts using pinacolborane. reaching the desired product with only low conversions. It is
Hartwig, Miyaura and co-workers established the first direct arene remarkable that the loading of 2a can be decreased to 1.5 mol%
borylation catalysed by an iridium catalyst in the presence of a without any loss in conversion to the desired product. In order to
bipyridine ligand using bis(pinacolato)diboron.8 The [Ir]/dtbpy compare the reactivity of our system with other ruthenium-based
system has been widely studied in various borylation reactions catalysts, dichloro(p-cymene)ruthenium dimer was tested (see
directed by steric6a,b,9 and/or electronic factors.6a,b,10 The use of ESI†). A less effective conversion of 76% was obtained under
numerous directing groups to achieve ortho selective arene borylation optimised reaction conditions (5 mol% of ruthenium and KOAc
has also been developed using Ir11 and Rh12 catalytic systems. Despite as additive), using the dimer, proving to be a less active system in
the wide interest in this transformation, the absence of less costly the arene borylation compared to the use of our well-defined
metal catalytic systems is noteworthy. Ruthenium has been found to catalyst (2a).
be highly active in a broad range of ortho arene functionalization
Under the optimized reaction conditions, the scope of a series of
through C–H activation reactions using different nitrogen- and substituted pyridyl and congener substrates was examined (Table 1).
oxygen-based directing groups.13 However, the ruthenium-catalysed The borylation of 2-phenylpyridine (3a) and benzo[h]quinolone (3b)
ortho-borylation of sp2 carbons is rare, and only a few examples have proceeded with very good isolated yields (84–90%). Substituted
been described,14 using a very specific substrate at very high catalyst pyridines with electron-donating or electron-withdrawing groups
loadings (10–50 mol%)14a and, more recently, in the borylation of were successfully borylated. The presence of either an electron-
indoles.14b
donating (Me or OMe) or an electron-withdrawing (F) group at the
Taking up this challenge, at first, we began studying the para or meta position of the 2-phenylpyridine did not affect the
solvent effect and the reactivity of the different ruthenium reactivity and the desired borylated adducts were always obtained in
dihydridosilyl complexes (2a–2d) in the direct ortho-borylation good yields (80–83%) (4c–e and 4g–h). The presence of an ester
of 2-phenylpyridine (Table 1), in the presence of 1 equiv. of group in either the para (3f) or meta (3i) position is also well
bis(pinacolato)diboron. We tested various solvents in the tolerated, leading to the desired products in good yields (4f and 4i)
presence of 2a (2.5 mol%) (see ESI†). No product formation accompanied with no change in regioselectivity. The ortho-borylation
was detected in NMP, DMF and DMAc, and only low conversions of 2-(naphthalene-1-yl)pyridine 3j did not take place to full conver-
were obtained when the reaction was carried out in toluene. sion (75%), however, the desired product was isolated in moderate
Noteworthy, 1,4-dioxane shows a greatly beneficial effect on the yield (65%) (4j).
reaction, leading to the formation of the desired borylated com-
Several experiments were conducted to probe the reaction
pound (3a) with full conversion. Lower temperatures (60 1C and mechanism. Unfortunately, we were unable to isolate the
80 1C) were also tested, but under these milder conditions the postulated ruthenium intermediate resulting from the activa-
tion of the phenylpyridine by 2. However, we were able to detect
the formation of triethylsilane by 1H-NMR and GC-MS in the
stoichiometric reaction between 2a and 2-phenylpyridine (3a).
In the course of our mechanistic studies, pinacolborane can be
spectroscopically identified (1H-NMR and 11B-NMR) after the
addition of (bis)pinacolatodiboron to the reaction mixture
depicted in reaction a of Fig. 1 (as well as in every other crude
reaction mixture generated).
Table 1 Catalysed borylation of pyridines using 2aa
On the basis of these observations and literature precedents,13,15
a mechanism involving reaction of the pyridine and 2a
generating the ruthenacycle I and HSiEt3 is proposed. The
ruthenium(IV) intermediate I then reacts with B2pin2 forming
a new ruthenium(IV) intermediate (II), with concomitant release
of HBpin.6e,16 Reductive elimination forming the desired borylated
adduct, and the subsequent regeneration of intermediate I in the
presence of a new molecule of pyridine closes the catalytic cycle.
Further investigations are currently ongoing in our laboratory to
better understand the mechanism of the hydrosilylation reaction
and how this sequence can suggest novel reactivity mediated by 2.
Finally, to test the compatibility of the method with other
transformations,11a the generated borylated product was directly
launched in a Suzuki–Miyaura reaction. As can be seen in
Scheme 2, the ruthenium-catalysed borylation and the palladium-
catalysed Suzuki–Miyaura cross-coupling procedures,17 in a one-pot
reaction, via sequential catalyst/substrate addition proceed very well
a
Reaction conditions: 3 (0.25 mmol), B2pin2 (0.25 mmol) and 2a (3.75 mmol,
1.5 mol%); 1,4-dioxane (0.5 mL). Isolated yields are average of at least two
runs. b 75% conversion.
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Fig. 1 Stoichiometric reactions and proposed catalytic cycle for the C–H
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Scheme 2 One-pot borylation/Suzuki–Miyaura sequence.
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Miyaura reaction is illustrative. Extensions of the methodology to
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6784 | Chem. Commun., 2014, 50, 6782--6784
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