A family of rhodium(i) NHC chelates featuring O-containing tethers for catalytic tandem alkene isomerization-hydrosilylation

Article abstract of DOI:10.1039/d0dt03698f

The rhodium complex Rh(HL)(COD)Cl, 1, L being a functionalized N-heterocyclic carbene (NHC) ligand with an oxygen-containing pendant arm, has been used as the entry point to synthesize a series of neutral and cationic Rh(i) O,C chelates. While the Rh-carbene interaction is similar in all these 16-electron complexes, structural analysis reveals that the strength of the Rh-O bond is greatly affected by the nature of the O-donor: R-O- > R-OH > R-OBF3. These subtle changes in the nature of the O-containing tether are found to be responsible for large differences in the alkene hydrosilylation catalytic activity of these compounds: the stronger the Rh-O interaction, the better the catalytic performances. The most active catalyst, [Rh(L)(COD)], 2, demonstrated good catalytic activity under mild reaction conditions for the hydrosilylation of a range of alkene substrates with the industrially relevant non-activated tertiary silane, 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM). Furthermore, this complex is an effective catalyst for the selective remote functionalization of internal olefins at room temperature via tandem alkene isomerization-hydrosilylation.

Full text of DOI:10.1039/d0dt03698f

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A family of rhodium(I) NHC chelates featuring
O-containing tethers for catalytic tandem alkene
Cite this: DOI: 10.1039/d0dt03698f
isomerization–hydrosilylation†
Ravi Srivastava, Martin Jakoobi,
and Clément Camp
Chloé Thieuleux,
Elsje Alessandra Quadrelli
*
The rhodium complex Rh(HL)(COD)Cl, 1, L being a functionalized N-heterocyclic carbene (NHC) ligand
with an oxygen-containing pendant arm, has been used as the entry point to synthesize a series of
neutral and cationic Rh(I) O,C chelates. While the Rh–carbene interaction is similar in all these 16-electron
complexes, structural analysis reveals that the strength of the Rh–O bond is greatly affected by the nature
of the O-donor: R-O− > R-OH > R-OBF
. These subtle changes in the nature of the O-containing tether
3
are found to be responsible for large differences in the alkene hydrosilylation catalytic activity of these
compounds: the stronger the Rh–O interaction, the better the catalytic performances. The most active
catalyst, [Rh(L)(COD)], 2, demonstrated good catalytic activity under mild reaction conditions for the
Received 26th October 2020,
Accepted 18th November 2020
hydrosilylation of a range of alkene substrates with the industrially relevant non-activated tertiary silane,
H
1
,1,1,3,5,5,5-heptamethyltrisiloxane (MD M). Furthermore, this complex is an effective catalyst for the
selective remote functionalization of internal olefins at room temperature via tandem alkene isomeriza-
tion–hydrosilylation.
DOI: 10.1039/d0dt03698f
rsc.li/dalton
tribution of positional isomers, combined with selective, irre-
Introduction
versible functionalization of the α-olefins yielding the desired
3
,4
Linear α-olefins are feedstocks of high industrial relevance, and linear functionalized products.
are used as high-value intermediates for the manufacture of
Alkene hydrosilylation, i.e. the addition of a hydrosilane
detergents, lubricants and polymers. Industrially, linear alpha onto an alkene, is a pertinent choice for such latter irreversible
olefins are produced mainly by dehydrogenation of alkanes functionalization due to its high industrial relevance.
from thermal cracking/reforming of oil or by oligomerization of Currently the silicone industry produces 2 billion tons of pro-
5
,6
ethylene. However, both processes yield mixtures of products, ducts/year for a wide range of applications.
Numerous
including a non-negligible amount of internal linear olefins, α-olefin hydrosilylation catalysts have been developed, yet
either directly in high-temperature dehydrogenation processes, these catalysts are generally unreactive towards internal olefins
1,2
or indirectly in the case of ethylene oligomerization. The under classical reaction conditions. Recently, a few examples
valorization of mixtures of internal olefins through the use of of tandem isomerization–hydrosilylation of internal olefins
transition metal catalysts can improve the olefin feedstock pool, promoted by a single or a dual metal catalytic system have
7
–17
yet classical methods for the valorization of these substrates emerged in the literature.
These pioneering studies are very
generally leads to the formation of branched products of lower promising, yet there is still room for improvement to strive
value or suffer from poor atom economy. To circumvent these toward ideal catalytic systems working under mild conditions,
issues, elegant tandem catalytic approaches were developed with non-activated silanes, no additives, low metal loadings,
through isomerization of the internal alkene moiety into a dis- and featuring large substrate scopes without selectivity issues.
This encouraged us to investigate new catalytic systems for the
catalytic tandem isomerization–hydrosilylation of internal
Université de Lyon, Institut de Chimie de Lyon, C2P2 UMR 5265 CNRS-UCBL-CPE
Lyon, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France.
E-mail: clement.camp@univ-lyon1.fr
alkenes.
NHC ligands currently play a ubiquitous role in organo-
metallic chemistry, including for the development of novel
†
Electronic supplementary information (ESI) available: NMR and IR spectra of
1
8–24
alkene hydrosilylation catalysts.
We recently developed a
new complexes and compounds, kinetic profiles. CCDC 2026410–2026412. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0dt03698f
new functionalized N-heterocyclic carbene (NHC) ligand with
an oxygen-containing pendant arm and demonstrated its inter-
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est to prepare a range of monometallic and heterobimetallic 2907.6 (s, νC–H), 2872.8 (s, νC–H), 2863.0 (s, νC–H), 2853.7 (m,
2
5–28
complexes.
Here, we use this ligand platform to generate a νC–H), 2925.6 (s, νC–H), 1641.4 (w), 1538.5 (m), 1519.8 (w),
family of Rh(I) O,C chelates and explore their catalytic poten- 1492.7 (s), 1466.8 (s), 1448.8 (m), 1443.0 (m), 1403.9 (s), 1384.7
tial in alkene hydrosilylation and in tandem alkene isomeriza- (s), 1362.2 (s), 1344.3 (m), 1322.3 (m), 1271.1 (s), 1214.2 (s),
tion/hydrosilylation reactions.
1204.6 (s), 1175.5 (s), 1142.4 (s), 1099.9 (w), 1080.4 (m), 1043.2
(
(
(
w), 1033.8 (m), 1000.6 (m), 982.1 (s), 951.7 (s), 983.9 (s), 875.3
m), 865.3 (m), 851.9 (m), 817.0 (m), 775.3 (s), 721.4(s), 717.4
s), 694.2 (m), 683.1 (m), 640.6 (m), 608.3 (m), 596.1 (s),
Experimental
General considerations
87.9 cm− (s). Elemental analysis calcd (%) for C24
1
H
N
ORh:
5
33
2
C 61.52, H 7.10, N 5.98; found: C 61.47, H 7.24, N 5.93.
The syntheses and manipulations described below were per-
formed using an MBraun inert atmosphere glovebox under an
Synthesis of 3. In the dark, silver tetrafluoroborate (185 mg,
.95 mmol, 1.0 eq.) was added to a 10 mL THF solution of
0
atmosphere of purified argon (<1 ppm O /H O). Glassware and
2
2
compound 1 (480 mg, 0.95 mmol, 1.0 eq.). The formation of a
white precipitate was observed. The reaction mixture was
stirred for 1 h at room temperature. The precipitate was
removed by filtration and the resulting yellow filtrate was con-
centrated to 3 mL, filtered again and stored at −40 °C for 12 h.
This yielded yellow crystals which were recovered and dried in
cannulae were stored in an oven at ∼110 °C for at least 12 h
prior to use. Pentane, diethylether and tetrahydrofuran (THF)
were purified by passage through a column of activated
alumina, dried over Na/benzophenone, vacuum-transferred to
a storage flask and freeze–pump–thaw degassed prior to use.
Deuterated THF (THF-d ) was dried over Na/benzophenone,
8
vacuo to give [Rh(HL)(COD)][BF ], 3, as a yellow crystalline
4
vacuum-transferred to a storage flask and freeze–pump–thaw
degassed prior to use. Dichloromethane and CDCl were dried
3
solid (408 mg, 0.73 mmol, 77%). Single crystals suitable for
1
X-Ray diffraction were grown similarly. H NMR (400 MHz,
3
over CaH and then vacuum-transferred to a storage flask and
2
CDCl , 296 K) δ 7.08 (d, J = 1.8 Hz, 1H, CHimid), 7.03 (s, 2H,
3
HH
3
freeze–pump–thaw degassed before use. Compounds Rh(HL)
m-CHmes), 6.78 (d, J = 1.8 Hz, 1H, CHimid), 5.75 (s, 1H, OH),
HH
2
6
t
t
29
(
COD)Cl (1, see below) and Ta(CH Bu)(CH
2
Bu)
3
were pre-
4.84 (s, 2H, COD), 4.33 (s, 2H, NCH
2
), 3.17–3.12 (br m, 2H,
pared using literature procedures. All other reagents were
acquired from commercial sources and used as received.
COD), 2.37 (s, 3H, p-CH3Mes), 2.29–2.19 (br m, 2H, COD), 2.16
(
2
s, 6H, o-CH3Mes), 2.05–1.95 (br m, 2H, COD), 1.81–1.73 (br m,
1
3
H, COD), 1.70–1.63 (br m, 2H, COD), 1.30 (s, 6H, CH
3
).
C
Synthetic procedures
1
{
H} NMR (100 MHz, CDCl , 296 K) δ 174.26 (d, JRh–C = 52 Hz;
3
Synthesis of 2. A colorless THF solution (6 mL) of KHMDS Rh–Ccarbene), 139.88 (C ), 135.42 (C ), 135.21 (C ), 129.37
Ar
Ar
Ar
(
47 mg, 0.24 mmol, 1.0 eq.) was added dropwise to a yellow (CHAr), 123.36 (CHimid), 122.92 (CHimid), 99.95 (d, JRh–C = 7 Hz,
THF solution (16 mL) of compound 1 (121 mg, 0.24 mmol, 1.0 CHCOD), 72.25 (OC), 68.14 (THF), 67.91 (d, JRh–C = 16 Hz,
eq.) under stirring. The yellow reaction mixture was stirred at CHCOD), 61.33 (NCH ), 32.97 (CH ), 28.10 (CH ), 25.78
2
2COD
3
1
9
room temperature for an extra 1 h. The crude reaction was (THF), 24.95 (CH2COD), 21.26 (o-CH3Mes), 18.51 (p-CH3Mes).
evaporated to dryness to give a yellow powder. The resulting { H} NMR (376 MHz, CDCl
solid was then dissolved in a minimum amount of THF (4 mL) −150.32 (s, BF4 ). B{ H} NMR (96 MHz, CDCl , 296 K) δ
and filtered. The filtrate was layered with pentane (10 mL) and −1.05 (s). DRIFT (298 K, cm ) 3349.4 (s, νO–H), 3157.7 (m,
stored at −40 °C. The thus obtained shiny rectangular shaped C–H), 3132.2 (m, νC–H), 2973.5 (s, νC–H), 2935.1 (s, νC–H), 2922.9
F
1
10
−),
3
, 296 K) δ −150.27 (s, BF
4
1
1
−
11
1
3
−1
ν
yellow crystals were recovered and dried in vacuo to yield (s, νC–H), 2874.3 (s, νC–H), 2829.3 (s, νC–H), 1608.7 (w), 1490.1
complex [Rh(HL)(COD)], 2 (70 mg, 0.15 mmol, 63%). Single (s), 1450.9(s), 1412.7 (s), 1392.0 (s), 1373.3 (m), 1348.5 (w),
crystals suitable for X-Ray diffraction were grown similarly. 1305.3 (w), 1259.1 (w), 1224.6 (m), 1192.4 (m), 1142.7 (s),
Complex 2 can be synthesized from 3 using the exact same syn- 1111.8 (s), 1084.1 (s), 967.9 (s), 912.9 (w), 887.2 (m), 865.6 (m),
1
−1
thetic procedure, with analogous performances. H NMR 775.7 (m), 762.5 (s), 703.2 (w), 587.2 (w), 518.8 (m), 494.2 cm
3
(
500 MHz, CDCl
3
, 296 K) δ 6.97 (s, 2H, m-CHmes), 6.89 (d, JHH (m). Elemental analysis calcd (%) for C24
1.7 Hz, 1H, CHimid), 6.64 (d, J = 1.7 Hz, 1H, CHimid), 4.60 51.82, H 6.16, N 5.04; found: C 51.93, H 6.25, N 5.12.
HH
H
34
N
2
ORhBF
4
C
3
=
(s, 1H, COD), 4.05 (s, 2H, NCH
2
), 2.64 (s, 2H, COD), 2.34 (s,
Synthesis of 4. A light yellow suspension of compound 3
3
H, p-CH3Mes), 2.19 (s, 6H, o-CH3Mes), 2.15–2.10 (br m, 2H, (250 mg, 0.45 mmol, 1.0 eq.) in 16 mL THF was added drop-
t
t
COD), 2.06–1.98 (br m, 2H, COD), 1.72–1.62 (br m, 4H, COD), wise to a stirring dark orange solution of Ta(CH Bu)(CH Bu)
2
3
1
3
1
0
.99 (s, 6H, CH
3
). C{ H} NMR (125 MHz, CDCl
3
, 296 K) δ (208 mg, 0.45 mmol, 1.0 eq.) in THF (8 mL). The reaction
1
79.53 (d, JRh–C = 56 Hz, Rh–Ccarbene), 138.60 (CAr), 136.36 mixture was stirred at room temperature for 6.5 h. After that
(
(
(
(
(
C ), 135.55 (C ), 128.76 (CH ), 121.74 (CHimid), 120.88 the solution was concentrated to half, filtered and stored at
Ar Ar Ar
CHimid), 97.62 (d, JRh–C = 8 Hz, CHCOD), 67.98 (s, THF), 67.79 −40 °C for 12 h. The resulting yellow solid was recovered and
OC), 67.01 (NCH ), 63.12 (d, JRh–C = 13 Hz, CHCOD), 33.46 dried in vacuo to give complex [Rh(L·BF )(COD)], 4 (115 mg,
CH2COD), 29.07 (CH ), 28.32 (CH ), 25.61 (s, THF), 21.09 0.21 mmol, 48%). The fate of the Ta-based coproduct was
2
3
3
2COD
o-CH3Mes), 16.82 (p-CH3Mes). DRIFT (298 K, cm− ) 3158.9 (m, further investigated by solution state NMR as discussed in the
C–H), 3128.4 (s, νC–H), 3010.9 (w, νC–H), 2970.0 (s, νC–H), 2957.3 results and discussion section. Single crystals of 4 suitable for
s, νC–H), 2942.1 (s, νC–H), 2932.1 (s, νC–H), 2919.8 (s, νC–H), X-Ray diffraction were grown by the slow diffusion of diethyl-
1
ν
(
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ether (5 mL) into a concentrated dichloromethane solution of in 500 µL of THF) were carried out similarly to reactions at
1
4
1
(2 mL) at −40 °C. H NMR (300 MHz, CDCl , 296 K) δ 7.11 (s, 0.1 mol% of catalyst loading.
3
3
H, m-CHmes), 7.03 (d, JHH = 1.8 Hz, 1H, CHimid), 6.91 (s, 1H,
Tandem isomerization/hydrosilylation catalysis. All the reac-
tions with internal alkenes were carried out similarly to those
4 Hz, 1H, NCH ), 5.40 (s, 1H, COD), 5.23 (s, 1H, OH), 5.27 (s, using the terminal alkene hydrosilylation procedure at
3
2
m-CHmes), 6.73 (d, JHH = 1.8 Hz, 1H, CHimid), 5.50 (d, JHH
=
1
1
2
2
2
2
2
H, COD), 3.95 (d, JHH = 14 Hz, 1H, NCH ), 3.35 (s, 1H, COD), 1.0 mol% of 2.
.65 (s, 1H, COD), 2.48 (br s, 1H, COD), 2.37 (s, 3H, p-CH3Mes),
.35 (s, 3H, o-CH3Mes), 2.04–1.99 (br m, 4H, COD), 1.88 (s, 3H,
Characterizations
o-CH3Mes), 1.76 (br m, 1H, COD), 1.61 (s, 3H, CH
3
), 1.44 (br m,
). H NMR (300 MHz, THF-d
96 K) δ 7.34 (d, JHH = 1.8 Hz, 1H, CHimid), 7.14 (s, 1H,
NMR spectroscopy. Multinuclei NMR spectra were recorded
on Bruker AV-300, AVQ-400 and AV-500 spectrometers. H and
1
1
2
2
H, COD), 1.21 (s, 3H, CH
3
8
,
3
13
C chemical shifts (δ) are given in parts per million (ppm)
3
1
m-CHmes), 7.00 (d,
JHH = 1.8 Hz, 1H, CHimid), 6.94 (s, 1H, referenced to the appropriate residual solvent peak ( H NMR:
1
3
m-CHmes), 5.38 (br m, 2H, COD), 5.33 (m, 1H, NCH
2
), 5.27 (br CDCl
3 8 3
at 7.26 ppm, THF-d at 3.58 ppm. C NMR: CDCl at
2
11
19
m, 1H, COD), 4.11 (d, J = 18 Hz, 1H, NCH ), 3.34 (br s, 1H, 77.16 ppm, THF-d at 67.21 ppm). B and F NMR chemical
HH
2
8
2
9
COD), 2.62 (br s, 1H, COD), 2.50 (br s, 1H, COD), 2.36 (s, 3H, shifts are reported relative to BF
3
·OEt
2
set at 0.00 ppm. Si
Si) O set at
.90 (s, 3H, o-CH3 ), 1.53 (s, 3H, CH ), 1.44 (br m, 2H, COD), 7.2 ppm. H and C NMR assignments were routinely con-
p-CH3Mes), 2.31 (s, 3H, o-CH3Mes), 2.01–1.95 (br m, 4H, COD), NMR chemical shifts are reported relative to (Me
3
2
1
13
1
1
1
Mes
3
1
3
1
1
1
1
13
.12 (s, 3H, CH
3 8
). C{ H} NMR (125 MHz, THF-d , 296 K) δ firmed by H– H COSY and H– C HSQC and HMBC experi-
78.91 (d, JRh–C = 51 Hz, Rh–Ccarbene), 139.42 (CAr), 137.71 ments. 1,2,4,5-tetramethylbenzene was used as internal stan-
1
(
C ), 136.40 (C ), 134.79 (C ), 130.04 (CH ), 128.88 (CH ), dard to measure yields via H NMR.
Ar Ar Ar Ar Ar
1
23.39 (CHimid), 123.10 (CHimid), 102.41 (d, JRh–C = 90 Hz,
Infrared spectroscopy. Samples were prepared in a glovebox,
COD), 72.09 (OC), 64.98 (s, N–CH
2
), 63.21 (d, JRh–C = 17 Hz, sealed under argon in a DRIFT cell equipped with KBr
CHCOD), 61.73 (d, JRh–C = 17 Hz, CHCOD), 34.94 (CH2COD), 31.49 windows and analyzed on a Nicolet 6700 FT-IR spectrometer.
s, CH2COD), 29.56 (s, CH2COD), 26.84 (s, CH2COD), 26.34 (s, Elemental analyses. Elemental analyses were performed at
CH ), 24.50 (s, CH ), 20.90 (o-CH3Mes), 19.25 (o-CH3Mes), 17.60 the School of Human Sciences, Science Center, London
p-CH3Mes). F{ H} NMR (282 MHz, CDCl , 296 K) δ −141.51 Metropolitan University.
(
3
3
1
9
1
(
(
−
3
ν
3
2
11
1
q,
0.53 (q, JF–B = 10 Hz). DRIFT (298 K, cm−1) 3172.2 (m, νC–H), measured on a Bruker QTOF Impact II at “Centre Commun de
142.7 (m, νC–H), 2971.0 (s, νC–H), 2930.0 (s, νC–H), 2872.3 (m, Spectrométrie de Masse” in Lyon, France.
C–H), 2829.2 (m, νC–H), 1608.7 (w), 1489.1 (s), 1474.7 (w), Gas chromatography. GC analyses were performed on HP
437.7 (m), 1412.9 (s), 1385.6 (s), 1366.6 (w), 1350.2 (w), 1333.3 6890 chromatograph with a HP5 (5% of phenylmethylsiloxane)
w), 1306.1 (w), 1271.4 (w), 1236.9 (m), 1224.7 (s), 1186.1 (s), column (30 m length, 320 µm of diameter, 0.25 nm of thick-
J
B–F = 10 Hz). B{ H} NMR (96 MHz, CDCl
3
, 296 K) δ
Mass spectrometry. High resolution mass spectra were
2
1
(
1
9
5
146.1 (s), 1091.7 (s), 1057.7 (s), 1039.1 (s), 984.2 (s), 968.5 (s), ness) equipped with flame ionization detector (FID).
15.6 (s), 866.0 (m), 812.3 (w), 751.6 (m), 727.6 (m), 698.9 (m), Mesitylene was used as internal standard to measure yields via
−
1
87.1 (w), 509.5 cm (m). Elemental analysis calcd (%) for GC. At the end of reactions, the crude mixture was passed
C
24
H
33
N
2
ORhBF
3
C 53.72, H 6.20, N 5.22; found: C 53.63, H through a short plug of silica (pentane was used as eluent) to
remove the catalyst. The resulting filtrate fractions were treated
6
.30, N 5.14.
under reduced pressure at 50 °C to remove the volatiles
Catalysis
(
solvent and remaining starting materials), yielding clear
Catalyst screening. The rhodium catalyst (0.0045 mmol, liquid products.
mol% per Rh-atom) was dissolved in a 0.5 mL THF-d solu- X-ray crystallography. X-ray structural determinations were
1
8
tion containing 3,3-dimethyl-1-butene (0.45 mmol, 1.0 equiv.), performed at the “Centre de diffractométrie Henri
H
MD M (0.45 mmol, 1.0 equiv.) and 1,2,4,5-tetramethylbenzene Longchambon, Université de Lyon”. A suitable crystal coated
(
0.45 mmol, 1.0 equiv., internal standard). The reaction was in Parabar oil was selected and mounted on a Gemini kappa-
1
monitored by H NMR at room temperature until completion geometry diffractometer (Rigaku Oxford Diffraction) equipped
conversion, yield and mass balance were determined by inte- with an Atlas CCD detector and using Mo radiation (λ =
(
1
gration of the H NMR signals).
0.71073 Å). Intensities were collected at 100 K or 150 K by
Terminal alkene hydrosilylation scope. A stock solution of 2 means of the CrysalisPro software. Reflection indexing, unit-
500 µL, 0.1 mol% of 2) was added to a solution containing the cell parameters refinement, Lorentz-polarization correction,
(
terminal alkene (0.45 mmol, 1.0 equiv., see Table 4 for list of peak integration and background determination were carried
H
alkenes investigated), MD M (122 µL, 0.45 mmol, 1.0 equiv.) out with the CrysalisPro software. An analytical absorption cor-
3
0
and mesitylene (63 µL, 0.45 mmol, 1.0 equiv., internal stan- rection was applied using the modeled faces of the crystal.
dard). The reaction was left to stir at 1000 rpm at room temp- The resulting set of hkl was used for structure determination
erature and the reaction progress was monitored by GC after 5 and refinement. The structures were solved by direct methods
3
1
2
and 24 h (conversion, yield and mass balance determined by with SIR97
and the least-square refinement on F was
3
2
GC). Reactions using 1 mol% of catalyst 2 (2.1 mg, 0.045 mmol achieved with the CRYSTALS software. All non-hydrogen
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Table 1 X-ray crystallographic parameters for complexes 2, 3 and 4
Compound
2
3
4
Formula
Cryst syst
Space group
Volume (Å )
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
24
H
33
N
2
ORh
C
24
H
34BN
2
4
OF Rh
24 2 3
C H33BN OF Rh
Triclinic
Pˉ1
Trigonal
R3
9813.7(19)
32.883(3)
32.883(3)
10.4799(11)
90
90
120
18
468.43
1.427
Monoclinic
C2/c
5787(3)
41.765(7)
7.8159(6)
24.836(4)
90
134.45(3)
90
8
3
1192.33(17)
7.6521(6)
12.0807(9)
14.3701(10)
65.159(7)
82.202(6)
83.726(7)
2
536.24
1.494
0.76
552
γ (°)
Z
−
1
Formula weight (g mol
Density (g cm
Absorption coefficient (mm
F (000)
)
556.25
1.277
0.63
−
3
)
−
1
)
0.80
4392
2288
θ
max (°)
29.615
100.0(1)
16 712
5342 [0.062]
29.704
100.0(1)
31 111
Temp. (K)
Total no. reflections
Unique reflections [R(int)]
Final R indices [I > 2σ(I)]
GooF
6159 [0.0792]
R
1
= 0.0753, wR
2
= 0.2150
2
= 0.1115
R
1
= 0.0631, wR
2
= 0.1314
0.970
1.125
atoms were refined anisotropically. The hydrogen atoms were
all located in a difference map, but those attached to carbon
atoms were repositioned geometrically. The H atoms were
initially refined with soft restraints on the bond lengths
and angles to regularize their geometry (C⋯H in the
range 0.93–0.98 Å, O⋯H = 0.82 Å) and Uiso(H) (in the range
1
.2–1.5 times Ueq of the parent atom), after which the posi-
tions were refined with riding constraints. Thermal ellipsoid
plots were created using Mercury supplied with Cambridge
Structural Database. CCDC 2026410, 2026411 and 2026412†
contain the supplementary crystallographic data for this
paper (Table 1).
Results and discussion
Synthesis of complexes and characterization in solution
Scheme 1 Synthetic routes to the Rh–NHC complexes 1–4.
The previously reported rhodium complex Rh(HL)(COD)Cl,
2
6
(
Scheme 1) 1, coordinated by a NHC ligand featuring an
oxygen-containing pendant arm (noted L), has been used as
the entry point to generate a series of neutral and cationic Rh
4
The cationic rhodium analogue [Rh(HL)(COD)][BF ], 3, is
(
I) O,C-chelates. Deprotonation of 1 with one equivalent of pot- isolated in 77% yield from treatment of 1 with the corres-
assium bis(trimethylsilyl)amide (KHMDS) yields the alkoxy- ponding AgBF salt, as shown in Scheme 1. The IR spectrum
NHC complex [Rh(L)(COD)], 2, in 63% isolated yield for complex 3 (Fig. S15†) displays an intense νOH signal around
4
1
−1
(
Scheme 1). In the H NMR spectrum of 2, the imidazolyl back- 3350 cm which confirms the presence of the hydroxyl group.
1
bone protons from the unsymmetrical NHC are observed as The H NMR spectrum for compound 3 (Fig. S4†), displays the
doublets ( JHH = 1.7 Hz) at δ = 6.89 and 6.64 ppm, respectively. hydroxyl proton signal at δ = 5.75 ppm which is fairly down-
3
The N–CH
protons are equivalent and observed as a singlet at field shifted as compared to the corresponding signal in 1
2
δ = 4.05 ppm integrating for two protons, which is indicative of where it is observed at δ = 3.17 ppm. This downfield shift is
3
3–35
fast conformational flipping of the ligand methylene bridge on most likely due to a hydroxyl-metal interaction in solution.
1
the NMR timescale. As a result of ligand deprotonation, the H Note that binding of the hydroxyl group to Rh is confirmed in
NMR spectrum for 2 does not contain a resonance corres- the solid-state (see below). As for 2, the NCH methylene frag-
ponding to the hydroxyl proton which is also verified by the ment resonance in 3 is a singlet found at δ = 4.33 ppm (2H).
2
−
11
absence of hydroxyl stretching frequency in IR spectroscopy The presence of the BF
4
counter anion is confirmed by
B
1
9
(Fig. S14†).
and F NMR spectroscopies (Fig. S6 and S7†).
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Treatment of compound 3 with 1 equivalent of KHMDS
Compound 4 can be seen as a boron trifluoride adduct of 2,
quantitatively affords compound 2, as an alternative synthetic as treatment of 2 with BF ·THF (Scheme 1) cleanly leads to 4
3
route to 2. The latter reactivity indicates that the hydroxyl group as well (Fig. S12†).
in 3 might react with basic metal derivatives to yield heterobi-
metallic assemblies. Accordingly, in an attempt to synthesize a Comparative structural analysis
2
5,26,36,37
tantalum/rhodium heterobimetallic species,
complex 3
at room
The crystallographic structures of 2, 3 and 4 were determined
by X-ray diffraction on single crystals (Fig. 1). The three com-
plexes adopt a typical square planar geometry at the Rh(I)
centre. The oxygen-functionalized NHC ligands interact with
rhodium in a (O,C) chelate fashion to form 6-membered metal-
lacycles featuring pseudo-boat conformations. The crystal
structure of 4 confirms that the BF moiety is bound to the
3
oxygen atom of the chelated alkoxy group, in agreement with
the NMR data.
t
t
2
was treated with 1 equivalent of Ta(CH Bu)(CH
Bu)
3
temperature in THF. After reaction completion, the crude
mixture was concentrated and stored at −40 °C to yield a new
product, 4, as a yellow microcrystalline powder in 48% yield.
The IR spectrum of complex 4 does not contain a hydroxyl
group stretching signal (Fig. S16†), which shows that the site of
reaction is the hydroxyl proton as anticipated. However, detailed
NMR studies reveal that the obtained product is not a Ta/Rh
heterobimetallic species, but rather the unexpected monometal-
3
)(COD)], 4, (Scheme 1). The presence of
the BF moiety is confirmed by the F NMR data for 4 which
displays a signal at δ = −141.51 ppm and by the B NMR spec-
trum for 4 featuring a characteristic quartet ( JB–F = 10 Hz) cen-
The analysis of the Rh–O distances (Table 2) shows a strong
disparity in the rhodium–oxygen interactions. The Rh–O bond
length in 2 (2.031(4) Å) is quite short and falls in the range of
lic complex [Rh(L·BF
1
9
3
11
reported
distances.
four-coordinate
The corresponding bond length in the cationic
rhodium(I)
alkoxy
bond
1
47,48
tered at δ = −0.53 ppm. As a result of borane coordination, the
alkoxy-NHC ligand backbone adopts a more rigid conformation,
methylene fragment H
NMR signal which splits into a pair of diastereotopic doublets
complex 3 (2.152(2) Å) is elongated, which is expected as a
result of protonation. Noticeably, the rhodium–oxygen bond
distance in 4 (2.212(3) Å) is much longer than in 2 but also in
1
which is reflected notably by the N–CH
2
3
. This suggests that coordination of the strong Lewis acidic
2
at δ = 5.50 and 3.95 ppm ( JHH = 14.0 Hz).
BF motif drastically reduces the nucleophilicity of the
3
Despite the low nucleophilicity of the tetrafluoroborate
anion, strong electrophiles such as early transition metal
O-donor site of the bifunctional NHC ligand. It is also possible
that this observed geometric distortion is a steric phenomenon
3
8–42
− 43,44
species
can abstract fluoride from BF4
.
Here the tan-
since the BF group is significantly more bulky than a proton
3
talum alkyl alkylidene derivative is proposed to act both as a
base to deprotonate the hydroxyl group in 3 and as an electro-
anion. This proposal
is based on NMR monitoring of the reaction, which was
substituent.
The trend in the Rh–O bonding strength is correlated with
the trend in ligand bite angle, O–Rh–CNHC, noted α (Fig. 2).
Complex 4 displays the most constrained ligand bite angle (α =
−
phile to abstract a fluoride from the BF
4
1
8
carried out in THF-d . As the reaction progressed, the H NMR
82.0(2)°); the corresponding metric is intermediate in 3 (α =
84.6(1)°), while 2 displays a ligand bite angle close to the ideal
90° angle for a square planar geometry (α = 89.8(2)°). This
signal corresponding to the acidic hydroxyl proton in 3 dis-
appeared, as a result of the 1,2-addition reaction with the tan-
talum alkylidene moiety, without the release of neopentane.
This was confirmed by the disappearance of the alkylidene
suggests an enhanced stability of the chelate ring, which is
more relaxed after deprotonation. The angle between the imi-
1
13
19
signal in H NMR and C NMR spectra. The F NMR spec- dazolyl ring plane and the square-planar coordination plane,
trum of the solution displays a singlet at δ = 73.69 ppm noted β, follows the inverse trend as the bite angle (Table 2).
4
5,46
(Fig. S13†) which is characteristic of tantalum fluorides,
The NHC ring is oriented close to perpendicular (β = 74.3(2)°)
and corroborates the formation of a tantalum neopentyl fluor- in 1 to minimize steric repulsion with the other ligands in the
t
ide derivative of general formula [Ta(CH Bu) F] . x,y square-plane, but this angle decreases significantly as a
2
4
n
Unfortunately, multiple attempts to obtain the crystal structure result of ligand chelation to reach 39.0(2)° in 2.
of this tantalum tetra-alkyl fluoride co-product failed because
The yaw distortion angle, θ (Fig. 2), was first introduced by
4
9
of the very high solubility of this species in non-polar solvents. Crabtree and coworkers to reflect the in-plane tilting of the
Based on all these observations, the following reaction scheme NHC ligand due to ring strain imposed by chelation in biden-
can be proposed (Scheme 2).
tate NHC species. Here (Table 2), the yaw angle values for 2
t
t
2 3
Scheme 2 Proposed synthesis of compound 4 from 3 upon deprotonation and fluoride abstraction by Ta(CH Bu)(CH Bu) .
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Fig. 2 Schematic representation of the geometric descriptors: ligand
bite angle (α) and in-plane distortion (yaw angle θ) as a result of ligand
chelation to the square-planar Rh(I) center.
and 3 (ca. 8°) are in the expected range for mesityl-substituted
4
9,50
NHCs engaged in a 6-membered ring chelate.
This value is
much higher in comparison to that of 1 (θ = 1.2°) where no
2
6,28
ring strain is imposed as expected.
The yaw distortion is
even more pronounced in 4 (θ = 11.6°), which we attribute to
increased steric pressure due to the presence of the BF moiety
3
and the more constrained bite angle. Nevertheless, Crabtree
4
9
has shown that yaw distortions do not seem to impact sig-
nificantly the donation properties of NHCs, which is reflected
here by typical Rh–CNHC bond distances (ca. 2.028 Å, Table 2)
1
3
1
and C{ H} NMR carbene signal values (δ = 174–182 ppm,
1
JRh–C = 51–56 Hz) which do not vary significantly across the
series, and without a clear trend (see Table 2). Note that com-
putational analyses from Dorta and coworkers suggested corre-
lation between NHC in-plane tilting and the chemical shift of
the carbene carbon in Ir(NHC)(COD)X species, yet on a larger
5
1
yaw angle range (θ = 0°–30°).
The Rh–CCOD bond distances in trans position with respect
to the NHC ligand (2.173(6) and 2.195(6) Å in 2; 2.178(4) and
2
.199(6) Å in 3; 2.175(5) and 2.225(5) Å in 4) are slightly
elongated compared to those trans to the oxygenated groups
2.097(6) and 2.134(5) Å in 2; 2.095(4) and 2.109(3) Å in 3;
.078(5) and 2.102(5) Å in 4), which is in agreement with the
stronger σ-donation of the NHC groups compared to the oxyge-
(
2
Fig. 1 Solid-state molecular structures of 2 (top), 3 (middle), and
4
(bottom) (30% probability ellipsoids). Hydrogen atoms, except that of
the hydroxyl group in 3 have been omitted for clarity. Selected bond nated moieties. The higher trans influence of the carbene
distances (Å) and angles (°) for 2: Rh1–O1 = 2.031(4); Rh1–C12 = 2.028 versus oxygen donors is also reflected by the increased olefinic
(
6); Rh1–C21 = 2.173(6); Rh1–C22 = 2.195(6); Rh1–C25 = 2.097(6);
cis C25vC26 bond lengths (1.403(9) Å in 2, 1.393(7) Å in 3;
.406(8) Å in 4) as compared with the C21vC22 bond lengths
1.379(8) Å in 2; 1.358(8) Å in 3; 1.357(8) Å in 4), which are
Rh1–C26 = 2.134(5); N1–C12 = 1.362(8); N2–C12 = 1.384(8), N1–C12–
N2 = 102.7(5); C12–Rh1–O1 = 89.9(2); for 3: Rh1–O1 = 2.152(3);
Rh1–C12
1
(
= 2.029(3); Rh1–C21 = 2.178(6); Rh1–C22 = 2.198(3);
Rh1–C25 = 2.095(4); Rh1–C26 = 2.109(4); N1–C12 = 1.355(4); N2–C12 alike for the whole series of these Rh COD species.
=
1.347(4), N1–C12–N2
=
104.9(3); C12–Rh1–O1
=
84.64(12);
In summary, the analysis of the structural properties in
complexes 1–4 shows analogous Rh(I)–carbene interactions but
high disparities in the rhodium–oxygen bonding which are
directly correlated with chelate geometry variations (α, β and θ
angles). Note that these structural trends are independent
for 4: Rh1–O1 = 2.212(3); Rh1–C12 = 2.012(5); Rh1–C21 = 2.175(5);
Rh1–C22
N1–C12
=
2.225(5); Rh1–C25
=
2.078(5); Rh1–C26
=
=
2.102(5);
104.6(4);
=
1.358(6); N2–C12
= 1.347(6), N1–C12–N2
C12–Rh1–O1 = 81.9(2).
Table 2 Selected metrical and NMR parameters for complexes 1–4
1
3
1
Rh–CNHC distance
Rh–O distance
Bite angle α
β angle
Yaw distortion θ
C NMR δCcarbene
J
Rh–Ccarbene
Compound
[Å]
[Å]
O–Rh–CNHC [°]
[°]
[°]
[ppm]
[Hz]
2
6
1
2
3
4
2.042(4)
2.028(6)
2.029(3)
2.012(5)
—
—
74.3(2)
39.0(2)
45.4(2)
49.6(2)
1.2
8.1
8.0
11.6
181.90
179.53
174.26
178.91
52
56
52
51
2.031(4)
2.152(3)
2.212(3)
89.8(2)
84.6(1)
81.9(2)
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from the charge of the complex since the metrical data for the no catalytic activity under this set of experimental conditions,
cationic species, 3, are comprised between those of the two even when the temperature was raised to 50 °C, (versus >90%
neutral species 2 and 4.
yield for 1 which is the poorest catalyst of the Rh–NHC series),
while RhCl(PPh exhibited no activity at r.t. but good activity
at 50 °C (>99% conversion after 24 h at 50 °C). Only at 100 °C
did [Rh(COD)Cl] and RhCl(PPh achieve similar activity to 2
>90% conversion after 1 h at 100 °C), demonstrating the posi-
tive role of NHC ligands to reach active Rh(I) catalysts even at
room temperature.
From this first set of experiments, several conclusions can
be drawn:
3 3
)
Catalyst screening
2
3 3
)
At first sight, compounds 1–4 might seem very similar. All
these 16-electron complexes feature a tetra-coordinated Rh(I)
center bound to a COD moiety and to a NHC ligand.
Furthermore, the first coordination sphere in compounds 2–4
is alike, with one COD ligand and one O,CNHC chelating
ligand. Nevertheless, the subtle changes in the nature of the
O-containing pendant arm are found to be responsible for the
substantial differences in the catalytic activity of these
compounds.
(
(
i) The NHC ligand enhances the catalytic alkene hydrosilyl-
ation performances of Rh(I).
ii) Subtle stereo-electronic effects have a dramatic impact
on the catalysts’ activity.
iii) The global charge of the complex is not a critical factor
(
Rh(I) NHC species have been used as catalysts in hydrosilyl-
(
2
2,52,53
54–57
21–23
ation of carbonyls,
alkynes
and alkenes.
for catalysis since the activity of the cationic species 3 is
average between that of 2 and 4, both of which being neutrally
charged.
Encouraged by these precedents, we decided to evaluate which
structural factors are critical for the alkene hydrosilylation per-
formances of the present family of catalysts. The Rh(I) species
(
iv) Oxygen binding to Rh is beneficial since complexes 2–4
are much more active than 1.
v) The stronger the oxygen binding to Rh, the better the
1
–4 were thus tested in the model reaction between 3,3-
dimethyl-1-butene (TBE) and an industrially relevant non-acti-
vated tertiary silane, namely 1,1,1,3,5,5,5-heptamethyl-
(
catalysis since the activity trend 2 > 3 > 4 ≫ 1 is directly corre-
lated to the Rh–O metric.
Note that hemilabile coordination of N- or O-donors
attached to functionalized NHCs can have a drastic impact in
the catalytic performances of Rh or Ir complexes (either ben-
H
1
trisiloxane (MD M). The reactions were monitored by H NMR
spectroscopy in THF-d8 under the same experimental con-
ditions (r.t., 1.0 mol% of catalyst). As shown in Table 3,
complex 2 displays the highest activity (100% yield within
1
5 min), while 3 and 4 required 5 h and 40 h respectively to
eficial or detrimental depending on the systems and catalytic
reach >90% yield. Complex 1 was the slowest catalyst at r.t. and
required heating (50 °C) to achieve complete hydrosilylation
under a reasonable time scale (20 h). Importantly, under these
experimental conditions, the anti-Markovnikov hydrosilylation
product was obtained with 100% selectivity in all cases (as con-
firmed by mass balance analysis), without evidence of parasitic
dehydrogenative silylation which was previously seen in other
33,35,55,58–60
reactions considered).
The decoordination of the
oxygen arm in complex 2 is unlikely, due to the strong inter-
action between Rh(I) and the charged alkoxy group. In the case
of complexes 3 and 4 the decoordination of the oxygenated
1
side-arm might occur in the course of catalysis, yet H NMR
studies on precatalyst 4, featuring inequivalent diastereotopic
N–CH2 protons, tend to suggest that fluxional hemilabile
coordination is not occurring at room temperature.
Optimization of the reaction conditions with catalyst 2
shows that excellent yields (94%) can be reached at room
temperature with catalyst loading as low as 0.01 mol% and
without any additives (TON = 9400, 100% selectivity, kinetic
profiles shown in Fig. S17†).
2
1
Rh(I)–NHC catalyzed alkene hydrosilylation reactions.
Additionally, we compared the alkene hydrosilylation activity
of 1–4 with two other Rh(I) complexes, namely with [Rh(COD)
Cl] and RhCl(PPh ) , to demonstrate the beneficial effect of
2
3 3
the bifunctional NHC ligand. Interestingly, [Rh(COD)Cl] had
2
Table 3 Catalyst screening for the hydrosilylation of 3,3-dimethylbut- Alkene hydrosilylation scope
H
1
-ene with MD M
With the best catalyst in hands, we then performed a substrate
screening for the catalytic hydrosilylation of terminal alkenes
H
with MD M. The results from Table 4 show that excellent cata-
lytic performances were achieved, with excellent selectivity
towards the hydrosilylation product. For simple unfunctiona-
lized terminal alkenes, 0.1 mol% of 2 can be used to convert
them to the corresponding anti-Markovnikov hydrosilylation
products (entries 1–3) in >92% yield at r.t after 5 h. However,
when functionalized alkenes were used, the catalyst loading
had to be increased to 1.0 mol% to obtain products in >90%
yields within 24 h. Entry 4 illustrates how the presence of a
phenyl group slows down the hydrosilylation rate in styrene to
13% after 5 h compared to TBE, which reached >95% product
Entry
Catalyst
Time to reach >90% yield
1
2
3
4
1
2
3
4
>40 h at r.t. 20 h at 50 °C
0.25 h
5 h
40 h
H
Conditions: 3,3-Dimethylbut-1-ene (0.45 mmol, 1.0 eq.), MD M
(
(
0.45 mmol, 1.0 eq.), durene as internal standard (1.0 eq.), catalyst
1.0 mol%), THF-d (0.5 mL). Conversions of starting materials and for-
8
1
mation of product are based on H NMR.
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Table 4 Catalytic performances of cat. 2 for the hydrosilylation of
H
various terminal alkenes with MD M
Cat. loading
(mol%)
Time
(h)
Scheme 3 Catalyst 2 promotes at the same time reversible olefin iso-
merization (blue) and regio- and chemo-selective α-olefin hydrosilyl-
ation (green).
Entry Product
Yield (%)
>95
1
2
0.1
0.1
0.1
1
5
5/24
5/24
5/24
92/>97
(
86)
95/>98
88)
strates were observed at short reaction times (<5 h). These
isomers are converted into the linear hydrosilylation product
at long reaction time (24 h), indicating that olefin isomeriza-
tion occurs hand in hand with α-olefin hydrosilylation in the
course of the reaction (Scheme 3). Overall, catalyst 2 is a versa-
tile alkene hydrosilylation catalyst under mild conditions (r.t.,
no additives) with excellent chemoselectivity (no O-silylation
or dehydrogenative silylation) and regioselectivity (only the
anti-Markovnikov product). These catalytic performances are
remarkable since numerous Rh-based alkene hydrosilylation
catalytic systems suffer from regio- and/or chemoselectivity
3
4
(
13/51 (37)
5
6
1
1
5/24
5/24
63/>93
(
80)
83/>93
85)
(
7
1
5/24
79/94 (85)
8
9
1
1
5/24
5/24
93/96 (84)
>94/>97
(
90)
>95/>98
85)
2
1,22,65–67
10
1
5/24
issues.
(
In order to get some hints on the active species, we investi-
H
gated the stoichiometric reaction between a THF solution of
Conditions: Alkene (0.45 mmol, 1.0 eq.), MD M (0.45 mmol, 1.0 eq.),
H
mesitylene as internal standard (1.0 eq.), catalyst (0.1–1.0 mol%), THF complex 2 and a THF solution of MD M. Upon mixing at 0 °C,
(
500 μL). Conversions of starting materials and formation of product
an instantaneous color change from light yellow to dark
are based on GC. The yields in brackets correspond to the isolated
yields at the end of the reaction.
1
orange is observed. H NMR reaction monitoring shows the
quick formation of a rhodium hydride intermediate species
1
with a characteristic resonance at δ = −13.04 ppm ( J
=
Rh–H
23.8 Hz, see Fig. S18†). However this intermediate is unstable
formation in 5 h (Table 4, entry 4). This surprising result is in at r.t. and degrades into a complex mixture of unidentified
sharp contrast to previously reported work, where >95% of species, which prevented further characterization of this tran-
styrene conversion was observed in the presence of a Rh(I)– sient compound. The involvement of a Rh–H intermediate,
NHC catalyst and various silane counterparts, but at the cost presumably formed via oxidative addition of the Si–H moiety
2
1
H
of low chemoselectivity. While the negative effect of the from MD M onto Rh(I), is not surprising since rhodium
phenyl group on the product formation was also seen for hydrides are known to be efficient alkene isomerization
6
8–70
longer chain allyl benzene (entry 5) and 4-phenyl-1-butene catalysts.
entry 6) hydrosilylation products (63% and 83% of respective
yields after 5 h), in both cases >93% hydrosilylation yields
were obtained when extending the reaction time to 24 h.
(
Tandem internal alkene isomerization–hydrosilylation
The transient formation of internal alkene isomers in the
Next, we tested allyl ethers, which are known to be capri- course of the catalytic tests described above with isomerizable
cious substrates as competing C–O bond cleavage and CvC α-olefin substrates suggested that catalyst 2 could be used to
6
1–64
bond isomerization are often observed.
Similarly to linear functionalize internal olefins. In our attempt to valorize sub-
alkenes containing a phenyl group (entries 4–6), benzyl allyl strates potentially relevant in silicon industry, we thus tested
ether (entry 7) led to only 79% expected product after 5 h, but internal octenes as tandem isomerization–hydrosilylation part-
H
reached 94% yield after 24 h. Two other allyloxy group contain- ners with MD M (Table 5). At 1.0 mol% catalyst loading,
ing substrates, allyloxytrimethylsilane (entry 8) and allyl glyci- complex 2 was able to effectively tandem isomerize–hydrosily-
dyl ether (entry 9), were well tolerated by the catalyst 2, as late in 5 h 2-trans- (entry 1), 3-trans- (entry 2), 4-trans- (entry 3)
>
90% yields were reached after 5 h. As a final example of func- and 4-cis-octene (entry 4) to the corresponding linear product
tionalized alkene, we chose ethyl-4-pentenoate (entry 10), in >92% GC yield in all cases. To mimic the industrially suit-
which demonstrated exclusive chemoselectivity towards CvC able conditions of using low-value internal olefins to generate
bond hydrosilylation product (>95% yield in 5 h) under linear silicon products, we demonstrated the possibility of
current reaction conditions.
using mixtures of the four previously mentioned internal
H
It should be noted that, throughout the course of hydro- octenes to generate the linear MD M derivative in 97% yield
silylation reactions, internal isomers of isomerizable sub- in only 5 h at room temperature (entry 5). These performances
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Table 5 Catalytic performances of cat. 2 for the tandem isomerization/ ligand and an oxygen-functionalized NHC ligand. These com-
H
hydrosilylation of various internal alkenes with MD M
plexes were tested in a model catalytic reaction: the hydrosilyl-
ation of terminal alkenes with a non-activated tertiary silane.
The results show that the global charge of the complex (cat-
ionic vs. neutral) is not a critical factor for catalysis. However,
this study suggests that the stronger the oxygen binding to Rh,
the better the catalytic performances since the activity trend is
directly correlated to the Rh–O metric. Excellent conversions
can be reached with the most active catalyst 2 for a scope of
Entry
1
Alkene
Product
Time (h)
5/24
Yield (%)
94/96 (84)
2
3
4
5
5/24
5/24
5/24
5/24
92/94 (88) terminal alkenes under mild conditions (room temperature,
no additives, catalyst loading as low as 0.01 mol%) with exclu-
94/95 (82)
sive regio- and chemo-selectivity. Furthermore, 2 exhibits excel-
lent tandem isomerization–hydrosilylation catalytic perform-
96/96 (86)
ances for the selective terminal hydrosilylation of mixtures of
internal alkenes, opening attractive perspectives for the chal-
97/97 (85)
lenging valorization of unconventional olefin feedstocks.
Conflicts of interest
6
7
No reaction
There are no conflicts to declare.
>98 (89)
5
Acknowledgements
Conditions: For entries 1–4 and 6–7: alkene (0.45 mmol, 1.0 eq.),
MD M (0.45 mmol, 1.0 eq.), mesitylene as internal standard (1.0 eq.),
1
H
We thank Lisa-Lou Gracia for the H NMR analysis of the
catalyst (0.1–1.0 mol%), THF (500 μL). For entry 5: for each of the hydride intermediate. We acknowledge the “Centre de
H
octene isomers (0.11 mmol, 0.25 eq.), MD M (0.45 mmol, 1.0 eq.),
diffractométrie Henri Longchambon, Université de Lyon”, and
mesitylene as internal standard (1.0 eq.), catalyst (0.1–1.0 mol%), THF
in particular Erwan Jeanneau for the single-crystal X-ray diffr-
(500 μL). Conversions of starting materials and formation of product
are based on GC. The yields in brackets correspond to the isolated action analyses. This research was funded by the “Programme
yields at the end of the reaction.
Avenir Lyon Saint-Etienne de l’Université de Lyon” as part of
the “Investissements d’Avenir” program (ANR-11-IDEX-0007),
the CNRS MOMENTUM 2017 program and the SINCHEM
program. SINCHEM is a Joint Doctorate program selected
under the Erasmus+Action 1 Program (FPA2013-0037).
7
,10–12,17
rivals those of the best catalysts described in literature,
and demonstrate the possibility of using industrially-relevant
mixtures of internal alkenes with the non-activated silane,
H
MD M, to form linear products in excellent yields.
The above results suggest that internal alkenes far less reac-
tive towards direct hydrosilylation than terminal olefins. In the
case of cyclic internal olefins where isomerization cannot lead
to exocyclic C–C double bond in terminal position, the results
were found dependent on the ring strain. Cat. 2 was found
inactive to hydrosilylate cyclooctene, even after prolonged
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