Cu(i)-SNS complexes for outer-sphere hydroboration and hydrosilylation of carbonyls

Article abstract of DOI:10.1039/c9cc07266g

Two new NHC-Cu(i)-[κ²-SNS] complexes were synthesized to directly compare the bifunctional catalytic activity of a hard amido vs. a soft thiolate donor. The Cu thiolate complex catalyzed ketone hydroboration but not hydrosilylation, while the C

Full text of DOI:10.1039/c9cc07266g

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Cu(I)–SNS complexes for outer-sphere
hydroboration and hydrosilylation of carbonyls†
Cite this: Chem. Commun., 2019,
55, 13574
Matthew R. Elsby and R. Tom Baker
*
Received 16th September 2019,
Accepted 18th October 2019
DOI: 10.1039/c9cc07266g
rsc.li/chemcomm
Two new NHC–Cu(I)-[j2-SNS] complexes were synthesized to directly and thiolate complexes as catalysts for hydroboration and hydro-
compare the bifunctional catalytic activity of a hard amido vs. a soft silylation of ketones and aldehydes.
thiolate donor. The Cu thiolate complex catalyzed ketone hydrobora-
There are many examples of Cu[NHC] complex-catalysed
tion but not hydrosilylation, while the Cu amido complex is a high- hydroboration and hydrosilylation, which have been the subject
performing outer-sphere carbonyl reduction catalyst using boranes of review.10 By combining the copper–NHC framework with
and silanes.
L1 and L2, we expected that the crowded and coordinatively
saturated metal centre may force an outer-sphere bifunctional
Transition-metal complexes that include redox1 or Lewis acid2 mechanism.11 While most Cu-catalysed carbonyl reductions
/base3 functionality in their ligands have extensive applications proceed via the in situ formation of a copper-butoxide base to
in homogeneous catalysis.4 Bifunctional catalysts based on generate the active copper-hydride species,12 the use of either
earth abundant first-row transition metals are especially sought the thiolate or amido acting as an internal base may function in
after for their economy and low toxicity, and have been shown a similar capacity.13
to facilitate difficult catalytic transformations.5 Cooperativity
The reaction of CuCl(IPr) with one equiv. of L1 and LiHMDS
between a ligand and metal centre to facilitate E–H bond in toluene afforded the 16e- Cu(k2-SMeNSMe)(IPr) (Cu-1) complex in
activation is often a crucial aspect of bifunctional carbonyl 92% yield (Scheme 1) (IPr = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-
reduction mechanisms,6 such as the hydrosilylation of ketones 2H-imidazol-2-ylidene; HMDS = hexamethyldisilazide, N[(SiMe3)2]À).
and aldehydes.7 A key strategy of this transformation is the use As shown in Fig. 1, the solid-state structure of Cu-1 features a
of a ligand that incorporates a Lewis basic donor site to form distorted trigonal planar geometry [N(1)–Cu(1)–S(1) = 85.14(8)1;
a reactive metal-hydride intermediate after activation of an
E–H bond.8 Comparing the bifunctional capacity of hard vs.
soft donors in these ligand frameworks may aid in the design of
future ligand frameworks for catalytic applications.
We have reported two easily prepared SNS ligands (Scheme 1)
that have both demonstrated bifunctional catalytic activity in their
first-row metal complexes.9 Upon deprotonation, L1 features a hard
nitrogen donor in a thioether–amido–thioether framework, while L2
incorporates a soft sulfur donor in a thiolate–imine–thioether frame-
work. Previous work has shown that the coordination chemistry of
L1 and L2 can differ greatly even under identical conditions. As part
of our efforts to compare the properties and capabilities of these two
ligands, we report herein the synthesis of NHC–Cu–[k2-SNS] amido
Department of Chemistry and Biomolecular Sciences and Centre for Catalysis
Research and Innovation, University of Ottawa, Ottawa, Ontario, K1N 6N5,
Canada. E-mail: rbaker@uottawa.ca
† Electronic supplementary information (ESI) available: Experimental details
including synthesis and characterization, 1H and 13C{1H} NMR spectra for all
complexes. CCDC 1952495 (Cu-1) and 1952496 (Cu-2). For ESI and crystallo-
Scheme 1 Syntheses of Cu-1 and Cu-2.
graphic data in CIF or other electronic format see DOI: 10.1039/c9cc07266g
13574 | Chem. Commun., 2019, 55, 13574--13577
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Communication
ChemComm
Fig. 1 ORTEP depiction of the solid-state molecular structures of (A) Cu-1: selected bond lengths (Å): Cu(1)–S(1) 2.4820(14), Cu(1)–N(3) 1.948(2), Cu(1)–
C(16) 1.919(3); (B) Cu-2: selected bond lengths (Å): Cu(1)–S(1) 2.1802(8), Cu(1)–N(1) 2.130(2), Cu(1)–C(15) 1.889(2). Hydrogen atoms and NHC-isopropyl
substituents are omitted for clarity.
Table 1 Substrate scope of hydroboration and hydrosilylation reactions
C(16)–Cu(1)–S(1) = 123.90(7)1; C(16)–Cu(1)–N(1) = 150.89(9)1],
of carbonyls with catalytic Cu-1 and Cu-2
due to the long Cu–Sthioether bond length [2.4820(14) Å]. The
other thioether moiety is directed away from the Cu-centre
Catalyst Substrate Carbonyl
Product
Yield (%)
rather than bonding to form an 18e- complex, presumably due
to the NHC steric bulk. The analogous reaction of CuCl(IPr)
with one equiv. of L2 and KtBuO gave the Cu(k2-SMeNS)(IPr)
(Cu-2) product in 88% yield. The 1H NMR spectrum showed two
sets of resonances (one broad and one sharp), presumably due
to E- and Z-isomers relative to the imine C–H; slow exchange
was confirmed by a 2D 1H-EXSY experiment. The solid-state
structure of Cu-2 (Fig. 1) shows a less distorted trigonal planar
geometry than Cu-1, with a more crowded Cu-centre due to the
short Cu–Sthiolate bond distance [2.1802(8) Å]. Furthermore,
while the NHC ligand in Cu-1 is in the same coordination
plane as the amido and thioether, in Cu-2, the N–C–N plane of
the NHC is twisted 81.31 relative to the S–Cu–N plane.
To investigate and directly compare the bifunctional activity
of the two ligand systems, a series of E–H bond activation
studies were carried out (Table 1). The reaction of benzaldehyde
and pinacolborane with 1 mol% Cu-1 gave quantitative con-
version to the hydroboration product after 5 min at room
temperature. The analogous reaction with acetophenone
provided the same results with no decrease in activity with this
less reactive substrate. Similar reactions with triethoxysilane
afforded quantitative hydrosilylation products in 5 min. Lowering
the catalyst loading to 0.1 mol% still gave quantitative conversion
in 15 min, demonstrating the high activity and stability of Cu-1.
Although sterically hindered ketones commonly present problems
for Cu-catalyzed hydrosilylations,14 benzophenone was efficiently
converted to the silyl ether product in 10 min at room tempera-
ture. Triethylsilane may also be used as the silane source, however
Cu-1
Cu-1
Cu-1
Cu-1
Cu-1
Cu-1
Cu-1
HBpin
499
HBpin
499
499
92
HSi(OEt)3
HSi(OEt)3
HSi(OEt)3
HSi(OEt)3
HSi(OEt)3
499
499
499
Cu-2
Cu-2
Cu-2
HBpin
HBpin
HBpin
499
499
499
Conditions: copper catalyst (1 mol%), room temperature, 5–15 min.
Yields were determined from 1H NMR relative to internal standard
mesitylene.
the reaction requires gentle heating and longer reaction times to solution (see ESI†). In addition, aliphatic 5-hexen-2-one gave
reach completion (5%, 40 1C, 30 min). In contrast, attempted exclusive and quantitative hydrosilylation product with no indica-
reactions with 4-tert-butyl styrene were unsuccessful and resulted tion of isomerization, providing further evidence that alkenes
in catalyst decomposition. To confirm selectivity for carbonyls cannot bind to the metal centre during catalysis.
over alkenes, trans-cinnamaldehyde was treated with triethoxy-
Since Cu-1 was found to be an efficient catalyst for both
silane and 1 mol% Cu-1, affording the silyl ether product with no hydroboration and hydrosilylation of carbonyls, the activity of
indication of the alkene hydrosilylation product. Interestingly, Cu-2 was also investigated. Reaction of benzaldehyde and
after 24 h we observed significant cis–trans equilibration in pinacolborane with 1 mol% Cu-2 afforded the hydroboration
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 13574--13577 | 13575

View Article Online
ChemComm
Communication
product in near quantitative yield after 5 min. Analogous
reactions with acetophenone, and even bulky benzophenone
also resulted in complete conversion. However, Cu-2 was not an
active catalyst for hydrosilylations. Attempts to facilitate the
hydrosilylation of either benzaldehyde or acetophenone were
unsuccessful, even with higher catalyst loadings, longer reaction
times, and elevated temperatures (see ESI†).
Several experiments were performed to gain insight into a
plausible reaction pathway. By NMR spectroscopy, neither Cu-1
nor Cu-2 showed any reaction with stoichiometric benzaldehyde
or with triethoxysilane at short reaction times; longer reaction
times with the latter led to decomposition. In contrast, both
complexes reacted with pinacolborane which has been shown
previously to undergo B–H bond activation at metal amido15 and
thiolate16 centres. Upon addition of 1 equiv. of pinacolborane to a
C6D6 solution of Cu-1, an immediate colour change from pale
yellow to dark yellow was observed. The 1H NMR spectrum
showed a new broad singlet at d 2.5 assigned as a Cu–H species,
as confirmed by the DBpin reaction with Cu-1 (Scheme 2A).
Further monitoring showed decomposition after approximately
Scheme 3 Proposed mechanism for E–H bond activation and reaction
with carbonyls.
1 h at room temperature, as evidenced by formation of a black via outer-sphere transition state D, to furnish the final reduced
precipitate and free L1. carbonyl and regenerate A. The carbonyl substrate does not
The instability of monomeric NHC Cu–H species has bind to the metal centre at any step throughout the catalytic
been reported previously.17 To establish the Cu–H species as an cycle, explaining the selectivity for carbonyls over alkenes and
intermediate in the hydroboration reaction, a solution of Cu-1H lack of isomerization of the latter. It should be noted that for
was prepared by reacting stoichiometric Cu-1 and pinacolborane. Cu-2 catalyzed hydroboration, the thiolate donor serves as the
The solution was then charged with stoichiometric acetophenone, Lewis base to promote B–H bond activation, as previously
resulting in the conversion to the hydroboration product, and established by Wang.18
reforming of Cu-1 (Scheme 2B). It should be noted that the
The original design of these Cu complexes was meant to
stoichiometric reaction of Cu-2 with pinacolborane resulted in crowd the Cu-centre with sufficient steric bulk to force an outer-
complete consumption of Cu-2 into a new complex with multiple sphere bifunctional mechanism. The fact that aldehydes and
isomers that were not characterized further (see ESI†).
ketones readily react while olefins are inactive supports the
Based on the above experiments, a plausible reaction path- envisioned outer-sphere mechanism, as the latter requires
way for the hydroboration/hydrosilylation of carbonyls using binding to the metal centre prior to activation. Complexes
Cu-1 is shown in Scheme 3. The reaction begins with E–H bond Cu-1 and Cu-2 also allowed a direct comparison for the bifunc-
activation through the proposed transition state, B, with the tional catalytic activity of the hard amido vs. the soft thiolate
amido acting as a Lewis base. This results in the formation of a donor in the ligands L1 and L2. While thiolate complex Cu-2 is
Cu–H species C, which then reacts with the carbonyl substrate an efficient catalyst for hydroboration of carbonyls using pina-
colborane, it was unable to effect the hydrosilylation. Amido
complex Cu-1 was shown to be superior for carbonyl reductions
using either pinacolborane or triethoxysilane, giving full con-
version at room temperature in less than 15 min, even with
catalyst loadings of 0.1%. Mechanistic studies support the
proposed reaction pathway in which the amido/thiolate serves
as a Lewis base to facilitate E–H bond activation, generating a
Cu–H intermediate that reacts with carbonyl substrates via an
outer-sphere mechanism. There are few instances of copper
being utilized in metal–ligand bifunctional catalysis, and most
examples employ dinuclear-copper complexes.19 Furthermore,
nearly all examples of Cu–NHC catalyzed hydroboration and
hydrosilylation reactions require either excess substrate, elevated
temperatures, longer reaction times, or the use of base to generate
the active Cu–H species (see Table S1 in ESI†).10a,b This work
presents an extremely efficient carbonyl reduction pathway via a
metal–ligand bifunctional outer-sphere mechanism that is unique
Scheme 2 Mechanistic studies performed with Cu-1.
13576 | Chem. Commun., 2019, 55, 13574--13577
for copper. Furthermore, installation of a chiral auxiliary in lieu of
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Communication
ChemComm
6 P. A. Dub and J. C. Gordon, ACS Catal., 2017, 7, 6635–6655.
the uncoordinated aryl-thioether may offer an opportunity for
stereospecific catalysis. Future work will further explore potential
reduction catalysis with other base metals.
We thank the NSERC and Canada Research Chairs program
for generous financial support and the University of Ottawa,
Canada Foundation for Innovation, and Ontario Ministry of
Economic Development and Innovation for essential infra-
structure. We also acknowledge Dr Jeff Ovens and Peter Pallis-
ter for XRD and NMR assistance.
7 (a) S. N. MacMillan, W. H. Harman and J. C. Peters, Chem. Sci., 2014,
5, 590–597; (b) M. A. Nesbit, D. L. Suess and J. C. Peters, Organo-
metallics, 2015, 34, 4741–4752; (c) R. H. Morris, Chem. Soc. Rev.,
2009, 38, 2282–2291.
8 M. Kanai, N. Kato, E. Ichikawa and M. Shibasaki, Synlett, 2005,
1491–1508.
9 (a) U. K. Das, S. L. Daifuku, S. I. Gorelsky, I. Korobkov, M. L. Neidig,
J. J. Le Roy, M. Murugesu and R. T. Baker, Inorg. Chem., 2016, 55,
987–997; (b) U. K. Das, S. L. Daifuku, T. E. Iannuzzi, S. I. Gorelsky,
I. Korobkov, B. Gabidullin, M. L. Neidig and R. T. Baker, Inorg.
Chem., 2017, 56, 13766–13776; (c) U. K. Das, C. S. Higman,
B. Gabidullin, J. E. Hein and R. T. Baker, ACS Catal., 2018, 8,
1076–1081.
10 (a) J. D. Egbert, C. S. Cazin and S. P. Nolan, Catal. Sci. Technol., 2013,
3, 912–926; (b) F. Lazreg, F. Nahra and C. S. Cazin, Coord. Chem.
Rev., 2015, 293, 48–79; (c) Y. Tsuji and T. Fujihara, Chem. Rec., 2016,
16, 2294–2313.
Conflicts of interest
There are no conflicts to declare.
11 O. Eisenstein and R. H. Crabtree, New J. Chem., 2013, 37, 21–27.
12 L. Dong, S. Qin, H. Yang, Z. Su and C. Hu, Catal. Sci. Technol., 2012,
2, 564–569.
Notes and references
1 (a) B. de Bruin, P. Gualco and N. D. Paul, Ligand Design in Metal 13 (a) T. He, N. P. Tsvetkov, J. G. Andino, X. Gao, B. C. Fullmer and
Chemistry: Reactivity and Catalysis, M. Stradiotto and R. J. Lundgren,
2016, p. 176; (b) V. Lyaskovskyy and B. de Bruin, ACS Catal., 2012, 2,
270–279; (c) H. Takeda, K. Koike, H. Inoue and O. Ishitani, J. Am. 14 S. Dıez-Gonzalez, H. Kaur, F. K. Zinn, E. D. Stevens and S. P. Nolan,
Chem. Soc., 2008, 130, 2023–2031. J. Org. Chem., 2005, 70, 4784–4796.
2 (a) W. H. Harman and J. C. Peters, J. Am. Chem. Soc., 2012, 134, 15 M. Pang, C. Wu, X. Zhuang, F. Zhang, M. Su, Q. Tong, C.-H. Tung
K. G. Caulton, J. Am. Chem. Soc., 2009, 132, 910–911; (b) M. Rakowski
Dubois and D. L. Dubois, Acc. Chem. Res., 2009, 42, 1974–1982.
´
´
5080–5082; (b) M. Devillard, G. Bouhadir and D. Bourissou, Angew.
Chem., Int. Ed., 2015, 54, 730–732.
3 (a) D. Sellmann, R. Prakash, F. W. Heinemann, M. Moll and
M. Klimowicz, Angew. Chem., Int. Ed., 2004, 43, 1877–1880; (b) M. L.
Helm, M. P. Stewart, R. M. Bullock, M. R. DuBois and D. L. DuBois,
Science, 2011, 333, 863–866.
4 (a) J. R. Khusnutdinova and D. Milstein, Angew. Chem., Int. Ed., 2015,
54, 12236–12273; (b) T. Ikariya and A. J. Blacker, Acc. Chem. Res.,
2007, 40, 1300–1308.
and W. Wang, Organometallics, 2018, 37, 1462–1467.
¨
16 (a) L. Omann, C. D. F. Konigs, H. F. Klare and M. Oestreich, Acc.
Chem. Res., 2017, 50, 1258–1269; (b) K. D. Hesp, R. McDonald,
M. J. Ferguson and M. Stradiotto, J. Am. Chem. Soc., 2008, 130,
16394–16406.
17 (a) N. P. Mankad, D. S. Laitar and J. P. Sadighi, Organometallics,
2004, 23, 3369–3371; (b) S. C. Schmid, R. Van Hoveln, J. W. Rigoli
and J. M. Schomaker, Organometallics, 2015, 34, 4164–4173.
18 H. Song, K. Ye, P. Geng, X. Han, R. Liao, C.-H. Tung and W. Wang,
ACS Catal., 2017, 7, 7709–7717.
5 (a) J. I. van der Vlugt, Eur. J. Inorg. Chem., 2012, 363–375; (b) R. H.
Morris, Acc. Chem. Res., 2015, 48, 1494–1502; (c) L. Alig, M. Fritz and 19 (a) J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young, T. D.
S. Schneider, Chem. Rev., 2018, 119, 2681–2751; (d) P. J. Chirik, Acc.
Chem. Res., 2015, 48, 1687–1695; (e) W. Zuo, A. J. Lough, Y. F. Li and
R. H. Morris, Science, 2013, 342, 1080–1083; ( f ) C. P. Casey and
H. Guan, J. Am. Chem. Soc., 2007, 129, 5816–5817.
Pawlik, D. J. Giesen, S. B. Harkins, A. J. Miller, S. F. Mickenberg and
J. C. Peters, J. Am. Chem. Soc., 2010, 132, 9499–9508; (b) R. Angamuthu,
P. Byers, M. Lutz, A. L. Spek and E. Bouwman, Science, 2010, 327,
313–315.
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 13574--13577 | 13577