Visible-Light Controlled Divergent Catalysis Using a Bench-Stable Cobalt(I) Hydride Complex

Article abstract of DOI:10.1002/chem.202000410

While the use of visible light in conjunction with transition metal catalysis offers powerful opportunities to switch between on/-off states of catalytic activity, the next frontier would be the ability to switch the actual function of the catalyst and resulting products. Here we report such an example of multi-dimensional catalysis. Featuring an easily prepared, bench-stable cobalt(I) hydride complex in conjunction with pinacolborane, we can switch the reaction outcome between two widely employed transformations, olefin migration and hydroboration, with visible light as the trigger.

Full text of DOI:10.1002/chem.202000410

A Journal of
Accepted Article
Title: Visible-light controlled divergent catalysis using a bench-stable
cobalt(I) hydride complex
Authors: Enrico Bergamaschi, Frédéric Beltran, and Christopher
Teskey
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202000410
Link to VoR: http://dx.doi.org/10.1002/chem.202000410
Supported by

10.1002/chem.202000410
Chemistry - A European Journal
DOI: 10.1002/ ((will be filled in by the editorial staff))
Divergent Catalysis
Visible-light controlled divergent catalysis using a bench-stable
cobalt(I) hydride complex
Enrico Bergamaschi†, Frédéric Beltran† and Christopher J. Teskey*
In this context, we were interested in the photochemistry of bench-
stable cobalt complex 1, CoH[PPh(OEt)2]4 which had been reported
by Onishi and co-workers to undergo reversible photodissociation of
a phosphonite ligand under irradiation from a high-pressure mercury
lamp.[14,15] In all catalytic transformations reported to date, complex
1 has been assigned the ‘off’ state, only switching ‘on’ in the light
when it becomes complex 2. Despite this, there were some reports
on stoichiometric redox reactions with 1 which encouraged us to
investigate the catalytic activity both in the light and dark.[16,17]
Given the absorption spectra of 1, we began our investigations
with commercial blue LEDs using a simple-laboratory set-up. We
elected to investigate the visible-light promoted hydroboration of
alkenes: an atom-economical, value-adding transformation[18] which
has provoked substantial research. Examples of both
Markovnikov[19–22] and anti-Markovnikov[23–29] cobalt-catalysed
hydroborations have been reported previously. Many of these
examples, however, rely on complex ligand systems or use an air
sensitive cobalt complex that must be manipulated in a glovebox.[30]
Alternatively, additives are used to form the active catalyst in situ.
Our approach would instead take an easily synthesised, non-
precious and bench-stable pre-catalyst.
Abstract: While the use of visible light in conjunction with
transition metal catalysis offers powerful opportunities to switch
between on / off states of catalytic activity, the next frontier would
be the ability to switch the actual function of the catalyst and
resulting products. Here we report such an example of multi-
dimensional catalysis. Featuring an easily prepared, bench-stable
cobalt(I) hydride complex in conjunction with pinacolborane, we
can switch the reaction outcome between two widely employed
transformations, olefin migration and hydroboration, with visible
light as the trigger.
V
isible light excitation of metal complexes, which can act as a
catalytic single electron reductant, single electron oxidant or an
energy transfer agent, has transformed the field of organic synthesis
in the last decades.[1,2] These modes of action mean that the vast
majority of reactions reside in the area of radical transformations.
However, interest in the direct visible light promotion of transition-
metal catalysed processes is growing.[3–7] Clearly, visible light, as a
widely-available, cheap, sustainable and non-invasive method of
energy delivery, can exert precise spatial and temporal control over
a chemical process. The use of light to switch a catalyst between
active and inactive states has been termed light-gated catalysis.[8]
Considerably rarer than the switching of a catalytic process
between active and inactive states, is switching selectivity between
two different pathways.[9] Indeed, within the area of transition metal
catalysis, only a handful of examples have been reported. Just
recently, Zhu, Sarina and co-workers reported a specific example
where gold-cobalt nanoparticles enabled the formation of imines
from anilines and arylalkynes under visible light irradiation.[10] In
contrast, without light, they observed only homocoupling of the
alkyne. Greaney and co-workers have reported an example where,
using a visible light absorbing copper catalyst, they could switch
between methoxyazidation and double azidation of alkenes in the
light and dark respectively.[11]
One method for activating transition metal complexes is to use
light to dissociate a ligand (Scheme 1). This increases reactivity by
opening up ligation sites at a coordinatively saturated metal centre in
a non-invasive and reversible manner. Many such methods have
been reported previously, particularly with metal carbonyl
complexes, however commonly rely on high energy UV light.[12,13]
[]
E. Bergamaschi,† Dr F. Beltran,† Dr C. J. Teskey
† These authors contributed equally.
Scheme 1. Visible light controlled catalysis and reaction discovery.
Institute of Organic Chemistry, RWTH Aachen University,
Landoltweg 1, 52074 Aachen (Germany).
E-mail: christopher.teskey@rwth-aachen.de
Homepage: https://www.teskeygroup.com
.
Supporting information for this article is available on the WWW
under http://dx.doi.org/
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202000410
Chemistry - A European Journal
Scheme 2. Substrate scope for switchable hydroboration/isomerisation reaction. Conditions A: 3 (0.2 mmol), CoH[PPh(OEt)2]4 (2 mol%), HBpin (2 equiv.), THF (0.2
M), Blue LEDs. Conditions B: 3 (0.2 mmol), CoH[PPh(OEt)2]4 (2 mol%), HBpin (1.1 equiv.), CH2Cl2 (1 M), dark. Yields and E/Z ratios measured by 1H NMR relative
to CH2Br2 as an internal standard. Isolated yields are in brackets. aKHBEt3 (5 mol%) was used as an additive. bThe product was obtained as a mixture of
diastereomers. cThe two-bond isomerised product was obtained as a minor product. d[5 mol%] of CoH[PPh(OEt)2]4 was used. eThe reaction was run at 30°C.
Our studies began with investigating the hydroboration of
allylpentafluorobenzene 3a under visible light irradiation, with blue
LEDs and at room temperature, using 2 mol% of complex 1. We
were delighted to observe hydroborated product 4a in excellent
yield (Scheme 1). Without pinacolborane in the light, starting
material 3a was recovered in almost quantitative yield. When
carrying out the reactions in the dark, we observed one-carbon
isomerised product, 5a as the main product with excellent E/Z ratio
(see supplemental information for full details). No conversion was
observed in the dark without pinacolborane. Although alkene
isomerisation has been reported with a wide array of metal catalysts
including non-noble metal examples with nickel[31] or cobalt,[32–39]
we believed that this method compares well, requiring no additional
co-catalysts, ligands or bases.[40] It is noteworthy that few existing
alkene isomerisation methods are reported to isomerise this
particular electron-deficient substrate.[41]
We then proceeded to carry out the substrate scope of these two
contrasting reactions, beginning with the visible-light mediated
cobalt catalysed hydroboration (Scheme 2, top). In the majority of
cases for hydroboration, addition of potassium triethylborohydride
increased the yield of products 4 by around 10 – 20%. In all cases,
only the anti-Markovnikov product was observed. Allylcyclohexane
3b and (+)-β-citronellene 3c gave hydroborated products in very
good yields. Substrates 3d – 3f, derived from indanones, also
yielded the hydroborated products in moderate to good yields. We
then trialled 4-aryl-1-butene substrates which were hydroborated to
give products 4g, 4h and 4i with complete terminal selectivity.
This is in contrast to a similar system, reported by Chirik and
co-workers, where the cobalt catalyst first isomerises the alkene into
conjugation with the aromatic group before hydroboration occurs.27
Longer chain alkene 3j was also hydroborated at the terminal
position in moderate yield. Cyclopropyl containing substrate was
hydroborated in to give 4k (with no traces of cyclopropyl ring-
opened products observed).
Next we turned to the method in the dark for alkene
isomerisation where better isolated yields were observed in
chlorinated solvents chlorobenzene or dichloromethane. One bond
isomerised product 5b was selectively isolated in good yield and
with good E/Z selectivity. (+)-β-citronellene gave trisubstituted
alkene 5c in moderate yield. Indanone derivatives also gave one
bond isomerised products 5d – 5f in good yields, however as a
mixture of E/Z products and diastereomers.§ Product 5g was
obtained with 5 mol% of CoH[PPh(OEt)2]4 at 30 °C as a result of
two bond isomerisation. Analogous naphthyl compound 3h,
however, gave mainly one bond isomerised products 5h under the
same conditions along with the two-bond isomerised compound as a
minor product. Starting material 3i did not undergo efficient
isomerisation under the reaction conditions with largely starting
material recovered. The longer chain substrate 3j gave only one
bond-isomerised product 5j in reasonable conversion but with some
starting material remaining. Finally, starting material 3k did not
undergo any isomerisation giving the first mechanistic hint for this
reaction.
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202000410
Chemistry - A European Journal
During the course of our investigations, we discovered that
certain substrates, that were unreactive in the light, still isomerised
in the dark. Firstly, benzyl protected alcohol 3m underwent two
bond alkene isomerisation to give the allyl ether 5m but with no E/Z
selectivity. Although the hydroboration method was only amenable
to terminal alkenes, exo-endo isomerisation was possible, giving
product 5n in almost quantitative yield. Varying the substitution
pattern on the ring did not affect this efficient exo-endo
To this end, we began by monitoring the isomerisation of allyl
pentafluorobenzene 3a with pinacolborane by NMR spectroscopy.
Here we observed formation of hydrogen gas at the start of the
reaction as well as a new signal by 11B NMR. Cognizant of work by
Norton34 and Shenvi33 on cobalt-hydride mediated H· addition to
alkenes, we trialled two substrates which we believed should
undergo cycloisomerisation or radical ring opening if this was the
mechanistic pathway (Scheme 4a and b). Only starting material was
observed in both cases, however. Additionally, we carried out the
isomerisation of 3a in the presence of different radical traps
(Scheme 4c). Excellent yields of product were observed in all cases
suggesting that a radical mechanism is unlikely to be operative.
We had observed during the course of our studies that the use of
complex 1 with visible light but without pinacolborane gave
isomerised products in select cases, consistent with previous work.14
However, we ruled out pinacolborane induced ligand dissociation in
isomerisation. Product 5o, containing
a useful aryl bromide
functionality for further derivatisation was obtained in excellent
yield. Finally, taking a substrate containing an alkyl boronic ester
(which had been produced first via selective hydroboration of a
terminal alkene under the light conditions) saw efficient
isomerisation of the exocyclic double bond in the dark (albeit as a
mixture of regioisomers) to give 5p.
Next, we looked to showcase the switchability of this reaction
system, beginning with a simple on-off experiment with substrate 3a. the dark to give complex 2 given that only starting material was
Monitoring the conversion into either 4a or 5a demonstrates clearly
that control over hydroboration or isomerisation could be exerted
solely by switching the light on or off (Scheme 3a). This clearly
distinguishes this work from other reported catalytic methods for
these transformations and opens up a unique opportunity to perform
both reactions in the same vessel with the reaction outcome dictated
by a simple switch of the light. We then exploited this by taking
substrates 3a and 3n in the same pot with CoH[PPh(OEt)2]4 and
pinacolborane in THF. First, we performed the hydroboration in the
light, selectively targeting the terminal alkene before turning off the
recovered with compound 3a under light irradiation with complex 1
without pinacolborane. Previous literature has reported that
CoH[PPh(OEt)2]4 can undergo single electron oxidation to the
corresponding cobalt(II) complex, [CoH[PPh(OEt)2]4]+ and also to
dihydride cobalt(III) complex [CoH2[PPh(OEt)2]4]+.16,17
Indeed, there is an isolated report by Onishi which reports a
moderate yield of alkene isomerisation using Co(II) species,42 and a
peak in the 1H NMR at -12.4 ppm suggests that Co(III) dihydride is
produced in small quantities during the course of the reaction.43 We
therefore elected to prepare both of these complexes in situ but in
neither case were more than traces of the product observed (Scheme
4d). Additionally, carrying out the isomerisation of 3n with 10
mol% of deuterated catalyst CoD[PPh(OEt)2]4 gave the product
without any deuterium incorporation (Scheme 4e). This suggests
that the catalytically active species is unlikely to be a cobalt hydride.
We then carried out a competition experiment between non-
deuterated compound 3n and deuterated compound 3q. Analysis by
1H and 2H NMR revealed approximately 30% deuterium
incorporation into the product 5n with only 60% deuterium at the
allylic position in 5q. However, when substrate 3q is isomerised by
itself, there is also approximately 30% loss of deuterium in the
product suggesting that there may be some exchange with the
pinacolborane. Given that we had ruled out a radical mechanism and
insertion of a cobalt-hydride species also seems unlikely, we
light and adding
a further equivalent of HBpin to allow
isomerisation of the exocyclic alkene to occur (Scheme 3b). This
one-pot, dual-function approach allows the same catalyst, added
from the start, to carry out two separate transformations.
Having demonstrated the unique synthetic utility of
CoH[PPh(OEt)2]4 for two valuable, atom-economical processes, our
attention turned to the mechanism of this unusual light switchable
process. Previous work by Onishi14,15 and similar catalytic systems27
give good evidence that the light mediated processes occur via a
mechanism which involves coordination of the alkene to the 16
electron, photogenerated, cobalt complex 2 (Scheme 4h, right). It
was not clear to us, however, what the role of the cobalt complex
was for the isomerisation that occurs in the dark.
therefore suggest that
a π-allyl-type mechanism could be
operative.44 1H NMR monitoring of the reaction indicates that the
reaction begins only once the signal corresponding to the cobalt
hydride of the starting complex 1 has decreased to a constant level,
implying initial reaction between pinacol borane and 1 to generate
the active catalytic species.
Taken together with the evolution of H2 gas that was observed,
we suggest that we may form the 18 electron complex 7.‡ This could
then, via ligand exchange and reversible HBpin formation, form a
cobalt π-allyl species from which isomerisation occurs. This is a
clear demonstration that modification of the active catalytic site
affects the reactivity of the system prompting
a different
mechanistic and reaction outcome.45 Alternatively, we cannot rule
out that a cationic cobalt(I) complex to which an alkene can
coordinate is formed before undergoing anion assisted cobalt π-allyl
formation. Further investigations to confirm this are currently
underway in our laboratory.
Scheme 3. Switchable catalysis.
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202000410
Chemistry - A European Journal
Scheme 4. Mechanistic experiments and tentative mechanism.
Conflict of interest
In conclusion, we have demonstrated switchable reactivity of
bench-stable cobalt hydride complex 1 for alkene isomerisation and
hydroboration. By controlling the coordination sphere and thus the
active site around the cobalt centre with visible light, different
mechanistic pathways result. Photodissociation of a phosphonite
ligand (carried out for the first time with visible light) gives 16
electron complex 2 with a free ligation site allowing insertion of the
alkene into the Co–H bond. Without light, we propose the 18
electron complex 1 instead reacts first with pinacol borane to form a
The authors declare no conflict of interests.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: cobalt catalysis
isomerisation • photoswitching
• photocatalysis • hydroboration •
new cobalt species which isomerises alkenes via
a π-allyl
mechanism. This conceptually new strategy opens up a new frontier
of switchable transition metal-catalysed processes in which the
reaction outcome can be controlled solely via visible light. We
anticipate this leading to development of more dual-function
catalytic systems.
§
As a result of this it was not possible to determine the E/Z ratio for 5e
and 5f.
‡
5a was obtained in 72% yield from substrate 3a (as determined by 1H
NMR) using only 20 mol% of HBpin, however, with other substrates,
isolated yields were lower when using less than 1 equiv. of HBpin.
C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013,
113, 5322–5363.
[1]
[2]
[3]
[4]
[5]
O. S. Wenger, J. Am. Chem. Soc. 2018, 140, 13522–13533.
M. Parasram, V. Gevorgyan, Chem. Soc. Rev. 2017, 46, 6227–6240.
K. E. Ruhl, T. Rovis, J. Am. Chem. Soc. 2016, 138, 15527–15530.
B. D. Ravetz, J. Y. Wang, K. E. Ruhl, T. Rovis, ACS Catal. 2019, 9,
200–204.
B. J. Shields, B. Kudisch, G. D. Scholes, A. G. Doyle, J. Am. Chem.
Soc. 2018, 140, 3035–3039.
S. M. Thullen, T. Rovis, J. Am. Chem. Soc. 2017, 139, 15504–15508.
R. S. Stoll, S. Hecht, Angew. Chem. Int. Ed. 2010, 49, 5054–5075;
Angew. Chem. 2010, 122, 5176–5200.
Acknowledgements
We thank the FCI (Liebig Fellowship to C.J.T., PhD studentship to
E.B) for financial support of this project. Dr Christoph Räuber is
thanked for assistance with NMR measurements. Prof. Dr. Franziska
Schoenebeck is gratefully acknowledged for her generous support
and advice.
[6]
[7]
[8]
[9]
A. J. Teator, H. Shao, G. Lu, P. Liu, C. W. Bielawski,
Organometallics 2017, 36, 490–497.
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202000410
Chemistry - A European Journal
[10] E. Peiris, S. Sarina, E. R. Waclawik, G. A. Ayoko, P. Han, J. Jia, H.-
Y. Zhu, Angew. Chem. Int. Ed. 2019, 58, 12032–12036; Angew.
Chem. 2019, 131, 12160–12164.
[11] G. Fumagalli, P. T. G. Rabet, S. Boyd, M. F. Greaney, Angew. Chem.
Int. Ed. 2015, 54, 11481–11484; Angew. Chem. 2015, 127, 11643–
11646.
[12] M. Wrighton, Chem. Rev. 1974, 74, 401–430.
[13] D. Lehnherr, Y. Ji, A. J. Neel, R. D. Cohen, A. P. J. Brunskill, J.
Yang, M. Reibarkh, J. Am. Chem. Soc. 2018, 140, 13843–13853.
[14] M. Onishi, K. Hiraki, M. Matsuda, T. Fukunaga, Chem. Lett. 1983,
12, 261–264.
[28] A. D. Ibrahim, S. W. Entsminger, A. R. Fout, ACS Catal. 2017, 7,
3730–3734.
[29] J. H. Docherty, J. Peng, A. P. Dominey, S. P. Thomas, Nat. Chem.
2017, 9, 595–600.
[30] J. V. Obligacion, P. J. Chirik, Nat Rev Chem 2018, 2, 15–34.
[31] A. Kapat, T. Sperger, S. Guven, F. Schoenebeck, Science 2019, 363,
391–396.
[32] H. Kumobayashi, S. Akutagawa, S. Otsuka, J. Am. Chem. Soc. 1978,
100, 3949–3950.
[33] S. W. M. Crossley, F. Barabé, R. A. Shenvi, J. Am. Chem. Soc. 2014,
136, 16788–16791.
[15] M. Onishi, S. Oishi, M. Sakaguchi, I. Takaki, K. Hiraki, BCSJ 1986,
59, 3925–3930.
[16] G. Albertin, P. Amendola, S. Antoniutti, E. Bordignon, J. Chem. Soc.,
Dalton Trans. 1990, 2979–2984.
[17] J. R. Sanders, J. Chem. Soc., Dalton Trans. 1973, 748–749.
[18] J. W. B. Fyfe, A. J. B. Watson, Chem 2017, 3, 31–55.
[19] S. W. Reilly, C. E. Webster, T. K. Hollis, H. U. Valle, Dalton Trans.
2016, 45, 2823–2828.
[34] G. Li, J. L. Kuo, A. Han, J. M. Abuyuan, L. C. Young, J. R. Norton,
J. H. Palmer, J. Am. Chem. Soc. 2016, 138, 7698–7704.
[35] Q.-Y. Meng, T. E. Schirmer, K. Katou, B. König, Angew. Chem. Int.
Ed. 2019, 58, 5723–5728; Angew. Chem. 2019, 131, 5779–5784.
[36] X. Liu, W. Zhang, Y. Wang, Z.-X. Zhang, L. Jiao, Q. Liu, J. Am.
Chem. Soc. 2018, 140, 6873–6882.
[37] A. Schmidt, A. R. Nödling, G. Hilt, Angew. Chem. Int. Ed. 2015, 54,
801–804; Angew. Chem. 2015, 127, 814–818.
[20] J. Peng, J. H. Docherty, A. P. Dominey, S. P. Thomas, Chem.
Commun. 2017, 53, 4726–4729.
[38] T. Kobayashi, H. Yorimitsu, K. Oshima, Chemistry – An Asian
Journal 2009, 4, 1078–1083.
[21] G. Zhang, J. Wu, M. Wang, H. Zeng, J. Cheng, M. C. Neary, S.
Zheng, Eur. J. Org. Chem. 2017, 2017, 5814–5818.
[22] S. R. Tamang, D. Bedi, S. Shafiei-Haghighi, C. R. Smith, C.
Crawford, M. Findlater, Org. Lett. 2018, 20, 6695–6700.
[23] J. V. Obligacion, P. J. Chirik, J. Am. Chem. Soc. 2013, 135, 19107–
19110.
[24] L. Zhang, Z. Zuo, X. Leng, Z. Huang, Angew. Chem. Int. Ed. 2014,
53, 2696–2700; Angew. Chem. 2014, 126, 2734–2738.
[25] L. Zhang, Z. Zuo, X. Wan, Z. Huang, J. Am. Chem. Soc. 2014, 136,
15501–15504.
[39] C. Chen, T. R. Dugan, W. W. Brennessel, D. J. Weix, P. L. Holland,
J. Am. Chem. Soc. 2014, 136, 945–955.
[40] J. J. Molloy, T. Morack, R. Gilmour, Angew. Chem. Int. Ed. 2019, 58,
13654–13664; Angew. Chem. 2019, 131, 13789–13800.
[41] S. Hanessian, S. Giroux, A. Larsson, Org. Lett. 2006, 8, 5481–5484.
[42] M. Onishi, K. Hiraki, Y. Ishida, K. Dakeshita, Chem. Lett. 1986, 15,
333–336.
[43] M. Onishi, J. Mol. Catal. 1993, 80, 145–149.
[44] K. Michigami, T. Mita and Y. Sato, J. Am. Chem. Soc. 2017, 139,
6094–6097.
[26] J. Chen, T. Xi, X. Ren, B. Cheng, J. Guo, Z. Lu, Org. Chem. Front.
2014, 1, 1306–1309.
[45] C. Weatherly, J. M. Alderson, J. F. Berry, J. E. Hein and J. M.
Schomaker, Organometallics 2017, 36, 1649–1661.
[27] M. L. Scheuermann, E. J. Johnson, P. J. Chirik, Org. Lett. 2015, 17,
2716–2719.
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202000410
Chemistry - A European Journal
Divergent catalysis
Enrico Bergamaschi, Frédéric Beltran
and Christopher J. Teskey*
Page – Page
Visible-light controlled divergent
catalysis using a bench-stable
cobalt(I) hydride complex
Night and Day: A rare example of switching product outcome depending on
light/dark is reported. Using a cobalt(I) hydride complex, hydroborated products
are obtained under visible-light irradiation whereas, in contrast, alkene
isomerisation occurs in the dark. This dual-function catalysis results from alteration
of the coordination sphere surrounding the metal centre.
6
This article is protected by copyright. All rights reserved.