Hydroboration of alkynes and nitriles using an α-diimine cobalt hydride catalyst

Article abstract of DOI:10.1039/c7cc02281f

Addition of NaEt₃BH to (^Ph2PPrDI)CoCl₂ affords the corresponding monohydride, (^Ph2PPrDI)CoH. X-ray diffraction and DFT calculations indicate that this compound possesses a radical monoanion α-DI chelate and a Co(ii) centre. Notably, (^Ph2PPrDI)CoH catalyzes the hydroboration of alkynes and dihydroboration of nitriles under mild conditions.

Full text of DOI:10.1039/c7cc02281f

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DOI: 10.1039/C7CC02281F
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Hydroboration of Alkynes and Nitriles Using an α-Diimine Cobalt
Hydride Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Hagit Ben-Daat,a Christopher L. Rock,a Marco Flores,a Thomas L. Groy,a Amanda C. Bowman,b and
Ryan J. Trovitch*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Addition of NaEt3BH to (Ph2PPrDI)CoCl2 affords the corresponding these examples, TOF has been inferred from general reaction
monohydride, (Ph2PPrDI)CoH. X-ray diffraction and DFT calculations conditions and may not represent the upper activity limit.
indicate that this compound possesses a radical monoanion α-DI
In this contribution, we describe the synthesis and
chelate and a Co(II) centre. Notably, (Ph2PPrDI)CoH catalyzes the characterization of an α-diimine (DI) cobalt hydride complex
hydroboration of alkynes and dihydroboration of nitriles under that mediates alkyne hydroboration with TOFs of up to 900 h-1
mild conditions.
at ambient temperature. Furthermore, we report modest TOFs
for Co-catalyzed dihydroboration of nitriles to yield
diborylamines.
First popularized by Miyaura and Suzuki,1 alkenyl boronate
esters have remained vital precursors for highly functionalized
organic molecules due to their utility in palladium-catalyzed
cross-coupling reactions.2 These reagents have traditionally
been prepared by adding Grignard or organolithium reagents
to trialkyl borates;3 however, the direct4 and metal-catalyzed5
hydroboration of alkynes have emerged as alternative, atom-
efficient synthetic routes. For precursors that do not easily
undergo direct hydroboration, it is desirable from a cost and
sustainability perspective to utilize hydroboration catalysts
which feature Earth abundant first transition series metals.6
There has been explosive growth in the development of
cobalt-catalyzed alkene hydroboration over the last three
years;7 however, relatively few examples of cobalt-mediated
alkyne hydroboration have been reported. In 2015, it was
determined that the bis(imino)pyridine cobalt alkyl complex,
An equimolar quantity of Ph2PPrDI11 was added to CoCl2 in
acetonitrile solution and stirring at 25 °C for 24 h afforded a
dark red product identified as (Ph2PPrDI)CoCl2 (
1, eqn 1). This
compound was found to exhibit broad 1H NMR resonances
that suggest two ligand coordination modes (one set
predominates at -20 °C, see ESI) and was determined to have a
magnetic moment of 2.8 B at 25 °C (by Evans method in
acetonitrile-d3 and magnetic susceptibility balance). Single
crystals of
1 grown at -35 °C were then analysed by X-ray
diffraction. Although high quality data could not be obtained
(R = 0.0984), two unique molecules in the unit cell were found
to possess a κ4-Ph2PPrDI chelate and cis-chloride ligands (Figure
S1 of the ESI). DI ligands including Ph2PPrDI,11,12 are known to
behave in a redox non-innocent manner when coordinated to
low-valent first row metals;13 however, the N=C [1.273(10),
(
2,6-iPr2PhPDI)CoCH3, affords E-alkenyl boronate esters following
terminal alkyne hydroboration while cyclohexyl-substituted
(CyPDI)CoCH3 yields Z-alkenyl boronate esters with turnover
frequencies (TOFs) of up to 6 h-1 at 23 °C.8 Zuo and Huang
subsequently reported that in situ activation of (IPO)CoCl2 with
2 equiv. of NaEt3BH allows for alkyne dihydroboration TOFs of
up to 3 h-1 (6 h-1 based on pinacol borane, HBPin).9 Most
recently, it was found that the supporting ligand can control
selectivity for either alkyl or alkenyl boronate esters during
cobalt catalysed 1,6-enyne cyclization (TOFs up to 4 h-1).10 For
Ryan received a B.S. in chemistry from King’s
College (PA) in 2004, and a Ph.D. in inorganic
chemistry from Cornell University in 2009.
Shortly thereafter, he moved to Los Alamos
National Laboratory as a postdoctoral
research associate. Ryan joined Arizona
State University’s Department of Chemistry
& Biochemistry (now the School of Molecular
Sciences) as an assistant professor in 2012
and is currently working with his students to
develop transition metal catalysts for
energy- and sustainability-focused initiatives.
Amanda is an emerging investigator at
Colorado College.
a School of Molecular Sciences, Arizona State University, Tempe, Arizona, 85287,
USA. Email: ryan.trovitch@asu.edu; Tel: +1 480 727 8930
b Department of Chemistry and Biochemistry, Colorado College, Colorado Springs,
Colorado, 80903, USA.
† Electronic Supplementary Information (ESI) available: X-ray crystallographic
information for
2 (CCDC 1539444), computational results, experimental details,
and supporting spectroscopic data. For ESI and crystallographic data in CIF format
see DOI: 10.1039/x0xx00000x
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1.303(10), 1.306(9), and 1.287(9) Å] and C-C [1.492(10) and
1.501(11) Å] distances determined for suggest an unreduced
View Article Online
DOI: 10.1039/C7CC02281F
1
Ph2PPrDI chelate and Co(II) centre.14 To confirm this assignment,
an electron paramagnetic resonance (EPR) spectrum of this
compound was collected at 9.4 GHz and 113 K, which revealed
a broad and nearly isotropic signal extending over 90 mT,
consistent with a low-spin 59Co(II) (d7, SCo= 1/2, ICo= 7/2)
electronic struture (Figure S3 of the ESI).
Adding 2.2 equiv. of NaEt3BH to
1
2
afforded a dark green
product identified as (Ph2PPrDI)CoH (
, eqn 1). The 1H NMR
spectrum of diamagnetic
2 was found to feature left-to-right
Fig. 2 Mulliken Spin density plot of BS(1,1) solution (left) and
updated electronic structure of 2 (right).
chelate inequivalence and a doublet of doublets at -19.80
ppm, indicative of a Co-H resonance that is split by two
inequivalent phosphine environments. This was corroborated
by the 31P NMR spectrum, which features two broadened
resonances at 75.33 and 50.59 ppm. Taken together, the data
of +0.74 on the metal and an overall charge of -0.72 on the DI
backbone (Fig. 2, left). The RKS and BS(1,1) solutions
reasonably match the experimental metrical parameters
suggest that complex
geometry consisting of apical and equitorial phosphine donors.
A single crystal of was then analysed by X-ray diffraction.
2 possesses a square pyramidal
determined for
2 (Table S5, ESI), and the BS(1,1) solution was
found to be 1.2 kcal/mol lower in energy. Performing single
point UKS and BS(1,1) calculations using the solid state
structure coordinates revealed a smaller preference for BS(1,1)
of 0.6 kcal/mol. Given this slight preference, the electronic
2
The solid state structure of this complex (Fig. 1) is consistent
with its observed solution phase behaviour. A pseudo square
pyramidal geometry is evident with P(1)-Co(1)-N(1), P(1)-
Co(1)-N(2), P(1)-Co(1)-P(2), and P(1)-Co(1)-H(1) angles of
93.82(5), 115.86(5), 114.437(19), and 78.9(7)°, respectively.
The Co-H distance of 1.439(19) Å is shorter than expected
based on the sum of low-spin Co and H covalent radii (1.57
Å).15 Importantly, the N(1)-C(2) and N(2)-C(3) distances of
1.347(2) and 1.357(2) Å, respectively, are elongated relative to
unreduced DI ligand values (1.29 Å).14 Moreover, contraction
of the C(2)-C(3) distance to 1.401(3) Å is suggestive of single
electron DI reduction and a Co(II) metal centre.14
structure of
2 is consistent with a low-spin Co(II) centre that is
antiferromagnetically coupled to a DI radical anion (Fig. 2,
right). Comparable DI ligand non-innocence has been observed
for
(
2,6-iPr2PhDI)Co(η3-allyl), which is an efficient alkene
hydroboration catalyst.7f
Having characterized
2
, the hydroboration activity of this
compound was evaluated (Table S6, ESI). Adding an equimolar
quantity of 1-hexyne and HBPin to 5 mol% in benzene-d6
2
solution resulted in 81% conversion (average of 5 trials) to the
E-alkenyl boronate ester after 2 h at 25 °C. Longer reaction
times did not improve the conversion rate, suggesting that
borane consumption was limiting turnover. Therefore, the
same transformation was conducted in the presence of 1.25
equiv. of HBPin and greater than 99% conversion was
observed. To optimize the reaction conditions, the catalyst
loading was lowered to 1.0 mol%, which also allowed for
To gain additional support for this electronic structure
assignment, DFT calculations were conducted on
2. An
unrestricted Kohn-Sham (UKS) calculation converged to the
restricted Kohn-Sham (RKS) solution, which features highly
mixed molecular orbitals (e.g., the HOMO possesses 28% Co
character, Fig. S4). A broken symmetry calculation [BS(1,1)]
was then performed, revealing a low-spin Co(II) metal centre
that is antiferromagnetically coupled to a DI based electron (S
= 0.66). The spin density plot for this solution features a charge
complete 1-hexyne hydroboration after 2 h (Table 1, entry a).
Table 1. Hydroboration of alkynes using 1.0 mol% 2.
Fig. 1 Solid state structure of 2 at 30% probability ellipsoids.
Hydrogen atoms except H1 are omitted for clarity. For complete
atom listing and metrical parameters see Fig S2 and Table S2, ESI.
aIsolated yields shown in parenthesis. bConversion after 6h.
2 | J. Name., 2012, 00, 1-3
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Table 2. Hydroboration of alkynes in neat HBPin using 0.1 mol% 2.
Table 3. Dihydroboration of nitriles using 1.0 mol% 2 at 60°C.
View Article Online
DOI: 10.1039/C7CC02281F
aConversion after 1 h. bIsolated yields shown in parenthesis.
Adding 2.2 equiv of HBPin did not result in further reduction of
the alkene and attempted hydroboration of 1-hexene revealed
no conversion after 24 h.
Under these conditions, the 2-catalyzed hydroboration of
seven additional substrates was conducted. Alkynes featuring
secondary alkyl substitution, cyclopropylacetylene and
cyclohexylacetylene (Table 1, b,c), were hydroborated in 2 h at
25 °C. Likewise,
2 was found to efficiently hydroborate
phenylacetylenes (d-f) in the presence of HBPin, affording the
respective E-alkenyl boronate ester in good yield. Lower
aIsolated yields in parenthesis. bTrial conducted with 3.3 equiv. HBPin.
conversion was noted for phenyl propargyl ether (
N-propargylphthalimide ( , 70%) after 2 h; however, the latter
was fully hydroborated after 6 h. For Table 1 entries a-f, the
TOF for
-mediated alkyne hydroboration is 49 h-1.
g, 58%) and
conditions, as were ether-, dimethylamine-, and
diphenylphosphine-substituted variants (d-f), demonstrating
h
that donor groups do not inhibit
2-catalyzed nitrile
2
dihydroboration. Interestingly, when 4-acetylbenzonitrile
hydroboration was attempted using 2.2 equiv. HBPin, an
exothermic reaction ensued. After 30 min, analysis by H NMR
To further optimize this reaction, the hydroboration of
aliphatic alkynes in neat HBPin was performed using 0.1 mol%
1
2
. Starting with 1-hexyne (Table 2, a), 90% conversion was
spectroscopy revealed complete ketone reduction with partial
noted after 1 h at 25 °C, equating to an alkyne hydroboration
TOF of 900 h-1. Allowing this reaction to stir for 2 h afforded
trans-1-hexen-1-ylboronic acid pinacol ester in 93% yield
following purification by silica gel column chromatography. No
conversion was noted in the absence of catalyst (Table S6,
entry 10). Similarly, the hydroboration of 1-octyne, 5-methyl-
1-hexyne, and 4-phenyl-1-butyne afforded the respective E-
nitrile dihydroboration, indicating that
2 is likely a highly active
carbonyl hydroboration catalyst. Repeating this trial with 3.3
equiv of HBPin allowed greater than 99% conversion to the
trihydroborated product (
yield was recorded following recrystallization. For each entry in
Table 3, was found to mediate nitrile dihydroboration with a
TOF of 4 h-1.
Although metal mediated imine hydroboration has been
g). As with a-f, a modest isolated
2
alkenyl boronate ester (b-d). For each entry in Table 2,
2-
mediated alkyne hydroboration was achieved with a TOF of
fairly well-documented,16 few catalysts for nitrile
dihydroboration have been described. In 2012, Nikonov and
co-workers reported that (2,6-iPr2C6H3N)MoH(Cl)(PMe3)3
catalyses the dihydroboration of acetonitrile and benzonitrile
in the presence of catechol borane (HBCat) at 5 mol% loading
after 12 h at 22 °C.17 Subsequently, this catalyst and related
495 h-1.
Knowing that
2 is capable of hydroborating alkynes, this
catalyst was also screened for nitrile hydroboration activity.
Adding an equimolar mixture of benzonitrile and HBPin to 1.0
mol %
2
in benzene-d6 and monitoring the reaction by 1H NMR
spectroscopy revealed partial conversion to a new, non-alkene
product after 6 h at 25 °C. Additional conversion was noted
after 24 h; however, it became clear that HBPin was being
consumed prior to complete benzonitrile reduction. Repeating
this experiment with 2.2 equiv. HBPin revealed selective
conversion to the N,N-diborylated product after 24 h at 60 °C
compounds,
(2,6-iPr2C6H3N)MoH2(PMe3)3 and
(η3-2,6-
iPr2C6H3NHBCat)MoH2(PMe3)3 were found to reduce an
expanded nitrile scope with HBCat.18 A loading of 5 mol% was
also used by Szymczak and co-workers when investigating the
substrate scope and functional group tolerance of
[(BH0PI)Ru(PPh3)2][K(18-crown-6)]
catalysed
nitrile
(Table 3, entry a), and recrystallization from pentane afforded
a modest 45% yield. No conversion was noted in the absence
of catalyst.
dihydroboration at 45 °C using HBPin.19 Several variants of this
catalyst also exhibited activity for this transformation. In 2016,
Hill and co-workers achieved nitrile dihydroboration using
HBPin and a butylmagnesium β-diiminate catalyst at 60 °C.20
Despite using a 10 mol% catalyst loading, propionitrile was
converted to the N,N-diborylpropylamine after only 30 min.
After determining that
dihydroboration, six additional substrates were screened.
Unfunctionalized examples such as acetonitrile ( ) and 4-
phenylbutyronitrile ( ) were efficiently converted under these
2 is active for benzonitrile
b
c
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2016, 138, 3241. (j) N. M. Weiliange, D. S. McGinness, M. G.
Gardiner and J. Patel, Dalton Trans., 2016,
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Guo and Z. Lu, Angew. Chem. Int. Ed., 2016, 55, 10835. (l) Z.
Zuo, J. Yang and Z. Huang, Angew. Chem, Int. Ed., 2016, 55
More recently, [(p-cymene)RuCl2]2 was found to mediate the
slow dihydroboration of benzylnitrile at ambient temperature,
with complete turnover after 24 h at 60 °C.21 A broad substrate
scope was effectively reduced with HBPin over 15-36 h. It can
,
10839. (m) W.-Y. Chu, R. Gilbert-Wilson, T. B. Rauchfuss, M.
van Gastel and F. Neese, Organometallics, 2016, 35, 2900.
be said that the nitrile dihydroboration activity achieved for
2
(n) H. Zhang and Z. Lu, ACS Catal., 2016,
Manna, P. Ji, Z. Lin, F. X. Greene, A. Urban, N. C. Thacker and
W. Lin, Nature Commun., 2016, , 12610. (p) H. A. Kerchner
6, 6596. (o) K.
is largely comparable to what has been reported for other
catalysts known to mediate this transformation.
In summary, the synthesis, electronic structure, and
catalytic hydroboration activity of a DI-supported Co hydride
catalyst, (Ph2PPrDI)CoH, has been described. This complex was
found to possess a distorted square pyramidal geometry in
7
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radical anionic DI chelate that is
8
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temperature alkyne hydroboration to yield the respective E-
alkenyl boronate esters with TOFs of up 900 h-1. The first
cobalt catalysed dihydroboration of nitriles to yield N,N-
diborylamines has also been reported.22
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‡ Acknowledgement is made to Arizona State University and
Colorado College for support in the form of start-up funding. The
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