Catalytic regeneration of a Th-H bond from a Th-O bond through a mild and chemoselective carbonyl hydroboration

Article abstract of DOI:10.1039/c8cc05030a

Here we present an unprecedented chemoselective hydroboration for aldehydes and ketones catalysed by actinides. The reaction features a very low catalyst loading (0.1-0.004 mol%) and quantitative product formation in less than 15 minutes, at room temperature. Thermodynamic and kinetic studies including stoichiometric and labeling studies with deuterated pinacolborane allow us to propose a plausible mechanism for this remarkable catalytic regeneration of a Th-H bond via carbonyl hydroboration.

Full text of DOI:10.1039/c8cc05030a

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Catalytic Regeneration of a Th-H Bond from a Th-O bond Through
a Mild and Chemoselective Carbonyl Hydroboration
Tapas Ghatak, Konstantin Makarov, Natalia Fridman and Moris S. Eisen*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we present an unprecedented chemoselective coordination numbers, which habitually influence their
hydroboration for aldehydes and ketones catalysed by stoichiometric and catalytic properties. These unique
actinides. The reaction features a very low catalyst loading properties are manifested by the reaction between Cp*
2 2
AnMe
(
0.1-0.004 mol%) and quantitative product formation in less (An = Th and U) with an excess of catecholborane (HBcat)
than 15 minutes, at room temperature. Thermodynamic and yielding a 15-membered macrocyclic organoactinide complex
kinetic studies including stoichiometric and labeling studies by the rapid opening of the dioxaborolane ring (Scheme S1,
with deuterated pinacolborane allow us to propose a plausible (1)).[
20]
An identical reaction with pinacolborane (HBpin)
2 3
mechanism for this remarkable catalytic regeneration of a Th- afforded a mixture of degradation products, B Pin and Me-Bpin
(Scheme S1, (2)).[ Hence, the utilization of these actinides as
21]
H bond via the carbonyl hydroboration.
hydroboration catalyst were found inappropriate and
During the past three decades, tremendous advances have been contesting, but at the same time, made this chemistry
made regarding the use of organo-5f-elements as catalysts for a wide
extremely challenging for us.
[1]
range of organic transformations.
Regarding actinides,
The research that is presented here was carried out aiming to
converge two requirements: (1) the fundamental challenge
associated with the catalytic transformation of oxygen-
containing substrates using highly oxophillic early actinide
complexes; (2) to investigate the possibility towards the
catalytically regeneration of an actinide (An) An-H-bond from a
8] Th-O bond, using specific actinide complexes. In addition, to
[2]
[3]
[4]
hydrosilylations,
hydroaminations,
hydrothiolation,
olefin/diene polymerizations, small-molecule activations, [6] etc.,
embrace some of these advances. However, in most of these
reactions, oxygen-containing substrates are excluded due to the high
[5]
[1a, 7]
oxophilicity of the actinides.
Hence, it is clear the challenge in
the quest of actinide systems able to catalytically transform
oxygenated substrates, which includes the Tishchenko reaction,
the hydroalkoxylation/cyclization of alkynyl alcohols,
ester/epoxide ring-opening polymerizations,
[
[9]
the cyclic incorporate the process in the catalytic hydroboration of
the intermolecular aldehydes and ketones using HBpin.
[
10]
[
11], [12], [13]
addition of alcohols to carbodiimides
and various In order to demonstrate the above hypotheses, we decided to
[14]
hydroelementation process.
Amongst the hydroelementation
use in the present study the complexes that were obtained from
processes, the hydroboration of C=O bonds is particularly impressive
and it is cementing its status as one of the utmost versatile and most
the protonolysis of the thorium and uranium metallacycles
2
[
{(Me
3
Si)N}
2
An{κ -C,N-CH
2
SiMe
2
N(SiMe
3
)}] (An = Th and
U) with
[15]
employed reactions for the pharmaceutical industry. The widely
employed carbonyl hydroboration catalysts include alkaline earth
a new family of highly basic seven-membered ring, N-
heterocyclic imine ligands.[ The reason we decided to use
these type of ligands is due to their analogy to the
cyclopentadienyl ligand with a 2σ, and 4π-electron donor
nature, forming a very strong high bond order metal-nitrogen
22]
[14]
[16]
[17]
metals,
transition metals,
and group 13 metal catalysts.
.
Interestingly is the emerging utility of light alkali metals borates in
[18]
the chemoselective carbonyl hydroboration. Moreover, recently
an example using organolanthanides was reported by Marks and co-
[19]
workers. Interestingly, for actinides, this is still a fascinating and bond. These ligand properties will reduce the oxophilicity of the
an unexplored research area.
metal centre and due to the steric nature of the ligand, will also
In general, actinide elements are distinguished by their large decrease the possibility for the formation of the macrocyclic
size, high oxophilicity, and the ability to display high actinide complexes. To our delight, here we report first
examples of actinides mediated the chemoselective
hydroboration of aldehydes and ketones with a very high
efficiency. In addition, we disclose the regeneration of a high-
a.Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Technion
†Electronic Supplementary Information (ESI) available. 1569927 (3) and 1569926 (4)
For ESI and crystallographic data in CIF or other electronic format see.
DOI: 10.1039/x0xx00000x
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energy An-H bond from a very stable An-O bond using HBPin, product or just the simple 1,2 hydroboration of the alkene.
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DOI: 10.1039/C8CC05030A
opening new frontiers for actinides in their use as catalysts.
The actinide (IV) complexes
reaction between the actinide metallacycles
equivalent of 5,7-diisopropyl-5,7-dihydro-6H-dibenzo- unsaturated and the distant C=C bonds unreacted (Table 1,
d,f][1,3]diazepin-6-imine LH), at room temperature, in entries 1k). Linear and cyclic aliphatic ketones were also
Encouraged by this result, we chose carvone, a naturally
3
–
4
were synthesized by the occurring terpenoid used as a flavoring agent, reducing
or with one chemoselectively the keto functionality and leaving the α,β
1
2
[
(
toluene, respectively (Scheme 1). The reaction mixture was hydroborated efficiently (Table 1, Entry 1l-n). For the keto-
stirred for 12 h and recrystallization from concentrated toluene alcohol substrate, 1m, the reaction required a longer amount of
solutions at ─35°C afforded X-ray quality crystals of both time (1 hour) and 0.1% catalyst loading, presumably due to the
complexes.
alcohol moiety interaction with the metal complex. To study the
The molecular structures of [(L)Th{N(SiMe
3
)
2 3
} ]
(3
)
and versatility of the actinides in this process, a compound
[
3 2 3
(L)U{N(SiMe ) } ] (4) (Figure S1) were determined by X-ray containing multiple functionalities was also examined. Methyl
diffraction. The X-ray structural analysis revealed alike jasmonate, a volatile organic compound used as a drug for
structures similar to those reported for other actinide leukemia and plant defense, which exhibits 100% selectively in
imidazoline iminato complexes.[
8b,12,13]
the ketone hydroboration with complete retention of the ester
and the olefinic functionalities (Table 1, entry 1o).
The scope of the reaction was also examined for a variety of
aldehydes (Table 2). The reaction showed excellent tolerance in
the presence of electron withdrawing groups (Table 2, Entry 2b-
d), halogens (Entry 2e-h), and electron donating groups (Entry
2
i-k).
In the hydroboration of 2-ethynyl benzaldehyde, trans-
cinnamaldehyde and 4-cyanobenzaldehyde, where the
substrates comprise both alkyne/alkene/nitrile and aldehyde,
the reaction proceeded selectively at the carbonyl position
leaving the alkyne/alkene/nitrile groups unreacted, even in the
presence of an excess of pinacolborane and with a high catalyst
loading (Entry 2l-n). This selectivity was further supported by
deuterium labeling studies with deuterated pinacolborane
Scheme 1: Synthesis of actinide (IV) complexes
3 − 4
We have started our investigations by comparing the activity of
the two complexes for the hydroboration of aldehydes and
ketones. Since both complexes showed similar activities, further
studies were performed with the diamagnetic complex 3.
Due to the rapid hydroboration rates observed, we decided to scale
down the catalyst loading for the hydroboration of
dicyclohexylketone with HBpin. We started from 0.1 mol% to 0.004
mol% to obtain in all cases quantitative conversions in a maximum
time of '15 minutes'. The remarkably high TOF > 100 000 h , or 28 s
1,
is to our knowledge, the highest ever TOF observed for a
hydroboration. A TOF of 84 000 h−1 was disclosed for a Mg-
functionalized MOF.[23] Interestingly, complexes 1 and 2 were also
investigated and quantitative conversions were achieved, however,
with lower reaction rates and higher catalyst loading as compared to
complexes 3 and 4. Table 1 summarizes the wide scope range of
(DBpin).
An identical reaction that was performed reacting trans-
cinnamaldehyde with 1 equiv of deuterated pinacolborane
affords exclusive the deuterium incorporation into the carbonyl
carbon (SI, Figure S69). Aldehydes containing polyaromatic
moieties were also studied (2o-2r). Interestingly, sterically
hindered 1-pyrenecarboxaldehyde was hydroborated however,
the reaction required longer amounts of time (30 minutes) and
a higher catalyst loading (0.1 mol%, Entry 2r).
−
1
-
A significant finding in the present study is the chemoselective
reduction of the carbonyl moiety in the all-trans-retinal to the
substrates for the hydroboration of ketones. The precatalyst 3 (0.001 corresponding boronate 'ester', a useful precursor for Vitamin
mmol) retained its activity even at high substrates loading (~ 1 mmol) A (Entry 2t). To evaluate the scope on the chemoselectivity of
when performing the reaction with dicyclohexylketone.
the hydroboration reaction promoted by the actinides, a series
of competitive experiments were also performed.
In stoichiometric mixtures of dicyclohexylketone with each one of
the compounds (carbonyl and non-carbonyl substrates) presented in
Scheme 2 (0.1 mol % of 3 or 4), the dicyclohexylketone was the only
In the hydroboration of aromatic ketones, benzophenone
(
entry 1a), 4-methylbenzophenone (Entry 1b), and 4,4
dimethyl benzophenone (Entry 1c), complex displayed a very
high efficiency giving quantitatively the corresponding borate
′-
3
esters in 10 minutes with 0.02 mol% catalyst loading (TOF ~30 compound that was selectively hydroborated while other substrates
-
1
0
00 h ). The reaction with electron-rich aromatic ketones, as in remain unreacted. Moreover, both catalysts displayed a “living”
Michler’s ketone, proceeds slower and generated the full behavior retaining almost an equal catalytic activity up to three
reduction product in 15 minutes, however, with a higher consecutive runs of the substrate in the same reaction mixture.
catalyst loading (0.1 mol%). It is interesting to note that, both Interestingly, a similar chemoselectivity was reported using an alkali
halogenated and nitro moieties were fully tolerated (Table 1,
Entry 1g-i). In addition, carbonyl functionalities containing a
conjugated double bond were also found to be compatible
metal hydride catalysts were benzophenone was chosen as a model
substrate.[18]
To continue the chemoselectivity studies,
intramolecular competing hydroboration experiments were
conducted with 4-acetyl benzaldehyde bearing an aldehyde and a
(Table 1, entry 1j). This latter substrate showed a complete
selectivity for the carbonyl hydroboration over the 1,4-addition
2
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ketone functionality in the same molecule. The aldehyde stoichiometric reactions between complex 3 and varViaiebwleArtricalteioOsnlinoef
hydroboration was obtained as the major product (75%), and the HBpin, starting from 3.5 equiv to 10 equiv. DOI: 10.1039/C8CC05030A
dihydro borated product was detected as the minor (15%) product.
Table 2. Scope in the aldehyde hydroboration promoted by complex 3[a][b]
Table 1. Product scope in the hydroboration of ketone with complex 3[a][b]
[
a] Reaction conditions: 0.373 mmol of ketone, 0.448 mmol of HBpin, and 0.02
mol% of catalyst in 550 μL C H
6
D
6
at 25 oC. The reactions are monitored by 1
NMR. [b] In all cases >99% NMR yields were obtained in 15 minutes. [c]
Required 0.1 mol% catalyst, 1 h. [d] 0.004 mol% catalyst.
[a] Reaction conditions: 0.373 mmol of aldehyde, 0.448 mmol of HBpin, 0.02
mmol of C
6
Me
6
(internal standard) and 0.02 mol% of catalyst in 550 μL C
6
D
6
o
1
at 25 C. The reactions are monitored by H NMR. [b] In all cases >99% yields
were obtained in 15 minutes. [c] Required 0.1 mol% catalyst, 30 min.
In all cases, the precatalyst reacts with HBpin replacing all the
coordinated amido groups to form one equivalent of (Me
3 2
Si) N-Bpin,
two equivalents of (Me Si) NH and likely the evasive transient
3
2
thorium monohydride species [A][24] (Scheme 4). With no catalyst,
there is no reaction. (ii) In-situ poisoning experiments were
performed between complex 3, 3.5 equiv of HBpin and variable ratios
of methanol (0.25, 0.5 and 1 equiv with respect to 3). A decrease in
activity of 25% and 50% and 100% were observed for the addition of
Scheme 2: Chemoselective stoichiometric hydroboration competition of dicyclohexyl
ketone and the compounds marked unreacted.
0
.25, 0.5 and 1.0 equiv of methanol, respectively. This result
Interestingly was the competing intermolecular hydroboration
between either acetophenone (not shown in the Scheme) or the
more reactive 4-nitro-acetophenone and benzaldehyde (Scheme 3
above). The aldehyde hydroborated was afforded as major product
whereas the ketone hydroborated product was obtained only in
trace amounts. For actinides, quantitative and exclusive aldehyde
hydroboration products is observed, however and for comparison,
corroborates with the formation of a monohydride complex, as the
plausible active intermediate.
Kinetic measurements indicate that the empirical rate law, observed
for the hydroboration of benzophenone catalysed by complex 3
exhibits a first-order dependence on the catalyst, HBpin and ketone
concentration (Eq. 1).
1
1
1
δp⁄δt= kobs× [
3
] [HBpin] [ketone]
(1)
when using
a lanthanide catalyst, acetophenone is partially
hydroborated (yield 9%).[19]
Activation parameters for the hydroboration of benzophenone were
To gain more detailed insights into the catalytic cycle and precatalyst
activation pathways, we carried out few controlled experiments: (i)
‡
‡
determined from the Eyring and Arrhenius plots with ΔS , ΔH , and
values of −25.4 (0.8) e.u., 13.7 (0.7) and 14.3 (0.7) kcal/mol,
E
a
respectively. Deuterium isotope studies show that the reaction
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(c) B. M. Gardner, J. C. Stewart, A. L. Davis, J. MVicewMAartsictleerO,nWline.
exhibits a KIE (kH/kD) of 2.51 (0.07), indicating that the hydride
transfer is the turnover-limiting step.
Lewis, A. J. Blake, S. T. Liddle, Proc. Nat. Acad. Sci., 2012, 109
,
DOI: 10.1039/C8CC05030A
9
2
265-9270; (d) T. L. Lohr, Z. Li, T. J. Marks, Acc. Chem., Res.,
016, 49, 824-834.
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3
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724-4741.
B. D. Stubbert, T. J. Marks, J. Am. Chem. Soc., 2007, 129, 6149-
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069−9078; (b) C. E. Hayes, D. B. Leznoff, Organometallics,
Scheme 3: (bottom) Competitive aldehyde/ketone hydroboration study: with 4-acetyl
benzaldehyde; (top) hydroboration study with benzaldehyde and 4-nitro acetophenone
mediated by complexes 3 or 4.
2
010, 29, 767−774; (c) S. M. Mansell, F. Bonnet, M. Visseaux,
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(a) V. Mougel, C. Camp, J. Pécaut, C. Copéret, L. Maron, C. E.
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2280−12284; (b) P. L. Arnold, S. M. Mansell, L. Maron, D.
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7
8
(
b) A. R. Fox, S. C. Bart, K. Meyer, C. C. Cummins, Nature, 2008,
Scheme 4: Stoichiometric reaction between complex
3
and an excess of HBpin.
455, 341−349; (c) J. C. Berthet, M. Ephritikhine, Coord. Chem.
Rev., 1998, 178−180, 83−116.
(a) T. Andrea, E. Barnea, M. S. Eisen, J. Am. Chem. Soc., 2008,
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30, 2454−2455; (b) I. S. R. Karmel, N. Fridman, M. Tamm, M.
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S. D. Wobser, T. J. Marks, Organometallics, 2013, 32,
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013, 52, 9077−9086.
(
2
1
1
1 R. J. Batrice, C. E. Kefalidis, L. Maron, M. S. Eisen, J. Am. Chem.
Soc., 2016, 138, 2114−2117.
2 (a) H. Liu, M. Khononov, N. Fridman, M. Tamm, M. S. Eisen,
Inorg. Chem., 2017, 56, 3153−3157; (b) T. Ghatak, N. Fridman,
M. S. Eisen, Organometallics, 2017, 36, 1296−1302.
Scheme 5: Proposed catalytic cycle for the actinide-catalysed hydroboration reaction.
A plausible mechanism is presented in Scheme 5.[25] It is crucial to
indicate that the catalytic regeneration of the Th-H bond, with a 13 H. Liu, N. Fridman, M. Tamm, M. S. Eisen, Organometallics,
2
017, 36, 3896-3903.
borane, without forming a macrocycle, is achieved by the high bond
order between the Th-N bond, and the large exothermicity (ΔHcalc.
25.0 kcal/mol) for the Th-H and B-O bond formations (Eq. 2). [26]
1
4 (a) G. Zhang, H. Zeng, J. Wu, Z. Yin, S. Zheng, J. C. Fettinger,
=
Angew. Chem. Int. Ed., 2016, 55, 14369-14372; (b) C. C. Chong,
−
R. Kinjo, ACS Catal., 2015, 5, 3238-3259.
1
1
1
1
1
2
2
2
2
2
2
2
5 M. Arrowsmith, T. J. Hadlington, M. S. Hill, G. Kociok-Kohn,
Chem. Commun., 2012, 48, 4567-4569.
6 C. Gunanathan, M. Holscher, F. Pan, W. Leitner, J. Am. Chem.
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7 Z. Yang, M. Zhong, X. Ma, K. Nijesh, S. De, P. Parameswaran,
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8 D. Mukherjee, H. Osseili, T. P. Spaniol, J. Okuda, J. Am. Chem.
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9 V. L. Weidner, C. J. Barger, M. Delferro, T. L. Lohr, T. J. Marks,
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0 T. A. Eyal Barnea, Moshe Kapon, and Moris S. Eisen, J. Am.
Chem. Soc., 2004, 126, 5066-5067.
In summary, here we present the first example for actinide-catalysed
chemoselective hydroboration of aldehydes and ketones. Aldehydes
are selectively hydroborated over ketones, and the actinide
complexes exhibited an unusual tolerance towards a large variety of
functional groups. The catalytic regeneration of the Th-H bond opens
new possibilities for actinides in catalysis.
1 R. K. Das, E. Barnea, T. Andrea, M. Kapon, N. Fridman, M.
Botoshansky, M. S. Eisen, Organometallics, 2015, 34, 742-752.
2 T. Ghatak, S. Drucker, N. Fridman, M. S. Eisen, Dalton Trans.,
2017, 47, 12005-12009.
Acknowledgments
This work was supported by the Israel Science Foundation administered
by the Israel Academy of Science and Humanities under Contract No.
3 K. Manna, P. Ji, F. X. Greene, W. Lin, J. Am. Chem. Soc., 2016,
1
38, 7488-7491.
7
8/14, and by the PAZY Foundation Fund (2015) administered by the
4 R. R. Langeslay, M. E. Fieser, J. W. Ziller, F. Furche, W. J. Evans,
J. Am. Chem. Soc., 2016, 138, 4036-4045.
Israel Atomic Energy Commission.
5 Detailed analysis of kinetic rate law can be found in Supporting
Information.
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Organoactinides mediated chemoselective hydroboration of aldehyde and ketones. The catalytic regeneration of a Th-H
bond from a Th-OR bond was achieved via the hydroboration with HBpin.

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