Actinide-Catalyzed Intermolecular Addition of Alcohols to Carbodiimides

Article abstract of DOI:10.1021/jacs.5b12731

The unprecedented actinide-catalyzed addition of alcohols to carbodiimides is presented. This represents a rare example of thorium-catalyzed transformations of an alcoholic substrate and the first example of uranium complexes showing catalytic reactivity with alcohols. Using the uranium and thorium amides U[N(SiMe₃)₂]₃ and [(Me₃Si)₂N]₂An[κ²-(N,C)-CH₂Si(CH₃)₂N(SiMe₃)] (An = Th or U), alcohol additions to unsaturated carbon-nitrogen bonds are achieved in short reaction times with excellent selectivities and high to excellent yields. Computational studies, supported by experimental thermodynamic data, suggest plausible models of the profile of the reaction which allow the system to overcome the high barrier of scission of the actinide-oxygen bond. Accompanied by experimentally determined kinetic parameters, a plausible mechanism is proposed for the catalytic cycle.

Full text of DOI:10.1021/jacs.5b12731

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Communication
Actinide-Catalyzed Intermolecular Addition of Alcohols to Carbodiimides
Rami J. Batrice, Christos E. Kefalidis, Laurent Maron, and Moris S. Eisen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b12731 • Publication Date (Web): 04 Feb 2016
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Actinide-Catalyzed Intermolecular Addition of Alcohols to Car-
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†
‡
‡
†,*
Rami J. Batrice, Christos E. Kefalidis, Laurent Maron, and Moris S. Eisen
†
Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Technion City, 3200008, Israel.
‡
LPCNO, CNRS, & INSA, Université Paul Sabatier, 135 Avenue de Rangueil, Toulouse, 31077, France.
KEYWORDS Actinide Catalysis ∙ Alcohol ∙ Addition ∙ Carbodiimide ∙ Insertion
ABSTRACT: The unprecedented actinide-catalyzed addition of alcohols to carbodiimides is presented. This represents a
rare example of thorium-catalyzed transformations of an alcoholic substrate, and the first example of uranium complexes
showing catalytic reactivity with alcohols. Using the uranium and thorium-amides U[N(SiMe
3
)
2
]
3
and
2
[
(Me
3
2
Si) N]
2
An[κ -(N,C)-CH
2
Si(CH
3
)
2
N(SiMe )] (An = Th or U), alcohol additions to unsaturated carbon-nitrogen bonds are
3
achieved in short reaction times with excellent selectivities and high to excellent yields. Computational studies, supported
by experimental thermodynamic data, suggest plausible models of the profile of the reaction which allow the system to
overcome the high barrier of scission of the actinide-oxygen bond. Accompanied by experimentally determined kinetic
parameters, a plausible mechanism is proposed for the catalytic cycle.
The chemistry of the actinides has drawn considerable at-
"
N
N"
Si
tention in recent years and given rise to impressive structures
U
1-13
and reactivities. As the development of organoactinides to
these ends has advanced, so too have their application in de-
manding stoichiometric and catalytic chemical transfor-
N"
"N An N
N"
SiMe3
(1)
1
4-19
20-22
N" = N(SiMe3)2 An = U (2), Th (3)
mations such as hydroaminations,
hydrosilylations,
oli-
23-25
26
gomerizations,
polymerizations of dienes, esters, and
and a host of small-molecule activations.
27-32
33-38
Figure 1. Amido-actinide complexes used in the catalytic addition
of alcohols to carbodiimides.
epoxides,
However, some of the most challenging transformations for
actinides are those involving oxygenated substrates. To date, a
very limited number of works exist which present the catalytic
reaction of oxygen-containing substrates, the first of which be-
ing the demonstration of a thorium-mediated catalytic Tish-
The catalytic investigation of these complexes have allowed for
a systematic study of the difference between a U and U
precatalysts, and the reactivity difference between analogous
III
IV
3
9
chenko reaction, with another recent example of (imidaz-
olin-2-iminanto)thorium catalyzed aldehyde dimerization,40
yet the most impressive of such reactions since has been an
IV
IV
U
and Th complexes. We predicted that opening the metal
coordination sphere around these actinide centers would aid
in facilitating challenging chemical transformations; these ex-
pectations were based on previous research which has shown
that increasing coordinative unsaturation yields higher reac-
41
intramolecular hydroalkoxylation, which was long thought to
be inaccessible due to the formation of an intractable actinide-
4
2
oxo species. This work challenged the conventional wisdom
of the rules of actinide catalysis, however it is of note that even
the remarkable intramolecular hydroalkoxylation was con-
fined to a limited substrate scope and only for organothorium
complexes.
46,47
tivity and selectivity for actinide-metallocene complexes.
The success in performing the aforementioned reactions us-
ing the actinide complexes (1-3) raised a significant conceptual
question as to whether actinide complexes possess the ability
to perform the catalytic transformation of alcohols. This is a
particularly challenging notion when considering the signifi-
cant bond strengths of the actinide-oxygen bonds and the dif-
ficulty in cleaving such species (Th-O = 208.0, U-O = 181.0 kcal
mol ). The most sensible approach to achieve this was deter-
mined to be the insertion of carbodiimides into various alco-
hols according to the following reaction (Equation 1); this pro-
cess has been shown to be efficiently catalyzed by transition
Most recently, we have reported interesting actinide-cata-
lyzed reactions mediated by actinide-amides, including termi-
43
nal alkyne cyclotrimerizations, and the addition of various E-
H nucleophiles into heterocumulenes.44,45 These studies re-
vealed distinctive modes of bonding for actinide chemistry
which facilitate the observed reactivity. The potential of this
catalysis is exemplified in the observed reactivity of actinide-
amide complexes (1-3) (Figure 1) as adept catalysts for de-
manding chemical transformations.
-
1 48
4
9-53
metal complexes, most notably those of copper,
however
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Page 2 of 5
no such reaction has been seen to be mediated by group 4 or
early actinide systems.
1,3-di-p-tolylcarbodiimide and tert-butanol occurred without
the need of catalyst.
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OR
A clear trend in the activity of insertion is observed when
comparing the nucleophilicity of the carbodiimide used, evi-
dent by the decreased reactivity of the more nucleophilic DIC.
Conversely, 1,3-di-p-tolylcarbodiimide, a more electrophilic
substrate, reacts much more rapidly, suggesting of a weaker,
more labile An-N bond formed after insertion. In addition, the
acidity and steric encumbrance of the alcohol used is a crucial
factor in determining the efficiency of the catalytic system. For
example, the percent conversion is seen to markedly drop us-
ing the thorium complex (3) when increasing the steric en-
cumbrance from methanol to iso-propanol (Table 1, entries 2,
8, and 14), with no catalytic turnover observed for tert-butanol.
In the reaction of DTC, the thorium complex shows the best
catalytic activity in the addition of aliphatic alcohols (Table 1,
entries 5, 11, 17, and 21), however in the reaction with phenol,
the uranium complexes (1 and 2) proved superior. These re-
sults illustrate the delicate interplay between the electronic
nature of the metal, the coordination sphere created around
the catalyst, and the stereoelectronic nature of the substrate of
the reaction.
N
cat. (1-3)
C D , 75 °C
6 6
H OR
+
R'
C
R'
R'
R' (1)
N
N
N
H
To our pleasure, it was seen that the addition of several alco-
hols into diisopropylcarbodiimide (DIC) and 1,3-di-p-tolylcar-
bodiimide (DTC) was efficiently catalyzed by complexes 1-3 in
benzene-d to generate the corresponding isoureas (Table 1).
6
While organocopper reagents are most widely used to form
isoureas, the investigation of this product class remains lim-
ited, surprising especially considering the utility of isoureas as
valuable biologically active molecules and versatile synthons
0
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4-56
for combinatorial total synthesis.
Table 1. Product scope of actinide-mediated intermolecular alcohol addition
a
to carbodiimides.
Entry
Catalyst
ROH
R’N=C=NR’
Time (h)
Conv (%)b
42
iPr
1
2
3
4
5
6
7
8
9
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
2
1
2
3
1
2
3
24
24
3
iPr
85
MeOH
p-tolyl
p-tolyl
>99
>99
>99
83
3
This transformation, occurring outside of the known capa-
bilities of actinide catalysis, stands in need of thorough mech-
anistic investigation. Knowing that some Brønsted acids are
capable of mediating insertions into carbodiimides, it was first
necessary to confirm that no such reaction was taking place;
performing this reaction in the presence a proton trap
(2,6-di-tert-butyl-4-methylpyridine) did not show any inhibi-
tion of the product formation. With Brønsted acid-mediated
insertion excluded, attention was diverted to experimentally
deriving the active catalytic cycle. Kinetic studies revealed
first-order behavior in catalyst and carbodiimide, with inverse
first-order kinetics in alcohol. The latter of these findings
demonstrates a rapid pre-equilibrium of alcohol coordination,
wherein the larger excess of alcohol saturates the metal center
and excludes nitrogen coordination by the carbodiimide sub-
strate, inhibiting the catalytic turnover.
p-tolyl
3
iPr
12
24
12
3
iPr
76
iPr
83
EtOH
p-tolyl
p-tolyl
p-tolyl
iPr
90
10
3
93
11
3
93
12
13
14
15
24
24
24
6
28
iPr
83
iPr
77
iPrOH
p-tolyl
p-tolyl
p-tolyl
iPr
83
16
3
93
17
18
19
3
93
Kinetic isotope studies using deuterated ethanol and tert-
butanol showed KIE values of 0.99 and 0.98, respectively,
showing that protonolysis by alcohol is not turnover limiting.
These data supply us with the kinetic rate law presented in
Equation 2 (see Scheme 1 for variables in equation):
24
12
12
12
6
17
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
88
tBuOH
20
97
21
22
23
88
휕푃
푘1[퐷퐼퐶][ퟏ]
95
=
휕푇 ꢀ + 퐾 [푅푂퐻]
(2)
PhOH[c]
푒푞
3
94
24
9
77
(
a thorough analysis of the kinetic equation can be found in
a
b
Reaction conditions: ~1.5 μmol catalyst (1 mol%), 600 μL C
6
D
6
, 75 °C. De-
the Supporting Information).
1
termined by H NMR of the crude reaction mixture. [c] Reaction of phenol
with DIC was the only reaction shown to proceed without the need of catalyst.
Thermodynamic parameters were experimentally calculated
from Eyring analysis using each of the three precatalyst com-
plexes. As a model system, the Eyring plot of the addition of
tert-butanol to DIC mediated by uranium complex 2 was in-
vestigated (see Supporting Information), revealing a moderate
Performing the reactions in THF-d and methylcyclohexane-d14
8
revealed inhibited catalytic activity, or complete deactivation
of the catalytic cycle, respectively. Blank reactions showed that
of the substrate combinations attempted, only the reaction of
‡
-1
enthalpic activation barrier (ΔH = 12.1 kcal mol ), and a highly
‡
negative entropy of activation (ΔS = -39.0 e.u.), the latter of
2
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nium(IV). In the uranium(IV)-mediated addition of tert-buta-
which is evocative of a highly ordered transition state, suggest-
1
2
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5
6
7
8
9
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
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4
4
5
5
5
5
5
5
5
5
5
5
6
-1
ing that a migratory insertion is likely in agreement with the
characteristic catalytic behavior of the actinides. These kinetic
and thermodynamic parameters provide sufficient data for the
proposal of a plausible catalytic mechanism. (Scheme 1).
nol (Figure 2), the disruption of the dimer costs 8.0 kcal mol
while the subsequent coordination of DIC is found to be exo-
-1
thermic, liberating -3.9 kcal mol (making an overall step
-1
slightly endothermic by 4.1 kcal mol ); the migratory insertion
U[N(SiMe3)2]3
H
is calculated as the rate-determining step of the reaction, re-
1
(tBuO)n [U]
t
-1
(O Bu)3
quiring 12.7 kcal mol from the DIC adduct and in excellent
t
3
HO Bu
u
t B
agreement with the experimentally found 12.1 kcal/mol. Inter-
estingly, in the case of uranium(III), the disruption of the di-
3
HN(SiMe3)2
O
u
H
t B
n
O
H
N iPr
+
H
-
n
-1
N
N
mer costs 27.6 kcal mol , making the overall reaction difficult
t
iPr
iPr
iPr
t
U
(O Bu)3
C
O Bu
N
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Product
CatA
in line with the experiment. The complete catalytic cycle is ac-
-
1
companied by a release of 18.0 kcal mol , and reveals the syn-
ergistic effects between nitrogen and oxygen coordination to
the actinide center making this reaction possible.
iPr
N
iPr
N
iPr
C
iPr
[U] O Bu
Int-1A
HN
C
N
t
t
O Bu
tBuO [U]
Int-4A
iPr
‡
iPr
‡
N i
Pr
C
turnover
limiting
N i
Pr
C
N
N
H
t
t
O Bu
[U] O Bu
TS1A-2A
tBuO [U]
TS3A-4A
iPr
N
iPr
iPr
N
iPr
N
C
N
C
tBuO [U]
H
O Bu
t
t
[U]
Int-2A
O Bu
t
Int-3A
t
[U] = U(O Bu)2
Figure 2. Plausible reaction profile of BuOH addition to DIC me-
t
H
O Bu
diated by complex 1. Values in parenthesis refer to complex 2.
Scheme 1. Proposed reaction mechanism of actinide-mediated
intermolecular alcohol addition to carbodiimides.
These results demonstrate an alternative approach to the
preparation of new isourea products, but more importantly
presents a novel reactivity for actinide-mediated catalysis. The
intermolecular addition of an alcohol substrate has only a sin-
gle predecessor for thorium catalysis which was limited to in-
tramolecular reactions, however no example prior to this work
exists for uranium-catalyzed conversion of alcohols. The find-
ings herein reopen a long avoided field of actinide catalysis and
provide a useful tool in the development of catalytic strategies
to facilitate challenging chemical transformations comple-
mentary to classical organic and/or late-transition metal
chemistry.
The activation of precatalyst 1 is achieved by protonolysis of
the amides by alcohol substrate to yield the uranium-alkoxide
1
(
Cat
A
); while the exact identity of this species is unknown, H
NMR analysis revealed complete displacement of the amides,
indicated by the loss of alcohol proton signals and appearance
of the N-H signal of HN(SiMe ) . The formation of CatA has
3
2
also been verified using theoretical approaches (see ESI) and
the sequential protonolysis of the amides are computed to be
both thermodynamically and kinetically favorable. When the
metallacycles (2 and 3) are used, the first equivalent of alcohol
cleaves the An-C bond, followed by displacement of the re-
maining amides. The catalytic cycle for each of the three com-
plexes is then proposed to follow a similar mechanism wherein
the active actinide-alkoxide complex undergoes coordination
by the nitrogen atom of the carbodiimide (Int-1A), followed by
migratory insertion of the C=N bond to form actinide-isou-
reate as the turnover limiting step (Int-2A). Coordination by
an additional equivalent of alcohol (Int-3A) followed by proto-
ASSOCIATED CONTENT
Supporting Information. Experimental details, characterization
data, reaction order plots, Eyring plots, discussion of kinetic rate
law, and DFT computation details. This material is available free
of charge via the Internet at http://pubs.acs.org.
nolysis regenerates Cat and liberates the desired isourea
A
product. The lack of any dimerization or amidine product fur-
ther supports this mechanism and excludes the likelihood of a
Lewis acid mediated process.
AUTHOR INFORMATION
Corresponding Author
*
chmoris@tx.technion.ac.il
In order to further support both the observed reactivity and
proposed mechanism, DFT calculations were carried out. The
reaction profiles were determined in silico for each catalyst for
the reaction of DIC with either tert-butanol or ethanol. The
nature of the resting state of Cat was first investigated, and
the formation of dimers are found to be thermodynamically
Author Contributions
All authors have given approval to the final version of the manu-
script.
A
Notes
favored over the monomers for both uranium(III) and ura-
The authors declare no competing financial interest.
3
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(
27)Karmel, I. S. R.; Botoshansky, M.; Tamm, M.; Eisen, M. S.
ACKNOWLEDGMENT
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Inorg. Chem. 2014, 53, 694.
(28)Hayes, C. E.; Sarazin, Y.; Katz, M. J.; Carpentier, J. F.; Leznoff,
D. B. Organometallics 2013, 32, 1183.
This work was supported by the Israel Science Foundation admin-
istered by the Israel Academy of Science and Humanities under
Contract No. 78/14; and by the PAZY Foundation Fund (2015) ad-
ministered by the Israel Atomic Energy Commission.
(
29)Barnea, E.; Moradove, D.; Berthet, J. C.; Ephritikhine, M.;
Eisen, M. S. Organometallics 2006, 25, 320.
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(31)Walshe, A.; Fang, J.; Maron, L.; Baker, R. J. Inorg. Chem. 2013,
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(
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