Actinide-Mediated Catalytic Addition of E-H Bonds (E=N, P, S) to Carbodiimides, Isocyanates, and Isothiocyanates

Article abstract of DOI:10.1002/anie.201502041

Unprecedented catalytic reactivity of actinide coordination complexes toward heterocumulenes, such as carbodiimides, isocyanates, and isothiocyanates is reported. The mono(imidazolin-2-iminato) thorium(IV) complex [Th(Im^DippN){N(SiMe₃)₂}₃] (1) was applied as a precatalyst for the addition of E-H (E=N, P, S) bonds to the Y≡C≡X core (Y=R₂N; X=NR₂, O, S) of carbodiimides, isocyanates, and isothiocyanates. The respective insertion products were obtained in high yields under mild reaction conditions, with complex 1 displaying high tolerance toward functional groups and heteroatoms. New reactivity: The mono(imidazolin-2-iminato) thorium complex [Th(Im^DippN){N(SiMe₃)₂}₃] was successfully applied as an active catalyst for the addition of E-H (E=N, P, S) bonds across the central Y≡N≡C≡X linkage of carbodiimides, isocyanates and isothiocyanates, yielding the respective insertion products in high yields and under mild reaction conditions.

Full text of DOI:10.1002/anie.201502041

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201502041
Actinide Catalysis
German Edition:
DOI: 10.1002/ange.201502041
À
Actinide-Mediated Catalytic Addition of E H Bonds (E = N, P, S) to
Carbodiimides, Isocyanates, and Isothiocyanates**
Isabell S. R. Karmel, Matthias Tamm,* and Moris S. Eisen*
Abstract: Unprecedented catalytic reactivity of actinide coor- functional groups and heteroatoms. Previous studies by Evans
dination complexes toward heterocumulenes, such as carbo- et al. have shown that carbodiimides can effectively insert
À
diimides, isocyanates, and isothiocyanates is reported. The into the An C (An = Th, U) bond of Cp*2An(CH3)2, yielding
mono(imidazolin-2-iminato) thorium(IV) complex [Th- mixed cyclopentadienyl actinide amidinate complexes in high
(ImDippN){N(SiMe3)2}3] (1) was applied as a precatalyst for yield.[16] Similarly, the synthesis of actinide complexes with
the addition of E H (E = N, P, S) bonds to the Y C X core various phosphido,[17] sulfido[18] and dithiolene[19] ligands was
(Y= R2N; X = NR2, O, S) of carbodiimides, isocyanates, and reported, and the reactivity of selected examples toward
isothiocyanates. The respective insertion products were stoichiometric amounts of carbon dioxide and carbon disul-
obtained in high yields under mild reaction conditions, with fide was studied.[20] It should be noted, however, that these
complex 1 displaying high tolerance toward functional groups actinide species have not been investigated as part of
À
= =
and heteroatoms.
a catalytic process.
We have recently shown that the mono(imidazolin-2-
M
etal-mediated catalytic hydroelementation reactions of iminato) thorium(IV) complex [Th(ImDippN){N(SiMe3)2}3]
C C multiple bonds, which are known as the addition of an (1), which can be obtained selectively in high yield by
À
À
À
E H (E = N, S, P, Si, O) bond across an unsaturated C C
a protonolysis reaction of the thorium metallacycle
bond, have gained popularity in the scientific community over [{(Me3Si)N}2Th{k2-C,N-CH2SiMe2N(SiMe3)}] with the neutral
the past decade, because they represent an atom-economical imidazolin-2-imine ligand ImDippNH [Eq. (1)], can be used as
route for the synthesis of various families of organic an active catalyst for the dimerization of aldehydes, displaying
molecules.[1] Hydroelementation reactions comprising the high catalytic activity and selectivity toward the formation of
hydroamination,[2,3] hydrosilylation,[4,5,2f] hydrophosphina- asymmetrically substituted esters.[21] Thus, we decided to
tion,[6,7] hydroalkoxylation,[8] and hydrothiolation[9,2f, 8b] of investigate the substantial question whether the stoichiomet-
alkenes and alkynes have been studied with a wide range of ric reactivity toward carbodiimides displayed by organome-
transition metal, lanthanide, and actinide catalysts, in which tallic actinide complexes[16,20] can be incorporated into
À
actinide coordination complexes often displayed a comple- a catalytic cycle by the protonolytic cleavage of the An N
mentary reactivity to their transition-metal and lanthanide bond, formed by the insertion of the carbodiimide, with
congeners.[10] In addition to hydroelementation reactions, different families of R E H (E = N, P, S; R = alkyl, aryl, H)
À À
lanthanide complexes have been applied as catalysts for the moieties, yielding actinide amido, phosphido, and sulfide
À
addition of E H bonds (E = N, P) to carbodiimides, isocya- intermediates. Therefore, we set out to study the catalytic
nates, and isothiocyanates, yielding the respective guanidine, activity of actinide coordination complexes toward different
phosphaguanidine, and thiourea derivatives, respectively,[11] families of heterocumulene systems and REH moieties, in
which have found wide application as ligands for coordination which E is a nucleophilic element, expanding the scope of
compounds,[12] in the field of medicinal[13] and materials accessible products. Owing to the high activity, selectivity, and
chemistry,[14] as well as synthons in organic chemistry.[15] tolerance toward functional groups displayed by 1 in the
Hence, developing atom-efficient, versatile catalytic systems Tishchenko reaction,[21] the thorium compound 1 was applied
for their preparation represents a major challenge in the field for all further catalytic studies with heterocumulenes pre-
of homogeneous catalysis, especially because the respective sented herein. In general, imidazolin-2-iminato ligands are
metal catalyst is required to display a tolerance toward considered as monodentate N-donor ligands, which can act as
2s,4p-electron donors to a metal atom,[22] especially when
[*] I. S. R. Karmel, M. S. Eisen
Schulich Faculty of Chemistry
coordinated to early transition metals in high oxidation
states[23] and lanthanides,[24] thus forming strong M N bonds
À
Technion—Israel Institute of Technology
Technion City, 32000 (Israel)
E-mail: chmoris@tx.technion.ac.il
with a potentially higher bond order. Due to the high
nucleophilicity and strong basicity of imidazolin-2-iminato
ligands, the electron density of highly electrophilic metal
centers, such as actinides and lanthanides, is slightly
increased, which in turn reduces their electrophilicity and
often enhances their catalytic activity toward highly nucleo-
philic substrates.[25] Thus, on the one hand, the open coordi-
nation sphere of the mono(imidazolin-2-iminato) thori-
um(IV) complex 1 should allow for a rapid insertion of the
M. Tamm
Institut für Anorganische und Analytische Chemie
Technische Universität Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
[**] This work was supported by the German Israel Foundation GIF
under Contract I-1264-302.5/2014.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502041.
À
heterocumulenes into the Th Namido bond. On the other hand,
12422
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Chemie
À
Table 1: Catalytic reactions of carbodiimides with E H moieties medi-
the increased electron density of 1 should lead to an increased
catalytic activity in the presence of amines, phosphines, and
thiols, as the formation of thermodynamically stable, catalyti-
ated by complex 1.[a]
Entry
RNCNR
HER1R2/HER3
Yield [%]
À
(7, 8, and 10)
cally inactive An E species is avoided.
1
2
3
4
5
6
7
8
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
iPrNCNiPr
(o-tol)NCN(o-tol)
(o-tol)NCN(o-tol)
(o-tol)NCN(o-tol)
(o-tol)NCN(o-tol)
PhNH2
92
90
54
90
79
97
99
88
96
95
97
95
o-OMe-C6H4NH2
o-Me-C6H4NH2
p-Cl-C6H4NH2
HN(C2H5)2
iPrNH2
HPPh2
HSCH2Ph
PhNH2
To study the reactivity of [Th(ImDippN){N(SiMe3)2}3] (1)
toward heterocumulenes, experiments with stoichiometric
amounts of carbodiimide, phenyl isocyanate, and phenyl
isothiocyanate were carried out, and the reactions were
monitored by 1H NMR spectroscopy, showing for all cases an
insertion of two equivalents of the heterocumulene into two
9
10
11
12
HN(C2H5)2
HPPh2
HSCH2Ph
[a] Reaction conditions: 4.48 mmol of catalyst 1; cat./RNCNR: 1/50; cat./
HER1R2: 1/50; 750 mL C6D6; 808C; yield was determined by 1H NMR
spectroscopy of the crude reaction mixture after 12 h.
À
Th Namido bonds of complex 1. After the addition of two
equivalents of the REH moieties, the respective insertion
(Scheme 2). The reaction mixture was then heated to
808C and monitored by 1H NMR spectroscopy until
full conversion of the starting materials was
observed (Tables 1 and 2). The reaction of primary
and secondary amines with carbodiimides selectively
afforded the guanidines 7 in moderate to high yields,
which underwent a 1,3-hydride shift when using
primary amines to yield the symmetric guanidines 8
(Table 1). Similarly, the addition of phosphines and
thiols to the NCN moiety of carbodiimides was
catalyzed by thorium complex 1, yielding phospha-
guanidines and thioureas 7 and 10, respectively
(Table 1). Furthermore, the addition of amines,
phosphines, and thiols to isocyanates and isothio-
cyanates was studied, affording the insertion prod-
ucts 9 and 11 in high yields (Table 2). Hence, the
mono(imidazolin-2-iminato) thorium(IV) complex
1 displays an unusual tolerance toward several heteroatoms,
such as oxygen, phosphorus, sulfur, and nitrogen, as well as
toward several functional groups, for example, OMe and Cl,
allowing for a large scope of products to be accessed.
To determine the percentage of active precatalyst in the
catalytic transformations, poisoning experiments were per-
Scheme 1. Reactions of [Th(ImDippN){N(SiMe3)2}3] (1) with stoichiometric
amounts of heterocumulenes.
products 2 and 5 were obtained (Scheme 1), as well as the
thorium amide, phosphide, and sulfide species 4. Catalytic
experiments were performed using 1 mol% of [Th-
(ImDippN){N(SiMe3)2}3] (1) and equimolar amounts of the
heterocumulenes and inserting-moieties REH, which were
added to a stock solution of the catalyst inside a glove box
À
Table 2: Catalytic reactions of isocyanates and thioisocyanates with E H
moieties mediated by complex 1.[a]
Entry
PhNCX
HER1R2/HER3
Yield [%]
(9 and 11)
1
2
3
4
5
6
7
8
PhNCO
PhNCO
PhNCO
PhNCO
PhNCS
PhNCS
PhNCS
PhNCS
PhNH2
HPPh2
HSCH2Ph
HN(C2H5)2
PhNH2
HPPh2
HSCH2Ph
HN(C2H5)2
76
78
89
83
82
78
88
87
[a] Reaction conditions: 4.48 mmol of catalyst 1; cat./PhNCX: 1/50; cat./
HER1R2: 1/50; 750 mL C6D6; 808C; yield was determined by 1H NMR
spectroscopy of the crude reaction mixture after 12 h.
À
Scheme 2. Catalytic additions of E H moieties to carbodiimides,
isocyanates, and isothiocyanates mediated by complex 1.
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Angewandte
Communications
afforded
a
kinetic isotopic
effect of KH/KD = 1.98, corrob-
orating that the rate determin-
ing step (r.d.s.) of the mecha-
nism (Scheme 3) is the proto-
nolysis of the phosphaguani-
dine.
@p
@t
1
1
¼ kobs Á ½iPrNCNiPrŠ Á ½HPPh2Š Á
½ThðImDippNÞðNðSiðCH3Þ3Þ2Þ3Š
1
ð4Þ
À
Scheme 3. Proposed mechanism for the thorium-mediated addition of E H moieties to carbodiimides.
@p
@t
1
1
¼ kobs Á ½PhNCOŠ Á ½HPPh2Š Á
½ThðImDippNÞðNðSiðCH3Þ3Þ2Þ3Š
1
formed with isopropanol. When the reactions were carried
out with a catalyst/isopropanol ratio of 1:0.25, a decrease in
the catalytic activity of 12.5% was observed. When a catalyst/
ð5Þ
The thermodynamic activation parameters (Figure 1)
were determined from the Eyring and Arrhenius plots
isopropanol ratio of 1:2 was used, no catalytic activity was
found, indicating that all catalyst 1 is active. Moreover,
experiments with stoichiometric amounts of carbodiimides
(Scheme 3) showed that two of the three amido groups in
complex 1 are activated, leading to the formation of
a bis(guanidinate) intermediate (2), which after the addition
of two further equivalents of the nucleophilic moiety, for
example, aniline, gave the thorium amido species 4. However,
when complex 1 was reacted with a stoichiometric amount of
aniline, intermediate 4 was not obtained, even upon heating
to 808C for prolonged periods of time. Therefore, the first
step in the catalytic cycle is the insertion of two equivalents of
À
carbodiimide into the Th Namido bonds of complex 1, forming
the bis(guanidinate) thorium species 2. Subsequent insertion
of two equivalents amine affords the active catalyst 4 under
elimination of two equivalents of N(SiMe3)2-substituted
guanidine 3. An additional insertion of two equivalents of
À
carbodiimide into the Th N bonds of compound 4 yields the
bis(guanidinate) thorium complex 12, which upon protono-
lysis with two equivalents of amine gives two equivalents of
guanidine 7 and regenerates the active catalyst 4. Guanidine 7
undergoes a subsequent 1,3-hydride shift forming the sym-
metric guanidine 8.
To shed light on the mechanism, kinetic experiments were
performed. Hence, in situ NMR experiments were carried out
for the addition of diphenyl phosphine to 1,3-diisopropylcar-
bodiimide (DIC) [Eq. (2)] or phenyl isocyanate [Eq. (3)],
revealing that the reactions display a first-order dependence
on 1,3-diisopropylcarbodiimide or phenyl isocyanate as well
as on diphenylphosphine and complex 1, giving rise to the
kinetic rate Equations (4) or (5). In addition, performing the
kinetic experiments with the deuterated phosphine DPPh2
Figure 1. Plot of reaction rate @p/@t against the concentration of
a) DIC for the reaction of DIC and HPPh2; b) HPPh2 for the reaction of
DIC and HPPh2; c) of complex 1.
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2013, 42, 5890; c) Z. Li, M. Xue, H. Yao, H. Sun, Y. Zhang, Q.
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[Eq. (2)] than for the analogous reaction with phenyl
isocyanate [Eq. (3)], with values of Ea = 20.7(2) kcalmolÀ1
(DH° = 19.9(2) kcalmolÀ1)
and
Ea = 17.6(2) kcalmolÀ1
(DH° = 16.9(2) kcalmolÀ1), respectively. The entropy of acti-
vation, determined from the Eyring plots, displays negative
values
of
DS° = À20.7(4) calmolÀ1 KÀ1
and
DS° =
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In summary, this work introduces new reactivity for
actinide coordination complexes, exemplified by the mono-
(imidazolin-2-iminato)
thorium(IV)
complex
[Th-
(ImDippN){N(SiMe3)2}3] (1). The reactivity of complex 1 was
studied for the catalytic addition of primary and secondary
= =
amines, phosphines, and thiols to the central Y C X linkage
of various heterocumulenes, for example, carbodiimides,
isocyanates, and isothiocyanates. The thorium compound
1 displayed an unusual tolerance toward heteroatoms and
functional groups, giving the respective insertion products in
high yields and selectivity. Kinetic studies together with
kinetic experiments using the deuterated substrate DPPh2
indicate that the protonolysis is the most plausible rate-
determining step of the reaction.
Keywords: actinide catalysis · carbodiimides ·
nucleophilic insertion · isocyanates · thioisocyanates
How to cite: Angew. Chem. Int. Ed. 2015, 54, 12422–12425
Angew. Chem. 2015, 127, 12599–12602
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Received: March 4, 2015
Revised: April 23, 2015
Published online: June 18, 2015
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