P. Ma, J.T. Spencer / Polyhedron 149 (2018) 148–152
149
Table 1
Data for the cyclodimerization of different isocyanates.
Entry
Isocyanate
Yield
1
2
3
4
5
Phenylisocyanate
n-butyl isocyanate
Benzyl isocyanate
Cyclohexyl isocyanate
Allyl isocyanate
85%
91%
89%
85%
92%
were recorded at 300.15 MHz with chemical shifts referenced to
Fig. 1. ORTEP drawing of the crystallographically-determined molecular structure
of 1,3-diphenyl-urea (2).
an internal standard of tetramethylsilane at d = 0.0 ppm. Carbon
(
13C) NMR spectra were obtained at 75.47 MHz. Unit resolution
mass spectra were obtained on a Hewlett Packard model 5989B
gas chromatograph/mass spectrometer (GC/MS) using an ioniza-
tion potential of between 11 and 70 eV. All photoreactions were
carried under a medium pressure mercury vapor lamp (Make:
ACE GLASS, Model: Hanovia, 7825-34, IMMERSION LAMP, Medium
Pressure, 450 W).
structure [19]. In comparison, without catalytic compound 1, no
reaction occurred under the same conditions.
Other substrates were also investigated using catalytic amount
of compound 1 under the same reaction conditions (Table 1). It can
be seen that compound 1 exhibits high catalytic activity for the
cyclodimerization of both aryl and alkyl isocyanates. Reaction
yields are comparable with literature reported catalytic systems.
For example, Li and coworker reported the yields in range of 60–
99% for a series of Ln(III) pyrimidine-2-thionate complexes [14].
Compared with their catalytic system, compound 1 has the advan-
tage of being more stable (r.t. and open air stable) thus making its
investigating and application more practical.
The formation of di-substituted urea from the reaction is
assumed to proceed through hydrolysis and elimination of CO from
the phenyl isocyanate cyclodimerization product (Fig. 2) [14]. It
was reported that tricarbonyl ligands of compound 1 underwent
photolysis with exchange of carbon monoxide for phosphine
[20]. Manganese metal has previously been shown to be capable
of catalyzing the synthesis of hydantoin derivatives from terminal
alkynes and isocyanates [21]. These indicates that good affinity of
isocyanate towards photo activated manganese center plays a key
role in compound 1’s catalytic reactivity. We therefore propose the
mechanism for this metallaborane catalyzed isocyanate
cyclodimerization as shown in Fig. 3. After the photo-activation,
active manganese center can coordinate one isocyanate and enter
the catalytic cycle. A second isocyanate molecule would then coor-
dinate to the metal center and the metallacyclic intermediate
formed by successive concerted oxidative coupling reactions. This
formation of a metallacyclic intermediate resembles those often
invoked for the cyclotrimerization of alkynes on metal centers
[22]. In the presence of another isocyanate molecule, the iso-
cyanate cyclodimerized product is then released via reductive
elimination process and the resulting activated metallaborane
begins another catalysis cycle.
4.2. Materials
All solvents used were reagent grade or better. Dichloro-
methane and hexane were distilled over potassium metal prior
to use. TLC plates were purchased from Fisher Scientific. The
nido-decaborane(14) (B10H14), was purchased from the Callery
Chemical Company and was purified by vacuum sublimation at
40 ꢁC prior to use. Appropriate care was taken in handling the
boron hydrides under inert atmosphere conditions [23]. All reac-
tions were done in an inert atmosphere (nitrogen) unless other-
wise noted. Deuterated NMR solvents were purchased from
Cambridge Isotope Laboratories, Inc. and dried over molecular
sieves before use unless otherwise noted. All other commercially
available reagents were used as received. The synthesis of com-
pound 1 ([nido-6-Mn(CO)3B9H13][NMe4]) were according to a pre-
vious procedure [15,17].
4.3. Synthesis
4.3.1. Phenylisocyanate with compound 1
Compound 1 (0.001 g, 0.003 mmol) and 0.016 ml (0.15 mmol)
phenylisocyanate in 5 mL dichloromethane were placed in a 10
mL flask and sonicated for 10 min before being put in the photore-
actor for 10 min. It was then hydrolyzed by 1 mL water, extracted
with diethyl ether (3 ꢂ 10 mL), dried over anhydrous MgSO4, and
filtered. After removal of the solvent from the extract, the solid
was re-crystalized in THF and filtered off. It gave 0.026 g (85%) col-
orless crystal of 1,3-diphenyl-urea. 1H NMR (300 MHz, CD2Cl2, d
(ppm)): 7.34–7.31 (8H, aromatic CH), 7.16–7.14 (2H, aromatic
CH), 6.51 (2H, NH). 13C NMR((75.47 MHz, CD2Cl2, d (ppm)):
151.7, 135.1, 131.5, 126.2, 117.3. MS: m/z = 212.10.
3. Conclusion
In summary, 10-vertex metallaborane compound 1 was found
to act as an efficient catalyst for isocyanate cyclodimerization
under photo-irradiation conditions. To our knowledge, such cat-
alytic activity of metallaborane represents first example of
employing metallaborane compounds to catalyze the cyclodimer-
ization of isocyanates. This finding not only represents a useful
approach to the synthesis of ureas from isocyanates, but also indi-
cates a new research avenue for metallaborane compounds.
4.3.2. n-Butylisocyanate with compound 1
Compound 1 (0.001 g, 0.003 mmol) and 0.017 ml (0.15 mmol)
of n-butylisocyanate in 5 mL dichloromethane were placed in a
10 mL flask and sonicated for 10 min before being put in the pho-
toreactor for 10 min. It was then hydrolyzed by 1 mL water,
extracted with diethyl ether (3 ꢂ 10 mL), dried over anhydrous
MgSO4, and filtered. After removal of the solvent from the extract,
the solid was re-crystalized in THF and filtered off. It gave 0.024 g
(91%) colorless crystal of 1,3-dibutyl-urea. 1H NMR (300 MHz, CD2-
Cl2, d (ppm)): 6.01 (2H, NH), 3.05 (4H, CH2), 1.53 (4H, CH2), 1.29
(4H, CH2), 0.89 (6H, CH3). 13C NMR(75.47 MHz, CD2Cl2, d (ppm)):
158.2, 40.2, 31.0, 20.8, 30.8. MS: m/z = 172.26.
4. Experimental
4.1. Physical measurements
All NMR spectra were recorded on a Bruker Avance 300 MHz
NMR equipped with a 5 mm OXI probe. Proton (1H) NMR spectra
Ma, Pei
