Z. Du, B. Behera, A. Kumar et al.
Journal of Organometallic Chemistry 950 (2021) 121982
Figure 3. Hydroboration of isocyanates with 3 equivalents of HBpin catalyzed by
LiHBEt3.
Figure 2. Hydroboration of isocyanates with equivalent of HBpin catalyzed by
LiHBEt3.
Table 2
Energies for the proposed catalytic cycle based on DFT calculations (The
values in parenthesis refers to the calculations where the fragments of
the substrates involved in the reactions was considered).
tive yield within 30 min. These results displays that the steric hin-
drance of the reactants possesses a great influence on the reaction
time in the process of the reduction.
Ester + 2HBpin → 2C
Ester + LiHBEt3 → A
A + HBpin → C + B
ꢀE = -53.7 (-56.3) kcal·mol−1
ꢀE = -29.1 (-23.1) kcal·mol−1
ꢀE = -28.4 (-27.6) kcal·mol−1
ꢀE = 3.8 (-5.6) kcal·mol−1
I
II
The highly unsaturated structure of isocyanate group (-NCO)
determines its high reactivity. With the perfect hydroboration of
ester in hand, we intended to extend the same protocol for the
hydroboration of isocyanate. Highly unsaturated isocyanates have
two unsaturated bonds C=N and C=O, and LiHBEt3 might exhibit
excellent chemo-selective catalytic properties.
III
IV
B + HBpin → C + LiHBEt3
firmed by 1H NMR spectroscopy. Notably, a predominant new or-
ganic product (4a) was formed, possessing a singlet resonance at
2.40 ppm in the 1H NMR spectrum. By analyzing the correspond-
ing 11 B NMR spectrum, it also revealed that there are two singlet
resonances at 23.77 and 20.86 ppm, respectively. In comparison
with the literature reports [16, 17, 23], it is confirmed that the res-
onance at 20.86 ppm is for O(Bpin)2. This indicates that the reduc-
0.1 mol% of LiHBEt3 was dropped to the mix-solution of 2,6-
dimethylphenyl isocyanate with equivalent of HBpin at ambient
temperature and air atmosphere, 15 seconds later the solution
changed to a thick slurry. The reaction was so quick that the B–
H bond of HBpin was cleaved, then selectively added to the C=N
double bond to afford the formamide derivatives in 99 % yield.
Spectroscopic data is consistent with the corresponding product
formed after the addition to C=N double bond, the 1H NMR spec-
trum exhibits a sharp peak around 8.55 ppm, while the 13C NMR
spectrum shows a signal around 163.6 ppm for the NCHO group.
Likewise, the 11 B NMR spectrum shows single broad peak at 25.3
ppm, which also indicates that the Bpin group is bound to the ni-
trogen atom instead of oxygen [8, 17, 22].
i
tion of PrNCO by HBpin has cleaved the C=O bond in the substrate
to form the N-borylated N-methyl isopropylamine iPrN(Bpin)CH3
(4a) and O(Bpin)2. The reactivity extends to other commercially
available alkyl isocyanates, as shown in Figure 3. All the reactions
were characterized by a new singlet methyl resonance from 2.4
ppm to 3.0 ppm in the1H NMR spectra. Meanwhile, the 11 B NMR
spectra showed a new single N-B resonance around 24 ppm for N-
borylated N-methyl, as well as the signal at 21 ppm for O(Bpin)2
(Figures S17 – S22 in SI).
The reaction was extended to other commercially available
aliphatic and aromatic isocyanates with different substituents, and
the results are shown in Figure 2. In comparison with the aliphatic
substrates the aromatic isocyanate reacted more absolutely with
higher yields and cleaner products (Figures S9 - S15 in SI).
The excellent chemo-selective catalytic reactivity of LiHBEt3
for the hydroboration of isocyanates with one equivalent HBpin
prompted us to investigate the conversion with two equivalents
of HBpin. To investigate this notion, controlled experiments were
performed by reacting 2,6-dimethylphenyl isocyanate with 2 equiv-
alents of HBpin for 12 h with catalyst LiHBEt3 in CDCl3. How-
In order to verify the versatility and the air tolerance of the sys-
tem, we scaled up the reduction of ethyl acetate with 2 equivalents
i
of HBpin, PrNCO with equivalent or 3 equivalents of HBpin. When
the ratio of the substrate was amplified to 10 mmol; 99%, 87%, and
95% of the corresponding products (1a, 2f, 4a) were obtained. The
yields of liquid 1a and 4a were obtained by 1H NMR, and 2f was
calculated by drying and weighing the obtained solid product after
washing it with cold npentane (1H NMR Figures S23 – S25 in SI).
However, product 4a possessed an excess of O(Bpin)2, which may
have formed because of the long reaction time which compromises
the air tolerance of the system.
ever, the product (3a) was obtained as
a mixture containing
RN(Bpin)CH2OBpin, RN(Bpin)CH(O), RN(Bpin)CH3, and O(Bpin)2.
Similar results were obtained after performing the reactions for
30 min (Figure S16.1 in SI). Next, we carried out the catalytic re-
action at elevated temperature (60°C), the product (3b) was still
a mixture of RN(Bpin)CH2OBpin, RN(Bpin)CH3, and O(Bpin)2, and
the yields of these products were similar (Figure S16.2 in SI). This
indicated that the C=O bond of isocyanates can be easily activated
by LiHBEt3 at ambient temperature. Elevated temperatures displays
simultaneous addition to C=O bond as well as its cleavage.
We calculated the enthalpy of all the reactions involved in the
proposed mechanism. To understand the reactions, we constructed
a potential energy surface (PES) of the possible reaction channel.
However, we only considered the fragment of the molecule which
are involved in the reactions to construct the PES. The connections
between TS and the reactant and product were established with
intrinsic reaction coordinate (IRC) calculations.
The proposed reaction mechanism for the hydroboration of
ethyl acetate (I) is presented in Figure 4. It involves three reac-
tion steps (II, III, IV) in Table 2. All the reactants, products, and
predicted transition states of molecules and reactive fragments are
In order to obtain a pure methyl amine, the reaction between
iPrNCO and 3 equivalents of HBpin was performed at 60 °C in
CDCl3. The conversion was completed within 3 h which was con-
3