Electrostatic Catalyst Generated from Diazadiborinine for Carbonyl Reduction

Article abstract of DOI:10.1016/j.chempr.2017.06.001

Since the seminal discovery by van der Waals in the late 19^th century that weak attractive forces exist between even electrically neutral atoms or molecules, a number of noncovalent interactions have been recognized. Among them, electrostatic interactions such as hydrogen bonds play pivotal roles in countless chemical processes and biochemical living systems. By mimicking biocatalysis, various organocatalysts equipped with hydrogen-bond functionality have been developed; however, a challenge has persisted in designing catalysts exploiting other types of noncovalent interactions. Here, we report metal-free hydroboration reactions of carbonyl compounds and CO₂ catalyzed by aromatic diazadiborinine. A joint experimental and computational study on the reaction mechanism suggests that adducts of diazadiborinine with carbonyl and CO₂ formed at the initial stage of the reactions serve as actual catalysts. The former stabilizes the transition state by using the electrostatic interaction between the hydride of borane and the polar, hole-shaped structure of the adduct.

Full text of DOI:10.1016/j.chempr.2017.06.001

Article
Electrostatic Catalyst Generated from
Diazadiborinine for Carbonyl Reduction
Di Wu, Ruixing Wang, Yongxin
Li, Rakesh Ganguly, Hajime
Hirao, Rei Kinjo
hhirao@cityu.edu.hk (H.H.)
rkinjo@ntu.edu.sg (R.K.)
HIGHLIGHTS
Aromatic diazadiborinine
promotes catalytic hydroboration
of carbonyls
Actual catalyst is the adduct of
diazadiborinine with carbonyl
derivatives
Mechanism involves a transition
state stabilized mainly by an
electrostatic effect
We show that, unlike other aromatic molecules, inorganic/organic hybrid benzene,
namely diazadiborinine, effectively accelerates hydroboration of carbonyl
derivatives. Experimental mechanistic investigations indicate that the adduct of
diazadiborinine with a carbonyl derivative formed at the initial step of the reaction
serves as the actual active catalyst. Computational studies suggest that the
transition state is stabilized by an electrostatic effect.
Wu et al., Chem 3, 134–151
July 13, 2017 ª 2017 Elsevier Inc.
http://dx.doi.org/10.1016/j.chempr.2017.06.001

Article
Electrostatic Catalyst Generated from
Diazadiborinine for Carbonyl Reduction
Di Wu, Ruixing Wang, Yongxin Li, Rakesh Ganguly, Hajime Hirao,1,2,* and Rei Kinjo1,4,
1
2
3
3
*
SUMMARY
The Bigger Picture
th
Since the seminal discovery by van der Waals in the late 19 century that weak
attractive forces exist between even electrically neutral atoms or molecules, a
number of noncovalent interactions have been recognized. Among them, elec-
trostatic interactions such as hydrogen bonds play pivotal roles in countless
chemical processes and biochemical living systems. By mimicking biocatalysis,
various organocatalysts equipped with hydrogen-bond functionality have
been developed; however, a challenge has persisted in designing catalysts ex-
ploiting other types of noncovalent interactions. Here, we report metal-free
Noncovalent interactions are
ubiquitously used for chemical
transformations in nature. For
instance, one of the major
catalytic factors of enzymes is the
electrostatic effect that stabilizes
transition states. By mimicking
geometric and electronic
hydroboration reactions of carbonyl compounds and CO
2
catalyzed by aro-
structures of enzymes, small
organic molecules featuring
similar functionality to enzymes
can be produced. This work
presents metal-free catalytic
hydroboration of carbonyl
compounds promoted by
matic diazadiborinine. A joint experimental and computational study on the
reaction mechanism suggests that adducts of diazadiborinine with carbonyl
2
and CO formed at the initial stage of the reactions serve as actual catalysts.
The former stabilizes the transition state by using the electrostatic interaction
between the hydride of borane and the polar, hole-shaped structure of the
adduct.
aromatic diazadiborinine. In stark
contrast to the reaction pathways
proposed for catalytic
INTRODUCTION
hydroboration with other main-
group catalysts, the mechanism
for the reaction reported here
does not involve bond formation
or dissociation between the
catalyst and substrate. The
computational analysis provides a
rationale for the catalytic action, in
which a low-energy pathway is
achieved by an electrostatic
effect.
Noncovalent interactions such as hydrogen bonding, halogen bonding, p-p stack-
ing, and ion/lone pair p interactions, are ubiquitously involved in a myriad of chem-
1
–5
ical transformation processes.
Notably, such interactions contribute significantly
to catalytic processes found in nature. For instance, enzymes use hydrogen bonding
as one of the major catalytic factors that stabilize transition states, thereby facili-
6
–8
tating chemical transformations of substrates.
catalysis, diverse small-molecule catalysts that exploit hydrogen bonds have
By mimicking enzymatic bio-
9
–14
emerged over the past several decades.
However, the development of cata-
lysts relying on electrostatic effects other than hydrogen bonding still remains
1
5–19
challenging.
Carbonyl reductions are useful methodologies for the synthesis of various products
and hence have been extensively used in organic syntheses and industrial pro-
2
0–24
cesses.
Among them, hydroboration has attracted considerable attention,
because boron hydrides are relatively stable and can easily be stored, which cir-
cumvents the drawbacks of the use of highly flammable and highly pressurized
2
5–30
hydrogen gas.
satile synthetic intermediates for the preparation of a wide range of chemicals and
In addition, the resulting boric and borinic esters serve as ver-
3
1–33
materials.
In this regard, a variety of transition-metal complexes capable of
promoting hydroboration of carbonyl derivatives have been developed over the
3
4
past two decades. By contrast, despite recent advances in main-group chemistry
3
5–37
made with these elements as alternatives to transition metals,
catalytic hydro-
boration reactions that operate with well-defined main-group catalysts are still
134 Chem 3, 134–151, July 13, 2017 ª 2017 Elsevier Inc.

Figure 1. Catalytic Cycles for Hydroboration of Carbonyls with Main-Group Catalysts
Commonly proposed hydride insertion / s-bond metathesis pathway for catalytic hydroboration
reactions with main-group hydride catalysts ([E] = Mg, Ge, Sn, Al, B, P).
limited, and in particular, the metal-free catalytic system remains virtually unex-
3
8,39
40–45
plored.
Thus far, main-group metal catalysts based on Mg,
Ge(II) and
and alkali metal borates have been reported, whereas our
group has demonstrated that even nonmetal 1,3,2-diazaphospholene can be
4
6–50
51,52
53
Sn(II),
Al,
5
4
used as a catalyst for hydroboration of carbonyl compounds. Remarkably, irre-
spective of the main-group element involved, a common reaction pathway for hy-
droboration is proposed involving the following steps (Figure 1): (1) insertion of the
C=O bond of a carbonyl compound into the [E]–H bond of main-group catalysts to
generate intermediate INT, (2) interaction between INT and HBpin to form four-
membered transition state TS, and (3) formation of the hydroboration product
via s-bond metathesis. As far as we are aware, a main-group catalytic system based
on a mechanism other than the insertion-metathesis pathway has never been
described.
Recently, we have reported the isolation of aromatic diazadiborinine derivatives 1a
and 1b (Figure 2A) and have shown that the two boron centers in these molecules
5
5–57
cooperatively activate unsaturated substrates such as alkynes and alkenes.
For instance, 1c can be obtained from the reaction of 1a with ethylene. Although
these findings demonstrate the potential of B,N-cyclic molecules for broad applica-
5
8–60
tions,
have never been used as promoters for catalytic reactions.
zadiborinine-catalyzed hydroboration of carbonyl derivatives as well as N-formyla-
tion of amines with the use of CO under ambient conditions. We demonstrate
to the best of our knowledge, aromatic six-membered B,N-heterocycles
6
1,62
Here, we report dia-
1Division of Chemistry and Biological Chemistry,
School of Physical and Mathematical Sciences,
Nanyang Technological University, 21 Nanyang
Link, 637371 Singapore, Singapore
2
that different pathways are used for these catalytic reactions, one of which exploits
a unique electrostatic effect.
2Department of Biology and Chemistry, City
University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong, China
3NTU-CBC Crystallography Facility, Nanyang
Technological University, 21 Nanyang Link,
RESULTS AND DISCUSSION
Catalytic Hydroboration of Ketones and Aldehydes
637371 Singapore, Singapore
Our preliminary studies showed that 1b activates both unsaturated bonds and E–H s
bonds (E = B, Si, P), whereas 1a reacts only with unsaturated bonds at ambient con-
dition (see below), which prompted us to use 1a as the catalyst for the investigation
of catalytic hydroboration. Initial assessments of the catalytic activity of 1a were
4Lead Contact
*Correspondence: hhirao@cityu.edu.hk (H.H.),
rkinjo@ntu.edu.sg (R.K.)
http://dx.doi.org/10.1016/j.chempr.2017.06.001
Chem 3, 134–151, July 13, 2017 135

A
B
C
D
E
Figure 2. Schematic Representation of Diazadiborinine Derivatives and Their Reactivity
(
(
(
(
(
A) 1,3,2,5-Diazadiborinine 1a, 1,4,2,5-diazadiborinine 1b, and 1a-ethylene adduct 1c.
B) Synthesis of 6.
C) Solid-state structure of 6.
D) Reaction of 6 with HBpin and 2a.
E) Reaction of 2g and HBpin in the presence of 6.
undertaken for the reduction of acetophenone 2a (Table 1). At first, the control
experiment confirmed that no reaction between acetophenone and HBpin pro-
ceeded in the absence of 1a (entry 1). With 5 mol % of 1a, acetophenone 2a was fully
converted to the corresponding boric ester PhMeHC-OBpin 3a after 2 hr (entry 2).
When the catalyst loading was decreased to 2 mol %, the boric ester 3a was still ob-
tained quantitatively within 2 hr (entry 3), whereas further reduction of the catalyst
loading to 1 mol % required longer reaction time (entry 4). Interestingly, the reaction
in CD
3
CN was completed within 30 min (entry 5). When 9-borabicyclo[3.3.1]nonane
0
(
9-BBN) was used, borinic ester 3a was formed in excellent yield after 6.5 hr (entry 6).
136 Chem 3, 134–151, July 13, 2017

Table 1. Optimization of the Reaction Conditions
a
Entry
Catalyst (mol %)
Borane (1.0 equiv)
HBpin
Solvent
Time (hr)
Yield (%)
1
2
3
4
5
6
0
5
2
1
2
2
C
6
C
6
C
6
C
6
D
D
D
D
6
6
6
2
0
HBpin
2
95
98
98
98
98
HBpin
2
HBpin
6
8
HBpin
CD
3
CN
0.5
6.5
9-BBN
C
6
D
6
Reaction conditions: acetophenone (0.269 mmol), H source (0.269 mmol), solvent (0.5 mL).
a
1
Yields were determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard.
3
The use of triethoxysilane ((EtO) SiH) did not afford the corresponding hydrosilyla-
tion products (see Supplemental Information and Figure S1). Note that the result
presents a rare example of metal-free organoboron-catalyzed hydroboration of
carbonyl derivatives.
With the optimized reaction conditions in hand, the scope of the catalytic reaction
was briefly examined with the use of various ketones (Table 2). Aliphatic ketones
2
b–2e afforded the corresponding boric esters 3b–3e in excellent yields. Ketone
derivatives 2f–2j involving electron-withdrawing as well as electron-donating aro-
matic groups were well tolerated. When a,b-unsaturated ketones 2k and 2l were
used, hydroboration occurred selectively only at the carbonyl functional groups.
2 6 4
Benzoquinone 2m furnished 1,4-(pinBO) C H 3m. The scope of the reaction was
further studied with aldehyde substrates (Table 3). Both aliphatic and aromatic alde-
hydes 4a–4h afforded the corresponding boric esters 5a–5h nearly quantitatively.
With 2 equiv of HBpin, hydroboration of terephthalaldehyde 4i proceeded smoothly
and bis-boric ester 5i was gained in excellent yield. Although a longer reaction time
was required, a similar process was observed for 4-acetylbenzaldehyde 4j, which
afforded 5j.
To gain insight into the reaction mechanism, we performed several control reactions
in a stoichiometric manner. Although compound 1a did not react with HBpin, 1a
cleanly underwent [4 + 2] cycloaddition with acetophenone 2a, and the correspond-
ing bicyclo[2.2.2] derivative 6 was obtained in 95% isolated yield (Figure 2B). Com-
pound 6 was fully characterized by standard spectroscopic methods and X-ray
diffraction analysis (Figure 2C). The formation of 6 via a regioselective [4 + 2] cyclo-
addition is in line with our previous observation for 1a. Thus, the B atom between two
carbon atoms serves as a Lewis basic center, whereas the B atom between two
5
5,56
N atoms acts as a Lewis acidic center.
addition of 6 did not take place even at 70 C. Surprisingly, compound 6 did not react
with HBpin (Figure S2). Addition of acetophenone 2a to a C solution of a mixture
We also confirmed that retro-[4 + 2] cyclo-
ꢀ
6
D
6
of 6 and HBpin afforded product 3a at ambient conditions (Figure 2D). These obser-
vations suggest that dissociation of the PhMeCO fragment from 6 concomitant with
the regeneration of 1a is not involved in the reaction to produce 3a. To confirm this
hypothesis, we performed a further reaction (Figure 2E). To the mixture of 6 and
Chem 3, 134–151, July 13, 2017 137

Table 2. Scope of Catalytic Hydroboration of Ketones
a
ꢁ1 b
)
2
2
Time
Product 3
3b
Yield (%)
TON
TOF (hr
b
2.75 hr
98
49
18
2
2
c
168 hr
3c
85
98
43
49
0.3
c
d
<10 min
3d
>288
c
2
2
e
f
2-adamantanone
<10 min
2 hr
3e
3f
98
98
49
49
>288
25
2
2
2
2
g
h
48 hr
2.5 hr
3 hr
3g
3h
3i
95
98
98
96
48
49
49
48
1
20
16
6
d
i
d
j
7.5 hr
3j
2
2
2
k
l
10 hr
3k
3l
98
98
98
49
49
49
5
3 hr
16
e
c
m
<10 min
3m
>288
a
1
Yields were determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard.
b
Turnover frequency (TOF) = turnover number (TON)/average reaction time.
c
TOF lower limit.
d
6 6
Reaction conducted with C D as solvent.
e
HBpin (2 equiv) was used in this reaction.
0
4
6 6
-methoxyacetophenone 2g in C D , 3 equiv of HBpin was added, and the reaction
was monitored by NMR spectroscopy. Within 50 min, full conversion of 2g to 3g was
observed without formation of a trace amount of 3a (Figure S4). We also confirmed
that compound 6 remained unchanged during the reaction, which led us to
138 Chem 3, 134–151, July 13, 2017

Table 3. Scope of Catalytic Hydroboration of Aldehydes
a
ꢁ1 b
)
4
4
Time
Product 5
5a
Yield (%)
TON
TOF (hr
a
1.83 hr
98
49
27
c
4
4
4
b
<10 min
15 min
5b
5c
5d
98
98
98
49
49
49
>288
c
196
c
d
<10 min
>288
c
4
4
4
e
f
<10 min
20 min
50 min
5e
5f
98
98
98
49
49
49
>288
148
59
g
5g
4
4
h
7 hr
1 hr
5h
5i
98
98
49
49
7
d
i
49
d
4
j
168 hr
5j
57
29
0.2
a
1
Yields were determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard.
b
Turnover frequency (TOF) = turnover number (TON)/average reaction time.
c
TOF lower limit.
d
HBpin (2 equiv) was used in this reaction.
postulate that 6 is the true active catalyst rather than the resting state in the hydro-
boration reaction.
Kinetic Study
To better understand the reaction pathway, we carried out further analysis of the
6 6
catalytic reaction. C D solutions of various concentrations of 1a, HBpin, and
Chem 3, 134–151, July 13, 2017 139

diisopropyl ketone 2c were prepared in a sealed NMR tube, and the reactions were
1
monitored at room temperature by H NMR at 2-min intervals. On the basis of the
rate constants kobs simulated from the kinetic conversion charts (see the Supple-
mental Information), we determined the rate law, which can be represented by
1
1
1
k
obs = k’[1a] [HBpin] [2c] . An Eyring plot was constructed for kinetic data obtained
ꢀ
ꢀ
ꢀ
at 10 C intervals from 35 C to 65 C, and activation parameters extracted from the
z
ꢁ1
z
Eyring analysis yielded values of DH = 8.6 G 1.0 kcal mol , DS = ꢁ50.7 G 3.1
z
ꢁ1
z
e.u., and DG (298) = 23.7 G 1.9 kcal mol . The large negative DS is in line with
the high molecularity of the rate law. Next, deuterated kinetic isotope effects (DKIEs)
on the reactions with diisopropyl ketone 2c were investigated with deuterated pina-
6 6
colborane (DBpin). To a C D solution of diisopropyl ketone 2c with 3 mol % of 1a,
excess amounts of deuterated pinacolborane (5.1 equiv) were added at room tem-
perature, and the reaction was monitored by NMR every 10 min. The DKIE value was
estimated on the basis of the rate constants, and a DKIE of 3.65 was obtained for the
1
8
reactions. We also performed the same reaction with O-labeled diisopropyl ke-
1
8
16
18
63,64
tone 2c- O, for which a O/ O KIE value of 1.83 was determined.
The double
1
8
KIE reaction using both DBpin and 2c- O provided the largest DKIE of 5.97, thus
suggesting that the rate-determining step could involve the dissociation of the
B–H bond in HBpin and the C=O bond in 2c and/or the formation of B–O and
C–H bonds.
DFT Study for the Reaction Profile of Hydroboration
Pathways for the reaction of acetophenone (2a) with pinacolborane (HBpin), cata-
lyzed by 6, were explored theoretically by density functional theory (DFT) calcula-
tions at the M06-2X(SCRF)/def2-TZVP//M06-2X(SCRF)/6-31G* level. The solvent
effect of acetonitrile was described by the SCRF (IEFPCM) solvent model. An ener-
getically feasible reaction pathway was obtained and is depicted in Figure 3A. The
ꢁ1
barrier for this pathway via TS was calculated as 27.1 kcal mol , which is reasonably
low and the lowest of all the barriers obtained (Figure S46). Interestingly, no intramo-
lecular bond dissociation/recombination of 6 is involved in this path. Instead, com-
pound 6 cuddles up to the migrating hydride and assists a direct insertion of the B–H
bond of HBpin into the C=O moiety of 2a at the transition state. Figure 3B (left and
middle) illustrates an electrostatic potential map at the transition state in which the
electrostatically positive and hole-shaped site of 6 interacts with the negatively
charged hydride of HBpin. Thus, 6 can be viewed as an electrostatic catalyst pos-
sessing a B–H hydride hole. The key interatomic distances at the transition state (Fig-
ure 3B, right) confirm that the hydride of HBpin is surrounded by the hydrogen atoms
carrying partial positive charges (highlighted in yellow; see Figure S47). In this tran-
sition state, which corresponds to the rate-determining step of this reaction, both
the cleavage of the B–H and C=O bonds and the formation of the B–O and C–H
bonds are involved, which is therefore consistent with the result of double KIE anal-
ysis. Note that the optimized structure of the transition state in the reaction with
9
-BBN is essentially identical to that obtained with HBpin (Figure S48). Other reac-
tion pathways that use the B–O–C–B moiety of 6 as the active site were found to
have much higher barriers (Figure S46), which indicates that these processes are un-
likely to occur.
Catalytic Hydroboration with 1c and Related Molecules
To further prove the catalytic mechanism based on electrostatic interaction and to
confirm the innocence of the B–O–C–B moiety of the 1a-carbonyl adducts in the cat-
alytic process, we carried out a catalytic reaction by using 1c. First, we confirmed that
there was no conversion of 1c to 6 in a C
6 6
D solution even in the presence of 2a
(
1.0 equiv) (Figure S28). A CD CN solution of 2a and HBpin (3 equiv) in the presence
3
140 Chem 3, 134–151, July 13, 2017

A
B
Figure 3. DFT-Calculated Free-Energy Profile of a Plausible Mechanism
ꢁ
1
(
A) Free-energy profile (in kcal mol ) for the reaction between 2a and HBpin catalyzed by 6 in
CH CN at 298 K, as determined at the M06-2X(SCRF)/def2-TZVP//M06-2X(SCRF)/6-31G* level of
theory.
B) Electrostatic potential map for the transition state.
3
(
of 2 mol % of 1c was prepared in a sealed NMR tube, and the reaction was monitored
by NMR spectroscopy. After 10 hr at room temperature, nearly quantitative forma-
tion of 3a was confirmed (Figures S29 and S30). With 2h under the same conditions,
3
h was obtained in 93% yield after 21 hr (Figures S31 and S32). These observations
demonstrate that the B–O–C–B moiety of 1a-carbonyl adducts does not serve as an
active site during the hydroboration reactions with 1a as pre-catalysts. Compared
with the reaction with 1a (Table 2), longer reaction periods were required with the
1
c catalyst for the quantitative formation of 3a and 3h, which is probably as result
of the less polar nature of 1c with respect to 1a-carbonyl adducts. Thus, a more polar
catalyst could stabilize the transition state more effectively, which is in line with the
computational result. In fact, the sum of natural population analysis charge (+0.65)
for the 1a fragment in 6 is found to be larger than that (+0.39) for the corresponding
moiety in 1c.
The electrostatic interaction is different from the charge-transfer interaction
via orbital interactions but can be accounted for by static coulombic-like
Chem 3, 134–151, July 13, 2017 141

Table 4. Examination of Catalytic Activity of the Related Compounds
a
Entry
Catalyst (mol %)
BPh (100)
B(C (100)
Solvent
Time (hr)
Yield of 3a (%)
1
2
3
3
CD
CD
CD
3
3
3
CN
CN
CN
12
12
12
0
0
0
6 5 3
F )
MIDA (2 or 100 )
4
5
CD
3
CN
12
0
0
[
2-Ph-N-MeOx]OTf (100 or 2)
CDCl
3
5
2-PhOx (100)
2
-PhOx-HBpin
b
6
7
CDCl
3
48
12
12 (100 mol % of catalyst)
6 (2 mol % of catalyst)
CD
CD
3
3
CN
3
N-MeOx∙BPh (100 or 2)
CN
0.5
97
1d (2)
b
8
CDCl
3
12
0
NDI (100 or 2)
Reaction conditions: acetophenone (0.269 mmol), HBpin (0.269 mmol), solvent (0.5 mL).
a
1
Yields were determined by H NMR spectroscopy.
b
3 3 3
Because of the poor solubility of N-MeOxBPh and NDI in CD CN, the reactions were performed in CDCl .
6
5–67
interactions.
To gain more insight into the catalytic mechanism, we further
investigated the hydroboration reaction of 2a by using several related compounds
as the catalysts (Table 4).
3 6 5 3
When a stoichiometric amount of BPh or B(C F ) was used, no reactions were
observed, even after 12 hr (Figures S5 and S6, entries 1 and 2), confirming that
typical Lewis acids do not catalyze the hydroboration reaction under the reaction
142 Chem 3, 134–151, July 13, 2017

conditions. Thus, unoccupied orbitals such as an empty p orbital on the B atom or
6
8
any s holes (s* orbitals) seem to be irrelevant to the hydroboration reaction
with 6. Neither a catalytic amount (2 mol %) nor a stoichiometric amount of 1-meth-
ylindole-2-boronic acid MIDA ester MIDA featuring a dative N:/B bond promoted
the reaction (Figure S7, entry 3). Analogously, 2-phenyl-N-methyloxazolium ion
[
2-Ph-N-MeOx]OTf was not effective as the catalyst at all (Figure S8, entry 4). We
also examined a combination of an oxazoline and a borane (entry 5). At first, we
confirmed that no adduct was formed between 2-phenyloxazoline 2-PhOx and
3 3
BPh in CD CN. Both 2a and HBpin were added to this solution, and the reaction
was monitored by NMR spectroscopy. 2a remained stable after 5 hr, but the hydro-
boration of 2-PhOx proceeded to afford 2-PhOx-HBpin cleanly (Figure S9). Thus, the
3
Lewis pair 2-PhOx/BPh does not serve as the catalyst for the hydroboration of 2a,
suggesting that the dissociation of the B–N bonds in 6 and 1c to function as if to
act as the frustrated Lewis pairs is unlikely during the hydroboration reaction of car-
6
9,70
bonyls in our system.
of N-methyloxazol-2-ylidene with BPh
amount of 3a was formed (12%) after 48 hr (Figure S10, entry 6). We confirmed
that the use of a catalytic amount (2 mol %) of N-MeOx$BPh also afforded 3a, albeit
in very low yield (6%) (Figure S11). Because no dissociation of N-Me$OxBPh to the
corresponding oxazol-2-ylidene and BPh was observed in either a CDCl or a
CD CN solution, N-MeOx$BPh is also likely to function as an electrostatic catalyst.
Remarkably, we found that compound 1d featuring a B six-membered ring
akin to those of 6 and 1c, effectively promoted the reaction (entry 7). With
mol % of 1d, nearly quantitative formation of 3a was observed after 30 min (Fig-
ure S12). From these results, the polarized B skeleton is putatively a key struc-
ture to enhance the catalytic activity as an electrostatic functionality. In addition, we
Interestingly, with a stoichiometric amount of the adduct
3
N-MeOx$BPh in a CDCl solution, a small
3
3
3
3
3
3
3
3
2 2 2
C N
2
2 2 2
C N
7
1
tested a p-acidic naphthalene diimide NDI, known for its ability for noncovalent
7
2,73
electrophilic activation of organic substrates as pioneered by Matile et al.,
but
no formation of 3a was observed (Figure S13, entry 8).
Catalytic N-Formylation of Amines with CO
Over the past few decades, various strategies have been developed for activating
thermodynamically highly stable carbon dioxide (CO ) and achieving its subse-
quent transformation into commodity chemicals with both metal and nonmetal
2
2
7
4–85
catalysts.
2
Among them, N-formylation of amines with CO as the carbon
source has been attracting great interest because the reaction products, that is,
formamides, are ubiquitous in organic syntheses as well as industrial products as
8
6–99
synthetic intermediates and solvents.
a reversibly undergoes a [4 + 2] cycloaddition reaction with CO
prompted us to investigate further the catalytic activity of 1a for functionalization
of CO
Previously, we showed that compound
5
6
1
2
,
which
2
.
We began our investigation by using N-methylaniline 7a as a test substrate for N-for-
mylation with CO . After a brief screening of the reaction conditions (Table S1), we
obtained the optimal conditions under which, with 1a (2 mol %) and HBpin (3 equiv)
under a CO atmosphere, 7a was transformed nearly quantitatively to N-methyl-N-
2
2
phenylformamide 8a after 5.5 hr at room temperature (Figure 4).
Next, the scope of the N-formylation reaction was studied with various amines
(
Table 5). Aromatic primary amines 7b–7g were well tolerated and the corresponding
formamides 8b–8g were obtained in good to excellent yields (76%–95%). Secondary
amines 7h–7n bearing an aromatic ring with varying degrees of electronic features af-
forded the corresponding N-formylated products 8h–8n in high yields (80%–98%).
Chem 3, 134–151, July 13, 2017 143

Figure 4. Catalytic N-Formylation with 1a
2
N-Formylation reaction of PhMeNH with CO catalyzed by 1a.
We observed that dialkylamines such as diethylamine and diispropylamine predom-
inantly underwent dehydrocoupling with HBpin. The strong nucleophilic nature of
dialkylamines probably accelerated the dehydrocoupling reaction before the N-for-
1
00
mylation, which is in line with the recent report by Romero et al.
lyzed dehydrocoupling reaction between amines and boranes.
on the noncata-
Mechanistic Study and a Proposed Catalytic Cycle
To identify a plausible reaction mechanism, we performed several control reactions.
In the absence of 1a, a mixture of N-methylaniline 7a and HBpin (3 equiv) afforded
N-formyl-N-methylaniline 8a only in 5% yield even after 20 hr, confirming the essen-
tial involvement of the catalyst in the N-formylation reaction (Table S1, entry 1).
Neither HBpin nor 7a reacted with 1a, suggesting that compound 10, an adduct
of 1a with CO
that 10, independently prepared from the reaction of 1a with CO
with 7a (Figure 5A, top). By contrast, treatment of a CD CN solution of 10 with HBpin
3 equiv) under an argon atmosphere produced HCOOBpin 11 as the major product
along with other minor side products, including O(Bpin) (Figure 5A, bottom; Fig-
ures S14 and S15). We found that with 2 mol % of 1a, hydroboration of CO pro-
2
, must be formed in the initial step of the reaction. We confirmed
2
, did not react
3
(
2
2
ceeded smoothly and 11 was formed in 48% yield after 34 hr (Figures 5B, S16,
and S17). The yield of 11 increased (70%) when 9 mol % 1a was used (Figures S18
and S19). Moreover, a stoichiometric reaction of HCOOBpin 11 and 7a with HBpin
(
2.5 equiv) afforded N-formyl-N-methylaniline 8a quantitatively, confirming that
this process does not require any catalysts (Figures 5C, S20, and S21). Interestingly,
the reaction of 7a (1 equiv) with HBpin (3 equiv) in the presence of 10 furnished
dehydrocoupling product PhMeNBpin 9a without formation of 8a (Figures 5D,
S22, and S23). This result indicates that 10 only promotes the dehydrocoupling re-
1
01–105
action
Figure 5A, bottom). In order to confirm whether dehydrocoupling product 9a is
involved as the key intermediate in the catalytic cycle, 9a was reacted with HBpin
under a CO atmosphere (Figure 5E). Without 1a, no reaction between 9a and HBpin
in the presence of amine rather than reacting with HBpin to afford 11
(
2
was observed, whereas the same reaction in the presence of 1a (2 mol %) afforded 8a
in 15% yield after 7 hr (Figure S24). Although the result shows that 1a can marginally
promote the formation of 8a from 9a, this reaction is much slower than that shown in
Figure 4, suggesting that the reaction pathway involving a dehydrocoupling inter-
mediate such as 9a is subordinate for N-formylation of amines. We synthesized
1
3
13
1
1
0-( CO
2
2
) by the treatment of 1a with CO
2
, and conducted the reaction of
(Figures 5F and S25–
1
3
0-( CO
) with 7a (1 equiv) and HBpin (3 equiv) under CO
2
S27). After 7 hr, we observed the formation of 8a in 95% yield, which is in stark
contrast to the exclusive formation of dehydrocoupling product 9a from the reaction
1
3
2
without CO (Figure 5D). The C NMR spectrum of formamide 8a confirmed that the
1
3
C atom was not incorporated in product 8a, indicating that transfer of the CO
2
1
3
fragment from 10-( CO
2
) to HBpin as observed in Figure 5A did not take place dur-
ing the catalytic N-formylation. Thus, compound 10 is not the resting state but is a
species that serves as the active catalyst in the N-formylation reaction.
144 Chem 3, 134–151, July 13, 2017

Table 5. Scope of Catalytic N-Formylation of Amines with CO
2
a
ꢁ1
)
7
7
Time (hr)
Product 8
8b
Yield (%)
TON
TOF (hr
b
10
95
48
5
7
7
c
23
16
8c
81
87
41
44
2
3
d
8d
7
7
e
f
20
12
8e
8f
94
87
47
44
2
4
7
7
g
h
22
11
8g
8h
76
85
38
43
2
4
7
i
j
5.5
4
8i
8j
98
93
49
47
9
7
12
7
k
l
21
8
8k
8l
80
94
40
47
2
6
7
7
m
n
21
5
8m
8n
80
83
40
42
2
8
7
a
1
Yields were determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard.
Chem 3, 134–151, July 13, 2017 145

A
B
C
D
E
F
G
Figure 5. Control Reactions for Mechanistic Studies
(
(
(
A) Reactions of 10 with 7a (top) and HBpin (bottom).
B) Catalytic hydroboration of CO using 1a.
C) Synthesis of 8a from 1a and 7a.
2
146 Chem 3, 134–151, July 13, 2017

Figure 5. Continued
(
(
(
(
D) Reaction of 7a and HBpin in the presence of 10.
2
E) Reactions of 9a and HBpin, with and without 1a under CO .
1
3
F) Reaction of 10-( CO
2
), 7a, and HBpin under CO
using 1c.
2
.
G) Catalytic hydroboration of CO
2
1
06–108
A series of recent seminal works
demonstrated that the formaldehyde
of ambiphilic phosphine-borane catalysts were generated in catalytic hy-
droboration of CO , and the CH O moiety remained in the ambiphilic system
1
09
adducts
2
2
throughout the catalysis. Thus, it is postulated that the formaldehyde adducts are
active catalysts in their catalytic hydroboration reaction. On the basis of the outcomes
2
of our control reactions (Figure 5), it is well conceivable that CO -adduct 10 could also
serve as the catalyst, which was confirmed by DFT calculations (Figure S49). Accord-
ingly, one of the most plausible reaction mechanisms is proposed in Figure 6. At the
initial stage of the reaction, compound 10 is formed in situ after the reaction of 1a with
CO
of the hydride to CO
0. The activation barrier (18.8 kcal mol ) of this process is lower than that
2
. Compound 10 can activate the B–H bond of HBpin, which is followed by transfer
2
to furnish HCOOBpin 11, concomitant with regeneration of
ꢁ1
1
ꢁ1
(
40.1 kcal mol ) of another path involving the transition state stabilized by an elec-
trostatic interaction of 10 (Figure S49). The non-catalytic reaction between 11 and
amines 7 should produce formamides 8 as well as HOBpin, which can undergo dehy-
drocoupling with HBpin to form O(Bpin)
2
. Note that although a number of reaction
mechanisms have been proposed for the p-block compound-catalyzed hydrobora-
3
4,110–114
92–99
tion of CO
our knowledge, none of these studies has assumed that main-group compound-
CO adducts play a role as the actual active catalysts.
2
and/or N-formylation of amines with CO
2
,
to the best of
2
Analogous N-formylation could take place when 1c was used as the catalyst. Thus,
under similar reaction conditions, amines 7a and 7i were cleanly transformed to
the corresponding formamides 8a and 8i, respectively (Figure 5G). Experimentally,
3 2
we confirmed that 1c cannot be converted to 10 in a CD CN solution under a CO
atmosphere (Figures S35 and S36). Because 1c does not possess a strong Lewis
basic site akin to the O–C=O moiety of 10 to activate HBpin, on the basis of the
Figure 6. Proposed Reaction Mechanism
2
Plausible catalytic cycle for the N-formylation of amines with HBpin under CO .
Chem 3, 134–151, July 13, 2017 147

above-mentioned results for the catalytic hydroboration of ketones (2a and 2h)
with 1c, we tentatively postulate that 1c could act as an electrostatic catalyst for
the formation of formamides (8a and 8i).
Conclusions
We have demonstrated that a catalytic amount of inorganic/organic hybrid benzene,
1
,3,2,5-diazadiborinine 1a, effectively promotes hydroboration of carbonyl deriva-
tives involving ketones, aldehydes, and CO under metal-free and ambient condi-
tions. Mechanistic studies suggest that the corresponding carbonyl adducts 6 and
0 are formed in the initial steps of those reactions. Significantly, neither of the cat-
2
1
alytic cycles seems to involve the regeneration of 1a, and thus, adducts 6 and 10
serve as the actual active catalysts rather than the resting states, which is in contrast
to the proposed mechanisms for other main-group compound-catalyzed reduction
of carbonyl derivatives. Computational studies suggest that among various possible
factors, a noncovalent interaction plays a critical role in facilitating some of the
reactions examined here. Thus, the electrostatic effect induced by the polar and
hole-shaped structures of the carbonyl adduct 6 helps decrease the activation
barrier for the hydroboration reactions of ketones and aldehydes. In contrast, the
CO
2
-adduct 10 serves as a Lewis base to activate the B–H bond of HBpin during
. We believe that the findings described here pave the way
the reduction of CO
2
for the design of novel nonmetal p-block catalysts.
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in the Supplemental Information.
ACCESSION NUMBERS
Crystallographic data have been deposited in the Cambridge Crystallographic Data
Center (CCDC) under accession number CCDC: 1523108. The data can be obtained
free of charge from the CCDC at http://www.ccdc.cam.ac.uk/data_request/cif.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 166
figures, 4 tables, and 1 data file and can be found with this article online at http://
dx.doi.org/10.1016/j.chempr.2017.06.001.
AUTHOR CONTRIBUTIONS
D.W. performed the synthetic studies. Y.L. and R.G. conducted single-crystal X-ray
diffraction studies. R.W. and H.H. performed the computational experiments. R.K.
directed the project and wrote the manuscript with input from all authors. All authors
analyzed the results and commented on the manuscript.
ACKNOWLEDGMENTS
We are grateful to Nanyang Technological University (NTU) and the Singapore Min-
istry of Education (MOE2015-T2-2-032) for financial support. H.H. acknowledges
financial support from City University of Hong Kong (CityU; 7200534 and 9610369)
and the supercomputer resources at NTU, CityU, and Hong Kong Baptist University.
Received: December 30, 2016
Revised: February 7, 2017
Accepted: June 5, 2017
Published: July 13, 2017
148 Chem 3, 134–151, July 13, 2017

REFERENCES AND NOTES
1
. Mahadevi, A.S., and Sastry, G.N. (2016).
Cooperativity in noncovalent interactions.
Chem. Rev. 116, 2775–2825.
Applications (Structure and Bonding)
(Springer).
34. Chong, C.C., and Kinjo, R. (2015). Catalytic
hydroboration of carbonyl derivatives, imines,
and carbon dioxide. ACS Catal. 5, 3238–3259.
1
9. Ma, J.C., and Dougherty, D.A. (1997). The
2
. Maharramov, A.M., Mahmudov, K.T.,
Kopylovich, M.N., and Pombeiro, A.J.L.
c1 a3 t2 i 4o .n ꢁp interaction. Chem. Rev. 97, 1303–
35. Stephan, D.W., and Erker, G. (2015).
Frustrated Lewis pair chemistry: development
and perspectives. Angew. Chem. Int. Ed. 54,
6400–6441.
(
2016). Non-Covalent Interactions in the
Synthesis and Design of New Compounds
John Wiley).
20. Zaidlewicz, M., and Pakulski, M.M. (2011).
Reduction of carbonyl groups: transfer
hydrogenation, hydrosilylation, catalytic
hydroboration, and reduction with
(
36. Martin, D., Soleilhavoup, M., and Bertrand, G.
(2011). Stable singlet carbenes as mimics for
transition metal centers. Chem. Sci. 2,
389–399.
3
4
5
. Baev, A.K. (2012). Specific Intermolecular
Interactions of Organic Compounds
borohydrides, aluminum hydrides, or
boranes. In Science of Synthesis,
Stereoselective Synthesis, Vol. 2, J.G. De
Vries, G.A. Molander, and P.A. Evans, eds.
(Springer).
. Baev, A.K. (2014). Specific Intermolecular
Interactions of Nitrogenated and Bioorganic
Compounds (Springer).
37. Power, P.P. (2010). Main-group elements as
transition metals. Nature 463, 171–177.
(Georg Thieme Verlag), pp. 59–131.
38. Blake, A.J., Cunningham, A., Ford, A., Teat,
2
2
1. Abdel-Magid, A.F. (1998). Reductions in
Organic Synthesis. Recent Advances and
Practical Applications; ACS Symposium
Series (American Chemical Society).
. Hobza, P., and M u¨ ller-Dethlefs, K. (2010).
Non-Covalent Interactions: Theory
and Experiment (Royal Society of
Chemistry).
S.J., and Woodward, S. (2000).
Enantioselective reduction of prochiral
ketones by catecholborane catalysed by
chiral group 13 complexes. Chem. Eur. J. 6,
3586–3594.
2. Hudlicky, M. (1984). Reductions in Organic
Chemistry (John Wiley).
6
. Hanoian, P., Liu, C.T., Hammes-Schiffer, S.,
and Benkovic, S. (2015). Perspectives on
electrostatics and conformational motions in
enzyme catalysis. Acc. Chem. Res. 48,
39. Ford, A., and Woodward, S. (1999). Catalytic
enantioselective reduction of ketones by a
chiral gallium complex and catecholborane.
Angew. Chem. Int. Ed. 38, 335–336.
23. Magano, J., and Dunetz, J.R. (2012). Large-
scale carbonyl reductions in the
482–489.
pharmaceutical industry. Org. Process Res.
Dev. 16, 1156–1184.
7
. Herschlag, D., and Natarajan, A. (2013).
Fundamental challenges in mechanistic
enzymology: progress toward understanding
the rate enhancements of enzymes.
Biochemistry 52, 2050–2067.
40. Arrowsmith, M., Hadlington, T.J., Hill, M.S.,
and Kociok-K o¨ hn, G. (2012). Magnesium-
catalysed hydroboration of aldehydes and
ketones. Chem. Commun. 48, 4567–4569.
24. Burkhardt, E.R., and Matos, K. (2006). Boron
reagents in process chemistry: excellent tools
for selective reductions. Chem. Rev. 106,
2617–2650.
4
1. Weetman, C., Anker, M.D., Arrowsmith, M.,
Hill, M.S., Kociok-K o¨ hn, G., Liptrot, D.J., and
Mahon, M.F. (2016). Magnesium-catalysed
nitrile hydroboration. Chem. Sci. 7, 628–641.
8
. Warshel, A., Sharma, P.K., Kato, M., Xiang, Y.,
Liu, H., and Olsson, M.H.M. (2006).
Electrostatic basis for enzyme catalysis.
Chem. Rev. 106, 3210–3235.
25. Arai, N., and Ohkuma, T. (2011). Reduction of
carbonyl groups: hydrogenation. In Science
of Synthesis, Stereoselective Synthesis, Vol. 2,
J.G. De Vries, G.A. Molander, and P.A. Evans,
eds. (Georg Thieme Verlag), pp. 9–57.
4
2. Intemann, J., Lutz, M., and Harder, S. (2014).
Multinuclear magnesium hydride clusters:
selective reduction and catalytic
9
. Jeffrey, G.A. (1997). An Introduction to
Hydrogen Bonding (Oxford University Press).
26. Cho, B.T. (2009). Recent development and
improvement for boron hydride-based
catalytic asymmetric reduction of
hydroboration of pyridines. Organometallics
33, 5722–5729.
1
0. Desiraju, G.R., and Steiner, T. (1999). The
Weak Hydrogen Bond in Structural Chemistry
and Biology (Oxford University Press).
unsymmetrical ketones. Chem. Soc. Rev. 38,
43. Arrowsmith, M., Hill, M.S., and Kociok-K o¨ hn,
G. (2013). Magnesium catalysis of imine
443–452.
1
1
1
1. Pihko, P.M. (2009). Hydrogen Bonding in
Organic Synthesis (Wiley-VCH).
2
2
7. Andersson, P.G., and Munslow, I.J. (2008).
Modern Reduction Methods (Wiley-VCH).
hydroboration. Chem. Eur. J. 19, 2776–2783.
4
4. Arrowsmith, M., Hill, M.S., Hadlington, T.,
Kociok-K o¨ hn, G., and Weetman, C. (2011).
Magnesium-catalyzed hydroboration of
pyridines. Organometallics 30, 5556–5559.
2. Gilli, G., and Gilli, P. (2009). The Nature of the
Hydrogen Bond (Oxford University Press).
8. Togni, A., and Gr u¨ tzmacher, H. (2001).
Catalytic Heterofunctionalization: from
Hydroamination to Hydrozirconation
3. Sohtome, Y., and Nagasawa, K. (2012).
Dynamic asymmetric organocatalysis:
(Wiley-VCH).
4
5. Fohlmeister, L., and Stasch, A. (2016). Ring-
shaped phosphinoamido-magnesium-
hydride complexes: syntheses, structures,
reactivity, and catalysis. Chem. Eur. J. 22,
10235–10246.
cooperative effects of weak interactions and
conformational flexibility in asymmetric
organocatalysts. Chem. Commun. 48, 7777–
2
9. Hayashi, T. (1999). Hydroboration of Carbon-
Carbon Double Bonds. In Comprehensive
Asymmetric Catalysis, Vol. 1, E.N. Jacobsen,
A. Pfaltz, and H. Yamamoto, eds. (Springer),
pp. 351–366.
7789.
1
4. Fu, X., and Tan, C.-H. (2011). Mechanistic
considerations of guanidine-catalyzed
reactions. Chem. Commun. 47, 8210–8222.
4
6. Hadlington, T.J., Hermann, M., Frenking, G.,
and Jones, C. (2014). Low coordinate
germanium(II) and tin(II) hydride complexes:
efficient catalysts for the hydroboration of
carbonyl compounds. J. Am. Chem. Soc. 136,
3028–3031.
3
0. Beletskaya, I., and Pelter, A. (1997).
Hydroborations catalysed by transition metal
complexes. Tetrahedron 53, 4957–5026.
1
5. Mahmudov, K.T., Kopylovich, M.N., Silva,
M.F.C.G.d., and Pombeiro, A.J.L. (2016). Non-
covalent interactions in the synthesis of
coordination compounds: recent advances.
Coord. Chem. Rev. http://dx.doi.org/10.
3
1. Lennox, A.J.J., and Lloyd-Jones, G.C. (2014).
Selection of boron reagents for Suzuki–
Miyaura coupling. Chem. Soc. Rev. 43,
47. Hadlington, T.J., Hermann, M., Frenking, G.,
and Jones, C. (2015). Two-coordinate group
412–443.
1016/j.ccr.2016.09.002.
14 element(II) hydrides as reagents for the
3
3
2. Hartwig, J.F. (2012). Borylation and silylation
of C–H bonds: a platform for diverse C–H
bond functionalizations. Acc. Chem. Res. 45,
864–873.
facile, and sometimes reversible,
hydrogermylation/hydrostannylation of
unactivated alkenes and alkynes. Chem. Sci.
6, 7249–7257.
1
1
6. Scheiner, S. (2015). Noncovalent Forces
(Springer).
7. Giese, M., Albrecht, M., and Rissanen, K.
(
2015). Anionꢁp interactions with
3. Soloway, A.H., Tjarks, W., Barnum, B.A., Rong,
F.-G., Barth, R.F., Codogni, I.M., and Wilson,
J.G. (1998). The chemistry of neutron capture
therapy. Chem. Rev. 98, 1515–1562.
48. Hadlington, T.J., Schwarze, B., Izgorodina,
E.I., and Jones, C. (2015). Two-coordinate
hydrido-germylenes. Chem. Commun. 51,
6854–6857.
fluoroarenes. Chem. Rev. 115, 8867–8895.
18. Metrangolo, P., and Resnati, G. (2010).
Halogen Bonding: Fundamentals and
Chem 3, 134–151, July 13, 2017 149

4
5
9. Hadlington, T.J., and Jones, C. (2014). A singly
bonded amido-distannyne: H activation and
isocyanide coordination. Chem. Commun. 50,
321–2323.
monomeric and dimeric ruthenium
complexes. Chem. Sci. 5, 1141–1152.
78. Aresta, M., Dibenedetto, A., and Angelini, A.
(2014). Catalysis for the valorization of exhaust
2
2
carbon: from CO to chemicals, materials, and
fuels. Technological use of CO . Chem. Rev.
2
114, 1709–1742.
2
64. Angeles-Boza, A.M., and Roth, J.P. (2012).
Oxygen kinetic isotope effects upon
catalytic water oxidation by a monomeric
ruthenium complex. Inorg. Chem. 51, 4722–
4729.
0. Hadlington, T.J., Hermann, M., Li, J.,
Frenking, G., and Jones, C. (2013). Activation
of H by a multiply bonded amido–
2
digermyne: evidence for the formation of a
hydrido–germylene. Angew. Chem. Int. Ed.
79. Appel, A.M., Bercaw, J.E., Bocarsly, A.B.,
Dobbek, H., DuBois, D.L., Dupuis, M., Ferry,
J.G., Fujita, E., Hille, R., Kenis, P.J.A., et al.
(2013). Frontiers, opportunities, and
65. von Hopffgartenm, M., and Frenking, G.
(2012). Energy decomposition analysis. Wires
Comput. Mol. Sci. 2, 43–62.
52, 10199–10203.
challenges in biochemical and chemical
2
catalysis of CO fixation. Chem. Rev. 113,
5
1. Yang, Z., Zhong, M., Ma, X., De, S., Anusha, C.,
Parameswaran, P., and Roesky, H.W. (2015).
An aluminum hydride that functions like a
transition-metal catalyst. Angew. Chem. Int.
Ed. 54, 10225–10229.
6621–6658.
66. Hirao, H. (2007). Reactive bond orbitals: a
localized resonance-structure approach to
charge transfer. Chem. Phys. Lett. 443,
141–146.
80. Lu, X.-B., Ren, W.-M., and Wu, G.-P. (2012).
CO copolymers from epoxides: catalyst
2
activity, product selectivity, and
stereochemistry control. Acc. Chem. Res. 45,
1721–1735.
5
2. Jakhar, V.K., Barman, M.K., and Nembenna, S.
67. Kitaura, K., and Morokuma, K. (1976). A new
energy decomposition scheme for molecular
interactions within the Hartree-Fock
(2016). Aluminum monohydride catalyzed
8
1. Lu, X.-B., and Darensbourg, D.J. (2012).
Cobalt catalysts for the coupling of CO
epoxides to provide polycarbonates and
cyclic carbonates. Chem. Soc. Rev. 41, 1462–
selective hydroboration of carbonyl
compounds. Org. Lett. 18, 4710–4713.
2
and
approximation. Int. J. Quant. Chem. 10,
325–340.
5
3. Mukherjee, D., Osseili, H., Spaniol, T.P., and
Okuda, J. (2016). Alkali metal
hydridotriphenylborates [(L)M][HBPh ]
3
1484.
6
8. Benz, S., L o´ pez-Andarias, J., Mareda, J.,
Sakai, N., and Matile, S. (2017). Catalysis with
chalcogen bonds. Angew. Chem. Int. Ed. 56,
8
8
2. Omae, I. (2012). Recent developments in
carbon dioxide utilization for the production
of organic chemicals. Coord. Chem. Rev. 256,
(M = Li, Na, K): chemoselective catalysts for
2
carbonyl and CO hydroboration. J. Am.
Chem. Soc. 138, 10790–10793.
812–815.
1384–1405.
6
9. Our system differs from the Corey-Bakshi-
Shibata system, which is historically the first
example of carbonyl hydroboration catalyzed
by an ambiphilic boron heterocycle. See
5
5
5
4. Chong, C.C., Hirao, H., and Kinjo, R. (2015).
Metal-free s-bond metathesis in 1,3,2-
diazaphospholene-catalyzed hydroboration
of carbonyl compounds. Angew. Chem. Int.
Ed. 54, 190–194.
3. Cokoja, M., Bruckmeier, C., Rieger, B.,
Herrmann, W.A., and K u¨ hn, F.E. (2011).
Transformation of carbon dioxide with
homogeneous transition-metal catalysts:
a molecular solution to a global challenge?
Angew. Chem. Int. Ed. 50, 8510–8537.
70
Corey et al.
70. Corey, E.J., Bakshi, R.K., and Shibata, S.
5. Wu, D., Ganguly, R., Li, Y., Hoo, S.N., Hirao,
H., and Kinjo, R. (2015). Reversible [4 + 2]
cycloaddition reaction of 1,3,2,5-
(1987). Highly enantioselective borane
reduction of ketones catalyzed by chiral
oxazaborolidines. Mechanism and synthetic
implications. J. Am. Chem. Soc. 109, 5551–
84. Riduan, S.N., and Zhang, Y. (2010). Recent
developments in carbon dioxide utilization
under mild conditions. Dalton Trans. 39,
3347–3357.
diazadiborinine with ethylene. Chem. Sci. 6,
7150–7155.
5553.
6. Wu, D., Kong, L., Li, Y., Ganguly, R., and Kinjo,
R. (2015). 1,3,2,5-Diazadiborinine featuring
nucleophilic and electrophilic boron centres.
Nat. Commun. 6, 7340.
7
1. Kobaisi, K.A., Bhosale, S.V., Latham, K.,
Raynor, A.M., and Bhosale, S.V. (2016).
Functional naphthalene diimides: synthesis,
properties, and applications. Chem. Rev. 116,
85. Sakakura, T., Choi, J.-C., and Yasuda, H.
(2007). Transformation of carbon dioxide.
Chem. Rev. 107, 2365–2387.
8
6. Nguyen, T.V.Q., Yoo, W.-J., and Kobayashi, S.
2015). Effective formylation of amines with
11685–11796.
5
5
5
7. Wang, B., Li, Y., Ganguly, R., Hirao, H., and
Kinjo, R. (2016). Ambiphilic boron in 1,4,2,5-
diazadiborinine. Nat. Commun. 7, 11871.
(
carbon dioxide and diphenylsilane catalyzed
by chelating bis(tzNHC) rhodium complexes.
Angew. Chem. Int. Ed. 54, 9209–9212.
7
2. Dawson, R.E., Hennig, A., Weimann, D.P.,
Emery, D., Ravikumar, V., Montenegro, J.,
Takeuchi, T., Gabutti, S., Mayor, M., Mareda,
J., et al. (2010). Experimental evidence for the
functional relevance of anion-p interactions.
Nat. Chem. 2, 533–538.
8. Wang, X.-Y., Wang, J.-Y., and Pei, J. (2015).
BN heterosuperbenzenes: synthesis and
properties. Chem. Eur. J. 21, 3528–3529.
8
7. Motokura, K., Takahashi, N., Miyaji, A.,
Sakamoto, Y., Yama-guchi, S., and Baba, T.
(
2014). Mechanistic studies on the N-
9. Campbell, P.G., Marwitz, A.J.V., and Liu, S.-Y.
formylation of amines with CO and
hydrosilane catalyzed by a Cu–diphosphine
complex. Tetrahedron 70, 6951–6956.
2
7
7
3. Zhao, Y., Domoto, Y., Orentas, E., Beuchat, C.,
Emery, D., Mareda, J., Sakai, N., and Matile, S.
(
2012). Recent advances in azaborine
chemistry. Angew. Chem. Int. Ed. 51, 6074–
092.
(2013). Catalysis with anion-p interactions.
6
Angew. Chem. Int. Ed. 52, 9940–9943.
8
8. Frogneux, X., Jacquet, O., and Cantat, T.
6
6
0. Bosdet, M.J.D., and Piers, W.E. (2008). B–N as
a C–C substitute in aromatic systems. Can. J.
Chem. 86, 8–29.
(
2014). Iron-catalyzed hydrosilylation of CO
CO conversion to formamides and
methylamines. Catal. Sci. Technol. 4, 1529–
533.
2
:
4. Liu, Q., Wu, L., Jackstell, R., and Beller, M.
2
(2015). Using carbon dioxide as a building
block in organic synthesis. Nat. Commun. 6,
5933.
1
1. Longobardi, L.E., Russell, C.A., Green, M.,
Townsend, N.S., Wang, K., Holmes, A.J.,
Duckett, S.B., McGrady, J.E., and Stephan,
D.W. (2014). Hydrogen activation by an
aromatic triphosphabenzene. J. Am. Chem.
Soc. 136, 13453–13457.
89. Gonz a´ lez-Sebasti a´ n, L., Flores-Alamo, M.,
and Garcıa, J.J. (2013). Nickel-catalyzed
75. Tlili, A., Blondiaux, E., Frogneux, X., and
Cantat, T. (2015). Reductive functionalization
hydrosilylation of CO in the presence of Et B
for the synthesis of formic acid and related
formates. Organometallics 32, 7186–7194.
2
3
2
of CO with amines: an entry to formamide,
formamidine and methylamine derivatives.
Green Chem. 17, 157–168.
6
2. Tsao, F.A., Cao, L., Grimme, S., and Stephan,
D.W. (2015). Double FLP-alkyne exchange
reactions: a facile route to Te/B heterocycles.
J. Am. Chem. Soc. 137, 13264–13267.
90. Itagaki, S., Yamaguchi, K., and Mizuno, N.
(2013). Catalytic synthesis of silyl formates
with 1 atm of CO2 and their utilization for
synthesis of formyl compounds and formic
acid. J. Mol. A Chem. 366, 347–352.
76. Yeung, C.S., and Dong, V.M. (2014). Making
C–C bonds from carbon dioxide via
transition-metal catalysis. Top. Catal. 57,
1342–1350.
6
3. Angeles-Boza, A.M., Ertem, M.Z., Sarma, R.,
Iba n˜ ez, C.H., Maji, S., Llobet, A., Cramer, C.J.,
and Roth, J.P. (2014). Competitive oxygen-18
kinetic isotope effects expose O–O bond
formation in water oxidation catalysis by
77. Maeda, C., Miyazaki, Y., and Ema, T. (2014).
Recent progress in catalytic conversions of
carbon dioxide. Catal. Sci. Technol. 4, 1482–
1497.
91. Motokura, K., Takahashi, N., Kashiwame, D.,
Yamaguchi, S., Miyaji, A., and Baba, T. (2013).
Copper-diphosphine complex catalysts for
N-formylation of amines under 1 atm
150 Chem 3, 134–151, July 13, 2017

of carbon dioxide with
polymethylhydrosiloxane. Catal. Sci. Technol.
of N–H bonds using carbon dioxide and
polymethylhydrosiloxane. J. Am. Chem. Soc.
134, 2934–2937.
107. Courtemanche, M.-A., L e´ gar e´ , M.-A., Maron,
L., and Fontaine, F.-G. (2014). Reducing
CO2 to methanol using frustrated Lewis
pairs: on the mechanism of phosphine–
borane-mediated hydroboration of CO2.
J. Am. Chem. Soc. 136, 10708–10717.
3
, 2392–2396.
9
9
2. Das, S., Bobbink, F.D., Bulut, S., Soudani, M.,
and Dyson, P.J. (2016). Thiazolium carbene
catalysts for the fixation of CO
100. Romero, E.A., Peltier, J.L., Jazzara, R., and
Bertrand, G. (2016). Catalyst-free
2
onto amines.
dehydrocoupling of amines, alcohols, and
thiols with pinacol borane and
Chem. Commun. 52, 2497–2500.
1
08. Courtemanche, M.-A., L e´ gar e´ , M.-A., Maron,
L., and Fontaine, F.-G. (2013). A highly active
phosphine–borane organocatalyst for the
reduction of CO2 to methanol using
hydroboranes. J. Am. Chem. Soc. 135, 9326–
9329.
9-borabicyclononane (9-BBN). Chem.
3. Chong, C.C., and Kinjo, R. (2015).
Hydrophosphination of CO and subsequent
formate transfer in the 1,3,2-
diazaphospholene-catalyzed N-formylation
of amines. Angew. Chem. Int. Ed. 54, 12116–
Commun. 52, 10563–10565.
2
1
1
1
01. Harinath, A., Anga, S., and Panda, T.K. (2016).
Alkali metal catalyzed dehydro-coupling
of boranes and amines leading to the
12120.
formation of a B–N bond. RSC Adv. 6, 35648–
109. Bontemps, S., and Sabo-Etienne, S. (2013).
Trapping formaldehyde in the
35653.
9
9
4. Frogneux, X., Blondiaux, E., Thu e´ ry, P., and
Cantat, T. (2015). Bridging amines with CO
organocatalyzed reduction of CO to aminals.
2
homogeneous catalytic reduction of carbon
dioxide. Angew. Chem. Int. Ed. 52, 10253–
2
:
02. Yang, Z., Zhong, M., Ma, X., Nijesh, K., De, S.,
Parameswaran, P., and Roesky, H.W. (2016).
An aluminum dihydride working as a catalyst
in hydroboration and dehydrocoupling.
J. Am. Chem. Soc. 138, 2548–2551.
10255.
ACS Catal. 5, 3983–3987.
1
10. Bontemps, S. (2016). Boron-mediated
activation of carbon dioxide. Coord. Chem.
Rev. 308, 117–130.
5. Zhou, H., Wang, G.-X., Zhang, W.-Z., and Lu,
X.-B. (2015). CO adducts of phosphorus
2
ylides: highly active organocatalysts for
carbon dioxide transformation. ACS Catal. 5,
03. Bola n˜ o, T., Esteruelas, M.A., Gay, M.P.,
O n˜ ate, E., Pastor, I.M., and Yus, M. (2015). An
acyl-NHC osmium cooperative system:
coordination of small molecules and
heterolytic B–H and O–H bond activation.
Organometallics 34, 3902–3908.
111. Fontaine, F.-G., Courtemanche, M.-A., and
L e´ gar e´ , M.-A. (2014). Transition-metal-free
catalytic reduction of carbon dioxide. Chem.
Eur. J. 20, 2990–2996.
6773–6779.
9
6. Hao, L., Zhao, Y., Yu, B., Yang, Z., Zhang, H.,
Han, B., Gao, X., and Liu, Z. (2015).
Imidazolium-based ionic liquids catalyzed
formylation of amines using carbon dioxide
and phenylsilane at room temperature. ACS
Catal. 5, 4989–4993.
1
12. Wolff, N.v., Lef e` vre, G., Berthet, J.-C., Thu e´ ry,
P., and Cantat, T. (2016). Implications of CO2
activation by frustrated Lewis pairs in the
catalytic hydroboration of CO2: a view using
N/Si+ frustrated Lewis pairs. ACS Catal. 6,
4526–4535.
1
1
04. Liptrot, D.J., Hill, M.S., Mahon, M.F., and
Wilson, A.S.S. (2015). Alkaline-earth-
catalyzed dehydrocoupling of amines and
boranes. Angew. Chem. Int. Ed. 54, 13362–
9
7. Blondiaux, E., Pouessel, J., and Cantat, T.
13365.
(2014). Carbon dioxide reduction to
methylamines under metal-free conditions.
Angew. Chem. Int. Ed. 53, 12186–12190.
05. Fern a´ ndez-Salas, J.A., Manzini, S., and Nolan,
S.P. (2013). Efficient ruthenium-catalysed S–S,
S–Si and S–B bond forming reactions. Chem.
Commun. 49, 5829–5831.
113. Tlili, A., Voituriez, A., Marinetti, A., Thu e´ ry, P.,
and Cantat, T. (2016). Synergistic effects in
ambiphilic phosphino-borane catalysts for the
hydroboration of CO2. Chem. Commun. 52,
7553–7555.
9
8. Gomes, C.D.N., Jacquet, O., Villiers, C.,
Thu e´ ry, P., Ephri-tikhine, M., and Cantat, T.
(2012). A diagonal approach to chemical
recycling of carbon dioxide: organocatalytic
transformation for the reductive
106. Declercq, R., Bouhadir, G., Bourissou, D.,
L e´ gar e´ , M.-A., Cour-temanche, M.-A., Nahi,
K.S., Bouchard, N., Fontaine, F.-G., and
Maron, L. (2015). Hydroboration of carbon
dioxide using ambiphilic phosphine–
borane catalysts: on the role of the
formaldehyde adduct. ACS Catal. 5, 2513–
2520.
114. Lafage, M., Pujol, A., Saffon-Merceron, N.,
functionalization of CO
2
. Angew. Chem. Int.
3
and M e´ zailles, N. (2016). BH activation by
phosphorus-stabilized geminal dianions:
synthesis of ambiphilic organoborane, DFT
studies, and catalytic CO reduction into
2
methanol derivatives. ACS Catal. 6, 3030–
Ed. 51, 187–190.
9
9. Jacquet, O., Gomes, C.D.N., Ephritikhine, M.,
and Cantat, T. (2012). Recycling of carbon and
silicon wastes: room temperature formylation
3035.
Chem 3, 134–151, July 13, 2017 151