A N-Phosphinoamidinato NHC-Diborene Catalyst for Hydroboration

A N ‑Phosphinoamidinato NHC-Diborene Catalyst for Hydroboration

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Jun Fan, Jian-Qiang Mah, Ming-Chung Yang, Ming-Der Su,* and Cheuk-Wai So*

Cite This: J. Am. Chem. Soc. 2021, 143, 4993 −5002

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sı Supporting Information

ABSTRACT: The use of the N-phosphinoamidinato NHC-diborene

catalyst 2 for hydroboration is described. The N-phosphinoamidine

tBu PN(H)C(Ph)N(2,6-iPr C H ) was reacted with nBuLi in Et O

to aﬀord the lithium derivative, which was then treated with

B Br (SMe ) in toluene to form the N-phosphinoamidinate-bridged

2 2

diborane 1. It was reacted with the N-heterocyclic carbene IMe

:C{N(CH )C(CH )} ) and excess potassium graphite at room

(

temperature in toluene to give the N-phosphinoamidinato NHC-

diborene compound 2. It can stoichiometrically activate ammonia−

borane and carbon dioxide. It also showed catalytic capability. A 2 mol

portion of 2 catalyzed the hydroboration of carbon dioxide (CO ) with pinacolborane (HBpin) in deuterated benzene (C D ) at

−

10 °C (conversion >99%), which aﬀorded the methoxyborane [pinBOMe] (yield 97.8%, TOF 33.3 h ) and the bis(boryl) oxide

[(pinB) O]. In addition, 5 mol % of 2 catalyzed the N-formylation of secondary and primary amines by carbon dioxide and

−

pinacolborane to yield the N-formamides (average yield 91.6%, TOF 25.9 h ). Moreover, 2 showed chemoselectivity toward

catalytic hydroboration of carbonyl compounds. In mechanistic studies, the BB double bond in compound 2 activated the

substrates, the intermediates of which then underwent hydroboration with pinacolborane to yield the products and regenerate

catalyst 2.

INTRODUCTION

(IPrMe)] (IPrMe = :C{N(iPr) C(Me)} ) was a precatalyst,

2 2

■

which reacted with CO to form a Al(μ-CO )(μ-O)Al six-

Multiply bonded main-group compounds containing an EE

double-bond (E = main-group element) or EE triple bond

are of particular interest, as they exhibit transition-metal-like

reactivity to activate and functionalize small molecules due to

the possession of a HOMO and LUMO with a small energy

membered ring. This acted as the active catalyst to react with

HBpin and CO to form formoxyborane. In these catalyses, the

AlAl and GeGe multiple bonds were not able to be

regenerated after the catalytic cycle. These results give rise to

the question of whether main-group multiple bonds can be

involved in a catalytic cycle.

diﬀerence. As an illustration, stable diboryne, diborene,

dialumene, disilene, digermyne, and distannyne were capable

of noncatalytically activating a diversity of small molecules:

namely, dihydrogen, ammonia, nitrous oxide, carbon mon-

To prove the concept, we were interested in synthesizing a

main-group-element multiply bonded delocalized compound

for catalytic organic reactions, because the delocalization eﬀect

could facilitate restoration of the main-group-element multiple

bond after a catalytic cycle. In this article, we report the

synthesis of a N-phosphinoamidinato N-heterocyclic carbene-

diborene catalyst and its application toward the catalytic

hydroboration of carbon dioxide and catalytic N-formylation

reactions of secondary and primary amines by carbon dioxide

and pinacolborane, as well as the chemoselective catalytic

hydroboration of carbonyl compounds. DFT calculations are

also reported to rationalize our experimental ﬁndings.

−4

oxide, and carbon dioxide.

However, reversible activation is

iPr

scarcely found, whereby only the distannyne [Ar SnSnAr ]

iPr

(

Ar = C H -2,6-Ar , Ar = 2,6-iPr C H ) reversibly under-

6 3 2 2 6 3

went cycloaddition with ethylene and norbornadiene, in

addition to the reversible oxidation with H . These results

indicate that small molecules have diﬃculty in dissociating

from the main-group-element centers after activation. As such,

catalytic organic transformations mediated by multiply bonded

main-group compounds are rare, with only two examples being

reported. The ﬁrst example was the digermyne-catalyzed

cyclotrimerization of terminal alkynes, where the digermyne

[

TbbGeGeTbb] (Tbb = 4-tBu-2,6-[CH(TMS) ]C H , TMS =

2 6 2

Received: December 4, 2020

Published: January 15, 2021

SiMe ) was a precatalyst (Figure 1) that reacted with the

alkyne to aﬀord the digermabenzene. It acted as the active

catalyst to react with three more molecules of alkynes to form

,2,4-triarylbenzene with high regioselectivity. The second

example was a dialumene-catalyzed CO hydroboration, where

the NHC-dialumene [(IPrMe)(tBu MeSi)AlAl(SiMetBu )-

2021 American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 4993−5002

993

Article

Figure 2. Molecular structure of 1 obtained by X-ray crystallography.

Thermal ellipsoids are set at the 50% probability level. All hydrogen

atoms are deleted for clarity. Selected bond lengths (Å) and angles

(

deg): B1−B2 1.704(5), P1−B2 1.940(4), N1−B1 1.428(5), N1−

C13 1.423(4), N2−C13 1.279(4), P1−N2 1.673(3), N1−B1−B2

C13 127.8(2), N2−C13−N1 126.3(3), C13−N1−B1 120.8(3).

24.3(3), B1−B2−P1 103.9(2), B2−P1−N2 104.53(15), P1−N2−

red precipitate, which was then reacted with excess KC in

toluene for 3 h at room temperature to yield a purple-red

mixture (Scheme 1). The precipitate (graphite and KBr) that

formed in the reaction mixture was removed by ﬁltration. After

that, the ﬁltrate was concentrated to aﬀord the phosphinoa-

midinato NHC-diborene compound 2 as a red crystalline solid.

The B{ H} NMR spectroscopy displays two signals at 28.8

and 10.5 ppm, indicating that compound 2 is composed of a

highly polarized BB double bond. The signals are in the

range of the B NMR signals (12−32 ppm) of similar Lewis

11−25

base-diborene complexes.

The P{ H} NMR signal (45.9

ppm) is shifted downﬁeld in comparison with that of 1. The

C{ H} NMR signal at 152.9 ppm is attributable to the

Figure 1. Multiply bonded main-group compounds for catalytic

organic reactions.

C_c_a_r_b_e_n_ecenter coordinating with the boron atom. The

molecular structure of compound 2 obtained by X-ray

crystallography is composed of a planar boron-containing six-

membered ring with a N2−C1−N1−B2 torsion angle of

−0.07° (Figure 3). This indicates that there is six-π-electron

delocalization along the N2−C1−N1−B1−B2 skeleton. The

N1 lone pair electrons delocalize along the amidinate skeleton,

RESULTS AND DISCUSSION

To begin with, the N-phosphinoamidine was reacted with

■

nBuLi in Et O for 3 h to aﬀord the lithium derivative, which

was then reacted with B Br (SMe )

in toluene for 12 h

Scheme 1). The LiBr that formed in the reaction mixture was

2 2

(

Scheme 1. Synthesis of 2

separated by ﬁltration. After that, the ﬁltrate was concentrated

to aﬀord the N-phosphinoamidinate-bridged diborane 1 as

colorless crystals. The ¹¹B{ H} NMR spectrum shows two

signals at 55.5 and −11.9 ppm for sp and sp boron centers,

respectively. The ³¹P{ H} NMR spectrum shows one signal at

5.9 ppm. X-ray crystallography of 1 shows that the ligand is

bridging between two boron centers, where the length of the

B1−B2 bond is 1.704(5) Å (Figure 2). Moreover, the length of

the N1−B1 bond is 1.428(5) Å, indicating that the N1 lone

pair of electrons delocalize to the vacant p orbital of the B1

center more than to the amidinate N1−C13−N2 skeleton

Figure 3. Molecular structure of 2 obtained by X-ray crystallography.

Thermal ellipsoids are shown at the 50% probability level. All

hydrogen atoms are removed for clarity. Selected bond lengths (Å)

and angles (deg): B2−B1 1.562(4), P1−B2 1.851(4), N1−B1

(

C13−N2 1.279(4) Å, C13−N1 1.423(4) Å).

.526(4), N2−P1 1.641(2), C1−N2 1.312(4), N1−C1 1.366(4),

Compound 1 was reacted with IMe (:C{N(CH )C-

N1−B1−B2 123.8(3), B1−B2−P1 112.5(2), B2−P1−N2

106.45(14), P1−N2−C1 129.8(2), N2−C1−N1 125.0(3).

(

CH )} ) at room temperature in toluene for 4 h to yield a

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from the mixing of B 2p orbitals (Figure 4 ). The HOMO

Article

as indicated by the length of the C1−N1 and C1−N2 bonds.

This results in lengthening of the N−B bond (N1−B1

Scheme 2. Synthesis of 3

.526(4) Å) in comparison with that of 1. The length of the

B2−B1 bond (1.562(5) Å) falls in the range of BB double-

bond lengths (1.546−1.625 Å) in similar Lewis base-diborene

1−25

complexes.

(

The NHC (B1−C20 1.590 Å) and phosphine

P1−B2 1.852(3) Å) of the phosphinoamidinate ligand are

bonded with the BB bond. Their bond lengths are similar to

those of other NHC/phosphine-diborene complexes (C−B

11−25

was isolated as a colorless crystalline solid from the

concentrated ﬁltrate. It is proposed that the reaction proceeds

.532−1.603 Å, P−B 1.863−1.929 Å).

DFT calculations (M06-2X/def2-TZVP, Figure S68) were

−

through the addition of H and H from NH BH to the BB

performed to elucidate the electronic structure of 2. The

HOMO-1 shows the σ orbital of the BB double bond arising

double bond in 2 to form the diborane intermediate I and

NH BH . The second pair of H and H from NH BH attacks

the N-phosphinoamidinate ligand and NHC-borane center,

respectively, to form the N-phosphinoamidine ligand, which

−

then displaces the Br moiety to form 3. Its H NMR spectrum

exhibits signals for the N-phosphinoamidine and IMe back-

bone. The B−H signal cannot be found in the spectrum due to

the quadrupolar boron nucleus. The P{ H} NMR spectrum

shows a multiplet at 85.6 ppm due to the coupling with N−H

and B−H protons. The B{ H} NMR spectrum shows two

broad signals for the boronium cation (−14.2 ppm) and

borane center (−36.2 ppm). The molecular structure of 3

obtained by X-ray crystallography (Figure 5 ) illustrates that the

Figure 4. Calculated frontier orbitals of compound 2 (B, pink; Br,

brown; C, yellow; H, white; N, blue; P, orange).

Figure 5. Molecular structure of 3 obtained by X-ray crystallography.

Thermal ellipsoids are displayed at the 50% probability level. All H

atoms are omitted for clarity. Selected bond lengths (Å) and angles

exhibits the π orbital of the BB double-bond, while the

LUMO represents the N 2p orbital conjugated with the π

system of the phenyl group. Accordingly, a natural bond orbital

(

deg): B2−B1 1.749(4), C28−B2 1.603(3), N1−B1 1.617(3), P1−

B1 1.970(2), P1−N2 1.6987(19), N2−C13 1.360(3), N1−C13

(

NBO, Table S8 ) analysis illustrates that the B1−B2 σ bond

.313(3), N1−B1−P1 96.19(14), N1−B1−B2 120.62(19), P1−B1−

arises from the mixing of sp hybrids on the boron atoms (B1,

B2 116.79(16), B1−B2−C28 114.2(2).

.31

1.41

sp ; B2, sp ). The B1−B2 π bond is slightly polarized

toward the B1 atom (54.2% B1 + 45.8% B2). It is generated by

the mixing of B p orbitals. The Wiberg bond index of 1.55

suggests that the B2−B1 bond is a double bond. In addition,

natural population analysis (NPA) shows that the charges on

the B1 and B2 atoms are −0.55 and 0.09 e, respectively. The

N-phosphinoamidine chelates to the boronium B1 cation,

while IMe coordinates to the borane B2 atom (C28−B2

1.603(3) Å). Both boron centers adopt a tetrahedral geometry.

The B1···Br distance is 9.331 Å, indicating that there is no

interaction between these atoms. The length of the B2−B1

bond is 1.749(4) Å. It is comparable to the B−B single-bond

length in compound 1 (1.704(5) Å). The N1−B1 bond

(1.617(3) Å) is considerably lengthened in comparison with

the N−B single-bond length (1.428(5) Å) in compound 1,

indicating that the former is a coordinative covalent bond. In

addition, the B1−P1 bond (1.970(2) Å) is comparable with

the B−P coordinative covalent bond (1.940(4) Å) in

dissected nucleus-independent chemical shift (NICS (1),

−

4.6 ppm; Table S9 ) shows that the diboron-containing six-

membered ring in 2 is slightly aromatic.

The BB double-bond character in 2 was illustrated by its

reactivity with NH BH in toluene at −78 °C (Scheme 2). The

reaction mixture was warmed to room temperature and stirred

for 3 h. The precipitate that formed in the reaction mixture was

discarded by ﬁltration. After that, the borylboronium cation 3

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compound 1. These data support that the N-phosphinoami-

dine ligand chelates the boronium B1 cation.

Compound 2 was capable of activating a small molecule,

fashion in reference to the boron-containing four-membered

ring. Compound 4 is stable in solution and the solid state and

does not decompose in reﬂuxing toluene. Recently, Braunsch-

weig et al. reported that the treatment of the NHC-diborene

[IPr(Br)BB(Br)IPr] (IPr = :C{N(2,6-iPr C H )CH} ) with

namely carbon dioxide (CO , 1 atm), in toluene at room

temperature (Scheme 3). The reaction mixture changed from

CO formed a [2 + 2] cycloaddition product, which was

Scheme 3. Formation of Compounds 4 and 5

thermally unstable and could not be isolated in quantity. It

rearranged to form the 2,4-diboraoxetan-3-one containing oxo

and carbonyl groups bridging between two boron centers.

It is anticipated that the activated CO moiety in compound

can be further functionalized. As such, compound 2 was

treated with CO₂and tris(pentaﬂuorophenyl)borane in

toluene at room temperature, where the reaction mixture

instantly changed from red-purple to colorless (Scheme 3).

After it was stirred for 30 min, the solution was ﬁltered.

Compound 5 was isolated as colorless crystals from the

concentrated ﬁltrate. The reaction proceeds through the

formation of compound 4, where the exocyclic CO double

bond further coordinates to B(C F ) to form compound 5. Its

reddish purple to colorless immediately. After the reaction

mixture was ﬁltered, compound 4 was isolated as a colorless

crystalline solid from the concentrated ﬁltrate. In the reaction,

the BB double bond in 2 undergoes a [2 + 2] cycloaddition

6 5 3

B{ H} NMR signals (−5.7, −20.3 ppm) are comparable with

with CO to give compound 4. Its H NMR spectrum displays

those of 4, indicating the presence of a B CO four-membered

signals of the N-phosphinoamidinate and IMe ligand. The

ring. Moreover, the B{ H} NMR signal of −26.9 ppm is

B{ H} NMR signals are at −5.5 and −17.9 ppm, which are

attributable to the four-coordinate B(C F ) . The molecular

shifted upﬁeld in comparison with those of compound 2,

6 5 3

structure of 5 obtained by X-ray crystallography illustrates that

demonstrating that the boron centers are four-coordinate.

the electronic structure of the B CO four-membered ring in 5

Moreover, the P{ H} NMR signal (39.7 ppm), which is

comparable with that of compound 1, is shifted upﬁeld in

comparison to that of compound 2. These spectroscopic data

indicate that the B−B bond in compound 4 is a single bond.

This is consistent with the molecular structure of 4 (Figure 6)

is diﬀerent from that of 4 (Figure 7 ). The O1 −C28 (1.292(4)

Figure 7. Molecular structure of 5 obtained by X-ray crystallography.

Thermal ellipsoids are set at the 50% probability level. All hydrogen

atoms are deleted for clarity. Selected bond lengths (Å) and angles

Figure 6. Molecular structure of 4 obtained by X-ray crystallography.

Thermal ellipsoids are illustrated at the 50% probability level. All

hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and

angles (deg): B2−B1 1.770(3), B2−C28 1.625(3), B1−O1 1.538(3),

O1−C28 1.349(3), O2−C28 1.218(3), B2−P1 1.940(2), B1−N1

(

deg): B1−B2 1.791(6), B2−C28 1.593(5), B1−O1 1.625(5), O1−

C28 1.292(4), O2−C28 1.280(4), O2−B3 1.551(5), B2−P1

.950(4), B1−N1 1.544(5), C29−B1 1.619(6), N1−B1−B2

21.2(3), O1−B1−B2 83.8(2), B1−B2−P1 106.6(2), B1−B2−C28

7.6(2), B2−C28−O1 104.2(3), C28−O1−B1 92.9(3), O1−C28−

.562(3), B1−C29 1.632(3), N1−B1−B2 118.67(17), B1−B2−P1

09.43(14), B1−B2−C28 78.73(15), B2−C28−O1 98.72(17), C28−

O2 122.7(3), B2−C28−O2 132.5(3), C28−O2−B3 128.1(3).

O1−B1 96.48(15), O1−B1−B2 86.06(15).

Å) and O2−C28 (1.280(4) Å) bond lengths are almost

identical, indicating that the double bond in the exocyclic C

O bond delocalizes along the O −C−O skeleton. The

B2−C28 (1.593(5) Å) bond is shortened and the B1−O1

(1.625(5) Å) bond is lengthened in comparison with those in

compound 4. The experimental results show that 5 should be

comprised of canonical forms, as illustrated in Scheme 4.

obtained by X-ray crystallography, where the length of the B−

B bond (1.770(3) Å) is comparable with that of the B−B

endo

exo

single bond in compound 1. The B CO four-membered ring is

planar with the endocyclic O1−C28 bond (1.349(3) Å) being

longer than the exocyclic O2−C28 bond (1.218(3) Å). In

addition, the Br and IMe moieties are positioned in a cis

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Scheme 4. Resonance Structures of 5

reaction was much more eﬃcient and extremely clean

(

[

conversion >99%), forming a mixture of the methoxyborane

pinBOMe] (6b, yield 97.8%, TOF 33.3 h ; Figure S22) and

−

c in a 1:1 ratio. The activity of compound 2 is outstanding

when the turnover frequency and product yield are considered.

The turnover frequency (TOF) far surpasses those of main-

group-element hydride compounds (TOF (h ): Mg, 0.07; Ca,

−

0.1; Ge, 2.1; Sn, 14.5) for such catalysis. Our results are

diﬀerent from thsoe for the NHC-dialumene-catalyzed hydro-

boration of CO with HBpin, where 80% conversion of CO to

[

pinBOC(O)H] was achieved at 80 °C for 3 h via the

It is anticipated that, if B(C F ) in 5 is replaced by HBpin,

5 3

dialuminum carbonate active catalyst.

the B−H bond could attack the CO bond to form the

formoxyborane [pinBOC(O)H] and regenerate compound 2.

As such, compound 2 should show catalytic reactivity toward

When the more sterically hindered 9-BBN (9-

borabicyclo[3.3.1]nonane) was used instead of HBpin (Table

S2 and Figure S25), 2 mol % of 2 catalyzed the hydroboration

of CO₂in C D at 110 °C to form small amounts of

formoxyborane 7a at time zero. When the reaction continued

to proceed, bis(boryl)acetal (7b, reaction time 1 h, yield

CO . To begin with, no reaction between carbon dioxide

(

CO ) and pinacolborane (HBpin) in deuterated benzene

C D ) at 110 °C was observed. A 2 mol % portion of 2 was

then used to catalyze the reaction of CO with HBpin in C D

7.5%) and methoxyborane (7c, reaction time 20 h, yield 92%)

at room temperature (Table S1). In the ﬁrst 1 h, a mixture of

the formoxyborane [pinBOC(O)H] (6a, reaction time 1 h,

were sequentially observed. The chronological formation of

hydroborated products suggest that the BB double bond was

recurrently restored in a catalytic cycle. Therefore, the BB

double bond in compound 2 is the active site in the catalytic

yield 7.5%; Figure S21) and bis(boryl)oxide [(pinB) O] (6c,

reaction time 1 h, yield 8.2%) were formed. After 20 h, the

conversion was still less than 30% and only 6c was observed

reaction time 20 h, yield 26%). When the reaction

temperature was increased to 110 °C (Scheme 5), the catalytic

^h^y^d^r^o^b^o^r^a^tⁱ^oⁿ^o^f^C^O2^.

(

To prove that compound 4 is an intermediate in the

catalysis, 2 mol % of 4 was utilized to mediate the catalytic

reaction of CO and HBpin in deuterated benzene (C D ) at

Scheme 5. Catalytic Hydroboration of CO₂

10 °C for 2 h to aﬀord a mixture of 6b (yield 92.7%, TOF

−

3.6 h ) and 6c in a 1:1 ratio. This illustrates that compound

further reacts with HBpin to form a zwitterionic moiety in

resemblance of compound 5.

On the basis of these experimental studies, the catalytic cycle

for the hydroboration of CO₂with HBpin is proposed

Scheme 6. DFT Calculations of the Catalytic Mechanism

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Scheme 6) and has been studied by DFT calculations (M06-

Article

(

Table 1. 2-Catalyzed N-formylation

X/def2-TZVP//M06-2X/def2-SVP/SMD(benzene)). The

BB double bond in 2 activates CO with HBpin via TS01

(

ΔG = 20.7 kcal/mol). Subsequently, the H−B bond

undergoes nucleophilic attack to the CO bond to form

Int01 (ΔG = −26.1 kcal/mol). It appears as a [2 + 2]

cycloaddition between 2 and the formoxyborane [HC(O)-

OBpin]. The second HBpin then coordinates with the acetal

oxygen atom in Int01 via TS02 (ΔG = −10.4 kcal/mol) to

give Int02 (ΔG = −11.0 kcal/mol). Its B−H bond attacks the

acetal carbon atom via TS03 (ΔG = −10.6 kcal/mol), which

induces nucleophilic attack of OBpin, leading to the formation

of pinBOBpin coordinated to the formaldehyde oxygen atom

in Int03 (ΔG = −39.0 kcal/mol). At this point, it is obvious

that the ﬁrst two C−H bond formations are kinetically facile

processes. The steric eﬀect between IMe and pinBOBpin leads

to dissociation of the latter to form Int04 (ΔG = −58.2 kcal/

mol). Int04 is considered as a [2 + 2] cycloaddition product

between 2 and a formaldehyde. The B−O bond in Int04 reacts

with the B−H bond of the third HBpin. It proceeds through σ-

Reaction conditions: Amine (0.10 mmol), HBpin (0.20 mmol, 2

bond metathesis at TS04 (ΔG = −30.2 kcal/mol) to form

equiv), 2a (5 mol %), C D (0.50 mL), and CO (1 bar). Yields were

Int05 (ΔG = −71.9 kcal/mol). The fourth HBpin then reacts

with Int05. The B−H bond attacks the methylene carbon at

calculated by H NMR spectroscopy on the basis of the amount of the

consumed amine with reference to cyclohexane (internal standard).

They are reported in parentheses. R R NC(O)H was identiﬁed as the

1 2

TS05 (ΔG = −46.7 kcal/mol), which results in cleaving the

sole product. All reactions were performed three times.

B−C

bond and forming the B−Bpin bond. These lead

methylene

to the formation of Int06 and MeOBpin (ΔG = −72.0 kcal/

mol). Int06 undergoes 1,2-elimination of HBpin via TS06

carbon dioxide. To understand the catalytic mechanism,

(

−

(

ΔG = −41.1 kcal/mol) to generate 2 and HBpin (ΔG =

piperidine 8e was reacted with HBpin in C D at 90 °C,

73.2 kcal/mol). The computed barriers of Int04→TS04

28.0 kcal/mol) and Int06→TS06 (30.9 kcal/mol) are

resulting in the formation of the borylamine and H . This

suggests that the formation of H in the above catalysis arose

from the coupling reaction of amines and HBpin. In this

context, a catalytic mechanism is proposed (Scheme 7).

consistent with the catalysis requiring 110 °C to occur. The

calculated results are in accordance with the experimental

conditions and observations. A reaction temperature of 25 °C

resulted in 6c with 26% yield (20 h), suggesting that the

reaction ends at Int04. A reaction temperature of 110 °C

provides suﬃcient energy to overcome the kinetic barrier

Scheme 7. Proposed Catalytic N-formylation Mechanism

(TS04 and TS06), and hence a mixture of 6b and 6c was

formed in a 1:1 ratio.

Then, the catalytic ability of 2 toward N-formylation of

primary and secondary amines by carbon dioxide and

pinacolborane was examined (Table 1 and Table S3 ). To

begin with, compound 2 (5 mol %) cannot catalyze the

reaction of 8e and carbon dioxide (CO ) at 90 °C without

pinacolborane (HBpin, Table S4). However, 5 mol % of 2

catalyzed the N-formylation of the primary aliphatic n-

butylamine 8b and 3-methoxypropan-1-amine 8c with CO₂

and HBpin in deuterated benzene (C D ) at 90 °C to yield the

−

corresponding N-formamides (yield (%) (TOF (h )): 9b, 95

40.0); 9c, 96 (40.0)), bis(boryl)oxide 6c, and H . Second, a

(

high yield and TOF were accomplished for the N-formylation

of aliphatic secondary diethylamine 8d (yield 92%, TOF 50

Compound 2 catalyzes the reaction of CO and HBpin to form

−

−1

h ), diisopropylamine 8a (yield 98%, TOF 13.3 h ), and

formoxyborane [pinBOC(O)H] (6a). This then undergoes a

substitution reaction with borylamine, which is formed by the

dehydrogenation reaction of amine and HBpin, to form N-

formamide and pinBOBpin 6c.

−

piperidine 8e (yield 98%, TOF 33.3 h ). Third, aromatic N-

methylaniline 8f (yield 81%, TOF 3.1 h ) and indoline 8g

yield 91%, TOF 1.4 h ) were less active in the catalyses. It is

−

(

noteworthy that compound 2 is one of the rare main-group

catalysts that mediate N-formylation using carbon dioxide and

pinacolborane, including the NHC-silyliumylidene cation

In addition to the catalytic hydroboration of carbon dioxide

and N-formylation, 2 (5 mol %) was able to catalyze the

chemoselective hydroboration of aldehyde and ketone 10 in

C D at room temperature to form borate esters (11, 16

[

(IMe) SiH]I. Moreover, 2 exceeds the latter in view of

−

both reaction time and TOF (example of [(IMe) SiH]I, 9a:

examples: aldehydes, yield = 99%, TOF = 4−120 h ; ketones,

−

−1

yield 68%, TOF 6.8 h ). Other main-group-element catalysts,

yield = 80−98%, TOF = 0.4−10 h ; Scheme 8 and Table S5).

namely proazaphosphatrane and carbodicarbene, can only

use the potent 9-BBN for the N-methylation of amines with

The results further support that compound 2 is an active

catalyst in >CO hydroboration.

998

J. Am. Chem. Soc. 2021, 143, 4993−5002

Journal of the American Chemical Society

Article

m/z calcd for C H B Br N P, 683.0744 [(M + H)] ; found,

Scheme 8. Chemoselective Hydroboration of Carbonyl

Compounds

27 41

683.0761.

Synthesis of 2. A toluene solution of 1,3,4,5-tetramethylimida-

zolin-2-ylidene (IMe, 0.149 g, 1.2 mmol) was placed in a 100 mL

Schlenk ﬂask containing 1 (0.685 g, 1 mmol) at room temperature.

After 10 min, the reaction mixture turned red with some precipitate.

The resulting suspension was stirred for 4 h. Subsequently, it was

ﬁltered and the pale red residue was then dried under vacuum for 2 h.

The red residue was the adduct 1·IMe adduct and was used without

further puriﬁcation. A 30 mL portion of toluene was placed in a 100

mL Schlenk ﬂask containing a mixture of 1·IMe (0.809 g, 1 mmol)

and excess potassium graphite (0.54 g, 4 mmol) at room temperature,

causing the reaction mixture to turn purplish red immediately. The

resulting suspension was stirred for 3 h and ﬁltered. The ﬁltrate was

concentrated to 5 mL and stored at −20 °C for 1 day to give 2 as a

CONCLUSION

In conclusion, the phosphinoamidinato NHC-diborene com-

pound 2 is a multiply bonded boron catalyst showing catalytic

capability toward the hydroboration of CO with HBpin and

■

red crystalline solid (0.24 g, 37% yield). Mp: 198 °C dec. H NMR

(

399.5 MHz, 25 °C, C D ): δ 7.66−7.64 (m, 2 H, ArH), 7.05−6.88

m, 6 H, ArH), 3.37 (sept, 2 H, CHMe , J = 7.2 Hz), 3.19 (s, 6 H,

N-formylation of primary and secondary amines by CO and

N-CH ), 1.71 (d, 18 H, tBu, J = 12.8 Hz), 1.23 (s, 6 H, C-CH₃),

HBpin, as well as chemoselective hydroboration of carbonyl

compounds. In particular, compound 2 eﬃciently reduces CO₂

with HBpin to form MeOBpin and pinBOBpin. Its catalytic

activity is superior to that of existing main-group-element

catalysts employed for such reactions. In the mechanistic

studies, compound 2 exhibits reactivity resembling that of a

.85 (d, 6 H, CHMe , J = 6.4 Hz), 0.80 (d, 6 H, CHMe , J = 6.8

Hz). B{ H} NMR (128 MHz, 25 °C, C D ): δ 30.64, 9.16. P{ H}

NMR (162 MHz, 25 °C, C D ): δ 45.93 (br). C{ H} NMR (101

MHz, 25 °C, C D ): 152.85 (C-carbene), 145.37 (CH-Ar), 144.46

(CH-Ar), 141.05 (d, J = 16.4 Hz, C-Ar), 130.72 (CH-Ar), 129.01

(CH-Ar), 128.25 (CH-Ar), 127.62 (CH-Ar), 126.90 (CH-Ar), 126.70

transition metal in the catalytic hydroboration of CO . Its B

(C-Ar), 126.65 (C-Ar), 124.13 (C-Ar), 123.04 (CCH

), 38.65 (d, J =

2.5 Hz, PC(CH )), 33.13 (NCH ), 28.47 (CH(CH ) ), 27.72 (d, J =

B double bond activated CO₂with HBpin, the initial

intermediate of which was further reacted with two HBpin

molecules to form MeOBpin and pinBOBpin, along with the

regeneration of 2. It is anticipated that compound 2 can

mediate a variety of other catalytic reactions, and they are

currently under investigation.

3 2

.9 Hz, PC(CH )), 24.95 (CH(CH ) ), 23.55 (CH(CH ) ), 7.82

3 2

(

CH ). HRMS(ESI): m/z calcd for C H B BrN P, 649.3377 [(M

3 34 53 2 4

H)] ; found, 649.3395.

Synthesis of 3. A 20 mL portion of toluene was placed in a 100

mL Schlenk ﬂask containing 2 (0.130 g, 0.2 mmol) and NH BH

(

0.007 g, 0.2 mmol) at −78 °C. The resulting solution was warmed to

room temperature and stirred for 3 h with the color changing from

red-purple to pale red. The reaction solution was ﬁltered. The ﬁltrate

was then concentrated and kept at room temperature to give 3 as a

EXPERIMENTAL SECTION

■

General Procedure. All experiments were performed under an

argon gas atmosphere by standard Schlenk procedures. Chemicals

were acquired from Sigma-Aldrich and utilized directly without

colorless crystalline solid (0.07 g, 54% yield). Mp: 147 °C. H NMR

(399.5 MHz, 25 °C, C D ): δ 7.78−7.76 (m, 2 H, ArH), 6.87−7.04

6 6

further puriﬁcation. All solvents were dried over K metal or CaH

(m, 4 H, ArH), 6.75−6.69 (m, 2 H, ArH), 3.29 (sept, 1 H, CHMe

prior to use. H, B{ H}, C{ H}, and P{ H} NMR spectroscopy

were performed using a JEOL ECA 400 MHz spectrometer. All NMR

spectra were recorded in deuterated benzene. The chemical shifts

HH = 7.2 Hz), 2.78 (sept, 1 H, CHMe

, JHH = 7.2 Hz), 2.61 (s, 6 H,

), 1.78 (dd, 18 H, tBu, JPH = 14.2 Hz, JPH = 15.1 Hz), 1.44 (d,

, JHH = 6.4 Hz), 1.27 (s, 6 H, C-CH ), 1.11 (d, 3 H,

, JHH = 6.8 Hz), 0.46 (d, 3 H, CHMe , JHH = 6.8 Hz), 0.12 (d,

N-CH

3 H, CHMe

CHMe

3 H, CHMe

δ −14.24 (br), −34.15 (br). P{ H} NMR (162 MHz, 25 °C, C

(

ppm) are respective to SiMe for C and H, BF ·Et O for B, and

5% H PO for P, respectively. HRMS spectrometry was carried out

, JHH = 6.8 Hz). B{ H} NMR (128 MHz, 25 °C, C

at the Division of Chemistry and Biological Chemistry (Mass

Spectrometry Laboratory), Nanyang Technological University.

Melting points were recorded using an OptiMelt automated melting

point instrument.

Synthesis of 1. A solution of nBuLi (2.6 M, 0.4 mL, 1.05 mmol)

in hexane was placed in a 100 mL Schlenk ﬂask containing a stirred

diethyl ether solution of N-phosphinoamidine (0.425 g, 1 mmol) at

δ 85.62 (br). C{ H} NMR (101 MHz, 25 °C, C D ): 167.13 (d, J =

6 6

11.5 Hz, NCN), 145.90 (C-carbene), 144.89 (CH-Ar), 143.81 (CH-

Ar), 139.24 (d, J = 8.6 Hz, C-Ar), 132.17 (CH-Ar), 131.83 (CH-Ar),

130.23 (CH-Ar), 126.70 (CH-Ar), 126.11 (CH-Ar), 125.25 (C-Ar),

124.63 (C-Ar), 124.38 (C-Ar), 123.74 (CCH

37.72 (d, J = 23.1 Hz, PC(CH )), 35.50 (d, J = 25.1 Hz, PC(CH

34.68 (NCH ), 31.50 (NCH ), 28.64 (CH(CH ), 28.08 (d, J = 28.9

Hz, PC(CH )), 27.74 (d, J = 28.9 Hz, PC(CH )), 25.48

(CH(CH ), 24.80 (CH(CH ), 23.94 (CH(CH ), 23.10 (CH-

(CH ), 22.67 (CH(CH ), 7.89 (CH ), 7.58 (CH ). HRMS (ESI):

P, 653.3690 [(M + H)] ; found,

), 121.75 (CCH

)),

−

78 °C, following which, the solution was warmed to room

)

3 2

temperature and stirred for 3 h. The resulting yellow solution was

dried under vacuum to remove all volatiles. A solution of

B Br (SMe ) (0.466 g, 1 mmol) in toluene was then slowly placed

)

3 2

)

2 2

in the Schlenk ﬂask at −78 °C. The reaction mixture was stirred at

room temperature for 12 h and then was ﬁltered. The ﬁltrate was then

concentrated and stored at 4 °C for 2 days to give 1 as a colorless

m/z calcd for C34H B BrN

57 2 4

653.3705.

Synthesis of 4. A toluene solution of 2 (0.130 g, 0.2 mmol) in a

Schlenk ﬂask was degassed by a freeze−pump−thaw method. Then,

CO (1 bar) was ﬁlled. The resulting solution changed from reddish

crystalline solid (0.52 g, 76% yield). Mp: 177 °C. H NMR (399.5

MHz, 25 °C, C D ): δ 7.33−7.30 (m, 2 H, ArH), 7.05−6.77 (m, 6 H,

ArH), 3.38 (sept, 2 H, CHMe , J = 6.8 Hz), 1.44 (d, 18 H, tBu, J

purple to colorless immediately. After 10 min of stirring, the solution

was ﬁltered. The ﬁltrate was concentrated and kept at −20 °C for 1

day to give 4 as a colorless crystalline solid (0.12 g, 88% yield). Mp:

14.2 Hz), 1.32 (d, 6 H, CHMe , J = 6.8 Hz), 0.93 (d, 6 H,

2 HH

11 1

CHMe , J = 6.8 Hz). B{ H} NMR (128 MHz, 25 °C, C D ): δ

5.50 (s), −11.94 (s). P{ H} NMR (162 MHz, 25 °C, C D ): δ

193 °C. H NMR (399.5 MHz, 25 °C, C D ): δ 7.66−7.64 (m, 2 H,

5.72 (br). ¹³C{ H} NMR (101 MHz, 25 °C, C D ): 168.14 (d, J =

ArH), 7.14−7.02 (m, 2 H, ArH), 6.92−6.88 (m, 3 H, ArH), 6.79−

6.75 (m, 1 H, ArH), 3.83 (sept, 1 H, CHMe , J = 7.2 Hz), 3.63 (s, 3

1.6 Hz, NCN), 145.41 (CH-Ar), 140.05 (CH-Ar), 138.74 (d, J =

3.5 Hz, C-Ar), 129.44 (CH-Ar), 128.89 (CH-Ar), 128.78 (CH-Ar),

28.45 (CH-Ar), 127.62 (CH-Ar), 126.99 (C-Ar), 124.45 (C-Ar),

8.28 (d, J = 34.8 Hz, PC(CH )), 28.62 (CH(CH ) ), 27.77

H, N−CH ), 2.83 (sept, 1 H, CHMe , J = 7.2 Hz), 2.61 (s, 3 H, N-

CH ), 1.69 (d, 3 H, CHMe , J = 7.3 Hz), 1.67 (dd, 18 H, tBu, J_P_H

7.3 Hz, J_P_H= 8.2 Hz), 1.37 (s, 3 H, C-CH ), 0.92 (d, 3 H, CHMe ,

(

PC(CH )), 25.14 (CH(CH ) ), 23.34 (CH(CH ) ). HRMS(ESI):

J_H_H= 6.8 Hz), 0.87 (s, 3 H, C-CH ), 0.74 (d, 3 H, CHMe , J = 6.4

3 2 HH

999

J. Am. Chem. Soc. 2021, 143, 4993−5002

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c12627.

Article

Hz), 0.09 (d, 3 H, CHMe , J_H_H= 6.8 Hz). B{ H} NMR (128 MHz,

were placed catalyst 2 (0.0032 g, 0.005 mmol), cyclohexane (internal

standard, 8.4 mg, 0.10 mmol), HBpin (0.0256 g, 0.20 mmol), amine

(0.10 mmol) and deuterated benzene (0.5 mL). Subsequently, it was

dipped in liquid nitrogen and the reaction mixture was frozen. It was

degassed using s freeze−pump−thaw method. Carbon dioxide (1 bar)

was subsequently ﬁ lled. The details of the catalytic settings are given

in Tables S3 and S4 in the Supporting Information. The catalysis was

5 °C, C D ): δ −5.47, −17.86. P{ H} NMR (162 MHz, 25 °C,

C D ): δ 39.66 (br). C{ H} NMR (101 MHz, 25 °C, C D ): 168.72

d, J = 8.7 Hz, NCN), 148.99 (C-carbene), 144.31 (CH-Ar), 142.42

CH-Ar), 141.10 (d, J = 15.4 Hz, C-Ar), 130.53 (CH-Ar), 128.52

CH-Ar), 127.28 (CH-Ar), 127.10 (CH-Ar), 126.76 (CH-Ar), 125.71

C-Ar), 124.72 (C-Ar), 124.45 (C-Ar), 123.88 (CCH ), 123.72

CCH ), 38.46 (d, J = 44.3 Hz, PC(CH )), 38.08 (d, J = 44.3 Hz,

PC(CH )), 34.37 (NCH ), 31.85 (NCH ), 28.64 (CH(CH ) ), 27.88

d, J = 28.9 Hz, PC(CH )), 26.71 (CH(CH ) ), 25.66 (CH(CH ) ),

13 1

(

checked by H NMR spectroscopy. The yields of products were

calculated on the basis of the integration of H NMR signals of

3 2

(

R R NC(O)H at ca. 8 ppm relative to that of cyclohexane (C H

3 2

5.09 (CH(CH ) ), 24.86 (CH(CH ) ), 22.87 (CH(CH ) ), 7.53

CH ), 7.31 (CH ). HRMS(ESI): m/z calcd for C H B BrN O P,

1.37 ppm). The NMR data are given in the Supporting Information.

3 2

(

Catalytic Reaction of Aldehydes and Ketones with HBpin:

35 53

General Procedure. Catalyst 2 (0.013 g, 0.02 mmol) and C D (0.5

93.3276 [(M + H)] ; found, 693.3302.

mL) were mixed in a J. Young NMR tube. Pinacolborane (0.052 g,

.41 mmol, 20.1 equiv) and carbonyl compounds (0.40 mmol, 20

Synthesis of 5. A toluene solution of 2 (0.130 g, 0.2 mmol) and

tris(pentaﬂuorophenyl)borane (0.110 g, 0.2 mmol) in a Schlenk ﬂask

equiv) were subsequently added. The catalytic settings are given in

Table S5 . The catalyses were checked by H NMR spectroscopy to

was degassed by a freeze−pump−thaw method. Subsequently, CO (1

bar) was ﬁlled. The resulting solution changed from red-purple to

colorless immediately. After 30 min of stirring, the solution was

ﬁltered. The ﬁltrate was concentrated and kept at −20 °C for 1 day to

calculate their yields. The chemical shifts of the products are

consistent with the reported values in the literature. All of the

catalytic trials were performed three times.

aﬀord 5 as a colorless crystalline solid (0.20 g, 84% yield). Mp: 161

C. H NMR (399.5 MHz, 25 °C, C D ): δ 7.27−7.25 (m, 2 H,

ArH), 7.08−6.90 (m, 2 H, ArH), 6.80−6.78 (m, 3 H, ArH), 6.64−

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge at

■

.61 (m, 1 H, ArH), 3.56 (s, 3 H, N-CH ), 2.82 (sept, 1 H, CHMe ,

sı

J_H_H= 7.2 Hz), 2.49 (sept, 1 H, CHMe , J = 7.2 Hz), 2.14 (s, 3 H,

N-CH ), 1.42 (dd, 18 H, tBu, J = 13.7 Hz, J_P_H= 15.1 Hz), 1.42 (s, 3

H, C-CH ), 1.13 (s, 3 H, C-CH ), 0.95 (d, 3 H, CHMe , J = 6.4

Hz), 0.81 (d, 3 H, CHMe , J = 6.8 Hz), 0.21 (d, 3 H, CHMe , J =

.8 Hz), 0.10 (d, 3 H, CHMe , J = 6.8 Hz). B{ H} NMR (128

Experimental procedures and DFT calculations (PDF)

2 HH

Accession Codes

MHz, 25 °C, C D ): δ −5.71, −20.32, −26.89. F{ H} NMR (376

CCDC 2047677−2047681 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

MHz, 25 °C, C D ): δ −131.31 (s, 1 F), −159.34 (t, 1 F, J = 21.7

Hz), −164.69 (dt, 1 F, J = 21.7, 21.7 Hz). P{ H} NMR (162 MHz,

5 °C, C D ): δ 37.44 (br). C{ H} NMR (101 MHz, 25 °C, C D ):

δ 168.48 (d, J = 8.7 Hz, NCN), 149.56 (C-F), 147.42 (C-carbene),

47.11 (C-F), 144.05 (CH-Ar), 141.74 (CH-Ar), 139.66 (d, J = 16.3

Hz, C-Ar), 138.33 (C-F), 137.58 (C-F), 135.73 (C-F), 130.89 (CH-

Ar), 129.27 (CH-Ar), 129.01 (CH-Ar), 128.24 (CH-Ar), 127.61 (CH-

Ar), 126.81 (C-Ar), 126.16 (C-Ar), 125.95 (C-Ar), 125.72 (C-Ar),

AUTHOR INFORMATION

Corresponding Authors

■

25.37 (CCH ), 123.58 (CCH ), 39.34 (d, J = 45.7 Hz, PC(CH )),

7.94 (d, J = 39.5 Hz, PC(CH )), 35.79 (NCH ), 32.10 (NCH ),

8.31 (d, J = 38.5 Hz, PC(CH )), 27.54 (CH(CH ) ), 26.82

Ming-Der Su − Department of Applied Chemistry, National

Chiayi University, Chiayi 60004, Taiwan; Department of

Medicinal and Applied Chemistry, Kaohsiung Medical

University, Kaohsiung 80708, Taiwan; orcid.org/0000-

3 2

(

CH(CH ) ), 25.03 (CH(CH ) ), 24.21 (CH(CH ) ), 22.20 (CH-

3 2 3 2 3 2

(

CH ) ), 21.11 (CH(CH ) ), 7.60 (CH ), 7.53 (CH ). HRMS(ESI):

m/z calcd for C H B BrF N O P, 1205.3129 [(M + H)] ; found,

1205.3145.

0002-5847-4271 ; Email: midesu@mail.ncyu.edu.tw

Synthesis of Compounds 6a−c. Catalyst 2 (0.026 g, 0.04

Cheuk-Wai So − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371;

orcid.org/0000-0003-4816-9801; Email: CWSo@

ntu.edu.sg

mmol), cyclohexane (internal standard, 0.168 g, 2 mmol, 50 equiv),

and deuterated benzene (0.5 mL) were placed in a J. Young NMR

tube. HBpin (0.256 g, 2 mmol, 50 equiv) was subsequently added.

After that, the J. Young NMR tube was dipped in liquiﬁed nitrogen,

which froze the reaction mixture. It was degassed using a freeze−

pump−thaw method. Carbon dioxide (1 bar) was then ﬁlled. The

Authors

reaction mixture was warmed from room temperature to 110 °C for

.5 h. The catalysis was monitored using H NMR spectroscopy. The

yields were calculated on the basis of the integration of H NMR

signals of pinBOMe (6b) at 1.05 and 3.51 ppm and (pinB) O (6c) at

.02 ppm relative to the −CH signal of cyclohexane (internal

standard) at 1.37 ppm. The chemical shifts of the products agree with

the reported values in the literature and independent in situ NMR-

Jun Fan − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, Singapore 637371

Jian-Qiang Mah − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371

Ming-Chung Yang − Department of Applied Chemistry,

National Chiayi University, Chiayi 60004, Taiwan

scale syntheses.

Synthesis of Compounds 7a−d. Catalyst 2 (1.3 mg, 0.002

mmol), 9-BBN (0.012 g, 0.1 mmol, 50 equiv), and C D (0.5 mL)

were placed in a J. Young NMR tube. The solution was degassed by a

freeze−pump−thaw method as described in the synthesis of 6a−c.

Carbon dioxide (1 bar) was then ﬁlled. After that, the resulting

mixture was warmed from room temperature to 110 °C. The catalysis

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c12627

Author Contributions

The manuscript was written through contributions of all

authors.

was checked by H NMR spectroscopy at time intervals of 20 min, 40

min, 1 h, 4 h, 12 h, and 20 h. The NMR data are given in the

Supporting Information.

Notes

Catalytic N-formylation of Secondary and Primary Amines

by CO and HBpin: General Procedure. In a J. Young NMR tube

The authors declare no competing ﬁnancial interest.

000

J. Am. Chem. Soc. 2021, 143, 4993−5002

Journal of the American Chemical Society

Article

6) Wang, S.; Sherbow, T. J.; Berben, L. A.; Power, P. P. Reversible

ACKNOWLEDGMENTS

■

Coordination of H by a Distannyne. J. Am. Chem. Soc. 2018, 140,

C.-W.S. thanks the Ministry of Education Singapore, AcRF

Tier 2 (MOE2019-T2-2-129), and the National Research

Foundation Singapore, NRF-ANR (NRF2018-NRF-ANR026

Si-POP), for the ﬁnancial support. M.-C.Y. and M.-D.S.

acknowledge the National Center for High-Performance

Computing of Taiwan for generous amounts of computing

time and the Ministry of Science and Technology of Taiwan

for ﬁnancial support.

7) Sugahara, T.; Guo, J.-D.; Sasamori, T.; Nagase, S.; Tokitoh, N.

90−593.

Regioselective Cyclotrimerization of Terminal Alkynes Using a

Digermyne. Angew. Chem., Int. Ed. 2018, 57, 3499−3503.

(8) Weetman, C.; Bag, P.; Szilvasi, T.; Jandl, C.; Inoue, S. CO

Fixation and Catalytic Reduction by a Neutral Aluminum Double

Bond. Angew. Chem., Int. Ed. 2019, 58, 10961−10965.

9) Ogawa, T.; Ruddy, A. J.; Sydora, O. L.; Stradiotto, M.; Turculet,

L. Cobalt- and Iron-Catalyzed Isomerization-Hydroboration of

Branched Alkenes: Terminal Hydroboration with Pinacolborane and

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NOTE ADDED AFTER ASAP PUBLICATION

■

After ASAP publication on January 21, 2021, DFT calculations

of the catalytic mechanism were revised, as a mechanistic

pathway with a lower kinetic barrier was found. The

calculations are consistent with experimental observations

and do not aﬀect the conclusions in the article. Scheme 6 and

the related text have been revised, and DFT data have been

added to Tables S19−S24 in the Supporting Information. The

corrected version was reposted on March 29, 2021.

002