Cobalt(II) Coordination Polymer as a Precatalyst for Selective Hydroboration of Aldehydes, Ketones, and Imines

Article abstract of DOI:10.1021/acs.joc.8b01094

Highly effective hydroboration precatalyst is developed based on a cobalt(II)-terpyridine coordination polymer (CP). The hydroboration of ketones, aldehydes, and imines with pinacolborane (HBpin) has been achieved using the recyclable CP catalyst in the presence of an air-stable activator. A wide range of substrates containing polar C=O or C=N bonds have been hydroborated selectively in excellent yields under ambient conditions.

Full text of DOI:10.1021/acs.joc.8b01094

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Note
Cobalt(II) Coordination Polymer as a Pre-catalyst for
Selective Hydroboration of Aldehydes, Ketones and Imines
Jing Wu, Haisu Zeng, Jessica Cheng, Shengping Zheng, James A. Golen, David R. Manke, and Guoqi Zhang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01094 • Publication Date (Web): 22 Jun 2018
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Cobalt(II) Coordination Polymer as a Pre-catalyst for Selective
Hydroboration of Aldehydes, Ketones and Imines
Jing Wu, †,‡ Haisu Zeng,†,‡ Jessica Cheng,† Shengping Zheng,*,‡ James A. Golen,§ David R. Manke§ and
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Guoqi Zhang,*,
†
† Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The Graduate Center of the City University of
New York, New York, 10019 NY, USA.
‡ Department of Chemistry, Hunter College, the City University of New York, New York, 10065 NY, USA.
§
Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA.
ABSTRACT: Highly effective hydroboration precatalyst is developed
based on a cobalt(II)-terpyridine coordination polymer (CP). The hy-
droboration of ketones, aldehydes and imines with pinacolborane
(HBpin) has been achieved using the recyclable CP catalyst in the pres-
ence of an air-stable activator. A wide range of substrates containing
polar C=O or C=N bonds have been hydroborated selectively in excel-
lent yields under ambient conditions.
Catalytic hydroboration of carbonyl compounds and imines
provides an important and convenient approach to functional-
ized alcohols and amines, respectively, comparing to the con-
ventional stoichiometric reduction using hydride reagents
(such as LiAlH4, or NaBH4), which usually suffer from poor
functional group tolerance and product selectivity (aldehydes
vs. ketones or over-reduction), modest reaction rates (with
hindered ketones and imines) and often harsh conditions.1
Recent efforts have been made to develop efficient catalysts
for the hydroboration of carbonyl compounds and imines with
relatively less-active reductants, such as HBpin. In a recent
review, Kinjo and coworker have documented the catalytic
methods for this transformation.2 A number of discrete metal
complexes have been utilized as catalysts for this reaction.3,4
Among the nonprecious metals explored, cobalt is quite attrac-
tive in catalytic hydrogenation and dehydrogenation process-
es.5,6 Major progress has been achieved for the observation of
well-defined homogeneous cobalt hydrogenation catalysts
under mild conditions, including the Co-PNP pincer complex
studied by our group,7,8 and some have also found applications
in catalytic hydroboration of alkenes, alkynes and nitriles. 8e,9
While homogeneous metal catalysts for hydroboration of
carbonyl compounds and imines have been extensively ex-
plored, heterogeneous approaches for this conversion were
little addressed. Mesoporous silica-supported amidozirconium
catalysts have been previously reported for the hydroboration
of carbonyl compounds.10 It was recently realized that incor-
porating active metal sites within a metal-organic framework
(MOF) could be a viable strategy for catalytic hydroboration
of unsaturated bonds. In 2016, Lin and coworkers revealed
that single-site magnesium alkyl supported on the metal nodes
of TPHN-MOF [UiO-69 topology, TPHN
=
4, 4-
bis(carboxyphenyl)-2-nitro-1,1′-biphenyl] cataylzed the hy-
droboration of carbonyl compounds and imines with high
turnover numbers (TONs), where the catalyst was post-
synthesized by reacting active Me2Mg with as-synthesized
MOF materials.11 More recently, Delferro et al. reported a
MOF-supported single-site Ti(IV) catalyst for the hydrobora-
tion of carbonyl compounds.12 This catalyst also utilizes the
post-synthetic modification on an existing Zr-MOF with
Ti(OiPr)4 to generate catalytically active Ti(IV) sites. Typical-
ly, in those limited examples the active hydroboration cata-
lysts were generated by modifying MOF materials (construct-
ed by complex organic linkers) with an air- and moisture-
sensitive metal reagent for immobilization of catalytically
active sites.
Herein, we report the first example of a readily available
cobalt(II) coordination polymer as a precatalyst for selective
hydroboration of aldehydes, ketones and imines under mild
conditions with high efficiency, in the absence of air- and
moisture-sensitive organometallic reagents.
Previously, well-defined cobalt(II) complexes based on
tridentate NNN ligands have found applications in the hydrob-
oration of a variety of nonpolar and polar multiple bonds, in
the presence of activators (NaHB(OEt)3 or LiCH2SiMe3).8e,9b,13
We also reported a manganese(II) dialkyl complex of
2,2′;6′,2′′-terpyridine (tpy) that catalyzed the Markovnikov-
selective hydroboration of alkenes.14 Significantly, Thomas
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and coworkers have revealed that air-stable cobalt(II) or
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iron(II) dihalide complexes of NNN ligands could act as effec-
tive precatalysts for alkene hydroboration and hydrosilylation
while being activated by sodium tert-butoxide.15 To design
extended coordination polymers as heterogeneous precatalysts,
we envisioned that anchoring an additional N coordination site
on the tpy backbone would form a ditopic ligand (1, Scheme
1) that is suitable for construction of extended coordination
framework upon coordinating with CoII dihalide.16 Thus, the
metal centers within the framework should adopt a coordina-
tion environment similar to that reported previously in discrete
complexes.15 To compare, the divergent terpyridine ligand, 4′-
(4-dimethylaminophenyl)-4,2′;6′,4′′-tpy (2), was also em-
ployed for similar coordination framework.17
4
4
Figure 1. The ORTEP structures of 3 and 4 plotted at 50% thermal ellip-
soid probability level and the ball-stick representation of the extended
network in 3 and 4, respectively. H-Atoms are omitted for clarity.
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To demonstrate the capability of CPs 3 and 4 in catalytic
hydroboration of carbonyl compounds with HBpin, initially
we employed similar reaction conditions from the known co-
balt-catalyzed hydroboration involving discrete complexes.15
The results are summarized in Table 1. When only 0.1 mol%
of 3 and potassium tert-butoxide (KOtBu, 2 mol %) were used,
the hydroboration of acetophenone proceeded well in THF at
room temperature, affording the hydroborated product 5 in
>99% GC yield after 4 h. In comparison, 4 is also an active
precatalyst under the same conditions, even though the yield
was slightly lower. Similar results were found when using
NaOtBu or KOH as an activator, while NaHB(OEt)3 acts as a
poorer activator for this reaction. Control experiments show
that the use of CP 3 was crucial for the excellent reactivity,
whereas the combination of CoCl2 (2 mol %) and KOtBu also
exhibited modest catalytic activity. Interestingly, the discrete
complex, Co(tpy)Cl2, displayed inferior efficiency compared
to the CP analogue. In addition, solvent effects were studied,
and the results indicated that THF is more suitable than several
other solvents tested for this reaction. When catecholborane
was used as a boron source under the optimized conditions (as
in entry 1, Table 1), the corresponding hydroborated product
was found in only 25% GC yield (entry 13, Table 1). It was
interesting to note that the hydroboration even proceeded
smoothly in the air, giving 5a in 87% yield (entry 14, Table 1).
Finally, when 0.01 mol % of 3 was used as the precatalyst,
product 5 was detected in 86% yield, corresponding to a turn-
over number of 8,600, comparable to the MOF-supported sin-
gle site catalysts reported previously.11,12
Scheme 1. Ligands
Terpyridine
1 and 2 Derived from 2,2′;6′,2′′-
Coordination-driving self-assembly of known 1 and 2 with
cobalt dichloride by a layering technique has afforded good-
quality crystals of 3 and 4 in high yields, respectively (see SI).
Both crystalline compounds were characterized by IR, ele-
mental analysis, and X-ray crystallography. Structural analysis
revealed that 3 crystallized in the orthorhombic space group
Pcca. As expected, the CoII center resides within the N3 cavity
of one ligand, and an extra Npyridyl atom from another ligand
links to the CoII center to extend the structure. Two coordinat-
ed chloride anions are located in the axial direction of CoN4
coordination sphere, leading to a pseudo-octahedral geometry
around CoII (Figure 1). The assembled structure is polymeric
and propagates along the crystallographic c axis to form a one-
dimensional (1D) coordination polymeric chain. The chains
are further packed by a strong interchain π-stacking interaction
(the shortest inter-atomic C…C distance between the adjacent
chains is 3.495 (5) Å), thanks to the perfect coplanarity of the
aromatic region. In contrast, 4 crystallized in the orthorhombic
space group Pbcn. CoII is hexacoordinate with four N atoms of
four distinct ligands and two Cl atoms to form an octahedral
coordination geometry. The structural expansion around CoII
centers results in the formation of a 1D coordination frame-
work composed of cage-like subunits formed by a [4 + 4] met-
al-ligand assembly (Figure 1). It is worth mentioning that alt-
hough numerous CPs derived from related tpy ligands have
been reported,18 3 and 4 are the first examples of structurally
characterized CPs from the assembly of tpy with CoCl2.
Table 1. Condition Screening for Cobalt(II) CP-Catalyzed
Hydroboration of Acetophenone with Pinacolborane.a
Entry Catalyst
Activator
KOtBu
KOtBu
NaOtBu
KOH
Solvent
THF
THF
THF
THF
THF
THF
Yield /%b
1
2
3
4
5
6
3
4
3
3
3
3
>99
95
98
97
70
0
NaHB(OEt)3
-
7
-
KOtBu
KOtBu
KOtBu
THF
THF
THF
8
8c
9d
CoCl2
Co(tpy)Cl2
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KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
Et2O
88
68
65
25
87
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nism was involved. In addition, relatively lower yields were
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14f
15g
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obtained for 2-acetylnaphthalene (6i) and diaryl ketones (6j
and 6k). However, the cyclic ketone, tetralone, was hydrobo-
rated completely (6l) to afford the corresponding cyclic alco-
hol in 97% yield. In addition, the steric hindered ketone, 2′-
methylacetophenone also proceeded well with 6m being iso-
lated in 90% yield. Unfortunately, ketones containing a reduc-
ible functional group were not suitable substrates for effective
ketone hydroboration under standard conditions. For example,
when 4′-nitroacetophenone was used, a complicated mixture
of hydroborated products was detected (~ 50% conversion of
ketone) and no desired alcohol product could be isolated. Fi-
nally, more challenging alkyl ketones proved to be suitable
substrates (6n-q).
The heterogeneous hydroboration of ketones catalyzed by
3 allows us to study the recycling and reuse of the catalyst. In
a recycling reaction using acetophenone as a model substrate
(Figure S3, see SI), catalyst 3 (2 mol %) was filtered out of the
reaction mixture and reused for 5 more reaction cycles (the
activator was reloaded in each reuse cycle). The results
showed no obvious drop in the yields determined by GC, indi-
cating excellent recyclability of the CP pre-catalyst.
Under the same conditions, we further investigated the
hydroboration of aldehydes to prepare primary alcohols. Gen-
erally, aldehyde hydroboration proceeded smoothly in 16 h,
with the alcohol products being isolated in good to excellent
yields after column chromatography (Scheme 3). Benzalde-
hydes bearing halogen, electron-withdrawing or electron-
donating groups were hydroborated to the corresponding
boronate esters in almost quantitative conversions, and alco-
hols were isolated in high yields (7a-i), except that in the case
of p-cyanobenzaldehyde the reaction was a little sluggish and
the desired product 7e was obtained in a modest yield. In addi-
tion, benzaldehyde containing an ester functional group was
selectively hydroborated on the aldehyde, affording 7j in a
reasonable yield. Furthermore, trans-cinnamaldehyde contain-
ing conjugated C=C and C=O bonds was selectively hydrobo-
rated on the C=O part (7k), even though 2.0 equiv. of HBpin
was used. While the heterocyclic aldehyde, 2-
thiophenecarboxaldehyde is a suitable substrate for the hy-
droboration (7l), 2-pyridinecarboxaldehyde did not react at all
(7m), probably due to the strong binding ability of the sub-
strate to metal centers, which deactivates the catalyst. Finally,
aliphatic aldehydes were found to be readily hydroborated in
high efficiencies (7n and 7o).
toluene
pentane
THF
THF
THF
a Conditions: acetophenone (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1
mol%), activator (2 mol%) and solvent (1 mL), 25°C, 1 h, N2. b Deter-
mined by GC analysis with hexamethylbenzene as an internal standard. c
2
mol% catalyst was used. d 1 mol% catalyst was used. e Catecholborane was
used as the boron source. f Reaction run in the air. g 0.01 mol% 3 was used
and reaction run for 24 h.
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Scheme 2. The CP-Catalyzed Hydroboration of Ketones.a
a Conditions: ketone (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol
%), KOtBu (2 mol %) and THF (1 mL), 25 C, 4 h, N2. Yields of isolated
°
products of alcohols (GC yields of boronate esters are shown in parenthe-
sis). b 10.0 mmol acetophenone was used.
Next, we sought to investigate the application of 3-
catalyzed hydroboration under optimized conditions. A variety
of ketones were first examined, and the results are summarized
in Scheme 2. For all reactions indicated here, the hydroborated
products were confirmed by GC-MS analysis. Purification of
the products through silica gel column chromatography af-
forded hydrolyzed products, the alcohols (6) in high yields,
consistent with the results reported previously.14 The hydrobo-
ration of acetophenone was readily approached in a gram
scale, yielding 1-phenylethanol (6a) in 94% isolated yield.
Functionalized aryl ketones were smoothly hydroborated un-
der the standard conditions. Halogenated acetophenones were
completely converted to the corresponding boronate esters and
isolated as alcohols in excellent yields (6b-d). Methyl phenyl
ketones containing electron-donating groups and a bulkier
isopropyl phenyl ketone also proceeded well (6e-g). Cyclo-
propyl phenyl ketone was used to produce 6h in a reasonable
yield without ring-opening, indicating that no radical mecha-
Scheme 3. The CP-Catalyzed Hydroboration of Aldehydes.a
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benzylideneaniline at room temperature, showing good
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chemoselectivity for ketones over imines. In addition, 4-
acetylbenzaldehyde was used to study the intramolecular se-
lectivity. It was found that the hydroboration reaction mainly
occurred at the aldehyde, affording 9 in 81% yield after col-
umn chromatography. The chemoselectivity on aldehydes over
ketones observed here is similar to those found in ruthenium-,
lanthanide-, or aluminum-catalyzed hydroboration of carbonyl
compounds.4,19
In summary, we report the first cobalt-based coordination
polymer-catalyzed selective hydroboration of ketones, alde-
hydes and imines under mild, heterogeneous conditions. In
comparison with other nonprecious metal catalysts, the CoII
CP catalysis system is easy-to-make, air-stable, highly effi-
cient, reusable, and tolerant of a wide range of substrates. This
work implies new catalytic applications of simple coordination
polymers based on Earth-abundant metals, paving the way for
development of practical and sustainable catalysts for key
organic transformations.
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a Conditions: aldehydes (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1
mol %), KOtBu (2 mol %) and THF (1 mL), 25°C, 16 h, N2. Yields of
isolated products of alcohols (GC yields of boronate intermediates are
shown in parenthesis). b 2.0 eq. of HBpin was used and ~34% starting
c
material was detected by GC. ~20% starting material was detected by
GC. d 2.0 eq. of HBpin was used. e n. r.: no reaction.
Encouraged by these results, we studied the hydroboration
of imines with the cobalt CP catalyst. While there was no reac-
tion observed when N-benzylideneaniline was mixed with
HBpin under the standard condition at room temperature, at
EXPERIMETAL SECTION
General Information. The catalytic reactions were carried out
under a nitrogen atmosphere using standard glove-box technique.
Anhydrous grade solvents and liquid reagents used were obtained
from Aldrich or Fisher Scientific and stored over 4 Å molecular
sieves. FT-IR spectra were recorded on a Shimadzu 8400S in-
strument with solid samples under N2 using a Golden Gate ATR
accessory. Elemental analyses were performed by Midwest Mi-
o
elevated temperature (70 C) complete hydroboration of the
imine occurred, with 8a being isolated in 96% yield (Scheme
4). Accordingly, several substituted aromatic imines contain-
ing fluoro, nitro or ester groups have been converted to the
corresponding amines (8b-d) in good yields by the same pro-
tocol, demonstrating better applicability of the CP precatalyst.
1
crolab LLC in Indianapolis. H NMR and 13C NMR spectra were
Scheme 4. The CP-Catalyzed Hydroboration of Imines.a
obtained at room temperature on a Bruker AV 500 or 600 MHz
NMR spectrometer, with chemical shifts (δ) referenced to the
residual solvent signal. GC-MS analysis was obtained using a
Shimadzu GCMS-QP2010S gas chromatograph mass spectrome-
ter. 2,2′;6′,2′′-Terpyridine was purchased from Sigma-Aldrich. 4′-
(4-Pyridyl)-2,2′;6′,2′′-tpy
(1),
4′-(4-dimethylaminophenyl)-
4,2′;6′,4′′-tpy (2) and Co(tpy)Cl2 were prepared according to the
literature procedure.8d, 16, 17
Experimental Procedure. Synthesis of 3. A solution of 1 (62.0
mg, 0.200 mmol) in MeOH/CH2Cl2 (10 mL, 1 : 3, v/v) was placed
in a test tube. A blank solution of MeOH/CH2Cl2 (6 mL, 1: 1, v/v)
was layered on the top of the ligand solution, followed by a solu-
tion of CoCl2·6H2O (47.4 mg, 0.200 mmol) in MeOH (10 mL).
The tube was sealed and allowed to stand at room temperature for
a week, during which time X-ray quality light brown crystals
grew on the wall and bottom of the tube. The crystals were col-
lected by decanting the solvent and washed with MeOH and then
dried in vacuo. Yield: 55.0 mg (89%). FT-IR (solid, cm-1) 3481br,
3057m, 1599s, 1569w, 1539s, 1472m, 1408s, 1346w, 1247s,
1169m, 1079m, 1012s, 897w, 842s, 792s, 748m, 730m. Anal.
Calcd. for C20H14Cl2CoN4·2H2O: C 50.44, H 3.81, N 11.77%.
Found C 50.31, H 3.79, N 11.52.
Synthesis of 4. A solution of 2 (70.4 mg, 0.200 mmol) in
MeOH/CH2Cl2 (10 mL, 1 : 3, v/v) was placed in a test tube. A
blank solution of MeOH/CH2Cl2 (6 mL, 1: 1, v/v) was layered on
the top of the ligand solution, followed by a solution of
CoCl2·6H2O (47.4 mg, 0.200 mmol) in MeOH (10 mL). The tube
was sealed and allowed to stand at room temperature for a week,
a Conditions: imines (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol
%), KOtBu (2 mol %) and THF (1 mL), 70°C, 16 h, N2. Yields of isolated
products of amines (GC conversion of imines are shown in parenthesis).
The chemoselectivity of CP-catalyzed hydroboration of
ketones and aldehydes was further examined. First, the hy-
droboration of equimolar acetophenone and benzaldehyde
with HBpin (1.0 equiv.) was conducted in a competition study
(Scheme S1), and the results indicated that the hydroboration
of aldehyde is much preferred, providing only hydroborated
product of benzaldehyde. In contrast, the discrete catalyst,
Co(tpy)Cl2/KOtBu is much less active, whereas the trend of
chemoselectivity remains the same (Scheme S1A). This trend
was further confirmed in a similar experiment using substitut-
ed ketone and aldehyde as substrates when 3 was the precata-
lyst. It was also observed that hydroboration of acetophenone
was not affected by the presence of equimolar N-
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during which time X-ray quality light brown crystals grew on the
none (120.0 mg, 1.0 mmol) and pinacolborane (140.8 mg, 1.1
mmol) were then added. The reaction mixture was allowed to stir
at room temperature for 4 h. Then, the solid was centrifuged out
of suspension and extracted with hexane three times. The com-
bined organic extracts were analyzed by GC-MS using hexa-
methylbenzene as an internal reference. The recovered solid cata-
lyst in 1.0 mL THF was charged into a small vial, to which
KOtBu (2.2 mg, 20 μmol) was added. Then, acetophenone (120.0
mg, 1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were
added. The resultant mixture was stirred at room temperature in
the glovebox for 4 h. The solid was centrifuged out of suspension
and extracted with hexane three times. The combined organic
extracts were analyzed by GC-MS. The results of five recycling
experiments are summarized in Figure S1.
1
2
3
4
5
6
7
8
bottom of the tube. The crystals were collected by decanting the
solvent and washed with MeOH and then dried in vacuo. Yield:
76.6 mg (92% based on 2). FT-IR (solid, cm-1) 3050m, 2900m,
1608s, 1589s, 1557w, 1525s, 1497m, 1434w, 1399s, 1363s,
1216m, 1168m, 1063s, 1012s, 946m, 844s, 817s, 727s, 674s.
Anal. Calcd. for C46H40Cl2CoN8: C 66.19, H 4.83, N 13.42%.
Found C 66.05, H 4.90, N 13.30.
General Procedure for CP-Catalyzed Hydroboration of Ke-
tones. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg, 1.0
μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0
mL) in a 3.8 mL glass vial equipped with a stir bar. Ketone (1.0
mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then added.
The reaction mixture was allowed to stir at room temperature for
4 h. After completion of the reaction, the solvent was evaporated
and the crude reaction mixture was first analyzed by GC-MS us-
ing a dilute CH2Cl2 solution, and then the product was isolated by
flash column chromatography with SiO2 using ethyl ace-
tate/hexane as eluent. The pure products were characterized by 1H
and 13C NMR spectroscopies.
General Procedure for CP-Catalyzed Hydroboration of Alde-
hydes. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg,
1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF
(1.0 mL) in a 3.8 mL glass vial equipped with a stir bar. Aldehyde
(1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then
added. The reaction mixture was allowed to stir at room tempera-
ture for 16 h. After completion of the reaction, the solvent was
evaporated and the crude reaction mixture was first analyzed by
GC-MS using a dilute CH2Cl2 solution, and then the product was
isolated by flash column chromatography with SiO2 using ethyl
acetate/hexane as eluent. The pure products were characterized by
1H and 13C NMR spectroscopies.
General Procedure for CP-Catalyzed Hydroboration of
Imines. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg,
1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF
(1.0 mL) in a 20 mL Schlenk tube equipped with a stir bar. Imine
(1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then
added. The reaction mixture was sealed and allowed to heat to 70
ºC for 16 h. After completion of the reaction, the solvent was
evaporated and the crude reaction mixture was first analyzed by
GC-MS using a dilute CH2Cl2 solution, and then the product was
isolated by flash column chromatography with SiO2 using ethyl
acetate/hexane as eluent. The pure products were characterized by
1H and 13C NMR spectroscopies.
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Spectroscopic Data for Isolated Products. 1-Phenylethanol
(6a). 14 Colorless oil. Yield: 117.0 mg (96%). 1H NMR (600 MHz,
CDCl3) δ 7.47 – 7.31 (m, 4H), 7.29 (ddd, J = 8.8, 4.8, 3.4 Hz, 1H),
4.85 (q, J = 6.5 Hz, 1H), 2.63 (s, 1H), 1.47 (d, J = 6.4 Hz, 3H)
ppm; 13C NMR (151 MHz, CDCl3) δ 145.9, 128.5, 127.4, 125.5,
70.3, 25.2 ppm.
1-(4-Fluorophenyl)ethanol (6b).14 Colorless oil. Yield: 133 mg
(95%). 1H NMR (600 MHz, CDCl3) δ 7.35 – 7.33 (m, 2H), 7.05 –
7.01 (m, 2H), 4.89 (q, J = 6.5 Hz, 1H), 1.82 (br, 1 H), 1.48 (d, J =
6.5 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 162.4 (d, J =
244.9 Hz), 141.9 (d, J = 2.4 Hz), 127.4(d, J = 8.1 Hz), 115.6 (d, J
= 21.2 Hz), 70.1 (d, J = 2.3 Hz), 25.6 ppm.
1-(4-Chlorophenyl)ethanol (6c). 14 Colorless oil. Yield: 150.3
1
mg (96%). H NMR (600 MHz, CDCl3) δ 7.33 – 7.28 (m, 4H),
4.86 (q, J = 6.5 Hz, 1H), 2.23 (br, 1H), 1.47 (d, J = 6.5 Hz, 3H)
ppm; 13C NMR (151 MHz, CDCl3) δ 144.3, 133.0, 128.6, 126.8,
69.7, 25.2 ppm.
1-(4-Bromophenyl)ethanol (6d). 14 Colorless oil. Yield: 191.0
1
mg (95%). H NMR (600 MHz, CDCl3) δ 7.46 – 7.43 (m, 2H),
7.22 – 7.20 (m, 2H), 4.81 (q, J = 6.6 Hz, 1 H), 2.34 (br, 1H), 1.43
(d, J = 6.6 Hz, 3 H) ppm; 13C NMR (151 MHz, CDCl3) δ 145.1,
131.8, 127.4, 121.4, 70.0, 25.5 ppm.
1-(4-Methoxyphenyl)ethanol (6e). 14 Yellowish oil. Yield: 149
mg (98%). 1H NMR (600 MHz, CDCl3) δ 7.30 (d, J = 8.7 Hz,
2H), 6.89 (d, J = 8.7 Hz, 2H), 4.84 (q, J = 6.5 Hz, 1 H), 3.81 (s,
1H), 2.27 (br, 1H), 1.45 (d, J = 6.5 Hz, 3 H) ppm. 13C NMR (126
MHz, CDCl3) δ 158.9, 138.1, 126.7, 113.8, 69.9, 55.3, 25.1 ppm.
1-(3-Methoxyphenyl)ethanol (6f).7a Yellowish oil. Yield:
1
144.6 mg (95%). H NMR (600 MHz, CDCl3) δ 7.29 – 7.27 (m,
1H), 6.97 – 6.96 (m, 2H), 6.84 – 6.82 (m, 1H), 4.88 (q, J = 6.7
Hz, 1H), 3.83 (s, 3H), 2.07 (br, 1H), 1.50 (d, J = 6.7 Hz, 3 H)
ppm; 13C NMR (151 MHz, CDCl3) δ 160.1, 147.9, 129.8, 118.0,
113.2, 111.2, 70.6, 55.5, 25.4 ppm.
2-Methyl-1-phenylethanol (6g).19a Colorless oil. Yield: 120 mg
(80%). 1H NMR (500 MHz, CDCl3) δ 7.42 – 7.32 (m, 4H), 7.32 –
7.28 (m, 1H), 4.39 (d, J = 6.7 Hz, 1H), 1.99 (qd, J = 6.8, 1.5 Hz,
1H), 1.89 (s, 1H), 1.03 (d, J = 6.7 Hz, 3H), 0.83 (d, J = 6.9 Hz, 3H)
ppm; 13C NMR (126 MHz, CDCl3) δ 143.7, 128.3, 127.5, 126.7,
80.2, 35.4, 19.1, 18.4 ppm.
Competing Experiment for Selective Hydroboration of Ketones
vs. Aldehydes. In a glove box under N2 atmosphere, catalyst 3
(0.44 mg, 1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved
in THF (1.0 mL) in a 3.8 mL glass vial equipped with a stir bar.
The Ketone (1.0 mmol), aldehyde (1.0 mmol) and pinacolborane
(128.0 mg, 1.0 mmol) were then added sequentially. The reaction
mixture was allowed to stir at room temperature for 16 h. The
reaction was exposed to the air and the solvent was evaporated.
The products were analyzed by GC-MS using hexamethylbenzene
as an internal reference.
1-Cyclopropyl phenylmethanol (6h).14 Yellowish oil. Yield:
1
106.7 mg (72%). H NMR (600 MHz, CDCl3) δ 7.43 – 7.42 (m,
2H), 7.37 – 7.35 (m, 2H), 7.31 – 7.28 (m, 1H), 4.00 (d, J = 8.3
Hz, 1H), 2.21 (br, 1H), 1.25 – 1.19 (m, 1H), 0.66 – 0.61 (m, 1 H),
0.58 – 0.53 (m, 1 H), 0.49 - 0.45 (m, 1 H), 0.40 – 0.35 (m, 1 H)
ppm; 13C NMR (151 MHz, CDCl3) δ 144.2, 128.6, 127.8, 126.3,
78.8, 19.4, 3.9, 3.1 ppm.
Recycling and Reusing Experiment for Hydroboration of Ace-
tophenone. In a glovebox under N2 atmosphere, catalyst 3 (4.4
mg, 10 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in
THF (1.0 mL) in a small vial equipped with a stir bar. Acetophe-
1-(2-naphthalenyl)ethanol (6i).20 White solid. M.p.: 69 ˗ 70
1
°C. Yield: 114 mg (66%). H NMR (600 MHz, CDCl3) δ 7.85 –
7.83 (m, 3H), 7.80 (s, 1H), 7.51 – 7.47 (m, 3H), 5.04 (q, J = 6.5
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Hz, 1H), 2.22 (br, 1H), 1.58 (d, J = 6.5 Hz, 3H) ppm; 13C NMR
(151 MHz, CDCl3) δ 143.5, 133.6, 133.2, 128.6, 128.2, 128.0,
126.4, 126.1, 124.1, 124.1, 70.8, 25.4 ppm.
4-(Dimethylamino)benzyl alcohol (7g). 21 Colorless oil. Yield:
137.6 mg (91%). H NMR (500 MHz, CDCl3) δ 7.27 (d, J = 8.5
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3
4
5
6
7
8
Hz, 2 H), 6.77 (d, J = 9.0 Hz, 2 H), 4.58 (s, 2 H), 2.98 (s, 6 H),
1.97 (br., 1 H) ppm; 13C NMR (126 MHz, CDCl3) δ 150.3, 129.0,
128.7, 112.7, 65.3, 40.8 ppm.
Diphenylmethanol (6j).4f White solid. M.p.: 64 ˗ 65 °C. Yield:
1
147.4 mg (80%). H NMR (600 MHz, CDCl3) δ 7.43 – 7.41 (m,
4H), 7.40 – 7.37 (m, 4H), 7.33 – 7.30 (m, 2H), 5.85 (s, 1H), 2.38
(br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 143.9, 128.5, 127.6,
126.6, 76.3 ppm.
Piperonol (7h).14 Colorless oil. Yield: 147.6 mg (97%). 1H
NMR (600 MHz, CDCl3) δ 6.85 (s, 1H), 6.79 – 6.76 (m, 2H), 5.94
(s, 2H), 4.55 (s, 2H), 1.95 (br, 1H) ppm; 13C NMR (151 MHz,
CDCl3) δ 148.1, 147.4, 135.2, 120.8, 108.5, 108.2, 101.3, 65.5
ppm.
Di-p-tolylmethanol (6k).20 White solid. M.p.: 66 ˗ 68 °C.
1
Yield: 157 mg (74%). H NMR (600 MHz, CDCl3) δ 7.28 (d, J =
8.4 Hz, 4H), 7.17 (q, J = 8.4 Hz, 4H), 5.78 (br, 1H), 2.36 (s, 7H)
ppm; 13C NMR (151 MHz, CDCl3) δ 141.4, 137.4, 129.4, 126.8,
76.2, 21.4 ppm.
2-(Methylthio)benzyl alcohol (7i).22 Colorless oil. Yield: 137.2
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mg (89%). H NMR (500 MHz, CDCl3) δ 7.15 (d, J = 7.5 Hz, 1
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H), 7.05 (m, 2 H), 6.96 (m, 1 H), 4.52 (d, J = 2 Hz, 2 H), 2.26 (d,
J = 1.5 Hz, 3 H), 2.18 (br., 1 H) ppm; 13C NMR (125 MHz,
CDCl3) δ 138.8, 136.6, 128.4, 127.9, 126.4, 125.5, 63.4, 16.0
ppm.
1,2,3,4-Tetrahydronaphthalen-1-ol (6l)20 Colorless oil. Yield:
1
143.7 mg (97%). H NMR (600 MHz, CDCl3): δ 7.44 – 7.42 (m,
1H), 7.22 – 7.20 (m, 2H), 7.12 – 7.11 (m, 1H), 4.77 – 4.75 (m,
1H), 2.86 – 2.81 (m, 1H), 2.76 – 2.71 (m, 1H), 2.11 (br, 1H), 2.02
– 1.95 (m, 1H), 1.92 – 1.87 (m, 1H), 1.82 – 1.76 (m, 1H). 13C
NMR (151 MHz, CDCl3): δ 139.1, 127.4, 129.2, 128.9, 127.8,
126.4, 68.4, 32.5, 29.5, 19.0 ppm.
Methyl 4-(hydroxymethyl)benzoate (7j). 21 Colorless oil. Yield:
118 mg (71%). 1H NMR (500 MHz, CDCl3) δ 8.00 (d, J = 8.0 Hz,
2 H), 7.41 (d, J = 8.0 Hz, 2 H), 4.74 (s, 2 H), 3.91 (s, 3 H), 2.52
(br., 1 H) ppm; 13C NMR (126 MHz, CDCl3) δ 167.1, 146.2,
129.8, 129.1, 126.5, 64.5, 52.2 ppm.
1-(2-Methylphenyl)ethanol (6m).20 Colorless oil. Yield: 122.0
1
mg (90%). H NMR (600 MHz, CDCl3) δ 7.54 (dd, J = 7.8, 1.7
Cinnamyl alcohol (7k).14 Yellowish oil. Yield: 104.6 mg
(78%). 1H NMR (500 MHz, CDCl3) δ 7.41 – 7.40 (m, 2H), 7.35 –
7.33 (m, 2H), 7.28 – 7.26 (m, 1H), 6.63 (d, J = 16 Hz, 1H), 6.97
(dt, J = 16, 5.8 Hz, 1H), 4.33 (dd, J = 5.8, 1.7 Hz, 2H), 2.49 (br,
1H) ppm; 13C NMR (126 MHz, CDCl3) δ 137.0, 131.2, 128.8,
127.9, 126.7, 63.8 ppm.
Hz, 1H), 7.27 (td, J = 7.5, 1.7 Hz, 1H), 7.21 (td, J = 7.4, 1.6 Hz,
1H), 7.17 (dd, J = 7.7, 1.8 Hz, 1H), 5.12 (q, J = 6.5 Hz, 1H), 2.37
(s, 3H), 2.26 (s, 1H), 1.49 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (151
MHz, CDCl3) δ 143.9, 134.2, 130.4, 127.2, 126.4, 124.6, 66.8,
24.0, 19.0 ppm.
1,3-diphenylpropan-2-ol (6n).14 Colorless oil. Yield: 201.7 mg
(95%). 1H NMR (600 MHz, CDCl3) δ 7.39 – 7.36 (m, 4H), 7.31 –
7.28 (m, 6H), 4.13 – 4.09 (m, 1H), 2.93 – 2.90 (m, 2H), 2.84 –
2.80 (m, 2H), 1.80 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ
138.8, 129.7, 128.8, 126.8, 73.8, 43.7 ppm.
2-Thiophenylmethanol (7l).14 Yellowish oil. Yield: 107.3 mg
(94%). 1H NMR (600 MHz, CDCl3) δ 7.28 – 7.27 (m, 1H), 7.01 –
6.97 (m, 2H), 4.81 (s, 2H), 2.07 (br, 1H) ppm; 13C NMR (150
MHz, CDCl3) δ 144.3, 127.2, 125.9, 125.8, 60.2 ppm.
N-benzylaniline (8a). 7a Yellowish oil. Yield: 176 mg (96%).
1H NMR (600 MHz, CDCl3) δ 7.55 – 7.43 (m, 4H), 7.42 – 7.36
(m, 1H), 7.29 (dd, J = 9.0, 7.2 Hz, 2H), 6.84 (t, J = 7.4 Hz, 1H),
6.78 – 6.70 (m, 2H), 4.42 (s, 2H) ppm; 13C NMR (151 MHz,
CDCl3) δ 148.2, 139.5, 129.4, 128.7, 127.6, 127.3, 117.7, 113.0,
48.4 ppm.
2-Heptanol (6o).20 Colorless oil. Yield: 72 mg (62%). 1H NMR
(500 MHz, CDCl3) δ 3.82 (m, 1 H), 1.47 (m, 4 H), 1.33 (m, 5 H),
1.21 (d, J = 6.0 Hz, 3 H), 0.91 (t, J = 7.0 Hz, 3 H) ppm; 13C NMR
(126 MHz, CDCl3) δ 68.2, 39.3, 31.9, 25.5, 23.5, 22.7, 14.1 ppm.
1
Benzyl alcohol (7a).14 Colorless oil. Yield: 102 mg (94%). H
NMR (600 MHz, CDCl3) δ 7.46 – 7.38 (m, 2H), 7.38 – 7.30 (m,
2H), 4.63 (s, 2H), 3.35 (s, 1H); 13C NMR (151 MHz, CDCl3) δ
140.9, 128.5, 127.5, 127.0, 64.9 ppm.
N-(4-fluorobenzyl)aniline (8b).7f Yellow oil. Yield: 145 mg
(72%). 1H NMR (600 MHz, CDCl3) δ 7.34 (ddd, J = 8.6, 5.4, 2.9
Hz, 2H), 7.27 (d, J = 5.3 Hz, 1H), 7.19 (td, J = 7.5, 2.7 Hz, 2H),
7.02 (td, J = 8.6, 3.1 Hz, 2H), 6.77 (t, J = 7.3 Hz, 1H), 6.68 (dd, J
= 8.3, 5.1 Hz, 2H), 4.31 (s, 2H) ppm; 13C NMR (151 MHz, CDCl3)
δ163.1, 161.5, 129.5, 129.4, 129.4, 118.7, 115.7, 115.5, 113.7,
48.3 ppm.
4-Chlorobenzyl alcohol (7b).14 Colorless oil. Yield: 135.4 mg
(95%). 1H NMR (600 MHz, CDCl3) δ 7.35 – 7.33 (m, 2H), 7.30 –
7.28 (m, 2H), 4.65 (s, 1H), 2.45 (s, 1 H), 2.12 (br, 1H) ppm; 13C
NMR (151 MHz, CDCl3) δ 139.6, 133.6, 129.0, 128.6, 64.7 ppm.
2-Chlorobenzyl alcohol (7c).21 Colorless oil. Yield: 136.9 mg
N-(4-nitrobenzyl)aniline (8c).23 Yellow oil. Yield: 180 mg
1
1
(96%). H NMR (600 MHz, CDCl3) δ 7.49 (d, J = 7.7 Hz, 1H),
(79%). H NMR (600 MHz, CDCl3) δ 8.23 – 8.17 (m, 2H), 7.54
7.38 (d, J = 7.7 Hz, 2H), 7.30 (t, J = 7.4 Hz, 1H), 7.25 (t, J = 7.4
Hz, 1H), 4.79 (s, 2H), 2.29 (s, 1H) ppm; 13C NMR (151 MHz,
CDCl3) δ 138.5, 133.0, 129.6, 129.1, 129.0, 127.3, 63.1 ppm.
4-Nitrobenzyl alcohol (7d).14 Yellowish oil. Yield: 140.9 mg
(d, J = 8.6 Hz, 2H), 7.24 – 7.15 (m, 2H), 6.77 (s, 1H), 6.61 (d, J =
8.0 Hz, 2H), 4.48 (s, 2H) ppm; 13C NMR (151 MHz, CDCl3) δ
147.3, 129.5, 128.0, 124.0, 118.8, 113.5, 100.1, 48.0 ppm.
Methyl 4-((phenylamino)methyl)benzoate (8d).24 Yellowish
1
1
(92%). H NMR (600 MHz, CDCl3) δ 8.21 (d, J = 8.4 Hz, 2H),
oil. Yield: 169 mg (70%). H NMR (600 MHz, CDCl3) δ 8.08 –
7.53 (d, J = 8.4 Hz, 2H), 4.83 (s, 2H), 1.97 (br, 1H) ppm; 13C
NMR (151 MHz, CDCl3) δ 148.5, 147.6, 127.3, 124.1, 64.3 ppm.
4-(Hydroxymethyl)benzonitrile (7e).21 Yellowish oil. Yield:
7.99 (m, 2H), 7.45 (d, J = 8.4 Hz, 2H), 7.18 (dd, J = 8.7, 7.2 Hz,
2H), 6.79 – 6.70 (m, 1H), 6.66 – 6.57 (m, 2H), 4.41 (s, 2H), 3.92
(s, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ167.1, 147.8, 145.0,
130.0, 129.4, 129.1, 127.2, 117.9, 113.0, 52.2, 48.0 ppm.
1
79.8 mg (60%). H NMR (600 MHz, CDCl3) δ 7.64 (d, J = 8.0
Hz, 2H), 7.47 (d, J = 8.0 Hz, 2H), 4.77 (s, 2H), 1.96 (br, 1H) ppm;
13C NMR (151 MHz, CDCl3) δ 146.6, 132.6, 127.3, 119.2, 111.5,
64.5 ppm.
4-Acetylbenzyl alcohol (9).25 Colorless oil. Yield: 121.5 mg
1
(81%). H NMR (600 MHz, CDCl3) δ 7.90 (d, J = 8.2, 2H), 7.42
(d, J = 8.2, 2H), 4.73 (s, 2H), 2.56 (s, 3H), 2.50 (br, 1H) ppm; 13C
NMR (151 MHz, CDCl3) δ 198.5, 146.8, 136.5, 128.9, 126.9,
64.7, 26.9 ppm.
4-Methoxybenzyl alcohol (7f).4f Colorless oil. Yield: 131.2 mg
1
(95%). H NMR (600 MHz, CDCl3) δ 7.28 (d, J = 8.6 Hz, 2H),
6.89 (d, J = 8.6 Hz, 2H), 4.60 (s, 2H), 3.80 (s, 3H), 1.81 (br, 1H)
ppm; 13C NMR (151 MHz, CDCl3) δ 159.5, 133.5, 129.0, 114.3,
65.3, 55.6 ppm.
AUTHOR INFORMATION
Corresponding Author
* Email address: szh0007@hunter.cuny.edu;
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The Journal of Organic Chemistry
of Base Metal Asymmetric Alkene Hydrogenation Catalysts. Science
2013, 342, 1076-1080.
guzhang@jjay.cuny.edu
ORCID
Guoqi Zhang: 0000-0001-6071-8469
Notes
1
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(6) Zhang, G.; Hanson, S. K. Cobalt-Catalyzed Acceptorless Alcohol
Dehydrogenation: Synthesis of Imines from Alcohols and Amines.
Org. Lett. 2013, 15, 650-653.
(7) (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Mild and Homoge-
neous Cobalt-Catalyzed Hydrogenation of C=C, C=O, and C=N
The authors declare no competing financial interest.
5
6
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8
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information for this article is available free of
charge via the Internet at http://pubs.acs.org.
CIF files of crystallographic data, additional experimental
details and copies of NMR spectra (PDF).
Bonds. Angew. Chem. Int. Ed. 2012, 51, 12102-12106; (b) Zhang,
G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Understanding
the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehy-
drogenation Reactions. J. Am. Chem. Soc. 2013, 135, 8668-8681;
(c) Yin, Z.; Zeng, H.; Wu, J.; Zheng, S.; Zhang, G. Cobalt-
Catalyzed Synthesis of Aromatic, Aliphatic, and Cyclic Secondary
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Amines via a “Hydrogen-Borrowing” Strategy. ACS Catal. 2016
, 6546-6550; (d) Zhang, G.; Yin, Z.; Zheng, S. Cobalt-Catalyzed
N-Alkylation of Amines with Alcohols. Org. Lett 2016 18, 300-
,
6
.
,
ACKNOWLEDGMENT
303; (e) Zhang, G.; Wu, J.; Zeng, H.; Zhang, S.; Yin, Z.; Zheng, S.
Cobalt-Catalyzed α-Alkylation of Ketones with Primary Alcohols.
Org. Lett. 2017, 19, 1080-1083.
(8) (a) Palmer, W. N.; Diao, T.; Pappas, I.; Chirik, P. J. High-
Activity Cobalt Catalysts for Alkene Hydroboration with Electron-
ically Responsive Terpyridine and α-Diimine Ligands. ACS Catal.
We are grateful to donors of the American Chemical Society Pe-
troleum Research Fund for partial support of this work (54247-
UNI3). We also acknowledge the support from the CUNY Col-
laborative Research Incentive Program, the PSC-CUNY awards
(69069-0047, 60328-0048) and the Seed grant from the Office for
Advancement of Research at John Jay College. The National Sci-
ence Foundation (CHE-1429086) is acknowledged for the X-ray
diffractometer.
2015
Cobalt-Catalyzed Enantioselective Hydroboration of 1,1-
Disubstituted Aryl Alkenes. J. Am. Chem. Soc 2014 136, 15501-
, 5, 622-626; (b) Zhang, L.; Zuo, Z.; Wan, X.; Huang, Z.
.
,
15504; (c) Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. Well-
Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence
for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation. J. Am.
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