Ir(NHC)-Catalyzed Synthesis of β-Alkylated Alcohols via Borrowing Hydrogen Strategy: Influence of Bimetallic Structure

Article abstract of DOI:10.1002/adsc.202100219

Multi N-heterocyclic carbene(NHC)-modified iridium catalysts were employed in the β-alkylation of alcohols; dimerization of primary alcohols (Guerbet reaction), cross-coupling of secondary and primary alcohols, and intramolecular cyclization of alcohols. Mechanistic studies of Guerbet reaction, including kinetic experiments, mass analysis, and density functional theory (DFT) calculation, were employed to explain the fast reaction promoted by bimetallic catalysts, and the dramatic reactivity increase of monometallic catalysts at the late stage of the reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.202100219

RESEARCH ARTICLE
DOI: 10.1002/adsc.202100219
Ir(NHC)-Catalyzed Synthesis of β-Alkylated Alcohols via
Borrowing Hydrogen Strategy: Influence of Bimetallic Structure
Kihyuk Sung,a Mi-hyun Lee,a Yeon-Joo Cheong,a Yu Kwon Kim,a Sungju Yu,a
and Hye-Young Janga,*
a
Department of Energy Systems Research, Ajou University, Suwon 16499, South Korea
Tel: +82-31-219-2555 Fax: +82-31-219-1615
E-mail: hyjang2@ajou.ac.kr
Manuscript received: February 18, 2021; Revised manuscript received: April 27, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100219
Abstract: Multi N-heterocyclic carbene(NHC)-modified iridium catalysts were employed in the β-alkylation
of alcohols; dimerization of primary alcohols (Guerbet reaction), cross-coupling of secondary and primary
alcohols, and intramolecular cyclization of alcohols. Mechanistic studies of Guerbet reaction, including kinetic
experiments, mass analysis, and density functional theory (DFT) calculation, were employed to explain the fast
reaction promoted by bimetallic catalysts, and the dramatic reactivity increase of monometallic catalysts at the
late stage of the reaction.
Keywords: iridium; multi-NHC; β-alkylation; borrowing hydrogen; Guerbet reaction
Introduction
essential properties for their application to lubricants
and detergents.
Transition metal-catalyzed cross-coupling of alcohols
Various homogeneous transition-metal catalysts
has been extensively studied to obtain β-alkylation of a have been reported for transition metal-catalyzed β-
variety of secondary and primary alcohols via the alkylation of alcohols via the BH strategy. Iridium,
borrowing hydrogen (BH) strategy.[1–4] Although alco- ruthenium, palladium, iron, copper, manganese, cobalt,
hols are not considered common electrophiles, the and nickel catalysts were employed for cross-coupling
transition metal-catalyzed BH route allows alcohols to of secondary and primary alcohols.[6–37] For the
undergo such a transformation. Transition metal- condensation of primary alcohols (Guerbet reaction),
catalyzed dehydrogenation of alcohols affords electro- iridium, rhodium, ruthenium, and manganese catalysts
philic carbonyl compounds that undergo subsequent were employed.[5,38–49] Based on studies of carbene-
aldol condensation to form α,β-unsaturated carbonyl ligand-modified iridium catalysts, the electronic and
products. Then, the carbonyl group and double-bond of steric properties of NHC ligands affect the catalytic
the aldol product are reduced by transition-metal activities of iridium catalysts forming IrÀ H from
hydrides to form β-alkylated alcohols. In particular, the alcohols, e.g., transfer hydrogenation and β-alkylation
condensation of primary alcohols through Guerbet of alcohols.[6–10,50–52] In this study, we employed mono-
reaction has received significant attention due to its and bimetallic Ir(NHC) catalysts for the synthesis of
environmental and economic benefits when utilized in high molecular weight branched alcohols through the
the reaction of bio-based alcohols for high-value Guerbet reaction and inter- and intramolecular cou-
chemical production.[5] There is an increasing interest pling of secondary and primary alcohols. The studies
in the investigation of efficient catalytic systems for of versatile catalysts working for all these reactions
the Guerbet reaction to produce commercially valuable have not been intensively conducted. The mono- and
high molecular-weight branched alcohols from small bimetallic Ir(NHC)-catalyzed kinetic studies on the
bio-alcohols to meet the environmental demands. High Guerbet reaction demonstrated that bimetallic catalysts
molecular-weight branched alcohols exhibit a low promote the reaction with a faster rate, compared to
melting point and good biodegradability, which are the monometallic catalysts (Scheme 1). Interestingly,
mechanistic studies indicated that the monometallic
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Table 1. Optimization of the reactions using 1a.
Scheme 1. Ir(NHC)-catalyzed hydrogen delivery.
catalysts might be converted to bimetallic catalysts
during the reaction. In accordance with mechanistic
study results, the bimetallic complex-catalyzed Guer-
bet reaction showed higher yields than monometallic
complex-catalyzed reactions for a short reaction time.
The conversion of monometallic catalysts to bimetallic
catalysts was also supported by the increased yields of
monometallic complex-catalyzed reactions for an in-
creased reaction time. Detailed catalytic reactions and
mechanistic studies are presented herein.
Results and Discussion
The reaction was optimized using heptanol 1a, as
listed in Table 1. In this transformation, bi- and
monometallic Ir(NHC) complexes A and B were
employed. The synthesis and characterization of the
Ir(NHC) complexes were reported in our previous
study.[51] The reaction mixture of 1a (1 mmol) and A
°
(0.0625 mol%) in toluene was heated at 120 C for
16 h, affording no product (entry 1). With KOH
(10 mol%), 1b was isolated with 54% yield (entry 2).
Bases including CsOH, Cs2CO3, and triethylamine
(TEA) were examined, and CsOH showing a high
yield of 1b was chosen as the base (entries 3–5).
Varying the amounts of bases and catalysts, and
solvents had no influence on the yield of 1b (entries may have played important roles to accelerate this
6–11). In the absence of the catalysts, the reaction transformation.
proceeded with 19% yield (entry 12). The reactions
To understand the reaction mechanism and behavior
with different stirring rates showed similar yields (see of each catalyst, we monitored the reaction mixture of
1
Supporting Information, Scheme S3). In addition to 1a, CsOH, and catalysts using H NMR spectroscopy,
bimetallic A, monometallic B was employed to as shown in Figure 1. Figures S4 illustrates the
demonstrate a slight increase in the yield of 1b magnified graphs showing the change in the concen-
(entry 13). The iridium catalyst without carbene tration of the intermediates 1c, 1d, and 1e, and
ligands exhibited a diminished yield of 1b (entry 14). Figure S5 shows the yield change of 1b–1e in the
To probe the structural effect of triscarbene ligands, presence of catalyst A (see Supporting Information).
Ir(biscarbene) catalyst C and Ir(monocarbene) catalyst Bimetallic catalyst A and monometallic catalyst B
D were subjected to the reaction conditions of entry 3 exhibited similar isolated yields of 1b; however, the
to form 1b with 30% and 36% yields, respectively reaction profiles indicated that catalyst A completed
(entries 15 and 16). The combination of catalysts C the reaction within 6 h while the reaction mediated by
and D also showed lower yields compared to catalysts catalyst B required 16 h. It is speculated that aldehyde
A and B, implying that the dangling imidazolium salt production occurs readily in the reaction sphere of
of catalyst B and the second iridium ion of catalysts A catalyst A, resulting in a facile aldol and subsequent
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Figure 2. TOF/min·Ir over catalysts A, B, C, D, and C+D
plotted against the concentration of 1a on a log scale. The TOF
values decrease as the reaction proceeds except for the case of
catalyst B, which is indicated by the red arrow.
the concentration of 1a. The initial TOF values were
approximately 4.7, 1.2, 0.3, 0.8, 0.7 minÀ 1 for the
catalysts A, B, C, D, and the mixture of C and D,
respectively. The TOF values decreased as 1a was
consumed and 1b was released into the reaction vessel.
The kinetic profiles in Figure 2 show the different
reaction orders for catalyst A and other catalysts. To
guide the eyes, the first- and second-order kinetics are
represented using dashed lines. It seems that the
reactions of catalyst A follow a reaction order closer to
1. For catalyst B (denoted by the hollow red circle),
the hollow red circle moves toward the dashed line of
the first-order kinetics from the second-order kinetics
Figure 1. Time course for the conversion of 1a to 1b, (a)
catalyst A, (b) catalyst B, (c) catalyst C, (d) catalyst D, (e)
mixture of catalysts C and D, and (f) reaction profiles forming
1b using catalysts A, B, C, D, and the mixture of C and D.
reductions. Based on Jiménez’s report regarding Ir- (red arrow), which is attributed to the partial trans-
(NHC)-catalyzed β-alkylation of alcohols,[8] aldol formation of monometallic catalyst B to bimetallic
condensation (conversion of 1c to 1d) occurs in the catalyst A (see Supporting Information, Scheme S4).
coordination sphere of the iridium ion. Catalyst B The higher TOFs of catalyst A can be explained
exhibited a slower conversion from 1a to 1b compared through the facile dehydrogenation of 1a, the subse-
to catalyst A. Compared to bimetallic catalyst A, the quent aldol condensation between the two aldehydes,
consumption of 1a and the formation of the final and reduction reactions in the close proximity of two
product 1b were much slower using catalyst B; iridium ions. Therefore, the overall reaction rate may
however, the final conversion and isolated yield of 1b be limited by the dehydrogenation of 1a. With the
were comparable with catalyst A. The final conversion other monometallic catalysts, the concentration of the
value and isolated yield of 1b in the presence of dehydrogenated product of 1a should increase for the
catalysts C, D, and the mixture of C and D were lower subsequent aldol condensation and thus, affects the
than those of catalyst B.
overall kinetics. Therefore, the apparent reaction order
Further insights into the reaction kinetics of 1a may appear to be different between catalyst A and the
over Ir catalysts were obtained from the analysis of other catalysts.
turnover frequency (TOFs, minÀ 1) divided by the
Based on the reaction results and reaction profiles,
number of Ir ions plotted against the concentration of a catalytic cycle involving bimetallic catalyst A is
1a for all the Ir catalysts (A, B, C, D, and mixture of proposed in Scheme 2. Iridium hydroxide I is gener-
C and D) on a log scale (Figure 2). In this plot, the ated from iridium catalyst A with CsOH.[53] Iridium
slop reflects the apparent reaction order with respect to hydroxide I’ was observed through the mass analysis
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reaction, the substitution of iridium hydroxides with
1a followed by β-hydride elimination yields IrÀ H
intermediate III. Aldehyde 1c readily undergoes aldol
condensation to afford intermediate IV, in which the
aldol condensation product 1d is reduced to give 1b.
The CsOH-catalyzed aldol reaction using 1c in the
absence of catalysts formed aldol product 1d with
37% yield (see Supporting Information, Scheme S5),
implying that the iridium catalyst is critical for
dehydrogenation of alcohols and hydrogenation of
aldol products.
Based on optimized conditions and mechanistic
results of the Guerbet reaction, primary alcohols were
subjected to the conditions using the catalyst (A
0.0625 mol%; B 0.125 mol%), CsOH (0.1 equiv) in
°
toluene at 120 C for 8 h and 16 h, respectively (Fig-
ure 4). The yields of 1b were 69% (catalyst A) and
41% (catalyst B) when the reactions were run for 8 h.
As we observed from Figure 1, the bimetallic catalyst
promoted the product formation with a higher yield
than the monometallic catalyst for a shorter reaction
time. The monometallic B-catalyzed reaction provided
Scheme 2. Catalytic cycle for the conversion of 1a to 1b.
of the mixtures of the catalysts (A or B), 1a, and 1b with a low yield (41%), but the yield was increased
CsOH in toluene (see Supporting Information, Figur- to 80% after 16 h of the reaction, implying the
es S1 and S2). The hydroxy group-bound iridium transition of monometallic to bimetallic catalysts. The
species were also observed in the reaction using reactions of 1-butanol afforded C8-alcohol 2b with
catalyst A with KOH by mass analysis (see Supporting 70% yield (catalyst A) and 30% yield (catalyst B) for
Information, Figure S3). In the case of monometallic 8 h, and 61% yield (catalyst A) and 70% yield (catalyst
catalyst B, an increase in the amount of intermediate I’
was detected through mass analysis over the course of
the reaction, implying the gradual formation of
bimetallic species. The structure of intermediate I was
studied using density functional theory (DFT) calcu-
lations. Two Ir-bound hydroxy groups were found to
interact with each other, resulting in OÀ H distances of
2.33 Å and 2.37 Å (Figure 3). These values are in the
range of the hydrogen bond.[54] The strong interaction
induces a short distance (5.83 Å) between two Ir ions.
Once iridium hydroxide I is formed during the
Figure 3. DFT-optimized molecular structures of iridium
hydroxide intermediate I. Arrows indicate atomic distances. Ir,
C, O, H, and N atoms are depicted as dark-blue, gray, red,
white, and blue spheres, respectively.
Figure 4. Examples of the Guerbet reaction.
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B) for 16 h. Further, 1-decanol participated in the
reaction to form 3b with 69% (catalyst A) and 45%
yields (catalyst B) for 8 h, and 76% (catalyst A) and
61% yields (catalyst B) for 16 h. Compared to 1b and
2b, 3b was formed along with aldehyde 3b’ (3%
yield) when the reaction was performed with catalyst
A for 16 h. 3-(4-Methoxyphenyl)propan-1-ol possess-
ing an aromatic group was converted to 4b with 89%
(catalyst A) and 68% yields (catalyst B) for 8 h, and
74% (catalyst 1) and 81% yields (catalyst B) for 16 h.
The conversions of 3,3-diphenylpropan-1-ol and 2-
cyclopentylethan-1-ol to 5b and 6b, respectively,
resulted in lower yields than the other alcohols
mentioned above. Due to the steric hindrance posed by
substituents, the reduced yields were observed. As the
reaction time becomes longer, the yields of 2b, 4b,
and 5b were decreased in the presence of catalyst A
due to the decomposition of products.
The reactions of 1-phenyl ethanol 2a (1 mmol) and
benzyl alcohol 3a (1.2 mmol) were performed using
catalyst A, forming cross-coupling products 7b and
7b’ with 91% yield (the ratio of 7b: 7b’=3.8:1) at
°
120 C, and 98% yield (the ratio of 7b: 7b’=6.5:1) at
°
100 C (Figure 5). It is assumed that benzaldehydes
generated by the dehydrogenation of benzyl alcohols
are not stable at a higher temperature, resulting in a
°
lower yield at 120 C. Due to the stability of
benzaldehyde, a slight excess of 3a was used. Because
the reactions of the secondary and primary alcohols
°
provided higher yields and selectivity at 100 C, the
°
subsequent reactions were performed at 100 C. The
reaction of 2a and 3a in the presence of catalyst B
afforded 7b and 7b’ with 94% yield (the ratio of 7b:
7b’=3.5:1). The reactions of 2a and 3a for 8 h
exhibited lower yields than those of 16 h reactions.
Similar catalytic activities of both catalysts A and B
were observed for the reactions of 8 h and 16 h. The
reaction of 2a and 4-methyl benzyl alcohol afforded
8b and 8b’ with 89% yield (the ratio of 8b: 8b’=
8.9:1) in the presence of catalyst A, and 96% yield (the
ratio of 8b: 8b’=7.7:1) in the presence of catalyst B.
The reactions of methoxy-substituted benzyl alcohols
showed a combined yield of alcohols and ketone with
Figure 5. Examples of the cross-coupling of secondary and
primary alcohols.
84–98% yields. In the reactions of 2a and benzyl due to the dehydrogenation of 11b to 11b’ (see
alcohol derivatives, both catalysts showed good yields, Supporting Information, Scheme S6). The reaction of
and the selectivity of alcohols over ketones was varied 1-heptanol and catalyst A provided 12b, 12b’, and
depending on substrates.
minor products 12b’’ and 12b’’’ with the total yield of
Further, reactions of 2a and aliphatic alcohols were 96% (the ratio of 12b: 12b’:12b’’:12b’’’=
°
also attempted at 120 C. The reaction using 1-hexanol 43:30:1:5.9). The reaction of 1-heptanol with catalyst
in the presence of catalyst A afforded 11b and 11b’ B showed a similar product distribution with catalyst
with 92% yield (the ratio of 11b: 11b’=0.77:1). The A. The reaction of 2a with 5-(4-methoxyphenyl)
catalyst B-catalyzed reaction produced 11b and 11b’ pentan-1-ol was performed to produce 13b and 13b’
with 98% yield (the ratio of 11b: 11b’=1.1:1). For a with 86% yield (the ratio of 13b: 13b’=2.3:1) using
reduced reaction time (4 h), both catalysts showed high catalyst A and with 92% yield (the ratio of 13b:
yields (90% for A and 95% for B) and even higher 13b’=2.1:1) using catalyst B.
selectivity for alcohols. The amount of alcohol was
Next, intramolecular reactions were attempted (Fig-
reduced for a prolonged reaction time (4 h vs. 16 h) ure 6). A diol as an alkylation precursor was used in
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alcohols, and intramolecular cyclized alcohols via the
BH strategy. The kinetic studies and catalytic reaction
results of the Guerbet reaction provided insight that
bimetallic catalysts promote this reaction faster than
monometallic catalysts. The bimetallic catalysts gen-
erated two aldehydes in the proximity of two iridium
ions, resulting in a facile aldol reaction and the
reduction of aldol products to complete the reaction
with a faster reaction rate. The low conversions of
monometallic catalysts B (the initial stage of the
reaction progress), C, and D support that the bimetallic
structure is a prerequisite for high yields of the Guerbet
reaction. Both bimetallic and monometallic catalysts
exhibited good yields of cross-coupling reactions; the
reactions using aliphatic alcohols were completed
much faster than those of benzyl alcohol derivatives.
Diols favor the dehydrogenation-intramolecular aldol-
hydrogenation route forming alcohols or ketones rather
than intramolecular Tishchenko lactone products.
Experimental Section
Figure 6. Intramolecular cyclization.
The catalytic procedure of Guerbet reaction. The catalyst (A
0.0625 mol%, B 0.125 mol%), CsOH (10 mol%), primary
alcohol (1 mmol), and toluene (1 ml) were added into the sealed
°
tube. The mixture was heated at 120 C for 16 h. After the
iridium and manganese-catalyzed cross-coupling of
ketones (sec-alcohols) and diols.[35,55,56] In this study,
the intramolecular cyclizations of diols in the absence
of enolate precursors were conducted. Diol 4a was
subjected to the optimized conditions listed under
Table 1 (entry 3), except for increased catalyst loadings
(catalyst A, 0.125 mol%), resulting in cyclized alcohol
14b and ketone 14b’ with 71% yield (the ratio of 14b:
14b’=4.9:1). With monometallic catalyst B, the
combined yield of 14b and 14b’ increased to 83%. In
the case of the reaction of 5a, monometallic catalyst B
showed a higher selectivity of 15b over 15b’ while
catalyst A promoted a higher combined yield with a
lower selectivity toward the formation of the alcohols.
Analogous to the cross-coupling reactions, the product
ratios of alcohols and ketones are varied depending on
the substrate. The reaction of 6a afforded four- and
six-membered ring products. The combined yield of
16b, 17b, and 17b’ was 63% with catalyst A, and the
combined yield of 16b, 17b, 17b’, and 17b’’ was 76%
with catalyst B. The reaction of 1-(2-(hydroxymethyl)
phenyl)ethan-1-ol provided lactone products under our
reaction conditions (intramolecular Tishchenko prod-
ucts, see supporting information Scheme S7). Other
than 1-(2-(hydroxymethyl)phenyl)ethan-1-ol, we did
not observe lactone product formation under our
catalytic conditions.
reaction, the mixture was cooled down to ambient temperature.
Product 1b–6b and 3b’ were isolated by flash column
chromatography with EtOAc/Hexane as eluent.
The catalytic procedure of cross-coupling of secondary
alcohols and benzyl alcohols. The catalyst (A 0.0625 mol%, B
0.125 mol%), CsOH (10 mol%), secondary alcohol (1 mmol),
benzyl alcohol (1.2 mmol), and toluene (1 ml) were added to
°
the sealed tube. The mixture was heated at 100 C for 16 h.
After the reaction, the mixture was cooled down to ambient
temperature. Product 7b-10b and 7b’-10b’ were isolated by
flash column chromatography with EtOAc/Hexane as eluent.
The catalytic procedure of cross-coupling of secondary
alcohols and aliphatic primary alcohols. The catalyst (A
0.0625 mol%, B 0.125 mol%), CsOH (10 mol%), secondary
alcohol (1 mmol), primary alcohol (1 mmol), and toluene (1 ml)
were added to the sealed tube. The mixture was heated at
°
120 C for 16 h. After the reaction, the mixture was cooled
down to ambient temperature. Product 11b–13b and 11b’–13b’
were isolated by flash column chromatography with EtOAc/
Hexane as eluent.
The catalytic procedure of intramolecular cyclization. The
catalyst (A 0.125 mol%, B 0.25 mol%), CsOH (10 mol%), diol
4a–6a (1 mmol), and toluene (1 ml) were added into the sealed
°
tube. The mixture was heated at 120 C for 16 h. After the
reaction, the mixture was cooled down to ambient temperature.
Product 14b–17b, 14b’–17b’, 17b’’, 17b’’’ were isolated by
flash column chromatography with EtOAc/Hexane as eluent.
Conclusion
Mono- and bimetallic Ir(NHC) catalysts were em-
ployed to produce Guerbet alcohols, cross-coupling
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Acknowledgments
This study was supported by the Carbon to X Program (No.
2020M3H7A1098283) and National Research Foundation Pro-
gram (No. 2019R1A2C1084021) by the Ministry of Science and
ICT, Republic of Korea.
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Ir(NHC)-Catalyzed Synthesis of β-Alkylated Alcohols via
Borrowing Hydrogen Strategy: Influence of Bimetallic
Structure
Adv. Synth. Catal. 2021, 363, 1–9
K. Sung, M.-h. Lee, Y.-J. Cheong, Y. K. Kim, S. Yu, H.-Y.
Jang*