Manganese-Catalyzed Multicomponent Synthesis of Pyrimidines from Alcohols and Amidines

Article abstract of DOI:10.1002/anie.201611318

The development of catalytic reactions for synthesizing different compounds from alcohols to save fossil carbon feedstock and reduce CO₂emissions is of high importance. Replacing rare noble metals with abundantly available 3d metals is equally important. We report a manganese-complex-catalyzed multicomponent synthesis of pyrimidines from amidines and up to three alcohols. Our reaction proceeds through condensation and dehydrogenation steps, permitting selective C?C and C?N bond formations. β-Alkylation reactions are used to multiply alkylate secondary alcohols with two different primary alcohols to synthesize fully substituted pyrimidines in a one-pot process. Our PN₅P-Mn-pincer complexes efficiently catalyze this multicomponent process. A comparison of our manganese catalysts with related cobalt catalysts indicates that manganese shows a reactivity similar to that of iridium but not cobalt. This analogy could be used to develop further (de)hydrogenation reactions with manganese complexes.

Full text of DOI:10.1002/anie.201611318

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Communications
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International Edition: DOI: 10.1002/anie.201611318
German Edition: DOI: 10.1002/ange.201611318
Heterocycles
Manganese-Catalyzed Multicomponent Synthesis of Pyrimidines from
Alcohols and Amidines
Nicklas Deibl and Rhett Kempe*
Abstract: The development of catalytic reactions for synthesiz-
ing different compounds from alcohols to save fossil carbon
feedstock and reduce CO2 emissions is of high importance.
Replacing rare noble metals with abundantly available 3d
metals is equally important. We report a manganese-complex-
catalyzed multicomponent synthesis of pyrimidines from
amidines and up to three alcohols. Our reaction proceeds
through condensation and dehydrogenation steps, permitting
À
À
selective C C and C N bond formations. b-Alkylation reac-
tions are used to multiply alkylate secondary alcohols with two
different primary alcohols to synthesize fully substituted
pyrimidines in a one-pot process. Our PN5P-Mn-pincer
complexes efficiently catalyze this multicomponent process.
A comparison of our manganese catalysts with related cobalt
catalysts indicates that manganese shows a reactivity similar to
that of iridium but not cobalt. This analogy could be used to
develop further (de)hydrogenation reactions with manganese
complexes.
T
he selective linkage of alcohols to important classes of
chemical compounds is an opportunity to develop more
sustainable chemistry.[1] Alcohols can be obtained from
indigestible and abundantly available lignocellulose bio-
mass,[2,3] and thus the development of alcohol re-functional-
ization reactions can contribute to the conservation of our
fossil carbon resources and the reduction of CO2 emissions. A
variety of reactions have been developed recently to catalyti-
cally synthesize aromatic N-heterocyclic compounds, such as
pyrroles,[4,5] pyridines,[6] pyrimidines,[7] and others,[8] from
alcohols.[9] These reactions have been catalyzed by rare
noble metals, mostly based on Ir and Ru. A more sustainable
approach would be the use of catalysts based on abundantly
available 3d metals, such as Co, Fe, and Mn (nonprecious or
base metals), to additionally conserve our rare noble metal
resources. Milstein and co-workers recently introduced
a cobalt-catalyzed synthesis of pyrroles from diols and
amines[10] (Scheme 1, top). This reaction was discovered by
Crabtree and co-workers using a Ru catalyst.[5a] The nonpre-
cious metal manganese, the third most abundant metal in the
earthꢀs crust, has been overlooked in recent years with regard
Scheme 1. Synthesis of aromatic N-heterocycles from alcohols cata-
lyzed by base-metal catalysts and corresponding methodology develop-
ment using noble metals.
report here on a manganese-catalyzed version of the multi-
component reaction of alcohols and amidines to form
pyrimidines[7] (Scheme 1, bottom). The reaction can be
carried out to give fully substituted pyrimidines in a 3-
component or a consecutive 4-component reaction. Multi-
component reactions are especially attractive in organic
chemistry since they allow the synthesis of large libraries of
diversely functionalized products from simple starting mate-
rials. Our synthetic method is especially useful for forming
selectively alkylated and/or arylated pyrimidines. Mn cata-
lysts stabilized by PN5P ligands[16] catalyze our reaction
efficiently. Related Co catalysts are nearly inactive.
to catalysis involving
a
(de)hydrogenation step.[11] We
recently introduced a variety of nonprecious metal catalysts
The reaction between 1-phenylethanol (1a), benzyl alco-
hol (2a), and benzamidine (3a) to pyrimidine 4a was
investigated to develop a base-metal-catalyzed version of
the 3-component pyrimidine synthesis (Table 1, top). After
optimization of common reaction parameters (solvent, base,
base amount, substrate ratio; see the Supporting Information
for details), a library of Mn complexes stabilized by PN5P or
PN3P ligands was tested to find the most active precatalyst
(Table 1, complexes A–G). The complexes stabilized with
for reactions involving (de)hydrogenation steps[11d,12–15] and
[*] N. Deibl, Prof. Dr. R. Kempe
Anorganische Chemie II—Katalysatordesign
Universitꢀt Bayreuth, 95447 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201611318.
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Table 1: Precatalyst screening of the model reaction.[a]
Table 2: Scope of the 3-component pyrimidine synthesis.[a]
Precatalyst
R
X
Yield[b]
of 4a
Entry
Product
R
4
Yield[b]
1
2
3
4
5
6
7
8
R1 =Ph
4b
4c
4d
4e
4 f
4g
4h
4i
79%
73%
71%
73%
73%
66%
70%
50%
R1 =PMP
A
B
C
D
E
Me
Ph
4-CF3(C6H4)-
H
C3H5NH-
H
Me
N
N
N
N
N
CH
CH
83%
96%
85%
92%
89%
48%
54%
R1 =3-Cl-(C6H4)-
R1 =2-thienyl
R1 =3-pyridyl
R1 =iso-propyl
R1 =cyclopropyl
R1 =H
F
G
[a] Reaction conditions: 1-phenylethanol (1.0 mmol), benzyl alcohol
(1.0 mmol), benzamidine (0.50 mmol), t-BuOK (0.55 mmol), precatalyst
(0.01 mmol, 2 mol%) 1,4-dioxane (1 mL), 1208C (oil bath temperature),
20 h. [b] Yield was determined by GC with dodecane as the internal
standard.
9
10
11
R2 =cyclohexyl
R2 =n-pentyl
R2 =H
4j
4k
4i
53%
62%
44%
12
13
14
R3 =PMP
R3 =Me
4l[c]
68%
57%
62%
4m[c]
4n[c]
R3 =NH2
triazine-based ligands bearing a phenyl (B) or H (D)
substituent in the 4-position gave the highest yield of 4a.
We also tested three cobalt complexes that were recently
reported by our group[12a,13a,14] as active catalysts for borrow-
ing hydrogen/hydrogen autotransfer (BH/HA) applications,
and only unreacted starting materials were obtained (see the
Supporting Information). In summary, the best yield was
obtained when precatalyst B (2 mol%) was applied, the
reaction was run in 1,4-dioxane with 1.1 equiv t-BuOK as the
base, and an excess of alcohols (1.5–2 equiv) with respect to
the amidine was used. The complexes used can be obtained on
a gram scale in high yields in two steps from commercially
available diamines and the corresponding Mn carbonyl
precursor. With these conditions in hand, we explored the
substrate scope of this 3-component reaction (Table 2). To
start with, different secondary alcohols were employed, and
aromatic (4b–d), heteroaromatic (4e,f), and aliphatic (4g,h)
moieties were tolerated to give the corresponding pyrimidines
in acceptable to good yields of isolated product (66–79%).
When ethanol (R1 = H) was used to contribute the C2 frag-
ment, the 2,4-substituted pyrimidine 4i was isolated in 50%
yield. Through variation of the primary alcohol, aliphatic
substituents were introduced to give the corresponding
products 4j,k. A secondary alcohol in combination with
methanol as a C1 building block[17] (instead of ethanol and
another primary alcohol) gave the 2,4-substituted pyrimidine
4i. A more electron-rich para-methoxyphenyl group (4l) as
well as a methyl (4m) and an amino group (4n) could be
installed in the 2-position when the corresponding amidine
(or guanidine) was used.
[a] Reaction conditions: Secondary alcohol (1.5 mmol), primary alcohol
(1.5 mmol), amidine/guanidine (1 mmol), t-BuOK (1.1 mmol), B
(0.02 mmol, 2 mol%) 1,4-dioxane (2 mL), 1208C (oil bath temperature),
20 h. [b] Yields of isolated products. [c] Corresponding amidine or
guanidine hydrochloride with 1 additional equiv of t-BuOK was used.
PMP=para-methoxyphenyl.
Table 3: Synthesis of pyrimidines with alkylation of methylene carbon
atoms.[a]
[a] Reaction conditions: Secondary alcohol (1.5 mmol), primary alcohol
(1.1 mmol), amidine (1.0 mmol), t-BuOK (1.5 mmol), B (0.02 mmol,
2 mol%) 1,4-dioxane (2 mL), 1208C oil bath, 20 h. [b] Yields of isolated
products. PMP=para-methoxyphenyl.
We next focused on the use of secondary alcohols with
a substituent in the b-position, which can give rise to fully
substituted pyrimidines. The alkylation of a secondary carbon
atom by BH/HA methods is known to be more difficult,[17,18]
but the corresponding pyrimidines 5 (Table 3) could be
isolated in moderate to good yields with a slight increase in
the base amount (1.5 equiv) and an adapted substrate ratio
(1.1 equiv primary alcohol). The use of cyclic alcohols, for
example, gave the corresponding products 5a–c, which
feature annulated aliphatic rings (ring size: 7, 8, or 12
carbon atoms). Two primary alcohols, of which one contrib-
utes the C2 building block, can give rise to 2,4,5-substituted
pyrimidines (e.g., 5d in good 75% yield). Fully and differ-
ently substituted pyrimidines can also be obtained as dem-
onstrated for 5 f.
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Finally, we became interested in whether the manganese
In summary, we report the first example of a Mn-catalyzed
catalyst is also able to catalyze a preceding b-alkylation
reaction between a secondary and a primary alcohol. A
manganese-catalyzed version of this reaction has not been
reported, but would lead to the corresponding b-alkylated
alcohol (or ketone). Subsequent addition of another primary
alcohol and an amidine would give the pyrimidine in a one-
pot process. Indeed, when we investigated the reaction
between 1-phenylethanol (1.0 equiv) and 1-propanol
(1.1 equiv) under the typical reaction conditions, the con-
version of 1-phenylethanol was quantitative after 5 h (see the
Supporting Information for details). Impressed by the good
activity of the catalyst, we decided to use it to develop
a consecutive 4-component reaction. The overall reaction to
give tetrasubstituted pyrimidines 5 (Table 4, top) gave the
synthesis of aromatic N-heterocycles from alcohols. This is
a multicomponent reaction in which selective dehydrogen-
À
À
ation and condensation steps lead to selective C C and C N
bond formations. Through a 3-component reaction, fully
substituted, 2,4- substituted, and 2,4,5-substituted pyrimidines
can be obtained. In combination with the b-alkylation of
secondary alcohols by primary alcohols, a reaction that has
not yet been described for Mn catalysts, a consecutive 4-
component process was developed to give fully substituted
pyrimidines in a one-pot procedure. Both multicomponent
methods are strong regarding the synthesis of selectively
alkylated and arylated products. Precatalysts stabilized by
PN5P ligands (triazine backbone) are about twice as efficient
as those stabilized by PN3P ligands (pyridine backbone).
Notably, both Ir[7] and Mn complexes catalyze the two distinct
reactions (BH/HA and ADC) efficiently under similar
reaction conditions. Co complexes stabilized by such ligands
are nearly inactive in the ADC step. The (double) diagonal
relationship Mn-Ru-Ir could be an explanation for the
analogous catalytic reactivity between Mn and Ir observed
here. Considering the many applications of Ir catalysts in
(de)hydrogenation reactions, we feel that manganese has
a great potential to partially replace Ir in such reactions.
Table 4: Synthesis of tetrasubstituted pyrimidines by a consecutive 4-
component reaction.[a]
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft, DFG, SFB
840 for financial support, Dr. Ulrike Lacher for HRMS
measurements, and Franziska Frielingsdorf for excellent help
in the laboratory.
Conflict of interest
The authors declare no conflict of interest.
Keywords: borrowing-hydrogen reactions ·
dehydrogenative coupling · maganese · pincer complexes ·
pyrimidines
[a] Reaction conditions: Secondary alcohol (2.0 mmol), primary alcohol
(2.2 mmol), t-BuOK (2.0 mmol), precatalyst B (0.05 mmol, 5 mol%) and
1,4-dioxane (1 mL) were heated for 5 h at 1208C (oil bath temp.).
Afterwards, amidine (1.0 mmol) and primary alcohol (1.1 mmol) were
added as a solution in 1,4-dioxane (2 mL) and the reaction was heated
under reflux for 20 h. [b] Yields of isolated products. [c] The b-alkylation
reaction was run in a closed system. PMP=para-methoxyphenyl.
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Manuscript received: November 18, 2016
Final Article published: && &&, &&&&
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Communications
Heterocycles
N. Deibl, R. Kempe*
&&& — &&&
Manganese-Catalyzed Multicomponent
Synthesis of Pyrimidines from Alcohols
and Amidines
Something borrowed: A manganese
complex stabilized by a PN5P-pincer
ligand catalyzes the multicomponent
synthesis of pyrimidines from amidines
and up to three (different) alcohols. The
consecutive 4-component reaction com-
bines the concept of borrowing hydrogen
or hydrogen autotransfer with dehydro-
genation condensation to permit selec-
À
À
tive C N and C C bond formation.
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These are not the final page numbers!