Functionalization of Highly Substituted 2,2′:6′,2″-Terpyridine Derivatives

Article abstract of DOI:10.1002/ejoc.201501299

On the basis of an easy access to 2,2′:6′,2″-terpyridine derivatives bearing 6- and 6″-methyl groups, the functionalization of these substituents was investigated. The direct oxidation or bromination of the methyl groups was not feasible on larger scale; however, a two-step process employing oxidation to the pyridine N-oxides followed by acetic anhydride promoted rearrangement (Boekelheide reaction) furnished the corresponding terpyridine derivatives with 6- and 6″-acetoxymethyl groups in reasonable overall yields. These are excellent precursors for the synthesis of tetra- and pentadentate ligands, as demonstrated by several examples. This approach allowed access to a library of new functionalized 2,2′:6′,2″-terpyridine derivatives.

Full text of DOI:10.1002/ejoc.201501299

DOI: 10.1002/ejoc.201501299
Full Paper
Terpyridine Derivatives
Functionalization of Highly Substituted 2,2′:6′,2′′-Terpyridine
Derivatives
Paul Hommes[a] and Hans-Ulrich Reissig*[a]
Dedicated to Professor Henning Hopf on the occasion of his 75th birthday
Abstract: On the basis of an easy access to 2,2′:6′,2′′-terpyr- responding terpyridine derivatives with 6- and 6′′-acetoxy-
idine derivatives bearing 6- and 6′′-methyl groups, the function-
alization of these substituents was investigated. The direct oxi-
dation or bromination of the methyl groups was not feasible
on larger scale; however, a two-step process employing oxida-
tion to the pyridine N-oxides followed by acetic anhydride pro-
moted rearrangement (Boekelheide reaction) furnished the cor-
methyl groups in reasonable overall yields. These are excellent
precursors for the synthesis of tetra- and pentadentate ligands,
as demonstrated by several examples. This approach allowed
access to a library of new functionalized 2,2′:6′,2′′-terpyridine
derivatives.
Introduction
pounds bear alkyl groups (in most cases methyl groups) at the
6- and 6′′-positions, which may hamper complexation of metal
ions with these tridentate ligands. On the other hand, these
alkyl substituents offer the opportunity for further functionali-
zation to introduce additional donor moieties, which can finally
lead to tetra- and pentadentate ligands that should be of con-
siderable interest for many applications. In this report, we de-
scribe our efforts to oxidize the methyl groups of compounds
of type A to approach novel hydroxy- and amino-substituted
terpyridine derivatives.
On the basis of a simple but highly flexible synthesis of pyridine
derivatives from β-ketoenamides,[1] we developed a new ap-
proach to functionalized 2,2′:6′,2′′-terpyridine derivatives A by
employing bis(β-ketoenamides). These precursors are easily ac-
cessible from β-diketones such as acetylacetone, ammonia, and
pyridine-2,6-dicarboxylic acids (Scheme 1).[2]
Results and Discussion
In our first report on the terpyridine synthesis, we described
the conversion of 1 into dialdehyde 3 by using selenium diox-
ide (1,4-dioxane, 110 °C).[1a] This direct oxidation would be an
ideal starting point to prepare the envisioned 6- and 6′′-substi-
Scheme 1. Approach to functionalized 2,2′:6′,2′′-terpyridine derivatives A with
highlighted methyl groups (green balls).
This route to terpyridines A has the advantage that the 4-,
4′-, and 4′′-positions of the three pyridine subunits may select-
ively be addressed, for example, by nucleophilic substitution or
by palladium-catalyzed coupling of the corresponding nona-
flates.[3] We thus easily prepared 4,4′,4′′-tris(dimethylamino)ter-
pyridine, a unique Lewis base with unprecedented methyl cat-
ion affinity, and novel unsymmetrically substituted terpyridine
derivatives.[2] A clear limitation of this method is that all com-
[a] Institut für Chemie und Biochemie, Freie Universität Berlin,
Takustrasse 3, 14195 Berlin, Germany
E-mail: hans.reissig@chemie.fu-berlin.de
http://www.bcp.fu-berlin.de/chemie/chemie/forschung/OrgChem/reissig/
index.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501299.
Scheme 2. Reaction of terpyridine derivative 1 with selenium dioxide to alde-
hyde 2 and dialdehyde 3.
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tuted compounds. Unfortunately, all attempts to perform this pounds were easily separated by column chromatography. The
oxidation on a reasonably large scale only led to mixtures of isolation of monoester 8 indicates that the oxidation of terpyr-
precursor 1, monoaldehyde 2, and the desired compound 3 idine derivative 1 to the desired bis-N-oxide was incomplete
(Scheme 2).[4]
and that considerable amounts of the corresponding mono-N-
oxide were formed. There was no evidence in these experi-
ments that the central ring of 1 was also oxidized. Apparently,
the two more electron-rich methoxy-substituted pyridine sub-
units are more reactive. The conditions as shown in Scheme 5
could not be improved by using hydrogen peroxide in the pres-
ence of acetic acid,[9] by employing hydrogen peroxide and
methylrhenium trioxide as the catalyst,[10] or by using dimeth-
yldioxirane as the oxidizing reagent.[10b] To check the electronic
effects of the two-step process, we also exposed terpyridine
derivative 9[2,6c,11] to mCPBA followed by acetic anhydride. In
this case, the desired diester 10 was isolated in 38 % yield over
two steps.
Because of this failure, we had a look at less-direct alterna-
tives and first checked bromination reactions. The reaction of
terpyridine 1 with bromine in the presence of acetate buffer[5]
only led to ring bromination, which provided 3,5′′-dibromoter-
pyridine derivative 4 in low yield as the main product in a mix-
ture of various differently brominated species (Scheme 3).
Scheme 3. Ring bromination of 1 leading to dibrominated terpyridine deriva-
tive 4.
On the other hand, conditions of a radical reaction with the
use of N-bromosuccinimide (NBS) and 2,2′-azobisisobutyro-
nitrile (AIBN) as initiator[6] caused the expected side-chain halo-
genation and afforded hexabromo compound 5 in a good yield
of 58 % (Scheme 4). Attempts to prepare dibromide 6 by using
a smaller amount of NBS or a different brominating agent failed.
Target compound 6 was only obtained as one component in a
complex mixture still containing starting material 1. We also
attempted to convert the tribromomethyl substituents of 5 into
carboxylate groups by treating the compound with strong base;
however, no defined products were isolated.[4]
Scheme 5. Two-step syntheses of diesters 7 and 10 by oxidation of 1 or 9
with mCPBA followed by Boekelheide rearrangement.
Although the two-step process to convert 1 and 9 into the
corresponding diesters 7 and 10, respectively, was satisfactory,
a more convenient and higher yielding process was neverthe-
less desirable. We therefore also examined the N-oxidation of
bisnonaflate 11.[2] Again, a very careful optimization was re-
quired to obtain reasonably high conversion and selectivity. The
best conditions were found by treating 11 repeatedly with fresh
mCPBA in dichloromethane at room temperature while using
washing steps in between to remove the m-chlorobenzoic acid
byproduct (Scheme 6). This allowed the isolation of mono-N-
oxide 12 (17 % yield) and bis-N-oxide 13 (51 % yield) in satisfac-
Scheme 4. Side-chain bromination of 1 leading to terpyridine derivative 5.
DCE = 1,2-dichloroethane.
Given that these methods failed, we finally investigated indi-
rect side-chain functionalization by oxidation of the pyridine
nitrogen atoms to N-oxides followed by rearrangement (Boekel-
heide reaction),[7] a two-step process successfully employed for
many other examples in our group.[1c,8] Upon treating ter-
pyridine 1 with an excess amount of m-chloroperoxybenzoic
acid (mCPBA) in dichloromethane and then with acetic anhy-
dride, the desired diester 7 together with monoester 8 was ob-
tained in satisfactory combined yield (Scheme 5). The two com-
Scheme 6. Conversion of bisnonaflate 11 into mono-, bis-, and tris-N-oxides
12, 13, and 14.
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tory combined yield. More forcing conditions involving the use ture of the desired dialdehyde 3[1a] together with hydroxy alde-
of a larger excess amount of mCBPA at elevated temperatures hyde 24. This transformation certainly requires additional opti-
with prolonged reaction times resulted in the decomposition mization or the use of a different oxidizing reagent.
of 12 and 13. To increase the yield of 13, precursor 11 was
also treated with the stronger oxidizing mixture of hydrogen
peroxide (35 %, as urea complex) and trifluoroacetic anhydride
(TFAA),[12] as the in situ formed trifluoroperacetic acid is more
electrophilic[13] and thus more reactive than mCPBA. The in-
creased reactivity, however, also led to N-oxidation of the cen-
tral pyridine ring, which thus delivered tris-N-oxide 14.
Unfortunately, none of these terpyridine N-oxides could be
converted into the corresponding acetoxymethyl-substituted
compounds upon heating with acetic acid anhydride.[4] Only
decomposition of the precursors was observed, very likely as a
result of side reactions of the nonafloxy groups, and this led to
intractable mixtures of undefined products. We therefore con-
verted mono-N-oxide 12 and bis-N-oxide 13 into phenyl-substi-
tuted terpyridine derivatives 15 and 16 by employing phenyl-
Scheme 8. Deprotection of 8 and 7 to terpyridines 21 and 22 followed by
boronic acid under standard Suzuki coupling conditions
(Scheme 7).[14] The two expected products were isolated in ex-
cellent yields, which demonstrates again the capacity of this
type of palladium-catalyzed reaction and the inertness of the
N-oxide function to the reaction conditions employed.
oxidation with IBX.
We previously demonstrated by the conversion of 3 into the
corresponding bisoxime[1a] that these aldehydes offer the possi-
bility for further functionalization to multidentate ligands. This
was confirmed by the conversion of 23 into terpyridine deriva-
tive 25 by reductive amination with benzylamine in very good
yield (Scheme 9).
Scheme 9. Reductive amination of 23 to tetradentate ligand 25.
Conclusions
In this report, we demonstrated that compounds of type A are
excellent precursors for the synthesis of a variety of functional-
ized 2,2′:6′,2′′-terpyridine derivatives. Whereas direct side-chain
oxidation and bromination did not provide the desired pro-
ducts, a “detour” via the corresponding pyridine N-oxides fol-
lowed by Boekelheide rearrangement with acetic anhydride
provided the expected 6- and 6′′-acetoxymethyl-substituted
terpyridines in reasonable overall yields. Depending on the oxi-
dation conditions, bis-nonafloxy-substituted derivative 11 could
be converted into mono-, bis-, and tris-N-oxides 12–14. These
nonafloxy-substituted compounds were unsuitable substrates
for the Boekelheide rearrangement; however, after
Scheme 7. Suzuki couplings of bisnonaflates 12 and 13 to phenyl-substituted
terpyridine derivatives 15 and 16, Boekelheide rearrangement to 17 and 18
and deprotection to terpyridine derivatives 19 and 20.
With terpyridine derivatives 15 and 16 in hand, we again
heated the two compounds with acetic anhydride and then
received the desired Boekelheide rearrangement products 17
and 18 in high yields (Scheme 7). Final cleavage of the acetoxy
group was efficiently achieved with methanol under mild basic
conditions to deliver tetradentate and pentadentate terpyridine
derivatives 19 and 20 in very good yields.
Acetoxy-substituted terpyridine derivatives 8 and 7 were Suzuki reaction and replacement of the nonafloxy groups by
similarly deprotected to furnish hydroxymethyl-substituted phenyl substituents, resulting N-oxides 15 and 16 also under-
terpyridine derivatives 21 and 22 almost quantitatively went the transformation into the expected acetoxymethyl-sub-
(Scheme 8). Finally, compound 21 was oxidized with 2-iodoxy- stituted terpyridines 17 and 18. Compounds such as 7, 8, 17,
benzoic acid (IBX, 1.4 equiv.)[15] to afford the expected aldehyde and 18 were deprotected to provide the hydroxymethyl-substi-
23 in excellent yield. Diol 22 was more resistant to complete tuted terpyridine derivatives. These constitute an interesting
oxidation. Even 3 equivalents of IBX only provided a 45:55 mix- class of tetra- and pentadentate ligands,[16] but they also allow
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1
100 mL), m.p. 222–224 °C. H NMR (500 MHz, [D6]DMSO): δ = 4.76
(d, J = 5.8 Hz, 4 H, CH2), 5.60 (t, J = 5.8 Hz, 2 H, OH), 7.52–7.61 (m,
6 H, 6 Ph), 7.85 (d, J = 1.4 Hz, 2 H, 5-H), 7.91–7.93 (m, 4 H, Ph), 8.12
(t, J = 7.8 Hz, 1 H, 4′-H), 8.47 (d, J = 7.8 Hz, 2 H, 3′-H), 8.79 (d, J =
1.4 Hz, 2 H, 3-H) ppm. 13C NMR (126 MHz, [D6]DMSO): δ = 64.4 (t,
CH2), 116.4 (d, C-3), 118.0 (d, C-5), 120.9 (d, C-3′), 126.8, 129.3, 129.4
(3 d, Ph), 137.8 (d, C-4′), 138.4 (s, Ph), 148.7, 154.8, 155.0, 162.6 (4 s,
subsequent transformations, as shown by oxidation of com-
pound 21 followed by reductive amination of 23 to furnish the
new tetradentate amino-substituted terpyridine derivative 25
in good overall yield.
Experimental Section
C-2, C-2′, C-4, C-6) ppm. IR (ATR): ν = 3240 (O–H), 3060 (=C–H), 2930,
˜
2870 (C–H), 1605, 1580, 1545, 1500 (C=C, C=N), 1465–1430, 1390,
1320, 1275 cm–1. HRMS (ESI-TOF): C29H23N3O2·Na+: calcd. 468.1682;
found 468.1684.
Typical Procedure for the Synthesis of 6,6′′-Bis(acetoxymethyl)-
4,4′′-dimethoxy-2,2′:6′,2′′-terpyridine (7) and 6-Acetoxymethyl-
4,4′′-dimethoxy-6′′-methyl-2,2′:6′,2′′-terpyridine (8): A mixture
of terpyridine 1 (216 mg, 0.57 mmol) and mCPBA (994 mg,
4.03 mmol, 70 %) in CH2Cl2 (12 mL) was stirred at room temperature
for 3 d. TLC analysis indicated incomplete conversion of 1. An addi-
tional amount of mCPBA (663 mg, 2.69 mmol, 70 %) and CH2Cl2
(5 mL) were added and stirring was continued at 45 °C under reflux
for 2 d. The mixture was diluted with CH2Cl2 (20 mL) and a solution
of satd. aq. Na2CO3 (30 mL). The layers were separated, and the
aqueous layer was extracted with CH2Cl2 (30 mL). The combined
organic layer was dried with Na2SO4, filtered, and concentrated un-
der reduced pressure. The remaining residue was dissolved in Ac2O
(5 mL) and stirred at 120 °C under reflux for 6 h. The solvent was
evaporated under reduced pressure, and the remaining residue was
slowly treated with a solution of satd. aq. Na2CO3 (15 mL) and ex-
tracted with CH2Cl2 (2 × 20 mL). The combined organic layer was
dried with Na2SO4, filtered, and concentrated under reduced pres-
sure. The obtained crude product was filtered through silica gel
(hexanes/EtOAc = 3:2) to provide a mixture of terpyridines 7 and 8
(159 mg). Flash column chromatography on aluminum oxide (activ-
ity grade III, hexanes/EtOAc = 6:1→4:1) provided terpyridine 8
(40 mg, 16 %) and 7 (117 mg, 40 %) as colorless solids. For elemen-
tal analysis, a sample of 7 was recrystallized from hexanes/EtOAc
(1:2). Data for 7: M.p. 105–108 °C. 1H NMR (500 MHz, CDCl3): δ =
2.19 (s, 6 H, CH3), 3.97 (s, 6 H, OMe), 5.27 (s, 4 H, CH2), 6.91 (d, J =
2.4 Hz, 2 H, 5-H), 7.91 (t, J = 7.8 Hz, 1 H, 4′-H), 8.05 (d, J = 2.4 Hz, 2
H, 3-H), 8.44 (d, J = 7.8 Hz, 2 H, 3′-H) ppm. 13C NMR (126 MHz,
CDCl3): δ = 21.0 (q, CH3), 55.3 (q, OMe), 66.9 (t, CH2), 105.8 (d, C-3),
107.7 (d, C-5), 121.6 (d, C-3′), 137.8 (d, C-4′), 154.8, 157.0, 157.6 (3 s,
Supporting Information (see footnote on the first page of this
article): Experimental details, characterization data, and copies of
the H NMR and 13C NMR spectra of all key intermediates and final
1
products.
Acknowledgments
Generous support of this work by the Deutsche Forschungs-
gemeinschaft (DFG) and Bayer HealthCare is most gratefully ac-
knowledged.
Keywords: Cross-coupling · Ligand design · Oxidation ·
Rearrangement · Nitrogen heterocycles · Terpyridines
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˜
3020 (=C–H), 2970, 2940, 2855 (C–H), 1735 (C=O), 1605, 1565 (C=C,
C=N), 1455, 1420, 1375, 1255–1160 cm–1
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1
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(s, 2 H, CH2), 6.72 (d, J = 2.4 Hz, 1 H, 5′′-H), 6.91 (d, J = 2.4 Hz, 1 H,
5-H), 7.91 (t, J = 7.8 Hz, 1 H, 4′-H), 7.94 (d, J = 2.4 Hz, 1 H, 3′′-H),
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=
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˜
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Received: October 9, 2015
Published Online: November 23, 2015
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