3-Aryl-2-phosphinoimidazo[1,2-
a]pyridine ligands were synthesized from 2-aminopyridine
via two complementary routes. The first synthetic route involves the copper-catalyzed iodine-mediated cyclizations of 2-aminopyridine with arylacetylenes followed by palladium-catalyzed cross-coupling reactions with phosphines. The second synthetic route requires the preparation of 2,3-diiodoimidazo[1,2-
a]pyridine or 2-iodo-3-bromoimidazo[1,2-
a]pyridine from 2-aminopyridine followed by palladium-catalyzed Suzuki/phosphination or a phosphination/Suzuki cross-coupling reactions sequence, respectively. Preliminary model studies on the Suzuki synthesis of sterically-hindered biaryl and Buchwald–Hartwig amination compounds are presented with these ligands.3-Aryl-2-phosphoimidazo[1,2-
a]pyridine ligands were prepared
via two complimentary synthetic routes and were evaluated in the Suzuki–Miyaura and Buchwald–Hartwig amination cross-coupling reactions.
Palladium-catalyzed cross-coupling reactions have revolutionized the formation of C–C and C–X bond formation in the academic and industrial synthetic organic chemistry sectors.
1,2 Applications such as synthesis of natural products,
3 active pharmaceutical ingredients (API),
4 agrochemicals,
5 and materials for electronic applications
6 are showcased. Snieckus described in his 2010 Nobel Prize review that privileged ligand scaffolds represented the “third wave” in the cross-coupling reactions where the “first wave” was the investigation of the metal catalyst-the rise of palladium and the “second wave” was the exploration of the organometallic coupling partner.
1 In the last twenty years, it was recognized that the choice of ligand facilitated the oxidative addition and reductive-elimination steps of the catalytic cycle of transition metal-catalyzed cross-coupling reactions, increasing the overall rate of the reaction. For example, bulky trialkylphosphines facilitated the oxidative addition processes of electron-rich, unactivated substrates such as aryl chlorides.
7,8 Sterically demanding ligands also provided enhanced rates of reductive elimination from [(L)
nPd(aryl)(R), R = aryl, amido, phenoxo,
etc.] species by alleviation of steric congestion.
9 Privileged ligands such as Buchwald''s biarylphosphines,
10,11 Fu''s trialkylphosphines,
7,8,12 Nolan–Hermann''s
N-heterocyclic carbenes (NHC),
13–15 Hartwig''s ferrocenes,
16,17 Beller''s bis(adamantyl)phosphines
18,19 and
N-aryl(benz)imidazolyl or
N-pyrrolylphosphines,
20,21 Zhang''s ClickPhos ligands,
22,23 and Stradiotto''s biaryl P–N phosphines,
24,25 to mention a few, have found wide-spread use in Suzuki–Miyaura, Corriu–Kumada, Heck, Negishi, Sonogashira, C–X (X = S, O, P) cross-coupling and Buchwald–Hartwig amination reactions (). Preformed catalysts with these ligands attached to the palladium metal center are also recognized as well-defined entities in cross-coupling reactions.
26Open in a separate windowPrivileged ligands for palladium-catalyzed cross-coupling reactions.The term privileged structure was first coined by Evans
et al. in 1988 and was defined as “a single molecular framework able to provide ligands for diverse receptors”.
27 In the last three decades, it is clear that privileged structures are exploited as opportunities in drug discovery programs.
28–31 For example, imidazo[1,2-
a]pyridines are privileged structures in medicinal chemistry programs ().
32 Imidazo[1,2-
a]pyridines are a represented motif in several drugs on the market such as zolpidem, marketed as Ambien™ for the treatment of insomnia,
33 minodronic acid, marketed as Bonoteo™ for oral treatment of osteoporosis,
34 and olprinone, sold as Coretec™ as a cardiotonic agent.
35Open in a separate windowImidazo[1,2-
a]pyridines as privileged structures in medicinal chemistry and in our cross-coupling reactions approach.Our group is interested in a long-term research program directed at the use of key privileged structures that are employed in drug discovery programs as potential phosphorus ligands for cross-coupling reactions. In our entry into the use of privileged structures from the medicinal chemistry literature for our investigation into new phosphorus ligands, we have developed two complementary synthetic routes for the preparation of 3-aryl-2-phosphinoimidazo[1,2-
a]pyridine ligands from 2-aminopyridine as our initial substrate.Our first synthetic route for the preparation of 3-aryl-2-phosphinoimidazo[1,2-
a]pyridine ligands 3a–3l required the copper(
ii) acetate iodine-mediated double oxidative C–H amination of 2-aminopyridine (1) with arylacetylenes under an oxygen atmosphere to give 3-aryl-2-iodoimidazo[1,2-
a]pyridines 2a–2d ().
36,37Open in a separate windowPreparation of 3-aryl-2-phosphinoimidazo[1,2-
a]pyridine ligands 3a–3l from 2-aminopyridine
via copper-catalyzed arylacetylene cyclizations/palladium-catalyzed phosphination reactions sequences.Phenylacetylene and 2-/3-/4-methoxyphenylacetylenes were commercially available reagents. With intermediates 2a–d in hand, we explored several cross-coupling phosphination reactions and we found that palladium-catalyzed phosphination with DIPPF ligand in the presence of cesium carbonate as the base in 1,4-dioxane under reflux provided twelve new ligands 3a–3l as shown in Moderate to good yields were obtained under these cross-coupling conditions. There are few commercially available dimethoxyphenylacetylenes, and most are prohibitively expensive, and so an alternative synthetic strategy was explored.Palladium-catalyzed phosphination of 3-aryl-2-iodoimidazo[1,2-
a]pyridines 2a–2d
aEntry | Ar | R | 3 (% yield) |
---|
1 | Ph (2a) | t-Bu | 3a (41) |
2 | Ph (2a) | Cy | 3b (50) |
3 | Ph (2a) | Ph | 3c (61) |
4 | 2-OMeC6H4 (2b) | t-Bu | 3d (53) |
5 | 2-OMeC6H4 (2b) | Cy | 3e (83) |
6 | 2-OMeC6H4 (2b) | Ph | 3f (69) |
7 | 3-OMeC6H4 (2c) | t-Bu | 3g (62) |
8 | 3-OMeC6H4 (2c) | Cy | 3h (72) |
9 | 3-OMeC6H4 (2c) | Ph | 3i (79) |
10 | 4-OMeC6H4 (2d) | t-Bu | 3j (73) |
11 | 4-OMeC6H4 (2d) | Cy | 3k (55) |
12 | 4-OMeC6H4 (2d) | Ph | 3l (59) |
Open in a separate windowaReaction conditions: 2a–2d (1 equiv.), HPR
2 (1 equiv.), Pd(OAc)
2 (2 mol%), Cs
2CO
3 (1.2 equiv.), DIPPF (2.5 mol%), 1,4-dioxane, 80 °C.2-Iodoimidazo[1,2-
a]pyridine (4) was conveniently prepared in three steps from 2-aminopyridine (1) following literature procedures, which was then converted into either iodo 5 or bromo 6 with NIS or NBS, respectively ().
39,40Open in a separate windowPreparation of 2,3-diiodoimidazo[1,2-
a]pyridine (5) and 3-bromo-2-iodoimidazo[1,2-
a]pyridine (6).When the phosphorus ligands 3 contained
tert-butyl or cyclohexyl groups, method 1 was followed where 2,3-diiodoimidazo[1,2-
a]pyridine (5) underwent Suzuki cross-coupling reactions with arylboronic acids to yield aryl intermediates 7a–7f, which was followed by palladium-catalyzed cross-coupling phosphination reactions with di-
tert-butylphosphine or dicyclohexylphosphine to give C-2 substituted phosphorus ligands 3m–3u in low to moderate yields (, The phosphorus ligands 3v–3ab were prepared from 3-bromo-2-iodoimidazo[1,2-
a]pyridine (6)
via a palladium-catalyzed phospination with diphenylphosphine (method 2) to give intermediate 8 (X = Br, I becomes PPh
2) followed by Suzuki palladium-catalyzed cross-coupling reactions with arylboronic acids. Note that the change in reactivity of the core when switching between bromo and iodo at C3 results in a change in the order of cross-coupling steps.
Open in a separate windowPreparation of 3-aryl-2-phosphinoimidazo[1,2-
a]pyridine ligands 3m–3ab from 2-iodo-3-iodo(or bromo)imidazo[1,2-
a]pyridines 5 or 6
via palladium-catalyzed Suzuki/phosphination or a phosphination/Suzuki cross-coupling reactions sequences.Palladium-catalyzed Suzuki/phosphination or phosphination/Suzuki reactions sequences of 2,3-diiodoimidazo[1,2-
a]pyridine (5) or 3-bromo-2-iodoimidazo[1,2-
a]pyridine (6)
aEntry | R | Ar | Method/substrate | Step 1 (% yield) | Step 2 (% yield) |
---|
1 | t-Bu | 2,3-diOMeC6H3 | 1, 5 | 7a (59) | 3m (64) |
2 | t-Bu | 3,4-diOMeC6H3 | 1, 5 | 7b (54) | 3n (31) |
3 | t-Bu | 2,5-diOMeC6H3 | 1, 5 | 7c (58) | 3o (61) |
4 | t-Bu | 3,4,5-triOMeC6H2 | 1, 5 | 7d (50) | 3p (62) |
5 | Cy | 2,3-diOMeC6H3 | 1, 5 | 7a (59) | 3q (46) |
6 | Cy | 2,6-diOMeC6H3 | 1, 5 | 7e (40) | 3r (52) |
7 | Cy | 3,4-diOMeC6H3 | 1, 5 | 7b (54) | 3s (52) |
8 | Cy | 2,3,4-triOMeC6H2 | 1, 5 | 7f (58) | 3t (21) |
9 | Cy | 3,4,5-triOMeC6H2 | 1, 5 | 7d (50) | 3u (55) |
10 | Ph | 2,3-diOMeC6H3 | 2, 6 | 8 (70) | 3v (52) |
11 | Ph | 2,5-diOMeC6H3 | 2, 6 | 8 (70) | 3w (68) |
12 | Ph | 3,4-diOMeC6H3 | 2, 6 | 8 (70) | 3x (67) |
13 | Ph | 2,3,4-triOMeC6H2 | 2, 6 | 8 (70) | 3y (52) |
14 | Ph | 3,4,5-triOMeC6H2 | 2, 6 | 8 (70) | 3z (64) |
15 | Ph | 4-FC6H4 | 2, 6 | 8 (70) | 3aa (40) |
16 | Ph | 3-F,5-OMeC6H3 | 2, 6 | 8 (70) | 3ab (39) |
Open in a separate windowaReaction conditions: 5, ArB(OH)
2, Pd(PPh
3)
4 (5 mol%), Na
2CO
3 (2 equiv.), 1,4-dioxane/H
2O (2 : 1) and HPR
2 (1 equiv.), Pd(OAc)
2 (2.5–5 mol%), Cs
2CO
3 (1.2 equiv.), DIPPF (2.5–10 mol%), 1,4-dioxane, 80 °C or 6, reverse sequence of reactions.With our library of functionalized imidazo[1,2-
a]pyridine phosphorus ligands 3a–3ab in hand, we began to screen these ligands in Suzuki–Miyaura cross-coupling reactions to prepare sterically-hindered biaryl compounds. We chose the Suzuki–Miyaura cross-coupling reactions of
m-bromo-xylene (9) and 2-methoxyphenylboronic acid (10) to give 2,6-dimethyl-(2-methoxy)biphenyl (11) as our model reaction as outlined in
|
---|
Entry | Ligand | Conditions | Conversiona (%) |
---|
1 | 3a | | 12 |
2 | 3m | | 20 |
3 | 3p | | 14 |
4 | 3r | | >99 b |
5 | 3t | | >99 |
6 | 3w | | 21 |
7 | 3y | | 55 |
8 | 3z | | 46 |
9 | 3ab | | 11 |
10 | 3r | K3PO4 was used as base reaction was performed at 25 °C reaction was stirred for 3 h no ligand | 91 |
11 | 3r | 4 |
12 | 3r | 39 |
13 | — | 0 |
14 | SPhos | | >99 |
15 | XPhos | | >99 |
Open in a separate windowaBased on GC analyses of consumed 9.
bIsolated yield of 96% was obtaisned.Furthermore, a Buchwald–Hartwig amination model study was investigated with our new imidazo[1,2-
a]pyridine phosphorus ligands 3a–3ab. The Buchwald–Hartwig amination reaction of 4-chlorotoluene (12) with aniline (13) to give 4-methyl-
N-phenylaniline (14) was screened with our ligands (