Key words
asymmetric catalysis - copper catalysis - fluorine - Mannich reaction - heterocycle
Organofluorine compounds generally exhibit distinctive chemical properties compared to their corresponding nonfluorinated analogues owing to the strong C–F bond and high electronegativity of fluorine.[1] The altered attributes are often beneficial for medicinal and agrochemical applications.[2] Therefore, the incorporation of fluorine and perfluoroalkyl groups such as CF3 into organic molecules has been a topic of the intensive research.[3] In addition to fluorinated aromatics, recent effort has also been dedicated to the preparation of fluorine-containing aliphatic compounds in enantioenriched form.[4] Two strategies exist for this purpose: fluorination/fluoroalkylation and building block approaches. Given the broad utility of enolate-based chemical transformations, α-CF3 enolates would seem one of the most ideal building blocks for the construction of a trifluoromethylated stereogenic carbon. Nevertheless, only limited chemistry has been explored with this class of nucleophiles due to their notorious instability associated with the high aptitude for β-fluoride elimination from the corresponding metal enolates (Scheme [1, a]).[5]
[6]
Scheme 1 (a) Known decomposition pathway for α-CF3 metal enolates. (b) Our chelated amide strategy.
As a part of our research program in direct enolization chemistry,[7] we have recently devised a chelated enolate strategy to tame otherwise unstable α-CF3 metal enolates (Scheme [1, b]).[8] The designed pronucleophile[9] contains a 7-azaindoline amide as a bidentate chelating unit that prevents unfavorable metal–fluorine interactions. The thus generated α-CF3 enolate has proven effective in the construction of CF3-containing stereogenic carbons in a wide range of Cu(I)-catalyzed asymmetric transformations.[10] The applications have, however, been limited to the construction of trisubstituted stereocenters at the β-position of the amide carbonyl group.[11]
[12] Facile Mannich addition of the α-CF3 amide to Boc-aldimines[8] prompted us to examine activated ketimines as potential reaction partners. Herein, we report the successful implementation of this strategy for the preparation of tetrasubstituted carbons by means of a direct catalytic asymmetric Mannich-type reaction to isatin imines. [13]
Our experience with 7-azaindoline amides has established a combined soft Lewis acid/Brønsted base system comprising Cu(I)/chiral bisphosphine ligand/Barton’s base as a particularly effective catalyst for direct enolization chemistry.[8]
[14] A recent systematic study has also found that the Ph-BPE ligand exhibits consistently high catalytic competency for a broad range of α-substituents of the amides including N3, Cl, and alkyl groups, but not fluoroalkyl groups such as CF3; biaryl-type phosphine ligands are preferred for the α-CF3 amide.[15] With these factors in mind, our optimization studies for the Mannich-type reaction of amide 2 to isatin imine 1a commenced with screening various biaryl-type ligands (Table [1]). A quick examination revealed that the desired product was indeed formed in the presence of 5 mol% Cu(I)/chiral biaryl ligand complex, although the enantioselectivities were low to moderate (Table [1], entries 1–4). Hence, we turned our attention to different ligand backbones, and surprisingly, Ph-BPE (L8) was found to perform the best among the ligands evaluated (Table [1], entries 5–8). The catalyst loading was reduced to as little as 1 mol% without sacrificing the reactivity and selectivities (Table [1], entry 9).
Table 1 Optimization Studiesa
|
Entry
|
Ligand
|
x (mol%)
|
y (mol%)
|
Yield (%)b
|
drb
|
ee (%)c
|
1
|
L1
|
5
|
5
|
93
|
91:9
|
–69
|
2
|
L2
|
5
|
5
|
70
|
60:40
|
21
|
3
|
L3
|
5
|
5
|
90
|
92:8
|
–49
|
4
|
L4
|
5
|
5
|
80
|
90:10
|
–23
|
5
|
L5
|
5
|
5
|
59
|
89:11
|
–95
|
6
|
L6
|
5
|
5
|
95
|
94:6
|
–70
|
7d
|
L7
|
5
|
5
|
88
|
88:12
|
31
|
8d
|
L8
|
5
|
5
|
98
|
>95:5
|
99
|
9d
|
L8
|
1
|
2
|
98
|
>95:5
|
99
|
|
a Reaction conditions: 1a (0.10 mmol), 2 (0.11 mmol), THF (0.1 M).
b Yield and diastereomeric ratio were determined by 1H NMR analysis of the unpurified reaction mixture using 3,4,5-trichloropyridine as an internal standard.
c Enantiomeric excess of (S,S)-isomer was determined with normal-phase HPLC on a chiral support.
d The reaction was performed on a 0.2 mmol scale in THF (0.2 M), and isolated yield was reported.
After the identification of a highly selective ligand for this transformation, a series of isatin imines 1 was evaluated with either 1 mol% or 3 mol% Cu catalyst (Table [2]). The Cbz-protected imine also proved suitable for this catalytic system, affording the corresponding product with almost the same level of selectivities (Table [2], entries 1, 2). Both electron-donating and electron-withdrawing substituents at the 5-position were tolerated (Table [2], entries 3–7). Positional isomers of 3d bearing a chlorine atom at different positions were obtained in comparable diastereo- and enantioselectivities (Table [2], entries 8, 9). Substituents on the oxindole nitrogen other than Me were also examined. While the PMB-protected substrate exhibited slightly lower reactivity and selectivities (Table [2], entry 10), the allyl-protected compound afforded results close to those of the Me-substituted one (Table [2], entry 11). The relative and absolute configurations of 3e were determined by X-ray diffraction, and those of the other compounds were assigned by analogy.[16]
Table 2 Substrate Scope of the Mannich-Type Reaction of α-CF3 Amide 2
a
|
Entry
|
R1
|
R2
|
PG
|
Product
|
Yield (%)b
|
erc
|
ee (%)d
|
1
|
H
|
Me
|
Boc
|
3a
|
98
|
>95:5
|
99
|
2
|
H
|
Me
|
Cbz
|
3b
|
91
|
>95:5
|
99
|
3
|
5-F
|
Me
|
Boc
|
3c
|
86
|
94:6
|
99
|
4
|
5-Cl
|
Me
|
Boc
|
3d
|
89
|
92:8
|
99
|
5
|
5-Br
|
Me
|
Boc
|
3e
|
90
|
>95:5
|
99
|
6
|
5-Me
|
Me
|
Boc
|
3f
|
99
|
>95:5
|
98
|
7
|
5-MeO
|
Me
|
Boc
|
3g
|
81
|
>95:5
|
99
|
8
|
6-Cl
|
Me
|
Boc
|
3h
|
86
|
>95:5
|
99
|
9
|
7-Cl
|
Me
|
Boc
|
3i
|
90
|
>95:5
|
96
|
10
|
H
|
PMB
|
Boc
|
3j
|
66
|
86:14
|
92
|
11
|
H
|
Allyl
|
Boc
|
3k
|
97
|
>95:5
|
97
|
a Reaction conditions: 1 (0.20 mmol), 2 (0.22 mmol), THF (0.2 M). For entries 1–4, [Cu(CH3CN)]PF6 (1.0 mol%), L8 (1.2 mol%), Barton’s base (2.0 mol%). For entries 5–11, [Cu(CH3CN)]PF6 (3.0 mol%), L8 (3.6 mol%), Barton’s base (3.0 mol%).
b Yield values refer to isolated yield.
c Diastereomer ratio was determined by 1H NMR and 19F NMR analysis of the unpurified reaction mixture.
d Enantiomeric excess of (S,S)-isomer was determined with normal-phase HPLC on a chiral support.
The reaction proceeded smoothly on a 3.0 mmol scale, producing 1.46 g of Mannich adduct 3a with almost perfect stereoselectivities, albeit a slightly higher catalyst loading was necessary for full consumption of the substrates (Scheme [2]).[17]
[18] We have previously shown that 7-azaindoline amides can provide an in situ chelating group when treated with an organometallic reagent in a manner similar to Weinreb amides, and thus prevent further sequential addition of the reagent.[8b,9,11b,14b] Mannich adduct 3a was reduced by the action of DIBALH to form a masked aldehyde accompanied by the formation of an aluminum alkoxide derived from reduction of the oxindole moiety, which cyclized presumably during the workup. This triple-bond-forming process (two reductions and one cyclization) furnished highly decorated tricycle 4 in 46% yield with excellent diastereoselectivity.[19]
Scheme 2 A large scale reaction and the transformation of its product into a tricyclic skeleton.
In summary, we developed the direct catalytic Mannich-type reaction of an α-CF3 amide to isatin imines. Enolization was promoted without decomposition by a proficient soft Lewis acidic Cu(I)/bisphosphine/Barton’s base catalytic system, and the generated enolate underwent a highly stereoselective addition, producing an α-tertiary amine with an adjacent trifluoromethylated stereogenic carbon. The Mannich adduct was smoothly transformed into a tricyclic framework by harnessing a unique property of the 7-azaindoline as a chelating unit in the reduction step.