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Acid-Assisted Ru-Catalyzed Enantioselective Amination of 1,2-Diols through Borrowing Hydrogen Li-Cheng Yang, Ya-Nong Wang, Yao Zhang, and Yu Zhao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02959 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016
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ACS Catalysis
Acid-Assisted Ru-Catalyzed Enantioselective Amination of 1,2-Diols through Borrowing Hydrogen Li-Cheng Yang,† Ya-Nong Wang,† Yao Zhang,*,§ and Yu Zhao*,† †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore, 117543
§
College of Chemistry, Liaoning University, Shenyang, 110036, People’s Republic of China
ABSTRACT: We present here a highly enantioselective synthesis of 1,2-amino alcohols from readily available racemic 1,2diols through a borrowing hydrogen process. An intriguing acid effect was discovered for this Ru-catalyzed amination reaction, which led to a significant improvement of the stereoselectivity of the process. Preliminary mechanistic studies suggest a scenario of Brønsted acid-assisted dynamic kinetic asymmetric amination of alcohols. KEYWORDS: amination, borrowing hydrogen, acid catalysis, amino alcohol, enantioselectivity
Due to the widespread use of chiral β-amino alcohols in asymmetric chemical synthesis as well as in fine chemicals and pharmaceutical industry,1 the efficient synthesis of them in enantiopure form has been an important target for organic chemists.2 In addition to the subclass that is easily accessible from reduction of α-amino acids, several synthetic protocols have also been developed to access other β-amino alcohols with good stereocontrol, including asymmetric aminohydroxylation of alkenes,3 ringopening of epoxides by amines4 and resolution of the racemic starting compounds.5 However, highly efficient and selective methods are still highly desired for the access of this important class of compounds. Amination of alcohols through borrowing hydrogen methodology has received much attention as a significantly atom-economical and environmentally friendly synthetic strategy for amine synthesis.6 In this overall redox-neutral process, the alcohol substrate is converted directly to the amine product without the need for any extra stoichiometric reagent, and water is generated as the sole side product. Great process has been made in this field of research from the efforts of many research groups.7-10 The development of asymmetric variants of this process, however, only emerged in the past few years.11 Our group reported the first enantioselective amination of secondary alcohols catalyzed by iridium and chiral phosphoric acid (Scheme 1a),11b and also applied the system to a highly stereo-convergent amination of α-
branched alcohols to prepare diastereo- and enantiopure α-branched amines.11c It is important to note that essentially all previous catalytic systems for amination of alcohols were carried out under basic or neutral conditions. In our system, on the other hand, the acid co-catalyst played a key role for promoting effective imine condensation as well as the reduction of imine, and the chirality of the acid was crucial in achieving high stereoselectivity of the process.
Scheme 1. Initial Attempts at Amination of Diols In an effort to extend the utility of this catalytic system, we were particular attracted to the amination of diols to access diamines which are important motifs in asymmetric synthesis. Initial attempts to obtain the di-amination of secondary diols such as meso-hydrobenzoin, unfortunately, met with limited success; only mono-amination
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took place to yield the amino alcohol with low efficiency and selectivity (Scheme 1b). The amination of primary, secondary diols such as 2 (Table 1) was pioneered by the group of Oe and Ohta using Ru-based catalyst; similarly, only amination of the primary alcohol took place to yield the amino alcohol products.11j The enantioselectivity of the reaction, however, was also far from synthetically useful level. Very recently the Beller group reported an elegant reaction between diols and urea through a sequence of nucleophilic substitution followed by intramolecular amination of alcohols to deliver oxazolidin-2-ones in high enantioselectivity.11g Based on our previous endeavours, we were curious whether the cooperative metal and acid catalysis could help deliver a higher enantioselective synthesis of this useful class of amino alcohols. Herein we report our finding of Brønsted acid-assisted Ru-catalyzed highly enantioselective amination of 1,2-diols. The amination of 2 with morpholine catalyzed by our previous iridium-based system unfortunately produced 3a with only moderate yield and er (entry 1, Table 1), and no further improvement was achieved after much optimization (results not shown). After systematic screening of different metal precursors and chiral ligands, the RuJosiphos system proved to be the optimal choice as reported previously by the group of Oe.11j To our excitement, while the control reaction using Ru-1 and Josiphos yielded 3a with only moderate er (entry 2), the addition of acid co-catalyst 1a led to a significant increase of the enantioselectivity of the reaction (93:7 er, entry 3). At this point, efforts were directed towards the systematic screening of various chiral phosphoric acids 1b-1d (entries 4-6), different chiral ligands 4b-4e (entries 7-10) or metal precursors Ru-2 and Ru-3 (entries 11-12). All these attempts, unfortunately, led to no further improvement on the enantioselectivity of this reaction.
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11
Ru-2
4a
1a
75
89:11
12
Ru-3
4a
1a
81
89:11
a General conditions: 2 (0.3 mmol), morpholine (0.1 mmol) and the catalysts were dissolved in toluene (0.1 M) and heated to 100 °C for 24 h. bNMR yield using 1,3,5-trimethoxybenzene as the internal standard. cDetermined by HPLC on a chiral stationary phase.
An interesting discovery was made when we used the achiral diphenyl phosphate as the acid co-catalyst, which provided a similar boost on the enantioselectivity (88:12 er, entry 1, Table 2). Realizing the chirality of the acid was not essential for the selectivity of this catalytic system, we moved on to screen a range of simple Brønsted acids for this reaction (entries 2-8). This acid effect turned out to be general, as a range of Brønsted acids all resulted in enhanced level of enantioselectivity (entries 1-8). The use of benzoic acid proved to be the optimal choice for both yield and selectivity (entry 6). Under these conditions, the loading of diol could be reduced to 1.5 equiv. with no erosion on the efficiency of the process (entry 9 vs. entry 10). Finally, extended reaction time of 48 h led to complete conversion to the desired product (entry 11). Table 2. Acid Effect in Borrowing Hydrogena
Table 1. Optimization of Amination of 1,2-Diola
entry
[M]
ligand
acid
yield(%)
1
[Ir]
\
1a
54
b
er
c
72:28
b
er
c
entry
acid
yield(%)
1
diphenyl phosphate
34
88:12
2
TFA
54
94:6
3
CH3CO2H
76
93:7
4
TfOH
83
90:10
5
TsOH
66
88:12
2
Ru-1
4a
\
90
72:28
3
Ru-1
4a
1a
83
93:7
4
Ru-1
4a
1b
41
89:11
6
PhCO2H
79
94:6
5
Ru-1
4a
1c
79
91:9
7
m-NO2PhCO2H
88
93:7
6
Ru-1
4a
1d
87
80:20
8
7 8
Ru-1 Ru-1
4b 4c
1a 1a
75 40
54:56
2,5-di-NO2PhCO2H
37
91:9
d
PhCO2H
79
94:6
e
PhCO2H
59
94:6
d,f
PhCO2H
99
94:6
9
20:80
10 11
9
Ru-1
4d
1a
29
89:11
10
Ru-1
4e
1a
80
90:10
a-c See Table 1. dUse of 1.5 equiv. diol. eUse of 1.0 equiv. diol. f48 h reaction.
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With the optimal conditions in hand, we examined the scope of amines for the amino alcohol synthesis (Scheme 2). In addition to morpholine 3a, the use of piperidine led to the formation of 3b in excellent yield and high er of 95:5. The seven-membered amine 3c was obtained in an even higher er of 96:4. The use of tetrahydroisoquinoline worked out similarly well (94.5:5.5 er for 3d). When an acyclic secondary amine was used, the enantioselectivity dropped significantly and 3e was generated with a moderate 79:21 er. It is noteworthy, however, a closely related product was obtained in an essentially racemic form in the absence of the acid co-catalyst.11j
a
See Scheme 2.
Scheme 3. Scope of 1,2-diols for 1,2-Amino Alcohol Synthesisa
a General conditions: 2 (0.15 mmol), amine (0.1 mmol) and the catalysts were dissolved in toluene (0.1 M) and heated to 100 °C for 48 h. b A mixture of amino alcohols were obtained. The isolated yield of the major isomer is shown.
Scheme 2. Scope of Amines for 1,2-Amino Alcohol Synthesisa Gratifyingly, this amination is not limited to the use of secondary amine substrates. When p-anisidine was used, product 3f could be obtained in reasonable yield of 70% with 91:9 er. When aniline was used for the reaction, however, a mixture of products including 3g and the corresponding isomer (with amination of the secondary alcohol) was produced in a ratio of 4:1. The major isomer 3g was obtained in 63% yield with an excellent 96.5:3.5 er. When electron-deficient aniline was used, amino alcohol 3h was also formed as a mixture with its isomer in a ratio of 3:1. There seemed to be some correlation of the electronic property of the anilines with the regioselectivity of this reaction. The same set of conditions could be applied to a wide range of diol substrates (Scheme 3). For aryl-containing diols, different substituents including electron-donating or electron-withdrawing ones at para- or meta-positions were all well-tolerated. Amino alcohols 3h-3n were obtained in uniformly good enantioselectivity and high yield. Notably, a highly sterically hindered diol was also tolerated to produce 3o in excellent yield and er. In addition to aryl-substituted diols, aliphatic diols could also be used to yield amino alcohols such as 3p and 3q in high yield and er, which greatly expanded the scope of this catalytic system.
While the exact nature of this intriguing acid effect is currently not clear, we propose the reaction pathway shown in Scheme 4 as a working hypothesis. The amination initiated with the oxidation of 2 to 5 (or possibly to the tautomer 6 as well, which is a tautomer of 5). In the original report by Oe and Ohta, it was proposed that 7 could be formed and converted to 8 via reductive amination (pathway A, Scheme 4a).11j The enantio-determining step was then proposed to be the asymmetric transfer hydrogenation (ATH) of ketone 8. To shed some light on the mechanism, we prepared amino ketone 8 and subjected it to the transfer hydrogenation condition using the same catalytic system in the absence or presence of acid. For this reaction, however, no improvement on enantioselectivity was observed at all (Scheme 4c), which was inconsistent with our observation of the acid effect. We therefore propose an alternative pathway B (Scheme 4b) in which 5 underwent imine condensation to generate 9, which should be more energetically favored over oxidation to 7, and the acid co-catalyst should significantly facilitate such a transformation. It is believed that the reduction of the most electrophilic iminium 9 should be highly favored over reduction of 10 and 8 to yield amino alcohol 3. We propose that this enantioselective process could be operative through a dynamic kinetic asymmetric reduction12 of 9: the two enantiomers of iminium 9 are expected to undergo facile racemization through the achiral enamine 10, while the chiral Ru-catalyst could differentiate the two enantiomers of 9 to promote a stereoselective reduction to deliver 3. It is important to note that while amino ketone 8 is also a tautomer of 10, the isomerization of ketone 8 to enol 10 is expected to be very slow, which might explain the lack of acid effect for the reduction of α-aminoketone 8 (Scheme 4c). Overall the benefi-
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cial effect of achiral Brønsted acids on the enantioselectivity of the reaction may come from an accelerated tautomerization/racemization. a) ATH of -amino ketone OH
OH
R
Pathway A
[O]
[O]
R1 HN
O
[O]
O
R
[email protected];
[email protected] Notes. The authors declare no competing financial interests.
ASSOCIATED CONTENT
6
2
OH
Corresponding Author
O
[O] OH
R
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O
R
5
Supporting Information O
R2 R
reductive amination
7
R1 N
OH
ATH of ketone R2
R
8
R1 N
R2
3
Experimental procedures and characterization data for all the products are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
b) Dynamic kinetic asymmetric reduction OH R
R1 N
[Ru-H]
[Ru]
R1 N
OH R
2
R
kfast
9
ACKNOWLEDGMENT
R2
3
We are grateful for the generous financial support from Singapore National Research Foundation (R-143-000-477-281 and R-143-000-606-281), the Ministry of Education (MOE) of Singapore (R-143-000-613-112), and the National Natural Science Foundation of China (21502082).
kfast OH
Pathway B
O
R
OH
facilitated by acid
5
R1 HN
R
R1 N
O R2
R
10
R1 N
R2
8
kfast R2
[Ru-H] OH R
R1 N
[Ru] R1 N
OH R
2
R
kslow
REFERENCES
R2
ent-3
ent-9
c) ATH of preformed -amino ketone O
O +
N
Ph 8
OH OH Ph 3 equiv.
5 mol % Ru-1, 6 mol % 4a
OH
toluene, 100 °C, 24 h
O N
Ph
3a without PhCO2H: 33%, 85:15 er with PhCO2H: 43%, 86:14 er
d) use of enantiopure diol OH
OH OH
Ph (S)-2
or
OH
standard conditions OH
Ph
O
Ph
O N 3a
(R)-2 N H
with (R)-2: >95% conv., 96.5:3.5 er with (S)-2: >95% conv., 94:6 er with rac 2: >95% conv., 94:6 er
Scheme 4. Mechanistic Studies and Proposed Reaction Pathway To shed more light on the mechanism, the use of enantiopure diol substrates were tested under the standard conditions (Scheme 4d). Interestingly the use of (S)-2 led to the same selectivity with the use of racemic 2, while the use of the matched substrate (R)-2 delivered 3a in higher er. This observation is consistent with the dynamic kinetic asymmetric reduction pathway, although we cannot rule out other possible reaction pathway or combination of different pathways for this transformation. In summary, we have developed a highly enantioselective synthesis of β-amino alcohols through monoamination of readily available racemic diols. The addition of a simple achiral acid co-catalyst proved to be the key factor to achieve high enantioselectivity in this Rucatalyzed process. A possible pathway of dynamic kinetic asymmetric amination of alcohols was proposed. The development of efficient methods to prepare other classes of amino alcohols and diamines are currently under investigation in our laboratories.
AUTHOR INFORMATION
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