Diastereoselectively switchable enantioselective trapping of carbamate ammonium ylides with imines

Article abstract of DOI:10.1021/ja201589k

The diastereoselectively switchable enantioselective trapping of protic carbamate ammonium ylides with imines is reported. The intriguing Rh ₂(OAc)₄ and chiral Bronsted acid cocatalyzed three-component Mannich-type reaction of a diazo compound, a carbamate, and an imine provides rapid and efficient access to both syn- and anti-α- substituted α,β-diamino acid derivatives with a high level control of chemo-, diastereo-, and enantioselectivity.

Full text of DOI:10.1021/ja201589k

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pubs.acs.org/JACS
Diastereoselectively Switchable Enantioselective Trapping of
Carbamate Ammonium Ylides with Imines
Jun Jiang,† Hua-Dong Xu,‡ Jian-Bei Xi,† Bai-Yan Ren,† Feng-Ping Lv,† Xin Guo,† Li-Qin Jiang,‡
Zhi-Yong Zhang,† and Wen-Hao Hu*,†,‡
†Department of Chemistry, ‡Institute of Drug Discovery and Development, East China Normal University,
3663 North Zhongshan Road, Shanghai 200062, China
S Supporting Information
b
ABSTRACT: The diastereoselectively switchable enantio-
selective trapping of protic carbamate ammonium ylides with
imines is reported. The intriguing Rh2(OAc)4 and chiral
Brønsted acid cocatalyzed three-component Mannich-type
reaction of a diazo compound, a carbamate, and an imine
provides rapid and efficient access to both syn- and anti-
R-substituted R,β-diamino acid derivatives with a high level
control of chemo-, diastereo-, and enantioselectivity.
Figure 1. Proposed mechanism for the three component reaction of 1a,
2a, and 3a catalyzed by Rh2(OAc)4.
econdary protic ammonium ylides1 accommodating both
S
acidic ammonium ion and basic carbon centers in close
Aware of the potential risks, we conducted the intermolecular
ylide trapping reaction. A solution of diazo compound 1a in 1,
2-dichloroethane (DCE) was slowly introduced into a mixture of
2a, 3a, and a catalytic amount of Rh2(OAc)4 in DCE over 1 h at
0 °C. We were gratified to find that the reaction gave 4a in 30%
yield with high diastereoselectivity (dr = 95:5), favoring the anti
isomer. A significant amount of NꢀH insertion product 6 was
isolated as a major side product. A control reaction indicated that
4a was not derived from the NꢀH insertion side product 6, since
under identical reaction conditions, except that 3a was added
to the reaction mixture 1 h later, after complete decomposition of
1a, only the NꢀH insertion product was obtained and no 4a was
detected.
The mechanism of rhodium carbenoid insertion into the
carbamate NꢀH bond remains a controversial subject, and both
concerted and stepwise (via metal-free or -associated ylide
intermediates) processes are possibilities.5 Our results strongly
suggest a stepwise ylide trapping mechanism for the formation of
4a and 6 (Figure 1). The protic carbamate ammonium ylides I/II
underwent either 1,2-proton transfer to form NꢀH bond inser-
tion product 6 or interception by an appropriate electrophile,
such as imine 3a in the current case, giving the first protic
carbamate ammonium ylide trapping product 4a via zwitterion
IV, a process of “delayed proton transfer”.
proximity are highly unstable and reactive intermediates, and
have been proposed as the intermediates in NꢀH insertion
reactions of amines with carbenes/carbenoids.2 Surprisingly,
recent studies in our laboratory showed that interception of
the in situ generated protic ammonium ylides with appropriate
carbon electrophiles before the 1,2-proton shift are viable.3
These intriguing findings have challenged and changed the
traditional perception of protic ammonium ylides, thus openning
up new research directions. Trapping the protic ammonium
ylides with appropriate electrophiles could provide quick ac-
cesses to a variety of structures.3 However, these reactions have
encountered several major restrictions: (1) only arylamines were
effective in forming the active ylides for electrophilic trapping in
catalytic settings, thus limiting their synthetic value; (2) poor
chemo- and/or diastereoselectivity were observed; (3) no asym-
metric version has been achieved. We now present our attempts
to resolve these issues via a dual/cooperative catalysis strategy.4
The methyl phenyldiazoacetate (1a)/benzyl carbamate (2a)/
phenylbenzylimine (3a) combination was chosen as the starting
point for the three-component reaction (Figure 1). The carbamate
2a was chosen instead of an arylamine as a component for the
following reasons: (1) easy deprotection to form free amino
compounds, and increased potential synthetic value; (2) the
less-coordinating carbamate should not poison the transition-
metal catalyst and should also be more compatible with other
electrophiles such as aldehydes. However, there is a significant risk
in using 2a in the desired three-component reaction due to the
decreased electron density of the nitrogen atom of 2a. A concerted
or near-concerted reaction pathway has been proposed in the
intramolecular NꢀH insertion of carbamate BocNHR.5 If this
concertedNꢀHinsertion occurred with2a inthe currentreaction,
no ylide trapping product would be obtained.
Pioneering work reported independently by the groups of
Terada6 and Akiyama7 indicated that bifunctional chiral phos-
phoric acids (PPAs) are excellent Brønsted acid catalysts to
activate imine substrates.8 Recent advances in Brønsted acid
activated electrophiles for carbonꢀcarbon bond formation with
Received: February 26, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Evaluation of Catalysts and Reaction Conditionsa
Figure 2. (a) Phosphoric acids. (b) Possible complex of phosphoric
acid with the diamine product.
T
yield
drc
Ee of
entry 5 (mol %) solvent (°C) (%)b anti/syn anti (syn) (%)c
diazoalkanes have highlighted the compatibility of diazo com-
pounds with Brønsted acid catalysis in organic reactions.9 We
envisioned that PPAs 5 (Figure 2a) would activate 3a to be a
more reactive electrophile toward carbamate ammonium ylides,
while minimizing the 1,2-proton shift side product. This proved
to be the case (Table 1); in the presence of 10 mol % 5 the
yields increased dramatically up to 86% with a broad spectrum
of reduced diastereoselectivities (83/17ꢀ15/85) (Table 1,
entries 1ꢀ7).
1
R-5a (10)
DCE
0
0
78
76
76
86
81
81
78
87
64
80
88
92
93
90
85
71
15:85
17:83
22:78
27:73
23:77
38:62
83:17
95:5
ꢀ9 (ꢀ94)
ꢀ4 (ꢀ78)
24 (79)
7 (77)
16 (79)
37 (89)
68 (79)
92
2
R-5b (10) DCE
3
S-5c(10)
S-5d (10)
S-5e (10)
S-5f (10)
S-5g (10)
S-5g (10)
S-5g (2)
S-5g (2)
S-5g (5)
S-5g (5)
S-5g (5)
R-5a (5)
S-5g (5)
ꢀ
DCE
DCE
DCE
DCE
DCE
PhMe
PhMe
PhMe
PhMe
PhMe
0
4
0
5
0
6
0
7
0
The PPAs with electron-deficient 2,20-aryl groups (5a, Ar = 3,
5-(CF3)2Ph; 5b, Ar = p-CF3Ph) afforded product mixtures
with anti-4a/syn-4a ratios of 15/85 and 17/83, respectively,
favoring the syn isomer. This is interesting in contrast with the
anti selectivity in the reaction without PPA cocatalysts. The
diastereoselectivity decreased slightly to 23/77 when a PPA with
an electron-donating 2,20-aryl group (5e, Ar = p-MePh) was
used. Compared with the moderate electronic effects of a 2,
20-aryl group in the PPA on the diastereoselectivities, the steric
effect was more pronounced when 5f (Ar = 9-phenanthryl)
catalyzed the reaction, furnishing diamine 4a with 38/62 dr, with
increased anti selectivity. This trend is amazing, as the extremely
bulky PPA 5g (Ar = triphenylsilyl) resulted in a reversal of the dr
to 83/17, favoring the anti diastereomer (Table 1, entry 7). The
enantioselectivities of all the PPA-promoted reactions for the
major isomer were quite high (68ꢀ94% ee).
8
0
9
0
98:2
11
10d
11d
12d
13d
14d
15e
16d
0
96:4
81
0
95:5
88
25
82:8
77
PhMe ꢀ20
PhMe ꢀ20
97:3
93
10:90
93:7
(ꢀ96)
90
PhMe
0
PhMe
0
89:11
0.7
a Standard conditions: Rh2(OAc)4/5/3a/2a = 0.004/0.01/0.2/0.26
mmol, 4 Å molecular sieve (MS) (0.1 g); 1a in 2.8 mL solvent was
added under an argon atmosphere at 0 °C for 1 h. b Isolated yield.
c Determined by chiral HPLC analysis. L-tartaric acid (20 mol %)
d
was used. e DL-tartaric acid (20 mol %) was used.
To improve the performance of these dual catalysts that
cooperatively catalyze multicomponent reactions, further opti-
mization of the conditions was carried out. Toluene was very
quickly identified as a better solvent than halogenated solvents
(Table 1, entry 8: 87% yield, 95:5 dr, 92% ee). When we tried to
decrease the catalyst loading of S-5g from 10 to 2 mol %, it was
surprising to find that the ee for the major isomer anti-4g
dropped significantly from 92% to 11% (Table 1, entry 9 vs 8).
Since more basic diamine products may play a negative role in the
reaction by sequestering the phosphoric acid via bidentate
coordination (Figure 2b),10 acidic additives should alleviate this
effect by regenerating the acid catalyst.11 L-Tartaric acid (20 mol %)
was identified to be the most effective additive to this end when
used along with 5g (2 mol %), evidenced by enhanced yield and
enantioselectivity (Table 1, entry 10 vs 9). Low temperatures
were better, especially for the ee values manifested by the
enantioselective outcomes of the reaction at 25 °C (77% ee)
and ꢀ20 °C (93% ee) (Table 1, entry 13 vs 12). Thus the optimal
conditions were established as 2 mol % Rh2(OAc)4, 5 mol %
phosphoric acid 5g, and 20 mol % tartaric acid in toluene at
ꢀ20 °C (entry 13). Under these conditions, the protic ylide
trapping reaction catalyzed by the less sterically hindered phos-
phoric acid 5a also gave diamine 4a in high yield and with
excellent ee, but with the diastereoselective control switched in
favor of the syn diastereomer (Table 1, entry 14). PPA only,
tartaric acid only, and the PPA/tartaric acid combination all failed
to promote this three-component reaction in the absence of the
Rh catalyst, indicating the critical role of the Rh catalyst in
generating the metal carbenoid intermediate. Although chiral
Brønsted acids have been shown to be effective in a number of
asymmetric Mannich reactions, there has been no paper describ-
ing this exceptional inversion of diastereoselectivity simply by
changing the steric nature of the 3,30-BINOL substituents.12
The role of tartaric acid as an additive which could promote
the turnover in the current reaction was further investigated.
A control reaction in which L-tartaric acid was replaced by rac-
tartaric acid delivered the same level of yield, dr, and ee (Table 1,
entry 15). In addition, in the absence of a phosphoric acid, the
reaction gave rise to negligible enantioselectivity and a decreased
yield (Table 1, entry 16), suggesting a unique function of the
phosphoric acids. In the absence of the acid additive, the reaction
catalyzed by 2% 5g gave only 11% ee; whereas, in the presence of
50% tartaric acid, the corresponding reaction delivered 81% ee.
This huge 70% ee divergence diminished quickly on increasing
the amount of 5g alone to 10 mol % and disappeared completely
when 20 mol % 5g was used (see Supporting Information (SI)).
These data indicated that the chirality-inducing agents in these
diamine-forming reactions were chiral phosphoric acids; the
acidic additive served as a proton source to neutralize the basic
diamine product, helping to release the phosphoric acid/Rh2-
(OAc)4 catalyst into the catalytic cycle.
Under the optimized conditions, a wide range of aromatic
imines, carbamates, and diazo compounds were evaluated with
5g as the cocatalyst (Table 2). In most cases, anti-diamino acid
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Journal of the American Chemical Society
COMMUNICATION
Table 2. anti-Selective Enantioselective Three-Component
Table 3. syn-Selective Enantioselective Three-Component
Mannich-Type Reactions with 5ga
Mannich-Type Reactions with Phosphoric Acid 5aa
yield %
(anti/syn)b,c
ee
(%)c
yield %
(syn/anti)b,c
ee %c
R1/R2
Ph/Me
R3
Ar1/Ar2
syn (anti)
entry
R1/R2
Ph/Me
R3
Ar1/Ar2
entry
1
Bn
Ph/Ph
4a, 92 (97:3)
4b, 77 (97:3)
4c, 81 (93:7)
4d, 89 (97:3)
4e, 87 (95:5)
4f, 85 (96:4)
4g, 75 (>99:1)
4h, 66 (97:3)
4i, 76 (98:2)
4j, 73 (>99:1)
4k, 72 (99:1)
93
93
87
90
89
90
90
92
94
93
95
96
89
90
ꢀ92
93
89
90
97
99
1
2
3
4
5
6
7
8
9d
Bn Ph/Ph
Bn Ph/Ph
Bn Ph/Ph
Bn Ph/PMP
4a, 90 (90:10)
4d, 79 (85:15)
4e, 88 (86:14)
4f, 78 (75:25)
96
93
2
Ph/Me
Ph/Me
CH2CCl3 Ph/Ph
p-BrPh/Me
p-FPh/Me
Ph/Me
3
CMe3
Ph/Ph
95
4
p-BrPh/Me Bn
Ph/Ph
95
5
p-FPh/Me
Ph/Me
Ph/Me
Ph/Me
Ph/Me
Bn
Bn
Bn
Bn
Bn
Ph/Ph
p-MePh/Me Bn m-BrPh/PMP 4j, 73 (75:25)
>99
97
6
Ph/PMP
m-Br/PMP
o-Br/PMP
p-BrPh/PMP
m-Br/PMP
m-Cl/PMP
Ph/Me
Ph/Me
Ph/Me
Me/Et
Et Ph/Ph
4m, 84 (87:13)
4n, 79 (89:11)
4p, 74 (87:13)
4s, 67 (35:65)
7
Bn Ph/p-BrPh
Bn Ph/m-BrPh
Bn Ph/Ph
96
8
99
9
88 (76)
a Standard conditions: Rh2(OAc)4/5a/3/2/1 = 0.004/0.01/0.2/0.26/
0.26 mmol, in 8.4 mL of PhMe, with 4 Å MS 100 mg at ꢀ20 °C.
b Isolated product yield. c Determined by chiral HPLC analysis. d Alkyl-
substituted diazoacetates (2 equiv) were used.
10
11
12
13
14
p-MePh/Me Bn
p-MePh/Me Bn
Ph/Me
Ph/Me
Ph/Me
Bn
Et
p-BrPh/p-ClPh 4l, 74 (98:2)
Ph/Ph
4m, 93 (97:3)
4n, 75 (91:9)
4n, 77(91:9)
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Ph/p-BrPh
Ph/p-BrPh
15 d Ph/Me
catalyzed three-component ylide trapping protocol. Catalytic
asymmetric synthesis of R,β-diamino acid derivatives through
carbonꢀcarbon bond-forming reactions represents a particular
challenge because it needs both diastereomeric and enantiomeric
control of the two newly generated vicinal stereogenic centers in
a flexible acyclic molecule. Even though great advances have been
made recently in this area,13 there are limited examples of
catalytic synthesis of optically active R,β-diamino acid derivatives
with an R-quaternary carbon center.13g,14 It is worth noting that
no efficient method has been documented previously for the
synthesis of optically active R-aryl-substituted-R,β-diamino acid
derivatives. The current methodology presents an efficient solu-
tion toward these challenges providing a novel entry to the
biologically important R,β-diamino acid derivatives.13k,l
16
Ph/Me
Ph/Me
Ph/Me
Bn/Et
m-ClPh/PMP 4o, 73 (>99:1)
17
Ph/m-BrPh
4p, 70 (96:4)
18
p-NO2/PMP
4q, 66 (>99:1)
19e
20e
p-BrPh/p-ClPh 4r, 71 (94:6)
Me/Et
Ph/Ph 4s, 88 (75:25)
a Standard conditions: Rh2(OAc)4/5g/3/2 = 0.004/0.01/0.2/0.26
mmol, with 4 Å MS 100 mg in 5.6 mL of PhMe; 0.26 of mmol 1 in
2.8 mL of PhMe were then added via a syringe pump at ꢀ20 °C.
b Isolated product yield. c Determined by chiral HPLC analysis. d (R)-5g
(5 mol %) was used to give (2S,3S)-4n. e Alkyl-substituted diazoacetates
(2 equiv) were used.
esters, anti-4, were obtained in high yields with excellent stereo-
selective control (Table 2, entries 1ꢀ19). Trichloroethyl carba-
mate and tert-butyl carbamate are equally good partners with
regard to yield and stereoselectivities (Table 2, entries 2 and 3).
Generally, imines with a halogenated Ar1 gave 10ꢀ20% lower
yields than did imines with just a phenyl group. It is noteworthy
that alkyl-substituted diazoacetates such as ethyl diazophenyl-
propanate and ethyl diazopropanate were feasible substrates in
this reaction, giving the desired products in good to high yields
and high to excellent enantioselectivities (entries 19ꢀ20).
Remarkably, all substrate combinations (except for alkyl diazo
compounds) switched their diastereomeric outcomes, namely
from anti-selectivity to syn-selectivity, when the phosphoric acid
catalyst 5g was replaced by 5aꢀe. Representative results are
tabulated in Table 3. The Rh2(OAc)4/5a-catalyzed three-com-
ponent coupling reactions of diazo esters, carbamates, and imines
delivered syn-4 in good yield, with good to high dr and excellent
ee. Thus, by judicious choice of chiral phosphoric acid catalysts,
each of the four stereoisomers of the R-substituted R,β-diamino
esters could be made at will using the dual catalyst cooperatively
The absolute configurations of anti-4h (2R,3R) (from S-5g)
and syn-4n (2R,3S) (from R-5a) were established by X-ray single
crystal analysis (see SI); by analogy to anti-4h and syn-4n, the
stereochemistries of other diamino products were assigned
correspondingly. These data also show that, for the use of a chiral
phosphoric acid cocatalyst bearing the same absolute configura-
tion, for example (R)-5g (Table 2, entry 15) and (R)-5a, the
switch of diastereoselectivity in 4 occurred at the quaternary C-2
carbon center.
The exact reasons for the stereoselective control are unclear at
the present time and deserve additional detailed study. The
proposed transition states in Figure 3 could be used to explain
theswitchofdiastereoselectivityinthe presence of differenttypeof
PPAs. The cis-enolate-like ammonium ylide is proposed due to the
intramolecular hydrogen bond.15 In the PPA cocatalyzed process,
the bifunctional PPA could form a larger NꢀHꢀOPOꢀHꢀN
bridge in a proposed transition state VI, leading to syn-selectivity.
In the case of an extremely sterically demanding PPA like 5g, the
restricted PPA space cannot accommodate both the imine and the
ylide; thus the open-chain anti-TS VII may operate, giving rise to
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Journal of the American Chemical Society
COMMUNICATION
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
characterization, analysis of enantioselectivities and X-ray crystal
structures. This material are available free of charge via the
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’ AUTHOR INFORMATION
Corresponding Author
whu@chem.ecnu.edu.cn
’ ACKNOWLEDGMENT
We are grateful for financial support from the National Science
Foundation of China (20932003, 20872036) and the MOST of
China (2011CB808600) and for sponsorship from Shanghai
(09JC1404901, 09ZZ45). We thank the Laboratory of Organic
Functional Molecules, Sino-French Institute of ECNU for
support.
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