Palladium-catalyzed diastereo- And enantioselective allylic alkylation of oxazolones with 1,3-dienes under base-free conditions

Article abstract of DOI:10.1039/d0cc00265h

Herein, we report a highly diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes by palladium-hydride catalyst under base-free conditions. With DTBM-SEGPHOS as the chiral ligand, a series of enantioenriched oxazolones bearing tertiary carbon centers were synthesized from substituted 1,3-dienes via exclusive 1,2-addition with moderate to good diastereoselectivities and high enantioselectivities. When simple 1,3-butadiene was used as the allyl precursor under this base-free catalytic system, 1,4-addition products were obtained in good yields with high regioselectivities.

Full text of DOI:10.1039/d0cc00265h

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Palladium-catalyzed diastereo- and enantioselective allylic
alkylation of oxazolones with 1,3-dienes under base-free
conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Haijian Yang and Dong Xing*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein, we report a highly diastereo- and enantioselective allylic alkylation of ketones with 1,3-dienes with a chiral SEGPHOS-type
alkylation of oxozolones with 1,3-dienes by palladium-hydride ligand.4h By using Pd-phosphinooxazoline (Pd-PHOX) catalysts,
catalyst under base-free conditions. With DTBM-SEGPHOS as the Malcolmson and co-workers accomplished the AAAs of various
chiral ligand, a series of enantioenriched oxazolones bearing activated carbon-based nucleophiles such as Meldrum’s acids or
tertiary carbon centers were synthesized from substituted 1,3- malononitriles with 1,3-dienes with high enantioselectivity.4i,4j Very
dienes via exclusive 1,2-addition with moderate to good recently, Zi and co-workers developed a stereodivergent AAA of
diastereoselectivities and high enantioselectivities. When simple aldimine esters by synergistically combining Pd-PHOX and Cu
1,3-butadiene was used as the allyl precursor under this base-free catalysts to access all four stereoisomers of the alkylation products.4k
catalytic system, 1,4-addition products were obtained in good For these palladium-catalyzed asymmetric transformations (Scheme
yields with high regioselectivities.
1b), the Pd-PHOX precatalyst has to be pre-formed and excess
amounts of amine-based organic bases (2–3 equiv) were essential.
With the aim to develop AAAs of versatile carbon-based
pronucleophiles with a broad scope of 1,3-dienes, the exploration of
mild and novel catalytic asymmetric system remains an urgent task
in this area.
Transition metal-catalyzed asymmetric allylic alkylation (AAA) of
carbon-based pronucleophiles is
a powerful tool for the
enantioselective construction of C–C bonds.1 In recent years,
significant progress has been made on the use of unsaturated
hydrocarbons, such as allenes, alkynes and 1,3-dienes, as atom-
economic allyl precursors for AAAs.2–4 Among them, 1,3-dienes have
gained considerable attention dating back to 1970s when Hata et al.
reported the palladium-catalyzed addition of 1,3-dicarbonyl
compounds to 1,3-dienes.5 Following this pioneering report,
different transition metal catalysts such as Pd, Ni, Rh etc have been
substantially used as the transition metal–hydride (M–H) precursors
to convert 1,3-dienes into electrophilic metal--allyl species, which
(a) 1,3-Diene serving as allylic electophiles
M
Nu
H
M
Nu
H
R
R
R
(Pd, Rh or Ni)
H
b
(
) Pd-catalyzed enantioselective coupling of carbon nucleophile with 1,3-diene
Nu
Pd-PHOX (cat)
+
Nu
H
R
base
(2-3 equiv)
R
*
O
O
NC
CN
Ar
N
CO2Me
then react with pronucleophiles for efficient C–C formations
Nu H:
O
O
4f-k
(Scheme 1a).2c,
In 2004, Hartwig’s group reported the first
R
R
Me Me
Malcolmson
(2018)
Malcolmson
(2019)
Zi (2019)
enantioselective example of such type on a palladium-catalyzed
addition of 1,3-diketones to 1,3-dienes with JosiPhos ligand.4f Since
then, AAAs of different carbon-based pronucleophiles with 1,3-
dienes have gained rapid progress. Meek et al. developed a chiral
(CDC)-Rh-catalyzed AAA of indoles with 1,3-dienes.4g Zhou and
coworkers reported a nickel-hydride-catalyzed asymmetric -allylic
c
This work
)
(
1
O
O
O
Pd(OAc)2 (5 mol%)
DTBM-SEGPHOS
(6 mol%)
2
2
R2
+
(1)
(2)
R
O
R
1
R2
N
N
acid (5 mol%)
no base required
1,2-addition
R1
R1
up to 13:1 dr
up to 95:5 er
O
1
O
2
as above
R2
Shanghai Engineering Research Center of Molecular Therapeutics and New Drug
Development, School of Chemistry and Molecular Engineering, East China Normal
University, 3663 North Zhongshan Rd., Shanghai, 200062, China.
E-mail: dxing@sat.ecnu.edu.cn
†Electronic Supplementary Information (ESI) available: Information regarding
materials and methods, characterization data of compounds from this study. See
DOI: 10.1039/c000000x/Footnotes relating to the title and/or authors should
appear here.
+
O
O
1
2
1,4-addition
R2
N
N
R1
R1
up to 81% yield
> 12:1 rr
Scheme 1 Palladium-catalyzed asymmetric allylic alkylation of various carbon-
based nucleophiles with 1,3-dienes
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J. Name., 2018, 00, 1-3 | 1
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Oxazolones have been widely used as good nucleophiles in a cocatalysts were firstly examined with THF as the solvent. When
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number of AAAs with either conventional allyl agents6 or atom- TsOH was employed as the Pd–H precuDrsOoIr:,10t.h10e39d/eDs0irCeCd00a2ll6y5liHc
economic allyl precursors.7 The resulted allylic alkylated oxazolones substitution product 3a was obtained in moderate yield with 2:1 dr
bearing tertiary carbon centers are valuable precursors of while the majority of the oxazolone substrate was recovered. To our
biologically important chiral quaternary amino acids.8 Although the delight, high enantioselective ratio (90:10 er) was observed for the
racemic example for the allylic alkylation of oxazolone with 1,3- major diastereomer (Table 1, entry 1). Using benzoic acid or
dienes have been reported by Meek and co-workers by using a (CDC)- diphenylphosphinic acid resulted improved yield and dr of 3a, but
Rh catalyst,9 the asymmetric version of such transformation has not with decreased er (Table 1, entries 2 and 3). When camphorsulfonic
been reported. During our exploration toward the different acid (CSA) was employed, 3a was obtained with 8:1 dr and 90:10 er
reactivities of 1,3-dienes under metal-hydride catalysis,10 we found (Table 1, entry 4). Changing the solvent from THF to 1,4-dioxane or
that a palladium(II) acetate & camphorsulfonic acid precatalytic CPME showed no improvement (Table 1, entries 5 and 6).
system, together with a chiral SEGPHOS ligand, has allowed an Interestingly, when the amount of CSA was decreased from 20 to 5
efficient AAA of oxazolones with substituted 1,3-dienes in an mol%, the yield of 3a was increased from 46% to 70% while the
exclusive 1,2-addition manner with moderate to good stereoselectivity remained unattached (Table 1, entry 7 vs entry 4).
diastereoselectivities and high enantioselectivities under base-free With CSA as the choice of acid, different chiral bidentate phosphine
conditions (Scheme 1c, eq 1). In addition, when simple 1,3-butadiene ligands were screened. While the BINAP-type ligand TolBINAP (L2),
was used as the allyl precursor under this base-free catalytic system, xylBINAP (L3) and the MeO-BIPHEP-type ligand L4 and L5 failed to
1,4-addition products were obtained in good yields with high give satisfactory results in balancing both the efficiency and the
regioselectivities (Scheme 1c, eq 2).
stereoselectivity, using SEGPHOS-type ligand such as L6 or L8 allowed
this transformation to proceed with high stereoselectivity while the
reaction efficiency was maintained (Table 1, entries 8–14). The best
result was achieved when DTBM-SEGPHOS (L8) was used as the
ligand, yielding 3a in 81% isolated yield with 10:1 dr and 93:7 er
(Table 1, entry 14). Finally, in the absence of the acid catalyst, the
formation of 3a was not observed, demonstrating the crucial role of
the palladium-hydride catalyst for this transformation (Table 1, entry
15).
Table 1 Condition Optimizationa
O
O
Pd(OAc)2 (5 mol%)
ligand (6 mol%)
n-Bu
O
Ph
O
+
n-Bu
acid (5 mol%)
Ph
N
N
solvent, 50 o
24 h
C
Ph
Ph
1a
2a
3a
O
O
O
PAr2
PAr2
MeO
MeO
PAr2
PAr2
PAr2
PAr2
O
O
Pd(OAc)2 (5 mol%)
(R)-DTBM-SEGPHOS
(6 mol%)
O
BINAP scaffold
MeO-BIPHEP scaffold
SEGPHOS scaffold
n-Bu
O
O
Ar
+
Ar
O
L1
L2
L3
L4
L6
L7
L8
: Ar = Ph
: Ar = p-Me-C6H4
: Ar = 3,5-Me2-C6H3
: Ar = p-Me-C6H4
: Ar = p-Me-C6H4
: Ar = 3,5-Me2-C6H3
: Ar = 3,5-(t-Bu)2-4-MeO-C6H2
n-Bu
N
CSA (5 mol%)
N
L5
: Ar = 3,5-Me2-C6H3
THF, 50 oC, 24 h
Ph
Ph
1a
2
3
O
O
entry ligand
acid
solvent
yield (%)b
drc
erd
O
O
O
n-Bu
n-Bu
n-Bu
N
N
N
1e
TsOH
THF
THF
31
50
59
46
59
70
70
84
85
55
63
67
2:1
6:1
6:1
8:1
6:1
8:1
8:1
8:1
8:1
7:1
5:1
10:1
90:10
63:37
78:22
90:10
78:22
75:25
90:10
73:27
61:39
90:10
60:40
90:10
Ph
Me
L1
Ph
Ph
Ph
3a
3b
3c
75% yield, 9:1 dr, 91:9 er
2e
PhCOOH
L1
81% yield, 10:1 dr, 93:7 er
80% yield, 11:1 dr, 91:9 er
3e
Ph2POOH
CSA
THF
THF
L1
O
O
O
O
O
OMe
4e
L1
5e
CSA
1,4-dioxane
CPME
THF
O
L1
n-Bu
n-Bu
n-Bu
N
N
N
6e
CSA
Cl
NO2
L1
Ph
Ph
Ph
3d
3e
3f
L1
L2
L3
L4
L5
L6
7
8
CSA
74% yield, 9:1 dr, 91:9 er
26% yield, 9:1 dr, 93:7 er
77% yield, 10:1 dr, 95:5 er
CSA
THF
O
O
O
9
CSA
THF
Me
O
O
O
n-Bu
n-Bu
n-Bu
10
11
12
CSA
THF
N
N
N
Ph
Ph
Ph
CSA
THF
3g
3h
41% yield, 4:1 dr
93:7 er; 93:7 er (minor)
O
3i
CSA
THF
70% yield, 8:1 dr, 91:9 er
83% yield, 9:1 dr, 91:9 er
L7
L8
L8
13
14
15
CSA
CSA
--
THF
THF
THF
70
81
7:1
10:1
--
58:42
93:7
--
O
O
O
O
O
O
C6H13
< 5
n-Bu
n-Bu
n-Bu
N
N
N
Ph
Ph
Ph
3k
3l
3j
a Unless otherwise noted, the reactions were run in 0.1 mmol scale of 2a. 1a (1.2
62% yield, 8:1 dr, 80:20 er
34% yield, 3:1 dr
66% yield, 10:1 dr, 89:11 er
b
85:15 er, 90:10 er (minor)
equiv), Pd(OAc)2 (5 mol%), ligand (6 mol%), acid (5 mol%), solvent (0.2 mL).
c
Scheme 2 Substrate scope of 1,3-dienes. All reactions were run in 0.1 mmol scale
of 2 with 3 parallel entries, 1a (1.2 equiv), Pd(OAc)2 (5 mol%), (R)-DTBM-SEGPHOS
(6 mol%), CSA (5 mol%), THF (0.2 mL); Isolated yield of the major diastereomer;
Diastereoselective ratio(dr) was determined by 1H NMR spectroscopy of the crude
reaction mixture; Enantioselective ratio (er) was determined by chiral HPLC
analysis.
Isolated yield of the major diastereomer. Diastereoselective ratio(dr) was
d
determined by 1H NMR spectroscopy of the crude reaction mixture.
Enantioselective ratio (er) was determined by chiral HPLC analysis. e 20 mol% of the
acid was used.
We started our initial investigation by using oxozolone 1a and 1-
phenylbutadiene 2a as the model substrates. In the presence of 5
mol% of Pd(OAc)2 and 6 mol% of (R)-BINAP (L1), several acid
With the optimized reaction conditions in hand, the generality of
this enantioselective allylation was investigated (Scheme 2).
2 | J. Name., 2018, 00, 1-3
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Different substituted 1,3-dienes were tested first. Aryl substituted benzoyl-protected amino acid 5 in 83% yield while the enantiomeric
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DOI: 10.1039/D0CC00265H
1,3-dienes bearing phenyl, methyl or chloro-substitutions at para level was maintained (Scheme 4).
position on the aryl ring gave the corresponding allylation products
O
O
in good yields with high dr and er (3b – 3d), however, strong electron-
withdrawing group such as nitro group resulted much diminished
yield but the stereoselectivity was maintained (3e). Aryl 1,3-dienes
bearing electron-donating meta-substitutions on the aryl ring also
yielded the desired product in good efficiency and high
stereoselectivity (3f, 3g). While 2-naphthyl substituted 1,3-diene led
to the corresponding product in good yield with high dr and er (3i),
1-naphthyl substituted one showed only moderate reactivity and
diastereoselectivity but both diastereomers were formed with high
enantiopurity (3h).11 2-Furyl substituted 1,3-diene reacted smoothly
with 1a to give desired product 3j in good yield with high dr and er.
Alkyl substituted 1,3-dienes were also suitable substrates. Hexyl
substituted 1,3-diene gave the product 3k in 62% yield with 8:1 dr
and 80:20 er. On the other hand, cyclohexyl substituted substrate
showed low efficiency, giving the corresponding product 3l in 32%
yield with 3:1 dr but again, with high enantioselective control for
both diastereomers.
K2CO3, MeOH
rt, 2 h
MeO
BzHN n-Bu
Ph
O
Ph
n-Bu
N
4
(91%)
Ph
3a
93:7 er
>20:1 dr, 92:8 er
O
HCl, 1,4-dioxane
80 oC, 8 h
HO
BzHN n-Bu
Ph
5
(83%)
>20:1 dr, 92:8 er
Scheme 4 Synthetic elaborations of compound 3a
Allylic alkylation reactions utilizing 1,3-dienes generally occurred
via 1,2-addition, and regioselective allylic alkylation via 1,4-addition
was seldom reported.12 As an important chemical feedstock, simple
1,3-butadiene has found wide applications in a number of useful
transformations.13 However, using 1,3-butadiene as the metal--allyl
precursor to react with pronucleophiles was challenging owing to
their nature to undergo rapid telomerization under metal catalysis.14
In view of the base-free nature of the current catalytic system, which
O
Pd(OAc)2 (5 mol%)
(R)-DTBM-SEGPHOS
(6 mol%)
O
might
reduce the tendency for 1,3-butadiene to undergo
R
O
O
Ar2
Ar2
+
telomerization, the reactivity between oxazolone and 1,3-butadiene
was explored. To our delight, under standard reaction conditions, the
1,4-addition product, rather than a 1,2-addition product generally
observed for substituted 1,3-dienes, were obtained with high
regioselectivities (Scheme 5).15 This regioselective transformation
showed a broad substrate scope of oxazolones. Various C4-alkyl
substituted oxazolones, as well as several C2-aryl substituted
oxazolones could all give corresponding 1,4-addition products in
moderate to good yields with high regioselectivities (7a – 7i). From
the mechanistic point of view, nucleophilic attack from oxazolone
toward the palladium-π-allyl species generated from 1,3-butadiene
at the less hindered terminal position would account for the high
regioselective control in a 1,4-addition manner. The excellent
regioselectivity for this transformation would offer a useful protocol
for the crotylation of other type of pronucleophiles.
N
R
CSA (5 mol%)
N
Ar1
THF, 50 oC, 24 h
Ar1
1
2
3
O
O
O
O
Ph
O
Ph
Ph
Et
O
Ph
n-Pr
Me
N
N
N
Ph
Ph
3m
3n
3o
63% yield, 8:1 dr, 91:9 er
52% yield, 8:1 dr, 92:8 er
72% yield, 9:1 dr, 91:9 er
O
O
O
O
Ph
O
Ph
O
Ph
Bn
N
N
N
S
Ph
Ph
Ph
3p
3r
3q
52% yield, 8:1 dr, 93:7 er
37% yield, 8:1 dr, 86:14 er
71% yield, 9:1 dr, 92:8 er
O
O
O
O
Ph
O
Ph
O
n-Bu
n-Bu
n-Bu
N
N
N
p-Me-C6H4
p-Cl-C6H4
p-Cl-C6H4
3s
3t
3u
45% yield, 8:1 dr, 93:7 er
46% yield, 13:1 dr, 92:8 er
41% yield, 8:1 dr, 93:7 er
Scheme 3 Substrate scope of oxazolones. All reactions were run in 0.1 mmol scale
of 2 with 3 parallel entries, 1 (1.2 equiv), Pd(OAc)2 (5 mol%), (R)-DTBM-SEGPHOS
(6 mol%), CSA (5 mol%), THF (0.2 mL); Isolated yield of the major diastereomer;
Diastereoselective ratio(dr) was determined by 1H NMR spectroscopy of the crude
reaction mixture; Enantioselective ratio (er) was determined by chiral HPLC
analysis.
O
Pd(OAc)2 (5 mol%)
(rac)-DTBM-SEGPHOS
(6 mol%)
O
1
2
1
R
O
O
+
2
N
R
CSA (5 mol%)
N
THF, 50 oC, 24 h
Ar
Ar
1
6
7
3.0 equiv
O
O
O
O
The substrate scope of oxazolones was then investigated (Scheme
3). Various alkyl substituents at the C4 position of oxazolones were
well tolerated to undergo the enantioselective allylic alkylation
reaction, providing the corresponding products bearing tertiary
stereogenic centers in moderate to good yields with high level of
diastereo- and enantioselectivities (3m – 3r). Oxazolones bearing
para-substituted phenyl group at the C2 position also underwent the
desired reaction with high dr and er but in moderate yields (3s – 3t).
The resulted optically active oxazolone products could be utilized
as precursors for the synthesis of ,-disubstituted unnatural amino
acids. To demonstrate their synthetic utilities, oxazolone 3a was
treated with K2CO3 in MeOH to access the benzoyl-protected
enantioenriched ,-disubstituted amino ester 4 in 91% yield. On
O
O
n-Pr
n-Bu
N
N
N
Ph
7a
Ph
7b
Ph
7c
, 74%, 13:1 rr
, 79%, 12:1 rr
, 76%, 13:1 rr
O
O
O
O
O
O
Et
Me
N
N
N
S
Ph
Ph
Ph
7e
7f
, 33%, 11:1 rr
7d
, 81%, 14:1 rr
, 58%, 7:1 rr
O
O
O
O
O
O
n-Bu
n-Bu
Bn
N
N
N
p-Me-C6H4
7h
p-Cl-C6H4
Ph
7g
, 66%, 14:1 rr
, 51%, 14:1 rr
7i
, 43%, 12:1 rr
Scheme 5 Allylic alkylation of oxazolones with 1,3-butadiene via 1,4-addition. All
the other hand, acid hydrolysis of 3a led to the corresponding reactions were run in 0.1 mmol scale of 1 with 3 parallel entries, 6 (3.0 equiv, 2.0
This journal is © The Royal Society of Chemistry 20xx
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M in THF), Pd(OAc)2 (5 mol%), (rac)-DTBM-SEGPHOS (6 mol%), CSA (5 mol%), THF
(0.15 mL); Isolated yield of the major regioisomer; Regioselective ratio(rr) was
determined by 1H NMR spectroscopy of the crude reaction mixture.
Malcolmson, Chem. Sci., 2019, 10, 5176-5182; (k) Q. Zhang, H.
Yu, L. Shen, T. Tang, D. Dong, W. ChaiDaOndI: 1W0..10Z3i,9J/D. A0CmC.0C0h2e6m5H.
Soc., 2019, 141, 14554-14559.
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5
6
G. Hata, K. Takahashi and A. Miyake, J. Org. Chem., 1971, 36,
2116-2123; (b) K. Takahashi, A. Miyake and G. Hata, Bull.
Chem. Soc. Jpn., 1972, 45, 1183-1191.
In conclusion, we have developed
a palladium-catalyzed
diastereo- and enantioselective allylic alkylation of oxazolones with
1,3-dienes under base-free conditions. A palladium-hydride catalyst
generated from Pd(OAc)2 and camphorsulfonic acid is response for
the activation of 1,3-dienes. With DTBM-SEGPHOS as the chiral
ligand, a series of oxazolones bearing tertiary carbon centers were
synthesized from substituted 1,3-dienes with moderate to good
diastereoselectivities and high enantioselectivities. This base-free
catalytic system is also applicable to simple 1,3-butadiene but
yielding 1,4-addition product exclusively. Further development of
asymmetric transformations with this catalytic system is currently
ongoing.
For selected examples, see: (a) B. M. Trost, C. Heinemann, X.
Ariza and S. Weigand, J. Am. Chem. Soc., 1999, 121, 8667-
8668; (b) B. M. Trost and X. Ariza, J. Am. Chem. Soc., 1999,
121, 10727-10737; (x) W. Chen and J. F. Hartwig, J. Am. Chem.
Soc., 2013, 135, 2068-2071; H. Zhou, H. Yang, M. Liu, C. Xia
and G. Jiang, Org. Lett., 2014, 16, 5350-5353; (x) W. Chen and
J. F. Hartwig, J. Am. Chem. Soc., 2014, 136, 377-382; X. Wei, D.
Liu, Q. An and W. Zhang, Org. Lett., 2015, 17, 5768-5771; X.-
D. Bai, Q.-F. Zhang and Y. He, Chem. Commun., 2019, 55, 5547-
5550.
(a) B. M. Trost, C. Jäkel and B. Plietker, J. Am. Chem. Soc., 2003,
125, 4438-4439; (b) J. Kuang, S. Parveen and B. Breit, Angew.
Chem. Int. Ed., 2017, 56, 8422-8425; (c) W. Ren, Q.-M. Zuo, Y.-
N. Niu and S.-D. Yang, Org. Lett., 2019, 21, 7956-7960; (d) H.-
C. Lin, P.-P. Xie, Z.-Y. Dai, S.-Q. Zhang, P.-S. Wang, Y.-G. Chen,
T.-C. Wang, X. Hong and L.-Z. Gong, J. Am. Chem. Soc., 2019,
141, 5824-5834.
7
8
We thank the National Natural Science Foundation of China
(21772043), the Shanghai Pujiang Program (No. 19PJ1403000)
and the Fundamental Research Funds for the Central
Universities for financial supports. We also thank Prof. Xiaoli
Zhao for the kind help in data analysis.
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Notes and references
‡ Footnotes relating to the main text should appear here. These
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matter under discussion, limited experimental and spectral data,
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9
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1
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