Palladium-catalyzed decarboxylative cycloaddition of vinylethylene carbonates with formaldehyde: Enantioselective construction of tertiary vinylglycols

Article abstract of DOI:10.1002/anie.201403754

An efficient method for the enantioselective construction of tertiary vinylglycols through a palladium-catalyzed asymmetric decarboxylative cycloaddition of vinylethylene carbonates with formaldehyde was developed. By using a palladium complex generated in situ from [Pd₂(dba) ₃]·CHCl₃ and a phosphoramidite ligand as a catalyst under mild reaction conditions, the process allows conversion of racemic 4-substituted 4-vinyl-1,3-dioxolan-2-ones into the corresponding 1,3-dioxolanes, as methylene acetal protected tertiary vinylglycols, in high yields with good to excellent enantioselectivities.

Full text of DOI:10.1002/anie.201403754

Angewandte
Chemie
DOI: 10.1002/anie.201403754
Asymmetric Catalysis
Palladium-Catalyzed Decarboxylative Cycloaddition of Vinylethylene
Carbonates with Formaldehyde: Enantioselective Construction of
Tertiary Vinylglycols**
Ajmal Khan, Renfeng Zheng, Yuhe Kan, Jiang Ye, Juxiang Xing, and Yong Jian Zhang*
Abstract: An efficient method for the enantioselective con-
struction of tertiary vinylglycols through a palladium-catalyzed
asymmetric decarboxylative cycloaddition of vinylethylene
carbonates with formaldehyde was developed. By using
Transition-metal-catalyzed enantioselective allylic substi-
tution with O-nucleophiles is a powerful method for the
synthesis of secondary allylic alcohol derivatives.[5] However,
regioselective construction of tertiary allylic ethers using this
method for 1,1- or 3,3-disubstituted allylic electrophiles is
difficult to achieve.[6,7] Although palladium-catalyzed asym-
metric etherification of 2-substituted-2-vinyloxiranes with
trialkylborane as a cocatalyst is an effective method for
furnishing tertiary allylic ethers regioselectively,[8] the reac-
tion conditions need to be carefully controlled to suppress the
potential of the primary alcohol of the product acting as
a nucleophile for the substrate. In addition, the process is less
effective for 2-aryl-substituted vinyloxiranes.[8a] Most recently,
palladium-catalyzed asymmetric interceptive decarboxylative
allylations of allylic partners by unsaturated electrophiles
have attracted a great deal of attention.[9] Inspired by this
synthetic strategy, we envisioned that substituted vinylethy-
lene carbonates[10] (VECs; 1) could undergo the decarbox-
ylative process to afford the zwitterionic p-allylpalladium
intermediates A and B (Figure 1). We also reasoned that
a
palladium complex generated in situ from [Pd2-
(dba)3]·CHCl3 and a phosphoramidite ligand as a catalyst
under mild reaction conditions, the process allows conversion
of racemic 4-substituted 4-vinyl-1,3-dioxolan-2-ones into the
corresponding 1,3-dioxolanes, as methylene acetal protected
tertiary vinylglycols, in high yields with good to excellent
enantioselectivities.
C
hiral tertiary alcohols are prevalent motifs in a variety of
important medicinally relevant agents and biologically active
natural products. Enantioselective construction of tertiary
alcohols is therefore an important objective in asymmetric
catalysis.[1] The most common approach to enantioenriched
tertiary alcohols includes transition-metal- or organocata-
lyzed asymmetric addition of carbon nucleophiles to keto-
nes.[1a] Nevertheless, some difficulties associated with the use
of ketone electrophiles for this approach have emerged,
difficulties such as relatively low reactivity and challenging
carbonyl enantiofacial differentiation. Asymmetric catalytic
dihydroxylation[2] or epoxidation[3] of 1,1-disubstituted
alkenes are useful methods for the synthesis of tertiary
alcohols. However, relatively low enantioselectivities are
usually obtained when the two alkene substituents are of
similar steric demand. Although some other approaches are
available to date,[4] the development of new efficient methods
for the construction of enantioenriched tertiary alcohol
derivatives is still a challenging task.
Figure 1. Strategy for palladium-catalyzed decarboxylative cycloaddition
of VECs with formaldehyde.
[*] A. Khan, Dr. R. Zheng, J. Ye, J. Xing, Prof. Dr. Y. J. Zhang
School of Chemistry and Chemical Engineering, and Shanghai Key
Laboratory of Electrical Insulation and Thermal Aging
Shanghai Jiao Tong University
formaldehyde, an abundant feedstock and a reactive one
carbon electrophile, could intercept the intermediates to form
favored five-membered 1,3-dioxolanes (2) regioselectively.
We hypothesized that if the isomerization of the diastereo-
meric p-allylpalladium intermediates A/B or C/D occurs
faster than subsequent nucleophilic cycloaddition, a dynamic
kinetic asymmetric transformation (DYKAT) could be fea-
sible.[11] Herein, we report the successful execution of these
ideals and present a palladium-catalyzed asymmetric decar-
boxylative cycloaddition of the VECs 1 with formaldehyde,
a first practical and efficient approach which allows con-
version of the racemic 4-substituted 4-vinyl-1,3-dioxolan-2-
ones 1 into the corresponding 1,3-dioxolanes 2, as methylene
acetal protected tertiary vinylglycols, in high yields with high
levels of enantioselectivities.
800 Dongchuan Road, Shanghai 200240 (P. R. China)
E-mail: yjian@sjtu.edu.cn
Prof. Dr. Y. Kan
Jiangsu Key Laboratory for Chemistry of Low-Dimensional
Materials, School of Chemistry and Chemical Engineering
Huaiyin Normal University, Huaian 223300 (P. R. China)
[**] This work was supported by the Innovation Program of Shanghai
Municipal Education Commission (14ZZ023), National Key Basic
Research Program of China (2013CB934101), Shanghai Pujiang
Program (11PJD012), and Shanghai Jiao Tong University. We thank
the Instrumental Analysis Center of Shanghai Jiao Tong University
for HRMS analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403754.
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Table 1: Optimization studies.[a]
reasonably in pure water to furnish 2a in 95% ee (entry 14).
To verify the water effect on the reaction, the reaction was
performed with paraformaldehyde, instead of the aqueous
solution, under otherwise identical conditions. As shown in
entry 15, the reaction proceeded to give 2a with a slightly
lower ee value. Treatment of 1a with [Pd2(dba)3]·CHCl3 and
(S,S,S)-L3 in water/THF without formaldehyde did not lead
to the formation of the corresponding diol. The starting
material 1a was recovered in practically quantitative yield.
These results imply that the reaction undergoes the inter-
ception of the zwitterionic p-allylpalladium intermediates by
formaldehyde with subsequent cycloaddition as illustrated in
Figure 1, and water has no effect on the reaction.[14]
With the optimized reaction conditions in hand, the
generality of this protocol was evaluated with a variety of
substituted VECs (1). Significantly, a wide range of aryl- and
alkyl-substituted VECs were tolerated under the reaction
conditions, thus affording the corresponding 1,3-dioxolanes 2
in high yields with high levels of enantioselectivities (Table 2).
Various substituted aryl VECs having different electronic and
steric properties were converted into the corresponding
products 2b–k in high yields (86–98%) with excellent
enantioselectivities (91–99% ee). In addition, the reactions
of VECs with naphthyl, heteroaromatic furan, and thiophene
moieties also proceeded smoothly to afford the corresponding
2l, 2m, and 2n in high yields with good to excellent
enantioselectivities. The process also worked well for alkyl
VECs, thus furnishing the 1,3-dioxolanes 2o–r in high yields
with high ee values (82–95% ee). Meaningfully, the cyclo-
addition reaction was also effective for more functionalized
VECs, thus providing cyclized products 2s and 2t bearing
three protected hydroxy groups and a vinyl group at one
carbon stereogenic center.
Entry
Ligand
Solvent
Yield [%][b]
ee [%][c]
1
(R)-binap
THF
THF
THF
THF
THF
THF
THF
THF
23
43
44
55
22
0
82
92
96
82
81
93
37
68
87
40 (R)
26 (S)
16 (S)
19 (S)
10 (S)
–
65 (R)
84 (R)
96 (R)
87 (R)
95 (R)
82 (R)
79 (R)
95 (R)
90 (R)
2
3
4
5
6
7
(S)-Tol-binap
(S)-xylyl-binap
(S)-Cl-MeO-biphep
(S)-Segphos
(R,R)-DACH-phenyl
L1
8
(S,R,R)-L2
9
(S,S,S)-L3
THF
10
11
12
13
14
15[d]
(S,S,S)-L3
(S,S,S)-L3
(S,S,S)-L3
(S,S,S)-L3
(S,S,S)-L3
(S,S,S)-L3
1,4-dioxane
toluene
CH2Cl2
CyH
H2O
THF
[a] Reaction conditions: [Pd2(dba)3]·CHCl3 (2.5 mol%), ligand (5 mol%
for bisphosphanes; 10 mol% for phosphoramidites), 1a (0.2 mmol),
formaldehyde (2.0 mmol, 37% aqueous solution), solvent (1.0 mL),
208C, 15 h. [b] Yield of isolated product. [c] Determined by HPLC using
a chiral stationary phase. The absolute configuration was confirmed by
the deprotection of 2a to the corresponding diol, and the comparison of
the sign of optical rotation with that reported in the literature.[13] [d] The
reaction was carried out with 5 equiv of paraformaldehyde. Cl-MeO-
biphep=5,5’-dichloro-6,6’-dimethoxy-2,2’-bis(diphenylphosphino)-1,1’-
biphenyl, binap=2,2’-bis(diphenylphosphino)-1,1’-binaphthyl, dba=di-
benzylideneacetone, DACH-phenyl=1,2-diaminocyclohexane-N,N’-
bis(2’-diphenylphosphinobenzoyl), Segphos=5,5’-bis(diphenylphos-
phino)-4,4’-bi-1,3-benzodioxole, THF=tetrahydrofuran, CyH=cyclohex-
ane.
The decarboxylative cycloaddition reaction of VECs with
other aldehydes was also examined. As shown in Scheme 1,
the reaction conditions were also effective for other alde-
hydes. Thus, the reaction of 1a with acetaldehyde (35%
aqueous solution) under our standard reaction conditions
gave the cyclized product 3a in high yield with a 1:1
diastereomeric ratio and good enantioselectivity. It is note-
worthy that the reactions also proceeded well with aromatic
aldehydes (1.2 equiv to 1a), thus furnishing the corresponding
1,3-dioxolanes 3b and 3c in high yields with excellent
enantioselectivities for both of the diastereomers. The
absolute configuration of quaternary carbon center of 3b is
Initial investigations focused on finding effective chiral
ligands for the palladium-catalyzed decarboxylative cyclo-
addition of the Ph-VEC 1a, as a standard substrate, with
formaldehyde (37% aqueous solution) in THF at 208C for
15 hours (Table 1). Firstly, we tried different binap-type
axially chiral bisphosphane ligands, but the reactions
showed low yields and poor enantioselectivities (entries 1–
5). Trostꢀs ligand, (R,R)-DACH-phenyl, was also ineffective
for the reaction (entry 6). Through replacement of the
bisphosphanes with Feringaꢀs phosphoramidite[12] ligand L1,
the reaction efficiency was significantly improved to give the
desired 2a in acceptable yield with 65% ee (entry 7). To our
delight, by employing the diastereomeric phosphoramidite
ligands (S,R,R)-L2 and (S,S,S)-L3 (entries 8 and 9), we found
that the reaction could be promoted dramatically using
(S,S,S)-L3 and it afforded 2a in 96% yield with excellent
enantioselectivity (entry 9). Although the reaction efficiency
did not improve further by using other solvents (entries 10–
14), it is noteworthy that the reaction could proceed
Scheme 1. Palladium-catalyzed asymmetric decarboxylative cycloaddi-
tion of 1a with aldehydes. [a] Using 10 equiv of acetaldehyde (35%
aqueous solution). [b] Using 1.2 equiv of aldehydes.
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Table 2: Substrate scope for palladium-catalyzed decarboxylative cyclo-
addition of VECs with formaldehyde.[a]
Scheme 2. Deprotection of 1,3-dioxolanes 2 to diols 4. TES=triethyl-
silyl.
triazole antifungal agents[18] such as Genaconazole, Ravuco-
nazole, and Albaconazole.
According to the proposed reaction pathway as revealed
in Figure 1, the final cycloaddition step might be the
stereochemistry-determining step. Although the true stereo-
chemical outcome of this cycloaddition reaction is compli-
cated, density functional theory (DFT) calculations using the
B3LYP/def2-TZVP method (see the Supporting Information
for details) were carried out for the geometry optimizations of
the four plausible conformers[5c,11] of the p-allylpalladium
intermediate C (R = Ph) to gain more information of the
stereochemistry. As shown in Figure 2, the p-allylpalladium
[a] Reaction conditions: [Pd2(dba)3]·CHCl3 (2.5 mol%), L3 (10 mol%),
1 (0.2 mmol), formaldehyde (2.0 mmol, 37% aqueous solution), THF
(1.0 mL), 208C, 15 h. Yields are of isolated materials. The enantiose-
lectivities were determined by HPLC using a chiral stationary phase.
[b] The ee value was determined by HPLC analysis of their diol mono- or
dibenzoyl esters.
Figure 2. Calculated structures of plausible four isomers of the p-
allylpalladium intermediate C and their relative energies.
same as that of 2a.[15] Although the catalytic system is less
effective for the control of the diasteroselectivity, these results
further proved that the reaction undergoes the interceptive
decarboxylative allylation pathway as illustrated in Figure 1.
The enantioenriched 1,3-dioxolanes 2 can be readily
converted into the corresponding tertiary vinylglycols in
good yields without the deterioration of enantiomeric purity
(Scheme 2).[16] More significantly, the 1,3-dioxolanes 2 could
be useful chiral building blocks for the synthesis of valuable
compounds. For example, the enantioenriched 2p could be
a useful compound for the synthesis of the natural product
tanikolide.[17] In addition, the 2,4-difluorophenyl-substituted
2j is an important chiral building block for the preparation of
intermediate C, having a syn-allylic unit (syn position
between alkoxymethoxymethyl group and C2-proton of allyl
group) revealed much lower energies than that with an anti-
allylic unit. The highly organized intermediate Cexo-syn is free
from steric repulsion between the phenyl ring of allylic group
and phenyl group of ligand, (S,S,S)-L3, thus revealing the
lowest relative energy, and delivering (R)-2a as the major
enantiomer through inner-attack for the cycloaddition.[19]
In conclusion, we have developed an efficient method for
the enantioselective construction of methylene acetal pro-
tected tertiary vinylglycols by palladium-catalyzed asymmet-
ric decarboxylative cycloaddition of VECs and formaldehyde.
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[7] For palladium-catalyzed asymmetric intermolecular allylic
etherification of phenols to 3,3-disubstituted allylic carbonates,
see: a) A. M. Sawayama, H. Tanaka, T. J. Wandless, J. Org.
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Toste, J. Am. Chem. Soc. 1998, 120, 12702 – 12703.
The reactions proceeded smoothly in the presence of [Pd2-
(dba)3]·CHCl3 and the phosphoramidite (S,S,S)-L3 under
mild reaction conditions, thus providing the 4-substituted 4-
vinyl-1,3-dioxolanes 2 in high yields (up to 98%) with good to
excellent enantioselectivities (82–99% ee). The protected
tertiary vinylglycols 2 could be useful chiral building blocks
for the synthesis of biologically active compounds and natural
products. Moreover, the stereochemical outcome of the
reaction has been explained by DFT calculations on the
geometry optimization of plausible reaction intermediates.
Further studies to extend the scope of the decarboxylative
cycloaddition of VECs are currently underway, and will be
reported in due course.
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J. A. Tunge, Chem. Rev. 2011, 111, 1846 – 1913. For recent
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[14] It is well known that water is a very poor nucleophile in
palladium-catalyzed allylic substitution, see: I. P. Beletskaya,
A. V. Cheprakov in Handbook of Organopalladium Chemistry
for Organic Synthesis, Vol. 2 (Eds.: E. Negishi, A. de Meijere),
Wiley, New York, 2002, pp. 2957 – 3006.
Received: March 26, 2014
Published online: May 14, 2014
Keywords: alcohols · asymmetric catalysis · cycloaddition ·
.
palladium · synthetic methods
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