Allylic Phosphorus Ylides Directly Generated from Alcohols with Water as the only Byproduct

Article abstract of DOI:10.1021/acs.orglett.9b01349

A novel strategy for the preparation of allylic phosphorus ylides directly from Morita-Baylis-Hillman (MBH) alcohols in an environmentally benign manner was developed. With the assistance of a calcium catalyst, the S_N2′ process between phosphin

Full text of DOI:10.1021/acs.orglett.9b01349

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Allylic Phosphorus Ylides Directly Generated from Alcohols with
Water as the Only Byproduct
Peizhong Xie,*,† Weishan Fu,† Ying Wu,† Xinying Cai,† Zuolian Sun,† Shuangshuang Li,†
Cuiqing Gao,‡ Xiaobo Yang,† and Teck-Peng Loh*,†,§
†School of Chemistry and Molecular Engineering, Institute of Advanced Synthesis, Jiangsu National Synergetic Innovation Center
for Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China
‡Co-Innovation Center for the Sustainable Forestry in Southern China, College of Forestry, Nanjing Forestry University, Nanjing,
210037, China
§Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: A novel strategy for the preparation of allylic
phosphorus ylides directly from Morita−Baylis−Hillman
(MBH) alcohols in an environmentally benign manner was
developed. With the assistance of a calcium catalyst, the SN2′
process between phosphines and allylic alcohols occurred
smoothly, delivering allylic phosphorus salts and calcium-stabilized hydroxide ions. Then, in situ deprotonation gave the allylic
phosphorus ylides with water as the only byproduct. Functionalized 1,3-diene moieties can be conveniently obtained by
trapping the ylides through a Wittig olefination.
hosphorus ylides have attracted increasing attention and
emerged as versatile building blocks in various trans-
P
formations1 beyond traditional olefinations.2 In classic
methods, ylides are prepared from the corresponding organic
halides using a stoichiometric base, producing stoichiometric
amounts of the halide salt, which is far from ideal based on
synthetic efficiency and environmental concerns (Figure 1a).3
In some special cases, allenoates,4 activated alkenes,5 and
alkynes6 have been identified as suitable precursors to produce
the corresponding phosphorus ylides;7 however, these
substrates were generally derived from organic halides and/
or required tedious preparation and purification processes.7
We envision that ideally, in terms of step and atom economy
and environmental impact, the synthetically suitable alcohols
could be directly converted into phosphorus ylides, delivering
water as the only byproduct (Figure 1b). In our design, the key
factor was to identify a catalyst that can efficiently activate the
C−OH bond and allow the subsequent SN2 process involving
the phosphine to occur smoothly. Following the SN2 process,
the in situ generated hydroxide ion (stabilized by the catalyst)
then deprotonates the phosphonium intermediate, delivering
the corresponding ylide with water as the only byproduct.
Since 2003, allylic phosphorus ylides I (Figure 1c) have been
demonstrated to be one of the most powerful and versatile
synthetic intermediates in diverse annulations7b,d,g,h in addition
to Wittig olefinations8 and Krische allylations.9 Pioneering 1,3-
dipolar [3 + n] (n = 2, 3, 4, and 6) cycloadditions10 with active
alkenes were independently developed by Lu10a and Tang.10b
Asymmetric versions have been developed by Tang,10e
Barbas,10i,o Lu,10h,j Shi,10g,k−m Liu,10n Zhong,10p Guo,10p,r,s
and Chen.10t A powerful [1 + 4] annulation11 was then
Figure 1. Strategies for preparing phosphorus ylides. (a) Traditional
methods with organic halides; (b) our environmentally benign
method; (c) this investigation: to obtain synthetically powerful allylic
phosphorus ylides.
reported by the Zhang,11a Huang,11b and He11c groups.
Huang’s group has explored the synthetic utilities of allylic
ylides in detail and realized various amazing [2 + 4],12 [3 +
3],13 [3 + 4],14 etc.15 annulations for accessing diverse,
Received: April 17, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01349
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
synthetically challenging frameworks with high efficiency.
Recently, asymmetric versions of [1 + 4] annulations have
also received great attention, and substantial contributions
have been made by the groups of Shi,16a−c Li,16d and He.16e
Asymmetric [3 + 3] annulations were also documented by
Guo17 and co-workers. Among these investigations, it is widely
recognized that only MBH adducts (including halides, acetates,
and tert-butyl carbonates) are suitable precursors for ylides I,
although these MBH adducts were derived from MBH
alcohols with tedious conversion and purification processes
in addition to the generation of stoichiometric byproduct.7b,d
This can be ascribed to the remarkably high activation barrier
of C−OH bond scission, which makes delivering phosphorus
ylides I directly from MBH alcohols quite challenging.8b
Furthermore, obstacles, such as the tendencies of MBH
alcohols to decompose and polymerize, limit their direct
synthetic utility. In our continuing effort to develop catalysts
consistent with green and sustainable chemistry,18 we selected
MBH alcohols as substrates (Figure 1c) to explore the
feasibility of our design, and to attempt to obtain allylic
phosphorus ylides I. Herein, we successfully developed a
calcium catalyst that enabled the desired protocol to occur
smoothly in isopropanol as the solvent with water as the only
byproduct. In this reaction, allylic phosphorus ylide I (Figure
1c) intermediates were trapped by a Wittig olefination,
producing synthetically useful 1,3-diene moieties in high to
excellent yields.
altering the nucleophilicity or hardness of the phosphine (SI,
Table S3, entries 10−13).
Under the optimized conditions, the substrate scope was
then investigated. As shown in Scheme 1, the in situ generated
a
Scheme 1. Substrate Scope of Aldehydes
We first explored the reaction between methyl 2-(hydroxy-
(phenyl)methyl)acrylate (1a) and triphenylphosphene (2a) in
toluene as the solvent. 4-Nitrobenzaldehyde was selected to
trap the desired allylic phosphorus ylides through a Wittig
olefination process. Given our interest in indium catalysis, the
strong Lewis acid In(OTf)3 was first introduced into the
reaction. After 50 h, the desired product, methyl (E)-2-((E)-
benzylidene)-4-(4-nitrophenyl)but-3-enoate19 (3a, CCDC:
1889054) was detected in 26% yield (Supporting Information
(SI), Table S1, entry 1). Much lower yields or no product
resulted when other traditional Lewis acids, such as InCl3,
Sc(OTf)3, Fe(OTf)2, FeCl3, and AlCl3, were used under the
same conditions (SI, Table S1, entries 2−6). Then, we
switched our attention to alkaline earth metals, which are
abundant and well established as a powerful catalyst in
Friedel−Crafts and related cationic cycloadditions20−22 by
Niggemann,20 Gandon,21 and other groups.22 To our delight,
Ca(NTf2)2 delivered a substantially better yield (45%) of 3a
(SI, Table S1, entry 7). Ca(OTf)2 and Ba(NTf2)2 give slightly
lower yields (SI, Table S1, entries 8−9). To improve the yield
of 3a, the effects of solvent and additives were then
investigated (SI, Table S2, entries 1−7). Screening solvents
revealed that isopropanol facilitated the transformation and
enabled the transformation in 3 h (SI, Table S2, entry 7).
However, the reaction mixture was complex, and 3a was
detected in comparable yields (38%). In further investigations,
we found that both a trace amount of water and a high reaction
concentration favored the transformation (SI, Table S3, entries
1−3), probably by accelerating the deprotonation in the
phosphorus ylide formation step. Negative effects were
observed upon increasing or decreasing the reaction temper-
ature (SI, Table S3, entries 4−7). The addition of CF3COOK
as an additive can alter the reaction by suppressing side
reactions (SI, Table S3, entries 8−9), increasing the yield of 3a
(up to 82% isolated yield); however, a longer reaction time was
required. No increase in yield or selectivity can be achieved by
a
Experimental conditions: 1a (0.6 mmol) and aldehyde (0.4 mmol),
Ca(NTf2)2 (15 mol %), PPh3(1.2 equiv), CF3COOK (30 mol %), and
water (1.3 equiv) in isopropanol (1.0 mL) under a N2 atmosphere for
24 h. Isolated total yield (major products (2E,3E)-diene (3) and
minor isomer (2Z,3E)-diene (3′) and (2E,3Z)-diene (3′′)). The ratio
1
of 3/3′/3′′ was determined by crude H NMR.
allylic ylide can be trapped by various aromatic aldehydes,
delivering 1,3-dienes in good to excellent yields with moderate
to high stereoselectivities. Both electron-withdrawing (3a−3g)
and electron-donating (3e−3f, 3i, and 3p) groups on the
phenyl ring were amenable to this protocol. The positions of
the substituents (at the para or meta positions) of the phenyl
ring have limited effects on the overall transformation (3b, 3j,
and 3k). Even 4-(tert-butyl)benzaldehyde was well tolerated
and delivered the corresponding product in excellent yield
(3h). However, the ortho-substituted phenyl aldehydes, such as
2,4-dichlorobenzaldehyde and 2-bromobenzaldehyde, afforded
the desired products (3i and 3j) in slightly lower yields. This
can be attributed to the steric hindrance disfavoring the Wittig
olefination. Monosubstituted and polysubstituted, as well as
fused-aromatic, aldehydes smoothly reacted with phosphorus
ylides under the identified conditions (3i, 3l, and 3m).
Moreover, thiophene-2-carbaldehyde was a suitable substrate
B
DOI: 10.1021/acs.orglett.9b01349
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Letter
for this reaction (3n). In the presence of aliphatic aldehydes
(butyraldehyde), the decomposition of methyl 2-(hydroxy-
(phenyl)methyl)acrylate overwhelmed the formation of the
allylic phosphorus species and thus disfavored the desired 1,3-
diene generation.
Subsequently, we explored the scope of this transformation
with respect to MBH alcohols. The result is shown in Scheme
2. A variety of MBH alcohols were effectively reacted with 4-
good yields. Sterically demanding MBH alcohols bearing
naphthalene-1-yl (3y), 2,4-dichlorophenyl (3aa), and 3,5-
dimethyl (3ab) moieties were compatible with this process. A
tert-butyl group, either on the phenyl ring (3ac) or as an ester
(3z), has little effect on the yield or selectivity. As expected,
heteroaryl substituted MBH alcohols, such as thiophen-2-yl
(3ad), furan-2-yl (3ae), and pyridien-2-yl (3af), were also well
tolerated. Remarkably, an MBH alcohol with an alkyl
substituent can also be successfully incorporated into the
desired 1,3-diene skeleton (3ag). Moreover, 2-(hydroxy-
(phenyl)methyl)acrylonitrile can deliver the corresponding
1,3-diene in moderate yield (3ah). This result indicated that
the ester group on the MBH alcohols was not required.
Unfortunately, allylic alcohols other than MBH derivatives
were inefficient under the developed conditions.
a
Scheme 2. Substrate Scope of the MBH Alcohols
To gain more insight into the catalysis, 31P NMR monitoring
experiments were conducted in an NMR tube (Figure 2). As
Figure 2. Stacking 31P NMR spectra. Experiments were conducted in
i-PrOH-d8.
shown in Figure 2, without the catalyst, there is no interaction
between PPh3 and 1a (stacking 31P NMR spectrum 2). In
addition, a coordination effect was detected for Ca(NTf2)2
with PPh3 (stacking 31P NMR spectrum 3). These results, to
some extent, revealed that PPh3 is free under these Lewis basic
reaction conditions. With the assistance of both PPh3 and
Ca(NTf2)2, 1a can then be consumed to generate the
corresponding ylide (stacking 31P NMR spectra, 5). By
introducing an aldehyde, the Wittig olefination proceeded
smoothly and served as the driving force for the conversion of
the allylic alcohols to the corresponding ylides (stacking 31P
NMR spectra 6−8).
a
Experimental conditions: 1 (0.6 mmol) and aldehyde (0.4 mmol),
The reaction mechanism shown in Figure 3 was then
proposed. Judging from the positive influence of water and
trifluoroacetate anions as well as the related investigation
concerning calcium coordination, the eight coordination sites
of calcium catalyst A23 may be occupied by water, NTf−, and
trifluoroacetate anions. Initially, the C−OH bond was
activated by calcium catalyst A through coordination to the
hydroxyl oxygen (B). Subsequently, the allylic SN2 process
between the phosphine and intermediate B gave ion pair C, in
which the hydroxyl ion was stabilized by calcium. Then, proton
transfer in the ion pair delivered the desired allylic ylide I and
regenerated the catalyst. The proton transfer process was
accelerated by the protic solvent (i-PrOH), which was identical
to what was observed in our previous study.24
Ca(NTf2)2 (15 mol %), PPh3(1.2 equiv), CF3COOK (30 mol %), and
water (1.3 equiv) in isopropanol (1.0 mL) under a N2 atmosphere for
24 h. Isolated total yield (major products (2E,3E)-diene (3) and
minor isomer (2Z,3E)-diene (3′), (2E,3Z)-diene (3′′)). The ratio of
1
3/3′/3′′ was determined by crude H NMR.
bromobenzaldehyde to afford the desired products in
moderate to excellent yields with good selectivities. The
electronic properties and the position of the substituents on
the phenyl ring of the MBH alcohols have limited effects on
the overall performance in terms of the yield and selectivity
(3q−3x). It should be noted that allylic alcohols bearing highly
active groups such as cyan (3t), nitryl (3u), and
trifluoromethyl (3s) could furnish the desired products in
C
DOI: 10.1021/acs.orglett.9b01349
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Accession Codes
CCDC 1889054 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: peizhongxie@njtech.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Peizhong Xie: 0000-0002-6777-0443
Teck-Peng Loh: 0000-0002-2936-337X
Notes
The authors declare no competing financial interest.
Figure 3. Proposed mechanism.
ACKNOWLEDGMENTS
■
The 1,3-diene moiety has been proven to be a versatile
synthetic intermediate for further elaboration. For instance, the
Diels−Alder reaction of 3o with DMAD in toluene afforded
dehydrobenzene derivative 4, a key intermediate for preparing
polysubstituted benzenes25a (Figure 4). In addition, an
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21702108),
the State Key Program of the National Natural Science
Foundation of China (21432009), the Natural Science
Foundation of Jiangsu Province, China (BK20160977), the
College Students’ Innovation and Open Experimentation Fund
Project (2019DC0416), Nanjing Tech University, and the
SCIAM Fellowship by Jiangsu National Synergetic Innovation
Center for Advanced Materials.
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DOI: 10.1021/acs.orglett.9b01349
Org. Lett. XXXX, XXX, XXX−XXX