Transition metal-free and regioselective vinylation of phosphine oxides and: H -phosphinates with VBX reagents

Article abstract of DOI:10.1039/d0cc05992g

A series of phosphine oxides and H-phosphinates were vinylated in the presence of the iodine(iii) reagents vinylbenziodoxolones (VBX), providing the corresponding alk-1-enyl phosphine oxides and alk-1-enyl phosphinates in good yields with complete chemo- and regioselectivity. The vinylation proceeds in open flask under mild and transition metal-free conditions.

Full text of DOI:10.1039/d0cc05992g

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DOI: 10.1039/D0CC05992G
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Transition Metal-Free and Regioselective Vinylation of Phosphine
Oxides and H-Phosphinates with VBX reagents
Laura Castoldi,a‡ Adam A. Rajkiewicza,b‡ and Berit Olofssona*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of phosphine oxides and H-phosphinates were vinylated in enzyme inhibitors.14 Alkenyl phosphine oxides and
the presence of the iodine(III) reagents vinylbenziodoxolones phosphonates are also valuable monomers in polymer and
(VBX), providing the corresponding alk-1-enyl phosphine oxides materials chemistry.15
and alk-1-enyl phosphinates in good yields with complete chemo-
Traditionally, the synthesis of alkenyl phosphorus
and regioselectivity. The vinylation proceeds in open flask under compounds relies on transition metal-catalyzed cross coupling
mild and transition metal-free conditions.
methodologies;16 and terminal alkenes can be prepared
through metal-catalyzed hydrophosphorylation of alkynes.17
Despite the utility of these methods, common drawbacks
include the removal of the transition metal from the final
product, the toxicity and the cost of the catalyst, and need for
preparation of non-commercial ligands.18 Metal-free
methodologies for P-C bond formation are hence highly
desirable.19 While several efficient methods for vinylation of
phosphine oxides to provide the internal alkene product have
recently been reported,20 the synthesis of the corresponding
terminal alkene product remains difficult. Methods utilizing
vinyl sulfones or nitro olefins as vinylation reagent have only
been demonstrated with a very limited phosphorus substrate
scope (Scheme 1b).21
Among the plethora of iodine(III) reagents,1 benziodoxolone
scaffolds have been introduced as more stable alternatives to
iodonium salts in alkynylation and trifluoromethylation
reactions.2 Vinylbenzoiodoxolones (VBX) were recently
introduced as efficient vinylating agents.3 They have since been
used in metal-catalyzed cross couplings and C-H vinylations,4 as
well as in several elegant metal-free reactions.4c, 4e, 5 We have
demonstrated the unique reactivity of VBX in the vinylation of
nitrocyclohexane, with regioselective formation of the terminal
alkene (Scheme 1a), whereas vinyliodonium salts gave the
internal alkene.3 To the contrary, the VBX vinylation of thiols
and mercaptothiazoles resulted in internal alkene products with
high E-selectivity (Scheme 1a),6 while vinyliodonium salts give
the Z-isomer through a vinylic SN2 mechanism.7
The synthesis of VBX reagents is straightforward, and can
either be performed in a one-pot reaction from 2-iodobenzoic
acid,3 or via a preformed iodine(III) species that is reacted with
a suitable vinylboronic acid.4d, 4g Heteroatom nucleophiles can
be added to EBX reagents to further increase the scope of VBX
reagents.4f, 4g, 8
Alkenylphosphorus compounds are primary building blocks
in
a
series of transformations, including reduction,9
epoxidation,10 Michael addition11 and cycloaddition.12
Moreover, vinylated phosphorus compounds have become of
interest for the development of pharmacologically important
and biologically active compounds such as antibiotics13 and
Scheme 1. Transition metal-free vinylations and alkynylations.
Vinyliodonium salts have been employed in a mild copper-
catalyzed procedure with H-phosphonates to afford (E) 2-
arylvinylphosphonates.22 In 2014, the benziodoxolone reagent
EBX was demonstrated to be efficient in metal-free alkynylation
of H-phosphites, H-phosphinates and secondary phosphine
oxides under mild conditions and short reaction times (Scheme
1c).23 We envisaged that a method involving the novel VBX
reagent could be attractive for the construction of C-P bonds.
a. Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University,
SE-106 91 Stockholm, Sweden. E-mail: berit.olofsson@su.se
b. Centre of New Technologies, University of Warsaw, Banacha 2C, 02-097 Warsaw,
Poland
‡ These authors contributed equally.
†
Electronic Supplementary Information (ESI) available: Experimental details,
analytical data and NMR spectra of novel compounds. See
DOI: 10.1039/x0xx00000x
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a mild and metal-free, open flask (3h), whereas electron withdrawing substituenVtsiewdArrtaicsletiOcnallinlye
Herein, we report
methodology to vinylate phosphine oxides and phosphinates reduced the reactivity. A modest yield ofD3OiIw: 1a0s.10o3b9t/aDi0nCeCd0u5n99d2eGr
with complete regioselectivity towards the terminal alkene.
standard conditions, which could be improved to 27% yield by
increasing the temperature and prolonging the reaction time.
Unsymmetric phosphine oxides were prepared from alkyl
phenyl-H-phosphinates by reaction with the corresponding
Grignard/organolithium reagent,24-25 and subsequently
examined in the vinylation. The unsymmetric product 3j,
bearing a thiophene moiety, and phenyl(p-tolyl)phosphine
oxide (3k) were obtained in good to high yields. The presence
of a CF3 group in product 3l decreased the yield to 41%.
Table 1: Optimization for phosphine oxides.a
Conc.
[mol/L] [h]
Time
Yield 3a
Entry Base
Solvent
[%]b
5
1
Et3N
THF
THF
0.1
0.1
0.1
0.1
0.1
0.1
0.1
20
20
20
20
20
20
20
20
20
1
2
3c
4
DMAP
Cs2CO3 THF
-
<5
62
0
88
90
40
64
76
84
80
78
93 (94)
THF
THF
THF
Et2O
5
6
7
8
DBU
BTMG
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Toluene 0.1
9
EtOAc
THF
THF
THF
THF
0.1
0.1
0.1
0.25
0.05
10
11d
12
13
1
1
1
a Reaction conditions: 1a (0.05 mmol) and base were stirred in solvent
b
for 5 min before addition of 2a (1.2 equiv). 1H-NMR yield using
trimethoxybenzene as internal standard, isolated yield in brackets. c at
d
60 °C. 1.0 equiv of 2a. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene;
BTMG=2-tert-butyl-1,1,3,3-tetramethylguanidine; DMAP=4-(dimethyl-
amino)pyridine.
Diphenylphosphine oxide (1a) was chosen as the model
substrate to study the vinylation of phosphorous compounds
with VBX 2a. Surprisingly, the vinylation proceeded with
complete regioselectivity in favor of the terminal alkene 3a
without formation of any internal alkenes 4 (Table 1).24 A series
of organic and inorganic bases were screened in THF at room
temperature (entries 1-6). The use of DBU and BTMG proved
superior to other bases (entries 5-6), while no reactivity was
detected in the absence of base (entry 4). DBU was selected for
further investigations as BTMG is more expensive, and THF was
found to be the best solvent (entries 5,7-9). Shortening the
reaction time to 1 h and reduced amount of VBX resulted in
small drops in yield (entries 10-11). Pleasingly, 3a was obtained
in 94% isolated yield by lowering the concentration of the
reaction to 0.05 M, likely due to increased solubility of VBX
(entry 13). We have previously demonstrated that the formed
iodobenzoic acid can be recovered and reused for VBX
synthesis, thus increasing the sustainability of the process.6
With the optimized conditions established, the substrate
scope was examined with symmetric and unsymmetric
diarylphosphine oxides 1 (Scheme 2a,b). Substrates with
electron donating substituents provided high yields of the
desired product (3a-g), including the sterically congested
products 3d and 3e. A fluorine substituent was also tolerated
Scheme 2: Scope with diaryl- and alkylaryl-phosphine oxides.
a At 60 °C, 20 h. b At –10 °C, 2 h. c At 60 °C, 2 h. d NMR yield.
The scope of VBX reagents was investigated by vinylation of
diphenylphosphine oxide (1a, Scheme 2c). VBX reagents with
both electron-donating and -withdrawing substituents on the
styryl moiety were well tolerated and gave the desired alk-1-
enyl phosphine oxides 3m-o in good yields. Aliphatic VBX
reagents have previously proved problematic and provided
2 | J. Name., 2012, 00, 1-3
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a
complex product mixtures.3, 6 Pleasingly, aliphatic VBX reagents Scheme 3: Scope with H-phosphinates. DBU used as a base.
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could be employed in the P-vinylation to provide the cyclohexyl b BTMG used as a base.
DOI: 10.1039/D0CC05992G
product 3p. Also the 3-chloropropenyl product 3q, with a
sensitive functional group, was successfully synthesized upon The different regiochemistry obtained in vinylations of
lowering the temperature. However, the Z-configured phosphorus and sulfur nucleophiles with VBX is intriguing, and
sulfonamide-substituted VBX reported by Waser4c and our core- preliminary tests with radical scavengers were conducted to
dimethylated Z-VBX derivative6 did not afford the desired investigate whether a radical pathway could be operating. The
product. Trisubstituted VBXs also proved unreactive.24
reaction of diphenylphosphine oxide (1a) with TEMPO under
Alkylarylphosphine oxides proved to be less reactive standard conditions afforded 82% yield of 3a, which is
towards VBX, and required additional optimization. Ethyl- comparable with the results without additive, indicating that a
phenylphosphine oxide could be vinylated to give 3r in excellent radical pathway is unlikely.24 The observed regiochemistry
yield within 2 hours at 60 °C (Scheme 2d). Other alkyl- and allyl would be consistent with a phospha-Michael-type addition28
derivatives were vinylated in the same manner to afford followed by proton shift and elimination, but alternative
products 3s-v. The successful synthesis of tert-butyl product 3w pathways cannot be excluded at this point.
illustrates the high tolerance for sterical hindrance. Dialkyl-
phosphine oxides were, on the other hand very unreactive, and
product 3x could only be obtained in modest yield upon employed to efficiently vinylate a wide range of phosphine
prolonged reaction time at 60 °C. oxides and H–phosphinates. The reactions were performed in
To conclude, the recently discovered reagent VBX was
We have recently investigated the effect of several core- open flask under mild and transition metal free conditions, and
substituted VBX reagents in the vinylation of thiols,6 and delicate functional groups were tolerated. Both aromatic and
substituent effects have previously been demonstrated with aliphatic vinyl moieties could be transferred, and the electronic
EBX reagents26 and other iodine(III) compounds.27 We hence and steric influences of core-substituted VBX derivatives were
investigated the influence of core-substituted VBX reagents in investigated. We are currently performing an extensive
P-vinylation of H-phosphinates, as they proved less reactive mechanistic investigation on the reactivity of VBX with several
than phosphine oxides. Phosphinate 5a was reacted with a nucleophiles under metal-free conditions, and will report the
series of core-substituted VBX reagents to investigate electronic results in due time.
and steric effects (see ESI). Me2-VBX, which we previously
reported to improve the S-vinylation yields,6 showed
comparable reactivity to the unsubstituted VBX, whereas other
Acknowledgements
changes in the core structure reduced the yield of 6a. A control
reaction with the corresponding acyclic vinyliodonium salt gave
only 10% yield of 6a.
We thank Ester Maria Di Tommaso for analytical support. We
acknowledge Carl Trygger Foundation (CTS 17:341, Fellowship
for
LC),
the
National
Science
Centre
Poland
The desired alkenylation of H-phosphinates proved viable at
60 °C for 20 h using either DBU or BTMG,24 and the scope was
examined with both bases as the outcome was substrate
dependent (Scheme 3).24 Ethyl phenylphosphinate and benzyl
phenylphosphinate gave 6a and 6b, respectively. Phenoxy and
allyloxy products 6c and 6d could also be obtained in this
fashion (the isolated yield of 6c is affected by the instability of
this compound). More complex substrates were also suitable
for this methodology, as exemplified by the vinylation of (R)-
phenyl menthoxy phosphinate 5e and glucose diacetonide-
derived H-phosphonate 5f to give products 6e and 6f in good
yields as a mixture of diastereoisomers. The vinylation of H-
phosphonates proved to be sluggish, despite optimization
attempts under various conditions.24
(2016/22/E/ST5/00566) and The Warsaw University’s
Integrated Development Programme (both Fellowships for
AAR), and the Swedish Research Council (2015-04404, 2019-
04232) for financial support.
Conflicts of interest
There are no conflicts to declare.
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