Mechanically induced solid-state generation of phosphorus ylides and the solvent-free wittig reaction

Article abstract of DOI:10.1021/ja017908p

We describe the nearly quantitative preparation of phosphorus ylides and the Wittig reaction occurring in the solid sate during high-energy mechanochemical processing. Initial insights into the details of the discovered chemical transformations indicate that high-energy mechanical processing supports the interaction of reacting centers by breaking crystallinity of the reactants and by providing mass transfer without a solvent. Copyright

Full text of DOI:10.1021/ja017908p

Published on Web 05/10/2002
Mechanically Induced Solid-State Generation of Phosphorus Ylides and the
Solvent-Free Wittig Reaction
Viktor P. Balema,*,† Jerzy W. Wiench,† Marek Pruski,† and Vitalij K. Pecharsky*,†,‡
Ames Laboratory, Iowa State UniVersity, Ames, Iowa 50011-3020, Department of Materials Science and
Engineering, Iowa State UniVersity, Ames, Iowa 50011
Received December 31, 2001
Scheme 1. Mechanochemical Generation of Phosphoranes
Mechanical alloying via high-energy ball-milling is a scalable
experimental technique, which is broadly used for the preparation
and modification of metals, hydrides, and other inorganic solids.1
In contrast, mechanochemistry is seldom attempted in organic
synthesis, and when it is, mechanical processing of organic reactants
is followed by additional treatment (usually heating), or it is carried
out in the presence of a solvent.2 Consequently, little is known about
chemical reactions taking place during mechanical milling of
organic solids. Furthermore, only indirect evidence confirming just
a few chemical transformations of solid organic compounds as a
result of milling has been presented to date.2e,f
We show that three major types of phosphorus ylides, that is,
stabilized, semistabilized and nonstabilized phosphoranes, can be
generated mechanochemically in a solid state. Stabilized ylides were
isolated in a pure form, while semistabilized and nonstabilized
phosphoranes were trapped by a solid organic carbonyl compound
and transformed into ethenes and triphenylphosphine oxide in a
solvent-free mechanochemical Wittig-type reaction. Conventionally,
phosphorus ylides are produced in a solution,3 and the solid-state
preparation of phosphoranes was not reported before.
Solid phosphonium salts, 1a-d, and an excess of anhydrous
K2CO3 were ball-milled in a hardened-steel vial with steel balls
under helium for various periods of time (Scheme 1 and Table 1).
Forced air-cooling of the vial was employed to prevent its heating
during ball-milling experiments. The obtained powders were
analyzed using solid-state 31P nuclear magnetic resonance (NMR)
spectroscopy, X-ray powder diffraction (XRD), and differential
thermal analysis (DTA) prior to any further treatment.
Table 1. Mechanochemically Prepared Compounds
compound
milling time, h
yield, %
E:Z ratioa
referenceb
2a
2b
3a
3b
3c
3d
4
3
4
7
99
96
-
c
-
3b, 8
7
9
10
11
6
85d
92d
70d
73d
99
1.6:1e
2:1
3.4:1
-
-
3.5:1
8
14
20
4
5
8
93d
7
a E:Z-isomer ratio as determined by the liquid-state (CDCl3) H NMR
spectroscopy. b References used for identification of the compounds.
c Identified by comparison with the commercial phosphorane. d Triphenyl-
phosphine oxide was isolated as a coproduct in the yields matching that of
ethene. e Estimated ratio based on the amount of the isolated E-isomer.
1
prepared mechanochemically from (carbethoxymethyl)triphenylphos-
phonium bromide (1b) and anhydrous K2CO3, see Table 1.
Semistabilized and nonstabilized phosphoranes were generated
from (benzyl)triphenylphosphonium bromide (1c) or (methyl)-
triphenylphosphonium bromide (1d) by mechanical processing with
an excess of K2CO3 in the presence of stoichiometric amounts of
solid organic carbonyl compounds. The latter served as traps for
highly reactive phosphoranes. Thus, we successfully carried out
several solvent-free Wittig-type reactions and prepared a series of
aromatic ethenes 3a-d (Scheme 1, Table 1) in high yields. Again,
the solid-state 31P MAS NMR confirmed the completion of all
performed reactions during ball-milling (e.g., Figure 1e-g).
Finally, we found that the mechanochemical preparation of
stabilized phosphorus ylides and the solvent-free Wittig reaction
can be successfully carried out as a “one-pot” process starting with
triphenylphosphine, an organic halogenide, and an organic carbonyl
compound in the presence of K2CO3 (Scheme 2, Table 1). In this
way, we synthesized the ylides 2a and 4, and the bromostilbene 5.
It is worth noting that 4 is usually prepared in a solution in a
multiple-step process,6 while in our approach the reactants were
simply ball-milled together for 4 h. In all cases, the completion of
Two stable phosphorus ylides, (benzoylmethylene)triphenylphos-
phorane (2a) and (carbethoxymethylene)triphenylphosphorane (2b),
were prepared in nearly quantitative yields (Scheme 1, Table 1).
To prepare ylide 2a, (phenacyl)triphenylphosphonium bromide (1a)
and anhydrous K2CO3, taken in a 1:1.2 molar ratio, were ball-milled
in a Spex 8000 mill for 1 and 3 h. The solid-state 31P magic angle
spinning (MAS) NMR spectrum of the powder sample after ball-
milling for 1 h consists of two peaks at δ31P ) 23 and 17 ppm
(Figure 1b). The peak at δ31P ) 23 ppm corresponds to the
unreacted 1a (Figure 1a) remaining in the reaction mixture. The
location of the second peak at δ31P ) 17 ppm is consistent with
pure 2a (Figure 1d). After ball-milling for 3 h, the spectrum of the
reaction mixture (Figure 1c) contains only one signal at δ31P ) 17
ppm, indicating a complete transformation of 1a into 2a. The sample
ball-milled for 1 h was reexamined 6 months after the completion
of the mechanical treatment. No continuing changes, that is, no
thermal solid-state reactions were detected. Similarly, 2b was
* Corresponding authors. E-mail: balema@ameslab.gov, vitkp@ameslab.gov.
† Ames Laboratory.
‡ Department of Materials Science and Engineering.
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J. AM. CHEM. SOC. 2002, 124, 6244-6245
10.1021/ja017908p CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Our experiments produced no evidence for melting of the studied
material below 180 °C, which is the melting point of the ylide 2a
formed during mechanical processing. Furthermore, DTA, XRD,
and NMR data (compare Figure 1, c and d) revealed the presence
of an amorphous phase in the ball-milled sample. Partial loss of
crystallinity was also observed during mechanical treatment of the
pure 1a and 2a for 1 h. According to DTA and in situ high-
temperature XRD, crystallization of the amorphous components in
the investigated powders occurs between 70 and 105 °C, also
indicating that the temperature of the reaction mixtures during
processing remained below ∼70 °C.
The acquired data confirm the solid-state character of the events
leading to the mechanochemical generation of phosphorus ylides.
They also allow us to explain the latter as a sequence of solid-state
processes, which include: (i) breaking the crystal lattice of a
phosphonium compound and the formation of an amorphous phase
and (ii) a deprotonation of an amorphous phosphonium salt by
microcrystalline K2CO3 in a heterogeneous solid-state reaction.
In conclusion, mechanically induced solid-state generation of
phosphorus ylides and the solvent-free Wittig reaction have been
observed for the first time. The completion of the reactions during
ball-milling has been directly confirmed by solid-state 31P MAS
NMR spectroscopy. High-energy mechanical processing is respon-
sible for both the amorphization of the reactants and mass transfer
in a solid state, thus playing a critical role in the discovered chemical
transformations.
Figure 1. Solid-state 31P MAS NMR spectra of: (a) crystalline 1a
(Lancaster synthesis); reaction mixtures after ball-milling of 1a with K2-
CO3 for (b) 1 h and (c) 3 h; (d) crystalline 2a (Alfa Aesar); (e) crystalline
1d (Aldrich); (f) reaction mixture after ball-milling of 1d with K2CO3 in
the presence of 2-naphthaldehyde for 10 h (the reaction is essentially
complete after 20 h); (g) crystalline triphenylphosphine oxide. Spectra were
obtained at 161.9 MHz under single-pulse excitation, 20 kHz MAS and
continuous wave 1H decoupling. The more efficient method of cross-
1
polarization from H nuclei yielded similar results.
Scheme 2. “One-Pot” Mechanochemical Synthesis
Acknowledgment. This work was supported by the Office of
Basic Energy Sciences, Materials Sciences Division (V.P.B. and
V.K.P.) and Chemical Sciences Division (J.W. and M.P.) of the
U.S. DOE. The authors thank to Mr. Kevin Dennis and Mr. Keita
Hosokawa for help with DTA measurements, and to Dr. Karl A.
Gschneidner, Jr., for discussions of the results.
mechanochemical transformations was confirmed by the solid-state
31P MAS NMR spectroscopy.
Two distinct dissimilarities between the conventional and the
mechanochemically induced Wittig reactions of semistabilized and
nonstabilized phosphorus ylides are worth noting. First, both
(benzylene)- and (methylene)triphenylphosphoranes can be gener-
ated mechanochemically using K2CO3, while their preparation in
a solution requires much stronger bases.3 As far as we are aware,
deprotonation of a (methyl)triphenylphosphonium salt with an alkali
metal carbonate was never reported in the literature. Second, while
the Wittig reaction between (benzylene)- or (2-naphthylene)tri-
phenylphosphoranes and aromatic carbonyl compounds in a solution
produces preferably Z-stilbenes or mixtures with nearly equal
content of Z- and E-isomers,3,7 the mechanochemical technique
discriminates between Z- and E-substituted products in favor of
more thermodynamically stable E-stilbenes.
To obtain preliminary insights into the processes resulting in the
mechanochemical generation of phosphorus ylides, the powder
obtained after ball-milling of the phosphonium salt 1a with K2CO3
for 1 h was studied using DTA and XRD. The major objective
was to explore the possibility of local melting of the reactants and,
hence, hidden liquid-state reactions during mechanical processing.
The data on the local temperature rise in different types of mills
during milling of inorganic solids1,4 and our results on mechanical
treatment of LiAlH4-based composites5 showed that the materials’
temperature during milling in a Spex 8000 mill do not exceed 110
°C. Since melting points of 1a and anhydrous K2CO3 are 265 and
891 °C, respectively, their melting during mechanical processing
is unlikely. On the other hand, the likelihood of the formation of
low-melting eutectics from these compounds during ball-milling
required further investigation.
Supporting Information Available: Experimental details, DTA
and XRD data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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