Wittig reactions of moderate ylides with heteroaryl substituents at the phosphorus atom

Article abstract of DOI:10.1016/j.tet.2005.05.002

The influence of various heteroaryl substituents at the phosphorus atom to the stereoselectivity of Wittig reactions of allylic and benzylic ylides has been studied. In the case of nitrogen bearing heteroaromatic ligands at the phosphorous atom of benzylidenephosphoranes high E-alkene selectivity's of up to 90:10 could be observed. NMR spectroscopic investigations revealed that substituents at the phosphorus have influences on the reactivity of ylides as well as the stability of reaction intermediates. Indications for chelation of lithium ions with ylides could also be detected and will be discussed in this article.

Full text of DOI:10.1016/j.tet.2005.05.002

Tetrahedron 61 (2005) 6764–6771
Wittig reactions of moderate ylides with heteroaryl substituents at
the phosphorus atom
*
Marco Ackermann and Stefan Berger
¨ ¨ ¨ ¨
´
Institut fur Analytische Chemie, Fakultat fur Chemie, Universitat Leipzig, Linnestr. 3, D-04103 Leipzig, Germany
Received 15 February 2005; accepted 2 May 2005
Abstract—The influence of various heteroaryl substituents at the phosphorus atom to the stereoselectivity of Wittig reactions of allylic and
benzylic ylides has been studied. In the case of nitrogen bearing heteroaromatic ligands at the phosphorous atom of benzylidenephosphoranes
high E-alkene selectivity’s of up to 90:10 could be observed. NMR spectroscopic investigations revealed that substituents at the phosphorus
have influences on the reactivity of ylides as well as the stability of reaction intermediates. Indications for chelation of lithium ions with
ylides could also be detected and will be discussed in this article.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
The Wittig reaction (Scheme 1) is known for more than 50
years and belongs to the most important carbon–carbon
double bond forming reactions in organic chemistry1 and is
also used in large scale in industry.2
The popularity of the Wittig reaction is mainly due to the
regioselective formation of the double bond at the position
of the former carbonyl group and the possibility to control
the stereoselectivity by applying special reaction con-
ditions.3 The reaction conditions needed for maximum Z-
Scheme 2. Examples for different classes of phosphorus ylides.
or E-alkene selectivity’s are strongly dependent on the
nature of the ylide used (Scheme 2).
observed. The negative influence of lithium ions on the
stereochemistry of Wittig reactions is concentration depen-
dent6 and is attributed to the fact, that lithium salts are at
least partially soluble in many organic solvents. Solvated
LiC is able to complex the carbonyl compound, which then
reacts faster with the ylide. This catalysed pathway of the
Wittig reaction is rather unspecific, explaining the reduced
selectivity’s in the presence of LiC.
For unstabilized, so called ‘reactive ylides’, the selective
formation of Z-alkenes is possible by applying ‘salt free’
reaction conditions.4,5 Salt free means performing the
reactions in the absence of lithium ions, although other
cations (e.g., NaC or KC) can be present. In the presence of
lithium ions most often reduced alkene selectivity’s are
Scheme 1. General overview of the Wittig reaction.
Keywords: Wittig reactions; NMR spectroscopy; Ylides; Configuration; Low-temperature chemistry.
*
Corresponding author. Tel.: C49 341 9736101; fax: C49 34197 11833; e-mail: stberger@rz.uni-leipzig.de
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.05.002

M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
6765
The selective formation of E-alkenes in the case of reactive
ylides is possible using the Schlosser methodology.7 For
stabilized ylides the selective formation of E-alkenes is
achieved using phosphorus ylides8 or PO-ylides within the
Horner–Wadsworth–Emmons reaction.9 Z-Alkene selective
variants of the Horner–Wadsworth–Emmons reaction with
stabilized ylides are also known, for example, the Ando-
methodology10 or the Still–Gennari-variant.11 Despite these
traditional and well established methods for the selective
formation of alkenes using reactive and stabilized ylides,
there is still a considerable lack of efficient methods for the
selective formation of alkenes in the case of moderate or
semistabilized ylides. Schlosser reported a Z-alkene selec-
tive variant of the Wittig reaction for moderate ylides using
‘methoxymethoxy-armed’ ylides.3,12 However, the yields
are below 50% in many cases due to the steric demand of the
2-methoxymethoxyphenyl groups. The E-alkene selective
formation of alkenes in the case of moderate ylides has been
reported in some cases.13,14,15
different heteroaromatic systems. The phosphonium salts
bearing heteroaromatic substituents at phosphorus are
accessible in two to three steps with an overall yield in
the range of 50%.
The following heteroaromatic systems have been
implemented in phosphonium salts (Scheme 3).
We were interested in the influence of heteroaromatic
substituents at the phosphorus center on stereochemistry and
reaction mechanism of Wittig reactions. Previously we
reported that reactive ylides bearing 2-furyl substituents at
the phosphorus atom react with greatly enhanced Z-alkene
selectivity’s.16 2-Pyridyl substituents also increase the
Z-alkene ratio.17
Scheme 3. Overview over the heteroaromatic substituents tested. R denotes
allyl or benzyl groups, L represents either a phenyl substituent or a
heteroaryl system.
The Wittig reactions with these heteroaromatic systems
were repeated two to three times to get reliable results. In
the following sections the experimental results of our
investigations will be presented, divided in two parts, for the
allylic and benzylic Wittig reaction, respectively.
Encouraged by this observation we wanted to test, if those
and other heteroaromatic substituents have similar effects of
increased alkene selectivity in Wittig reactions of moderate
ylides.
2.1. Results of allylic Wittig reactions
Here we want to report our investigations of the influence of
a number of different heteroaromatic substituents at the
phosphorus to selectivity and yield of Wittig reactions with
moderate ylides. The preparative results will be analysed by
means of NMR spectroscopy.
The Wittig reaction of allylidenetriphenylphosphorane with
benzaldehyde is well investigated and results were pub-
lished by a number of different authors.18,19,20,21 The
reported yields vary from 58 to 95% and the stereochemical
outcome spread from 44:56 Z:E to 75:25 Z:E. The results of
our investigations concerning Wittig reactions between
allylic ylides bearing heteroaromatic systems and benzal-
dehyde are presented in Table 1.
2. Results and discussion
For our studies we chose the well investigated Wittig
reaction of benzaldehyde with allylidenetriphenylphosphor-
ane or benzylidenetriphenylphosphorane in THF. For
deprotonation of the corresponding phosphonium salts
either NaHMDS or n-BuLi was used. We investigated the
influence of the heteroaromatic systems at phosphorus by
replacing one or all three standard phenyl substituents by
The standard reaction with three phenyl substituents at the
phosphorus atom is indeed rather unspecific yielding Z:E-
alkene ratios of 55:45 and 73:27 with n-BuLi and NaHMDS
as base, respectively. The introduction of different hetero-
aromatic substituents has only minor influences of the
stereochemical outcome of allylic Wittig reactions. Inter-
estingly, Wittig reactions with one 2-pyridyl ring at
Table 1. Yield and stereochemical results of Wittig reactions in THF using allylideneheteroaryl-phenylphosphoranes and benzaldehyde with either NaHMDS
or n-BuLi as base
Compound
n-BuLi
NaHMDS
Z:Ea
Yield (%)b
Z:E
Yield (%)
AllylidenePCPh3
55:45
66:34
70:30
66:34
61:39
37:63
66:34
63
68
59
63
33
32
19
73:27
74:26
75:25
67:33
63:37
44:56
54:46
68
46
50
68
62
12
20
AllylidenePC(2-thienyl)Ph2
AllylidenePC(2-furyl)Ph2
AllylidenePC(2-furyl)3
AllylidenePC(benzo[b]furyl)Ph2
AllylidenePC(2-pyridyl)Ph2
AllylidenePC(2-pyridyl)3
a Z:E ratios were determined on isolated products using NMR-spectroscopy and GC.
b All yields were determined on isolated products.

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M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
Table 2. Yield and stereochemical results of Wittig reactions in THF using benzylideneheteroaryl-phenylphosphoranes and benzaldehyde with either
NaHMDS or n-BuLi as base
Compound
n-BuLi
NaHMDS
Z:Ea
Yield (%)b
Z:E
Yield (%)
BenzylidenePCPh3
65:35
79:21
67:33
69:31
55:45
46:54
40:60
49:51
24:76
66
89
82
92
85
87
79
65
58
61:39
53:47
42:58
51:49
30:70
32:68
14:86
51:49
11:89
81
91
65
92
81
86
85
69
84
BenzylidenePC(2-thienyl)Ph2
BenzylidenePC(2-furyl)Ph2
BenzylidenePC(2-furyl)3
BenzylidenePC(benzo[b]furyl)Ph2
BenzylidenePC(benzo[b]furyl)3
BenzylidenePC(2-pyridyl)Ph2
BenzylidenePC(2-pyridyl)3
BenzylidenePC(2-chinolyl)Ph2
a Z:E ratios were determined on isolated product using NMR-spectroscopy and GC.
b All yields were determined on isolated products.
phosphorus yield a slight excess of the E-alkene in contrast
to the standard reaction. The yield of this Wittig reaction
drops from above 60% to less than 20% with nitrogen
bearing ligands present in the phosphonium salt. In these
cases a side product could be isolated, which is formed via
g-substitution of the allylic ylides.19
2.3. NMR investigations
We rationalized three kinds of possible influences of the
different heteroaryl groups at phosphorus on the stereo-
chemistry of the investigated Wittig reactions.
(a) Steric influences
(b) Chelation of metal ions with ylides
(c) Electronic influences on ylide reactivity
It has to be summarized that the introduced heteroaromatic
substituents at phosphorus had not the anticipated influences
on Wittig reactions of allylic ylides. Instead of increased
stereoselectivity only minor influences on the stereo-
chemistry of Wittig reactions could be observed. The
decreased yield in some cases could be attributed to a
concurrence reaction of allylic ylides.
The stereoselectivity of benzylic Wittig reactions was
changed the most, when one 2-chinolyl group was present
in the ylide. The question arose, if this influence is due to the
increased steric demand of the 2-chinolyl group in
comparison to the phenyl substituent. We therefore prepared
a 2-naphthyl group bearing ylide. The 2-naphthyl group has
almost the same steric demand as the 2-chinolyl group, but
lacks the heteraoatom. The Wittig reaction of benzyl-
idene(2-naphthyl)diphenylphosphorane with benzaldehyde
yielded Z:E-alkene ratios of 73:27 with n-BuLi as base and
67:33 with NaHMDS. These values are almost identical to
the results of the similar Wittig reaction with three phenyl
groups at phosphorus presented in Table 2 (entry 1).
Furthermore, the stereochemistry of benzylic Wittig reac-
tions is similar with one or three benzo[b]furyl groups at
phosphorus (Table 2, entries 5 and 6). We therefore rule out
any significant steric influence of the investigated hetero-
aromatic systems on the stereo selectivity of the presented
Wittig reactions with benzylic ylides.
2.2. Results of benzylic Wittig reactions
The Wittig reaction of benzylidenetriphenylphosphorane
with benzaldehyde is extraordinary well investigated and
many results have been published in literature.19,22,23,24,25,26
The Z:E alkene ratios reported vary from of 68:32 to 47:53
with a yield typically above 90%. Our results for Wittig
reactions between different benzylidenephosphoranes and
benzaldehyde are shown in Table 2.
Our results for the standard reaction with three phenyl
groups at phosphorus are in accordance with reported values
in literature and proof that this reaction is unspecific. Only a
slight preference for the Z-alkene could be observed.
Introduction of heteroaromatic systems changes the stereo-
chemical results of this Wittig reaction fundamentally. In
most cases with NaHMDS as base the E-alkene gets the
preferred product. With one 2-pyridyl or one 2-chinolyl
group present, the E-alkene is even formed in ratios of
almost 90:10. With n-BuLi as base the tendency to form the
E-alkene is also present, although to a lesser extent. The
yield remains in the order of 80% in most cases.
Another possible influence on stereochemistry of Wittig
reactions might be the chelation of solvated metal ions
between the carbanion site and the heteroatom of the
heteroarylsubstituent. Such chelated species might possess a
preferred conformation in the transition state of the Wittig
reaction, which could be an explanation for the selective
formation of one alkene.17 To investigate the possible
presence of such species we switched our attention to allylic
ylides. Although the stereo selectivity of Wittig reactions
with allylic ylides was rather unaffected by heteroaromatic
systems, these species in contrast to benzylic ylides showed
dynamic behaviour at different temperatures. This circum-
stance allowed NMR studies concerning chelation. In
Scheme 4 31P NMR spectra of allylidenetriphenyl-
phosphorane in THF are shown, which were recorded at
Heteroaromatic substituents at the phosphorus atom have at
least in benzylic Wittig reactions significant influences on
the stereoselectivity. Often an inversion of the stereo-
selectivity is obtained, leading to high E-alkene selectivity’s
in two cases. The origin of these influences was unknown at
this point and numerous NMR experiments where initiated
to clarify the role of those heteroaryl groups at phosphorus.

M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
6767
Scheme 4. 31P NMR spectra of allylidenetriphenylphosphorane at different temperatures.
different temperatures between K80 and K15 8C. The ylide
has been prepared using NaHMDS as base.
could be detected, the coalescence temperature lies also at
K50 8C. It has to be stressed, that here the dynamic process
has been detected via 7Li NMR spectra, although the lithium
ions are not covalently bound to the phosphorus ylide. This
result therefore is a strong indication for the presence of
lithium–ylide complexes, which have been postulated
above. The two different 7Li NMR-signals at lower
temperatures do not represent different lithiated species,
since the same splitting can be observed in Scheme 4, were
At higher temperatures one sharp signal represents the
phosphorus ylide. At temperatures below K50 8C two
different conformers are present in the solution. The
coalescence temperature for this dynamic and reversible
process is at K50 8C. The calculated free activation
enthalpy is 40.8 kJ/mol.27 To the best of our knowledge
such dynamic behaviour of allylidenephosphoranes has so
far not been described in literature.
7
no Li-ions were present. Unfortunately, numerous other
methods, including several HOESY experiments28,29,30 and
the detection of 15N-chemical shifts in possibly chelated
structures, failed to provide any further evidence for the
existence of such lithium–ylide complexes.
In Scheme 5 7Li NMR spectra of allylidene(2-pyridyl)-
diphenylphosphorane are shown, which were recorded at
temperatures between K80 and K10 8C. This ylide was
prepared using n-BuLi as base.
Nevertheless it has to be concluded, that these investigations
give a clear indication for the presence of lithium ion–ylide
chelate complexes, since the conformational changes in the
Again a dynamic interconversion of the two conformers
Scheme 5. 7Li NMR spectra of allylidene(2-pyridyl)diphenylphosphorane at different temperatures.

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M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
ylide structure can be detected via 31P NMR spectroscopy as
well as Li NMR.
denoted in Scheme 6. The temperature was kept for 20 min
and then warmed further. At a temperature of K40 8C the
reaction was finished. All the ylide had been reacted into
alkene and phosphane oxide.
7
Finally, we wanted to investigate the possible influence of
heteroaryl groups at phosphorus on ylide reactivity. Again
NMR is the most valuable tool for this purpose. Different
phosphorus ylides bearing heteroaromatic substituents were
prepared in NMR tubes. All ylides were prepared using the
base NaHMDS, since the highest stereoselectivity’s could
be observed in this case. Furthermore, with the base n-BuLi
often several peaks are observed for one species, which are
broadened at lower temperatures, most likely due to the
formation of different lithiated species. The Wittig reaction
was started by adding 1.5 equiv of benzaldehyde at low
temperatures. The reaction was then monitored with 31P
NMR spectra at temperatures between K80 8C and room
temperature. For these experiments we switched our
attention back to benzylic ylides, since they were generally
more influenced by heteroaryl substituents than allylic
ylides.
In conclusion, the standard benzylic Wittig reaction with
three phenyl rings at the phosphorus atom proceeds at
temperatures between K80 and K40 8C. At very low
temperatures of K80 8C the reaction gets very slow, after
50% of the starting material is consumed. The mixture had
to be warmed, to let the reaction proceed further. This might
be an explanation, why Wittig reactions of moderate ylides
are rather unspecific, since the reaction takes place over a
temperature span of 40 8C. Different thermal influences act
to the transition state of the reaction, yielding different
alkene ratios at each temperature. These different alkene
ratios add to a rather unspecific overall reaction.
The highest E-alkene selectivity for benzylic Wittig
reactions could be observed, when one 2-pyridyl substituent
was bound to the phosphorus atom. This reaction was also
monitored by 31P NMR spectroscopy. The result of this
experiment is shown in Scheme 7.
First of all we investigated the standard reaction with
benzylidenetriphenylphosphorane and benzaldehyde in
THF. The reaction was started by addition of the
benzaldehyde at K80 8C and after that the NMR tube was
put into the NMR spectrometer, which was precooled to
K80 8C. The first 31P NMR spectrum could be recorded ca.
1 min after addition of the benzaldehyde. In the first
spectrum a sharp signal at 8 ppm was detected representing
the phosphorus ylide. At 24 ppm a signal slowly arose,
corresponding to the phosphane oxide. Since, here moderate
ylides are under investigation, the reaction from the ylide to
the phosphane oxide is rather slow at low temperatures of
K80 8C. No oxaphosphetane species could be detected in
the expected chemical shift range, in accordance with earlier
investigations concerning benzylic Wittig reactions.31 As
expected, the reaction was slow at K80 8C. After 20 min ca.
45% of the ylide had reacted to phosphane oxide and alkene.
After that point the reaction rate dramatically decreased, so
that no further decay of the ylide signal could be detected at
K80 8C. The reaction mixture had to be warmed to K70 8C
in order to let the reaction proceed any further. The progress
of the reaction at temperatures between K80 and K40 8C is
shown in Scheme 6.
Scheme 7. Progress of the Wittig reaction between benzylidene(2-
pyridyl)diphenylphosphorane and benzaldehyde.
All shown spectra were recorded at a temperature of
K80 8C. Again no oxaphosphetane species could be
detected. More importantly, the reaction is finished after
40 min at K80 8C. The introduction of one 2-pyridyl
substituent thus greatly increases the ylide reactivity. This
might explain the high alkene selectivity in this reaction.
The reaction proceeds completely at low temperatures,
therefore a change of temperature does not lower the alkene
selectivity. However, unexplained remains the fact, that the
E-alkene is the preferred product, which is formed with an
excess of almost 90:10 in this reaction.
Each spectrum was recorded after 5 min at the temperature
Finally, we want to present the effect, which 2-furyl
substituents have in this benzylic Wittig reaction. In
Scheme 8 31P NMR spectra of the Wittig reaction between
benzylidenetris-(2-furyl)phosphorane with benzaldehyde
are shown.
Like in the standard case with three phenyl rings at
phosphorus, the reaction mixture has to be warmed to let
the reaction proceed. But in contrast to the case above, here
Scheme 6. Progress of the Wittig reaction between benzylidenetriphenyl-
phosphorane and benzaldehyde.

M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
6769
3. Conclusion
This work investigated the influence of heteroaryl sub-
stituents at phosphorus on the stereochemistry of Wittig
reactions of moderate ylides. With standard phenyl ligands
at phosphorus Wittig reactions of allylic and benzylic ylides
are rather unspecific. While the introduction of heteroaryl
substituents does not significantly change the stereo-
chemistry of allylic Wittig reactions, the situation is
different with benzylic Wittig reactions. With the introduc-
tion of heteroaryl substituents the E-alkene gets the
predominant product in most reactions. With one nitrogen
containing heteroaromatic substituent the E-alkene
selectivity almost reaches 90:10.
The reasons for these influences have been investigated by
three different approaches. First, steric interactions of the
heteroaromatic systems can be ruled out, since the
stereochemistry of benzylic Wittig reactions is not changed
if one phenyl or one bigger naphthyl ligand is present.
Second, we investigated the possibility of a metal ion
chelation with ylides. We observed a conformational
Scheme 8. Progress of the Wittig reaction between benzylidenetris(2-
furyl)phosphorane and benzaldehyde at K80 8C.
we were able to detect oxaphosphetane intermediates. They
are formed by the reaction of benzaldehyde with the ylide
and remain stable until ca. K20 8C. This is the first time
well characterized oxaphosphetane signals can be observed
in the case of benzylic Wittig reactions. Furthermore, the
two isomers cis and trans could be resolved in the chemical
shift region of K100 ppm. The two peaks were assigned to
the cis- and trans-oxaphosphetane taking into account, that
the cis-oxaphosphetane is less stable then the trans-
isomer.31 This explains why at 5 8C the signal for the cis-
isomer is almost gone, whereas still significant amounts of
the trans-oxaphosphetane are present. In the Wittig reaction
of benzylidene-(2-furyl)diphenylphosphorane, where only
one 2-furyl ring is present, also oxaphosphetanes could be
observed. However, in this case, only one signal was
resolved for the two oxaphosphetane species. Furthermore,
the oxaphosphetanes were only semistable in this case. They
were formed up to a maximum concentration, which
remained constant until the beginning of decomposition at
K10 8C. We can conclude that 2-furyl substituents stabilize
oxaphosphetane intermediates and the amount of stabilis-
ation is higher the more 2-furyl rings are present within the
phosphorane. Additionally 2-furyl systems have a dramatic
shielding effect on the phosphorus nucleus. These two
effects have been observed previously for reactive ylides.16
In the case of reactive ylides the stabilization of oxa-
phosphetanes was strong enough that these intermediates
could be isolated at ambient temperatures allowing the
recording of a crystal structure.
7
equilibrium for allylic ylides in 31P NMR as well as Li
NMR spectra, indicating some kind of contact between both
nuclei. The third investigation concerned the influence of
heteroaryl substituents on ylide reactivity. We could show,
that one 2-pyridyl substituent greatly increases the reactivity
of benzylic ylides, so that the Wittig reaction proceeds
completely at very low temperatures. This circumstance
favours high alkene selectivity’s. We could show, that
2-furyl substituents greatly increase the stability of oxa-
phosphetane intermediates. The reasons for this stabiliz-
ation and the unusual values of 31P NMR chemical shift will
be discussed elsewhere.
4. Experimental
4.1. General remarks
All reactions were carried out with exclusion of air and
moisture. THF was dried over sodium/benzophenone and
freshly distilled prior to use. Benzaldehyde was also
distilled. All other reagents were bought from commercial
sources and were used without further purification.
NMR spectra were recorded with a Bruker DRX 400
spectrometer in CDCl3, with TMS as internal shift
reference. 31P- and 7Li NMR spectra were calibrated
according to the X-scale, in which nitromethane is at
0 ppm.32 NMR investigations of Wittig reactions were
performed in [D8]THF, shifts being referred to traces of
undeuterated solvent. High resolution mass spectra were
recorded with a Bruker FT-ICR-MS apex II with ESI
technique.
In summary we could show, that heteroarylsubstituents at
the phosphorus nucleus change the reactivity of phosphorus
ylides, which has consequences for the reaction inter-
mediates and most important for the observed alkene ratio.
2-Pyridyl substituents increase the ylide reactivity so, that
the Wittig reaction is finished in less than 1 h at K80 8C.
This results in a high E-alkene selectivity. In contrast 2-furyl
substituents increase the stability of reaction intermediates,
allowing for the first time the detection of oxaphosphetanes
in the case of moderate ylides, which are stable until
K20 8C.
4.2. Synthesis of phosphanes
Phosphanes with 2-furyl, 2-thienyl or 2-benzo[b]furyl
substituents were prepared according to a procedure
described by Allen et al.33 2-Pyridyl bearing phosphanes
were prepared by a procedure starting from 2-bromo-
pyridine.34 Phosphanes with 2-chinolyl substituents were

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M. Ackermann, S. Berger / Tetrahedron 61 (2005) 6764–6771
prepared by the same procedure, however 2-bromochinolin
is not commercially available and had to be synthesised.35
4.4.12. Benzyl(2-chinolyl)diphenylphosphonium bromide.
Yield 61%; mp ca. 265 8C. 31P NMR (CDCl3): dZ19.5. HR-
MS: m/zZ404.15630 [C28H23NPC], Calcd 404.15626.
4.3. Synthesis of phosphonium salts
4.4.13. Benzyl(2-pyridyl)diphenylphosphonium bromide.
Yield 89%; mp 217–220 8C. 31P NMR (CDCl3): dZ19.6. HR-
MS: m/zZ354.14010 [C24H21NPC], Calcd 354.14061.
One equivalent phosphane was dissolved in toluene and
stirred with 2 equiv allyl bromide or benzyl bromide at
temperatures of 80 8C until a white precipitate had formed.
If this method was unsuccessful, the reaction was performed
in neat allyl bromide or benzyl bromide. The precipitate was
filtered off, washed and dried. If necessary recrystallisation
was performed in ethanol/ethyl acetate.
4.4.14. Benzyltris(2-pyridyl)phosphonium bromide.
Yield 54%; 31P NMR (CDCl3): dZ9.8. HR-MS: m/zZ
356.13122 [C22H19N3PC], Calcd 356.13111.
4.4.15.
Benzyl(2-naphthyl)diphenylphosphonium
bromide. Yield 71%; mp 253–255 8C. 31P NMR (CDCl3):
dZ23.4. HR-MS: m/zZ403.16108 [C29H24PC], Calcd
403.16101.
4.4. Wittig reactions
Wittig reactions were performed as described previously.16
4.4.1. Allyl(2-thienyl)diphenylphosphonium bromide.
Yield 78%; mp 178–182 8C. 31P NMR (CDCl3): dZ16.7.
MS (FAB): m/zZ389.0 and 387.0 (1:1).
Acknowledgements
We thank the Verband der Chemischen Indutrie for the
financial support within a Chemiefonds-Stipendium grant
number 168291.
4.4.2. Allyl(2-furyl)diphenylphosphonium bromide.
Yield 88%; mp 180–185 8C. 31P NMR (CDCl3): dZ11.4.
HR-MS: m/zZ293.10874 [C19H18OPC], Calcd 293.10898.
4.4.3. Allyltris(2-furyl)phosphonium bromide. Yield
45%; mp 153–157 8C. 31P NMR (CDCl3): dZK14.3. HR-
MS: m/zZ273.06735 [C15H14O3PC], Calcd 273.06751.
References and notes
¨
1. Nicolaou, K. C.; Harter, M. W.; Gunzner, J. L.; Nadin, A.
Liebigs Ann. Chem. 1997, 1283–1301.
4.4.4. Allyl(2-benzo[b]furyl)diphenylphosphonium
bromide. Yield 76%; mp 152–156 8C. 31P NMR (CDCl3):
dZ12.8. HR-MS: m/zZ343.12465 [C23H20OPC], Calcd
343.12463.
2. Pommer, H. Angew. Chem. 1977, 89, 437–443. Angew.
Chem., Int. Ed. Engl. 1977, 16, 423–430.
3. Wang, Q.; El Khoury, M.; Schlosser, M. Chem. Eur. J. 2000, 6,
420–426.
4.4.5. Allyl(2-pyridyl)diphenylphosphonium bromide.
Yield 78%; mp 187–190 8C. 31P NMR (CDCl3): dZ17.0.
HR-MS: m/zZ304.12452 [C20H19NPC], Calcd 304.12496.
4. Schlosser, M.; Schaub, B.; de Oliveira-Neto, J.; Jeganathan, S.
Chimia 1986, 40, 244–245.
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7. Schlosser, M.; Christmann, K. F. Angew. Chem. 1966, 78, 115.
Angew. Chem., Int. Ed. Engl. 1966, 5, 126.
4.4.6. Allyltris(2-pyridyl)phosphonium bromide. Yield
83%; mp 106–110 8C. 31P NMR (CDCl3): dZ9.8. HR-MS:
m/zZ306.11548 [C18H17N3PC], Calcd 306.11546.
8. Speziale, A. J.; Bissing, D. E. J. Am. Chem. Soc. 1963, 26,
3878–3884.
4.4.7. Benzyl(2-thienyl)diphenylphosphonium bromide.
Yield 89%; mp O2608C. 31P NMR (CDCl3): dZ18.4.MS
(FAB): m/zZ439.3.
9. Wadsworth, W. E. Org. React. 1977, 25, 73–253.
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4405–4408.
4.4.8. Benzyl(2-furyl)diphenylphosphonium bromide.
Yield 84%; mp 260–263 8C. 31P NMR (CDCl3): dZ13.2.
HR-MS: m/zZ343.12403 [C23H20OPC], Calcd 343.12463.
12. Tsukamoto, M.; Schlosser, M. Synlett 1990, 605–608.
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4.4.9. Benzyltris(2-furyl)phosphonium bromide. Yield
90%; mp 218–220 8C. 31P NMR (CDCl3): dZK12.7. HR-
MS: m/zZ323.08282 [C19H16O3PC], Calcd 323.08316.
¨
17. Schroder, U.; Berger, S. Eur. J. Org. Chem. 2000, 2601–2604.
4.4.10. Benzyl(benzo[b]furyl)diphenylphosphonium
bromide. Yield 98%; mp 165–168 8C. 31P NMR (CDCl3):
dZ14.5. HR-MS: m/zZ393.140347 [C27H22OPC], Calcd
393.14028.
¨
18. Wittig, G.; Schollkopf, U. Chem. Ber. 1954, 87, 1318–1330.
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4.4.11. Benzyltris(benzo[b]furyl)phosphonium bromide.
Yield 83%; mp 180–185 8C. 31P NMR (CDCl3): dZK6.8.
HR-MS: m/zZ473.1303 [C31H22O3PC], Calcd 473.13011.
21. Schlosser, M.; Schaub, B. Chimia 1982, 36, 396–397.
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