New reactions and reactive intermediates in the pyrolysis of cyclic phosphonium ylides

Article abstract of DOI:10.24820/ark.5550190.p010.208

Pyrolysis, either neat or in diphenyl ether solution, results in the conversion of both 4-triphenylphosphoranylidenetetrahydrofuran-2,3,5-trione and 4-triphenylphosphoranylidenetetrahydrothio-phene-2,3,5-trione into 3,5-bis(triphenylphosphoranylidene)cyclopentane-1,2,4-trione. These reactions involve extrusion of CO₂ or COS to give 3-triphenylphosphoranylidenecyclopropane-1,2-dione which further loses CO to give triphenylphosphoranylideneketene. The precise way in which these two reactive phosphorus compounds combine to give the observed product has been examined by chemical and isotopic labelling studies. Cyclotrimerization of triphenylphosphoranylideneketene upon thermolysis in diphenyl ether has also been observed for the first time. The erroneous literature interpretation of the ¹³C NMR spectrum for triphenylphosphoranylideneketene is corrected.

Full text of DOI:10.24820/ark.5550190.p010.208

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Arkivoc 2017, part iii, 293-301
Organic Chemistry
New reactions and reactive intermediates in the pyrolysis of cyclic
phosphonium ylides
R. Alan Aitken,*a Vidar Bjørnstad,b Tracy Massil,a Jan Skramstad,b and Robert J. Youngc
a EaStCHEM School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, UK
b Department of Organic Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315, Oslo, Norway
c GSK Medicines Research Centre, Gunnels Wood Road, Stevenage, SG1 2NY, UK
Email: raa@st-and.ac.uk
Dedicated to Professor Oleg A. Rakitin on the occasion of his 65th birthday
Received 06-04-2017
Accepted 07-13-2017
Published on line 08-31-2017
Abstract
Pyrolysis, either neat or in diphenyl ether solution, results in the conversion of both 4-
triphenylphosphoranylidenetetrahydrofuran-2,3,5-trione and 4-triphenylphosphoranylidenetetrahydrothio-
-phene-2,3,5-trione into 3,5-bis(triphenylphosphoranylidene)cyclopentane-1,2,4-trione. These reactions
involve extrusion of CO2 or COS to give 3-triphenylphosphoranylidenecyclopropane-1,2-dione which further
loses CO to give triphenylphosphoranylideneketene. The precise way in which these two reactive phosphorus
compounds combine to give the observed product has been examined by chemical and isotopic labelling
studies. Cyclotrimerization of triphenylphosphoranylideneketene upon thermolysis in diphenyl ether has also
been observed for the first time. The erroneous literature interpretation of the 13C NMR spectrum for
triphenylphosphoranylideneketene is corrected.
O
O
Ph2O, 260
°
or neat, 250
C
Ph3P
O
°
C
Ph3P
PPh3
X
−
COX
O
O
O
X = O, S
Keywords: Cyclic ylides, pyrolysis, phosphoranes, reactive intermediates
DOI: https://doi.org/10.24820/ark.5550190.p010.208
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Introduction
We recently introduced the value of the two-bond NMR coupling constant 2JP–CO as a diagnostic parameter for
the reactivity of β-oxophosphonium ylides 1 towards thermal extrusion of Ph3PO to give alkynes (Scheme 1).1
All known ylides that do undergo such extrusion to form an alkyne have 2JP–CO < 11 Hz, while those with values
above this do not. The latter group includes formyl and alkoxycarbonyl ylides (R2 = H, O-Alkyl) where the
failure of extrusion as well as the high J value is associated with the C=P and C=O functions being aligned anti
rather than syn to one another. However the rule also applies to ylides where the functions are constrained
syn such as the cyclic examples 2 and 3. With a J value of 11.3 Hz, 2 does not undergo extrusion under any
circumstances whereas 3 with a value of 3.7 Hz readily eliminates Ph3PO upon flash vacuum pyrolysis (FVP) at
750 °C to give products derived from cyclohexyne.1
R1
O
O
R1
R2
Ph3P
O
Ph3P
O
heat
Ph3P
O
Ph3P
O
?
X
X
– Ph3PO
R2
O
O
3
1
2
4
5
X = O
X = S
Scheme 1
2
A wide range of heterocyclic phosphonium ylides are known,2,3 and among those with published JP–CO
values, our attention was drawn to ylide 4 and its thio-analogue 5. These are readily prepared from
2
dichloromaleic (thio)anhydride and, with JP–CO values for the ketone carbonyl of 8 Hz4 and 7 Hz5 respectively,
seemed possible precursors for thermal generation of dehydromaleic anhydride and its thio analogue. Such
heterocyclic alkynes are elusive and highly reactive compounds that have attracted considerable interest,6 and
we describe here for the first time the thermal decomposition of 4 and 5.
Results and Discussion
In fact, FVP of both 4 and 5 gave disappointing results, with the only products obtained in the cold trap being a
trace of Ph3PO in the first case and a mixture of Ph3PO, Ph3PS, Ph3P and triphenylphosphoranylideneketene 11
in the second. However there was also substantial decomposition in the inlet tube and analysis of the residue
from attempted FVP of 4 at 200 ˚C showed it, most surprisingly, to be the trioxo bis(ylide) 6 (10%) previously
reported by Bestmann and co-workers.7 This must obviously be formed by an intermolecular process and,
once this was clear, much better results could be obtained by heating 4 or 5 in a small volume of boiling
diphenyl ether (bp 260 ˚C) for 24 h or in the case of 4 by heating the neat material at 250 ˚C for 3 h. In each
case the major product was the trioxo bis(ylide) 6 which was readily identified by its characteristic NMR
spectra [δP +9.0; δC 196.4 (t, J 10) and 187.6 (dd, J 20, 6)] and also by comparison with an authentic sample.
There are several mechanistic possibilities for the formation of this unexpected product (Scheme 2). It
seems likely that either CO2 or COS is first lost to give the triphenylphosphoranylidenecyclopropanedione 7
which may then dimerise probably by way of the dipolar ring-opened forms 9 or 10 to give 8 and this could
then lose CO to give 6. Of course the diradical intermediates corresponding to 9 or 10 could also be considered
but these seem less likely in view of the highly polar groups present. A second major possible route is for 7 to
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O
Ph2O, 260 °
or neat, 250
C
O
Ph3P
°
C
Ph3P
PPh3
X
O
O
O
O
Ph3P
O
O
O
O
– CO
6
4
or
5
PPh3
H2O2
KOH7
– COX
Ph3P
8
x 2
O
O
O
O
+
–
Ph3P
O
Ph3P
O
PPh3
or
7
O
10
[O]7
O
Ph3P
O
O
+
–
O
– CO
13
Ph3P
O
PPh3
O
9
+
neat, 200 °C
Ph3P C C O
PPh3
12
11
Scheme 2
lose CO to give 11 which can then undergo direct cycloaddition with 9 or 10 to give 6. Evidence in favour of the
latter pathway was provided by heating an equimolar mixture of 4 and 118,9 in boiling diphenyl ether for 2 h,
which led to formation of 6 as the only phosphorus-containing product. Interestingly a control experiment of
heating 11 alone for 30 min under these conditions led to formation of its cyclic trimer 12 (δP +13.9) which was
previously obtained by Bestmann and co-workers by a different route.7 We can thus conclude that whatever
intermediate (9 or 10) is being formed by 4 in its decomposition to 6 reacts efficiently with added 11 to give
the same product. Compound 6 was previously prepared by HCl catalysed trimerisation of 11 to give 12 which
was then subjected to oxidation by 2-tosyl-3-phenyloxaziridine to afford the tetraoxo bis(ylide) 13. This
underwent oxidative ring-contraction upon treatment with hydrogen peroxide and KOH to afford 6. However
an additional lower-yielding route to 6 is the treatment of ethyl triphenylphosphoranylidenepyruvate 14 with
sodium hexamethyldisilazide to give the product in 4% isolated yield (Scheme 3).10 This was carried out in the
expectation of isolating 7 and, although the author proposed that 6 was formed by a different route not
involving 7, the new evidence presented here makes it likely that this route may also proceed by initial base-
induced elimination of ethanol from 14 to give 7 which thereafter reacts as shown in Scheme 2 to give 6.
O
O
O
Ph3P
PPh3
NaN(TMS)2
Ph3P
O
Ph3P
H
OEt
O
O
O
7
14
6
Scheme 3
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In order to differentiate between the possible modes of bond breaking in 7, we first attempted
chemical labelling by preparing a tri-p-tolyl analogue. We were unable to obtain the tri-p-tolyl analogue of 4
but the tri-p-tolyl analogue of 11, compound 17, was prepared as shown in Scheme 4. Ethyl bromoacetate was
reacted with tri-p-tolylphosphine11 to give the salt 15 and treatment of this with sodium hydroxide gave the
new stabilised ylide 16, which was fully characterised.
Br
O
O
O
p-Tol3P
NaNH2
OMe
NaOH
p-Tol3P C C O
p-Tol3P
Br
p-Tol3P
OMe
OMe
17
16
15
Scheme 4
When this was reacted with sodium amide in toluene a new product with characteristic NMR signals at
δP +5.3 and δC 142.6 (d, J 39, P=C=C=O) attributed to 17 was formed. However heating this under a wide
variety of conditions, either with or without added 4, did not lead to formation of a p-tolyl analogue of 6 and
tri-p-tolylphosphine oxide was the only identifiable product derived from 17.
We then resorted to isotopic labelling and prepared 20% 13C-labelled methoxycarbonyl ylide 19 by the
literature method12 starting from labelled methyl bromoacetate via the phosphonium salt 18 (Scheme 5).
Br
O
C
O
C
O
C
Ph3P
NaNH2
OMe
NaOH
*
Ph3P C C O
*
*
Ph3P
*
Br
Ph3P
OMe
OMe
20
19
18
* = 20% 13
C
Scheme 5
When this was heated in diphenyl ether with 4 the resulting sample of the trioxo bis(ylide) proved to
be the isomer 21 with the label at the isolated carbonyl (ca. 20 x enhancement of signal at C 196.4 compared
δ
to δC 187.6) and not the isomer 22, thus showing conclusively that it is the intermediate 9 or its diradical
analogue rather than 10 which reacts with 11 to give 6.
O
O
*
O
Ph3P
O
Ph3P
PPh3
Ph3P
PPh3
Ph2O, 260 °C
*
+ Ph3P C C O
O
not
*
20
O
O
O
O
O
22
21
4
Scheme 6
We cannot however exclude the possibility that, in the absence of added 11, formation of 6 is by
dimerisation of an intermediate, probably 9, to give 8 which then loses CO. Whichever mechanism is
operating, it should be possible to trap the intermediates with other added dipolarophiles leading to new
types of stabilised ylides and this is currently being examined.
Finally, in the course of this study we have become aware of conflicting information regarding the 13C
NMR spectrum of triphenylphosphoranylideneketene 11. This compound was first reported by Birum and
Matthews,13,14 but was later obtained in a more convenient way by base-induced elimination of methanol
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from methoxycarbonylmethylenetriphenylphosphorane as shown in Scheme 5.8,9 There are three literature
reports of its 13C NMR spectrum and there is widespread agreement on the rather remarkable chemical shifts
of the ketene carbons [δC –10.5 (d, J 189, P=C), 145.6 (d, J 43, P=C=C=O)]. The situation for the P-phenyl signals
is however confused as summarised in Table 1 with erroneous data shaded. Analysis of the 13C NMR spectra of
nearly 200 stabilised triphenylphosphonium ylides,1 shows that the P-phenyl signals form a regular pattern
with the ipso-C having a large coupling constant to phosphorus of between 90 and 100 Hz, the ortho-C coming
around 132-133 ppm with a coupling constant of 9–11 Hz, the meta-C coming around 128–129 ppm with a
larger coupling constant of 12–13 Hz, and the para-C around 132 ppm with a coupling constant of 0–2 Hz. The
main problem in correctly interpreting the spectrum of 11 is the close proximity of the ortho and para signals
and partial or complete overlap of one half of the ortho doublet with the para singlet giving two apparent
singlets. The previous reports have all made this mistake,9,15,16 with two also interchanging the ortho and meta
signals.9,16 The corrected data as obtained in our work is shown and this agrees with the revised data newly
acquired by one of the original authors.17
Table 1. 13C NMR data for the P-phenyl group of 11
Frequency Solvent ipso-C
ortho-C
129.6 (d, J 98.5) 128.8 (d, J 12.9) 132.3 (s)
129.6 (d, J 98) 132.3 (s) 128.8 (d, J 13)
129.6 (d, J 98.7) 128.8 (d, J 12.9) 132.3 (s)
meta-C
para-C
Ref.
100 MHz
125 MHz
100 MHz
75 MHz
CDCl3
C6D6
132.2 (s)
132.2 (s)
132.2 (s)
132.1 (s)
132.2 (s)
9
15
CDCl3
CDCl3
CDCl3
16
129.6 (d, J 99)
129.6 (d, J 99)
132.2 (d, J 11)
132.3 (d, J 11)
128.8 (d, J 13)
128.9 (d, J 13)
this work
17
125 MHz
Conclusions
The result of the isotopic labeling experiment indicates that formation of the trioxo bis(ylide) 6 from both 4
and 5 proceeds by initial loss of CO2 or COS respectively to give ylide 7. This undergoes both loss of CO to
afford the ketene ylide 11 and ring-opening to the dipolar species 9 which then combine in a cycloaddition
reaction to form 6. Purely thermal cyclotrimerisation of the ketene ylide 11 to give 12 has been observed and
correct interpretation of the 13C NMR data for 11 shows this to be consistent with the usual pattern of
chemical shifts and P–C coupling constants for triphenylphosphonium ylides.
Experimental Section
General. Melting points were determined using a Reichert hot-stage microscope and are uncorrected. NMR
1
spectra were recorded using a Varian Gemini 2000 instrument at 300 MHz for H, 75 MHz for 13C, and 121
1
MHz for 31P. All spectra were recorded on solutions in CDCl3 with internal Me4Si as reference for H and 13C
and external H3PO4 as reference for 31P. Chemical shifts (δ) are given in ppm to high frequency from the
reference and coupling constants (J) are in Hz. Flash vacuum pyrolysis was performed using the set-up
previously described.18 Compounds 4,4 5,5 and 118,9 were prepared by the published methods.
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Flash Vacuum Pyrolysis of 4. FVP of 4 (36.5 mg) at 400 °C and 10–2 Torr gave a brown solid at the furnace exit
that was mainly Ph3PO (δP +29.2) and gaseous products in the cold trap that condensed as a white solid.
Addition of methanol to the cold trap followed by warming to RT did not result in trapping of any reactive
products and no phosphorus compounds were present.
Repeat FVP of 4 (320 mg) at 200 °C and 10–2 Torr gave a white solid at the furnace exit that proved to be a 5:3
mixture of Ph3PO and Ph3P (δP –5.0). In the inlet tube a pink solid was left which proved to be 3,5-
1
bis(triphenyphosphoranylidene)cyclopentane-1,2,4-trione 6 (22 mg, 9%). H NMR: δH 7.40–7.75 (30H, m). 13C
NMR: δC 67.6 (dd, J 112, 4, P=C), 124.6 (d, J 92, C-1 of Ph), 128.5 (d, J 13, C-3 of Ph), 132.3 (d, J 2, C-4 of Ph),
134.1 (d, J 11, C-2 of Ph), 187.6 (dd, J 20, 6, 1,2-CO), 196.4 (t, J 10, 4-CO). 31P NMR: δP +9.0 (Lit.,7 +8.8).
Flash Vacuum Pyrolysis of 5. FVP of 5 (29 mg) at 600 °C and 10–2 Torr gave a white solid at the furnace exit
1
which was shown by H and 31P NMR to consist of small amounts of Ph3PO, Ph3PS (δP +43.6) and mainly
ketenylidenetriphenylphosphorane 11 (δP +5.7).
Solution Pyrolysis of 4. A solution of 4 (44 mg) in diphenyl ether (2 mL) was heated under reflux for 30 min.
1
The solvent was removed by kugelrohr distillation to leave a dark coloured residue shown by H and 31P NMR
to consist mainly of bis(ylide) 6 together with a little Ph3PO.
Solution Pyrolysis of 5. A solution of 5 (20 mg) in diphenyl ether (1 mL) was heated under reflux for 4 h. The
1
solvent was removed by Kugelrohr distillation to leave a dark coloured residue shown by H and 31P NMR to
consist mainly of bis(ylide) 6 together with a little Ph3PO.
Neat Pyrolysis of 4. A sample of 4 (20 mg) was heated in a kugelrohr distillation apparatus at 250 °C for 1.5 h.
The resulting dark coloured solid was shown by 1H and 31P NMR to consist mainly of bis(ylide) 6.
Neat Pyrolysis of 5. A sample of 5 (10 mg) was heated in a kugelrohr distillation apparatus at 250 °C for 3 h.
The resulting dark coloured solid was shown by 1H and 31P NMR to consist mainly of unchanged 5.
Solution Pyrolysis of 4 and 11 Together. A solution of 4 (40 mg, 0.11 mmol) and 11 (32 mg, 0.11 mmol) in
diphenyl ether (1 mL) was heated under reflux for 2 h. The resulting dark coloured solution was shown by 31P
NMR to contain 6 (20%) in addition to Ph3PO and Ph3P.
Solution Pyrolysis of 11. A solution of 11 (46 mg) in diphenyl ether (1.5 mL) was heated under reflux for 30
min. The dark solution was shown by 31P NMR to contain Ph3PO, Ph3P and 2,4,6-
tris(triphenylphosphoranylidene)cyclohexane-1,3,5-trione 12 (δP +13.9).
Neat Pyrolysis of 11. Compound 11 (250 mg) was heated in a kugelrohr distillation apparatus at 200 °C for 3 h.
After this time a vacuum was applied and a colourless oil distilled off which proved to be Ph3PO. The dark
residue was mainly 2,4,6-tris(triphenylphosphoranylidene)cyclohexane-1,3,5-trione 12 (60%). 13C NMR: δC 74.3
(dt, J 116, 10, P=C), 127.6 (d, J 12, C-3 of Ph), 128.3 (d, J 91, C-1 of Ph), 130.4 (C-4 of Ph), 133.6 (d, J 10, C-2 of
Ph), 184.3 (CO). 31P NMR: δP +13.4 (Lit.,7 +13.7).
(Methoxycarbonylmethyl)tri-p-tolylphosphonium bromide (15). To a stirred solution of tri-p-tolylphosphine11
(2.0 g, 6.6 mmol) in dry toluene (40 mL) a solution of methyl bromoacetate (1.0 g, 6.6.mmol) in dry toluene
(20 mL) was added dropwise. After stirrring at RT for 18 h, the resulting white precipitate was filtered off and
1
washed with diethyl ether to give the product (2.6 g, 87%) as a white powder, mp 187–189 °C. H NMR: δH
2.48 (9H, s, Me), 3.59 (3H, s, OMe), 5.37 (2H, d, J 12, P-CH2), 7.39–7.50 (6H, m), 7.66–7.80 (6H, m). 13C NMR: δC
21.8 (Me), 33.0 (d, J 58, P-CH2), 53.2 (OMe), 114.6 (d, J 92, C-1 of Tol), 130.9 (d, J 14, C-3 of Tol), 133.7 (d, J 11,
C-2 of Tol), 146.3 (d, J 3, C-4 of Tol), 165.2 (C=O). 31P NMR: δP +20.2.
(Methoxycarbonylmethylene)tri-p-tolylphosphorane (16). A solution of the salt 15 (2.5 g, 5.5 mmol) in water
(20 mL) was stirred rapidly while NaOH (0.22 g, 5.5 mmol) was added. The resulting mixture was extracted
with ethyl acetate (3 x 10 mL) and the extract was dried and evaporated to give the title compound (1.6 g,
77%) as a yellow solid, mp 85–87 °C. IR (Nujol, νmax, cm–1): 1728, 1271, 1118, 808. 1H NMR: δH 2.38 (9H, s, Me),
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3.40 (1H, br s, =CH), 3.52 (3H, s, OMe), 7.20–7.36 (6H, m), 7.48–7.65 (6H, m). 13C NMR: δC 21.4 (Me), 30.4 (d, J
119, P=C), 49.8 (OMe), 123.5 (d, J 96, C-1 of Tol), 129.4 (d, J 13, C-3 of Tol), 132.8 (d, J 10, C-2 of Tol), 142.5 (C-
4 of Tol), 170.8 (d, J 11, C=O). 31P NMR: δP +16.7. MS (CI) m/z 377 (M+, 34%), 361 (6), 345 (24), 321 (100), 305
(11). HRMS: C24H26O2P (M+H) requires 377.1670. Found: 377.1665.
Tri-p-tolylphosphoranylideneketene (17). A solution of ylide 16 (0.95 g, 2.5 mmol) was stirred in dry toluene
(10 mL) under nitrogen while sodium amide (0.34 g, 8.7 mmol) was added. The mixture was heated under
reflux under nitrogen for 24 h then cooled and filtered under nitrogen. Evaporation of the filtrate gave a solid
containing tri-p-tolylphosphine oxide (δP +29.4) and the title product (0.4 g, 46%). 13C NMR: δC 142.6 (d, J 39,
P=C=C=O). 31P NMR: δP +5.3.
Attempted Pyrolysis of 17 with 4 or on its own. All attempts at solution pyrolysis of 17, either with or without
added 4, in boiling diphenyl ether, as well as neat pyrolysis of 17 in a kugelrohr oven at 170 °C, gave tri-p-
tolylphosphine oxide as the only significant phosphorus-containing product.
13C-Labelled Methyl Bromoacetate. Thionyl chloride (9.3 g, 5.7 mL, 78 mmol) was added dropwise to a
solution of 20% 13C-CO-labelled bromoacetic acid (5.0 g, 36 mmol) in methanol (50 mL) and the mixture was
stirred at RT for 2 h. Evaporation followed by distillation at atmospheric pressure gave the product as a
colourless liquid, bp 135–140 °C. 1H NMR: δH 2.92 (3H, s, OMe), 2.95 (2H, s, CH2). 13C NMR: δC 25.5 (CH2), 53.2
(OMe), 167.7 (CO, 20 x enhanced compared to unlabelled material).
13C-CO Labelled (Methoxycarbonylmethylene)triphenylphosphonium Bromide (18). A solution containing
labelled methyl bromoacetate (5.0 g, 33 mmol) and triphenylphosphine (8.56 g, 33 mmol) in dry toluene (30
mL) was stirred at RT for 48 h. The resulting solid was filtered off and dried to give the title product (11.03 g,
81%) as colourless crystals, mp 161–162 °C (Lit.,19 163 °C). 13C NMR: δC 32.7 (d, J 57, CH2), 53.2 (OMe), 117.5
(d, J 89, C-1 of Ph), 130.1 (d, J 13, C-3 of Ph), 133.7 (d, J 11, C-2 of Ph), 135.0 (d, J 2, C-4 of Ph), 164.8 (d, J 3,
C=O, 20 x enhanced). 31P NMR: δP +22.8 (Lit.,20 +20.3).
20% 13C-CO Labelled (Methoxycarbonylmethylene)triphenylphosphorane (19). A solution of the salt 18 (4.0
g, 9.6 mmol) in water (30 mL) was stirred rapidly while NaOH (0.39 g, 9.6 mmol) was added. The resulting
mixture was extracted with ethyl acetate (3 x 10 mL) and the extract was dried and evaporated to give the title
compound (2.6 g, 81%) as a white solid, mp 163–164 °C (Lit.,19 162–163 °C). 13C NMR: δC 29.7 (d, J 127, P=C),
49.6 (OMe), 127.7 (d, J 91, C-1 of Ph), 128.6 (d, J 12, C-3 of Ph), 131.8 (C-4 of Ph), 132.8 (d, J 10, C-2 of Ph),
171.5 (d, J 12, C=O, 20 x enhanced). 31P NMR: δP +18.2 (Lit.,13 +17.6).
20% 13C-CO Labelled Triphenylphosphoranylideneketene (20). A solution of labelled ylide 19 (1.4 g, 4.2 mmol)
was stirred in dry toluene (7 mL) under nitrogen while sodium amide (0.40 g, 10 mmol) was added. The
mixture was heated under reflux under nitrogen for 48 h then cooled and filtered under nitrogen. Evaporation
of the filtrate gave a solid containing a 1:2 mixture of triphenylphosphine oxide (δP +29.2) and the title product
(0.63 g, 50%). 13C NMR: δC –10.5 (d, J 189, P=C), 128.8 (d, J 13, C-3 of Ph), 129.6 (d, J 99, C-1 of Ph), 132.1 (C-4
of Ph), 132.2 (d, J 11, C-2 of Ph), 145.6 (d, J 43, P=C=C=O, 20 x enhanced). 31P NMR: δP +5.6 (Lit.,21 +5.4).
Solution Pyrolysis of 4 and 20 Together. A solution of 4 (210 mg, 0.56 mmol) and 20 (170 mg, 0.56 mmol) in
diphenyl ether (4 mL) was heated under reflux for 2 h. The resulting dark coloured solution was shown by 31P
NMR to contain (a labelled version of) 6 (δP +9.1) in addition to Ph3PO and Ph3P. In the carbonyl region of the
13C NMR spectrum the signal at δC 196.0 (t, J 9, 4-CO) showed a 20 x enhancement compared to the signal at
δC 187.2 (dd, J 20, 6, 1,2-CO), i.e. corresponding to structure 21 and not 22.
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Acknowledgements
We are grateful to the late Professor H. J. Bestmann (Erlangen) for helpful discussions and for providing
authentic samples of compounds 6 and 12, to Professor Rainer Schobert (Bayreuth) for confirming our
conclusions on the published 13C NMR data for 11 and to EPSRC and Glaxo-Wellcome (now GSK) for their
support of this work through a CASE Studentship to TM.
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