Kinetics and mechanism of gas-phase pyrolysis of ylides. Part 3. 1 Thermal reactivity of α-carbonyl- and thiocarbonyl-stabilized methylenetriphenylphosphoranes

Article abstract of DOI:10.1002/poc.1752

Fourteen ketone/thione-stabilized triphenylphosphonium methylides were subjected to conventional gas-phase and flash vacuum pyrolysis (FVP). The kinetics of the first-order thermal gas-phase reactions of all these compounds were investigated over 360-653K temperature range. The values of the Arrhenius logA and energy of activation of these ylides averaged 11.52±0.34s ^-1 and 133.20±3.14kJmol^-1, respectively. The products of sealed-tube (static) and FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate constants [k (s^-1)] of the substrates at 500K were calculated and used to substantiate the proposed mechanism of pyrolysis, and to rationalize the thermal gas-phase reactivities of the ylides under study.

Full text of DOI:10.1002/poc.1752

Research Article
Received: 11 January 2010,
Revised: 31 March 2010,
Accepted: 10 May 2010,
Published online 15 July 2010 in Wiley Online Library: 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1752
Kinetics and mechanism of gas-phase
pyrolysis of ylides. Part 3.1 Thermal reactivity
of a-carbonyl- and thiocarbonyl-stabilized
methylenetriphenylphosphoranes
a
b
b
*
Rasha F. Al-Bashir , Nouria A. Al-Awadi and Osman M. E. El-Dusouqui
Fourteen ketone/thione-stabilized triphenylphosphonium methylides were subjected to conventional gas-phase and
flash vacuum pyrolysis (FVP). The kinetics of the first-order thermal gas-phase reactions of all these compounds were
investigated over 360–653 K temperature range. The values of the Arrhenius log A and energy of activation of these
ylides averaged 11.52 W 0.34 sS1 and 133.20 W 3.14 kJ molS1, respectively. The products of sealed-tube (static) and
FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate
constants [k (sS1)] of the substrates at 500 K were calculated and used to substantiate the proposed mechanism of
pyrolysis, and to rationalize the thermal gas-phase reactivities of the ylides under study. Copyright ß 2010 John Wiley
& Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: gas-phase; kinetics; mechanism; pyrolysis; ylides
pyrolysis of these ylides and their arsonium analogs have
INTRODUCTION
shown that the stabilized monosubstituted phosphonium ylide is
less reactive than the disubstituted counterparts, and that the
monosubstituted arsonium analog changes mechanism from a
dipolar transition state (TS) to a pathway involving a reactive
carbene intermediate.[1,16–20] The present study investigates the
kinetics and mechanism of the thermal gas-phase elimination
reactions of fourteen a-carbonyl- and thiocarbonyl-stabilized
triphenylphosphonium methylides to assess the magnitude of
change in reactivity with change in the structure of the stabilizing
ketone/thione moiety. Seven of these substrates have an addi-
tional phenyl substituent on the ylide carbon, one has a methyl
group, and four are unsubstituted. The study further combines
analysis of the products of reaction from sealed-tube (static)
pyrolysis (STP) and flash vacuum pyrolysis (FVP) with results from
a kinetic investigation of these reactions using STP. It is of interest
to note that in both STP and FVP reactions no reagents, solvents,
or catalysts are used, as a result of which the thermal gas-phase
reactivities and the structural properties of the ylides may be
deemed intrinsic.
The chemistry of ylides is rich and diverse covering all aspects of
synthesis, structure and properties, theoretical calculations,
stability and reactivity, reactions, qualitative and quantitative
correlations, and applications.[1–17] The structure of phosphorus
ylides is characterized by a P/C bond which is highly polar with
partial double bond character.[15] The barrier to rotation of this
bond is low (4.5 kJ molꢀ1) and the bond order is >1 and <2. The
electron pair on the carbanion forms a back-bond by overlap with
an unoccupied orbital at the phosphorus moiety, which gives
the ylide bond its ylene character and results in an approximately
planar geometry around carbon. These considerations allow
the structure of the unsubstituted triphenylphosphonium meth-
ylide to be represented as either A (dipolar ylide form) or B (ylene
form), but (A) is considered a more favorable description, and
hence an appropriate model for the discussion and rationaliz-
ation of ylide chemical reactivity.[15,17]
*
Correspondence to: N. A. Al-Awadi, Chemistry Department, Kuwait University,
P. O. Box 5969, Safat 13060, Kuwait.
E-mail: n.alawadi@ku.edu.kw
Replacement of one of the (H) atoms particularly by an
electron-withdrawing group (EWG) leads to molecular stability,
and the planar conformation at (C) provides the ideal geometry
for conjugation and hyperconjugation effects to dominate the
molecular reactivity of stabilized triphenylphosphonium methy-
lides. Our pioneering studies on the kinetics of the gas-phase
a
b
R. F. Al-Bashir
Applied Science Department, College of Technological Studies, Public
Authority for Applied Education and Training, Kuwait
N. A. Al-Awadi, O. M. E. El-Dusouqui
Chemistry Department, Kuwait University, Safat 13060, Kuwait
J. Phys. Org. Chem. 2011, 24 311–319
Copyright ß 2010 John Wiley & Sons, Ltd.

R. F. AL-BASHIR, N. A. AL-AWADI AND O. M. E. EL-DUSOUQUI
RESULTS AND DISCUSSION
Pyrolysis: Products and reaction pathway
FVP was conducted at either 500 or 750 8C and a pressure of
10ꢀ2 Torr, for a residence time of the sample passing the reactor
hot zone estimated at ca. 10ꢀ3 s. The pyrolysates are collected in a
trap cooled in liquid nitrogen. The STP was carried out in a Pyrex
sealed-tube reactor at temperatures exceeding those required for
the complete pyrolysis of the ylides as established from kinetic
runs. The products of the thermal gas-phase elimination react-
ions by FVP and STP were analyzed and characterized; the two
techniques gave identical results for each of the 12 substrates
under investigation. With the exception of 2c,d, the pyrolysate
from each ylide consisted of two fragments only: an organoph-
osphorus fragment (Ph3PO or Ph3PS), and an alkyne or its
rearranged product (Scheme 1). The novel ylide 1a pyrolysed to
Scheme 2. TS of gas-phase pyrolysis pathway
the negative entropy of activation observed in this study and
its predecessor is indicative of the extent of conformational
pre-organization and an ordered TS.[17]
The degree of non-bonding interaction between (Pþ) and (Xꢀ),
—
—
S
—
the relative thermodynamic stability/liability of the C O/C
—
bond, the relative size of the oxide and sulfide moieties, the
inductive, conjugative and hyperconjugative effects of the ylide-
stabilizing group (R), and the presence or absence of an added
substituent (G) represent the major factors affecting the negative
charge on (X), and hence ylide relative reactivity. It is also useful to
recall that bonding in triphenylphosphonium ylides is analogous
to bonding in Ph3PX (X ¼O,S).[15,23,24]
— —
C CHPh) and
— —
give the symmetric 1,3-diphenylallene (PhCH
not its 1,3-diphenylpropyne precursor. The rearrangement of
pyrolysates into more stable tautomers is a common feature
—
—
of thermal gas-phase elimination reactions. The alkyne (PhC
—
CꢀNHPh) from ylides 1d and 1e also rearranged into the more
[21]
— —
stable ketenimine tautomer (PhCH
C
NPh).
It is possible
— —
that symmetry factors may contribute to the formation of
these products. Characterization of the ylides and their pyroly-
sates included elementary analysis, MS/GC-MS spectrometric and
Kinetics and thermal gas-phase reactivity
The present reactions followed a first-order kinetic equation, and
the kinetic results for the fourteen ylides under study are given in
Table 1. The rate constant of each substrate at each reaction
temperature is an average from at least three runs in agreement
to within ꢁ2% rate spread. Rates were obtained by monitoring
the disappearance of substrates. For reliable kinetic data, the
pyrolysis of each substrate was followed over a temperature
range of ca. 48 ꢁ 8 K and ꢂ95 ꢁ 4% reaction progress.[1,17–20,23,24]
The kinetic data from Table 1 were used to calculate the Arr-
henius log A (sꢀ1) factor, the energy of activation [Ea (kJ molꢀ1)],
the rate constant [k (sꢀ1)] at 500 K, and the entropy of activation
[DS (J Kꢀ1 molꢀ1)]. The Arrhenius plots were strictly linear over >
92% reaction, and the correlation coefficients were in the order of
0.998 ꢁ 0.001. Typical plots are shown in Fig. 1 for substituted
1
FT-IR and H/13C NMR spectroscopic analyses.
Besides, Scheme 1 contains ylides (4a,b), included here to
extend the range of structure-reactivity correlation; the kinetics
and the analysis of the products of pyrolysis of these two ylides
have been reported in an earlier communication.[17]
A mechanism is available (Scheme 2) which provides an
adequate account of the observed products of pyrolysis of the
ylides under consideration. It involves conformational pre-organi-
zation of the ylide, and a four-membered TS leading to the
products of thermal elimination.[15–17] In the ylide form, the
—
a-ketone/thione (C X) group is arranged syn to the phos-
—
phonium moiety; and in the resonance dipolar enolate form, the
oxide/sulfide moiety is consequently arranged in a conformation
ideal for the subsequent extrusion of triphenylphosphine oxide
(Ph3PO) and its sulfide analog (Ph3PS).[9,15–22] The magnitude of
—
ylides 1a and 3 [G (Ph) and (CH ), respectively], and unsub-
—
3
stituted ylide 2a; the correlation coefficients in all three cases is
0.999. The values of the energy of activation and pre-exponential
factor indicate a homogeneous unimolecular thermal gas-phase
elimination reaction.[1,17,25–28] The limits of error in Table 1
represent the correlation statistical values. The rate constants
used for comparing thermal reactivities were calculated at 500 K
for two valid reasons: (i) this temperature lies well within the
range over which the kinetic measurements were made; (ii) the
rates calculated at this temperature allow the correlation and
comparison of molecular reactivity of the present and other
compatible ylides.[17]
The rate constants [k (sꢀ1)] calculated at 500 K for the 14
ketone/thione stabilized triphenylphosphonium methylides
under study are given in Fig. 2. The interpretation and rationali-
zation of these results follow the mechanism suggested for the
thermal gas-phase elimination of these ylides (Scheme 2).
Ylide 4a is ca. 22-times more reactive than 4b, while ylide 3 is
even more reactive than 2a by a factor of 347. In both cases,
higher reactivity is associated with additional substitution of (Ph)
and (CH3) at the ylide carbon, respectively. In the present study, all
the monosubstituted ylides display lower thermal reactivity than
the fully substituted triphenylphosphonium methylides. The
effect of the degree of substitution on the molecular stability of
Scheme 1. Ylides and elimination fragments
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 311–319

KINETICS AND MECHANISM OF GAS-PHASE PYROLYSIS OF YLIDES
Table 1. Rate constants (k), Arrhenius log A, and Ea, entropy of activation DS#, and rate constants at 500 K of gas-phase pyrolysis of
ketone/thione stabilized methylides 1, 2, and 3
Log A
(sꢀ1
104 k (500 K)
(sꢀ1
DS# (500 K)
Ylide
T (K)
104 k (sꢀ1
)
)
Ea (kJ molꢀ1
134.3 ꢁ 1.7
)
)
(J Kꢀ1 molꢀ1
)
1a
402.45
409.15
417.85
425.55
434.15
442.35
481.15
492.85
502.85
513.25
523.35
476.35
483.25
490.45
497.45
504.55
511.35
519.65
320.65
330.00
335.35
344.35
350.65
360.00
330.95
341.05
343.55
349.75
353.75
360.15
368.16
426.40
435.10
443.35
451.25
459.15
467.15
432.62
438.95
452.55
459.55
466.55
477.75
483.25
587.90
602.45
610.55
627.65
644.65
652.65
503.85
515.35
1.394
2.831
6.094
12.79
25.19
54.51
1.410
3.202
8.539
16.06
30.56
1.849
3.443
4.905
9.356
15.97
25.22
45.95
0.890
2.245
3.775
7.949
13.79
29.56
1.653
4.271
5.564
10.03
13.56
23.47
40.06
2.161
3.266
6.122
10.23
15.03
27.62
1.071
1.552
4.990
8.004
12.87
28.10
39.43
1.943
4.331
5.840
12.90
26.41
37.67
1.761
4.970
13.58 ꢁ 0.21
3560
ꢀ10.75
1b
1c
13.01 ꢁ 0.64
12.93 ꢁ 0.38
155.3 ꢁ 6.2
152.0 ꢁ 3.6
6.062
11.28
ꢀ23.01
ꢀ16.07
1d
1e
9.806 ꢁ 0.10
9.970 ꢁ 0.29
85.02 ꢁ 0.63
87.04 ꢁ 1.9
83920
75730
ꢀ61.50
ꢀ58.35
1f
9.033 ꢁ 0.46
11.20 ꢁ 0.21
104.0 ꢁ 3.9
121.7 ꢁ 1.8
149.5
114.0
ꢀ76.31
ꢀ34.74
1g
2a
2b
9.11 ꢁ 0.18
144.2 ꢁ 2.1
147.0 ꢁ 8,3
0.0112
1.552
ꢀ74.84
ꢀ28.15
11.55 ꢁ 0.82
(Continues)
J. Phys. Org. Chem. 2011, 24 311–319
Copyright ß 2010 John Wiley & Sons, Ltd.
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R. F. AL-BASHIR, N. A. AL-AWADI AND O. M. E. EL-DUSOUQUI
Table 1. (Continued)
Log A
(sꢀ1
104 k (500 K)
(sꢀ1
DS# (500 K)
(J Kꢀ1 molꢀ1
)
Ylide
T (K)
104 k (sꢀ1
)
)
Ea (kJ molꢀ1
)
)
524.25
531.05
538.45
545.55
496.25
506.15
514.75
524.15
533.40
542.65
493.45
507.55
515.65
522.85
529.55
536.95
549.35
478.05
486.15
499.25
506.25
513.15
526.15
532.35
453.70
462.60
471.40
479.35
488.60
497.75
500.25
511.05
521.15
533.35
544.25
9.043
12.33
19.68
26.58
1.962
4.522
7.605
14.09
22.54
37.04
0.949
2.514
4.931
7.918
12.87
20.96
47.23
0.611
1.224
3.695
6.232
10.81
28.56
47.66
3.420
6.100
10.33
20.22
38.96
66.54
3.384
7.406
11.74
25.19
45.21
2c
11.05 ꢁ 0.46
12.75 ꢁ 0.17
139.8 ꢁ 4.6
2.796
1.517
ꢀ37.63
ꢀ51.29
2d
159.6 ꢁ 1.7
168.5 ꢁ 1.2
3
14.19 ꢁ 0.12
3.890
ꢀ22.55
4a
4b
11.37 ꢁ 0.40
10.29 ꢁ 0.40
129.1 ꢁ 3.7
131.7 ꢁ 4.0
76.28
3.442
ꢀ31.55
ꢀ52.21
phosphonium ylides has been reported.[15] The results of the
present study further confirm the importance of conjugative and
hyperconjugative effects in promoting ylide reactivity. These
substituent effects acquire special significance in the context of
gas-phase pyrolysis, since in gas phase reactions, there are no
solvation effects to help stabilize the incipient elimination
fragments or their precursors, and hence reactivity is primarily
dependent on intrinsic structural effects. For example, C—H
hyperconjugation involving the sp3 carbon of the methylene
moiety inserted between the carbonyl and phenyl groups of ylide
4a has resulted in an increase in the reactivity of ylide 1a over
4a by a factor of 46.7. A more dramatic rate enhancement is
observed when conjugation involves the delocalization of lone
pairs of electrons: ylides 1d and 1e are ca. 103-fold more reactive
than 4a, and are also 23.4- and 21.3-fold more reactive than 1a. In
all cases, the overall higher thermal reactivities of ylides 1d and
1e are associated with the lone pair of electrons on (N) being
available for conjugative delocalization leading to a substantial
increase in negative charge at the oxide/sulfide centers of
the ylides (Scheme 2). The lower rates of thermal gas-phase
3.6
2.4
1.2
0
-1.2
-2.4
-3.6
-4.8
-6
-7.2
-8.4
-9.6
-10.8
1.432 1.532 1.632 1.732 1.832 1.932 2.032 2.132 2.232 2.332 2.432 2.532
1000/T
Figure 1. Arrhenius plots of log k versus 103 Tꢀ1 for ylides 1a (&, inter-
cept ¼ 13.58, slope¼ ꢀ7.014, r2 ¼ 0.999); 2a (~, intercept ¼ 9.11, slope ¼
ꢀ7.530, r2 ¼ 0.999); 3 (*, intercept ¼ 14.19, slope ¼ ꢀ8.803, r2 ¼ 0.999)
wileyonlinelibrary.com/journal/poc
Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 311–319

KINETICS AND MECHANISM OF GAS-PHASE PYROLYSIS OF YLIDES
Ph
Ph
C
Ph
C
Ph
Ph3P
C
Ph3P
C
Ph3P
C
Ph3P
C
C
CPh
C
CH=CHPh
NHPh
C
CH2Ph
O
O
O
O
1a: 3.560 x 103
1b: 6.062
1c: 11.28
1d: 8.342 x 104
H
Ph
C
Ph
C
Ph
Ph3P
CH2(CH2)4CH3
C
Ph3P
CH2(CH2)4CH3
Ph3P
Ph3P
C
C
C(CH3)3
C
C
C
NHPh
O
O
S
S
1f: 1.495 x 102
1g: 1.140 x 102
2a: 1.120 x 10-2
1e: 7.573 x 104
H
C
H
H
C
Ph3P
C
Ph3P
C
Ph3P
C
CH3
C
CH3
C(CH3)3
S
O
S
2b: 1.552
2c: 2.796
2d: 1.517
H
C
CH3
C
Ph
Ph3P
Ph3P
Ph3P
C
C
Ph
C
C(CH3)3
C
Ph
O
O
O
3: 3.890
4a: 76.28
4b: 3.442
Figure 2. Rate constants [104 k (sꢀ1)] at 500 K of ylides 1–4
elimination reactions of substrates 1b and 1c compared with 1a
is attributed to the inductive electron-withdrawing effects asso-
ciated with the structural change to the sp2 and sp hybridized
states of the carbon atoms of the multiple bonds, reinforcing the
electron-withdrawing power of the phenyl group.
moiety, its greater nucleophilicity, and the soft-base character
arising from the negative charge being diffused over the large
size of the (S) anion. Besides, the positive charge on phosphorus
(Pþ) would help the development of partial positive charge and
its delocalization over the incipient phenyl group, and the latter
would thus be expected to behave as soft acid. Size, proximity,
and the soft acid/base character emerge as factors in favor of
an attack by the sulfide center on the phenyl group, a process
reminiscent of the behavior of the phenyldiarsonium cation in its
replacement reactions.
The effect of the relative thermodynamic stability and the
—
—
consequent basicity of the C S/C O bonds towards the posi-
—
—
tively charged (Ph3Pþ) moiety (Scheme 2) is reflected in the rate
ratio of about one for the ylide pairs 1e/1d and 2d/2b, ca. 1.3 for
1f/1g, and ca. 250 for the tert-butyl substituted ylide pair 2c/2a.
This effect of structure on rates of thermal gas-phase elimination
reactions has been reported.[18–20,29] Electron-donating C—H and
C—C hyperconjugation effects are well documented, and are
known to be subject to steric and solvation factors.[30] The results
of the present kinetic investigation indicate that both modes of
hyperconjugation seem to contribute to the thermal gas-phase
reactivity of triphenylphosphonium methylides, as evidenced by
the rate ratios for the reaction of ylides 1f, 1g and 2a, 2d; the
most striking example being a rate factor of ca 139 in favor of
CONCLUSION
The present study and the earlier publications in this series
have demonstrated for the first time the accessibility and range of
utilizing kinetic investigations in the study of thermal gas-phase
elimination reactions of stabilized triphenylphosphonium methy-
lides.[1,17,19,20] The kinetic data provided valuable support and
insight into the mechanism of pyrolysis and a quantitative
basis for structure/molecular reactivity correlations. The pyrolysis
—
C—H hyperconjugation for the (C O) ylide pair 2b/2a.
—
A pyrolysis product, unusual in the gas-phase reactions of
ylides, is that obtained from the novel rearrangement reported by
Aitken et al.[31] and also observed and documented in the present
study for substrates 2c,d. These ylides pyrolyzed to give the
reactions of triphenylphosphonium ylides stabilized by (SO2) and
—
C
X (X ¼O, S) groups represent alternative and environmentally
—
friendly routes to the synthesis of novel alkene and alkyne
compounds. The results of the present study and that of Aitkin’s
and co-workers have revealed the unique combined effect of a
hydrogen atom on the ylide carbon (absence of stabilizing
—
phenylthioalkenes [PhSCR CHPPh ; R: CH or tert-butyl] as the
—
major products of pyrolysis, rather than the usual alkynes [HC
2
3
—
—
—
CR], as is the case of the other fully or partially substituted
carbonyl and thiocarbonyl ylides. An explanation for this behavior
of the sulfur ylides 2c,d based on the mechanism outlined in
Scheme 2, invokes the comparatively large size of the sulfide
—
substituent) and the thione sulfur (C S moiety) in providing a
—
novel rearrangement pathway in the gas-phase pyrolysis reaction
of triphenylphosphonium methylides.[31]
J. Phys. Org. Chem. 2011, 24 311–319
Copyright ß 2010 John Wiley & Sons, Ltd.
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R. F. AL-BASHIR, N. A. AL-AWADI AND O. M. E. EL-DUSOUQUI
—
—
CPh),
—
135.57 (d, J 5, C-2,6 of
CPh), 138.41 (d, J 12, C-1 of
—
EXPERIMENTAL
139.45 (C-1 of CH2Ph), 188.26 (d, J 5.5, CO). 31P NMR(CDCl3): d 16.9.
IR(KBr): ymax (cmꢀ1) 1592, 1534, 1480, 1436, 1361, 1224, 1196,
1105, 975, 761, 746, 714, 699, 536. MS: m/z (%) ¼ 470 (Mþ, 8), 428
(38), 379 (100), 351 (20), 277 (68), 262 (38), 191 (48), 183 (46), 105
(86), and 91 (96). Anal. Calcd. for C33H27OP (470): C, 84.26; H, 5.74.
Found: C, 83.88; H, 5.81.
N,2-Diphenyl-2-(triphenylphosphoranylidene)acetamide (1d). A
solution of phenylisocyanate (1.7 g, 14 mmol) in 20 mL dry THF
was slowly added dropwise at room temperature to a stirred
solution of equivalent amounts of benzyltriphenylphosphonium
bromide (6.06 g, 14 mmol) and 1.6 M solution butyllithium (0.92 g,
14 mmol), the latter solution having been admixed for 3 h prior to
use. The reaction mixture was kept stirring at room temperature
overnight. The precipitate was filtered and dissolved in dich-
loromethane (75 mL) followed by extraction with water (2 ꢃ
100 mL), dried and evaporated to give a yellow solid which was
recrystallized from dichloromethane/hexane to yield yellow
General
Melting points were recorded on a Shimadzu-Gallenkamp appa-
ratus. Mass spectra were obtained using a VG Auto-Spec-Q high
resolution spectrometer coupled to a DEC 3000 Alpha AXP
workstation and OPUS data system, as well as a Finigan Mat
INCOSXL instrument for GC-MS studies (EI at 70 eV). Elemental
analyses were performed on a LECO CHNS-932 analyzer. Fourier
Transform FT-IR Perkin Elmer 2000 spectrophotometer, and a
Bruker DPX 400 MHz spectrometer with H, 13C, and 31P probes
1
were used for the infrared and NMR analyses. All the NMR spectra
were run in solution in CDCl3 as solvent, TMS as reference for 1H
and 13C, and external 85% H3PO4 for 31P NMR. HLPC analysis was
carried out on a Waters probe (pump model 515, and Schimadzu
2487 UV detector).
1
Pyrolysis
crystals (3.2 g, 49%), m.p. 164–166 8C; H NMR(CDCl3): d 6.74 (s,
1H, exchangeable H (NH), 6.85 (t, 1H, J 8), 7.02–7.18 (m, 7H),
7.36–7.42 (m, 8H), 7.47–7.51 (m, 3H), 7.65 (dd, 6H, J 7.4, 12). 13C
STP reactions were conducted in a custom-made Chemical Data
System (CDS) pyrolyzer comprising an aluminum block grooved
to accommodate the Pyrex sealed-tube reactor, and fitted with a
platinum-resistance thermometer and thermocouple connected
to a Comark microprocessor thermometer. The temperature of
the block was controlled by a digital Eurotherm 093 precision
temperature regulator. Aluminum was chosen for its low temp-
erature gradient and resistance to elevated temperatures. FVP
was conducted in a horizontal fused-quartz tube (30 cm ꢃ 2.5 cm)
housed in Bu¨chi Kugelrohr sublimation oven. Heating was by
means of a carbolite Eurotherm tube furnace MTF-12/38A, and
the temperature is monitored by a Pt/Pt–13% Rh thermocouple
situated at the center of the furnace. The pressure which is
measured by a Pirani gauge is maintained at 1 ꢃ 10ꢀ2 Torr using
an Edwards E2M5 high capacity rotary oil pump. The sample
passing through the hot zone is estimated to have a residence
time of ca 10ꢀ3 s. The pyrolysate leaving the heated tube is
collected in a U-shaped trap cooled in liquid nitrogen. A short
residence time is deemed necessary to prevent secondary
decomposition.
—
—
NMR(CDCl )(J P—C): d 52.65 (d, J 131.6, P
C), 118.60 (CH),
3
121.13 (CH), 125.88 (CH), 128.55 (d, J 91, C-1 of P—Ph), 128.67
(CH), 128.83 (d, J 12, C-3 of P—Ph), 128.98 (CH), 131.77 (d, J <2,
C-4 of P—Ph), 134.05 (d, J 8, C-2 of P—Ph), 135.95 (CH), 138.07 (d,
J 12), 141.54 (d, J 2), 168.62 (d, J 12.7, C O). 31P NMR(CDCl3): d
—
—
19.53. IR(KBR): ymax (cmꢀ1) 3413, 3052, 1606, 1580, 1568, 1503,
1425, 1311, 1257, 1219, 1103, 1069, 974, 743, 689. MS: m/z
(%) ¼ 471 (Mþ, 18), 454 (75), 378 (95), 278 (75), 262 (70), 183 (70),
152 (30), 108 (32), 91 (100). Anal. Calcd. for C32H26NOP (471): C,
81.53; H, 5.52; N, 2.97. Found: C, 81.24; H, 5.52; N, 3.03.
N,2-Diphenyl-2-(triphenylphosphoranylidene)ethanethioamide
(1e). Solution of BuLi (0.92g, 14 mmol) was added under nitrogen
atmosphere at room temperature to a 1.6 M stirred suspension
of benzyltriphenylphosphonium bromide (6.06 g, 14 mmol) in
50 mL dry THF. The reaction mixture was stirred for 3 h followed
by dropwise addition of phenylisothiocynate (1.90 g, 14 mmol).
After 12 h stirring, the reaction mixture was poured into water
and extracted twice with dichloromethane. The extract was dried,
evaporated and recrystallized from dichloromethane/hexane to
give yellow crystals (3.95 g, 58%), m.p. 139–142 8C;[32] 1H NMR
Synthesis
(CDCl3):
exchangeable H (NH)], 7.30–7.36 (m, 2H), 7.38–7.41 (m, 5H),
7.45–7.49 (m, 6H), 7.54–7.52 (m, 3H), 7.75 (dd, 6H, J 7.2, 13.3). 13
d 7.12–7.16 (m, 2H), 7.19–7.21 [m, 2H, includes
Ylides 1b, 1c, 1f, 1g, 2a, 2b, 2c, 2d, and 3 were made available by
the laboratory of Professor R. A. Aitken, University of St. Andrews,
UK. All ylides were purified and fully characterized before use.
1,3-Diphenyl-3-(triphenylphosphoranylidene)propan-1-one (1a).
Solution (1.6 M) of BuLi (0.83 g, 13 mmol) was added at room
temperature from a syringe to a stirred suspension of benzyl
(triphenyl)phosphonium bromide (Ph3PþCH2PhBrꢀ) (5.41 g, 12.5
mmol) in 60 mL dry THF. The reaction was carried out for 3 h
under nitrogen atmosphere before a solution of phenylacetyl
chloride (0.97 g, 6.25 mmol in 10 mL dry THF) was added drop-
wise. The reaction mixture was kept stirring overnight, and then
extracted with water (3 ꢃ 150 mL), dried, evaporated, and crys-
tallized from ethyl acetate/hexane to give colorless crystals of the
ylide (0.53 g, 18%), m.p. 177–179 8C; 1H NMR(CDCl3): d 3.58 (s, 2H),
6.94 (m, 2H), 7.02 (m, 3H), 7.15–7.22 (m, 5H), 7.30–7.35 (m, 6H),
7.44–7.50 (m, 9H). 13C NMR(CDCl3)(J P—C): d 44.93 (d, J 10, CH2),
C
—
NMR(CDCl )(J P—C): d 75.74 (d, J 141.7, P C), 122.61, 123.09,
—
3
127.03 (d, J 2), 127.08 (d, J 92, C-1 of P—Ph), 128.41, 128.67 (d, J 12,
C-3 of P—Ph), 129.00, 131.45 (d, J 2.5, C-4 of P—Ph), 134.09 (d, J
10, C-2 of P—Ph), 135.25 (d, J 4), 137.61 (d, J 15.3), 141.75 (d, J 2),
181.80 (d, J 20, C S). 31P NMR(CDCl3): d 9.09. IR(KBR): ymax (cmꢀ1
—
)
—
3360, 3053, 1589, 1530, 1493, 1434, 1358, 1335, 1265, 1176, 1156,
1099, 1069, 1036, 1015, 888, 872, 745, 708, 648. MS: m/z (%) ¼ 487
(Mþ, 15), 454 (22), 418 (80), 294 (82), 262 (75), 225 (30), 194 (70),
183 (90), 165 (30), 152 (25), 121 (40). Anal. Calcd. for C32H26NPS
(487): C, 78.85; H, 5.34; N, 2.87; S, 6.57. Found: C, 78.83; H, 5.50; N,
2.92; S, 6.32.
1,2-Diphenyl-2-(triphenylphosphoranylidene)ethanone (4a). Sol-
ution of Buli (0.83 g, 13 mmol) was slowly added with a syringe
under nitrogen atmosphere at room temperature to a 1.6 M
stirred suspension of benzyltriphenylphosphonium bromide
(5.40 g, 12.5 mmol) in dry THF (60 mL). The mixture was stirred
for 3 h, then a solution of benzoylbromide (1.16 g, 6.25 mmol) in
dry THF (10 mL) was added dropwise to the solution. The reaction
—
—
72.30 (d, J 106, P C), 125.79 (d, J 2, C-3 of CPh), 125.86 (C-4 of
—
—
—
CH Ph), 127.17 (d, J 90, C-1 of P—Ph), 127.95 (d, J 1, C-4 of CPh),
—
2
128.19 (2C of CH2Ph), 128.79 (d, J 12, C-3 of P—Ph), 129.70 (2C, of
CH2Ph), 131.85 (d, J 3, C-4 of P—Ph), 134.00 (d, J 10, C-2 of P—Ph),
wileyonlinelibrary.com/journal/poc
Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 311–319

KINETICS AND MECHANISM OF GAS-PHASE PYROLYSIS OF YLIDES
mixture was stirred over night, filtered and evaporated. The
residue was dissolved in dichloromethane (150 mL), dried and the
solvent was evaporated. The residue was recrystallized from ethyl
1-(Triphenylphosphoranylidene)propan-2-one (2b). 1H NMR(CDCl3):
d 2.12 (s, 3H), 3.72 (brs, 1H), 7.45–7.50 (m, 6H), 7.54–7.58 (m, 3H),
7.65–7.70 (m, 6H). 31P NMR(CDCl3): d 15.58. Anal. Calcd. for
C21H19OP (318): C, 79.24; H, 5.97. Found: C, 78, 96; H, 5.94.
3,3-Dimethyl-1-(triphenylphosphoranylidene)butan-2-thione
(2c). 1H NMR(CDCl3): d 1.42 (s, 9H), 5.20 (brs, 1H), 7.45–7.49 (m, 6H),
7.54–7.57 (m, 3H), 7.71–7.76 (m, 6H). 31P NMR(CDCl3): d 4.36. Anal.
Calcd. for C24H25SP (376): C, 76.59; H, 6.65; S, 8.51. Found: C, 76.47;
H, 6.33; S, 8.26.
1-(Triphenylphosphoranylidene)propan-2-thione (2d). 1H NMR
(CDCl3): d 2.65 (s, 3H), 5.19 (brs, 1H), 7.48–7.52 (m, 6H), 7.57–7.61
(m, 3H), 7.70–7.79 (m, 6H). 31P NMR(CDCl3): d 8.11. Anal. Calcd. for
C21H19SP (334): C, 75.45; H, 5.69; S, 9.58. Found: C, 75.28; H, 5.64; S,
9.19.
1
acetate to give white crystals (1.91 g, 67%), m.p. 130–132 8C; H
NMR (CDCl3): d 7.43–7.58 (m, 15H), 7.64–7.72 (m, 10H). Anal. Calcd.
for C32H25OP (456): C, 84.21; H, 5.48. Found: C, 83.96; H, 5.29.
1-Phenyl-2-(triphenylphosphoranylidene)ethanone (4b). The
quaternary salt for this ylide, phenacyltriphenylphosphonium
bromide (Ph3PþCH2COPh Brꢀ), was prepared by refluxing trip-
henylphosphine (10.49 g, 0.04 mol) with 2-bromoacetophenone
(8.00 g, 0.04 mol) in 60 mL toluene for 4 h. After cooling, the
formed precipitate was filtered and washed with ether to obtain
18.20 g (99%) of the salt, m.p. 298–300 8C;[33,34] 1H NMR(CDCl3): d
6.40 (d, 2H, J 12), 7.51 (t, 2H, J 7.7), 7.64–7.69 (m, 7H), 7.75–7.78 (m,
3H), 7.93–7.98 (m, 6H), 8.39 (d, 2H, J 7.7). 13C NMR(CDCl3)(J P—C): d
39.43 (d, J 61, P—CH2), 119.54 (d, J 88.9, C-1 of P—Ph), 129.58,
130.58, 130.70 (d, J 13, C-3 of P—Ph), 134.65 (d, J 10.5, C-2 of
P—Ph), 135.27 (d, J 3, C-4 of P—Ph), 135.41, 135.76 (d, J 5.5),
192.79 (CO). 31P NMR(CDCl3): d 23.18. MS: m/z (%) ¼ 381 (Mþ).
Anal. Calcd. for C26H22BrOP (461): C, 67.68; H, 4.77. Found: C, 67.64;
H, 4.88.
2,2-Dimethyl-4-(triphenylphosphoranylidene)pentan-3-one (3).
1H NMR(CDCl3): d 1.27 (s, 9H), 1.73 (d, J 4.6, 3H), 7.40–7.57 (m,
15H). 31P NMR(CDCl3): d 12.88. Anal. Calcd. for C25H27OP (360): C,
80.21; H, 7.22. Found: C, 79, 82; H, 6.98.
Pyrolysates and elimination fragments
The salt (12 g, 0.026 mol) was treated with aqueous NaOH
(1.2 g. 0.03 mol). The reaction mixture was extracted two times
with dichloromethane. The extract was dried, evaporated, and
recrystallized from ethyl acetate to yield the target ylide (5.53 g,
56%), m.p. 180–182 8C;[11–14] 1H NMR (CDCl3): d 4.46 (d, 1H, J 24),
7.37 (t, 3H, J 3), 7.48–7.51 (m, 6H), 7.56–7.60 (m, 3H), 7.72–7.77 (m,
6H), 7.99–8.01 (m, 2H).[11–14] 13C NMR(CDCl3)(J P—C): d 51.29 (d, J
Triphenylphosphine oxide (Ph3PO) (from ylides 1a, 1b, 1c, 1d, 1g,
2a, 2b, 3, 4a, and 4b), 1H NMR(CDCl3): d 7.46–7.49 (m, 6H),
7.53–7.56 (m, 3H), 7.66–7.71 (m, 6H). 31P NMR(CDCl3): d 30.42. MS:
m/z (%) ¼ 279 (15), 278 (Mþ, 62), 277 (100), 265 (39), 264 (15), 263
(13), 262 (18), 201 (29), 152 (17).
— —
CH–C H ) (from ylide 1a)
— —
6 5
1,3-Diphenylallene (C H CH
C
6
5
—
111.4, P C), 127.52, 127.66 (d, J 90.6, C-1 of P—Ph), 128.34,
was isolated in pure form, this allene is formed as a result of
the rearrangement of the initially formed 1,3-diphenyl-1-propyne
—
129.48 (d, J 12, C-3 of P—Ph), 129.96 (2C), 132.65 (d, J 2.7, C-4 of
P—Ph), 133.75 (d, J 10, C-2 of P—Ph), 141.84(d, J 14.5) 185.48 (d, J
3.3, C O). 31P NMR(CDCl3): d 17.84. IR(KBr): ymax (cmꢀ1) 1587,
(PhC CCH Ph), 1H NMR(CDCl3): d 6.65 (s, 2H), 7.29–7.40
—
—
—
2
(m, 10H).[38] 13C NMR(CDCl ): d 98.88 (terminal C of C
—
—
— —
C C),
— —
3
—
1525, 1483, 1438, 1386, 1300, 1184, 1104, 1052, 890, 873, 712, 514.
MS: m/z (%) ¼ 380 (Mþ, 66), 379 (100), 351 (18), 303 (78), 277 (37),
262 (10), 202 (39), 183 (60), 165 (39), 152 (20), 107 (15. Anal. Calcd.
for C26H21OP (380): C, 82.10; H, 5.52. Found: C, 81.61; H, 5.58.
127.3 (4C), 127.5 (C-4 of Ph), 128.6 (4C), 133.5 (C-1 of Ph), 207.7 (
[38]
—
—
—
C
).
—
—
—
E-1,4-Diphenylbut-1-en-3-yne (C H C C—CH CHC H ) (from
—
—
6
5
6 5
ylide 1b) as a yellow solid, 1H NMR (CDCl3): d 6.40, 7.06 (2d, 2H, J 16),
7.10–7.55 (m, 10H).[35] 13C NMR (CDCl3): d 88.9, 91.7, 108.1, 123.4,
126.0, 128.1, 128.6, 128.6, 128.73, 131.50, 136.50, 141.30.[35] MS:
m/z (%) ¼ 204 (Mþ, 73 abundance), fragment ions at 127 (12), 103
(22), 91 (21), 77 (6), and base peak at 27 (100).
Characterization
—
—
Diphenylbutadiyne (C H C C—C CC H ) (from ylide 1c) as
— —
6 5 6 5
—
—
1,4-Diphenyl-1-(triphenylphosphoranylidene)but-3-en-2-one (1b). m.p.
228–229 8C;[35] 1H NMR(CDCl3): d 6.93 (d, 1H, J 16), 7.02 (s, 5H)
7.10–7.75 (m, 21H).[35] 31P NMR(CDCl3): d 15.95.[35] Anal. Calcd. for
C34H27OP (482): C, 84.65; H, 5.60. Found: C, 84.38; H, 5.46.
1,4-Diphenyl-1-(triphenylphosphoranylidene)but-3-yn-2-one
(1c). m.p. 238–239 8C;[36] 1H NMR(CDCl3): d 7.00–7.20 (m, 10H),
7.25–7.75 (m, 15H).[36] 31P NMR(CDCl3): d 16.3.[36] Anal. Calcd. for
C34H25OP (480): C, 85.00; H, 5.21. Found: C, 84.73; H, 5.18.
1-Phenyl-1-(triphenylphosphoranylidene)octan-2-thione (1f). 1H
NMR(CDCl3): d 0.83(t, 3H, J 7), 1.15–1.23 (m, 6H), 1.70–1.74 (m, 2H),
2.58 (t, 2H, J 7), 6.98–7.02 (m, 5H), 7.46–7.75 (m, 15H). 31P
NMR(CDCl3): d 5.97. Anal. Calcd. for C32H33SP (480): C, 80.00; H,
6.87; S, 6.67. Found: C, 79.64; H, 6.48; S, 6.44.
1-Phenyl-1-(triphenylphosphoranylidene)octan-2-one (1g). 1H
NMR(CDCl3): d 0.86 (t, 3H, J 7), 1.18–1.29 (m, 6H), 1.57–1.61 (m,
2H), 2.28 (t, 2H, J 7), 7.01–7.02 (m, 5H), 7.35–7.59 (m, 15H). 31P
NMR(CDCl3): d 15.93. Anal. Calcd. for C32H33OP (464): C, 82.76; H,
7.11. Found: C, 82.53; H, 6.97.
colorless crystals, 1H NMR (CDCl3): d 7.20–7.30 (m, 6H), 7.40–7.50
(m, 4H).[36] 13C NMR (CDCl3): d 73.90, 81.50, 121.80, 128.40 (2C),
129.20, 132.5 (2C).[36]
— —
NC H ) (from
— —
6 5
N-(2-Phenylethenylidene)aniline (C H CH
C
6
5
ylides 1d and 1e). The formation of this ketenimine could be
attributed to the tautomerization of the initially formed pheny-
lamine (C6H5 C—CNHC H ) which is less stable tautomer.[21] 1H
NMR(CDCl3): d 5.52 (s, 1H), 7.22 (d, 4H, J 7.5), 7.32–7.38 (m, 6H).[39]
IR(CDCl3): ymax (cmꢀ1) 2016, 1595, 1487, 758, 694.[39] MS: m/z
(%) ¼ 195 (40), 194 (100), 193 (Mþ, 46), 180 (40), 167 (12), 165 (22),
—
—
—
6
5
117 (78), 105 (36), 91 (88).
1
—
—
Phenyloctyne (Ph—C C—C H ) (from ylides 1f and 1g), H
—
6
13
NMR (CDCl3): d 0.90 (t, 3H, J 7), 1.26–1.50 (m, 6H), 1.54–1.65 (m,
2H), 2.39 (t, 2H, J 7), 7.23–7.30 (m, 3H), 7.37–7.40 (m, 2H).[40] 13
C
NMR (CDCl3): d 14.0, 19.4, 22.6, 28.6, 28.7, 31.4, 80.6, 90.4, 124.2,
127.4, 128.1, 131.5.[40]
—
—
5 3
Triphenylphosphine sulfide [(C H ) P S] (from ylides 1e and
6
3,3-Dimethyl-1-(triphenylphosphoranylidene)butan-2-one (2a). m.p.
183–185 8C;[36] 1H NMR(CDCl3): d 1.23(s, 9H), 3.82 (brs, 1H),
7.50–7.96 (m, 15H).[37] 31P NMR(CDCl3): d 17.27.[37] Anal. Calcd. for
C24H25OP (360): C, 80.00; H, 6.94. Found: C, 79, 78; H, 6.86.
1f), 1H NMR(CDCl3) d: 7.45–7.48 (m, 6H), 7.52–7.55 (m, 3H),
7.72–7.77 (m, 6H).[41] 31P NMR(CDCl3): d 44.58. MS: m/z (%) ¼ 296
(7), 295 (21), 294 (Mþ, 100), 293 (79), 262 (20), 261 (14), 185 (68),
183 (81), 152 (18).
J. Phys. Org. Chem. 2011, 24 311–319
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

R. F. AL-BASHIR, N. A. AL-AWADI AND O. M. E. EL-DUSOUQUI
Table 2. Conditions for kinetics and HPLC analysis of ylides 1–4
Ylide
Internal standard
UV detector l (nm)
T range (K)
Eluent (CH3CN/H2O)
1a
1b
1c
1d
1e
1f
1g
2a
2b
2c
2d
3
1,2,4-Trichlorobenzene
1,3-Dichlorobenzene
Chlorobenzene
210
256
256
256
256
210
256
256
256
256
256
256
210
210
402–442
481–523
476–520
321–360
331–369
426–467
433–483
588–653
504–546
496–543
493–549
478–548
454–498
500–544
65:35
70:30
70:30
60:40a
60:40a
70:30
70:30
60:40a
60:40a
70:30a
50:50
60:40a
65:35
60:40
1,3-Dichlorobenzene
1,3-Dichlorobenzene
1,2,4-Trichlorobenzene
1,2,4-Trichlorobenzene
1,3-Dichlorobenzene
1,3-Dichlorobenzene
1,2,4-Trichlorobenzene
1,3-Dichlorobenzene
1,3-Dichlorobenzene
1,2,4-Trichlorobenzene
1,3-Dichlorobenzene
4a
4b
a With addition of 0.1% acetic acid.
[3,3-Dimethyl-2-(phenylthio)but-1-enyl]diphenylphosphine
t
probe with the UV detector at wavelength of 256 or 210 nm, and
the standardization value (A0) was then calculated. Several
HPLC measurements were obtained with an accuracy of ꢂ2%.
The temperature of the pyrolysis block was then raised until
approximately 10% pyrolysis was deemed to occur over 900 s.
This process was repeated after each 10–15 8C rise in the temp-
erature of the pyrolyzer until >90% pyrolysis was achieved. The
relative ratios of the integration values of the sample and the
internal standard (A) at the pyrolysis temperature were then
calculated. A minimum of three kinetic runs were carried out
at each reaction temperature following each 10–15 8C rise in
the pyrolyzer temperature to ensure reproducible values of (A).
Treatment of the kinetic data and calculation of Arrhenius
parameters and reaction rate constants have been detailed else-
where.[44] The experimental conditions for kinetic and HPLC
analysis of the ylides 1–4 are summarized in Table 2.
1
—
(Ph P—CH C(Bu )—SPh) (from ylide 2c), H NMR (CDCl ): d 1.24
—
2
3
(s, 9H), 6.96 (s, 1H), 7.09–7.12 (m, 1H), 7.16–7.20 (m, 2H), 7.22–7.24
(m, 2H), 7.31–7.36 (m, 10H).[31] 13C NMR (CDCl3): d 29.8 (3C), 41.5
(d, J 3, C—Me3), 125.2 (C-4 of SPh), 127.6 (C-3 of SPh), 128.3 (d, J
11, C-3 of PPh), 128.3 (C-4 of PPh), 128.6 (C-2 of SPh), 132.7 (d, J 19,
C-2 of PPh), 137.3 (C-1 of SPh), 137.8 (d, J 4, P—C), 139.4 (d, J 12,
[31]
31P NMR (CDCl3): d
—
C-1 of PPh), 158.9 (d, J 25, P—C C).
—
ꢀ19.79. MS: m/z (%) ¼ 376 (Mþ, 100).
(Z) and (E) Isomers of diphenyl[2-(phenylthio)prop-1-enyl]
—
phosphine (Ph P—CH C(Me)—SPh) (from ylide 2d), MS: m/z
—
2
(%) ¼ 334 (Mþ, 100) base peak. For the Z isomer, 1H NMR (CDCl3): d
2.00 (s, 3H), 6.33 (s, 1H), 7.25–7.50 (m, 13H), 7.60–7.75 (m, 2H).[31]
31P NMR (CDCl3): d ꢀ22.7. The (E) isomer showed the following
signals for 1H NMR (CDCl3): d 2.25 (s, 3H), 5.95 (s, 1H), 7.25–7.50 (m,
13H), 7.60–7.75 (m, 2H).[31] 31P NMR (CDCl3): d ꢀ25.4.
t
—
—
4,4-Dimethyl-2-pentyne (CH C CBu ) (from ylide 3) as a
—
3
colorless liquid together with triphenylphosphine oxide, 1H NMR
(CDCl3): d 1.20 (s, 9H), 1.77 (s, 3H).[42] 13C NMR (CDCl3): d 3.40,
27.33, 31.32, 73.80, 88.00.[43]
Acknowledgements
Kuwait University Research Grant Sc 01/99 and ANALAB and SAF
Grant 01/01 and GE 03/01 are gratefully acknowledged.
Kinetic measurements: Stock solution (7 mL) was prepared by
dissolving 6–10 mg of the ylide in acetonitrile as solvent to give a
concentration of 1000–2000 ppm. Internal standard was then
added, the amount of which was adjusted to give the desired
peak area ratio of ylide to standard (2.5:1). The solvent and the
internal standard were selected on account that both are stable
under the conditions of pyrolysis, and because they do not react
with either ylide or product. The internal standards used in this
study are chlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trich-
lorobenzene. Each solution is filtered before use to ensure a
homogenous solution.
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