2,6-Dimethoxyphenylphosphirane oxide and sulfide and their thermolysis to phosphinidene chalcogenides - Kinetic and mechanistic studies

Article abstract of DOI:10.1016/S0040-4020(99)00779-6

2,6-Dimethoxyphenylphosphirane is readily converted to its oxide and sulfide whose pyrolysis and (perhaps) photolysis lead to the generation of phosphinidene chalcogenides Ar-P=Z (Z=O,S). Trapping experiments were carried out under conditions similar to the kinetic studies of the phosphirane chalcogenide pyrolyses that confirmed the formation of free Ar-P=Z. The trapping experiments were in accord with carbene-like character for Ar-P=Z, and the activation parameters suggest a non-least motion pathway for the addition of Ar-P=Z to olefins. This represents quantitative evidence for the validity of the predictions of frontier-orbital theory for species that undergo reactions with small (or no) enthalpic barriers.

Full text of DOI:10.1016/S0040-4020(99)00779-6

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 105–119
2,6-Dimethoxyphenylphosphirane Oxide and Sulfide and their
Thermolysis to Phosphinidene Chalcogenides—Kinetic and
Mechanistic Studies
a
a
a
a
Peter P. Gaspar,a, Hu Qian, Alicia M. Beatty, D. Andre d’Avignon, Jeff L.-F. Kao,
*
´
Jesse C. Watta and Nigam P. Rathb
aDepartment of Chemistry, Washington University, St. Louis, MO 63130-4899, USA
bDepartment of Chemistry, University of Missouri—St. Louis, St. Louis, MO 63121, USA
Received 10 May 1999; accepted 2 July 1999
Abstract—2,6-Dimethoxyphenylphosphirane is readily converted to its oxide and sulfide whose pyrolysis and (perhaps) photolysis lead to
the generation of phosphinidene chalcogenides Ar–PyZ (ZˆO,S). Trapping experiments were carried out under conditions similar to the
kinetic studies of the phosphirane chalcogenide pyrolyses that confirmed the formation of free Ar–PyZ. The trapping experiments were in
accord with carbene-like character for Ar–PyZ, and the activation parameters suggest a non-least motion pathway for the addition of
Ar–PyZ to olefins. This represents quantitative evidence for the validity of the predictions of frontier-orbital theory for species that undergo
reactions with small (or no) enthalpic barriers. ᭧ 1999 Elsevier Science Ltd. All rights reserved.
Introduction
necessary to interconnect their positions in the non-least
motion transition state with their positions in the corre-
sponding silirane and phosphirane chalcogenide adducts.
It was recognized several years ago that differences in
frontier-orbital symmetry between the ‘carbene family’
and ‘nitrene family’ of six-valence electron species might
allow a fundamental question regarding chemical reactivity
to be answered: does maximization of bonding in the transi-
tion state of a reaction, as embodied in frontier-orbital (FO)
theory, correctly predict reaction pathways, even in the
absence of a significant energy barrier?1 Frontier orbitals
of closed-shell singlet silylenes and phosphinidenes, and of
a phosphinidene chalcogenide are shown in Scheme 1.2
It is more than 40 years since Skell proposed that stereo-
specific addition of carbenes to olefins was due to a
concerted process involving a singlet species.3 It has been
pointed out that singlet carbenes or silylenes could undergo
stepwise, non-stereospecific addition, and triplet species
might add stereospecifically if conformational changes
were slow compared with intersystem crossing and intramo-
lecular radical coupling.4,5 Nevertheless the reactant orien-
tations and transition state structures shown in Scheme 2
should describe the maximization of bonding during
concerted addition (and retroaddition) processes of closed-
shell singlet species.
The HOMO of the silylene is aligned with the molecular
axis (and the HOMO of the phosphinidene chalcogenide has
the same ‘local symmetry’) while the HOMO of the phos-
phinidene is perpendicular to the molecular axis. This differ-
ence in symmetry will lead to a non-least motion transition
state for concerted addition of a silylene or a phosphinidene
chalcogenide to an olefin, if FO overlap is maximized, but
permits a least motion transition state for concerted addition
of a phosphinidene. In Scheme 2, orientations of a silylene,
phosphinidene and phosphinidene chalcogenide that maxi-
mize overlap of their HOMO’s with an olefin LUMO are
shown at the left. Curved arrows indicate the motions of
substituents on a silylene and a phosphinidene chalcogenide
In 1963 Moore suggested that concerted addition by a sin-
glet carbene to an olefin implied a maximization of what we
now call FO-interactions, half of which are shown in
Scheme 2.6 By 1968 Roald Hoffmann had pointed out that
the ‘least motion’ transition-state for carbene addition was
forbidden by symmetry, and he predicted, on the basis of
extended-Hu¨ckel calculations, that there would be a signifi-
cant energy barrier along this pathway but none along the
non-least motion pathway depicted in Scheme 2.7 There was
no experimental evidence for non-least-motion addition.
Keywords: phosphorus heterocycles; strained compounds; phosphine
oxides; phosphine sulfides.
* Corresponding author. Tel.:ϩ314-935-6568; fax: ϩ314-935-4481;
e-mail: gaspar@wuchem.wustl.edu
There have been many calculations at various levels of
theory on the addition of carbenes to olefins,8 and they are
all in accord with Hoffmann’s view that a non-least motion
0040–4020/00/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(99)00779-6

106
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Scheme 1.
pathway is preferred. A similar picture emerges from calcu-
lations on the pathways for the silylene–olefin addition
reaction. Gordon has predicted that along the minimum
energy approach of SiH2 to ethylene the H–Si–H plane
remains parallel to the plane containing the ethylene nuclei
until late in the reaction when there is an abrupt change in
the angle between the planes, as shown in Scheme 2.9
Houk has proposed entropic control of the free energy
barrier for carbene addition reactions.12 A consequence of
entropic rather than enthalpic control of addition rates
would be that maximization of bonding, an enthalpic
factor, might fail as a predictive model for transition state
structure.
The non-least motion transition states predicted by the FO
model, and supported by calculations of minimum energy
pathways, for addition reactions of the ‘carbene family’
should lead to a lower entropy of activation than would a
least-motion transition state for both the forward and reverse
reactions. This is due to the ‘extra’ steric interactions
between substituents on the olefin and those on the
attacking species that are not present in the adduct or the
free reagents.
In accord with these theoretical results is the qualitative
picture of approach control governed by orbital overlap
that emerges from the FO model: HOMO–LUMO mixing
lowers the energy of the transition-state, and hence reaction
occurs by the pathway that maximizes FO overlap.
To be sure, these are enthalpic considerations, and transition
states are related to free energy barriers. Given the tremen-
dous success of FO theory in organic chemistry,10 it may
seem wrongheaded to question its predictive powers in the
realm of highly reactive species. But most carbene and
silylene reactions are highly exothermic, and laser-flash
kinetic spectroscopy has indicated that their addition
reactions are very rapid, some occurring at the diffusion-
controlled rate. Thus activation barriers are very small, and
indeed often negative,11 and little bonding has occurred
when the transition state is reached.
We favor kinetic studies of the retro-additions, shown in
cartoon form in Scheme 3, because their reasonably
large activation barriers considerably ease the task of
measuring their rates and of obtaining their activation
parameters from the temperature dependence of their
rate constants. The addition (and indeed all other)
reactions of silylenes have been found to be cleanly
reversible,13 and this prompted us to develop the
Scheme 2.

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
107
Scheme 3.
retroaddition route to phosphinidenes.14 Arylphosphir-
anes have been found to undergo clean pyrolysis and
photolysis to phosphinidenes R–P.15
Scheme 4.
conversion of R–P to R–PyZ changes the symmetry of the
HOMO making R–PyZ isolobal with a singlet carbene or
silylene.16 This is shown in Scheme 4 in an MO interaction
diagram.
Our interest in phosphinidene chalcogendides R–PyZ
(ZˆO, S, Se, Te) has been kindled by the recognition that
Figure 1. Reactions that have been reported to yield phosphinidene chalcogenides R–PyZ.

108
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Figure 2. Reactions that have been attributed to phosphinidene chalcogenides R–PyZ.
Carbene-like reactions have long been attributed to phos-
phinidene chalcogenides, but evidence for their generation
has been sparse.17 Fig. 1 displays eleven reactions that have
been proposed, largely on the basis of product studies, to
generate R–PyZ. The reactions that have been attributed to
R–PyZ are summarized in Fig. 2.
methanol H–O insertion product (reaction e).24 From a
series of elegant fluorescence and quenching experiments
Tomioka concluded that this photofragmentation occurred
via a singlet excited state.25 The non-stereospecific frag-
mentation could be explained by competing disrotatory
chelotropic processes forming Ph–PyO, or by stepwise
fragmentation which would not necessarily lead to free
R–PyO.
The formation of R–PyO and R–PyS upon the dehalogena-
tion of the corresponding phosphonic or phosphonothioic
dichlorides 1 was deduced by Inamoto and his distinguished
coworkers in the early 1970s from the products formed by
carbene-like additions18 and insertions from benzil (reaction
a) and diethyl disulfide (reaction b), respectively.19–21 Formal
1,4-addition to 2,3-diphenylbutadiene was also attributed to
Ph–PyO in these experiments (reaction c), but a correspond-
ing reaction was not found in experiments in which the inter-
mediacy of Ph–PyS was inferred, and instead 2ϩ4
cycloaddition of the PyS bond was proposed (reaction d).22
Product studies suggested the possible generation of
phosphinidene chalcogenides from bicyclic and tricyclic
molecules in which a strained 3-phospholene chalcogenide
unit is present as a substituted 7-phosphanorbornene
chalcogenide. Tomioka reported that direct irradiation of
1-phenyl-3-phospholene oxide dimer 4 leads to an apparent
H–O insertion product of Ph–PyO (reaction e).26 Mathey
found that heating exo-dimer 5 to 150ЊC in the presence of
2,3-dimethylbutadiene led to the apparent 1,4-adduct of
Ph–PyS and the diene (reaction c).27 When the bicyclics
6 were irradiated in the presence of MeOH the products
expected from trapping of R–PyS (RˆPh, Bu, Br(CH2)n;
nˆ4,5,6) by H–O insertion (reaction e) were obtained, but
Mathey did not suggest phosphinidene sulfide intermedi-
ates.28 Tomioka reported the trapping (via reaction e) of
the phosphinidene selenide as well as the oxide and sulfide
when the phosphole chalcogenide-cyclopentadiene adducts
7 were irradiated in MeOH.29
It was again the formation of insertion products from
EtSSEt (reaction b) and MeOH (reaction e) that supported
Stille’s 1975 suggestion that Ph–PyO was extruded upon
pyrolysis of a 7-phosphanorbornadiene oxide 2.23 No
adducts were obtained when the pyrolysis was carried out
in the presence of alkenes or alkynes, and Stille recognized
the possibility that addition was occurring to 2 before loss of
tetraphenylnapthalene.
In 1974 Tomioka had suggested that Ph–PyO is formed by
photolysis of a 3-phospholene oxide 3 based on the apparent
However Quin et al. found that 1-phenyl-3-phospholene
oxide
3
(R,R0ˆH, R00ˆMe), bicyclic
6
(RˆPh,

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
109
Scheme 5.
Y,Y0ˆNPh), and its P-sulfide analog did not undergo
cleavage when irradiated in the absence of MeOH.30 It
was suggested that photo-induced addition of MeOH to 7-
phospha-norbornene and 3-phospholene chalcogenides may
form five-coordinate intermediates that fragment to the
observed products.30
the conditions that the liberation of the free phosphinidene
chalcogenides was verified. Finally, if free phosphinidene
chalcogenides are formed by concerted thermolysis of phos-
phirane chalcogenides, that is by retroaddition reactions,
then classical kinetic studies of the thermolysis might
allow the elucidation of the retroaddition/addition mechan-
ism and experimentally answer the question whether a non-
least motion pathway operates as predicted by the frontier
orbital model.
In 1978 Quast synthesized the first stable phosphirane oxide
8 by a cyclization that resembles the first step in the
31,32
¨
Ramberg–Backlund reaction.
Formation of tBu–PyO
upon the thermolysis of 8 above 60ЊC was deduced from
the cis-di-tert-butylethylene coproduct and the formation of
products presumed to arise by H–O-insertion (reaction a)
and formal 1,4-addition to quinones (reaction f ). In 1982
Quast reported that the disappearance of 8 appeared to be a
first-order process whose rate was similar in benzene and
dioxane and did not appear to be accelerated by the presence
of trapping agents.33 No adducts were obtained from benzil,
biacetyl, 2,3-dimethylbutadiene, or bis(trimethylsilyl)-
acetylene.
Results
The synthetic sequence in Scheme 5 was followed for the
synthesis of the phosphirane oxide 14 and sulfide 15. 2,6-
Dimethoxyphenylphosphirane 13 was prepared from 2,6-
dimethoxyphenylphosphine38 12 by Li’s modification39 of
a procedure devised by Oshikawa and Yamashita.40 Two-
phase oxidations (H2O2–CH2Cl2 and S8–CH2Cl2) gave the
chalcogenides in high yield. The X-ray crystal structures of
13, 14, and 15 are presented in Figs. 3–5, respectively.
Crystal data and structure refinement parameters are given
in Table 1. Selected bond-lengths and angles are given in
Table 2.
Quin suggested that the slow decomposition of the bicyclic
phosphirane oxide 9 liberated an arylphosphinidene oxide
that was trapped by residual water as a phosphinic acid
anhydride (reaction g).34
Recently deselenization of diselenoxo- and selenoxothioxo-
phosphoranes 10 yielded the corresponding phosphinidene
selenide35 and sulfide.36 The selenide Ar–PySe (Arˆ2,4-
(tBu)2-6-(Me2N)C6H2) is metastable and its 31P NMR spec-
trum was recorded; it underwent disproportionation (reac-
tion h).35 The corresponding sulfide Ar–PyS was longer-
lived and did not undergo reactions a and b.36 Yoshifuji
suggested that the N coordinates to the P-atom.
Most recently Cowley reported the pyrolysis of a P(III)
oxalate 11 leading to a product that appears to be due to a
carbene-like intramolecular C–H insertion by the phos-
phorus center, reaction i.37
Against this background, our goal was to simplify the
synthesis of phosphirane chalcogenides, and to examine
their thermal and photochemical decomposition to deter-
mine whether free phosphinidene chalcogenides were
formed. If so, the reactivity of free phosphinidene chalco-
genides could be explored by carrying out reactions under
Figure 3. ORTEP drawing of a crystal structure of 13.

110
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Figure 5. ORTEP drawing of a crystal structure of 15.
Figure 4. ORTEP drawing of a crystal structure of 14.
undertaken of the thermolysis of 14 and 15. The disappear-
ance of 14 and 15 were clean first-order processes over at
least three half-lives in the temperature ranges 44–80 and
44–75ЊC, respectively.41 Table 3 compares the first-order
rate constants for thermolysis of 14 in benzene with that in
acetonitrile, and in benzene with and without EtSSEt
(12 vol%, 10× precursor concentration) and CD3OD (5 vol%,
10× precursor concentration) trapping agents. The rate
In preliminary thermolysis experiments, heating degassed
benzene solutions of 14 and 15 to 80ЊC in sealed NMR
tubes for 3 h led to quantitative yields of ethylene as deter-
1
mined by H NMR spectroscopy.
In order to establish the intermediacy of phosphinidene
chalcogenides in these reactions, kinetic studies were
Table 1. Crystal data and structure refinement parameters
13
14
15
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
C10H13O2P
196.2
296
monoclinic
P21/c
C10H13O3P
212.2
296
triclinic
P1 (bar)
C10H13O2PS
228.23
298(2)
triclinic
P1 (bar)
ꢀ
ꢀ
ꢀ
aˆ12:578ꢀ3† A
aˆ8:762ꢀ2† A
aˆ8:2937ꢀ7† A
ꢀ
ꢀ
ꢀ
ꢀ
bˆ5:4450ꢀ10† A
bˆ11:235ꢀ2† A
bˆ12:1787ꢀ9† A
ꢀ
ꢀ
cˆ15:038ꢀ3† A
cˆ11:992ꢀ2† A
cˆ18:6125ꢀ11† A
aˆ99:353ꢀ6†Њ
bˆ94:314ꢀ6†Њ
gˆ108:289ꢀ8†Њ
1745.2(2)
bˆ103:51ꢀ3†Њ
aˆ76:97ꢀ3†Њ
bˆ75:00ꢀ3†Њ
gˆ72:80ꢀ3†Њ
3
3
˚
˚
Volume
1015.9(4) A
1075.1(4) A
Z
4
4
6
Density (calculated)
Absorption coefficient
F(000)
1.283 g/cm3
21.24 cmϪ1
416
1.311 g/cm3
21.21 cmϪ1
448
1.303 g/cm3
35.64 cmϪ1
720
Crystal size (mm3)
u range for data collection
Index ranges
0:18×0:28×0:33
3.0–113.0Њ
0ՅhՅ13
0ՅkՅ5
Ϫ16Յ1Յ15
1405
0:06×0:44×0:30
3.0–113.5Њ
0ՅhՅ9
Ϫ11ՅkՅ11
Ϫ12Յ1Յ12
3063
0:42×0:33×0:30
4.86–123.98Њ
Ϫ9ՅhՅ9
Ϫ14ՅkՅ14
0Յ1Յ22
Reflections collected
Independent reflections
Absorption correction
Refinement method
14,643
1337 (Rintˆ6.50%)
2837 (Rintˆ3.63%)
6000 (Rintˆ4.8%)
none
none
none
Full-matrix
Least-squares
5.3:1
Full-matrix
Least-squares
7.0:1
Full-matrix
Least-squares
15.3:1
Data-to-parameter ratio
Goodness-of-fit on F2
Final R indices ‰IϾ2sꢀI†Š
1.16
1.05
1.079
Rˆ4:56%
wRˆ5:35%
Rˆ10:72%
wRˆ6:88%
–
Rˆ4:87%
wRˆ6:62%
Rˆ8:10%
wRˆ8:27%
–
R1ˆ7:62%
wR2ˆ21:75%
R1ˆ9:30%
wR2ˆ26:63%
0.0036(7)
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
0.18 and Ϫ0.24
0.29 and Ϫ0.32
0.440 and Ϫ0.736
Ϫ3
˚
(e A
)

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Table 2. Selected bond distances (A) and angles (Њ) for compounds 13, 14 and 15
111
˚
13, Zˆ:
14, ZˆOa
15, ZˆSb
P–Z
P–Cring
–
1.473(4), 1.479(4)
1.739(6)–1.759(5)
1.552(9), 1.555(8)
1.792(5), 1.793(6)
1.362(7)–1.375(6)
1.402(9)–1.424(9)
116.4(2), 117.4(2)
123.7(2)–125.0(3)
52.5(3)–53.0(3)
1.9365(15)–1.945(2)
1.761(4)–1.786(4)
1.538(6)–1.558(7)
1.782(4)–1.791(4)
1.351(6)–1.377(5)
1.417(5)–1.434(5)
118.51(13)–119.13(15)
123.2(2)–124.3(2)
51.5(2)–53.0(3)
1.820(7), 1.828(7)
1.488(9)
1.826(6)
1.365(8), 1.365(9)
1.414(9), 1.417(8)
–
Cring–Cring
P–Cipso
O–Cortho
O–CMe
Z–P–Cipso
Z–P–Cring
–
Cring–P–Cring
48.1(3)
Cring–P–Cipso
O–Cortho–Cipso
CMe–O–Cortho
101.4(3), 101.5(3)
114.9(6), 115.7(6)
118.8(5), 119.3(5)
111.1(3)–111.6(3)
114.6(5)–115.4(5)
117.6(5)–119.6(5)
110.1(2)–111.6(2)
114.1(4)–115.5(4)
118.5(4)–119.7(5)
a There are two nearly identical molecules of 14 in the unit cell.
b There are three nearly identical molecules of 15 in the unit cell.
Table 3. First-order rate constants for the pyrolysis of 14 and 15
Precursor
Solvent
Temperature (ЊC)
Trapping agent
Rate constant (10Ϫ5 sϪ1
)
14
14
14
14
15
15
C6D6
CD3CN
C6D6
C6D6
C6D6
C6D6
75:0^0:1
75:0^0:1
75:0^0:1
75:0^0:1
65:0^0:1
65:0^0:1
none
none
EtSSEt
CD3OD
none
7:99^0:15
12:1^0:4
8:02^0:15
16:4^0:3
68:3^0:19
69:0^0:17
EtSSEt
constant for disappearance of 14 was 50% larger in aceto-
nitrile than in benzene. The presence of EtSSEt did not
accelerate the disappearance of 14 or 15, but the presence
of methanol at one molar concentration doubled the rate
of disappearance of 14. The yields of the expected
trapping products of the phosphinidene chalcogenide
(vide infra) corresponding to 14 in these kinetic experi-
ments were 79^3% (EtSSEt) and 70^2% (CD3OD) and
that corresponding to 15 was 86^1%; as determined by
NMR integration.
Fig. 6 displays the Eyring plot for the temperature depen-
dence of the first-order rate constants for the thermolysis of
14 in C6D6 and Fig. 7 displays the Arrhenius plot for 15. The
activation parameters derived from these plots are: 14
Eaˆ32.2^1.3 kcal/mol, log Aˆ16.1^0.8, DH݈31.4^
1.3 kcal/mol, DS݈12.9^3.6 eu; 15 Eaˆ25.8 kcal/mol,
log Aˆ13.5^1.1, DH݈25.1^1.6 kcal/mol, DS݈1.1^
4.8 eu.
Fig. 8 displays the results of RHF/6-31Gء ab initio
Figure 6. Eyring plot for the pyrolysis of 14 in C6D6.

112
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Figure 7. Arrhenius plot for the pyrolysis of 15 in C6D6.
ء
˚
Figure 8. Structures (A, deg) predicted by SCF 6/31G calculations for (a) 1-methylphosphirane oxide and (b) the transition state for pyrolysis of (a) to C2H4
and Me–PyO.
calculations on a model compound, 1-methylphosphirane
oxide and the transition state for its pyrolysis to methyl-
phosphinidene oxide Me–PyO and ethylene.42 In the tran-
sition state (b) the angle between the plane containing the O,
P, and CMe atoms and the plane containing the four ring
H-atoms is 36Њ. This angle is 90Њ in the intact molecule (a).
presence of various trapping agents gave rise to products
that had previously been attributed to phosphinidene
chalcogenides. The photolyses were carried out with low-
pressure mercury lamps.
S,S-Diethyl 2,6-dimethoxyphenylphosphonodithioate 16 and
the corresponding trithioate 18 were purified and gave satis-
factory elemental analyses. Their 31P chemical shifts d
31Pˆ49.1 16, 72.4 18 are similar to those of related
As indicated in the reactions in Schemes 6 and 7, thermal
and photochemical decomposition of 14 and 15 in the
Scheme 6.

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
113
Scheme 7.
compounds: PhP(O)(SEt)2 d 31Pˆ52.5 ppm,18 PhP(S)(SEt)2
and photochemical generation of a range of phosphinidene
chalcogenides R–PyZ.
d
31Pˆ80.5 ppm.20 The instability of even purified methyl
2,6-dimethoxyphenylphosphinate 17 and the corresponding
O-methyl thiophosphinate 19 precluded a satisfactory
elemental analysis, but the distinctive phosphinate and
thiophosphinate NMR signals made their identification
unambiguous: 17 31P dˆ28.3 ppm, JP–Hˆ564 Hz (cf.
PhPH(O)OMe 31P dˆ29.3 ppm, JP–Hˆ576 Hz),43 19 31P
dˆ55.9 ppm, JP–Hˆ559 Hz (cf. PhPH(S)OMe 31P
dˆ69.3 ppm, JP–Hˆ528 Hz).30
The present kinetic studies indicate clearly that 2,6-
dimethoxyphenylphosphinidene oxide and sulfide were
generated by pyrolysis of phosphirane oxide 14 and sulfide
15. The disappearance of 14 and 15 are clean first-order
processes whose rates were unchanged in the presence of
a high concentration of a trapping agent EtSSEt that led to
the formation of 79 and 86%, respectively, of the product
expected from insertion of R–PyZ into the S–S bond.
These results preclude induced decomposition as a contrib-
utor to the disappearance of 14 and 15 and are consistent
with a concerted extrusion of R–PyZ. A long-lived revers-
ibly-formed intermediate that can transfer a phosphinidene
chalcogenide unit to a substrate is also excluded. A stepwise
decomposition to free phosphinidene chalcogenide with a
fast second step is rendered unlikely by the activation
parameters, the small entropies of activation being consis-
tent with the non-least-motion concerted extrusion depicted
earlier but inconsistent with the cleavage of a single P–C
bond. There has been little experimental evidence that
speaks to the question of whether non-least-motion addition
occurs for carbenes and carbene-like species.47
2,6-Dimethoxyphenyl-3,4-dimethyl-3-phospholene sulfide
20 was identified by comparison with an authentic sample
synthesized by condensation of 2,6-dimethoxyphenyl-
lithium with 1-bromo-3,4-dimethyl-3-phospholene44 fol-
lowed by treatment with sulfur.
The structure of 3,6-dihydro-4,5-dimethyl-2-(2,6-dimethoxy-
phenyl)-1-thiophosphorin-2-oxide 21 was deduced from its
mass spectrum (Mϩˆ298), its 31P chemical shift 31P
dˆ60.1 ppm (cf. tBu(Ph)P(O)SMe 31P dˆ63.3 ppm),45
and from Heteronuclear Multiple Bond Correlation
(HMBC) and Heteronuclear Multiple Quantum Coherence
(HMQC) NMR experiments.46 (2,6-Dimethoxyphenyl)-3,4-
diphenyl-2,5-dioxa-3-phospholene sulfide 22 was identified
on the basis of its NMR spectra, 31P dˆ96.8 ppm (cf. 1,3,4-
triphenyl-2,5-dioxa-3-phospholene sulfide 31P dˆ105 ppm).20
Pure 22 could not be obtained.
The results of ab initio calculations on the dissociation of a
model compound 1-methylphosphirane oxide are shown in
Fig. 8.42 A concerted extrusion of Me–PyO is predicted by
the calculations, and a non-least-motion transition state is
clearly seen. For this model system the calculations predict
an entropy of activation of ϩ0.8 eu which compares favor-
ably with the experimental DS‡ for 14 and 15 of 12:9^3:6
and 1:1^4:8 eu; respectively. The experimental activation
parameters for the presumed non-least-motion extrusion of
dimethylsilylene Me2Si from hexamethylsilirane are
DH‡ˆ31.6^2.7 kcal/mol, DS‡ˆ11.2 eu.48
Discussion
The improvements in the synthesis of phosphiranes
previously reported from this laboratory14,15,39 and their
direct oxidation to phosphirane chalcogenides described
here promise ready access to precursors for the thermal

114
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Scheme 8.
The operation of a concerted non-least-motion retroaddition
(and addition) pathway that is suggested by the present
results supports the applicability of frontier-orbital predic-
tions to reactions with small enthalpic barriers.
chemistry. R–PyZ and the synthetically useful electrophilic
two-electron phosphinidene-transition metal complexes
developed by the Mathey laboratory49 are isolobal. When
the electrophilicity of phosphinidene chalcogenides is
increased, the accessibility of their precursors and the ease
of their generation should ensure their widespread use. It is
interesting that the reaction with 2,3-dimethylbutadiene
suggests that phosphinidene sulfide 23 is more reactive
than the corresponding phosphinidene oxide generated
from 14. The corresponding selenide may prove to be
even more reactive, and the electrophilicity of R–PyZ can
be increased by making R more electron-withdrawing than
the 2,6-dimethoxyphenyl group employed here.
That a phosphirane chalcogenide can undergo pyrolysis
without participation by a trapping agent does not guarantee
that it will always do so. The pyrolysis rate for 14 doubled in
the presence of CD3OD, and thus the role of free phos-
phinide oxide in these reactions remains to be determined
by future experiments. The same caution must be exercised
regarding the mechanisms of the photolysis of 14 and 15 in
the presence of methanol, the photolysis of 15 in the
presence of 2,3-dimethylbutadiene and the pyrolysis of 15
in the presence of benzil. The products observed, 17, 19–22,
can be rationalized in terms of reactions previously
suggested for phosphinidene chalcogenides (Fig. 2,
reactions a, c–e), but kinetic or spectroscopic studies are
needed to probe the intermediacy of free phosphinidene
chalcogenides in these reactions.
Conclusion
Kinetic studies of the pyrolysis of 2,6-dimethoxyphenyl-
phosphirane oxide 14 and sulfide 15 suggest that the corre-
sponding phosphinidene chalcogenide R–PyO and R–PyS
are formed by a concerted, non-least-motion extrusion. The
pyrolysis rate is not accelerated by the presence of EtSSEt
and RP(Z)(SEt)2 products 16 and 18 are formed in high
yield, confirming S–S insertion by the free phosphinidene
chalcogenides. The kinetic studies provide quantitative
evidence for the validity of predictions of frontier-orbital
theory for species that undergo reactions with small enthal-
pic barriers. The products from thermal and photochemical
reactions in the presence of other trapping agents are in
accord with previously suggested carbene-like reactions of
phosphinidene chalcogenides, but a rate acceleration
observed for pyrolysis of 14 in the presence of CD3OD
dictates caution in the acceptance of such intermediates
without complete mechanistic studies.
The results reported here do confirm the insertion of phos-
phinidene chalcogenides into the S–S bond of EtSSEt
previously suggested by Inamoto et al. (Fig. 3, reaction
b).20 This reaction places the participation of phosphinidene
chalcogenides in carbene-like reactions on a firm basis, and
their scope will surely be extended.
The formation of both 20 and 21 from photolysis of 15 in the
presence of 2,3-dimethylbutadiene is of interest in suggest-
ing that a carbene-like phosphorus center and a PyS p-bond
may compete as reaction sites on a phosphinidene sulfide.
When PhP(S)Cl2 (1 RˆPh, ZˆS) was dehalogenated with
magnesium in the presence of 2,3-dimethyl-1,3-butadiene,
the product obtained by Inamoto et al. was attributed to
cycloaddition of the PyS p-bond of Ph–PyS.19,22 That
thiophosphorin was converted by air to its oxide.22 The
phospholene sulfide analogous to 20 was not found in that
experiment,22 but it was found by Mathey et al. from
pyrolysis of 5.27 Thus 20 and 21 could arise from competing
reactions of a phosphinidene sulfide 23 as shown in
Scheme 8.
Experimental
General data
All preparative reactions were carried out in flame-dried
glassware under an atmosphere of dry nitrogen or argon.
Photolyses were carried out in a Rayonet RS photochemical
reactor (model RPR-208, 120 W radiated power) equipped
While the intermediacy of free 23 in the formation of 20 and
21 remains unproved, it may be confidently predicted that
the reactions of phosphinidene chalcogenides and their
mimics will soon enter the synthetic arsenal of phosphorus
1
with low-pressure Hg lamps (254 nm radiation). H, 13C,
and 31P NMR spectra were recorded on Varian Unity
Plus-300, Gemini-300 (13C 75.4 MHz, 31P 121.4 MHz),

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
115
Unity Plus-500 (13C 125 MHz, 31P 202.3 MHz) and Unity-
600 (13C 250 MHz) spectrometers. 85% H3PO3 was
employed as an external 31P standard. Recorded yields are
based on starting materials consumed and were determined
by integration of 31P spectra. Combined gas chromato-
graphy–mass spectrometry was performed on a Hewlett–
Packard Model 5890 Series II instrument fitted with a
Quadrex 25-m×0:32 mm capillary column coated with
0.5 mm polymethyl(5% phenyl)silicone bonded phase
connected to a Model 5971 mass selective detector (quad-
rupole mass filter), mass spectra recorded above m/e 40,
ionization energy 70 eV.
163.6 (d, JP–Cˆ5.9 Hz, o-C); elemental analysis, calcd for
C10H13O2P C 61.22%, H 6.68%; obsvd C 60.98%, H 6.85%.
2,6-Dimethoxyphenylphosphirane oxide 14. To 0.99 g
(5.00 mmol) 13 dissolved in 30 mL CH2Cl2 in a 100 mL
flask equipped with a magnetic stirrer bar was added
2.8 mL (25.0 mmol) 30% H2O2. After 48 min of stirring a
31P NMR spectrum of an aliquot indicated that 13 was
consumed and 14 was formed. The reaction mixture was
extracted with water (3×20 mL) and brine (3×20 mL).
The colorless organic phase was dried with Na2SO4, filtered,
and volatiles removed under vacuum, leaving 1.01 g (95%
crude yield) light-yellow oily product. This was purified by
chromatography on silica gel with 10:10:2 CH2Cl2–
EtOAc–EtOH eluent, yielding 14 as a white solid (0.51 g,
48%) that was recrystallized from EtOAc: 31P NMR
(121.4 MHz, CD2Cl2) d Ϫ50.3; 1H NMR (500 MHz,
CD2Cl2) d 1.16–1.25 (m, 2H, Hsyn), 1.92–2.03 (m, 2H,
Hanti), 3.90 (s, 6H, CH3), 6.63 (dd, 2H, JP–Hˆ5.7 Hz,
JH–Hˆ8.7 Hz, m-H), 7.48 (t, 1H, JH–Hˆ8.7 Hz, p-H);
13C{1H} (150.9 MHz, CD2Cl2) d 9.9 (d, JP–Cˆ26.0 Hz,
CH2), 56.5 (s, CH3), 104.1 (d, JP–Cˆ4.5 Hz, m-C), 108.4
(d, JP–Cˆ122.1 Hz, ipso-C), 135.5 (s, p-C), 162.2 (s, o-C);
elemental analysis, calcd for C10H13O3P C 56.59%, H
6.18%; obsvd C 56.44%, H 6.23%.
Materials
Benzene, THF, ether, and pyridine were dried by and
distilled from sodium benzophenone ketyl under a nitrogen
atmosphere immediately before use. CaH2 was the drying
agent for pentane, cyclohexane, and hexane. CH2Cl2 was
distilled from P2O5. Reagent chemicals were employed as
received except as noted: C6D6 (Aldrich, 99.6%), n-BuLi
(Fisher, 1.6 M or Aldrich, 2.5 M in hexane), chloroform-d
(Aldrich, 99.8%), CD2Cl2 (Aldrich, 99.95%), 1,3-dimethoxy-
benzene (Aldrich, 99%, distilled under N2 from Na before
use), 2,3-dimethyl-1,3-butadiene (Aldrich, 98%, distilled
under N2 before use), ethylene glycol (Sigma, 99%), H2O2
(Aldrich, 30%), LiAlH4 (Aldrich, 95%), CD3OD (Aldrich,
99.8%), PBr3 (Aldrich, 97%, distilled under N2 before use),
p-toluenesulfonyl chloride (Aldrich, 98%).
2,6-Dimethoxyphenylphosphirane sulfide 15. To 0.99 g
(5.00 mmol) 13 dissolved in 30 mL CH2Cl2 in a 100 mL
flask equipped with a magnetic stirrer bar was added
1.00 g (31 mmol) sublimed sulfur. After 45 min of stirring
a
31P NMR spectrum of an aliquot indicated the disappear-
2,6-Dimethoxyphenylphosphine was synthesized by the
ance of 13 and the formation of 15. Solvent was removed
under vacuum and the residue mixed with 5 mL CH2Cl2 and
filtered to remove excess sulfur. Evaporation of solvent left
1.12 g (98% crude yield) yellow oil that was purified by
chromatography on silica gel with CH2Cl2 eluent, yielding
0.75 g (65%) 15 as a yellow solid. X-Ray quality crystals
were obtained by recrystallization from 1:4 EtOAc–hexane.
15: 31P NMR (121.4 MHz, CD2Cl2) d Ϫ90.7; 1H NMR
(300 MHz, CD2Cl2) d 1.47–1.57 (m, 2H, Hsyn), 1.72–1.82
(m, 2H, Hanti), 3.92 (s, 6H, CH3) 6.60 (dd, 2H, JP–Hˆ5.7 Hz,
JH–Hˆ8.4 Hz, m-H), 7.48 (t, JH–Hˆ8.4 Hz, p-H);
method of Qian et al.38
Ethylene glycol ditosylate was synthesized by the method
of Corey and Mitra.50
2,6-Dimethoxyphenylphosphirane
13.
To
2.20 g
(5.94 mmol) ethylene glycol ditosylate in a N2-flushed
250 mL round-bottomed Schlenck flask equipped with a
magnetic stirrer bar and a septum were added 150 mL
THF and 1.00 g (5.88 mmol) 2,6-dimethoxyphenyl-
phosphine. The flask was cooled to 0ЊC, and 7.5 mL
(12.0 mmol) 1.6 M n-BuLi was added dropwise via syringe.
The reaction mixture was then stirred for 45 min. A
31P NMR spectrum of an aliquot indicated disappearance
of the primary phosphine (31P d Ϫ172) and the appearance
of the product (31P d Ϫ254). Excess n-BuLi was quenched
by addition of 0.1 mL (0.78 mmol) Me3SiCl. After solvent,
excess Me3SiCl and other volatiles were removed under
vacuum, 150 mL pentane was added to precipitate lithium
salts. Filtration and concentration under vacuum gave a
slightly yellow liquid, 0.98 g (85% crude yield) suitable
for oxidation. Pure product was obtained by column chro-
matography on silica gel with 1:1 CH2Cl2-hexane eluent
followed by recrystallization from 1:9 CH2Cl2-hexane at
Ϫ20 ЊC. 13: mp 89.5–90.5ЊC 31P NMR (202.6 MHz,
13C{1H} NMR (150.9 MHz, CD2Cl2) d 13.5 (d, JP–C
ˆ
11.5 Hz, CH2), 56.7 (s, CH3), 104.3 (d, JP–Cˆ6.2 Hz,
m-C), 109.3 (d, JP–Cˆ97.6 Hz, ipso-C), 134.9 (s, p-C),
161.8 (s, o-C); elemental analysis, calcd for C10H13O2PS
C 52.62%, H 5.75%; obsvd C 52.55%, H 5.99%.
1-Bromo-3,4-dimethyl-3-phospholene was synthesized by
the method of Gruneich and Wisian–Neilson.44
1-(2,6-Dimethoxyphenyl)-3,4-dimethyl-3-phospholene
sulfide 20. A 100-mL three-necked flask equipped with a
pressure-equalizing dropping funnel, a magnetic stirring
bar, and an N2-inlet was charged with 3.165 g
(22.9 mmol) 1,3-dimethoxybenzene and 50 mL THF. The
reaction mixture was cooled to 0ЊC, and 9.15 mL
(22.9 mmol) 2.5 M n-BuLi was added dropwise. The reac-
tion mixture was than allowed to warm to room temperature
and stirred for 4 h. To this yellowish turbid solution, cooled
to Ϫ78ЊC, 3.00 g (15.5 mmoles) 1-bromo-3,4-dimethyl-3-
phospholene in 25 mL THF was added slowly, and the
reaction mixture was allowed to warm to room temperature.
1
CD2Cl2) d Ϫ253.7; H NMR (500 MHz, CD2Cl2) d 1.13–
1.17 (m, 4H, CH2), 3.84 (s, 6H, CH3), 6.47 (dd, 2H
JP–Hˆ2.0 Hz, JH–Hˆ8.0 Hz, m-H), 7.02 (t, 1H,
JP–Hˆ8.1 Hz, p-H); 13C{1H} NMR (125.7 MHz, CD2Cl2)
d 10.0 (d, JP–Cˆ40.2 Hz, CH2), 56.1 (s, CH3), 103.8 (s,
m-C), 116.8 (d, JP–Cˆ45.1 Hz, ipso-C), 130.0 (s, p-C),

116
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
The reaction mixture turned from yellow to orange and then
to brown in 2 h. A 31P NMR spectrum of an aliquot revealed
a strong new peak at 31P d Ϫ33 (Cf. 1-mesityl-3-phospho-
lene 31P d Ϫ31).14 Solvent was removed under vacuum, and
80 mL pentane was added to precipitate salts. The reaction
mixture was stirred 10 min., filtered, and the pentane
removed under vacuum. The orange residue was crude 1-
T
Azeotrope
79:9^0:1 ЊC
75:0^0:1 ЊC
70:0^0:1 ЊC
65:0^0:1 ЊC
60:0^0:1 ЊC
55:0^0:1 ЊC
44:0^0:1 ЊC
iPrOH:H2Oˆ88:12
CH3CN:EtOAcˆ23:77
cyclohexane:H2Oˆ92:8
CH3CN:CCl4ˆ17:83
CHCl3:hexaneˆ72:28
ClCH2CH2Cl:EtOHˆ86:14
CS2:iPrOHˆ72:28
(2,6-dimethoxyphenyl)-3,4-dimethylphospholene
which
was dissolved in 40 mL CH2Cl2 and filtered through a
small pad of silica gel. 1.5 g (46.9 mmol) S8 was added to
this solution, which was then stirred for 1 h. The 31P d Ϫ33
signal had been replaced by one at 31P d 41. Volatiles were
removed under vacuum, another 4 mL CH2Cl2 was added,
and residual sulfur was removed by filtration. Volatiles were
removed at 90ЊC/1 torr, and the 2.7 g residue was chro-
matographed on silica gel with CH2Cl2 eluent, giving a
white solid that was recrystallized from 1:9 CH2Cl2–hexane
at Ϫ20ЊC, yielding 2.3 g (53% based on 1-bromo-3,4-
dimethyl-3-phospholene) pure 20: mp 187.5–188.5ЊC;
The temperature of each vapor bath was measured over a
week with a 0.01ЊC precision thermometer, and the vari-
ation was less than 0.1ЊC. 31P NMR spectra were recorded
at evenly spaced intervals (the reaction mixtures were
quenched in cold water), and the concentrations of the phos-
phirane chalcogenide (and of the product of reaction with
trapping agent when present) were determined by compar-
ison of integrated peak areas with those of the ‘external’
standard. In each kinetic run three measurements were
taken at each time interval, which was measured with a
stop-watch. The plots of ln[Co/C] versus time were linear
in each case, and first-order rate constants and their
uncertainties were calculated from the slopes of the linear-
least squares fitted lines and their standard deviations. Rate
constants for pyrolysis of 14 at 75ЊC in C6D6 and in CD3CN
with no added trapping agent and in C6D6 in the presence of
EtSSEt and CD3OD and for pyrolysis of 15 at 65ЊC in C6D6
without and with added EtSSEt are displayed in Table 3.
31P NMR (121.4 MHz, CD2Cl2)
d
41.4; 1H NMR
(300 MHz, CD2Cl2) d 1.77 (s, 6HˆCCH3), 2.85 (bt, 2H
Jˆ16.5 Hz, CHH syn to S), 3.41(dm, 2H, Jˆ17.2 Hz,
CHH, anti to S), 3.89 (s, 6H, CH3), 6.08 (dd, 2H,
JP–Hˆ4.2 Hz, JH–Hˆ8.4 Hz, m-H), 7.40 (t, 1H,
JH–Hˆ8.4 Hz, p-H); 13C{1H} NMR (125.9 MHz, CD2Cl2)
d 16.4 (d, JP–Cˆ13 Hz, yCCH3), 47.7 (d, JP–Cˆ58 Hz,
CH2), 56.3 (s, OCH3), 104.7 (d, JP–Cˆ6 Hz, m-C), 111.8
(d, JP–Cˆ74 Hz, ipso-C), 127.4 (d, JP–Cˆ9 Hz, CyC),
133.8 (s, p-C), 160.9 (s, o-C); elemental analysis, calcd
for C14H19O2PS C 59.56%, H 6.78%; obsvd C 59.41%, H
6.92.
Rate constants for the disappearance of 14 in C6D6 without
trapping agent were determined at all seven temperatures,
while for 15 in C6D6 without trapping agent rate measure-
ments were made at 44, 55, 65, and 75ЊC. Plots of ln k and ln
k/T versus 1/T were linear. From these Arrhenius and
Eyring plots the activation parameters and their uncertain-
ties included in the discussion section were computed from
the slopes and intercepts of the linear least-squares fitted
lines and their standard deviations. The Eyring plot for 14
is shown in Fig. 6 and the Arrhenius plot for 15 in Fig. 7.
The original data including all the first-order kinetic plots
may be consulted.51
Single crystal X-ray analysis of 13, 14, and 15
The data were collected using a Siemens R3m/V diffracto-
meter (13, 14)with a graphite monochrometer and a Bruker
SMART Charge Coupled Device (CCD) Detector System
X-Ray Diffractometer (15), using Cu Ka radiation
ꢀ
(lˆ1:54178 A). The structures were solved by direct
methods. Structure solution and refinement were carried
out employing SHELXTL PLUS. The hydrogen atoms
were treated using the appropriate Riding model. Projection
views of 13, 14, and 15 are shown in Figs. 3–5, respectively
with non-hydrogen atoms represented by 25% probability
ellipsoids.
Thermal and photochemical trapping experiments
Pyrolysis of 14 in the presence of EtSSEt. 50 mg
(0.24 mmol) 14 and 0.30 mL (2.4 mmol) EtSSEt were
dissolved in 2.1 mL C6D6 producing a solution 0.1 M in
14 and 1.0 M in EtSSEt. This mixture in a Pyrex tube was
degassed to 10Ϫ5 torr via three freeze-pump–thaw cycles,
and the tube was sealed with a torch and heated at 75ЊC for
3 h. 31P NMR spectroscopy indicated 95% conversion of 14
and a 90% yield of 16. Volatiles were removed from the
slightly yellow reaction mixture by heating at 60ЊC/1 torr.
The residue was chromatographed on silica gel with 1:50
EtOH–CH2Cl2 eluent yielding 22 mg (30%) pure 16 as
colorless liquid: 31P NMR (121.4 MHz, CDCl3) d 49.5;
1H NMR (300 MHz, CDCl3) d 1.23 (t, 6H, JH–Hˆ7.2 Hz,
SCH2CH3), 2.80–3.00 (m, 4H, SCH2), 3.76 (s, 6H OCH3),
6.46 (dd, 2H, JP–Hˆ5.7 Hz, JH–Hˆ8.4 Hz, m-H), 7.53 (t, 1H,
JH–Hˆ8.4 Hz, p-H); 13C{1H} NMR (125.7 MHz, CDCl3) d
16.1 (s, SCH2CH3), 25.0 (s, SCH2), 56.1 (s, OCH3), 104.7 (s,
Kinetic studies of the pyrolysis of 14 and 15
A sealed capillary tube containing a C6D6 solution of
MeP(O)(OMe) used as an ‘external’ standard for
31P NMR integrations was placed in a 4 mm o.d.×150 mm
pyrex tube. The reaction mixtures were 0.1 M solutions of
14 or 15 (5.000 mg, 0.02356 mmol, 14 in C6D6 or CD3CN,
5.000 mg, 0.02191 mmol, 15 in C6D6) to which trapping
agents, when employed, were added (0.030 mL,
0.022 mmol, EtSSEt; 0.010 mL, 0.24 mmol, CD3OD). The
reaction mixtures were degassed at 10Ϫ5 torr via a freeze-
pump–thaw three-cycle process, and then the tubes were
sealed with a torch. The samples were thermostatted at
various temperatures by immersion in the vapors of
azeotropic mixtures whose boiling points are given below.

P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
117
m-C), 112.0 (d, JP–Cˆ103.9 Hz, ipso-C), 134.5 (s, p-C),
162.0 (s, o-C); MS (EI) m/e (relative intensity) 306 (Mϩ,
5), 277 (28), 245 (100), 217 (39), 199 (33), 187 (15), 185
(57), 170 (13), 169 (10), 155 (30), 141 (10), 139 (13), 138
(11), 123 (13), 109 (13), 107 (19), 95 (15), 92 (13), 79 (12),
77 (21), 76 (15), 64 (15), 47 (10); elemental analysis, calcd
for C12H19O3PS2 C 47.04%, H 6.25%; obsvd C 46.72%, H
6.52%.
o-C); MS (EI) m/e (relative intensity) 322 (M, 14), 294 (14),
262 (52), 261 (32), 229 (30), 201 (16), 200 (42), 185 (52),
170 (32), 169 (25), 167 (17), 153 (10), 151 (12), 139 (19)
138 (100), 123 (18), 109 (18), 108 (10), 107 (14), 95 (19), 77
(12), 63 (33); elemental analysis, calcd for C12H19O2PS2 C
44.70%, H 5.94%; obsvd C 44.61%, H 6.32%.
Pyrolysis of 15 in the presence of MeOH. 55 mg
(0.24 mmol) 15 and 0.50 mL (12.3 mmol) MeOH were
dissolved in 1.9 mL CD2Cl2 to produce a solution 0.1 M
in 15 and 5 M in MeOH. This mixture was placed in a
Pyrex tube, degassed, and sealed as above, and heated at
65ЊC for 3 h. 31P NMR spectroscopy indicated 99% conver-
sion of 15 and a 90% yield of 19. Volatiles were removed
from the light yellow reaction mixture by heating at
69ЊC/l torr, yielding 36.7 mg (66%) 19 as a light yellow
solid that decomposes above 75ЊC and slowly at room
temperature. A satisfactory elemental analysis was not
obtained. 19: 31P NMR (121.4 MHz, CDCl3) d 51.9 (d,
Pyrolysis of 14 in the presence of MeOH. 50 mg
(0.24 mmol) 14 and 0.50 mL (12.3 mmol) MeOH were
dissolved in 1.9 mL CD2Cl2 to produce a solution 0.1 M
in 14 and 5 M in MeOH. This mixture was placed in a
Pyrex tube, degassed and sealed as above, and heated at
75ЊC for 3 h. 31P NMR spectroscopy indicated 99% conver-
sion and an 85% yield of 17. Volatiles were removed from
the slightly yellow reaction mixture by heating at
75ЊC/l torr; yielding 31.5 mg (61%) 17 as a white solid
that decomposes above 80ЊC and slowly at room tempera-
ture. A satisfactory elemental analysis was not obtained. 17:
31P NMR (121.4 MHz, CDCl3) d 28.3 (d, JH–Pˆ564 Hz);
1H NMR (300 MHz, CDCl3) d 3.87 (s, 3H, POCH3), 3.90
(s, 6H, COCH3), 6.56 (dd, 2H, JH–Hˆ8.4 Hz, m-H), 7.41 (t,
1H, JH–Hˆ8.4 Hz, p-H), 8.62 (d, 1H, JP–Hˆ564 Hz, PH);
13C{1H} NMR (75.4 MHz, CDCl3) d 54.5 (s, POCH3),
56.0 (s, COCH3), 105.5 (d, JP–Cˆ8.4 Hz, m-C), 112.7 (d,
JP–Cˆ104 Hz, ipso-C), 133.9 (s, p-C), 161.1 (s, o-C); MS
(El) m/e (relative intensity) 217 (21), 216 (M, 100), 197
(46), 185 (16), 184 (14), 183 (63), 138 (39), 137 (34), 121
(19), 109 (40), 107 (19), 79 (33), 78 (15), 77 (33), 76 (15),
65 (14), 64 (10), 63 (16), 51 (13), 47 (21).
1
JH–Pˆ559 Hz); H NMR (300 MHz, CDCl3) d 3.80 (s, 3H,
POCH3), 3.86 (s, 6H, COCH3), 6.54 (dd, 2H, JH–Hˆ8.4 Hz,
m-H), 7.37 (t, 1H, JH–Hˆ8.4 Hz, p-H). 8.58 (d, 1H, JP–H
ˆ
559 Hz, PH); 13C {1H} NMR (75.4 MHz, CDCl3) d 52.6 (s,
POCH3), 56.4 (s, COCH3), 105.4 (d, JP–Cˆ5.6 Hz, m-C),
111.7 (d, JP–Cˆ94 Hz, ipso-C), 133.7 (s, p-C), 161.0 (s,
o-C); MS (EI) m/e (relative intensity) 232 (M, 85), 200
(61), 199 (51), 185 (100), 169 (35), 168 (11), 167 (45),
155 (11), 153 (26), 139 (21), 138 (10), 137 (13), 109 (14),
107 (11), 95 (11), 93 (23), 91 (11), 77 (11), 63 (26), 47 (10).
Photolysis of 15 in the presence of 2,3-dimethyl-1,3-buta-
diene. 55 mg (0.24 mmol) 15 and 0.27 mL (2.4 mmol) 2,3-
dimethyl-1,3-butadiene were dissolved in 2.13 mL CD2Cl2
producing a solution 0.1 M in 15 and 1 M in diene. This
mixture was placed in a quartz tube, degassed and sealed as
above, and irradiated at 15ЊC with 254 nm radiation for
100 min. A 31P NMR spectrum indicated 99% conversion
of 15 and a 60% yield of 20 together with a 15% yield of a
second product 21. Volatiles were removed under vacuum
from the light yellow reaction mixture, and the residue was
chromatographed on silica gel with CH2Cl2 eluent to give
two fractions. The second consisted of 40.6 mg (60%) 20,
Photolysis of 14 in the presence of MeOH. 50 mg
(0.24 mmol) 14 and 0.50 mL (12.3 mmol) MeOH were
dissolved in 1.9 ml CD2Cl2 to produce a solution 0.1 M in
14 and 5 M in MeOH. The mixture was placed in a quartz
tube, degassed and sealed as above, and irradiated at 254 nm
for 100 min. 31P NMR spectroscopy indicated 99% conver-
sion of 14 and an 80% yield of 17. Volatiles were removed
by heating at 75ЊC/1 torr leaving 25.1 mg (48%) 17 as a
white solid whose 31P, 1H, and 13C NMR spectra were iden-
tical with those obtained in the thermolysis experiment
reported above.
1
identical in its 31P, H, and 13C NMR spectra with those
reported above for an authentic sample. The first fraction
consisted of 10.7 mg (15%) 21 that could not be brought to
analytical purity: 31P NMR (121.4 MHz, CD2Cl2) d 60.1;
1H NMR (500 MHz, CD2Cl2) d 1.87 (d, 3H, JP–Hˆ5.2 Hz,
PCH2CCH3), 1.92 (s, 3H, SCH2CCH3), 3.17–3.57 (m, 4H,
Pyrolysis of 15 in the presence of EtSSEt. 55 mg
(0.24 mmol) 15 and 0.30 mL (2.4 mmol) EtSSEt were
dissolved in 2.1 mL C6D6 producing a solution 0.1 M in
15 and 1.0 M in EtSSEt that was placed in a Pyrex tube
and degassed and sealed as above. After the reaction
mixture was heated at 65ЊC for 3 h, 31P NMR spectroscopy
indicated 99% conversion of 15 and a 96% yield of 18.
Volatiles were removed from the slightly yellow reaction
mixture by heating at 60ЊC/l torr. The residue was chroma-
tographed on silica gel with 1:50 EtOH–CH2Cl2 eluent
yielding 32 mg (41%) pure 18 as a light yellow liquid:
SCH2 and PCH2), 3.90 (s, 6H, OCH3), 6.62 (dd, JP–H
ˆ
5.0 Hz, JH–Hˆ8.4 Hz, m-H), 7.38 (t, 1H, JH–Hˆ8.7 Hz,
p-H); 13C{1H} NMR (125.9 MHz, CD2Cl2) d 19.2 (s,
PCH2CCH3), 21.5 (s, SCH2CCH3), 34.5 (d, JP–Cˆ7.8 Hz,
SCH2), 45.0 (d, JP–Cˆ43.1 Hz, PCH2), 56.3 (s, OCH3),
105.1 (d, JP–Cˆ7.8 Hz, m-C), 114.7 (d, JP–Cˆ84 Hz, ipso-
C), 126.5 (s, SCH2), 132.0 (d, JP–Cˆ15.7 Hz, PCH2), 133.6
1
31P NMR (121.4 MHz, CDCl3)
(300 MHz, CDCl3)
d
72.4; 1H NMR
(s, p-C), 159.9 (s, o-C); The H–13C correlations observed
d
1.37 (t, 6H, JH–Hˆ6.6 Hz,
in HMQC and HMBC 2-D NMR experiments are listed
in Table 4. MS (EI) m/e (relative intensity) 298 (M, 1),
283 (14), 282 (84), 267 (13), 250 (13), 249 (75), 186
(10), 185 (100), 170 (28), 167 (25), 139 (14), 138 (16),
123 (18), 113 (13), 111 (24), 109 (20), 107 (21), 97 (10), 95
(23), 91 (13), 83 (13), 79 (14), 77 (23), 53 (15), 51 (11), 41
(25).
SCH2CH3), 2.98–3.12 (m, 4H, SCH2) 3.84 (s, 6H, OCH3),
6.61 (dd, 2H, JP–Hˆ5.9 Hz, JH–Hˆ8.4 Hz, m-H), 7.40 (t,
1H, JH–Hˆ8.4 Hz, p-H), 13C{1H} NMR (75.4 MHz,
CDCl3) d 15.1 (d, JP–Cˆ7.1 Hz, SCH2CH3), 28.1 (s,
SCH2), 56.0 (s, COCH3), 105.2 (d, JP–Cˆ7 Hz, m-C),
113.8 (d, JP–Cˆ97.4 Hz, ipso-C), 133.9 (s, p-C), 161.1 (s,

118
P. P. Gaspar et al. / Tetrahedron 56 (2000) 105–119
Table 4. Hydrogen–carbon correlations observed in 2-D-NMR experi-
ments on 21
Carbene Chemistry; Kirmse, W., Ed; 1st ed.; Academic: New
York, 1964; pp235–274; Spin states in carbene chemistry. In
Carbenes Vol. II, Moss, R. A., Jones, M. Jr., Eds.; Wiley: New
York, 1975; pp 207–362.
HMQC correlation
(one-bond)
HMBC correlations
(multiple-bond)
5. Gaspar, P. P.; Beatty, A. M.; Chen, T.; Haile, T.; Lei, D.;
Winchester, W. R.; Braddock-Wilking, J.; Rath, N. P.; Klooster,
W.T.; Koetzle, T. F.; Mason, S. A.; Albinati, A. Organometallics.
1999, 18, 3921–3932.
6. Moore, W. R.; Moser, W. R.; Laprade, J. E. J. Org. Chem. 1963,
28, 2200–2205; see also: Jerosch Herold, B.; Gaspar, P. P.
Fortschr. Chem. Forssch. 1965, 5, 89–146.
7. Hoffmann, R. J. Am. Chem. Soc. 1968, 90, 1475–1485.
8. Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J. Am.
Chem. Soc. 1997, 119, 10805–10809 and earlier references
contained therein.
1H d
13C d
19.2
21.5
34.5
34.5
45.0
45.0
56.3
105.1
133.6
13C d
1.87 (3H)
1.93 (3H)
3.21 (1H)
3.31 (1H)
3.31 (1H)
3.54 (1H)
3.90 (6H)
6.62 (2H)
7.38 (1H)
34.5, 126.5, 132.0
45.0, 126.5, 132.0
19.2, 126.5, 132.0
21.5, 126.5, 132.0
21.5, 126.5, 132.0
21.5, 126.5, 132.0
159.9
105.1, 114.7
105.1, 114.7, 159.9
9. Anwari, F.; Gordon, M. S. Isr. J. Chem. 1983, 23, 129–132.
10. Fleming, I. Frontier Orbitals and Organic Chemical Reac-
tions, Wiley: London, 1976.
11. Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. Rev. 1995, 95,
1203–1228.
12. Houk, K. N.; Rondan, N. G.; Mareda, J. Tetrahedron 1985, 41,
1555–1563.
13. Lei, D.; Gaspar, P. P. Organometallics 1985, 4, 1471–1473.
14. Li, X.; Lei, D.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc.
1992, 114, 8526–8530; Phosphorus, Sulfur Silicon Relat. Elem.
1993, 76, 71.
15. Li, X. Stereospecific Synthesis of Phosphiranes and Mechan-
isms of Phosphinidene Reactions; Washington University: St.
Louis, May 1994.
16. That phosphinidene chalcogenides are isolobal with singlet
carbenes was first pointed out by Schoeller and Niecke in a seminal
communication: Schoeller, W. W.; Niecke, E. J. Chem. Soc.,
Chem. Commun. 1982, 569–570.
Pyrolysis of 15 in the presence of benzil. 55 mg
(0.24 mmol) 15 and 0.5 mL (2.4 mmol) benzil were
dissolved in 2.4 mL C6D6 producing a solution 0.1 M in
15 and 1 M in benzil. This mixture was placed in a Pyrex
tube, degassed, and sealed as above, and heated at 65ЊC for
3 h. A 31P NMR spectrum indicated 99% conversion of 15
and a 70% yield of 22. Volatiles were removed from the
yellow reaction mixture under vacuum. The residue was
chromatographed on silica gel with CH2Cl2 eluent, yielding
20 mg (20%) 22 as a light yellow solid with a broad melting
point, 198–220ЊC that did not give a satisfactory elemental
analysis. 22: 31P NMR (121.4 MHz, CDCl3) d 96.8;
1H NMR (300 MHz, CDCl3) d 3.83 (s, 6H, CH3), 6.25 (d,
2H, JH–Hˆ8.4 Hz, Ar-m-H), 6.79–6.89 (m, 4H), 6.92–7.05
(m, 4H), 7.87–7.91 (m, 4H); 13C{1H} NMR (75.4 MHz,
CDCl3) d 56.5 (s, OCH3), 104.6 (d, JP–Cˆ12 Hz, Ar-m-C),
117.5 (d, JP–Cˆ88 Hz, Ar-ipso-C), 125.3 (s, O–Cy), 129.0
(s, Ph-o- or m-C), 130.0 (s, Ph-o- or m-C), 130.2 (s, Ph-ipso-
17. For a review of work up to 1990, see: Quin, L. D.; Szewczyk,
J. Oxo-, thioxo-, and selenoxophosphines. In Multiple bonds and
low coordination in phosphorus chemistry, Regitz, M., Scherer, O.
J., Eds. Georg Thieme: Stuttgart, 1990; pp 352–366.
18. 1,4-Addition to dienes is not commonly observed for carbenes
themselves, but silylenes and germylenes have been shown to
undergo their frequently found formal 1,4-additions via 1,2-addi-
tion followed by a vinylheterocyclopropane to heterocyclopentene
rearrangement See: Lei, D.; Gaspar, P. P. J. Chem. Soc., Chem.
Commun. 1985, 1149–1151; Bobbitt, K. L.; Maloney, V. M.;
Gaspar, P. P. Organometallics 1991, 10, 2772–2777.
19. Nakayama, S.; Yoshifuji, M.; Okazaki, R.; Inamoto, O.
J. Chem. Soc., Chem. Commun. 1971, 1186–1187.
20. Yoshifuji, M.; Nakayama, S.; Okazaki, R.; Inamoto, N.
J. Chem. Soc., Perkin Trans. I. 1973, 2065–2068.
21. Nakayama, S.; Yoshifuji, M.; Okazaki, R.; Inamoto, N.
J. Chem. Soc., Perkin Trans. I. 1973, 2069–2071.
22. Nakayama, S.; Yoshifuju, M.; Okazaki, R.; Inamoto, N. Bull.
Chem. Soc. Jpn. 1975, 48, 546–548.
C), 133.7 (s, Ar-p-C), 134.5 (s, Ph-p-C), 161.6 (d, JP–C
5 Hz, Ar o-C).
ˆ
Acknowledgements
This work has received financial support from National
Science Foundation Grant CHE-9632897. NMR instrumen-
tation was made available in part through NIH Shared
Instrument Grants RR-02004, RR-05018 and RR-07155.
Dr Frank Borkenhagen provided valuable experimental
advice.
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