The deoxygenation of phosphine oxides under green chemical conditions

Article abstract of DOI:10.1002/hc.21249

The deoxygenation of a few diaryl-phenylphosphine oxides, dimethyl-phenylphosphine oxide, and 3-methyl-1-phenyl-3-phospholene 1-oxide was studied by phenylsilane, tetramethyldisiloxane (TMDS), and polymethylhydrosiloxane (PMHS) under conventional or microwave (MW) heating, in toluene or in the absence of any solvent at different temperatures. It was found that the deoxygenation with TMDS or PMHS under MW and solvent-free conditions may be the method of choice and provides a green chemical approach.

Full text of DOI:10.1002/hc.21249

Heteroatom Chemistry
Volume 00, Number 00, 2014
The Deoxygenation of Phosphine Oxides
under Green Chemical Conditions
Gyo¨rgy Keglevich, Tamara Kova´cs, and Flo´ra Csatlo´s
Department of Organic Chemistry and Technology, Budapest University of Technology and
Economics, 1521 Budapest, Hungary
Received 5 September 2014; revised 16 October 2014
wide range of phosphine oxides including mainly
ABSTRACT: The deoxygenation of a few diaryl-
phospholene- and phospholane derivatives [7–10].
phenylphosphine oxides, dimethyl-phenylphosphine
In these cases, the phosphoranes were formed in
oxide, and 3-methyl-1-phenyl-3-phospholene 1-oxide
situ by the quaternization of the phosphine by the
was studied by phenylsilane, tetramethyldisiloxane
α-haloacetic ester [7–9] or even an unactivated alkyl
(TMDS), and polymethylhydrosiloxane (PMHS) un-
halide [8], followed by dehydrohalogenation by
der conventional or microwave (MW) heating, in
Na2CO3 [7, 9], diisopropylethylamine [8], or in situ
toluene or in the absence of any solvent at different
generated tBuONa [10].
temperatures. It was found that the deoxygenation
The most commonly used method for the
with TMDS or PMHS under MW and solvent-free con-
deoxygenation of phosphine oxides involves the
ditions may be the method of choice and provides
application of silanes [11–14]. Beside the silanes,
ꢀC
a green chemical approach.
2014 Wiley Period-
other reducing agents, such as lithium aluminum
hydride [12], sodium borohydride [15], diisobutyla-
luminum hydride (DIBAL) [16,17], or triphenylphos-
phine [18] were also described. Phenylsilanes, such
as PhSiH3, Ph2SiH2, and Ph3SiH represent an im-
portant group within silanes [19, 20]. Phenylsilane
(PhSiH3) was used over a temperature range of
26–150°C [19,20], whereas triphenylsilane (Ph3SiH)
was used at 300°C [19]. The application of PhSiH3
is quite advantageous, as principally 0.33 equiv is
enough due to the presence of the three H atoms.
However, PhSiH3 is rather expensive. Cl3SiH is
widely used in synthetic organic chemistry [21, 22].
The reduction of phosphine oxides with Cl3SiH
may be performed in an autoclave at 200°C [21],
in boiling benzene in the presence of 1 equiv of
triethylamine [21], or together with 3 equiv of
pyridine [22]. In the latter case, the P-configuration
was preserved [22]. Stereochemical retention was
also justified by theoretical calculations [23]. It
is a disadvantage that Cl3SiH is quite volatile
(bp 31.8°C) and corrosive. Alkyldichlorosilanes
(RSiCl2H, where R = C1–C4 and C6 alkyl) and
phenyldichlorosilane (PhSiCl2H) were also used,
icals, Inc. Heteroatom Chem. 00:1–7, 2014; View
this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21249
INTRODUCTION
The reduction of tertiary phosphine oxides to
phosphines is applied to many syntheses, e.g., in the
final step of the preparation of P-ligands [1–5] or
during the regeneration of triphenylphosphine from
triphenylphosphine oxide formed in the Wittig- or
the Appel reaction. It has been a challenge to develop
catalytic versions of these reactions involving the
recycling of the waste cyclic phosphine oxides in
in situ reduction [6]. O’Brian et al. have developed
catalytic versions of the Wittig reaction involving
in situ reduction, often using diphenylsilane, of a
Correspondence to: Gyo¨rgy Keglevich; e-mail: gkeglevich@
mail.bme.hu.
Contract grant sponsor: Hungarian Scientific Research Fund
(OTKA). Contract grant number: K83118.
ꢀC
2014 Wiley Periodicals, Inc.
1

2
Keglevich, Kova´cs, and Csatlo´s
At the same time, under solvent-free conditions, the
deoxygenation was practically quantitative after 3 h
(Table 1, entry 2). On increasing the temperature to
150°C, a 1-h reaction time was sufficient (Table 1,
entry 3). The best variation is to carry out the 1 → 2
transformation at 150°C under solvent-free and MW-
assisted conditions. In this case, the deoxygenation
was complete already after 30 min. The workup in-
volved essentially a filtration via a thin silica layer
using hexane–ethyl acetate to give triphenylphos-
phine (2) in a yield of 95% (Table 1, entry 4).
Then, we changed to the less reactive TMDS that
was applied to a 10-equiv quantity (that is really a 5
molar equivalent quantity) at 175°C using 1,4-xylene
as the solvent, or performing the reduction in the ab-
sence of solvent. After a reduction time of 48 and 24
h, the conversion was 53% and 100%, respectively
(Table 1, entries 5 and 6). Changing to MW heat-
ing, at 175°C/6 h and 200 °C/6 h, the conversion to
phosphine 2 was 47% and 93%, respectively (Table 1,
entries 7 and 8). In the latter case, the preparative
yield of phosphine 2 was 89%.
T, t (N2)
O
silane
..
Y
P
Y
Y
P
Y
solvent
Ph
Ph
Y
1
3
5
7
Ph
4-MePh
4-ClPh
Me
2
4
6
8
SCHEME 1
but these reagents have not proved popular [24].
Perchlorosilanes (e.g., Si2Cl6) were also applied
to deoxygenations [25], but they remained of lim-
ited use [25]. The group of user friendly silanes
include (EtO)3SiH [26, 27], (EtO)2MeSiH [28],
1,1,3,3-tetramethyldisiloxane (TMDS), and poly-
methylhydrosiloxane (methylpolysiloxane [PMHS])
that has the structure of [–O–SiH(Me)–]n [19,26,28].
These silanes are not as reactive as, e.g., PhSiH3
or Cl3SiH. The deoxygenations by TMDS were
performed in the presence of a catalytic amount of
titanium(IV) isopropoxide [29]. (EtO)3SiH was also
used together with Ti(OiPr)4 as the catalyst [26,27].
Deoxygenations by TMDS and PMHS were also cat-
alyzed by copper salts and Brønsted acid catalysts
(e.g., by Cu(OTf)2 or the diphenyl ester of phosphoric
acid, respectively) [28,30]. The authors reported that
these catalysts were helpful at 100–110°C in toluene.
PMHS was also tried, being applied to a quan-
tity of 5 equiv at 175°C. To carry out the reduction
in 1,4-xylene for 48 h, without any solvent for 16 h
and under solvent-free MW conditions for 7.5 h, the
conversion was 85%, 100%, and 98%, respectively
(Table 1, entries 9–11). In the last case, phosphine 2
was isolated with a yield of 90%.
If was
a challenge for us to study the
The best experiments were those marked by
entries 4, 8, and 11 in Table 1.
deoxygenations of phosphine oxides by TMDS and
PMHS that are relatively cheap and user friendly
reagents. In this article, the optimum conditions for
the reductions by TMDS and PMHS are described.
Our preference was to carry out the reductions with-
out the use of any catalyst.
The reductive experiments with phenyl-di(4-
methylphenyl)phosphine oxide (3) led essen-
tially to results similar to those obtained with
triphenylphosphine oxide (1) (Scheme 1, Table 2).
Mostly solvent-free deoxygenations were studied
with the three silanes (PS, TMDS, and PMHS). In
respect of the MW-assisted variations, the reduc-
tions with TMDS at 200°C for 6 h and with PMHS
at 175°C for 7.5 h (Table 2, entries 5 and 8) are good
alternatives of the accomplishment by PS at 150°C
for 0.5 h (Table 2, entry 2). On conventional heat-
ing at lower or at the same temperature, the com-
pletion required a longer period of time (Table 2,
entries 1, 3, and 7). The isolated yields of phenyl-
di(4-methylphenyl)phosphine 4 from the best MW-
promoted reductions amounted to ca. 87% (Table 2,
entries 2, 5, and 8).
Our next model reaction was the reduction
of di(4-chlorophenyl)-phenylphosphine oxide (5)
(Scheme 1). The experimental data are listed in
Table 3. PS could be used as in the deoxygenation
of the other diaryl-phenylphosphine oxides 1 and 3.
It can also be seen that in a reaction with TMDS
and PMHS, di(4-chlorophenyl)-phenylphosphine
RESULTS AND DISCUSSION
The
phosphine
oxides
selected
(diaryl-
phenylphosphine oxides, dimethyl-phenylphosphine
oxides, and 3-methyl-1-phenyl-3-phospholene 1-
oxide) were reduced by phenylsilane (PS), TMDS,
and PMHS under different conditions in an aromatic
solvent, or without any solvent, using conventional
heating or on microwave (MW) irradiation. The
reaction mixtures were analyzed by 31P NMR.
The first model reaction was the deoxygena-
tion of triphenylphosphine oxide (1) (Scheme 1).
Experimental details of the optimizations are listed
in Table 1.
The reduction of triphenylphosphine oxide 1
with 9 equiv of PS at 110°C in toluene was slow.
After a 12-h reaction time, the conversion to triph-
enylphosphine (2) was only 14% (Table 1, entry 1).
Heteroatom Chemistry DOI 10.1002/hc

Deoxygenation of Phosphine Oxides under Green Chemical Conditions
3
TABLE 1 Deoxygenation of Triphenylphosphine Oxide (1) (0.36 mol) by Different Silanes on Conventional Heating or on MW
Irradiation
Mode of
Heating
Conversion
(%)
Entry Silane Equivalent
T (°C)
t (h)
Solvent
Yield (%)
Comment
1
2
3
4
5
6
7
8
9
PhSiH3
PhSiH3
PhSiH3
PhSiH3
TMDS
TMDS
TMDS
TMDS
PMHS
9
9
9
ꢀ
ꢀ
ꢀ
MW
ꢀ
ꢀ
MW
MW
ꢀ
110
110
150
150
175
175
12
3
1
0.5
48
2 mL toluene
14
97
100
99
53
100
47
–
–
–
86
92
95
9
10
10
10
10
5
2 mL 1,4-xylene
24
–
–
–
91
89
a
175 6 (textrap ꢀ14)
200
175
6
48
93
85
2 mL 1,4-xylene
Solid crude
mixture
Solid crude
mixture
Solid crude
mixture
10
11
PMHS
PMHS
5
5
ꢀ
175
175
16
–
–
100
98
MW
7.5
90
atextrap = Extrapolated reaction time.
ꢀ = Conventional heating.
TABLE 2 Deoxygenation of Phenyl-di(4-methylphenyl)phosphine Oxide (3) by Different Silanes on Conventional Heating or
on MW Irradiation
Mode of
Heating
Conversion
(%)
Entry Silane Equivalent
T (°C)
t (h)
Solvent
Yield (%)
Comment
1
2
3
4
5
6
7
PhSiH3
PhSiH3
TMDS
TMDS
TMDS
PMHS
PMHS
9
9
10
10
10
5
ꢀ
MW
ꢀ
MW
MW
ꢀ
110
150
175
3
0.5
–
–
–
–
–
98
97
94
46
93
76
100
91
89
85
24
a
175 6 (textrap ꢀ14)
200
175 48 (textrap ꢀ60) 2 mL 1,4-xylene
175
6
86
93
88
a
5
ꢀ
20
–
Solid crude
mixture
Solid crude
mixture
8
PMHS
5
MW
175
7.5
–
92
atextrap = Extrapolated reaction time.
ꢀ = Conventional heating.
TABLE 3 Deoxygenation of Di(4-chlorophenyl)-phenylphosphine Oxide (5) by Different Silanes on Conventional Heating or
on MW Irradiation
Mode of
Heating
Conversion
(%)
Entry
Silane
Equivalent
T (°C)
t (h)
Yield (%)
Comment
1
2
3
4
5
6
7
PhSiH3
PhSiH3
TMDS
TMDS
TMDS
PMHS
PMHS
9
9
10
10
10
5
ꢀ
MW
ꢀ
MW
MW
ꢀ
110
150
175
175
200
175a
175b
3
0.5
24
6
8
24
7.5
97
92
74
36
90
51
54
91
87
86
Solid crude mixture
Solid crude mixture
5
MW
a50 μL of toluene was added as a homogenizing additive.
bThe addition of 50 μL of toluene did not have any impact on the course of the reaction.
ꢀ = Conventional heating.
oxide (5) is less reactive than the previous model
compounds (1 and 3). Using TMDS at 175°C, the
conversions were lower both on conventional heat-
ing (24 h) and under MW (6 h) conditions as com-
pared to the unsubstituted case as shown by the con-
versions of 74% versus 100% and 36% versus 47%,
respectively (Table 3, entry 3 vs. Table 1, entry 6
and Table 3, entry 4 vs. Table 1, entry 7). The MW
Heteroatom Chemistry DOI 10.1002/hc

4
Keglevich, Kova´cs, and Csatlo´s
Me
Me
Me
variation at 200°C required 8 h to obtain a conver-
sion of 90% (Table 3, entry 5). The yield of phosphine
6 was almost the same (86%), as that obtained in the
case, when PS was applied at 150°C for 0.5 h un-
der MW conditions (87%) (Table 3, entries 5 and 2).
At the same time, chlorophenyl derivative 5 could
not be deoxygenated quantitatively with PMHS due
to the heterogeneity of the reaction mixture. There
was practically no difference between the outcome
of the traditional thermal and the MW-assisted re-
actions (Table 3, entries 6 and 7). The addition of
a small quantity of toluene as a “cosolvent” did not
help either.
25 °C/12 h
T, t (N2)
silane
S8 (1.2 equiv.)
P
P
P
solvent
.
PhMe
.
O
Ph
Ph
S
Ph
9
10
11
SCHEME 2
reductions. In this case, the phosphine (10) was
quite sensitive to air oxidation, and so this was pre-
vented by an in situ reaction with sulfur to give the
corresponding phospholene sulfide (11) for identifi-
cation (Scheme 2).
Earlier, the reduction of 3-methyl-1-phenyl-
3-phospholene 1-oxide 9 by silanes was studied
in toluene solutions [31]. Applying PS, TMDS,
and PMHS in boiling toluene, completion of the
deoxygenation required 6–8 h. The experimental de-
tails of the present study are listed in Table 5. The
enhanced reactivity of the phospholene oxide (9)
and the oily consistency made possible the use of
the silanes in a smaller quantity. Hence, PS, TMDS,
and PMHS were applied to a quantity of 3, 4, and 2
equiv, respectively. Using PS at 80°C without any sol-
vent, the completion of the deoxygenation required
2 h (Table 5, entry 1). Under solvent-free MW con-
ditions, the reaction time was 1 h (Table 5, entry
2).
Changing to TMDS and selecting 110°C, under
thermal and MW conditions, the reaction time was
5 and 3 h, respectively (Table 5, entries 3 and 4).
Finally, the experiments with PMHS performed at
110°C showed that the thermal deoxygenation took
4 h, whereas the MW variation only 2 h (Table 5,
entries 5 and 6).
One can see that the more reactive, but expen-
sive PS can be replaced well by the less reactive, but
cheaper TMDS or, with the exception of the case of
the chlorophenyl derivative, PMHS. The preparative
yields of triarylphosphines (1–5) were around 90%.
It is also noteworthy that the deoxygenations were
accomplished under solvent-free and MW-assisted
conditions, providing an advantage from the point
of view of green chemistry techniques.
Then, reduction of an obviously more reactive
phosphine oxide, dimethyl-phenylphosphine oxide 7
was investigated (Scheme 1). As can be seen from
Table 4, all the three kinds of silanes could be
used at somewhat milder conditions, than in the
case of triarylphosphine oxides (1, 3, and 5). The
deoxygenation of phosphine oxide 7 by PS at 110°C
required 2 h on conventional heating and 1 h under
MW irradiation (Table 4, entries 1 and 2). In the
presence of only 3 equiv of PS, quantitative deoxy-
genation was achieved at 130°C after a 1 h of MW
irradiation (Table 4, entry 3). Using TMDS at 150°C,
completion of the thermal and the MW-assisted
variation required 15 and 9 h, respectively (Table 4,
entries 4 and 5). On increasing the temperature of
the MW-promoted reaction to 175°C, the deoxy-
genation was complete after 4 h even in the presence
of only 4 equiv of TMDS (Table 4, entry 6). Applying
PMHS at 150°C and depending on whether conven-
tional or MW heating was used, completion of the
reaction required 10 and 4.5 h, respectively (Table 4,
entries 7 and 8). At 175°C, the MW variation com-
pleted after 2 h even in the presence of only 2 equiv of
PMHS (Table 4, entry 9). The best experiments were
again the MW-promoted deoxygenations giving
dimethyl-phenylphosphine 8 in 88–94% yield after a
filtration through a thin silica gel layer (Table 4, en-
tries 3, 6, and 9) and, for these reactions, the cheaper
TMDS and PMHS may well replace the more expen-
sive PS. MW irradiation made it possible to shorten
reduction times, and there was no need for a solvent.
Finally, the deoxygenation of 3-methyl-1-phenyl-
3-phospholene 1-oxide (9) was studied in silane
The yields of the MW-assisted reactions
amounted to 91–92%, and TMDS along with
PMHS were again fully comparable with PS. MW
activation, together with solvent-free conditions,
providing an attractive alternative for the deoxy-
genations with silanes.
On the basis of our experiences, the order of re-
activity of the phosphine oxides tested is as follows:
Me
(4-ClPh)2P(O)Ph
< Ph3P(O) ~ (4-MePh)2P(O)Ph < Me2P(O)Ph <
P
O
Ph
The trialkylphosphine oxides are less reactive as
a consequence of the steric hindrance around the
P=O moiety. 3-Phospholene 1-oxide 9 may be more
reactive than PhP(O)Me2, as on the one hand, the
ring system results in some decrease in the steric
hindrance. On the other hand, unsaturation may also
promote reactivity.
In summary, the deoxygenation of phosphine
oxides by the cheaper reagents TMDS and PMHS
Heteroatom Chemistry DOI 10.1002/hc

Deoxygenation of Phosphine Oxides under Green Chemical Conditions
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TABLE 4 Deoxygenation of Dimethyl-phenylphosphine Oxide (7) by Different Silanes on Conventional Heating or on MW
Irradiation
Mode of
Heating
Conversion
(%)
Entry
Silane
Equivalent
T (°C)
t (h)
Yield (%)
Comment
1
2
3
4
5
6
7
8
9
PhSiH3
PhSiH3
PhSiH3
TMDS
TMDS
TMDS
PMHS
PMHS
PMHS
9
9
3
10
10
4
5
5
2
ꢀ
110
110
130
150
150
175
150
150
175
2
1
1
100
97
99
97
70
98
100
87
93
90
94
89
MW
MW
ꢀ
MW
MW
ꢀ
15
a
6 (textrap ꢀ9)
4
88
95
10
Solid crude mixture
Solid crude mixture
Solid crude mixture
a
MW
MW
4 (textrap ꢀ4.5)
2
100
93
atextrap = Extrapolated reaction time.
ꢀ = Conventional heating.
TABLE 5 Deoxygenation of 3-Methyl-1-phenyl-3-phospholene 1-Oxide (9) without a Solvent by Different Silanes on Conven-
tional Heating or MW Irradiation
Mode of
Heating
Conversion.
(%)
Entry
Silane
Equivalent
T (°C)
t (h)
Yield (%)a
1
2
3
4
5
6
PhSiH3
PhSiH3
TMDS
TMDS
PMHS
PMHS
3
3
4
4
2
2
ꢀ
MW
ꢀ
MW
ꢀ
80
80
110
110
110
110
2
1
5
3
4
2
100
100
100
100
100
100
95
91
92
92
91
92
MW
aThe phosphine (10) was identified as its sulfide (11).
ꢀ = Conventional heating.
may offer a good alternative under MW and solvent-
free conditions to replace the more expensive and
more reactive PS. This is the first case that TMDS
and PMHS turned out to be generally usable
deoxygenation agents. The application of MW and
solvent-free conditions offers a greener chemical ap-
proach.
General Procedure for the Deoxygenation of the
Phosphine Oxides 1, 3, 5, and 7
Using PS in a Quantity of 9 Equiv. A mixture of
0.36 mmol of phosphine oxide (1: 0.10 g, 3: 0.11 g,
5: 0.098 g, 7: 0.13 g) and 1.1 mmol (0.13 mL) of PS
was heated under nitrogen atmosphere using an oil
bath or a MW oven at the appropriate temperature
in a glass bomb (a thick-wall glass tube that can be
closed) or in a commercial MW vial, respectively, for
the appropriate time. Then, the reaction mixture was
cooled to room temperature and after taking up the
oily mixture in some ethyl acetate, it was absorbed
on a 2-cm layer of silica gel. Then, the phosphine was
washed off using hexane–ethyl acetate, 9:1 to afford
the corresponding phosphine (2: as a white solid, or
4, 6, and 8: as colorless oils). (For the details, see
Tables 1–4.) As a matter of fact, the main fraction
was collected after a smaller prefraction.
EXPERIMENTAL
General
1H, 13C, and 31P NMR spectra were obtained
in a CDCl3 solution on a Bruker AV-300 spec-
trometer operating at 300, 75.5, and 121.5 MHz,
respectively. Chemical shifts are downfield relative
to 85% H3PO4 and TMS. Couplings are given in
hertz. High-resolution molecular weights were ob-
tained using a Q-TOF Premier mass spectrometer
in the positive electrospray mode. PMHS with an
average molecular weight 1700–3200 was used. The
reactions were carried out in a 300-W CEM Discover
focused MW reactor equipped with a pressure con-
troller applying 50–80 W under isothermal condi-
tions.
Using TMDS in a Quantity of 10 Equiv. The re-
ductions were carried out as those with PS shown
above using 1.8 mmol (0.32 mL) of TMDS. The
phosphines (2, 4, 6, and 8) were obtained as a white
solid (2) or as colorless oils (4, 6, and 8). (For the
details, see Tables 1–4.)
Heteroatom Chemistry DOI 10.1002/hc

6
Keglevich, Kova´cs, and Csatlo´s
Using PMHS in a Quantity of 5 Equiv. The re-
raphy (hexane–ethyl acetate, 9:1 as an eluent) of the
residue afforded the sulfide (11) as a dense oil. (For
the details, see Table 5.)
ductions were carried out as those with PS shown
above using 1.8 mmol (0.068 mL) of PMHS. The
phosphines (2, 4, 6, and 8) were obtained as a white
solid (2) or as colorless oils (4, 6, and 8). (For the
details, see Tables 1–4.)
3-Methyl-1-phenyl-3-phospholene 1-sulfide (11).
31P NMR (CDCl3) δ: 55.3, δ (CDCl3) [35]: 55.3;
[M + H]+
= 209.0554, C11H14PS requires [M +
found
The following phosphines were prepared:
H]+ = 209.0554.
Triphenylphosphine (2). Based on the experimental
details presented in Table 1 (entry 3), yield: 95%;
white solid, mp: 81–82°C; mp. [32]: 79–81°C; 31P
NMR (CDCl3) δ: –5.0, δ: (CDCl3) [32]: –5.1; [M +
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ArH); [M + H]+
= 331.0212, C18H14Cl2P re-
found
quires: 331.0205.
Dimethyl-phenylphosphine (8). Based on the exper-
imental details presented in Table 4 (entry 8), yield:
95%; colorless oil; 31P NMR (CDCl3) δ: –43.2, δ
(CDCl3) [34]: –42.4; [M + H]+
= 139.0682,
found
C8H12P requires: 139.0677.
General Procedure for the Deoxygenation of
1-Phenyl-3-methyl-3-phospholene 1-Oxide (9),
and for the Trapping of the Phosphine (10) so
Obtained
A mixture of 0.11 g (0.55 mmol) of 1-phenyl-3-
methyl-3-phospholene 1-oxide (9) and 0.068 mL
(0.55 mmol) of PS or 0.19 mL (1.1 mmol) of TMDS
or 0.042 mL (1.1 mmol) of PMHS was heated under
nitrogen atmosphere at the appropriate temperature
in a glass bomb immersed in an oil bath or in a com-
mercial MW vial in the MW oven for the appropriate
time. Then, the reaction mixture was cooled to room
temperature to afford phosphine 10 that was reacted
further immediately to form the corresponding sul-
fide (11).
To the ꢀ0.55 mmol of the corresponding
phosphine (10) in 2 mL of toluene, 20.7 mg
(0.65 mmol) of powdered sulfur was added under
nitrogen. The mixture was stirred at 26°C overnight
and then evaporated to dryness. Column chromatog-
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