Thermal rearrangement of thiocarbonyl-stabilised triphenylphosphonium ylides leading to (Z)-1-diphenylphosphino-2-phenylsulfenylalkenes

Article abstract of DOI:10.1039/b913107h

While thermolysis of thiocarbonyl-stabilised phosphonium ylides generally results in extrusion of Ph₃PS to give alkynes, those with a PCH function instead undergo a novel P to S transfer of a phenyl group to give (Z)-configured 1-phosphino-2-su

Full text of DOI:10.1039/b913107h

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Thermal rearrangement of thiocarbonyl-stabilised triphenylphosphonium
ylides leading to (Z)-1-diphenylphosphino-2-phenylsulfenylalkenesw
R. Alan Aitken,* Graham Dawson, Neil S. Keddie, Helmut Kraus, Alexandra M. Z. Slawin,
Joanne Wheatley and J. Derek Woollins
Received (in Cambridge, UK) 1st July 2009, Accepted 20th October 2009
First published as an Advance Article on the web 5th November 2009
DOI: 10.1039/b913107h
While thermolysis of thiocarbonyl-stabilised phosphonium ylides
generally results in extrusion of Ph3PS to give alkynes, those
with a PQCH function instead undergo a novel P to S transfer
of a phenyl group to give (Z)-configured 1-phosphino-2-sulfenyl-
alkenes of interest as bidentate ligands.
Table 1 Preparation and pyrolysis of thiocarbonyl ylides
Ylide
Yield (%)
dP
FVP product(s)
Yield (%)
5
6
7
8
9
42
40
30
55
55
ꢀ12.3
+6.7
+6.1
+4.4
+8.1
10 (+Ph3PS)
11 (+Ph3PS)
12 (+Ph3PS)
13
52
40
39
82
49
14
Thermal extrusion of triphenylphosphine oxide from
carbonyl-stabilised phosphonium ylides 1, particularly using
the technique of flash vacuum pyrolysis (FVP), provides a
useful synthesis of functionalised alkynes 2,1 and recent
applications include synthesis of acetylenic amino acid
analogues2 and cascade reactions leading to polycyclic hetero-
aromatic compounds.3 In contrast, the thermolysis of the
corresponding thiocarbonyl-stabilised ylides has scarcely been
examined, although Bestmann and Schaper found that heating
the ylides 3 above their melting point resulted in extrusion of
triphenylphosphine and triphenylphosphine sulfide to give the
thiophenes 4 (Scheme 1).4 In the context of our recent studies
on the kinetics and mechanism of the thermal reactions of
stabilised ylides,5 we were interested to compare the behaviour
of analogous carbonyl and thiocarbonyl ylides and we report
here the behaviour of thiocarbonyl-stabilised phosphonium
ylides under FVP conditions.
Thiocarbonyl-stabilised ylides have been prepared by
thioacylation of non-stabilised ylides with dithioesters or
dithiocarbonates,6 and also by activation of the corresponding
carbonyl-stabilised ylides with either triflic anhydride7 or
phosphoryl chloride8 followed by treatment with sodium
sulfide. However we opted for the more direct method of
O–S exchange using Lawesson’s reagent in boiling toluene as
reported by Capuano and co-workers,9 and in this way the
known thiocarbonyl ylides 5–7,7 as well as the previously
unknown compounds 8 and 9,z were prepared from their
carbonyl analogues (Table 1).
When compounds 5–7 were subjected to FVP in
a
conventional flow system, complete reaction occurred at 650 1C
and the products were triphenylphosphine sulfide and alkynes
10–12. This behaviour is entirely analogous to that of the
carbonyl-stabilised ylides 1, although the latter require a
temperature of 750 1C for complete reaction.10 We attribute
the higher reactivity of the thiocarbonyl compounds, a feature
already noted in the pyrolysis of thiocarbamoyl vs. carbamoyl
ylides,5 to the greater nucleophilicity of sulfur.
The two ylides 8 and 9 also underwent complete reaction
upon FVP at 650 1C, but the process observed was entirely
different. In each case, only a single product was obtained
whose analytical and spectroscopic data showed it to be
isomeric with the starting material, but to have a 31P NMR
signal in the range dP ꢀ20 to ꢀ25 characteristic of phosphines.
The spectroscopic data were fully consistent with the
1-diphenylphosphino-2-phenylsulfenylalkene structures 13
and 14 and this was proved beyond doubt by a single crystal
X-ray diffraction study on 13 (Fig. 1). Compound 13 was
obtained in good yield and in pure form by recrystallisation
(EtOAc) of the material that collected at the furnace exit, and
was entirely the (Z)-isomer [dH 6.85 (PQCH); dC 137.8
Scheme 1
EaStCHEM School of Chemistry, University of St Andrews, North Haugh,
St Andrews, Fife, UK KY16 9ST. E-mail: raa@st-and.ac.uk;
Fax: +44 (0)1334 463808; Tel: +44 (0)1334 463865
w CCDC 738768. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b913107h
2
(d, 1JP–C 6, C-1), 158.9 (d, JP–C 25, C-2), 41.6 (d, 3JP–C 3, C-3)].
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Fig. 1 X-Ray structure of compound 13. Selected bond lengths, non-
bonded distance, angles and torsion angle; P(1)–C(1) 1.818(4), C(1)–C(2)
1.316(6), C(2)–S(2) 1.788(4), P(1)–S(2) 3.146 A; C(1)–P(1)–C(7)
99.15(19)1, C(1)–P(1)–C(13) 103.60(19)1, C(7)–P(1)–C(13) 102.03(18)1,
P(1)–C(1)–C(2) 125.5(3)1, C(1)–C(2)–S(2) 115.1(3), C(1)–C(2)–C(3)
125.0(4)1, S(2)–C(2)–C(3) 118.9(3)1; P(1)–C(1)–C(2)–S(2) 9.8(5)1.
In contrast, compound 14 was obtained in lower yield as a
colourless liquid which was mainly the (Z)-isomer [dH 2.02
(3H, s, Me), 6.34 (1H, s, PQCH); dC 132.2 (d, 1JPꢀC 10, C-1),
Scheme 2
2
3
148.8 (d, JPꢀC 27, C-2), 25.7 (d, JPꢀC 4, Me); dP –22.7],
but upon repeated distillation (bp 90 1C/0.1 Torr) was
progressively converted into the (E)-isomer [dH 2.24 (3H, s, Me),
1
5.93 (1H, s, PQCH); dC 132.1 (d, JPꢀC 10, C-1), 149.7
2
3
(d, JPꢀC 30, C-2), 20.9 (d, JPꢀC 23, Me); dP –25.4].
Despite the obvious potential of alkenes bearing a (Z)-1,2-
arrangement of phosphine and sulfide functions as bidentate
ligands, few such compounds appear to be known. The
(E)-compound Bu2P–CHQCH–SBu has been reported,11 as
well as a variety of chlorinated analogues as mixtures of (E)
and (Z) isomers,12 and more recently Braga and co-workers
have obtained alkenes with
a (Z)-1,2-arrangement of
phosphonate and sulfide, selenide and telluride,13 as well as
phosphine oxide and telluride functions.14
Fig. 2 X-Ray structure of compound 8. Selected bond lengths, non-
bonded distance, angles and torsion angle; P(1)–C(1) 1.739(2), C(1)–C(2)
1.373(3), C(2)–S(2) 1.708(2), P(1)–S(2) 3.268 A; P(1)–C(1)–C(2)
124.05(16)1, C(1)–C(2)–S(2) 122.46(16)1, C(1)–C(2)–C(3) 118.96(18)1,
S(2)–C(2)–C(3) 118.56(15)1; P(1)–C(1)–C(2)–S(2) 7.0(3)1.
Some analogy for this unprecedented rearrangement may be
found in the transformation of 15 into 16, postulated by Zbiral
as the final step in the reaction of ylides with benzyne,15 and in
the rearrangement of the ylide-containing N-heterocyclic
carbene 17 to give 18.16 However the discrepancy in pyrolysis
behaviour between 5–7 and 8 and 9 is hard to explain.
We believe that both processes involve the intermediate
l5-thiaphosphetes and, while those derived from 5 to 7 fragment
as shown in 19, behaviour consistent with that of the only
isolable thiaphosphete 21 (Scheme 2),17 the relief in steric
congestion when R1 = H results in a change in behaviour to
what is effectively a reductive elimination at phosphorus as
shown in 20.
The X-ray structure of 8, the first for a thiocarbonyl-stabilised
ylide, was also determined (Fig. 2). This confirmed both the
significant contribution from the phosphonium enethiolate
zwitterionic form, resulting in lengthening of P(1)–C(1) and
C(2)–S(2) and shortening of C(1)–C(2) as compared to standard
values,18 and the relative (Z)-configuration about the partial
double bond. While these features are consistent with the
structures of ketone-stabilised ylides,19 they offer no hint as to
the reason for the anomalous reaction observed upon pyrolysis.
The (Z)-configured 1-phosphino-2-sulfenylalkenes, readily
accessible for the first time from this novel rearrangement, are
Fig. 3 X-Ray structure of complex 22. Selected bond lengths, angles
and torsion angle; Pt(1)–Cl(1) 2.364(6), Pt(1)–Cl(2) 2.313(5), Pt(1)–P(1)
2.216(6), Pt(1)–S(1) 2.259(5), P(1)–C(1) 1.773(14), C(1)–C(2) 1.34(2),
C(2)–S(1) 1.825(14) A; P(1)–Pt(1)–Cl(2) 91.2(2)1, P(1)–Pt(1)–Cl(1)
174.8(3)1, P(1)–Pt(1)–S(1) 88.59(19)1, Cl(1)–Pt(1)–Cl(2) 92.81(19)1,
S(1)–Pt(1)–Cl(1) 87.4(2)1, S(1)–Pt(1)–Cl(2) 178.8(3)1, Pt(1)–P(1)–C(1)
106.6(6)1, P(1)–C(1)–C(2) 121.4(11)1, C(1)–C(2)–S(1) 118.0(10)1,
C(2)–S(1)–Pt(1) 105.3(5)1; P(1)–C(1)–C(2)–S(2) 3.4(17)1.
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of considerable interest as bidentate ligands, and treatment of
13 with PtCl2(cod), for example, led to formation of the
square-planar platinum complex 22 in 79% yield (Fig. 3).
Further results on their coordination chemistry will be
reported shortly.
(Rint = 0.058), R1 = 0.0406, wR2 = 0.1058 for 2345 data with
I 4 2s(I) and 262 parameters.
1 Reviews: R. A. Aitken and A. W. Thomas in Chemistry of the
Functional Groups, Supplement A3, ed. S. Patai, Wiley, Chichester,
1997, pp. 503–507; F. Symery, B. Iorga and P. Savignac, Synthesis,
2000, 185–213.
2 R. A. Aitken, N. Karodia, T. Massil and R. J. Young, J. Chem.
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Notes and references
3 R. A. Aitken and L. Murray, J. Org. Chem., 2008, 73, 9781–9783.
4 H. J. Bestmann and W. Schaper, Tetrahedron Lett., 1979, 20,
243–244.
5 (a) R. A. Aitken, N. A. Al-Awadi, M. E. Balkovich,
H. J. Bestmann, O. Clem, S. Gibson, A. Gross, A. Kumar and
z Compound 8: pale yellow crystals, mp 200–202 1C (found: C, 76.5;
H, 6.4%. C24H25PS requires C, 76.6; H, 6.7%); nmax/cmꢀ1 1572, 1260,
1205, 1160, 1105, 978, 880, 792, 751, 722, 713, 691 and 620; dH 1.45
(9H, s), 5.20 (1H, d, J 35, PQCH), 7.40–7.65 (9H, m) and 7.70–7.80
(6H, m); dC 31.6 (3C), 43.5 (d, J 14, CMe3), 81.5 (d, J 118, PQC),
125.8 (d, J 92, C-1 of Ph), 128.8 (d, J 12, C-3 of Ph), 131.8 (d, J 2, C-4
of Ph), 133.1 (d, J 10, C-2 of Ph) and 214.7 (d, J 4, CQS); dP +4.4;
m/z (EI) 376 (M+, 16%), 343 (9), 319 (100), 294 (7), 262 (12) and 183
(23). Compound 9: pale yellow crystals, mp 172–174 1C (found: C,
75.2; H, 5.3%. C21H19PS requires C, 75.4; H, 5.7%); nmax/cmꢀ1 1585,
1270, 1175, 1106, 993, 872, 763, 747, 723, 686 and 660; dH 2.65 (3H, s),
5.20 (1H, br d, J 35, PQCH), 7.45–7.55 (6H, m), 7.55–7.60 (3H, m)
and 7.70–7.80 (6H, m); dC 36.8 (d, J 18, Me), 84.1 (d, J 113, PQC),
124.6 (d, J 92, C-1 of Ph), 128.8 (d, J 12, C-3 of Ph), 132.3 (d, J 3, C-4
of Ph), 133.3 (d, J 10, C-2 of Ph) and 200.5 (CQS); dP +8.1; m/z (EI)
334 (M+, 85%), 319 (16), 301 (40), 262 (14), 225 (30), 183 (38) and 43
(100). Compound 13: pale yellow plates, mp 121–123 1C (found: C,
76.3; H, 6.7; S, 8.2%. C24H25PS requires C, 76.6; H, 6.7; S, 8.5%);
T. Roder, Eur. J. Org. Chem., 2003, 840–847; (b) R. A. Aitken,
¨
N. A. Al-Awadi, G. Dawson, O. El-Dusouqi, D. M. M. Farrell,
K. Kaul and A. Kumar, Tetrahedron, 2005, 61, 129–135; (c) R. A
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(d) R. A. Aitken, N. A. Al-Awadi, G. Dawson, O. El-Dusouqi,
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6 H. Yoshida, H. Matsuura, T. Ogata and S. Inokawa, Bull. Chem.
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125–129.
n
max/cmꢀ1 1582, 1551, 1303, 1181, 1118, 1027, 960, 740, 720 and 694;
10 R. A. Aitken and J. I. Atherton, J. Chem. Soc., Perkin Trans. 1,
1994, 1281–1284.
d
H 1.20 (9H, s), 6.85 (1H, s) and 6.92–7.28 (15H, m); dC 29.9 (3C), 41.6
(d, J 3, CMe3), 125.3 (C-4 of SPh), 127.6 (C-3 of SPh), 128.3 (C-4 of
PPh), 128.3 (d, J 11, C-3 of PPh), 128.6 (C-2 of SPh), 132.7 (d, J 19,
C-2 of PPh), 137.3 (d, J 2, C-1 of SPh), 137.8 (d, J 6, P–CHQ), 139.4
(d, J 12, C-1 of PPh) and 158.9 (d, J 25, P–CQC); dP ꢀ19.8; m/z (CI)
377 (MH+, 100%), 319 (9) and 279 (10). Compound 22: off-white
crystals (found: C, 44.8; H, 3.6, S, 4.8%. C24H25Cl2PPtS requires C,
44.9; H, 3.9; S, 5.0%); nmax/cmꢀ1 1665, 1437, 295; dH 1.20 (9H, s),
6.70 (1H, d, JH–P 10, JH–Pt 67) and 7.5–8.0 (15H, m); dP ꢀ19.8
(JP–Pt 3524); m/z (ESꢀ) 641 (M ꢀ Hꢀ, 100%). Crystallographic data
for 13: C24H25PS, Mr = 376.47, colourless prism, monoclinic, space
group P21/c; a = 12.645(7), b = 9.394(6), c = 17.830(9) A,
b = 91.586(9)1, V = 2117(2) A3, Z = 4, Dc = 1.181 Mg mꢀ3, T =
173(2) K, 7903 reflections of which 3356 unique (Rint = 0.043), R1 =
0.0696, wR2 = 0.1649 for 1949 data with I 4 2s(I) and 236
parameters. Crystallographic data for 8: C24H25PS, Mr = 376.47,
yellow prism, monoclinic, space group P21/c; a = 12.8861 (12), b =
10.0602 (9), c = 15.8756 (14) A, b = 91.836 (2)1, V = 2057.0(3) A3,
Z = 4, Dc = 1.216 Mg mꢀ3, T = 125(2) K, 10 638 reflections of which
2910 unique (Rint = 0.055), R1 = 0.0344, wR2 = 0.0781 for 2306 data
with I 4 2s(I) and 236 parameters. Crystallographic data for 22:
C24H25Cl2PPtS, Mr = 642.46, colourless prism, triclinic, space group
P1; a = 8.371(3), b = 9.178(3), c = 9.282(3) A, a = 103.715(5)1,
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b = 116.135(4)1, g = 103.015(4)1, V = 575.8(3) A3, Z = 1, Dc
1.853 Mg mꢀ3, T = 125(2) K, 2888 reflections of which 2346 unique
=
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 7381–7383 | 7383