Thermal Rearrangement of Thiocarbonyl-Stabilised Triphenylphosphonium Ylides Leading to (Z)-1-Diphenylphosphino-2-(phenylsulfenyl)alkenes and Their Coordination Chemistry

Abstract

While thiocarbonyl-stabilised phosphonium ylides generally react upon flash vacuum pyrolysis by the extrusion of Ph₃PS to give alkynes in an analogous way to their carbonyl-stabilised analogues, two examples with a hydrogen atom on the ylidic carbon are found to undergo a quite different process. The net transfer of a phenyl group from P to S gives (Z)-configured 1-diphenylphosphino-2-(phenylsulfenyl)alkenes in a novel isomerisation process via intermediate λ⁵-1,2-thiaphosphetes. These prove to be versatile hemilabile ligands with a total of seven complexes prepared involving five different transition metals. Four of these are characterised by X-ray diffraction with two involving the bidentate ligand forming a five-membered ring metallacycle and two with the ligand coordinating to the metal only through phosphorus.

1. Introduction

The thermal extrusion of Ph₃PO from carbonyl-stabilised triphenylphosphonium ylides 1 is a well-established synthetic route to functionalised alkynes 2 (Scheme 1) [1,2,3]. The process proceeds particularly well using flash vacuum pyrolysis (FVP), and we have found that the phosphorus to carbonyl coupling constant ²J_P–CO provides a diagnostic parameter for the likely success of the reaction, with ylides for which ²J_P–CO < 11 Hz usually providing the alkynes in high yield [4]. By way of contrast, the thermal behaviour of the corresponding thiocarbonyl-stabilised ylides 3 has only been examined in a few cases and Bestmann and Schaper found that heating the ylides 3 above their melting point resulted in a bimolecular process with the loss of Ph₃P and Ph₃PS to give tetrasubstituted thiophenes 4 [5]. Some time ago, we described a preliminary study in which FVP of the ylides 5 was found to proceed as expected by analogy with the carbonyl analogues 1 with the loss of Ph₃PS to give alkynes 2 for R¹ ≠ H, but when R¹ was hydrogen, a quite different process was observed: rearrangement with the transfer of a phenyl group from P to S giving the potentially useful bidentate proligands 6 [6]. In this paper, we describe in more detail the synthesis and structure of these novel phosphinovinyl sulfides as well as their coordination chemistry.

2. Results and Discussion

Synthetic access to thiocarbonyl-stabilised ylides 5 is available using various methods including the treatment of non-stabilised ylides with dithioesters or dithiocarbonates [7], or activation of the corresponding carbonyl-stabilised ylides with triflic anhydride [8] or POCl₃ [9] followed by treatment with sodium sulfide. For the current study, we used the direct reaction of carbonyl-stabilised ylides 1 with Lawesson’s reagent introduced by Capuano and coworkers [10], and in this way, we prepared the five examples 7–11 (Figure 1) from their carbonyl analogues.

Compounds 7–9 are already known [8] andgave analytical and spectroscopic data in agreement with the reported values. The new compounds 10 and 11 were fully characterised and showed distinctive NMR signals confirming the presence of P=CH–C=S [10 δ_P +5.0; δ_H 5.22 (d, ²J 34 Hz, P=CH); δ_C 81.3 (d, ¹J 118 Hz, P=CH), 214.4 (d, ²J 4 Hz, C=S). 11 δ_P +8.1; δ_H 5.18 (d, ²J 32 Hz, P=CH); δ_C 84.1 (d, ¹J 113 Hz, P=CH), 200.5 (d, ²J 4 Hz, C=S)]. In fact, the phosphorus coupling extended throughout the structures with all carbon signals except the CH₃ of t-butyl observed as doublets in the ¹³C NMR spectra. This is discussed further below in the context of a comparison of the coupling pattern before and after the thermal rearrangement. The structure of 10 was also confirmed by X-ray diffraction (CSD RefCode: AJOMUI) and this was described in our earlier communication [6]. We might also note at this point that compound 10 has a particularly pungent and unpleasant smell. Although it is not very volatile, it is extremely persistent and inadvertent contact with equipment or work surfaces contaminated with 10, even after several months, results in the release of its characteristic smell. We speculate that this may be due to slow hydrolysis of the P=C bond to give Ph₃PO and release thiopinacolone, t-BuC(=S)Me.

When the thiocarbonyl ylides 7, 8 and 9 were subjected to FVP, there was complete reaction at a furnace temperature of 650 °C to give Ph₃PS (δ_P +43) at the furnace exit and the expected alkynes 2, 2,2-dimethylpent-3-yne, 1-phenylpropyne and 1-phenyloctyne, in the cold trap. Thus, these compounds behave in a similar way to their carbonyl analogues but react more readily than the latter, which require a temperature of 750 °C for complete reaction [11]. The higher reactivity of thiocarbonyl- as compared to carbonyl-stabilised ylides is a feature that we have already noted and quantified in a series of kinetic studies on the pyrolysis of carbamoyl and thiocarbamoyl ylides [12,13,14].

When the two ylides 10 and 11 were subjected to FVP, the reaction was also complete at 650 °C, but the process involved turned out to be completely different. In each case, only a single main product was obtained, which was isomeric with the starting material. In the case of 10, the product 12 was obtained in good yield and in a pure form as a crystalline solid after preparative TLC. This showed a ³¹P NMR signal at −19.8 ppm, in the region expected for an alkyldiphenylphosphine, and the single P–CH= hydrogen gave a ¹H NMR singlet at 6.94 ppm (Scheme 2).

The ¹³C NMR spectrum, and particularly the pattern of P–C coupling, was particularly informative and showed major changes from the values in 10 (Figure 2).

The values of J_P–C for the thiocarbonyl ylides 10 and 11 are consistent with those well established for the corresponding carbonyl ylides [11] and carbamoyl/thiocarbamoyl ylides [12,13,14]. However, in the rearranged product 12, the values around P-Ph are more similar to those in Ph₃P, and the much higher coupling to P–C=C as compared to P–C=C as well as the absence of coupling to P–CH are surprising features. As already mentioned in our preliminary communication [6], the structure of 12 was confirmed by X-ray diffraction (CSD RefCode: AJOMOC).

When we examined the corresponding pyrolysis of the thioacetyl ylide 11, the corresponding reaction occurred at the same temperature, but the product was now a liquid formed in lower yield and containing some impurities including Ph₃P, Ph₃PO and Ph₃PS. More interestingly, while it was predominantly (6.5:1) the (Z)-isomer 13 (δ_P −22.7), signals attributed to the (E)-isomer 14 (δ_P −25.4) were also apparent (Scheme 3).

Attempts to purify this by repeated Kugelrohr distillation under reduced pressure led instead to isomerisation to give a 1:1 mixture of 13 and 14. At first sight, it might seem surprising that the product is obtained mainly as the less thermodynamically stable isomer from pyrolysis at 650 °C but then isomerises to the more stable isomer upon simple distillation at 90 °C, but this only serves to emphasise the mild nature of the flash vacuum pyrolysis technique. In addition, it has been shown that under FVP conditions, (Z)-alkenes do not normally isomerise to the (E)-isomer to any great extent at temperatures as low as 650 °C with the degree of conversion of (Z)- to (E)-stilbene, for example, being determined as 12%, in good agreement with our results [15]. The presence of aromatic impurities even after distillation prevented full assignment of the ¹³C NMR spectra for 13 and 14, but the key signals and the values of the phosphorus coupling constants (Figure 2) were in good agreement with those for 12 while also showing significant differences between the two isomers. The detailed form of the signals for P–CH and P–C=C–Me in the two isomers was at first sight surprising. However, this could be explained by coincidental equivalence of some H–H and P–H coupling constants, and the observed patterns (see Supplementary Materials, Figure S14) could be satisfactorily simulated using the values shown in Figure 3.

Although the 1,2-arrangement of phosphine and sulfide functions on an alkene double bond, particularly in the (Z)-configuration, gives potentially valuable “hemilabile” proligands, few such compounds seem to be known (Figure 4). Chlorinated compounds such as 15 [16], 16 [17] and 17 [18] have been prepared as mixtures of (E)- and (Z)-isomers by the addition of phosphorus compounds to alkynes. The simpler disubstituted alkenes 18 [19], 19 [20] and 20 [21] have also been prepared but these are the (E)-isomers as shown. More recently, the (Z)-configured vinylphosphonates 21 containing sulfide, selenide and telluride functions [22] as well as the tellurovinylphosphine oxides 22 [23] have also been reported.

In terms of the mechanism of this new thermal rearrangement, we envisage attack of the nucleophilic sulfur at phosphorus to give a transient λ⁵-1,2-thiaphosphete, which is of course the same intermediate involved in the extrusion of Ph₃PS to give alkynes as observed for 7–9. This behaviour is also consistent with that of the isolable thiaphosphete 23, which fragments with the loss of an alkyne to give the benzoxaphosphole P-sulfide [24]. However, perhaps due to the relief of steric congestion, the thiaphosphetes derived from 10 and 11 instead undergo what is effectively a reductive elimination at phosphorus to give the (Z)-phosphinovinyl sulfides 12 and 13 (Scheme 4).

Somewhat similar processes are the transfer of Ph from Se to O in acylselenonium ylides such as 24 to give 25 observed by Rakitin [25] (Scheme 5), the transformation of 26 into 27 postulated by Zbiral in the interaction of ylides Ph₃P=CHR with benzyne [26] and the rearrangement of the ylide-containing N-heterocyclic carbene 28 via 29 to give 3-phosphinoindole 30 [27].

With the two new hemilabile proligands in hand, we now set about exploring their coordination chemistry. In our preliminary communication [6], the formation of the square planar platinum complex 31 from 12 was described along with its X-ray structure determination (CSD RefCode: AJONAP). We were also successful in obtaining complexes of 12 with a wide range of other standard transition metal reagents (Scheme 6).

Reaction of the starting materials in CH₂Cl₂ followed by partial evaporation and precipitation with diethyl ether gave the new complexes 32–36 in moderate to good yield as crystalline solids, giving the expected microanalytical data and ¹H and ³¹P NMR spectra. From the analytical data, it was clear that the complexes had formed with the expected stoichiometry according to the metal source employed, with 12 acting as a bidentate ligand in the square planar platinum(II) and palladium(II) complexes 31 and 32 and the cationic ruthenium(II) complex 36, but as a monodentate ligand through the more strongly donating phosphorus atom in palladium(II) complex 33, gold(I) complex 34 and iridium(III) complex 35. In addition to the compound 31 already confirmed by X-ray diffraction [6], we were able to obtain X-ray structures for gold complex 34 and iridium complex 35 (see below).

Although the methyl proligand 13 was available in lower quantity and purity, we were able to prepare its palladium dichloride complex 37 analogous to 32 and also determined its X-ray structure (Scheme 7).

The structures of the complexes 34, 35 and 37 together with the numbering systems used are shown below (Figure 5) with that of 31 [6] also included for comparison.

For comparison, we also show (Figure 6) the previously reported structures of 10 [6] and 12 [6], and the iron(II) complex 38 [28], which, although made in a quite different way, contains the phosphinovinylthiolate corresponding to 12 as an anionic bidentate ligand.

The key structural parameters for complexes 34, 35 and 37 and, for comparison, those for 10, 12, 31 and 38 are presented in

Table 1. Geometric parameters for 10 [6], 12 [6], 31 [6], 34, 35, 37 and, for comparison, 38 [28].

	Bond lengths (Å)
Compound	P–CH	CH=C	=C–S	P–M	M–S
10	1.739(2)	1.373(3)	1.708(2)	—	—
12	1.818(4)	1.316(6)	1.788(4)	—	—
31	1.773(14)	1.34(2)	1.825(14)	2.216(6)	2.259(5)
34	1.795(12)	1.332(17)	1.782(13)	2.229(3)	—
35	1.824(5)	1.318(7)	1.787(5)	2.3076(12)	—
37	1.798(8)	1.322(11)	1.796(8)	2.227(2)	2.252(2)
38	1.770(6)	1.346(8)	1.764(5)	2.256(1)	2.307(2)
	Angles (°)
Compound	P–C=C	C=C–S	=C–P–M	P–M–S	M–S–C=
10	124.05(16)	122.46(16)	—	—	—
12	125.5(3)	115.1(3)	—	—	—
31	121.4(11)	118.0(10)	106.6(6)	88.59(19)	105.3(5)
34	128.7(10)	118.9(9)	121.2(4)	—	—
35	131.7(4)	119.8(4)	109.27(15)	—	—
37	119.6(6)	119.1(6)	107.2(3)	87.07(8)	106.8(3)
38	118.3(2)	120.7(2)	108.4(1)	85.1(0)	106.3(1)

. In all seven structures, the P–C–C–S fragment is quite accurately planar with a torsion angle of <10° in every case.

If we first compare 10 and 12, the major change in geometry associated with the transformation of P=C–C=S into P–C=C–S is clear. However, comparing the structural parameters of 12 with those of its complexes 31, 34 and 35 as well as the related thiolate complex 38 shows a remarkable degree of consistency. As expected, the complexes 34 and 35 where the metal binds only to phosphorus have bond lengths around the =C–SPh that are relatively unaffected, while the bidentate binding to platinum in 31 results in the significant lengthening of C–S and shortening of C–P. While the smaller size of AuCl means the ligand 12 can retain the orientation of the PPh₂ group, coordination to the much larger Cp*IrCl₂ requires the PPh₂ group to rotate, placing the phenyl groups facing towards SPh. The similarity between the geometry of the neutral ligand 12 in complexes such as 31 and the anionic enethiolate in 38 is also notable, with only the =C–S length being significantly shorter in the latter. The angles within the five-membered ring in complexes 31, 37 and 38 are also remarkably consistent.

3. Experimental Section

3.1. General Experimental Details

NMR spectra were recorded on solutions in CDCl₃ unless otherwise stated using Bruker instruments, and chemical shifts are given in ppm to high frequency from Me₄Si for ¹H and ¹³C and H₃PO₄ for ³¹P with coupling constants J in Hz. The ¹³C NMR spectra are referenced to the solvent signal at 77.0 (CDCl₃). IR spectra were recorded on a Perkin Elmer 1420 instrument. Elemental analysis was conducted using a Carlo Erba CHNS analyser. Mass spectra were obtained using a Micromass instrument and the ionisation method used is noted in each case. Preparative TLC was carried out using 1.0 mm layers of Merck alumina 60 G containing 0.5% Woelm fluorescent green indicator on glass plates. Melting points were recorded on a Gallenkamp 50W melting point apparatus or a Reichert hot-stage microscope.

Flash vacuum pyrolysis (FVP) was carried out in a conventional flow system by subliming the starting material through a horizontal quartz tube (30 × 2.5 cm) externally heated by a tube furnace to 650 °C and maintained at a pressure of 2–5 × 10⁻² Torr by a rotary vacuum pump. Products were collected in a liquid N₂ cooled U-shaped trap and purified as noted.

General organic and inorganic reagents and solvents were obtained from standard suppliers and used as received. Dry THF was prepared by storage over sodium wire. Starting transition metal complexes [AuCl(tetrahydrothiophene)] [29], [PdCl₂(cyclooctadiene)] [30], [PtCl₂(cyclooctadiene)] [31], [{RuCl(μ-Cl)(η⁶-p-MeC₆H₄ⁱPr)}₂] [32], [{IrCl(μ-Cl)(η⁵-C₅Me₅)}₂] [33] and [{Pd(m-Cl)(η³-C₃H₅)}₂] [34] were prepared by the reported methods.

3.2. Preparation of Thiocarbonyl Ylides

3.2.1. Preparation of Thiopivaloylmethylenetriphenylphosphorane 10

A solution of pivaloylmethylenetriphenylphosphorane (5.0 g, 13.9 mmol) and Lawesson’s reagent (2.81 g, 6.9 mmol) in toluene (300 mL) was heated under reflux under nitrogen for 3 h. The mixture was allowed to cool to RT and the solution was poured off leaving an insoluble oily residue and evaporated. Recrystallisation of the resulting solid from ethyl acetate gave the product (2.86 g, 55%) as pale-yellow crystals, mp 200–202 °C; (Found: C, 76.5; H, 6.4. C₂₄H₂₅PS requires C, 76.6; H, 6.7%); ν_max/cm⁻¹ 1572, 1260, 1205, 1160, 1105, 978, 880, 792, 751, 722, 713, 691 and 620; ¹H NMR (300 MHz) δ_H 1.40 (9H, s), 5.22 (1H, d, J 34, CH=P), 7.40–7.48 (6H, m), 7.48–7.52 (3H, m) and 7.67–7.80 (6H, m); ¹³C NMR (75 MHz) δ_C 31.3 (3C), 43.2 (d, J 14, CMe₃), 81.3 (d, J 118, P=C), 125.3 (d, J 92, C-1 of Ph), 128.5 (d, J 12, C-3 of Ph), 131.6 (d, J 2, C-4 of Ph), 132.8 (d, J 10, C-2 of Ph) and 214.4 (d, J 4, C=S); ³¹P NMR (121 MHz) δ_P +5.0; MS (EI) m/z 376 (M⁺, 16%), 343 (9), 319 (100), 294 (7), 262 (12) and 183 (23).

3.2.2. Preparation of Thioacetylmethylenetriphenylphosphorane 11

A solution of acetylmethylenetriphenylphosphorane (8.0 g, 25 mmol) and Lawesson’s reagent (5.1 g,12.6 mmol) in toluene (300 mL) was heated under reflux under nitrogen for 3 h. The mixture was allowed to cool to RT and the solution was poured off leaving an insoluble oily residue and evaporated. Recrystallisation of the resulting solid from ethyl acetate gave the product (4.62 g, 55%) as pale-yellow crystals, mp 172–174 °C; (Found: C, 75.2; H, 5.3. C₂₁H₁₉PS requires C, 75.4; H, 5.7%); ν_max/cm⁻¹ 1585, 1270, 1175, 1106, 993, 872, 763, 747, 723, 686 and 660; ¹H NMR (300 MHz) δ_H 2.63 (3H, s), 5.18 (1H, d, J 32, CH=P), 7.45–7.55 (6H, m), 7.55–7.65 (3H, m) and 7.70–7.80 (6H, m); ¹³C NMR (75 MHz) δ_C 36.8 (d, J 18, Me), 84.1 (d, J 113, P=C), 124.6 (d, J 92, C-1 of Ph), 128.9 (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 (d, J 4, C=S); ³¹P NMR (121 MHz) δ_P +8.1; MS (EI) m/z 334 (M⁺, 85%), 319 (16), 301 (40), 262 (14), 225 (30), 183 (38) and 43 (100).

3.3. Thermal Rearrangement of Thiocarbonyl Ylides

3.3.1. Preparation of (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12

FVP of the ylide 10 (0.50 g, 1.33 mmol) was performed at 650 °C and 3.8 × 10⁻² Torr and was complete within 1 h. Preparative TLC (silica, diethyl ether) of the crude material gave the product 12 (0.41 g, 82%) as pale-yellow plates, mp 121–123 °C; (Found: C, 76.3; H, 6.7; S, 8.2. C₂₄H₂₅PS requires C, 76.6; H, 6.7; S, 8.5%); ν_max/cm⁻¹ 1582, 1551, 1303, 1181, 1118, 1027, 960, 740, 720 and 694; ¹H NMR (300 MHz) δ_H 1.21 (9H, s), 6.94 (1H, s), 7.05–7.25 (5H, m) and 7.25–7.40 (10H, m); ¹³C NMR (75 MHz) δ_C 29.8 (3CH₃), 41.5 (d, J 3, CMe₃), 125.2 (C-4 of SPh), 127.5 (C-3 of SPh), 128.24 (C-4 of PPh), 128.25 (d, J 11, C-3 of PPh), 128.6 (C-2 of SPh), 132.6 (d, J 19, C-2 of PPh), 137.2 (C-1 of SPh), 137.8 (d, J 6, P–CH=), 139.4 (d, J 12, C-1 of PPh) and 158.8 (d, J 23, S–C=); ³¹P NMR (121 MHz) δ_P −19.8; MS (CI) m/z 377 (M+H⁺, 100%), 319 (9) and 279 (10).

3.3.2. Preparation of (Z)-1-Diphenylphosphino-2-phenylthiopropene 13

FVP of the ylide 11 (0.50 g, 78.8 µmol) was performed at 650 °C and 3.8 × 10⁻² Torr and was complete within 1 h. NMR analysis of the crude product (0.245 g, 49%) showed a 6.5:1 ratio of (Z)- and (E)-13. Repeated Kugelrohr distillation of this (bp 90 °C/0.1 Torr) in an attempt to remove trace impurities of Ph₃P, Ph₃PO and Ph₃PS resulted in isomerisation to afford a 1:1 ratio of (Z) and (E)-13. By comparing the NMR data before and after distillation, the following assignments could be made (owing to peak overlap, definite assignment of the remaining aromatic ¹³C NMR signals was not possible):

(Z)-13: ¹H NMR (300 MHz) δ_H 2.02 (3H, t, J 1.5), 6.33 (1H, qd, J 1.5, 0.8) and 7.25–7.75 (15H, m); ¹³C NMR (75 MHz) δ_C 148.8 (d, J 27, =C–S), 139.0 (d, J 9.5, PPh C-1), 132.2 (d, J 11, P–CH) and 25.7 (d, J 4.5, CH₃); ³¹P NMR (121 MHz) δ_P −22.7.

(E)-13: ¹H NMR (300 MHz) δ_H 2.23 (3H, d, J 0.9), 5.94 (1H, dq, J 2.0, 0.9) and 7.25–7.75 (15H, m); ¹³C NMR (75 MHz) δ_C 149.8 (d, J 30, =C–S), 138.9 (d, J 9.8, PPh C-1), 122.0 (d, J 12, P–CH) and 20.9 (d, J 23, CH₃); ³¹P NMR (121 MHz) δ_P −25.4.

For the isomer mixture (Found: C, 75.2; H, 5.7. C₂₁H₁₉PS requires C, 75.4; H, 5.7%); ν_max/cm⁻¹ 1584, 1478, 1435, 1184, 1109, 1026, 999, 743, 719 and 694; HRMS (EI) m/z calcd for C₂₁H₁₉PS (M⁺) 334.0945. Found 334.0960.

3.4. Formation of Transition Metal Complexes

3.4.1. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Platinum Dichloride Complex 31

A solution of [PtCl₂(cod)] (66 mg, 0.18 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (66 mg, 0.18 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 1 h, the mixture was reduced to 1 mL by evaporation, and the addition of diethyl ether (15 mL) led to precipitation of the product as an off-white solid (89 mg, 79%), which was isolated by filtration. (Found: C, 44.8; H, 3.6; S, 4.8. C₂₄H₂₅Cl₂PPtS requires C, 44.9; H, 3.9; S, 5.0%); ν_max/cm⁻¹ 1665, 1437, 295; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 8.0–7.5 (15H, m), 6.70 (1H, dd, ³J_Pt-H 67, ²J_P-H 10) and 1.20 (9H, s); ³¹P NMR (121 MHz, CD₂Cl₂) δ_P +29.4 (d, ¹J_P-Pt 3524); MS (ESI⁻) m/z 641 (M–H).

3.4.2. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Palladium Dichloride Complex 32

A solution of [PdCl₂(cod)] (38 mg, 0.13 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (50 mg, 0.13 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 2 h, the mixture was reduced to 1 mL by evaporation, and the addition of diethyl ether (15 mL) led to precipitation of the product as a yellow solid (63 mg, 79%), which was isolated by filtration. (Found: C, 49.7; H, 4.4. C₂₄H₂₅Cl₂PPdS•0.5 CH₂Cl₂ requires C, 49.4; H, 4.4%); ν_max/cm⁻¹ 1575, 1436, 289; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 8.0–7.5 (15H, m), 6.70 (1H, d, ²J_P-H 8) and 1.20 (9H, s); ³¹P NMR (121 MHz, CD₂Cl₂) δ_P +50.4; MS (ESI⁺) m/z 553 (M).

3.4.3. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene η³-allyl Palladium Chloride Complex 33

A solution of [Pd(η³-allyl)Cl] (32 mg, 0.22 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (66 mg, 0.18 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 2 h, the mixture was reduced to 0.5 mL by evaporation, and the addition of diethyl ether (10 mL) led to precipitation of the product as a yellow solid (54 mg, 55%), which was isolated by filtration. (Found: C, 56.7; H, 4.6. C₂₇H₃₀ClPPdS•0.4 CH₂Cl₂ requires C, 56.4; H, 5.3%); ν_max/cm⁻¹ 1599, 1435, 296; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 8.0–7.5 (15H, m), 6.70 (1H, d, ²J_P-H 8) and 1.20 (9H, s); ³¹P NMR (121 MHz, CD₂Cl₂) δ_P +40.9; MS (ESI⁺) m/z 523 (M–Cl).

3.4.4. (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Gold Chloride Complex 34

A solution of [Au(tht)Cl] (18 mg, 0.06 mmol) and (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (21 mg, 0.06 mmol) in CH₂Cl₂ (2 mL) was stirred for 18 h. The mixture was reduced to 0.5 mL by evaporation, and the addition of diethyl ether (10 mL) led to precipitation of the product as a white solid (21 mg, 62%), which was isolated by filtration. (Found: C, 47.2; H, 4.1. C₂₄H₂₅AuClPS requires C, 47.3; H, 4.1%); ν_max/cm⁻¹ 1577, 1436, 253; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 7.5–7.0 (15H, m), 6.70 (1H, d, ²J_P-H 12) and 1.20 (9H, s); ³¹P NMR (121 MHz, CDCl₃) δ_P +18.2; MS (ESI⁺) m/z 631 (M+Na).

3.4.5. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Pentamethylcyclopentadienyl Iridium Dichloride Complex 35

A solution of [{IrCl(μ-Cl)(η⁵-C₅Me₅)}₂] (75 mg, 0.1 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (71 mg, 0.19 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 2 h, the mixture was reduced to 0.5 mL by evaporation, and the addition of diethyl ether (20 mL) led to precipitation of the product as a yellow solid (84 mg, 57%), which was isolated by filtration. (Found: C, 48.1; H, 4.65. C₃₄H₄₀Cl₂IrPS•1.25 CH₂Cl₂ requires C, 48.1; H, 4.9%); ν_max/cm⁻¹ 1648, 1437, 290; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 8.0–7.5 (15H, m), 6.70 (1H, d, ²J_P-H 8), 1.20 (9H, s) and 1.00 (15H, s); ³¹P NMR (121 MHz, CD₂Cl₂) δ_P −8.7; MS (ESI⁺) m/z 739 (M).

3.4.6. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene p-Cymene Ruthenium Dichloride Complex 36

A solution of [{RuCl(μ-Cl)(η⁶-p-MeC₆H₄ⁱPr)}₂] (26 mg, 0.04 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene 12 (32 mg, 0.08 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 18 h, the mixture was reduced to 0.5 mL by evaporation, and the addition of diethyl ether (10 mL) led to precipitation of the product as an orange solid (39 mg, 67%), which was isolated by filtration. (Found: C, 56.9; H, 4.0. C₃₄H₃₉Cl₂PRuS•0.5 CH₂Cl₂ requires C, 57.1; H, 5.6%); ν_max/cm⁻¹ 1637, 1436, 291; ¹H NMR (300 MHz) δ_H 8.0–7.5 (19H, m), 6.70 (1H, d, ²J_P-H 8), 2.50 (1H, m), 1.80 (3H, s), 1.20 (9H, s) and 0.70 (6H, m); ³¹P NMR (121 MHz) δ_P +14.6; MS (ESI⁺) m/z 647 (M–Cl).

3.4.7. (Z)-1-Diphenylphosphino-2-phenylthiopropene Palladium Dichloride Complex 37

A solution of [PdCl₂(cod)] (33 mg, 0.1 mmol) in CH₂Cl₂ (5 mL) was stirred while a solution of (Z)-1-diphenylphosphino-2-phenylthiopropene 13 (64 mg, 0.2 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 30 min. After 2 h, the mixture was reduced to 0.5 mL by evaporation, and the addition of diethyl ether (10 mL) led to precipitation of the product as a yellow solid (43 mg, 73%), which was isolated by filtration. (Found: C, 49.5; H, 3.2. C₂₁H₁₉ClPPdS requires C, 49.3; H, 3.7%); ν_max/cm⁻¹ 1576, 1435, 296; ¹H NMR (300 MHz, CD₂Cl₂) δ_H 8.0–7.5 (15H, m), 6.30 (1H, d, ²J_P-H 8) and 2.00 (3H, s); ³¹P NMR (121 MHz, CD₂Cl₂) δ_P +52.4; MS (ESI⁺) m/z 532 (M+Na).

3.5. X-ray Structure Determination of Complexes

Data were collected on a Bruker SMART diffractometer using graphite monochromated Mo Kα radiation λ = 0.71075 Å. The data were deposited at the Cambridge Crystallographic Data Centre and can be obtained free of charge via http://www.ccdc.cam.ac.uk/getstructures (accessed on 13 December 2023). The structure was solved by direct methods and refined by full-matrix least-squares against F² (SHELXL, Version 2018/3 [35]).

3.5.1. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Gold Chloride Complex 34

Crystal data for C₂₄H₂₅AuClPS, M = 608.91, colourless prism, crystal dimensions 0.13 × 0.03 × 0.03 mm, monoclinic, space group P2₁/c (No. 14), a = 18.290(6), b = 7.007(2), c = 17.660(6) Å, β = 96.966(6)°, V = 2246.7(13) ų, Z = 4, D_c = 1.800 g cm⁻³, T = 125 K, R1 = 0.0622, Rw2 = 0.1417 for 2869 reflections with I > 2σ(I) and 253 variables. CCDC 2298238.

3.5.2. (Z)-1-Diphenylphosphino-3,3-dimethyl-2-phenylthiobut-1-ene Pentamethylcyclopentadienyl Iridium Dichloride Complex 35

Crystal data for C₃₄H₄₀Cl₂IrPS, M = 774.85, yellow prism, crystal dimensions 0.30 × 0.20 × 0.20 mm, triclinic, space group P-1 (No. 2), a = 10.2041(15), b = 10.2862(15), c = 16.724(3) Å, α = 80.903(2), β = 82.655(2), γ = 65.666(2)°, V = 1575.5(4) ų, Z = 2, D_c = 1.633 g cm^–3, T = 125 K, R1 = 0.0263, Rw2 = 0.0717 for 4283 reflections with I > 2σ(I) and 354 variables. CCDC 2298239.

3.5.3. (Z)-1-Diphenylphosphino-2-phenylthiopropene Palladium Dichloride Complex 37

Crystal data for C₂₁H₁₉Cl₂PPdS, M = 511.69, orange prism, crystal dimensions 0.30 × 0.15 × 0.10 mm, triclinic, space group P-1 (No. 2), a = 8.677(3), b = 11.063(4), c = 11.665(4) Å, α = 76.460(6), β = 87.468(6), γ = 71.174(5)°,V = 1029.8(6) ų, Z = 2, D_c = 1.650 g cm^–3, T = 125 K, R1 = 0.0519, Rw2 = 0.1335 for 2660 reflections with I > 2σ(I) and 235 variables. CCDC 2298237.

4. Conclusions

While thiocarbonyl ylides with other groups on the ylidic carbon undergo thermal extrusion of Ph₃PS upon FVP at 650 °C, two examples, 10 and 11, with hydrogen on the ylidic carbon instead undergo a novel isomerisation under the same conditions to afford useful (Z)-configured 1-diphenylphosphino-2-phenylsulfenylalkenes. The t-butyl compound 12 is obtained in good yield as the pure (Z)-isomer and behaves well as a ligand, forming a range of transition metal complexes with both bidentate binding via P and S and monodentate binding via only P. The methyl compound is obtained in lower yield mainly as the (Z)-isomer 13 but with a significant proportion of (E)-14, which increases upon distillation. A more limited study of its coordination chemistry resulted in the isolation of a bidentate bonded palladium complex. It is clear that while seven new complexes involving the two ligands have been isolated and characterised, including in four cases by X-ray diffraction, much more work needs to be carried out to fully exploit the potential of these simple yet versatile proligands, which are now readily available thanks to this unusual thermal rearrangement.