HETEROCYCLES

Communication | Special issue | Vol. 90, No. 1, 2015, pp. 144-149
Received, 27th June, 2014, Accepted, 14th July, 2014, Published online, 16th July, 2014.
DOI: 10.3987/COM-14-S(K)45

■ Palladium(0)-Mediated C−S and O−H Bonds Cleavage Reaction in 2-Hydroxybenzyl Phenyl Sulfide: Formations of Oxyphosphorane and 1,2-Oxapalladacycle

Norio Nakata, Noriyuki Furukawa, Hiroki Kobayashi, Izuru Suzuki, and Akihiko Ishii*

Department of Chemistry, Faculty of Science, Saitama University, Urawa, Saitama 338-8570, Japan

Abstract

Treatment of 2-hydroxybenzyl phenyl sulfide 3 with 1.5 equiv. of [Pd(PPh3)4] in toluene led to the unanticipated formation of oxyphosphorane 4 in 30% isolated yield. In contrast, the reaction of 3 with Pd(0) complex bearing PMe3 ligands in toluene afforded the corresponding 1,2-oxapalladacycle 7 and oxyphosphorane 9 in addition to dithiolato complex trans-[Pd(SPh)2(PMe3)2] 8.

Oxametallacycles have received remarkable attention in past years because of their unique structures and behaviors as key intermediates in catalytic reactions.1 In particular, reactions involving oxapalladacycles have proved to be useful for the synthesis of various types of organic compounds and heterocycles.2,3 For example, Malinakova and co-workers have demonstrated the synthesis of a series of five-membered 1,3-oxapalladacycles featuring a Pd-bonded sp3-hybridized stereogenic carbon and phosphine or N,N-chelating ligands.4 The stoichiometric reactions of these oxapalladacycles with alkynes or allenes efficiently afforded 2H-1-benzopyrans.4 However, investigations of five-membered 1,2-oxapalladacycles with a Pd−O bond were quite limited probably due to a lack of suitable synthetic methods.5 Meanwhile, we have recently found the Pt(0)-promoted C(sp3)−S and O−H bonds cleavages of 2-hydroxybenzyl sulfide derivatives (1) to lead to an unexpected formation of novel five-membered 1,2-oxaplatinacycles (2) (Scheme 1).6 In this contribution, we present the sequential bond cleavage reaction of 2-hydroxybenzyl phenyl sulfide with Pd(0) complexes.

When 2-hydroxybenzyl phenyl sulfide 3 was allowed to react with 1.5 equiv. of [Pd(PPh3)4] in toluene at room temperature for 36 h, the five-coordinated phosphine compound, oxyphosphorane 4, instead of the expected corresponding five-membered 1,2-oxapalladacycle, was produced in 30% isolated yield together with polymeric palladium clusters as insoluble materials (Scheme 2).7 The molecular structure of 4 was characterized by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, and finally verified by X-ray crystallographic analysis. In the 1H NMR of 4, the characteristic benzylic protons were observed as a doublet signal at δ 2.77 with the 1H-31P coupling constant of 13 Hz. In the 31P{1H} NMR spectrum of 4, a sharp singlet appeared at δ −41.5, which is comparable to those of the reported oxyphosphoranes (δ −60.8 to −39.8).8

Colorless single crystals of 4 suitable for X-ray crystallography were grown from a concentrated CH2Cl2/hexane solution at −18 °C. As illustrated in Figure 1,9 the phosphorus center adopts a slightly distorted trigonal bipyramidal geometry that has a phenoxide oxygen (O1) and a phenyl carbon (C4) atoms at apical positions and a benzylic carbon (C1) and two phenyl carbons at equatorial positions. The P−O bond length is 1.8738(17) Å, which is slightly elongated in comparison with those of the previously reported oxyphosphoranes within the range of 1.75 to 1.85 Å.8 As expected,10 three equatorial P−C bonds [1.816(2)−1.841(2) Å] are remarkably shorter than the apical P1−C4 bond [1.912(2) Å].
The formation mechanism of 4 is reasonably explained as shown in Scheme 3. In the first stage, the C−S and O−H bonds cleavage of 3 mediated by Pd(0) complex would occur to furnish the corresponding 1,2-oxapalladacycle 5 as an initial intermediate similarly to the formation of 1,2-oxaplatinacycle 2.6 Then, nucleophilic attack of the phenoxide oxygen in 5 at the coordinated cis phosphorus center takes place to give a six-membered palladaphosphorane intermediate 6.11 Finally, the coordination of PPh3 to the palladium center in 6 followed by the reductive elimination of [Pd(PPh3)2] or direct elimination of Pd(0) species from 6 leads to the formation of 4.

We next carried out the reaction of 3 with Pd(0) complex bearing much strong σ-donor PMe3 ligands. Treatment of 3 with [Pd(PMe3)4] in toluene at room temperature for 42 h, which is generated in situ by the reaction of 2.0 equiv. of [Pd(η-C3H5)(η-C5H5)] with 9.6 equiv. of PMe3, afforded a mixture of 1,2-oxapalladacycle 7 and dithiolato complex trans-[Pd(SPh)2(PMe3)2] 85b in the ratio of 1.6:1.0 based on the 31P{1H} NMR integral ratio (Scheme 4). After rinsing with Et2O and hexane, 7 and 8 were isolated in 6% and 7% yields, respectively.12 In the 1H NMR spectrum of 7, a characteristic doublet of doublets signal due to benzylic protons was observed at δ 3.16 with the 31P coupling constants of 9 and 5 Hz, and two methyl protons for PMe3 ligands appeared at δ 0.72 (3JP−H = 10 Hz) and 1.01 (3JP−H = 7 Hz) as doublet signals. The 31P{1H} NMR spectrum of 7 displayed two nonequivalent doublet signals with 31P coupling constant of 35 Hz at δ −19.3 and −8.7. The molecular structure of 7 was supported by the X-ray diffraction, but we are not able to discuss any structural parameters owing to the low quality of crystal data.13

In relation to the formation mechanism for 4, we examined the reaction of 3 with [Pd(η-C3H5)(η-C5H5)] in the presence of an excess amount of PMe3 (20 eq.) in toluene for 60 h. As the result, the corresponding trimethyl-substituted oxyphosphorane 9 together with complexes 7 and 8 were mainly produced in the ratio of 0.2:1.0:0.1 judged by the 31P{1H} NMR (Scheme 5).14 In the 1H NMR of 9, the benzylic protons were exhibited as a doublet signal at δ 2.47 (3JP−H = 14 Hz), which is somewhat high-field shifted compared with that of the above mentioned 4. The three methyl protons in 9 were observed equivalently as a doublet signal at δ 0.97 (3JP-H = 11 Hz) due to Berry pseudorotation in the NMR time scale.10,15 The 31P{1H} NMR spectrum of 9 showed a broad singlet at δ −53.3. The formation of 9 suggests that the coordination of free phosphine to the palladium center of palladaphosphorane intermediates like 6 induces the reductive elimination of Pd(0) species to give oxyphosphoranes 4 and 9.

In summary, we have demonstrated the unique reactivity of 2-hydroxybenzyl phenyl sulfide 3 with Pd(0) complexes via unusual C(sp3)−S and O−H bonds cleavage giving the novel oxyphosphoranes 4 and 9 or 1,2-palladacycle 7. Further investigations on the bond cleavage of 3 are currently in progress using other transition metal complexes.

ACKNOWLEDGEMENTS
Part of this study was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” from the Ministry of Education, Culture, Science and Technology of Japan.

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7. A solution of 3 (90.2 mg, 0.28 mmol) in toluene (6 mL) was added to a solution of [Pd(PPh3)4] (501.0 mg, 0.43 mmol) in toluene (15 mL) at room temperature. The mixture was stirred for 36 h at room temperature, and then the solvent was removed under reduced pressure. The residue was dissolved in a mixed solvent of hexane and CH2Cl2, and insoluble PPh3=O and palladium black were removed by filtration. The filtrate was evaporated to dryness and the residue was washed with MeOH to give oxyphosphorane 4 (39.9 mg, 32%) as colorless crystals. 4: Mp 155−156 ˚C; 1H NMR (400 MHz, CDCl3) δ 0.98 (s, 9 H), 1.29 (s, 9 H), 3.75 (d, 2JP−H = 13 Hz, 2 H), 7.01 (s, 1 H), 7.09 (s, 1 H), 7.23−7.35 (m, 15 H); 13C{1H} NMR (101 MHz, CDCl3) δ 28.9 (CH3), 31.9 (CH3), 33.5 (d, 1JP−C = 96 Hz, CH2), 34.18 (C), 34.23 (C), 118.7 (C), 118.8 (d, 3JP−C = 19 Hz, CH), 121.6 (CH), 127.5 (d, 2JP−C = 17 Hz, CH), 128.5 (d, 4JP−C = 2 Hz, CH), 131.0 (d, 3JP−C = 9 Hz, CH), 132.7 (d, 2JP−C = 10 Hz, C), 132.3 (C), 137.5 (C), 154.2 (d, 2JP−C = 4 Hz, C−O); 31P{1H} NMR (202 MHz, CDCl3) δ −41.5. HRMS (ESI-TOF) Calcd for [C33H36OP + H]+: 481.26548. Found: 481.26565.
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12. 7: 1H NMR (300 MHz, C6D6) δ 0.70 (d, 2JP−H = 10 Hz, 9 H), 1.00 (d, 2JP−H = 7 Hz, 9 H), 1.58 (s, 9 H), 1.92 (s, 9 H), 3.17 (dd, 3JP−H = 9, 5 Hz, 2 H), 7.31 (s, 1 H), 7.46 (s, 1 H); 31P{1H} NMR (202 MHz, C6D6) δ −19.3 (d, 2JP−P = 35 Hz), −8.7 (d, 2JP−P = 35 Hz).
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