C−H Activation in Phosphonium Salts Promoted by Platinum(II) Complexes

Abstract

The reaction of PtCl2(NCPh)2 with the bis-phosphonium salts {[R2PhPCH2]2C(O)}Cl2 (R2 = Ph2, PhEt, Et2) (1:1 molar ratio) in refluxing 2-methoxyethanol affords the C,C-orthometalated complexes [PtCl2(C6H4-2-PR2C(H)C(O)CH2PPhR2)] (R2 = Ph2 1a, PhEt 1b, Et2 1c). The reaction of PtCl2 with the bis-phosphonium salt {[Ph3PCH2]2C(O)}Cl2 (1:1 molar ratio) gives {[Ph3PCH2]2C(O)}[PtCl4], 2, while treatment of PtCl2 with the perchlorate salts {[R2PhPCH2]2C(O)}(ClO4)2 (1:1 molar ratio) results in the formation of the cationic dinuclear orthometalated derivatives [Pt(μ-Cl)(C6H4-2-PR2C(H)C(O)CH2PPhR2)]2(ClO4)2 (R2 = Ph2 3a, PhEt 3b, Et2 3c). The cycloplatination of the bis-phosphonium salts to give the C,C-orthometalated derivatives implies two C−H bond activation processes at two types of carbon atoms, one arylic and one alkylic. The reaction of PtCl2(NCPh)2 or PtCl2 with the allyl-phosphonium salts [PhR2PCH2CHCH2]Cl (R2 = Ph2, Me2) or [Ph3PCH2CHCHMe]Cl (1:1 molar ratio) in refluxing 2-methoxyethanol gives the orthometalated vinyl-phosphonium derivatives [PtCl2(C6H4-2-PR2-η2-E-C(H)C(H)CH3)] (R2 = Ph2 4a, Me2 4b) or [PtCl2(C6H4-2-PPh2-η2-E-C(H)C(H)CH2CH3)], 7, respectively, while the reaction of PtCl2 with [Ph3PCH2CHCH2]ClO4 (1:1 molar ratio) gives the η2-olefin-bonded derivative [Cl3Pt(η2-CH2CH−CH2PPh3)], 5. The reaction of [Ph3PCH2CHCHPh]Cl with PtCl2 (1:1 molar ratio, 2-methoxyethanol, reflux) gives an easily separable mixture of products: the η2-olefin-bonded [Cl3Pt(η2-CHPhCH−CH2PPh3)], 8, and the orthometalated [PtCl2(C6H4-2-PPh2-η2-E-C(H)C(H)CH2Ph)], 9, while the reaction of [Ph3PCH2CMeCH2]Cl with PtCl2 (1:1 molar ratio) gives a mixture of the cycloplatinated [PtCl2(C6H4-2-PPh2-η2-C(H)CMe2)], 10, and the isomerized vinyl-phosphonium salt [Ph3P−C(H)CMe2]Cl, 11. The synthesis of complexes 4, 7, 9, and 10 implies the activation of one C(aryl)−H bond (instead of the more active methylene group adjacent to the phosphonium unit P−CH2) and the rearrangement of the allyl group into a vinyl group by 1,3-prototropic shift, this shift being produced in the absence of external base. The complexes 4a·CH2Cl2 and 8 have been characterized by X-ray diffraction methods.