Mechanistic aspects of the cyclometalation of quinoline-8-carboxaldehyde with platinum(II)

Article abstract of DOI:10.1021/om00156a041

Rates of reaction at 61°C in CDCl₃ for the cyclometalation of quinoline-8-carboxaldehyde (1) with trans-PtCl₂(1)L (L = PEt₃, PBuⁿ₃, PPh₃, PTol₃, AsEt₃, AsPrⁱ₃, AsTol₃) are reported. Starting from trans-PtCl₂(1)(PEt₃), there is a kinetic isotope effect k_H/k_D = 1.4. Modification of the Pt(II) complex to trans-PtBr₂(1)L enhances the rate by a factor of ca. 70. The use of sym-trans-[Pt(μ-Cl)(SnCl₃)(PTol₃)]₂ and 2 equiv of 1 affords facile cyclometalation at room temperature within the mixing time. The direct generation of cationic complexes via cis-PtCl₂L₂ plus Ag(CF₃SO₃) followed by addition of 1 is also shown to lead to facile cyclometalation. All of these reactions are discussed in terms of the importance of cationic metal compounds. Synthetically, the most efficient cyclometalation of 1 can be accomplished by using [Pt(μ-Cl)(η³-CH₂C(CH₃)CH ₂)]₂. ¹H and ³¹P NMR data are given for selected examples.