Ring expansion of platinacyclobutanes: The scope and mechanism of the reaction

Article abstract of DOI:10.1021/om00138a005

New platinacyclobutylcarbinyl esters of formula Cl₂L₂PtCH₂CR¹(CHR²OR ³)CH₂] (R¹ = H, Me, or Ph; R² = H or Me; R³ = methanesulfonyl or 4-nitrobenzoyl; L = pyridine or L₂ = 2,2′-bipyridine) have been prepared and characterized. On solvolysis in aqueous acetone these complexes give the ring-expanded platinacyclopentanol products [Cl₂L₂PtCHR²CR¹(OH)CH ₂CH₂], which have been isolated and characterized. The ring expansions occur with greater selectivity than in the corresponding solvolyses of cyclobutylcarbinyl esters, but the scope of the reactions is limited by the low thermal stability of the platinacyclobutane precursors. It was not possible to catalyze the ring expansion of platinacyclobutylcarbinols to the corresponding platinacyclopentanols. The solvolyses of [Cl₂py₂PtCH₂CH(CH₂OMs)CH ₂] (3a, OMs = mesylate), [Cl₂(bpy)PtCH₂CH(CH₂OMs)CH₂] (3b), and [Cl₂py₂PtCH₂CMe(CH₂OMs)CH ₂] (3c) in acetone-water (60% v/v) at 36°C follow good first-order kinetics and give the products [Cl₂L₂PtC¹H₂C²R(OH)C ³H₂C⁴H₂]. The solvolysis of 3a is retarded by the presence of free pyridine, and the limiting rate in the presence of excess pyridine is almost the same as for 3b. The solvolysis of 3c is affected only slightly by the presence of free pyridine. Solvolysis of analogous complexes labeled at the CH₂O center with ¹³C (3a* and 3b*) or ²H (3a**, 3b**, 3c**) gave the products with the label at C¹ or C³. In the absence of pyridine, 3a** and 3c** gave the corresponding products labeled at C¹, 86% and 10%, respectively, and at C³, 14% and 90%, respectively, while in the presence of pyridine they gave the label at C¹, 32% and 0%, and at C³, 68% and 100%, respectively. Complex 3b** gave the label at C¹, 27%, and C³, 73%, as determined by integration of ²H{¹H} NMR spectra. The data are rationalized in terms of a mechanism in which skeletal isomerization of the fragment PtCH₂CR(CD₂OMs)CH₂ (6), to give [PtCH₂CH₂CR(CD₂OMs)] (7), may occur in the absence of free pyridine for 3a** and 3c** but not in its presence and cannot occur for 3b**. Solvolysis of 6 gives exclusively (3c**) or largely (3a**, 3b**) the product labeled at C³ whereas solvolysis of 7 gives exclusively the product labeled at C¹. Similarities with and differences between these reactions and the analogous organic ring expansions are discussed.