Solvolysis of 1-pent-3-ynyl triflate. Mechanism of the homopropargyl rearrangement

Article abstract of DOI:10.1021/ja00399a036

1-Pent-3-ynyl triflate (1b) was solvolyzed (25°C, 24 h) in ethanol-water with 2,6-lutidine as the buffer. Products were formed predominantly (97-98.5%) through direct substitution (k_s processes) and elimination. As the water content increases, the yields of 2-methylcyclobutanone (7b) (formed through k_Δ processes) also increase from 0.2% (100% ethanol) to 2.8% (50% ethanol). 1-Pent-3-ynyl triflate was solvolyzed in anhydrous trifluoroethanol (25°C, 24 h) in nine different experiments with nine different buffers. Sodium and calcium carbonate, 2,6-lutidine, pyridine, and quinoline all favored k_Δ processes (88-65%), whereas potassium carbonate, triethylamine, and sodium trifluoroethoxide suppressed the formation of rearranged products. The products of the solvolyses include: 2-methylcyclobutenyl trifluoroethyl ether (5b); cyclopropyl methyl ketone (6b); 2-methylcyclobutanone (7b); pent-1-en-3-yne (13); 1-pent-3-ynol (15); 2-methylcyclobutanone bis(trifluoroethyl) acetal (16), and 1-pent-3-ynyl trifluoroethyl ether (17). In 80% trifluoroethanol-20% water (sodium carbonate buffer) the yields of rearranged products (k_Δ) dropped to 46% - as expected for an increase in nucleophilicity of the solvent. A quantitative correlation exists between percent rearrangement of 1b and nucleophilicity of the solvent, as demonstrated by use of the Winstein-Grunwald-Swain equation. 1-Pent-3-ynyl triflate was synthesized with carbon-14 in the 1 position (1b-1-¹⁴C) and, separately, in the 3 position (1b-3-¹⁴C). 1-Pent-3-ynyl triflate was also prepared doubly labeled both with carbon-14 and with deuterium (1b-3-¹⁴C-1,1-d₂). Isotope effects were determined for all isotope position isomers. These are: k/*k = 1.048 ± 0.003 (1b-1-¹⁴C); k/*k = 0.990 ± 0.005 (1b-3-¹⁴C); and ^Hk/^Dk = 1.098 ± 0.004 (1b-1,1-d₂). In addition, 1b-1,1-d₂ on solvolysis in trifluoroethanol-sodium carbonate yields 2-methylcyclobutanone (7b) containing equal fractions of 7b-3-d₂ and 7b-4-d₂. The tracer and isotope effect experiments confirm the mechanistic conclusions arrived at through product distribution studies and, in addition, offer strong evidence for significant anchimeric assistance during the k_Δ processes investigated.