SYNTHESIS, STEREOSTRUCTURE, PYRAMIDAL INVERSION, AND ALKYLATION OF 1-THIONIABICYCLO[4.4.0]DECANE SALTS

Eastman's sulfonium salt, 1-thioniabicyclo[4.4.0]decane tetrafluoroboratc, has been prepared by an improved process and shown to be an equimolar mixture (molecular compound) of “trans” and “cis” stereoisomers la and lb, each of which has been isolated in pure form. The 6-mcthoxycarbonyl analogue 9 has also been prepared, Stereostructures have been assigned by a combination of 13C NMR, 360-MHz 1H NMR, and single-crystal X-ray analysis of salt 9 (X- = I-). Crystals of 9 are monoclinic, space group P21/n, a = 8.890 (3) Å, b = 20.120 (7) Å, c = 8.150 (3) Å, ʲ = 110.17 (4) °, Z = 4, t = 20 °C, R = 0.042 for 1432 X-ray reflections with F2 > σ. Although stereoisomer la is favored over lb by 0.64 kcal mol-1 at I 10°C, the 6-methoxycarbonyl analogue 9 prefers the “cis” configuration by more than 2.2 kcal mol-1 at this temperature. The unexpectedly large effect of the angular substituent is shown to be the result of the small C-S-C bond angles, which cause an angular substituent to have a larger effective size in this system than in decalin. Force-field calculations are presented to support this view. Activation parameters for inversion of salt la are found to be ΔH± = 28.0 ± 0.9 kcal mol-1 and ΔS± = -3.8 ± 2.5 eu in the range 90.07-105.80 °C. Unstabilized sulfonium ylides 15 and 16 have been prepared and their interconversion by pyramidal inversion of sulfur has been investigated; activation parameters for conversion of 15 to 16 are found to be ΔH± = 20.5 kcal mol-1 and ΔS± = 6 eu. The “cis” ylide 16 is found to be much more stable than the “trans” form 15, and the effect has been explained in terms of interactions of the vicinal lone pairs on sulfur and carbon. © 1979, American Chemical Society. All rights reserved.