Synthesis, stereostructure, pyramidal inversion, and alkylation of 1-thioniabicyclo[4.4.0]decane salts

Eastman's sulfonium salt, 1-thioniabicyclo[4.4.0]decane tetrafluoroborate, has been prepared by an improved process and shown to be an equimolar mixture (molecular compound) of trans and cis steroisomers 1a and 1b, each of which has been isolated in pure form. The 6-methoxycarbonyl 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- = 1-). Crystals of 9 are monoclinic, space group P21/n, a = 8.890 (3) A, b = 20.120 (7) A, c = 8.150 (3) A, β = 110.17 (4)0, Z = 4, t = 200C, R = 0.042 for 1432 x-ray reflections with F2 > 2 sigma. Activation parameters for inversion of salt 1a are found to be ΔH+ = 28.0 +- 0.9 kcal mol-1 and ΔS+ = -3.8 +- 2.5 eu in the range 90.07 to 105.800C. Unstabilized sulfonium ylides were prepared and their interconversion by pyramidal inversion of sulfur has been investigated; activation parameters for the conversion were found to be ΔH+ = 20.5 kcal mol-1 and ΔS+ = 6 eu. Three of the four stereoisomeric 2-methyl-1-thioniabicyclo[4.4.0]decanes have been prepared and their sterostructures assigned by 13C NMR. Evidence is adduced that both ylides react with electrophiles in a stereoselective manner. Treatment of salt 1 with tert-butyllithium at -720C yields the corresponding ylide anions, which undergo stereoselective methylation to provide the 2,10-dimethyl-1-thioniabicyclo[4.4.0]decanes, stereostructures of which are assigned by 13C NMR. The ring fusion bond of 9 undergoes reductive cleavage, leading to an interesting synthesis of 6-methoxycarbonylthiacyclodecane