Protiodesilylation of 3-trimethylsilyl-2-thiabicyclo[2.2.1]hept-5-enes and derivatives: a stereochemical study (original) (raw)

endo-and exo-trimethylsilyl-3-phenyl-2-thiabicyclo[2,2.1 ]hept-5-enes, the endotrimethylsilyl-exo-S-oxide and their S,S-dioxide derivatives were protiodesilylated with fluoride ion. Stereoconvergent reactions were found, affording the endo-phenyl derivatives. These results can be rationalised with the hypothesis of a common carbanionic intermediate. In contrast, the desilylation of 3-endo-trimethylsilyl-3-exo-phenyl-2-thiabicyclo[2.2.1]heptane gave a mixture of endo-and exoisomers, thus proving the involvement of the double bond in the control of the reaction stereoselectivity. Introduction. Silyl thioketones are a class of compounds characterised by a particularly high reactivity of the carbon-sulfur double bond in either nucleophilic or electrophilic additions as well as in cycloaddition reactions. In this way a variety of compounds containing the Si-C-S unit can be synthesised I. Subsequent protiodesilylation of these compounds gives products formally arising from thioaldehydes 2. During the last decade there has been a large growth in the use of fluoride-promoted desilylation as a methodology for the generation under very mild conditions of species endowed with carbanionic reactivity 3. The nature of the intermediates and the stereochemistry of the process although investigated2b,2c,2e,2f, 4,5,6, is still under discussion. The high affinity of the fluoride ion towards the silicon atom is the driving force of the reaction, giving rise to a pentacoordinated silicon anion. The substitution of this hypervalent silicon by an electrophile can then proceed either via a concerted reaction or via the formation of a free carbanion, depending on the nature of the silicon-bonded groups. In this context, the stereochemical course of the desilylation in rigid cyclic systems could shed light on the mechanism. In this paper we report an investigation on the stereochemical outcome and the nature of the intermediates involved in the fluoride-promoted protiodesilylation of a series of diastereomeric 3-trimethylsilyl-2-thiabicyclo[2.2.1]hept-5-enes. In these compounds the sulfur atom a to silicon can be oxidised to sulfoxide or sulfone, thus changing its electronwithdrawing (carbanionic stabilising) power. Results and Discussion. In a previous work 2a, we reported that the reaction of trimethylsilyl phenyl thioketone with cyclopentadiene at 0°C, gave only the endo-silyl derivative la. The reaction performed at higher temperature (25°C) revealed, at a more accurate 1H NMR investigation (200 MHz) on the crude, a 8:1 ratio of 3-endo la and 3-exo-trimethylsilylphenyl-2-thiabicyclo[2.2.1]hept-5-enes lb (Scheme 1). The two products were separated by crystallisation and chromatography (see experimental). The oxidation of la and lb to the corresponding sulfones 2a and 2b was easily performed with oxone (KHSO5). The oxidation of la with m-chloroperbenzoic acid at-50°C led to the exo sulfoxide 32a (Scheme l). For this reason we did not