Achiral and Racemic Silaketals

Hoye and Promo examined the effect of ring size and alkene substitution on /Z-selectivity and rate of reaction (Table 8.2) [15]. This study also employed the diphenylsilane as the linker for the construction of symmetrical his-alkoxysilanes. 

The optimal reaction conditions involved the slow addition of catalyst [Ru]-I via a syringe pump at room temperature to avoid isomerization of the alkene, which led to truncated ring systems after RCM. Although the reaction favors the formation of the Z-isomer, the selectivity does diminish with increasing ring size (Table 8.2, entries 1-3), whereas substituted aUcenes result in lower efficiency and selectivity (entry 1 vs 4). Additional studies examined the effect of a racemic secondary and achiral tertiary allylic alcohol in the TST-RCM process. Interestingly, the former affords an equal mixture of meso- and c ,I-silaketals, indicating no obvious diastereoselectivity for this particular tether (entry 5), whereas the latter proved completely unreactive (entry 6). 

---