Intramolecular Reaction - Molecular Tethers

In a niche concept called molecular tethers, otherwise-intermolecular reactions can be made temporarily intramolecular by anchoring both reactions by a tether with all the advantages associated to it. Popular choices of tether contain a carbonate ester, boronic ester, silyl ether, or a silyl acetal link (silicon tethers) which are fairly inert in many organic reactions yet can be cleaved by specific reagents. The main hurdle for this strategy to work is selecting the proper length for the tether and making sure reactive groups have an optimal orientation with respect to each other. An examples is a Pauson-Khand reaction of an alkene and an alkyne tethered together via a silyl ether

In this particular reaction, the tether angle bringing the reactive groups together is effectively reduced by placing isopropyl groups on the silicon atom via the Thorpe-Ingold effect. No reaction takes place when these bulky groups are replaced by smaller methyl groups.

Another example is a photochemical cycloaddition with two alkene groups tethered through a silicon acetal group (racemic, the other enantiomer not depicted), which is subsequently cleaved by TBAF yielding the endo-diol.

Without the tether, the exo isomer forms.