Abstract
1,3-Diols, which are a frequent motif in biologically active molecules, can be prepared
from readily available allylic alcohols via formal anti-Markovnikov hydration. The
commonly employed hydroboration–oxidation sequence for the synthesis of terminal alcohols
is challenging for allylic alcohols, and O-protection of the alcohol can be necessary.
To increase atom economy, we explored the use of silane protecting groups that can
be engaged in intramolecular hydrosilylation. Oxidative cleavage of the cyclized product
yields the desired 1,3-diol and obviates the need for super-stoichiometric borane
reagents. Based on a detailed study of O-silylation conditions, a protocol is presented
that furnishes quantitative yields of a wide range of O-silylated alcohols which contain
Si–H bonds for further functionalization. We show that a MOF-based Rh(II) porphyrin
can furnish efficient intramolecular hydrosilylation, while the corresponding homogeneous
analogue proved unreactive. Radical trapping studies suggest that silyl radicals constitute
key intermediates in Rh(II)-catalyzed intramolecular hydrosilylation. Preferential
5-endo-trig versus 6-exo-trig cyclization and 5-exo-trig versus 6-endo-trig cyclization of the silyl radical intermediates led to chemoselective 1,3-diol formation
for substrates containing multiple olefins.
Key words
rhodium(II) metalloradical - 1,3-diol synthesis - silyl radical - radical cyclization
- MOF catalysis