Supersonic expansions of organic molecules in helium carrier gas mixtures are used
to synthesize model (pre)reactive complexes at low temperature. Whether or not barriers
for hydrogen bond rearrangements can be overcome in this collisional process is not
well understood. Using the example of alcohols inserting into intramolecular hydrogen
bonds of α-hydroxy esters, we explore whether dispersion energy donors can assist
the process in a systematic way. Bromo, iodo, and tert-butyl substitution of benzyl alcohol in the para-position is used to show that the insertion process into methyl glycolate is controllable,
whereas it is largely avoided for the chiral methyl lactate homologue. Methyl lactate
appears to steer the transient chirality of benzyl alcohol derivatives in a uniform
direction relative to the lactate handedness for the OH∙∙∙O=C insertion product, as
well as for the competing attachment to the hydroxy group of the ester. A simple rule
based on the total binding energy in relation to the rearrangement barrier is tentatively
proposed to estimate whether the insertion is feasible or not in such molecular complexes
during expansion.
Key words
spectroscopy - chirality induction - gas phase reaction - hydrogen bond topology -
halogenation - α-hydroxy esters
