The Importance of Conformation in the Ozonolysis of Acetals

Abstract

Six gauche conformations are theoretically possible for an acetal function and only three of them are reactive toward ozone. Each oxygen of the acetal function must have an electron pair orbital oreitned antiperiplanar to the C—H bond in order to be reactive toward ozone.

Whither enzyme mechanisms?

“…The fact that a very small movement of the iron atom out of the plane of the heme in hemoglobin [23] is coupled with much more major changes in protein structure should be a warning to bioorganic chemists wielding their curly arrows [24]. The stereo-electronic restrictions on reaction pathways as exemplified by Dunitz's view of the attack on a carbonyl group [25] or Deslongchamps' theory of how a tetrahedral intermediate collapses [26] will surely prove crucial to our understanding not only of the pathways of catalysis but also of the rates of the steps involved. It may be that, to discern such stereoelectronic restrictions, we shall need structural detail at very high resolution.…”

An isokinetic relationship in the oxidation of acetals by ozone. Evidence for rotation before the oxidation of acyclic acetals

“…Previous studies had shown (2,3,9) that electron donors accelerate the reaction. However, one consequence of hkving a reaction series in a domain of temperature where reactivity is dominated by entropy factors, is to reverse the sign of p (1 3).…”

“…This relationship is called the isokinetic relationship (or the compensation effect) and may be written in the form of eq. [2] where p is the isokinetic temperature and 12, a constant. When such a relationship exists for a particular reaction series, P represents a point of inversion of reactivity., When P is higher than the experimental temperature range, then this represents a domain of temperatures where reactivity depends essentially on AH*.…”

“…Proportionality between AH* and AS* as determined by plotting eq. [2] however, is not a criterion to prove that an isokinetic relationship holds. Exner (14) and others (15) have criticized the procedure because AH* and AS* are determined from the same equation and are therefore necessarily interdependent.…”

“…The rates at which different acetals are oxidized, however, vary considerably and this led to the proposal of a stereoelectronic theory for the oxidation of acetals by ozone (2). Basically, it was proposed that in order for oxidation to proceed, it is required that one nonbonded electron pair on each oxygen atom of the acetal function lies antiperiplanar to the C-H bond of the acetal function (Scheme I).…”