II. Structural Formulas

The structural formula for a radical often, but not always, can be deduced from a combination of different types of information. This infor­mation includes the structure of the radi­cal precursor, the method of radical for­ma­tion, and the identity of the reaction products. For example, in the reaction shown in Scheme l the structure of the deoxyiodo sugar 1, the known reac­tivity of alkyl iodides with the tri-n-butyltin radical, and the structure of the product 3 together provide enough information to assign the basic structural formula 2 to the inter­mediate radical.¹ At this point, the config­ur­ation at the radical center in 2 and the conformation of this radical remain to be determined. The way in which radical configuration and confor­mation are assigned is discussed in Sections III and IV, respectively, in this chapter.

The same type of information that effectively establishes the structure of the radical 2 (Scheme 1) is insufficient for determining the structures of the radicals produced by hydrogen-atom abstraction from simple sugars. Due to the large number of hydrogen atoms present in even simple sugars, know­ing the structure of the starting material has limited value in establishing the identity of any particular intermediate radical. Product structures also are of limited usefulness due to the large number of compounds generated by hydrogen-atom abstraction (at least twenty-five from D-glu­cose²), and the probability that molec­ular rear­range­ment has occurred during formation of some of these products.^2,3 Pro­posing struc­tures for the radicals generated by hydrogen-atom abstraction from even a simple sugar, such as D‑glucose, can involve a good deal of spec­u­lation, but such spec­u­la­tion can be reduced by using electron spin reson­ance (ESR) to observe radicals directly.

It is possible to identify six, first-formed radicals in the ESR spectrum of the mixture pro­duced by reaction of α-D-gluco­pyranose (4) with the hydroxyl radical (eq 1).³ These six radicals are the ones generated by hydro­gen-atom abstraction from of the six carbon atoms present in 4. (Hydro­gen-atom abstraction from the oxygen atoms is too slow to be com­pet­itive.) Identification of first-formed radicals is possible because when the structure of the radi­cal precursor is combined with information from its ESR spectrum, the combination provides a basis for assigning a struc­tural form­ula to each radical.

Whenever a radical reaction is encountered for the first time, the struc­ture of any intermedi­ate radical is naturally a topic of primary interest. Once the basic structure of a radical has been es­ta­blished, the unknowns that usually remain are the configuration at a radical center and confor­ma­tion of the radical. Establishing this configuration and determining radical confor­mation often involve both experimental findings and molecular-orbital calcu­lations.