Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 20: Reactions of Samarium(II) Iodide with Carbohydrate Derivatives

Chapter 20 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 20 button below. The drawing underneath the button shows the minimum energy needed for electron transfer from SmI2 to a carbohydrate sulfone. Beside the drawing is a description of the reaction. Below the drawing and its description is a summary of Chapter 20.

Chapter 20: Reactions of Samarium(II) Iodide with Carbohydrate Derivatives

This drawing shows the effect of HMPA on th HOMOP evergy os samarium(II) iodide.

Drawing Description

When HMPA complexes with SmI2, it raises the HOMO energy level in the resulting complex and, in so doing, reduces the energy required for electron transfer to the σ* orbital (LUMO) of the sulfone This difference in energy requirement is shown in the drawing on the left, which is Figure 1 in Chapter 20. The energy reduction that takes place translates into a larger rate constant for reaction when HMPA is present.

Summary of Chapter 20

Transfer of an electron from samarium(II) iodide to a carbohydrate with an electron-accept­ing substituent produces a radical anion. Dissociation of this radical anion generates a carbohydrate radical along with an anion derived from the substituent group. A carbohydrate radical formed in this way reacts rapidly with a second molecule of SmI2 to produce an organosamarium compound. This organometallic compound can undergo reactions that include addition to a compound containing a carbonyl group, proton capture, and elimination of a samarium-containing leaving group. In order for a radical formed from reaction of a carbohydrate derivative with SmI2 to avoid immediate reaction with a second molecule of SmI2, a rapid, radical process must inter­vene. The one of greatest interest is radical cyclization; thus, a carbohydrate derivative that has a properly placed multiple bond and an electron-accepting substituent reacts with samarium(II) iodide to a form radical that cyclizes. Carbohydrates that contain a pair of properly positioned aldehydo or keto groups cyclize to form pinacols. The intermediate in the cyclization step leading to a pinacol is a radical anion. Organochromium complexes form and react in a manner similar to organosamarium compounds.