Stereochemistry Grob fragmentation, mechanisms

Since the mid 1950s Grob has investigated these fragmentations in detail, and provided very much experimental material for the scope, mechanism and stereochemistry of this reaction type, summarized in some reviews.Therefore, the term Grob fragmentation is justified. 

Mechanism and Stereochemistry of Heterolytic Fragmentation C. A. Grob, Angew. Chem., Int. Ed. Engl., 1969, 8, 535-546. 

The evidence for such a mechanism results from both the reaction stereochemistry and also from the observation of minor diphenyltoluene by-products (Scheme 8). The major pathway is outlined using heavy arrows. This can be seen to afford the stereospecificity of equations 14a and 14b. Additionally, each of the diradical intermediates and intermediate excited states—B, C, E, F and G—undergo a minor extent of internal bond fission [i.e. Grob or 2,3- (1,4) fragmentation] to afford a diphenyltoluene with the corresponding ring skeleton. The basis for the choice of a main pathway versus the minor ones comes from the observed stereochemistry. 

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