Diols Grob fragmentations

If we now consider a 1,3-diol (3), accepting a similar mechanism to the abovementioned for 1,2-diols, in acid conditions the fragmentation of the molecule will take place according to a process known as "Grob fragmentation" [2] to give water as the "nucleofuge", an alkene (4) and a carbonyl compound (5) as the... 

There are three potential options for the mechanism of Grob fragmentation reactions, one concerted and two stepwise. The stepwise reactions, which result from initial loss of either the nucleofuge or electrofuge, are less useful in synthesis since they often promote side reactions. However, one highly efficient stepwise transformation was reported by Kato. Treatment of diol 6 with pyridinium chloride results in formation of benzyl carbocation 8. Cleavage of this alcohol yields monocycle 9 and, ultimately, the ketone product 10. ... 

Another variation involves metal-catalyzed Grob fragmentations. Tamam reported nickel and palladium catalysts for the fragmentation of 1,3-diols and 1,3-diol derivatives. Early research by the Tamaru group... 

The bis-trimethylsilyl ethers of stereoisomeric 5,7-undecanediol 78 (as well as the free diols) give upon chemical ionization (protonation with QH9) MH+ ions, which show two major processes (reaction 41). One corresponds to a Grob-type fragmentation 05-cleavage) of the ion 79 (formed by loss of Me3SiOH from protonated 78) to yield the silylated aldehyde 80. The second major reaction path involves loss of CH4 from MH+, followed by cyclization of the intermediate 81 (siloxy transfer) to 82 and ft-cleavage (loss of 1-hexene) to generate 8467. This reaction sequence, in which a centrally located structural unit rather than a peripheral one is eliminated, is rarely observed under chemical ionization and deserves further study. 

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