Novel Migrating Group in 1,2-Anionotropic Reactions:  Cobalt Complexation Facilitates 1,2-Shift of Alkynyl Groups

Recent advances in the semi-pinacol rearrangement of α-hydroxy epoxides and related compounds

“…Their findings show that cobalt complexation does indeed facilitate the 1,2-shift of alkynyl groups. 70 They were able to demonstrate this phenomenon by successfully rearranging 138 (R = n-Bu) in a clean 61% yield (55 uC, benzene, EtMgBr); the un-complexed analogue does not undergo the 1,2-shift under these conditions. 29 The reaction was also shown to proceed at lower temperatures (220 to 20 uC) in the presence of Me 3 Al.…”

“…In 1996 a communication by Suzuki and co-workers successfully demonstrated an improved procedure for enhancing the migratory aptitude of alkynyl groups by complexing the triple bond to cobalt, and demonstrated the ease with which it underwent a 1,2-shift, in contrast to the free alkynes which are known not to migrate readily. 70 The poor migratory aptitude of the alkynyl group is thought to derive from the higher activation energy associated with its migration compared to alkenyl (and other) group(s) migration. The large difference in activation energies seems to derive from the stability of the two-electron three-centred system formed when it migrates.…”

Group-Selective Approaches to Complex Natural Product Synthesis: Three Examples of Diastereotopos-Selective Reactions

“…However, a different idea came to us, focusing on bisalkynyl alcohol C by our incidental discovery of a diastereotopos selective reaction as shown in Scheme 6. 11 Hydroalumination 12 of bisalkynyl alcohol 2, derived from (S)-lactate, with LiAlH 4 stopped at the monoreaction stage. Importantly, one of the two diastereotopic 1-hexynyl groups was predominantly reacted, giving diol 3a after deprotection.…”

3.16 The Semipinacol Rearrangements

“…Suzuki and coworkers successfully developed a stereospecific semipinacol rearrangement of alkyne-Co complex substrates. 101 Among them, substrate 130 and 131 were subjected to this protocol to give a-quaternary b-hydroxy carbonyl compounds 132 and 133, respectively (Scheme 82).…”