BH3-catalyzed oligomerization of ethyl diazoacetate: the role of C-boron enolates.

In contrast to trialkyl boranes, the reaction of borane (BH3) and ethyl diazoacetate (EDA) generates dimer, trimer, and oligomers of EDA. The products arise from double, triple, and multiple insertions of CHCO2Et groups in B-C bonds. On the basis of NMR spectroscopic data, trapping experiments, and computational studies, a novel C-boron enolate has been identified as a key intermediate in this reaction. This C-boron enolate species is calculated to be 7.1 kcal/mol (gas phase) more stable than its isomeric O-boron enolate form. Both spectroscopic data and trapping results also reveal the formation of a doubly borylated enolate generated as a side product by a proton transfer between the C- and O-boron monoenolates.