Regioisomeric and Substituent Effects upon the Outcome of the Reaction of 1‑Borodienes with Nitrosoarene Compounds

A study
of the reactivity of 1-borodienes with nitrosoarene compounds
has been carried out showing an outcome that differs according to
the hybridization state of the boron moiety. Using an sp2 boron substituent, a one-pot hetero-Diels–Alder/ring contraction
cascade occurred to afford N-arylpyrroles with low
to good yields depending on the electronic properties of the substituents
on the borodiene, whereas an sp3 boron substituent led
to the formation of stable boro-oxazines with high regioselectivity
in most of the cases, in moderate to good yields. 1H and 11B NMR studies on two boro-oxazine regioisomers showed that
selective deprotection can be performed. Formation of either the pyrrole
or the furan derivative is pH- and regioisomer-structure-dependent.
The results obtained, together with previous B3LYP calculations, support
mechanistic proposals which suggest that pyrrole, or furan, formation
proceeds via oxazine formation, followed by a boryl rearrangement
and an intramolecular addition–elimination sequence.