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

Article abstract of DOI:10.1021/acs.joc.5b00593

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.