Observation of a Thermally Induced Bora-Nazarov Cyclization at a Phosphole Framework

The reaction of the bis(enynyl)phosphanes 6 a,b with the electrophilic borane reagents RB(C6F5)2 (R=C6F5, CH2CH2Ph, CH3) gave phospholes cleanly in a 1,1‐carboboration reaction sequence. Depending on the steric bulk, the resulting 2,5‐alkenylphospholes underwent a thermally induced bora‐Nazarov type cyclization. The equilibrium situation of these examples of a bora‐Nazarov type cyclization was investigated in detail by NMR spectroscopy, X‐ray crystal structure analysis, and DFT calculations. Ringing the changes: 2,5‐Alkenyl‐substituted phospholes bearing an adjacent electrophilic borane can undergo a thermally induced bora‐Nazarov type ring closure. Tipp=2,4,6‐triisopropylphenyl.