The reaction of the bis(enynyl)phosphanes 6 a, b with the electrophilic borane reagents RB(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. [ABSTRACT FROM AUTHOR]