NEW PATHWAYS TO PRECURSORS OF PENTALENE

Pentalene dimers 2 and 3 are easily available in moderate yields by Cu Cl2-induced oxidative coupling of dilithium-pentalenediide (5) (Scheme 1). On the other hand, NBS bromination of 1,5-dihydropentalene (4) or of 1,2-dihydropentalene (8) gives unstable 1-bromo-1,2-dihydropentale ne (9), while subsequent in-situ elimination with Et3N exclusively giv es syn-cis-pentalene dimer 2 in moderale yields (Scheme 3). NMR-Spectr oscopic evidence for compounds 2, 3, and 9 is presented, and mechanist ic alternatives for the formation of pentalene dimers 2 and 3 are disc ussed.