Syn-cis-(2) and anti-cis-(3) pentalene dimers are easily available by CuCl2-induced oxidative coupling of dilithium-pentalenediide (5). On the other hand, NBS-bromination as well of 1,5-dihydro-pentalene (4) as of 1,2-dihydropentalene (6) gives unstable 1-bromo-1,2-dihydropentalene (7), while subsequent in situ elimination with Et3N exclusively leads to syn-cis-pentalene dimer 2 in moderate yields.
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You et al. (1996) studied this question.
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