Syntheses, structures, and reactions of cyrhetrenylphosphines; applications in palladium catalyzed Suzuki cross-coupling reactions

Reaction of bromocyrhetrene (η5-C5H4Br)Re(CO)3 with lithium tetramethylpiperidide (LiTMP; 1.0 equiv, −78 °C) and then ClPR2 (R = a, Ph; b, Cy) gives (η5-1,2-C5H3BrPR2)Re(CO)3 (5a, 48%; 5b, 80%). Analogous reactions with 2.0 equiv of LiTMP and ClPR2 give the 1-bromo-2,5-diphosphidocyclopentadienyl complexes [η5-1,2,5-C5H2Br(PR2)2]Re(CO)3 (6a, 60%; 6b, 80%). These rhenium containing or cyrhetrenylphosphines are combined with Pd(OAc)2 (2:1 mol ratio) to give catalysts for Suzuki couplings of phenylboronic acid and p-bromotoluene or p-bromoacetophenone (1 mol%; Cs2CO3, toluene, 100 °C). The catalyst with 5b exhibits much higher activities than the others, with conversions of 92–64% after 20 min. The crystal structures of 5b and 6a,b are determined.