Cross-dehydrocoupling: A Novel Synthetic Route to P–B–P–B Chains

Transition-metal-catalyzed dehydrocoupling of tert-butylferrocenylphosphine–borane (2) with [{Rh­(μ-Cl)­(1,5-cod)}₂] (cod = cyclooctadiene) as the catalyst gave the homocoupled product [Fc­(tBu)­(H)­P­(BH₂)­P­(Fc)­(tBu)­(BH₂X)] [3; Fc = Fe­(C₅H₅)­(C₅H₄), X = H/Cl], while cross-dehydrocoupling with the tertiary phosphine–boranes P­(tBu)­(nBu)₂(BH₃) (2a) and PPh­(nBu)₂(BH₃) (2b) using [Rh­(1,5-cod)₂]­OTf (OTf = trifluoromethanesulfonate) gave the first cross-dehydrocoupled products reported to date, [Fc­(tBu)­(BH₃)­P­(BH₂)­P­(tBu)­(nBu)₂] (4) and [Fc­(tBu)­(BH₃)­P­(BH₂)­PPh­(nBu)₂] (5), in moderate yields. Compounds 2–5 were characterized by NMR spectroscopy (¹H, ¹³C, ³¹P, and ¹¹B), IR spectroscopy, mass spectrometry, and single-crystal X-ray structure determination.