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Effect of the phosphine steric and electronic profile on the Rh-promoted dehydrocoupling of phosphine-boranes

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Published copy (DOI)

Author(s)

  • Thomas N. Hooper
  • Miguel A. Huertos
  • Titel Jurca
  • Sebastian D. Pike
  • Andrew S. Weller
  • Ian Manners

Department/unit(s)

Publication details

JournalInorganic Chemistry
DatePublished - 7 Apr 2014
Issue number7
Volume53
Number of pages14
Pages (from-to)3716-3729
Original languageEnglish

Abstract

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine-boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF 3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound linear diboraphosphines H3B·PR2BH 2·PR2H and H3B·PRHBH 2·PRH2, respectively, are reported. Reaction of [Rh(L)(η6-FC6H5)][BArF 4] [L = Ph2P(CH2)3PPh2, ArF = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords [Rh(L)(H)(σ, η-PR2BH3)(η1-H3B· PR2H)][BArF4]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ, η2-PR2·BH2PR2· BH3)][BArF4]. With electron-withdrawing groups on the phosphine-borane there is the parallel formation of the products of B-P cleavage, [Rh(L)(PR2H)2][BArF4], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B·P(Ad)2H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine-borane complex is formed, [Rh(L){ η2-H3B·P(Ad) 2H}][BArF4], that undergoes B-P bond cleavage to give [Rh(L){ η1-H3B·P(Ad) 2H}{P(Ad)2H}][BArF4]. The relative rates of dehydrocoupling of H3B·PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P-B bond cleavage. H3B·PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P-H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.

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