Effect of the Phosphine Steric and Electronic Profile on the Rh-Promoted Dehydrocoupling of Phosphine–Boranes

Abstract

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine–boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound linear diboraphosphines H3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively, are reported. Reaction of [Rh(L)(η6-FC6H5)][BArF4] [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.

Umpolung at Boron: Ancillary‐Ligand‐Induced Formation of Boron‐Based Donor Ligands from Phosphine‐Boranes

“…The determined molecular structure in the solid state confirms the structural assignment (Figure ). The Fe−H1 B1 , B−H1 B1 and B−P bond lengths are in the same range as those in related complexes …”

“…The FeÀH1 B1 ,B ÀH1 B1 andB ÀPb ond lengths are in the same range as those in related complexes. [3,7,13,14,16,[29][30][31][32][33][34][35][36] Isocyanides such as tBuNC are neutrala nd more bulky than the other ligands, but they are also good s-donorl igands. The reaction of I with tBuNC resulted in the formation of a1:1 mixture of two different products according to the 31 P{ 1 H} NMR spectrum of the reaction mixture (Scheme 2).…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…This provides evidence for the mechanism proposed in Scheme 69 in which the Rh I σ-bound linear diboraphosphine complex is in equilibrium with the P-H-activated Rh III octahedral complex as such a process would allow interconversion of the Rh III diastereomers. Such a diastereomeric bias may afford some control of polymer tacticity in dehydropolymerisation reactions [175]. Interestingly, the use of a chiral chelating phosphine ligand on rhodium resulted in a further bias towards one diastereoisomer, but the absolute configuration was not determined.…”

“…When electron-withdrawing substituents are present on phosphorus, the P-H bond more readily undergoes P-H oxidative addition, making Rh III species favoured [174]. In a follow-up report, secondary phosphine-boranes bearing fluorinated substituents H 3 B · P( p-F 3 C-C 6 H 4 ) 2 H and H 3 B · P(m-(F 3 C) 2 C 6 H 3 ) 2 H were found to dehydrocouple at a faster rate than H 3 B · PPh 2 H using the same [Rh(dppp)] + system [175]. However, stoichiometric reactions showed that the weakening of the P-B bond by the presence of the electron-withdrawing groups caused P-B bond cleavage and hence catalyst deactivation by the formation of [Rh(dppp)(PR 2 [176].…”

Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh(xantphos)]+

“…TBE in 1,2-F 2 C 6 H 4 solvent, by addition of the alkene to in situ generated 6, Scheme 9. After 45 minutes a new peak is apparent in the 1 40,42,43 After a further 12 hours all of 6 was consumed, but a greater proportion of 8 (ca. 33%) was also present.…”