Rhodium-Catalyzed Dehydrocoupling of the Sterically Encumbered Phosphine−Borane AdducttBu₂PH·BH₃:  Synthesis of the Linear DimerstBu₂PH−BH₂−tBu₂P−BH₃andtBu₂PH−BH₂−tBu₂P−BH₂Cl

Abstract

The dehydrocoupling of the sterically hindered phosphine-borane adduct tBu(2)PH.BH(3) above 140 degrees C is catalyzed by the rhodium complexes [Rh(1,5-cod)(2)][OTf] or Rh(6)(CO)(16) to give the four-membered chain tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), which was isolated in 60% yield and characterized by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. Thermolysis of 1 in the temperature range 175-180 degrees C led to partial decomposition and the formation of tBu(2)PH.BH(3). When the dehydrocoupling of tBu(2)PH.BH(3) was performed in the presence of [[Rh(mu-Cl)(1,5-cod)](2)] or RhCl(3) hydrate, the chlorinated compound tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) was formed which could not be obtained free of 1. The molecular structures of tBu(2)PH.BH(3), tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), and tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) together with 1 were determined by single-crystal X-ray diffraction studies.

Rhodium‐Catalyzed Dehydrocoupling of Fluorinated Phosphine–Borane Adducts: Synthesis, Characterization, and Properties of Cyclic and Polymeric Phosphinoboranes with Electron‐Withdrawing Substituents at Phosphorus

“…Characterization of 1 and 2 was achieved by 1 H, 31 P, 13 C, 19 F and 11 B NMR spectroscopy and mass spectrometry, which afforded spectra completely consistent with the assigned structures. For example, the 31 P NMR spectrum of 1 displayed a doublet ( 1 J PH = 386 Hz) due to coupling to one PH proton while 2 exhibited a triplet ( 1 J PH = 370 Hz) due to the presence of the PH 2 moiety when the 1 H coupled spectrum was recorded.…”

“…The 11 B NMR spectrum of 5 showed a single broad resonance at d À34.3, which is consistent with a four-coordinate boron center attached to two phosphorus atoms. [26] The 19 F NMR resonance for 5 at d À62.5 ppm differs slightly from that of monomer 2 (d À63.7 ppm). As previously reported for other polyphosphinoboranes, [15,16] polymer 5 is also air-and moisture-stable in the solid state.…”

“…[18] The electronic nature of the substituents at phosphorus in phosphine-borane adducts might be expected to significantly influence both the rate of dehydrocoupling and the polymer properties. The observation that the catalytic dehydrocoupling of PhPH 2 ·BH 3 and Ph 2 PH·BH 3 is more facile compared with that of iBuPH 2 ·BH 3 and iBu 2 PH·BH 3 [16] as well as tBu 2 PH·BH 3 [19] is likely associated with differences in the polarity of the PÀH bonds. The strong (+)-inductive effect of the alkyl group attached to phosphorus is expected to decrease the polarity of the P-H functionality, which in turn, would be anticipated to render the bond less reactive for catalytic dehydrocoupling with B d + ÀH dÀ bonds.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…The alkyl-substituted secondary phosphine-borane H 3 B · P t Bu 2 H could be dehydrocoupled by similar rhodium-based precatalysts under melt conditions at elevated temperatures. Full conversion of the phosphine-borane was not achieved for any of the catalysts although the major product formed was the diboraphosphine, H 3 B · P for the previous systems [154]. Another report from Manners and co-workers focussed on the formation of polymers from the dehydrocoupling of primary phosphine-boranes; aryl-substituted phosphine-boranes H 3 B · P( pn Bu-C 6 H 4 )H 2 and H 3 B · P( p-(C 12 H 25 )-C 6 H 4 )H 2 were polymerised by [Rh(1,5-cod)(μ-Cl)] 2 under melt conditions [155].…”

A Study of the Reactivity of Secondary Phosphanes with Radical Sources: A New Dehydrocoupling Reaction

“…The stoichiometric reduction using alkali metals is commonly used, although PÀC bond cleavage of aryl phosphanes has been observed [2]. The catalytic dehydrocoupling reaction to form R 2 PÀPR 2 , RHPÀPHR, or cyclo-(PR) n (n ¼ 5, 6) is gaining in prominence using a Ti- [3], Zr- [4], and Rh-based [5] catalysis, whilst Manners and co-workers have extensively published [6] on the use of Rh catalysis for the hetero-dehydrocoupling of phosphane -borane adducts to form PÀB containing rings, chains, and polymers from phenyl [7], t Bu [8], and fluorous phosphanes [9], along with mechanistic studies [10]. The use of B(C 6 F 5 ) 3 as a catalyst has also been reported to dehydrocouple H 3 P · BH 3 and PhPH 2 · BH 3 with low catalyst loadings [11].…”