Transition Metal-Catalyzed Formation of Phosphorus−Boron Bonds:  A New Route to Phosphinoborane Rings, Chains, and Macromolecules

Abstract

A novel catalytic dehydrocoupling route for the synthesis of linear, cyclic, and polymeric phosphinoboranes has been developed. The dehydrocoupling of neat Ph 2 PH‚BH 3 , which is otherwise very slow below 170 °C, is catalyzed by [{Rh(µ-Cl)(1,5-cod)} 2 ] or [Rh(1,5-cod) 2 ][OTf] (0.5-1 mol % Rh) to give the linear compound Ph 2 PH-BH 2 -PPh 2 -BH 3 (1) at 90 °C, and a mixture of the cyclic trimer [Ph 2 P-BH 2 ] 3 (2a) and tetramer [Ph 2 P-BH 2 ] 4 (2b) at 120 °C. In addition, the catalytic potential of other (e.g., Ti, Ru, Rh, Ir, Pd, Pt) complexes toward the dehydrocoupling of Ph 2 PH‚BH 3 was investigated and was in many cases demonstrated. The molecular structures of 1 and 2b, and of the primary phosphine-borane adduct PhPH 2 ‚ BH 3 , were determined by single-crystal X-ray analysis. The dehydrogenative coupling of PhPH 2 ‚BH 3 gave low-molecular-weight poly(phenylphosphinoborane) [PhPH-BH 2 ] n (3) when performed in toluene (110 °C) with ca. 0.5 mol % [Rh(1,5-cod) 2 ][OTf] as catalyst. The absolute weight-average molecular weight was determined by static light scattering (SLS) in THF which showed that M w ) 5600. Samples of high-molecularweight polymer 3 (M w ) 31 000 or 33 300 by SLS) were synthesized using neat conditions at 90-130 °C in the presence of [{Rh(µ-Cl)(1,5-cod)} 2 ], anhydrous RhCl 3 , or RhCl 3 hydrate (ca. 1 mol % Rh). Poly-(phosphinoborane) 3 was thereby obtained in ca. 75% yield as an air-stable, off-white solid and was structurally characterized by 1 H, 11 B, 13 C, and 31 P NMR and IR spectroscopy and elemental analysis. The hydrodynamic diameters for polymers 3 in THF were also determined by dynamic light scattering (DLS). Catalytic dehydrocoupling of the alkyl-substituted phosphine-borane adduct iBuPH 2 ‚BH 3 was also investigated and was found to be much slower than that of PhPH 2 ‚BH 3 . This produced poly(isobutylphosphinoborane) [iBuPH-BH 2 ] n (4) under neat conditions at 120 °C in the presence of [{Rh(µ-Cl)(1,5-cod)} 2 ] in 80% yield. Multinuclear NMR spectroscopy and DLS were also used to characterize polymer 4, and the latter indicated that M w ) ca. 10 000-20 000. Prolonged heating of polymers 3 and 4 at elevated temperatures in the presence of catalyst led to insoluble but solvent-swellable gels possibly due to light interchain cross-linking through P-B bonds. In the absence of rhodium catalyst thermally induced dehydrocoupling of PhPH 2 ‚BH 3 and iBuPH 2 ‚BH 3 proceeds very slowly and forms only low-molecular-weight materials with complex NMR spectra and which probably possess a branched structure.

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

“…[15,16] The synthesis of these polymers was most effectively achieved using rhodium (pre)catalysts [{Rh(m-Cl) ( B NMR spectroscopy that in the absence of Rh catalyst, the reactions are very slow and yield poorly defined oligomeric materials that appear to have branched structures. [15] To investigate whether aryl-substituted monomers bearing electronegative groups are indeed more reactive than those previously studied, we synthesized and investigated the para-substituted phosphine-borane adducts (p-CF 3 C 6 H 4 ) 2 PH·BH 3 1 and (p-CF 3 C 6 H 4 )PH 2 ·BH 3 2. The secondary phosphine-borane adduct 1 was isolated as a colorless oil in 72 % overall yield in four steps from PCl 3 by using a protection-deprotection sequence (Scheme 1).…”

“…This has resulted in the development of new synthetic routes to phosphineborane rings, chains and macromolecules. [14][15][16][17] In addition, the catalytic dehydrocoupling approach has been extended to amine-borane adducts to yield cyclic products with BÀN skeletons. [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.…”

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

“…[66] [70] Sie schlagen einen Reaktionsweg vor (Schema 44), der jenem der asymmetrischen Rutheniumkomplex-katalysierten "Noyori-Hydrierung" [71] prochiraler Ketone ähnelt. [78] 7.…”

Polymer science with transition metals and main group elements: Towards functional, supramolecular inorganic polymeric materials

“…62 Recently, we have shown that the dehydrocoupling process is effectively catalyzed by late transition-metal complexes (especially Rh species), which has permitted the formation of high molecular weight polyphosphinoboranes 23 (M w Ͼ 30,000; Scheme 8). 63 The properties of these interesting, recently prepared polymers are now beginning to be explored. Polyphenylphosphinoborane, for example, is a white, air-stable powder and is formally an analogue of PS with a phosphorus-boron backbone.…”

“…Weak BOH•••HOP interactions have been detected in the solid-state structure of monomers such as PhPH 2 •BH 3 with X-ray crystallography. 63 Such interactions might be productively used to promote self-assembly processes in solution or the solid state. 64 Also, improved dehydropolymerization catalysts for POB bond formation are highly desirable to further raise molecular weights and hopefully allow more controlled polymerizations.…”