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

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

doi:10.1021/ic500032f

Thomas N Hooper, Andrew S Weller + Show 4 more

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https://doi.org/10.1021/ic500032f

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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.

Highlights

The development of efficient catalytic methods for the formation of bonds between main group elements is of considerable interest for the continued development of main group chemistry
Bonds, and development in the area has been spurred on by the potential for ammonia−borane to act as a hydrogen carrying vector.[9−11] In addition, polymeric materials that can arise from dehydropolymerization of primary analogues are of significant interest as they are valence isoelectronic with technologically ubiquitous polyolefins
Decomposition routes in Rh-systems for phosphine−borane dehydrocoupling to form bis(phosphine)boronium salts have recently been discussed.[36]. In this Article, we report an extension of our investigations into the mechanism of phosphine−borane dehydrocoupling using the {Rh(Ph2P(CH2)3PPh2)}+ fragment, by varying the electronic and steric profile of the secondary phosphine− boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p(OMe)C6H4; adamantyl], as well as investigations with the Inorganic Chemistry primary phosphine−borane H3B·PCyH2

Summary

Introduction

The development of efficient catalytic methods for the formation of bonds between main group elements is of considerable interest for the continued development of main group chemistry. Catalytic dehydrocoupling[5,7,8] of amine− and phosphine−boranes is one method that has emerged for the formation of B−N and B−P bonds, and development in the area has been spurred on by the potential for ammonia−borane to act as a hydrogen carrying vector.[9−11] In addition, polymeric materials that can arise from dehydropolymerization of primary analogues are of significant interest as they are valence isoelectronic with technologically ubiquitous polyolefins. Proposed Catalytic Cycle for the Dehydrocoupling of H3B·PR2H To Give H3B·PR2BH2·PR2Ha

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