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

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.

本文报道了二级与一级膦硼烷（phosphine–borane）H3B·PR2H [R = 3,5-双(三氟甲基)苯基、对三氟甲基苯基、对甲氧基苯基、金刚烷基(Ad)] 以及H3B·PCyH2的化学计量脱氢偶联反应中的电子效应与空间效应，两类反应的产物分别为金属键合的直链二硼磷烷H3B·PR2BH2·PR2H与H3B·PRHBH2·PRH2。
将[Rh(L)(η⁶-FC6H5)][BArF4] [L = Ph2P(CH2)3PPh2，ArF = 3,5-(CF3)2C6H3] 与2当量的H3B·PR2H反应，可得到配合物[Rh(L)(H)(σ,η-PR2BH3)(η¹-H3B·PR2H)][BArF4]。此类配合物可发生脱氢偶联反应，生成二硼磷烷配合物[Rh(L)(H)(σ,η²-PR2·BH2PR2·BH3)][BArF4]。
当膦硼烷底物连有吸电子基团时，会同时生成B-P键断裂的副产物[Rh(L)(PR2H)2][BArF4]；而当底物连有给电子基团时，则无此类副产物生成。
对于位阻庞大且富电子的H3B·P(Ad)2H，未观察到脱氢偶联反应，仅形成了Rh(I)的σ型膦硼烷中间体配合物[Rh(L){η²-H3B·P(Ad)2H}][BArF4]，该中间体可发生B-P键断裂，生成[Rh(L){η¹-H3B·P(Ad)2H}{P(Ad)2H}][BArF4]。
芳基取代的H3B·PR2H的脱氢偶联相对速率显示，取代基的吸电子能力越强，脱氢偶联速率越快，但同时也会伴随P-B键断裂副产物的生成。
H3B·PCyH2可发生类似的脱氢偶联过程，其产物为金属键合的二硼磷烷立体异构体混合物，该混合物源于前手性P-H键的活化，其中一种立体异构体占主导。通过使用手性膦配体，还可对该非对映异构体混合物的组成进行调控。
本文还简要讨论了所得催化脱氢偶联反应的选择性与效率。