Tetraphosphine boranes

Imamoto, Gridnev and co-workers" reacted the secondary diphosphine borane 71 (Section 5.2.1.3) with tosylate 85 (Section 5.2.1.1) yielding the expected tetraphosphine borane 86 in 65% yield (Scheme 5.36). 

Deprotonation of t-Bu-BisP (BH3)2 with 1.2 equivalents of s-BuLi/( )-sparteine formed a mixture of mono- and dilithiated t-Bu-BisP (BH3)2 whose - intermolecular oxidative coupling with Cu(II) afforded a mixture of tetraphosphine borane 86 and hexaphosphine borane 87. The use of ( )-sparteine may surprise the reader, as the reaction does not involve any enantioselective deprotonation. However, it was found that its presence was beneficial to activate the -BuLi, showing that ( )-sp is not only a chiral inductor, but also modifies the reactivity of the organolithium reagents. After separation by column chromatography, 86 and 87 were obtained in 26% and 14% isolated yields respectively. Tetraphosphine borane 86 was subjected to similar reactions, affording compounds 88 (29% yield) and 90 (8% yield), which contain 8... 

Oligophosphine boranes t-Bu-BisP, 86, 87 and 90 have been also prepared in optically inactive form, i.e. synthesised without the presence of sparteine. Comparison of these diastereomeric mixtures with the corresponding optically active oligophosphine boranes revealed that the latter are more crystalhne. For example, the optically inactive tetraphosphine borane exhibits DSC thermograms and XRD profiles similar to the optically pure hexaphosphine borane 87. 

The same method, applied to the tetraphosphine borane 86, led to the 12-phosphacrown-4 skeleton (Scheme 5.40). 

---