Diphosphine boranes DiPAMP

With this strategy, tert-butylmethylphenylphosphine borane and PAMP-BH3 were obtained, although the latter presented low optical purity (41% ee) due to racemisation in the decarboxylation step, which had been performed in xylene at 130 °C for 1 h. The menthyl ester intermediate 60, for R = o-An, was also reduced to the alcohol, which was mesylated and iodinated affording the (I-iodophosphine borane 62 in 62% combined yield. This compound was reductively dimerised by activated copper to diphosphine borane 63 (64% yield), which is similar to DiPAMP but with a butyl bridge. 

Some of the methylphosphine boranes presented in the previous tables have been used to prepare several diphosphine boranes with an ethyl bridge, i.e. analogues of DiPAMP via Cu(II) promoted dimerisation (Scheme 2.23 and Table 2.14). 

All the phosphine boranes in Table 2.14 were obtained as optically pure compounds after removal of the small quantity of meso compound, if any. Compounds of entries 1-4 bear an ort/zo-substituted phenyl group and were prepared in order to compare their performance in Rh-catalysed hydrogenation with DiPAMP and to shed light into the role of the methoxy group in the catalysis. The menthyldiphosphinite borane of entry 7 is interesting, because it has been used to prepare other diphosphine boranes (Table 2.11, entry 11, and Table 2.12, entry 19). 

From a-lithiated carbanions 72, formylated (73), carboxylated (74), silylated (75) and aminated (76) P-stereogenic phosphine boranes have been prepared without loss of optical purity. Some of them can be further functionalised to obtain other interesting compounds. Furthermore, oxidative coupling with Cu(II) salts affords C2 diphosphine boranes of the DiPAMP family (77) and... 

The Juge-Stephan methodology has been used to prepare many Cj diphosphine boranes bearing at least one aryl group at each phosphorus atom. In particular, it is probably the best method to prepare not only DiPAMP itself, but also a large family of analogues with dilferent substituents at the phosphorus atoms and/or different bridges, which have found application in many catalytic processes. 

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