Reactivity of cyclooligophosphanes: synthesis and structural characterisation of cyclo-1,4-(BH3)2(P4Ph4CH2) and cyclo-1,2-(BH3)2(P5Ph5)

Abstract

The borane complexes cyclo-1,4-(BH₃)₂(P₄Ph₄CH₂) (3) and cyclo-1,2-(BH₃)₂(P₅Ph₅) (4) were prepared by reaction of cyclo-(P₄Ph₄CH₂) and cyclo-(P₅Ph₅) with BH₃(SMe₂). Only the 2 : 1 complexes 3 and 4 were isolated, even when an excess of the borane source was used. In solution, 3 exists as a mixture of the two diastereomers (R_P*,S_P*,S_P*,R_P*)-(±)-3 and (R_P*,R_P*,R_P*,R_P*)-(±)-3. However, in the solid state the (R_P*,S_P*,S_P*,R_P*)-(±) diastereomer is the major stereoisomer. Similarly, while only one isomer of 4 is observed in its X-ray structure, NMR spectroscopic investigations reveal that it forms a complex mixture of isomers in solution. 3 may be deprotonated with ^tBuLi to give the lithium salt cyclo-1,4-(BH₃)₂(P₄Ph₄CHLi) (3·Li), though this could not be isolated in pure form.