Rh-Catalysed Desymmetrisation of Dialkynylphosphine Oxides

The reactions discussed in the last sections produce enantioemiched P-stereo-genic compounds by metal-catalysed enantioselective resolutions of chiral but racemic secondary phosphines and derivatives. Another set of methods is based on desymmetrisations of achiral (but prochiral) tertiary phosphine derivatives bearing two enantiotopic groups amenable to functionahsation by chiral transition metal catalysts. 

In this reaction, both triple bonds in 1,6-diynes 100 and one of the prochiral alkynes in the phosphine oxides 101 react enantioselectively in a Rh-catalysed [2- -2-h2] cycloaddition, generating an aromatic ring attached to the newly 

Previous studies showed that biaryl phosphines are suitable ligands for this reaction dtbm-segphos provided the best results under the reaction 

Diyne 103 and tetrayne 104 reacted as expected, giving diphosphine dioxide 105 as a mixture of diastereomers (C2 Awe5o = 60 40 71% ee) in quantitative yield. A single recrystallisation furnished the C2 compound in 33% yield with 97% ee. 

The spatial arrangement in 106 results from the steric interaction between the alkynyl group and the bulky PAr2 groups of (P)-dtbm-segphos. Insertion of this alkynyl into the rhodium metallacycle and subsequent reductive elimination gives (.S)-107. Although mentioned as a possibility, no reduction of the phosphine oxides to the parent phosphines was reported. 

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