Ru- and Mo-Catalysed Metathesis

Alkene metathesis has been successfully employed to prepare P-stereogenic phosphine oxides and phosphine boranes. Although most of the work has 

It was demonstrated that the reaction proceeds without racemisation of the stereogenic phosphorus atom and with total selectivity towards the ( )-alke-nylphosphine oxide. Unexpectedly, it was also possible to dimerise phosphine oxide 109 (with R = Me) with 5% of 112b to obtain the corresponding optically pure ( )-diphosphine oxide in 85% yield. The crystal structure of this compound has been determined by X-ray diffraction and has been used as dipolarophile in 1,3-dipolar cycloadditions with nitrones, yielding several optically pure diphosphine oxides. Similar homometathesis reactions have been investigated in more detail by Grela, Pietrusiewicz, Butenschon and co-workers with other (racemic) substrates such as 109 and different catalysts. Gouverneur and co-workers studied a similar dimerisation of 

These desymmetrisations by CM are challenging because selectivity has to be controlled at several levels. There are many alkenes in the reaction mixture that can undergo undesired self-metathesis reactions. In addition, double metathesis yielding achiral products has to be minimised. Finally, there is the issue of EjZ selectivity. Despite these hurdles, several examples of F-stereogenic (but racemic) phosphine oxides 115 were obtained from prochiral phenyl divinylpho-sphine oxide (114) in 47 6% yield. A three-fold excess of 114 was used to minimise double metathesis. In all but one case none of the E isomer of 115 is formed. The vinyl group in 115 was further functionalised by CM with styrene. 

Remarkably, two successive CM reactions (R = (CH2)9CH3, R = Ph and vice versa) allowed the preparation of 118, whose vinyl group was subjected again to CM with 3-phenylpropene yielding a phosphine oxide with three different E-alkenyl groups. 

Starting trienes 124 fulfil two essential conditions the two diastereotopic alkenes at the chirotopic phosphorus atom do not react with each other and the metathesis catalyst attacks the double bond closest to the stereogenic carbon atom first, since the two identical alkenes are conjugated with the electron-withdrawing phosphinate. Yields of cyclic phosphinates 125 were typically above 60% and with de up to 86%. As expected, both the geometry and substitution at the double bonds are prominent factors in the selectivity of the reaction. 

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