## Results and Discussion
### Synthesis of Liquid-Phase Phosphine Reagent (2)
#### Poly(ethylene glycol) as a Soluble Polymer Matrix for the Stille Cross Coupling Reaction  
The Stille [39,40] cross-coupling reaction possesses wide scope and functional group tolerance. In addition, the requisite tin reagents are easily prepared and relatively stable when compared to most organometallic reagents. These properties make this particular process very attractive as a route to many carbon-carbon bond forming reactions under relatively ambient conditions. The importance of this reaction within the sphere of polymer-supported chemistry has been demonstrated with the recent emergence of solid- [41-43] and fluorous-phase [44,45] approaches. The defining feature of the Stille coupling is the use of a trialkyltin reagent in a palladium-catalyzed coupling with either a halide or triflate. As with all tin chemistry, the benefits associated with these reagents are to some extent outweighed by the problems associated with their removal after reaction completion. The tri-\(n\)-butyl derivatives are more difficult to remove from a reaction mixture than the methyl congeners due to their low volatility and poor water solubility. Therefore there is an increasing need for methodologies which facilitate the separation of the side-product tributyltin derivatives from the required adducts following the Stille reaction. Our aim was to study the potential of poly(ethylene glycol) as a soluble matrix for the electrophile component of the reaction. The tributyltin derivative and 'other' components of the reaction being in solution could, theoretically, be separated from the polymer-supported products by a simple precipitation of the support into a suitable solvent and recovery by filtration. MeO-PEG\({}_{5000}\) was initially esterified with either _para-_ or _ortho_-iodobenzoic acid using DCC and DMAP to give _para-_ and _ortho_-polymer-supported iodides, **15a-b** respectively, with polymer recoveries being \(>95\%\)[46]. The conversion was quantitative based upon \({}^{1}\)H NMR spectroscopic analysis of **15a-b**. \begin{table}
\begin{tabular}{c c c c} \hline
**Entry** & **Reagent** & **Conditions** & **P(V) to P(III)\({}^{a}\)/\%** \\ \hline
1 & PMHS\({}^{b}\), CH\({}_{2}\)Cl\({}_{2}\) & CH\({}_{2}\)Cl\({}_{2}\)/ r.t. & 20 \\
2 & PMHS & Benzene/ reflux 20 h & 33 \\
3 & PMHS & THF/ reflux 20 h & 40 \\
4 & HSiCl\({}_{3}\)\({}^{c}\) & Benzene/ reflux 20 h & 56 \\
5 & AlH\({}_{3}\bullet\)THF & THF/ r.t. 30 min & 100 \\ \hline \end{tabular} \({}^{a}\)Measured by \({}^{31}\)P NMR of the reaction mixtures. \({}^{b}\)Polymethylhydrosiloxane. \({}^{b}\)With triethylamine.  
\end{table} 
Table 3: Regeneration of PEG-supported triarylphosphine **2**. A number of efficient reaction conditions for solution-phase Stille couplings have been developed [39,40,47,48] and initially we utilized a number of these for the reaction between PEG-iodide **15a** and tributyl phenyltin (**16**) to explore the viability of PEG as a soluble polymer support for this reaction and then to identify the optimal conditions necessary for the PEG-supported Stille variant (Scheme 3 and Table 4). Previous work has revealed conflicting evidence regarding the potential for coordination of the PEG polyether backbone to transition metals [49, 50, 51]. Initially therefore, we had serious concerns that the PEG-backbone may serve to either retard or completely inhibit the Stille reaction _via_ complexation of either the palladium catalyst, the alkali metal halide or the organostannane reagent.