## Results and Discussion
### Synthesis of Liquid-Phase Phosphine Reagent (2)
#### Poly(ethylene glycol) as a Soluble Polymer Matrix for the Stille Cross Coupling Reaction
In addition, the homocoupled biaryl side-product **20**, an incontrovertible product in the solution-phase reaction [39, 40], does not contaminate the soluble-polymer supported biaryl product **17a**, because the homocoupling side-reaction takes place in solution and thus the contaminant is removed during the precipitation step. Having optimized the liquid-phase Stille reaction conditions on monomethoxy-PEG\({}_{5000}\), we then sought to examine the scope of this cross coupling process in a parallel format. A range of tributyl stannanes, **16** and **21**-**28**, were reacted with the PEG-supported iodides **15a**-**b** in a parallel fashion, furnishing the library of biaryl, heterobiaryl and styryl derivatives **18a**-**b** and **28a**-**b** to **34a**-**b** (Table 5). The isolated yields of all library members were good to excellent (69 - 99%) showing that under our pre-optimized conditions _vide supra_, that the scope of the PEG-supported variant of the Stille reaction is quite broad. Following passage down a short pad of silica gel, the library members (**18a**-**b**, **28a**-**b** to **34a**-**b**) were each isolated in \(>\) 95% purity. \begin{table}
\begin{tabular}{c c c c} \hline Entry & Bu\({}_{3}\)Sn-R & Products\({}^{b}\) & Yield/\%\({}^{c}\) \\  & & **a** & **b** \\ \hline
1 & & 98 & 82 \\
2 & & 99 & 78 \\
3 & & 21 & 28a-b \\
3 & & 23 & 30a-b \\
4 & & 96 & 78 \\
5 & & 90 & 69 \\
7 & & 98 & 90 \\
8 & & 73 & 99 \\
99 & & 82 \\ \hline \end{tabular} \({}^{\rm a}\)Reactions were conducted under the conditions described _vide supra_.  
\({}^{\rm b}\)Compound descriptors **a** and **b** represent the _para_- and _ortho_-isomers respectively. All the products were characterized by \({}^{\rm 1}\)H, \({}^{\rm 13}\)H NMR spectroscopy and HRFABMS and gave satisfactory spectroscopic data when compared to authentic samples or to literature data. See the supporting information for more detail.  
\({}^{\rm c}\)Isolated yields after the two steps.  
\({}^{\rm d}\)Yields of _ortho_-product **30b** was not determined because instability of the stannanes **23** precluded its efficient synthesis.  
\end{table} 
Table 5: Parallel liquid-phase Stille couplings of iodides **15a**-**b** with tributyl stannanes **16** and **21**-**27\({}^{\rm a}\)**. In each case (Table 5, entries 1-8) the yield of the _ortho_-isomer was lower than its corresponding _para_-congener. We rationalize that this phenomenon may be a result, at least in part, of the steric effect imparted by the PEG-backbone polymer chain during ligand transfer. However this effect followed no definitive trend related to the size of the moiety being transferred. suggests that the process involved may be more complex than simple steric retardation. While, in this report, no attempt has been made to re-optimize the reaction conditions to improve the yield of the _ortho_-isomers, it is speculated that future studies directed to resolve this issue would involve simply increasing the number of equivalents of the stannane and/or palladium catalyst, whilst perhaps using more dilute reaction conditions.