Grignard reagents react with epoxides but dissolve in THF, what happens in oxetane (four membered ether ring)?
In a 1979 paper entitled Copper-catalysed reactions of Grignard reagents with epoxides and oxetane, Huynh, Derguini-Boumechal, and Linstrumelle report that the copper-catalysed ring opening of oxetane by various Grignard reagents gives the alcohols "in acceptable yields":



$$ \begin{array}{cccc}
\ce{R} &amp; \ce{X} &amp; \text{Solvent} &amp; \text{Yield (%)} \\
\hline
n\text{-}\ce{C4H9} &amp; \ce{Cl} &amp; \text{Ether} &amp; 75 \\
\ce{C6H5} &amp; \ce{Br} &amp; \text{THF} &amp; 52 \\
\ce{CH2=CH-CH2} &amp; \ce{Br} &amp; \text{Ether} &amp; 50 \\
\ce{(CH3)2C=CH-CH2} &amp; \ce{Cl} &amp; \text{THF} &amp; 50
\end{array} $$